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Sometimes—but not by default. A magnesium alloy can be appropriate for a structural part only when the exact alloy, temper, product form, geometry, loads, joints, environment, fire exposure, inspection plan, and approval requirements have been evaluated together. A test or material specification for one alloy and use case does not make every magnesium component safe.
What “safe” means for a magnesium structure
Structural safety is a system property, not a label attached to the word “magnesium.” The same alloy family can behave differently as sheet, plate, extrusion, forging, casting, chips, or powder. Temper, thickness, heat treatment, surface condition, joining method, and service temperature also change performance.
A defensible decision answers five questions:
- Does the selected product form retain adequate strength and ductility under the design loads and temperatures?
- Can the geometry, joints, and adjacent materials tolerate corrosion, vibration, fatigue, and damage?
- Could the component ignite or worsen a fire in the actual exposure scenario?
- Can inspection and maintenance detect degradation before failure?
- What code, certification, or customer approval route applies to the finished structure?
These questions must be answered for the real part. General statements such as “magnesium is safe” or “magnesium is unsafe” are technically misleading.
The main variables that control risk
Alloy, temper, and product form
AZ31B sheet in a specified temper is not interchangeable with a die casting, an extrusion, or a forging. Composition limits, grain structure, porosity, heat treatment, and directional properties vary by product form. Use design allowables and test data that identify the exact grade, temper, thickness or section size, manufacturing route, and temperature range.
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Geometry and loading
Thin walls, sharp transitions, holes, threads, and contact surfaces can concentrate stress. Load cases should include static strength, fatigue, impact, vibration, creep where temperature makes it relevant, and credible damage or manufacturing defects. A design that is adequate in a smooth laboratory coupon may not be adequate at a fastener hole or rubbing interface.
Environment and adjacent materials
Moisture, chloride or sulphate salts, condensation, temperature cycling, and contaminants can accelerate corrosion. Contact with dissimilar metals can create galvanic couples, particularly where an electrolyte bridges the surfaces. Sealants, isolation washers, coatings, drainage, and inspection access must be designed rather than added as an afterthought.
Fire exposure
Ignition behavior depends on surface area, thickness, heat flux, ventilation, and whether the material is solid or finely divided. A finished solid component and a pile of machining chips are different fire hazards.
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Can magnesium alloys catch fire?
Why chips and powder are especially hazardous
FAA Advisory Circular AC 33.17-1B, dated May 20, 2025, notes that many magnesium engine alloys are highly combustible when finely divided, including chips and powder. Fine particles have a much larger surface area than a solid part, so they can ignite and burn rapidly under suitable conditions. Machining areas therefore need controls for chip accumulation, hot work, friction, ignition sources, and housekeeping.
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Thin sections, rubbing, and hot gases
The same FAA guidance calls particular attention to thin sections and to parts that rub or operate at high scrubbing speeds. Frictional heating, contact with hot gases, and a geometry that exposes a large surface area can increase vulnerability. The circular recommends evaluating the overall engine design so that a sustained magnesium fire does not create a hazardous condition. This is aircraft-engine guidance, not a universal prohibition on magnesium structures in other industries.
What aircraft-seat testing does—and does not—show
FAA programs reported in DOT/FAA/AR-11/3 and DOT/FAA/TC-13/52 examined magnesium-alloy aircraft seat frames under laboratory and full-scale aircraft fire scenarios. SAE AIR6160A, stabilized January 9, 2020, summarizes the FAA work and reports that the specific alloys tested did not increase hazard potential in the tested passenger-cabin post-crash fire scenario. FAA material also notes that susceptibility to ignition differs among magnesium alloys and that an ignited magnesium fire can be difficult to handle with aircraft fire extinguishers.
Those findings apply to the named alloys, seat-frame designs, quantities, and test conditions. They do not establish safe behavior for a building frame, vehicle component, pressure vessel, machine guard, or another alloy exposed to a different fire. A project must define its own fire scenario and acceptance criteria.
Fire questions to resolve in a design review
- What heat source, duration, ventilation, and fuel load are credible?
- Are there thin walls, exposed edges, rubbing interfaces, or high-speed contacts?
