the chemical combination of a substance with oxygen. All petroleum products are subject to oxidation, with resultant degradation of their composition and performance. The organic acids formed by oxidation are corrosive to metals. The process is accelerated by heat, light, metal catalysts and the presence of water, acids, or solid contaminants. The basic oxidation process for hydrocarbons is summarized as:

Organic peroxides form as the first reaction products
Peroxides catalyze continued formation of alcohols, aldehydes, ketones, and organic acids
Alcohols, etc. further oxidize to form high molecular-weight, oil insoluble polymers;
Polymers settle out as sludges, varnishes and gums.

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