Blistering at Weldments

Q. We are electrocoating cold-rolled-steel parts. The problem we are having is blistering around the weld areas. We are using an iron phosphate. We need some suggestions. J.I.

 

A. A few things that could be causing your problem are listed below. These suggested causes should give you something to look for in the search for the cause of the blistering problem.

1. By-products of the welding process are not being removed by the pretreatment. These materials may be contaminants to the electrocoat process. If that is the case, special care must be taken to remove them.
2. The weldments are so rough that the cleaner is not impinging on the entire surface, and, therefore, not removing all the oily soils. When the parts are baked, the paint blisters in those areas where paint has bridged across the soils.
3. The weldments are porous enough that the phosphating solutions are trapped in the interstices and cause blistering of the paint film during the baking cycle.
4. Because of the aforementioned porosity, rinse water trapped in the weldments is not removed by the dry-off oven, if one is used.