- Can chips, dust, or powder accumulate during manufacturing or maintenance?
- How will ignition be detected, isolated, suppressed, and cleaned up?
- Would a sustained fire threaten occupants, escape paths, critical systems, or neighboring materials?
Corrosion and stress-corrosion cracking
General corrosion is not the only concern
Loss of section from corrosion reduces load-carrying area, while pits and surface defects can become fatigue-crack starters. Water traps, damaged coatings, blocked drainage, and unsealed joints are common design concerns. Galvanic attack can occur where magnesium contacts a more noble metal through a conductive electrolyte. Electrical isolation, compatible finishes, sealants, drainage, and a defined inspection interval are part of the structural design.
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What ISO 20728:2018 actually establishes
ISO 20728:2018 specifies a method for determining resistance of magnesium alloys to stress-corrosion cracking in structural applications. It permits relative assessment in chloride- or sulphate-containing environments when the failure mechanism remains unchanged. The standard expressly states that the method does not qualify a material or product for service application.
Therefore, a passing result is comparative evidence under the test conditions, not a service approval. Engineers still need representative stress levels, surface condition, environment, duration, residual stresses, and consequences of cracking considered for the real part. If the failure mechanism changes between the test and service, the relative comparison may no longer be valid.
Practical corrosion controls
- Specify the alloy, temper, coating system, preparation, and allowable coating damage.
- Prevent stagnant water with drainage, ventilation, and accessible joint details.
- Isolate dissimilar metals electrically and verify sealant compatibility.
- Control chloride and sulphate exposure where feasible and define cleaning procedures.
- Inspect pits, coating blisters, fastener areas, crevices, and crack-like indications at intervals tied to risk.
- Reassess the design if service conditions, coatings, fasteners, or maintenance chemicals change.
Strength, standards, and the limits of a material specification
Example: ASTM B90/B90M-21
ASTM B90/B90M-21 covers AZ31B magnesium-alloy sheet and plate in tempers O, H24, and H26. It specifies chemical composition and tensile properties for the applicable thickness. Those requirements help control incoming material, but they do not by themselves demonstrate fire safety, corrosion resistance, fatigue life, joining performance, or code compliance for a finished structure.
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Why allowables matter more than a catalog strength
Design values must reflect orientation, thickness, temperature, manufacturing variability, joining details, fatigue spectrum, and the required reliability. A nominal tensile value from a room-temperature coupon cannot substitute for allowables or qualification data for the finished part. Casting porosity, weld or fastener effects, surface damage, and anisotropy can govern performance even when the base material meets its specification.
A project workflow for deciding whether magnesium is appropriate
- Define the part and consequences. Record function, load paths, failure consequences, occupants or equipment at risk, required life, inspection access, and whether failure is fail-safe, contained, or catastrophic.
- Freeze the material identity. Name the alloy, temper, product form, thickness or section, heat treatment, supplier controls, and allowable substitutions. Do not use a family name as the material definition.
- Map every load case. Include static, fatigue, impact, vibration, thermal gradients, assembly loads, accidental damage, and manufacturing tolerances. Identify stress concentrations and rubbing contacts.
- Characterize the environment. Specify humidity, immersion, salt or sulphate exposure, contaminants, temperature range, cleaning agents, galvanic partners, and coating damage that must be tolerated.
- Define the fire scenario. Set credible ignition sources, heat flux, duration, ventilation, suppression method, and consequences. Test representative geometry when fire behavior is a governing requirement.
- Select appropriate tests. Use stress-corrosion testing such as the method in ISO 20728:2018 for relative resistance, while recognizing its service-qualification limitation. Add fatigue, fracture, corrosion, fire, joining, and environmental tests as the application requires.
- Qualify manufacturing and joints. Control casting quality, machining, forming, heat treatment, fasteners, adhesive or mechanical joints, coatings, and repair processes. Verify that production variation is covered by inspection.
- Check the approval route. Identify jurisdictional codes, customer specifications, airworthiness or vehicle requirements, and documentation needed for traceability and continued-airworthiness or maintenance programs.
