Technical Article: Powder Coating Defects

What Went Wrong . . .
A component used on recreational vehicles was experiencing sporadic defects in the powder coat finish. The part was reported to be phosphate pretreated using a spray application process, and was subsequently coated with a polyester-based powder paint material. The defects became apparent during the final quality inspection after the powder coated part had been subjected to the required oven cure process.

Inspection and Testing . . .
A visual examination of the powder coating defects revealed characteristics of outgassing. Pretreated parts exhibited features indicative of non-uniform phosphate application. Microscopic inspection of the coated parts revealed that the defect center exhibited a region void of coating. Phosphate coating weight was determined on pretreated parts, and confirmed that the coating weight was below that typically specified for this type of application. As-fabricated, pretreated, and coated parts were further examined in the scanning electron microscope (SEM) to more fully characterize the respective surfaces. The morphology of the defect center was consistent with the as-fabricated and pretreated steel part surfaces, and exhibited apparent porosity in the steel surface. Energy dispersive x-ray spectroscopy (EDS) identified the presence of carbon in the surface pores, likely in the form of retained fabrication oils. Fourier transform infrared spectroscopy (FTIR) confirmed the powder coating material was polyester-based, and did not reveal evidence of bulk contamination within the coating on the parts. Analysis of the as-fabricated parts determined that hydrocarbon-based oils, likely containing wax, were used during the manufacturing operation. Using an acetone extraction technique, pretreated parts were further evaluated by FTIR. The presence of hydrocarbon-based materials, such as mineral or paraffinic oils and wax, was confirmed.

Conclusion . . .
It was the conclusion of the investigation that the powder coating defects were consistent with outgassing. Specifically, the steel used to fabricate the components exhibited surface porosity which entrapped fabrication oils. The spray pretreatment process utilized was unable to remove the trapped oil, and was found to produce a very non-uniform, less than ideal phosphate coating. This oil subsequently volatilized during the powder coating cure process, causing a rupture in the applied powder film which was unable to recover.

	Electron image of a typical defect showing regions of coating voids along with apparent steel substrate roughness. (SEM Original Mag. 100X)
	Pretreated part surface exhibiting non-uniform phosphate coating application. (Original Mag. 7.5)
	The acetone extract FTIR results of the pretreated parts produced results consistent with a hydro-carbon-based material, such as mineral oil or wax.