Corrosion Documents

Galvanic Corrosion

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Abstract: Galvanic processes occur between different metals and between different areas of the same metal in the water environment. Water is an electrolyte, a poorly conductive one at the low dissolved solids content of fresh waters, and a highly conductive one at the high dissolved solids content of sea water. When two different metals are immersed in an electrolyte and connected through a metallic path, current will flow. Oxidation occurs at the anode and reduction (normally oxygen reduction) occurs at the cathode.

Galvanic processes occur between different metals and between different areas of the same metal in the water environment. Water is an electrolyte, a poorly conductive one at the low dissolved solids content of fresh waters, and a highly conductive one at the high dissolved solids content of sea water.

When two different metals are immersed in an electrolyte and connected through a metallic path, current will flow. Oxidation occurs at the anode and reduction (normally oxygen reduction) occurs at the cathode. These reactions and the hydrogen reduction reaction that occurs in deaerated waters are represented in the usual form below.

Oxidation (corrosion) M → M+ + 2e Reduction (deaerated waters) O2 (dissolved) + 2H2O + 4e → 4(OH-) Reduction (deaerated waters) 2H+ + 2e → 2e

The electrons flow through the metal path from the anode to the cathode. The circuit is completed by transport (migration) of the ionic species (OH) from the vicinity of the cathode to the vicinity of the anode. In the absence of other species, the rate at which these reactions occur, and consequently the rate at which the anode corrodes, is controlled by the rate at which oxygen can be reduced at the cathode.

The rate of reduction of oxygen at the cathode in turn is determined primarily by the resistance to electron flow in the circuit, the cathodic surface area available for oxygen reduction and the amount of oxygen available at the cathodic area. The galvanic current (corrosion) is directly proportional to the cathodic area when the cell is under cathodic control as it normally is in water.

Conductivity plays a major role by limiting galvanic corrosion to the immediate area of contact in low conductivity fresh water and by spreading the galvanic effect over rather large areas in highly conductive waters such as seawater.

Painting the anode requires all of the anodic corrosion to occur in the very small areas where coating breakdown at scratches, welds, etc. occurs and exposes the steel. Painting the cathode reduces the area available for the rate-controlling oxygen reduction reaction

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and the amount of oxidation (corrosion) that can occur at the anode. Stainless steels are positioned towards the cathodic end of the galvanic series in seawater.

The potential range shown for each alloy should be interpreted as the range within which the metal to sea water potential is likely to vary for each alloy, not as an indication that alloys close to each other in the series are likely to change position. This rarely happens. The second more anodic potential band for stainless steels should be interpreted as the potential that develops in a shielded area where crevice corrosion has initiated.

Lee and Tuthill have developed quantitative guidelines for the amount of carbon steel or Ni-Resist required suppressing crevice corrosion of types 304 and 316 stainless steels in seawater.

These data indicate that carbon steel is very effective in suppressing crevice corrosion of 304 and 316 stainless steel in up to 100:1 SS to CS area ratios at 14°C (57°F) in seawater. At 28°C there is complete protection at 10:1 and a tenfold reduction in the percentage of sites where initiation occurs at 50:1 SS: CS area ratios. Ni-Resist (NR) is found to provide full protection for 316 and substantial protection for 304 at 50:1 SS: NR area ratios at temperatures up to 28°C.

The position of the copper alloys in the galvanic series suggests that copper alloys will not suppress crevice corrosion in stainless steels and, in fact, may accelerate crevice attack once it has started. Experience indicates copper alloys provide no useful galvanic protection for stainless steels.

Type 304 is the least noble of the nickel stainless steels and alloy 825 the most noble, being separated by about 0.05 volt. The various nickel stainless steels are generally coupled mechanically to each other and to nickel-base alloys without serious galvanic effects. There are two major qualifications:

Should type 316L inadvertently be welded with type 308L filler metal instead of 316L, the weld metal will suffer severe localized corrosion. Hard facing overlays for rotating seal faces and weld overlay of tail shafts are other applications where close attention must be given to the position of individual alloys with respect to each other in the galvanic series in order to avoid costly failures.

Both type 303 and 303Se suffer extraordinarily severe corrosion in seawater. The high density of manganese sulfide or selenide inclusion in these free-machining alloys creates a surface with numerous built in austenite-to-inclusion galvanic cells.

Carbon is 0.2-0.3 volts or more positive than the nickel stainless steels. Carbon in the form of graphite; containing gaskets, packing and lubricants has been responsible for serious galvanic corrosion of stainless steels in seawater. Graphite in any form should never be used in contact with stainless steels in brackish or seawater.

Carbon filled rubber O-rings and gaskets are widely used in contact with stainless steels in seawater. The corrosion that occurs under O-rings and black rubber gaskets is normally crevice corrosion. However, in some instances acids used for chemical cleaning have

softened these carbon-filled rubbers sufficiently to release carbon and set up adverse galvanic cell action, greatly accelerating the crevice attack that occurs in these rubber-to-stainless crevices.

Galvanic corrosion is a two way street and the effect on the other material coupled to stainless steels must always be considered. Investigating why copper alloy tube sheets were being so severely corroded when copper alloy condensers were re-tubed with stainless steel and titanium tubes, Gehring, Kuester and Maurer found that the whole inside surface area of the tube of the more noble alloys became effective as a cathode in copper alloy to stainless steel or titanium couples.

The more noble materials are so easily polarized that the cathodic area available for the reduction reaction (the rate controlling process) is multiplied far beyond the old two or four diameters rule of thumb, which was based on copper alloy behavior. Later work by Gehring and Kyle indicated that the intensity of the galvanic effect decreased with salinity. The increasing resistance of the lower salinity waters limits the effective cathodic area.

Galvanic corrosion can occur between different constituents of the same metal as well as between different metals. Iron embedded in the surface of stainless steel, manganese sulfide stringers and less highly alloyed weld metal are common examples.

Summary of galvanic corrosion of nickel stainless steels Galvanic corrosion occurs between weld metal and base metal, between

different areas of the same metal and between different metals in water. The intensity of galvanic corrosion is determined by the conductivity, oxygen

content and the effective anode-to-cathode area ratio. Galvanic effects are spread over a large area in brackish and seawater; are

confined to the immediate area of the junction in fresh water; and are often negligible in deaerated brines.

Steel, Ni-Resist, zinc and aluminum are very effective in suppressing crevice corrosion on stainless steels except types 303 and 303Se.

Carbon, graphite-lubricated gaskets, packing, greases etc. are very effective in initiating severe corrosion of stainless steels.

Galvanic effects can be significantly reduced by removing coatings from the anode and by coating the more noble (cathodic) material.

Stainless steel (or titanium) tubing increases copper alloy tube sheet attack to the point where impressed current cathodic protection is normally required to control tube sheet corrosion.

Galvanic corrosion between different grades of nickel stainless steels mechanically joined is rare, but can be severe when welded. Caution and exposure tests are suggested.

Avoid use of types 303 and 303Se.

Crevices and Corrosion

A crevice is a narrow gap between a piece of metal and another piece of metal or tightly adhering material like plastic or a film of bacterial growth.

Many metals and alloys are susceptible to crevice corrosion, but in stainless steel, crevices are the first and most common place for corrosive attack to begin. With a little understanding, crevice corrosion can either be avoided or minimized.

Crevices can be:

The space under a washer or bolt head. The gap between plates bolted together. The gap between components intermittently welded. The space under a sticky label. The space between a gasket and the metal in a flange (especially if the

gasket is absorbent). Any other tight gap.

Crevices can be designed into the structure; they can be created during fabrication or can occur during service.

Prevention measures should therefore also aim at design, fabrication and service.

Why crevices can corrode

To work at its best, stainless needs free access to oxygen. Crevices are wide enough to permit entry of moisture, but narrow enough to prevent free circulation.

The result is that the oxygen in the moisture is used up. In addition, if chlorides are present they will concentrate in the stagnant conditions and, by a combination of reactions, the moisture can become acidic.

These are all conditions that can lead to the breakdown of the passive film on the stainless. Attack can then progress rapidly.

Crevices can create conditions much more aggressive than on adjacent surfaces. Having crevices builds in weak spots where attack can begin and begin in much less severe conditions than anticipated for the remainder of the structure.

Table one shows laboratory measurements of critical temperatures needed to cause pitting on an open surface (CPT) and crevice (CCT) attack of a metal plate beneath a PTFE washer in a 10% ferric chloride solution.

The CCT is at least 20 degrees C lower than the temperature to cause pitting corrosion in this aggressive liquid. (Ferric chloride solution is an aggressive co rodent and is used because it is similar to the liquid in a pit when it is actively corroding.)

Factors influencing crevices:

Crevice Shape

The geometry of the crevice will influence its susceptibility to attack and the speed of progress. The narrower and deeper (relative to its width) a crevice is the worse attack will be.

Metal to flexible plastic crevices tend to be narrower than rigid metal to metal gaps so metal to plastic joints provide more aggressive crevices.

Environment

The more aggressive the liquid outside the crevice, the more likely it is that the crevice will be attacked.

This is why crevice attack can be a problem in a salty swimming pool but not in a fresh water tank.

In the atmosphere, crevices beside the sea give more problems than in rural environments. If the liquid outside the crevice is very oxidizing, e.g. with bleach, hydrogen peroxide or ozone, then crevice attack will tend to be more severe.

Temperature

Once the CCT is exceeded, then as with pitting corrosion, higher temperatures mean corrosion is more rapid. The rule of thumb is that a 10 degrees C rise in temperature wills double the corrosion rate.

This means that when comparing Far North Queensland to Tasmania, not only are crevices more likely to start corroding but also that once they do, they will corrode faster because the temperature is consistently higher.

Alloy Resistance

Using a more corrosion resistant alloy gives less crevice attack. For example, in seawater at ambient temperature, crevices will form on 304 if there is a 0.9mm gap, on 316 if there is a 0.4mm gap and on 904L (similar corrosion resistance to 2205) if there is a 0.15mm gap.

Minimizing the risk of crevice corrosion

Good design, fabrication and operating practices will anticipate and hence minimize crevice corrosion.

Design

Design to minimize the occurrence of crevices. If a crevice is a necessary part of a component's design - can it be made wider?

Full penetration butt welds are best for joints. Seal lap joints and avoid gaps between pipes and fittings.

Minimize use of bolted connections and other fasteners. Where crevices can't be avoided use steel grade resistant to crevice corrosion in the operating environment. It is also possible to seal the crevices to keep out corrosive liquids, but care must be taken that the seal is permanent.

Be careful that the sealant 'wets' the surface. If it doesn't it may form its own crevice. Sealants that dry and shrink can form their own crevices.

Gaskets between flanges will probably form a slight crevice, but if the gasket does not absorb the liquid and is compressed between the surfaces (and not bulging around the flange), then the crevice is usually shallow enough so that crevice corrosion is not a problem.

Fabrication

Ensure full root penetration of welded joints with smooth weld bead. Avoid under cut and cracks in welding. Use of sticky labels or markers of various kinds (such as crayons) should be avoided, as should smears of grease or oil.

'Smooth and clean' at all times. ASSDA Accredited Fabricators are assessed on their

knowledge of crevice corrosion.

Operation

Sediment and scale can both result in crevices. If the problem can't be designed out, routine maintenance will minimize risk. Crevice corrosion under bacteria film can occur. Maintaining circulation reduces the risk that debris will collect and form crevices in dead legs or low flow areas.

Further Reading

The Nickel Institute's free publication #11021 'High Performance Stainless Steels' contains much of the information used in this article.

This publication and a mathematical model useful for assessing crevice corrosion risk can be downloaded from the Nickel Institute website - www.nickelinstitute.org.

If more detailed corrosion mechanism information is required, then 'Corrosion of Stainless Steel' by A. John Sedriks is a good intermediate point.

Credits

The Australian Stainless Steel Development Association (ASSDA) would like to acknowledge the contribution of the following Technical Committee members for their contribution to the production of this article.

