Aqueous corrosion

9.2.2 Aqueous corrosion

9.2.2.1 Electrochemistry of corrosion

Metals corrode in aqueous environments by an electrochemical mechanism involving the dissolution of the metal as ions (e.g. Fe → Fe 2 + + 2e). The excess electrons generated in the electrolyte either reduce hydrogen ions (particularly in acid solutions) according to

2H + + 2e → H 2 so that gas is evolved from the metal, or create hydroxyl ions by the reduction of dissolved oxygen

according to O 2 + 4e + 2H 2 O → 4OH − . The corrosion rate is therefore associated with the flow of electrons or an electrical current. The two

reactions involving oxidation (in which the metal ionizes) and reduction occur at anodic and cathodic sites, respectively, on the metal surface. Generally, the metal surface consists of both anodic and cathodic sites, depending on segregation, microstructure, stress, etc., but if the metal is partially immersed there is often a distinct separation of the anodic and cathodic areas with the latter near the waterline where oxygen is readily dissolved (differential aeration). Figure 9.6 illustrates the formation

492 Physical Metallurgy and Advanced Materials

Air Iron

Waterline

O 2 dissolved

Cathode area OH ⫺ ions

Water

Rust

Fe(OH) 2

Fe 2⫹ ions Anode area

Figure 9.6 Corrosion of iron by differential aeration. of such a differential aeration cell; Fe 2 + ions pass into solution from the anode and OH − ions from

the cathode, and where they meet they form ferrous hydroxide, Fe(OH) 2 . However, depending on the aeration, this may oxidize to Fe(OH) 3 , red-rust Fe 2 O 3 ·H 2 O or black magnetite, Fe 3 O 4 . Such

a process is important when water, particularly sea water, collects in crevices formed by service, manufacture or design. In this form of corrosion the rate-controlling process is usually the supply of oxygen to the cathodic areas and, if the cathodic area is large, can often lead to intense local attack of small anode areas, such as pits, scratches, crevices, etc.

In the absence of differential aeration, the formation of anodic and cathodic areas depends on the ability to ionize. Some metals ionize easily, others with difficulty, and consequently anodic and cathodic areas may be produced, for example, by segregation or the joining of dissimilar metals.

When any metal is immersed in an aqueous solution containing its own ions, positive ions go into solution until the resulting electromotive force (EMF) is sufficient to prevent any further solution; this EMF is the electrode potential or half-cell potential. To measure this EMF it is necessary to use

a second reference electrode in the solution, usually a standard hydrogen electrode. With no current flowing, the applied potential cancels out the extra potential developed by the spontaneous ionization at the metal electrode over and above that at the standard hydrogen electrode. With different metal

electrodes a table of potentials (E 0 ) can be produced for the half-cell reactions M

→M n + + ne, (9.8) where E 0 is positive. The usual convention is to write the half-cell reaction in the reverse direction

so that the sign of E 0 is also reversed, i.e. E 0 is negative; E 0 is referred to as the standard electrode potential. It is common practice to express the tendency of a metal to ionize in terms of this voltage, or potential, E 0 = −nFE 0 for the half-cell reaction with nF coulombs of electrical charge transported per mole. The half-cell potentials are given in Table 9.2 for various metals, and refer to the potential developed in a standard ion concentration of one mole of ions per liter (i.e. unit activity), relative to a standard hydrogen electrode at 25 ◦

C, which is assigned a zero voltage. The voltage developed in any galvanic couple (i.e. two half-cells) is given by the difference of the electrode potentials. If the activity of the solution is increased, then the potential increases

according to the Nernst equation, E =E 0 + (RT /nF) ln a.

The easily ionizable ‘reactive’ metals have large negative potentials and dissolve even in concen- trated solutions of their own ions, whereas the noble metals have positive potentials and are deposited from solution. These differences show that the valency electrons are strongly bound to the positive core in the noble metals because of the short distance of interaction, i.e. d atomic ≃d ionic . A metal will therefore displace from solution the ions of a metal more noble than itself in the series. When two dissimilar metals are connected in neutral solution to form a cell, the more metallic metal becomes

Oxidation, corrosion and surface treatment 493 Table 9.2 Electrochemical Series.

Electrode reaction Standard electrode electrode

potential E 0 (V) Cs = Cs + +e

+e metals

the anode and the metal with the lower tendency to ionize becomes the cathode. The Electrochemical Series indicates which metal will corrode in the cell, but gives no information on the rate of reactions.

