Review-Passive Film Properties and Electrochemical Response of Different Phases in a Cu-Alloyed Stainless Steel after Long Term Heat Treatment

In this work the influence of copper (0-4 wt%) on the microstructure, passive film properties and local electrochemical response of 25Cr7Ni-type duplex stainless steel is investigated after long term heat-treatment at 800 degrees C for 6 months. This heat-treatment was done to promote the formation of different phases which could be studied in terms of passive film properties and electrochemical response. The unique microstructures of the alloys comprise austenite, sigma phase, Cr2N nitrides and, for the 2 wt% and 4 wt% Cu alloys, epsilon-Cu phase. The results show that alloying with Cu increases slightly the amount of isothermal Cr2N nitrides and epsilon-Cu phase, but decreases the sigma phase fraction. The location of pitting corrosion as well as the Electrochemical Potential (EP), or electron work function, measured with Scanning Kelvin Probe Force Microscopy (SKPFM) show that the epsilon-Cu phase has the lowest corrosion resistance. The EP appears to depend more on the composition of the underlying phase than on the thickness of the passive film. Cr-nitrides have the highest EP followed by sigma phase, austenite and epsilon-Cu phase. There is a clear decrease of EP of the austenitic phase when 2 wt% Cu is added in the alloy. (C) 2016 The Electrochemical Society. All rights reserved.