Discoloration of Metal and Corrosion
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.
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
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
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Update: June 2012