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Corrosivity surveys have demonstrated that in Australia marine environments represent by far the main corrosion hazard to metals and coated metal products. Also field inspection of some commercial product has revealed cases of very significant early corrosion in unwashed areas. Existing published data on the relative performance of metals and prepainted coated metals, especially as fabricated building products, is inadequate to meet the needs of those faced with decisions regarding product selection. A ten-year industrially funded project has been initiated to address this by studying the comparative performance of generic classes of metal products and alternative finishes, and this paper presents some interim results. Twenty-two commercial products based on galvanised steel, 5% and 55% aluminium-zinc coated steel, stainless steels and aluminium, were obtained from five countries, some prepainted with acrylic, polyester, silicon-modified polyester (SMP), or polyvinyldifluoride (PVF2) topcoats, and two with a polyvinylflouride (PVF) or polyvinylchloride (PVC) film laminate. All have been fabricated into ‘bent’ specimens, the test piece designed to simulate the types of distress metal products encounter during manufacture and building. Initial microcracking at bends and Erichsen domes was recorded photographically. A complex aluminium extrusion in four finishes was also exposed and measurements of initial colour and gloss were made for all specimens. Materials exposed for mass loss measurements include ‘mild’ steel, low-alloy copper-steel, zinc, 5% and 55% aluminium-zinc coated steel, and ‘wire-on-bolt’ assemblies. Specimens are exposed at three marine sites (severe, moderate and mild) both in the open and under a shelter designed to permit deposition of salt and particulates but prevent rainwashing. A glass canopy roof enables some UV degradation of the organic coatings. After one year the shelters have produced a remarkable deterioration of the coated steel products at the severe and moderate sites, the sheltered corrosion rates being up to seven times those in the open. Of the three coatings, the 55% aluminium-zinc consistently has the lowest corrosion rate which on average is about one-third that of zinc. Strong relationships have been established between the corrosion rates and the measured levels of airborne chloride.
atmospheric corrosion, corrosivity, marine environment, prepainted coated product, formed testpiece, microcracking, semi-sheltered exposure, ‘mild’ steel, zinc, aluminium-zinc alloy, aluminium extrusion
Senior Research Scientist, Commonwealth Scientific and Industrial Research Organisation (CSIRO), Division of Building Construction & Engineering (DBCE), Highett, Victoria
Technical Officer, CSIRO DBCE, Highett, Victoria