Microelectrochemical identification of the submicron-sized initiation site for pitting corrosion in 17–4PH stainless steel

The initiation site of pitting corrosion of 17–4PH stainless steel in 1 M NaCl was identified by SEM/EDS analysis after micro-scale galvanostatic polarization. A commercially available 17–4PH stainless steel was solution treated at 1040°C for 1 h and subsequently tempered, and as-quenched, 482°C-tempered, and 621°C-tempered specimens were made. All specimens contained MnS and NbC particles with a size of approximately 500 nm. In addition, the precipitation of ε-Cu was observed in the tempered specimens. In 1 M NaCl at 25°C, the pitting potential measured in conventional potentiodynamic polarization decreased by tempering, suspecting that ε-Cu was the initiation site for pitting corrosion. However, micro-scale galvanostatic polarization at 3 mA m⁻² on a 500 μm × 500 μm electrode allowed the pit diameter to be controlled to approximately from 500 nm to 1 μm, and SEM/EDS analysis after the polarization revealed that the initiation site of pitting of the tempered specimens was MnS, not ε-Cu. The results provide significant insight into the improvement of the pitting corrosion resistance of 17–4PH stainless steel and indicate that the pitting corrosion resistance can be improved by surface treatments to remove MnS or by controlling the MnS composition with trace amounts of alloying elements. Fabricating a small working electrode and performing galvanostatic polarization are promising techniques for identifying submicron-sized initiation sites for pitting corrosion.