Higgins R.A. Engineering Metallurgy: Applied Physical Metallurgy

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of the latter tend also to ionise thus increasing the hydrogen ion concen-

tration, pH.

The connection between electrode potential (E

) and the hydrogen ion

concentration (pH) was investigated in Belgium by M. Pourbaix. He

derived diagrams which connect these values with the electrochemical reac-

tions between metals and water which was made either acid or alkaline.

Both electrode potential, Eh, and hydrogen ion concentration, pH, influ-

ence the transfer of electrons in the system so that hydrogen evolution and

oxygen absorption depend on these factors and these reactions are the

basis of electrolytic corrosion. The effects of E

and pH on the extent to

which water will liberate either hydrogen or oxygen during what we term

'electrolysis' are illustrated in Fig. 21.7. Hydrogen will only be liberated

under conditions of Eh and pH which are represented by a point below

'ab\

and oxygen can only be liberated if E

and pH are represented by a

point above 'cd\ Hence there is a domain (between ab and cd) on the

EJpH diagram where the electrolysis of water is impossible. As a result

the electrolysis of water cannot take place, regardless of the pH of the

electrolyte, unless the electrodes have a minimum potential difference of

at least 1.2 volts, that is the distance apart of ab and cd in terms of Eh.

21.38 In Fig. 21.7 the equilibrium of hydrogen and oxygen are rep-

resented in terms of E

and pH. However, equilibrium of a metal in contact

Fig.

21.7 The Eh/pH diagram for water.

with water can also be represented using a Pourbaix diagram. Since the

corrosion of iron (or mild steel) is important this is represented as a simpli-

fied diagram in Fig. 21.8. The complete Pourbaix diagram for iron is more

complex since several chemical reactions are involved. However, a simpli-

fied diagram can be drawn based on a study of the following chemical

equilibria:

(i) Fe ^ Fe

+ 2e~ Iron atoms form soluble iron(II) (Fe

) ions,

(ii) Fe

^± Fe

+++

+ e" Iron(II) ions are oxidised to iron(III) ions.

ELECTRODE

POTENTtAL

(volts)

OXYGEN LIBERATED

WATER NOT

ELECTROLYSED

HYDROGEN LIBERATED

acid

neutral

alkaline

Fig.

21.8 A simplified Pourbaix diagram for the Fe/H

O system.

(iii) Fe

+ 3OH~ ^± Fe(OH)

+ e" Insoluble iron(III) hydroxide pre-

cipitates.

(iv) Fe

+++

H- 3H

O ^± Fe(OH)

+ 3H

Insoluble iron(III) hydroxide

precipitates.

(v) Fe + 3H

O ^± Fe(OH)

+ 3H

+ 3e~ Insoluble iron(III) hydroxide

precipitates.

(vi) Fe + 2H

O ^± FeO

H" + 3H

+ 2e~ Ferrite ions go into solution.

(vii) FeO

H" + H

O ^± Fe(OH)

+ e" Soluble ferrite ions precipitate

as iron(III) hydroxide.

When the electrode potential (E

= -0.44 volts) of iron is measured in

pure water (pH = 7) it will be represented on the Pourbaix diagram (Fig.

21.8) by the point O. This indicates that corrosion can take place under

such conditions of Eu and pH. The diagram suggests three ways in which

the extent of corrosion could be reduced:

(a) The potential can be changed in the negative direction (X) so that

it enters the domain of immunity. This is the normal 'sacrificial' or

cathodic protection such as is offered by zinc or aluminium.

(b) The potential can be changed in a positive direction (Y) into the

domain of passivity by applying a suitable external EMF (21.110).

