Hydrogen Embrittlement In Cadmium Deposit

Question:

I am having a problem with hydrogen embrittlement of fasteners on which I deposit cadmium, then bake. What can I do to correct this?

Answer:

It has been demonstrated that detrimental hydrogen embrittlement may occur from a cadmium-cyanide plating process, and that it is a function of cathode efficiency. The cathode efficiency controls the introduction of hydrogen to the surface of the part. The structure of the coating controls the rate of removal of the hydrogen during postplating baking and high-temperature treatments. Optimization of the cadmium plating process, minimizes the hydrogen embrittlement of the surface. Recent specifications call for the exclusion of organic or metallic brighteners in the plating process in order to maximize the efficiency of the solution and leave a surface that can be relieved of hydrogen.

Brighteners tend to suppress the ability of the surface to loose hydrogen, because of the tight crystal structures formed. In addition, you must keep the bath free from contaminants, especially nickel, as well as other heavy metals that will also detrimentally affect the efficiency of the process.

Brightener-free cadmium cyanide plating solution will deposit a surface structure that will allow the removal of hydrogen from the surface and decrease the effects of embrittlement. Another area to watch is the carbonate concentration of the plating solution. Carbonate concentrations of more than four oz/gal will drastically reduce the efficiency of the plating solution and will increase the propensity of the bath to cause embrittlement. Constant carbon treatment of the solution is appropriate for eliminating the chance of organic contamination. A dedicated line for production of cadmium plated parts is essential for the elimination of the contamination problems associated with the process.

To keep the plating solution at peak performance, allowing you the opportunity to check the embrittlement tendencies of your process, l recommend the use of a notched tensile strength sample on a regular basis, to monitor the potential embrittlement of your system.

In addition, keep in mind that, when calculating the applied current density of the operation, barrel plating is a combination of high- and low-current densities, depending on the position of the part within the load and merely an average of the current applied. How the parts behave in the barrel, as well as the load size, is important to making a determination of the applied current. Experimentation is important to optimizing the process, as to the proper load size and the current density to be used.