Study on the Cut Edge Corrosion Behavior of Organic Coated Galvanized Steel by Scanning Electrochemical Technique

Abstract

The organic coated galvanized steels are widely used for automotive, home appliance and architectural industry due to their excellent corrosion resistance and cost effectiveness. The overall corrosion resistance of galvanized steel has been greatly improved by surface treatment and application of various organic coatings. Although the corrosion resistance of general surface is improved by coatings, the sheared edges of the coated steel sheet can be easily corroded because of direct exposure of bare metal at the edge to the corrosive environment. Corrosion at cut edges is the most critical failure mechanism of the organic coated galvanized steel. Since the corrosion mechanism of the cut edge of galvanized steel is quite different from that of the general surface, it is important to understand the cut edge corrosion behavior of galvanized steel with organic coatings. For the study of localized processes such as the galvanic corrosion at the galvanized steel cut edge, the modern electrochemical techniques such as scanning vibrating electrode technique (SVET) and scanning Kelvin probe (SKP) were used. These techniques are powerful tools to better understand the fundamental processes of corrosion at defects and underneath coatings. Basically, electrogalvanized steel (EG) sheets were used in this study. To investigate the effect of organic coating on the cut edge corrosion, polyester and polyvinyl-butyral (PVB) coatings were applied to EG. Phosphate pre-treated galvanized steels were used to examine the effect of interfacial properties on the cut edge corrosion. Time to red rust forming at the cut edge of each specimen were evaluated by a cyclic corrosion test (CCT) and the corrosion potential change in the surface of organic coated samples was measured by SKP. The current density change in the cut-edge measured by SVET was monitored to analyze the rate of metal loss at the cut edge. The basic analyses on the microstructure and morphology of samples were also carried out by optical microscope (OM), scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS).Firstly, to evaluate the effect of organic coatings on the cut edge corrosion, EG specimens with/without the organic coating were compared. The organic coated one shows longer time for appearance of red rust than the one without organic coating at the edge. The current density distribution at the edge of one-side organic coated EG was monitored by SVET to investigate the electrochemical activities at the edge. Zn dissolution rate at the edge of organic coated side was smaller than that of the other side without organic coating. The sacrificial protection for steel by Zn dissolution can last long with organic coating.Secondly, to investigate the effect of phosphate treatment on the cut edge corrosion, the organic coated samples with/without phosphate pre-treatment were evaluated. Without the organic coatings, the phosphate treated sample showed a poor corrosion resistance at the cut edges because the phosphate accelerated steel corrosion at the cut edge by inhibiting sacrificial zinc dissolution. On the other hand, when the organic coating was applied on the galvanized steel, the phosphate treatments provided the beneficial effect on the cut edge corrosion and organic coating delamination resistance since the phosphates improved adhesion between metal/organic coatings and inhibited transport of electrolyte cations through the metal/organic coating interface.In conclusion, this study has shown that the scanning electrochemical techniques of SVET and SKP are very valuable tools to understand clearly the local corrosion process occurring at the cut-edge of the galvanized steel. The phosphate treatment alone does not provide any beneficial effect on the cut-edge corrosion of the galvanized steel, but the phosphate treatment with organic coatings provides better corrosion resistance against the cut edge corrosion and organic coating delamination since the phosphate treatment imparts a strong adhesion between the galvanized steel and organic coating.