Protecting Mg Alloys from Corrosion with PEO Keronite Whitepaper Protecting Mg alloys from corrosion with PEO © Keronite 2020 | keronite.com 1 Protecting Mg Alloys from Corrosion with PEO Contents 2 - Introduction 3 - Developing an advanced coating for lightweight magnesium components 6 - Further developments in coatings for extreme corrosion resistance 7 - Duplex PEO coatings in salt-spray testing © Keronite 2020 | keronite.com 1 Protecting Mg Alloys from Corrosion with PEO Introduction As weight becomes ever more important when considering the design of new products, the interest in magnesium continues to increase, particularly in sectors such as aerospace, automotive, oil & gas. In many ways, magnesium is the ideal material for applications in these industries – it has a low density, it is easy to process, and has high mechanical stability and strength. However, there are two key challenges with using magnesium - its low wear resistance and susceptibility to corrosion. Keronite’s magnesium coatings offer a solution to these problems via the conversion of reactive and soft magnesium to stable and hard magnesium oxide. Coatings formed on magnesium by plasma electrolytic oxidation broadly consist of three layers: a thin but dense interface layer between the coating and the underlying alloy, a compact and hard middle layer, and a porous outer layer. These separate layers provide a magnesium PEO coating with its characteristic high adhesion, high wear and corrosion resistance, and excellent binding to topcoats. Due to the manner in which the coating forms, (see ‘What is PEO?’) the resulting coating is also flexible, allowing for increased mechanical loading without the cracking and flaking associated with other coating processes. © Keronite 2020 | keronite.com Figure 1. Magnesium component being PEO coated with G3Mag. Figure 2. Cross section of G3Mag coating on magnesium component. 2 Protecting Mg Alloys from Corrosion with PEO Keronite’s coatings don’t just convert the underlying alloy. By using our proprietary electrolytes and regimes, we are able to incorporate other elements into the ceramic in order to produce wear and corrosion resistant surfaces e.g. by forming very hard Al/Mg spinel structures. The highly conformal nature of Keronite’s process also reduces the problem of thin and thick patches in a coating associated with spray or electrochemical deposition techniques. Developing an advanced coating for lightweight magnesium components Keronite’s primary magnesium coating is G3Mag, and has long been used to protect magnesium components in the aerospace sector from both mechanical and chemical attack. Highly conformal, G3Mag is capable of providing extremely dimensionally precise coatings which allow for the design of components with the tight dimensional tolerances required in precision aerospace applications. Immersion in 5% NaCl at 80°C Before After A coating of only 5μm is sufficient to protect ZE41A alloy unscathed for 6 hours in 5% NaCl at 80°C for 6 hours (Table 1). G3Mag is further complemented by the use of impregnation sealing. The surface of a PEO coating has numerous pores, formed from the plasma discharge events. Keronite has developed © Keronite 2020 | keronite.com Table 1. Salt corrosion test of 5μm G3Mag coating on ZE41A alloy. 3 Protecting Mg Alloys from Corrosion with PEO techniques which allow for a variety of materials to be bonded to the surface. These include glass ceramic, polymer resin, ceramic polymer matrix, fluoropolymers and silicones. As can be seen in Table 2, both the unsealed and sealed 8μm coating of Keronite G3Mag is broadly unaffected by immersion in 5% NaCl at 80°C for 6 hours. Unsealed Glass ceramic Polymer resin Ceramic polymer matrix Fluropolymer Silicone Before immersion in 5% NaCl at 80°C After 6 hours Table 2. Salt corrosion test of 8um G3Mag coating on AZ91 alloy using different topcoat sealers. © Keronite 2020 | keronite.com 4 Protecting Mg Alloys from Corrosion with PEO Glass ceramic Polymer resin Ceramic polymer matrix Fluropolymer Silicone 1 min 240 min 240 min 23 min 35 min Table 3. Acid corrosion test of 8um G3Mag coating on AZ91 alloy using different topcoat sealers. Even in 15% HCl, – traditionally immediately destructive to magnesium