Blistering Around Seams

Q. I work at a large automotive manufacturer and was recently assigned to the phosphate and E-coat process. Occasionally we are getting blisters on the top right corners of the hatches. The blisters form right where the weld seam is. We have some thoughts about the cause. Could you give us some suggestions for things to consider? S.P.

 

A. That kind of problem can really be aggravating. If it’s not happening every time then there must be some variation in the manufacturing process. Without actually seeing the condition you’re describing, it is difficult to comment on it, but let’s give it a try. Here are some things you might think about:

If there is a void or a slight opening in the perimeter seam, you might be trapping chemicals inside the fold. Once they are there it is very difficult to get adequate rinsing. Even if the chemicals are rinsed out you still have rinse water and perhaps permeate trapped inside. Any trapped liquids will be expelled by the heat in the cure oven causing defects in the E-coat film.

Another thought is the possibility of oxidation at the weld affected zone. That might cause the problem you describe. If that is occurring, you may have to take some type of action to have it removed, either abrasive or chemical with an acid etch.