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 acidizing treatments on the fatigue behavior of welded C-Mn line pipe steels. This paper describes the results of fatigue crack growth rate (FCGR) tests on as-welded (i.e. unstrained) pipe.
FCGR tests were conducted at room temperature (RT) in three different acid conditions: fresh acid with corrosion inhibitor, spent acid with corrosion inhibitor and spent acid without corrosion inhibitor. Frequency scan FCGR tests were performed on compact tension (CT) specimens notched in the parent pipe (PP), heat affected zone (HAZ) and weld centerline (WCL). The FCGRs in all three environments were higher than in air and exhibited a frequency dependence. Tests in fresh acid with inhibitor exhibited plateau FCGR values around 20–30 times higher than in air. Tests in spent acid with inhibitor exhibited a strong frequency dependence with plateau FCGR values approximately 80–100 times higher than in air. In spent acid without inhibitor, the plateau FCGR was around 20 times higher than in air, however at the lowest frequencies the FCGR decreased, most likely due to crack closure/blunting effects. This behavior is consistent with the higher corrosion rate in this uninhibited environment.
The role of corrosion products in causing crack closure/blunting was further evidenced in tests performed at elevated temperature (165°F / 74°C), where the FCGR at 1Hz was significantly higher than at RT. The plateau FCGR in fresh acid and spent acid with inhibitor was approximately 40–50 times higher than in air, but the FCGR decreased at lower frequency. This is similarly believed to be due to the higher corrosion rates at elevated temperature causing crack closure/blunting. The FCGR in spent acid without inhibitor at 165°F (74°C) was high initially at 1Hz but then decreased sharply, which is consistent with the highest corrosion rates expected at elevated temperature and in the absence of corrosion inhibitor.
Paris curve FCGR tests were subsequently conducted at 0.1Hz. Tests were performed in the worst case combinations of microstructure/environment/temperature identified from the frequency scan tests.
Near- threshold Fatigue Crack Growth Behavior and Crack Closure of Natural Gas Pipeline Steels
