Automotive Corrosion Environment

8/11/2019 Automotive Corrosion Environment

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Chapter 9 Automotive Corrosion Environment

9.1 Conductivity of Water

Water is modelled as a covalently bonded molecule H2O. Transition of covalently bondedwater to ionically bonded water is expressed as the ionization constant K

w for water.

H2O <-> (H+) + OH-

Covalent bond Ionic bond

The value of this constant is temperature dependent, but at 25 °C it is 1.01x10-14. The pH is

the square root of the ionization constant K w . If the H+ ions exceed the OH- ions the medium

is acidic and is represented by a pH from 1 to 7. If OH- ions exceeds the H+ ions the

medium is alkaline (pH from 7 to 14).

The resistivity of water varies depending on its source. Approximate value of the resistivity

(ohm meters) are given. The effect of this resistivity has a marked effect on the corrosion

profile in a given system as shown in Fig 10.1.

Table 9.1 Resistivity

Pure water 20,000,000Distilled water 500,000

Rain water 20,000

Tap water 1-5000

River water (brackish) 200

Sea water (coastal) 30Open sea 20-25



9.1 Conductivity of Water

Water, however, is not pure. It contains gases which dissolve in the water to producecharge carriers, which upsets the equilibrium between the H+ ions and OH- ions. Thus,

when rain water falls through the atmosphere, it dissolves carbon dioxide and both sulphur

dioxide and sulphur trioxide. These are acidic gases and dissolve in water to form a weak

acid but the number of ions is significantly increased by this up-take. Hence such as solution

will greatly assist the corrosion reaction in providing more charge carrier for the liquidenvironment.

9.1.1 Dissolved gases and solidsThe solubilities of oxygen, carbon dioxide and sulphur oxides in water are temperature

dependent as shown in Table 9.2 and dependent on the salinity of the water.

Table 9.2 The solubility (cubic metres per cubic metre of water under 1 standard

atmosphere) of various gases in water will affect the resistivity and aggressiveness of the

medium.

Temperature of water (0C)0 20 40 60

CO2 1.676 0.848 0.518 0.360

N2 0.0230 0.0152 0.0119 0.010

O2 0.047 0.03 0.022 0.019

Sulphur oxide 79.8 39.4 18.8 -




These oxides of carbon and sulphur are responsible for the industrial aggressive

environments, while the dependence of the salinity of the solubility of oxygen is thought toaccount for the effect of salt concentration on corrosion rate for steel as shown in Fig 9.1

and 9.2.

Solids must ionize for the solid to be dissolved in water. Hence a salt, such as the road de-

icing salts (NaCl), will produce ions (Na+ and Cl-) which will again act as charge carriers in

the liquid environment without affecting the pH. The corrosion rates show the marked effectof the addition of these charged carriers, which increase the aggressive nature of the liquid

medium. Note that the industrial atmospheres containing sulphur dioxides are more

aggressive.

Fig 9.1 A schematic diagram indicating the

effect of good and bad resistivity of the

aqueous phase on the surface of mild steel.Fig 9.2 The effect of salt concentration on the

corrosion rate of mild steel was studied by

using intermittent salt spray at 5 °C.




In Fig 9.2, the reason for the decrease in corrosion rate with the increase in salt

concentration is due to the decrease in oxygen uptake with increase in salinity.

9.2 Road De-Icing Salts: Their equivalence to sea water Consider an ideal situation where a piece of steel can corrode in two media: pure water and

sea water (Fig 9.1). Both sea water and road de-icing salts contain sodium and calcium

chlorides. The effect of the chloride ion upon the corrosion rate is immense.In the pure water without chloride, the Fe2+ ion is taken out of solution by the OH- ions from

the water. The ‘insoluble’ ferrous hydroxide Fe(OH)2 initially forms a film over the steel

which act as a good barrier layer through which oxygen must diffuse to get to the steel

surface so that the cathodic reaction can take place. The pH of a saturated solution of

ferrous hydroxide is 10.5 and so the steel is also in an alkaline environment. The rate ofcorrosion in alkaline media is low owing to the presence of a protecting passive film on the

surface of steel.

