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Passivity. In some cases the surface of a metal is naturally
covered by a film of reaction products. Stainless steels and titanium alloys are
resistant to corrosion in many environments due to the presence of such passive
films. In the case of stainless steel and titanium, these films are oxides and are
naturally formed during the manufacture of the materials. This film causes a high
level of polarization and can result in excellent resistance to corrosion. In some
environments, however, these passive films can break down and result in localized
attack. |
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Pitting is the attack of a metal
where the corrosion rate is substantially higher at some exposed areas than at others.
When the maximum penetration due to corrosion is less than two times the average
rate the corrosion is considered to be uniform, when it is greater than two times
the average rate, the attack is considered to be pitting. |
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Outokompu Handbook for the pickling and cleaning of stainless steel |
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In electrochemical reactions, chemical activity at or
near the metal surface can have a significant effect on the reaction rates.
These reactions often limit the flow of current in the reactions and result in polarization,
or resistance to flow of current across the metal-electrolyte interface. |
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