The basics of metal coating through moisture, salts and residual treatment solution

Introduction

A common question is heard in metal coating operations.

“Rust appeared even though we used a rust-preventive coating.”
“Why does it rust even with a rust-preventive primer?”
“Could the coating performance be insufficient?”

Selecting a rust-preventive coating is indeed important.
However, such a coating cannot completely prevent rust under every possible condition.

Rust is not determined by the coating material alone.
It develops through a combination of the metal surface before coating, surface preparation, rinsing, drying, film thickness, product shape, storage environment and service environment.

In particular, if moisture, salts, acids, alkalis, treatment solution, oil or contamination remain on the substrate before coating, rust can progress beneath the film even with a rust-preventive coating.

This article examines why rust can occur despite rust-preventive coatings from the entire process, rather than the coating material alone.

1. Rust-preventive coatings are important, but not a universal solution

Rust-preventive coatings are designed to protect metal from rust.
Adhesion to the metal substrate, corrosion-inhibiting pigments and the film’s barrier properties help limit moisture and oxygen reaching the metal.

However, rust-preventive coatings also have limitations.

For example, they cannot fully deliver their performance under the following conditions.

• Rust remains before coating
• Salts or perspiration remain
• Oil or rust-preventive oil remains
• Surface preparation solution has not been rinsed away
• Coating is applied while moisture remains
• Film thickness is insufficient
• Drying or curing is insufficient
• Treatment solution remains in gaps or joints
• The service environment is harsher than the coating specification allows

Thus, rust-preventive coatings function only with the correct substrate condition and process conditions.

Rather than relying on the coating alone, it is important not to leave causes of rust before coating.

2. Rust does not suddenly begin after coating

When rust is found after coating, it is tempting to assume that the problem started afterward.
In reality, the cause may have remained from before coating.

For example, salts may have remained on the metal surface.
Even if there is no visible problem immediately after coating, corrosion may gradually progress beneath the film.

The same applies when surface preparation chemicals have not been adequately rinsed away.
Acids, alkalis or treatment solution components left on the substrate react with moisture and cause rust or blistering beneath the film.

Marks from operators’ bare-hand contact may also become starting points for rust later.
The salts in perspiration are invisible.
However, if left beneath the film, they may appear as fingerprint-shaped rust over time.

Rust does not suddenly emerge after coating.
Often, small causes left before coating become apparent afterward.

3. Residual moisture allows rust to progress more readily under the film

Moisture is closely involved in metal rusting.
Coating a metal surface while moisture remains traps that moisture beneath the film.

Moisture tends to remain in locations such as the following.

• Around holes
• Joints
• Gaps
• Welds
• Bent areas
• Pocket-shaped structures
• Jig contact points
• Shapes that are difficult to drain

Even when the surface appears dry, moisture may remain inside gaps or joints.
Coating them in that condition can result in rust or blistering later.

In hot, humid environments such as Vietnam, inadequate drying after surface preparation and condensation during temporary storage create rust risks.

Moisture control requires checking areas likely to retain water according to the product shape, rather than simply putting products into an oven.

4. Salts, perspiration and residual treatment solution become starting points for rust

Salts and residual treatment solution are also important factors that promote rust.

Salts promote corrosion when combined with moisture.
They may remain on metal surfaces because of coastal exposure, perspiration, contaminated water or inadequate surface preparation.

Surface preparation includes degreasing, acid pickling, chemical conversion treatment and rinsing.
These processes are themselves necessary for better coating quality.

However, residual treatment solution can instead cause rust.

The following conditions, for example, create risks.

• Acid remains after pickling
• Alkaline components remain after alkaline degreasing
• Chemical conversion solution has not been rinsed away
• Rinse water is contaminated
• Drainage after rinsing is inadequate
• White residue or run marks remain after surface preparation

A metal surface before coating should do more than simply look clean.
It needs to be checked for less visible residues such as moisture, salts, treatment solution and oil.

5. Residual oil or rust-preventive oil leads to poor adhesion and rust

Metal substrates may carry rust-preventive oil.
It is effective for preventing rust during storage, but needs to be properly removed before coating.

If coating is applied over residual oil or rust-preventive oil, it cannot adhere firmly to the substrate.
Moisture enters areas with weak adhesion more readily.
Rust can then progress beneath the film, or blistering and peeling may occur.

Compatibility between the rust-preventive oil type and degreasing conditions is important here.

A strong rust-preventive oil is used.
The degreaser is unsuitable.
Degreasing temperature is low.
Degreasing time is short.
The degreasing solution is dirty.
Rinsing is inadequate.

Oil may remain under these conditions.

Before using a rust-preventive coating, it is important to check that rust-preventive oil has been properly removed.

6. Insufficient film thickness weakens rust protection

Rust-preventive coatings deliver their performance only when applied at an appropriate film thickness.

If the film is too thin, moisture and oxygen reach the metal surface more readily.
Pinholes or missed areas also become starting points for rust.

The following locations require particular attention.

• Corners
• Edges
• Around holes
• Welds
• Around bolts
• Jig contact points
• Uneven areas
• Around joints

Even with sufficient thickness on flat areas, films may be thin at edges or corners.
Thickness control therefore needs to cover weak areas prone to rust, not only average thickness.

Even if the film thickness is said to be within specification, measuring only flat areas may overlook the actual risk.

7. Incomplete drying or curing reduces film protection

A film delivers its performance only after proper drying and curing.

Incomplete drying or curing may leave adhesion, water resistance, chemical resistance and rust protection insufficient.

Common points to check include the following.

