What a coating failure tells you
A coating failure is a symptom, not a diagnosis. Blistering, delamination, cracking, chalking, rust spots and application defects each point to a short list of causes, and a coating failure analysis confirms which one with field measurements before anyone recoats. Repainting over an unresolved cause reproduces the same defect, usually faster.
Two questions narrow the search: where is the failure plane (coating to steel, between coats, or within one coat), and when did it appear. A defect within weeks of application usually traces back to preparation or application conditions; after years of service, to ageing, the environment or damage. Degradation is rated with the ISO 4628 series, which designates the quantity and size of defects so that two inspectors, or two shutdowns, give comparable results.
Blistering: osmotic and cathodic
What it looks like. Dome-shaped swellings of the film, intact or broken, often filled with liquid. ASTM D714 and ISO 4628-2 rate blister size and frequency against reference photographs.
Probable causes.
- Osmotic blistering. Soluble salts left on the steel, mainly chlorides and sulfates, draw water through the film until it lifts. It is the classic failure of immersed linings applied without salt testing.
- Retained solvent. Solvent trapped by excessive thickness, a coat applied too soon over the previous one, or cold conditions.
- Cathodic blistering. Alkalinity produced by cathodic reactions under the film, around a defect or under cathodic protection, pushes the coating off.
How to confirm it. Open a few blisters and check the fluid with pH paper: strongly alkaline fluid points to cathodic blistering, near-neutral fluid with rust underneath is consistent with osmosis. Test the bare steel for salts (ISO 8502-6 sampling, ISO 8502-9 conductivity) and measure thickness around the blisters.
How to repair and prevent it. Remove blistered coating back to sound edges, blast, wash where salts exceed the specified limit, retest, recoat. Prevention: a salt limit in the specification, verified before each coat, and the overcoat times on the data sheet.
Delamination: between coats and from the substrate
What it looks like. The coating peels in sheets or flakes (ISO 4628-5). The back of a detached chip is the key clue: rust or mill scale on it means a substrate failure; primer on both the chip and the steel means an intercoat failure.
Intercoat delamination.
- Recoat window exceeded. Past its maximum recoat window, an epoxy no longer bonds chemically to the next coat without abrasion.
- Amine blush. A waxy film formed when amine-cured epoxy cures in cold, humid air, invisible on dark colours.
- Contamination or incompatibility. Dust, overspray, oil, salts or condensation on the first coat, or a topcoat whose solvents attack it.
Delamination from the substrate. Cleanliness grade or surface profile below specification, flash rust or condensation before priming, and mill scale left on the steel: the coating holds to the scale, which later cracks off and takes it along.
How to confirm it. Tape adhesion to ASTM D3359 (X-cut or cross-cut) and the pull-off adhesion test to ASTM D4541, recording the failure mode — adhesive, cohesive or glue — to locate the interface that gave way. Application logs show recoat intervals, temperatures and dew point readings.
How to repair and prevent it. Intercoat: remove the loose topcoat, wash off blush with water and scrubbing, abrade the sound primer, recoat within the window. Substrate: spot repair rarely holds; return the area to the specified blast grade, as described in our NACE No. 2 surface preparation guide.
Cracking and mud cracking
What it looks like. Hairline to open cracks through one or more coats (ISO 4628-4). Mud cracking is a network of irregular cracks like dried mud, typical of thick, highly pigmented films such as zinc silicate primers.
Probable causes. Thickness above the data sheet maximum, which builds stress during cure; ageing of the binder; thermal cycling; a rigid coating on a flexing substrate; too many maintenance coats.
How to confirm it. Measure total thickness; a destructive cut through the film shows the number and thickness of the layers. Check whether cracks reach the steel.
How to repair and prevent it. Remove cracked coating rather than overcoat it: a new coat inherits the cracks. Control wet film thickness and choose a system flexible enough for the substrate and temperature range.
Chalking
What it looks like. A powdery, faded surface that comes off on a finger or cloth (ISO 4628-6, tape method).
Probable causes. Ultraviolet light breaks down the binder at the surface. Aromatic epoxies chalk outdoors as a property of the resin, not an application fault.
How to repair and prevent it. Mostly cosmetic while enough film remains, which a thickness survey confirms. Wash and lightly abrade before overcoating, and finish exterior epoxy systems with a UV-resistant topcoat such as an aliphatic polyurethane or a polysiloxane.
Under-film and filiform corrosion
What it looks like. Rust creeping under an apparently intact film from a scratch, edge or pinhole (rated around damage with ISO 4628-8). Filiform corrosion forms thin, thread-like tracks under thin films, most often on aluminum (ISO 4628-10).
Causes, confirmation and repair. Unrepaired damage, poor preparation, soluble salts and humidity. Cut the coating back until it is adherent and test the steel for salts. Remove all undercut coating, prepare beyond the rusted area, feather the sound edges and recoat.
Pinpoint rust, holidays and thin edges
What it looks like. Scattered rust specks, or rust lines along edges, welds and bolts. ASTM D610 and ISO 4628-3 rate the degree of rusting; ASTM D610 also records whether it is spot, general or pinpoint.
Probable causes. Film below the specified minimum, profile peaks left uncovered, holidays that expose the steel, and coating pulling thin over sharp edges.
How to confirm it. Dry film thickness per SSPC-PA 2 or ISO 19840, and a holiday test: low-voltage wet sponge for thin films, high-voltage spark for thick linings.
How to repair and prevent it. Spot-prepare and recoat to full thickness. Specify the ISO 8501-3 preparation grade for edges and welds, stripe-coat them, and check thickness and holidays before acceptance.