(Z) so that the metal becomes passive. Inhibitors have this effect but

need careful control because the passivity domain extends over a very

narrow pH range, making constant surveillance of the electrolyte compo-

sition necessary.

incrcsingly

cathodic

incrcsingly

anodic

ELECTRODE

POTENTIAL

(volts)

soluble Fe"* ions

CORROSION

Fc*

ions dissolve from anode

metallic Fc stable

IMMUNITY

PASSIVITY

Fe(OHl

precipitous

CX)RROSION

Fe forms

soluble

^FeCVH

ions

acid

neutral

alkaline pH

Electrolytic Action or Wet Corrosion Involving

Mechanical Stress

21.40 It has already been shown (21.35 and 21.36) that electrolytic cor-

rosion can occur as a result of different parts of a microstructure being of

different compositions and, hence, different electrode potentials. Thus

intergranular corrosion can occur in a cast metal since impurities, segre-

gated at grain boundaries, have different electrode potentials to the rest of

the structure. Nevertheless a perfectly uniform wrought metal may corrode

more rapidly if it is in the cold-worked condition.

As we have seen cold-working causes a movement of dislocations

towards crystal boundaries (4.19). This 'pile-up' of dislocations results in

the formation of a region of elastic strain in the vicinity of the crystal

boundary and the iocked-up' stresses associated with this strain constitute

a region of higher energy level than is present in the remainder of the

crystal. If an electrolyte is present then energy can dissipate itself by the

grain-boundary atoms going into solution as ions. That is the high-energy

grain-boundary material is anodic to the rest of the crystal.

21.41 This type of intergranular corrosion manifests itself in many

instances. The 'season cracking' of 70-30 brass has been mentioned

(16.33),

but of greater significance in the modern world is the accelerated

corrosion which occurs in mild steel which has been cold formed. In many

instances it is possible to apply a stress-relieving annealing process to the

cold-worked material but in other cases this is inappropriate either because

mechanical distortion of the structure would result or because the results

of cold-work are necessary to give rigidity and strength. Adequate protec-

tion of the surface by some form of coating is then the only way to prevent

corrosion.

Consider the cold-formed rim of a motor-car 'wing' (Fig. 21.9). The

oxygen

cathodic

region

anodic

region

Fig.

21.9 Corrosion due to local cold-work in the rim of a motor car wing.

forming operation will have cold-worked the rim more severely than the

rest of the structure making it anodic to its surroundings so that, assuming

that the paint work is defective, corrosion will be accelerated there particu-

larly when salt-laden moisture collects during winter. This type of corrosion

in metals is generally termed stress corrosion and may not necessarily be

caused by cold-work. For example, locked-up stresses may be set up near

to welded joints as a result of non-uniform cooling. Similar situations may

occur during the cooling of a casting. It must be admitted, however, that

in both welded joints and castings galvanic corrosion is much more likely

to be the result of variations in composition rather than variations in strain

energy.

21.42 Corrosion assists in other forms of failure which plague the

engineer. Thus corrosion fatigue refers to the failure of a component in

which the propagation of a fatigue crack is helped by corrosion, or indeed

initiated by corrosion. A surface flaw may be accentuated by corrosion and

then act as a focal point for the initiation of a fatigue crack (5.65). Once

such a fatigue crack has started its progress will be accelerated by corrosion

due to the anodic effect of high stress concentrations always present at the

root of the crack.

21.43 Impingement corrosion describes a process involving electro-

lytic action following mechanical abrasion of the surface. Thus particles of

grit or entrained air bubbles carried in a flow of liquid may impinge upon

an oxide-coated surface, wearing away the oxide skin. Electrolytic action

between the cathodic oxide skin and the anodic metal beneath follows (Fig.

21.10) leading to solution of the metal. This form of attack is prevalent in

tubes carrying sea water but also occurs with fresh water, particularly when

the flow of water is so rapid that the rate of damage to protective films

exceeds the rate of repair by oxidation. Corrosion in this instance is not a

result of mechanical working but of removal of an oxide film so that simple

galvanic action can occur between the rest of the oxide film and the exposed

metal.

Fig.

21.10

'Impingement corrosion' results from

combination

mechanical abrasion

and

electrolytic action.

The

corrosion product will

flushed away.

oxide film

(cathodic)

water flow

mild steel

(anodic)

Electrolytic Action or Wet Corrosion Involving

Electrolytes of Non-uniform Composition

21.50 Electrolytic action may occur between two electrodes of precisely

the same composition if these electrodes are in contact with an electrolyte

which is different in composition at the surface of each electrode. This

situation can be described by reference to what is termed a concentration

cell (Fig. 21.11). Here both electrodes are of pure iron but the electrolyte,

Fig.

