Q. We are facing the serious problem of powder adhesion loss. We have followed required procedures for powder coating but with our best practices we still have a problem. Why is the powder coating chipping off? In one component the adhesion is good in all places but one particular place where it is developing a crack after one or two days. Is it because of material problem or of improper powder coating process? Is there any possibility of getting powder peel off due to stresses developed during metal forming? If yes please advise the solution. Adhesion loss is basically on a radius area where there are stresses during post forming. H.M.
A. Adhesion loss is usually due to inadequate pretreatment or under cure. Your surface is not clean enough or it is not prepared with enough roughness or a conversion coating that will enhance adhesion or you are not in the oven long enough. If the adhesion is inadequate, you will see more trouble along stress lines from post forming but you have a problem that is revealed by the forming, the stress is not the root cause. You need to check that your substrate is clean and properly prepared. That means you need to know the condition of the metal before coating and what steps are needed to make it receptive to bonding of the powder coating. It must be clean and the surface must have some texture imparted by a conversion coating or mechanical abrasion. If you are washing the parts make sure the water does not bead up when the part exists the washer. Check the surface with a white glove to make sure it does not have a lot of residual smut. You can test adhesion using ASTM D3359 and a tape pull test. You can get more quantitative results using a pull-off tester and dolly. You can also bend panels in a mandrel or do an impact test to see if it will take the stress of post forming. You should also make sure you have a good, flexible coating material that can take the post forming operation. Use some Bonderite 1000 pretreat panels to compare results to your own steel. Finally, check gloss, chemical resistance and hardness to make sure you have full cure.
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