Chromate Conversion Problems—One

Question:

Our company does not do any plating, but we do buy zinc plated steel enclosures. The plater uses a chloride zinc plating bath and has an ongoing problem with the after treatment of the zinc plate. The coating seems to slough off on many lots. The plater has told us that this is normal and can’t be helped. We are not platers but feel that we should not have this problem. Can you give us any insight into this? S.J.

Answer:

Your problem is not unusual. Many platers seem to have the same problem and seem to be at a loss as how to solve it. Not surprisingly, the problem you have can be solved! The major causes, as with most problems in the plating shop, have to do with poor surface preparation prior to plating and chromating and poor process control measures. In your situation, the problem is most likely caused by both of these. Classically, the electroplater paid very little attention to his chromate conversion coatings and, as a result, chromate conversion coatings were variable in their properties.

While you do not mention it in your e-mail, no doubt you are specifying a bright, shiny finish for an eye-pleasing appearance. The zinc chloride plating bath is very good at doing this. In order to get this finish, various additives (brighteners, etc.) are added to the plating bath. These additives are usually organic in nature and become part of the zinc plated layer. If too much of these brighteners are present in the zinc plating bath, the plated layer does not offer a good surface for the chromate conversion coating to “grip.” The result is the sloughing off that your plater is experiencing.

Your plater has to reduce the amount of organics in the bath. By doing so, the finished part may not have quite the same bright finish, but I doubt your customers will notice the difference.

Some vendors of chromate conversion coatings offer “hardeners” to help harden up the chromate film. They also offer chromate conversion baths specifically formulated for zinc chloride plating baths.

Another problem when using acid chloride zinc baths is drag out from the acid chloride plating tank into the chromate conversion tank. The chloride from this particular plating bath tends to “thicken up” the chromate conversion coating. This thickening causes the chromate coating to slough off. The solution to this problem does not involve rocket science just better rinsing between the plating step and the chromating step.

Last, but-not-least, your plater has to get his act together regarding control of the chromating process.