DECORATIVE CHROMIUM and CADMIUM SUBSTITUTES

Although alloy tin plating has been known and used for many years, this paper will describe the use of a new non-cyanide tin/zinc alloy as a possible replacement coating for cadmium and the use of a tin/nickel alloy as a possible substitute for barrel decorative chromium plating. A review on the corrosion properties of tin, zinc and cadmium compared to tin/zinc alloys and review of the properties, hardness, wear resistance, corrosion and color of the tin/nickel alloy versus electrodeposited chromium. INTRODUCTION TO TIN/ZINC Combining the barrier protection offered by tin with the galvanic protection of zinc, without the bulky corrosion products associated with pure zinc or high percentage zinc alloy deposits, tin/zinc electrodeposits containing 20-30% zinc offer outstanding corrosion protection for steel and other substrates. Tin/zinc alloys have been plated from cyanide electrolytes beginning in the 1940's and it is not surprising that a number of applications for this alloy have been known for many years.' However, since the mid 1960's the use of tin/zinc has declined considerably, and the coating is now not wed extefisively. A'ltlioiigh pari is due io the unpopularity of cyanide plating solutions, it is more probable that cyanide tin/zinc plating solutions are very difficult to operate and require significant control and service.* In order to improve and update the process of tin/zinc plating a non-cyanide process was developed. This development was important in view of the drive to reduce the use of cadmium coatings, for which tin/zinc offers an alternative. PLATING PROCESS In general, alloy plating processes are harder to control than single metal plating processes. This tin/zinc process follows the same tendencies. Statistical experimental design techniques in process development have made this alloy process easier to control than most others. The electroplating solution is a mixture of potassium stannate, potassium zincate, potassium hydroxide and complexing agent. Tables 1 and 2 give typical electrolyte compositions and operating parameters required to give a deposit containing 20-25% zinc. The concentrations of tin and zinc in the solution have a direct effect on the composition of the deposit. An increase in either one is reflected in an increase in the percentage of that metal in the deposit. Figure 1 shows the effect of zinc concentration on the alloy at different temperatures but at a constant current density, 40 ASF. At lower temperature more zinc is deposited inconsequential of the amount of zinc in solution. At higher temperatures more tin is deposited. Figure 2 shows a more detailed effect of temperature and current density on the composition of the alloy. At higher temperature and cunent density the tin concentration in the deposit increases. Unlike the cyanide process where free caustic has a significant effect on the zinc content in the alloy, the non-cyanide alkaline