When an anode M corrodes, its ions enter into the solution initially low in M + ions, but as current flows the concentration of ions increases. This leads to a change in electrode potential known as polarization, as shown in Figure 9.7a, and corresponds to a reduced tendency to ionize. The current density in the cell is a maximum when the anode and cathode potential curves intersect. Such a condition would exist if the two metals were joined together or anode and cathode regions existed on the same metal, i.e. differential aeration. This potential is referred to as the corrosion potential and the current as the corrosion current.

In many reactions, particularly in acid solutions, hydrogen gas is given off at the cathode rather than the anode metal deposited. In practice, the evolution of hydrogen gas at the cathode requires a smaller additional overvoltage, the magnitude of which varies considerably from one cathode metal to another, and is high for Pb, Sn and Zn and low for Ag, Cu, Fe and Ni; this overvoltage is clearly of importance in electrodeposition of metals. In corrosion, the overvoltage arising from the activation energy opposing the electrode reaction decreases the potential of the cell, i.e. hydrogen atoms effectively shield or polarize the cathode. The degree of polarization is a function of current density and the potential E

to drive the reaction decreases because of the increased rate of H 2 evolution, as shown in Figure 9.8 for the corrosion of zinc and iron in acid solutions. Corrosion can develop up to a rate given by the current when the potential difference required to drive the reaction is zero; for zinc this is i Zn and for

iron i Fe . Because of its large overvoltage zinc is corroded more slowly than iron, even though there is

494 Physical Metallurgy and Advanced Materials

Cathode polarization

High O 2

Low O

Anode 2 polarization

Anode polarization curve

Current i

i corr

Current i

(a)

(b)

Figure 9.7 Schematic representation of cathode and anode polarization curves (a) and influence of oxygen concentration on cathode polarization (b).

Zn→Zn 2⫹

⫺ ve

i Zn

i Fe i Zn (Pt)

Current i

Figure 9.8 Corrosion of zinc and iron and the effect of polarization.

a larger difference between zinc and hydrogen than iron and hydrogen in the Electrochemical Series. The presence of Pt in the acid solution, because of its low overvoltage, increases the corrosion rate as it plates out on the cathode metal surface. In neutral or alkaline solutions, depolarization is brought about by supplying oxygen to the cathode area, which reacts with the hydrogen ions as shown in Figure 9.7b. In the absence of oxygen both anodic and cathodic reactions experience polarization and corrosion finally stops; it is well known that iron does not rust in oxygen-free water.

It is apparent that the cell potential depends on the electrode material, the ion concentration of the electrolyte, passivity and polarization effects. Thus, it is not always possible to predict the precise electrochemical behavior merely from the Electrochemical Series (i.e. which metal will be anode or cathode) and the magnitude of the cell voltage. Therefore, it is necessary to determine the specific behavior of different metals in solutions of different acidity. The results are usually displayed in Pourbaix diagrams, as shown in Figure 9.9. With stainless steel, for example, the anodic polarization curve is not straightforward as discussed previously, but takes the form shown in Figure 9.10, where the low-current region corresponds to the condition of passivity. The corrosion rate depends on the position at which the cathode polarization curve for hydrogen evolution crosses this anode curve, and can be quite high if it crosses outside the passive region. Pourbaix diagrams map out the regions of passivity for solutions of different acidity. Figure 9.9 shows that the passive region is restricted to certain conditions of pH; for Ti this is quite extensive, but Ni is passive only in very acid solutions and Al in neutral solutions. Interestingly, these diagrams indicate that for Ti and Ni in contact with each other in corrosive conditions then Ni would corrode, and that passivity has changed their order

Oxidation, corrosion and surface treatment 495

otential ( P

P otential (

otential ( P

P otential (

(d) Figure 9.9 Pourbaix diagrams for: (a) Ti, (b) Fe, (c) Ni, (d) Al. The clear regions are passive, the

(c)

heavily shaded regions corroding and the lightly shaded regions immune. The sloping lines represent the upper and lower boundary conditions in service.

in the Electrochemical Series. In general, passivity is maintained by conditions of high oxygen concentration but is destroyed by the presence of certain ions such as chlorides.

The corrosion behavior of metals and alloys can therefore be predicted with certainty only by obtaining experimental data under simulated service conditions. For practical purposes, the cell potentials of many materials have been obtained in a single environment, the most common being sea water. Such data in tabular form are called a Galvanic Series, as illustrated in Table 9.3. If a pair of metals from this series were connected together in sea water, the metal which is higher in the series would be the anode and corrode, and the further they are apart, the greater the corrosion tendency. Similar data exist for other environments.

