Inspecting Magnesium Parts and Castings Safely

Magnesium is the most chemically active structural metal, and bare magnesium corrodes fast, so nearly every magnesium part depends on a protective surface treatment, and verifying that treatment is the central quality activity. The common systems are chromate conversion coatings (per older MIL specs), chrome-free conversion coatings, and anodize-type treatments. Inspection confirms coating presence, coverage, and often a salt-spray performance test per ASTM B117, since a magnesium part with a missed or thin conversion coating will corrode in weeks rather than years. Galvanic corrosion is the design-level trap that inspection has to flag. Magnesium is anodic to nearly everything, so a magnesium part bolted to steel or aluminum with the wrong fasteners or no isolation corrodes at the joint. Inspection of magnesium assemblies should verify isolation provisions (coatings, isolating washers, compatible fasteners) are present, because a perfectly coated magnesium part still fails galvanically if assembled wrong. The coating-thickness and adhesion checks mirror other coated metals but with higher stakes. For aerospace WE43 and AZ91 parts, the conversion coating spec and its verification are called out explicitly, and a NADCAP chemical-processing accreditation backs the treatment. A supplier that machines magnesium but treats corrosion protection as an afterthought is the most common path to field corrosion failures on these parts.

Most magnesium parts are castings (AZ91D dominates high-pressure die casting; sand and investment casting cover larger aerospace parts), and casting porosity is the primary internal-defect concern. High-pressure die casting traps gas, producing subsurface porosity that a dimensional check never sees but that becomes a leak path or fatigue initiator when machining cuts into it. X-ray (radiographic) inspection per ASTM E155 reference radiographs is the standard for grading magnesium casting porosity against a defined acceptance class on critical parts. The machining-exposes-porosity problem hits magnesium castings hard. A die-cast AZ91D housing can gauge perfectly until a machined boss or sealing face cuts into a gas pocket, creating a leak path on a pressurized part. Inspection of machined casting features includes a visual and sometimes penetrant check (using approved, non-reactive penetrant systems) for exposed porosity on sealing and bearing surfaces. For aerospace sand and investment magnesium castings, the inspection escalates to full radiography, fluorescent penetrant for surface cracks, and sometimes dimensional layout of the casting envelope. Porosity acceptance classes are called out by zone, with critical zones (around bolt holes and high-stress regions) held tighter than non-critical areas. This zoned inspection is specialized work, and the qualified supplier base is narrower than for aluminum castings.

The reference grades span a wide range. AZ31B is a wrought magnesium-aluminum-zinc alloy for sheet and extrusion; AZ91D is the workhorse die-casting alloy; WE43 is a magnesium-yttrium-rare-earth alloy developed for elevated-temperature aerospace and high-performance applications, with much better high-temperature strength and creep resistance. Inspection rigor tracks the grade: AZ31B and AZ91D get standard alloy and coating verification, while WE43 aerospace parts get full metallurgical scrutiny including microstructure and mechanical verification. WE43's properties depend on a solution-and-aging heat treatment that precipitates the rare-earth strengthening phases, so for aerospace WE43, hardness and microstructure verification confirm the heat treatment took, similar in spirit to aluminum and superalloy aging checks. A WE43 part machined from un-aged stock gauges fine and underperforms in strength and temperature capability. Alloy verification by spectrometry confirms the grade, important because the rare-earth content of WE43 makes it expensive and a substitution with cheaper AZ91 would be a serious escape on a part chosen for high-temperature service. Mill and foundry certs anchor the chemistry, and on aerospace work incoming verification plus full traceability to the heat is standard. WE43 is also used in bioabsorbable medical applications, where purity and microstructure control are even tighter, a specialized inspection tier that few suppliers can certify.