Humidity and Condensation Testing

Humidity and condensation testing exposes coated specimens to very high humidity or continuous condensation, so that the effect of water on the coating and its adhesion can be judged in a shorter time than outdoor exposure. The main methods are ASTM D2247 (water resistance of coatings in 100% relative humidity), ASTM D4585 (controlled condensation, the Cleveland cabinet), and ISO 6270-1 and ISO 6270-2. These tests reveal blistering, softening, adhesion loss, color change and corrosion caused by moisture.

Why water exposure tests matter

Water is involved in most coating failures. It diffuses through films, collects at the interface, and drives coating blistering, osmotic blistering, and corrosion. Contaminants such as soluble salts under the coating greatly increase the effect. Humidity and condensation tests help detect:

• Poor surface preparation or contamination
• Under-cured films
• Water-sensitive formulations
• Intercoat adhesion problems
• Inadequate film thickness

Test methods

ASTM D2247: 100% relative humidity

Panels are placed in a cabinet in which heated water produces a saturated atmosphere, typically at about 38 °C (100 °F). Condensation forms on all surfaces of the panels. The test runs for a specified number of hours, after which panels are evaluated.

ASTM D4585: controlled condensation

In the Cleveland condensing cabinet, the coated panels form the roof or walls of a chamber above heated water. The coated face is exposed to warm water vapor while the back of the panel is cooled by room air. The temperature difference causes continuous condensation on the coated face. This closely represents conditions on tank roofs, cold-water pipes and building walls where warm, moist air meets a cool surface.

ISO 6270

• ISO 6270-1: continuous condensation, similar in principle to the Cleveland cabinet.
• ISO 6270-2: condensation atmospheres in a chamber, with constant and alternating conditions.

ISO 12944-6 uses ISO 6270-1 among its laboratory tests for qualifying protective paint systems (see ISO 12944 corrosivity categories).

Water immersion

ASTM D870 covers immersion in water. Immersion is more severe for some coatings and is used for linings and submerged service.

Specimen preparation

• Use the substrate, preparation and coating system of interest.
• Measure and record dry film thickness.
• Cure panels as specified before testing. Testing under-cured panels gives misleading results.
• Seal panel edges and backs if required, so that failure does not start from uncoated areas.
• Include a scribe if undercutting is to be evaluated.

Evaluation

After exposure, panels are examined promptly, since some effects such as blistering and softening can recover as the panel dries. Typical assessments include:

• Blistering by ASTM D714 or ISO 4628-2 (see rust and blister rating).
• Rusting by ASTM D610 or ISO 4628-3.
• Adhesion after a recovery period by tape or pull-off methods (see adhesion testing).
• Color and gloss change (see gloss and color measurement).
• Softening or wrinkling.

Interpreting results

• Comparative, not predictive. These tests rank coatings or preparations against each other. They do not translate directly into years of service.
• Blisters with clear fluid often suggest osmotic effects or contamination; the location of the blister (within the coating, between coats, or at the substrate) points to the cause.
• Adhesion loss without blisters may indicate interfacial weakness, such as poor intercoat adhesion (see intercoat delamination).
• Control panels with known performance help calibrate interpretation.

Practical uses

• Product qualification for specifications and owner approval programs.
• Quality control of factory-applied coatings.
• Failure investigation, by reproducing a field failure on laboratory panels.
• Surface preparation studies, for example comparing salt levels or abrasive types.

Limitations

• Constant high humidity does not reproduce wet and dry cycling, UV, or temperature extremes. Cyclic tests such as ISO 12944-9 or accelerated weathering testing add those factors.
• Results depend strongly on cure, film thickness and panel preparation.
• Some coatings perform well in humidity but poorly in immersion, and the reverse.

In practice

In practice, humidity and condensation tests are most useful as screening tools. A coating that blisters, softens or loses adhesion quickly in a condensing cabinet is likely to have trouble on cold water pipes, chilled tanks or the underside of roof decks. Passing the test does not guarantee field performance, because real exposures add UV, chemicals and temperature cycling. Specifiers often combine a humidity test with salt fog or cyclic corrosion testing and with adhesion testing before and after exposure.

Key terms

• Condensation. Water forming on a surface cooler than the dew point of the surrounding air.
• Cleveland cabinet. A test apparatus in which panels form one wall of a heated water chamber so condensation forms on the coating.
• Constant humidity. A test in which panels are held at high relative humidity and constant temperature, such as ASTM D2247.
• Dew point. The temperature at which air becomes saturated and water condenses (see environmental conditions for coating).
• Recovery period. Time allowed after exposure before rating, since some blisters and softening reverse as panels dry.

Frequently asked questions

How long should a humidity test run?

The specification or qualification standard sets the duration. Common durations range from a few hundred to over a thousand hours.

Why do blisters disappear after the test?

Water in the film evaporates and the blister can flatten. That is why ratings are taken soon after removal.

Is condensation testing the same as salt spray?

No. Condensation uses pure water and focuses on moisture effects. Salt spray uses a salt fog and focuses on corrosion.

Related entries

• Salt spray and cyclic corrosion testing
• Accelerated weathering testing
• Coating blistering
• Water vapor transmission of coating films
• Rust and blister rating (ASTM D610 and D714) (new)
• Filiform corrosion (new)