Explainer: Why Magnesium Die Castings Are Corrosive

What drives the corrosion behavior of magnesium alloy?

Magnesium die casting corrodes for two distinct reasons. The first is electrochemical. Magnesium sits at the negative end of the electrochemical series. If a magnesium part touches a less active metal, such as steel, aluminum, or zinc, the magnesium becomes the anode. It oxidizes to protect the other metal. The result is localized material loss on the magnesium side.

The second reason is internal. Die casting alloys are not pure magnesium. They contain additions of aluminum, manganese, copper, and iron. These additions improve strength and fluidity. They also introduce impurities. Tiny inclusions of iron or manganese can form galvanic cells within the material itself. When moisture reaches these inclusions, corrosion starts at the inclusion boundary and spreads into the surrounding matrix.

This combination makes magnesium more reactive than zinc die casting alloys. A zinc part may sit in a damp environment for years without visible damage. A magnesium part in the same environment may develop white powder or pitting within months. The difference is not usually manufacturing error. It is the fundamental chemistry of the base metal.

How does the galvanic effect change design choices?

The galvanic effect dictates how magnesium parts interact with other components. If a magnesium housing holds a steel bolt, the steel is the cathode. The magnesium is the anode. Moisture from the environment or internal condensation completes the circuit. The magnesium corrodes around the bolt head. The steel remains intact.

This creates a specific design constraint. Magnesium parts should not be mechanically fastened to dissimilar metals without a barrier. Engineers use plastic washers, painted surfaces, or isolating gaskets to break the electrical connection. The barrier must be continuous. A small gap in paint or a scratched coating can create a point where corrosion accelerates.

The size of the cathode also matters. A large steel surface accelerates corrosion on a small magnesium surface. A small steel screw on a large magnesium bracket can cause rapid local attack. Designers must account for this asymmetry. The ratio of cathode area to anode area is a key factor in predicting service life.

What role do internal inclusions play?

Internal inclusions are microscopic particles trapped in the solidified metal. In magnesium die casting alloys, these often consist of iron, manganese, or aluminum oxide. They are not always harmful. In some cases, they improve strength. In the context of corrosion, they are weak points.

An inclusion acts as a local cathode or anode depending on its composition. When water reaches the inclusion-matrix boundary, a small electrical potential difference drives ions. Magnesium atoms dissolve from the matrix near the inclusion. This creates a small pit. The pit deepens as the process continues. The inclusion itself may remain intact while the surrounding metal disappears.

The density of inclusions affects the rate of pitting. Higher alloy content can increase inclusion density. This does not necessarily mean the part is defective. It means the part may require a thicker protective coating. A die caster will often use a coating that builds up a thicker film to cover more surface area and seal potential pits.

How do standard coatings address magnesium corrosion?

Coatings are the primary defense against magnesium corrosion. They must block moisture and isolate the magnesium from other metals. The most common coatings are chromate conversion coatings, zinc plating, and anodizing. Each has different properties and limitations.

Chromate conversion coatings create a thin, porous film on the surface. They provide a sacrificial barrier and inhibit corrosion. However, they are thin. A scratch or chip exposes the base metal. They are often used as a base coat before painting. The paint provides the bulk of the protection.

Zinc plating adds a sacrificial layer. The zinc corrodes before the magnesium does. This is effective, but the zinc layer must be thick enough and free of porosity. If the zinc coating has holes, moisture reaches the magnesium directly. The zinc then protects the surrounding area, but the pitting at the holes continues.

Anodizing is less common for magnesium die casting because magnesium does not form a dense oxide layer like aluminum. Chemical anodizing can be used, but it requires careful control. The resulting layer is porous and must be sealed. If the seal is not uniform, moisture passes through the pores to the base metal.

How do alloy selections affect corrosion resistance?

Not all magnesium die casting alloys are equal in their susceptibility to corrosion. The choice of alloy grade influences the internal structure and the behavior of inclusions.

The table below compares common magnesium die casting alloy behaviors. The data describes general trends, not specific performance metrics for a single application.

