Evaluation of the Effect of Molybdenum on the Pitting Corrosion Behavior of Austenitic Stainless Steels Using Electrochemical Noise Technique

Electrochemical noise (EN) studies were conducted on three austenitic stainless steels (SSs) with different molybdenum contents, 0.02 wt% Mo (Type 304LN SS), 2.53 wt% Mo (Type 316LN SS), and 3.58 wt% Mo (Type 317LN SS), in 0.01 M FeCl3 solution at the corrosion potential (Ecorr) and at a sampling frequency of 1 Hz. The EN data were analyzed using shot noise and wavelet analysis techniques. Current transient analysis showed that the total number of current transients, as well as transients with high current amplitude, decreased with increase in Mo content indicating increased resistance to pitting corrosion. Shot noise analysis revealed higher normalized characteristic charge (q) N at low frequency in Type 304LN SS as compared to Type 316LN SS and Type 317LN SS, implying increase in pitting corrosion resulting from the absence of Mo in this steel. Pit current decreased substantially with increase in Mo content. These results were supported by the standard deviation of partial signal (SDPS) values generated from wavelet analysis. High SDPS values for low-frequency components (d(6) to d(8)) for Type 304LN SS indicated increased pit propagation rate. Higher pitting potentials (E-pit) observed for high Mo-containing SSs revealed improved pitting corrosion resistance in these SSs. X-ray photoelectron spectroscopic and laser Raman spectroscopic analyses confirmed the presence of Mo oxides, Cr enrichment (through various Cr spinels), and polymolybdates in the passive films of Type 316LN and Type 317LN SSs, which imparted increased pitting resistance to these steels.