The basics of stainless steel coating through oxide films and adhesion

Introduction

Stainless steel is widely used as a rust-resistant metal.

It is used for food-processing equipment, kitchen equipment, building materials, medical devices, machine parts, decorative parts, precision parts and many other applications.
It has an attractive appearance, high corrosion resistance and a clean look.

This sometimes leads to the following assumptions on the coating shop floor.

“Stainless steel resists rust, so coating it should be no problem.”
“The surface is clean, so perhaps it can be coated as it is.”
“It is a higher-grade material than iron, so it should also be easier to coat.”

However, these assumptions require caution.

Stainless steel resists rust.
However, it is not necessarily easy to coat.

In fact, the stability of its surface can make adhesion difficult.
Using the same surface preparation and coating specification as iron without changes can cause poor adhesion or peeling on stainless steel.

This article explains why adhesion becomes an issue in stainless steel coating and what needs to be checked.

1. Why stainless steel resists rust

Stainless steel resists rust because its surface has a very thin protective film.

This protective film is generally called an oxide film.
Chromium in the stainless steel reacts with oxygen in the air to form a stable surface film.

This film protects the metal surface from the external environment.
Stainless steel therefore resists rust better than ordinary iron.

In other words, its corrosion resistance is supported by a stable film on the substrate surface.

From a coating perspective, however, this surface film can create a different problem.

2. A rust-resistant surface can be difficult for coating materials to adhere to

For a coating material to adhere firmly, it must interact well with the substrate surface.

The metal surface has suitable roughness.
It is chemically compatible with the coating material.
There is no oil or contamination.
The coating material wets and spreads over the surface.
The primer connects the substrate and topcoat.

Meeting these conditions produces a consistent coating film.

However, stainless steel surfaces are chemically stable.
This is an advantage for corrosion resistance, but not necessarily for coating adhesion.

A highly stable surface can make it difficult for the coating to grip firmly.
Thus, even stainless steel that looks clean may develop poor adhesion if coated as it is.

This is the challenge of stainless steel coating.

Rust resistance and ease of coating are not the same thing.

3. Problems commonly encountered in stainless steel coating

Common problems include the following.

• Poor adhesion
• Film peeling
• Cratering (fisheyes)
• Blistering
• Peeling initiated by a scratch
• Peeling from bends or edges
• Peeling in a tape test
• Localized peeling after use

Poor adhesion is particularly common.

The coating looks good immediately after application but peels over time.
It peels through rubbing during transport.
A scratch during assembly initiates spreading damage.
Applying and removing tape pulls off the film.
Peeling begins at bends or edges.

These defects may involve the stainless steel surface condition, surface preparation, primer and drying conditions, not only the coating material.

4. Check the stainless steel surface condition

The first important step in stainless steel coating is checking the surface condition.

Stainless steel comes in various surface conditions.

• 2B finish
• BA finish
• Hairline finish
• Mirror finish
• Polished material
• Pickled material
• Degreased material
• Material with oil
• Material after removal of a protective film

A different surface finish changes coating wettability and adhesion.

A surface close to a mirror finish is smooth and may make mechanical anchoring more difficult.
Sanding scratches or contamination, on the other hand, cause appearance defects or poor adhesion.

Adhesive or oil may also remain after protective film removal.
Even a clean-looking surface is not necessarily suitable for coating.

Stainless steel coating requires checks of surface finish and contamination, not just the material name.

5. Degreasing alone may be insufficient

Degreasing before coating is important for stainless steel.

Residual oil, fingerprints, protective film residue, fabrication oil or sanding dust make adhesion difficult.

However, degreasing alone may not be sufficient for stainless steel.

Degreasing removes surface oil and contamination.
However, if the stainless steel surface itself is difficult to adhere to, removing oil alone may not provide enough adhesion.

Additional measures such as the following may then be needed.

