Article 18: Coating Defects and Failure Diagnosis

Separating the Visible Symptom from the Actual Cause

Defect, Failure, and Normal Change

Not every visible change is a coating failure, and not every serious failure is immediately visible.

• Defect: A departure from the intended coating condition, appearance, or specification.
• Failure: Loss of the coating system's ability to perform its required function.
• Normal change: An expected change that does not necessarily prevent performance, such as controlled weathering or limited epoxy chalking.

A cosmetic color variation may be unacceptable on a highly visible structure but have little immediate effect on corrosion protection. A tiny holiday may be nearly invisible while presenting a serious problem in immersion service.

The acceptance decision must be based on the specification, service environment, coating system, and effect of the condition on performance.

Symptoms Are Not Root Causes

One defect can have several possible causes. The same cause can also produce several different defects.

Blistering, for example, may involve soluble salts, moisture, trapped solvent, chemical exposure, osmotic pressure, cathodic-protection conditions, or loss of adhesion. Looking at a blister does not automatically identify which mechanism produced it.

Prematurely naming a cause can lead to the wrong repair. If the real cause remains, the repaired coating may fail in the same way.

Begin with a Structured Investigation

A coating-failure investigation should begin before cleaning, scraping, cutting, or repairing changes the evidence.

A practical investigation sequence is:

1. Protect the area and preserve evidence.
2. Document the visible condition before disturbing it.
3. Map the location, pattern, and extent of the defect.
4. Review the coating specification and product data.
5. Review surface-preparation, environmental, mixing, and application records.
6. Examine coating thickness, adhesion, cure, and discontinuities.
7. Open selected defects carefully and examine the layers.
8. Collect samples when laboratory analysis may be needed.
9. Develop and test possible causes.
10. Establish corrective action only after the evidence is evaluated.

Preserve the Evidence

Pressure washing, solvent wiping, scraping, sanding, and coating removal can destroy information needed to determine what happened.

Before disturbing the area, document:

• Overall appearance
• Close-up appearance
• Location and orientation
• Relationship to welds, edges, bolts, repairs, drains, and water traps
• Wet, dry, shaded, heated, submerged, or chemical-exposure conditions
• Coating color and layer sequence
• Presence of corrosion products, liquid, deposits, or contamination

Photographs should include an overall view, close-up view, scale, location identification, and date.

Look for the Pattern

The distribution of a defect often provides more information than its appearance.

Ask whether the condition:

• Occurs only on edges or welds
• Follows spray passes or overlap bands
• Appears only on one coating batch
• Is limited to a particular shift or applicator
• Occurs near drains, leaks, vents, chemicals, or heat sources
• Appears only in immersion or splash zones
• Follows areas of repair or field touch-up
• Occurs near cathodic-protection components
• Is isolated or widespread

A defect following spray passes suggests a different investigation than a defect concentrated beneath insulation or around an electrical isolation joint.

Blistering

Blisters are raised areas where the coating has separated locally from the substrate or from another coating layer. They may contain liquid, gas, corrosion products, or no visible material.

Blisters should be described by their size, frequency, distribution, location, and the coating layer in which separation occurred.

Possible causes include:

• Soluble salts beneath the coating
• Moisture contamination
• Solvent or water trapped within the film
• Applying coating over a porous layer
• Excessive coating thickness
• Premature immersion or chemical exposure
• Osmotic pressure
• Cathodic disbondment
• Loss of adhesion between coating layers

Opening a Blister

When authorized, carefully open selected blisters and observe their contents, odor, location of separation, condition of the substrate, and appearance of each coating layer.

Use appropriate protective equipment. Blister liquid may contain chemicals, corrosion products, uncured coating components, or service material.

Opening one blister may not represent the complete condition. Compare blisters from different locations and include apparently sound control areas.

Cathodic Disbondment

Cathodic disbondment is loss of coating adhesion associated with electrochemical conditions at exposed steel beneath or adjacent to the coating.

