ABSTRACT
Sea salt deposition is the principal factor causing atmospheric corrosion of metals in coastal regions of Australia. In rain-sheltered positions of buildings and structures situated close to breaking surf very corrosive conditions may develop with time. A long-term industrially funded project is currently studying the comparative performance of generic classes of metals and coatings exposed to marine environments. The exposures involve the use of semi-enclosed glass shelters which prevent the specimens from being rain-washed. The exposure phase now exceeds 4.5 years and some products have already failed, particularly those exposed under the shelters. In order to more closely study the early stages of corrosion, complementary short-term (1–6 months) measurements of corrosion rates and microclimate were recently initiated at the most severe marine site employed, in the open and under the shelters. The materials exposed for corrosion rate measurements include ‘mild’ steel, zinc, and 5% and 55% aluminium-zinc alloy coated steel. In general, the results show significant increases in corrosion rate for sheltered specimens compared with those exposed in the open. This tendency can be observed even after one month. The corrosion rates decrease with time and this trend is in good agreement with earlier data for the same materials after 1 and 2 years. Relative humidity, temperature and time of wetness (TOW) according to ISO 9223 do not reveal any difference between the open and sheltered environments. In contrast, direct measurements of TOW using the WETCORR-technique clearly demonstrate the gradual accumulation of sea salt, particularly on sheltered surfaces, resulting in significantly longer TOW compared with the open surfaces. Additional tests have involved zinc panels that were sprayed once a week with natural sea water and exposed under the shelter at the mild site at CSIRO in Highett south of Melbourne.