9.2.2.2 Protection against corrosion

The principles of corrosion outlined above indicate several possible methods of controlling corro- sion. Since current must pass for corrosion to proceed, any factor such as cathodic polarization which

496 Physical Metallurgy and Advanced Materials

E Passive

P otential

loop

Active loop

Log i

Figure 9.10 Anode polarization curve for stainless steel. Table 9.3 Galvanic Series in sea water.

Anodic or most reactive

Mg and its alloys

Cu

Zn

Ni (active)

Galvanized steel

Inconel (active)

Mild steel

Ni (passive)

Cast iron

Inconel (passive)

easing

easing

Stainless steel (active)

Stainless steel (passive)

Brass

Cathodic or most noble

reduces the current will reduce the corrosion rate. Metals having a high overvoltage should be utilized where possible. In neutral and alkaline solution, deaeration of the electrolyte to remove oxygen is bene- ficial in reducing corrosion (e.g. heating the solution or holding under a reduced pressure, preferably of an inert gas). It is sometimes possible to reduce both cathode and anode reactions by ‘artificial’ polar- ization (for example, by adding inhibitors which stifle the electrode reaction). Calcium bicarbonate, naturally present in hard water, deposits calcium carbonate on metal cathodes and stifles the reaction. Soluble salts of magnesium and zinc act similarly by precipitating hydroxide in neutral solutions.

Anodic inhibitors for ferrous materials include potassium chromate and sodium phosphate, which convert the Fe 2 + ions to insoluble precipitates stifling the anodic reaction. This form of protection has no effect on the cathodic reaction and hence if the inhibitor fails to seal off the anode completely, intensive local attack occurs, leading to pitting. Moreover, the small current density at the cathode leads to a low rate of polarization and the attack is maintained. Sodium benzoate is often used as an anodic inhibitor in water radiators because of its good sealing qualities, with little tendency for pitting.

Some metals are naturally protected by their adherent oxide films; metal oxides are poor electrical conductors and so insulate the metal from solution. For the reaction to proceed, metal atoms have to diffuse through the oxide to the metal/liquid interface and electrons back through the high-resistance oxide. The corrosion current is very much reduced by the formation of such protective or passive oxide

Oxidation, corrosion and surface treatment 497 Table 9.4 Pre-coated automotive strip steel.

Coated product

Typical applications Hot-dip zinc coated

Main characteristics

Mainly non-visible parts Galvannealed/iron–zinc alloy

Standard hot-dip product

Body panels and non-visible (heated to 500 ◦

Good weldability and

parts interdiffusion of Fe and Zn)

C to allow

paintability

Electro-zinc coated

Equivalent range of properties

Body panels

to CR; single-sided coatings are available

Electro-zinc/nickel coated

Body panels Electro-zinc/nickel + organic

Improved weldability

Improved corrosion resistance

Body panels

films. Al is cathodic to zinc in sea water even though the Electrochemical Series shows it to be more active. Materials which are passivated in this way are chromium, stainless steels, Inconel and nickel in oxidizing conditions. Reducing environments (e.g. stainless steels in HCl) destroy the passive film and render the materials active to corrosion attack. Certain materials may be artificially passivated by painting. The main pigments used are red lead, zinc oxide and chromate, usually suspended in linseed oil and thinned with white spirit. Slightly soluble chromates in the paint passivate the underlying metal when water is present. Red lead reacts with the linseed oil to form lead salts of various fatty acids, which are good anodic inhibitors.

Sacrificial or cathodic protection is widely used. A typical example is galvanized steel sheet when the steel is protected by sacrificial corrosion of the zinc coating. Any regions of steel exposed by small flaws in the coating polarize rapidly since they are cathodic and small in area; corrosion products also tend to plug the holes in the Zn layer. Zinc coatings are very important for corrosion protection in the car industry, as shown in Table 9.4. Zinc coating also acts as an important barrier layer in the use of steel panels for building cladding, one of the largest markets for steel. The zinc layer is followed by pretreatment layers of phosphate or chromate before a primer and decorative top coat (PVC plastisol paint up to 200 µm thick, polyester 25 µm or fluorocarbon 25 µm) is applied. Cathodic protection is also used for ships and steel pipelines buried underground. Auxiliary sacrificial anodes are placed at frequent intervals in the corrosive medium in contact with the ship’s hull or pipe. Protection may also

