Powder coatings have attracted attention as environmentally friendly coatings. Development of powder coatings with an excellent corrosion resistance and weathering resistance and determination of a method of evaluating these coatings must take place rapidly. In this research, the effects of different kinds of coating films (resins and pigments) on the corrosion resistance of powder-coated steel sheets were evaluate using an Ac impedance method after conducting accelerated weathering tests and outdoor weathering tests.
By means of Cole-Cole plots, the size of the capacitive semi-circle was reduced as the coating films continued to deteriorate, and the effects of corrosion below film caused by the diffusion of dissolved oxygen were noted. The resistance of powder-coated steel sheets after accelerated weathering tests and outdoor weathering tests dropped with a long exposure time. The drop rate was lower with resin specimens for coating films with substantial weathering resistance than with resin specimens for coating films with slight weathering resistance. Furthermore, the rate was lower for specimens to which TiO2
white pigments had been added than for specimens of azo red pigment. When the moisture permeability and oxygen permeability increased, the resistance of the powder-coated steel sheets dropped. Furthermore, the coating film surface displayed progressive deterioration with a long exposure time, and the deterioration for coating films containing red pigments was more noticeable than for coating films containing white pigments. In accordance with the XPS method, the production of ketone, carboxylic acid, etc. can be expected to occur as the deterioration of the coating film surfaces progresses.