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CHAPTER I
I n t r o d u c t i o n t o S u r f a c e
E n g i n e e r i n g f o r
C o r r o s i o n a n d W e a r
R e s i s t a n c e
SURFACE ENGINEERING is a multidisciplinary activity intended
totailor the properties of the surfaces of engineering components
so thattheir function and serviceability can be improved. The ASM
Handbook de-fines surface engineering as "treatment of the surface
and near-surfaceregions of a material to allow the surface to
perform functions that aredistinct from those functions demanded
from the bulk of the material"(Ref 1). The desired properties or
characteristics of surface-engineeredcomponents include:
Improved corrosion resistance through barrier or sacrificial
protectionImproved oxidation and/or sulfidation resistanceImproved
wear resistanceReduced frictional energy lossesImproved mechanical
properties, for example, enhanced fatigue ortoughnessImproved
electronic or electrical propertiesImproved thermal
insulationImproved aesthetic appearance
As indicated in Table 1, these properties can be enhanced
metallurgically,mechanically, chemically, or by adding a
coating.
The bulk of the material or substrate cannot be considered
totally inde-pendent of the surface treatment. Most surface
processes are not limitedto the immediate region of the surface,
but can involve the substrate by
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Table 1 Surface engineering options and property benefitsSurface
treatment/coating type
Changing the surface metallurgyLocalized surface hardening
(flame, induction, laser,
and electron-beam hardening)Laser melting
Shot peening
Changing the surface chemistryPhosphate chemical conversion
coatings
Chromate chemical conversion coatings
Black oxide chemical conversion coatingsAnodizing
(electrochemical conversion coating)
Steam treating
Carburizing
Nitriding
CarbonitridingFerritic nitrocarburizingDiffusion (pack
cementation) chromizingDiffusion (pack cementation)
aluminizingDiffusion (pack cementation) siliconizingBoronizing
(bonding)Ion implantationLaser alloying
Adding a surface layer or coatingOrganic coatings (paints and
polymeric or elastomeric
coatings and linings)Ceramic coatings (glass linings, cement
linings,
and porcelain enamels)Slip/sinter ceramic coatingsHot-dip
galvanizing (zinc coatings)Hot-dip aluminizingHot-dip lead-tin
alloy-coatings (terne coatings)Tin plate (continuous
electrodeposition)Zinc-nickel alloy plate (continuous
electrodeposition)Electroplating
Electroless plating
Mechanical platingWeld overlays
Thermal spraying
Cladding (roll bonding, explosive bonding, hotisostatic
pressing, etc.)
Laser claddingCarbide (salt bath) diffusionChemical vapor
deposition (CVD)
Physical vapor deposition (PVD)
Primary property benefits
Improved wear resistance through the development of a hard
martensitic surface
Improved wear resistance through grain refinement and the
formation of finedispersions of precipitates
Improved fatigue strength due to compressive stresses induced on
the exposedsurface, also relieves tensile stresses that contribute
to stress-corrosion cracking
Used primarily on steels for enhanced corrosion resistance,
increased plating or paintadhesion, and for lubricity (e.g., to
increase the formability of sheet metals)
Enhanced bare or painted corrosion resistance, improved adhesion
of paint orother organic finishes, and provides the metallic
surface with a decorative finish
Used for decorative applications, e.g., the "bluing" on steel
gun barrelsUsed primarily for aluminum for increased corrosion
resistance, improved decorative
appearance, increased abrasion resistance (hard anodizing),
improved paint adhe-sion, and improved adhesive bonding (higher
bond strength and durability)
Used on ferrous powder metallurgy parts to increase wear
resistance and transverserupture strength
Used primarily for steels for increased resistance to wear,
bending fatigue, androlling-contact fatigue
Used primarily for steels for improved wear resistance,
increased fatigue resistance,and improved corrosion resistance
(except stainless steels)
Used primarily for steels for improved wear resistanceImproved
antiscuffing characteristics of ferrous alloysImproved molten-salt
hot corrosionImproved oxidation resistance, sulfidation resistance,
and carburization resistanceImproved oxidation resistanceImproved
wear resistance, oxidative wear, and surface fatigueImproved
friction and wear resistance for a variety of substratesImproved
wear resistance
Improved corrosion resistance, wear resistance, and aesthetic
appearance
Improved corrosion resistance
Improved wear resistance and heat resistanceImproved corrosion
resistance via sacrificial protection of steel substrateImproved
corrosion and oxidation resistance of steel substrateImproved
corrosion resistance of steel substrateImproved corrosion
resistance of steel substrateImproved corrosion resistance of steel
substrateDepending on the metal or metals being electrodeposited,
improved corrosion
resistance (e.g., nickel-chromium multilayer coatings, and
cadmium and zinc sacri-ficial coatings), wear resistance (e.g.,
hard chromium coatings), electrical proper-ties (e.g., copper and
silver), and aesthetic appearance (e.g., bright nickel or
deco-rative chromium plating)
Improved corrosion resistance (nickel-phosphorus) and wear
resistance(nickel-phosphorus and nickel-boron)
Improved corrosion resistanceImproved wear resistance
(hardfacing alloys) and corrosion resistance (stainless steel
or nickel-base overlays) and dimensional restoration (buildup
alloys)Primarily used for improved wear resistance (many coating
systems including ceramics
and cermets), but also used for improved corrosion resistance
(aluminum, zinc, andtheir alloys) and oxidation resistance (e.g.,
MCrAlY), thermal barrier protection(partially stabilized zirconia),
electrically conductive coatings (e.g., copper and sil-ver), and
dimensional restoration
Improved corrosion resistance
Improved wear resistanceUsed primarily for steels for improved
wear resistance in tooling applicationsImproved wear (e.g., tools
and dies), erosion, and corrosion resistance; also used for
epitaxial growth of semiconductorsImproved wear (e.g., tools and
dies) and corrosion resistance, improved optical and
electronic properties, and decorative applications
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exposure to either a thermal cycle or a mechanical stress. For
example,diffusion heat treatment coatings (e.g.,
carburizing/nitriding) often havehigh-temperature thermal cycles
that may subject the substrate to temper-atures that cause phase
transformations and thus property changes, orshot-peening
treatments that deliberately strain the substrate surface to
in-duce improved fatigue properties. It is the purpose of this
book, and inparticular Chapters 4 to 6, to review information on
surface treatmentsthat improve service performance so that
metallurgists, chemists, me-chanical engineers, and design
engineers may consider surface-engi-neered components as an
alternative to more costly materials.
Surface Engineering to Combat Corrosion and Wear
The Economic Effects of Corrosion and Wear. The progressive
deteri-oration, due to corrosion and wear, of metallic surfaces in
use in major in-dustrial plants ultimately leads to loss of plant
efficiency and at worst a shut-down. Corrosion and wear damage to
materials, both directly and indirectly,costs the United States
hundreds of billions of dollars annually. For exam-ple, corrosion
of metals costs the U.S. economy almost $300 billion per yearat
current prices. This amounts to about 4.2% of the gross national
product.
However, about 40% of the total cost could be avoided by proper
corro-sion prevention methods. Table 2 provides a breakdown of the
cost ofmetallic corrosion in the United States. Similar studies on
wear failureshave shown that the wear of materials costs the U.S.
economy about $20billion per year (in 1978 dollars) compared to
about $80 billion annually(see Table 2) for corrosion during the
same period. Table 3 illustrates theextent of wear failures by
various operations within specific industrialsegments. Highway
vehicles alone use annually 14,600 X 1012 Btu/ton ofenergy
represented in lost weight of steel and 18.6% of this energy
couldbe saved through effective wear-control measures.
Table 2 Cost of metallic corrosion in the United StatesBillions
of U.S. dollars
Industry 1975 1995
All industriesTotal 82.0 296.0Avoidable 33.0 104.0
Motor vehiclesTotal 31.4 94.0Avoidable 23.1 65.0
AircraftTotal 3.0 13.0Avoidable 0.6 3.0
Other industriesTotal 47.6 189.0Avoidable 9.3 36.0
Source: Ref 2
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Table 3 Industrial operations with significant annualwear
economic consequencesIndustry Operation Loss mass (a), 1012
BtuUtilities (28% total U.S. Seate 185
consumption) Accessories 120Bearings 55Reliability 145Total
505
Transportation (26% total Brakes (b)U.S. consumption) Valve
trains (b)
Piston ring assemblies (b)Transmission (b)Bearings (b)Gears
(b)Total (b)
Mining Ore processing 22.80Surface mining 13.26Shaft mining
10.70Drilling 5.58Total 52.34
Agriculture Tillage 16.85Planting 2.47Total 19.32
Primary metals Hot rolling 14.30Cold rolling 0.14Total 14.44
(a) Assumes 19.2 X 106 Btu per ton of energy represented in lost
weight of steel.(b) Lost mass not estimated. Source: Ref 3
Corrosive Wear. Complicating matters is the fact that the
combined ef-fects of wear and corrosion can result in total
material losses that are muchgreater than the additive effects of
each process taken alone, which indi-cates a synergism between the
two processes. Although corrosion canoften occur in the absence of
mechanical wear, the opposite is rarely true.Corrosion accompanies
the wear process to some extent in all environ-ments, except in
vacuum and inert atmospheres. Corrosion and wear oftencombine to
cause aggressive damage in a number of industries, such asmining,
mineral processing, chemical processing, pulp and paper
produc-tion, and energy production. Corrosion and wear processes
involve manymechanisms, the combined actions of which lead to the
mutual reinforce-ment of their effectiveness. As listed in Table 4,
17 synergistic relation-ships among abrasion, impact, and corrosion
that could significantly in-crease material degradation in wet and
aqueous environments have beenidentified.
The combined effects of corrosion and wear can also lead to
galvaniccorrosion in some applications, such as crushing and
grinding (comminu-tion) of mineral ores. Wear debris and corrosion
products that are formedduring comminution affect product quality
and can adversely affect sub-sequent benefication by altering the
chemical and electrochemical proper-ties of the mineral system (Ref
5-8). Electrochemical interactions be-tween minerals and grinding
media can occur, causing galvanic couplingthat leads to increased
corrosion wear. More detailed information on gal-vanic corrosion
can be found in Chapter 2.
