Evidence of gas permeation in electrostatically painted steel cylinders

The process of gas generation, entrapment and subsequent permeation through coatings applied on the surface of metals for corrosion protection purposes is the cause for the development of a defect known as “fish-scaling”. Particularly, in operations like enamelling or painting of metals, this phenomenon affects strongly the surface quality of the items and the development of further processing operations. In this work, the reasons that promote the development of defects in electrostatically painted steel during the manufacture route of fire extinguishers are analysed. The surface and cross section analyses of the samples conducted by optical and scanning electron microscopy suggest that the degradation of the coating is caused by gas migration from the metal-coating interface to the surface of the cylinders. Water used during cleaning and descaling operations acted as a potential gas source being hydrogen the element responsible for the formation of both, structural defects in the coating and cracks in the steel substrate. The degradation mechanisms of the steel and the coatings are found and explained in terms of manufacturing process of the cylinders.