Inhibition of pitting corrosion of AISI 420 (40X13) stainless steel in low-mineralized water by sulphate ions

Peculiarities of the pitting corrosion occurrence on AISI 420 (40X13) ferritic stainless steel in low-mineralized water with a composition close to river and tap water are considered.Using the potentiodynamic polarization curves measurements (0.1 mV/s), the dependences of the pitting potentials on the chloride concentration were obtained, as well as in the presence of sulfate and carbonate ions in a solution.In a certain area of concentrations, the dependence of pitting formation potentials on the logarithm of chloride ions concentration in the electrolyte has been found.An insignificant effect of bicarbonate on pitting corrosion of ferritic stainless steel was discovered.On the contrary, the presence of sulfate ions in solution together with chlorides shifts the pitting potential to more positive values.Their inhibitory effect is the stronger, the lower the concentration of the initial chloride in the aqueous medium.Moreover, the repassivation potential is practically independent of the sulfate concentration.It has been found that the maximum safe chloride concentration for solutions containing 50 and 100 mg/L [ 2 4 SO -] can be considered, respectively, 10 mg/L and 20 mg/L [Cl -], which is an order of magnitude greater than in the case of pure chloride solutions without additives.This dependence indicates the existence of a thermodynamic adsorption equilibrium in the case of competing adsorption of chloride and sulfate on the oxide surface.A significant part of the researches are devoted to the inhibition of pitting corrosion of structural alloys by organic inhibitors [1][2][3][4].However, inorganic anions can act as inhibitors for stainless steels [5].In the current research, the possibility of neutralizing of chloride ions negative effects on pitting corrosion by other components of river water, for example, sulfate and carbonate ions, has been studied.