Indium Alloy as Cadmium Brush Plating Replacement
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Indium Alloy as Cadmium Brush Plating Replacement Elizabeth Berman1, Paul Brezovec2, Melissa Klingenberg2, Thomas Naguy1, Eileen Schmura3, and Natasha Voevodin1, 4 1 Air Force Research Laboratory, Materials and Manufacturing Directorate, 2179 12th Street, Wright-Patterson Air Force Base, OH 45433, U.S.A. 2 Concurrent Technologies Corporation, 100 CTC Dr., Johnstown, PA 15904, U.S.A. 3 Concurrent Technologies Corporation, 425 6th Avenue, Pittsburgh, PA 15219, U.S.A. 4 University of Dayton Research Institute, 300 College Park, Dayton, OH 45469, U.S.A. ABSTRACT It is well-known that cadmium and its corrosion product (cadmium oxide) are carcinogenic and toxic. Consequently, efforts to eliminate cadmium from original equipment and repair processes have been on-going. One potential replacement is indium, which is a soft posttransition metal, whose primary use is in the form of indium tin oxide, e.g., in liquid crystal displays (LCDs). The semi-conductive properties of indium oxide alloys make it possible to use these for cadmium brush plating replacement in applications where contact resistance and impedance are critical parameters. Critical requirements of an alternative to cadmium brush plating in a corrosive industrial atmosphere are (1) be sacrificial to mild steel and (2) provide good electrical conductivity. Cadmium oxide remains semi-conductive, while most other pure metal oxides are electrical insulators, such as aluminum oxide, nickel oxide, and zinc oxide, and therefore, fail in meeting the requirement for bonding and grounding. Similar to cadmium oxide, indium oxide is semi-conductive, but indium exhibits other properties such as cold welding. This paper discusses indium- electroplating approaches to overcome the insulating limitation of pure metal plating and to replace cadmium brush plating. Test results are given for the brushplated indium-tin alloy as compared to cadmium brush plating and conclusions on such replacement feasibility are provided. INTRODUCTION Cadmium (Cd) coatings provide desirable properties to metal parts including improved chemical and corrosion resistance, paint adhesion, lubricity, electrical conductivity, and wear resistance. However, Cd and its oxide (CdO) are carcinogenic and toxic. Therefore, it is desirable to find a Cd replacement that retains dimensional tolerances and electrical conductivity, and provides sacrificial corrosion protection to 10xx mild steel during the service life of the part. Cd replacements often provide corrosion protection, but form insulating oxides and those which could change part dimensions. CdO is semi-conductive (1 x 10-3 to 7.56 x 10-3 Ω-cm electrical resistivity [1-3] vs. 6.83 x 10-6 Ω-cm for Cd [4]) and forms non-voluminous corrosion products (8.15 g/cm3 [3] vs. 8.65 g/cm3 for Cd [4]) ensuring form and fit of the part over time. To find potential alternatives, different galvanic series were reviewed to identify metals that were anodic to mild steel and electrical conductivity and density values were reviewed for metals and their oxides. Small changes
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