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The Impact of Chlorides on Cooling Water System M t ll i d Pitti Metallurgies and Pitting Alf Pl Alfonso Palazzo Feb 2009
13

Cl on Metals in CWS ALP 2009

Jul 20, 2016

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Page 1: Cl on Metals in CWS ALP 2009

The Impact of Chlorides on Cooling Water System M t ll i d PittiMetallurgies and Pitting

Alf P lAlfonso PalazzoFeb 2009

Page 2: Cl on Metals in CWS ALP 2009

Stainless SteelStainless Steel• Stainless steel has provided good service in many applications. However,

f th i t d i i t t t th lti t f thuse of the appropriate grade is important to the ultimate success of the steel.

• Corrosion should be an expected by-product of any steel use, including p y p y , gstainless steel. The question is, how much corrosion is acceptable in the intended application?

• Corrosion is the deterioration of a metal or alloy as the result of exposure• Corrosion is the deterioration of a metal or alloy as the result of exposure to and reaction with its environment. It is an electrochemical process

• The first equation is anodic and the rest are cathodic. Corrosion of stainless steels is dependent almost entirely on chlorides, sulphates and dissolved oxygen content. Chlorides also can be adsorbed or chemisorbed in films on the metal surfaces (silicates, ( ,aluminates, iron oxides and calcareous deposits).

The typical reactions areFe0 Æ Fe2+ + 2e–2H+ + 2e Æ H2O2H+ + 2e– Æ H2O

2H+ + 1/2O2 + 2e– Æ H2OH2O + 1/2O2 + 2e– Æ 2OH–

Page 3: Cl on Metals in CWS ALP 2009

Chl id i i Wh h hl id d 50 /L 304 i l• Chloride pitting—When the chlorides exceed 50 mg/L, 304 stainless steel should be used instead of carbon steels.

• Where the chlorides exceed 200 mg/L, 316L would be preferred.g , p• Manufacturers often recommend a chloride ceiling for Type 316L

of 500 mg/L. This level is below all brackish water sources. Chl id i d d itti i th bl l d b t l• Chloride induced pitting is another problem resolved by metal selection. Inappropriate metal selection can lead to serious damage to the pipe, as well as microbial colonization

• The conditions that lead to pitting are moderately high temperatures, high concentrations of chloride ions (from salt), other halides such as fluorides bromides and iodides Acidic conditions (low pH) alsoas fluorides, bromides and iodides. Acidic conditions (low pH) also aid the growth of the pits, which are themselves acidic.

• This acidity within the pit is the reason why pits can continue to grow th h b i iti t donce they have been initiated.

Page 4: Cl on Metals in CWS ALP 2009

PREN – the Pitting Resistance Equivalent Number

• The PREN number is a useful guide to the tendency of certain stainless steels to pit It should be used as a guide only &pit. It should be used as a guide only & not as a totally predictive measure that will totally guarantee pitting resistance in all i t All th t h hi h

Typical values of PREN are:-circumstances. Alloys that have high

concentrations of nitrogen (N), chromium (Cr) and Molybdenum (Mo) have been

are:Grade.........PREN430............16444.............25

found to show good resistance to pitting. The relative effect of these elements in combination is given by:

444.............25304.............19304LN..........21316.............26g y

PREN = (%Cr) + (3.3x%Mo) + (16 x %N) • (note that some variants use 32 x %N )

316LN.........27.5904L...........36Zeron 100...41

• High PREN numbers therefore mean higher resistance to pitting corrosion.

SAF 2507.....42

Page 5: Cl on Metals in CWS ALP 2009

Larson Skold IndexLarson-Skold Index• The Larson-Skold index describes the corrosivity of water towards mild steel.

The inde is the ratio of eq i alents per million (epm) of s lphate (SO4) and chloride• The index is the ratio of equivalents per million (epm) of sulphate (SO4) and chloride (Cl) to the equivalents per million of alkalinity in the form bicarbonate plus carbonate (HCO3 + CO3).

Larson-Skold index = (epm Cl + epm SO4)(epm HCO3 + epm CO3)

• It must be noted that the Larson-Skold relationship is based upon Great Lakes• It must be noted that the Larson-Skold relationship is based upon Great Lakes waters. Extrapolation to other waters, such as those of low alkalinity or extreme alkalinity, goes beyond the range of the original data.

• The index might be interpreted by the following guidelines:g p y g gLarson-Skold Indication

<< 0.8 Chlorides and sulphate probably will not interfere with natural film formation.

>> 0.8 but << 1.2 Chlorides and sulphates may interfere with natural film formation. Higher than desired

corrosion rates might be anticipated.

>> 1.2 The tendency towards high corrosion rates of a local type should be expected as the index increases.

Page 6: Cl on Metals in CWS ALP 2009

Crevice Corrosion Of Stainless Steels

• Crevice corrosion is the second most common modem failure of stainless steels with pitting corrosion being the most commonstainless steels, with pitting corrosion being the most common.

