Cadmium coated bolts / nuts vs. stainless steel bolts for seawater exposure
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?
Aquatic Design & Engineering, Inc. - Oakland, Florida, USA
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.
- Goleta, California
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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
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