• Provide suitable surface roughness
• Carry out chemical surface preparation
• Carry out mechanical sanding or blasting
• Use a primer for stainless steel
• Use a coating with improved adhesion
• Perform adhesion tests before mass production

Stainless steel coating therefore requires cleaning and adhesion assurance to be considered separately.

6. Mechanical and chemical surface preparation

Surface treatment may be considered to improve adhesion on stainless steel.

There are two broad categories: mechanical and chemical surface preparation.

6-1. Mechanical surface preparation

Mechanical preparation uses sanding, blasting or similar methods to create fine surface irregularities.

Suitable roughness helps the coating anchor mechanically to the surface.
This is sometimes called the anchoring effect.

However, mechanical preparation requires attention to certain points.

• Appearance changes
• Surface scratches may be conspicuous
• Thin parts may deform
• Sanding dust or contamination may remain
• Additional process steps are needed
• Excessive roughening creates appearance defects

For decorative stainless steel parts, excessive roughening can harm appearance.
Controlling treatment intensity according to the application is therefore important.

6-2. Chemical surface preparation

Chemical preparation uses treatment solutions to condition the surface for coating.

It can provide uniform treatment relatively easily, but requires chemical bath management and wastewater treatment.

The following points require attention.

• Treatment solution concentration control
• Treatment time
• Rinsing condition
• Drying condition
• Wastewater treatment
• Work safety
• Uneven treatment
• Effects of residue on subsequent processes

Chemical treatment needs to be considered together with wastewater, environmental and safety requirements, as well as coating quality.

7. The role of primers for stainless steel

Primer selection can be important in stainless steel coating.

A primer is an undercoat between the substrate and topcoat.
Its role is to improve adhesion to the substrate and provide consistent bonding with the topcoat.

Primers for stainless steel have purposes such as the following.

• Improve adhesion to the stainless steel substrate
• Provide compatibility with the topcoat
• Prevent peeling
• Stabilize film thickness and appearance
• Supplement durability for the service environment

However, using a primer does not necessarily solve the problem.

If oil remains on the surface, the primer will not adhere either.
If drying conditions are insufficient, the primer will not deliver its performance.
Poor compatibility with the topcoat may cause intercoat delamination.

A primer needs to be considered together with the substrate, surface preparation, topcoat and drying conditions, rather than on its own.

8. Adhesion tests are important for stainless steel coating

Adhesion tests before mass production are important for stainless steel coating.

A coating can look good immediately after application but have inadequate adhesion.
Starting mass production in that condition may lead to peeling complaints later.

Methods for checking adhesion include the following.

• Cross-cut test
• Tape peel test
• Bend test
• Impact test
• Adhesion checks after damp-heat testing
• Evaluation simulating actual service conditions

It is important to align test conditions with the service environment.

Indoor decorative use?
Outdoor use?
Water exposure?
Contact with chemicals?
Bending or impact?
Rubbing?

The required adhesion evaluation changes with the service environment.

Standardizing adhesion tests as well as appearance checks contributes to consistent stainless steel coating quality.

9. Basic items to check for stainless steel coating

When considering stainless steel coating, check the following items.

About the substrate

• What type of stainless steel is it?
• What is its surface finish?
• Is it polished, mirror-finished or hairline-finished?
• Is a protective film used?
• Is there residual oil or adhesive on the surface?
• Are there differences between substrate batches?

About the purpose of coating

• Decoration?
• Identification?
• Corrosion protection?
• Chemical resistance?
• Scratch prevention?
• Adding a functional property?

About surface preparation

• Is the degreasing method appropriate?
• Is surface roughening necessary?
• Is chemical treatment necessary?
• Are rinsing and drying sufficient?
• Is there recontamination after preparation?

About the coating specification

• Is a stainless steel primer needed?
• Is compatibility with the topcoat good?
• Are drying conditions appropriate?
• Is film thickness appropriate?
• Can it withstand the service environment?

About evaluation

• Has an adhesion test been performed?
• Has adhesion after damp-heat exposure been checked?
• Has resistance to bending or impact been checked?
• Have tests simulating actual service conditions been performed?

