Stainless Passivation impaired by plating?

[Paskenell]

Thank you for allowing me to ask this question. On a commerical 18-8 stainless steel fastener, does the addition of oxide, xylan, or zinc plating have any long term affects on the stainless' ability to self passivate? I am working on the assumption the part received proper passivation after manufacturing and before plating.

I have heard 2 theories. #1 Since the part was passivated and then was plated, any damage to the plating would expose the stainless, thus allowing it to form a new self protective layer. #2 The plating starves the stainless' ability to self passivate and thus may increase the rate of corrosion.

I apologize if the question is too general. I see more and more factories asking for plated stainless and a little voice in the back of my head says it may have long term problems.

 

[mcguire]

This question needs to be posed broken down to be answered clearly.
Case 1. metallic coatings:
The unexposed stainless may not be passive, but when it is exposed by any means, the exposed surface will behave as part of a galvanic couple. If the plated metal is anodic, then it corrodes rather than the stainless. If it's more noble, the stainless passivates anodically and no corrosion occurs.
( one wonders what the point of the coating is, anti-galling?)
Case 2. non-metallic coatings:
A breach in the coating exposes the surface and it behaves as it would were there no coating. Whether it was previously passivated is irrelevant. It must be able to form and maintain a passive film in the environment it finds itself.
If the breach forms a crevice, it would accelerate attack on the stainless. A partial coating is worse than no coating in this case.
If the coating provides any soluble ions, they must be taken into account. E.g. chlorides released from a caoting would be bad.
Of the two, the first theory is closer to being right, but it over-simplifies and could lead to errors if applied to all cases.
If you respond to this, tell me what the purpose of the coating of stainless is.

 

[Paskenell]

Thank you for responding. In 90 - 95% of the applications the plating is added for decorative purposes, to color match the fastener to the material. A common application would be the black console on a boat. By adding black xylan to the stainless screw, the fastener now blends in with the console.

Some applications are added to lessen the chances of galling, but primarily on larger sizes of bolts. One customer in particular has found galling to be such a problem that they are having their larger 18-8 fasteners (3/8 & 1/2 diameters) cad plated.

 

[mcguire]

There are several experts, TVP and CoryPad, who know a lot about anti-galling. Pose the issue for them to answer.