Effect of structural defects on corrosion initiation of TiN nanocrystalline films

TiN thin films were deposited on AISI304 stainless steel using a DC reactive magnetron sputtering process. An in situ observation is carried out in order to investigate the relationship between the corrosion initiation and the structural defects such as pores and pinholes by using atomic force microscopy (AFM). It is found that the corrosion initiates at some larger structural pores in the form of the detachment of the particles which will plug the transport path for the corrosion products, whereas the small enough pinholes are easily filled in by corrosion products at the beginning of corrosion. Also, the surface roughness of the corroded film is improved with increasing the corrosion time. The corrosion morphology after polarization test shows that fewer and large pits appear on the TiN-coated substrates probably associated with large structural defects such as high density area of through film pores or pinholes present in the films, which is in accord with those results obtained by in situ AFM observation. Additionally, the effect of bias voltages on corrosion resistance of the films is also involved because the structural defects are strongly associated with the bias voltages.