Effect of Steel Surface Variation on Electrochemical Heterogeneity and the Protection Afforded By Organic Coatings

A well-prepared surface is the foundation on which an effective protective paint system is built. Most adhesion theories consider a rough surface to be favourable for achieving a high adhesive strength with a given organic coating. However, results of some case studies have found no relationship^1,2 or inverse relationship^3,4 between corrosion protection properties of the organic coating and the surface roughness. In the present work four conventional surface preparation methods including ultra high pressure (UHP) hydroblasting, wet abrasive blasting, acid pickling and abrasion with emery were used and compared to an as received control surface. Scanning vibrating electrode technique (SVET) was used to study the formation of active anodic sites on a local scale. Multiple repetitive scans were performed to study the dynamic of anodic activity. Figure 1 shows the variation of anodic activity by type of surface preparation in the early stages of corrosion. The level of surface activity was compared with the surface roughness and surface free energy. Electrochemical Impedance spectroscopy (EIS) and Open Circuit Potential (OCP) methods were used to study the effect of surface preparation on electrochemical properties at the surface as well as at the interface.

Results revealed the important role of the native oxide film and the deleterious effect of surface contaminants on the protective performance of the subsequently applied organic coating. Also it was shown that a large surface profile combined with a highly active surface can be deleterious if an appropriate interaction between organic coating and metal is not achieved. This is due in part to surfaces with coarse surface profile presenting a larger active area. It appears that the number of initial anodic sites plays a key role in the long term anodic/cathodic activity and this number varies by the surface preparation method.

Figure 1 Local distribution of current density for (a) as received (control), (b) abraded, (c) acid treated, (d) hydroblasted and (e) abrasive blasted samples after 5 min immersion in 1 mM NaCl.

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2.            A. Momber, S. Koller, H. Dittmers, J. Protect. Coat. Lin., 44–50 (2004).

3.            D.M. Santágata, P.R. Seré, C.I. Elsner, A.R. Di Sarli, Prog. Org. Coat., 33, 44–54 (1998).

4.            D.J. Mills, K. Schaefer, Prog. Org. Coat. 69, 193–198 (2010).