Cadmium plating

Attack on metals can be a function of fuel components as well as of water and oxygen. Organic acids react with cadmium plating and 2inc coatings. Traces of H2S and free sulfur react with silver used in older piston pumps and with copper used in bearings and brass fittings. Specification limits by copper and silver strip corrosion tests are requited for fuels to forestall these reactions. 

Cadmium. In 1989, U.S. consumption of cadmium for coatings was 1474 t (7), compared to 1552 t in 1970, 2089 t in 1979, and 1230 t in 1985. Cadmium plating amounts to about 15% of total cadmium production (see Cadmiumand cadmium alloys). Of the cadmium being plated in 1989, 30% was for automotive parts, over 22% for electronics, and 18% for industrial fasteners. Because of cadmium s high and weU-pubHcized toxicity and very tight waste restrictions, there are considerable efforts to develop alternative materials, and the quantities of cadmium used in electroplating ate expected to decrease. The price of cadmium anodes in early 1993 was about l/kg. 

Analysis methods for hydrogen absorbed in the deposit have been described (65), and instmments are commercially available to detect hydrogen in metals. Several working tests have been devised that put plated specimens under strain and measure the time to failure. A method for cadmium-plated work has been described (66) as has a mechanical test method for evaluating treatments on AlSl 4340 Steel (67). Additional information on testing for hydrogen embrittlement is also available (68). 

The future of cadmium plating is in question because of cadmium toxicity (83). Cyanide-free cadmium plating systems have experienced some growth. Acid cadmium, based on cadmium sulfate compositions, is replacing some cyanide baths in the United States. The fluoborate cadmium is reported in use in the UK, especially in barrel plating (84). Cadmium plating is covered by ASTM (85), U.S. government, and ISO specifications (86). 

D. Berman, "Effect of Baking and Stress on the Hydrogen Content of Cadmium Plated High Strength Steels," paper 192 presented at Corrosion 85 NMCE, Materials Peformance, Boston, Mass., Sept. 1985. 

For the torque coefficient, K, reported values range from 0.153 to 0.328 for cadmium plated parts, with a mean of 0.24 (Shigley and Mischke, 1996). Therefore, applying the same reasoning as above ... 

RTJ gaskets are normally cadmium-plated, soft iron or low carbon steel. Soft iron is used for ANSI 600 and 900 classes, and 304 or 316 stainless steel for higher classes. 

Some liquid metals have been observed to cause embrittlement in many titanium alloys. In mercury, for example, Ti-8Al-lMo-lV exhibits both intergranular and transgranular fracture with velocities as high as lOcm/s. Heat treatment affects this behaviour in a manner similar to that observed in aqueous and methanolic solutions. Some alloys are embrittled by liquid cadmium and zinc. More surprising, perhaps, is the observed solid metal embrittlement which has been found on titanium alloy components coated with cadmium, silver or zinc Service failures of cadmium-plated Ti-6A1-4V fasteners have been reported , and cracking of this alloy and... 

Additional metal layers can create bimetallic corrosion cells if discontinuities appear in service. The layer of copper beneath cadmium plate on aluminium (using a zincate plus cuprocyanide deposit technique) can cause corrosion troubles. When aluminium is plated with nickel and chromium, rapid service corrosion in the zinc layer causes exfoliation. 

Ion Vapour Deposition A variant of the process is ion vapour deposition, in which a high negative potential is applied to the workpiece during chemical vapour deposition. The process has been employed on a commercial scale chiefly for depositing aluminium on to steel and titanium in the aerospace industry as an alternative to cadmium plating, which is liable to cause hydrogen embrittlement, especially of high tensile steel components. The aluminium is evaporated from a wire-fed resistance-heated boat . 

Zinc plating Cadmium plating 25-50 56-112 40-80 32 (optimum) 1-2 (vat) 0-3-0-7 (barrel)... 

Cadmium plating for general engineering use is covered by BS 1706 1960 and BS 3382 1961, and for aircraft parts by Ministry of Aviation Supply Specification DTD 904. Special requirements for very strong steels are given in Defence Standard 03-4 (Directorate of Standardisation, Ministry of Defence). 

Health and Safety Cadmium metal and its compounds are toxic and are injurious to health, and for this reason, cadmium is being replaced by other forms of coating wherever possible. For a number of important applications, however, no suitable alternatives have yet been identified. Where cadmium plating continues to be used, it is essential to comply with the regulations covering the use of cadmium. 

Standard probe body from cadmium-plated mild steel (other metals available on special order)... 

Fairly stable in steel containers at 65°C. Stability improves with increasing purity. Attacks tin, magnesium, cadmium plated steel, some aluminums slight attack on copper, brass, lead practically no attack on 1,020 steel, Inconel and K-monel. 

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