In Situ AFM Study of Localized Corrosion Processes of Tempered AISI 420 Martensitic Stainless Steel: Effect of Secondary Hardening

Abstract

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.

Redefining passivity breakdown of super duplex stainless steel by electrochemical operando synchrotron near surface X-ray analyses

“…However, many advanced alloys nowadays contain multi-elements and consist of multi-phase microstructures, where different kinds of precipitates and phase/grain boundaries may trigger localized corrosion. [16][17][18][19][20][21][22][23] Further studies are necessary to better understand the passivity and its breakdown of such alloys.…”

Accessing the full spectrum of corrosion behaviour of tempered type 420 stainless steel

“…[4][5][6][7] Different austenitisation temperatures, holding times, quench rates and tempering treatments affect the microstructure condition, corrosion behaviour and mechanical properties. [8][9][10][11] The subsequent tempering treatments influence the formation of chromium carbides, typically nucleating from M 3 C to M 7 C 3 and then transforming into M 23 C 6 . [12][13][14] These chromium carbides influence the hardness.…”

On the role of transmission electron microscopy for precipitation analysis in metallic materials

“…[24][25][26][27] TEM is the most versatile out of these tools for precipitation characterization and it is capable of quantitatively characterizing, for example, structure, chemistry, morphology and dispersion of precipitates. [28] By measuring and controlling these precipitation parameters, it is possible to tune and optimize properties of the material, not only the more standard mechanical properties such as strength and ductility but also creep resistance, [29,30] wear resistance, [31] functional [32] and degradation [33,34] properties (see schematic illustration in Figure 1).…”