A GFRP tank used for 20 years at hot water was investigated and observed at the surface and cross section of tank wall. The investigation of so long time used sample is very rare case. We can see the strange phenomenon that corrosion occurred the surface layer than the inner layer of tank wall. In generally, severe corrosion occur in the surface that contact with environmental agents. In this case, high performance matrix resin used in surface layer and something low performance matrix resin used in inner layer. This deference of corrosion performance for matrix resin is cleared for long time use.
