Magnesium Corrosion: Causes, Types & Prevention Methods

Understanding Magnesium Corrosion: From Mechanisms to Proven Protection Strategies

Magnesium is the lightest structural metal to be handled by engineers – about 33% lighter than aluminum, 75% than steel. However, magnesium’s corrosion rate remains the single greatest impediment to increased use in, aerospace, automotive, electronics and medical devices manufacture. It has a standard electrode potential of 2.37 V (vs.

SHE) and remains at the active (anodic) end of the galvanic series.

This guide to corrosion of magnesium provides a detailed comparison of the mechanism of Mg degradation, common alloy grades, compares the corrosion resistance of common Mg alloy grades, and guides you through proven protection techniques. Whether you are choosing a magnesium alloy for a new design or investigating corrosion failures on present parts, this information and hands-on experience will enable you to make smarter engineering choices.

Contents show

Why Magnesium Corrodes — The Electrochemical Reality

magnesium corrosion is electrochemical. If magnesium comes into contact with water or an electrolyte, it will oxidize (remove electrons) dissolving as Mg ions. Oxidation occurs at the anode forming Mg(OH)₂ and hydrogen – this is why you sometimes see very small bubbles coming off a corroding magnesium surface.

At its core, this is a question of thermodynamics. The Standard Electrode Potential for magnesium is the most negative of all of the structural metals, 2.37 volts against the SHE. Its galvanic series potential (in a seawater ASTM G82 galvanic series) has been determined as being 1.60 volts against an Ag/AgCl reference, more than 600mV more negative than zinc, the most active engineering metal.

−2.37 V

Standard Electrode Potential (vs. SHE)

1.74 g/cm³

Density — Lightest Structural Metal

600+ mV

More Negative Than Zinc in Galvanic Series

Unlike aluminum which creates a very well-bonded oxide AlO layer that prevents further attack; the corrosion film of magnesium hydroxide that develops on Mg is porous and poorly adherent. It only provides partial protection and is also very easily pitted away in chloride environments. This explains the sudden escalation of magnesium corrosion in the marine or salt-spray environments, whereas aluminum is fairly unaffected.

In our CNC machining workshops, it is repeatably seen that simply sitting in humid shop air causes surface tarnishing on freshly machined magnesium parts indicating that the corrosion mechanism of Mg occurs almost as soon as the native oxide layer is disrupted.

Types of Magnesium Corrosion in Real-World Applications

magnesium and its alloys can undergo several types of corrosion modes, each one respectively caused by different climate and metallurgical conditions. The importance of the corrosion behaviour of each type is, because the protection system is different for each case.

Corrosion Type	Mechanism	Visual Appearance	Risk Level
Galvanic Corrosion	Contact with a more noble metal creates an electrochemical cell; Mg dissolves as the anode	Accelerated attack at the contact zone, white corrosion product buildup	High
Pitting Corrosion	Chloride ions break down the surface film at localized weak points	Small, deep cavities on the surface	Medium–High
General (Uniform) Corrosion	Even dissolution across the surface in acidic or neutral electrolytes	Uniform surface roughening and thinning	Medium
Stress Corrosion Cracking (SCC)	Combined effect of tensile stress + corrosive environment initiates crack propagation	Branching cracks, often intergranular; may show no visible surface corrosion	Critical
Filiform Corrosion	Thread-like corrosion under coatings or paint; moisture penetrates at defects	Worm-track patterns visible under transparent coatings	Medium

⚠️ Common Mistake

The original corrosion problem we have observed most often with magnesium assemblies is a form of galvanic corrosion resulting from direct contact with steel fasteners. Engineers tend to fasten magnesium housings to steel frames without using any form of insulation or barrier coating and white corrosion product appears in the circumference of every fastener within a matter of weeks. The potential difference between magnesium and steel/mild steel is sufficient to produce rapid localised attack in a relatively high humidity indoor environment.

Stress corrosion cracking (SCC) is another topic that warrants special consideration for load-carrying Mg parts. As indicated in research published in the Journal of The Minerals, Metals & Materials Society (JOM), the stress corrosion cracking threshold of AZ91 in distilled water and in 5 g/L NaCl is 55-75 MPa. parts operating close to these levels of stress must only be designed with an SCC-resistant alloy grade.