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Methods to Control Corrosion. Owing to its many favorable
charac-teristics, steel is well suited and widely used for a broad
range of engi-neering applications and is referenced here to
demonstrate the various cor-rosion-control steps that can be
considered. Steel has a variety of excellentmechanical properties,
such as strength, toughness, ductility, and dent re-sistance. Steel
also offers good manufacturability, including
formability,weldability, and paintability. Other positive factors
include its availability,ferromagnetic properties, recyclability,
and cost. Because steel is suscep-tible to corrosion in the
presence of moisture, and to oxidation at elevatedtemperatures,
successful use of these favorable characteristics generallyrequires
some form of protection.
Methods of corrosion protection employed to protect steel
include:
Altering the metal by alloying, that is, using a more highly
alloyed andexpensive stainless steel rather than a plain carbon or
low-alloy steelChanging the environment by desiccation or the use
of inhibitorsControlling the electrochemical potential by the
application of ca-thodic or anodic currents, that is, cathodic and
anodic protectionApplying organic, metallic, or inorganic (glasses
and ceramics) coat-ings
Application of corrosion-resistant coatings is one of the most
widely usedmeans of protecting steel. As shown in Table 1, there
are a wide varietyof coatings to choose from, and proper selection
is based on the compo-nent size and accessibility, the corrosive
environment, the anticipated
Table 4 Synergistic relationships between wear and corrosion
mechanismsAbrasionRemoves protective oxidized metal and polarized
coatings to expose unoxidized metal, in addition to removing metal
particles.Forms microscopic grooves and dents for concentration
cell corrosion.Increases microscopic surface area exposed to
corrosion.Removes strain-hardened surface layers.Cracks brittle
metal constituents forming sites for impact hydraulic
splitting.Plastic deformation by high-stress metal-mineral contact
causes strain hardening and susceptibility to chemical attack.
CorrosionProduces pits that induce microcracking.Microcracks at
pits invite hydraulic splitting during impact.Roughens surface,
reducing energy needed to abrade away metal.May produce hydrogen
with subsequent absorption and cracking in steel.Selectively
attacks grain boundaries and less noble phases of multiphase
microstructures, weakening adjacent metal.ImpactPlastic deformation
makes some constituents more susceptible to corrosion.Cracks
brittle constituents, tears apart ductile constituents to form
sites for crevice corrosion, hydraulic splitting.Supplies kinetic
energy to drive abrasion mechanism.Pressurizes mill water to cause
splitting, cavitation, and jet erosion of metal and protective
oxidized material.Pressurizes mill water and gases to produce
unknown temperatures, phase changes, and decomposition or reaction
products from ore and water
constituents.Heats ball metal, ore, fluids to increase corrosive
effects.
Source: Ref 4
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Thickness, mmFig. 1 Approximate thickness of various surface
engineering treatments
temperatures, component distortion, the coating thickness
attainable(Fig. 1), and costs. Many of these selection criteria are
addressed in Chap-ters 6 to 8 in this book.
Painting is probably the most widely used engineering coating
used toprotect steel from corrosion. There are a wide variety of
coating formula-tions that have been developed for outdoor
exposure, marine atmospheres,water immersion, chemical fumes,
extreme sunlight, high humidity, andmoderately high temperatures
(less than about 200 0C, or 400 0F).
The most widely used corrosion-resistant metallic coatings are
hot-dipped zinc, zinc-aluminum, and aluminum coatings. These
coatings ex-hibit excellent resistance to atmospheric corrosion and
are widely used inthe construction, automobile, utility, and
appliance industries.
Other important coating processes for steels include
electroplating,electroless plating, thermal spraying, pack
cementation aluminizing (forhigh-temperature oxidation resistance),
and cladding (including weldcladding and roll-bonded claddings).
Applications and corrosion perform-ance of these coatings are
described in Chapter 6 in this book.
Methods to Control Wear. As is described in Chapter 3 in this
book,there are many types of wear, but there are only four main
types of wearsystems (tribosystems) that produce wear and six basic
wear control steps(Ref 9). The four basic tribosystems are:
Weld overlayFriction surfacing
Thermal sprayingCarburizing
CarbonitridingNitrocarburizing
NitridingMechanical working
Electrochemical plate + diffusionTransformation hardening
Surface alloyinglasersHot dipping (galvanizing and aluminum)
Mechanical platingElectroless plating
Electrolytic platingChemical vapor deposition
Physical vapor depositionResin or laquer"bonding
Ion implantation
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Relatively smooth solids sliding on other smooth solidsHard,
sharp substances sliding on softer surfacesFatigue of surfaces by
repeated stressing (usually compressive)Fluids with or without
suspended solids in motion with respect to asolid surface
As shown in Fig. 2, the wear that occurs in these tribosystems
can be ad-dressed by coatings or by modifications to the substrate
metallurgy orchemistry.
The six traditional techniques applied to materials to deal with
wearproduced in the preceding tribosystems include:
Separate conforming surfaces with a lubricating film (see
Chapter 3 inthis book).Make the wearing surface hard through the
use of hardfacing, diffu-sion heat treatments, hard chromium
plating, or more recently devel-oped vapor deposition techniques or
high-energy processes (e.g., ionimplantation).Make the wearing
surface resistant to fracture. Many wear processesinvolve fracture
of material from a surface; thus toughness and frac-ture resistance
play a significant role in wear-resistant surfaces. Theuse of very
hard materials such as ceramics, cemented carbides, andhard
chromium can lead to fracture problems that nullify the benefitsof
the hard surface.Make the eroding surface resistant to corrosion.
Examples include theuse of cobalt-base hardfacing alloys to resist
liquid erosion, cavitation,and slurry erosion; aluminum bronze
hardfacing alloys to prevent cav-itation damage on marine
propellers or to repair props that have
Substrate treatments toreduce wearThrough hardeningSurface
hardening (flame,induction, EB, laser)Diffusion of a
hardeningspecies (carburizing,nitriding, etc.)Laser/EB alloyingIon
implantationWork hardening
Coatings to reduce
wearPolymers/elastomersElectrochemical(plating, etc.)Chemical
(CVD,electroless plating)Thermal sprayingFusion weldingThin films
(PVD,sputtering, ion plating)Wear tilesCladding (cast,
explosion,hot rolling)Lubricants
Wear-causingeffects
Tribosystem SurfacewearFlC. 2 Surface engineering processes used
to prevent wear. CVD, chemical vapor deposition; PVD,
" physical vapor deposition; EB, electron beam
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suffered cavitation damage; nickel-base hardfacing alloys to
resistchemical attack; and epoxy-filled rebuilding cements used to
resistslurry erosion in pumps.Choose material couples that are
resistant to interaction in sliding(metal-to-metal wear
resistance). Hardfacing alloys such as cobalt-base and
nickel-chromium-boron alloys have been used for manyyears for
applications involving metal-to-metal wear. Other
surface-engineering options include through-hardened tool steels,
diffusion(case)-hardened surfaces, selective surface-hardened alloy
steels, andsome platings.Make the wearing surface fatigue
resistant. Rolling-element bear-ings, gears, cams, and similar
power-transmission devices oftenwear by a mechanism of surface
fatigue. Repeated point or line con-tact stresses can lead to
subsurface cracks that eventually grow toproduce surface pits and
eventual failure of the device. Prevention ispossible through the
use of through-hardened steels, heavy case-hardened steels, and
flame-, induction-, electron beam-, or laser-hardened steels.
More details on these surface-engineering techniques can be
found inChapters 5 through 8 in this book.
Material/Process Selection (Ref 10). Faced with the wide range
ofpossibilities indicated in Table 1 and the discussions on
"Methods to Con-trol Corrosion" and "Methods to Control Wear,"
selection of surface engi-
Predict workingenvironment fromconsideration of design
Proceed with one-piececonstruction (see notebelow)Note:
One-piece construction is often leastexpensive for small parts as
some surfacing alloysare available as castings machined to finished
sizeor as powder metallurgical parts.
Identify materialrequirements forstructure and surface
Consider one-piececonstructionYes No
Select surfacing materialto suit requirements
Analyze service failuresto assist selection ofbetter
materials
Select substrate materialto suit strength, heat, andcorrosion
needs
Select from surfacing processessuitable for chosen material and
job,(must satisfy needs for coating density,thickness, dilution,
etc.)
Reconsidermaterials
Decide if chosen processsuits substrate materialand design
(adhesion,HAZ, access, distortion,etc.)
Yes
Identify quality assuranceand control needs Finalize choice
ofmaterials and process
Decide manufacturingdetails, procedures,health and
safetyrequirements, etc.
Fig, 3 Checklist for surface engineering material/process
selection. HAZ, heat-affected zone
Reconsider processand/or material
None
Yes No
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neering material and process may seem difficult, but it is
normallystraightforward. Often there are constraints placed on the
choice becauseof availability (e.g., laser melting and/or alloying
are not widely used, andthese processes can only be obtained by a
special arrangement with laserjob shops). In many cases there is a
precedent, but when considering anew problem it helps to follow a
checklist of the type shown in Fig. 3.
The sequence of decisions to be made covers several
fundamentalpoints. The first is the need to be clear about service
conditions, based onexperience or plant data. This is the key to
material selection. The seconddecision is the choice of application
process for the material. This involvesthe question of
compatibility with the coating material; that is, not all
ma-terials can be applied by all processes. A further question of
compatibilityarises between both material and process with the
substrate, for example,whether distortion from high-temperature
processes be tolerated. All theseissues are covered in subsequent
chapters in this book (see, in particular,Chapters 7 and 8).
