Corrosion 4

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CHAPTER 4: LOCALISED CORROSION

Introduction

As indicated initially in Figure 3 of Chapter 1, there are a number of manifestations of thesituation where the anodic and cathodic reactions are permanently separated spatially on acomponent surface. Three such types of localised corrosion are depicted in Figure 1; these(and others) are described in the following sections.

Figure 1: Some types of localised corrosion



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Galvanic (“Bimetallic”) corrosion

Galvanic corrosion may occur when two different metals are in contact (Figure 1), say bywelding or bolting, or connected by an electrical conductor, and are exposed to an aqueousenvironment. We have already seen in Figure 1 of Chapter 1 how such an arrangement

constitutes an electrochemical cell in which one of the metals becomes the anode and thuscorrodes. What we have not considered thus far is what determines which of the twometals becomes the anode and which becomes the cathode.

The answer to this has been hinted at in point 3 in the section, "Electrode potential as anindicator of corrosion tendencies," towards the end of Chapter 1 but is more clearlydeduced as follows. We need to consider the electrode potentials that would be exhibitedby the two metals if exposed to the same environment separately (i.e. without the externalconnecting wire in Figure 1 of Chapter 1). The rule is that the more electronegative of thetwo metals (i.e the metal which has the more negative electrode potential if measured as inFigure 6 of Chapter 1) becomes the anode (iron in Figure 1 of Chapter 1) and the other

metal becomes the cathode (copper in Figure 1 of Chapter 1). The result of this is that themore-electronegative metal now suffers more corrosion and the less electronegative(sometimes referred to as “more noble”) metal component (providing sites for a cathodicreaction such as oxygen reduction) suffers less corrosion than if they were placedunconnected in the same environment. The current flowing between the two componentsrepresents the additional corrosion experienced by the anodic metal. The driving force forgalvanic corrosion, that is for the intensification of attack on one of the two connectedmetals, is the difference in electrode potentials of the two components when exposedseparately in the same environment.

An indication of the factors which determine the extent of accelerated corrosion on the

anodic component of a bi-metallic couple is given in the three points which follow.

1. Difference between uncoupled corrosion potentials

As implied above, the greater this is, the more likely the accelerated corrosion on the“anodic” component will be severe. A very approximate guide to this factor can beobtained from the table of Standard Electrode Potentials (Table 1) but it is important tonote that this table lists the equilibrium electrode potentials of pure metals in standardlaboratory solutions containing ions at unit concentrations.

In practice, engineering materials are usually complex alloys exposed to environments which are very different from the standard laboratory solutions on which Table 1 isbased and are not at equilibrium .

Therefore, more reliable guides to possible galvanic corrosion problems are obtained fromrelevant “galvanic series” determined in an environment simulating, as closely as possible,the practical one under consideration. These are available in the literature for someenvironments, notably seawater. A version of such a galvanic series is presented in Table2 and indicates, for example, that coupling of carbon steel to type 316 stainless steel or tocopper-nickel alloys makes the carbon steel susceptible to severely-enhanced corrosion.

Notice in comparison of Tables 1 and 2 how, for example, titanium occupies a position atthe noble end of the seawater galvanic series in contrast with its location at the active end



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of Table 1. This is due to the influence of the passive oxide film which is present on thesurface of titanium and its alloys in seawater and many other environments and whicheffectively transforms this metal from an active to a noble material. However, in someconditions (e.g. non-oxidising acids), the passive film on titanium is not stable and themetal behaves as an extremely active metal with an electronegative electrode potential (as

indicated in Table 1). Other materials, such as stainless steels and aluminium alloys,exhibit similar characteristics, i.e. being passive (noble) in some environments but activein others.

Care is still required in interpreting even such tables based on one general environment(e.g. seawater, Table 2) because changes, in say temperature, pH or turbulence, maychange the relative positions of materials in the series. Moreover, particular materials canexist over a wide range of potentials as is indicated for Inconel 625 and 316 stainless steelin Table 2.

Notice also the noble position of graphite in Table 2. This means that particular care is

needed when many materials are likely to be in contact with carbon components, e.g sealsor carbon-fibre reinforced polymers.

