Hard Chrome on 17-4 pH steel
Q. We are hard chrome plating the bore of a part made of 17-4 pH steel. The Bore is 3" in diameterand has a very small (1/32") chamfer. We activate the part with anodic sulfuric/HF acid and going into a standard 40 oz/gal hard chrome solution with 1 1/2 volts. The deposit looks great.
We can disc sand it with 120 grit with a hand grinder with no problems. When my customer grinds it with a grinding wheel the chrome on the chamfer chips off. Do you know of a more robust activation procedure?
A. Let me ask you, do you use a robber, thieve or other device near the chamfer?? How about the masking (if any) might it be robbing the current at the chamfer? Does your anode conform to the shape of the ID? Does it stick out past the end of the part or is it shorter? Is the non conformance occur at both ends of the ID? Describe the part overall shape and size.
17-4 contains a relatively high ferrous alloy content and we usually got away with treating it like high strength steel. The HF etch is a plus if the chemistry is fresh and bath is clean (no oil, sludge or floaters). The downside is that HF tends to wick into tiny cracks and openings and will leach out slowly over time. Make sure HF is thoroughly and vigorously rinsed. HF will drag out very slowly and can tend to wick out from behindmasking, shields or tooling on the chamfer area while transferring the part into the CR bath. Even after immersion into Cr, HF isn't as soluble as chromic acid and may temporarily inhibit the reaction of the chromic acid and the base metal.
Regarding your procedure, the 1.5 volts seems low to affect an adequate reverse etch, especially in an ID?? (ID's react differently than outside surfaces because of the shape, surface area and ratio of the anode to cathode, usually opposite of an OD) You didn't mention the length of time of reverse but I'm betting you're probably etching for 70 seconds or something like that if following a spec? If less? An increase in duration may help.
It's worth noting that 17-4 is technically a "corrosion resistant alloy" and it might be necessary to not only warm the parts in the CR bath, but to step the plating voltage up from zero, .2 of a volt per 10 seconds until you reach the desired plating current. I've gotten away with not stepping voltage on 17-4 when using conforming anodes.
PS: Ron, I design and build hard chrome tooling for a living so I am pretty sure I can help you with this if you decide to get more in depth with the discussion. Let me know if I can help you further.