Burnt Plating Deposits

Q. Our company is primarily research-oriented and is interested in learning more about electroplating. Currently we are using a Hull cell to test various types of plating processes. We have observed that a black deposit is obtained on the cathode on the end nearest the anode. On the other end of the cathode there is barely any type of deposit. Based on my reading, the black deposit is usually referred to as a burnt or an oxide. Can you give me a better understanding of this phenomenon?—N.S.

A. This phenomenon has not been completely explained. A burnt deposit is usually observed on edges or areas of your cathode that have a current density higher then the average current density on the cathode. In the practical world of plating, precautions must be used to prevent this. Platers will use shields and fixturing to reduce the current density on areas that otherwise would receive higher current densities then the average.

Some researchers believe that the so-called burning is attributed to high rates of hydrogen discharge. This causes the pH to increase in the solutions surrounding these areas that in turn precipitate metal hydroxide or basic salts. These hydroxides or basic salts become part of the plated deposit. It has also been demonstrated that burnt deposits can contain water molecules and organic materials.

A couple of common methods for eliminating of burnt deposits are to lower the current density, and increase metal ion transfer and diffusion rates. The latter method is achieved by increasing agitation, raising the temperature of the bath or increasing the concentration.