Coating quality fundamentals through anodizing, sealing, and chromium-free treatment

Introduction

Aluminum is a lightweight metal that is easy to process and offers excellent corrosion resistance.

It is used in many fields, including automotive parts, motorcycle parts, electronic equipment, building materials, decorative components, and machinery parts. Its use is increasing in products where weight reduction and appearance quality are priorities.

However, being lightweight and easy to handle does not necessarily make aluminum easy to coat.

Before selecting paint for aluminum, it is necessary to establish the condition of the substrate surface.

Has it been anodized? Has it been sealed? Has it received a conversion coating? Was chromate or chromium-free treatment used? Will the coating be clear or pigmented? Will it be baked or dried at room temperature?

These conditions significantly affect paint adhesion, appearance, corrosion resistance, and stability during baking.

This article organizes surface preparation precautions for aluminum coating and the points to check in daily operations.

1. Aluminum resists corrosion but is not necessarily easy to coat

Aluminum does not develop red rust like iron. It forms a surface oxide film that protects the substrate.

This oxide film allows aluminum to be used as a material with relatively high corrosion resistance.

From a coating perspective, however, this surface film requires attention. A stable surface can be difficult for paint to adhere to strongly.

The statement that aluminum resists corrosion is valid in some respects.

However, assuming that aluminum can therefore be coated as it is is risky.

Aluminum coating requires the surface condition to be prepared so that the paint can adhere firmly to the substrate.

2. Typical problems in aluminum coating

Typical problems in aluminum coating include the following.

• Poor adhesion
• Coating film peeling
• Cissing
• Blistering
• Discoloration
• Appearance changes after baking
• Uneven substrate appearance beneath clear coatings
• Peeling from edges or machined areas
• Insufficient corrosion resistance
• Quality variation after a change in surface treatment

Many of these problems relate to the condition of the aluminum surface treatment as well as the paint.

Aluminum’s coatability changes significantly with surface treatment, not only with the material itself. The first step is therefore to establish what kind of aluminum surface is being coated.

3. Checking the surface treatment is the starting point for aluminum coating

Aluminum substrates can have various surface treatments.

Typical examples include the following.

• Untreated aluminum
• Degreased products
• Conversion-coated products
• Chromate-treated products
• Chromium-free-treated products
• Anodized products
• Sealed products
• Colored aluminum
• Polished products
• Blasted products

Even when they look similar, their compatibility with paint differs.

For example, anodized products have a surface oxide film. Whether sealing has been performed also changes paint penetration and adhesion.

For conversion-coated products, adhesion and corrosion resistance vary with the treatment solution and treatment condition.

The material name alone is insufficient for aluminum coating. The type of surface treatment must always be checked.

4. What is anodizing?

Anodizing is a treatment that artificially forms an oxide film on aluminum. It is also referred to as alumite treatment.

This treatment can improve the aluminum surface’s corrosion resistance, wear resistance, and appearance.

Anodized surfaces may receive clear or pigmented coatings. Clear coating is sometimes used to retain aluminum’s metallic luster.

However, coating anodized products requires attention to several points.

• Oxide film thickness
• Whether sealing has been performed
• Surface absorption
• Oxide film behavior during heating
• Paint adhesion
• Uneven substrate appearance beneath clear coatings
• Baking conditions

Changes in the oxide film during heating require particular attention when baking paints are used.

5. Sealing changes coatability

Anodic oxide films can have a porous structure. Sealing is the process of treating and closing these fine pores.

Sealing improves corrosion resistance and resistance to contamination. It can also affect coatability.

Paint may have difficulty penetrating a sealed surface. An unsealed surface allows easier penetration, but can also produce variations in absorption and appearance.

Sealing therefore should not simply be classified as good or bad. Its compatibility with the coating specification must be assessed.

Check the following points.

• Has the surface been sealed?
• Is it unsealed?
• What sealing treatment was used?
• Will the coating be clear or pigmented?
• Are the baking conditions appropriate?
• Have adhesion tests been performed?

Defining coating specifications without information about sealing can lead to poor adhesion or appearance defects.

6. Consider the effects of heating on anodic oxide films when baking

An aluminum anodic oxide film can behave differently from the underlying aluminum during heating.

The aluminum itself and its surface film do not expand in exactly the same way when heated. Processes involving heat, such as baking, can therefore cause fine cracks or appearance changes.

Particular care is needed in the following cases.

