Soluble Salts: The Contaminant That Passes Visual Inspection

Steel blasted to a clean SSPC-SP 10, profile on target, primed the same day. Two years later the lining is covered in blisters. The inspection report never tested for the thing that caused the failure.

What soluble salts are

Soluble salts, mostly chlorides and sulfates, arrive on sea air, road salt, process chemicals and old rust. They cannot be seen, and they survive abrasive blasting. The blast can even drive some of them deeper into the pits.

Under a coating, a small amount of salt pulls moisture through the film by osmosis. The film lifts, and the result is an osmotic blister filled with salty liquid. Seawater immersion rarely causes this, because the water is salty on both sides of the film. Potable water, deionized water and condensate are the worst cases.

How to test

The test is inexpensive and takes about ten minutes: a Bresle patch per ISO 8502-6 and 8502-9, distilled water, a conductivity meter and a short calculation to get micrograms per square centimeter.

Acceptance limits are set by the owner or the coating manufacturer, not by the test standard. Tnemec Technical Bulletin 99-19 R3 publishes Tnemec's limits for chloride:

• 7 µg/cm² for atmospheric service

• 5 µg/cm² for potable water immersion

• 3 µg/cm² for chemical immersion

The same bulletin notes that blasting alone usually will not get a contaminated surface below those numbers. Washing or a salt remover is often needed.

Where it matters most

Immersion work in coastal areas, wastewater plants and industrial facilities with chemical exposure. A lining spec in one of those settings that never mentions soluble salts has a gap in it. Raising it before the blast costs very little. Raising it after the blisters appear costs a great deal.

What to check in your spec

• A soluble salt test method (ISO 8502-6/9 or SSPC-Guide 15).

• A numeric acceptance limit matched to the service.

• Test frequency and locations.

• A remediation step if results exceed the limit.

Standards: SSPC-Guide 15, ISO 8502-6, ISO 8502-9, SSPC-SP 10.

Talk to a consultant. Atlantic Coating Consultants helps owners, engineers and contractors in PA, NJ, DE, MD, DC, WV and OH with specification writing, on-site condition assessment, training, product recommendations and field support. Call 484-644-5303 or email atlantic@tnemec.com.