In any case, the difference in the uncoupled potentials (as indicated in galvanic seriestables) represents only one factor determining the severity of attack on the susceptiblemetal. Two other factors are discussed in the following sections 2 and 3.



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TABLE 1: Standard Electrode Potentials(Electrochemical Series)

ELECTRODE REACTIONEQUILIBRIUM ELECTRODE POTENTIAEoo, at 25°C (SHE)

Au 3+ + 3e - = Au +1.43 (Noble metal end)

Cl2 + 2e - = 2Cl - +1.36

O2 + 4H + + 4e - = 2H 2O +1.23

Ag + + e - = Ag +0.80

O2 + 2H 2O + 4e - = 4(OH) - +0.40

Cu2+ + 2e - = Cu +0.34

2H + + 2e - = H 2 ZERO

Pb2+ + 2e - = Pb -0.13

Sn2+ + 2e - = Sn -0.14

Ni2+ + 2e - = Ni -0.25

Cd2+ + 2e - = Cd -0.40

Fe2+ + 2e - = Fe -0.44

Cr3+ + 3e - = Cr -0.74

Zn 2+ + 2e - = Zn -0.76

Ti 2+ + 2e - = Ti -1.63

Al3+ + 3e - = Al -1.66

Mg 2+ + 2e - = Mg -2.40

Na+ + e - = Na -2.71 (Base {"reactive"} metal end)



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GraphiteTitaniumHastelloy CMonelCu/Ni alloys(60-90Cu/10-40NiBronzes (Cu/Sn)CopperBrasses (Cu/Zn)Tin

LeadNi-Resist cast ironCast ironCarbon steelAl-base alloys(2000 serieswith Cu/Mg/Mn)CadmiumPure aluminiumZincMagnesium and alloys

ACTIVE

(ANODIC)END

NOBLE(CATHODIC)END

I n c o n e

l a l l o

y

3 1 6

- t

y p e

s t

a i n l e

s s

s

t e e

l

Table 2. Galvanic series for ambienttemperature seawater

2. Relative areas of the two metals

A much larger area of the noble (cathodic), compared to the more-electronegative (anodic)metal (depicted in Figure 1), will accelerate the attack on the anodic component and viceversa. This combination of small anode/large cathode is deleterious in any corrosionsituation because the basic requirement for an equality of the total cathodic and anodic

currents on a component means that a modest cathodic reaction rate (manifested by asmall cathodic current density) acting over a large area can support a very high anodiccurrent density (i.e. high rate of attack) over a small area - as outlined below.

Total rate of production of electrons (i.e. anodic current, I a) must equal the total rate ofconsumption of electrons (i.e. cathodic current, I c).

i.e. i a x A a = i c x A c

where “i” = current density in say amp/cm 2, and “A” = component area.

Thus if A a << A c i.e. small anode with large cathode,



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then i a / i c is large causing intense attack on the anode.

3. Progress of reaction on cathode

Again since during corrosion I a must equal I c, if the cathodic reaction is restricted then I a is small and the galvanic corrosion rate is low. One example of this is that in deaeratedseawater, galvanic corrosion is much less of a problem on account of the retardation of theoxygen reduction cathodic reaction.

Moreover, coatings or deposits on the cathodic element of a bimetallic couple alsodramatically reduce galvanic corrosion rates because the coating will restrict the cathodicreaction and hence the galvanic corrosion rate. One “natural” source of such a coatingarises from calcareous scale which can deposit on the cathode in some circumstances onaccount of the increased cathodic reaction rates generating high-pH conditions on thecathode via the cathodic reaction:-

O2 + 2H 2O + 4e - --> 4(OH) - (1)

The effect of pH on calcareous scale deposition was alluded to in Chapter 2; thus inseawater (and hard-water groundwater), it is possible that, in some circumstances,galvanic corrosion rates may decline somewhat with time after the establishment of acalcareous coating on the cathodic component.