oxide coatings – several of the PEO and sealer combinations showed very impressive chemical resistance (Table 3). © Keronite 2020 | keronite.com 5 Protecting Mg Alloys from Corrosion with PEO Further developments in coatings for extreme corrosion resistance In order to explore more extreme corrosion resistance, particularly for reactive alloys, Keronite developed G3Mag-C. G3Mag-C combines multiple developments to improve the corrosion resistance of even the most reactive of magnesium alloys. A new electrolyte formulation, advanced electrical regime and co-developed sealer options allow for the use of reactive magnesium even in applications where it was previously thought impossible. While initially developed from Keronite’s previous G2Mag, G3Mag-C is an entirely different formulation which creates different phases to adapt to the varied corrosion mechanisms present in different locations within the ceramic and the underlying metal. As crevice corrosion is of key concern when using reactive alloys, G3Mag-C solves this issue by creating a stabilised layer at the This advanced corrosion interface between the ceramic and protection has already underlying alloy. This layer inhibits garnered significant corrosion at sites adjacent to inter-meinterest, in particular tallic particles in the underlying within the energy and alloy by limiting localised pH swings, automotive sectors. essentially forming a localised passivation effect for those sites. This advanced corrosion protection has already garnered significant interest, in particular within the energy and automotive sectors. © Keronite 2020 | keronite.com 6 Protecting Mg Alloys from Corrosion with PEO 20 hours G2Mag G3Mag-C 92 hours G2Mag G3Mag-C 164 hours G2Mag G3Mag-C Table 4. Salt fog test of G3Mag-C versus G2Mag. Duplex PEO coatings in salt-spray testing As can be seen in Table 4, this new formulation is incredibly corrosion resistant, even when unsealed. Combining G3Mag-C with Keronite’s new fluoropolymer sealer blends immensely improves the corrosion resistance even on alloys containing extremely reactive elements such as lithium, as used in the hydraulic fracturing industry. Part of the secret of these new “duplex” coatings is eliminating the formation of pinholes in the top-coat. In solvent based sealers, carrier solvent and other chemicals must be evaporated during the © Keronite 2020 | keronite.com 7 Protecting Mg Alloys from Corrosion with PEO curing process. As this is not a wholly uniform process throughout the sealer layer, pinholes form to allow the vapours to escape. These pinholes have traditionally been the bane of the corrosion protection due to the formation of moisture pockets and development of a crevice corrosion redox cell which accelerates the rate of corrosion. The Keronite process helps solve this issue due to the formation of an oxidatively stable ceramic layer at the base of the pinholes. Keronite scientists and engineers have developed blends and curing processes that eliminate these types of pinholes altogether. The resulting surface is both smooth and extremely corrosion resistant, as shown by Table 5. Dissolvable Magnesium Alloy Before immersion in 5% NaCl at 80°C 30 minutes Keronite Coated Dissolvable Magnesium Alloy After 6 hours 7 hours Table 5. Salt corrosion test of dissolvable magnesium alloy vs G3Mag-C fluoropolymer Independent testing has shown duplex coated dissolvable magnesium alloy. that these new duplex coatings are capable of withstanding extreme corrosion environments on reactive magnesium alloys e.g. 95°C 3% KCl for 7 days without failure. © Keronite 2020 | keronite.com 8 Talk to our experts about PEO’s corrosion resistance potential on Mg alloys Advanced surface coatings are creating new opportunities for manufacturers to exploit the opportunities provided by magnesium alloys. They are helping reduce weight, improve fuel efficiency and enhance the performance of all sorts of equipment. At Keronite, our materials scientists have developed advanced corrosion-resistant coatings for magnesium. We also have a range of research projects underway, in partnership with our customers, to offer uniquely advantageous coatings. To learn more, get in touch with our materials science experts. Get in touch