An examination of the corrosion resistance of zinc-magnesium and zinc-aluminium-magnesium coated steels

摘要

This project has investigated the development of the optimum combination levels and processing conditions for zinc and magnesium coatings deposited by a Physical Vapour Deposition (PVD) process under investigation by Tata Steel Europe Ltd. Temperatures in the range of 100°C to 350°C and times ranges of 2-10 hours and 30 - 300 seconds were investigated and the coatings characterised by optical microscopy, scanning electron microscopy (SEM) with energy dispersive x-ray analysis, (EDX), also known as electron probe microanalysis (EPMA), glow discharge optical emission spectrophotometry (GDOES) and x-ray diffraction (XRD), and examined for corrosion resistance using the scanning vibrating electrode technique (SVET) and accelerated weathering cabinet tests; prohesion and salt spray. The work has shown that the initial zinc magnesium coatings prepared via the developing Tata Steel Europe PVD process did not perform as well as the current and developing hot dipped zinc-aluminium or zinc-aluminium-magnesium alloys in continuously submerged sodium chloride solutions. In support of this work, and to contrast the coatings prepared via the PVD process the project investigated changes to the coating composition and substrate gauge of a range of conventionally prepared hot dip galvanised samples. This part of the project has looked at variations in the alloying additions to zinc using magnesium in the range of 0-2wt% and aluminium in the range of 0-5wt%. Changing coating composition and/or changing processing conditions produced coatings with dendrites per mm2 (dendrite number) ranging from 350 to 7600. Primary zinc% has also been found to vary in the range of 29% to 95% and the relative corrosion rates have been found to vary between 49% and 477% that of benchmark samples of conventional hot-dip galvanised steel of 275g/m2. Conventional and more recently developed coatings have been studied in mildly alkaline environments by immersion mass loss and SVET testing, as part of a study to investigate if Tata Steel Europe can substitute a lower coating weight, newly developed MagiZinc metallic coating into construction industry in the UK. It was found that both conventional and the new alloy chemistries were seen to have sufficient coating weight after the 28 days, the total cement curing time, to allow the substitution.