Practical aspects of Automotive corrosion

8/11/2019 Practical aspects of Automotive corrosion

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6.1 Corrosion in the Motor Car

Chapter 6 Automotive Corrosion due to Build-up of the Micro-climate

Fig. 6.1c When plants photosynthesize sugars from

carbon dioxide and water, oxygen is given off. This

oxygen does not accelerate the corrosion rate of the car

when it is parked over grass or under trees. The trees and

grass will produce a higher humidity which affects dryingand allows the winter de-icing salts to absorb water.

Fig 6.1b The earth contains magnetic North and

South poles. Some engineers postulate that it is the

car cutting this magnetic filed which acts as a smallgenerator or dynamo. The electric current

generated is considered as the cause of corrosion.

Fig 6.1a A metal is not porous, nor does

it sweat water. Corrosion is not due to

residual root from previous recycled

corroded vehicles.


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6.2 Demonstration of the battery process the differential aeration cell

This potential difference or voltage of the equivalent battery due to the difference in theoxygen availability can be shown by taking two pieces of steel and immersing them in a

salt solution as shown in Fig. 6.4. The two pieces of steel are connected to a potential-

measuring instrument a digital voltmeter (DVM). One side of the U-tube is oxygenated by

blowing air through the liquid, while the other side remains relatively de-oxygenated. The

cotton wool prevents the two liquid from mixing, while at the same time allowing thecorrosion process to take place between the two liquids in either arm. Potential reversals or

anodes changing to cathodes and vice versa may be obtained depending on which arm is

oxygenated. For this reason the cell or battery is called the differential aeration cell.


Fig. 6.2 Two pieces of mild steel connected to a DVM

and immersed in a saline electrolyte which is

oxygenated differentially will show a potential difference.


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6.3 Where are these cells and their electrodes (equivalent batteries)

These cells form short-circuited batteriesbecause the car body is one integral electrical

conductor, which is unlike the two pieces of

steel used above. These equivalent batteries

occur at all places where there is a difference

in the availability of oxygen.


Fig. 6.3 Schematic view of the road de-icing

salts (a) on the surface of the car body, (b)

absorbing water from the atmosphere to form

the differential aeration cell and (c)concentrating at the base of the road

aggregate on drying. (d) the short circuited

equivalent battery is generated by the

difference in oxygen availability. The

oxygenated region is the cathode; thedeoxygenated region and the area which

dissolves is anode. (e) The corrosion (metal

penetration by pitting) is initiated owing to the

lack of oxygen replenishment beneath these

deposits. (f) A stone chip doesnt cause

severe corrosion in the early stages of life

(Why?).


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6.3 Where are these cells and their electrodes (equivalent batteries)

The equivalent batteries can be formed at the following locations.1)Area of bad design as they harbour road dirt which restrict natural drying.

(corrosion can only proceed in the presence of a moist environment. Bad design, in the

corrosion sense, is any design which allows the accumulation)

of a corrosive micro0climate, the presence of which the lay public are unaware of but

the motor manufacturers should be aware of in view of their expertise in the subjectmatter.

2) Beneath road dust dirt (Fig 6.3a) or other surface deposits.

3) In inter-welds or clinch flanges or hemming folds. These also form stagnant liquid

environments which presents a small liquid surface to the atmosphere and so drying by

evaporation is severely restricted. More importantly, oxygen replenishment at the metalsurface which is furthest from the liquid surface is restricted or denied.

4) Beneath mud poultices or layers

5) At the corrosion front or pit which will explain the localized pitting nature and speed of

rust through corrosion. This is often termed inside-out corrosion owing to the fact that

it is initiated by the deposition of salts on the inner panels of box members which thencorrode through by the localized pitting to the outer surface.

6) Beneath non-adherent but coherent surface coatings

7) At threaded screw sections. Corrosion here means that very high torques have to be

applied to loosen corroded studs.



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More technical chapters are intended for further studying to bring the students up to a

greater understanding of motor vehicle corrosion in conjunction with the associatedchemical and physical sciences. The remainder of this section poses some questions and

their answers are given in section Chapter 9

Motor vehicle corrosion is the application of the differential aeration cell to the micro-

climate which forms during the life of the vehicle. This micro-climate becomes a free-riderand the subsequent sections will develop a theory which will enables the starter to

understanding and even to predict the area of corrosion. In order to test and concentrate

your mind the following questions are concerned with Fig. 6.3

Fig. 6.3(b) shows the road de-icing salt adsorbing atmospheric water.

Fig. 6.3(c) gives a schematic view of the road de-icing salts concentrating at the base ofthe road aggregate, which helps to fix the aggressive saline environment upon vertical

surfaces. The liquid envelop which is produced by the uptake of water on humid days is

also shown. As an exercise, try to indicate which is the region or area which has most

oxygen replenishment.

Fig. 6.3(d) shows the short circuit which is developed by this form of corrosion. There is noway of breaking this electrical circuit as there was in the case of the torch battery and

circuit.

Fig. 6.3(e) shows diagrammatically the difference between internally initiated pitting

corrosion, causing metal penetration, and scab corrosion (surface corrosion beneath a

painted surface). Indicate which are the areas of oxygen denudation on Fig. 6.3(e).