Application defects: runs, orange peel, pinholes, craters and bubbles
- Runs and sags. Excess wet film on vertical surfaces, over-thinning or cold product. Sand once cured and check thickness: the bottom of a run can be thick enough to crack.
- Orange peel. Dimpled surface from spray technique, viscosity or fast solvent evaporation; cosmetic unless the low points are thin.
- Craters and fisheyes. The film pulls away from oil, silicone or grease. Clean, abrade, recoat.
- Pinholes and bubbles on concrete. Air expelled from the concrete pores rises through the wet coating (outgassing), worst when the slab is warming. Prime or seal first and apply while the slab temperature is stable or falling.
- Bubbles in thick films. Air whipped in by mixing or trapped by rolling; follow the data sheet's induction time and roller type.
Cathodic disbondment
What it looks like. Loss of adhesion spreading from a holiday on a buried or immersed structure under cathodic protection, the steel beneath often still bright.
Causes and confirmation. Alkalinity generated at the cathode attacks the bond at the edge of the holiday; excessively negative potentials accelerate it. Lift the coating around the holiday and measure the disbonded radius. Cathodic disbondment resistance is a laboratory property checked at product selection.
Repair and prevention. Remove the disbonded coating, prepare and apply a repair coating compatible with cathodic protection, and review potential settings with the cathodic protection specialist.
Coating defects at a glance: cause, test and remedy
| Defect | Probable cause | Confirming test | Remedy |
|---|---|---|---|
| Osmotic blistering | Soluble salts, retained solvent | Blister fluid pH, salt test, DFT | Remove, blast, wash, retest, recoat |
| Cathodic blistering / disbondment | Alkalinity at cathodic sites, high CP potentials | Alkaline fluid, disbonded radius | CP-compatible repair, review potentials |
| Intercoat delamination | Recoat window exceeded, amine blush, contamination | Back of chip, pull-off failure mode | Wash, abrade, recoat within window |
| Substrate delamination | Poor preparation, mill scale, flash rust | Rust or scale on chip, adhesion test | Return to specified blast grade |
| Cracking, mud cracking | Excess thickness, ageing, flexing | Total and per-coat thickness | Remove, control thickness |
| Chalking | UV on aromatic epoxy | Tape test, DFT | Clean, UV-resistant topcoat |
| Under-film / filiform corrosion | Damage, salts, poor preparation | Cut-back, salt test | Remove undercut coating, recoat |
| Pinpoint rust, rusted edges | Low DFT, holidays, thin edges | DFT survey, holiday test | Stripe coat, recoat to thickness |
| Runs, orange peel | Technique, thinning | Visual, DFT | Sand, recoat where needed |
| Pinholes, craters, bubbles | Concrete outgassing, contamination, air | Visual, holiday test | Seal, clean, abrade, recoat |
How to run a coating failure analysis on site
- Survey and map. Locate each defect relative to welds, edges, waterline, sun-exposed faces and supports. The pattern is a clue: defects on edges suggest thin film; defects in a tank's liquid-vapour band suggest service conditions.
- Photograph before touching. Overall views and close-ups with a scale, located on a sketch.
- Measure. Dry film thickness, adhesion with failure mode, soluble salts on exposed steel, and steel temperature, air temperature, humidity and dew point if work is recent or about to restart.
- Sample. Keep chips and blister fluid in clean, labelled containers. A laboratory can identify layers in cross-section, the generic coating type and contaminants at the failure plane.
- Read the records. Specification, data sheets, batch numbers, daily logs.
- Treat the cause before recoating. A local defect calls for spot repair; a systemic one — salts across the surface, a wrong system, a substrate failure — calls for removal of the affected area. When the steel itself is the problem, identify the mechanism with our guide to types of industrial corrosion.
Choosing between touch-up, overcoat and full removal is covered in maintenance painting of steel structures; how failures shorten lining life in epoxy coating service life; and the condensation and blush risks of winter repairs in cold-weather epoxy coating application.
FAQ
What are the most common causes of coating failure? Inadequate surface preparation, soluble salts on the steel, film thickness outside the specified range, application outside the temperature, humidity and dew point limits, and a system unsuited to the service.
What causes paint blistering on industrial steel? Most often soluble salts under the film, drawing water through it by osmosis. Retained solvent and cathodic reactions are the other usual causes; blister fluid pH and a salt test tell them apart.
How do I tell intercoat from substrate delamination? Look at the back of a detached chip. Primer on both chip and steel means the topcoat let go of the primer; rust, mill scale or bare steel on the chip means the coating let go of the substrate.
Can I paint over a failed coating? Only where the existing coating is sound, adherent and compatible, and only once the cause is dealt with. Painting over blisters, cracks or salt-contaminated steel buries the problem.
Which standards rate coating defects? The ISO 4628 series covers blistering, rusting, cracking, flaking, chalking and filiform corrosion. ASTM D714 rates blistering, ASTM D610 rusting, and ASTM D3359 and D4541 cover adhesion.
In practice
A coating repair holds when the failure has been explained first. Induscoat carries out on-site coating assessments for plants in Quebec and Ontario — mapping, thickness and adhesion readings, salt testing — then removes the failed coating and applies a system suited to the service through its protective coatings service and its industrial corrosion protection work, including cold-bonded rebuilding of steel pitted under the failed film. Photos of the defect, the age of the coating and the service conditions are enough to start: request a quote. Technical data sheets for the Induscoat range are published on the brand site, induscoat.com.
Hicham M, P Eng, PMP
Engineer and project manager (PMP) at Induscoat. Over 16 years of experience in industrial coatings, composite repairs and wear protection on mining, energy and petrochemical sites in Canada and internationally.