Letter 41071

Special nickel plate requirements

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PROBLEM :
I wish to coat mild steel burglar guards to protect them from coastal atmospheric corrosion. I have a little experience of nickel electroplating and it is a fairly simple process that does not require major equipment investment. As a bright finish is not required and I estimate that I will need a plating thickness of 30 micron I am inclined to use a nickel sulfamate bath. However, I believe that a Watts solution made up with Phosphoric acid in place of Boric acid and topped up with Phosphorous acid will produce a nickel-phosphorus alloy with enhanced corrosion resistance, similar to electroless nickel.
QUESTIONS:
1. Will a 30 micron nickel sulfamate coating offer sufficient corrosion protection for me to offer, say, a 10 year warranty against corrosion? (About 1000 hours Salt Spray)
2. Is it possible to produce a high phosphorous (10% or more)nickel coating using Phosphoric/Phosphorous acid? What would the solution composition be?
3. Are there any special precautions I should take?

Many thanks to you all.

Chas King
Manufacturer - Port Shepstone, KZN, South Africa

1. Single layer nickel, 30 microns, will not likely last 10 years. Depending on surface preparation, you may get six months to a year before rusting begins. For 10 year life you'll need meticulous surface preparation and a layer of low sulfur nickel overplated with higher sulfur nickel.

2. If you substitute phosphoric acid for boric acid, the bath will be so acidic that no nickel will deposit at all.

3. Phosphate in the nickel bath will not result in any phosphorous in the deposit.

And as far as your opinion that "it is a fairly simple process that does not require major equipment investment", be advised that obtaining ten year exposure life will require high technical skill, stringent control and significant capital investment.

Jeffrey Holmes, CEF - Spartanburg, SC, USA

I fully agree with Jeff. It's not easy to get 10 years life in coastal environments. Not even for a fully installed and experienced plater. On the other hand, if you want to use a nickel-phos electro bath you will get highly stressed deposits which mean possible cracks and early failure. In that respect it will be easier and safer to electroless nickel plate it. In any case surface preparation and selected thickness are crucial.

Guillermo Marrufo
Monterrey, NL, Mexico

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