Black chrome solar selective coatings optimized for high temperature applications

Abstract

A statistical design approach has been used on a specified set of plating parameters to optimize the thermal stability of electrodeposited black chrome solar coatings. Plating parameters studied included the bath composition (chromic acid, acetic acid, trivalent chromium and iron), current density, plating time, substrate and bath temperature. The optimization procedure consisted of maximizing the solar absorptance values after thermal aging at high temperatures (up to 450°C in air). The estimated functional dependence of the thermally aged solar absorptance values on the four bath constituents was determined in the region of most stable coatings. This relationship was used to specify a range of bath compositions that provide coatings with optimum thermal stability. It was also found that chloride ion contamination of the black chrome plating solution significantly degraded the thermal stability of plated coatings. Fortunately, the chloride ions could be effectively removed through the addition of silver oxide, which precipitates silver chloride. When the plating variables were properly controlled, electrodeposited black chrome coatings were obtained which should maintain solar absorptance values of 0.96 or higher during years of operation at 300°C in air.