What Affects the Corrosion Rate of Mg Alloys

Corrosion rates in magnesium alloys can vary by 6 orders of magnitude based on 5 main influences. Getting the design right is paramount to avoiding most problem field corrosion issues.

Five Factors That Accelerate Magnesium Corrosion

Heavy-metal impurities (Fe, Ni, Cu) – Iron, nickel, and copper develop at very low solid-solubility in Mg. When their levels exceed the tolerance threshold they form cathodic intermetallic particles to promote micro-galvanic corrosion. Of the three, nickel can be three to five times more damaging than iron at equimolar concentrations, and copper the least damaging.
Alloy composition and microstructure – The volume fraction, location, and composition of second phase particles (such as -MgAl in AZ-Series alloys) all induce micro-galvanic cells. Grain size, temper, and process history also influence the corrosion properties of the finished part.
Environment – chlorides, humidity, pH – Chloride ions (Cl⁻) are the primary aggressors. Marine atmospheres, road salt, and even fingerprint residue (which contains NaCl) speed up pitting and general corrosion. Highly alkaline conditions (pH greater than 10.5) actually produce lower corrosion rates because the Mg(OH)₂ surface film becomes more stable.
Contact with dissimilar metals – Contact directly with steel, copper, brass, or other noble metal produces a micro-galvanic cell. The corrosion potential difference drives rapid anodic dissolution of the Mg component.
Surface condition – Machined surfaces with embedded iron particles from cutting tools show higher corrosion rates than clean, polished surfaces. Residual machining fluids will also catalyze corrosion if not properly washed away.

💡 Pro Tip — Impurity Tolerance Limits

In the case of AZ-Series magnesium alloys, the critical Fe/Mn weight ratio that initiates rapid corrosion ranges from 0.010 to 0.032 depending on the exact alloy. For example, AZ91 with 0.15% Mn has an iron limit of around 0.0048% (0.032 x 0.15%). When purchasing Mg alloy stock, make sure to obtain the mill certificate and check that Fe, Ni, Cu levels are below their individual threshold limits.

At Lecreator, our alloy selection process for magnesium CNC projects begins by examining the material certification for impurity levels. We have refused incoming Mg billets that passed dimensional specifications but exceeded the Fe/Mn limit – a detail that would have caused field corrosion of Mg alloy components within months of deployment. This upstream control has prevented several clients from warranty failures.

Magnesium Alloy Corrosion Resistance — Comparing Common Grades

Not all magnesium alloys corrode similarly. Your choice of alloy grade will impact corrosion performance, strength, and machinability. Here’s a comparison of the popular machined alloys.

Property	AZ31	AZ91	AZ80	WE43
Al Content	3%	9%	8%	0% (RE-based)
Corrosion Resistance	Moderate	Good (short-term); degrades over time	Moderate–Good	Good (consistent long-term)
Corrosion Behavior	Uniform surface attack	Initially low rate; accelerates with β-phase undermining	Similar to AZ91 with fewer β precipitates	Micro-galvanic around RE intermetallics
Strength (UTS)	255–290 MPa	230–275 MPa	340–380 MPa	250–295 MPa
CNC Machinability	Excellent (wrought sheet/plate)	Very Good (die-cast or gravity cast)	Good (forged/extruded)	Good (requires sharper tooling)
Typical Applications	Sheet enclosures, laptop cases, brackets	Die-cast housings, engine blocks, covers	Forged wheels, high-strength structural parts	Aerospace fittings, medical implants (biodegradable)
Relative Cost	$	$	$$	$$$

One of the main conclusions from the extensive corrosion testing published in the Journal of Magnesium and Alloys: in 3.5% salt solution, AZ91 had a slightly higher initial rate of corrosion resistance then the az31 magnesium alloy in the first few hours, but on long term immersion (>3 hours) was far more higher corrosion due to undermining of the -MgAl phase—i.e. short term salt spray tests could give a false impression of AZ91’s actual in service performance.

WE43 is a less rare earths alloy which gives more predictable long term corrosion performance and is being currently used in alloys for biomedical applications where there is a need for controlled and predictable degradation – for example biodegradable Mg bone implants in simulated body fluid.

