Pitting problem in hard chrome plating

A. Hi Alberto. I confess to having no idea what I'm looking at in that photo :-(

But when you have an infrequent, random, problem in hard chrome plating the first thing to eliminate from the discussion is whether any specialty step may have been selectively performed. After the parts have been sandblasted, I assume there would be some sort of inspection? Does that inspection ever call for a manual intervention like burr removal, sanding, or polishing?

Luck & Regards,

A. If surfactants are used in the chrome bath this can aggravate the problem. Suggest you contact Enthone OMI whose personnel have written a number of papers on this subject.

A. It is difficult to comment when we have no clue as to the chrome plating process that the vendor is using.

From the size of the pit, I will make a guess that if you look in the very bottom of the pit, you will find a "tree stump". You need something to hold a gas bubble of that size. A stray piece of blasting media, imbedded in the base metal will tree, hold a gas bubble and then break off leaving a stump and a large gas pit. This pit can/will plate over so you do not see it until it is ground.

I used to preach multiple water-break checks, which will spot most, but not all of the problem children. Pumice [affil links] , really fine, or whiting may help reduce the problem.

A. Hi Tony. It doesn't necessarily sound like the most likely cause, but according to Guffie's "Hard Chromium Plating" [on Amazon affil link] , it's certainly a possibility.

Regards,

A. Hi Mislan. Those defects are occurring because you're not doing it right :-)

I'd suspect inadequate cleaning; cleaning is most difficult to do at the grooves. Chrome plating is unfortunately a difficult process though where a hundred different things must be exactly right before you will get satisfactory plating. The subject is broad enough that I have 3 books just on chrome plating (Dubpernells's Electrodeposition of Chromium from Chromic Acid Solutions [affil link on Amazon], Weiner & Walmsley's Chromium Plating [affil link on Amazon] , and the previously mentioned Guffie book).

So, unfortunately, it is really difficult to help from this distance with a fairly vague problem and situation. If you give us data on your operation, though, people can probably steer you in a likely direction.

But one hard chrome plating expert often addressed trouble-shooting forums on chrome plating by saying this: "Step 1: go to every toolbox, locker, and junk drawer in the shop and throw out every scrap of old emery cloth and sandpaper before proceeding to step 2. Offer a bounty of a quarter for each piece so you don't miss any". The reason is that when people don't have the fine grade sandpaper they need (perhaps to remove a tiny burr at a ring groove), they'll use a worn piece of a courser grade, and transfer oil and other contaminants to the part at just the wrong time. This might be a problem in those grooves where cleaning is less likely to be good.

And the thing is, if oily emery cloth isn't the problem, you'll no sooner fix the problem this time than have to worry about pits starting from oily emery cloth at any time :-)

Please complete step 1, and give us some data like Sergeant, Fluoride, or HEEF-25 bath and its analysis; and the cleaning and etching sequence. Best of luck!

A. I must say I like Mr. Mooney's answer :)

Mr. Mislan, The current, trivalent chrome contaminants, and sulphate ratio to the chrome, temperature. Also control the hardness of the part you like to hard chrome. I would also recommended a book about trouble shooting problems.

Regards,

A. Dear Sir,

The Chemical composition is Okay.

These are the following reasons for Pit Marks.

1) If Bath Temperature is very High
2) If the Bath has Organic Contamination
3) If the substrates got over etching.

Please check the bath operating conditions.

Thanking you,

A. Hi Vathana. We appended your inquiry to a thread which may offer some hints. This is just one of more than a dozen threads on this site about pitting in hard chrome plating -- which means there are many potential causes. And we'll need some clarification to understand your comments about "corrosion issue" and "thickness is lower than specification".

I have nothing against EDX, SEM, or other inspections of plated parts, but relying on laboratory tests of the defects to figure out the cause of a problem is always a longshot; things usually go much better when you can "walk the line" and actually examine the plating process, not just the defective parts. Good luck.

Regards,