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-----Chromate conversion coating on zinc discolors at 210 degrees
We have been zinc plating an electrical switchboard for a telecomunications company for about 2 years . their specs require that the item is zinc plated and gold pass to 12mic.that seems to be the easy part, when the company recieves the goods back, they then partly powdercoat it masking off any areas that they wish to use for earthing purposes . they bake the switch board to 210 degrees then demask , by this time the zinc passivate has discoloured and is showing signs of oxidizing . AT THIS POINT the customer is saying that the zinc is a problem.Should the zinc pass not discolour? I have tried to find info on temperature limits for zinc passivates and not been successful also I have tried to make my customer aware that he is reducing the protection of the pass layer by heating when he powdercoats. Am I liable for his product if it fails after he heats it? Is it possible that the passivate layer can be protected during powdercoating? Thanks for your time.
Craig Andersonwestern australia
1998
1998
Craig,
By passivate, I assume that you are talking about a gold (yellow) chromate conversion coating on the zinc.
The chromate is really a very very thin gel. Heating above 140F degrades the chromate's corrosion resistance because it dehydrates it too much.
For powder coat, 210 degrees, I assume that you are talking centigrade. At that temp, you will definitely discolor a chromate coating.
Are you liable? Only if it is in the contract. Of course, you can be "right" and lose a customer.
Suggestions. Try an olive drab or possibly a very dark green. The OD is a better coating because it is thicker. Because it is so ugly, any slight color change will probably not be noticed. I think that there is a black chromate available, but I have never used it, so can not comment on what it will do at an elevated temp.
PS, the customer MIGHT have to go to star washers to get a satisfactory electrical connection thru an OD coating.
James Watts- Navarre, Florida
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