Effect of Sulfate Ion on Corrosion of Carbon Steel in Carbonate/Bicarbonate Solutions

We have investigated the effect of sulfate ion on corrosion behavior of SM 400B carbon steel in carbonate/bicarbonate solutions at 343K by using a conventional potentiokinetic method with potential scan rate of 0.1mV·s-1. In the solution containing sulfate less than 0.001kmol·m-3, there are two active anode peaks and passive region. Addition of sulfate more than 1.0kmol·m-3 gave catholic current between the two active peaks and lowered pitting potential. The active peak current density at lower potential increased linearly with increase in equilibrium concentration of bicarbonate. This suggests to result from the formation of iron complex with bicarbonate ion for the active dissolution of iron. Cathodic polarization curves of Pt in the test solution showed that the increase in the concentration of sulfate led to the linear increase in catholic current around the potential between two active peaks. Accordingly, it is concluded that sulfate ion can act as oxidant against the natural corrosion of the steel in this environment. Pitting potential of the steel was lowered with increase in the concentration of sulfate in concentrated region. In the same concentration of sulfate, the pitting potential increase with increase in pH. The trend suggests the retardation of the pitting is due to the solution properties of high pH and high buffer capacity.