Q: Water-jet metal cutting uses silica sand (silicon dioxide, SiO2), and I have adhesion problems around the edges of steel that have been cut this way. The flat areas are fine, no issues with cure and adhesion, but the cut edges are 10-mm thick and peeling off.
The metal was washed with hot iron phosphate water solution, rinsed, dried, and primed before the topcoat! It was not rusty and came from the water-jet cutter clean and rust free and was powder coated the same day. Could this type of silicon cause adhesion problems (and fisheye craters)?
A: It requires a bit of digging to understand what the cause of the adhesive failures and fisheyes could be in your powder coating. On the surface (pun intended), what could be a problem? Water shouldn’t cause an issue with adhesion or cratering. In addition, silicon dioxide is inert and is not known to cause fisheyes or craters. And you report that the surface is not corroded when you apply the powder coatings.
Here’s where I would look for the root cause and hopefully the eventual solution: dispersion of the SiO2 in the water most probably requires the use of a surfactant; otherwise, the silicon dioxide would not mix well into the water and the process would be ineffective. In addition, the use of a surfactant to disperse the abrasive may require a defoamer to minimize foaming. Surfactants and defoamers are known to cause adhesion and cratering issues in powder coatings. The use of a high-pressure stream of abrasive slurry would only exacerbate the problem by embedding the surfactant/defoamer into the steel surface.
I would guess that the surfactant cannot be eliminated from the slurry. Perhaps there is an alternate surfactant that doesn’t cause this problem, but I would not expect this to be the case. Consequently, you will need to identify a method that can aggressively remove residual surfactant on the machined edges. Two paths are possible: (1) use mechanical means to abrade the edges (sanding, grinding, etc.) or (2) use a strong acid to etch the edges.
The mechanical approach is either labor intensive or costly to automate. The chemical approach is expensive and requires the handling of a hazardous material. No easy answers here. Alternately, you can consider using a different cutting technique, but you’re probably also aware that laser cutting can cause similar adhesive failures for a different reason.