Environmental aspects of near-neutral pH stress corrosion cracking of pipeline steel

Abstract

The severity of four different soil environments toward the development of near-neutral pH stress corrosion cracking (SCC) of pipeline steel was evaluated using slow strain-rate testing (SSRT). These soils were collected from pipeline sites where near-neutral pH SCC has been observed. It was demonstrated in this investigation that SSRT can differentiate the severity of various soil electrolytes to near-neutral pH SCC. For different soils, the relative susceptibility was found to be determined by the pH values of the soil electrolytes in equilibrium with a given CO₂/N₂ gas mixture. The higher the pH value up to ∼7, the more conducive the soil electrolyte was to near-neutral pH SCC. The pH value in a soil electrolyte was found to depend on the level of CO₂ in the soil solution and the initial HCO ⁻₃ concentration before the introduction of CO₂. For a given soil, the susceptibility depends on the actual level of CO₂ in the soil electrolyte. Higher levels of CO₂ lower the pH in the soil electrolyte and tend to increase the susceptibility to SCC. In laboratory tests, cathodic polarization was found to increase the susceptibility to failure, possibly by inhibiting general corrosion, which otherwise removed discrete stress-raising pits and defects from the specimen surface that acted as crack initiation sites or by increasing the extent of hydrogen-induced crack initiation or propagation. In the field, cathodic polarization is likely to prevent near-neutral pH SCC by increasing the pH at the pipe surface to values greater than 7.5. The pH was maintained near-neutral in the lab tests by continuous purging of the test solution with CO₂/N₂. A method is proposed for assessing the relative aggressiveness of various soil extracts to near-neutral pH SCC. Aggressive soil extracts appear to exhibit a narrower variation in pH between solutions purged with N₂ and with CO₂ than that for less-aggressive soil extracts purged with the same gases.