The basics of coating lightweight metals through safety, surface preparation and adhesion

Introduction

Magnesium is known as a very lightweight metal.
It is sometimes used for electronic equipment, precision parts, automotive parts, motorcycle parts and other applications requiring weight reduction.

Lightweight.
Easy to fabricate.
High functionality.
Used in high-value parts.

These characteristics make magnesium an attractive material.

From a coating perspective, however, magnesium is not an easy substrate.

Magnesium coating is prone to problems such as the following.

• Poor adhesion
• Corrosion
• Bubbling
• Residue after surface preparation
• Seepage of treatment solution
• Dimensional changes caused by acid pickling
• Ignition risk during sanding or blasting
• Dust explosion risk
• Poor compatibility between primer, filler and topcoat

Selecting a coating material alone is therefore insufficient for magnesium.
Substrate properties, safety, surface preparation, surface conditioning, primer, drying and adhesion evaluation need to be considered together.

This article explains why magnesium is difficult to coat and what should be checked on the shop floor.

1. Magnesium is light, but difficult to prepare for coating

Magnesium offers major advantages as a lightweight metal.
On the other hand, its surface is reactive and prone to corrosion.

Appropriate surface preparation is therefore needed before coating.

However, this preparation is not straightforward.

Treatment that is too mild makes adhesion difficult.
Treatment that is too aggressive damages the substrate.
Inadequate cleaning leaves treatment residue that causes poor adhesion.
Inadequate drying can lead to bubbling or blistering later.
An inappropriate sanding method increases safety risks.

Simply cleaning the surface is not enough for magnesium coating.
The surface needs to support adhesion without substrate damage, while allowing safe work.

2. Safety management is a prerequisite for quality management on magnesium

Safety management is one of the most important concerns in magnesium coating.

Magnesium has increased ignition and explosion risks in dust form.
Adequate safety measures are needed when sanding, machining or blasting generates fine powder.

The surface needs roughening to improve adhesion.
Sanding or blasting is used for this purpose.
However, this treatment generates dust and increases safety risks.

This is a difficulty of magnesium coating.

From coating quality alone, surface roughening may be effective.
However, surface preparation that ignores operator safety should not be adopted.

Check the following safety items.

• Is dry sanding being performed?
• Does dry blasting generate dust?
• Is dust collection equipment appropriate?
• Are exhaust and ventilation sufficient?
• Are ignition sources strictly controlled?
• Do operators understand the hazards?
• Is a method that accounts for safety, such as wet blasting, needed?
• Is the dust cleaning method appropriate?

For magnesium, a safe process needs to be established before quality improvement.

3. Alkaline degreasing and acid pickling also require care

Degreasing or acid pickling may be considered for magnesium surface preparation.
However, chemical treatment also requires care.

Different magnesium alloys react differently to treatment solutions.
A treatment suitable for one alloy may cause corrosion or dimensional changes in another.

Acid pickling in particular reacts strongly with the substrate surface and may change dimensions.
Even small dimensional changes can create quality problems in precision parts.

An unsuitable degreaser may damage the surface or fail to remove enough oil.

Check the following items.

• Magnesium alloy type
• Compatibility with the degreaser
• Alkaline degreasing conditions
• Need for acid pickling
• Dimensional changes caused by acid pickling
• Treatment time
• Treatment temperature
• Rinsing conditions
• Surface condition after treatment

More aggressive chemical treatment is not necessarily better for magnesium.
Conditions need to be selected for the purpose while checking their effects on the substrate.

4. Chemical conversion treatment improves adhesion, but residue needs attention

Chemical conversion treatment is sometimes used to prepare magnesium surfaces for coating.

It is expected to create fine surface irregularities that make mechanical anchoring easier.
It may also be used to improve corrosion resistance.

However, residue left after conversion treatment leads to poor adhesion.

Insoluble residue generated during preparation is sometimes called smut.
If smut remains on the surface, it creates something like a contamination layer between the coating and substrate.
The film then fails to adhere firmly, potentially causing peeling or blistering.

Check the following points for conversion treatment.