Richard Matheson - Executive Director, ASSDA Graham Sussex - Technical Specialist, ASSDA Peter Moore - Technical Services Manager, Atlas Specialty Metals

This article featured in Australian Stainless magazine - Issue 29, September 2004.

Corrosion Control - Galvanic TableLee Erb

Originally published August 1997

Listed below is the latest galvanic table from MIL-STD-889. I have numbered the materials for future discussion of characteristics. However, for any combination of dissimilar metals, the metal with the lower number will act as an anode and will corrode preferentially.

The table is the galvanic series of metals in sea water from Army Missile Command Report RS-TR-67-11, "Practical Galvanic Series."

The Galvanic Table

Active (Anodic)

1. Magnesium 2. Mg alloy AZ-31B 3. Mg alloy HK-31A 4. Zinc (hot-dip, die cast, or plated) 5. Beryllium (hot pressed) 6. Al 7072 clad on 7075 7. Al 2014-T3 8. Al 1160-H14 9. Al 7079-T6 10. Cadmium (plated)

11. Uranium 12. Al 218 (die cast) 13. Al 5052-0 14. Al 5052-H12 15. Al 5456-0, H353 16. Al 5052-H32 17. Al 1100-0 18. Al 3003-H25 19. Al 6061-T6 20. Al A360 (die cast) 21. Al 7075-T6 22. Al 6061-0 23. Indium 24. Al 2014-0 25. Al 2024-T4 26. Al 5052-H16 27. Tin (plated) 28. Stainless steel 430 (active) 29. Lead 30. Steel 1010 31. Iron (cast) 32. Stainless steel 410 (active) 33. Copper (plated, cast, or wrought) 34. Nickel (plated) 35. Chromium (Plated) 36. Tantalum 37. AM350 (active) 38. Stainless steel 310 (active) 39. Stainless steel 301 (active) 40. Stainless steel 304 (active) 41. Stainless steel 430 (active) 42. Stainless steel 410 (active) 43. Stainless steel 17-7PH (active) 44. Tungsten 45. Niobium (columbium) 1% Zr 46. Brass, Yellow, 268 47. Uranium 8% Mo. 48. Brass, Naval, 464 49. Yellow Brass 50. Muntz Metal 280 51. Brass (plated) 52. Nickel-silver (18% Ni) 53. Stainless steel 316L (active) 54. Bronze 220 55. Copper 110 56. Red Brass

57. Stainless steel 347 (active) 58. Molybdenum, Commercial pure 59. Copper-nickel 715 60. Admiralty brass 61. Stainless steel 202 (active) 62. Bronze, Phosphor 534 (B-1) 63. Monel 400 64. Stainless steel 201 (active) 65. Carpenter 20 (active) 66. Stainless steel 321 (active) 67. Stainless steel 316 (active) 68. Stainless steel 309 (active) 69. Stainless steel 17-7PH (passive) 70. Silicone Bronze 655 71. Stainless steel 304 (passive) 72. Stainless steel 301 (passive) 73. Stainless steel 321 (passive) 74. Stainless steel 201 (passive) 75. Stainless steel 286 (passive) 76. Stainless steel 316L (passive) 77. AM355 (active) 78. Stainless steel 202 (passive) 79. Carpenter 20 (passive) 80. AM355 (passive) 81. A286 (passive) 82. Titanium 5A1, 2.5 Sn 83. Titanium 13V, 11Cr, 3Al (annealed) 84. Titanium 6Al, 4V (solution treated and aged) 85. Titanium 6Al, 4V (anneal) 86. Titanium 8Mn 87. Titanium 13V, 11Cr 3Al (solution heat treated and aged) 88. Titanium 75A 89. AM350 (passive) 90. Silver 91. Gold 92. Graphite

End - Noble (Less Active, Cathodic)

NotesAC43.13, starting at Par 247, briefly covers several types of corrosion and corrosion protection. The grouping of materials is an early method of MS33586 which was superseded in 1969 by MIL-STD-889.

More on Galvanic Table (Almost straight from MIL-STD-889)

GeneralThe Galvanic Table lists metals in the order of their relative activity in sea water environment. The list begins with the more active (anodic) metal and proceeds down the to the least active (cathodic) metal of the galvanic series.

A "galvanic series" applies to a particular electrolyte solution; hence for each specific solution which is expected to be encountered for actual use, a different order or series will ensue. The sea water galvanic series is the most complete series that I know and I have not seen another series published by either the Army, Navy, or Air Force. Civilian aircraft encounter moisture and a salt of some kind.

Galvanic series relationships are useful as a guide for selecting metals to be joined, will help the selection of metals having minimal tendency to interact galvanic ally, or will indicate the need or degree of protection to be applied to lessen the expected potential interactions.

Generally, the closer one metal is to another in the series, the more compatible they will be, i.e., the galvanic effects will be minimal. Conversely, the farther one metal is from another, the greater the corrosion will be.

Notice that graphite is at the bottom of the table. Think of the corrosion potential if you put a big hunk of graphite on a small piece of magnesium.

In a galvanic couple, the metal higher in the series (or the smaller the number I have given it) represents the anode, and will corrode preferentially in the environment.

Types of ProtectionMetals widely separated in the galvanic series must be protected if they are to be joined. Appropriate measures should be taken to avoid contact. This can be accomplished by several methods:

1. Sacrificial - by applying to the cathodic member a sacrificial coating having a potential similar to or near that of the anodic member. If you are designing for a sacrificial element, the sacrificial element should be on the anodic side and smaller. Cadmium plate (No. 10) on steel bolts (No. 81) holding 2024-T4 (No. 25) plates will sacrifice the cadmium instead of corroding the Aluminum. This is one reason for using new bolts that have the Cad plate intact. (Don't use Cad plate with Titanium (No. 82 through 88). But that's another story.)

2. Sealing - by sealing to insure that faying surfaces are water-tight. (We have "talked" about this before.)

3. Resistance - by painting or coating all surfaces to increase the resistance of the electrical circuit. (We have "talked" about this only in terms of primer and sealant on fayed surfaces. There is still more that can be done by design selection.)

The (Non-Aerodynamic) Area RuleTo avoid corrosion, avoid a small anodic area relative to the cathodic area.

Corollary I - Use LARGE ANODE AREA.

Corollary II - The larger the relative anode area, the lower the galvanic current density on the anode, the lesser the attack.

Corollary III - The amount of galvanic corrosion may be considered as proportional to the Cathode/Anode area ratio.

Corollary IV - Design for a SMALL Cathodic/Anodic Ratio (CAR). (When designing, remember your small CAR.)

Corollary V - The same metal or more noble (cathodic, higher number in the table) metals should be used for small fasteners and bolts.

Sea Water EnvironmentsMetals exposed to sea water environments shall be corrosion and stress corrosion resistant or shall be processed to resist corrosion and stress-corrosion. Irrespective of metals involved, all exposed edges should be sealed with a suitable sealant material conforming to MIL-S-8802. When non-compatible materials are joined, an interposing material compatible with each shall be used.

Non-Metallic MaterialsMaterial other than true metals, i.e., non-metallic materials which must be considered as metallic materials, unless there is supporting evidence to the contrary. If these material are essentially free of corrosive agents (salts), free of acid or alkaline materials (neutral pH), and free of carbon or metallic particles, not subject to biodeterioration or will not support fungal growth, and do not absorb or wick water, then these may be considered non-metallic suitable for joining to metals.

Many materials classed non-metallic will initiate corrosion of metals to which they are joined, e.g., celluloid reinforced plastics, carbon or metal loaded resin materials, asbestos-cement composites.

More Precautions for JoiningWhere it becomes necessary that relatively incompatible metals must be assembled, the following precautions and joining methods are provided for alleviation of galvanic corrosion.

For Electrical Connection - Select materials which are indicated to be more compatible in accordance with the galvanic series.

Design metal couples so that the area of the cathode is smaller (appreciably) than the area of the anodic metal. For example, bolts or screws of stainless steel for fastening aluminum sheet, but not reverse.

Interpose a compatible metallic gasket or washer between the dissimilar metals prior to fastening.

Plate the cathodic member with a metal compatible to the anode.

Select a electrically conductive sealant. (More on these later.)

Not For Electrical Conductors - Interpose a non-absorbing, inert gasket material or washer between the dissimilar materials prior to connecting them.

Other ApproachesSeal all faying edges to preclude the entrance of liquids.

Apply corrosion-inhibiting pastes or compounds under heads of screws or bolts inserted into dissimilar metal surfaces whether or not the fasteners had been previously plated or otherwise treated. In some instances, it may be feasible to apply an organic coating to the faying surfaces prior to assembly. This would be applicable to joints which are not required to be electrically conductive.

Where practicable or where it will not interfere with the proposed use of the assembly, the external joint should be coated externally with an effective paint system.

Galvanic/Dissimilar Metal Corrosion

Contact between dissimilar metals occurs frequently but is often not a problem. The aluminum head on a cast iron block, the solder on a copper pipe, galvanizing on a steel purlin and the steel fastener in an aluminium sheet are common examples.

What causes galvanic corrosion?

For galvanic or dissimilar or electrolytic corrosion to occur, three conditions must be met:

the metal join must be wet with a conductive liquid there must be metal to metal contact the metals must have sufficiently different potentials

Wetting the join

The conductive liquid or electrolyte could be rainwater or even water from condensation. The greater the conductivity the more severe the galvanic effects. Salt or industrial pollution significantly increases the conductivity of water so

galvanic effects are normally more severe near the coast or in heavy industrial areas. Low conductivity, pure rainwater will only cause slight galvanic effects. One complication is that during evaporation, water films become more conductive so initially benign water may cause quite active galvanic effects as the liquid in the crevice under a bolt or clamp becomes more concentrated. Water may be excluded by design or the use of adhesive sealants.

Metal to metal contact

Galvanic corrosion can only occur if the dissimilar metals are in electrical contact. The contact may be direct or by an external pipe or wire or bolt. If the dissimilar metals are insulated from each other by suitable plastic strips, washers or sleeves then galvanic corrosion cannot occur. Paint is not a reliable separator from direct contact although painting all of the noble metal is quite effective. Painting the active metal causes drill holes at coating defects.

Potential differences

All metals dissolve to some extent when they are wetted with a conductive liquid. The degree of dissolution is greatest with active or sacrificial metals such as magnesium and zinc and they have the most negative potential. In contrast, noble or passive metals such as gold or graphite are relatively inert and have a more positive potential. Stainless steel is in the middle although it is nobler than carbon steel. The potential can be measured with a reference electrode and used to construct a galvanic series as shown in the table below (ASTM Standard G82).

When two metals are connected and in contact with a conducting liquid, the more active metal will corrode and protect the noble metal. Zinc is more negative than steel and so the zinc coating of galvanized steel will corrode to protect the steel at scratches or cut edges. The stainless steels, including 304 and 316, are more positive than zinc and steel, so when stainless steel is in contact with galvanized steel and is wet, the zinc will corrode first, followed by the steel, while the stainless steel will be protected by this galvanic activity and will not corrode. The rate of galvanic attack is governed by the size of the potential difference.

As a rule of thumb, if the potential difference is less than 0.1 volt, then it is unlikely that galvanic corrosion will be significant.

If all three conditions are met then galvanic corrosion is probable and the rate of corrosion will be influenced by the relative area and the current density delivered by the noble metal.

Relative wetted surface area

If a noble metal like stainless steel has a large surface area in contact with the electrolyte while the sacrificial metal (such as galvanized steel) has a very small surface area in contact with the electrolyte, then the stainless steel will generate a large corrosion current which will be concentrated on a small area of sacrificial metal. The galvanized steel will corrode quickly? First the zinc then the underlying steel? and so galvanized fasteners in stainless steel are not acceptable.

However, a stainless screw in galvanized steel is frequently used although a mound of zinc corrosion product will accumulate around the fastener. This is because the ratio of wetted noble fastener in an active metal might change from a 1:50 ratio to 1:1 during drying after a rainstorm. If contaminants are significant this means that avoiding dissimilar metal pairs may be a preferred option to prevent galvanic attack.