10. Points to note in Vietnamese factories

Certain points require particular attention when coating stainless steel in Vietnamese factories.

10-1. Storage in high temperature and humidity

Although stainless steel resists rust, its surface is not entirely free of concerns.
Contamination, oil, moisture, handling marks or dust deposited during storage affects coating adhesion.

10-2. Operators’ hand contact

Because stainless steel looks clean, it is often handled with bare hands.
However, fingerprints, perspiration or skin oils can cause poor adhesion or appearance defects.

10-3. Inadequate cleaning after sanding or blasting

Even when roughening is intended to improve adhesion, residual sanding dust or other dust is counterproductive.
Cleaning and drying after preparation are important.

10-4. Reusing a coating specification for iron

Using a coating specification for iron unchanged on stainless steel can cause poor adhesion.
Coating specifications need to be checked for each substrate.

10-5. Omitting adhesion tests

Omitting adhesion tests creates a risk even if the initial sample looks good.
Before mass production in particular, adhesion tests need to be performed and agreed with the customer.

11. Questions for customer visits

For the factory manager

• Are there complaints about peeling or poor adhesion on coated stainless steel parts?
• Is the coating intended for decoration, identification, corrosion protection or adding a function?
• Are coated stainless steel products used indoors or outdoors?
• Will they encounter water, chemicals, oil or cleaning agents in service?

For production engineering

• What is the stainless steel surface finish?
• What degreasing is performed before coating?
• Is surface roughening or blasting performed?
• Is a stainless steel primer used?
• Is the time between surface preparation and coating controlled?

For quality assurance

• Are adhesion tests performed?
• Where does peeling occur?
• Is adhesion after damp-heat exposure or service checked?
• Is the relationship between surface finish, substrate batches and defects recorded?

For purchasing

• Is stainless steel surface finish information shared with the coating process team?
• Is the presence of protective film or rust-preventive oil checked?
• Is the impact on coatability checked when the substrate changes?
• Is the surface finish being changed based on price alone?

For coating shop-floor managers

• Is stainless steel being touched with bare hands?
• Are oil and fingerprints checked before coating?
• Is dust adequately removed after sanding?
• Are primer thickness and drying conditions controlled?
• When peeling occurs, is surface preparation checked as well as the coating material?

12. How CoatureVietnam can help

CoatureVietnam can support stainless steel coating from substrate checks through surface preparation and adhesion evaluation, as well as propose coating materials.

Specifically, the following support is available.

• Checking stainless steel surface condition
• Clarifying the coating purpose
• Considering surface preparation methods
• Selecting a stainless steel primer
• Clarifying adhesion test conditions
• Analyzing defect photographs
• Organizing peeling locations
• Preparing process checklists
• Designing pre-production evaluation items

What matters in stainless steel coating is not simply a clean-looking surface.
It is whether the coating adheres reliably and withstands the service environment.

Conclusion

Stainless steel resists rust.
However, it is not necessarily easy to coat.

The oxide film supporting its corrosion resistance can create adhesion challenges.
Using the same coating specification as iron without changes may therefore cause poor adhesion or peeling.

The following points need to be checked in stainless steel coating.

• Surface finish
• Presence of oil or fingerprints
• Surface preparation method
• Need for surface roughening
• Need for a primer
• Drying conditions
• Adhesion testing
• Service environment

Rather than assuming the substrate is acceptable because it resists rust,
check whether its surface allows the coating to adhere readily.

CoatureVietnam’s principle, “Coating the Future with Science and Care,” means understanding these substrate properties correctly and creating processes that help the shop floor avoid failures.

Science means examining the stainless steel oxide film, surface treatment, adhesion and surface preparation separately.
Care means creating clear check items and test criteria that reduce peeling and rework.

Stainless steel resists rust.
However, it is not necessarily easy to coat.
Recognizing this is the first step toward consistent stainless steel coating quality.