Correlation of Electrochemical Noise Parameters with Degree of Sensitization in AISI Type 316LN Stainless Steel

Abstract

Degree of sensitization in 316LN stainless steel (SS) specimens sensitized at 898, 923 and 948 K for 500 h was obtained using double loop electrochemical potentiokinetic reactivation (DLEPR) technique as 7.4, 14.5 and 9.3% respectively. The sensitized specimens were pulsed polarized so that only Cr-depleted regions of the sensitized grain boundaries contributed to the electrochemical noise (EN) study. The DOS values evaluated from DLEPR technique were correlated with the EN parameters viz. standard deviation of current, σ_I and characteristic charge, q and characteristic frequency, f _n , obtained from shot noise analysis in order to assess the extent of intergranular corrosion (IGC) attack in 316LN SS. The plot of σ_I versus time showed highest σ_I values for the specimen sensitized at 923 K for 500 h, indicating high grain boundary dissolution and hence, severe IGC attack, whereas the specimen sensitized at 898 K for 500 h showed the least σ_I values indicating lower dissolution and least IGC attack. 316LN SS specimen heat-treated at 948 K for 500 h showed intermediate grain boundary dissolution rate. The charge q, determined from σ_I versus time plot showed a good correlation (>99%) with the DOS values obtained from DLEPR experiments. The power spectral density values of the current signal in the frequency independent region were found to have excellent correlation with these observations. The above findings were further supported by scanning electron microscopic examination which showed an increase in grain boundary width in the sensitized specimens when the heat-treatment temperature was raised from 898 to 923 K and on further increasing the temperature to 948 K, a marginal decrease in the grain boundary width was observed.