References
1. CM. Cotell and J.A. Sprague, Preface, Surface Engineering,
VoI 5,ASM Handbook, ASM International, 1994, p v
2. Economic Effects of Metallic Corrosion in the United States,
BattelleColumbus Laboratories and the National Institute of
Standards andTechnology, 1978 and Battelle updates in 1995
3. "Tribological Sinks in Six Major Industries," Report Number
PNL-5535, Sept 1985, Pacific Northwest Laboratory, Richland, WA,
oper-ated for the U.S. Department of Energy by Battelle Memorial
Institute(NTIS No. DE86000841)
4. DJ. Dunn. Metal Removal Mechanisms Comprising Wear in
MineralProcessing, Wear of Materials, K.C. Ludema, Ed., American
Societyof Mechanical Engineers, 1985, p 501-508
5. R.L. Pozzo and I. Iwasaki, Pyrite-Pyrrhotite Grinding Media
Interac-tions and Their Effects on Media Wear and Flotation, /.
Electrochem.Soc, VoI 136 (No. 6), 1989, p 1734-1740
6. R.L. Pozzo and I. Iwasaki, Effect of Pyrite and Pyrrhotite on
the Cor-rosive Wear of Grinding Media, Miner. Metall. Process., Aug
1987, p166-171
7. K.A Natarajan, S.C. Riemer, and I. Iwasaki, Influence of
Pyrrhotite onthe Corrosive Wear of Grinding Balls in Magnetite Ore
Grinding, Int.J. Miner. Process., VoI 13 1984, p 73-81
8. R.L. Pozzo and I. Iwasaki, An Electro-chemical Study of
Pyrrhotite-Grinding Media Interaction Under Abrasive Conditions,
Corrosion,VoI 43 (No. 3), 1987, p 159-169
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9. K.G. Budinski, Surface Engineering for Wear Resistance,
Prentice-Hall, Inc., 1988, p 6-10
10. Engineering CoatingsDesign and Application, 2nd ed., S.
Graingerand J. Blunt, Ed., Woodhead Publishing Ltd., 1999, p 7
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CHAPTER Mm
P r i n c i p l e s o f C o r r o s i o n
CORROSION of metal is a chemical or electrochemical process
inwhich surface atoms of a solid metal react with a substance in
contact withthe exposed surface. Usually the corroding medium is a
liquid substance,but gases and even solids can also act as
corroding media. In some in-stances, the corrodent is a bulk fluid;
in others, it is a film, droplets, or asubstance adsorbed on or
absorbed in another substance.
All structural metals corrode to some extent in natural
environments(e.g., the atmosphere, soil, or waters). Bronze, brass,
most stainless steels,zinc, and pure aluminum corrode so slowly in
service conditions that longservice life is expected without
protective coatings. Corrosion of struc-tural grades of cast iron
and steel, the 400 series stainless steels, and somealuminum
alloys, however, proceeds rapidly unless the metal is
protectedagainst corrosion. As described in Chapter 1, corrosion of
metals is of par-ticular concern because annual losses in the
United States attributed tocorrosion amount to hundreds of billions
of dollars.
Although emphasis in this Chapter has been placed on irons and
steels,the electrochemical corrosion basics and the forms of
corrosion describedare applicable to all metallic materials. For
more detailed information onthe corrosion resistance of various
metals and their alloys, the readershould consult the selected
references listed at the conclusion of thisChapter, as well as
Corrosion, VoI 13, of the ASM Handbook or Corro-sion: Understanding
the Basics, published by ASM International in 2000.
Electrochemical Corrosion Basics
Electrochemical corrosion in metals in a natural environment,
whetheratmosphere, in water, or underground, is caused by a flow of
electricityfrom one metal to another, or from one part of a metal
surface to anotherpart of the same surface where conditions permit
the flow of electricity.
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Fig. 1 Simple electrochemical cell showing the components
necessary for corrosion
For the flow of energy to take place, either a moist conductor
or anelectrolyte must be present. An electrolyte is an
electricity-conducting so-lution containing ions, which are atomic
particles or radicals bearing anelectrical charge. Charged ions are
present in solutions of acids, alkalis,and salts. The presence of
an electrolyte is necessary for corrosion tooccur. Water,
especially salt water, is an excellent electrolyte.
Electricity passes from a negative area to a positive area
through theelectrolyte. For corrosion to occur in metals, one must
have (a) an elec-trolyte, (b) an area or region on a metallic
surface with a negative charge,(c) a second area with a positive
charge, and (d) an electrically conductivepath between (b) and (c).
These components are arranged to form a closedelectrical circuit.
In the simplest case, the anode would be one metal, suchas iron,
the cathode another, perhaps copper, and the electrolyte might
ormight not have the same composition at both anode and cathode.
Theanode and cathode could be of the same metal under conditions
describedlater in this article.
The cell shown in Fig. 1 illustrates the corrosion process in
its simplestform. This cell includes the following essential
components: (a) a metalanode, (b) a metal cathode, (c) a metallic
conductor between the anode andthe cathode, and (d) an electrolyte
in contact with the anode and the cath-ode. If the cell were
constructed and allowed to function, an electrical cur-rent would
flow through the metallic conductor and the electrolyte, and ifthe
conductor were replaced by a voltmeter, a potential difference
betweenthe anode and the cathode could be measured. The anode would
corrode.Chemically, this is an oxidation reaction. The formation of
hydrated rediron rust by electrochemical reactions may be expressed
as follows:
Metallic conductor betweenthe anode and the cathodeCurrent flow
in
conductor
Metal anode
Oxidation reactionoccurs at anode
Metal cathodeOxygen or otherdepolarizer in
electrolyte
Electrolyte, water containingconductive salts
Reduction reactionoccurs at cathode
Current flow throughthe electrolyte
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(EqI)
(Eq 2)
During metallic corrosion, the rate of oxidation equals the rate
of re-duction. Thus, a nondestructive chemical reaction, reduction,
would pro-ceed simultaneously at the cathode. In most cases,
hydrogen gas is pro-duced on the cathode. When the gas layer
insulates the cathode from theelectrolyte, current flow stops, and
the cell is polarized. However, oxygenor some other depolarizing
agent is usually present to react with the hy-drogen, which reduces
this effect and allows the cell to continue to func-tion.
Contact between dissimilar metallic conductors or differences in
theconcentration of the solution cause the difference in potential
that resultsin electrical current. Any lack of homogeneity on the
metal surface or itsenvironment may initiate attack by causing a
difference in potential, andthis results in localized corrosion.
The metal undergoing electrochemicalcorrosion need not be immersed
in a liquid but may be in contact withmoist soil or may have moist
areas on the metal surface.
Corrosive Conditions
If oxygen and water are both present, corrosion will normally
occur oniron and steel. Rapid corrosion may take place in water,
the rate of corro-sion being accelerated by several factors such
as: (a) the velocity or theacidity of the water, (b) the motion of
the metal, (c) an increase in tem-perature or aeration, and (d) the
presence of certain bacteria. Corrosioncan be retarded by
protective layers or films consisting of corrosion prod-ucts or
adsorbed oxygen. High alkalinity of the water also retards the
rateof corrosion on steel surfaces. Water and oxygen remain the
essential fac-tors, however, and the amount of corrosion is
generally controlled by oneor the other. For example, corrosion of
steel does not occur in dry air andis negligible when the relative
humidity of the air is below 30% at normalor lower temperatures.
This is the basis for prevention of corrosion by
de-humidification.
Water can readily dissolve a small amount of oxygen from the
atmos-phere, thus becoming highly corrosive. When the free oxygen
dissolved inwater is removed, the water becomes practically
noncorrosive unless itbecomes acidic or anaerobic bacteria incite
corrosion. If oxygen-freewater is maintained at a neutral pH or at
slight alkalinity, it is practically
-
noncorrosive to structural steel. Steam boilers and water supply
systemsare effectively protected by deaerating the water.
Additional informationon corrosion in water can be found in Ref
1.
Soils. Dispersed metallic particles or bacteria pockets can
provide a nat-ural electrical pathway for buried metal. If an
electrolyte is present and thesoil has a negative charge in
relation to the metal, an electrical path fromthe metal to the soil
will occur, resulting in corrosion. Differences in soilconditions,
such as moisture content and resistivity, are commonly re-sponsible
for creating anodic and cathodic areas (Fig. 2). Where a
differ-ence exists in the concentration of oxygen in the water or
in moist soils incontact with metal at different areas, cathodes
develop at points of rela-tively high-oxygen concentrations and
anodes at points of low concentra-tion. Further information on
corrosion in soils is available in Ref 2.
Chemicals. In an acid environment, even without the presence of
oxy-gen, the metal at the anode is attacked at a rapid rate. At the
cathode,atomic hydrogen is released continuously, to become
hydrogen gas. Cor-rosion by an acid can result in the formation of
a salt, which slows the re-action because the salt formation on the
surface is then attacked.
Corrosion by direct chemical attack is the single most
destructive forceagainst steel surfaces. Substances having chlorine
or other halogens in theircomposition are particularly aggressive.
Galvanized roofing has beenknown to corrode completely within six
months of construction, the build-ing being downwind of an aluminum
ingot plant where fluorides were al-ways present in the atmosphere.
Consequently, galvanized steel should nothave been specified.
Selection of materials and evaluation of service con-ditions are
extremely important in combating corrosion. The response ofvarious
materials to chemical environments is addressed in Ref 3 and 4.
Atmospheric corrosion differs from the corrosion action that
occurs inwater or underground, because sufficient oxygen is always
present. In at-
pjo 2 A metal pipe buried in moist soil forming a corrosion
cell. A difference^* in oxygen content at different levels in the
electrolyte will produce a
difference of potential. Anodic and cathodic areas will develop,
and a corrosioncell, called a concentration cell, will form.
Oxygen diffusing into earthfrom ground surfaceElectrolyte 1
(soil withground water high in
oxygen content)Current flow
Electrolyte 2 (soil withground water deficient
in oxygen content)Fe2+ (rust)
Anodic area (steelat bottom of pipe)
Buried pipe
Cathodic area (steelat top of pipe)
-
mospheric corrosion, the formation of insoluble films and the
presence ofmoisture and deposits from the atmosphere control the
rate of corrosion.Contaminants such as sulfur compounds and salt
particles can acceleratethe corrosion rate. Nevertheless,
atmospheric corrosion occurs primarilythrough electrochemical means
and is not directly caused by chemical at-tack. The anodic and
cathodic areas are usually quite small and close to-gether so that
corrosion appears uniform, rather than in the form of
severepitting, which can occur in water or soil. A more detailed
discussion on at-mospheric corrosion can be found in Ref 5.
