Effect of different surface preparations prior to painting on the corrosion behaviour and surface activity of mild steel

Previous work in our laboratory 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. Particularly water jetting led to better performance. It is generally accepted that factors such as removal of impurities and introducing surface roughness to steel to obtain better adhesion through the mechanical interlocking of polymeric structure and the metal surface play a part. However it is postulated that the ease of breakdown of the oxide film is also critically important and varies between surface preparations. This was investigated in the present study where 5 different surface preparation methods (abrasive blasting, water-jetting, abrasion, acid pickling and degreasing) were applied to the mild steel surface and each resultant surface was characterised using electrochemical methods. The Scanning vibrating electrode technique (SVET) was performed in dilute saline water to detect anodic and cathodic sites on the surface and also to determine the intensity of electrochemical activity at these sites. Scanning electron microscope (SEM) together with energy dispersive X-ray spectroscopy (EDX) was used to characterise the surface structure and its chemical composition. Changes in open circuit potential have been monitored during the time of exposure to electrolyte in order to investigate the general surface activity. The electrochemical impedance spectroscopy (EIS) was employed to analyse the charge transfer situation on the metal surface. Results using these techniques will be presented and discussed. An anticipated outcome from this work is development of a simple electrochemical method to assess that a metal surface is in a condition suitable for coating. This could also be used as a way of checking that standard pretreatments had been effective and assist in the development of new ones