Failure of Zinc Plating
Q. Our company produces steel components that are welded and sent out to a plating vendor for zinc plating and a trivalent chromium chemical conversion coating. On some of these parts, the zinc plate deteriorates after short periods of storage. Corrosion appears along welds and close to the weld areas. We know that the plater is using an acid chloride zinc plating bath. I’ve attached a process sequence given to me by the plating vendor. Do you have any suggestions for eliminating this corrosion problem? S.J.
A. Based on the information you have provided I am convinced that the issue you are facing has to do with cleaning of the welded parts prior to the electroplating process. Most likely all of the fluids that the parts come in contact with prior to the welding step have not been completely removed from the surface of the parts. The fluids contain both organic and inorganic materials. If these materials are not completely removed, they can be converted to materials that have very different properties when exposed to high temperatures required during the welding process. Some of the simple organic materials, such as stearic acid, will polymerize at higher temperatures. These polymers are very difficult to remove in most cases and require more vigorous cleaning.
The best way to handle this is to thoroughly clean the parts prior to the welding step. This should make the plater’s job easier. If this is not possible, then you have to go to plan B. Plan B is to work with your plating vendor to develop a more robust cleaning process.
The cleaning process outlined in your process flow diagram is probably adequate for many types of soils, but apparently is not working properly in your particular case. What you should do is go to your plating vendor and work with him developing a better cleaning process. I suggest you talk to the vendors of the various fluids used in the manufacturing of the parts and determine what type of cleaning process they recommend to remove these materials. You can then take that information to your plater and hopefully develop a better cleaning process. You mentioned in your e-mail that you have sent samples out for surface analysis, and I would suggest that after you experiment with the cleaning process, send samples to the laboratory again to determine if all of the organics and other materials have been completely removed.