Forms of Corrosion
The differing forms of corrosion can be divided into the
following eightcategories based on the appearance of the corrosion
damage or the mech-anism of attack:
Uniform or general corrosionGalvanic corrosionPitting
corrosionCrevice corrosion, including corrosion under tubercles or
deposits, fil-iform corrosion, and poultice
corrosionErosion-corrosion, including cavitation erosion and
fretting corrosionIntergranular corrosion, including sensitization
and exfoliationDealloyingEnvironmentally assisted cracking,
including stress-corrosion crack-ing (SCC), corrosion fatigue, and
hydrogen damage (including hydro-gen embrittlement,
hydrogen-induced blistering, high-temperature hy-drogen attack, and
hydride formation)
Figure 3 illustrates schematically some of the most common forms
of cor-rosion. More detailed information pertaining to recognition
and preven-tion of these forms of corrosion can be found in Ref 6
and 7.
Uniform CorrosionGeneral Description. Uniform or general
corrosion, as the name im-
plies, results in a fairly uniform penetration (or thinning)
over the entireexposed metal surface. The general attack results
from local corrosion-cellaction; that is, multiple anodes and
cathodes are operating on the metalsurface at any given time. The
location of the anodic and cathodic areascontinues to move about on
the surface, resulting in uniform corrosion.Uniform corrosion often
results from atmospheric exposure (especiallypolluted industrial
environments); exposure in fresh, brackish, and saltwaters; or
exposure in soils and chemicals.
-
Flg. 3 Schematics of the common forms of corrosion
Metals Affected. All metals are affected by uniform corrosion,
al-though materials that form passive films, such as stainless
steels or nickel-chromium alloys, are normally subjected to
localized forms of attack. Therusting of steel, the green patina
formation on copper, and the tarnishingof silver are typical
examples of uniform corrosion. In some metals, suchas steel,
uniform corrosion produces a somewhat rough surface by re-moving a
substantial amount of metal, which either dissolves in the
envi-ronment or reacts with it to produce a loosely adherent,
porous coating ofcorrosion products. In such reactions as in the
tarnishing of silver in air,the oxidation of aluminum in air, or
attack on lead in sulfate-containingenvironments, thin, tightly
adherent protective films are produced, and themetal surface
remains smooth.
Prevention. Uniform corrosion can be prevented or reduced by
propermaterials selection, the use of coatings or inhibitors, or
cathodic protec-tion. These corrosion prevention methods can be
used individually or incombination.
Galvanic CorrosionGeneral Description. The potential available
to promote the electro-
chemical corrosion reaction between dissimilar metals is
suggested by thegalvanic series, which lists a number of common
metals and alloysarranged according to their tendency to corrode
when in galvanic contact(Table 1). Metals close to one another on
the table generally do not havea strong effect on each other, but
the farther apart any two metals are sep-arated, the stronger the
corroding effect on the one higher in the list. It ispossible for
certain metals to reverse their positions in some environ-ments,
but the order given in Table 1 is maintained in natural waters
andthe atmosphere. The galvanic series should not be confused with
the sim-
Pitting Exfoliation Dealloying lntergranular
Stress-corrosioncracking
Corrosionfatigue
Tensile stress Cyclic stress
CreviceFrettingErosionGalvanicUniformNo corrosion
More noblemetal
Flowingcorrodent
Cyclicmovement Metal ornonmetal
-
Table 1 Galvanic series in seawater at 25 0C (77 0F)Corroded end
(anodic, or least noble)MagnesiumMagnesium alloysZincGalvanized
steel or galvanized wrought ironAluminum alloys 5052, 3004, 3003,
1100, 6053, in this orderCadmiumAluminum alloys 2117, 2017, 2024,
in this orderLow-carbon steelWrought ironCast ironNi-Resist
(high-nickel cast iron)Type 410 stainless steel (active)50-50
lead-tin solderType 304 stainless steel (active)Type 316 stainless
steel (active)LeadTinCopper alloy C28000 (Muntz metal, 60%
Cu)Copper alloy C67500 (manganese bronze A)Copper alloys C46400,
C46500, C46600, C46700 (naval brass)Nickel 200 (active)Inconel
alloy 600 (active)Hastelloy alloy BChlorimet 2Copper alloy C27000
(yellow brass, 65% Cu)Copper alloys C44300, C44400, C44500
(admiralty brass)Copper albys C60800, C61400 (aluminum
bronze)Copper alloy C23000 (red brass, 85% Cu)Copper C! 1000 (ETP
copper)Copper alloys C65100, C65500 (silicon bronze)Copper alloy
C71500 (copper nickel, 30% Ni)Copper alloy C92300, cast (leaded tin
bronze G)Copper alloy C92200, cast (leaded tin bronze M)Nickel 200
(passive)Inconel alloy 600 (passive)Monel alloy 400Type 410
stainless steel (passive)Type 304 stainless steel (passive)Type 316
stainless steel (passive)Incoloy alloy 825Inconel alloy
625Hastelloy alloy CChlorimet
3SilverTitaniumGraphiteGoldPlatinum
Protected end (cathodic, or most noble)
ilar electromotive force series, which shows exact potentials
based onhighly standardized conditions that rarely exist in
nature.
The three-layer iron oxide scale formed on steel during rolling
varieswith the operation performed and the rolling temperature. The
dissimilar-ity of the metal and the scale can cause corrosion to
occur, with the steelacting as the anode in this instance.
Unfortunately, mill scale is cathodicto steel, and an electric
current can easily be produced between the steeland the mill scale.
This electrochemical action will corrode the steel with-out
affecting the mill scale (Fig. 4).
A galvanic couple may be the cause of premature failure in metal
com-ponents of water-related structures or may be advantageously
exploited.
-
Fig, 4 Mill scale forming a corrosion cell on steel
Galvanizing iron sheet is an example of useful application of
galvanic ac-tion or cathodic protection. Iron is the cathode and is
protected against cor-rosion at the expense of the sacrificial zinc
anode. Alternatively, a zinc ormagnesium anode may be located in
the electrolyte close to the structureand may be connected
electrically to the iron or steel. This method is re-ferred to as
cathodic protection of the structure. Iron or steel can become
theanode when in contact with copper, brass, or bronze; however,
iron or steelcorrode rapidly while protecting the latter metals.
Also, weld metal may beanodic to the basis metal, creating a
corrosion cell when immersed (Fig. 5).
While the galvanic series (Table 1) represents the potential
available topromote a corrosive reaction, the actual corrosion is
difficult to predict.Electrolytes may be poor conductors, or long
distances may introducelarge resistance into the corrosion cell
circuit. More frequently, scale for-mation forms a partially
insulating layer over the anode. A cathode hav-ing a layer of
adsorbed gas bubbles, as a consequence of the corrosion
cellreaction, is polarized. The effect of such conditions is to
reduce the theo-retical consumption of metal by corrosion. The area
relationship betweenthe anode and cathode may also strongly affect
the corrosion rate; a highratio of cathode area to anode area
produces more rapid corrosion. In thereverse case, the cathode
polarizes, and the corrosion rate soon drops to anegligible
level.
The passivity of stainless steels is attributed to either the
presence of acorrosion-resistant oxide film or an oxygen-caused
polarizing effect,
FlC, 5 Weld metal forming a corrosion cell on steel. Weld metal
may be an-^* odic to steel, creating a corrosion cell when
immersed.
Electrolyte (water) (rust)Current flow
Cathode (steel)Anode(weld metal) \
Electrolyte (water)(rust) Cathode(broken mill
scale)Current flow
Anode(steel)
-
durable only as long as there is sufficient oxygen to maintain
the effect,over the surfaces. In most natural environments,
stainless steels will re-main in a passive state and thus tend to
be cathodic to ordinary iron andsteel. Change to an active state
usually occurs only where chloride con-centrations are high, as in
seawater or reducing solutions. Oxygen starva-tion also produces a
change to an active state. This occurs where the oxy-gen supply is
limited, as in crevices and beneath contamination onpartially
fouled surfaces.
Prevention. Galvanic corrosion can be prevented or reduced by
propermaterials selection (i.e., select combinations of metals as
close together aspossible in the galvanic series), insulating
dissimilar metals, applying abarrier coating to both the anodic
(less noble) and cathodic (noble) metal,applying a sacrificial
coating (aluminum, zinc, or cadmium) to the ca-thodic part,
applying nonmetallic films (e.g., anodizing aluminum alloys),and by
providing cathodic protection.
PittingGeneral Description. Pitting is a type of localized cell
corrosion. It is
predominantly responsible for the functional failure of iron and
steelwater-related installations. Pitting may result in the
perforation of waterpipe, rendering it unserviceable, even though
less than 5% of the totalmetal has been lost through rusting. Where
confinement of water is not afactor, pitting causes structural
failure from localized weakening whileconsiderable sound metal
still remains.
Pitting develops when the anodic or corroding area is small in
relationto the cathodic or protected area. For example, pitting can
occur wherelarge areas of the surface are covered by mill scale,
applied coatings, ordeposits of various kinds and where breaks
exist in the continuity of theprotective coating. Pitting may also
develop on bare, clean metal surfacesbecause of irregularities in
the physical or chemical structure of the metal.Localized,
dissimilar soil conditions at the surface of steel can also
createconditions that promote pitting.
Electrical contact between dissimilar materials or concentration
cells(areas of the same metal where oxygen or conductive salt
concentrationsin water differ) accelerates the rate of pitting. In
closed-vessel structures,these couples cause a difference of
potential that results in an electric cur-rent flowing through the
water or across the moist steel from the metal-lic anode to a
nearby cathode. The cathode may be copper, brass, millscale, or any
portion of a metal surface that is cathodic to the more activemetal
areas. In practice, mill scale is cathodic to steel and is found to
bea common cause of pitting. The difference of potential generated
betweensteel and mill scale often amounts to 0.2 to 0.3 V. This
couple is nearlyas powerful a generator of corrosion currents as is
the copper-steel cou-ple. However, when the anodic area is
relatively large compared with the
-
cathodic area, the damage is spread out and usually negligible,
but whenthe anode is relatively small, the metal loss is
concentrated and may bevery serious.