21.11 A'concentration'cell.

iron(II) chloride solution, is more concentrated on the right of the cell

than on the left. The cell is divided by a porous plate—similar to that in

an old-fashioned Leclanche cell—which prevents the two solutions from

mixing freely but is sufficiently porous to allow the passage of ions and so

maintain 'electrical contact'. The main reaction in the cell is represented

by:

Fe ^± Fe

4- 2e"

and the direction in which it proceeds at any point in the cell is governed

by the concentration of the reactants. Therefore, on the left-hand side, the

relatively low concentration of Fe

ions will cause iron atoms to form

ions,

releasing electrons to the external circuit:

Fe -» Fe

+ 2e"

On the right-hand side of the cell the already high concentration of Fe

ions will tend to make the reaction go in the opposite direction as these

ions receive electrons from the external circuit:

pure iron

anode

porous plate

pure iron

cathode

dil. ironin) chloride solution

cone, ironin) chloride solution

Since iron atoms are entering solution as Fe

ions at the left-hand elec-

trode this is obviously the anode, whilst the electrode on the right-hand

side becomes the cathode as is indicated by the movement of electrons in

the external circuit.

21.51 Electrolytic action of this type is the basis of corrosion which

occurs in a variety of different instances in engineering practice. For

example, so-called 'crevice corrosion' is caused by a non-uniform distri-

bution of dissolved oxygen in water thus producing a 'concentration cell'

situation. In Fig. 21.12 two steel plates of similar composition are bolted

together and, although the gap between them is small, water will be drawn

into the gap by capillary action. If, for a short time, the concentration of

dissolved oxygen increases in that water exposed to the atmosphere, then

the region 'C of the plate will become cathodic to that at 'A\ The plate

in the vicinity of A is covered by water containing less oxygen and so

becomes anodic. As soon as this happens iron atoms will begin to ionise

in the anodic area of the crevice forming Fe

ions, and the electrons so

released will travel quickly through the steel plate to the cathodic area to

form OH" ions:

O + O

+ 4e" -> 4OH

As the concentration of OH" ions in the cathodic area is thus increased so

a persistent concentration cell is set up since oxygen is unable to dissolve

in the water trapped in the crevice, whereas oxygen is able to dissolve

continuously from the atmosphere in the cathodic area.

Having the greater mobility, Fe

ions travel outwards to the cathodic

area more quickly than the OH" ions travel to the anodic region. Therefore

the reaction:

+ 2OH" -> Fe(OH)

occurs mainly outside the crevice. Fe(OH)

is rapidly oxidised to Fe(OH)

'rust'. Thus the formation of a concentration cell

accelerates

corrosion but

accelerates

it where the concentration of oxygen is lower. This explains

Fig.

21.12 The 'crevice corrosion' of steel plates.

ahodic

cathodic

rust deposit

oxygen

why steel corrodes most

places where

would seem

to be

better

pro-

tected from

the

atmosphere.

21.52

The

fairly rapid rusting which occurs

in the

vicinity

defective

paintwork

on a

motor

car is

explained

in Fig.

21.13.

Water begins

penetrate beneath

the

loose paint film

but

oxygen does

not and so the

metal beneath

the

paint becomes anodic

that which

exposed

to the

atmosphere. Crevice corrosion then proceeds

above.

Fig.

21.13

Sub-paint rusting

of a

motor

car

body. Another case

crevice corrosion.

One frequently hears

motor-car owner bemoaning

the

fact that 'rust

is eating

its way

beneath

the

painting work'.

fact

course

the

reverse

is true—Fe

ions

are

moving outwards from beneath

the

paint

form

rust

at the

cathodic surface. Some

of the

ions will meet

with

OH"

ions

these move slowly inwards beneath

the

paint.

As a

result some rust

forms beneath

the

paint producing blisters which lift

it off.

