Cracks & Pitting in Chrome
Q. I have just chrome plated an a320 slider. The base material was high tensile steel with two nickel patches. I cleaned and loaded the component live into the vat myself. After a 22 hour run and a deposit of o.o20'', a small area about six inches long came out with a dozen small nodules on the surface. After heat treatment craking was then very clearly seen in the chrome around that area only.Where the nickel patches were the chrome was fine.Any help would be of much help.
A. Without seeing it for myself, my best guess is, If the base metal in the discrepant area has no other known rework history and was not visually detected before chrome you may be describing a base metal burn. This can be caused by aggressive polishing or belting to remove corrosion and defects. It could also be cause by a pre-grinding operation, i.e., poor cooling, wrong wheel type, worn or improperly dressed grinding wheel, wrong speeds and feeds, etc.
Some questions to ponder:
1.) Was the part chemically stripped or did they grind the old chrome off prior to repairs?
2.) Hopefully the chrome was chemically stripped in Sodium Hydroxide. If so, was the part re-shot peened before it came to you for nickel and re-chrome?
3.) was the part cylindrically ground between centers at anytime "after" the initial chrome was removed?
4.) hopefully the part was properly magnetic particle inspected (NDI) prior to rework which could have caught a burn.
5.) did you "pumice scrub" and thoroughly rinse the entire OD surface prior to activating for chromium plating?
These are all the standard questions to ask your self. If the answers make sense and all the proper procedures were followed, then you have to dig deeper. You said you went in "live". I assume you mean with a current cable attached to the cathode while immersing the slider into the chrome path??
We typically reverse etched in sulfuric/HF which properly activated both the steel and the nickel surfaces. We wouldn't go in live, but rather immerse the part into the chromic acid plating tank, down into the conforming anode. Then connect the cathode as quickly as possible and cathodize for 1-2 minutes. Cathodize means to start the rectifier (power supply) at 2 volts (no significant amps) and hold at that 2 volts for a period of time. Millions of small hydrogen bubbles are generated at the cathode and as they are released they scrub the surface of the part as they escape, further cleaning the part surface. After about 2 minutes, the operator will step the voltage up in 2/10th increments every 15 seconds until full voltage/amperage is reached.
Were you using conforming anodes? Or individual "stick" style anodes? If stick anodes were used, check to see if any one was too close to your part, maybe had gotten bent and was not hanging straight and spaced equally.
I could speculate even more if you care to provide some additional information including, a digital photo of the defect area. Let me know if I can help you further.
Best of luck.