Osmotic blistering in immersion coatings: causes, diagnosis and repair

Osmotic blistering occurs when water passes through the coating and collects under the film, drawn by soluble salts or solvents left at the interface, often helped by a wall that is colder than the liquid. It is diagnosed by rating per ASTM D714 and by analyzing the blister fluid and the substrate; it is repaired by removing the affected coating, decontaminating the steel and applying a system suited to the service.

Blisters on an immersion coating are always a concern, and rightly so: they show that water has crossed the barrier. Not all blisters share the same cause, though, and the repair depends on that diagnosis. This guide is written for maintenance and integrity managers responsible for tanks, process vessels, clarifiers and lined piping in Quebec and Ontario. It complements our coating inspection page and our general article on coating failure modes.

What is osmotic blistering?

It is the formation of liquid-filled blisters under an immersed coating, caused by water moving through the film toward an area with a higher concentration of soluble matter. The film acts as a semi-permeable membrane: water gets in, salts stay, pressure builds and the film lifts.

All organic coatings let a small amount of water vapour through. As long as the steel-coating interface is clean and adhesion holds, that water does not accumulate. It accumulates when it finds, under the film:

• Soluble salts. Chlorides, sulfates or nitrates left on the steel before application, invisible to the eye.
• Retained solvent. A thick coat applied too fast, or overcoated before its solvents evaporated, keeps some of them.
• Hydrophilic components. Some pigments or fillers, or incomplete cure, make the film more sensitive to water.

Osmotic blistering differs from cathodic disbondment, which is driven by cathodic protection and produces an alkaline fluid.

Why do soluble salts under the film cause blisters?

Because a very small amount of salt on the steel is enough to create a concentration difference with the tank water, and water naturally migrates toward the more concentrated solution. Salts cannot be seen and are not removed by dry blasting alone: they stay lodged at the bottom of the profile.

Where do they come from?

• The steel itself. Steel corroded in service, especially in salt water or chemical service, holds salts in its pits.
• The site environment. Sea spray, de-icing salt, industrial dust.
• Abrasive or wash water. Contaminated recycled abrasive or uncontrolled rinse water.

They are measured on the prepared surface, for example by sampling per ISO 8502-6 and conductometry per ISO 8502-9; SSPC-Guide 15 describes field methods. The acceptable limit comes from the specification or the coating data sheet, not from a universal value.

How do temperature gradients make blistering worse?

When the liquid is warmer than the wall, water moves through the film toward the cold side and condenses at the interface: this is the cold wall effect. It greatly speeds up blistering, even on a correctly applied coating if that coating was not selected for this service.

Typical situations:

• Uninsulated hot water or heated process tanks exposed to Quebec winters.
• Exchangers, water boxes and vessels whose contents are heated while the wall is cooled from outside.
• Tank floors in contact with cold ground.

System selection must account for this difference. One-side immersion cell tests (for example per ASTM C868 or NACE TM0174) reproduce this condition and are used to compare coatings before selection.

How are blisters diagnosed per ASTM D714?

Blisters are rated with ASTM D714, which describes size by a number (from 10, no blistering, to 2, the largest) and frequency by a letter (F, M, MD or D, from few to dense), using reference photographs. ISO 4628-2 offers an equivalent rating. The rating describes the defect; the cause is found by investigation.

A complete diagnosis includes:

1. Mapping. Blister locations: waterline, floor, welds, areas near heat sources.
2. Rating. Size and frequency per ASTM D714, with photos and location markers.
3. Opening blisters. Measuring the pH of the fluid: a neutral fluid points to osmosis, a strongly alkaline fluid to cathodic disbondment.
4. Substrate examination. Clean bright steel or corroded steel under the blister, salts at the interface, failure plane (between coats or on the steel).
5. Additional tests. Thickness, adhesion around blistered areas, holiday testing of sound areas.
6. Record review. Preparation, salt readings, ambient conditions, recoat intervals and service conditions.

Our article on coating inspection details the field tests.

How is a coating with osmotic blistering repaired?

The blistered coating and a band of sound coating around it are removed, the steel is decontaminated to the required salt level, blast cleaned again to the specified grade and coated with a system suited to the service. Puncturing and filling blisters does not address the cause: the water comes back.

Choices depend on the extent:

• Localized blisters, cause identified. Repair by areas, with feathered edges and an overcoat compatible with the existing coating.
• Widespread blistering. Full removal and a new system, often better suited than the old one to the temperature or chemistry of the service.
• Attacked steel. Remaining thickness measured and metal repaired before coating.

Decontamination often combines water washing, blasting and repeated salt testing until the required result is reached. Abrasive blasting then restores the profile.

How do you prevent osmotic blistering on a new coating?

Prevention relies mostly on a clean interface and on system selection: salts measured and removed before application, thicknesses and intervals respected, full cure before filling, and a coating chosen for the real temperature and chemistry of the service.

Useful control points:

• Clear specification. Salt limit, blast grade, profile, thicknesses and inspection method written into the spec.
• Application conditions. Surface temperature above the dew point with the required margin, relative humidity tracked.
• Intervals. Recoat and cure per the data sheet; our article on pot life and recoat windows explains what is at stake.
• Holiday testing before return to service on critical areas.
• Quality records archived, for comparison if a defect appears.

For tank interiors, our tank lining page and our guide to epoxy service life in immersion give context for the choice. These systems are part of our protective coatings.

Where do we work and how do you get a quote?

Our crews and qualified subcontractors work from LaSalle across Quebec and Ontario, including Bécancour and Montérégie, for inspection, diagnosis and repair. When a root cause analysis is needed before choosing a repair, our strategic advisory service can take over.

Pricing is on consultation: it depends on the affected area, access, the shutdown window, required testing and the scope of the repair. Send photos, drawings and service conditions through the quote request: reply within 24 business hours.

Frequently asked questions

Is an unbroken blister already steel corrosion?

Not necessarily. Under a recent osmotic blister, the steel is often still clean. Over time, the water under the blister ends up attacking it, which is why early action pays.

Can a tank return to service with a few blisters?

It depends on their rating, how they evolve and the service. A few stable blisters can be monitored until the next shutdown; blisters that grow or break open call for repair.

Is osmotic blistering a product defect?

Rarely on its own. It most often results from a contaminated interface, a missed interval or a system unsuited to the service. The diagnosis is what separates these causes.

Should salts be measured on every immersion project?

Yes, it is the safest approach, especially on steel that has already been in service. Testing is done after blasting and before the first coat is applied.