Implications of CaSO₄ scale growth on the corrosive response of carbon steel in acid media

A detailed experimental study has been conducted with the aim to examine the corrosion effects on a common C-steel electrode in H₂SO₄, in the absence and presence of CaSO₄-scale formation. The redox alterations are examined using electrochemical techniques, while the morphology and composition of the electrode surface were analyzed by scanning electron microscopy/energy dispersive X-Ray spectroscopy (SEM/EDS) and glow-discharge optical emission spectroscopy (GD-OES). The corrosion process in Ca⁺²-solution involves both the formation of a corrosion layer and CaSO₄-scales over C-steel surface, as confirmed by SEM-EDS analysis. The impedance results reveal that the accumulation of CaSO₄-scales does not modify the corrosion mechanism if compared against its absence. The interaction between Ca⁺² and SO₄ ²⁻ does not generate a sufficient protective coating of CaSO₄ along with the corrosion products at early stages of the growing scale, whereby there is a more important corrosion in the presence than in the absence of Ca⁺². After approximately 7.4 h, the stabilization of the scale formation provides a more compact and protective film, which increases the corrosion resistance if compared against the pure corrosive solution. Nevertheless, the films formed in both media gradually corrode with time due to the accumulation of ions producing more porous and less protective films regardless of the presence of the scale. These findings provide a new approach to account for mineral-scaling using electrochemical techniques, and open the possibility to link the growth crystals with the spontaneous electrochemical process.