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White rust formation on Zinc + Cr+3 finished threads


I was doing a research in the web and found this excellent discussion website.
I'm working with some metal connectors which is being used for hydraulic brake hoses. Even though parts are from one end to another of this world by sea, I do suspect that the supplier is having some difficult to keep the finishing quality of the parts.

These metal connectors are actually like couplings formed of a female connector and a big mass bracket brazed to it. The finishing (Zn / Cr+3) is applied over after the brazing process of these parts. The rust occurs inside the female connector, in other words, it happens along the thread.

We have another part which is just a female connector only (don't have any bracket brazed to it) and we don't have rust problem in this one.

So, I'm wondering in the first connector with bracket, if the big metal mass brazed to it kind of "steal" more protection to it, while the threads inside the female connector end up the process with a weaker finishing over it. Does anybody knows if this has some explanation?

Also, I've heard that Cr+3 finishing is more difficult to control when compared to Cr+6. I've used Cr+6 parts for years and with ZERO problems with rust such as I'm having now.

I appreciate any comments.

Nelson S. Nishimura

I tend to think it's not the chromate that is the problem, but the zinc plating (or lack thereof on the inside of the female part). If the zinc thickness can be measured (perhaps by cross-sectioning ), I think you'll be able to quantify the problem and to look squarely at it. The unbrazed connector may be barrel plated and be benefitting from it, or the brazed connector may be acid zinc plated and be suffering for it.

Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey

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