Global and local corrosion of welded joints of high-strength low-alloy automotive steel

Global and local corrosion techniques were used to study the corrosion behaviour of weld joints of a high strength low alloy steel (LNE500), typically employed in the automotive industry, in Brazil. The welded joints were prepared by gas metal arc welding (GMAW). Two welding transfer modes were used in order to obtain different heat inputs (HI): pulsed (PUL) and cold metal transfer (CMT). Local and global corrosion analysis techniques presented complementary information. While the local in-situ analysis (scanning vibrating electrode technique) revealed the weld metal (WM) as the region where corrosion started, conventional electrochemical techniques (Potentiodynamic polarization (PP) and electrochemical impedance spectroscopy (EIS)) revealed a higher corrosion rate in the coarse-grained heat affected zone (CGHAZ), which was preferentially corroded. A superior corrosion resistance of the weld metal obtained using CMT over the PUL transfer mode was revealed by EIS and PP. In addition, the results from CMT and PUL samples show that austenite grain size affects the propagation of the corrosion process. The results are discussed based on microstructural and compositional aspects of the different regions that characterize the welded joints.