Q. We are using a number of different plating vendors with the same result: delayed blisters and stains. The spec is ASTM B 633 zinc with a trivalent chrome chemical conversion coating. The base materials are 1010 to 1060 steels, and the parts are stamped. The blistering and staining does not occur all the time, and none of our suppliers has been able to define root cause. What are your thoughts on this? —G.H.
A. Attempting to determine the root cause of problems that you encounter only occasionally requires good detective work. As most experienced platers know, it is a lot easier to solve a plating problem if the whole process goes south at once. Not knowing the complete details of your plating process, I can only make some suggestions for you to follow up on:
1. Determine the zinc plating process that is used at each of the plating facilities. There are three main types: acid zinc, cyanide zinc and alkaline non-cyanide. More than likely, your vendors are using the acid zinc process.
2. Examine the failed parts and determine if the blisters and stains occur in the same locations on the majority of the rejected parts. Do they occur on the flat surfaces or in areas with indentations?
3. Assuming that your plating vendors are using the acid zinc process, the cause of staining could be poor rinsing after the plating step and or the trivalent chromium conversion step. Another common cause of staining is storing parts in a humid, damp environment.
4. The number one cause of blisters in almost all types of plating is poor cleaning and preparation of the base metal prior to plating. What might cause the infrequent appearance of blisters is something as simple as an acid pickling bath not being up to snuff or a rinse tank that is not functioning as it should. Other causes of blisters can be problems with the plating bath itself, including low chloride content, low temperature, excess brightener and low wetting agent.
5. Consider the human factor. No matter how automated a plating process may be, you still have people involved. In my experience, looking at that people factor is usually an important part of solving a problem. Individuals may have had a bad day, or may have been daydreaming and not performing a procedure as required.
As you can see, solving your problem requires gathering information and using it to try to pin down the root cause or causes. Root-cause analysis is a good approach to solving many issues in the electroplating arena, but it typically is a time-consuming process with a problem of the type you are experiencing. Your plating vendors may not have the wherewithal to undertake this or may not want to unless you are a significant customer.