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topic 43834

Cadmium coated bolts / nuts vs. stainless steel bolts for seawater exposure


A discussion started in 2007 but continuing through 2019

2007

Q. We are trying to determine the best / longest lasting materials for metallic hardware items (nuts, bolts, washers) in an underwater sea-water / salt water exposure (a pipe line).

Will a cadmium coated steel perform better or worse underwater than will a stainless steel material? Should the stainless steel material be coated as well?

Kenneth R. Martin
Aquatic Design & Engineering, Inc. - Oakland, Florida, USA


2007

A. Much worse. Cadmium plating 0.0005" thick, yellow chomated, may last 1 week before white rust and 1-6 months before red rust. Somewhat illegal due to toxicity.

No, use a suitable grade of stainless. For fasteners, 2205 (UNS S31803) is better than 316 with respect to strength and resistance to crevice & pitting corrosion & SCC. What is the pipeline material -- coated steel, stainless, reinforced plastic...? Galvanic compatibility may be an issue.

The best long-term material for both pipeline and hardware is probably titanium.

Ken Vlach
- Goleta, California

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2007

A. Cadmium plated steel bolts are not suitable for seawater usage, and neither are most stainless steels. The proper choice of fastener depends a great deal on what is being joined (geometry, materials, stresses, etc.). Nickel-based alloys like MONEL K-500 or INCONEL 686 or Titanium alloys like TIMETAL 5111 are the most resistant to corrosion and offer high strength and toughness. However, if the stresses are not too high, a duplex stainless steel grade like 254SMO (UNS S31254) may be acceptable.

Toby Padfield
Automotive module supplier - Michigan



October 28, 2019

Q. Hello, I am also considering using cadmium coating threaded rods for anchoring into concrete in submerged saltwater application. Per your previous comment, cadmium is no good for saltwater. Also, looked at Aluminum-Bronze but very expensive. My base material as aluminum, as well as all other existing metal framing. Galvanic action has been huge issue in the past and we must eliminate all stainless steel and zinc materials. Titanium and Nickel also on the wrong end of the spectrum for Aluminum. Do you have any other recommendations or I just bite the bullet with AL-Bronze?

Tirrell Day
Engineering - Tampa, Florida, United States

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