Adhesion to Stainless Steel

Q. I am the design and quality assurance engineer at my plant. We are a door and window manufacturer that utilizes a Type 409 stamped stainless steel sheet for our door kick panels for dent resistance. We perform a prewash to clean with RO water, etch the panel with a commercially available chemical to roughen the surface, a second rinse and then apply a dry-on adhesion promoter. Polyester paint is applied by an electrostatic wet spray process. The panels are then run through a bake oven for?10–12 min at 350ºF.

Initial AAMA testing shows that good adhesion is achieved. Ongoing testing reflects satisfactory results. We have always experienced field failures of this product. We have made some process improvements over the last year but are still plagued with paint peeling from the kick panels. These panels are siliconed into a painted aluminum frame before snap-in aluminum rails are installed to retain the panels. We have in the past seen a silicone contamination issue and it did cause some fish-eye problems in our aluminum painting process, but nothing that affected adhesion. Our paint and pre-treatment suppliers are in agreement with our current processes. They feel that our process is sound and have no explanation for why we would be having difficulty.

Could the issue be with the dissimilar metals even though there is theoretically no direct surface contact between the stainless and the aluminum? They are both coated with the same material.

Dyne testing shows that the panels after treatment are at a 70. Our coating supplier reveals that a 20–30 is all that’s need for good initial adhesion.

Do you have any suggestions as to what the cause might be? Anything new to the industry that we should investigate? We are looking for longer-term adhesion. Would sanding the surface with 220-grit sand paper help? Sanding would be done before application of the adhesion promoter and perhaps without its use. T.B.

A. You seem to be pretreating the stainless steel correctly. The only addition I would suggest is an aqueous alkaline or detergent cleaner stage before the RO water wash, to remove any oily soils. Sanding instead of chemical etching would not help because both are done to roughen the metal surface and the etching process is less labor-intensive than sanding. Furthermore, if you sanded the surface after application of the adhesion promoter, it would be removed. I don’t know what you are using, but do you really need the dry-on adhesion promoter? If your polyester enamel is formulated to be applied direct-to-metal and doesn’t need a primer, why use an adhesion promoter? Obviously it’s not helping. It may be contributing to or causing the problem.

The other thing that concerns me is the use of a silicone adhesive. If this operation is anywhere near the paint spray booth, you could have silicone contamination on the stainless steel surface. Worse than that, a little silicone goes a long way. I remember a situation in which airborne silicone micro- particles were transported several hundred feet through a shop by the ventilation system and drawn into a paint booth, thereby contaminating surfaces to be painted. As you pointed out, you did have fisheyes in the paint on aluminum parts. If silicone got on the aluminum parts, what makes you think it wouldn’t get on the stainless steel parts?

If there is any surface contamination which acts as an interference coating (interfering with adhesion), the Dyne test fluid should show it. When you do the Dyne test, do you check the same area on every panel or do you check multiple areas? Do you test immediately before painting? If not, you should. Although this test was designed to test plastic surfaces, it should work on metals. After all, a Dyne/cm is a Dyne/cm; surface tension is surface tension; and surface energy is surface energy.
For the electrochemical reaction (between dissimilar metals) to occur you must have contact and/or an electrolyte.