Unplated area inside blind holes is rusting

A. Hi Kaleb. I know of shops who did nickel-chrome plating on tubing and finished the cycle by phosphatizing the I.D. of the tubing. The advantage of that approach was that the phosphate reacts with the bare steel but does not affect the nickel-chrome plated area, so after rinsing there is no residue to cause staining. But they did not have to bake like you do.

Exactly what would happen to the phosphatized area during baking I don't know. Phosphatizing and oiling after the baking sounds more straight forward, but may involve more than your shop wants to deal with. A high-pH rinse may not do as much as you'd like, and sounds like, by definition, it would have to leave an alkali residue on the parts.

An important thing about plating for you to learn as an intern, which you may already know but which your bosses may not have learned, is that you try these chemistry ideas in the lab first. I'm pretty serious! -- you can't imagine how many people write to us saying that they added a thousand pounds of 'x' to their plating tank and it didn't work, or they tried to treat 6,000 gallons of waste by adding 750 gallons of sulfuric acid or sodium hydroxide, so what now :-)

Luck & Regards,

A. Kaleb,

Unplated area parts rusty because of this area oxidized; if you want to prevent rust, you can do wax of this area or dip this part into rust inhibitor liquid.

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A. This problem may be more about the plating shop managing your expectations than their technique, Ronny.

In typical commercial zinc plating, rust inside tapped holes like this is a commonplace because there is no plating inside the holes. The plating will deposit in proportion to the current flowing to that area, and essentially no current will flow into the hole. Some platers (with their customer's permission) stopper the holes during processing to minimize dragover and contamination rather than pretend there will be plating there. If the parts were treated with a rust inhibitor at some point prior to plating, plugging the holes would retain the rust inhibitor in them, and might help.

If the inside of the holes MUST be plated, they can be. But this involves detailed handwork like putting a piece of wire (auxiliary anode) into the hole, and inverting the part in every tank so that neither air pockets nor cupped solution interfere with the process. But this is certainly not the usual commercial practice, and would be very expensive. Sorry to be the messenger.

The only cost-effective encouragement I can offer besides plugging the holes is that: 1. alkaline zinc and cyanide zinc plating solutions have greater covering power than acid zinc, and you may see at least some improvement if the plating shop uses one of those processes instead of acid zinc. 2. barrel plating may help a bit if the parts are amenable to barrel processing. Good Luck.

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A. Hi cousin Arijit. You probably realize that there are entire books about troubleshooting plating problems, like Durney's "Trouble in Your Tank", and entire books just about zinc plating like Geduld's "Zinc Plating". I mention this so you can have some reasonable expectations about people's ability to help you through this forum, and how much work it is going to necessitate on your end. If you need speedy and closely focused help, you need to call in the suppliers of your plating processes and have experienced eyes look it over, or retain a plating consultant.

But to get started here, please tell us more about these bent pipes, whether the problem is at the inside or outside bend, how you rack them, and what the plating thickness is in the rusty area. Photos of the parts and the racking would help. If the problem occurs after packaging, the packaging material may be an issue too.

Regarding the green chromate rubbing off, please tell us the maximum temperature the parts see at any point after chromating, and whether you are able to prevent any abrasion for a 24-hour curing time. Thanks!

Regards,

A. Hi. That is extremely heavy zinc plating -- 7-1/2 to 9 ten thousandths of an inch -- and you should not be encountering problems. How do you know the zinc plating thickness? Are the pipes bent before or after plating (a bender could damage them)? When does the rust develop -- immediately after plating or months later? I find it hard to believe you could encounter such heavy red rust within a reasonable time.

Are the parts stored in a plating shop laden with acid fumes?

Rub-off of the chromate can occur for a number of reasons, but foremost is exceeding a drying temperature of 140 °F. It's hard to trust the color tone of photographs without well-known things like trees, sky, and human skin in them, but the chromate looks black, not olive green. Good luck.

Regards,

A. Economical is a matter of need or perception.
Cleaning is mandatory and nearly impossible. This by and large makes tank plating impossible.

A grease like cosmoline is probably the best answer.

If You have to plate it, then a motorized and flow thru brush plating is about the only way that I can think off. The chemicals will mess up the existing plating requiring a difficult masking job. If you brush plate the holes first, then you are going to need an excellent plug for the holes when you clean and activate the rest of the part.

You might be able to sell your customer on using a good anti-seize ⇦ this on eBay or Amazon [affil links] material after plating and after excellent flushing of the hole.

A. Hi Stuart. I'm glad you are enjoying at least some small measure of success in plating from the vinegar & epsom salt. My great friend and former partner Tom Pullizzi invented this solution 20 years ago as a safer way for elementary school children to actually see a little something happen as they learned the principles of electroplating. It was never intended to be stable, robust, or usable for actual parts :-)

A concave area will get very little current (most of the current will take the shortest path), and thus little plating unless you can fashion an anode to follow that contour. Still, when plating doesn't stick, it's usually best to assume that the area was not properly cleaned and activated. Good luck!

Regards,

A. Hi Stuart. No pics received. Please attach them in email to

Yes, I can see from googling that a large number of auto enthusiasts are now using Pullizzi's vinegar & epsom salt solution for actual home plating. It is gratifying, but I wish they'd occasionally attribute it to him; I guess it's just the new way.

Regards,

Hi Jim. As the owner, over time, of many 10-12 year old cars I wish they'd still use metal :-)
The underhood plastic seems to be fine for 8 or 10 years but finally loses the last of its plasticizers, becoming weak & brittle and unrepairable.

A. I'm perhaps misunderstanding you, but you want a D-I-Y plating that is more robust than OEM plating? Sorry, that's not going to happen. You can send the clips to a plating shop for zinc alloy plating, or you can try to do the very easy vinegar-based zinc plating described on this letter and in our FAQ. As noted, this process was intended for grade schoolers to play with plating relatively safely, but auto enthusiasts have reported some success with it on actual parts.

Spring clips are hardened steel which is very subject to hydrogen embrittlement, but on the other hand they don't seem to be life-critical parts, so it seems you could try the D-I-Y plating if you wished. Just wear safety glasses when flexing them because without a hydrogen de-embrittlement baking they might be prone to shattering. Good luck.

Regards,

A. Hi Ramnarayanan.
a. Although it can depend on the diameter and depth of the blind holes, it is generally true that you can't expect plating inside them.
b. An alkaline zinc plating process would exhibit somewhat better coverage and leave behind somewhat less corrosive contaminants.
c. Surely the process doesn't end with plating, rinsing, and drying but rather with plating, rinsing, chromating, rinsing, and drying?
d. 80-100 °C drying is too hot, as chromates should only be exposed to about 60 °C.

But it seems that you are now doing nothing at all to the bare steel inside the blind holes and expecting bare unprotected steel not to rust? That doesn't sound to me like a reasonable expectation. I don't fully understand your fabrication sequence, but I think at some point you need to clean those blind holes and apply some sort of preservative. Maybe run them through the electrocleaner, squirt a dap of rust preservative in as you plug the holes, then run them through the whole process beginning with cleaning again?

Regards,

A. Perhaps some kind of conforming anode?