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Multi-layer structures including zigzag sculptured thin films for corrosion protection of AISI 304 stainless steel

To increase corrosion resistance of the sample, its electrical impedance must be increased. Due to the fact that electrical impedance depends on elements such as electrical resistance, capacitance, and inductance, by increasing the electrical resistance, reducing the capacitance and inductance, electrical impedance and corrosion resistance can be increased. Based on the fact that these elements depend on the type of material and the geometry of the material, multilayer structures with different geometries are proposed. For this purpose, conventional multilayer thin films, multilayer thin film including zigzag structure (zigzag 1) and multilayer thin film including double zigzag structure (zigzag 2) of manganese nitride are considered to protect AISI 304 stainless steel against corrosion in salt solution. These multilayer coatings including zigzag structures are prepared by alternately using the conventional deposition of thin film and glancing angle deposition method. After deposition, the samples are placed in a furnace under nitrogen flux for nitriding. The cross sections of the structures are observed by field emission scanning electron microscopy (FESEM). Atomic force microscope (AFM) is used to make surface analyses of the samples. The results show that the multilayer thin films including zigzag structures have smaller grains than conventional multilayer thin films, and the zigzag 2 structure has the smaller grain than the other two samples, which is attributed to the effect of shadowing and porosity on the oblique angle deposition method. Crystallography structures of the samples are studied by using x-ray diffraction (XRD) pattern and the results show that nitride phase formation in zigzag 2 structure is better than that in zigzag 1 structure and conventional multilayer thin film. To investigate the corrosion resistances of the structures, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests are performed. The results reveal that the multilayer thin films with zigzag structures have better corrosion protection than the conventional multilayer thin films, and the zigzag structure 2 has the smallest corrosion current and the highest corrosion resistance. The electrical impedances of the samples are investigated by simulating equivalent circuits. The high corrosion resistance of zigzag 2 structure as compared with conventional multilayer structure and zigzag 1 structure, is attributed to the high electrical impedance of the structure due to its small capacitance and high electrical resistance. Finally, the surfaces of corroded samples are observed by scanning electron microscope (SEM).