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Problems in zinc plating spotwelded chassis

1995

I have a problem with rust appearing near spotwelded joints of a zinc plated chassis. This five-sided chassis is 14" wide x 8" high x 10" deep where the top, rear, and bottom is one piece of cold rolled steel 0.060" thk. The left and right side panels are spotwelded on.   My problem is two-fold and maybe related:
1) zinc plated thickness on the interior of the shelf is critically low, and
2) rust appeared from the joint less than one month after plating.  

Does anyone out there have any experience with such a problem, or can anyone zinc plate these chassis for me?      

Manny Stathopoulos
- California

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The plating of chassis requires good throwing power which usually dictates using alkaline or cyanide zinc. If you are trying to use acid zinc, investigate changing over to alkaline zinc.

Even with alkaline zinc, the deposit will be thin in the corners, and many chassis platers find that they need to use auxiliary anodes to produce successful work. Are you already using them?

The bleedout problem can be addressed via an anaerobic sealant used in the area of the spot welds. although I don't have a copy, I believe that there is an IBM spec which describes what this sealant is, and how to apply it.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
1995

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I have seen a shop where just as good results were obtained from acid zinc as from an adjacent cyanide line, as regards thickness. I know they run their chloride quite a bit higher than the spec, also, the owner is a fanatic about cleanliness and contamination control. They love their acid zinc line, in fact, talk about replacing their cyanide line with another.  

Dave Wichern
Consultant - The Bronx, New York
1995

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Hello David.

Acid zinc is a fine product and I've helped several shops convert from cyanide zinc to acid zinc. I believe that surveys show that more acid zinc is in use than cyanide and alkaline put together. I'd never knock it

But it is simply not true that you can get as equal thickness distribution from acid zinc as you can from cyanide. The function of the cyanide is to tie up the zinc, to limit its ionization, so that the concentration of zinc ion in solution is extremely low. By so doing, you limit the deposition rate (it becomes limited by the re-ionization rate of additional zinc). This makes the zinc deposition efficient in the LCD areas and relatively inefficient in the HCD areas--thus helping toward building a more even plating thickness.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
1995

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Of course, you are right. But the fellow I mentioned didn't mind if more than the spec thickness was applied to the outside, or corners, as long as there was no burning. So, who knows, maybe we were putting 4 or 5 tenths on the outside. But at 65 cents a pound, a little extra zinc isn't much of an expense. And the customer *loved* the way the parts looked - you know how people like em shiny.

Dave Wichern
Consultant - The Bronx, New York
1995

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1995

Manny: We do work like you describe every day … Ken

Ted: There are also a number of tricks that can be done by the plater to minimize bleedout.

David: If you want coverage on the inside of the box and '4 or 5 tenths on the outside', you had better stick with cyanide zinc and auxiliary anodes. With acid zinc, you're probably looking at .001" - .003" on the outside to get enough zinc on the inside corners to provide corrosion protection.

Ken Rosenblum
finishing shop - Minneapolis, Minnesota

Thanks for the reply.   We have not yet tried to use auxiliary anodes but we are investigating that avenue presently; we have also found a plater using a cyanide solution as opposed to our first prototypes which were acid. We are also investigating the possibility of nickel plating.   I have found your service very helpful and I hope others profit from it also.      

Manny Stathopoulos
- California
1995