Corrosion Behavior of High Velocity Oxy-Fuel Sprayed Composite Ni-/Fe-Based Self-Fluxing Alloy – Cermet Coatings (Pages 1-9)

Heikki Sarjas1, Andrei Surzhenkov2, Janis Baroninš2, Mart Viljus2, Rainer Traksmaa2 and Priit Kulu2

1Ensto Ensek AS, Paldiski Maantee 35, 76606 Keila, Estonia; 2Department of Mechanical and Industrial Engineering, Tallinn University of Technology, Ehitajate Tee 5, 19086 Tallinn, Estonia

Abstract: The paper focuses on the corrosion behavior of Ni-/Fe-based self-fluxing alloy – WC-Co/Cr3C2-Ni cermet composite coatings manufactured by high velocity oxy-fuel spraying (HVOFS). Microstructure and phase composition of the coatings were studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The potentiodynamic tests were conducted in the 3.5 wt.% NaCl solutions under room temperature using a three-electrode cell setup (sample was applied as the working electrode, Pt as the counterelectrode, and Ag/AgCl – as the reference electrode) after 10 min and 24 h of immersion. Stainless steel AISI 316L was used as a reference. Tafel polarization plots were obtained, and corrosion potential Ecorr and corrosion current density icorr were determined. All the studied coatings exhibited more negative Ecorr values than the reference steel, both after short and long immersion time, whereas in the latter case, the difference in the corrosion potentials increased remarkably. The reason was the expansion of corrosion products at the self-fluxing alloy–cermet interface that led to cracking and penetration of electrolyte deeper into the coating. Ni-alloy based coatings demonstrated better resistance to corrosion during short immersion time due to a nobler matrix; Fe-based coatings, vice-a-versa, exhibited better corrosion resistance during long immersion time due to a larger amount of amorphous phase. No influence of porosity and cermet reinforcement type on the corrosion resistance of the HVOFS coatings was observed. The principal corrosion mechanism of the coatings was crevice corrosion, accompanied with binder depletion in the cermet reinforcement.

Keywords: HVOF spraying, Ni-based alloy, Fe-based alloy, cermet, corrosion resistance, corrosion mechanism.