Electrochemical characterization of mild steel after different surface preparations

Chemical and mechanical surface preparation methods have been commonly used to remove impurities and oxide layers from the steel surface prior to painting and enhance the protection afforded by coating. The level of adhesion between the coating and substrate is a key factor in maintaining protection. Mechanical interlocking of polymeric structure into the metal surface and electrostatic bonds between coating and substrate are two of the mechanisms involved. The surface structure resulting from each surface preparation method gives different geometry and electrochemical properties to the surface. Recent studies (by Chen et al [1]) have shown that the geometry and depth of surface irregularities of steel play an important role in pit forming and the localized corrosion. Also previous work in our laboratory [2] showed that the type of surface preparation of the steel prior to coating had a dramatic effect on the subsequent corrosion protection afforded in immersion tests and in salt spray testing. In the present work 5 different surface preparation methods (abrasive blasting, water-jetting, abrasion, acid pickling and degreasing) have been applied on the mild steel surface. The electrochemical activity of the resulting surface in the dilute saline water has been characterised using scanning vibrating electrode technique (SVET) to locally detect anodic and cathodic sites on the surface. Scanning electron microscope together with energy dispersive X-ray spectroscopy has been used to characterise the surface structure and its chemical composition. Also the general corrosion behaviour and the overall susceptibility to localised corrosion have been studied using electrochemical noise measurement. Results have shown significant variability between the different surface preparations. Some explanations for these differences will be advanced.