be achieved by impressing a d.c. voltage to make it a cathode, with the negative terminal of the d.c. source connected to a sacrificial anode. Anodizing, the formation of a protective oxide, ∼30 µm, may be achieved electrolytically with aluminum as the anode and electrolytes of either sulfuric acid or phosphoric acid. The anodic film produced may then be colored in a second-stage electrolytic process using metal salt solutions. The structure of the film produced by sulfuric acid consisting of long narrow pores is different to that pro- duced by phosphoric acid, which is widened at the base of the pore (see Figure 9.11). The metal deposit in the wide pores gives rise to interference coloring as compared to conventional light scattering.

9.2.2.3 Corrosion failures

In service, there are many types of corrosive attack which lead to rapid failure of components.

A familiar example is intergranular corrosion and is associated with the tendency for grain boundaries to undergo localized anodic attack. Some materials are, however, particularly sensitive. The common example of this sensitization occurs in 18Cr–8Ni stainless steel, which is normally protected by a pas-

C and slowly cooling. During cooling, chromium near the grain boundaries precipitates as chromium carbide. As a consequence, these regions are depleted

sivating Cr 2 O 3 film after heating to 500–800 ◦

498 Physical Metallurgy and Advanced Materials

Metal deposit

Metal deposit

Deposit height of the order

of 1⫺5 µm

Separation of the order of 0.05⫺0.3 µm

Figure 9.11 Schematic microstructure of sulfuric acid (a) and phosphoric acid (b) anodizing treatment with metal deposits for coloration.

in Cr to levels below 12% and are no longer protected by the passive oxide film. They become anodic relative to the interior of the grain and, being narrow, are strongly attacked by the corrosion current gen- erated by the cathode reactions elsewhere. Sensitization may be avoided by rapid cooling, but in large structures that is not possible, particularly after welding, when the phenomenon (called weld decay) is common. The effect is then overcome by stabilizing the stainless steel by the addition of a small amount (0.5%) of a strong carbide former such as Nb orTi, which associates with the carbon in preference to the Cr. Other forms of corrosion failure require the component to be stressed, either directly or by residual stress. Common examples include stress corrosion cracking (SCC) and corrosion fatigue. Hydrogen embrittlement is sometimes included in this category, but this type of failure has somewhat different characteristics and has been considered previously. These failures have certain features in common. SCC occurs in chemically active environments; susceptible alloys develop deep fissures along active slip planes, particularly alloys with low stacking-fault energy with wide dislocations and planar stack- ing faults, or along grain boundaries. For such selective chemical action the free energy of reaction can provide almost all the surface energy for fracture, which may then spread under extremely low stresses.

Stress corrosion cracking was first observed in α-brass cartridge cases stored in ammoniacal envi- ronments. The phenomenon, called season cracking since it occurred more frequently during the monsoon season in the tropics, was prevented by giving the cold-worked brass cases a mild anneal- ing treatment to relieve the residual stresses of cold forming. The phenomenon has since extended to many alloys in different environments (e.g. Al–Cu, Al–Mg, Ti–Al), magnesium alloys, stainless steels in the presence of chloride ions, mild steels with hydroxyl ions (caustic embrittlement) and copper alloys with ammonia ions.

Stress corrosion cracking can be either transgranular or intergranular. There appears to be no unique mechanism of transgranular stress corrosion cracking, since no single factor is common to all susceptible alloys. In general, however, all susceptible alloys are unstable in the environment concerned but are largely protected by a surface film that is locally destroyed in some way. The variations on the basic mechanism arise from the different ways in which local activity is generated. Breakdown in passivity may occur as a result of the emergence of dislocation pile-ups, stacking faults, microcracks or precipitates (such as hydrides in Ti alloys) at the surface of the specimen, so that highly localized anodic attack then takes place. The gradual opening of the resultant crack occurs by plastic yielding at the tip and as the liquid is sucked in also prevents any tendency to polarize.

Many alloys exhibit coarse slip and have similar dislocation substructures (e.g. co-planar arrays of dislocations or wide planar stacking faults) but are not equally susceptible to stress corrosion. The observation has been attributed to the time necessary to repassivate an active area. Additions of Cr and Si to susceptible austenitic steels, for example, do not significantly alter the dislocation distribution but are found to decrease the susceptibility to cracking, probably by lowering the repassivation time.

