Cathodic protection (CP) shielding and the presence of carbon dioxide (CO2) in the soil environment are necessary for the occurrence of near-neutral pH stress corrosion cracking (NNPHSCC). Quantitative understanding of the relationship between external conditions, coating deterioration and NNPHSCC is emerging but needs further improvements in modeling and experimental tool. This paper is aimed at understanding the environments initiating the NNPHSCC. New experimental results are presented on crevice chemistry due to the degradation of the mastic of a commercial high-density polyethylene (HDPE) coating and due to CO2 penetration into a disbonded crevice through the coating. Also presented are the results obtained from a comprehensive model, TECTRAN, on the effect of CO2 penetration into a crevice through the holiday alone and through both the holiday and the coating. The experimental results show that as the coating mastic degrades in the soil solution, the solution pH decreases within a few days from about 9 to a steady-state value of about 7.5. The Co2 diffusion through a 0.3 mm commercial HDPE coating is rapid, with a decrease of the soil solution pH from 9 to 5 within a matter of days (external CO2 pressure is 1 atm). The model results show that the presence of CO2 in the soil (0.05 atm partial pressure) can reduce the crevice solution pH to near neutral due mainly to its penetration through the coating, confirming previous hypotheses regarding its role in initiating NNPHSCC.

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