Different metals have different cathodic polarisation characteristics; hence when acting ascathode in a bi-metallic cell will (all else being equal) support larger galvanic corrosionrates on the anodic component

EXAMPLE: from data in seawater experiments:

Material E corr (using SCE ref electrode)

Uncoupled carbon steel about -700 mV

Uncoupled stainless steel about -280 mV

Uncoupled 90%Cu-10%Ni about -240 mV

Bi-metallic couple Galvanic corrosion rate

(µAmp/cm 2) Exposure time

carbon steel coupled 90 2 days,

to stainless steel 45 922 days

carbon steel coupled 112 2 days

to/90%Cu-10%Ni 70 922 days



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Thus a similar driving force for galvanic corrosion on the carbon steel (i.e. a similardifference in electrode potentials (E corr ) between carbon steel and stainless steel as forcarbon steel and 90%Cu-10%Ni) does not result in similar galvanic corrosion rates on the

carbon-steel component due to the ability of 90%Cu-10%Ni to support more rapidcathodic reaction rates than does the stainless steel.

Galvanic corrosion in ships

Bi-metallic corrosion is not prevalent in external ship’s hulls.

Most galvanic corrosion problems occur in seawater tanks and piping systems andancillary equipment.



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Pitting corrosion and crevice corrosion

These are the types of corrosion in which there is intense attack at localised sites on thesurface of a component (see Figure 1) whilst the rest of the surface is corroding at a muchlower rate - either because of an inherent property of the component material (such as the

formation of a protective oxide film) or because of some environmental effect. Indeed themain surface may be essentially under satisfactory corrosion control. In suchcircumstances, if corrosion protection breaks down locally then corrosion may be initiatedat these local sites.

If this event occurs under a deposit on the surface (perhaps a weld deposit or some foulingagent from the water) or at the joint of a bolted assembly etc, the attack is termed, “ crevicecorrosion ”. If the attack initiates on the free surface of a component, it is termed“pitting ”. The resistance to these two types of localised corrosion varies greatly betweendifferent materials and is extremely dependent upon environmental factors.

Initiation of attack

PITTING: by:-

1. Localised chemical or mechanical damage to protective oxide film; water chemistryfactors which can cause breakdown of a passive film are: low pH, low dissolved oxygenconcentrations (which tend to render a protective oxide film less stable) and highconcentrations of chloride (as in seawater).

2. Localised damage to, or poor application of, a protective coating.

3. Presence of non-uniformities in the metal structure of the component, e.g. non-metallicinclusions.

CREVICE CORROSION: by changes in local chemistry within the crevice, such as:-

1. Depletion of inhibitor in crevice

2. Depletion of oxygen in crevice

3. Decrease in pH in crevice

4. Build-up of aggressive ion species (e.g. chloride) in crevice

Propagation

Once the localised attack has initiated and progressed somewhat, the distinction betweenpitting and crevice corrosion diminishes because of deposition of insoluble corrosionproducts at the mouth of the pit. Propagation can be accelerated by formation of amacrocell between the small pit (acting as anode) and the external surface (acting as large

cathode). - as shown in Figure 2.



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Figure 2: Pitting/crevice corrosion propagation (involving a macrocell betweenpit or crevice interior (at relatively negative potential) and externalsurface (at more positive potential)

One particularly common form of macrocell is an “oxygen concentration cell” whichresults when there is a region (such as a crevice) in which the dissolved oxygenconcentration is low adjacent to a part of the component which is exposed to, say, fullyaerated water. In such a situation, the cathodic reaction:-

O2 + 2H 2O + 4e - --> 4(OH) - (1)

is stimulated at the high-oxygen locations whereas the anodic reaction is concentrated inthe (often relatively small) oxygen-depleted zone. Such corrosion is sometimes called“differential aeration”. The resulting, often severe, attack at the anodic sites is stimulatedby the difference in electrode potentials in the two regions.

