The Big End - Understanding The Dreaded Metal Worm
Corrosion is something most car owners witness, and the effects it has on a vehicle. In the case of classic cars, expensive repairs are all too often necessary.A lot of unhelpful information on this subject is very often bandied about and the market is plagued by false claims of instant remedies.
Really, what is corrosion. In definition it can be described broadly as the result of environmental chemical action on metals which frequently lead to deterioration and eventual destruction. Many metals are inherently unstable and tend to revert to a more stable state of which the metallic ores found in nature are typical examples. It is a proportional deterioration of the bare metal exposed to contamination and to the atmosphere. Fog, rain and high humidity may initiate a progressive rate in corrosion, whilst an atmosphere containing more severe corrodants such as salt water or toxic fumes etc will accelerate the process. Anyone living by the sea can confirm that vehicles rust well there and of course in our winter environment much salting of our roads occurs.
Living near to factories and industrial areas can also add to the toll.
A corrosive attack may take place over a large area frequently sprayed by water and the additional accelerative products. Other factors come into play such as electrolysis, which is in effect similar to that of a battery cell which is anodic and cathodic in operation.
Just to demonstrate this, you will have seen spots of red rust appearing on your chrome bumper well this is a good example of Galvanic corrosion the attempt by the lesser of the metals which is the steel to plate onto the chrome by the electrometallic action.
Other interesting types of corrosion can be readily found on any motor vehicle and are categorized as follows.
- Ferrous metals (Iron and steel)
- Aluminium and Magnesium alloys
- Copper alloys
Surface corrosion: The best known type readily found on bodywork and in the areas most prone to water spray and contamination of dirts and readily open to oxygen because of unpainted or unprotected surfaces. Identified by etching of the metal surface.
Dissimilar metal corrosion: (sometimes called electrolytic or galvanic action). This is one of the most common forms of corrosion it is caused by the electrolytic action between two dissimilar metals in contact in the presence of a conducting liquid dissimilar metals have different electro potential and when in the presence of moisture the difference in the potential anode cathode will produce a small electrical cell and one metal is attacked at the expense of the other. Can be readily identified by visual inspection.
Pitting corrosion: localized attacks to surface anywhere and is due to the breakdown of the protective coating such as paint, plating or underseal, and is readily identified by small spots of corrosion.
Crevice corrosion: areas containing small entrapments which act like a cup and hold liquids, which if left will attract a corrosive condition by anodic and cathodic action and can be identified by areas left in a contaminated state and by localized attacks in prone areas.
Intergranular corrosion: small flaws concentrated in the metals grain boundaries and can produce a network of corrosion or cracking of the metal surface. Sometimes hard to identify as no visible sign is shown on external examination by the naked eye.
Stress corrosion: A combination of static stresses and a corrosive environment may produce a form of metal failure known as stress corrosion cracking. The stress may be wholly internal and may be the direct fault of external stress loading. It can sometimes only be found when the part fractures or fails in operation.
Fatigue corrosion: It is a form of stress corrosion induced by cyclic stressing mainly by external applied loads. Can be identified by cracks appearing from shallow pits and extending much the same as fatigue cracks
In identifying the type metal that is corroding, as a rule
- With iron and steel most generally a red coating or flaking surface will be seen.
- In the case of alloys most generally a white or grey powder deposits and sometimes by black patching will indicate this type of corrosion.
- With copper it is usual to see a blue green salt deposit.
Treatment of corrosion is a specialized area and in my next article I will be giving tips and suggestions on the ways to tackle this obnoxious problem
Next Month: How to tackle and prevent corrosion.
© Chris Street, May 1999