Advanced Bipolar Model Based on the Semiconductive Properties of the Passive Film in Stainless Steel
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This study evaluated the electrochemical and semiconductive properties of passive films on austenitic stainless steels with varying pitting resistance equivalent (PRE) values in deaerated HCl solutions. The electrochemical analysis indicated that higher PRE values resulted in a lower passive current density (ip) and an increased polarization resistance (Rp). XPS depth profiling showed that the passive film had a bilayer structure: the inner layer primarily consisted of Cr2O3/ Cr(OH)3, while the outer layer was enriched with oxyanion species such as CrO42− and MoO42−. As PRE increased, the proportion of Cr2O3 in the inner layer rose, while the outer layer had a higher concentration of oxyanions. These alterations led to a simul- taneous increase in both p-type and n-type slopes. To quantify this effect, the ‘Bipolar Index (|p-type slope| + |n-type slope|)’ was applied, and an advanced bipolar model of the passive film was proposed. An increase in the bipolar index was associated with a decrease in passive current density (ip), an increase in polarization resistance (Rp), and a reduction in the total defect density within the passive film.
Abstract
This study evaluated the electrochemical and semiconductive properties of passive films on austenitic stainless steels with varying pitting resistance equivalent (PRE) values in deaerated HCl solutions. The electrochemical analysis indicated that higher PRE values resulted in a lower passive current density (ip) and an increased polarization resistance (Rp). XPS depth profiling showed that the passive film had a bilayer structure: the inner layer primarily consisted of Cr2O3/ Cr(OH)3, while the outer layer was enriched with oxyanion species such as CrO42− and MoO42−. As PRE increased, the proportion of Cr2O3 in the inner layer rose, while the outer layer had a higher concentration of oxyanions. These alterations led to a simul- taneous increase in both p-type and n-type slopes. To quantify this effect, the ‘Bipolar Index (|p-type slope| + |n-type slope|)’ was applied, and an advanced bipolar model of the passive film was proposed. An increase in the bipolar index was associated with a decrease in passive current density (ip), an increase in polarization resistance (Rp), and a reduction in the total defect density within the passive film.