The effect of post-cure annealing on the protective properties of polyimides on chromium substrates

The effect of post-cure annealing on the protective properties of thin (2.6-2.8 μm) polyimide films on chromium metallic substrates was investigated in aerated neutral 0.05, 0.5, and 5 M aqueous NaCl solutions at ambient temperature (≈20°C). The study was carried out using the technique of electrochemical impedance spectroscopy to monitor film degradation as a function of immersion time in the test solutions. Results obtained indicate that when exposed to aqueous NaCl solutions for extended periods, a majority of the nonannealed coated substrates failed; however, the performance of the annealed samples was significantly better. At face value, these results suggested improved protective properties of the polyimide due to annealing. However, a more in-depth analysis of the data revealed that post-cure annealing leads to the development of a thicker oxide on the chromium substrate. The change in the oxide occurring during the post-cure annealing treatment contributes to the improved corrosion resistance of the polyimide/Cr system.