Microstructure and micro-electrochemical properties of the heat affected zone in flash butt welded joints of corrosion resistant steel for marine flexible risers

In this study, flash butt welding was employed to join 4 mm thick high-strength corrosion-resistant steel. The electrochemical measurements and immersion test in different heat-affected zones (HAZ) with upset allowances of 2 mm and 4 mm were investigated, specifically focusing on the coarse grain HAZ (CGHAZ) and fine grain HAZ (FGHAZ). Electrochemical properties were evaluated using open circuit potential (OCP), potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS) and scanning vibrating electrode technique (SVET). The results indicate that the base material (BM) and HAZ microstructures exhibit varying corrosion behaviors: CGHAZ < FGHAZ < BM, the CGHAZ is electrochemically the weakest region. The CGHAZ of upset allowance with 2 mm exhibits a coarse grain size and wide lath bainite structure, leading to more severe uneven current density distribution, increasing susceptibility to anodic dissolution, and higher corrosion sensitivity. In contrast, the large plastic deformation and reduced heat input result in the upset allowance with 4 mm has finer grain sizes and narrower lath bainite spacing in the CGHAZ, reducing electrochemical activity, the local pitting depth in the D 4 -CGHAZ region is 3 μm less than that of the D 2 -CGHAZ samples after 168 h, showing superior pitting corrosion resistance.