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Fig. 6.3(f) also shows diagrammatically the effect of a stone chip on the paintwork of a

vehicle. Such paint chipping is relatively harmless. Why? This is because the metalsurface is subjected to an equal amount of oxygen and so the corrosion battery gives zero

potential difference, i.e., it is like a run-down battery lifeless. What will happen if the

corrosion process starts to undercut the paint film as shown in Figs 8.18 and 8.12? There

will be a voltage generated as the area beneath the paint film will be denuded of oxygen.

Fig. 6.4 shows an overlap produced by a bolted or inter-weld section of the motor vehicle.

Crevices can be formed whenever two materials are joined.

Chapter 6 Corrosion mechanism and build-up of the micro-climate

Fig. 6.4 There are two surfaces to a metal inter-weld or overlap which can be formed

either by a bolted arrangement or by resistance spot welding.


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6.4 How can this corrosion be stopped?

The battery in the toy to torch is switched off to stop the flow of electricity and so preventsthe unnecessary run-down of the battery. This breaks the metallic part of the circuit. This

method of switching off the corrosion process is not available in the case of the motor car,

because to use it we would have to cut the car up. In corrosion the flow of electricity by

electrical charges (called electrons) in the metal must be balanced by the flow of charged

entities in electrolyte or liquid environment (the salt droplet). Therefore, if there is no waterenvironment outside the metal, there can be no flow of these charged entities and so there

is no corrosion.

Hence, if one uses the same concept in the case of the motor car, we must

isolate the metal surface from the agent causing the corrosion, i.e., the electrolyte which

contains the road de-icing salts in solution. This isolation means that we shall be lookingfor a physical barrier such as a paint film. However, a painted film is permeable to both

water and oxygen, and so does not function when wet for long periods and breaks down

beneath mud poultices.

There are four options open to stop corrosion.1) Keep the vehicle dry, thus breaking the contact in the liquid environment, and/or prevent

stagnant deoxygenated water from forming a water trap.

2) Have a highly electrically insulating film on the metal to separate the metal from the

environment. This is one of the functions of a paint film

3) Keep the vehicle clean, thus eliminating road de-icing salts which are naturallyhygroscopic and absorb water from the atmosphere. The presence of chlorides can

Chapter 6 Corrosion mechanism and build-up of the micro-climate


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accelerate the breakdown of the protective paint film and destroy the effectiveness of any

corrosion inhibitors and allow corrosion to proceed.4) Ensure equal oxygen supply or replenishment at all surfaces. This cannot be achieved

with the restriction placed on motor vehicle manufacturing methods. One of the functions of

a paint film is to provide a uniform oxygen replenishment over the whole metallic surface.

This area will be discussed again later with more details. Vehicle design and corrosionprotection are inter-related properties and one cannot be achieved without the other.

6.5 Road Spray, Mud Compaction and MomentumIn order to understand how a motor vehicle corrodes, it is necessary to understand

something about the spray generated by a vehicle moving at speed along a wet road. Thetyre effectively recycles the rain and increase any affect due to rain. The origin of the

water spray is given in Fig. 6.5

Chapter 6 Corrosion causing by build-up of the micro-climate

Fig 6.5 Analysis of spray generation


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It is helpful to define the size of a spray droplet by its momentum. The particle size, and

hence momentum in the spray generated by a rotating wheel, is small the faster the speedof the car (see the Table). The size of droplet, often termed a microdroplet, is given in the

literature as varying inversely with the third power of the vehicle speed. Hence a car

travelling at 60 mph creates a very fine spraying of mud and water. For this high velocity

water spray to form a microdroplet, nature dictates, by the surface free energy theory, that

there must be a nucleus upon which the water can form its droplet. The nucleus acting formicro-droplet formation is road dirt, which may be worn road surfaces, tyre dust and de-

icing salts.

The small amount of water present upon each road dirt particle will have been observed on

motorway of free-way driving, where there is often insufficient water for the wiper blades toclear the windscreen.


largerLow speed

smallHigh speed

MomentumRatio of Water/Dirt


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The lower water content of each microplet also means that the associated road dirt is

artificially densely compacted by the phenomenon of surface tension into a high densitymud pocket upon ledges or in areas which are out of the way of direct washes from the

wheel, a steam cleaner or a garden hose.

To summarize: the slower the car travels, the larger will be these water droplets and the

higher the ratio of water vs dirt which they can contain. The faster the car travels, thesmaller will be these spray droplets and the less water they can contain. The wheel spray

from slowly moving vehicles has a high momentum and so travels in straighter lines. The

spray is not affected by pressure eddies beneath the car body.

SummaryRoad dirt throw off the tyre from any point will impact the car body at the vehicle speed.

Road dirt is thrown forward of the vehicle and will collect over lights, air vent pipes to petrol

tank and inlet hose and associate ledges.

The faster the vehicle speed, the less water is associated with the dirt and the more the dirt

is compacted.The faster the vehicle speed, the less is the momentum of the road dirt and the more likely

it is to alter direction in the pressure eddies around car.

The vehicle traveling at speed produces various pressure eddies which result in turbulence

beneath and alongside the vehicle. It is the turbulence which causes the micro-climate to

build-up in box members.


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Practical aspects of Automotive corrosion