Epoxy resin systems with different amounts of hardener were prepared. Bisphenol A type epoxy resin prepolymer was used and the hardener was m-xylylenediamine.Specimens were immersed in water, 10wt% resistance to water and sodium hydroxide solution regardless of the amount of hardener.On the other hand, color changed layers were observed on the both surfaces of specimen uniformly in sulfuric acid solution. Sulfuric acid did not penetrate into color unchanged layer. From the results of weight change in sulfuric acid solution, it was suggested that the larger the amount of hardener, the faster was the penetration of sulfuric acid solution. Especially for specimen cured with the largest amount hardener, cracks and defects were observed.The flexural strength, which was measured immediately after taking from sulfuric acid solution, decreased as immersion time increased. The flexural strength recovered by drying. However, the specimen with the largest amount of hardener indicated irreversible degradation due to formation of cracks. The results suggested that excess hardener promote the penetration of sulfuric acid solution. The amount of hardener must be determined carefully in order to avoid corrosion.

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