As a rule of thumb, if the wetted area of the corroding metal is 10 times the wetted area of the noble metal, then galvanic effects are not serious although the larger the ratio the less the effect.

Available current density

Stainless steel has an effective passive film so the available corrosion current is quite low. If the behavior of a copper/steel and a stainless steel/steel couple is compared, the copper/steel coupling is a more significant galvanic problem despite the similar potential separation of 0.35 volts.

Examples of acceptable galvanic pairs include:

Galvanized steel pipe hangers are used to hang stainless steel piping externally around chemical plants. The surface area ratio is bad with large area of stainless steel to small area of active zinc/steel but the rainwater is usually of quite low conductivity and 20 year service life is normal.

In the water industries, galling between stainless steel threads and nuts has been avoided by using aluminium bronze nuts on stainless steel studs or bolts. Although aluminium bronze is more active than stainless steel, the conductivity of the water, and hence the corrosion rate, is generally quite low. The nuts will require replacement but only at times of major overhaul.

One unacceptable case was a gasket with a carbon black loading so high it was conductive and caused severe galvanic attack of a 316 stainless lug. Graphite gaskets have caused similar problems.

Important Disclaimer

The technical recommendations contained in this publication are necessarily of a general nature and should not be relied on for specific applications without first securing competent advice. Whilst ASSDA has taken all reasonable steps to ensure the information contained herein is accurate and current, ASSDA does not

warrant the accuracy or completeness of the information and does not accept liability for errors or omissions.

Grade Selection

Stainless steels have diverse properties which provide viable and cost-effective solutions to a vast range of applications. In practice, development of a hierarchy of required properties reduces the range to a manageable number of highly versatile materials (ASSDA's Reference Manual lists about 30 grades commonly available in Australia), enabling users to choose the most appropriate grade for the particular application.

The family of stainless steels can be divided into five basic alloy groups (see Table 1):

Table 1: Family of Stainless Steels

Alloy Group Common Grades

Austenitic

UNS No. S30100 S30400 S3403 S31600 S31603Commonly 301 304 304L 316 316LUNS No. S32100 S31000 S30815 N08904 S31254

Commonly 321 310 253MATM or2111HTRTM 904L 254SMOé

Ferritic

UNS No. S40900 S43000 * S44400 S44600

Commonly 409 430DIN 1.40033CR12TM or5CR12TM

444 446

Martens ticUNS No. S41000 S42000 S43100 S44004 Commonly 410 420 431 440C

Duplex

UNS No. S32304 S31803 S32750 S32550 S32760

Commonly 2304 2205 2507 Ferrallium255TM Zeron 100TM

PrecipitationHardening

UNS No. S17400 S17700 S15500

Commonly 17-4PHor 630

17-7PHor 631 15-5PH

* UNS No. not yet allocatedNote: UNS Numbers are used throughout this article for consistency

Grade S30400 Stainless Steel

S30400 is the most specified grade - it accounts for more than 50% of stainless steel produced in the world and services a wide range of applications. It withstands ordinary rusting in architecture, is resistant to most food processing

environments, and resists organic chemicals, dye stuffs and a wide variety of inorganic chemicals. It is used extensively in consumer products and appliances, and equipment for domestic and commercial kitchens, hospitals, transportation and waste water treatment. S30400 is also available in

virtually all product forms and finishes.

When selecting a stainless steel, first consider the fundamental "competitive advantage" properties required. These basic properties can initially be viewed according to the five basic alloy groups (see Table 2).

For corrosive environments, take advantage of past experience. Even small amounts of some impurities and/or changes in temperature of flow conditions can have a significant effect on corrosion resistance. Specialist suppliers and other independent experts, who often provide advice at no cost, are contactable through ASSDA for assistance with a recommendation.

For resistance to environments such as strong acids, where uniform general corrosion is the controlling mechanism, refer to published tables of recommended grades and Iso-corrosion curves (usually comparing several grades) that indicate the rate at which the stainless can be expected to corrode. It is again stressed that minor differences between apparently similar environments, e.g. presence of chloride, can significantly alter the corrosion rates. Local corrosion, such as the related mechanisms of pitting and crevice corrosion, is largely controlled by the presence of chlorides in the environment and exacerbated by elevated temperature and surface deposits. Keeping a stainless steel surface clean is an important way of minimizing corrosion. Surface roughness can also help in initiating corrosion, so the smoother the surface, the better. A grade's resistance to pitting and crevice corrosion is indicated by its Pitting Resistance Equivalent number (PRE = %Cr + [3.3 x %Mo] + [16 x %N]) (see Table 3).

Grades with high levels of each element are more resistant. Hence, S31600 (2% Mo) is standard for marine fittings. Grade S31803 (3% Mo and 0.15% N) is more resistant to higher temperature chloride environments and the "super duplex" (e.g. S32750 and S32760) and "super austenitic" (e.g. S31254) grades have very high levels of each element and can withstand high chloride environments up to nearly boiling point.

Common austenitic grades (e.g. S30400 and S31600) may suffer from stress corrosion cracking (SCC) in chloride-containing environments, particularly in temperatures above about 50C and when a tensile stress is present in the steel. The Ferritic and duplex grades are highly resistant, though not immune, and should be selected if SCC is a possibility.

Selection for Mechanical and Physical Properties

Martens tic (e.g. S43100) and precipitation hardening (e.g. S17400) grades are often preferred materials for products requiring high strength, e.g. shafts and valve spindles.

Often, a grade is selected for the required corrosion resistance, and the mechanical and physical properties of the grade are only considered in the subsequent design process. However, these properties are fundamental and should be considered as early as possible in the selection process. For example, while a duplex grade like S31803 is highly corrosion resistant, it may also be more cost-effective in the long term for a given application due to its high strength.

Selection for Fabric ability:

Fabric ability should be considered early in the grade selection process - it greatly influences the cost of the product. Table 4 lists some common grades and compares their relative fabric ability on a scale of 1 to 10, with 10 indicating excellent fabric ability by this method:

A compromise between desirable properties of certain grades may be necessary. For example, grade S30300 (austenitic steel known as 303) has excellent machinability, but the high sulphur content which dramatically increases the cutting speed also substantially reduces the grade's weldability, formability and corrosion resistance (its PRE is wrong because the negative effect of sulphur is omitted, making it totally inappropriate for applications where there is a likelihood of corrosive conditions of even a mild nature).

Conclusion

Before selecting a grade of stainless steel, it is essential to consider the required properties such as corrosion resistance, but it is also important to consider the secondary qualities such as physical and mechanical properties and fabric ability of any competing grade. The appropriate choice will ultimately provide both short- and long-term benefits: cost-effective fabrication and installation and a long, trouble-free life.

Specifiers of stainless steel can obtain experienced help from independent and supplier/fabricator sources, often at no cost. Corrosion performance, physical and mechanical properties, surface finishes and a range of information comparing stainless steels are available in ASSDA's Reference Manual. Call ASSDA for the right help for your needs.

References

ASM International (1989) Corrosion Data.

ASM International (1994) ASM Specialty Handbook - Stainless Steels.

Sedriks, A.J. (1996) Corrosion of Stainless Steels. John Wiley & Sons.

304: the place to start

UNS S30400 (grade 304) is the greatest stainless success story. It accounts for more than 50% of all stainless steel produced, represents between 50 and 60% of Australia's consumption of stainless materials and finds applications in almost every industry.

304 is not the only stainless steel and is not appropriate in every application. However, an understanding of the attributes of 304 provides an excellent base

for comparing members of the austenitic family of stainless steels and a practical base for determining the appropriateness of stainless steel in a given application.

You already have substantial experience of 304 and its properties on which to draw. Chances are some of your cutlery (look for the telltale 18/8 or 18/10 designation), your saucepans, your sink or, even, the shutter on your floppy disk are 304 stainless.

Composition

Grade 304L (see Table 1) is a low carbon 304 often used to avoid possible sensitization corrosion in welded components. Grade 304H (see Table 1) has a higher carbon content than 304L, which increases the strength (particularly at temperatures above about 500oC). This grade is not designed for applications where sensitization corrosion could be expected.

Table 1: Composition of 304 and related grades

Grade C% Si% Mn% P% S% Cr% Ni%

UNS S30400 304 0.08 1.00 2.00 0.045 0.03 18.0-20.0 8.0-10.5

Related GradesUNS S30403 304L 0.03 1.00 2.00 0.045 0.03 18.0-20.0 8.0-12.0UNS S30409 304H 0.04-0.10 1.00 2.00 0.045 0.03 18.0-20.0 8.0-12.0

1. Single values are maximum specification limits.2. These limits are specified in ASTM A240 for plate, sheet and strip. Specifications for some other products may vary slightly from these vales.

Both 304L and 304H are available in plate and pipe, but 304H is less readily available ex-stock. 304L and 304H are sometimes stocked as standard 304 (test certificates will confirm compliance with the 'L' or 'H' specification).

Corrosion resistance

Grade 304 has excellent corrosion resistance in a wide range of media. It resists ordinary rusting in most architectural applications. It is also resistant to most food processing environments, can be readily cleaned, and resists organic chemicals, dye stuffs and a wide variety of inorganic chemicals.

In warm chloride environments, 304 is subject to pitting and crevice corrosion and to stress corrosion cracking when subjected to tensile stresses beyond about 50oC. However, it can be successful in warm chloride environments where exposure is intermittent and cleaning is a regular event (such as saucepans and some yacht fittings). Descriptions of these mechanisms may be found in ASSDA's Reference Manual.

Heat resistance

304 has good oxidation resistance in intermittent service to 870oC and in continuous service to 925oC. Continuous use of 304 in the 425-860oC range is not recommended if subsequent exposure to room temperature aqueous environments is anticipated, but it often performs well in temperatures fluctuating above and below this range. Grade 304L is more resistant to carbide precipitation and can be used in the above temperature range. Where high temperature strength is important, higher carbon values are required. For example, AS1210 Pressure Vessels Code limits the operating temperature of 304L to 425oC and restricts the use of 304 to carbon values of 0.04% or higher for temperatures above 550oC.

304 has excellent toughness down to temperatures of liquefied gases and finds application at these temperatures.

Physical and mechanical properties (see Tables 2 and 3)

Table 2: Mechanical properties of grade 304 (annealed condition) given in ASTM A240M

Table 3: Physical properties of grade 304 (typical values in annealed condition)

Tensile strength 515MPa min Density 8,000kg/m 3

0.2% proof stress 205MPa min Elastic modulus 193GPa

Elongation 40% min Mean coefficient of thermal expansion

Brinell hardness 201HB max 0-100oC 17.2µm/m/ oCRockwell hardness 92HRB max 0-315oC 17.8µm/m/ oCVickers hardness 210HV max 0-538oC 18.4µm/m/ oC

Note: Slightly different properties are given in other specifications.

Thermal conductivity

at 100oC 16.2W/m.K

at 500oC 21.5W/m.K

Specific heat 0-100oC 500J/kg.K

Electrical conductivity 720nOhms.m

Like other austenitic grades, 304 in the annealed condition is virtually non-magnetic (i.e very low magnetic permeability). After being cold worked, however, it can become significantly attracted to a magnet (reversible by annealing).

Like other austenitic steels, 304 can only be hardened by cold working. Ultimate tensile strength in excess of 1,000MPa can be achieved and, depending on

quantity and product form required, it may be possible to order to a specific cold-worked strength (see ASTM A666 or EN10088-2).

Annealing is the main heat treatment carried out on grade 304. This is accomplished by heating to 1,010-1,120oC and rapidly cooling - usually by water quenching.

Fabric ability:

Grade 304 has excellent forming characteristics. It can be deep drawn without intermediate heat softening - a characteristic that has made this grade dominant in the manufacture of drawn stainless parts, such as sinks and saucepans. It is readily brake or roll formed into a variety of other parts for application in the industrial, architectural and transportation fields.

Grade 304 has outstanding weldability and all standard welding techniques can be used (although oxyacetylene is not normally used). Post-weld annealing is often not required to restore 304's corrosion resistance, although appropriate post-weld clean-up is recommended. 304L does not require post-weld annealing and finds extensive use in heavy gauge fabrication.

