Investigating changes in corrosion mechanism induced by laser welding galvanised steel specimens using scanning vibrating electrode technique

Abstract

A novel three-dimensional scanning vibrating electrode technique (3D SVET) apparatus is described, which uses a bifunctional probe to record topographical and current density data. This apparatus is used to investigate the localised corrosion occurring on 2 cm² exposed areas of flat specimens of electroplated zinc and galvannealed (Zn-Fe alloy coated) 1·2 mm sheet steel and specimens of the same substrates laser welded together, freely corroding in near neutral, aerated, aqueous chloride electrolyte. On flat galvannealed (IZ) specimens anodic events are highly localised and occur at random over the exposed specimen surface during a 24 h immersion period. This reflects the progressive dezincification of zinc rich areas of the iron zinc intermetallic coating. By contrast on flat electroplated zinc (EZ) specimens anodic activity is localised but corrosion initiates at a single anodic centre, eventually spreading out to form a scar on the metallic surface. This concentration of anodic activity on the specimen leads to greater dezincification than for the IZ coating. The SVET data was used to provide an estimate of the total zinc loss from the 2 cm² exposed area on the coupons of 544 μg for EZ and 236 μg for IZ respectively. The close physical proximity of anodic and cathodic events in the latter substrate is believed to lead to greater zinc (hydr) oxide formation and hence lower measured zinc loss. Laser welded specimens were prepared by joining IZ to IZ and IZ to EZ coated 1·2 mm steel panels. A 2 cm² exposed area was investigated using SVET with ca. 1 cm² exposed either side of the weld. The joining of IZ specimens together using a laser weld changes the localisation of anodic activity in neutral aerated sodium chloride solution dramatically. In this instance focal anodes initially concentrate next 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 next to the weld reduces the anodic damage on the IZ remote to the heat affected zone. When specimens of EZ and IZ are laser welded together all anodic activity becomes focussed on the EZ specimens with a total zinc loss over 24 h from the 1 cm² exposed area measured as 489 μg, very close to that of the zinc loss from the EZ specimen (2 cm²) alone. By contrast there is no measurable zinc loss from the IZ portion specimen under these conditions. The increase in zinc loss per unit area from the EZ reflects the additional cathodic area provided by the connected IZ coupon and bimetallic coupling of the metallic coatings.