Corrosion and cracking behavior in an API X-80 steel exposed to sour gas environments

In this work, self-loaded linear elastic fracture mechanics (modified wedge opening loading, M-WOL) specimens were used to investigate the stress sulfide cracking (SSC) susceptibility of a microalloyed API X-80 pipeline steel. For this purpose, M-WOL specimens were exposed to hydrogen sulfide (H(2)S)-saturated NACE solutions at room temperature under applied stress intensity values, K(I), from 35 MPam(1/2) to 50 MPam(1/2). Under all the applied K(I) values, crack growth was exhibited, but the propagation rates were relatively slow (<10(-7) m/s). In both orientations, crack propagation was predominantly transgranular across ferrite grains. Apparently, myriads of minute cracks developed at the crack tip plastic zone, which, in turn, gave rise to the growth of a main crack. Crack growth in the rolling direction (transverse-longitudinal [T-L]) was slightly higher than in the transverse direction (L- T). Seemingly, the presence of a banded structure containing pearlite forced the development of a discontinuous path in the transverse direction stowing down crack growth. A threshold stress intensity factor, K(ISSC), of 28 MPam(1/2) was found experimentally, which suggests that under the applied KI values the steel is susceptible to SSC, even though the steel hardness is 13 HRC (i.e., meets NACE standards for applications in sour gases). In the X-80 steel, the rates of crack growth can be directly related to the rates of metal dissolution. Apparently, the conditions that promote crack growth from the crack tip plastic zone are strongly influenced by the rate of hydrogen released through the metal dissolution reaction. Also, additions of 5% sodium chloride (NaCl) to the NACE solution further exacerbate crack growth as dissolution from the crack tip is enhanced.