STRESS-CORROSION CRACKING OF X-60 LINE PIPE STEEL IN A CARBONATE-BICARBONATE SOLUTION

An experimental system was developed to reproduce stress corrosion cra cking (SCC) of API X-60 line pipe steels in highly alkaline (pH = 10) carbonate-bicarbonate (1 N sodium carbonate [Na2CO3] + 1 N sodium bica rbonate [NaHCO3]) solutions. Intergranular fracture developed in precr acked specimens under very low frequency (40 cycles/day to 400 cycles/ day) cyclic loading conditions (minimum to maximum stress ratio [R] of 0.82) at 75 degrees C and -650 mV(SCE) that simulated actual pipeline conditions. Crack growth rates were determined following fractographi c examination, and a maximum average growth rate of 2.7 x 10(-7) mm/s was measured. The minimum threshold stress intensity for SCC (K-ISCC) was 25 MPa root m. These findings supported the view that rupture of t he passive film is responsible for SCC of pipeline steels under highly alkaline field conditions.