Environmental effects on near-neutral pH stress corrosion cracking in pipelines

Abstract

Stress corrosion crack (SCC) growth mechanisms in pipeline steels exposed to near-neutral pH environments are not well understood, although cracking is generally thought to result from some combination of crack-tip dissolution and hydrogen embrittlement. This chapter discusses how electrochemical factors in the environment influence the dissolution and cracking rates in near-neutral pH SCC highlighting the factors that influence the tendency of cracks to become dormant. The synthetic near-neutral pH soil solutions are designed on the basis of compositions of electrolytes extracted with distilled water from soils removed from near-neutral pH SCC sites. The corrosion resistance of X-65 steel in various synthetic solutions is determined using weight loss coupons of ∼ 15 × 15 × 1 mm suspended in test solution. The compact toughness specimens are machined from the X-65 pipe with the crack plane normal to the circumferential (hoop) direction. Each specimen is polished to produce a scratch-free surface prior to tests. Specimens are then prefatigued in air to produce a sharp crack tip from the machined notch in accordance with ASTM E647. A possible explanation for SCC morphology in the NOVA Trapped Water solution might be related to creep deformation at the crack tip. The pipeline steels are susceptible to creep deformation at room temperature, which can produce several percent strain under a constant stress above the yield strength.