Study on corrosion protection of organic coatings using electrochemical techniques: developing electrochemical noise method, effective of surface preparation and inhomogeneity of organic coatings
This study looks into two important aspects of corrosion protection of steel by organic coatings, steel surface preparation and ionic conduction through the coating, as well as development of the electrochemical noise method as an effective assessment method. Surprising and somehow controversial previous findings at the University of Northampton showed an inverse relationship between the roughness of metal substrate and performance of paint coating. So this study was initially launched to further study the effect of metal surface preparation. Four conventional surface preparation methods including ultra high pressure (UHP) hydroblasting, wet abrasive blasting, acid pickling and emery abrasion were studied and compared to an as received control surface. A particular interest of this work was the high demand for an environmentally friendly surface preparation method, e.g. as afforded by UHP hydroblasting, to replace the traditional wet abrasive blasting method. Results of this study revealed the important role of the innate native oxide film and the deleterious effect of contaminants on the protective performance of organic coating. Also it was shown that a highly active surface and large surface profile can be deleterious if an appropriate interaction between paint and metal is not achieved. Results of this study confirmed the earlier findings and suggested the UHP hydroblasting is a successful, cost effective and environmentally friendly surface preparation method and a modern replacement for wet abrasive blasting method. In addition to the effectiveness of metal surface preparation, the ability of organic coating in preventing ions access to metal plays an equally important role in defining the anti-corrosion performance of a coated metal. Hence the mechanism of ionic conduction through organic coatings and their inhomogeneity which are normally formed in crosslinking systems was extensively studied with the aim of finding the cause of formation of the more permeable areas and the ways by which they can be prevented. Several structural and environmental parameters were examined including the coating thickness, multi-layer paint application, curing temperature, partially non-functional resin, pigmentation and solvent. Experimental results showed that the solvent degree to which can escape, the non-functional polymer parts and inherently hydrophilic functional groups of organic coatings are the main parameters causing inhomogeneity and highly ion permeable areas. A statistical model was also developed that can be used to estimate number of permeable areas or corrosion initiation sites in a large area of coating. A particular concern of this work throughout the entire study was development of the electrochemical noise measurement (ENM) in the sense of a good assessing technique for protection efficiency of a coating system. Previous studies have shown great potential of ENM as a practical technique in the field. However, the technique always involved measuring the electrochemical noise between two or three isolated electrodes which cannot be easily provided in certain applications such as submerged structures or inside storage tanks. Also involvement of two or three electrodes in the measurement induces an ambiguity in regards to which electrode dominates the result. Here an attempt was made to perform the noise measurement on a single working electrode so that it can be used in more practical situations. Preliminary results indicate this approach holds promise.
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