Aluminum Adhesion

I read with interest Rodger Talbert's response to the individual having difficulty with powder adhesion on aluminum (Powder Q&A, June 2006). Talbert is correct in referencing conversion coatings as the preferred chemical pretreatment, as well as the wet adhesion test as an indicator for pretreatment/powder performance. However, he missed several important factors that can influence the adhesion of the topcoat on aluminum.

Aluminum cannot be converted with an iron phosphate because it does not contain sufficient Fe to complete the process. The purpose of the phosphate is to micro-etch, or deoxidize, the surface to form a profile for the powder to adhere to the aluminum substrate. Using fluoride within the product controls the etch rate on aluminum. The problem with using a floride additive is in the control; use too much, and you can smut the aluminum, which can lead to premature powder failure.

As described, aluminum cannot be converted with an Fe phosphate, so the unreacted phosphate salts remain on the substrate. Unless sufficient rinsing follows the "etch" process, the unreacted salts will interfere with the adhesion of the powder.

Compounding this issue is the use of a seal as the final rinse. Seals inherently raise the total dissolved solids (TDS), which directly impacts the adhesion of the powder. If the seal is alkaline in nature, it then becomes "powder-specific," meaning that not all powders adhere to a alkaline surface. Since the user in this case is not converting the substrate, the seal is unnecessary. If the process is correct, seals should not have to be used; a deionized (DI) halo rinse should give substantially better adhesion performance than any seal.

Talbert indicates that a high level of TDS remains on the aluminum prior to powder coating. The basis for this is that the powder coming off in "large pieces." In an undercure situation, a higher gloss in areas on the part would be evident. Conversely, overcuring would be indicated by "dulling" of the powder.

Marc Oprean, Sales Engineer, Hawkins, Inc.