Studying Tribocorrosion Mechanisms of Aged 2507 Super Duplex Stainless Steel By Scanning Probe Microscopy
Authors: Shockley J. Michael , So Christopher R , Policastro Steve A , Horton Derek J. , Wahl Kathryn J. 
Published: February 2020

The phenomenon of tribocorrosion is created when the surface degradation processes of wear and corrosion are combined. The tribocorrosion-induced material losses may greatly exceed those of either individual process combined. In one synergistic mechanism the increased material loss results from the stripping of the passivating oxide layer of a corrosion-resistant material through mechanical surface contact, exposing unpassivated material to a corrosive environment. The state of the art of sliding tribocorrosion testing (as opposed to abrasive and erosive tribocorrosion) is to employ an electrochemical cell holding a bulk (i.e., centimeter-scale) specimen, equipped with an alumina or zirconia ball which abrades the surface in reciprocating or unidirectional kinematics [1-4]. The width of the Hertzian ball-on-flat contact in a conventional tribocorrosion cell is on the order of dozens to hundreds of micrometers. However, microstructural features may be considerably smaller than this contact width and, thus far, few attempts have been made to achieve tribocorrosion testing at smaller length scales. Surface probe microscopy techniques, notably the AFM, provide an attractive opportunity to study the links between local microstructural features, the electrochemical response before and during abrasion, and the material loss. Such in situ, microscale tribocorrosion studies in the AFM present a further opportunity in that local material losses can be quantified spatially and in real time, rather than the ex situ characterization required for conventional tribocorrosion testing.
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 [5]. 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 [7]. However, when subject to tribocorrosion testing, the aged microstructure was shown to undergo extensive pitting in the chromium-depleted secondary austenite [7]. 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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Figure 1