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Cracking problem in chrome plating on berylium copper

July 2, 2008

Our company is experiencing cracking in chrome plating. The cracking could be described as macro-cracking vs. microcracking. The chrome is being deposited out of a proprietary self- regulating mixed catalyst solution. Analysis of solution looks good, we tried adjusting ratio closer to 240:1 to reduce cracking with only marginal improvement. There is no trivalent or chloride present. Previously we were low on fluoride @ 1.5 g/l and added fluoride catalyst and also noted some improvement but did not completely eliminate cracking. Fluoride from recent analysis is good @ 2 g/l. We do a 2 hour after plate bake @ 500 F.

The chrome is being deposited on berylium copper. Thickness is ~.006/.008". Part is ground to size after plate by customer. Sometimes at grind operation when cracking is present, chrome is breaking off causing part to be rejected. There is no evidence of an adhesion failure from cleaning/activation cycle, rather the chrome breaks off where cracked. Customer appears to use acceptable grinding practices, i.e. wheel selection, lubrication and amount of chrome removed per grinding pass.

I would appreciate any input into solving this problem, thanks.

Peter Cox
Plating facility technical Director - No. Andover, Massachusetts

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July 3, 2008

As soon as you say "proprietary", there is no one that should be more knowledgeable about the problem than the tech services for that product.
1. Can you see cracks after plate?
visually
with a 3 power glass
with a 10 power loop?
2. Why are you baking chrome over BeCu?I am not familiar with that operation, but it does not have a Hydrogen embrittlement problem.
3. If the part is fine when it leaves your shop, then you can bet a Starbucks that the grinding operation has a problem. Since you offer zero information other than it seems OK, who can comment. Also, who says that the operators are following their instructions.
4. I have only seen macro cracking on as plated chrome a couple of times and it was traced back to the plating operator not following his instructions.

James Watts
- Navarre, Florida

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Hi James,

Thanks for the response. Yes I said proprietary and we are wroking with chemical supplier but I was also hoping to get input from the finishing community.You can not see the cracks after plate @ 3x or 10 x but the cracks do then appear after bake. The reason we are baking is it is a purchase order requirement. I agree it is not a high strength steel and hydrogen embrittlement should not be a concern, but on the other hand I am not a Metallurgist. I can confirm the Operators are following instructions because I have witnessed the parts being processed.

Peter Cox
- No. Andover , Massachusetts
July 7, 2008

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First of two simultaneous responses -- July 7, 2008

OK, no cracks before bake, and cracks after bake, so the problem has to be in the bake process. If I remember correctly, chrome does not change structure until you get to 600F. So my guess is that it has to do with the coefficient of thermal expansion. 6 x 10^6 for chrome and 17 x 10^6 for copper. This means that the substrate is expanding at nearly three times as much as the chrome is. Therefore, if there is any way around this, it will require a slower warm up and a slower cool down.It is more ductile as it is warmer if the rate of change is not too great. I would certainly try it to see if it makes a difference. Thermocouple the core of a part and the surface of a part to monitor the respective temperatures.
For comparison, iron is nearly identical to chrome and SS is about 13, so they do not have the problem that you do.
Standard post plate bake is 375F. I would certainly request a change from 500 to 375 with an extended bake time to compensate. 3 - 4 hours being normal.

James Watts
- Navarre, Florida

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Second of two simultaneous responses --

Please consider that chrome has around one third the coefficient of thermal expansion of copper (4 microinches/inch/degF versus 11 for copper) and that it also has almost zero ductility (less than 1%)and a very high internal stress. To aggravate things, it tends to shrink when first heated instead of dilate. So, your thermal cycle may be stressing it beyond its limit. Try adjusting the temperature curve and see what happens.

Guillermo Marrufo
Monterrey, NL, Mexico
July 7, 2008