Using Macro and Micro Electrochemical Methods to Understand the Corrosion Behavior of Stainless Steel Thermal Spray Coatings & Mg/Al Diffusion Couples

Abstract

High-velocity oxygen fuel (HVOF) thermal spray coatings are desirable for their excellent erosion resistance. However, the fabrication process can lead to a decrease in corrosion resistance in comparison to the original bulk material. Stainless steel coatings on stainless steel substrates were produced using varying deposition parameters to investigate the corrosion properties of the resulting composite steels and elude the corrosion mechanism. Potentiodynamic polarization measurements carried out in corroding environments demonstrated the rate of degradation of the Fe-Cr alloy. After short immersion periods, both the coatings and sintered powder pellets for the same alloy showed iron-like active corroding behavior and no passivation regions on the anodic branch. Over time, the coating’s corrosion behavior began to change to signify similar results to that of pure chromium. Ex-situ electron microscopy and elemental composition revealed a Cr oxide rich layer left on the coating’s surface. Furthermore, chemical etching and digestion revealed microstructural changes between the powdered material and the bulk sheet metal such as Cr rich inclusions, known to be responsible for initiating intergranular corrosion. Micro electrochemical techniques including scanning electrochemical microscopy and scanning micro capillary method were employed over the coatings and powdered material, respectively, to prove that the lack of protective passivity the thermal spray coatings possess is mostly inherited from the atomized powdered stainless steel material.