The candidate material for this study is grade 2507 super duplex stainless steel, which possesses a good combination of mechanical properties and corrosion resistance due its high chromium content and two-phase ferrite-austenite microstructure . However, when subject to aging heat treatments in the 600-900° C range, this alloy is known to develop secondary phases such as sigma, secondary austenite, chi, and Cr2N phases [5, 6]. Sigma phase, an Fe-Cr-Mo intermetallic, contains elevated chromium concentrations compared to the local microstructure and forms in lamellar structures with a spacing on the order of 100 nm – 3 µm. Between the sigma phase lamellae, the locally Ni-rich but Cr-poor microstructure transforms into secondary austenite. In various studies it has been shown that the presence of these phases has not been shown to greatly affect the corrosion resistance in room temperature 0.6 M NaCl during conventional potentiodynamic polarization testing . However, when subject to tribocorrosion testing, the aged microstructure was shown to undergo extensive pitting in the chromium-depleted secondary austenite . However, many questions remain regarding the mechanism of pitting initiation and its link to the mechanical sliding contact.
In the present study, an in situ tribocorrosion testing method was developed by using a commercial potentiostat in combination with an AFM fitted with a diamond coated fluid cell tip. By abrading the surface with the AFM tip and monitoring the corrosion current in real time, the electrochemical changes can be mapped along with the changes in height, and can thus be linked to microstructural features. In doing so, an understanding of the spatial and temporal material loss rates and electrochemical activity was developed that complements the information gained from conventional, macro-scale tribocorrosion testing.
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