• Thick oxide films
• A high degree of sealing
• High baking temperatures
• Rapid heating or cooling
• Clear coatings intended to retain the substrate’s appearance
• Strict appearance requirements

Clear coatings make the substrate condition readily visible. Even minor unevenness or cracks can appear as visible defects.

Before mass production of baked coatings on anodized products, appearance and adhesion after heating must therefore be checked.

7. Chromate and chromium-free treatments

Conversion treatment may be used to prepare aluminum for coating. Chromate treatment is a typical example.

Chromate treatment helps improve adhesion and corrosion resistance. However, treatments containing hexavalent chromium can be restricted for environmental, safety, or regulatory reasons.

Chromium-free treatment has therefore become increasingly important in recent years.

Chromium-free treatment is an environmentally responsive surface treatment that does not use hexavalent chromium. It offers environmental benefits, but coating quality requires attention.

When changing from chromate to chromium-free treatment, the following checks are necessary.

• Is adhesion equivalent?
• Is corrosion resistance equivalent?
• Is compatibility with the paint good?
• Are treatment conditions stable?
• Are rinsing and drying appropriate?
• Will variations occur during mass production?

Environmental performance is important. However, prioritizing it alone while allowing insufficient adhesion or corrosion resistance creates coating problems.

Conversion to chromium-free treatment requires both environmental performance and stable quality.

8. Rinsing and drying controls are also important in aluminum coating

Aluminum surface preparation requires rinsing and drying as well as chemical treatment.

Residual preparation chemicals can cause poor adhesion, blistering, discoloration, or reduced corrosion resistance after coating. Insufficient rinsing leaves treatment solution components on the surface. Insufficient drying leaves moisture beneath the coating film.

Complex aluminum parts have areas where moisture can remain easily.

• Holes
• Gaps
• Folded sections
• Ribs
• Hollow sections
• Fixture contact areas
• Areas that drain poorly

Aluminum parts are lightweight and often formed into complex shapes, making drainage and drying design important.

Even when the surface appears dry after preparation, moisture can remain inside gaps and holes.

9. The role of primers for aluminum

Primers can be important in aluminum coating.

A primer connects the aluminum substrate with the topcoat. It is used to improve adhesion and supplement corrosion resistance and film stability.

A primer for aluminum should be considered in cases such as the following.

• Poor adhesion is occurring
• The service environment is severe
• The product is exposed to water or moisture
• The product is used outdoors
• Compatibility with the topcoat is inconsistent
• The surface treatment has changed
• The process has changed to chromium-free treatment
• Peeling is likely from machined areas or edges

However, using a primer does not solve every problem.

A primer will not adhere if oil or treatment chemicals remain on the surface. Its performance will not develop with inadequate drying conditions. Poor topcoat compatibility can also cause intercoat delamination.

Primers must be considered together with surface preparation, coating specifications, and drying conditions.

10. Checking substrate appearance is important for clear coating

Clear coatings are sometimes used on aluminum to retain its metallic luster.

Because the paint is transparent, the substrate condition remains visible. Unevenness, scratches, polishing marks, uneven oxidation, or uneven treatment on the substrate will therefore remain visible after coating.

Check the following points for clear coating.

• Substrate scratches
• Uneven polishing
• Uneven treatment
• Fingerprints
• Oil
• Water stains
• Surface oxidation
• Color changes after baking
• Appearance differences caused by film thickness differences

A clear coating cannot hide these conditions. The substrate appearance before coating strongly influences the final appearance.

Substrate appearance checks before clear coating are therefore more important than with conventional pigmented coatings.

11. Basic checks for aluminum coating

The following items should be checked when considering aluminum coating.

Substrate

• What type of aluminum is used?
• What surface treatment has been applied?
• Is the product anodized?
• Has sealing been performed?
• Type of conversion treatment
• Chromate or chromium-free treatment
• Is there variation between substrate lots?

Purpose of coating

• Is it decorative?
• Is it for corrosion protection?
• Is it for identification?
• Is it for protection?
• Is it to provide a functional property?
• Will the coating be clear or pigmented?

Surface preparation

• Is degreasing sufficient?
• Is rinsing sufficient?
• Is drying sufficient?
• Are treatment chemicals remaining?
• Is surface roughening required?
• Is a primer required?

Baking conditions

• Will the coating be baked or dried at room temperature?
• Is the baking temperature appropriate for the substrate?
• Will the anodic oxide film be affected?
• Is the actual product temperature checked?
• Are there effects from rapid heating or cooling?

