Electrochemical Corrosion Behavior Of Carbon Steel Under The Combined Electrolyte Layer Of Open And Confined Layer

Atmospheric corrosion is the most common form that metal suffers corrosion and degradation and its essence is an electrochemical corrosion process occurred under a thin electrolyte layer. In order to prevent the metal from atmospheric corrosion, most of metallic materials or constructions have been applied some anti-corrosion method, among them organic coating is the protective approach that has been most used and is most efficient and economical. However, organic coating is easily broken during the transport and application, which can cause the local exposure of metal matrix under the coating. On the surface of exposed metal the electrolyte layer formed is an open form due to its surface being directly exposed to atmosphere, but because of capillary effect the surface of metal under the coating around the exposed metal can be also covered by an electrolyte layer that is a confined form, therefore the region around the broken coating, there is a combined layer of electrolyte consisting of an open layer connecting a confined one. However, it is still lack of systematic study on the corrosion behavior of metal under this kind of combined layer of electrolyte, by far.Basis on the analysis above, by designing and building up a novel electrochemical cell, and combined the electrochemical impedance spectroscopy, polarization curve, morphology observation and composition analysis of corrosion products, the corrosion behavior of carbon steel under the combined layer of electrolyte of 3 wt% NaCl consisting of an open electrolyte connecting with a confined one was investigated as well as the corrosion behavior of epoxy coating/carbon steel under the dry-wet cycle. The main results as follows:(1)A novel electrochemical cell that can simulate the corrosion environment of metal under the combined layer of electrolyte consisting of an open layer connecting a confined one was designed and built up, which can control both of the thickness of open and confined layer, and be used for electrochemical measurements of corrosion of metal under the combined layer of electrolyte.(2)By using the electrochemical cell the electrochemical corrosion behavior of carbon steel under the combined layer of electrolyte of 3 wt% NaCl was studied. In the range of thickness of layer investigated (50μm-410μm), the corrosion of carbon steel under the open layer and the confined layer were both controlled by the oxygen diffusion. Corrosion rate of carbon under the open layer increased firstly with the decrease of layer thickness and reached the maximum value at the 80μm, and then decreased with the thinning of layer. Under the confined layer, corrosion rate of carbon steel increased with decreasing the thickness of layer.(3)The dry-wet cycle corrosion behavior of carbon/epoxy coating system under the combined layer of 3 wt% NaCl electrolyte was studied. The corrosion rate of the exposed carbon steel reached the maximum at the 4th cycle, but the corrosion rate of carbon under the coating reached the maximum at 6th cycle, and corrosion rate of carbon steel under the coating was always lower than that of the exposed carbon steel.(4)In the system of epoxy coating/carbon steel, corrosion products of exposed steel were the layered structure. The outer layer containedδ-FeOOHå'ŒÎ³-FeOOH, but the inner, besidesγ-FeOOH there wasα-FeOOH. Theα-FeOOH in the inner layer was transmitted form theγ-FeOOH due to that Fe2+ could be adsorbed on theγ-FeOOH and causedγ-FeOOH to beα-FeOOH in the acid environment.(5)Because the exposed steel and the steel under the coating was respectively under the open layer and confined layer, the difference in corrosion behavior and ions migration and diffusion between the layers resulted the different corrosion morphologies between the steel under the open layer and confined layer. The corrosion products onto the exposed steel present the layered ladder morphology from the centre of exposed steel to its periphery, however, on the steel under the coating the corrosion product has a radial ripple morphology, probably owing to the directional diffusion and migration of ions appearance.