In situ health monitoring of corrosion resistant polymers exposed to alkaline solutions using pH indicators

Polymers and their composites are widely used as corrosion resistant materials in chemical plants. It is required to establish a health monitoring method for chemically exposed fiber reinforced plastic (FRP) structures. In this paper, authors proposed a new health monitoring technology that could simply detect the penetration of corrosive environment solution into plastics by using a pH indicator. At first, FRP laminates for corrosion sensing were manufactured and tested. The laminate consisted of three layers; two surface layers and one sensing layer. Each layer was made of glass mat reinforcement and unsaturated polyester (UP) resin. In the sensing layer, phenolphthalein was added to the UP resin. The laminate was immersed in sodium hydroxide (NaOH) solution. The sensor layer was colored reddish purple and it was easy to distinguish from the color before immersion. Then, detection of NaOH penetration into the UP resin was carried out by using an optical fiber and a spectrophotometer. A narrow slit was machined in an optical fiber and the slit was filled by phenolphthalein containing UP resin. This optical fiber sensor was immersed in NaOH solution. The optical fiber connected to a spectrophotometer and the transmittance was measured. It was found that the penetration of NaOH could be detected by the change in spectrum. This system may be easily applied to online health monitoring of chemical apparatus.