Distinguishing filiform corrosion, fingerprint rust, rust streaks, and spot rust

Introduction

When rust occurs on a coated metal product, the first priority is not to jump to a conclusion about the cause.

The paint is bad.
The surface preparation is bad.
The operator made a mistake.
The material is bad.

Assigning responsibility too early can prevent the true cause from being found.

To assess a rust problem correctly, first observe the rust pattern.

Rust does more than turn a surface red.
It can extend like threads.
It can appear in fingerprint patterns.
It can form lines resembling water flow.
It can appear as scattered spots.
It can emerge from joints and gaps.
It can start where different metals contact each other.

The way rust appears provides clues to its cause.

This article organizes rust on coated metal by pattern and explains how to consider causes on the shop floor.

1. Rust patterns are traces of causes left on site

When rust appears, the paint name is not the first thing to examine.
First examine the rust pattern and location.

Rust patterns reflect substrate condition before coating, surface preparation, operator handling, drying, storage, and the service environment.

For example, causes can differ greatly even though all are rust.

Fingerprint-shaped rust may be caused by sweat or salt from bare-hand contact.
Rust streaks may relate to rinse-water or treatment-solution flow marks.
Rust emerging from joints may be caused by moisture or treatment solution trapped in gaps.
Spot rust may relate to localized contaminants, machining debris, burrs, or pinholes.

Observing the rust pattern is therefore the starting point for cause analysis.

A record stating only that rust occurred is insufficient.
It is necessary to identify its pattern, location, and timing.

2. Filiform corrosion: Fine threads progressing beneath the coating

Filiform corrosion progresses beneath the coating as fine, thread-like traces.
It may appear as branching thin lines.

In this type of corrosion, moisture may enter beneath the coating and react with corrosive substances remaining on the metal surface, allowing corrosion to progress.

Possible causes include the following.

• Salt remaining before coating
• Residual surface preparation solution
• Insufficient rinsing
• Insufficient drying
• Insufficient adhesion beneath the coating
• Insufficient film thickness
• Moisture entry through scratches or pinholes
• Use or storage in high humidity

The concern with filiform corrosion is that it advances beneath the film, not only on its surface.
A small visible trace may conceal more extensive corrosion underneath.

When filiform corrosion occurs, check cleanliness before coating, rinsing after surface preparation, drying, film thickness, and the service environment, as well as the paint’s rust resistance.

3. Fingerprint rust: Bare-hand contact can be a cause

As its name suggests, fingerprint rust appears in patterns resembling finger contact marks.

Touching metal with bare hands deposits sweat and skin oils.
Sweat contains salts and other components that may initiate rust after coating.

Even if invisible before coating, sweat or salt beneath the film can later become evident as fingerprint rust.

Metal surfaces after surface preparation require particular attention.

After surface preparation, metal should be clean and suitable for coating.
Bare-hand contact at that point redeposits sweat and dirt onto the prepared surface.

When fingerprint rust occurs, check the following.

• Are products touched with bare hands after surface preparation?
• Are gloves used?
• Are gloves dirty?
• Is the way operators hold products standardized?
• Is there recontamination in temporary storage before coating?
• Is there bare-hand contact during inspection or handling?

Fingerprint rust can sometimes be addressed through work standards and training rather than changing paint.

4. Rust streaks: Suspect flow marks from moisture or treatment solutions

Rust streaks are long, narrow rust patterns.
They may follow product hanging orientation or water-flow direction.

These patterns may relate to flow marks from rinse water, treatment solutions, acidic or alkaline components, salt, or moisture after surface preparation.

For example, insufficient draining after surface preparation may allow moisture to flow over the product and remain in particular areas.
Treatment solution or salts left in those areas can later appear as rust streaks after coating.

When rust streaks occur, check the following.

• Product hanging orientation
• Water-flow direction
• Draining condition after surface preparation
• Whether air blowing is used
• Residual moisture before entering the oven
• Rinse-water management
• Surface preparation solution concentration control
• Whether flow marks remain during drying

For rust streaks, checking water movement after surface preparation, drying conditions, and product orientation is more important than treating the issue simply as a paint problem.

