Cathodic disbondment of coatings: mechanism, tests and system selection

Cathodic disbondment is the loss of coating adhesion that spreads from a defect under the effect of cathodic protection: the cathodic reaction creates a strongly alkaline environment at the defect, sometimes with hydrogen, that attacks the bond between coating and steel. It is measured with standard tests such as those in CSA Z245.20, CSA Z245.30, ASTM G8 and ASTM G95, and limited by choosing a coating qualified for cathodic protection, getting surface preparation right and avoiding overprotection.

Coatings and cathodic protection work together: the coating isolates most of the steel, cathodic protection protects the defects. That partnership has a cost, because the current that protects steel at the defect also weakens adhesion all around it. This guide is written for owners and integrity leads of pipelines, tanks and submerged structures in Quebec and Ontario who must select or repair a coating on an asset under cathodic protection.

What is cathodic disbondment?

Cathodic disbondment is coating separation that starts at a defect (holiday, scratch, missed area) and spreads under the film as long as cathodic protection is active. The steel at the defect stays protected, but the disbonded area grows over time.

The problem is not only cosmetic. A disbonded coating lets water in between the film and the steel; depending on the coating type, it can then shield the protective current. The steel under the film is then protected neither by the coating nor by cathodic protection. On buried pipelines, that combination of trapped water and missing current is one of the conditions linked to external corrosion and cracking.

How does cathodic protection disbond a coating?

At the defect, the steel acts as the cathode: dissolved oxygen and water are reduced to hydroxide ions there and, at more negative potentials, hydrogen is released. The environment at the edge of the defect becomes strongly alkaline.

Three effects combine:

• Alkaline attack at the interface. Alkalinity weakens the bond between the coating and the steel's surface oxide, and can degrade some binders.
• Water and ion migration. Cations from the environment migrate toward the cathodic area to balance charges, which sustains the alkaline environment under the film edge.
• Hydrogen evolution. With overprotection, gas formed at the interface pushes the film and accelerates disbondment.

Disbondment rate increases with temperature, with an overly negative potential and with weak surface preparation or soluble salt contamination.

Field conditions add their own variables. Wet clay soils, seasonal changes in the water table and stray currents from nearby rail or power systems can shift potentials locally, so two sections of the same line may behave very differently. That is why disbondment found at one dig is not automatically representative of the whole asset, and why potential surveys are read together with coating defect surveys.

Which tests measure cathodic disbondment resistance?

Cathodic disbondment tests create an artificial holiday in the coating, apply a cathodic potential in an electrolyte for a set duration and temperature, then measure the disbonded radius around the holiday. Each standard sets its own conditions and criteria.

Durations, temperatures, potentials and allowable radii are read in the standard and the project specification. Comparing two results only makes sense when they come from the same test under the same conditions. Our article on CSA Z245.30 details the qualification of field systems.

Which coating should you choose under cathodic protection?

Choose a coating qualified by a cathodic disbondment test under conditions close to service, especially operating temperature. The system must bond strongly, resist alkalinity and, ideally, not shield current if it ever disbonds.

• Fusion bond epoxy (FBE) and dual-layer FBE. Good adhesion and good cathodic disbondment resistance; disbonded FBE generally still lets protective current through.
• Multilayer polyolefin systems. Strong mechanical and moisture barrier; the epoxy base layer provides adhesion and disbondment resistance.
• Liquid epoxies and polyurethanes. Common for joints, repairs and complex shapes; their performance varies widely by formulation.
• Tapes and sleeves. Practical in the field, but some can shield current if they disbond: qualification and installation matter a great deal.

For tanks and submerged structures protected by anodes, the internal lining must also be compatible with cathodic protection; our guide to industrial tank lining selection covers this point. All of this falls under our protective coatings work.

Why are field joints the weak point?

Field joints are the weak point because they are coated on site, under less controlled conditions than in a plant, and must bond both to the steel and to the mainline coating. A poorly prepared joint disbonds faster than the rest of the line.

The checkpoints are well known:

• Steel preparation. Blasting to the required grade, adequate profile, no salts or flash rust; see our NACE 2 / SSPC-SP 10 surface preparation guide.
• Overlap zone. Preparation of the adjacent mainline coating to secure adhesion of the joint system.
• Preheat and conditions. Steel temperature, dew point and humidity per the system data sheet.
• Compatibility. Joint system qualified to CSA Z245.30 for the mainline coating in place and the service temperature.

Our article on pipeline coatings and field joints in Canada completes these points.

How do you limit disbondment in service?

Limit disbondment in service by reducing the number of defects from the start and keeping cathodic protection within its intended range. The two levers work together.

• Film continuity. Spark testing for holidays over the full surface before burial or immersion; see the holiday test.
• Cathodic protection adjustment. Potentials monitored and kept within the range of the selected criterion; overprotection speeds disbondment without adding protection.
• Handling and backfill. Mechanical protection of the coating during installation, backfill free of sharp rock.
• Monitoring. Potential surveys and coating defect surveys to locate areas that draw current.

The choice between sacrificial anodes and impressed current also changes the overprotection risk, as our cathodic protection comparison explains.

How do you repair a disbonded area?

Repair by removing all disbonded coating back to firmly adherent material, then preparing the steel and applying a qualified repair system compatible with the existing coating. Coating over a disbonded film only moves the problem.

1. Delimit. Probe adhesion around the defect over a sufficient distance.
2. Remove. Take off the disbonded film and feather the adherent edges.
3. Prepare. Cleaning, blasting and soluble salt check.
4. Apply. Qualified repair system, thickness measured.
5. Inspect. Holiday and adhesion testing per the specification.

Our coating inspection service documents these checks, described in our article on DFT, adhesion and holiday testing.

Frequently asked questions

Does cathodic disbondment mean the cathodic protection is poorly designed?

Not necessarily. All coatings show some disbondment around defects. Fast disbondment points instead to overprotection, insufficient preparation or a coating poorly suited to the temperature.

Why does temperature matter so much?

Because it speeds up interface reactions and water diffusion. That is why the standards include elevated-temperature tests for hot assets.

Does a disbonded coating always shield current?

No. It depends on the coating: some let protective current through once disbonded, others block it. That behaviour is an important selection criterion.

Do ASTM G8 and ASTM G95 give the same result?

Not necessarily. The methods differ in setup and exposure. Only compare results obtained with the same method under the same conditions.

Need to select, qualify or repair a coating on a pipeline, tank or submerged structure under cathodic protection? Our protective coatings service defines and applies the system with our crews and qualified subcontractors, in Quebec and Ontario, including pipe coating. Send photos, drawings and service conditions through the request a quote form: pricing on consultation, reply within 24 business hours.