Electrochemical investigation on effect of sodium thiosulfate (Na2S2O3) and ammonium chloride (NH4Cl) on carbon steel corrosion

The effect of sodium thiosulfate (Na2S2O3) concentrations (0.01, 0.1 and 1 M) on carbon steel corrosion behavior in 3.75 M ammonium chloride (NH4Cl) solution was investigated. The potentiodynamic polarization results show that the i(corr)-increases from 8.7 x 10(-6) A cm(-2) to 7.2 x 10(-4)A cm(-2) when thiosulfate (S2O32-) concentration increases from 0 to 1 M. Electrochemical impedance spectroscopy measurements also mimic the same trend. However, it is also observed that the i(corr)-value starts decreasing when S2O32- concentration equals or exceeds the NH4Cl concentration (Cl-:S2O32- > 1:1) in the electrolyte solution. The polarization behavior and field-emission scanning electron microscopy images reveal that the carbon steel undergoes general corrosion at lower concentrations (0.01 M S2O32- and 0.1 M S2O32-) and, both pitting and general corrosion at higher concentration (1 M S2O32-). Based on the experimental results, the mechanistic reaction pathway, which involves two dissolution paths for carbon steel dissolution is suggested. (C) 2018 Brazilian Metallurgical, Materials and Mining Association. Published by Elsevier Editora Ltda.