- Set maintenance and retirement criteria. Define inspection methods, intervals, allowable corrosion or damage, repair limits, chip and dust housekeeping, and conditions that require removal from service.
- Have an independent engineering review. A qualified materials, structural, fire, and corrosion team should review the evidence before release. The sources cited here do not qualify any particular project.
How to compare magnesium with another structural material
Do not rank materials using density or one tensile-strength number. Compare candidates for the same function, geometry, manufacturing route, and service conditions.
| Comparison axis | What to compare | Why it changes the decision |
|---|---|---|
| Strength and life | Allowables by product form and temperature; fatigue, fracture, impact, and creep data where applicable | The governing failure mode may not be static tensile strength. |
| Fire behavior | Ignition and burning for the actual thickness, surface area, heat flux, ventilation, and suppression scenario | Solid parts, thin sections, chips, and powder can behave very differently. |
| Corrosion | General corrosion, pitting, galvanic interactions, stress-corrosion behavior, coatings, and inspection burden | Environment and maintenance can dominate life-cycle risk. |
| Joining and production | Fasteners, welding or bonding process, porosity, residual stress, repairability, and quality-control capability | Joint or manufacturing defects often govern structural reliability. |
| Compliance | Applicable code, certification evidence, traceability, and failure-consequence requirements | A technically sound material may still be unusable without an approval path. |
Common failure modes and ways to reduce them
| Failure mode | Typical contributors | Controls |
|---|---|---|
| Ignition or sustained burning | Fine chips or dust, thin exposed sections, rubbing, hot gases, inadequate fire isolation | Housekeeping, friction and temperature control, geometry review, representative fire testing, detection and suppression planning |
| Stress-corrosion cracking | Tensile residual stress plus chloride or sulphate exposure and susceptible alloy condition | Material selection, stress reduction, protective systems, environmental control, inspection, and application-specific testing |
| Galvanic attack | Dissimilar-metal contact bridged by moisture or salts | Electrical isolation, compatible coatings and sealants, drainage, and inspection of interfaces |
| Fatigue cracking | Notches, holes, rough surfaces, vibration, corrosion pits, or underestimated load spectrum | Detail design, surface control, fatigue allowables, damage tolerance, and scheduled inspection |
| Hidden manufacturing weakness | Porosity, inclusions, wrong temper, heat-treatment variation, or unapproved repairs | Supplier qualification, traceability, nondestructive examination where justified, and change control |
Bottom line for engineers and specifiers
Magnesium alloys are neither automatically safe nor automatically unsuitable for structural applications. They can be viable when the exact alloy and product form are supported by appropriate strength, corrosion, fire, joining, manufacturing, and qualification evidence. Treat finely divided material as a distinct fire hazard, use ISO 20728:2018 as a test method rather than a service approval, and treat ASTM product specifications as material controls rather than complete structural safety cases. Final selection should follow a documented, project-specific engineering review and the requirements of the applicable jurisdiction or certification authority.
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Frequently Asked Questions
Can a magnesium alloy that passed a fire test be used in any structure?
No. Fire-test results apply to the named alloy, geometry, quantity, exposure, and acceptance criteria. Aircraft seat-frame results summarized by SAE AIR6160A do not qualify unrelated structures or fire scenarios.
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Does ISO 20728:2018 certify an alloy for service?
No. It provides a method for relative stress-corrosion-cracking assessment in specified environments and expressly states that it does not qualify a material or product for service application.
Is AZ31B sheet covered by a general magnesium specification?
ASTM B90/B90M-21 specifically covers AZ31B sheet and plate in O, H24, and H26 tempers with stated composition and tensile requirements. Other forms, such as castings, forgings, and extrusions, use different specifications.
What information should a supplier receive before quoting a structural magnesium part?
Provide the exact alloy and temper, product form and dimensions, loads and temperatures, environment, fire scenario, joining method, coating requirements, inspection needs, traceability, and applicable code or approval route.
Are machining chips handled like a finished magnesium component?
No. Chips and powder are finely divided and can be highly combustible under suitable conditions. Manufacturing controls must address accumulation, ignition sources, friction, hot work, storage, and cleanup separately from the finished-part design.
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