Influence of annealing temperature on corrosion inhibition and nanostructure of nitride Ni and Ni/Ti coatings on AISI stainless steel

The corrosion behaviour of Ni/(AISI 304) and Ni/Ti/(AISI 304) annealed at different temperatures (623 K to 1073 K) with flow of nitrogen gas is investigated in the 3.5 wt% (0.6 M) NaCl corroding medium. X-ray diffraction analysis of the samples confirmed formation of different phases of nickel nitride for Ni/(AISI 304) samples and nickel nitride and NiTi for the Ni/Ti/(AISI 304) samples depending on the annealing temperature. Surface morphology of the samples was obtained by means of atomic force microscopy from which average grain size, and the surface roughness values were obtained and their relationship with the corrosion results are discussed. Corrosion resistance of the samples was studied by the electrochemical impedance spectroscopy (EIS) and polarization measurements. The results showed highest corrosion resistance for both types of samples at a critical annealing temperature of 773 K. An improvement of 98 times for the Ni/Ti/(AISI 304) and 24 times for the Ni/(AISI 304) relative to the bare AISI 304 was obtained from the polarization results while the EIS analysis gives the enhancement factor of 90% for the Ni/Ti/(AISI 304) and 64% for the Ni/(AISI 304) samples annealed at 773 K temperature which is a high enhancement in particular in case of Ni/Ti/(AISI 304) sample. The equivalent circuits for both types of samples annealed at different temperatures were obtained, using the EIS data which showed strong dependence of the equivalent circuit elements on the surface morphology of the sample. The better performance of the former sample may be attributed to the reaction at the interfaces and to the presence of oxide particularly at the film grain boundaries as well as the thickness of this sample. Scanning electron microscope (SEM) and energy dispersive spectroscope (EDS) analyses of the samples also confirm these results. Graphical abstract