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

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Passive film formation and passivation

dear all;
My question is about passive film formation on stainless steel. If we scratch the surface, in how much time will passive film re-form and what about its thickness?
What if we don't use passivation, rather we do fine grinding and proper cleaning afterwards. Passive film will form automatically and if surface was clean, there should not be any difference in its ability to resist corrosion?

Passivation with citric acid or nitric, will mainly clean the surface and expose more Cr to be oxidized, but if we manage the cleanliness by alkaline cleaning and water jetting and make sure about that, what is your opinion?
Thanks

It depends a lot on what grade of stainless you're talking about. Alloys high in Cr are better able to self passivate without a chemical treatment. When you scratch, grind, or otherwise damage a stainless passive layer, you are exposing iron to the environment, and it is this iron that will be the source of any rust that may develop. Chemical passivation with citric or nitric acid will remove the iron, leaving a Cr-enriched surface that forms a better Cr oxide layer. Mere alkaline or water jet cleaning will remove grease, oil, and dirt, yes, but not specifically the surface iron. Ray Kremer Stellar Solutions, Inc. McHenry, Illinois March 28, 2008 March 29, 2008 Ihsan As you are aware, stainless steel will form a passive surface on its own. You can accelerate this by heating the metal. However, neither addresses the removal of free iron from the surface. If it was as simple as cleaning the surface, anyone could say they do passivation. Willie Alexander - Colorado Springs, Colorado

Ihsan,

It but natural that there is going to a passive fil formation by the Cr in the steel, but it also tru that you will be exposing Fe to the atmosphere in which the free energy of FeO is higher that any of carbides of cr with fe so there will be corrosion and that is why we have to do the passivation treatment to mask the surface with Cr oxides, through passivation solution and clean the surface also.
This is just an opinion.

This is a recurring subject. Yes, SS naturally forms the passive layer by simply exposing it to air at room conditions. But passivation process besides cleaning out iron impurities from the manufacturin ops. also depletes the iron from the alloy to several angstrom deep down the surface. This theoretically creates a better an resilent passive layer as the surface is enriched with chromium. wish you good luck.

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