by Austin Jack Smith
[See also: Aircraft Corrosion Control by Crane, Dale]
In order for corrosion to get a start , it must penetrate any corrosion shield that has been used to protect the vehicle from the various types of corrosive forces. Corrosion can be classified into 9 categories:
This occurs when two atoms of Iron or Steel(Fe) jointhree atoms of oxygen producing iron oxide or rust. 2Fe+3O -> Fe2O3 Essentially the same formula holds true for Aluminum and Magnesium as well except it occurs at a much slower rate in Aluminum.
2. Uniform Surface Corrosion
When an area of unprotected metal is exposed to atmospheric contaminates there will a uniform attack over the entire surface. This dulling or miniature pitting of the metal is caused by microscopic amounts of the metal being converted into corrosion salts. Which will accelerate the corrosion until the metal is destroyed.
Pitting is a continuation of surface corrosion. Pitting may be detected by the appearance of a white powder on the surface of the metal.
4. Intergranular Corrosion
This occurs mainly in non-ferris metal or various alloys. Microscopic examination of most alloys reveal that they are made up extremely small grains of the various components of the alloy held together by chemical bonds. When this type of metal is heated and cooled improperly these grains grow until the grains will reach such a size that areas of dissimilar metal will form efficient cathodes and anodes for corrosion formation. Your vehicle could look like a dry cell battery that has been left in a flashlight way to long.
5. Exfoliation Corrosion
This is a type of Intergranular corrosion that occurs in extruded materials such as angles and channels where the grain structure is more laminated. This type of corrosion along grain boundaries and causes the metal to separate or delaminate with the resulting structure weakness.
6. Galvanic Corrosion
This is one of the most common type of corrosion and will occur when these two conditions are present.
A. Two dissimilar metals are connected in such a way as to provide a path for electron flow.
B. their common surface are covered with some material that will serve as electrolyte. In other words conduct electricity.
This means that a steel fitting on a alloy crankcase or transmission can form a very primitive battery. The less noble metal in this case the alloy will form corrosion products and will be eaten away, like the outer zinc case of a dry cell battery. It must be noted that both of the conditions above must be present. Thosed of you that use outboard engines in salt water have or should have a small blocj of "sacrifice metal" on your engine part that is in water, the sacrafice piece is designed to be eaten away before your engine replace it when it start to have lost one half of its size.
7. Concentration Cell Corrosion
This occurs when water covers the surface of a metal some seeps in between joints, spot welds, and seams. This condition can cause Concentrated Cell Corrosion. Water in open air readily absorbs oxygen (O) from the air, it then attracts electrons from the metal to form negative hydroxide ions. 2H2O & O2 & 4 electrons-> 4(OH) This is a type of corrosion that you would find underneath sealant in the various joints and seams and also in closed areas like the hollow side beams.
8. Stress Corrosion
This is another type of Intergranular corrosion found in the present of stress from poor quenching in heavily welded areas such as the transmission engine horn areas, axles, steering gear, etc. . Tensile stress occurs in the presence of a corrosion environment. The bad part of this type of corrosion causes cracks and grow very rapidly as the corrosive attacks the end of cracks rather than the sides. In a high stress area shot peening is used to provide uniformly compressive stressed surface. Careful visual inspections may reveal this type of corrosion, but to be able to find the actual extent of the cracks requires dye penetrant inspection.
9. Fretting Corrosion
This occurs when two surfaces fit tightly together but can move relative to one another. This type of corrosion acts very rapidly as any protective coating is rapidly removed. This is another reason to properly torque all fasteners.
Corrosion occurs when the metal is unprotected by proper coatings. All good protection methods are used as a “System” that is a series of treatments that end up by having several compatible coatings on the metal to be protected. All of the major paint companies that sell to industry have such systems. I am particularly impressed with the POR-15 system as it includes the basic treatments ie:
One last point reference to painting the engine and transmission. True it used to be thought by many well meaning people that painting an engine could cause heat build up, but I have checked with many aircraft engine overhaul companies and engine manufactures, they all report that they paint their engines and the FAA requires it. I have corresponded with a friend in Europe who reports that even VW paint their overhauled engines to indicate the engine built up for leaded and unleaded gas. I have written Bob Hoover (Sermons) and he paints all of his engines. I have included his references above. A flat black paint is nearly transparent to heat transmission. Aircraft engines are subject to the same heat dissipation problem as our engines and it would seem logical to follow their recommendations.
I hope this may help you with your vehicle protection, especially those
of you who live where they use salt on the road. If you are unfortunate
to live in that environment you should wash your vehicle every time that
it is exposed to salt, a good hose down especially in the undercarriage
area. When I was stationed at Omaha, I rigged a 7 ft. pipe with drilled
hole pointed up attached by hose to the hot water tank drain. I could
drive slowly over before I put my car into the garage. I had a Mark VII
Jag and a 1960 Bus at the time and they had a very poor rust prevention
coating. I used about 15 gal. of hot water and it drove my wife crazy,
but I didn't have a corrosion problem.