Pitting corrosion behaviour of AISI 316L steel in chloride containing solutions

Electrochemical potentiodynamic measurements have shown that the critical pitting potential E_pit of AISI 316L steel in aqueous solutions decreases linearly with increasing chloride ion concentration in the range 10⁻³–2·4M NaCl and with increasing temperature from 30 to 70°C. The critical chloride concentration for pitting at 30°C is 10⁻³M in air saturated solutions, but this increases to 10⁻²M in solutions saturated by nitrogen or oxygen. Additions of either sodium thiosulphate or sodium sulphide promote chloride induced pitting corrosion in 0·6M NaCl solution. This contrasts with the effect of increasing sodium sulphate additions, which result in increased resistance to pitting. Indeed, with Na₂SO₄ concentrations of 0·6M and above there is complete immunity to pitting in 0·6M NaCl solution. The chlorination of natural sea water by the addition of 100 ppm sodium hypochlorite promotes the nucleation of pitting by increasing the mixed potential to above E_pit. Scanning electron microscopy has revealed that corrosion pits with a lacelike appearance are formed in AISI 316L steel in both sodium chloride solutions and sea water.