It is a concern on buried or submerged structures protected by cathodic-protection systems. Disbondment may begin around a holiday or damaged area and spread beneath the coating.

Possible contributing factors include:

• Coating chemistry not suitable for the service
• Poor surface preparation or contamination
• Coating holidays or mechanical damage
• Elevated temperature
• Excessive or improperly controlled cathodic-protection conditions
• Extended exposure time

A coating contractor should not diagnose overprotection from coating appearance alone. Cathodic-protection measurements and evaluation by qualified personnel are required.

Delamination and Peeling

Delamination is separation of coating from the substrate or between coating layers. Peeling is a visible form of separation in which coating lifts in sheets, strips, or flakes.

Possible causes include:

• Oil, grease, dust, salts, moisture, or corrosion products
• Inadequate surface profile
• Applying over rust-back
• Incompatible coating layers
• Exceeded recoat interval
• Amine blush, chalk, or zinc salts between coats
• Improper cure
• Moisture or chemicals penetrating beneath the film

Examine the back of the detached coating and the exposed surface. The material present on each side helps identify the plane of separation.

Rusting

Rusting indicates that moisture and oxygen have reached carbon steel. The pattern of rusting can point toward the failure mechanism.

Pinpoint Rusting

Small rust points may occur where surface-profile peaks were not adequately covered, where pinholes formed, or where small contamination sites remained.

Edge and Weld Rusting

Corrosion along edges, welds, bolts, and corners may indicate inadequate edge preparation, insufficient stripe coating, thin film, missed areas, or moisture-retaining geometry.

Rusting at Damage

Local rust at scratches, impact marks, attachment points, or handling damage indicates that the coating barrier was broken after application.

Widespread Rusting

Widespread rusting may indicate insufficient system thickness, extensive porosity, coating deterioration, poor surface preparation, severe exposure, or a system that has exceeded its useful life.

Underfilm Corrosion

Underfilm corrosion develops beneath coating that may initially appear intact. Corrosion products occupy more volume than the original steel and can push the coating away from the substrate.

Moisture and contaminants may enter through holidays, damaged edges, cracks, joints, or areas of poor adhesion. Corrosion can then spread laterally beneath the coating.

The visible defect may represent only a portion of the affected area. Sounding, adhesion checks, careful probing, or removal of selected coating may be necessary to define the repair boundary.

Cracking and Checking

Cracking is a break in the coating film. Checking is a pattern of smaller surface cracks that may or may not extend through the complete system.

Possible causes include:

• Excessive coating thickness
• Internal stress during cure
• Applying a hard coating over a softer layer
• Temperature cycling
• Substrate movement or vibration
• Weathering and aging
• Incorrect mix ratio or cure
• Applying another coat before the underlying layer was ready

Determine whether cracks are limited to the finish coat, extend through several layers, or reach the substrate.

Mud Cracking

Mud cracking creates a pattern resembling dried mud. It is often associated with excessive film thickness, uneven drying, internal stress, or susceptible coating types such as improperly applied inorganic zinc primer.

Applying another coat over mud cracking does not restore a sound foundation. The affected material may require removal and reapplication within the approved thickness range.

Flaking and Scaling

Flaking or scaling occurs when pieces of coating detach from the surface. The pieces may be small and scattered or form larger sheets.

Examine whether failure occurs at the substrate, between coats, or within a brittle coating layer. Flaking can be associated with poor adhesion, aging, excessive thickness, loss of flexibility, underfilm corrosion, or incompatible overcoating.

Pinholes and Porosity

Pinholes are small openings. Porosity is a condition containing multiple small voids or passages through or within the coating.

Possible causes include:

• Air entrained during mixing
• Outgassing from the substrate or underlying coating
• Excessive coating thickness
• Rapid solvent release
• Poor atomization
• Coating applied to a hot surface
• Failure to seal a porous primer or substrate
• Incorrect application technique

Holiday testing may reveal through-film discontinuities, but it does not identify every enclosed void within the coating.

