pH Measurement Under a Simulated Disbonded Coating Using an Optical Fiber Chemical Sensor

Abstract

The present paper is concerned with pH measurement under the disbonded coating of steel, pH has been monitored at different locations down the crevice using optical fibre chemical sensor. The physical model was developed to understand the chemical and electrochemical conditions that created within a crevice beneath a disbonded coating. The physical model has used a polyethylene film covering a crevice of controlled thickness, with controlled gas composition (air with 1% CO₂) on the exterior of the film. The crevice was filled with a simulated ground water and pH measurement was carried out at a controlled potential of -900 mV (SCE). The permeability of the coating plays an important role in controlling the conditions at the end of the crevice, with permeable coating where the gas transport through the coating creates a more alkaline solution inside the crevice, the generation of hydroxyl ions due to the oxygen reduction reaction increasing the pH.

Keywords: optical fibre sensor, pH, physical model