Machinability of 304 is lower than most carbon steels. The standard austenitic grades like 304 can be readily machined, provided that slower speeds and heavy feeds are used, tools are rigid and sharp, and cutting fluids are used. An 'improved machinability' version of 304 also exists.

Cost comparisons

'First cost' cost comparisons can only be approximate, but the guidelines in Table 4 are suggested for sheet material in a standard mill finish suitable for construction projects. Lifecycle cost parameters will, in many applications, dramatically increase the appeal of stainless over its first cost competitors.

Table 4: First cost comparisons

Material ApproximatePrice ($/kg)

Glass (clear ann.) 0.2Mild steel 1.0-1.5Hot dipped galvanized steel 1.5-2.5

304 stainless 4.0-5.0Aluminium alloy (extruded) 4.0-5.5

316 stainless 5.0-6.0Copper 8.0Brass 8.5Bronze 10.0

Source: Facet Consulting Engineers, Brisbane

Forms available

Grade 304 is available in virtually all stainless product forms, including coil, sheet, plate, strip, tube, pipe, fittings, bars, angles, wire, fasteners, castings and some others. 304 is also available with virtually all surface finishes produced on stainless steel, from standard to special finishes.

Applications

Alternative grades to 304 should be considered in certain environments and applications, including marine conditions, environments with temperatures above 50-60oC and with chlorides present, and applications requiring heavy section welding, substantial machining, high strength or hardness, or strip with very high cold-rolled strength.

However, typical applications for 304 include holloware, architecture, food and beverage processing, equipment and utensils, commercial and domestic kitchen construction, sinks, and plant for chemical, petrochemical, mineral processing and other industries.

With this breadth of application, grade 304 has become a fundamental alloy in modern industry and is certainly worth committing to your materials knowledge base.

Table 5: Some approximate equivalent designations

Wrought productStandard UNS ASTM British German Swedish Japanese

Specification S30400 304 BS 304S15En 58E

W. No 1.4301DIN X5CrNi 18 9 SS 2332 JIS SUS 304

Cast productStandard UNS ASTM BS3100 German AS2074

Specification J92600 A743 CF-8 304C15 STD No. 4308DIN G-X6CrNi 18 9 H5A

Note: For fasteners manufactured to ISO3506, 304 is included in the 'A2' designation. 316: the first step up

If a job requires greater corrosion resistance than grade 304 can provide, grade 316 is the 'next step up'. Grade 316 has virtually the same mechanical, physical and fabrication characteristics as 304 with better corrosion resistance, particularly to pitting corrosion in chloride environments.

Grade 316 (UNS S31600) is the second most popular grade in the stainless steel family. It accounts for about 20% of all stainless steel produced.

Composition

Table 1 compares three related grades - 316, 316L and 316H.

Grade 316L is a low carbon 316 often used to avoid possible sensitization corrosion in welded components.

Grade 316H has a higher carbon content than 316L, which increases the strength (particularly at temperatures above about 500oC), but should not be used for applications where sensitization corrosion could be expected.

Table 1 - Composition on 316 and related gradesGrade C% Mn% Si% P% S% Cr% Ni% Mo% N%

UNS 31600 316 0.08 2.0 0.75 0.045 0.03 16.0-18.0

10.0-14.0 2.0-3.0 0.10

Related GradesUNS S31603 316L 0.03 2.0 0.75 0.045 0.03 16.0-

18.010.0-14.0 2.0-3.0 0.10

UNS S31609 316H 0.04-

0.10 2.0 0.75 0.045 0.03 16.0-18.0

10.0-14.0 2.0-3.0 -

Both 316L and 316H are available in plate and pipe, but 316H is less readily available ex-stock. 316L and 316H are sometimes stocked as standard 316 (test certificates will confirm compliance with the 'L' or 'H' specification).

Corrosion resistance

Grade 316 has excellent corrosion resistance in a wide range of media. Its main advantage over grade 304 is its increased ability to resist pitting and crevice corrosion in warm chloride environments. It resists ordinary rusting in virtually all architectural applications, and is often chosen for more aggressive environments such as sea-front buildings and fittings on wharves and piers. It is also resistant to most food processing environments, can be readily cleaned, and resists organic chemicals, dye stuffs and a wide variety of inorganic chemicals.

In hot chloride environments, grade 316 is subject to pitting and crevice corrosion and to stress corrosion cracking when subjected to tensile stresses beyond about 50oC. In these severe environments duplex grades such as 2205 (UNS S31803) or higher alloy austenitic grades including 6% molybdenum (UNS S31254) grades are more appropriate choices.

The corrosion resistances of the high and low carbon versions of 316 (316L and 316H) are the same as standard 316. They are mostly chosen to give better resistance to sensitization in welding (316L) or for superior high temperature strength (316H).

Descriptions of these corrosion mechanisms are in ASSDA's Reference Manual.

Heat resistance

Like grade 304, 316 has good oxidation resistance in intermittent service to 870oC and in continuous service to 925oC. Continuous use of 316 in the 425-860oC range is not recommended if subsequent exposure to room temperature aqueous environments is anticipated, but it often performs well in temperatures fluctuating above and below this range.

Grade 316L is more resistant to carbide precipitation than standard 316 and 316H and can be used in the above temperature range. However, where high temperature strength is important, higher carbon values are required. For example, AS1210 Pressure Vessels Code limits the operating temperature of 316L to 450oC and restricts the use of 316 to carbon values of 0.04% or higher for temperatures above 550oC. 316H or the titanium-containing version 316Ti can be specified for higher temperature applications.

Like other austenitic stainless steels 316 has excellent toughness down to temperatures of liquefied gases and has application at these temperatures, although lower cost grades such as 304 are more usually selected for cryogenic vessels.

Physical and mechanical properties (see Tables 2 and 3)

Table 2: Mechanical properties of grade 316 (annealed condition) given in ASTM A240M

Table 3: Physical properties of grade 316 typical values in annealed condition)

Tensile strength 515MPa min Density 8,027kg/m3

0.2% proof stress 205MPa min Elastic modulus 193GPa

Elongation 40% min Mean coefficient of thermal expansion

Brinell hardness 217HB max 0 - 100oC 15.9µm/m/oC

Rockwell hardness 95HRB max 0 - 315oC 16.2µm/m/oC

Note: Slightly different properties are given in other specifications

0 - 538oC 17.5µm/m/ oC

0 - 649oC 18.6µm/m/ oC

0 - 815oC 20.0µm/m/ oC

Thermal conductivity

at 100oC 16.3W/m.K

at 500oC 21.5W/m.K

Specific heat 0 - 100oC 500J/kg.G

Electrical resistivity 20oC 740 nOhm.m

Like other austenitic grades, 316 in the annealed condition is virtually non magnetic (ie very low magnetic permeability). While 304 can become significantly attracted to a magnet after being cold worked, grade 316 is almost always virtually totally non-responsive. This may be a reason for selecting grade 316 in some applications.

Another characteristic that 316 has in common with other austenitic steels is that it can only be hardened by cold working. An ultimate tensile strength in excess of 1,000MPa can be achieved and, depending on quantity and product form required, it may be possible to order to a specific cold-worked strength (see ASTM A666 or EN10088-2).

Annealing (also referred to as solution treating) is the main heat treatment carried out on grade 316. This is done by heating to 1,010 1,120oC and rapidly cooling - usually by water quenching.

Fabric ability:

Like other austenitic stainless steels, grade 316 has excellent forming characteristics. It can be deep drawn without intermediate heat softening enabling it to be used in the manufacture of drawn stainless parts, such as sinks and saucepans. However, for normal domestic articles the extra corrosion resistance of grade 316 is not necessary. 316 is readily brake or roll formed into a variety of other parts for application in the industrial and architectural fields.

Grade 316 has outstanding weldability and all standard welding techniques can be used (although oxyacetylene is not normally used). Although post weld annealing is often not required to restore 316's corrosion resistance, making it suitable for heavy gauge fabrication, appropriate post-weld clean-up is recommended.

Machinability of 316 is lower than most carbon steels. The standard austenitic grades like 316 can be readily machined if slower speeds and heavy feeds are used, tools are rigid and sharp, and cutting fluids are used. An 'improved machinability' version of 316 also exists.

Cost comparisons

The guidelines in Table 4 are approximate 'first cost' comparisons for sheet material in a standard mill finish suitable for construction projects. The appeal of stainless over its first cost competitors dramatically increases when lifecycle costs are considered.

Table 4: First cost comparisons

Material Approximate Price ($/kg)

Glass (clear annealed) 0.2Mild steel 1.0-1.5Hot dip galvanized steel 1.5-2.5304 stainless 4.0-5.0Aluminium alloy (extruded) 4.0-5.5

316 stainless 5.0-6.0Copper 8.0Brass 8.5Bronze 10.0

Source: Facet Consulting Engineers, Brisbane

Forms available

Grade 316 is available in virtually all stainless product forms including coil, sheet, plate, strip, tube, pipe, fittings, bars, angles, wire, fasteners and castings. 316L is also widely available, particularly in heavier products such as plate, pipe and bar. Most stainless steel surface finishes, from standard to special finishes, are available.

Applications

Typical applications for 316 include boat fittings and structural members; architectural components particularly in marine, polluted or industrial environments; food and beverage processing equipment; hot water systems; and plant for chemical, petrochemical, mineral processing, photographic and other industries.

Although 316 is often described as the 'marine grade', it is also seen as the first step up from the basic 304 grade.

Alternatives

Alternative grades to 316 should be considered in certain environments and applications including:

strong reducing acids (alternatives might be 904L, 2205 or a super duplex grade),

environments with temperatures above 50-60oC and with chlorides present (choose grades resistant to stress corrosion cracking and higher pitting resistance such as 2205 or a super duplex or super austenitic), and

applications requiring heavy section welding (316L), substantial machining (an improved machinability version of 316), high strength or hardness (perhaps a martens tic or precipitation hardening grade).

Specifications

Table 5: Some approximate equivalent designations

Wrought productStandard UNS ASTM British German Swedish Japanese

Specification S31600 316 BS 316S16En 58H, 58J

W. No 1,4401DIN X5CrNiMo 18 10 SS 2347 JIS SUS 316

Cast productStandard UNS ASTM BS3100 German AS2074

Specification J92900 A743CF-8M 316C16 STD 1,4408

DIN G-X6CrNiMo 18 10 H6B

Note: For fasteners manufactured to ISO3506, 316 is included in the "A4" designation.

Grade 2205 for High Corrosion Resistance and Strength

Combining many of the beneficial properties of both Ferritic and austenitic steels, 2205 is the most widely used duplex stainless steel grade. Its high chromium and molybdenum content gives the stainless steel excellent corrosion resistance. The microstructure provides resistance to stress corrosion cracking and ensures high strength.

The grade is generally not suitable for use at temperatures above 300oC or below -50oC because of reduced toughness outside this range.

You are most likely to encounter 2205 stainless steel being used in industrial environments such as petrochemical, chemical, oil, gas and paper plants.

Alternative Grades

2205 has been available for several years - in general this complies with UNS grade designation S31803. More recently, product has become available complying with the higher corrosion resistant composition UNS S32205, as in table 1. Both these alternatives are known as 2205.

Composition

Grade 2205 has a micro structure of roughly equal amounts of ferrite and austenite, hence the 'duplex' description. The duplex structure of 2205 has the following properties:

High strength. Lower thermal expansion coefficient than austenitic steels but greater than carbon

steels. High resistances to corrosion, particularly stress corrosion cracking, corrosion fatigue

and erosion.

The high content of chromium and molybdenum and the addition of nitrogen gives the steel further beneficial characteristics:

High general corrosion resistance. High pitting and crevice corrosion resistance. Good sulphide stress corrosion cracking resistance.

The addition of nitrogen gives a further increase in pitting and crevice corrosion resistance.