Fe2+ + 2OH- <-> Fe (OH)2

The iron ion Fe2+ becomes more soluble and mobile in the presence of chlorides and so can

move away from the metal-liquid interface and react with the hydroxyl ions OH- away fromthe dissolving area. This prevents the ferrous hydroxide film Fe(OH)2 from being deposited

on the surface of the steel and thus there is no barrier film through which the oxygen has to

diffuse. The road de-icing salts therefore make the liquid environment more aggressive by

providing more mobility to the Fe2+ ions, and more charge carrier (Na+ and Cl- ions) in the

liquid environment. Thus the corrosion reaction is accelerated.




9.3 Effect of Dissolved Oxygen

Oxygen is removed from water by the following:1) Corrosion of steel

2) Dissolved salts

3) Biological decay

4) Increase in temperature.

The corrosion of iron in oxygenated water is a parabolic curve. The initial corrosion rate may

reach a figure of 100 mg dm-2 day-1, which rapidly falls as the ferrous hydroxide is deposited

on the steel surface (as low as 10-25 mg dm-2 day-1). The corrosion reaction rate is

controlled mainly by the diffusion of oxygen to the steel surface. The corrosion rate reaches

a maximum with high oxygen levels because the excess of oxygen required by the cathodicreaction is available to form a passive oxide film on the steel. This effect is not often

observed in automotive corrosion because most corrosion cells are formed by differences in

the replenishment rates of oxygen. Under these conditions of very high oxygen

replenishment, the metal is very susceptible to pitting corrosion, which once started will set

up and can not be destroyed once they have been set up. Localized metal penetration is theresult.




9.4 Effect of Dissolved Salts on Oxygen Uptake in Water

The amount of oxygen which water can dissolve is dependent on the concentration ofdissolved salt as shown in Fig 9.2. Here the corrosion rate at first increase with salt content

and then at 3% of sodium chloride in solution it falls to below the corrosion rate for pure

water when over 20% salt solution is dissolved. This is because the oxygen solubility falls

with salt content. The initial rise in corrosion rate with salt content, shown in Fig 9.2, is

almost certainly due to the fact that the anodes and cathodes can be further away fromeach other when the medium contains salt than when the steel is in pure water. In pure

water the anodes must be near to cathodes owing to the poor conductivity of the medium.

As the salt content rises, the conductivity rises and so the distance between the local

electrodes may increase. Because of this increase in distance between anodes and

cathodes, the protective ferrous hydroxide Fe(OH)2 is no longer deposited as the barrierlayer.

9.5 Hygroscopic Salts, Deliquescent SaltsThe air which we breath consists of oxygen (23% by weight), nitrogen (75% by weight),

carbon dioxide (0.04%) and water vapour (0.7% by weight at 10°C). This water vapour is‘invisible’ in the air and only shows its presence when the air becomes cold, whereupon the

water condenses out as dew, fog or clouds. The amount of water taken in by the air is

expressed as the relative humidity:




9.5 Hygroscopic Salts, Deliquescent Salts

Relative humidity = (amount of water vapour in the air)/ (amount of water vapour required tosaturate the air) X 100

This is expressed as a percentage. A relative humidity of 30% means that the air is very dry,

while a relative humidity of 95% means that the air is very damp. Cold steel will reduce the

temperature of the adjacent air and water will condense onto the metal, giving the concept

of a metal “sweating”. Water vapour in the colder layer of air adjacent to the car body in thelate evenings and early mornings condense out on the car body and in crevices and on

hygroscopic salts. The main road de-icing salt are hygroscopic NaCl and CaCl2. Saturated

solution of salts have an equilibrium relative humidity. The relative humidity of the air in

equilibrium with saturated solution at 20°C for NaCl and CaCl2 is 76% and 36%,

respectively.

A hygroscopic salt is a compound, a salt, which absorbs water from the atmosphere to

produce an electrolyte. In some case the compound will make a pool of water at its site and

the materials is said to be deliquescent. A hygroscopic salt therefore can be considered to

be artificially increasing the relative humidity of its immediate environment. Thus thehygroscopic salt produces an environment which is wetted earlier as the relative humidity

rises and remains wet for a longer period after the relative humidity falls. Hence the road de-

icing salts NaCl and CaCl2 will not start to dry out until the relative humidity falls to 76% and

32%, respectively.




