Letter 8095

Bright Chrome dilema  

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I want to replace a Bright Chrome finish that include cooper as base bright Nickel and B556 Chromium, because the parts are failing the ASTM B 368 CASS test before 60 hr. This is happen due to Faraday Cage or 3/4" rule. Can you recommend a softer metal coverage that will hold better? The hole 1 1/2 diameter throw rule seems to be in place too. The x-ray test is showing very thin plating and the microscopic images shows a lot of cracks on the surface.

Any advice please!?!

George Serb
- Newmarket Ont., Canada

It is true that ions in a plating bath prefer to take the path of least resistance so that the plating thickness is greater at corners than in recesses.

But I don't think the term 'Faraday Cage' is applicable to this phenomenon, and if we use the wrong terminology it may lead us away from the correct solutions. I've never heard of a '3/4 inch rule', and your '1-1/2 diameter throw rule' is probably optimistic when it comes to chrome plating.

I would start with the chrome coverage problem and figure out what you need to do. A Hull Cell test will reveal whether your plating solution is deficient or whether you need an auxiliary anode to get chrome coverage in the recesses. It should not be too hard to gauge progress because you will be able to see from the yellowish color where you are failing to get chrome coverage.

Once you have good chrome coverage, if you still are failing the CASS test, you need to get better nickel thickness distribution. Again a Hull Cell will warn you if the solution is deficient. If it isn't, your experience with auxiliary anodes will guide you toward one way to solve the current distribution problem, and the use of simple polypropylene shields to block the high current density areas is another.

Can you expound on these 'lot of cracks'? I don't know quite what you are talking about there, but if the plating is cracked, it may be further indication of something amiss with the solutions. Don't discard the ideal solution, copper-nickel-chrome plating, just because it has to be done right

 
Ted Mooney, P.E. - finishing.com Inc. - Brick, NJ

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