In this work, the corrosion behavior of tempered AISI 420 martensitic stainless steel (MSS) was studied by in-situ atomic force microscopy (AFM) in 0.1 M NaCl and correlated with the microstructure. Thermocalc simulation, dilatometry, and X-ray diffraction (XRD) were performed to investigate phase transformation which showed the formation of M 3 C, M 7 C 3 , and M 23 C 6 type of carbides and also retained austenite. Optical microscopy, scanning electron microscopy (SEM), and AFM characterization revealed undissolved carbides and tempering carbides in the martensitic matrix. Volta potential mapping measured by scanning Kelvin probe force microscopy (SKPFM) indicated higher electrochemical (practical) nobility of the carbides with respect to the martensitic matrix whereas regions adjacent to carbides showed lower nobilities due to chromium depletion. Open circuit potential and cyclic potentiodynamic polarization measurements showed metastable corrosion activities associated with a weak passive behavior and a risk for localized corrosion along certain carbide boundaries. In-situ AFM measurements revealed selective dissolution of certain carbide interphases and martensitic inter-lath regions indicating higher propensity to localized corrosion.

The effect of secondary hardening of tempered AISI 420 martensitic stainless steel on the corrosion behavior in aqueous 0.01 M NaCl has been studied, in-situ, using atomic force microscopy (AFM) to monitor real-time localized corrosion processes. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy, and X-ray diffraction analyses confirmed the presence of undissolved and secondary carbides (Cr 23 C 6 , Cr 7 C 3 , Cr 3 C 2 , Cr 3 C, Cr 2 C, and CrC) as well as retained austenite, all finely dispersed in the tempered martensitic matrix. Electrochemical measurements, consisted of monitoring of the open-circuit potential vs. time and cyclic polarization in 0.01 M NaCl solution, were performed to evaluate the passivity and its breakdown, and it was seen that initiation sites for localized corrosion were predominantly peripheral sites of carbides. In-situ AFM measurements revealed that there was a sequence for localized corrosion in which the neighboring matrix next to secondary carbides dissolved first, followed by corrosive attack on regions adjacent to undissolved carbides. Tempering at 500 • C reduced the corrosion resistance and the ability to passivate in comparison to tempering at 250 • C.

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