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Ted Mooney, P.E. RET
Pine Beach, NJ

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  for Metal Finishing since 1989
  

Passivation Theory

I am interested in the theory behind passivation. Is the mechanism known? I know that an chromium oxide is prevalent in the passive layer, but in what percentage? Is there any documents on the theory behind passivation?

Will:

There are a number of papers written on the subject of passivation, and several standards that try to describe it. However, there is not agreement among the metallurgists or chemists as to exactly what is going on in the process. The key factor, in the end, is whether the metal is corrosion resistant after treatment. There are many factors that cause the disagreements, but most everyone agrees with three factors: In order for passivation to occur, you must have a clean surface, you must remove free iron from the surface, and you want to get chromium enrichment on the surface. To measure this, many use ESCA and AES testing to measure the physical content of the surface layer and the depth of the chromium oxide layer. This gives some measure of the success of the passivation, but the test is tricky and expensive to run.

We have quite a bit of data on chromium content of the passive layer. I would be happy to talk to you or anyone further about this, but the subject is too long for this forum. See ASTM A967.

Regards,

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