Effect of Reeling and pH on the Fatigue Crack Growth Behavior of C-Mn Line Pipe Steels Exposed to Acidizing Environments

Abstract

Acidizing treatments are typically performed intermittently during the life of a well. However, more recently there has been a desire to perform an increased number of acidizing treatments in order to improve production. The acidizing treatments typically involve highly corrosive acids, such as hydrofluoric (HF), hydrochloric (HCl) and acetic acid, which are known to cause significant corrosion, but could also lead to environmentally assisted fatigue and fracture. A study was performed to evaluate the effect of cyclic plastic strains associated with reeling installation on the subsequent fatigue crack growth rate (FCGR) behavior of welded C-Mn line pipe steel in acidizing environments. The influence of the pH of the acidizing environment on the FCGR performance was also investigated as part of this study. This paper compares the results of FCGR tests on as-welded (i.e. unstrained) pipe with those from strained and aged welds, as well as quantifying the effect of the pH of the acidizing treatments. Strained and aged welds were obtained by subjecting the as-welded pipe to 4 cycles of full-scale reeling simulation, with each cycle corresponding to 1% strain. Small-scale compact tension (CT) specimens were then extracted from the strained welds and aged at 250°C for one hour to simulate strain aging. FCGR tests were performed in spent acid with corrosion inhibitor on specimens notched in the parent pipe (PP), heat affected zone (HAZ) and weld centerline (WCL) in both the as-welded and strained and aged condition. The majority of the tests were conducted at room temperature (RT) along with a select few tests at elevated temperature (165°F / 74°C). Overall, the results of frequency scan tests indicated that reeling did not have a significant effect on the FCGR behavior of welded C-Mn line pipe steel in spent acid with inhibitor, regardless of which microstructure was sampled. Frequency scan FCGR tests were also performed on strained and aged samples extracted from the intrados side of the strained welds and notched in the PP, HAZ and WCL to investigate the influence of pH on FCGR behavior. Tests were performed in spent acid with inhibitor at RT, with the pH ranging from 3.7 to 6. The observed FCGRs were higher than in air and all microstructures exhibited a frequency dependence (i.e. the FCGR increased with decreasing frequency). At pH = 3.7, the maximum FCGR was approximately 30 times higher than in air and at pH = 5 the FCGR increased to approximately 80 times higher than in air. However, a further increase in pH to 6 produced a decrease in FCGR. The increase in the maximum FCGR is believed to be due to the decrease in corrosion rate with increasing pH leading to reduced crack closure/blunting. However, as the pH increased to around 6, the corrosion rate decreased substantially, which is likely due to a substantial decrease in the concentration of hydrogen being generated, resulting in a lower FCGR. Paris curve FCGR tests were subsequently conducted on strained and aged samples at 0.1Hz.