Solvent Entrapment

Solvent entrapment occurs when solvent remains within the coating because it cannot escape at the required rate.

Contributing factors may include:

• Excessive film thickness
• Applying coats too quickly
• Insufficient ventilation
• Low temperature
• Incorrect thinner or excessive thinning
• Rapid surface skinning

Symptoms may include soft film, blistering, bubbling, pinholes, lingering solvent odor, delayed cure, or poor intercoat adhesion.

Dry Spray and Overspray

Dry spray occurs when coating droplets partially dry before reaching or merging into the wet film. The resulting surface may feel rough, powdery, or weakly bonded.

Possible causes include excessive gun distance, high airflow, hot steel, rapid solvent release, incorrect pressure, poor gun angle, or spraying complex steel from one direction.

Dry spray can reduce intercoat adhesion and create a porous or irregular surface. It should be evaluated and corrected before another coat is applied.

Runs and Sags

Runs and sags occur when wet coating moves downward before it develops enough resistance to flow.

Possible causes include:

• Excessive wet film thickness
• Slow gun movement
• Gun held too close
• Excessive overlap
• Excessive or incorrect thinner
• Material or surface temperature outside the approved range

A cured sag may contain excessive thickness and trapped solvent. It should not be accepted solely because it no longer moves.

Orange Peel and Poor Flow

Orange peel is an uneven texture resembling the surface of citrus skin. It can result from poor atomization, high viscosity, cold material, incorrect pressure, unsuitable tip or nozzle selection, rapid solvent release, or inadequate flow and leveling.

Orange peel may be primarily cosmetic in some services, but severe texture can create thin peaks, contamination traps, poor cleanability, or difficulty obtaining uniform subsequent coats.

Amine Blush

Some amine-cured epoxy coatings may develop a surface film commonly called amine blush. It can appear oily, waxy, cloudy, or greasy and may not always be visually obvious.

Cool, damp, or high-humidity conditions can increase the likelihood of blush in susceptible materials.

Blush can interfere with intercoat adhesion. It should be removed using the coating manufacturer's approved procedure before recoating, abrasion, or adhesion testing.

Chalking and Fading

Chalking is the formation of a powdery surface as the coating binder weathers. Fading is a change or loss of color.

Exterior epoxies commonly chalk under ultraviolet exposure. Limited chalking may not mean the barrier has immediately failed, but progressive erosion can reduce thickness and affect adhesion of maintenance coats.

Chalking must be removed before recoating. Applying a new coating over loose chalk bonds the new system to weak surface material rather than sound coating.

Soft, Tacky, or Uncured Coating

A coating that remains soft or tacky beyond its expected cure period may have been affected by:

• Incorrect mix ratio
• Incomplete mixing
• Wrong curing agent
• Low temperature
• Excessive film thickness
• Excessive or incorrect thinner
• Contamination or chemical interference
• Material beyond shelf life or pot life

Waiting longer may help when cure is merely delayed by temperature, but additional time cannot correct an incorrect component ratio or wrong curing agent.

A Defect-to-Evidence Guide

Review the Project Records

A failure investigation becomes much stronger when reliable project records are available.

Useful records include:

• Coating specification and approved submittals
• Product and safety data sheets
• Batch numbers and storage records
• Surface-preparation and cleanliness records
• Surface-profile measurements
• Soluble-salt and dust-test results
• Environmental readings
• Mixing, thinning, induction, and pot-life records
• Wet and dry film thickness records
• Holiday and adhesion test results
• Repair records and photographs
• Service history and operating changes
• Cathodic-protection readings when applicable

Missing records do not prove that work was performed incorrectly, but they make it more difficult to confirm what happened.

Collecting Coating Samples

Laboratory examination may be needed when the cause cannot be established in the field. Samples should be selected, labeled, protected, and documented so their location and condition remain known.