This situation is depicted schematically in the polarisation diagrams in Figure 3 which canbe understood by recalling (Figure 10 of Chapter 1) that the point of intersection of anodicand cathodic polarisation curves represents the electrochemical conditions prevailing onthe surface of a component. In the present context (Figure 3), because of the physicalseparation of the pit/crevice from the external surface, the electrochemical conditionsdiffer in the two regions. The electrode potential within the crevice (or pit), E crev , is morenegative than that, E ext , on the free external surface because the passive film has becomede-stabilised within the occluded region thus depolarising the anodic reaction. In contrast,on the external surface, the component is protected by a passive oxide which stifles theanodic reaction (i.e. severely polarises it). As discussed in Chapter 1 (in point 3 in thesection, "Electrode potential as an indicator of corrosion tendencies," towards the end ofChapter 1) and, as also described in relation to bimetallic corrosion in this chapter, aregion of a component at more-negative electrode potential acts as anode and isvulnerable to enhanced corrosion. Accelerated attack within a pit/crevice is further

exacerbated by the small anode / large cathode situation.



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{Additional note: “differential aeration cells” are not restricted to crevice corrosionsituations; they can also operate over longer distances - for instance between well-oxygenated soil and poorly-oxygenated soils in underground pipelines.}

Figure 3: Schematic representation of different electrode potentials within, andexternal to, a pit or crevice



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Intergranular corrosion

The grain boundaries in a material are sometimes attacked by a corrodant preferentially tothe interior of the grain (Figure 1). The attack is usually related to the segregation ofspecific elements or the formation of a compound in the boundary. Corrosion then occurs

by preferential attack on the grain-boundary phase, or in a zone adjacent to it that has lostan element necessary for adequate corrosion resistance - thus making the grain boundaryzone anodic relative to the remainder of the surface. The attack usually progresses along anarrow path along the grain boundary and, in a severe case of grain-boundary corrosion,entire grains may be dislodged due to complete deterioration of their boundaries. In anycase the mechanical properties of the structure will be seriously affected.

Many aluminium-base alloys are susceptible to intergranular corrosion on account ofeither phases anodic to aluminium being present along grain boundaries or due to depletedzones of copper adjacent to grain boundaries in copper-containing alloys. In somesusceptible wrought aluminium-base alloys containing elongated grains parallel to the

surface, intergranular attack can cause a form of deterioration often termed, exfoliationcorrosion , which takes the form of layers of material “peeling off” the surface on accountof wedging forces of corrosion product situated along the grain boundaries just under thecomponent surface.

Susceptibility to intergranular attack can be a bi-product of some heat treatment (e.g awelding or stress-relieving operation) and can usually be corrected by another heattreatment or use of a modified alloy. This is the case with “intergranular corrosion” ofstainless steels.

Intergranular Corrosion of Stainless Steel

(a) Weld Decay of Stainless Steels

This type of intergranular corrosion can occur in the heat-affected zone (HAZ) of weldedstainless steel components due to precipitation, during cooling after the welding operation,of chromium carbides at the grain boundaries (and hence loss of chromium in theimmediately-adjacent zone, see Figure 4). The local loss in corrosion resistance arisesbecause the chromium is crucial in promoting the formation of a Cr-rich passive film onthe surface of stainless steels.

Figure 4: Cross-section of Stainless Steel showing Carbide Precipitation in GrainBoundary (GB) of HAZ after Welding Dashed lines show chromium concentrationwhich falls to low values in region adjacent to GB

18%Cr

Grain interior

Grainboundary

Cr carbide(formed in GB byreaction of Cr + Cduring slow

%C

%Cr in carbide >> %Cr in steel



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(b) Stainless Steel Castings

These components can also be susceptible to intergranular corrosion due to the grainboundary precipitation of chromium carbides in instances where the cooling rate in thecasting is low. Thus this type of intergranular corrosion of stainless steels is similar toweld decay except that carbide precipitation occurs very locally in the HAZ of a weldedcomponent but may be more widespread in a casting (especially in the interior of a largecasting).

Intergranular corrosion of stainless steel castings can also be associated with theprecipitation of a high-chromium-containing intermetallic phase called “sigma phase”.This phase is not normally found in most stainless steel grades but can precipitate at grainboundaries if the component (e.g. a casting) has spent a relatively long period of time inthe temperature range 560-900 oC. Sigma phase has a relatively higher chromium content(greater than 30%) than the overall chromium content of the stainless steel (typically 12-20% in most commercial stainless steels). Thus sigma precipitation causes chromiumdepletion in the region immediately adjacent to the grain boundary – in a similar mannerto that shown in Figure 4 except that the GB precipitate is now sigma rather thanchromium carbide.

