CORROSION CONTROL/INSPECTION continued
1.49.5. General Information
Corrosion is the deterioration of a substance, usually a metal, because of a reaction with its environment. Corrosion
reactions are said to be electrochemical because it involves the transfer of electrons. Four conditions must exist for
corrosion to occur:
a. A metal known as the anode must be present, which is the corroding metal.
b. A dissimilar conductive material, the cathode which has less tendency to corrode.
c. A conductive liquid, the electrolyte, must be in contact with the anode and cathode to carry the electric current.
d. Electrical contact between the anode and cathode (dissimilar metals) must exist.
The elimination of any one of the above four conditions that create corrosion, will stop corrosive action.
1.49.6. Development of Corrosion
All corrosive attacks begin on the surface of metals, and if allowed to progress can penetrate into the metal. When
corrosion products form, they often precipitate onto the corroding surface as a powdery deposit. This film of
corrosion products may reduce the rate of corrosion, if the film acts like a barrier to electrolytes. Metals such as
stainless steel and titanium may produce corrosion products, that if bound tight enough to the corroding metal, forms
a passive film preventing further corrosion. When the corrosion product film is loose and porous, such as aluminum
and magnesium, the electrolyte will penetrate and continue the corrosion process.
1.49.7. Types of Corrosion
a. Uniform surface corrosion
Uniform surface corrosion results from a direct chemical attack on a metal surface and involves only the metal
surface. On a polished surface, this type of corrosion is first seen as a general dulling or etching of the surface and
if the attack is allowed to continue, the surface becomes rough and possibly frosted in appearance. This type of
corrosion appears uniform because the anodes and cathodes are very small and constantly shift from one area of
the surface to another. An example is the etching of metals by acids. The discoloration or general dulling of metal
created by exposure to elevated temperatures is not considered to be uniform surface corrosion.
The most common corrosion of aluminum and magnesium alloys is pitting. It is first noticeable as a white or gray
powdery deposit, similar to dust, which blotches the surface. When the deposit is cleaned away, tiny pits or holes can
be seen in the surface. Pitting corrosion may also occur in other metal types or alloys. The combination of small
active anodes to large passive cathodes causes severe pitting.
Galvanic corrosion occurs when different metals are in contact with each other and an electrolyte, such as salt water,
is present. It is usually recognizable by the presence of a buildup of corrosion at the joint between the metals. Two
dissimilar metals joined together form a galvanic couple if moisture and contamination are present.
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