Localized Electrochemical Study on Corrosion Behavior of Nitride Thin Films

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 a laboratory environment. A broad oxygen O 1s peak suported this argument. However, insignificant change in the oxygen content was found for TiN exposed for more than 60 days. Oxide film formation on ZrN but not on TiN was proposed as driven by the potential difference between coating and steel substrate. Lower potential of ZrN than steel promotes oxidation of the nitride to form oxide film as a protective layer. Higher potential of TiN than the substrate will not promote oxidation of the nitride. The substrate oxidizes and its corrosion protective ability will depend on the passive film on the substrate. Thermodynamic calculations were carried out to construct the pH-potential or Pourbaix diagram for ZrN in water which potentially can be used as protective coating. Equilibrium potential for oxide formation from nitride is higher than from its base metal. The shape of immunity, passivity and corrosion regions of nitride follow closely that of the base metal. Theoretial prediction for choosing good protective coatings for particular metal or alloys can be made using this diagram. A local electrochemical impedance spectroscopy (LEIS) technique was developed to enhance understanding of overall impedance of passive systems like stainless steel and aluminum alloys. Local impedance of passive area and pits were measured by this technique. Contribution of these local areas to the overall impedance measured by EIS were identified. Pit may not significantly change the overall impedance because the current from the small pit spread laterally to the larger passive area. The pit response is masked by the response of the passive area. Information on the local corrosion rate of pit and passive area, and pit growth behavior are obtainable from this local impedance technique which is not available from the overall impedance which only provide surface averaged response. Local charge transfer resistance and capacitance values can be used to justify the active pit model for pit in pasive system. Application of LEIS technique to monitor damage in carbon/ glass/vinyl ester composite under simulated galvanic coupling in 0.5N NaCl was carried out. Differences of local impedance over blisters and polymer removal regions were measured. Under open circuit condition, significant variation in local impedanec over glass and carbon regions were identified suggesting differences in electrolyte diffusion into the composite at these regions. LEIS technique was found as very useful to measure local corrosion rate and monitor damage. Further investigations on application of this technique to other corrosion system should be conducted to fully utilized its capability to map the impedance of entire corroding surface