Use of electrochemical methods to examine different surface preparation methods for organic coatings on steel

For preparation of steel surfaces before applying an organic coating, waterjetting is a modern, effective and environmentally friendly method. However there is a lack of data on its performance in terms of the subsequent protection afforded by the paint compared with more conventional methods. Hence previously exposed steel panels had their surfaces prepared in a variety of ways (abrasion, blasting with garnet entrained in water (10 K psi) and waterjetting without garnet at two velocities (20 K psi and 40 K psi)) prior to coating with two typical maintenance coatings. These were then exposed under immersion condition in the laboratory and monitored using the DC resistance method. Another set of samples were exposed in a salt spray test in accordance with ASTM B 117 for 500 h. At the end of both tests DC measurements were carried out and the samples were inspected visually in terms of blistering and rust. Results showed that both the immersion test and the salt spray test differentiated the surface preparations methods in the same order. Thus control panels performed the best. Nearly as good though were the two (20 K psi and 40 K psi) sets of waterjetted samples. The abraded samples were variable but overall provided less protection. The panels blasted with garnet entrained in water (10 K psi) samples proved to be the worst. Characterisation of the surfaces prior to coatings using the scanning electron microscope was also conducted. A tentative explanation for the differing protection afforded involving the integrity of the oxide film was suggested. Further work is needed to substantiate this. Overall, with samples in the immersion test, results using electrochemical methods proved themselves to be very effective in assessing and even predicting performance in a relatively short period of time. The results also correlated with the salt spray test. Also the work showed that waterjetting is a very effective way of preparing the surface prior to maintenance painting