(c) Avoidance of Intergranular Corrosion in Stainless Steels

The susceptibility to weld decay or intergranular corrosion in castings can be counteractedby carrying out a suitable (post-weld or post-casting) heat treatment to restore a uniformcomposition at the grain boundaries. This involves heating to a high temperature (typicallyto about 1000 oC) in order to eliminate the carbide or sigma phase (and hence restore theuniform chromium content adjacent to the grain boundaries) followed by rapid cooling toroom temperature. Such a heat treatment operation is clearly often not a practicableproposition. Consequently the usual strategy in combating weld decay and intergranularcorrosions of castings, involving chromium carbide precipitation, is by the choice ofstainless steel with either of the two following features.

1) Specification of a stainless steel with low carbon content (<0.03%); this willclearly decrease the likelihood of carbide formation in the steel. Such low-carbon gradesof stainless steel are often designated by a ‘L’ in their code; for instance the “316” gradeof steel (18%Cr/10Ni/2.5Mo) is designated as “316L” when its carbon content has beenlimited in this way.

2) Specification of a stainless steel containing a small amount (say a few tenths of apercent) of either titanium or niobium; these elements have a higher affinity than does Crfor carbon and hence carbides of these elements tend to form instead of chromiumcarbides, thus avoiding the Cr-depletion problem. Such steels are usually termed“stabilised stainless steels”.

Strategies 1 and 2 above do not work when the grain boundary precipitate is sigma phasein which case the heat treatment operation described above is the only option.



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De-alloying (selective leaching)

In some alloys (notably copper-base alloys), corrosion of the component may take theobserved form of one or more of the major alloying constituents being seemingly“selectively dissolved” leaving other alloy elements behind often as a kind of spongy mass

on the component surface. The detailed mechanisms involved to produce this observedphenomenon may sometimes be rather more complex than simple selective dissolution.

De-alloying of copper-base alloys

The most well-known example of de-alloying is “dezincification” of brass (copper-zincalloys) in which, as the name implies, the zinc constituent (typically 20 - 40% incommercial brasses) is lost leaving the copper behind as an often readily-visible copper-coloured porous mass on the surface. However, the process can also remove other alloyingelements as well as the zinc; e.g. aluminium in ‘aluminium brass (a Cu-28%Zn-2%Alalloy).

Other copper-base alloys are susceptible to de-alloying; e.g. “denickelification” of copper-nickel alloys.

De-alloying of copper-base alloys is often encountered in marine equipment.

Graphetisation of cast iron

This phenomenon, again prevalent in seawater conditions, comprises the selective removalof the major iron constituent of cast irons leaving the carbon (originally present in the castiron at around 4-5 % concentration) behind as a network of graphite flakes or nodules.The original shape of the component (e.g. a pipe) may appear unchanged but, of course,the load-bearing capacity of the component is drastically reduced.



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Microbiologically-induced corrosion (“MIC”)

We have already discussed, in Chapter 2, the propensity for biological species to invade

the surfaces of equipment exposed to water and especially seawater with its abundance ofnutrients. Biological fouling of surfaces can be a source of trouble in many respects, suchas reduction of thermal efficiency of heat-exchangers, blockage of pipes, interference withthe operation of oceanographic sensors and optical devices, but it is influence on corrosionwhich is of interest in this course.

Biological organisms do not generally cause “new” forms of corrosion but, rather,stimulate some of the types of corrosion that we have already discussed. Corrosionproblems promoted by biological organisms usually take the form of extremely localisedattack (e.g. pitting) rather than general-surface deterioration. Microbiological organisms(rather than larger bio-species) are most-often implicated in corrosion problems. Two

mechanisms by which bio-organisms can stimulate corrosion are:-1. Promotion of "differential aeration cells" Thus, metal surfaces beneath somemicrobiological (and also larger, e.g. macro-algal and fungal) colonies can be depleted inoxygen either by:-

• consumption of oxygen by the metabolic processes of the organism, or

• as a result if the shielding effect of the deposit

The subjacent oxygen-depleted zones then act as anodes and hence suffer pitting corrosionwith neighbouring, well-aerated, less-colonised regions providing the necessary sites for

the cathodic reaction.2. In other cases, corrosion under biofouling layers can be due to the production ofcorrosive organic and inorganic acids as metabolic by-products.

