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)-2.4 M NaCl and with increasing temperature from 30 to 70 degrees C. The critical chloride concentration for pitting at 30 degrees C is 10(-3) M in air saturated solutions, but this increases to 10(-2) M in solutions saturated by nitrogen or oxygen. Additions of either sodium thiosulphate ol sodium sulphide promote chlorine induced pitting corrosion in 0.6 M NaCl solution. This contrasts with the effect of increasing sodium sulphate additions, which result in increased resistance to pitting. Indeed, with Na2SO4 concentrations of 0.6 M and above there is complete immunity to pitting in 0.6 M 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.