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Electroless nickel acid resistance

I have an electroless nickel plated shaft. It is part of an assembly. The remaining assembly is steel and flash rusts during strorage. Prior to painting we use a phosphoric acid to remove the rust. This is applied to the steel portion of the assembly. It is possble, of course, that the operator inadvertantly splash some Phosphoric acid on the EN. We are at about 20% acid. Would this damage the plate? The dwell time is only 5-10 mins.

Kelly Draper
- West Plains, Missouri, USA

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It sounds like you are applying a phosphate coating prior to painting. If not, it might be a good idea for paint adhesion. It should be fine, but you could try a sample, and inspect the EN after painting. How thick is the EN coating, does it get masked before painting?

  
Tom Pullizzi
Falls Township, Pennsylvania

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Tom,

We are not trying to phosphate, just trying to remove rust. There are no accelerators or other additives in the phos. (We do use a organic vynil wash primer pretreatment prior to paint.) The EN coating is 2 mil thick, and yes it does get masked before coating. A visual inspection does not indicate any difference before to after. I have also had an SEM-EDS performed on the exposed side and the interface side and there is a higher phos concentration on the interface side, so we are not getting any diffusion into the surface during the process. (I believe...)

Thanks for your input!

Kelly Draper
- West Plains, Missouri, USA

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Kelly:

High phos electroless nickel (10% min P) dissolves at a rate of approx. 0.1 mil per year in phosphoric acid at 85% at 20 °C. Lower phos nickels are not as resistant. I hope this will help you find yourself the answer?

Guillermo Marrufo
Monterrey, NL, Mexico