INVESTIGATING CHANGES IN THE CORROSION MECHANISM INDUCED BY LASER WELDING GALVANISED STEEL SAMPLES USING SVET

摘要

A novel three-dimensional scanning vibrating electrode technique incorporating a bi-functional probe, to record topographical and current density data, has been used to investigate the localised corrosion occurring on 2cm~2 exposed samples of galvanised steel freely corroding in aqueous sodium chloride. On galvannealed (IZ) samples localised anodes activate and deactivate over the entire exposed sample surface during a 24-hour immersion period due to progressive de-zincification of the iron/zinc intermetallic coating. On electroplated zinc (EZ) samples localised anodic initiates at a single point, eventually spreading out to form a scar on the metallic surface leading to greater zinc loss. The joining of IZ samples together using a laser weld causes focal anodes to initially concentrate proximal to the weld area in a zone enriched in zinc. When samples of EZ and IZ are laser welded together all anodic activity is focussed on the EZ due to bimetallic coupling of the metallic coatings lading to accelerated zinc loss.