Influence of environmental conditions on MIC

In common with all living matter, bio-organisms require water to survive.

Aerobic organisms are those that require oxygen for their metabolic (life-growing)

processes conditions whereas other types are adapted to the opposite, anaerobic environments.

MIC is often encountered in circumstances where stagnant or low-flow conditions occurbecause these promote attachment of bacteria to component surfaces. Crevices andgasketed joints are attractive sites for attachment and growth of microbial colonies. MICpenetration rates of up to 3 mm per month have been recorded on 18%Cr-10%Ni stainlesssteel.

Some common MIC problems associated with specific types of bacteria

Corrosion can be promoted by specific microbiological organisms. Some of the mostcommon types are described below.



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“IRON-OXIDISING BACTERIA”

These grow in aerobic waters that contain Fe 2+ ions which the bacteria oxidise to Fe 3+

ions with the result that the filamentous bacteria are surrounded by a sheath of ferricoxide. This process consumes oxygen and thus causes corrosion by the differentialaeration mechanism mentioned above. The resulting corrosion is often manifested by thepresence of tubercles consisting, primarily of hard black magnetite (Fe 3O4) often withred-rust coloured ferric oxide (Fe 2O3) stalks or rosettes above or surrounding thetubercles, and is prevalent in cooling-water circuits and water supply systems.

Corrosion involving iron bacteria is more a problem in freshwater conditions.

SULPHUR OXIDISING BACTERIA

These are aerobic bacteria that oxidise sulphur or its compounds (e.g. in some crude oils)to produce sulphuric acid, i.e. produce extremely corrosive conditions.

SULPHATE REDUCING BACTERIA (SRB)

Deterioration promoted by sulphate-reducing bacteria is a potential issue in anyenvironment (e.g. seawater) that contains sulphate, (SO 4),2-, which fuels SRB metabolicactivity. As mentioned in Chapter 2, in order to thrive SRB require anaerobic conditions

and their metabolic activity results in the reduction of sulphate , (SO 4)2-, ions tosulphides and especially hydrogen sulphide which are extremely corrosive to manymaterials including stainless steels. These bacteria can be present in the locally anaerobicconditions prevailing in wet clay, polluted harbours or at the base of deposits includingother biological colonies. Aspects of deterioration associated with SRB are:-

• This type of corrosion can often be recognised by the rotten-egg odour of hydrogensulphide exuding from black corrosion product which may be "capped" by a layer oforange / red rust

• The detailed corrosion mechanisms involved are complex and not the subject of

universal agreement but it is likely that H 2S gas or solid sulphides can stimulate thecathodic reaction and, thereby, increase the corrosion rate of steels

• A particularly unfortunate characteristic of SRB is that they promote corrosion incircumstances (neutral pH / low oxygen) in which it would not usually be a problem.

• SRB can also promote hydrogen ingress into materials and this can be implicated instress corrosion cracking and hydrogen embrittlement damage (see later).

• In some circumstances, SRB and sulphur oxidising bacteria can become involved incyclic deterioration processes in situations where the environment is alternately

oxygen rich and oxygen denuded. Thus, in periods when oxygen is excluded, SRBthrive and cause corrosive attack to be followed, if the conditions become aerobic, by





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It is considered that SRB are involved which thrive in the low-oxygen conditions andproduce iron sulphide corrosion product

PREVENTION

Use of conventional corrosion control tactics (see later) – protective paint coatings andcathodic protection .



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Stray current corrosion

Stray current corrosion is possible whenever a metallic component or structure is located inan electrically conducting environment (e.g. water, moist soil and concrete) in which there isa potential gradient associated with strong currents flowing due to current “leakage” from a

nearby current-carrying component. In this situation, a potential difference develops acrossthe metal which is thereby likely to suffer accelerated corrosion at certain points (see below)on the surface.

