Metal finishing Q&As since 1989

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Pinholes on an ENP 50 microns layer

2004

I'm a technician of an italian metallurgical laboratory. I'm requested to evaluate the quality of an ENP (thickness 50-60 microns). The plating has been applied with a traditional process over a spherical steel surface and complies with all the specified requirements (adhesion, hardness, etc.), with the exception of the look of the Ni-P layer, which appears almost totally affected by very little pinholes (like "needle-tips"). Your opinion would be greatly appreciated about the source of such a defect (hydrogen evolution?) and the impact of it on the performance of the protective layer. Could the defect be really detrimental for the behaviour in service (contact with water) or it can be considered only an aesthetic problem, being the pinholes not penetrating the coating to the base metal? Could the ferroxyl test be significant? Is there a non-destructive system (apart from the observation with a magnifier) for evaluating the extent of the defect (for examples, replicas)! ?

Finally, if it is a sign of an incorrect process condition, which are possible reliable interventions? Thanks a lot for your attention.

Best regards.

Fabrizio Fantechi
metallurgical testing and control lab - Carpi, Modena, Italy

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2004

If the pits are very small and universally over the surface of the part, they are probably the result of an organic contaminant (like a solvent) in the plating bath. If the pits do not extend through the coating to the substrate, a ferroxyl test will not reveal them. The pits may or may not effect the performance of the part. If the EN coating is not attached by the "water", then the pits probably will not get any deeper. However, if the "water" does corrode ENP, then the life of the coating will be much shorter.

The substrate has been ground or lapped, an alternative cause of the pitting might be embedded media on the surface which is initiating small gas pits from hydrogen evolution.

Ron Duncan [deceased]
- LaVergne, Tennessee
It is our sad duty to note Ron's passing on Dec. 15, 2006. A brief obituary opens Episode 13 of our Podcast.