Stress Corrosion Cracking of X-60 Line Pipe Steel in a Carbonate-Bicarbonate Solution Available to Purchase

An experimental system was developed to reproduce stress corrosion cracking (SCC) of API X-60 line pipe steels in highly alkaline (pH = 10) carbonate-bicarbonate (1 N sodium carbonate [Na₂CO₃] + 1 N sodium bicarbonate [NaHCO₃]) solutions. Intergranular fracture developed in precracked 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°C and –650 mV_SCE that simulated actual pipeline conditions. Crack growth rates were determined following fractographic 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√m. These findings supported the view that rupture of the passive film is responsible for SCC of pipeline steels under highly alkaline field conditions.