Simulation of Long Term Pipe Exposure to Disbondment With an Advance Permeable Coating Model (PCM3.0)

Abstract

The Permeable Coating Model (PCM) is a mathematical model which has been developed to predict the generation and evolution of environments under a disbonded permeable coating as a consequence of the action of CP. The early version of the PCM was presented at IPC 2004, which focused on the prediction of the environment under a disbonded permeable coating in a fully water-saturated soil without including the generation of CO2 in the soil. As a consequence, the model predicted the generation of a high-pH environment for NaOH-based solution rather than a concentrated HCO3−/CO32− trapped water. The advanced version of PCM takes into account the generation of CO2 in soil by both microbial activity and plant roots respiration. Also, the concept of degree of saturation was introduced, which enables the PCM to predict the pipe surface conditions for situations in which the pipeline is either permanently above or below the water table. The simulation results from the advanced version of PCM show that the concentrated carbonate (i.e, 0.1 to 1 M) and high pH (> 9) environment required for high pH SCC, can be developed within 10 years with a CP level of −1.5VCSE and T > 25°C. For low temperatures (i.e., T ≤ 25°C) a time longer than 10 years is necessary to establish this concentrated carbonate and high pH environment. The results also suggest that although the necessary environment can be generated through the application of CP = −1.5 VCSE, the selected CP level does not cause the potential on the pipe surface to reach the critical potential range (i.e., −750 mVCSE to −600 mVCSE) required for high pH SCC. As expected, the loss of CP after an application of CP for 10 years could provide the environment needed for near-neutral pH SCC to occur.