The saltspray test is still today often utilized to control the anticorrosive performance of paints applied on metallic substrates. The time required for this test method is much shorter than that necessary to carry out the atmospheric exposure test some weeks instead of many years. Nevertheless the results provided by the salt spray test are only qualitative and their extrapolability to the real behaviour of the paint system's filmmetallic substrate is very difficult. Moreover, in some cases there is a discrepancy between the results of the salt spray test and the reality for instance, it was observed some time ago that hotdip galvanized steel constitutes a better substrate for paints than bare steel, whereas the salt spray test results indicate the contrary. The present work represents only a part of a more conspicuous set of observations devoted to a comparison of laboratory and field results. In this first stage we examined, by means of salt spray and total immersion tests, the behaviour of bare steel and hotdip galvanized steel substrates, both phosphatized and coated by the same paint. The aim of this work was to ascertain if the results of the two methods are in accordance and, moreover, to compare the qualitative information given by the salt spray tests with those, quantitative, obtained by the electrode impedance technique on specimens immersed in 3 NaCl solution. By means of this technique it is possible to determine at the same time the values of the paint film resistance, of its capacitance, of the polarisation resistance and of the double layer pseudocapacitance related to the corrosion taking place on the metallic substrate and to ascertain the eventual intervention of diffusive phenomena into the corrosive process. Finally, this electrochemical method, because its nondestructive nature, permits one to observe the variation of the values of overmentioned parameters as a function of time and therefore to interpret the evolution of the corrosion process.
Anti-Corrosion Methods and Materials – Emerald Publishing
Published: Jun 1, 1991