Role of sulphate ions on the formation of calcareous deposits on steel in artificial seawater; the formation of Green Rust compounds during cathodic protection

Cathodic protection of steel in seawater could be optimized by taking into account the calcareous deposits forming on the steel surface. The aim of this work was to study the influence of sulphate ions on the kinetics and mechanisms of formation of these deposits. The experiments were performed at 20 °C, with an applied potential of −1.0 V/saturated calomel electrode (SCE), in artificial seawater-like solutions with various SO42− concentrations. The deposition was monitored by chronoamperometry and electrochemical impedance spectroscopy (EIS). Micro-Raman analyses were performed in situ in a specific electrochemical cell to identify the solids forming on the steel surface. It could be demonstrated that sulphate ions had an important effect on the formation of both Ca(II)- and Mg(II)-containing phases. In the solution enriched with sulphate ions, the deposition of CaCO3 was almost totally inhibited. Experiments performed in Ca2+-free solutions demonstrated that the Mg-based deposit was, in contrast, favoured by the increase of the sulphate concentration. In situ Raman spectra of the solid forming at the early stages of the cathodic protection proved to be characteristic of Green Rusts (GRs). This compound was favoured by the presence of Ca2+ and/or Mg2+ cations, and is more likely a GR-like M(II)-Fe(III) hydroxysulphate, with M = Fe, Mg and Ca.