• Treatment solution type
• Treatment concentration
• Treatment time
• Treatment temperature
• Surface roughness
• Residual smut
• Rinsing condition
• Drying condition
• Adhesion test results
• Corrosion resistance evaluation

Conversion treatment is an important process for improving adhesion.
However, inadequate management can make it a cause of defects instead.

5. Roughening the surface does not necessarily improve adhesion

Moderate surface roughening can improve coating adhesion.
The coating enters surface irregularities and anchors mechanically.

However, excessively aggressive treatment also requires caution on magnesium.

Excessive treatment may cause the following problems.

• Excessive surface roughness
• More residue
• Seepage of treatment solution
• Excessive attack on the substrate
• Changes in dimensional accuracy
• Poorer appearance
• Adhesion becomes less consistent instead

Improving adhesion requires an appropriate surface condition.
Aggressive treatment is not always better.

Magnesium coating requires simultaneous checks of surface roughness, cleanliness, residue removal, dimensional accuracy and safety.

6. Porosity, cracks and bubbling require countermeasures

Depending on the forming method and substrate condition, magnesium parts may have surface pores, fine cracks or unevenness.

These substrate defects lead to bubbling or appearance defects after coating.

For example, air or treatment solution remains in surface pores.
Coating is applied over them.
During drying or baking, the trapped air or liquid expands.
The film then develops bubbles or blisters.

Preventing these problems may require surface conditioning through filler, primer or undercoats.

Check the following items.

• Presence of porosity
• Presence of surface cracks
• Forming method
• Effects of mold release agents
• Need for filler application
• Need for a primer
• Bubbling during drying
• Whether sanding exposes the substrate
• Surface smoothness before the topcoat

Trying to obtain appearance through the topcoat alone can make magnesium coating inconsistent.
The underlying process that conditions the substrate surface is important.

7. Consider filler, primer and topcoat as a system

It is risky to assume that one coating material alone can create quality on magnesium.

Consider a coating system such as the following where needed.

• Surface conditioning
• Chemical conversion treatment
• Filler application
• Primer
• Sanding
• Intermediate coat
• Topcoat

Filler corrects pores and unevenness.
Primer improves adhesion to the substrate.
Topcoat provides appearance, weather resistance, chemical resistance, abrasion resistance and other properties.

These processes do not exist independently; together they function as one coating system.

Even a good primer will not adhere with poor surface preparation.
Even a good topcoat will have appearance defects if the underlying layers bubble.
Exposing the substrate through sanding can create a starting point for defects.

Designing the complete film system is important for magnesium coating.

8. Chromium-free treatment and environmental considerations

Environmental considerations are also important in magnesium coating.

Treatments containing chromium have traditionally been used for corrosion resistance or adhesion.
However, environmental, safety and regulatory considerations are increasing demand for chromium-free treatments.

Chromium-free treatment offers environmental advantages.
On the other hand, consistent quality requires adequate verification.

Check the following points.

• Is adhesion sufficient?
• Is corrosion resistance sufficient?
• Are treatment conditions stable?
• Can the treatment solution be managed?
• Are rinsing and drying appropriate?
• Is there variation in mass production?
• Is compatibility with the coating material good?

Environmental measures are important.
However, prioritizing them alone while coating quality becomes inconsistent leads to customer complaints and recoating.

CoatureVietnam needs a perspective that balances environmental measures with consistent quality.

9. Basic items to check for magnesium coating

When considering magnesium coating, the following items need to be checked.

About the substrate

• Pure magnesium or a magnesium alloy
• Alloy type
• Forming method
• Presence of pores or cracks
• Surface condition
• Dimensional accuracy requirements
• Service environment

About safety

• Does sanding or blasting generate dust?
• Is the treatment dry or wet?
• Are dust collection and exhaust equipment appropriate?
• Are ignition sources controlled?
• Are operators trained?
• Is the cleaning method safe?

About surface preparation

• Is the degreaser appropriate?
• Is acid pickling necessary?
• Does acid pickling change dimensions?
• Are conversion treatment conditions appropriate?
• Is smut left on the surface?
• Are rinsing and drying sufficient?

