IMPACT OF THE MARINE ENVIRONMENTON THE SCALE DEPOSIT FORMATION:ROLE OF SULFATE AND THIOSULFATE

摘要

In seawater, during the application of cathodic protection on carbon steel, a calcareousrndeposit builds on its surface. This deposit formation is interesting because it contributes to thernprotective effect of cathodic protection; it is a barrier for oxygen diffusion.rnThe understanding of deposit formation and the influence of various parameters on itsrnproperties, like biological and chemical, is essential in the search for the improvement of thernperformances of the systems of cathodic protection.rnIn marine corrosion, principal biological parameters are due to the bacteria reducingrnsulfate and thiosulfate presence. To have a huge development, they use sulfate or thiosulfate.rnBacteria pollution depends on sulfate and thiosulfate concentration.rnSo, the aim of this study is to precise the role of sulfate and thiosulfate concentrationrnon deposit formation and its properties. The experiments are realised in artificial seawater tornset working conditions.rnWhatever the sulfate concentration is, calcium carbonate precipitates like aragonite.rnAfter three days, the deposit chemical composition is the same. In the first instants, arnmagnesium layer precipitates, on which the calcium carbonate settles. Only depositrngermination and growth, and thus the covering, differ with sulfate concentration. So, if thernsulfate concentration increases, the germ number decreases but the growth kinetic increases.rnIn the thiosulfate case, the same conclusions are established: no modification ofrnchemical composition, the calcium carbonate is present like aragonite on steel surface. Alsorncalcium carbonate germination and growth are changing with thiosulfate concentration.rnIn these two cases, sulfate and thiosulfate concentration modify the current density.rnThese chemical components contribute to oxygen barrier diffusion efficacity.rnThe barrier of the most effective oxygen diffusion is supplied by the deposit formed inrnASTM with a thiosulfate concentration of 4.09 g/L (Na2S2O3).