Effect of the Coating Type on Cut Edge Corrosion Resistance of Galvanized Steel

Abstract

The overall performance of galvanized steel has been greatly improved by different surface treatments, various bath chemical compositions and application of organic coatings. In spite of improved surface corrosion resistance, the sheared edges of the steel sheet still suffer from corrosion attack. Since the involved mechanism in the cut edge corrosion is different from that of general surface, the approach of minimizing this problem has to be adjusted in order to satisfy both surface and edge resistance.When the cut edge corrosion resistance is evaluated, it can be interpreted by three main characteristics such as the time of corrosion commencement, degree of galvanic protection, and initiation site which gives an insight of further corrosion propagation. Therefore, the effect of the coating manufacturing method, aluminum addition, and phosphate pretreatment was investigated by employing potentiostatic polarization test to reveal coating dissolution behavior along with the initiation site of cut edge corrosion. The results of coating resistance were obtained from exposure to simulated marine environment in cyclic corrosion chamber. The following analysis of the coating deterioration caused by cut edge corrosion was performed with the aid of scanning electron microscope and Energy-dispersive X-ray spectroscopy.Consequently, it has been revealed that galvannealing process resulted in substantially reduced cut edge corrosion resistance due to the high iron content at the outer surface and presence of innate cracks in the coating. On the other hand, 55% of aluminum addition showed good corrosion resistance owing to slow degradation rate of the coating. Further, phosphate was found to inhibit zinc dissolution from the surface of electrogalvanized steel, which led to the accelerated zinc dissolution from the edges. In addition, the geometry of the sheared edge of organic coated specimen was shown to affect cut edge corrosion resistance through initial adhesion loss caused by cutting process.