Short term predictive testing for cut edge corrosion resistance in zinc–aluminium alloy galvanised steels

Abstract

The scanning vibrating electrode technique (SVET) is shown to be a reasonably reliable and very rapid test method for ranking the cut edge corrosion resistance of zinc aluminium alloy galvanised steels. The data obtained in 24 h relates directly to the organic coating delamination, determined by 5 years external weathering, and zinc metal ion runoff resistance, determined by up to 24 months external exposure. By altering the chemical composition and the processing conditions of the 4·5%Al containing zinc galvanising layer on 0·7 mm gauge steel, the volume percentage of primary zinc evolved during solidification has been carefully controlled to increase from 27(±0·25)% to 30(±0·25)%. In addition, the number of primary zinc dendrites was also significantly altered from ca 600 to ca 1600 mm⁻². SVET corrosion tests performed on the cut edge of organically coated samples of these materials revealed that as the volume percentage of primary phase and number of dendrites increased the intensity of corrosion increased (with SVET determined current density maxima increasing from 8 A m⁻² to 13 A m⁻²). The average total zinc loss (tzl) measured using SVET in the 24 h exposure also increased (from 166 μg to 309 μg). Zinc ion enriched rain water collected as runoff from externally weathered samples (industrial/marine location) revealed similar performance trends to those from the SVET, with the primary zinc volume fraction controlling the zinc ion release in each case. This is repeated for zinc runoff measurements recorded between 3 and 24 months exposure. Analysis of the total organic coating delamination area after five years external weathering showed direct correlation with zinc ion runoff levels from the coating over the first 24 months and also to the SVET zinc losses measured in 24 h. Hence, the SVET appears to be a reliable and rapid tool for metallic coating optimisation, and associated runoff measurements can be used as an environment specific corrosion test with an accurate performance indication within 3 months.