|Content Type||Conference Paper|
|Title||Evaluating the Physical Properties of a Cadmium Replacement Coating|
|Authors||David A. Shifler, Regis K. Conrad, and Harry L. Archer, Naval Surface Warfare Center|
|Source||CORROSION 2003, March, 2003 , San Diego Ca|
|Copyright||2003. NACE International|
|Keywords||cadmium plating, zinc nickel plating, corrosion, hydrogen embrittlement, gravelometer testing|
Cadmium plating has been used to protect steel by sacrificial galvanic action, particularly in conjunction with steel components in marine environments. Cadmium has been used because of its hardness, close dimensional tolerance, and its ability to restrict hydrogen permeation into or out of steels. However, cadmium is highly toxic to the environment and is severely detrimental to occupational safety, thereby necessitating the search for alternative corrosion control coating systems.
Alternative plating/coatings to replace cadmium plating have been developed and evaluated for some period of time. In this paper, the mechanical properties of zinc-nickel plated 4130 steel panels are evaluated and compared to cadmium plated 4130 steel panels. The presence of hydrogen embrittlement on zinc-nickel plated 4340 steel test strips was compared to cadmium-plated 4340 steel test strips.
Cadmium plating is applied mainly to steels, copper and copper alloys, and to a smaller extent aluminum and zinc alloys . Cadmium is usually electrodeposited from cyanide baths. Cadmium plating provides an anodic plating to steel that has greater chemical resistance to alkalis, seawater wet spray, and alternate immersion in seawater. Cadmium plating has slower formation of corrosion products and tarnish films which can interfere with moving parts in mechanical and electrical systems. Reduced incidents of gouging, seizure of moving parts, and hydrogen ernbrittlement of medium and high-carbon steels have been observed from using cadmium cyanide electroplating baths than that associated with zinc cyanide electroplating solutions . Cadmium plating is frequently used for fasteners and other very tight tolerance parts because of the dual qualities of lubricity at minimal thickness and superior sacrificial corrosion protection. Cadmium plating functions as a very effective barrier coating, particularly in marine environments . The degree of protection by cadmium is conveyed by the barrier coating properties and the sacrificial action of the plating in area of damage.
However, cadmium is highly toxic to occupational safety and is harmful to the environment, thereby necessitating the search for alternative corrosion control coating systems. The use of cadmium has been regulated under Part 1910 of Title 29 of the Code of Federal Regulations Section 1027 (29 CFR, § 1910.1027) by the Occupational Safety and Health Administration (OSHA) since December 1992 . The regulation reduced the permissible exposure limits (PELs) twenty fold to the current PEL of 5 micrograms of cadmium per cubic meter of air (5 gg/m 3) for all compounds, dust and fumes of cadmium. The Department of Defense has listed cadmium on it's "List of Toxic Chemicals, Hazardous Substances, & Ozone-Depleting Chemicals" SD-141.
Alternative cadmium plating/coating systems require mechanical and performance properties similar to cadmium plating over the full spectrum of applications and exposures for which they are currently used. Zinc plating offers the desirable property of being a sacrificial coating, but a drawback of zinc plating is that it generates voluminous corrosion products that cause seizure and wedging of fasteners  and adhesive failure of organic coatings.
It is possible to maintain the sacrificial protection of zinc plating over steel by alloying it with another metal more noble than zinc such as nickel or tin. As a result, the alloy corrodes at a much slower rate than zinc alone, affording better corrosion protection. Both tin-zinc and zinc nickel plating processes provide characteristics that will meet the majority of the corrosion resistance and lubricity requirements imposed by MIL-SPECs on cadmium-plat
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