Aesthetic and performance enhancements of ZMA coated steels
Authors:   , Malla Amar D. 
Published: June 2022

The motivation for the investigations performed in this thesis is the aspiration to develop a metallic coating and an environmentally friendly duplex coating system with superior corrosion & mechanical properties. This in turn will reduce the raw material usage, prolong the structural integrity and save money to the manufacturers and customers. To this end, a quaternary metallic coating alloy system and a duplex coating system with a synergy between the metallic and organic layers with enhanced corrosion properties are developed. A combination of novel techniques such as; Scanning vibrating electrode technique (SVET) and Time-lapse microscopy (TLM) are used to understand and establish the corrosion mechanism of a quaternary metallic alloy and duplex coating system. Miniature testing techniques such as Nanoindentation, Vicker hardness and Small punch tensile test are used to understand the mechanical properties of the quaternary metallic alloy. The original contributions to knowledge are the establishment and validation that a magnesium source such as Mg2Ge and Mg2Si could perform as a corrosion inhibitor and provide a transient level of protection when embedded in the metallic coating systems. A varying amount of germanium (Ge) was introduced into the ternary Zinc Magnesium Aluminium system (ZMA- 0 Ge). The introduction of Ge had a significant impact leading to the formation of Mg2Ge. The area fraction of the eutectic phase diminished with increasing Ge addition, accompanied by a corresponding increase in area fraction of primary zinc and Mg2Ge. These microstructural changes have significantly enhanced the corrosion performance and altered the corrosion mechanism, particularly on the highest Ge addition (ZMA-1.8 Ge). Both SVET and TLM showed a significant delay in visible anode formation however SEM-EDS analysis and TLM using indicator revealed the phenomenon of Mg2+ ions discharge from Mg2Ge during this delayed period, which is associated with the enhanced corrosion protection. Nanoindentation revealed a significant difference in hardness between the zinc phase and the eutectic phase of ZMA-0 Ge. All Ge containing alloys demonstrated a significant decline in plastic deformation however the magnitude of declination was similar. Mg2Si particulates embedded as a source of magnesium in a zinc-rich powder-based galvanising system (Zn) perform as corrosion inhibitors and reduce the SVET measured aqueous corrosion metal loss exponentially. Similar to Mg2Ge, Mg2Si particles preferentially anodically dissolved discharging Mg2+ ions galvanically protecting the zinc surface consequently reducing the initial Zn corrosion. In addition, the presence of Mg2Si delayed the initiation and diminished the cathodic delamination rate of Zn. In both cases (Mg2Ge and Mg2Si), once exposed to the corrosive electrolyte the initial discharge of Mg2+ ions enabled the pH to rise above 8 as Mg2+ ions will not hydrolyse. This rise in pH will encourage the precipitation of Mg(OH)2 and also stabilise pre-existing Zinc oxide/hydroxides on the sample surface. The addition of 1 wt.% of calcium (Ca) to Zn led to the formation of intermetallic CaZn13. SVET and LPR measurements demonstrated a significant enhancement in corrosion resistance. TLM demonstrated that corrosion initiated and preferentially grew via CaZn13. The addition of Na3PO4 enhanced the corrosion resistance significantly of both systems with the effect being much higher in presence of Ca. TLM revealed a significant variation in the corrosion mechanism of Zn and Zn-1Ca in presence of Na3PO4. For Zn, the growth of the anode was restricted by the precipitation of insoluble corrosion product in the vicinity of the anode whereas for Zn-1Ca it leads to a formation of a protective film that covers the whole of the exposed surface. It is postulated that the superior corrosion resistance offered by Zn-1Ca in the presence of Na3PO4 could be due to the formation of Ca3(PO4)2 and Zn2Ca(PO4)2.2H2O with a more compact structure and better preventive ability.