Adhesion Loss

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. 

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. 

Sealing Before Coating

Q: I manage a custom powder coating shop that does many different sizes and types of parts. We have a very large capacity 42×10-ft batch. We specialize in very heavy or large parts, although we do a lot of the same types of parts other batch shops do. 

The problem I would like to address is that of sealing the parts after blasting and before coating. We have a chemist here who made an acrylic sealer for us that we could spray on with an airless sprayer, which was very nice because we could work it in all of the cracks and joints and it didn’t tie up the same room in our shop as the actual powder coating. We felt like it had solved a lot of problems for us until we got complaints of chipping. Sure enough we went back and re-tested and we had adhesion loss. 

We then got back with the chemist who put two and two together and realized that it was not going to work, so he suggested an iron phosphate that they make for the shops that use steam cleaners. Our hope was that we could pre-heat the part and spray the phosphate on to it with the airless. The problem with this was that the phosphate solution would boil out of the cracks under the coating and make ugly marks so we had to quit using it as well. 

We have not pursued it any further to this point, but we really need something that we can spray on to seal the metal from exposure, especially in the cracks and joints, and that will give us an added layer of protection without double coating or priming, as the vast array of fabricators out there leave joints unsealed or if we happen to have a spot that is under mil thickness.We want to provide a top-notch product for our customers! Do you have any suggestions? 

A: Your first problem was asking a chemist to add 2 and 2. His answer was 2+2= “iron phosphate.” Funny, I always thought 2+2=4. In this case your chemist was right; iron phosphate is your answer. However, your method of application was all wrong.

Your problem was caused by using an airless paint sprayer to apply the undiluted chemical directly to the part’s surface. Without dilution, the iron phosphate coating would be streaky, at best. Without rinsing the part afterwards, the iron phosphate would leave a powdery residue on the surface. These results would not meet your top-notch product goal.

You want to buy a cleaner/coater with iron phosphate that can be used in a spray wand (hot-water pressure washer, not steam cleaner) pretreatment machine. This chemical is designed to be applied through the pump suction siphon (chemical or soap siphon) and mixed with water and then heated by the coil. All this occurs within the pressure washer unit. This chemistry will remove any oils and apply an even iron phosphate coating to the part. Finally, rinse the part with water, using the same pressure washer, by turning off the chemical siphon. Lastly, place this part into your batch oven for at least 30 minutes at 250ºF to completely dry the part before applying the powder coating.

Following this recipe will ensure that your customer obtains a high-quality product with good corrosion protection.