Open circuit potentials of metallic chromium and austenitic 304 stainless steel in aqueous sulphuric acid solution and the influence of chloride ions on them

Open circuit potential measurements and cyclic voltammetry of chromium and 304 stainless steel in deaerated aqueous H₂SO₄ solution of pH 1, without and containing NaCl in the concentration range 1–4 M revealed that chromium exhibits two stable open circuit potentials both having the character of a Wagner–Traud corrosion potential. One, E_corr.1, was established on the passive surface formed by previously exposing Cr to air or by potentiostatic passivation in a controlled manner, and the second one, E_corr.2, at the bare Cr surface obtained by prolonged cathodic activation. There was a small difference in the E_corr.1 values depending on the properties of the passive layer. Addition of NaCl accelerates to some extent, the hydrogen evolution reaction on the passive surface, while the same reaction on the bare surface was somewhat inhibited by NaCl. On the other hand, presence of NaCl accelerates the anodic reaction on the bare surface, and it activates the dissolution of the passive layer so that the passivation currents increase with addition of NaCl. This effect is so large that at concentrations of NaCl larger than 3 M, the destruction of the passive layer was so fast that in a matter of seconds the Cr is activated, and the only stable corrosion potential observed was E_corr.2. No pitting of Cr in the presence of NaCl was observed up to the transpassive potentials. 304 stainless steel could not be activated in sulphuric acid solution, and its open circuit potential, unlike the corrosion potential of the 400 types of stainless steel, was established by the hydrogen evolution reaction on the passive steel surface. The small anodic peak often observed on 304 stainless steel if the metal had been cathodically pre-treated is a pseudo-peak due to the anodic oxidation of hydrogen absorbed inside the metal. This finding should be elaborated more in recommendations (e.g. ASTM standards) for the application of electrochemical corrosion rate measurement to 304 stainless steel corrosion. Addition of NaCl activates the anodic dissolution of steel with the current of the passivation peak being proportional to the NaCl concentration. Unlike chromium, austenitic 304 stainless steel achieves only one corrosion potential in sulphuric acid, both in the presence and absence of NaCl, with the value of ca. ?0.200 to ?0.350 V (SCE) in the absence and ?0.450 V (SCE) in the presence of NaCl, when steel corrodes as the active metal.