Corrosion inhibition of mild steel by sodium tungstate in neutral solution Part 2: Behaviour in NaCl and Na 2 SO 4

Abstract

The corrosion behaviour of mild steel in freely aerated NaCl and 10⁻⁵–10⁻¹M NaCl and Na₂SO₄ solutions was studied by means of open circuit potential and other electrochemical measurements. The final steady state potentials (E_st) varied with the logarithm of molar concentration of aggressive ion according to E_st = a − b log C. The effect of adding concentrations C_inh of Na₂ WO₄ on the corrosion of the metal in both aggressive media was investigated at 25°C. The E_st–log C_inh curves in the presence of an increasing amount of aggressive ions were constructed. These showed two inflections in the case of NaCl and one inflection with Na₂SO₄, and followed the relation log C_inh =k + n log C_agg, where C_agg is the concentration of aggressive species. This reveals the occurrence of competitive adsorption of the two ions. It is argued that SO²⁻ ₄ ions are more aggressive than the Cl⁻ ion. Electrochemical measurements indicate that the log–log variation of corrosion current with inhibitor concentration at the points of inflection in the case of Cl⁻ additions also gives two steps, corresponding to those of the E_st–log C_inh curves. The effect of increasing temperature was to promote corrosion and retard the adsorption of the inhibitor. The low activation energy of inhibition supports the idea that the inhibitor is physically adsorbed.