About the coating specification

• Is a primer needed?
• Is filler application needed?
• Are there measures to prevent bubbling?
• Does the topcoat suit the application?
• Is film thickness appropriate?
• Do drying conditions suit the substrate?

About evaluation

• Are adhesion tests performed?
• Are corrosion resistance tests performed?
• Is bubbling evaluated?
• Are dimensional changes checked?
• Are small-batch trials performed before mass production?

10. Points to note in Vietnamese factories

The following points require particular attention when coating magnesium in Vietnamese factories.

10-1. Do not postpone safety measures

Dust and ignition risks must not be underestimated with magnesium.
Sanding or blasting to improve quality may create occupational safety risks.

A process that cannot protect operators’ safety cannot function as a quality process in the long term either.

10-2. Clarify substrate information

The magnesium alloy type changes its response to surface preparation.
Setting preparation conditions with unclear substrate information is risky.

10-3. Avoid excessively aggressive chemical treatment

Degreasing, acid pickling and conversion treatment are needed, but overly aggressive treatment damages the substrate.
Dimensional changes, residue and surface roughening require attention.

10-4. Do not omit underlying surface conditioning

Substrates with pores, cracks or unevenness need conditioning through filler or primer.
Trying to obtain appearance through the topcoat alone makes bubbling and appearance defects more likely.

10-5. Always test before mass production

Magnesium is a high-risk substrate to move directly into mass-production coating.
It is important to check surface preparation, adhesion, corrosion resistance, bubbling, dimensional changes and safety in small batches.

11. Questions for customer visits

For the factory manager

• Do you have experience coating magnesium parts?
• Are there complaints about poor adhesion, bubbling or corrosion?
• Are safety measures taken during sanding or blasting?
• In what service environment are the magnesium parts used?

For production engineering

• Is the magnesium alloy type known?
• How is surface preparation performed?
• Is blasting dry or wet?
• Are dimensional changes from acid pickling checked?
• Is residual smut checked after conversion treatment?
• Are filler and primer used?

For quality assurance

• Are adhesion tests performed?
• Are corrosion resistance tests performed?
• Are bubbling locations recorded?
• Are surface preparation conditions linked to defect occurrence?
• Are dimensional and appearance changes evaluated?

For purchasing

• Is the magnesium alloy type shared with the coating process team?
• Are forming methods and mold release agent information checked?
• Is the impact on coatability checked when the substrate supplier changes?
• Are surface preparation or primer being omitted based on cost alone?

For coating shop-floor managers

• Are sanding debris and dust managed safely?
• Is there residue on the surface after preparation?
• Is sanding exposing too much substrate?
• Are filler and primer drying conditions followed?
• When bubbling occurs, are substrate defects and surface preparation checked too?

12. How CoatureVietnam can help

CoatureVietnam can support magnesium coating through safety, surface preparation and adhesion evaluation, as well as material selection.

Specifically, the following support is available.

• Organizing magnesium substrate information
• Checking surface preparation methods
• Identifying safety risks
• Checking chemical treatment conditions
• Checking residual smut
• Considering primer, filler and topcoat systems
• Designing adhesion test conditions
• Organizing bubbling causes
• Preparing small-batch trial conditions
• Preparing improvement proposal materials for customers

In magnesium coating, process design needs to include operator safety alongside coating quality.

Conclusion

Magnesium is a lightweight, highly functional material.
However, coating it requires considerable care.

The following three factors need to be considered together in magnesium coating.

1. Safety
2. Surface preparation
3. Adhesion

Dust and ignition risks must not be ignored.
More aggressive chemical treatment is not necessarily better.
Roughening the surface does not necessarily improve adhesion.

Magnesium coating requires checks of substrate type, preparation conditions, residual smut, dimensional changes, filler and primer, bubbling, adhesion tests and safety equipment.

This is exactly the thinking behind CoatureVietnam’s principle, “Coating the Future with Science and Care.”

Science means considering magnesium properties, preparation reactions, surface condition, film system and safety risks separately.
Care means creating reproducible shop-floor process conditions without exposing operators to hazards.

Magnesium coating should be considered not merely as coating a lightweight substrate, but as
coating that designs safety, surface preparation and adhesion together.