Accumulations

chemically-inert dirt

on the

surface

exposed metal

will also give rise

this form

corrosion. Such encrustations allow moist-

ure

penetrate

and

this moisture becomes increasingly deficient

oxygen

thus giving rise

anodic spots beneath

the

dirt. This situation

made

worse

by the

salt used

roads

winter since

Cl"

ions will tend

congregate along with

OH"

ions outside

the

dirt encrustations

and con-

siderably increase

the

concentration gradient within

the

electrolyte.

It is

far more important

wash away accumulated dirt from

the

hidden parts

motor-car—particularly during winter—than

it is to

waste time

pol-

ishing

the

areas which

are

visible. 'Wings' invariably rust from beneath

particularly where

'mud

traps' have been incorporated into

the

design.

21.53 Finally

will consider

why a

sheet

high-quality mild steel

rusts under

the

action

of a

simple rain drop

(Fig.

21.14). Here

the

concen-

Fig.

21.14 The

rusting

mild steel

in the

presence

of a

rain drop containing dissolved

oxygen.

Note

the

position

of the

anulus

rust—near

the

outer

rim

because

of the

lower mobility

the

OH"

ions compared with those

oxydcn

rain drop

rust deposits

here

cathodic

anodic

mild steel

cathodic

oxygen

rust deposit

paint film

anodic

cathodic

tration of dissolved oxygen in the rain drop will be higher at 'C than at

'A'

where the depth of water is greater, so that dissolved oxygen reaches

the metal surface at A more slowly. A concentration cell is therefore set

up so that atoms of iron at A go into solution as Fe

ions releasing

electrons which pass through the 'external circuit' (the mild stbel sub-

surface) to C where OH" ions form. A deposit of Fe(OH)

is produced

mainly around the periphery of the rain drop and this oxidises immediately

to form Fe(OH)

or 'rust'.

This type of corrosion would appear to be important on the massive scale

with the many off-shore structures associated with oil and gas operations in

the North Sea, for as water depth increases so the oxygen content decreases

as in the case with the droplet of water described above. However the rate

of corrosion is probably influenced more directly by factors such as sea

water velocity (impingement corrosion—21.43), chlorine ion

(-Cl")

con-

centration, marine organisms, temperature and the rise and fall of the

tides.

The Prevention of Corrosion

21.60 There are two principal methods by which corrosion may be pre-

vented or minimised. First, the metallic surface can be insulated from the

corrosive medium by some form of protective coating. Such coatings

include various types of paints and varnishes, metallic films having good

corrosion-resistance and artificially thickened oxide films. All of these are

generally effective in protecting surfaces from atmospheric corrosion. Zinc

coatings are used to protect iron and steel from the rusting action of moist

atmospheres and though zinc offers its 'sacrificial protection' (21.33) ^s a

second line of defence, it should be clearly understood that the main objec-

tive is to produce a sound continuous film of zinc which will seal off the iron

completely from atmospheric action. Sacrificial protection is a temporary

phenomenon and is only effective for a limited time since the zinc dissolves

quickly once electrolytic action begins.

Tin coatings offer protection against most animal and vegetable juices

encountered in the canning industry but due to current very high costs of

tin extremely thin coatings of the metal are used on the mild-steel cans.

This tin coat is now generally covered by a film of some organic polymer

(plastics material) as the main protection, so that modern tin cans tend to

rust on the outside rather than the inside.

21.61 In circumstances where corrosive action is severe, or where

mechanical abrasion is likely to damage a surface coating, it may be neces-

sary to use a metal or alloy which has an inherent resistance to corrosion.

Such corrosion-resistant alloys are relatively expensive, so that their use is

limited generally to chemical-engineering plant, marine-engineering equip-

ment and other special applications.

The Use of a Metal or Alloy Which Is Inherently

Corrosion-resistant

21.70 The corrosion-resistance of a pure metal or a homogeneous solid

solution is generally superior to that of an alloy in which two or more

phases are present in the microstructure. As mentioned above, the exist-

ence of two phases leads to electrolytic action when the surface of the alloy

comes into contact with an electrolyte. The phase with the lower electrode

potential will behave anodically and dissolve, leading to pitting of the alloy

surface; and the greater the difference in electrode potentials between the

phases, the more rapid will be corrosion. Resistance to corrosion will

generally be at a minimum when the second phase is segregated at the

crystal boundaries, particularly if this phase is anodic to the matrix. In

such circumstances serious intercrystalline corrosion will occur.

