Localized Electrochemical Study on Corrosion Behavior of Nitride Thin Films

Abstract

Eletrochemical studies were conducted on TiN and ZrN coated 304 stainless steels in 0.5N chloride containing solution to identify the effect of film thickness and Ti or Zr interlayer between TiN or ZrN coatings, respectively, on their corrosion properties. Time dependent corrosion behavior was monitored utilizing the widely used electrochemical impedance spectroscopy (EIS) technique. The corrosion resistance values were justified by the polarization resistance obtained from linear polization. Active to passive transition behavior was studied utilizing the potentiodynamic cyclic polarization test. The charge transfer resistance values obtained from both EIS and linear polarization indicated higher corrosion resistance of ZrN coated steels than the TiN coated steels; justifying our previous findings. It was approximately 106 ohm.cm2 for the former and 6x105 ohm.cm2 for the latter. Higher resistance of ZrN coated steels was attributed to formation of a passive film on the coating. Increasing the film thickness from 5 to 10 μm and laying a metal interlayer between two coating layers did not significantly change the charge transfer resistance suggesting the mechanism for protection is dominated by surface phenomena like formation of an oxide film. Cyclic polarization scans indicated that the corrosion potential of ZrN coated steels was lower than the bare steel and that of TiN coated steels slightly higher than the bare steel. The critical current density for film formation was an order of magnitude lower for ZrN coated steels than TiN coated steels; approximately 10-3 for the former and 10-2 for the latter. This suggested easier formation of oxide film on ZrN than on TiN. The passive films were in the form of ZrO2.2H2O for the former and TiO2.H2O for the latter. Increasing the coating thickness and laying an interlayer between the coating layers increased the coating breakdown potential where pits formed. The higher corrosion resistance of ZrN was then attributed to the easier formation of oxide film on its surface as suggested by the lower critical current density for film formation. Formation of passive oxide film on ZrN was investigated utilizing electron spectroscopy for chemical analysis (ESCA). A layer of approximately 1000 Angstrom thick containing oxygen formed on the ZrN surface after exposure for more than 60 days. This layer was identified by ZrO2 from the binding energy value of the core electron of Zr. It was suggested that this oxide existed in the hydrated form during exposure to aqueous environment but dehydrated after removal and exposure to