Eventos Anais de eventos
COBEM 2019
25th International Congress of Mechanical Engineering
CHARACTERIZATION OF CORROSION WITHIN FRICTION STIR WELD ZONES OF AN API X-70 STEEL
Submission Author:
Juliane Ribeiro da Cruz Alves , SP
Co-Authors:
Juliane Ribeiro da Cruz Alves, Rodnei Bertazzoli
Presenter: Juliane Ribeiro da Cruz Alves
doi://10.26678/ABCM.COBEM2019.COB2019-2409
Abstract
The susceptibility of API-X70 friction stir weld zones to corrosion in Na2SO4 acid media is investigated using an electrochemical microcell. Potential galvanic couples between the weld zones and the preferential corrosion sites were identified. The most anodic zone was base metal that, with a banded microstructure of ferritic and perlitic grains, developed a potential difference of up to 45 mV in comparison to the adjacent heat affected zone. Hard zone was the second most anodic region and, with fine bainitic structure, developed a corrosion potential difference of 37 mV toward the adjacent thermomechanical affected zone (TMAZ), the most cathodic zone. The best combination of parameters for corrosion resistance was obtained at microstructures with both acicular ferrite and coarse bainite, as TMAZ and stir zone, that presented high corrosion potentials, polarization resistances and Tafel slopes. Although fine bainitic structure of hard zone developed the smallest corrosion current densities, its lower corrosion potential and higher sensitivity to potential variation (small Tafel slopes) eventually led to pitting corrosion at grain boundaries. Corrosion current density slightly decreased from base metal toward stir zone, while polarization resistance slightly increased. Microstructure refinement and homogenization accounted for this improvement with a more uniform distribution of corrosion products and possible reduction of galvanic corrosion between ferrite and cementite.
Keywords
Pipeline steel, Mild steel, capillary-based droplet microcell, weld zones, FSW processing