On surfaces having some mill scale, the total metal loss is
nearly con-stant as the anode is decreased, but the degree of
penetration increases.Figure 4 shows how a pit forms where a break
occurs in mill scale. Whencontact between dissimilar materials is
unavoidable and the surface ispainted, it is preferred to paint
both materials. If only one surface ispainted, it should be the
cathode. If only the anode is coated, any weakpoints such as
pinholes or holidays in the coating will probably result inintense
pitting.
As a pit, perhaps at a break in mill scale, becomes deeper, an
oxygenconcentration cell is started by depletion of oxygen in the
pit. The rate ofpenetration by such pits is accelerated
proportionately as the bottom of thepit becomes more anodic.
Fabrication operations may crack mill scale andresult in
accelerated corrosion.
Metals Affected. Pitting occurs in most commonly used metals and
al-loys. Iron buried in the soil corrodes with the formation of
shallow pits,but carbon steels in contact with hydrochloric acid or
stainless steels im-mersed in seawater characteristically corrode
with the formation of deeppits. Aluminum tends to pit in waters
containing chloride ions (for exam-ple, at stagnant areas), and
aluminum brasses are subject to pitting in pol-luted waters.
Despite their good resistance to general corrosion, stainless
steels aremore susceptible to pitting than many other metals.
High-alloy stainlesssteels containing chromium, nickel, and
molybdenum are also more re-sistant to pitting but are not immune
under all service conditions.
Pitting failures of corrosion-resistant alloys, such as
Hastelloy C,Hastelloy G, and Incoloy 825, are relatively uncommon
in solutions thatdo not contain halides, although any mechanism
that permits the estab-lishment of an electrolytic cell in which a
small anode is in contact with alarge cathodic area offers the
opportunity for pitting attack.
Prevention. Typical approaches to alleviating or minimizing
pittingcorrosion include the following:
Use defect-free barrier coatingsReduce the aggressiveness of the
environment, for example, chlorideion concentrations, temperature,
acidity, and oxidizing agentsUpgrade the materials of construction,
for example, use molybdenum-containing (4 to 6% Mo) stainless
steels, molybdenum + tungstennickel-base alloys, overalloy welds,
and use corrosion-resistant alloyliningsModify the design of the
system, for example, avoid crevices and theformation of deposits,
circulate/stir to eliminate stagnant solutions,and ensure proper
drainage
-
Crevice CorrosionGeneral Description. Crevice corrosion is a
form of localized attack
that occurs at narrow openings or spaces (gaps) between
metal-to-metal ornonmetal-to-metal components. This type of attack
results from a con-centration cell formed between the electrolyte
within the crevice, which isoxygen starved, and the electrolyte
outside the crevice, where oxygen ismore plentiful. The material
within the crevice acts as the anode, and theexterior material
becomes the cathode.
Crevices may be produced by design or accident. Crevices caused
bydesign occur at gaskets, flanges, rubber O-rings, washers, bolt
holes,rolled tube ends, threaded joints, riveted seams, overlapping
screen wires,lap joints, beneath coatings (filiform corrosion) or
insulation (poulticecorrosion), and anywhere close-fitting surfaces
are present. Figure 6shows crevice corrosion in a riveted assembly
caused by concentrationcells. Occluded regions are also formed
under tubercles (tuberculation),deposits (deposit corrosion), and
below accumulations or biological ma-terials (biologically
influenced corrosion). Similarly, unintentionalcrevices such as
cracks, seams, and other metallurgical defects could serveas sites
for corrosion.
Metals Affected. Resistance to crevice corrosion can vary from
onealloy-environment system to another. Although crevice corrosion
affectsboth active and passive metals, the attack is often more
severe for passivealloys, particularly those in the stainless steel
group. Breakdown of thepassive film within a restricted geometry
leads to rapid metal loss andpenetration of the metal in that
area.
Low metal ion concentration Metal ion concentration cell
High metal ion concentration
Oxygen concentration cellHigh oxygen concentration
Low oxygen concentrationFig. 6 Corrosion caused at crevices by
concentration cells. Both types of con-
centration cells shown sometimes occur simultaneously as in a
reentryangle in a riveted seam.
-
Prevention. Crevice corrosion can be prevented or reduced
through im-proved design to avoid crevices, regular cleaning to
remove deposits, byselecting a more corrosion-resistant material,
and by coating carbon steelor cast iron components with epoxy or
other field-applied or factory-applied organic coatings.
Erosion-CorrosionGeneral Description. Erosion-corrosion is the
acceleration or increase
in the rate of deterioration or attack on a metal because of
mechanicalwear or abrasive contributions in combination with
corrosion. The combi-nation of wear or abrasion and corrosion
results in more severe attack thanwould be realized with either
mechanical or chemical corrosive actionalone. Metal is removed from
the surface as dissolved ions, as particles ofsolid corrosion
products, or as elemental metal. The spectrum of erosion-corrosion
ranges from primarily erosive attack, such as sandblasting,
fil-ing, or grinding of a metal surface, to primarily corrosion
failures, wherethe contribution of mechanical action is quite
small.
All types of corrosive media generally can cause
erosion-corrosion, in-cluding gases, aqueous solutions, organic
systems, and liquid metals. Forexample, hot gases may oxidize a
metal then at high velocity blow off anotherwise protective scale.
Solids in suspension in liquids (slurries) areparticularly
destructive from the standpoint of erosion-corrosion.
Erosion-corrosion is characterized in appearance by grooves,
waves,rounded holes, and/or horseshoe-shaped grooves. Analysis of
these markscan help determine the direction of flow. Affected areas
are usually free ofdeposits and corrosion products, although
corrosion products can some-times be found if erosion-corrosion
occurs intermittently and/or the liquidflow rate is relatively
low.
Metals Affected. Most metals are susceptible to
erosion-corrosionunder specific conditions. Metals that depend on a
relatively thick protec-tive coating of corrosion product for
corrosion resistance are frequentlysubject to erosion-corrosion.
This is due to the poor adhesion of thesecoatings relative to the
thin films formed by the classical passive metals,such as stainless
steels and titanium. Both stainless steels and titanium
arerelatively immune to erosion-corrosion in many environments.
Metals that
Water flowImpingementcorrosion pits Original
metalsurfaceCorrosion film
Metal tube wall
Fig, 7 Schematic of erosion-corrosion of a condenser tube
-
are soft and readily damaged or worn mechanically, such as
copper andlead, are quite susceptible to erosion-corrosion. Even
the noble or pre-cious metals, such silver, gold, and platinum, are
subject to erosion-cor-rosion. Figure 7 shows a schematic of
erosion-corrosion of a condensertube wall. The direction of flow
and the resulting attack where the protec-tive film on the tube has
broken down are indicated.
Prevention. Erosion-corrosion can be prevented or reduced
through im-proved design (e.g., increase pipe diameter and/or
streamline bends to re-duce impingement effects), by altering the
environment (e.g., deaeration andthe addition of inhibitors), and
by applying hard, tough protective coatings.
CavitationGeneral Description. Cavitation is a form of
erosion-corrosion that is
caused by the formation and collapse of vapor bubbles in a
liquid againsta metal surface. Cavitation occurs in hydraulic
turbines, on pump im-pellers, on ship propellers, and on many
surfaces in contact with high-ve-locity liquids subject to changes
in pressure. The appearance of cavitationis similar to pitting
except that surfaces in the pits are usually muchrougher. The
affected region is free of deposits and accumulated
corrosionproducts if cavitation has been recent.
Figure 8 is a simplified representation of the cavitation
process. Figure8(a) shows a vessel containing a liquid. The vessel
is closed by an airtightplunger. When the plunger is withdrawn
(Fig. 8b), a partial vacuum iscreated above the liquid, causing
vapor bubbles to form and grow within
Partialvacuum
Pressurized
(a) RestQuiescent liquid
at standardtemperatureand pressure
(b) ExpansionLiquid boiling
at roomtemperature
(c) CompressionCollapse of
vapor bubbles
Metal
(d)
Metaloxide
Approachingmicrojettorpedo
Destruction ofmetal oxideon impact
Repair ofmetal oxide at
expense of metal
P J o- 8 Schematic representation of cavitation showing a cross
section through a vessel and plunger enclosing a f luid.
" (a) Plunger stationary, l iquid at standard temperature and
pressure, (b) Plunger withdrawn, l iquid boils at roomtemperature,
(c) Plunger advanced, bubbles collapse, (d) Disintegration of
protective corrosion product by impacting mi-crojet "torpedo."
Source: Ref 8
-
SurfaceOxide
Fig, 9 Schematic of the fretting process
BareMetalMetal andOxide Debris
the liquid. In essence, the liquid boils without a temperature
increase. Ifthe plunger is then driven toward the surface of the
liquid (Fig. 8c), thepressure in the liquid increases, and the
bubbles condense and collapse(implode). In a cavitating liquid,
these three steps occur in a matter of mil-liseconds. As shown in
Fig. 8(d), implosion of a vapor bubble creates amicroscopic
"torpedo" of water that is ejected from the collapsing bubbleat
velocities that may range from 100 to 500 m/s (330 to 1650 ft/s).
Whenthe torpedo impacts the metal surface, it dislodges protective
surface filmsand/or locally deforms the metal itself. Thus, fresh
surfaces are exposedto corrosion and the reformation of protective
films, which is followed bymore cavitation, and so on. Damage
occurs when the cycle is allowed torepeat over and over again.
Prevention. Cavitation can be controlled or minimized by
improving de-sign to minimize hydrodynamic pressure differences,
employing stronger(harder) and more corrosion-resistant materials,
specifying a smooth finishon all critical metal surfaces, and
coating with resilient materials such asrubber and some
plastics.
Fretting CorrosionGeneral Description. Fretting corrosion is a
combined wear and cor-
rosion process in which material is removed from contacting
surfaceswhen motion between the surfaces is restricted to very
small amplitudeoscillations (often, the relative movement is barely
discernible). Usually,the condition exists in machine components
that are considered fixed andnot expected to wear. Pressed-on
wheels can often fret at the shaft/wheelhole interface.