Some of the earliest - and present-day - examples of this phenomenon relate to electrictraction systems where underground pipelines running underneath the tram- or rail lines arefound to be vulnerable to corrosion associated with current leaking into the ground and thenonto the pipeline (because this provides a better conducting path than the soil) along which itruns for a distance before returning to the tramway rails. Other sources of stray current attackare traction systems associated with mining, industrial plant, where equipment such as DCcranes, welding machines, elevators, electroplating are used, and in cathodic protection

systems.

In the context of marine equipment, the mechanism of stray current corrosion is illustrated inFigure 5 in which the arrows show the flow of positive electrical charge. Consider currentleakage emanating, for example, from some shore-based welding operation. The currentflows through the seawater and then onto a metallic structure/component (e.g. a jetty or partof a ship’s hull) because the metal provides a better conductive path than the sea. The currentruns along the metal object before returning to the sea.

Where the positive current enters the metal (CC), the transfer of positive charge from sea tometal involves a cathodic reaction:-

e.g. 2H + + 2e - ! 2H or O 2 + 2H 2O + 4e - ! 4(OH) -

The positive current leaves the metal at (AA) via the anodic reaction:

M ! M n+ + ne - i.e. by a metal loss (corrosion) reaction.

Thus, in Figure 5, sites where the positive current enters the structure are un-affected but siteswhere the positive current leaves the structure suffer metal loss by corrosion.



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Figure 5: Detailed mechanism of stray current corrosion. Arrows show flow ofpositive current.



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Erosion corrosion and cavitation corrosion

Erosion Corrosion

This occurs in turbulent conditions which can increase material-loss rates by:-

• increasing corrosion rates, for instance by facilitating the plentiful supply of corrodantto the metal surface or by tearing away of protective oxide films

• causing direct erosive damage to the metal surface. In some circumstances, theerosion is accentuated by suspended solid particles, such as sand, in the liquid.

Erosion corrosion often occurs in pipes - particularly at bends, sudden changes in sectionor around valves and inserts. If this form of attack is highly localised by the action of a jetof liquid, it is usually termed “impingement attack”.

Even when the liquid contains high burdens of solids (“slurries”), in a corrosive mediumsuch as seawater, the overall amount of material loss can involve a substantial contributionfrom corrosion mechanisms. Moreover, the influence of corrosion processes upon erosioncorrosion is quite complex. Detailed laboratory studies have demonstrated that the overallmaterial loss (say as weight loss, W, in grams) is composed of several contributions:-

W = E + C + S

where E is the weight loss due to pure mechanical erosion (caused by a high-velocitywater stream containing suspended solids such as sand),

C is the weight loss due to pure, electrochemical corrosion

And S is an additional contribution to weight loss caused by (often complex) interactionsbetween corrosion and erosion; it is often considered to be the influence of corrosion onincreasing material loss by erosion.

Cavitation Corrosion

This type of attack is often linked with erosion corrosion and indeed the two phenomenahave features in common. Cavitation occurs in fluid flow systems in which large changesin pressure occur. Thus, in a flowing liquid, if the local static pressure falls below thevapour pressure of the liquid, vapour bubbles form in the liquid (Figure 6). Passage of theliquid, to a region in which the pressure increases, leads to collapse of the bubbles.Unfortunately, for reasons to do with surface energies, the collapse process is muchfavoured on solid surfaces rather than within the bulk liquid. Moreover, the collapseprocess takes place extremely rapidly (about a micro-second) and results in a strong shockwave (involving enormous instantaneous stresses) which destroys passive films anddamages the metal surface by a kind of hammering action.

This process can lead to straightforward localised mechanical cavitation damage ofmaterials or to a combined cavitation-corrosion attack in corrosive environments. Pumpcomponents are especially subject to this form of deterioration. For instance, there is alarge change in pressure as the liquid moves from the (low pressure) pump suction into the



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impellor where the pressure rises and possible cavitation damage occurs. Other locationswhere cavitation can occur are valves and obstructions.