From the point of view of the CNC machining, the exact machinability of each grade is clearly illustrated at the spindle. While the AZ31 wrought plate is generated by long, entangled chips that are dangerous if not kept under control with respect to the fire factor, the die-cast AZ91 billets are machined by shorter chips, finer and easier to evacuate while microporosity may occur to entrapped cutting fluid leading to faster corrosion after machining if not clean and dry.

Magnesium Corrosion Protection: Proven Methods That Work

Corrosion protection for magnesium must be used in a stratified manner. No single technique works for every application — the right choice depends on the service environment, required lifespan, and cost constraints. Here are five proven corrosion protection methods ranked by durability.

Micro-Arc Oxidation (MAO / Plasma Electrolytic Oxidation) is a high-voltage plasma discharge process in an alkaline electrolyte to produce an oxide coating (10-100 m) directly on the Mg surface. MAO produces dense, well-adhered coatings and provides the highest corrosion protection out of the electrochemical treatments. Recent research from PMC (2024) confirms that MAO-treated magnesium alloys show dramatically improved corrosion resistance compared to untreated surfaces.
Anodizing- electrochemically forms a thin oxide layer (5-25 m) over the material using lower applied voltage. It is less strong than MAO and inexpensive, but useful for housings for consumer electronics and for use indoors.
Chemical Conversion Coatings – Chromate free conversion coatings (permanganate stannate or rare-earth based) form a very thin layer. Usually used as a preparatory step before painting. May have a lower cost but provides only moderate protection.
Organic Coating (Paint/Powder Coat/E-coat) – applied on top of a conversion coating for the best corrosion protection. E-coat (electrodeposition) gave the most even coverage on complex shapes. The weak point is any scratch, chip, or imperfection that exposes bare Mg – hence the value of a good conversion coating underneath.
Design-Level Prevention — Avoid dissimilar metal contact. Use insulating washers (nylon, PEEK), barrier tapes, or apply sealant at all metal-to-metal joints. Design drainage paths so water cannot pool on Mg surfaces. This costs almost nothing but prevents the most common corrosion failures.

✔
Choose the Mg alloy grade depending on your corrosion environment (see table above)
✔
Apply surface treatment immediately after machining — do not leave bare Mg exposed
✔
Isolate all dissimilar metal joints with insulating barriers
✔
Verify impurity levels in raw material certificates before machining
✔
Specify salt spray testing (ASTM B117) and corrosion measurements in your part qualification requirements

Here at Lecreator, working with various partners including surface treatment, we are able to provide MAO, anodizing and chemical conversion as post-machining finishes for our precision machining services for corrosion-sensitive magnesium components. Which finish you prefer (or requires) will be dictated by the working environment – outdoor marine will use MAO plus topcoat, while electronics indoors usually only need anodizing.

💡 Pro Tip

No coating lasts forever. All coatings break down in time (and any mechanical damage exposes bare magnesium for further corrosion. When designing your corrosion protection system, plan for the coating to be left behind – which makes alloy selection and design-level separation almost as important as the coating itself.

CNC Machining and Magnesium Corrosion — What Engineers Must Know

CNC machining a magnesium component requires knowledge of both corrosion rejection and fire safety. First, the corrosion concerns – and second, the fact that magnesium is flammable and aluminum or steel is not.

Machining-Induced Corrosion Risks

During machining – carbon particles from the cutting tools can embed into the magnesium surface. These carbon inclusion sites form micro-galvanic cells that make corrosion progress much faster. Lecreator uses special carbide and PCD (polycrystalline diamond) tooling for magnesium jobs (no steel/iron tooling is used) to prevent contaminating the material.

Also important is coolant type. Water based or water soluble machining fluids will react with magnesium chips and cut surfaces – generating hydrogen as well as instigating corrosion. We prefer light mineral oil or compressed air for mag machining (as lubricant), and immediately dry the workpieces after machining.

Fire Safety Protocol

⚠️ Critical Safety Warning

Chip handling is important. Our chips are stored only in closed steel drums, separated from other steels and chips every step of the way. Magnesium scrap is very flammable, and cannot use a standard ABC fire extinguisher or extinguishing medium. Only a Class-D fire extinguisher or a dry (sand) extinguishing media are effective. Per OSHA combustible dust regulations, facilities machining magnesium must maintain proper dust collection and emergency response procedures.