• Is the oven set temperature appropriate?
• Does the actual product temperature reach the specified level?
• Is drying time sufficient?
• Is a thick coating delaying internal drying?
• Does temperature distribution vary within the oven?
• Does product placement change how heat reaches it?
• Does incomplete curing occur at low temperatures?

In baking coatings in particular, the temperature of the product itself needs to be checked, not only the oven setpoint.
Even with the specified oven air temperature, product thickness or shape can delay the actual temperature rise.

An insufficiently dried film may look dry yet lack adequate rust protection.

8. Joints and gaps are likely weak points in rust protection

Metal parts may have sheet-metal joints, welds, bends, gaps or pocket-shaped structures.

These locations readily become weak points in rust protection.

Treatment solution and moisture enter them easily and are difficult to dry out.
Coating material may also have difficulty entering them, making adequate film thickness hard to achieve.

If rust appears from joints or gaps, changing the coating material alone may not improve it.
Measures such as the following need to be considered where necessary.

• Provide rust protection before assembly
• Apply sealant
• Design the product to retain less water
• Review the drainage orientation
• Add an air-blow step
• Extend drying time
• Check for residual moisture before coating

Rust at joints relates to product design and assembly sequence as well as the coating process.

9. Harsher service environments require changes to the coating specification

Even with rust-preventive coatings, rust becomes more likely in a harsh service environment.

For example, the following environments require care.

• Outdoor use
• Rainwater exposure
• High humidity
• Condensation
• Coastal salt exposure
• Chemical atmosphere
• Contact with cleaning agents
• Contact with oil
• Heat exposure
• Abrasion or impact

Using an indoor coating specification unchanged outdoors or in high humidity may cause early rust.

The primer, intermediate coat and topcoat system, film thickness, coating type and surface preparation conditions need to be reviewed for the service environment.

Particular care is needed when omitting a primer to cut costs.
Although initial costs may fall, rust, recoating and complaint handling can create substantial losses later.

10. Points for consistent rust protection in Vietnamese factories

Rust protection in Vietnamese factories needs to account for high temperature, humidity and the rainy season.

The following points are particularly important.

10-1. Shorten waiting time after surface preparation

Metal surfaces should preferably be coated as soon as possible after preparation.
Long waiting periods can allow recontamination by humidity, dust, oil or hand contact.

10-2. Manage temporary storage after surface preparation

Leaving prepared products in humid areas or outdoors increases the risk of renewed rust.
Humidity, dust, rainwater and condensation in temporary storage need to be managed.

10-3. Manage rinse water and drying conditions

Contaminated rinse water can leave contamination even when the surface is thought to have been washed.
Inadequate drying also leaves moisture under the film.

It is important to record and stabilize rinse water, drying temperature and drying time.

10-4. Prevent operators’ hand contact

Bare-hand contact causes rust.
Not only glove use, but also glove cleanliness, holding methods and placement methods need to be standardized.

10-5. Record where rust occurs

When rust appears, record its location and pattern.
Is it at an edge, a flat area, a joint or a hole?
This makes the likely direction of the cause easier to see.

11. Questions to check before selecting a rust-preventive coating

Before considering a change or selection of rust-preventive coating, it is important to check the following questions.

About the substrate

• Is the substrate iron, galvanized steel or stainless steel?
• Does the substrate surface have rust-preventive oil?
• Does rust occur during substrate storage?
• Is there variation in surface roughness or substrate batches?

About surface preparation

• Are degreasing conditions appropriate?
• Is rust-preventive oil sufficiently removed?
• Is rinse water clean?
• Is the concentration of surface preparation solution controlled?
• Is drying after surface preparation sufficient?

About coating

• Is the specified film thickness achieved?
• Is film thickness checked at edges and corners?
• Are drying and curing conditions followed?
• Is the actual product temperature checked?

About the environment

• Is the service environment indoors or outdoors?
• Is there high humidity, condensation, salt, chemicals or water exposure?
• Does condensation or wetting occur during storage or transport?

Changing only the rust-preventive coating without these checks may leave the cause in place and allow recurrence.

12. How CoatureVietnam can help

CoatureVietnam can support processes that make rust less likely, as well as propose rust-preventive coatings.

For example, we can support the following activities.

• Checking the current coating specification
• Organizing rust locations
• Checking surface preparation conditions
• Reviewing film thickness measurement points
• Checking drying conditions
• Checking substrate storage conditions
• Selecting rust-preventive coatings and primers
• Preparing operator checklists
• Preparing material for preventing recurrence of rust problems

What matters is not the material change itself.
It is establishing a process that leaves no causes of rust.

Conclusion

Rust can occur even when a rust-preventive coating is used.

This does not mean rust-preventive coatings are ineffective.
It means the conditions needed for them to perform may not have been established.

Is the substrate free from moisture, salts, oil, treatment solution and contamination before coating?
Are surface preparation, rinsing and drying appropriate?
Is the required film thickness achieved?
Are drying and curing conditions sufficient?
Does moisture remain in joints or gaps?
Does the coating specification suit the service environment?

Only after these points are checked can the performance of rust-preventive coatings be fully used.

Rust protection of coated metal is not created by coating material alone.
It becomes consistent only through integrated management of the substrate, surface preparation, coating, drying, storage and service environment.

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

Science means examining rust causes separately through substrate, moisture, salts, treatment solution, film thickness, drying and environment, rather than looking only to the coating material.
Care means creating clear check items and process standards so the shop floor does not face the same rust problem repeatedly.

Correctly understanding why rust occurs despite rust-preventive coatings is the first step toward consistent metal coating quality.