Table 1: Composition of 2205 and Alternative Grades (Single Values are Maximum)Grade Common Name C% Mn% Si% P% S% Cr% Ni% Mo% N%S31803 2205 0.030 2.00 1.00 0.030 0.020 21.0-23.0 4.5-6.5 2.5-3.5 0.08-0.20S32205 2205 0.030 2.00 1.00 0.030 0.020 22.0-23.0 4.5-6.5 3.0-3.5 0.14-0.20

Corrosion Resistance

The grade has excellent corrosion resistance and is superior to grade 316, performing well in most environments where standard austenitic grades may fail. 2205's low carbon content gives the grade a high resistance to intergranular corrosion and has better resistance to uniform, pitting and crevice corrosion due to its high chromium and molybdenum content.

As 2205 is a duplex stainless steel, the grade is also less sensitive to stress corrosion cracking in warm chloride environments, unlike austenitic stainless steels. The grade also has good resistance to stress corrosion cracking when exposed to hydrogen sulphide in chloride solutions.

High mechanical strength combined with excellent corrosion resistance gives 2205 high corrosion fatigue resistance.

© Copyright ASSDA 2004. Privacy

Questions of Carbon

The common austenitic grades of stainless steel, 304 and 316, are also available with controlled low or high carbon contents, known as "L" and "H" variants, with particular applications.

Low carbon or "L" grades are used to prevent or delay sensitization of stainless steel at elevated temperatures and the resulting lower corrosion resistance. The problematic temperature zone is 450-850oC, encountered during welding or specific application environments. "L" grades are often available in thicker selection sizes, greater than about 5mm in flat products.

High carbon or "H" grades are used for higher strength.

Substitution between standard, "L" and "H" grades is often possible allowing many specifications to be met from existing stock.

What "L" Grades are used and why are they used?

The low carbon "L" grades are used where high temperature exposure will occur, including welding of medium or heavy sections. The low carbon is one way of delaying or preventing grain boundary carbide precipitation (often referred to as sensitization) which can result in intergranular corrosion in corrosive service environments. As shown in the time-temperature-sensitization curve (below), there is an incubation time before the precipitation of carbides at temperatures in the range of about 450-850oC. The time for precipitation to occur is highly dependent upon the amount of carbon present in the steel, so low carbon content increases resistance to this problem. Because of their application area the "L" grades are most readily available in plate and pipe, but often also in round bar. In the absence of heavy section welding, or of high temperature exposure, the corrosion resistances of the standard and "L" grades are usually identical.

What "H" Grades are used and why are they used?

"H" grades are higher carbon versions of standard grades and have increased strength, particularly at elevated temperatures (generally above 500oC). Long term creep strength is also higher. "H" grades are primarily available in plate and pipe. Applicable grades are most commonly 304H and

316H, but high carbon versions of 309, 310, 321, 347 and 348 are specified in ASTM A240/A240M. These grades are susceptible to sensitization if held in the temperature range of 450-850oC. Once sensitized, impaired aqueous corrosion resistance and some reduction in ambient temperature ductility and toughness will result (usually irrelevant in high temperature applications).

What are the differences?

1. Composition limits for 304 and 304L are identical except for carbon content (304L does





Alternative Grades


Composition




and erosion.









http://www.assda.asn.au/asp/index.asp?pgid=17958


permit up to 12.0%Ni, compared to 10.5% max for 304 - but given the cost of nickel it is usual for both grades to have close to the minimum of 8.5%, so there is no practical difference). Neither grade has a minimum carbon content specified. A carbon content of 0.02% for example complies with both 304 and 304L specifications.

2. 304H has the same composition specification as 304 except for the carbon range of 0.04-0.10% (note the minimum limit for carbon) and that the 304H does not have the 0.10% nitrogen maximum limit which applies to both standard and "L" grades. Also, all austenitic "H" grades must have a grain size of ASTM No. 7 or coarser.

3. The relationship between 316, 316L and 316H is the same as that between the 304 series of stainless steels. Only the carbon contents differentiate 316, 316L and 316H grades (and the nitrogen and grain size limits mentioned above). Carbon contents are listed in Table 1 (from ASTM A240/A240M). Specifications for some other products, particularly tube and pipe, have a carbon limit of 0.035% or 0.040% maximum for 304L and 316L, but are otherwise the same.

4. Mechanical property specification differences are illustrated in Table 2 (from ASTM A240/A240M). In practice, steel mills generally ensure that the "L" grade heats meet the strength requirements of standard grades, ie all 304L will have yield/tensile properties above 205/515MPa, so will meet both standard and "L" grade requirements.

5. Dimensional and other requirements are the same for standard, "L" and "H" grades.

6. Pressure Vessel codes (e.g. AS1210) and Pressure Piping codes (e.g. AS4041) give allowable working pressures for each of the grades at nominated elevated temperatures. These codes allow higher pressure ratings for standard grades than for "L" grades. The codes do not permit the use of "L" grades above 525oC (AS4041) or 425oC (AS1210). Both codes include a clause stating that for use above 550oC the standard grades must contain at least 0.04% carbon. 304 or 316 material with 0.02% carbon are therefore not permitted for these elevated temperatures, whether called "L" or not. At temperatures from ambient up to this high temperature cut-off "L" grade heats with the standard grade pressure ratings would be permitted, so long as the material was in full compliance with the standard grade composition and mechanical property specifications. As discussed above, it is normal practice for this condition to be met.

7. The pressure vessel codes give the same allowable pressure rating for "H" grades as for standard grades - this is logical as the "H" grades are simply the standard grades with their carbon contents controlled to the top half of the range, or slightly above.

Table 1: Carbon Content (ASTM A240/A240M)Grade UNS Number Specified Carbon Content (%)304 S30400 0.08 max304L S30403 0.030 max304H S30409 0.04-0.10316 S31600 0.08 max316L S31603 0.030 max316H S31609 0.04-0.10





Alternative Grades


Composition




and erosion.









Table 2: Mechanical Property Specification Differences (ASTM A240/A240M)

GradeUNS Strength (MPa) min

Tensile Strength(MPa) min

Yield (%) min

Elongation Hardness (HB) max

Brinell Hardness (HRB) max

Rockwell

304 S30400 515 205 40 201 92304L S30403 485 170 40 201 92304H S30409 515 205 40 201 92316 S31600 515 205 40 217 95316L S31603 485 170 40 217 95316H S31609 515 205 40 217 95

Alternative Grade Usage

Because of availability issues it is sometimes desirable to use a product labeled as a standard grade when an "L" or "H" grade has been specified, or vice versa. Substitution can be made under the following conditions:

1. "L" grades can be used as standard grades so long as the mechanical properties (tensile and yield) conform to the standard grade requirements and high temperature strength is not a requirement. "L" grades usually comply with standard grade requirements, but Mills' test certificates need to be checked on a case by case basis. It is common for steel mills to supply "L" heats when standard grades have been ordered. The practice is legitimate and should not present problems to fabricators or end users.

2. Standard grades can be used as "L" grades as long as their carbon content meets the "L" grade maximum limits.

3. It is increasingly common for "dual certified" products to be stocked - particularly in plate, pipe and bar. These materials fully comply with both 304 and 304L or 316/316L. Dual certified product is deliberately intended to fulfill requirements for both standard and "L" grades, but cannot be used in applications for "H" grade. If an application requires an "H" grade, this must be specified at time of order. Standard grades can often be used in place of "H" grades so long as their carbon contents meet the "H" limits (generally 0.04-0.10%). Grain size requirements may have to be satisfied by extra testing. The product and its test certificate may describe it as a standard 304 or 316 unless it was originally manufactured as an "H" grade. Details of the test certificate will confirm grade compliance.

4. "H" grades can be used as standard grades so long as their carbon contents are 0.08% maximum, and nitrogen 0.10% maximum. This is likely, but would need to be checked.

References for Further Reading

AS 1210: Pressure Vessels

AS 4041: Standard Specification for Pressure Piping

ASTM A240/A240M: Heat-resisting Chromium and Chromium-Nickel Stainless Steel Plate, Sheet and Strip for Pressure Vessels

© Copyright ASSDA 2004. Privacy Statement | DisclaimerLow Nickel Austenitic Stainless Steels

The most common grades of stainless steel are 304 and 316, which are particularly popular because their austenitic microstructure results in an excellent combination of corrosion resistance, mechanical and physical properties and ease of fabrication.

The austenitic structure is the result of the addition of approximately 8-10% nickel. Nickel is not alone in being an austenite former; other elements that are used in this way are manganese, nitrogen, carbon and copper.

The Cost of Nickel and Its Addition to Stainless Steel

The cost of the common stainless steels is substantially determined by the cost of ingredients. The cost of the chromium that is the essential "stainless ingredient" is not high, but additions of elements that improve the corrosion resistance (especially molybdenum) or that modify the fabrication properties (especially nickel) add very much to the cost.

Costs for nickel have fluctuated from US$5,000 or US$6,000 in 2001 to US$15,000 per tonnes in 2004.

Similarly, molybdenum has dramatically increased from approximately US$8,000 per tonnes in 2001 to around US$50,000 per tonnes in 2004.

These costs impact directly on the two most common grades: 304 (18%Cr, 8%Ni) and 316 (17%Cr, 10%Ni, 2%Mo). The impact is most keenly felt in grade 316, which has suffered an increase to its cost premium above 304.

Other grades such as the duplex 2205 (22%Cr, 5%Ni, 3%Mo) and all more highly alloyed stainless steels are also affected.



Relative costs of the ingredients are shown in Figure 1, but these do vary widely and sometimes rapidly over time. These costs were correct in late 2004.

Alloying Additions - Manganese Replacing Nickel

The point of the alloying elements is that they achieve certain changes to the corrosion resistance or to the microstructure (which in turn influences the mechanical and fabrication properties).

Chromium is used to achieve corrosion resistance, and molybdenum adds to this.

A common evaluation of corrosion resistance of stainless steel grades is the Pitting Resistance Equivalent (PRE), where this is usually evaluated as PRE = %Cr + 3.3 x %Mo + 16 x %N. A neat equation, but unfortunately only a guide.

The PRE gives a guide to ranking of grades, but is not a predictor of resistance to any particular corrosive environment. What is apparent is that pitting corrosion resistance can be increased by molybdenum, but also by chromium or by nitrogen additions. These are much cheaper than molybdenum. Despite its high PRE factor, nitrogen has limited effect on corrosion resistance because of low solubility, i.e. <0.2%.

The microstructure of the steel is largely determined by the balance between austenite former elements and ferrite former elements.

On the austenite former side carbon, manganese, nitrogen and copper are all possible alternatives to nickel. All these elements are lower cost than nickel.

As is the case for the PRE, the Ni-equivalence formulas are a guide but do not tell the full story; each element acts in slightly different ways, and it is not possible to fully remove nickel and replace it with, for example, copper or nitrogen.

Manganese acts as an austenite former but is not as effective as nickel, and Cr-Mn steels have higher work hardening rates than do apparently equivalent Cr-Ni steels.

Carbon is a very powerful austenite former, but has only limited solubility in austenite, so is of limited value in a steel intended to be fully austenitic.

Although not recognized by the PRE formula, nickel has positive effects on resistance to some corrosive environments that manganese does not duplicate.

There can also be synergy between the elements. Addition of nitrogen has the double effect of forming austenite and of increasing the pitting corrosion resistance. And manganese is a strong austenite former in its own right, but also has the effect of increasing the solubility of nitrogen.

The Rise of the "200 Series" Steels

Manganese is therefore a viable alternative to nickel, ranging from a minor addition to an almost complete replacement.

The development of high manganese austenitic grades first occurred about fifty years ago, during one of the (several) previous periods of high nickel cost.

At that time some Cr-Mn-Ni grades were sufficiently developed to be allocated AISI grade numbers Ü 201 (17%Cr, 4%Ni, 6%Mn) and 202 (18%Cr, 4%Ni, 8%Mn) are high Mn alternatives to the straight chromium-nickel grades 301 and 302, and are still included in ASTM specifications as standard grades.

Their consumption over the following decades has been low relative to their Cr-Ni equivalents. The reasons for the poor take-up of these lower cost grades have been:

Very high work hardening rate (this can be an advantage in some applications).

Slightly inferior surface appearance Ü considered unacceptable for certain applications.