Oxidation is the most common element in the fretting process. In
oxi-dizing systems, fine metal particles removed by adhesive wear
are oxi-dized and trapped between the fretting surfaces (Fig. 9).
The oxides actlike an abrasive (such as lapping rouge) and increase
the rate of materialremoval. This type of fretting in ferrous
alloys is easily recognized by thered material oozing from between
the contacting surfaces.
Fretting corrosion takes the form of local surface dislocations
and deeppits. These occur in regions where slight relative
movements have oc-curred between mating, highly loaded
surfaces.
-
Prevention. Fretting corrosion can be controlled by lubricating
(e.g.,low-viscosity oils) the faying surfaces, restricting the
degree of movement,shot peening (rough surfaces are less prone to
fretting damage), surfacehardening (e.g., carburizing and
nitriding), anodizing of aluminum alloys,phosphate conversion
coating of steels, and by applying protective coat-ings by
electrodeposition (e.g., gold or silver plating), plasma spraying,
orvapor deposition (Ref 9).
lntergranular CorrosionGeneral Description. lntergranular
corrosion is defined as the selec-
tive dissolution of grain boundaries, or closely adjacent
regions, withoutappreciable attack of the grains themselves. This
dissolution is caused bypotential differences between the
grain-boundary region and any precipi-tates, intermetallic phases,
or impurities that form at the grain boundaries.The actual
mechanism differs with each alloy system. Although a wide va-riety
of alloy systems are susceptible to intergranular corrosion under
veryspecific conditions, the majority of case histories reported in
the literaturehave involved austenitic stainless steels and
aluminum alloys and, to alesser degree, some ferritic stainless
steels and nickel-base alloys.
Precipitates that form as a result of the exposure of metals at
elevatedtemperatures (for example, during production, fabrication,
and welding)often nucleate and grow preferentially at grain
boundaries. If these pre-cipitates are rich in alloying elements
that are essential for corrosion re-sistance, the regions adjacent
to the grain boundary are depleted of theseelements. The metal is
thus sensitized and is susceptible to intergranularattack in a
corrosive environment. For example, in austenitic stainlesssteels
such as AISI type 304, the cause of intergranular attack is the
pre-cipitation of chromium-rich carbides ((Cr5Fe)23C6) at grain
boundaries.These chromium-rich precipitates are surrounded by metal
that is depletedin chromium; therefore, they are more rapidly
attacked at these zones thanon undepleted metal surfaces.
Impurities that segregate at grain boundaries may promote
galvanic ac-tion in a corrosive environment by serving as anodic or
cathodic sites.Therefore, this would affect the rate of the
dissolution of the alloy matrixin the vicinity of the grain
boundary. An example of this is found in alu-minum alloys that
contain intermetallic compounds, such as Mg5Al8 andCuAl2, at the
grain boundaries. During exposures to chloride solutions,the
galvanic couples formed between these precipitates and the alloy
ma-trix can lead to severe intergranular attack. Susceptibility to
intergranularattack depends on the corrosive solution and on the
extent of intergranu-lar precipitation, which is a function of
alloy composition, fabrication, andheat treatment parameters.
Prevention. Susceptibility to intergranular corrosion in
austenitic stain-less steels can be avoided by controlling their
carbon contents or by
-
adding elements (titanium and niobium) whose carbides are more
stablethan those of chromium. For most austenitic stainless steels,
restrictingtheir carbon contents to 0.03% or less will prevent
sensitization duringwelding and most heat treatment.
Intergranular corrosion in aluminum alloys is controlled by
material se-lection (e.g., the high-strength Ixxx and Ixxx alloys
are the most suscep-tible) and by proper selection of thermal
(tempering) treatments that caneffect the amount, size, and
distribution of second-phase intermetallic pre-cipitates.
Resistance to intergranular corrosion is obtained by the use ofheat
treatments that cause precipitation to be more general throughout
thegrain structure (Ref 10).Exfoliation
General Description. Exfoliation is a form of macroscopic
intergran-ular corrosion that primarily affects aluminum alloys in
industrial or ma-rine environments. Corrosion proceeds laterally
from initiation sites onthe surface and generally proceeds
intergranularly along planes parallel tothe surface. The corrosion
products that form in the grain boundaries forcemetal away from the
underlying base material, resulting in a layered orflakelike
appearance (see, for example, the schematic shown in Fig. 3).
Prevention. Resistance to exfoliation corrosion is attained
throughproper alloy and temper selection. The most susceptible
alloys are thehigh-strength heat-treatable Ixxx and Ixxx alloys.
Exfoliation corrosion inthese alloys is usually confined to
relatively thin sections of highlyworked products. Guidelines for
selecting proper heat treatment for thesealloys can be found in Ref
10.
Dealloying CorrosionGeneral Description. Dealloying, also
referred to as selective leaching
or parting corrosion, is a corrosion process in which the more
active metalis selectively removed from an alloy, leaving behind a
porous weak de-posit of the more noble metal. Specific categories
of dealloying oftencarry the name of the dissolved element. For
example, the preferentialleaching of zinc from brass is called
dezincification. If aluminum is re-moved, the process is called
dealuminification, and so forth. In the case ofgray iron,
dealloying is called graphitic corrosion.
In the dealloying process, typically one of two mechanisms
occurs:alloy dissolution and replating of the cathodic element or
selective disso-lution of an anodic alloy constituent. In either
case, the metal is leftspongy and porous and loses much of its
strength, hardness, and ductility.Table 2 lists some of the
alloy-environment combinations for which deal-loying has been
reported. By far the two most common forms of dealloy-ing are
dezincification and graphitic corrosion.
Copper-zinc alloys containing more than 15% zinc are susceptible
todezincification. In the dezincification of brass, selective
removal of zinc
-
leaves a relatively porous and weak layer of copper and copper
oxide.Corrosion of a similar nature continues beneath the primary
corrosionlayer, resulting in gradual replacement of sound brass by
weak, porouscopper.
Graphitic corrosion is observed in gray cast irons in relatively
mild en-vironments in which selective leaching of iron leaves a
graphite network.Selective leaching of the iron takes place because
the graphite is cathodicto iron, and the gray iron structure
establishes an excellent galvanic cell.
Prevention. Dezincification can be prevented by alloy
substitution.Brasses with copper contents of 85% or more resist
dezincification. Somealloying elements also inhibit dezincification
(e.g., brasses containing 1%tin). Where dezincification is a
problem, red brass, commercial bronze, in-hibited admiralty metal,
and inhibited brass can be successfully used.
Attack by graphitic corrosion is reduced by alloy substitution
(e.g., useof a ductile or alloyed iron rather than gray iron),
altering the environment(raise the water pH to neutral or slightly
alkaline levels), the use of in-hibitors, and avoiding stagnant
water conditions.
Stress-Corrosion CrackingGeneral Description. Stress-corrosion
cracking (SCC) is a cracking
phenomenon that occurs in susceptible alloys and is caused by
the con-joint action of a surface tensile stress and the presence
of a specific cor-rosive environment. For SCC to occur on an
engineering structure, threeconditions must be met simultaneously,
namely, a specific crack-promot-ing environment must be present,
the metallurgy of the material must besusceptible to SCC, and the
tensile stresses must be above some thresholdvalue. Stresses
required to cause SCC are small, usually below the macro-scopic
yield stress. The stresses can be externally applied, but
residualstresses often cause SCC failures. This cracking phenomenon
is of partic-ular importance to users of potentially susceptible
structural alloys be-cause SCC occurs under service conditions that
can result, often with nowarning, in catastrophic failure. Failed
specimens exhibit highly branched
Table 2 Combinations of alloys and environments subject to
dealloying and elements preferentiallyremovedAlloy
BrassesGray ironAluminum bronzesSilicon bronzesTin
bronzesCopper-gold single crystalsMonelsGold alloys with copper or
silverTungsten carbide-cobaltHigh-nickel alloysMedium- and
high-carbon steelsIron-chromium alloysNickel-molybdenum alloys
Environment
Many waters, especially under stagnant conditionsSoils, many
watersHydrofluoric acid, acids containing chloride
ionsHigh-temperature steam and acidic speciesHot brine or
steamFerric chlorideHydrofluoric and other acidsSulfide solutions,
human salivaDeionized waterMolten saltsOxidizing atmospheres,
hydrogen at high temperaturesHigh-temperature oxidizing
atmospheresOxygen at high temperature
Element removedZinc (dezincification)Iron (graphitic
corrosion)Aluminum (dealuminification)Silicon
(desiliconification)Tin (destannification)CopperCopper in some
acids, and nickel in othersCopper, silverCobaltChromium, iron,
molybdenum, and tungstenCarbon (decarburization)Chromium, which
forms a protective filmMolybdenum
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Table 3 Some environment-alloy combinations known to result in
stress-corrosion cracking (SCC)
Environment
Alloy system
Aluminumalloys
Carbonsteels
Copperalloys
Nickelalloys
Stainless SteelsAustenitic Duplex Martensitic
Titaniumalloys
Zirconiumalloys
Amines, aqueousAmmonia, anhydrousAmmonia,
aqueousBromineCarbonates, aqueousCarbon monoxide, carbon
dioxide, water mixtureChlorides, aqueousChlorides,
concentrated,
boilingChlorides, dry, hotChlorinated solventsCyanides,
aqueous,
acidifiedFluorides, aqueousHydrochloric acidHydrofluoric
acidHydroxides, aqueousHydroxides, concentrated,
hotMethanol plus halidesNitrates, aqueousNitric acid,
concentratedNitric acid, fumingNitrites, aqueousNitrogen
tetroxidePolythionic acidsSteamSulfides plus chlorides,
aqueousSulfurous acidWater, high-purity, hot
X, known to result in SCC
Stress-corrosion cracking control
Mechanical Metallurgical Environmental
Avoid stressconcentrators
Change alloycomposition
Modifyenvironment
Relieve fabricationstresses
Change alloystructure
Apply anodic orcathodic protection
Introduce surfacecompressfve
stresses
Use metallicor conversion
coatingAdd inhibrtor
Reduce operatingstresses
Use organiccoating
Nondestructivetesting implications
for design
Modifytemperature F i g . 1 0 Me thods used to control SCC.
