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Name:  Joe
Status: other
Grade: other
Country: N/A
Date: 9/25/2005

Can discoloration of a metal be considered the first step in the corrosion process?

Discoloration usually indicates the formation of an oxide (or sometimes a sulfide) layer on the surface. So yes, discoloration is indicative but not proof that a reaction has occurred, i.e. corrosion.

Vince Calder

Sure, Joe.

It is definitely an oxidation film, but one that is thin and fairly adherent, not eating up too much of the metal or flaking off fast to make way for more such oxidation. In later steps, the oxidation film gets thick, differently organized, and crumbly. Then it is definitely the degradation we want to call "corrosion". But the crusty corrosion is only different than the discoloration in small details like crystal structure, pH, water, or salty-impurity content..

For some metals, in many situations, the thin discoloration film is considered inevitable or uneconomical to prevent. Then people settle for keeping that film stable, trying to prevent evolution to the next stage. In these situations they call the next stage "corrosion", and the first stage "passivation". Both are oxidation. If one could manage to be completely pristine, one would prevent both.

There are also passivation films which have no color. They are transparent but so thin one cannot see any glossiness or surface-crystallinity from them. Aluminum in air always has this kind of film, but it is not considered "corrosion" and it is highly inevitable.

It is all in the connotation and situation, but your insight can be very useful for maintaining certain metals. Brass might be a good example. Stainless even better. It should have only a transparent film. If stainless gets discolored, oxidation is probably going farther than it should be allowed to, and faster too. The slightly thicker discolored film is usually less resistant to late-stage corrosion than the thin invisible film. Discoloration of stainless is often a sign that corrosive impurities such as the chloride in salt are present, accelerating oxidation of your metal. Without such salts, stainless' passivation film usually remains clear for many years.

Jim Swenson

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