9.5 Hygroscopic Salts, Deliquescent SaltsThere are many salts which are hygroscopic, but the main ones in the corrosion of the

motor vehicle are phosphoric acid and its salts, which arise from the metal pretreatment

process and the road de-icing salts. The excess or unreacted phosphoric acid from these

preparation should be removed with distilled water in order to make sure that the

hygroscopic contaminants are not trapped beneath the new paint films. Tap water is

unsuitable as the water contains several dissolved salts. Research has shown that

sulphates are main cause of paint breakdown by blistering. The sulphates may arise from

industrial atmospheres.

9.6 Critical Humidity

Water molecules are chemically bound to thesurface of the metal by weak van der waals

bonds. It is estimated that at a relative humidity of

55% there is a surface film some 15 molecules

thick upon the surface of mild steel, and this

increase to 90 molecular layers as the relativehumidity rises to 100%. These surface-adsorbed

layers are capable of supporting the corrosion

process above 60% relative humidity. This is very

pronounced if the atmosphere contains sulphur

dioxided (Fig 9.3). Fig 9.3 The corrosion rate of iron varies markedly with SO2, humidity

and particles, accelarating quickly at a critical humidity of over 70%.




9.7 Effect of Capillary

CondensationSome important consequences for water

ingress into porous materials such as

paint films or the narrow crevices are

as follows:

1) Water will condense more easilyupon a concave surface than on a

flat surface.

2) Water will evaporate from a convex

surface and condense upon a flat

surface or concave surface.3) Once capillaries contain water it is

very difficult to dry them out as their

surface profile is always concave.

4) Protective coatings (e.g., paints and

chromium plating) contain micro-capillaries which take a long time to

dry and act as ‘preferential sites’ for

atmospheric condensation.Fig 9.4 Capillary rise is inversely proportional to the

crevice gap or capillary radius r.




9.8 Effects of Surface Tensionwater

Two dirt particles with a small amount of water will be attracted towards each other to

gain the minimum surface area for the amount of water present. This will ensure that thedirt particles are in contact in three dimensions (i.e., compacted). As the amount of water

with each dirt particle increases, the minimum surface area for the associated water can

only be obtained by pushing the dirt particles apart as shown in (b). This will not be ‘self-

compacting’.

Dirt particle

water

Small amount of water leads tocompact by capillary action

Larger amount of water leads to an

open structure

(b)

(a)

1) The compact of road dirt by the phenomenon of surface tension is important in the

building-up of the aggressive micro-climate within the car body structure.

2) The small amount of water present upon each road dirt particle means that there is

insufficient water to wash away and clear the road dirt which is deposited out of

wheel spray.3) The slower the car travel, the larger will be these water droplets, and the higher the

ratio of water to dirt which they can contain.

Fig 9.5 Effect of surface tension




9.9 Road De-icing Salts

Road de-icing consist of naturally occurring sodium and calcium chlorides mixed with finesands which also act as an abrasive owing to the relative speed of the car body and road.

This mixture, is spread onto the roads as a safety measure.

In some countries these salts (especially calcium chloride) are made use of to keep down

the dust on minor (dirt) roads. They keep the dust down because they are hygroscopic anddeliquescent thus absorbing the atmospheric water vapour at low humidities.

The road de-icing salt melt the snow and ice because they depress the equilibrium

temperature at which ice co-exists with its liquid (saline) state. If these The maximum

freezing point decrease is concentration dependent. Thus the freezing point of water with

soluble additions are usually shown in a phase diagram. One section of the phase diagramfor both sodium chloride and calcium chloride is shown in Fig 9.6(a) and 9.6(b) respectively.

This shows the maximum freezing point depression being -21°C for sodium chloride and -

55°C for calcium chloride. The addition of sodium chloride to crushed ice cubes from the

refrigerator at 0°C will actually produce this temperature of -21°C (if 23.31% sodium chloride

in the mixture; -55°C is for 30% calcium chloride). Hence it would appear that by theaddition of road salts to our roads we are making the road and ice-colder. In actual fact we

do make them colder, but then the road can take up heat from the surrounding atmosphere

which may be at -5°C. More and more ice will melt in trying to keep tehice-salt mixture at -

21°C until all the ice has melted. Hence the addition of road de-icing salts to our roads will

only work if the ambient temperature is above -21°C. CaCl2 must be used where theambient temperature can be as low as -55°C.




