Direct Observation of Corrosive Wear byIn SituScanning Probe Microscopy

Tribocorrosion involves mechanical wear in a corrosive environment, damaging the protective oxide layer of passivating alloys and increasing material loss rates. Here, we develop a nanoscale, in situ technique using scanning probe microscopy in an electrochemical cell to explore the phase-by-phase tribocorrosion behavior of a heat-treated duplex stainless-steel alloy with secondary phases. We found that under anodic potentials well within the passive oxide region, sliding mechanical contact initiated pitting corrosion and increased electrochemical cell current localized to regions undergoing pitting. Secondary phases were most vulnerable to pitting corrosion during sliding, particularly secondary austenite which is chromium-depleted relative to the matrix steel phases. Under certain conditions, even sigma phases of high nobility were damaged from pits that originate from chromium nitrides. Initiation sites coincide with nanoscale surface voids created at chromium nitride inclusions under a threshold contact stress. Below the initiation stress, no pitting or corrosive wear was observed on sensitized phases. Material loss ceased to propagate when sliding stresses were removed but accelerated when sliding contact stresses were increased. Wear rates and current in the cell were both linearly correlated with material loss. Electrochemical current data were used to monitor oxide penetration spatially but could not be used to quantify material loss. In situ tribocorrosion using a scan probe tip is a viable platform to resolve mechanisms of failure that originate at the nanoscale on actively passivated metal surfaces.