Magnesium Anodizing: Plasma/Conversion Coatings That Stop Corrosion

Yes, magnesium genuinely anodizes, it's one of only a few structural metals (with aluminum and titanium) that forms a real anodic coating. But the processes and purpose differ from aluminum. The modern premium process is plasma electrolytic oxidation (PEO), known commercially as Tagnite, Keronite, or Magoxid, which uses high-voltage micro-arc discharges to convert the surface into a hard ceramic magnesium-oxide coating, typically 10-50 microns, with excellent wear and corrosion resistance and no hexavalent chromium. The legacy anodic processes are chromic-acid types like Dow 17 (green-black) and HAE (hard alkaline) under MIL-M-45202, which work well but use regulated hex-chrome and are being phased out. The big practical difference from aluminum: aluminum anodize is often the complete final finish, but magnesium is so chemically active that its anodic coating is usually just one layer in a full protective system (conversion/anodic base, primer, topcoat, sealed edges). You almost never leave magnesium with just an anodic coating and call it done, the coating is mandatory for corrosion survival, not a cosmetic choice, and it's typically sealed and painted over.

For AZ91D, the high-purity die-casting alloy used in automotive and consumer housings, the typical robust finish is a conversion or anodic base coat followed by a primer-and-topcoat paint system, with PEO (plasma electrolytic oxidation) being the premium option where wear and maximum corrosion resistance are needed. AZ91D itself is already a big improvement over older magnesium alloys because it's controlled for low iron, nickel, and copper, the impurities that catalyze magnesium corrosion, so high-purity AZ91D has far better salt-spray performance than standard-purity material. But even high-purity AZ91D needs a coating system in any humid, salt, or galvanic-exposure environment. A common stack is a chrome-free conversion coating, an epoxy primer, and a durable topcoat, with all edges, holes, and machined surfaces sealed because bare spots initiate corrosion. For under-hood or structural aerospace parts, PEO plus seal plus paint gives the best durability. The other critical measure is galvanic isolation: AZ91D fasteners and inserts must be isolated from steel and aluminum hardware with coatings, sealants, or compatible washers, because magnesium will sacrificially corrode at any bare contact with a more noble metal. Cost and process complexity scale with the coating stack, from simple conversion-plus-paint to full PEO systems.

Magnesium is the most anodic (least noble) structural metal in common use, sitting below aluminum, zinc, and steel in the galvanic series, which means whenever magnesium is electrically connected to a more noble metal in the presence of moisture, the magnesium corrodes sacrificially and rapidly. A steel or even aluminum fastener threaded into a magnesium housing creates a galvanic cell that can eat away the magnesium around the fastener, a classic field-failure mode. This is handled through deliberate galvanic isolation: using compatible fasteners (or coating/plating the fasteners), installing insulating washers and sleeves, applying sealant at the interface, and ensuring the magnesium's protective coating system extends into and around the joint so there's no bare magnesium at the contact. Designers also locate dissimilar-metal joints away from moisture traps and seal them thoroughly. Coating the magnesium fully (PEO or conversion plus primer plus topcoat) and sealing all fastener holes is essential, because any breach exposes bare metal that becomes a corrosion site. This galvanic sensitivity is why magnesium finishing is never just a single anodic coat, it's a complete protective and isolation strategy, and it's a major reason magnesium is harder to design with than aluminum despite its lower density.

WE43 (magnesium with yttrium and rare-earth elements like neodymium) is a high-performance alloy with notably better corrosion resistance and high-temperature strength than conventional AZ-series magnesium, so it's used in aerospace gearbox housings, missile and aircraft components, and even bioresorbable medical implants. Its rare-earth additions improve the inherent corrosion behavior, but it still needs protective finishing for most service, and the same family of coatings applies: PEO (plasma electrolytic oxidation) for hard ceramic wear-and-corrosion protection, conversion coatings, and full paint systems on aerospace structural parts. For aerospace WE43, PEO plus seal plus primer/topcoat is common, often under AS9100 and sometimes ITAR control given the defense applications. The interesting outlier is the bioresorbable medical use, where WE43-based alloys are designed to corrode in the body in a controlled, beneficial way (the implant dissolves as bone heals), so there the surface treatment goal is to control and tune the corrosion rate rather than stop it entirely, using specialized surface modifications rather than barrier coatings. For structural and aerospace uses, treat WE43 like other magnesium for finishing purposes, coat it fully, seal edges, and isolate galvanic couples, while benefiting from its better baseline corrosion resistance. Its higher alloy cost and aerospace pedigree mean it's usually finished to demanding qualified specs rather than commercial conversion coatings alone.