Additional production costs Ü higher refractory wear in melting in particular.

Corrosion resistance is lower in some environments, compared to Cr-Ni grades.

An additional issue is that Cr-Ni and Cr-Mn-Ni austenitic scrap is all non-magnetic, irrespective of nickel content, but scrap merchants evaluate scrap on the basis of assumed nickel content.

This has the potential to destabilize scrap markets. The upshot is that the cost reduction (due to lower ingredient cost) has been generally insufficient to move applications from the traditional Cr-Ni grades.

Take-up has often been because of technical advantages of 200-series in niche applications, not cost-driven.

Some New Contenders

The last decade has seen the rise of some new contenders in the Cr-Mn-Ni austenitic group. The main development work has been in India and the principal application has been kitchen ware Ü cooking utensils in particular.

The very high work hardening rate of the low nickel / high manganese grades has been acceptable to a point in this application, but additions of copper have also been made to reduce this problem.

India has been a fertile development and production venue for these grades because of local economic factors.

Other Asian countries have also become strong markets and more recently also producers. The Chinese market is particularly strong, and there is substantial demand in China for the Cr-Mn-Ni grades, often referred to generically as ?200-seriesî stainless steels.

Other centers of production are Taiwan, Brazil and Japan. Alloy development has resulted in a range of austenitic grades with nickel contents ranging from 1% to 4% and up to over 9% manganese. None of these grades are included in ASTM or other internationally recognized standards as yet.

The growth rate in production of these low-nickel austenitic grades has been very rapid. The most recent data published by the International Stainless Steel Forum (ISSF) shows that in 2003 as much as 1.5 million tonnes (7.5% of the worlds stainless steel) was of this type. In China the proportion has been estimated to be 25% in 2004.

Problems still exist however, and large-scale conversion of Cr-Ni applications to Cr-Mn-Ni-(Cu) grades is not likely.

The principal issue is lack of control ‹ unscrupulous suppliers misrepresenting low-nickel product as grade 304 with some resultant service corrosion failures, and degradation of scrap due to contamination by low-nickel material.

As at the start of 2005 the future is unclear. Although it seems logical that there should be a place for the low nickel austenitic grades, the practical issues may mean that grade selectors will instead choose to either continue to use the higher cost Cr-Ni grades, or to seek lower cost alternatives amongst the Ferritic or duplex grades.

Credits

ASSDA would like to thank Mr. Peter Moore, Technical Services Manager of Atlas Specialty Metals, for the contribution of this article.

This technical article was featured in Australian Stainless magazine # 32 - Winter 2005 and is an extract from the Grade Selection section of the Australian Stainless 2005 Reference Manual.

To order a copy of this essential technical resource for the stainless steel industry, download an order form from the ASSDA website - www.assda.asn.au or phone the ASSDA office on 07 3220 0722.

Properties of Stainless Steel

Mechanical Properties of Wrought Stainless Steel* (All Properties Specified, usually Flat Products)

CommonType

UNS No

Condition

0.2% ProofStrengthMPa

TensileStrengthMPa

TensileElongation %

BrinellHardnessHB

RockwellHardnessHRB

VickersHardnessHV Max

ASTMSpecification

Min Min≤1.2mmMin

>1.2mmMin Max

[HRC]Max

201 S20100 Annealed 310 655 40 40 217 100 240

A240

202 S20200 Annealed 260 620 40 40 241 - -

A240

301 S30100 Annealed 205 515 40 40 217 95 210

A240

301 S30100 1/4 hard 515 860 25 25 - - -

A666

301 S30100 1/2 hard 760 1035 15 18 - - -

A666

301 S30100 3/4 hard 930 1205 10 12 - - -

A666

301 S30100 Full hard 965 1275 8 9 - - -

A666

302 S30200 Annealed 205 515 40 40 201 92 210

A240

302HQ S30430 Annealed - 605

max - - - - -A493

303 S30300 Annealed 240 585 50 50 - - -

A582

304 S30400 Annealed 205 515 40 40 201 92 210

A240

304L S30403 Annealed 170 485 40 40 201 92 210

A240

305 S30500 Annealed - 585

max - - - - -A493

+ S30815 Annealed 310 600 40 40 217 95 -

A240

309S S30908 Annealed 205 515 40 40 217 95 225

A240

310S S31008 Annealed 205 515 40 40 217 95 225

A240

316 S31600 Annealed 205 515 40 40 217 95 225

A240

316L S31603 Annealed 170 485 40 40 217 95 225

A240

316Ti S31635 Annealed 205 515 40 40 217 95 225

A240

317 S31700 Annealed 205 515 35 35 217 95 -

A240

317L S31703 Annealed 205 515 40 40 217 95 -

A240

321 S32100 Annealed 205 515 40 40 217 95 210

A240

347 S34700 Annealed 205 515 40 40 201 92 -

A240

904L N08904 Annealed 215 490 35 35 - 90 -

B625

409 S40900 Annealed 205 380 20 22 179 88 -

A240

430 S43000 Annealed 205 450 20 22 183 88 210

A240

444 S44400 Annealed 275 415 20 20 217 96 200

A240

446 S44600 Annealed 280 480 20 - - - -

A580

1.4003 S41003 Annealed 280 460 18 18 220 - -

-

2205 S31803 Annealed 450 620 25 25 293 [31] -

A240

+ S32304 Annealed 400 600 25 25 290 [32] -

A240

+ S32750 Annealed 550 795 15 15 310 [32] -

A240

+ S32760 Annealed 550 750 25 25 270 [28] -

A240

+ S32550 Annealed 550 760 15 15 302 [32] -

A240

410 S41000 Annealed 205 450 20 22 217 96 210

A240

416 S41600 Annealed 276 517 30 30 - - -

A582

420 S42000

Cold Finished - 520 - 42 255 - -

A276

431 S43100

Cold Finished - 965

max - - 285 - -A580

440A S44002

Cold Finished - - - - 285 - -

A580

440C S44004

Cold Finished - - - - 285 - -

A580

17-4PH S17400 H1025 1000 1070 12 12 401 [42] -

A564

+ Proprietary alloy names apply.* Consult the relevant product standard for definitive values.

Typical Physical Properties - Annealed Condition

Mean Coefficient of Thermal Expansion (b)

Thermal Conductivity

Grade or type UNS No Density

(kg/m3)

Elastic Modulus (a) GPa

0-100oC µm/m/ oC

0-315oC µm/m/ oC

0-538oC µm/m/ oC

At 100oC W/m.K

At 500oC W/m.K

Specific Heat 0-100oC J/kg.K

Electrical Resistivity nOhms.m

201 S20100 7800 197 15.7 17.5 18.4 16.2 21.5 500 690

202 S20200 7800 - 17.5 18.4 19.2 16.2 21.6 500 690

301 S30100 8000 193 17.0 17.2 18.2 16.2 21.5 500 720

302 S30200 8000 193 17.2 17.8 18.4 16.2 21.5 500 720

302B S30215 8000 193 16.2 18.0 19.4 15.9 21.6 500 720

303 S30300 8000 193 17.2 17.8 18.4 16.2 21.5 500 720

304 S30400 8000 193 17.2 17.8 18.4 16.2 21.5 500 720

304L S30403 8000 193 17.2 17.8 18.4 16.3 21.5 500 720

302HQ S30430 8000 193 17.2 17.8 - 11.2 21.5 500 720

304N S30451 8000 196 17.2 17.8 18.4 16.3 21.5 500 720

305 S30500 8000 193 17.2 17.8 18.4 16.2 21.5 500 720

308 S30800 8000 193 17.2 17.8 18.4 15.2 21.6 500 720

309 S30900 8000 200 15.0 16.6 17.2 15.6 18.7 500 780

310 S31000 8000 200 15.9 16.2 17.0 14.2 18.7 500 780

314 S31400 7800 200 - 15.1 - 17.5 20.9 500 770

316 S31600 8000 193 15.9 16.2 17.5 16.2 21.5 500 740

316L S31603 8000 193 15.9 16.2 17.5 16.3 21.5 500 740

316N S31651 8000 196 15.9 16.2 17.5 14.4 - 500 740

317 S31700 8000 193 15.9 16.2 17.5 16.2 21.5 500 740

317L S31703 8000 200 16.5 - 18.1 14.4 - 500 790

321 S32100 8000 193 16.6 17.2 18.6 16.1 22.2 500 720

329 S32900 7800 186 10.1 11.5 - - - 460 750

330 N08330 8000 196 14.4 16.0 16.7 - - 460 1020

347 S34700 8000 193 16.6 17.2 18.6 16.1 22.2 500 730

384 S38400 8000 193 17.2 17.8 18.4 16.2 21.5 500 790

409 S40900 7800 200 11.7 12.0 12.4 24.9 - 460 -

410 S41000 7800 200 9.9 11.4 11.6 24.9 28.7 460 570

416 S41600 7800 200 9.9 11.0 11.6 24.9 28.7 460 570

420 S42000 7800 200 10.3 10.8 11.7 24.9 - 460 550

430 S43000 7800 200 10.4 11.0 11.4 26.1 26.3 460 600

430F S43020 7800 200 10.4 11.0 11.4 26.1 26.3 460 600

431 S43100 7800 200 10.2 12.1 - 20.2 - 460 720

434 S43400 7800 200 10.4 11.0 11.4 - 26.3 460 600

436 S43600 7800 200 9.3 - - 23.9 26.0 460 600

440C S44004 7800 200 10.2 - - 24.2 - 460 600

444 S44400 7800 200 10.0 10.6 11.4 26.8 - 420 620

446 S44600 7800 200 10.4 10.8 11.2 20.9 24.4 500 670

630 S17400 7500 196 10.8 11.6 - 18.3 23.0 460 800

631 S17700 7800 204 11.0 11.6 - 16.4 21.8 460 830

+ N08904 7900 195 16.0 17.5 - 14.0 - 500 950

+ S30815 7800 200 16.3 17.3 18.0 14.0 18.0 500 -

+ S31803 7800 200 13.7 14.7 - 19.0 - 480 850

+ S32304 7800 200 13.0 - - 16.0 - 470 850

+ S32750 7800 200 13.0 14.0 - 17.0 - 470 -

3CR12 S41003 7800 205 10.8 11.3 12.0 31.0 32.0 480 570

4565S S34565 8000 190 14.5 16.3 17.2 14.5 - 510 920

+ S32760 7600 190 12.8 13.8 - 14.4 - 480 850

(a) 1GPa = 1000MPa(b) µm/m/oC = x 10-6/oC(c) 1% flow in 10,000 hours at 540oCMagnetic permeability of all 300 series austenitic steels in the annealed condition is approximately 1.02+ Proprietary alloy names apply

Surface Finishes

Surface finish is an important element in any specification or purchase order for stainless steel regardless of the intended end use. For those applications in which appearance is important, finish is a design element and must be specified. For those applications where finish is not so important, it should still be specified to avoid receiving a finish that is inappropriate for the application.

