Adhesion Problem

Question:

We are an automotive ancillary supplier, catering to various automotive OEMs in India. We are brazing brushes to steel material sheets. The jobs are subsequently powder coated using epoxy. After complete curing of the powder at specified temperatures, the powder coating peels off in and around the brazed areas by just rubbing it by hand. We do not face similar problems when we MIG weld similar brushes on the same jobs. We tried two different types of brazing rods, but the problem is persistent. R. S.

Answer:

The problems you have are a direct result of the flux contained in brazing rods or used in the brazing process. These inorganic soils must be removed prior to applying the powder coating. Contacting your pretreatment chemical supplier for their recommendations will go a long way to solving your problem. They will probably recommend an acid-based chemical to etch this material from the steel substrate (known as pickling). If this chemical approach is undesirable, then you must remove the soil using either grit blasting or abrasion (wire brush or sanding). This mechanical cleaning method will of course remove some base metal along with the soil.