Addition of Alloying Agents to Improve the Performance of Zn/Al Metallic Coated Steel

This work set out to investigate the microstructural changes and corrosion mechanisms of several zinc-based sacrificial metallic coatings which are used within the strip steel industry. The assessment of alloying additions at various compositions to determine what microstructural features and phase formations lead to superior performance in a corrosive environment. This has the potential to reduce raw material usage and improve the environmental impact / carbon footprint of galvanised coatings, extend the surface life and save the manufacturer money by reducing maintenance costs. Chapter 3 uses Scanning Electron Microscopy (SEM) and Energy Dispersive Spectrometry (EDS) to evaluate the microstructure and phase morphologies present within several industry-made metallic coatings. Time-lapse microscopy (TLM) and Scanning Vibrating Electrode Technique (SVET) are used to understand and determine the corrosion mechanism of each coating when immersed in salt solution. Both TLM and SVET determined that higher weight percentages of aluminium in zinc-aluminium (ZA) coatings led to enhanced corrosion protection. Higher weight percentages of aluminium and magnesium in the zinc-aluminium-magnesium (ZMA)system also led to improved corrosion performance in salt solution. Chapter 4 investigated, using TLM and Linear Polarisation Resistance (LPR) the corrosion rate of the same industrially made metallic coatings but in both 0.45 wt.%NaCl and Prohesion solution (0.4 wt.% Ammonium sulphate, 0.05 wt.% NaCl). This assessed the effect of ammonium ions on ZMA coatings when compared to non-magnesium-containing. TLM illustrated a change in corrosion mechanism from local corrosion to general corrosion when immersed in a dilute salt solution compared with Prohesion solution respectively. LPR experiments confirmed that the presence of magnesium had a detrimental effect on the corrosion rate of metallic coatings and that aluminium offered resistance to corrosive ammonium ions. The addition of Benzotriazole into Prohesion solution offered protection against ammonium ions for ZMA coatings. Chapter 5 utilises microstructural analysis techniques, TLM, SVET and LPR techniques to assess the effect of coating weight (g.m-2) on the microstructure and corrosion performance of MagiZinc samples produced using the ZODIAC (Zinc and Other Developments in Alloy Coatings) galvanising line. It was found that the volume fraction of the primary Zn phase reduced with increased coating weights do to varying cooling rates of the ZMA coatings. SVET and LPR showed an increase in coating thickness resulting in a significant increase in performance due to a change in corrosion mechanism. The anodic growth mechanism for ZMA310 (310 g.m-2) was more localised and constrained compared to ZMA80 (80 g.m-2).Chapter 6 followed on from the previous chapter and investigated the effect of cooling rate on MagiZinc samples. Samples were produced using a hot dip simulator at several cooling rates and microstructural analysis techniques, TLM, SVET and LPR techniques were used to assess changes in microstructure and corrosion performance. It was found that faster-cooled samples lead to a reduction in the formation of binary eutectic and an increase in the formation of ternary eutectic. This results in an improved corrosion rate when more ternary eutectic is present within the microstructure. In summary, the diverse behaviours observed in different coatings underscore the importance of understanding their microstructures, corrosion mechanisms, and the influence of alloying elements. These insights are crucial for the development of advanced coatings with enhanced corrosion resistance which contribute to the longevity and durability of materials in various environmental conditions.