Immersion Service: Why the Rules Get Stricter

A coating on the outside of a tank has an easy life compared with the one on the inside. Outside, water arrives, sits for a while and evaporates. Inside, the water never leaves.

Why immersion is unforgiving

In atmospheric service, small defects are tolerated. The coating dries out between rain events and damage progresses slowly. In immersion, water is working to get through the coating, around it and under it all the time.

• Every defect becomes a pathway.

• Every soluble salt left on the steel becomes an osmotic blister.

• Every pinhole becomes a corrosion cell.

There is a subtler risk too. When a warm liquid sits against a cold steel wall, the temperature difference drives vapor through the film and can blister even a well-applied lining. This is the cold wall effect, described in Tnemec Technical Bulletin 04-44.

How immersion specs respond

Immersion specifications tighten everything at once:

• Surface prep moves from SSPC-SP 6 to SP 10 or SP 5.

• Soluble salt testing is added.

• Coats go from one to two or three, often in contrasting colors so coverage can be checked.

• Holiday testing becomes mandatory.

• Cure means full chemical cure before the tank sees water, not dry to touch.

• The coating itself must be tested and proven for immersion, not simply a good exterior epoxy.

Budgeting for it

An owner comparing the cost of a lining to the cost of exterior painting is comparing two different jobs. One is a maintenance coat. The other is a barrier that has to hold back water continuously for years. The stricter steps are what make the service life possible.

What to check in your spec

• SSPC-SP 10 or better, with soluble salt testing and a stated limit.

• Holiday testing per ASTM D5162.

• A full-cure requirement before fill, tied to temperature.

• Consideration of cold wall conditions for heated or chilled contents.

Standards: SSPC-SP 10, SSPC-Guide 15, ASTM D5162, AWWA D102, NACE SP0178.

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.