21.71 Most of the alloys which are used because of their high corrosion-

resistance exhibit solid-solution structures. Aluminium-magnesium alloys

(17.30) containing up to 7.0 % magnesium fulfil these conditions and are

particularly resistant to marine atmospheres. 18-8 stainless steel is com-

pletely austenitic when correctly heat-treated but faulty heat-treatment

may lead to the precipitation of chromium carbide at the grain boundaries.

This gives rise to a zone adjacent to the grain boundary which is impover-

ished in chromium (Fig. 21.15(i)) so that it becomes anodic to the remain-

der of the crystal which is still rich in chromium. Consequently in the

presence of a strong electrolyte (mineral acids and the like) intergranular

corrosion will occur (Fig. 21.15(ii)). 'Weld decay' occurs in 18-8 stainless

steels for similar reasons (20.93).

Fig.

21.15 'Weld decay

in austenitic stainless steel.

21.72 As might be expected, coring in a cast alloy of the solid-solution

type gives rise to a quicker rate of corrosion than that which obtains when

the same solid solution has been cold-worked and annealed. This treatment

produces greater homogeneity in the structure and reduces the extent to

which electrolytic action will take place between the core and outer fringes

of individual crystals. Similarly, the presence of impurities segregated at

the crystal boundaries of a metal used in either the alloyed or unalloyed

state will accelerate corrosion by setting up electrolytic action with the

metal when an electrolyte is present.

electrolyte

chromium-rich

austenite

(cathodic)

chromium-

depleted

zone (anodic)

chromium-rich

austenite

(cathodic)

precipitated

chromium

carbide

grain

boundary

chromium-depleted

zone (anodic)

Fig.

21.16 The relative corrosion-resistance of types of microstructure.

21.73 Although a homogeneous single phase metal or alloy will tend

to be less vulnerable to simple electrolytic corrosion, it will of course

succumb to attack which is caused by mechanical stress (21.40) or by

local variations in electrolyte composition (21.50). Thus stainless steel is

susceptible to crevice corrosion since it relies for its corrosion resistance

on a supply of oxygen to maintain a dense protective film of chromium

oxide on the surface (13.16). For similar reasons stainless steel may suffer

impingement corrosion when abrasive particles damage the oxide skin in

the presence of strong electrolytes such as acid chloride solutions.

The effects of cold-work in producing regions of high-energy value which

accelerate corrosion have already been mentioned. 70-30 brass, normally

a reasonably corrosion-resistant single-phase alloy when of high purity,

nevertheless suffers 'season cracking' (16.33) as a result of intergranular

corrosion associated with deep drawing and the pile-up of dislocations at

grain boundaries so produced.

Protection by Metallic Coatings

21.80 Protection afforded by metallic coatings can be either 'direct' or

'sacrificial'. Direct protection depends on an unbroken film of some cor-

rosion-resistant metal covering the article, and if the film becomes broken,

corrosion may be accelerated by electrolytic action between the film and

the metal beneath. In the case of sacrificial protection, however, the met-

allic film becomes anodic in the event of a break in the film, and thus

dissolves in preference to the cathodic surface beneath. It follows that,

when protection is limited to the direct type, as in the case of tin coatings

on steel, the quality of the coating is most important, since acceleration,

and not inhibition, of corrosion would follow a break in the film. In both

cases,

of course, protection of the direct type is the fundamental aim of

the metal-coating process, and it is only in the possibility of the coating

becoming broken that the effects of electrolytic action must be considered.

INCREASE IN RESISTANCE TO INTERCRYSTALLINE CORROSION

THE PRESENCE OF A

GRAIN-BOUNDARY

PRECIPITATE WHICH IS

ANODIC TO THE MATRIX.

(CORROSION RESISTANCE - POOR.)

UNIFORMLY DISTRIBUTED

RESISTANCE - FAIR )

HOMOGENEOUS SINGLE

PHASE (CORROSION-

RESISTANCE - GENERALLY

GOOD)