Metal finishing Q&As since 1989
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Passivation problem
We have a situation that our customer has complained about. We furnish an assembly made from 17-4 s/s block with a 420 s/s spring pin inserted per customer design. Both are separately passivated. Cracking and rusting are evident from one pin manufacturer but not another. Both sources pass the metallurgical tests, but one fails(the one that cracks) a copper sulphate [on eBay or Amazon]test. Does anyone have any idea as to why pins that are made from the specified material and passivated properly crack and rust while the other pin source does not?
Thanks for any ideas.
Ronald E. Cochrane - Q.A. Mgr- Lyndonville, Vermont, USA
2003
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I would only guess. One supplier is using a "pickling" operation to first remove rust and scale. This can cause Hydrogen embrittlement. Suggest bake after passivation 375 F 4 hour min. Better yet what is the other source doing? Suspect conditioning of any scale and rust to allow easy removal in inhibited acid. Then preferred electropolish and passivation next preferred just passivation. Is the alloy 420 pure or does it have S or SE? Means can be used to selectivly remove these from the surface. A side story to this but rings a bell is that many years ago the aircraft industry noted that springs from one company had a life cycle more than the gemeral industry standard. Upon an investigation they found that one dilligent employee at company X was sandblasting the springs prior to plating as the springs had a surface skin not akin to plating. This blasting operation was causing a compressivly stress layer to the surface and resisted crack propagation. What followed was shotpeening as we know it today. Jon Quirt- Minneapolis, Minnesota Ronald, I'm surprised that the one 420 pin PASSES the copper sulphate test. The martensitic stainless steels are on the edge with respect to having sufficient corrosion resistance for sopper sulphate to be a worthwhile test. Anyway, to get to your question, it's readily believable to me that EITHER the material could be sufficiently different, or the processing (how much cold heading, etc) could be sufficiently different to account for why one supplier would be successful and another not. Or some combination. Remember your sources of variation-  Lee Gearhart metallurgist - E. Aurora, New York |
I agree with both of the above answers. There are MANY reasons why they could be different. 420 is a very borderline "corrosion resistant" steel that can be very tempermental. Each lot can be different even though the analysis looks the same. All of the things in Lee Gearhart's comments can affect it, and the pretreatment before passivation can be critical. There are ways to improve your chance of success dramatically by pretreatment and passivation properly. copper sulphate is not an acceptable test for 420ss, but it you can make it pass the test usually.
Lee Kremer
Stellar Solutions, Inc.
McHenry, Illinois
2003
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