Corrosion behavior of TiN-coated 304 stainless steel

Titanium nitride (TiN) was deposited on AISI 304 stainless steel using a hollow cathode discharge ion-plating technique. Corrosion behavior of the TiN-coated stainless steel was studied on the specimens with controlling TiN film thickness. The composition depth profiles of TiN films were determined using a secondary ion mass spectrometer. The N/Ti ratios were measured using both X-ray photoelectron spectrometer and Rutherford backscattering spectrometer (RBS). From the results of RBS, packing factors of the TiN films can be obtained. The corrosion resistance was evaluated by standard salt spray test, and by potentiodynamic polarization scan of the specimens in two kinds of solutions, 5% NaCl and 1 N H2SO4 + 0.05 M KSCN, respectively. The surface morphology of the specimens after corrosion tests was observed using both FEG-SEM and optical microscope. The corrosion resistance was correlated to film thickness, packing factor, and thickness x packing factor. There is a good agreement between the results of salt spray tests and potentiodynamic polarization scans. In the case of 5% NaCl solution, the critical thickness is about 0.7 mum, while in 1 N H2SO4 + 0.05 M KSCN, the critical thickness is about 0.3 mum. As the film thickness was smaller than the critical thickness, the corrosion current increased abruptly. It is also found that when the value of thickness x packing factor > 0.6. there is only minor increase in corrosion resistance for the TiN-coated specimen in 5% NaCl solution. This index becomes 0.3 for specimen in 1 N H2SO4 + 0.05 M KSCN. (C) 2001 Elsevier Science Ltd. All rights reserved.