Investigating changes in the corrosion mechanism induced by welding galvanised steel samples using SVET

摘要

A novel three dimensional scanning vibrating electrode technique (3-D SVET) apparatus is described, which uses a bi-functional probe to record topographical and current density data. This apparatus was used to investigate the localised corrosion occurring on 2cm~2 exposed areas of flat samples of galvannealed (Zn-Fe alloy coated) 1.2 mm sheet steel and samples joined using spot welding and laser welding, freely corroding in near neutral, aerated, aqueous sodium chloride electrolyte. On flat galvannealed (IZ) samples anodic events were highly localised with many active anodes at any instant reflecting progressive de-zincification of zinc rich areas of the intermetallic coating leading to a zinc loss of 118 μg cm~(-2) estimated from the SVET data. The influence of electrode life on spot weld quality in terms of corrosion performance was investigated using 1.2 mm galvannealed coupons joined using a 5 mm domed copper electrode and welding up to 1200 different joints. Exposure of a 2 cm~2 area of the welded sample revealed a change in the localisation of corrosion following welding. For the first welds with the fresh electrode, significant copper deposition was observed leading to localisation of cathodic activity within the spot weld. Throughout the exposure anodic activity was distributed at a number of anode sites on the metal. As the number of welds increased brass formation in the weld became less noticeable as did the focus of cathodic activity. In addition, typical measured zinc losses dropped from 91 μg cm~(-2) for the first weld to values of 36 and 38 μg cm~(-2) after 200 and 800 welds respectively, despite the persistent localisation of activity in zinc enriched zones. As weld quality failed (at 1000-1200 welds) localisation of anodic activity became far more apparent in the zinc enriched zone around the weld leading to focal anodic attack and a zinc loss > 100 μg cm~(-2). Laser welded samples were prepared by joining IZ to IZ coated 1.2 mm steel panels. A 2 cm~2 exposed area was investigated using SVET with ca 1cm~2 exposed either side of the weld. The joining of IZ samples together in this instance again lead to focal anodes initially concentrated proximal to the weld area in a zone enriched in zinc (and depleted in iron) as a result of the welding process. This localisation of anodic and cathodic activity proximal to the weld reduces the anodic damage on the IZ remote to the heat affected zone but anodic activity is overall maintained with a zinc loss recorded of 76 μg cm~(-2).