Useful samples may include failed coating, apparently sound coating, corrosion products, deposits, blister liquid, unused product, and material from a control area.

Sampling should not begin until the investigation team agrees on the locations and information required.

Corrective Action Must Address the Cause

Spot repair is appropriate only when the surrounding coating is sound and the cause is local and controllable.

Widespread contamination, incomplete cure, extensive porosity, poor adhesion, or incompatible layers may require removal of a larger area or the complete system.

Before repair begins, define:

• The cause or most-supported failure mechanism
• The full extent of affected coating
• The removal and surface-preparation boundaries
• The approved repair coating system
• Environmental and curing requirements
• Inspection and retesting requirements
• Actions needed to prevent recurrence

Common Failure-Investigation Mistakes

• Naming the cause from a photograph: Similar-looking defects can have different causes.
• Cleaning before documenting: Important deposits, patterns, and evidence are removed.
• Examining only failed areas: No sound control area is available for comparison.
• Assuming one defect has one cause: Several conditions may work together.
• Ignoring the failure plane: Separation location is not identified.
• Ignoring project records: The investigation relies on memory and appearance.
• Testing only convenient areas: Results do not represent the failure pattern.
• Assigning blame too early: The investigation becomes defensive instead of evidence-based.
• Repairing before finding the extent: Unsound coating remains around the repair.
• Failing to correct the source: Leaks, salts, heat, movement, or electrical conditions continue.

Contractor's Failure-Investigation Checklist

• The affected area has been protected from unnecessary disturbance.
• Overall and close-up photographs have been taken.
• The location, pattern, and extent have been mapped.
• The defect has been described without prematurely naming the cause.
• The coating system and layer sequence have been identified.
• Surface-preparation and application records have been reviewed.
• Service exposure and operating history have been reviewed.
• Apparently sound control areas have been examined.
• The plane of separation has been identified where possible.
• Required field tests and samples have been planned.
• Cathodic-protection data have been reviewed when applicable.
• The repair boundary includes all unsound coating.
• Corrective action addresses both the symptom and the cause.
• Repair inspection and retesting requirements are documented.

Key Takeaways

• Visible coating defects are symptoms, not automatic proof of their cause.
• Evidence should be preserved before cleaning or repair begins.
• The location and pattern of a defect can reveal important clues.
• Blistering can have several moisture, chemical, application, and electrochemical causes.
• The separation plane helps identify whether failure occurred at the substrate, between coats, or within a coating layer.
• Project records can be as important as physical testing.
• Cathodic disbondment cannot be diagnosed from coating appearance alone.
• Corrective action must address the root cause and the complete affected area.

Knowledge Check

1. Why should an investigator describe a defect before naming its cause?

View Answer

Similar-looking defects can have different causes. Describing the evidence first reduces the risk of reaching a premature conclusion.

2. Why is the pattern of a defect important?

View Answer

Its relationship to edges, welds, spray passes, repairs, exposure zones, shifts, or coating batches can provide clues about the failure mechanism.

3. Does the presence of blistering automatically prove soluble-salt contamination?

View Answer

No. Blistering can involve salts, moisture, trapped solvent, chemical exposure, cathodic conditions, excessive thickness, or other causes.

4. Why should apparently sound coating be examined during a failure investigation?

View Answer

Sound areas provide a control for comparing thickness, adhesion, layer condition, contamination, and exposure with failed areas.

5. Can cathodic overprotection be diagnosed from coating appearance alone?

View Answer

No. Coating evidence must be evaluated together with cathodic-protection measurements and other service information by qualified personnel.

6. Why can spot repair be inappropriate for widespread defects?

View Answer

Widespread contamination, porosity, incomplete cure, poor adhesion, or incompatibility may leave unsound coating around every spot repair.

Tags: coating defects, coating failure analysis, blistering, delamination, underfilm corrosion, cathodic disbondment, coating cracks, pinholes, dry spray, industrial coatings, failure diagnosis, corrosion protection