Source:
Ket I I
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cracks (see Fig. 3) that propagate intergranularly and/or
transgranularly,depending on the metal-environment combination.
Table 3 lists some of the alloy-environment combinations that
result inSCC. This table, as well as others published in the
literature, should beused only as a guide for screening candidate
materials prior to further in-depth investigation, testing, and
evaluation.
Prevention. Figure 10 summarizes the various approaches to
control-ling SCC. Surface engineering treatments like shot peening,
metallic coat-ings, and organic coatings play a key role in
controlling SCC.
Corrosion FatigueGeneral Description. Corrosion fatigue is a
term that is used to de-
scribe the phenomenon of cracking, including both initiation and
propa-gation, in materials under the combined actions of a
fluctuating or cyclicstress and a corrosive environment. Corrosion
fatigue depends strongly onthe interactions among the mechanical
(loading), metallurgical, and envi-ronmental variables listed in
Table 4.
Corrosion fatigue produces fine-to-broad cracks with little or
no branch-ing (see Fig. 3); thus, they differ from SCC, which often
exhibits consid-erable branching. They are typically filled with
dense corrosion product.The cracks may occur singly but commonly
appear as families or parallelcracks. They are frequently
associated with pits, grooves, or some otherform of stress
concentrator. Transgranular fracture paths are more com-mon than
intergranular fractures.
Table 4 Mechanical, metallurgical, and environmental variables
thatinfluence corrosion fatigue behaviorVariable TypeMechanical
Maximum stress or stress-intensity factor, amax or Kmax
Cyclic stress or stress-intensity range, ACT or AKStress ratio,
RCyclic loading frequencyCyclic load waveform (constant-amplitude
loading)Load interactions in variable-amplitude loadingState of
stressResidual stressCrack size and shape, and their relation to
component size and geometry
Metallurgical Alloy compositionDistribution of alloying elements
and impuritiesMicrostructure and crystal structureHeat
treatmentMechanical workingPreferred orientation of grains and
grain boundaries (texture)Mechanical properties (strength, fracture
toughness, etc.)
Environmental TemperatureTypes of environments: gaseous, liquid,
liquid metal, etc.Partial pressure of damaging species in gaseous
environmentsConcentration of damaging species in aqueous or other
liquid environmentsElectrical potentialpHViscosity of the
environmentCoatings, inhibitors, etc.
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Prevention. All metals and alloys are susceptible to corrosion
fatigue.Even some alloys that are immune to SCC, for example,
ferritic stain-less steels, are subject to failure by corrosion
fatigue. Both temporaryand permanent solutions for corrosion
involve reducing or eliminatingcyclic stresses, selecting a
material or heat treatment with higher corro-sion fatigue
strengths, reducing or eliminating corrosion, or a combina-tion of
these procedures. These objectives are accomplished by changesin
material, design, or environment and by the application of
surfacetreatments. Shot peening, nitriding of steels, and organic
coatings cansuccessfully impede corrosion fatigue. Noble metal
coatings (e.g.,nickel) can be effective, but only if they remain
unbroken and are of suf-ficient density and thickness. The
relatively low corrosion-fatiguestrength of carbon steel is reduced
still further when local breaks in acoating occur.
Hydrogen DamageGeneral Description. The term hydrogen damage has
been used to
designate a number of processes in metals by which the
load-carrying ca-pacity of the metal is reduced due to the presence
of hydrogen, often incombination with residual or applied tensile
stresses. Although it occursmost frequently in carbon and low-alloy
steels, many metals and alloysare susceptible to hydrogen damage.
Hydrogen damage in one form or an-other can severely restrict the
use of certain materials.
Because hydrogen is one of the most abundant elements and is
readilyavailable during the production, processing, and service of
metals, hydro-gen damage can develop in a wide variety of
environments and circum-stances. The interaction between hydrogen
and metals can result in theformation of solid solutions of
hydrogen in metals, molecular hydrogen,gaseous products that are
formed by reactions between hydrogen and ele-ments constituting the
alloy, and hydrides. Depending on the type of hy-drogen/metal
interaction, hydrogen damage of metal manifests itself inone of
several ways.
Specific types of hydrogen damage, some of which occur only in
spe-cific alloys under specific conditions include:
Hydrogen embrittlement: Occurs most often in high-strength
steels,primarily quenched-and-tempered and precipitation-hardened
steels,with tensile strengths greater than about 1034 MPa (150
ksi). Hydro-gen sulfide is the chief embrittling
environment.Hydrogen-induced blistering: Also commonly referred to
as hydro-gen-induced cracking (HIC), it occurs in lower-strength
(unhardened)steels, typically with tensile strengths less than
about 550 MPa (80ksi). Line pipe steels used in sour gas
environments are susceptible toHIC.
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As described in the previous section, surface treatments, and in
particu-lar protective coatings, are widely used to control
corrosion in its varyingforms. The problems of corrosion should be
approached in the designstage, and the selection of a protective
coating is important. Paint systemsand lining materials exist that
slow the corrosion rate of carbon steel sur-faces. High-performance
organic coatings such as epoxy, polyesters,polyurethanes, vinyl, or
chlorinated rubber help to satisfy the need for cor-rosion
prevention. Special primers are used to provide passivation,
gal-vanic protection, corrosion inhibition, or mechanical or
electrical barriersto corrosive action.
Corrosion Inhibitors. A water-soluble corrosion inhibitor
reduces gal-vanic action by making the metal passive or by
providing an insulatingfilm on the anode, the cathode, or both. A
very small amount of chromate,polyphosphate, or silicate added to
water creates a water-soluble inhibitor.A slightly soluble
inhibitor incorporated into the prime coat of paint mayalso have a
considerable protective influence. Inhibitive pigments in
paintprimers are successful inhibitors except when they dissolve
sufficiently toleave holes in the paint film. Most paint primers
contain a partially solu-ble inhibitive pigment such as zinc
chromate, which reacts with the steel
Cracking from precipitation of internal hydrogen: Examples
includeshatter cracks, flakes, and fish eyes found in steel
forgings, weld-ments, and castings. During cooling from the melt,
hydrogen diffusesand precipitates in voids and
discontinuities.Hydrogen attack: A high-pressure, high-temperature
form of hydro-gen damage. Commonly experienced in steels used in
petrochemicalplant equipment that often handles hydrogen and
hydrogen-hydrocar-bon streams at pressures as high as 21 MPa (3
ksi) and temperaturesup to 540 0C (1000 0F)Hydride formation:
Occurs when excess hydrogen is picked up duringmelting or welding
of titanium, tantalum, zirconium, uranium, andthorium. Hydride
particles cause significant loss in strength and largelosses in
ductility and toughness.
Prevention. The primary factors controlling hydrogen damage are
ma-terial, stress, and environment. Hydrogen damage can often be
preventedby using more resistant material, changing the
manufacturing processes,modifying the design to lower stresses, or
changing the environment. In-hibitors and post-processing bake-out
treatments can also be used. Bakingof electroplated high-strength
steel parts reduces the possibility of hydro-gen embrittlement (see
Chapter 8 for additional information).
Coatings and Corrosion Prevention
-
substrate to form the iron salt. The presence of these salts
slows corrosionof steel. Chromates, phosphates, molybdates,
borates, silicates, andplumbates are commonly used for this
purpose. Some pigments add alka-linity, slowing chemical attack on
steel. Alkaline pigments, such as metab-orates, cement, lime, or
red lead, are effective, provided that the environ-ment is not too
aggressive. In addition, many new pigments have beenintroduced to
the paint industry such as zinc phosphosilicate and zincflake.
Barrier coatings are used to prevent the electrolyte from
reaching thecomponent surface. Examples of barrier coatings include
painted steelstructures, steels lined with thick acid-proof brick,
steels lined with rub-berlike materials, or steels electroplated
with a noble (see Table 1) metal(e.g., chromium, copper, or
nickel). Protection is effective until the coat-ing is penetrated,
either by a pit, pore, crack, or by damage or wear. Thesubstrate
will then corrode preferentially to the coating (since it is
anodicto the coating material), and corrosion products will lift
off the coatingand allow further attack (Fig. 11).
Generally, electroplated coatings that are completely free of
pores andother discontinuities are not commercially feasible. Pits
eventually format coating flaws, and the coating is penetrated. The
resulting corrosion cellis shown in Fig. 12. The substrate exposed
at the bottom of the resultingpit corrodes rapidly. A crater forms
in the substrate, and because of the
p jo "I \ Illustration of the mechanism of corrosion for painted
steel, (a) A void" in the paint results in rusting of the steel,
which undercuts the paint
coating and results in further coating degradation, (b)
Photograph showing blis-tering and/or peeling (undercutting) of
paint where exposed steel is rusting.
(b)
(a)
PaintSteel
Rust
-
f\a 1 2 Crater formation in a steel substrate beneath a void in
a noble metal" coating, for example, passive chromium or copper.
Corrosion pro-
ceeds under the noble metal, the edges of which collapse into
the corrosion pit.
Noble metal coating(cathode)Moist air
Steel substrate(anode)
Coating (M1)Water drop
Coating (M2)Substrate (M3)FlC, 1 3 Corrosion pit formation in a
substrate beneath a void in a duplex
^* noble metal coating. The top coating layer (M1) is cathodic
to thecoating underlayer (M2), which is in turn cathodic to the
substrate (M3). As inFig. 12, the coating tends to collapse into
the pit.
large area ratio between the more noble coating and the anodic
crater, thecrater becomes anodic, and high corrosion current
density results. Elec-trons flow from the substrate to the coating
as the steel dissolves. Hydro-gen ions (H+) in the moisture accept
the electron and, with dissolved oxy-gen, form water at the noble
metal surface near the void. Use of anintermediate coating that is
less noble than a surface coating but morenoble than the base metal
can result in the mode of corrosion shown inFig. 13. This would be
typical of a costume jewelry item with a brass sub-strate, an
intermediate nickel coating, and a tarnish-resistant gold top
coat.It is also exemplified by nickel-chromium coating systems.