5. Spot rust: Localized contamination or insufficient film thickness can be causes

Spot rust occurs as isolated local spots.
It may look minor at first, but there are various causes.

Possible causes include the following.

• Adhering machining debris
• Remaining burrs
• Localized acid or alkali residues
• Residual moisture
• Residual rust-preventive oil
• Pinholes
• Insufficient film thickness
• Adhering contaminants
• Small scratches on the substrate surface
• Casting voids or pits

Localized weak points are the key concern with spot rust.
The whole coating film may not be defective; rust may start only at particular small locations.

For example, machining debris left beneath a coating can initiate rust.
A small pinhole can allow moisture in and cause a rust spot.

When spot rust occurs, inspect the surrounding area under magnification and consider contaminants, pinholes, insufficient film thickness, and substrate defects.

6. Rust at joints: Trapped moisture or treatment solution can be a cause

Rust may emerge from joints and gaps in sheet-metal and assembled parts.

Joints and gaps may look satisfactory from the outside.
However, they readily retain moisture and treatment solution and are difficult to dry.

Rinse water or treatment solution remaining inside a joint after surface preparation makes rust after coating particularly likely.

When rust occurs at joints, check the following.

• Does the product have overlaps or gaps?
• Does the structure readily admit surface preparation solution?
• Are draining and drying sufficient?
• Does air blowing reach the area?
• Does paint reach inside the gap?
• Is rust-prevention treatment needed before assembly?
• Is sealing needed?

Rust at joints may not be solvable through the coating process alone.
Product design, assembly sequence, surface preparation conditions, and rust-protection specifications need to be considered together.

7. Corrosion between dissimilar metals: Do not overlook galvanic corrosion

Where different metals contact each other, galvanic corrosion can occur in addition to ordinary rusting.

Galvanic corrosion is a phenomenon in which, in the presence of moisture, the potential difference between different metals makes one metal more susceptible to corrosion.

For example, corrosion can progress at contact points between steel and aluminum or stainless steel and steel.

Changing paint alone may not address the root cause.
Component construction, electrical insulation, sealing, contact-area protection, and corrosion-protection design need to be considered.

When corrosion occurs at dissimilar-metal contacts, check the following.

• Are different metals in contact?
• Can moisture enter the contact area?
• Is electrical insulation provided?
• Is sealing needed?
• Does the service environment involve wetting or condensation?
• Which metal is corroding?

Dissimilar-metal corrosion should be examined by design, purchasing, and quality assurance, as well as the coating shop floor.

8. Checks by rust pattern

The following hypotheses can be developed from the rust pattern.

Filiform corrosion

Checks:

• Has moisture entered beneath the coating?
• Are salt or treatment-solution residues present?
• Is film thickness insufficient or adhesion poor?
• Is the product used or stored in high humidity?

Fingerprint rust

Checks:

• Has there been bare-hand contact?
• Are gloves clean?
• Has recontamination occurred after surface preparation?
• Is the surface touched during handling or inspection?

Rust streaks

Checks:

• Are there flow marks from rinse water or treatment solution?
• Is draining sufficient?
• Does moisture remain before drying?
• Is the pattern related to hanging or product orientation?

Spot rust

Checks:

• Are contaminants or machining debris remaining?
• Are there pinholes?
• Is film thickness locally insufficient?
• Are there scratches or pits on the substrate surface?

Rust at joints

Checks:

• Is moisture remaining in gaps?
• Has surface preparation solution entered?
• Does drying reach the area?
• Is rust-prevention treatment needed before assembly?

Corrosion between dissimilar metals

Checks:

• Are different metals in contact?
• Is moisture present between them?
• Is insulation or sealing needed?
• Are corrosion countermeasures needed at the design stage?

9. Recording rust problems

The recording method matters when using rust problems to prevent recurrence.

A record stating only that rust is present cannot support cause analysis.
Retain the following information.

