Investigating Protective Coating Failure

Introduction

When a protective coating fails, the visible defect is often only the starting point of the investigation. Blistering, flaking, cracking, rusting, delamination or loss of adhesion describes what has happened to the coating; it does not necessarily explain why. A coating failure can result from one cause or several contributing factors involving surface preparation, contamination, application, environmental conditions, coating compatibility, film thickness, curing, workmanship, exposure or the condition of an existing coating system. The objective is to gather and evaluate sufficient evidence to understand the most technically supportable explanation for the failure.

Defect Description Is Not Root-Cause Analysis

If a coating is blistering, blistering is an observation. If it is peeling from the substrate, loss of adhesion describes the behaviour or failure interface. Neither statement establishes root cause. The investigation must consider physical evidence and relevant project information before conclusions are reached. Jumping directly from appearance to a cause can lead to an incorrect diagnosis and inappropriate repair strategy.

Start by Defining the Failure

Establish the nature and extent of the problem: the defect type, location, distribution, whether deterioration is localised or systematic, the coating layer that appears to have failed, corrosion beneath the coating, affected structural details, behaviour of repaired versus original areas, relevant exposure, and when the problem was first observed. Mapping the distribution can reveal patterns not apparent from an isolated defect.

Visual Examination and Defect Classification

Structured visual examination provides the foundation for further investigation. Record rusting, blistering, cracking, flaking, peeling, delamination, chalking, colour change, underfilm corrosion, mechanical damage, pinholing and breakdown around edges, welds and connections. Where appropriate, the ISO 4628 series can provide consistent methods for describing particular forms of deterioration. Classification makes records more repeatable, but still does not establish cause.

Look at Where the Coating Failed

Where coating has detached, consider whether failure occurred between the substrate and first coat, between coating layers, cohesively within a layer, within corrosion products, within a historical coating, or through a combination of interfaces. Separation at an intercoat interface raises different questions from failure at the steel substrate. The interface is evidence and must be considered alongside the rest of the investigation.

Surface Preparation, Salts and Profile

Investigate initial steel condition, achieved cleanliness, surface profile, residual contamination, dust, soluble salts, deterioration between preparation and coating, and contamination introduced afterwards. Historic records for blast cleanliness, profile, salts and dust can be valuable because those conditions are often impossible to assess directly after coating. If contamination is suspected, testing may assist, but a post-failure salt result does not prove the level present at application: exposure after application or deterioration may have changed the surface. Surface profile may also be relevant where adhesion to blast-cleaned steel is in question, but corrosion, coating removal and later preparation can alter the substrate.

Coating Thickness and Adhesion Testing

DFT measurements may identify unusually low or excessive build, significant variation, differences between failed and sound areas, or multiple historical layers. Compare measurements with applicable requirements where available, but thickness alone does not establish why coating failed. Adhesion testing can be valuable, particularly when comparing failed and sound areas. Select a method suited to the coating system and question. Pull-off testing may provide a quantitative result while other methods provide classification or comparison; failure mode and location are essential evidence alongside any value.

Coating Identity and FTIR

On existing structures, original records may be incomplete and maintenance campaigns may have introduced different layers. FTIR analysis can assist with generic binder identification where samples are available. It can help establish types of coating that may be present and support investigation of compatibility or intercoat problems. FTIR does not identify an exact commercial product, prove compatibility, or determine root cause by itself.

Review Application and Environmental Records

Where available, review product and batch information, mixing and thinner use, application times, environmental conditions, substrate temperature, humidity, dew point, wet and dry film thickness, recoat intervals, cure records, inspection points, photographs and non-conformance records. Compare these with manufacturer requirements and project documentation. Temperature, humidity, condensation risk and ventilation can affect application and cure, but an environmental cause should not be assumed merely because failure occurred in a particular season or weather condition.

Cure, Overcoating and Compatibility

For intercoat failure, review application dates, environmental conditions, cure requirements, recoat windows, surface condition before overcoating, contamination between coats and any specified preparation between layers. For applicable ethyl silicate inorganic zinc-rich primers, MEK resistance testing may have provided evidence of cure before overcoating. Compatibility investigation may also consider binder chemistry, retained-coating condition, historical-layer adhesion, solvent sensitivity, preparation, contamination, manufacturer recommendations and prior compatibility trials. Generic binder identification alone does not prove compatibility or incompatibility.

Laboratory Analysis and Comparison Areas

Some failures need laboratory work in addition to field inspection, such as coating-layer examination, FTIR, microscopy, contamination or deposit examination, and other techniques selected to answer a defined question. Testing everything simply because a technique is available can create data without improving understanding. Compare failed and apparently sound locations for DFT, adhesion, layer structure, exposure, records, application dates, environmental conditions, batches and structural details. Differences can help identify factors that correlate with failure.

Be Careful with Cause and Correlation

An abnormal result does not automatically establish that it caused the failure. Several deficiencies may coexist, some with little relationship to the observed defect; conversely, several minor factors may combine. A robust investigation distinguishes observed facts, test results, possible contributing factors and supported conclusions, especially where findings may influence contractual, financial or remedial decisions.

What Should a Failure Investigation Report Contain?

A useful report may include background and objective; documentation reviewed; inspected areas; access and limitations; defect description and distribution; photographs; DFT and adhesion results with failure modes; surface or contamination testing; samples and laboratory findings; application and environmental record review; comparison of failed and sound areas; relevant requirements; evidence; interpretation; conclusions; and limitations or further-investigation needs. Make clear where conclusions are supported directly and where uncertainty remains.

Practical Inspection Note

Do not begin a coating failure investigation with a predetermined cause. Start with the evidence. Record the defect, establish its distribution, examine the failure interface, review available project records and select testing that can answer specific technical questions. Most importantly, distinguish between describing the failure and establishing why it occurred.

Why Independent Investigation Matters

Coating failures can involve asset owners, contractors, manufacturers, applicators and designers. An independent investigation should document and interpret available technical evidence. Paint Inspection Ltd can combine site inspection, testing, sampling, laboratory analysis and project-document review. The purpose is not to allocate blame; it is to establish what the available evidence supports and provide a stronger basis for further investigation, repair or maintenance.

Conclusion

A visible coating defect tells you that something has happened, not necessarily why. Effective failure investigation considers the defect, coating system, substrate, application history, environment and test evidence together. Adhesion testing, DFT, soluble-salt testing, FTIR and laboratory examination can all contribute, but none is a standalone answer. Good investigation follows the evidence rather than forcing it to fit an assumed cause.