Rust bleeds from welded seams after zinc plating

A. Hi Jason. Although it is important that parts be thoroughly dry before shipment, no amount of drying will solve the problem of solution entrapment because the dried salts in the crevices will be hygroscopic and will absorb humidity from the air and start bleeding again.

Rinsing can be improved by alternating between hot & cold dips, and possibly via a dilute nitric acid dip between the plating and the chromating. Still, as you'll read on this thread, it may be necessary to seal those crevices via better welding or the application of an anaerobic sealant before the plating process.

Luck & Regards,

A. I can't envision the structural integrity of the rack being compromised by rusting in a temperature and humidity controlled environment; so I'd consider it an aesthetic problem. Nonetheless, the plating and powder coating is not satisfactory and the problem should be corrected through better welding, better rinsing, or by sealing the joint with anaerobic adhesive sealant before plating.

A. Hello Mr Doolan,

It looks like a simple Press fit assembly; can't you assemble them together after plating them separately?

Or else choose a Dip-Paint or E-Coat process where the bleed out won't attack the coating.

regards

Thank you for your response.

Unfortunately the components are welded together (on the underside not shown on the photo, so plating separately is not an option.
The part is used to weld onto steel posts for gate hinges and so zinc plating is desirable for these reasons.

Any other thoughts?

Regards,
Bill

A. This is a common problem with your type of parts. Start out in a hot rinse tank then go into a cold rinse tank and then do your normal plating process with a long rinse in the last rinse tank with intense air agitation. Then go into the chromate tank and rinses. End with a very warm (110F) rinse and blow dry immediately. It might work on some of the parts. Baking with a slow temperature rise may work until they are rained on the first time in outside use.

A. Hi again,

You can try the process alternatives all which work for us, In addition to the above I would also recommend a long immersion in de-watering oil, if you don't mind having an oil film over the part.

Regards,

A. Hi, J.

A picture would help us understand, but there will be no plating in crevices because plating thickness is proportional to electrical current density, and electricity essentially takes the path of least resistance such that no electricity will flow into crevices. You may need to use some sort of organic sealant that can flow into those crevices and/or you may need (as environmentally unfriendly as it sounds) a vapor degreaser/drier to drive capillary-entrained acid and water out of those crevices.

Regards,

A. I agree a sealer is the best way to cover a crevice. The other thing is weld spots tend me be the hardest spot on a part to clean. Try cleaning a load with a heavy organic cleaner by hand and see what kind of results you get. It should tell you if it's a cleaning problem or a throw problem.

A. Zinc is a relatively lower priced plating and thus does not qualify for a lot of individual rinsing, namely pressure spray.
The higher surface tension of the chemical is enough to keep a dip rinse, even 2 or 3, from clearing out the chemicals.
The brown is rust, so there is not a lot that can be done for a permanent fix. Naval jelly will attack the zinc faster than the rust.
Now you see why platers hate skip weld seams.

A. Hi Philip. Improving the welds is possible but expensive. The weld I see in your first pic is apparently single sided, on the outside of the part only. Anaerobic adhesive sealant would probably solve the problem but the issue is back to "Zinc is a relatively lower priced plating ..." Manual application of a sealant before plating could conceivably double, triple, or quadruple the cost. Most people in most cases consider zinc plating functional rather than decorative, and it becomes an issue when parts have stitch welds, one-sided welds, crimped areas, etc., so a down-&-dirty process doesn't work well enough. But yes, the plater probably could do a better job dealing with these issues.

Regards,

A. Hello Scott,
It's a little hard to see from the photo whether the raised plate that is welded is partially open at the bottom. If it is partially open you are most likely experiencing a degree of solution entrapment. This could be happening from your pre clean and or dips. The entrapped solution could very well be leaching out in that area and affecting your final finish. Try to increase your rinsing times and give the part the proper agitation whereby the entrapped solution will be removed. As far as calling it corrosion or staining, I would lean towards staining. Hope this helps you.

A. Hi Scott
I would agree with Mark that you have a classic case of solution entrapment. However it is such an obvious fault that I would expect the plater to refuse to process it as he will inevitably be blamed when it fails later.
The weld on the visible side is only partial. Even if the cavity is cleaned by extended rinsing, the inside of the void will not be plated and will be a long term source of rust in service.
The weld needs to be uninterrupted all round the flange. It is not just there for strength.

Thank you for your advice and comments. They have been very helpful. This is a stitch or Skip weld on the part as you have suspected. The brackets are as flat as a sheet steel can get, but there is room for fluid to be entrapped. This looks like a design solution would be best to eliminate the opportunity for fluid entrapment and/or allow for plating between the welded parts.

Thanks again.