Q: We are powder coating mild steel articles as follows: mild steel article + zinc plating (10 microns) + zinc phosphating + pure polyester powder (70-80 microns). Adequate quality parameters are maintained for plating, phosphating, and the powder coating line.
The primary adhesion is satisfactory. Components when exposed to a Salt Spray Test (ASTM B 117) or to a humidity test (46 ± 2°C, 95% relative humidity) show blisters on the powder after about 150 hours. The secondary adhesion fails in this case.
What are the probable reasons and solution(s) to this problem?
A: It would be helpful to know if you are using a TGIC-based polyester or an HAA-based polyester, as these products behave differently. TGIC polyesters typically exhibit better corrosion resistance than HAA polyesters.
First of all, adhesion to zinc plating is more difficult than adhesion to mild steel. The plating is less reactive and less porous than conventional steel. The use of zinc phosphate may not be the best to achieve adhesion of the polyester powder.
A chromate pretreatment will probably produce better results; however, there may be toxicity issues and local emissions regulations that may affect your decision to use chromate. Before using the chromate, you may have to activate the surface. Cyanide deposition techniques usually call for a nitric acid dip activation, whereas chloride zinc processes work better with a hydrochloric acid activation. I would check with your chemical pretreatment supplier for their recommendations on products they offer for activation and chromating.
If you cannot use chromate due to environmental regulations, I would approach a couple powder coating suppliers for their solution to this problem. In general, TGIC-based polyesters work better than HAA types. And epoxy-based powders work better than polyesters.
In addition, higher grade powders (costlier) may work better. Low-cost powders that contain high levels of cheap fillers will undoubtedly fail adhesion and salt fog resistance.
One other approach to consider would be preheating the parts prior to powder coating application. This can sometimes eradicate an adhesion problem.