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Corrosion resistance of dichromated cadmium plated vs. zinc plated U-bolt

I have encountered a situation where the vendor suggested me to use zinc plated instead of bichromated cadmium plated U-bolt to be used on topside. May I know which one has better corrosion resistance? Is bichromated cadmium plating still common in Oil & Gas industry?

Yu Suen Foo
Piping Engineer - Ipoh, Perak, Malaysia
February 10, 2011

There are many factors that would have an influence on the answer. For a maritime environment, chromated zinc should work. Who knows what it would be subjected to for oil drill operations. The best way to find out is to put some out there with an offer to replace them with the cad parts if they are not suitable. Zinc is cheaper than cad.

James Watts
- Navarre, Florida
First of three simultaneous responses -- February 10, 2011

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If it is still legal to use cadmium in your country you will find it gives better service than zinc, chromate or not.

Jeffrey Holmes, CEF
Spartanburg, South Carolina
Second of three simultaneous responses -- February 10, 2011

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The cadmium plated part will have better performance in a salt spray corrosion test chamber which possibly translates to better corrosion performance in the real world. But to answer your second question, cadmium has been abandoned in favor of zinc (or zinc-nickel) for most non-aerospace applications due to various hazards associated with cadmium. It is reasonable to consider zinc + a suitable chromate conversion as a reasonable substitute for your previous coating.

Jon Barrows, MSF, EHSSC
GOAD Company

Independence, Missouri

Third of three simultaneous responses -- February 10, 2011

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Hi, Yu. Cadmium is not specified these days for its corrosion resistance alone, because you can get better corrosion resistance at lower cost by using a thicker plating of zinc or a zinc alloy. It is specified when its unique combination of properties is required: galvanic compatibility with aluminum, freedom from stick-slick, malleability, lack of voluminous and sticky corrosion products, etc. For a U-bolt, my first choice to try as a drop-in replacement would be tin-zinc plated with a chromate conversion coating.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
February 12, 2011

I received a requirements from my customer asking for the bolting come with Cadmium Plated + Fluorocarbon coated. From my experience, normally client will ask for either 1, not for both. Do you have any idea the reason why they asked for it?

Ryn Saad
- Selangor, Malaysia
June 15, 2011

Thank you everyone for the advice. My client approved the proposed zinc plated. :)

Yu Suen, Foo
- Kuala Lumpur, Malaysia
October 13, 2011