BRASS PLATING BASIC CONCEPT

1. BRASS PLATING BASIC CONCEPT

2. Brass plating is the most widespread alloy plating method due to its color and decorative outlook. Applications vary from fasteners, lighters, and fittings to other hardware and even functional applications such as corrosion-resistant coatings in marine environments. Usually, brass plating is post-treated with lacquer to prevent any tarnishing or discoloration. A brass-plating process is done mainly from cyanide solutions. The solution contains a mixture of copper and zinc cyanides, although zinc can exist in other forms as well. The amount of the bath constituents can vary in great amounts. The copper to zinc ratio is the most important parameter to control the deposit composition (46), which is typically held at about 4:1. The free cyanide and pH affect the color of the deposit and coating uniformity. Ammonia can be used to enhance the throwing power and color of the coating. Organic or metallic brighteners are used to increase brightness and improve grain refinement. Tartrates or citrates are used to prevent anode polarization.

3. An example of a solution used to make yellow color deposit is a bath containing about 30 g l−1 CuCN, 10 g l−1 Zn(CN)2, 50 g l−1 NaCN, 8 g l−1 Na2CO3, 10 g l−1 NaHCO3 and 3–5 ml l−1 ammonia (47). pH is maintained around 10, temperature at 25–35 °C, and current density at 0.3–1 Adm−2. If higher current density, that is, the production rate is desired, a bath at an elevated temperature above 60 °C with higher metal concentrations may be used. Plating can be done by rack or barrel operation. Typically, barrel solutions are more dilute than the solutions used in rack operations. Anode material can be as close to the desired coating composition as possible, for example, 75% Cu and 25% Zn. Also, separate anodes with separate current control can be used. Anode material should be clean from impurities, and the surface area is such that the current density will be under 0.5 Adm−2. Noncyanide formulations will need to be developed to be more stable in order to gain further acceptance by operators. A number of such formulations have been developed, for example, sulfate-based formulas containing glycerol. Another trend is a substitution of brass plating by electrolytic dyes and resins.