9.9 Road De-icing Salts

Fig 10.6 (a) and (b) show one section of the phase diagram for both sodium chloride and

calcium chloride, respectively.

(a) (b)




9.10 Concentration of the Road De-icing Salt within CrevicesThe road de-icing salts concentrate within the last areas to dry. Among the last areas to dry

are (1) hemming folds or clinches, (2) metal overlap, (3) at or between the motor vehicle

panel and decorative brightwork, plastic guards or any attachment which forms a crevice,

(4) behind the door-tailgate hinges, (5) beneath areas which have suffered ‘paint stripping’,

(6) around the door-glass rubbers and (7) around the door return, crevices and folds.

Fig 9.7 and 9.8 show diagrammatically what happens as water evaporates from a crevice

which contains a very dilute solution of road de-icing salts within the crevices area. This is

compared with a similar concentration of saline solution on a flat vertical panel Fig 8.7(b). If

the vehicle is stationary, then after, say 1 h of drying, the water profile would shrink as

shown in Fig 9.7(b). If the vehicle is now considered in motion, then a sudden ‘bump’ in the

road can displace the droplet on the vertical panel to some other location.




Fig 9.7 This shows a water droplet drying (a) with and (b) a dirt particle as a

locating nucleus for water droplet. Vibration will therefore mean that waterdroplet will run down the vertical panel as a result of gravity. (C) A water droplet

drying within an inter-weld which acts as a locating nucleus for the water droplet.

9.10 Concentration of the Road De-icing Salt within Crevices




Fig 9.8 the same diagrams as Fig. 9.7 except that now the substrate is coated

with a wax or oil which presents a hydrophobic surface to the saline droplet. Theright one shows a water droplet excluded from the wax-oil-coated crevice. Any

dissolved salts such as the road de-icing salts will be concnetrated outside the

crevice at the last area to dry, which is now a well oxygenated area and so the

corrosion rate will be very low.

9.10 Concentration of the Road De-icing Salt within Crevices




9.10 Concentration of the Road De-icing Salt within Crevices Any saline solution left on the panels will dry first. The last area to dry will be the inter-welds

and so these regions will suffer an increase in the concentration of road de-icing salts within

the inter-welds.

9.11 The Motor Industries Acceleration Corrosion TestsOver the years the raw products used in motor vehicle manufacture have undergone

accelerated corrosion tests based on the traditional laboratory tests. These acceleration test

are numerous in number and some of them are as follows: (1) The ASTM 117B salt spray

test; (2) the cupric chloride accelerated acetic acid salt spray (CASS) test; (3) cyclic

humidity test, which are probably the most reliable; (4) the Corrodkote test, where ferric

chloride is added to the acetic acid salt spray. These accelerated test do not detect amechanism of corrosion.

Another series can be used where the corrosive effects are much less and therefore a

longer time is required to show up corrosion and these are the field or service tests. In these

test samples are exposed to the condition which are met in practice. The corrodkote testwas the near service condition that was reached in that a corrosive acetic acid salt spray,

modified with cupric chloride and ferric chloride, was splashed onto the specimen by

rotating paddles and the soiled specimen were then exposed to a humid environment at

high temperatures.




10.11 The Motor Industries Acceleration Corrosion Tests

When it came to putting the finished motor car under an accelerated test cycle, theconditions chosen were not the ideal to simulate service conditions. The accelerated

tests conventionally used to drive the vehicle through a salt ford or dip at low speeds (20

mph) and then to place the vehicles in a humidity chamber (100% relative humidity at

49°C) and allow the vehicle to sweat for several hours. Other manufacturers have various

test methods. None of these methods will produce the small spray droplets required toform the low momentum micro-droplets which will so easily alter direction as a result of

the pressure eddies around the vehicle body.

The large diameter spray from this 20 mph encounter with the salt ‘dip’ does not produce

the (low momentum) fine particles of spray which will be ‘self-compacting’ on out of sight(or out of trajectory) ledges. Also, the high momentum (20 mph) droplet will not be draw

through the manufacturer’s process drainage holes by the small pressure changes

around the vehicle. This procedural test therefore leads to the acceptance of bad design

faults which lead to premature failure by corrosion as described throughout this book.


Automotive Corrosion Environment

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