• Affected product name
• Substrate material
• Surface treatment
• Coating specification
• Occurrence location
• Rust pattern
• Occurrence timing
• Affected quantity
• Surface preparation conditions
• Drying conditions
• Storage environment
• Photographs
• Relationship to the rainy season and humidity
• Operator and line conditions

Photographs are particularly important.
Keep both overall and close-up photographs.
Show the rust pattern, direction of occurrence, and surrounding areas clearly.

Recording rust locations on a product drawing or simple sketch also makes trends easier to see.

10. Rust assessment points requiring particular attention at factories in Vietnam

The hot, humid environment increases rust-problem risks at factories in Vietnam.

Pay particular attention to the following.

• Do not leave products for long periods after surface preparation
• Do not touch prepared metal with bare hands
• Drain thoroughly after rinsing
• Check residual moisture in joints and holes
• Review storage conditions during the rainy season and high humidity
• Avoid outdoor storage
• Keep gloves and wiping cloths clean
• Define clear acceptance and rejection criteria for substrates before coating

Rust can relate to small shop-floor habits.
Touching with hands.
Leaving moisture.
Setting a product aside briefly.
Coating it because it looks dry.

These small decisions can later become evident as rust.

11. Questions for customer visits

The following questions are useful when investigating rust problems on site.

For the factory manager

• Which products have the most rust?
• Does rust appear immediately after coating or after shipment?
• Does it increase during the rainy season or high humidity?
• Has rust caused recoating or delivery delays?

For production engineering

• Is rust concentrated at edges, joints, holes, or welds?
• Is the time from surface preparation to coating controlled?
• Are there product shapes that are difficult to drain or dry?
• Is there a relationship between hanging orientation and rust direction?

For quality assurance

• Are rust patterns classified and recorded?
• Are rust locations linked to surface preparation conditions?
• Are both overall and close-up rust photographs retained?
• Is rust occurrence timing classified?

For purchasing

• Are effects on coating quality checked when materials or surface treatments change?
• If substrates carry rust-preventive oil, are degreasing conditions compatible?
• Are any parts made from combinations of dissimilar metals?

For the coating shop-floor supervisor

• Is prepared metal touched with bare hands?
• Are gloves and wiping cloths clean?
• Are moisture, flow marks, and white residues checked before coating?
• When rust occurs, are its pattern and location recorded?

12. How CoatureVietnam can help

CoatureVietnam can help organize rust causes across the entire process, rather than treating rust simply as a paint complaint.

Specific support can include the following.

• Classifying rust patterns
• Organizing defect photographs
• Creating occurrence-location maps
• Checking surface preparation conditions
• Reviewing film thickness measurement locations
• Checking drying conditions
• Creating SOPs for bare-hand contact prevention and glove management
• Creating rust-problem interview sheets
• Reviewing rust-protection specifications
• Preparing customer improvement proposal materials

Reading rust patterns correctly improves the accuracy of cause analysis.
More accurate cause analysis makes countermeasures more specific.

Summary

Rust is more than a red defect.
Its pattern provides clues to its cause.

For filiform corrosion, suspect moisture and residues beneath the coating.
For fingerprint rust, suspect bare-hand contact and sweat.
For rust streaks, suspect flow marks from rinse water or treatment solution.
For spot rust, suspect contaminants, pinholes, or insufficient film thickness.
For rust at joints, suspect moisture or treatment solution trapped in gaps.
For corrosion between dissimilar metals, suspect galvanic corrosion.

Resolving rust problems requires checking the pattern, location, timing, surface preparation, drying, storage, and service environment in sequence, rather than examining paint alone.

This approach reflects CoatureVietnam’s guiding principle, “Coating the Future with Science and Care.”

Science means observing rust by pattern and separating its causes.
Care means translating findings into clear checking criteria and SOPs so that shop-floor operators and managers do not face the same problems repeatedly.

When you see rust, first examine its pattern.
That is the starting point for approaching the true cause of a coating problem.