Oxidation, corrosion and surface treatment 499

III 10 ⫺ 7 1 ⫺ ) II

10 ⫺ 9 I Crack velocity (m s

Figure 9.12 Variation of crack growth rate with stress intensity during corrosion.

The susceptibility to transgranular stress corrosion of austenitic steels, α-brasses, titanium alloys, etc., which exhibit co-planar arrays of dislocations and stacking faults may be reduced by raising the stacking-fault energy by altering the alloy composition. Cross-slip is then made easier and deforma- tion gives rise to fine slip, so that the narrower, fresh surfaces created have a less severe effect. The addition of elements to promote passivation or, more importantly, the speed of repassivation should also prove beneficial.

Intergranular cracking appears to be associated with a narrow soft zone near the grain boundaries. In α-brass this zone may be produced by local dezincification. In high-strength Al alloys there is no doubt that it is associated with the grain boundary precipitate-free zones (i.e. PFZs). In such areas the strain rate may be so rapid, because the strain is localized, that repassivation cannot occur. Cracking then proceeds even though the slip steps developed are narrow, the crack dissolving anodically as discussed for sensitized stainless steel. In practice there are many examples of intergranular cracking, including cases (1) that depend strongly on stress (e.g. Al alloys), (2) where stress has a comparatively minor role (e.g. steel cracking in nitrate solutions) and (3) which occur in the absence of stress (e.g. sensitized 18Cr–8Ni steels); the last case is the extreme example of failure to repassivate for purely electrochemical reasons. In some materials the crack propagates, as in ductile failure, by internal necking between inclusions, which occurs by a combination of stress and dissolution processes. The stress sensitivity depends on the particle distribution and is quite high for fine-scale and low for coarse-scale distributions. The change in precipitate distribution in grain boundaries produced, for example, by duplex ageing can thus change the stress dependence of intergranular failure.

In conditions where the environment plays a role, the crack growth rate varies with stress intensity K in the manner shown in Figure 9.12. In region I the crack velocity shows a marked dependence on stress, in region II the velocity is independent of the stress intensity and in region III the rate

becomes very fast as K 1C is approached. K ISC is extensively quoted as the threshold stress intensity below which the crack growth rate is negligible (e.g. ∼ < 10 −10 ms −1 ) but, like the endurance limit in fatigue, does not exist for all materials. In region I the rate of crack growth is controlled by the rate at which the metal dissolves and the time for which the metal surface is exposed. While anodic dissolution takes place on the exposed metal at the crack tip, cathodic reactions occur at the oxide film on the crack sides, leading to the evolution of hydrogen, which diffuses to the region of triaxial tensile stress and hydrogen-induced cracking. At higher stress intensities (region II) the strain rate is higher, and then other processes become rate controlling, such as diffusion of new reactants into the crack tip region. In hydrogen embrittlement this is probably the rate of hydrogen diffusion.

500 Physical Metallurgy and Advanced Materials

SELECTIVE

HIGH- SURFACE

LOW-

TEMPERATURE HARDENING

TEMPERATURE

THERMOCHEMICAL (THERMAL)

plus Gas additions

‘Cyaniding’ Salt Figure 9.13 Established methods of surface heat treatment. The influence of a corrosive environment, even mildly oxidizing, in reducing the fatigue life has

Salt

been briefly mentioned in Chapter 6. The S–N curve shows no tendency to level out, but falls to low S-values. The damage ratio (i.e. corrosion fatigue strength divided by the normal fatigue strength) in salt water environments is only about 0.5 for stainless steels and 0.2 for mild steel. The formation of intrustions and extrusions gives rise to fresh surface steps which form very active anodic sites in aqueous environments, analogous to the situation at the tip of a stress corrosion crack. This form of fatigue is influenced by those factors affecting normal fatigue but, in addition, involves electrochemical factors. It is normally reduced by plating, cladding and painting, but difficulties may arise in localizing the attack to a small number of sites, since the surface is continually being deformed.

Anodic inhibitors may also reduce the corrosion fatigue but their use is more limited than in the absence of fatigue because of the probability of incomplete inhibition, leading to increased corrosion.

Fretting corrosion, caused by two surfaces rubbing together, is associated with fatigue failure. The oxidation and corrosion product is continually removed, so that the problem must be tackled by

improving the mechanical linkage of moving parts and by the effective use of lubricants.

th is reduced and the rate of crack propagation is usually increased by a factor of 2 or so. Much larger increases in crack growth rate are

produced, however, in low-frequency cycling when stress corrosion fatigue effects become important.