Corrosion Behavior of Co-Polyacetal Resin in Nitric Acid Solutions

Abstract

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.