Particlesizeinches

Particlesizemicrons

All product other than emery EmeryGrading system Comparable

grit symbolPolishingpaper Cloth

CAMI FEPA0.00026 6.5 1200 - - 4/0 -0.00035 9.0 - - - - -0.00036 9.2 1000 - - 3/0 -0.00047 12.0 - - - - -0.00048 12.2 800 - - - -0.00059 15.0 - - - - -0.00060 15.3 - P1200 - - -0.00062 16.0 600 - - 2/0 -0.00071 18.3 - P1000 - - -0.00077 19.7 500 - - 0 -0.00079 20.0 - - - - -0.00085 21.8 - P800 - - -0.00092 23.6 400 - 10/0 - -0.00098 25.0 - - - - -0.00100 25.75 - P600 - - -0.00112 28.8 360 - - - -0.00118 30.0 - P500 - - -0.00137 35.0 - P400 - - -0.00140 36.0 320 - 9/0 - -0.001575 40.0 - - - - -0.00158 40.5 - P360 - - -0.00172 44.0 280 - 8/0 1 -

0.00177 45.0 - - - - -0.00180 46.2 - P320 - - -0.00197 50.0 - - - - -0.00204 52.5 - P280 - - -0.00209 53.5 240 - 7/0 - -0.00217 55.0 - - - - -0.00228 58.5 - P240 - - -0.00230 60.0 - - - - -0.00254 65.0 - P220 - - -0.00257 66.0 220 - 6/0 2 -0.00304 78.0 180 P180 5/0 3 -0.00363 93.0 150 - 4/0 - Fine0.00378 97.0 - P150 - - -0.00452 116.0 120 - 3/0 - -0.00495 127.0 - P120 - - -0.00550 141.0 100 - 2/0 - Medium0.00608 156.0 - P100 - - -0.00749 192.0 80 - 0 - Coarse0.00768 197.0 - P80 - - -0.01014 260.0 - P60 - - -0.01045 268.0 60 - 1/2 - -0.01271 326.0 - P50 - - -0.01369 351.0 50 - 1 - Ex. Coarse0.01601 412.0 - P40 - - -0.01669 428.0 40 - 1-1/2 - -0.02044 524.0 - P36 - - -0.02087 535.0 36 - 2 - -0.02426 622.0 - P30 - - -0.02488 638.0 30 - 2-1/2 - -0.02789 715.0 24 - 3 - -0.02886 740.0 - P24 - - -0.03530 905.0 20 - 3-1/2 - -0.03838 984.0 - P20 - - -0.05148 1320.0 16 - 4 - -0.05164 1324.0 - P16 - - -0.06880 1764.0 - P12 - - -0.07184 1842.0 12 - 4-1/2 - -

The purpose of this section is to help designers and specifiers achieve a better understanding of stainless steel finishes. The text describes and illustrates standard industry finishes and how finishes are applied to stainless steel.

Designers, specifiers and purchasing officials are also encouraged to contact ASSDA, a local stainless steel service centre or fabricator concerning any surface finish requirements.

Finish - A Design Element

There are several good reasons for paying close attention to the finish designation. In architecture or other highly visible applications, the appearance of stainless steel is a critical design element and a misunderstanding or the wrong finish can alter the desired effect. In consumer products such as appliances, cookware and automobiles, the gleam of well polished stainless steel has strong sales appeal. In commercial equipment, such as used in institutional kitchens, restaurants and hospitals, properly finished stainless steel helps to emphasize the feeling of cleanliness.

In addition to the visual appeal of polished stainless steel, there are a number of functionally important purposes served by properly prepared stainless steel surfaces. In sanitary applications, polished stainless steel not only looks clean, it is easy to clean and keep clean.

In aggressive environments, the smoother the surface, the better the corrosion resistance. A smooth surface is less susceptible to an accumulation of deposits which may become focal points for localized corrosion.

Finish and Fabrication

Some fabrication operations in manufacturing stainless steel products, such as deep drawing, may yield better results if the metal surface has a slightly rough texture to hold lubricants. Proper lubrication minimizes tool wear, and it helps to reduce the severity of tool marks. In wire, the finish and coating serve to facilitate further steps in manufacturing such as cold heading of fasteners.

There are also economic considerations in specifying finish. For example, a cold rolled bright annealed finish might be specified instead of a more expensive No. 8 polished finish.

Relative comparison of grit size

Chemical Surface Treatments

Successfully using stainless steel depends on environment, grade selected, surface finish, the expectations of the customer and the maintenance specified.

Stainless steels provide robust solutions, but in harsh or borderline environments with high expectations for durability, surface finish will have a substantial impact on

performance. Surface finishes can be applied mechanically (usually with abrasives) and chemically.

Understanding how chemical and mechanical treatments will affect the characteristics of the surface and will enable the best possible outcome for the client and the structure.

Chemical treatment can be used to improve the corrosion performance of the steel, and hence its appearance in service.

Stainless steels resist corrosion best if they are clean and smooth. Clean means being free of contaminants on or in the surface that can either react with the steel (like carbon steel

or salt) or that create crevices or other initiation points where corrosion can start.

Smooth means having a low surface area at the 'micro' level. Mechanically abrading the surface can roughen the steel's surface and may also embed unwanted particles.

The common feature of chemical treatments is that they all clean the surface of the steel. They may also smooth or roughen the steel surface, or leave it unaffected depending on

which process is chosen. But if carried out properly, they all increase the corrosion resistance.

Corrosion resistance improves as you go to the right of this graph. The graph shows the relative importance of the smoothness of the surface and chemical treatment of the surface. They can be used together to get the best corrosion resistance. The study

reported by G. Coates (Materials Performance - August 1990) looked at the effect of various methods of treating an artificial welding heat tint on grade 316, 2B surface.

Stainless Steel Products

During steel making, sulphur in the steel is controlled to very low levels. But even at these levels sulphide particles are left in the steel, and can become points of corrosion attack.

This 'achilles heel' can be improved greatly by chemical surface treatment.

Most bar products will be slightly higher in sulphur when produced, so chemical treatment to remove inclusions in the surface of these products becomes more important.

Generally mill finishes for flat products (sheet, plate and strip) will be smoother as their

thickness decreases.

A No 1 finish on a thick plate may have dimples or other imperfections and a surface roughness of 5 to 6 micrometers Ra.

A typical 2B cold rolled finish on 1.7mm thick sheet might have a surface roughness of 0.2 micrometers Ra or better as shown in Mill Forms.

New surfaces will be created during fabrication processes, (e.g. cutting, bending, welding and polishing). The corrosion performance of the new surfaces will generally be lower than the mill supplied product because the surface is rougher, or sulphide inclusions sitting just

under the surface have been exposed or mild steel tooling contamination may have occurred.

Chemical treatments correctly performed can clean the surface and ensure the best possible corrosion performance.

Chemical surface treatments can be grouped into four categories:

Pickling - acids that remove impurities (including high temperature scale from welding or heat treatment) and etch the steel surface. 'Pickling' means some of the

stainless steel surface is removed. Passivation - oxidizing acids or chemicals which remove impurities and enhance the

chromium level on the surface. Chelating agents are chemicals that can remove surface contaminants.

Electro polishing - electrochemical treatments that remove impurities and have the added beneficial effect of smoothing and brightening the surfaces.

Pickling

Mixtures of hydrofluoric (HF) and nitric acid are the most common and are generally the most effective. Acids are available as a bath, a gel or a paste.

Commercially available mixtures contain up to about 25% nitric acid and 8% hydrofluoric acid. These chemicals etch the stainless steel which can roughen and dull the surface.

Care is required with all these chemicals because of both occupational health and safety and environmental considerations. HF is a Schedule 7 poison which has implications for

sale or use in most states. See ASSDA's Technical Bulletin on this subject.

Passivation

Nitric acid is most commonly used for this purpose. Passivation treatments are available as a bath, a gel or a paste. Available formulations contain up to about 50% nitric acid and may also contain other oxidizers such as sodium dichromate. Used correctly, a nitric acid treatment should not affect the appearance of the steel although mirror polished surfaces

should be tested first.

Passivation works by dissolving any carbon steel contamination from the surface of the

stainless steel, and by dissolving out sulphide inclusions breaking the surface.

Nitric acid may also enrich the proportion of chromium at the surface - some chelants are also claimed to do this.

Pickling and passivation (L-R): before and after treatment of fuel tanks for storing

helicopter fuel on ships. Photos courtesy of Alloy Engineers and MME Surface Finishing.

Chelants

Chelants have chemical 'claws' designed to selectively clean the surface.

The carboxylic acid group COOH is the basis for many chelants which are used in cleaners, water softening and lubricants. The pH and temperature must be correct for the chelant to

do its job. Turbulent rinsing of pipes and vessels afterwards is important.

Cleaning by chelating agents tends to be based on proprietary knowledge and systems, and is less standardized than the other methods described.

The successful use of these systems needs to be established on a case by case basis.

Electro polishing

Most commonly phosphoric and sulphuric acids are used in conjunction with a high current density to clean and smooth (by metal removal) the surface of the steel.

The process preferentially attacks peaks and rounds valleys on the surface and raises the proportion of chromium at the surface.

The technique can have substantial effect on the appearance increasing luster and brightness while only changing the measured roughness by about 30%.

Electro polishing of the example on the right effectively removed contamination including heat tint and smoothed the surface lifting luster and reflectivity compared with the

untreated example on the left.

Precautions

For chemical processes that etch the stainless steel, reaction times will increase with increasing grade.

More care is required with 'free machining' grades and these will usually require substantially less aggressive chemicals. The sulphur addition in these steels makes them readily attacked by chemical treatments. Care is also required when treating martens tic

or low chromium Ferritic stainless steels.

Detailed recommendations for each grade of stainless steel are given below.

The four categories of treatment are detailed in a number of Standards, but the most commonly used are:

ASTM A380 Cleaning, Descaling and Passivation of Stainless Steel Parts, Equipment and Systems.

ASTM A967 Chemical Passivation Treatments for Stainless Steel Parts. ASTM B912 Passivation of Stainless Steels using Electro polishing.

These very useful documents give detailed recommendations on many aspects of selection, application and evaluation of these treatments. Highly recommended reading.

Dirt and grease will mask the surface from treatments outlined above. Therefore, the steel surfaces must be free of these agents before applying chemical treatments.

Many of the chemical treatments described contain strong acids. Before disposal they will require neutralization. Check with your local authority concerning the requirements for

trade waste, neutralization and disposal.

Many of the chemicals described above will be classified as hazardous substances under State OHS legislation, with implications for purchasing, transport, storage and handling.

Chemical treatments are useful tools in cost effectively achieving peak performance with stainless steels. With appropriate training, hazards associated with their use can be

managed.

FEPA - Federation of European Producers of AbrasivesCAMI - Coated Abrasive Manufacturers Institute (USA)

© Copyright ASSDA 2004. Privacy Statement | Disclaimer

SUBJECT: Corrosion problems associated with stainless steel 4-1

The rotating equipment business uses a great deal of 300 series stainless steel, and as a result we often experience corrosion that is described in a variety of technical terms that include:

General corrosion Galvanic corrosion Pitting Inter granular corrosion Stress corrosion cracking Erosion- corrosion Fretting Concentrated cell or crevice corrosion Selective leaching Micro organisms

The last page of this report is a list titled "The Galvanic Series Of Metals and alloys". I will be referring to this chart during our discussion.

The basic resistance of stainless steel occurs because of its ability to form a protective coating on the metal surface. This coating is a "passive" film which is resistant to further "oxidation" or rusting. The formation of this film is instantaneous in an oxidizing atmosphere such as air, water, or many other fluids that contain oxygen. Once the layer has formed we say that the metal has become "passivated" and the oxidation or "rusting" rate will slow down to less than 0.002" per year (0,05 mm. per year).

Unlike aluminum or silver this passive film is invisible in stainless steel. It is due to the combining of oxygen with the chrome in the stainless to form chrome oxide which is more commonly called "ceramic". This protective oxide or ceramic coating is common to most corrosion resistant materials.



Halogen salts, especially chlorides easily penetrate this passive film and will allow corrosive attack to occur. The halogens are easy to recognize because they end in the letters "ine". Listed in order of their activity they are: fluorine, chlorine, bromine, iodine and astatine. These are the same chemicals that will penetrate Teflon and cause trouble with Teflon coated or encapsulated O-Rings and/ or similar coated materials. Chlorides are one of the most common elements in nature and if that isn't bad enough they are also soluble, active ions; the basis for good electrolytes, the best conditions for corrosion or chemical attack.

GENERAL OR OVERALL CORROSION.

This type of corrosion occurs when there is an overall breakdown of the passive film formed on the stainless steel. It is the easiest to recognize as the entire surface of the metal shows a uniform "sponge like" appearance. The rate of attack is affected by the fluid concentration, temperature, fluid velocity and stress in the metal parts subject to attack. As a general rule the rate of attack will double with an eighteen degree Fahrenheit rise in temperature (10° C.) of either the product or the metal part.

If the rotating portion of the seal is rubbing against some stationary component, such as a protruding gasket or fitting the protective oxide layer will be polished off and the heat generated will increase the corrosion as noted above. This explains why corrosion is often limited to only one portion of the metal case.

