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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...

Local dissolution behavior of duplex stainless steel UNS S32304, UNS S31803, and UNS S32750 in 1MH2SO4+1MnormalNaCl was studied in situ by electrochemical scanning tunneling microscopy (STM). Submicrometer features could be resolved and events occurring at phase/grain boundary regions could be monitored in the solution. By STM imaging, severe local dissolution was observed on UNS S32304 in the form of both pitting-like dissolution occurring at the active potential region, and selective dissolution of ferrite phase that already started at the corrosion potential. On UNS S31803, no pitting-like corrosion was observed, but a small amount of selective dissolution occurred at anodic potentials in the active region. The phase boundary region seemed to be prone to local dissolution. No noticeable local dissolution was observed on UNS S32750 in this solution in active and passive potential regions. The results show that a sufficient amount of both Mo and N in duplex stainless steel resulted in more homogeneous dissolution of the two phases and strengthened phase boundaries, and thus decreased the risk for local dissolution. © 2002 The Electrochemical Society. All rights reserved.

Scanning Auger electron spectroscopy (SAES) has been used to investigate the phase boundary region between ferrite and austenite in three duplex stainless steels. Of the elements investigated Cr and Mo are partitioned to the ferrite phase, while Ni and N are partitioned to the austenite phase. The composition gradient across the phase boundary occurs within a few micrometers. The results are in accordance with previous results of the same phase boundary region obtained with complementary techniques. They form evidence of galvanic interaction between the ferrite and austenite phases and suggest this to be the main cause of the higher initial dissolution rate of ferrite adjacent to the austenite phase. The addition of alloying elements improves corrosion resistance of both the ferrite and austenite phases, the weaker sites are comparatively more strengthened. In the highly alloyed duplex stainless steel, the alloying elements are also partitioned, but in such a way that the corrosion resistance of the two phases is very similar, which results in a homogeneous dissolution behavior. © 2004 The Electrochemical Society. All rights reserved.