Painted Stainless Steel and Passivation

Q1. I am writing because I believe the input supplied by G.D. in a previous Painting Clinic may be in error.

I do not believe that the nascent protective (chromium) oxide layer on the stainless surface needs continuous regeneration so long as nothing damages the surface—and particularly if it is protected from any electrolyte by a paint.

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There is no reason for the oxide to fail. If there were mechanical damage and the paint coating prevented oxygen from getting to the surface to reform the oxide, there might be some possibility of corrosion. If there were holidays in the paint, I suppose there could be some argument for a differential-oxide cell being established and corrosion from that mechanism. D.H.

Q2. Just as you, I find it interesting that folks want to paint stainless steel. I gather the grades being considered are Types 304 and 321. It seems like folks would likely be better off using 7XXX series anodized aluminum. K.B.

Q3. Yes, it’s true that wire brushing can leave iron on the surface of stainless steel (or aluminum). But that doesn’t mean that you must not wire brush—just use a brush made of compatible material (stainless steel for example). K.G.

A. In a previous Painting Clinic, I asked for opinions on the input from G.D. on the subject of passivated stainless steel. G.D. believes stainless steel must be exposed to air to reinforce the passivated surface. He went on the say that if oxygen flow to the surface is interrupted by sealing it with a coating, degradation of the surface begins. I got some interesting comments as shown in the three responses above. Concerning the aforementioned comments, I can only say, in the words of a wise man: “Where masters disagree, it is wiser to withhold assent.”