• Crevice corrosion occurs at locations where there is a small gap, or crevice, between the stainless steel article and another item. This second item can commonly be an insulator such as a gasket or a rubber separator although it could be metallic. The actual geometry of the small gap is critical for the onset of crevice corrosion. The gap g p g phas to be just big enough to allow an aggressive liquid to penetrate into the gap but not big enough to allow material to be flushed out of the gap due to flow or confection in the fluid.the gap due to flow or confection in the fluid.

• The mechanism of crevice corrosion is well-known, with the first stage being the accumulation of aggressive ions (such as chloride i ) i th d th l i f f th l ti ithiions) in the gap and the exclusion of oxygen from the solution within the gap. This results in a fixed anode forming in the crevice with the material outside of the crevice becoming a fixed cathode.

Page 7: Cl on Metals in CWS ALP 2009

Stress Corrosion CrackingStress Corrosion Cracking

Page 8: Cl on Metals in CWS ALP 2009
Page 9: Cl on Metals in CWS ALP 2009

Prevention of SCC1. Lowering the stress below the threshold value if one exists. This may be d b li i th f id l t thi k i th tidone by annealing in the case of residual stresses, thickening the section, or reducing the load. Plain carbon steels may be stress-relief annealed at 590 to 650°C, and the austenitic stainless steels are frequently stress-relieved at t t i f 820 t 930°Ctemperatures ranging from 820 to 930°C.

2. Eliminating the critical environmental species by, for example, de-gasification, demineralization, or distillation.

3. Changing the alloy is one possible recourse if neither the environment nor 3 C a g g t e a oy s o e poss b e ecou se e t e t e e o e t ostress can be changed. For example, it is common practice to use Inconel (raising the nickel content) when type 304 stainless steel is not satisfactory. Although carbon steel is less resistant to general corrosion, it is more resistant to stress-gcorrosion cracking than are the stainless steels. Thus, under conditions which tend to produce stress-corrosion cracking, carbon steels are often found to be more satisfactory than the stainless steels. For example, heat exchangers used in

9contact with seawater or brackish waters are often constructed of ordinary mild steel.

Page 10: Cl on Metals in CWS ALP 2009

4. Applying cathodic protection to the structure with an external power supply or pp y g p p pp yconsumable anodes. Cathodic protection should only be used to protect installations where it is positively known that stress-corrosion cracking is the cause of fracture, since hydrogen embrittlement effects are accelerated by impressed cathodic currentscathodic currents.

5. Adding inhibitors to the system if feasible. Phosphates and other inorganic and organic corrosion inhibitors have been used successfully to reduceand organic corrosion inhibitors have been used successfully to reduce stress-corrosion cracking effects in mildly corrosive media. As in all inhibitor applications, sufficient inhibitor should be added to prevent the possibility of localized corrosion and pitting.p g

6. Coatings are sometimes used, and they depend on keeping the environment away from the metal - for example, coating vessels and pipes that are covered with y p g p pinsulation. In general, however, this procedure may be risky for bare metal.

7. Shot-peening (also known as shot-blasting) produces residual compressive stresses in the surface of the metal. Very substantial improvement in resistance to stress corrosion found as a result of peening with glass beads. Type 410 stainless was exposed to 3% NaCl at room temperature; type 304 to 42% MgCI2 at 150°C; and aluminum alloy 7075 T6 to a water solution of K2Cr2O7 CrO3 NaCl at room

10

and aluminum alloy 7075-T6 to a water solution of K2Cr2O7-CrO3-NaCl at room temperature.

Page 11: Cl on Metals in CWS ALP 2009

AluminiumAluminium• Aluminium alloys are widely used structural materials in

the industry because of their high strength-to-weight ratio and good fracture toughnessand good fracture toughness.

• Their resistance to corrosion depends upon the rapidly formed stable oxide filmformed stable oxide film.

• However, in the presence of aggressive anions, such as halide, which can destabilize the passive film over the a de, c ca des ab e e pass e o e ematrix, aluminium alloys are subject to localized corrosion.

Page 12: Cl on Metals in CWS ALP 2009

Aluminium continAluminium contin.Al i i d i i i i hl id i l l• Aluminium tends to pit in waters containing chlorides, particularly at crevices or at stagnant areas where passivity breaks down through the action of differential aeration cells.

• The mechanism is analogous to that for stainless steel.

Page 13: Cl on Metals in CWS ALP 2009

Copper and its alloysCopper and its alloysC i f i i ll i h b f• Corrosion of copper-containing alloys in the absence of complicating, aggressive variables decreases as the system pH increases. Variables affecting copper corrosion and its alloys are ammonia and ammonia ions, hydrogen sulphide and other sulphur compounds, and aerated acidic waters.

• Waters high in oxygen and carbon dioxide and low in calciumWaters high in oxygen and carbon dioxide and low in calcium and manganese hardness, waters high in chlorides and hypochlorite ion and waters high in bicarbonate alkalinity also affect copper corrosionaffect copper corrosion.

• The corrosion resistance of copper metal in cooling water systems depends on the stability of the protective oxide film. Any contaminant that interferes with the protective oxide film formation or stability will increase copper attack.