Effects of microstructure alteration on corrosion behavior of welded joint in API X70 pipeline steel

In the present work, a heat treatment process was used to modify corrosion behavior of heat affected zone (HAZ) and weld metal (WM) in welded pipe steel of grade API X70. A one-step austenitizing with two-step quenching and subsequent tempering treatment was performed to alter the microstructure of HAZ and WM. The hardness and strength values were controlled to be in the standard range after the heat treatment process. In order to investigate the effect of the heat treatment on the corrosion properties of welded joint, the samples were immersed in a mixture of naturally aerated 0.5 M sodium carbonate (Na2CO3) and 1 M sodium bicarbonate (NaHCO3) solution with pH of 9.7 for 45 days. The electrochemical impedance spectroscopy (EIS) measurements were carried out then to study the protective properties of the corrosion products layer. The X-ray diffraction (XRD) investigation depicted that the corrosion products layer composition includes FeCO3, FeO(OH), Fe3O4 and Fe2O3. The EIS results showed that, the corrosion resistance of HAZ and WM increased after heat treatment. This can be attributed to formation of uniformly distributed polygonal ferrite (PF) and to the decrease in the volume fraction of bainite (B) after heat treatment.