✔
Use sharp tooling. (hot spots and heat) generates fine particles & deforms the chip.
✔
Use discontinuous chips (by adjusting feed rate) not long ribbon chips.
✔
Store chips in closed steel drums, separated from other chips & metals.
✔
Keep Class-D fire extinguisher close at hand.
✔
Apply surface treatment or some other coating within 24 hours of machining operation.

With 17 years and over 1,000 magnesium machining projects completed, our team at Lecreator’s magnesium machining capabilities has built these protocols into every production run. Knowing how to practically handle the material, from the machine tool to the shelf, prevents much of the magnesium corrosion problems we see in the field.

Frequently Asked Questions About Magnesium Corrosion

Q: Is magnesium highly corrosive?

View Answer

Magnesium itself is not corrosive—but it is very easy to corrode. With a standard electrode potential of 2.37 V, magnesium is the most electrochemically active of all standard structural metals. It corrodes faster than aluminum, zinc, or even steel in most environments—especially if moisture, salt, or contact with other dissimilar metals is present. However, with proper alloy choice and surface treatment, magnesium parts can provide a service life acceptable even in moderately harsh environments.

Q: How to stop magnesium from corroding?

View Answer

Five proven ways to improve corrosion resistance: (1) Protect magnesium by applying surface treatments as micro-arc oxidation (MAO) or anodizing. (2) Coat with chemical conversion coatings to prepare for painting. (3) Protect magnesium from contact with other dissimilar metals with separators like insulating washers and gaskets, or with barrier coatings. (4) Choose high-purity alloy grades that contain minimal Fe, Ni, and Cu impurities. (5) Design assemblies that shed water away from magnesium surfaces and prevent drainage.

Q: Does magnesium rust?

View Answer

No. Rust specifically means iron oxide (Fe₂O₃), which only forms on iron and steel. Magnesium corrodes differently—it produces magnesium hydroxide (Mg(OH)₂), a white powder on the surface.

Q: What does magnesium corrosion look like?

View Answer

Magnesium hydroxide (Mg(OH) ) deposits out onto the surface as white or pale gray powder—this is magnesium hydroxide. In more advanced cases, dark gray or black spottiness may be evident, and pitting or large blisters occurring under paint, coating, or plating. In the case of galvanic attack from a dissimilar metal, all such attack is localized to the mating surface area, with plentiful white oxide buildup around fastener holes.

Q: Does magnesium chloride cause corrosion?

View Answer

Yes. One of the most aggressive corrosion accelerants for magnesium alloys is magnesium chloride (MgCl). This compound is abundant in road de-icing agents and marine environments. The chloride anion (Cl) penetrates the protective oxide film of magnesium and triggers pitting corrosion. This is why magnesium parts in cars that are used in colder climates typically require very durable surface protection—like MAO or multi-layer coatings.

Q: How resistant are magnesium alloys to corrosion in harsh environments?

View Answer

Bare magnesium alloys show poor corrosion resistance in aggressive environments—marine atmospheres, chemical plants, or any setting with high chloride or moisture exposure. But the right combination of alloy grade and surface finish changes the picture entirely. WE43 with MAO plus a sealed topcoat, for instance, can push salt spray endurance past 500 hours under ASTM B117 testing. Match the protection system to the actual service conditions. Indoor electronics enclosures need far less armor than an underbody automotive bracket exposed to road salt year-round.

Need Corrosion-Resistant Magnesium Parts?

Lecreator machines magnesium alloys with dedicated tooling, fire-safe procedures, and post-machining surface treatment choices. Send us your drawing for a free DFM consultation.

Get a Magnesium Machining Quote

About This Analysis

This knowledge piece was prepared by Lecreator’s engineering department, utilizing 17 years of CNC machining know-how spanning all magnesium alloy materials—AZ31, AZ91, WE43, and others. The corrosion data cited here originates from published research, in-house testing, ASTM standards, and OSHA sources. Our surface treatment recommendations are what we have tried and tested in actual production environments and know to work in more than 1,000 magnesium jobs.