Keywords: Coil coating , Cut edge corrosion , Field Performance , Chromate free Primers, Cyclic tests , Continuous Salt Fog , Service life prediction methodology.
Since most coil-coated products are used in non-marine environments it is clear that traditional methodologies should not solely be relied on as an assessment tool for coating systems. Further examples from natural exposure tests provide evidence that the nature of electrolytes, the influence of wet/dry cycling stresses, coating photodegradation and coating physicomechanical changes can all participate in corrosion failure modes and mechanisms. The importance of understanding the application and design variables for the coated product is also discussed and consideration given to how to include these in the laboratory testing protocols. It is crucial both to identify and test the role of each of these parameters in establishing the overall corrosion performance of a system. This necessitates adopting accelerated tests, which focus on one or another of the specific degradation mechanisms. It is felt unlikely that any single accelerated test will be able to provide a complete and realistic prediction of service life despite the moves within some current and proposed international standards to try to do just that. Today, new demands in the industry mean the development of more environmentally friendlier coatings which, in turn mean a move away from traditional pretreatments, and both hexavalent chromium primers and thick film polyvinylchloride systems which have for so long been the mainstay for corrosion protection. Indeed one of the features of hexavalent chromium is its excellent performance in the salt fog test and one of the drawbacks of the newer chromate-free primers, is their often, poorer performance on the same test thus condemning a new technology. To avoid erroneous and untimely conclusions being drawn a new approach to performance assessment has had to be derived alongside the development of these new technologies. The inclusion of cyclic testing techniques in addition to the continuous tests is shown to be an important feature of this new approach. Techniques such as Prohesion®, PUCAT testing , CCT testing  and a combined Salt Fog/UV Exposure test  help to broaden the means by which to simulate exterior performance. However, like a jigsaw puzzle, it is the combination of the results from all tests (including salt fog !), that enable the formulator to gain the overall picture of performance. Each piece is vital to complete the picture, but on its own, it can not only be uninstructive, but potentially misleading. Finally a methodology for corrosion performance prediction is proposed based on one suggested for the automotive industry . This entails the integration of results from three sources ; first that from standard, natural exposure sites, second, from a broad spectrum of laboratory accelerated performance tests and third, that from fundamental property measurements. The latter may involve measurement of the electrochemical, adhesive, or film porosity/barrier characteristics to name but a few. By integrating the responses from all three sources into a central model, it then becomes possible to both predict service life in untested environments and to understand and rectify early failures. This approach represents a significant change from the traditional one which regarded a limited number of severe outside tests as the final arbiter of performance and in which the accelerated test was often seen as a means of representing the real world in all its many guises. The objective of this paper is not to promote any single new test to replace the salt fog procedure, but rather to argue for this more coherent approach. Once the behaviour and responses to the various outdoor corrosion mechanisms have been understood and simulated, a model can be constructed which then has the potential to provide a rapid, realistic and reliable prediction of performance.
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