DETERMINATION OF THE CRITICAL PITTING TEMPERATURE AND THE CRITICAL ION (Cl^–) CONCENTRATION INDUCING PITTING OF AISI 304L STAINLESS STEEL IN 0.5 M H₂SO₄

Regina Fuchs-Godec and Valter Doleček
Faculty of Chemistry and Chemical Engineering, University of Maribor,
Smetanova 17, 2000 Maribor, Slovenia

Abstract

Corrosion of AISI 304L stainless steel in aqueous solutions of 0.5 M H₂SO₄ + 0.01 M KCNS + x M NaCl was studied at different temperatures (25, 40, and 50 °C). The critical ion (Cl^-) concentration (CIC) and the critical pitting corrosion temperature (CPT) were determined on the basis of characteristic corrosion parameters (i_crit, i_pass, E_pp), which were obtained through potentiodynamic scanning experiments. The activation energy of the process was found to be equal to –42.0 (1 ± 0.08 ) kJ mol^-1. As the temperature increased, the pitting potential, E_pp, was shifted towards more negative values linearly with the reciprocal of temperature, when the concentration of added NaCl was above the CIC. In contrast, at concentrations very close to the CIC, the straight lines changed slopes. This result may indicate of the approximate boundary between metastable pitting and the beginning of stable pitting.

Key words: austenitic stainless steel, critical ion (Cl^-) concentration, critical pitting corrosion temperature, metastable pitting.