The Critical Pitting Chloride Concentration of Various Stainless Steels Measured by an Electrochemical Method

Since chloride ion is one of the most dangerous accelerators for pitting, the chloride concentration allowed for stainless steels is always the concern of scientists and engineers. At present, a novel electrochemical method was designed to efficiently and precisely measure the critical pitting chloride concentration (CPCC) of various stainless steels in a buffered chloride solution. The current flowing from the working electrode was recorded at a constant potential in the buffered solution with an almost linearly increased chloride concentration. The given chloride concentration when the current density reached 10 μA cm−2 or 1000 μA cm−2 was defined as the CPCC of stainless steels. Abnormal corrosion morphologies were observed on the sample surface after the CPCC test and a pitting-induced crevice corrosion mechanism was introduced. The logarithm of the CPCC values showed an approximately linear relationship with both the temperature and the testing potential. The potentiodynamic polarization curves of AISI 304 samples in buffered solutions with various chloride concentrations at 50°C also certified the reliability of CPCC results. Furthermore, the pitting resistance of AISI 304, AISI 201 and AISI 316L stainless steels ranked by the CPCC results agreed well with the data trend derived from traditional electrochemical methods.