There are many good publications available to help you select the proper metal for any given mechanical seal application. As a general rule, if the wetted parts of the equipment are manufactured from iron, steel, stainless steel or bronze, and they are showing no signs of corrosion, grade 316 stainless is acceptable as long as you do not use stainless steel springs. (see chloride stress corrosion)

GALVANIC CORROSION

If you put two dissimilar metals or alloys in a common electrolyte, and connect them with a voltmeter, it will show an electric current flowing between the two. (This is how the battery in your automobile works). When the current flows, material will be removed from one of the metals or alloys (the ANODIC one) and dissolve into the electrolyte. The other metal (the CATHODIC one) will be protected.

Now let's take a look at the Galvanic Series chart that is attached to this report. The further apart the materials are located on this chart the more likely that the one on the ANODIC end will corrode if they are both immersed in a common fluid considered to be an electrolyte. water, containing chlorides, is one of the best.

Example #1.

A ship has lots of bronze fittings and a steel hull. Note that steel is located seven lines from the ANODIC end, and bronze is listed at twenty seven rows from the same end. Sea

water is a perfect electrolyte so the bronze fittings would immediately attack the steel hull unless something could be done to either protect the steel or give the bronze something else to attack. The classic way to solve this problem is to attach sacrificial zinc pieces to the hull and let the bronze go after them. Again, looking at the chart, you will note that zinc is found on line three from the top of the chart. In other words the zinc is further away from the bronze than the iron is, so the galvanic action takes place between the zinc and the bronze, rather than between the steel and the bronze. Zinc paint is used for the same reason in many applications.

Example #2

Nickel base tungsten carbide contains active nickel. When this face material is used in dual seal it is common to circulate water or antifreeze containing water between the seals (as mentioned in the beginning of this report, water can be an excellent electrolyte because of the addition of chlorine and fluorine). You will note that active nickel is located twenty one rows from the top of the chart. Passivated 316 stainless steel is positioned nine rows from the bottom. This means that the stainless steel can attack the nickel in the tungsten carbide causing it to corrode. Many of you have run into this problem already.

The rate at which corrosion takes place is determined by:

The distance separating the metals on the galvanic series chart The temperature and concentration of the electrolyte. The higher the temperature,

the faster it happens. Any stray electrical currents in the electrolyte will increase the corrosion also.

The relative size of the metal pieces. A large cross section piece will not be affected as much as a smaller one.

Many metal seal components are isolated from each other by the use of rubber O-Rings or similar materials and designs. Shaft movement that causes fretting of the 316 stainless steel rubs off the passivated layer and exposes the active stainless to the electrolyte until the metal part becomes passivated once more. This is one of the reasons we see corrosion under O-rings, Teflon, and similar materials. In the next paragraph I will be discussing another cause of corrosion under rubber parts.

PITTING

This is an accelerated form of chemical attack in which the rate of corrosion is greater in some areas than others. It occurs when the corrosive environment penetrates the passivated film in only a few areas as opposed to the overall surface. As stated earlier the halogens will penetrate passivated stainless steel. Referring to the galvanic chart you will note that passivated 316 stainless steel is located nine lines from the bottom and active 316 stainless steel is located thirteen lines from the top. Pit type corrosion is therefore simple galvanic corrosion, as the small active area is being attacked by the large passivated area. This difference in relative areas accelerates the corrosion causing the pits to penetrate deeper. The electrolyte fills the pits and prevents the oxygen from

passivating the active metal so the problem gets even worse. This type of corrosion is often called "concentrated cell corrosion". You will also see it under rubber parts that tend to keep oxygen away from the active metal parts, retarding its ability to form the passivated layer.

INTERGRANULAR CORROSION

All Austenitic stainless steels (the 300 series, the types that "work harden", is one of them) contain a small amount of carbon in solution in the austenite. Carbon is precipitated out at the grain boundaries, of the steel, in the temperature range of 1050° F. (565° C) to 1600° F. (870° C.). This is a normal temperature range during the welding of stainless steel.

This carbon combines with the chrome in the stainless steel to form chromium carbide starving the adjacent areas of the chrome they need for corrosion protection. In the presence of some strong corrosives an electrochemical action is initiated between the chrome rich and chrome poor areas with the areas being low in chrome becoming attacked. The grain boundaries are then dissolved and become non existent. There are three ways to combat this:

Anneal the stainless after it has been heated to this sensitive range. This means bringing it up to the proper annealing temperature and then quickly cooling it down through the sensitive temperature range to prevent the carbides from forming.

When possible use low carbon content stainless if you intend to do any welding on it. A carbon content of less than 0.3% will not precipitate into a continuous film of chrome carbide at the grain boundaries. 316L is as good example of a low carbon stainless steel.

Alloy the metal with a strong carbide former. The best is columbium, but sometimes titanium is used. The carbon will now form columbium carbide rather than going after the chrome to form chrome carbide. The material is now said to be "stabilized"

CHLORIDE STRESS CORROSION.

If the metal piece is under tensile stress, either because of operation or residual stress left during manufacture, the pits mentioned in a previous paragraph will deepen even more. Since the piece is under tensile stress cracking will occur in the stressed piece. Usually there will be more than one crack present that causes the pattern to resemble a spider's web. Chloride stress cracking is a common problem in industry and not often recognized by the people involved. In the seal business it is a serious problem if you use stainless steel springs or stainless steel bellows material. This is the main reason that Hastelloy C is recommended for spring material. Here are some additional thoughts about chloride stress cracking that you will want to consider:

Chlorides are the big problem when using the 300 series grades of stainless steel. The 300 series is the one most commonly used in the process industry because of its good corrosion resistant proprieties. Outside of water chlorides is the most common chemical found in nature and remember that the most common water treatment is the addition of chlorine.

Beware of insulating or painting stainless steel pipe. Most insulation contains plenty of chlorides and piping is frequently under tensile stress. The worst condition would be insulated steam traced, stainless steel piping.

If it is necessary to insulate stainless steel pipe a special chloride free insulation can be purchased or the pipe can be coated with a protective film prior to insulating.

Stress cracking can be minimized by annealing the metal, after manufacture to remove residual manufactured stresses.

Never replace a carbon steel bolt with a stainless steel one unless you are sure there are no chlorides present. Bolts can be under severe tensile stress.

No one knows the threshold values for stress cracking to occur. We only know that you need tensile stress, chlorides, temperature and the 300 series of stainless steel. We do not know how much chloride, stress or temperature.

Until I figured out what was happening I had trouble breaking stainless steel fishing hooks in the warm water we have in Florida.

Many cleaning solutions and solvents contain chlorinated hydrocarbons. Be careful using them on or near stainless steel. Sodium hypochlorite, chlorethene. methylene chloride and trichlorethane are just a few in common use. The most common cleaner used with dye checking material is trichloroethane accounting for the reason we sometimes experience cracks after we weld stainless steel and die check it to check the quality of the weld.

EROSION CORROSION

This is an accelerated attack resulting from the combination of mechanical and chemical wear. The liquid velocities in some pumps prevent the protective oxide passive layer from forming on the metal surface. The suspended solids also remove some of the passivated layer increasing the galvanic action. You see this type of corrosion very frequently at the eye of the pump impeller.

FRETTING CORROSION

This type of corrosion is easily seen on the pump shaft or sleeve. You will see the damage beneath:

The grease or lip seal that is supposed to protect the bearings. The packing used to seal the fluid. The dynamic Teflon or elastomer used in most original equipment seals. The vibration damper used in rotating metal bellows seals.

Under the rubber boot used in low cost seals, if they did not attach them selves to the shaft properly.

As mentioned earlier, 300 series stainless steel passivates its self by forming a protective chrome oxide layer when ever it is exposed to free oxygen. This oxide layer is very hard and when it imbeds into a soft elastomer it will cut and damage the shaft or sleeve rubbing against it. The mechanism works like this:

Oxygen passivates the active stainless steel forming a protective ceramic layer. The seal or packing removes the oxide layer as the shaft or sleeve rubs against it. The ceramic sticks into the soft elastomer turning it into a "grinding surface". The oxide reforms when the active metal is exposed and the process starts all over

again. A visible groove is cut into the shaft or sleeve that will cause seal leakage and

"hang up".

CONCENTRATED CELL OR CREVICE CORROSION

This corrosion occurs any time liquid flow is kept away from the attacked surface. It is common between nut and bolt surfaces, under O-rings and gaskets, and between the clamps and stainless steel shafts we find in many split seal applications. Salt water applications are the most severe problem because of the salt water low PH (8.0&endash; 9.0). Here is the mechanism:

Chlorides pit the passivated stainless steel surface. The low PH salt water attacks the active layer that is exposed Because of the lack of fluid flow over the attacked surface oxygen is not available

to re passivate the stainless steel. Corrosion continues unhampered under the rubber and tight fitting clamp. The inside of the O-ring groove experiences the same corrosion as the shaft or

sleeve.

SELECTIVE LEACHING

The process fluid selectively removes elements from the piping or any other part that might be exposed to the liquid flow. The mechanism is:

Metals are removed from the liquid during a de-ionization or de-mineralizing process.

The liquid tries to replace the missing elements as it flows through the system. The un-dissolved metals often coat them selves on the mechanical seal faces or

the sliding components and cause a premature seal failure. Heat accelerates the process.

MICRO ORGANISMS

These organisms are commonly used in sewage treatment, oil spills and other cleaning processes. Although there are many different uses for these "bugs", one common one is for them to eat the carbon you find in waste and other hydrocarbons, and convert it to carbon dioxide. The "bugs" fall into three categories:

Aerobic, the kind that need oxygen. Anaerobic, the kind that do not need oxygen. Facultative, the type that goes both ways.

If the protective oxide layer is removed from stainless steel because of rubbing or damage, the "bugs" can penetrate through the damaged area and attack the carbon in the metal. Once in, the attack can continue on in a manner similar to that which happens when rust starts to spread under the paint on an automobile.

CORROSION DOE-HDBK-1015/1-93 Corrosion CH-02 Rev. 0 Page 32 The most effective means for preventing SCC are proper design, reducing stress, removing critical environmental contributors (for example, hydroxides, chlorides, and oxygen), and avoiding stagnant areas and crevices in heat exchangers where chlorides and hydroxides might become concentrated. Low alloy steels are less susceptible than high alloy steels, but they are subject to SCC in water containing chloride ions. Nickel based alloys are not affected by chloride or hydroxide ions. Two types of SCC are of major concern to a nuclear facility.

Chloride Stress Corrosion Cracking (Stainless Steels)

The three conditions that must be present for chloride stress corrosion to occur are as follows. Chloride ions are present in the environment Dissolved oxygen is present in the environment Metal is under tensile stress Austenitic stainless steel is a non-magnetic alloy consisting of iron, chromium, and nickel, with a low carbon content. This alloy is highly corrosion resistant and has desirable mechanical properties. One type of corrosion which can attack austenitic stainless steel is chloride stress corrosion. Chloride stress corrosion is a type of intergranular corrosion. Chloride stress corrosion involves selective attack of the metal along grain boundaries. In the formation of the steel, a chromium-rich carbide precipitates at the grain boundaries leaving these areas low in protective chromium, and thereby, susceptible to attack. It has been found that this is closely associated with certain heat treatments resulting from welding. This can be minimized considerably by proper annealing processes. This form of corrosion is controlled by maintaining low chloride ion and oxygen content in the environment and the use of low carbon steels. Environments containing dissolved oxygen and chloride ions can readily be created in auxiliary water systems. Chloride ions can enter these systems via leaks in condensers or at other locations where auxiliary systems associated with the nuclear facility are cooled by unpurified cooling water. Dissolved oxygen can readily enter these systems with feed and makeup water. Thus, chloride stress corrosion cracking is of concern, and controls must be used to prevent its occurrence.

Corrosion Documents

Documents

galvanic current corrosion

rate of reduction

galvanic series

cathodic area

oxygen reduction reaction

galvanic processes

severe corrosion

anodic corrosion