Quality evaluation

• Have adhesion tests been performed?
• Have corrosion resistance tests been performed?
• Has appearance after heating been checked?
• Has adhesion after damp heat exposure been checked?
• Have samples been checked before mass production?

12. Points to watch in Vietnamese factories

The following points require attention when coating aluminum in Vietnamese factories.

12-1. Storage in hot, humid environments

Although aluminum is corrosion resistant, dirt or moisture on the surface before coating affects adhesion and appearance. Particular care is needed when waiting times after surface preparation are long.

12-2. Insufficient surface treatment information

Substrate suppliers may not provide sufficient information on anodizing, sealing, conversion treatment, or chromium-free treatment. Coating without knowledge of the surface treatment can lead to problems.

12-3. Balancing chromium-free requirements with quality

Environmental requirements may call for chromium-free treatment. However, adhesion and corrosion resistance must be checked when changing the treatment.

12-4. Baking oven temperature control

Aluminum has high thermal conductivity, but heat input varies with product shape and sheet thickness. Inappropriate baking conditions can cause appearance changes or poor adhesion.

12-5. Operator handling and residual oil

Fingerprints and oil can readily affect the appearance of aluminum surfaces. Bare-hand contact and water stains are particularly noticeable beneath clear coatings.

13. Questions for customer visits

For factory managers

• Have there been complaints about poor adhesion or peeling on coated aluminum products?
• Is the purpose of aluminum coating decoration, corrosion protection, protection, or a functional property?
• Are there environmental requirements for chromium-free treatment or VOC reduction?
• Are the coated aluminum products used indoors or outdoors?

For production engineering

• What surface treatment does the aluminum have?
• Is the product anodized?
• Has it been sealed?
• Does it have chromate or chromium-free treatment?
• Are degreasing, rinsing, and drying conditions controlled before coating?
• Do you check the baking temperature and actual product temperature?

For quality assurance

• Do you perform adhesion tests?
• Do you evaluate coatability when changing surface treatments?
• Do you check appearance changes after heating?
• Do you perform corrosion resistance or damp heat tests?
• Do you record the locations of peeling or discoloration?

For purchasing

• Do you share aluminum surface treatment information with the coating operation?
• Do you check the effect on coatability when changing substrate suppliers?
• Do you check chromium-free requirements together with quality requirements?
• Are surface treatment specifications being changed solely on price?

For coating operation supervisors

• Do you check aluminum surfaces for oil, fingerprints, and water stains before coating?
• For clear coating, do you check substrate appearance in advance?
• Is drying after surface preparation sufficient?
• Do you control primer film thickness and drying conditions?
• When peeling occurs, do you trace the investigation back to the surface treatment?

14. How CoatureVietnam can help

For aluminum coating, CoatureVietnam offers support that includes surface preparation, surface treatment, and adhesion evaluation as well as paint recommendations.

Specifically, we can provide the following support.

• Organizing aluminum surface treatment information
• Confirming the purpose of coating
• Checking surface preparation conditions
• Selecting primers for aluminum
• Checking quality when meeting chromium-free requirements
• Designing adhesion test items
• Checking baking conditions
• Evaluating appearance for clear coatings
• Organizing defect locations
• Preparing improvement proposal materials for customers

Correctly understanding the substrate surface is the starting point for stable aluminum coating quality.

Summary

Aluminum is lightweight and offers excellent corrosion resistance. However, it is not necessarily easy to coat.

Surface treatment condition has a major influence on aluminum coating quality.

Anodizing. Sealing. Conversion treatment. Chromate treatment. Chromium-free treatment. Clear coating. Baking conditions.

Coating without checking these factors can lead to poor adhesion, peeling, discoloration, appearance defects, and insufficient corrosion resistance.

Three points are important in aluminum coating.

1. Check the surface treatment
2. Design surface preparation and primer use appropriately
3. Evaluate baking conditions and adhesion before mass production

CoatureVietnam’s principle, “Coating the Future with Science and Care,” means correctly understanding these material properties and translating them into processes that can be reproduced in daily operations.

Science means analyzing aluminum surface treatment, oxide films, sealing, chromium-free treatment, and baking conditions separately. Care means creating clear checking standards for operators and preventing peeling and appearance defects before they occur.

Aluminum coating is not determined by paint selection alone. Examine surface treatment and surface preparation. This is the first step toward stable aluminum coating quality.