Scanning Kelvin Probe Force Microscopy and Magnetic Force Microscopy for Characterization of Duplex Stainless Steels

Abstract

In this study, the Volta potential distribution over the surface of duplex stainless steels ͑DSSs͒ has been mapped for the first time with submicrometer resolution by scanning Kelvin probe force microscopy ͑SKPFM͒. The different magnetic properties of ferrite and austenite enable the utilization of magnetic force microscopy ͑MFM͒ for visualizing their surface distribution without the need of surface etching. The combined MFM and SKPFM mapping of the same area makes it possible to associate the variation in the Volta potential to the phase distribution and phase boundaries. The difference in potential between the two phases is measurable and significant. Generally, the ferrite phase was associated to regions of lower potential, and the austenite phase to regions of more noble potential. This can be regarded as direct evidence of galvanic interactions between the two phases. The phase boundary regions often exhibited a lower potential in the ferrite phase, indicating a higher tendency to corrosion. The high lateral resolution of SKPFM provides the possibility of comparing these results with those obtained from other localized techniques, a necessary step for a deeper understanding of the local corrosion processes in DSSs.Duplex stainless steels ͑DSSs͒ are designed to have a microstructure consisting of ferrite and austenite in approximately equal volume fractions. Modern DSSs offer both high mechanical strength and superior resistance to corrosion and stress corrosion cracking especially in chloride environments, and are increasingly used in marine environments, in chemical, petrochemical, pulp and paper, oil and gas industries. 1-3 Due to the partitioning of alloying elements ͑Cr, Mo, N, and Ni͒ in the two phases, there is an intrinsic heterogeneity in the phase composition, which may result in a difference in their corrosion behavior. 4,5 This heterogeneity in microstructure may lead to galvanic effects between the two phases and cause a preferential corrosion attack of the weakest phase. 6,7 Localized corrosion on DSSs most commonly occurs at the ferrite/austenite phase boundaries, which also provide initiation sites for fatigue cracks and stress corrosion cracks. 4 The composition and processing parameters are vital for the control of the microstructure and, thus, the final industrial application. Quantitative metallography and microhardness are the common methods for characterization of DSSs. Conventional electrochemical techniques such as potentiostatic and potentiodynamic polarization measurements and surface analysis are often used in corrosion studies of DSSs. [6][7][8][9][10][11] During recent years, in situ high-resolution microscopy has been applied for studies of localized corrosion behavior of DSSs. 12 In situ local studies of DSSs were reported by the present authors in earlier works [13][14][15] in which electrochemically controlled scanning tunneling microscopy ͑EC-STM͒ was used to investigate the dissolution behavior of three DSSs with different alloying composition. The EC-STM images revealed het...

Study of corrosion behavior of a 22% Cr duplex stainless steel: Influence of nano-sized chromium nitrides and exposure temperature

“…In general, Figure 4a shows the different tendency to corrosion of the ferrite and austenite, with ferrite expected to be more prone to corrosion (darker in Figure 4a) compared to austenite (brighter in Figure 4a). This is in agreement with previous studies where a higher susceptibility to dissolution of ferrite has been observed [8,17,18]. Moreover, a high resolution image in Figure 4b shows the Volta potential variation within a ferrite grain, with finely dispersed quenched-in Cr 2 N exhibiting higher Volta potential (nobler) than the ferrite matrix.…”

Effect of Plastic Deformation on the Corrosion Behavior of a Super-Duplex Stainless Steel

“…7(b) and (c) with ferrite (a) and austenite (c) phases marked. This is a well recorded phenomenon in duplex stainless steels in chloride bearing solutions and is caused by a galvanic couple that forms between the two phases as a result of chemical partitioning; Cr, Mo, W concentrating in ferrite and Ni, N in austenite (Ref [14][15][16][17]. The high nickel and nitrogen give austenite a more noble potential in chloride solutions making it the cathode to the more negative ferrite (the anode).…”

Microgalvanic Corrosion Behavior of Cu-Ag Active Braze Alloys Investigated with SKPFM

“…Importantly, the VPD observed was an effective indicator of how corrosion developed due to microstructural features. Subsequent to these findings, several research groups [31][32][33][34][35][36][37][38][39] have been able to verify that SKPFM is both reliable and effective in characterizing various alloy systems to accurately explain corrosion initiation behavior.…”