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Question on Chromium Plating on Piston Crown

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We carry out chromium plating on the grooves of piston crown (for cargo vessel's diesel engine). We have encountered a re-work problem. After the old layer of Chromium on the grooves is machined by lathe, we then put the piston in the Chromium bath (temperature about 50-52 deg C). When the plating process finishes, small holes are seen along the grooves. We have no other choices but have to repeat the machining and plating again. We don't know what the root causes are. Can anyone give any insights?

Ernest LEE
Piston Crown Reconditioning - Hong Kong
2006

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First of two simultaneous responses --

This letter is nearly identical to one about a year ago. Use the search engine to find it and see what it said. If my bad memory is correct, it might not have been answered.
Pits, voids, skip plate and holes are similar but not the same.
My first guess is that there was burs or carbide grit that formed trees that broke off. The tree robs the plate area below it. With a 10 X glass, look at the pit. If it has a tiny tree stump near the center, that is your problem. Solution is to make sure there are no burrs or trash in the groove. Pumice step might help.
If the pit has a really nice round shape and no tree stump, then you have a gas bubble problem. Increasing agitation may be a solution. I liked conforming anodes as I could use a higher voltage/amperage and plate fast enough that the bubbles were self clearing. A conforming anode for a large piston can be a real trick to make and maintain, but the reward is faster plating, less grind after plate and less rejects.
Where do you need the chrome? bottom only, sides only or bottom and sides? How thick? There is a small possibility that one of the high velocity plasma or flame spray has a unit that could do the job. Normally, they do not work well in tiny spaces, but the improvements in the last few years have been great.

James Watts
- Navarre, Florida
2006

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Second of two simultaneous responses --

Are you sure the pores were not there before? Hot gas corrosion causes this type of problem. Machining with a light cut will not clean them totally but will conceal them since the surrounding metal smears over them thus, making it impossible to detect visually. A dye check after machining will reveal if this is the case.

Guillermo Marrufo
Monterrey, NL, Mexico
2006

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