The corrosion mechanism and its modeling of Co-polyacetal (Co-POM) under nitric acid (HNO 3 ) solution were investigated. The degree of degradation was evaluated by the changes in weight and molecular weight. The weight change in HNO 3 solution was linearly decreased after the incubation period. From SEC measurements, it was observed that the occurrence of degradation of Co-POM by HNO 3 solution was limited near the surface on a specimen, and the inside had remained original material. From this result, it was inferred that the degradation type of Co-POM by HNO 3 solution is the corroded layer-forming type. The rate equation for corrosion by hydrolysis reaction between Co-POM and HNO 3 solution was proposed with the rate constant for reaction which was calculated from molecular weight change of the surface on a specimen. The diffusion rate of nitric acid concentration was calculated from weight change (Wet Dry in condition). Finally, the corrosion rate of Co-POM was estimated numerically by combining the concentration of diffused HNO 3 solution into Co-POM over time and the previous equation for corrosion. This result of simulation agreed well with the experimental ones.

We investigated the corrosion behavior of Co-polyacetal (Co-POM) in nitric acid (HNO3) solution and evaluated the degree of degradation by following the changes in weight, dimension, color, flexural strength, and molecular weight. After immersion, the surface of the specimens changed from white to yellow. The specimen weight in HNO3 solution decreased after an incubation period. When the specimens immersed in distilled water were dried, the flexural strength recovered to nearly its initial value; however, this was not observed for specimens that were immersed in HNO3 solution and then dried. From size exclusion chromatography measurements, it was observed that the degradation of Co-POM by the HNO3 solution was limited to near the surface of the specimen, whereas the interior had remained nearly intact. The corrosion rate was calculated from the thickness of the corrosion layer, and the residual thickness of the uncorroded layer and the flexural strength after immersion could be predicted according to the estimated corrosion rate. From this study, it was inferred that the degradation of Co-POM by HNO3 solutions is of the corroded layer-forming type.

The corrosion mechanism and its modeling of Polyamide (PA) in nitric acid (HNO3) solution were investigated. The appearances on specimens in HNO3 solution were changed with discoloration while not happen in distill water (DH). The weight change on wet specimens in nitric acid solution linearly increased after the incubation period. The weight change showed that the diffusion of DH and HNO3 solution into PA obeyed Fickian diffusion model except in HNO3 30mass%. Thus, the diffusion rate of each nitric acid concentration and DH were calculated from weight change with wet specimens. From SEC measurements, it was observed that PA by HNO3 solution degraded gradually in the specimen wholly with increasing immersion time. The reaction rate of each HNO3 concentration was calculated from molecular weight change at the surface of the specimen. The corrosion rate equation between PA and HNO3 solution was estimated with the previous diffusion rate and reaction rate. This equation showed that the calculated molecular weight change agreed well with the experimental ones. From this result, it was observed that the degradation of PA by HNO3 solution was of the corroded layer-forming type in the early immersion stage and of the penetration type in the later stage.