Effect of surface preparation on the protective efficiency and adhesion of organic coatings applied to steel

Surface preparation of metal prior to painting is one of the key factors in order to ensure consequent protective properties of coating. This is mainly done to produce effective adhesion between the coating and substrate. Interfacial adhesion can be improved through two general mechanisms; electrostatic bonds and mechanical interlocking. Electrostatic bonds between the polar coating and metal substrate can be improved by removing any significant (eg millscale) oxide layer and impurities. Surface roughening enhances the mechanical interlocking of paint within the surface irregularities. In general surface preparation of any kind is believed to improve the adhesion and consequent protection afforded by organic coatings. However recent findings have shown that the electrochemical properties of oxide film play an important role and also vary by the preparation method. In the present study 5 methods of surface preparation are applied to mild steel and the resulting surface has been characterized using various electrochemical techniques. Scanning vibrating electrode technique (SVET) has been used to characterize the metal surface. Metal surfaces are then coated with different coating systems and exposed to corrosive environment. Anti-corrosive properties were examined as a function of time. DC resistance method was used as the basic method for electrochemical resistance measurement. The mechanical wet and dry adhesion strength has been examined using pull-off adhesion method. Results have indicated that surface geometry governs the electrochemical activity of metal. Also it is suggested that surface activation without providing effective metal-coating bonds promotes corrosion. Theories of this breakdown mechanism are tentatively advanced.