Sacrificial coatings, which corrode preferentially to the
substrate, in-clude zinc, aluminum, cadmium, and zinc-rich paints.
Initially these sac-rificial coatings will corrode, but their
corrosion products are protectiveand the coating acts as a barrier
layer. If the coating is damaged or defec-tive, it remains
protective as it is the coating that suffers attack and not
thesubstrate. Figure 14 shows the sacrificial (galvanic) protection
offered bya zinc coating to a steel substrate.
Cathodic protection involves the reversal of electric current
flowwithin the corrosion cell. Cathodic protection can reduce or
eliminatecorrosion by connecting a more active metal to a metal
that must be
-
FlC. 14 Principles and mechanism of galvanic protection of a
substrate by a^* coating. Galvanic protection of a steel substrate
at a void in a zinc
coating. Corrosion of the substrate is light and occurs at some
distance from thezinc.
protected. The use of cathodic protection to reduce or eliminate
corrosionis a successful technique of long-standing use in marine
structures,pipelines, bridge decks, sheet piling, and equipment and
tankage of alltypes, particularly below water or underground.
Typically, zinc or magne-sium anodes are used to protect steel in
marine environments, and the an-odes are replaced after they are
consumed.
Cathodic protection uses an impressed direct current (dc)
supplied byany low output voltage source and a relatively inert
anode. As is the casein all forms of cathodic activity, an
electrolyte is needed for current flow.Cathodic protection and the
use of protective coatings are most often em-ployed jointly,
especially in marine applications and on board ships whereimpressed
current inputs do not usually exceed 1 V. Beyond 1 V, manycoating
systems tend to disbond. Current source for cathodic protection
insoils is usually 1.5 to 2 V.
Choice of anodes for buried steel pipe depends on soil
conditions. Mag-nesium is most commonly used for galvanic anodes;
however, zinc canalso be used. Galvanic anodes are seldom used when
the resistivity of thesoil is over 30 fl m (3000 ft cm); impressed
current is normally usedfor these conditions. Graphite,
high-silicon cast iron, scrap iron, alu-minum, and platinum are
used as anodes with impressed current. Theavailability of low-cost
power is often the deciding factor in choosing be-tween galvanic or
impressed current cathodic protection. Figure 15 illus-trates both
types of galvanic protection systems.
Protective coatings are normally used in conjunction with
cathodic pro-tection and should not be disregarded where cathodic
protection is con-templated in new construction. Because the
cathodic protection currentmust protect only the bare or poorly
insulated areas of the surface, coat-ings that are highly
insulating, very durable, and free of discontinuitieslower the
current requirements and system costs. A good coating also en-ables
a single-impressed current installation to protect many miles of
pip-ing. Coal-tar enamel, epoxy powder coatings, and vinyl resin
are exam-
steel substrate(cathode)
Water drop Zinc coating(anode)
-
Fig, 1 5 Cathodic protection for underground pipe, (a)
Sacrificial or galvanic anode, (b) Impressed-cur-^* rent anode, ac,
alternating current
pies of coatings that are most suitable for use with cathodic
protection.Certain other coatings may be incompatible, such as
phenolic coatings,which may deteriorate rapidly in the alkaline
environment created by thecathodic protection currents. Although
cement mortar initially conductsthe electrical current freely,
polarization, the formation of an insulatingfilm on the surface as
a result of the protective current, is believed to re-duce the
current requirement moderately.
Cathodic protection is used increasingly to protect buried or
submergedmetal structures in the oil, gas, and waterworks
industries and can be usedin specialized applications, such as for
the interiors of water storage tanks.Pipelines are routinely
designed to ensure the electrical continuity neces-sary for
effective functioning of the cathodic protection system.
Thus,electrical connections or bonds are required between pipe
sections in linesusing mechanically coupled joints, and insulating
couplings may be em-ployed at intervals to isolate some parts of
the line electrically from otherparts. Leads may be attached during
construction to facilitate the cathodicprotection installation when
needed.
Corrosion Testing
Many tests exist for establishing the reliability of protective
coatings onmetal substrates. Existing tests and standards are under
continuous devel-opment, and new tests are being designed.
Organizations active in the de-velopment and standardization of
corrosion tests for coatings includeASTM, NACE International, the
Society of Automotive Engineers (SAE),the National Coil Coaters
Association (NCCA), the InternationalStandards Organization (ISO),
international systems (e.g., DIN), andcommercial (e.g., automotive,
architectural, electronics), proprietary, and
Insulatedcopper wire
Anode
BackfillCurrent
ac line
Rectifier.
(b)
Pipeline
Soil
Insulated copper wire
Soil
Pipeline Activemetalanode
CurrentBackfill
(a)
-
military organizations. This section provides a brief review of
the mostwidely used test methods including:
Field testsSimulated service testsLaboratory (accelerated) tests
(e.g., salt spray tests, humidity tests,and electrochemical
tests)
Table 5 lists selected tests used for determining the
effectiveness of pro-tective coatings in corrosive
environments.
More detailed information on testing of coated specimens can
befound in several excellent sources. Gaynes (Ref 13) and Munger
(Ref14) give descriptions and the framework for effective use of
tests andstandards. Gaynes provides detailed descriptions including
photo-graphs, cross-listing ASTM to federal tests and a broader
perspectiveencompassing the federal standard, miscellaneous tests,
and somecaveats of traditional testing. Munger offers practical
material directedtoward large structures and provides a listing
based on ASTM stan-dards. Altmayer (Ref 15) compiled a table of 13
applicable corrosiontests for 30 metallic, inorganic, and organic
coating/substrate combina-tions. Other useful sources of
information can be found in review arti-cles by Simpson and
Townsend (Ref 16) and Granata (Ref 12), whichdescribe tests for
metallic coatings and nonmetallic coatings, respec-tively.
Field TestsThe most reliable performance data are obtained by
field tests/surveys.
One example would be to monitor and test the corrosion of
autobody pan-els that sit in junkyards. Another example of
in-service testing would beto monitor the behavior of the materials
in a fleet of captive vehicles. Thisenables better control and
recording of the exposure and driving condi-tions. The use of fleet
vehicles also makes it possible to test coupons rep-resenting a
larger database of materials.
Simulated Service TestsThe most widely used simulated service
test for static atmospheric test-
ing is described in ASTM G 50, "Practice for Conducting
AtmosphericCorrosion Tests on Metals." It is used to test coated
sheet steels for a va-riety of outdoor applications. Test
materials, which are in the form of flattest panels mounted in a
test rack (Fig. 16), are subjected to the cycliceffects of the
weather, geographical influences, and bacteriological factorsthat
cannot be realistically duplicated in the laboratory. Test
durations canlast from several months up to many years. Some
zinc-coated steel speci-mens have undergone testing for more than
30 years.
-
Table 5 Widely used tests for determining the corrosion
resistance of protective coatingsTest
Salt spray (ASTM B 117)
100% relative humidity (ASTM D 2247)
Acetic acid-salt spray ASTM G 85, Al (formerlyASTM B 287)
Sulfur dioxide-salt spray (ASTM G 85, A 4)
Copper-accelerated salt spray, or CASS(ASTM B 368)
FACT (formerly ASTM B 538)
Accelerated weathering
Lactic acid
Acidified synthetic seawater testing or SWAAT(ASTM G 85, A3;
formerly ASTM G 43)
Electrographic and chemical porosity tests
Adhesion (ASTM D 3359-90)
T-bend adhesion (ASTM D 4145)
Description and remarks
Most widely specified test. Atomized 5% sodium chloride (NaCl),
neutral pH, 35C (95 0F)(a), follow details of ASTM B 117, Appendix
Xl. Emphasizes wet surfaces (nondrying),high oxygen availability,
neutral pH, and warm conditions. Control of comparative speci-mens
should be run simultaneously. Corrosivity consistency should be
checked as de-scribed in ASTM B 117, Appendix X3. Notes: May be the
most widely misused test. Re-quires correlation to service tests
for useful results. Do not assume correlation exists.
Widely used test. Condensing humidity, 100% RH, 38 0C (100 0F).
Emphasizes sensitivity towater exposure
Widely used test. Atomized 5% NaCl, pH 3.2 using acetic acid, 35
0C (95 0F). More severethan ASTM B 117. The lower pH and the
presence of acetate affect the solubility of corro-sion products on
and under the protective coatings.
Atomized 5% NaCl, collected solution pH = 2.5-3.2, 35 0C (95
0F), SO2 metered (60 min 35 cm3/min per m3 cabinet volume) 4 times
per day
Atomized 5% NaCl, pH 3.2 with acetic acid, 0.025% cupric
chloride-dihydrate, 35 0C (95F).Galvanic coupling due to copper
salt reduction to copper metal. More severe than ASTM B117
Testing anodized aluminum specimens. Electrolyte as in salt
spray or CASS test. Specimen ismade the cathode to generate high pH
at defects.
Exposure of coated specimens to effects of ultraviolet radiation
experienced in outdoor sunlightconditions, which may be combined
with other exposures such as moisture and erosion.Exposure cabinets
use carbon arc (ASTM D 822), xenon lamp (ASTM G 26), or
fluores-cent lamp (ASTM G 53).
On substrates of brass and copper alloys, determines coatings
porosity and resistance to handling(perspiration). Consists of
immersion in 85% lactic acid solution, drying, and incubatingabove
acetic acid vapors for 20 h to reveal discoloration spots at
failure points or delami-nations
Atomized synthetic seawater (ASTM D 1141) with 10 mL glacial
acetic acid per L of solution,pH 2.8 to 3.0, 35 0C (95 0F). More
severe than ASTM B 117. The lower pH and the pres-ence of acetate
affect the solubility of corrosion products on and under the
protective coat-ings.
Pores and active defects in nonmetallic coatings can be revealed
by color indication or depositformation. On nickel substrates,
dimethylglyoxime, or steel, potassium ferricyanide (fer-roxyl test)
indicator can be applied to surface on filter paper while substrate
is made theanode. Alternatively, a substrate immersed in acidic
copper sulfate can be made the cath-ode to form copper nodules at
conductive coatings d