Alloy Type	Typical Additions	Corrosion Behavior	Design Implication
Magnesium 5 (Mg5)	Aluminum, Manganese	High reactivity, pitting prone	Requires thick coatings, avoid steel contact
Magnesium 6 (Mg6)	Aluminum, Manganese	Moderate reactivity, stable structure	Standard coating systems, good for indoor parts
Magnesium 9 (Mg9)	Aluminum, Manganese	Lower reactivity, denser structure	Better for outdoor or damp environments
Magnesium 17 (Mg17)	Aluminum, Manganese	High strength, moderate corrosion	Requires careful plating, check for porosity

Magnesium 5 is a common general purpose alloy. It has high fluidity and good surface finish. It is also highly reactive. In a wet environment, it pits quickly. Magnesium 9 offers better corrosion resistance due to a denser matrix structure. It is often selected for parts that will see outdoor exposure or high humidity.

The selection is not just about the alloy. It is about the combination of alloy, coating, and environment. A Magnesium 9 part with a thin, scratched paint coating may fail faster than a Magnesium 5 part with a thick, intact epoxy coating. The engineer must evaluate the system, not just the base metal.

How does porosity interact with corrosion?

Porosity is a common defect in die casting. It appears as small voids or gas pockets in the solidified metal. In magnesium, porosity is particularly dangerous for corrosion. The voids act as reservoirs for moisture. Water gets trapped inside the cavity. The trapped water does not drain. It stays in contact with the magnesium walls of the void.

This creates a continuous source of electrolyte. The trapped water facilitates the internal galvanic reactions described earlier. Corrosion starts from the inside of the void and spreads to the surface. The part may look fine on the outside, but the internal structure is degrading. This is a hidden failure mode.

Porosity control is a critical part of the die casting process. It involves vacuum holding, proper mold temperature, and careful gating. A die caster who controls porosity reduces the risk of internal corrosion. The buyer should ask for porosity specifications in the drawing. Visual inspection is not enough. X-ray or ultrasonic testing may be required for critical parts.

What are the practical implications for sourcing?

Sourcing magnesium die casting parts requires a different level of detail than zinc die casting. The buyer must specify the alloy, the coating, and the environmental exposure. A simple request for “magnesium die casting” is not sufficient. The specification must define the corrosion protection requirements.

The drawing should state the coating type, thickness, and adhesion requirements. It should also specify the alloy grade. If the part will be in contact with steel, the specification should require an insulating coating or a non-galvanic assembly method. The supplier should be able to explain how they control porosity and how they test for coating porosity.

Quality control is more complex for magnesium. The parts must be tested for coating adhesion, porosity, and salt spray resistance. The salt spray test is a standard method for evaluating coating performance. The part is exposed to a salt solution in a fog chamber. The test measures how long the coating takes to show rust or pitting. A higher time indicates better protection.

The supplier should provide test reports for each lot. These reports should include the alloy certification, coating thickness measurements, and salt spray results. The buyer should verify that the coating system meets the project requirements. If the coating fails a test, the part is rejected. The cost of rework is higher than the cost of specifying the correct coating initially.

How to specify magnesium die casting in a drawing

A clear drawing specification prevents most corrosion issues. The drawing should include the following elements:

1. Alloy designation: Specify the exact alloy, such as ASTM B270 or equivalent.
2. Coating specification: State the type, thickness, and color of the coating.
3. Environmental rating: Indicate if the part is for indoor or outdoor use.
4. Assembly constraints: Note if the part contacts dissimilar metals.
5. Testing requirements: Require salt spray, adhesion, and porosity testing.

The drawing is the contract between the buyer and the supplier. It defines what “good” means. Without it, the supplier will use standard practices, which may not be enough for a specific application. The buyer must define the risk profile and the acceptable level of protection.

Corrosion is not a single event. It is a slow process that starts with a small defect and grows over time. The goal of design and sourcing is to delay that start and limit the growth. By understanding the material science, the engineer can make informed decisions that extend the service life of the part. The choice of alloy, coating, and assembly method are all part of the same system. They must work together to protect the magnesium from its own reactivity.