Question:

We use brass ball anodes in titanium anode baskets, and we have a problem with a black film over the anodes. This prevents operations at normal current densities. Sometimes the black film breaks away from the anode and causes roughness. What can we do to prevent this from happening?

Answer:

This problem is common in brass plating, and may be traced to several difficulties that may be occurring simultaneously. The black film is probably an oxide film of copper oxide, which forms on the surface of poorly manufactured anodes, or in systems that are operating at less-than-ideal efficiencies.

The first method I would use to evaluate the problem is to examine the plating solution, itself. Is it within the ideal operating parameters that you are trying to maintain? Is the free cyanide high enough to efficiently dissolve the anodes, without causing the oxide surface to form? Is the pH of the solution within operating range, and is it low enough to allow the film to dissolve into the plating bath? Are the concentrations of the components high enough to effect the anodes in a positive way?

In addition to the components you can control, you must analyze for contaminants that are manufactured by the operation of the plating solution specifically, the concentration of carbonates. Carbonates that are formed by the breakdown of cyanide at the anode surface severely affect the anodic efficiency and cause the inordinate production of carbonates in the plating solution. The production of these carbonates is self-perpetuating, and causes the increased rate at which they are produced in the bath.

The higher the carbonate levels are, the lower the anodic efficiency is, and the propensity of the solution to produce carbonates in operation increases. This is definitely a situation to avoid. Treatment to remove carbonates and control them at low levels is very important. Levels should be maintained at not more than 10 oz/gal.

Last, but not least: The total anodic current density must be monitored and controlled, and a ratio of 2:1 anode-to cathode surface should be sustained. An anode current density that is too high will also cause polarization of the anodes. You have mentioned that the anode baskets are made from titanium. Titanium metal may become inactive and polarized in low-level alkaline solutions, if the current densities are high enough. This happens often in barrel plating operations, where the anode current densities tend to be higher than in rack operations. You should perform an anodic electrical profile of your system to make certain that the anodic current density is distributed evenly in the system, and that there are no points of extraordinarily high anodic current densities.

If anodic hot spots in the tank are found, perform the necessary changes in bussing to eliminate them; change them, prior to bussing; or change the primary bussing; or reinforce the electrical transport through the titanium baskets; or replace the baskets with mild steel ones, or use brass bar anodes, which have further advantages.

I hope that these suggestions help you. It has been my observation that brass baths that are highly sophisticated alloy-plating solutions are not controlled as such, because of the relatively low functionality requirement of the parts produced. Brass is a high-tech, complex system, and should be treated as such. As always, the quality and process control systems that are established should reflect the complexity of the system and the requirements of the parts that are being processed.