Effect of STL6/TiN Composite Coating on Corrosion Resistance of F347 Stainless Steel
The corrosion resistance of ASTM A182 F347 austenitic stainless steel was enhanced via a two-stage surface engineering protocol: plasma transferred arc deposition of Stellite 6 (STL6) followed by chemical vapor deposition of titanium nitride (TiN). Microstructural characterization confirmed a metallurgically bonded interface with elemental interdiffusion; the STL6 layer exhibited a graded structure from planar/equiaxed grains at the substrate to columnar dendrites and fine equiaxed grains at the surface, with minimal oxide content. Electrochemical testing in 3.5% NaCl solution revealed that the STL6/TiN composite coating reduced the corrosion current density to 0.37 μA/cm², the lowest among the three sample types (F347 substrate, F347-STL6, and F347-STL6-TiN). The composite coating also demonstrated the highest passivation potential (939 mV) and the largest impedance modulus |Z|, indicating superior passive film stability. Post-corrosion analysis showed that the F347 substrate suffered extensive deep pitting, while the STL6 coating exhibited intergranular corrosion with an oxide film. In contrast, the STL6/TiN composite coating displayed only sparse shallow pits. The improved performance is attributed to the formation of a dense Cr₂O₃ passive film on the STL6 layer and the additional barrier provided by the TiN topcoat. These findings offer a viable route for extending the service life of F347 stainless steel in aggressive environments.