Validation and development of an electroplating process to deposit a black chromium coating from a trivalent chromium electrolyte

SAAB AB coats a part of their magnetron with black chromium to enhance its ability to radiate thermal radiation. Today an electrodeposition process that has hexavalent chromium as its main component is used, but hexavalent chromium is carcinogenic and will be prohibited. This project examines if an electrolyte based on trivalent chromium can result in a black chromium coating. The project was divided into in four experimental parts: investigation of the adhesion on copper, the effect on color if copper was added to the electrolyte and investigation of the process parameters with and without cooling of the electrolyte. It was concluded that a black chromium coating can be deposited from a trivalent electrolyte. Heating the sample after plating and addition of iron or copper in the electrolyte darkens the color but addition of copper can not produce a coating on copper substrates with good adhesion. To examine how the coating thickness and emissivity vary with the current density, electrolyte temperature and plating time, the coatings thickness and emissivity were measured for different settings. It was shown that the coating thickness increased with plating time and current density until a critical value was reached and the coating started to peel off. No correlation between the emissivity and process parameters could be shown. It is suggested that further experiment are conducted to investigate if a variation in pH- value effects the emissivity. Based on the results and conclusions it is recommended that the addition of iron to the electrolyte is further examined