Figure 6: Schematic representation of cavitation phenomenon



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Stress corrosion cracking and corrosion fatigue

Both these phenomena involve eventual failure of a component by the initiation and

growth of cracks during service.Stress-corrosion cracking (SCC) occurs in the presence of a steady applied or residualtensile stress and is a very specific type of phenomenon in that a particular material willonly be susceptible to SCC in certain environments and another material is likely to beimmune to SCC in a similar environment but itself prone to SCC in a different set ofconditions. The environmental requirements for SCC may be extremely specific. e.g. anarrow pH range or a certain temperature range. The progress of SCC comprises an initialphase in which one or more cracks initiate on the surface of the component; this isfollowed by a period of slow crack growth and finally by component “failure” by fastfracture or leak.

There is a greater tendency for SCC to occur in high-strength alloys exposed toenvironments in which passive behaviour is exhibited. Most SCC failures emanate fromresidual stresses - a feature which adds to the difficulties in prediction of problems.

Prediction of SCC susceptibility is complicated when components are likely to containsurface flaws, such as cracks, before they enter service. In such situations, the timerequired for the first stage of SCC, namely crack initiation, may be drastically reducedwith obvious implications for reduced time to failure. One design approach, whichattempts to take into account pre-service cracking, makes use of "fracture mechanics" (thebasic principles of which are covered in the other part of Module B4). As summarised inFigure 7, this approach involves calculating the initial stress intensity factor associatedwith the crack- and stress geometries and using data that shows the relationship betweenthe initial value of K 1 and component time-to-failure. In some circumstances, it ispossible to identify a "threshold value" of stress intensity factor, K 1SCC , below which thematerial is immune from SCC failure in the specific aqueous environment for which thedata have been determined.



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Figure 7: Crack growth in SCC

In contrast to SCC, corrosion fatigue involves deterioration under fluctuating loadingsand can occur in any environment. Fatigue is covered in detail in the other part ofModule B4 but it is relevant, here, to state that the effect of a corrosive environment is tocause a decrease in the fatigue life at any specific applied stress range. This feature isillustrated schematically in Figure 8 in the form of "S/N" curves which are conventionallyemployed for representing fatigue data for specific materisls.

Nonmetallic materials are susceptible to SCC and fatigue deterioration but, in suchcircumstances the term “ environmental cracking ” is often employed.

Figure 8: Schematic "S/N" curves for fatigue lives



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Hydrogen embrittlement

This is a type of deterioration which can be linked to corrosion and corrosion-control

processes. It involves the ingress of hydrogen into a component which can seriouslyreduce the ductility and load-bearing capacity of susceptible materials. Hydrogenembrittlement occurs in a number of forms but the common features are an applied tensilestress and hydrogen dissolved in the metal. Examples of hydrogen embrittlement arecracking of weldments or hardened steels when exposed to conditions which injecthydrogen into the component. Hydrogen embrittlement is not a practical problem with allmetallic materials; the ones most vulnerable are high-strength steels, titanium alloys andaluminium alloys. Hydrogen entry - the obvious pre-requisite of embrittlement - can befacilitated in a number of ways summarised below.

• By some manufacturing operations such as welding, electroplating and pickling; if a

material subject to such operations is susceptible to hydrogen embrittlement then afinal, baking heat treatment to expel any hydrogen is employed.

• As a by-product of a corrosion reaction such as in circumstances when the hydrogenproduction reaction (equation 2, Chapter 1) acts as the cathodic reaction since some ofthe hydrogen produced may enter the metal in atomic form rather than be all evolvedas a gas into the surrounding environment. In this situation, cracking failures can oftenbe thought of as a type of stress corrosion cracking. If the presence of hydrogensulphide causes entry of hydrogen into the component, the cracking phenomenon isoften termed “sulphide stress cracking (SSC)”.

• As an unwanted consequence of the use of cathodic protection (CP) for corrosionprotection if the CP process is not properly controlled. CP is described in a later partof this course but it is worth stating at this juncture that. if CP potentials are allowed togo very negative, say past -1V (silver/silver chloride reference electrode)), then evenstructural carbon-manganese steels can become susceptible to cracking damageespecially welded joints experiencing fatigue stresses.

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Corrosion 4