Tank Coating Failure: Reading a Diver's Condition Report
By Marisol Vega, Municipal water operations manager. Reviewed by Dana Whitcombe, technical reviewer.
Commercial Diving Contractors Alabama | Commercial Diving Contractors Alaska I was halfway through a Thursday afternoon when the email came in with the diver's condition report for tank 7. The first photo made me stop: a section of the concrete floor where the coating had let go entirely, rust-colored water weeping through the exposed steel. That tank holds 2 million gallons of finished drinking water, and we operate 40 of these things. We put a diver inside each tank every five years per our state permit. That report is the only look we get between full drain-downs. It needs to tell me whether a tank can make another five years or whether we're about to schedule an emergency re-coat. Learning to read one of these reports takes practice, and I've learned the hard way what matters and what's noise. A water tank coating inspection doesn't start on the tank. It starts by reading the diver's report the way a detective reads a case file.
Start with the overall condition rating
Dive companies usually give each tank an overall condition rating: 'Good,' 'Fair,' or 'Poor.' That rating is a starting point, not a verdict. I've seen a 'Fair' rating hide a 4-foot square of delamination behind a baffle. And I've seen a 'Poor' rating drive a needless and expensive re-coat of a tank that just needed to be cleaned out and patched. Look at the rating, then flip to the photos. Every report should have photos with captions. I want to know exactly where the worst spots are: floor, walls, roof, or around the pipe penetrations. Those locations tell me a story. Roof failures usually mean condensation and poor ventilation. Floor failures often come from floor-standing supports or from the tank sitting on a wet surface. Wall failures near the waterline? That's nearly always a coating that wasn't applied thick enough.
Know your failure terms
Your diver's report is full of coating vocabulary that can create more questions than answers. Let's cut through it. Blistering means water is getting under the coating and pushing it up into bumps. Catastrophic blisters are the size of dinner plates and they pop under any pressure. Cracking is what it sounds like: the coating splits, often because it's too rigid for the tank's flexing. Rust bleeding is orange or brown staining on the surface. Delamination is the big one - the coating peels off in sheets, exposing bare steel or concrete. Cathodic disbondment happens when a cathodic protection system's current works against the coating, lifting it off the substrate. A good report will reference the SSPC or AMPP visual standards for describing these defects. If you see 'blistering' and 'rust bleeding' on the same report, that's a warning sign that the coating system has failed and water is in contact with the substrate. If you see 'delamination' anywhere, you are looking at exposed material that's already corroding. I'll pause here for a real-world digression: I once inherited a tank that had been coated in a coal-tar epoxy that was never meant for potable water. The report called it 'saponified.' That's not in most textbooks. The coating had reacted with the water chemistry and turned to a white mush. That was a 100% re-coat job, and we learned to ask which specific coating was used on every tank after that.
Check dry film thickness and adhesion numbers
The report should include dry film thickness (DFT) readings from a magnetic or ultrasonic gauge. We spec a two-part epoxy system from Tnemec or a comparable supplier at 12 to 16 mils, give or take. If the diver's readings come in below 6 mils, you have essentially no protection left. But those numbers need context. I always compare the diver's readings to what was specified at the last re-coat. If we expect 14 mils and the report says an average of 8 mils, someone under-applied the coating, and that tank is going to fail early. The other key number is adhesion. Divers often use a pull-off adhesion tester that measures pounds per square inch (psi). I like to see 300 psi or better for a shop-applied system on steel. For a field-applied system on concrete, I'll accept 200 psi. If the report shows adhesion values below 150 psi, you've got a coating that's barely clinging. Even if the visual photos look fine, that tank is a time bomb.
Ask the dive company the right questions
Don't stop at the report. Call the diver's supervisor and ask questions. First: 'Which areas were you not able to inspect?' Every tank has zones that are tough to reach, so you need to know where you're flying blind. Ask about the floor-to-wall joint, the waterline, and the roof seam. Ask about the condition of the perimeter caulking at the floor joint, because that's a common leak path. Ask about the clarity of the water. If the diver couldn't see anything beyond three feet, the report is incomplete. If your tank has a sacrificial anode or an impressed current system, ask for the cathodic protection reading. A sudden current demand can mean the coating is failing and the anodes are working overtime. I also ask the diver to compare the current reading to the previous inspection's number. Did it climb dramatically? That's a red flag. The last question is the simplest and the most important: 'If you owned this tank, when would you re-coat it?' That gets you a straight answer from someone who has no interest in selling you a coating job.
Turn the report into a decision
Once you've digested the report, you need to plan. If the failure is isolated, you can often get away with a spot repair: a diver with a grinder and an underwater patching compound can fix blisters smaller than a few square inches. But if the report shows widespread blistering, delamination, or rust bleeding, you're looking at a tank re-coating. That means draining the tank, hydro blasting, applying a new coating system, and curing. For a 2-million-gallon tank, that's weeks of work, and the tank is out of service. The other factor is risk tolerance, and that varies from plant to plant. We had a tank with a 'Fair' rating that we nursed for six years with cathodic protection and annual diver inspections because we couldn't afford the capital outlay that year. It was a gamble. It paid off, but I wouldn't recommend it. If you have a report that says coating failure and your tank is supplying a hospital or a school district, you schedule the re-coat. Period. A good condition report is the cheapest insurance you'll ever buy. It's the difference between a planned re-coat and an unplanned water outage. And once you learn to read it, the diver's report stops being a mystery and starts being a management tool.
Frequently Asked Questions
What does 'blistering' mean in a diver's condition report for a water tank? Blistering is the formation of bumps or domes in the coating, caused by water migrating into the coating system. Large blisters mean the coating is losing adhesion and the tank is at risk for corrosion. What is a typical dry film thickness for a potable water tank coating? For a two-part epoxy system, you'll see a spec of 12 to 16 mils dry film thickness. The diver's report should show average readings; anything below 6 mils is thin and worth watching. How often should a water tank be inspected by a diver? Many state permits require a diver inspection every five years. That's the common cycle for finished-water storage tanks, but you may need to do annual inspections if a previous report showed concerns. Should I drain the tank for a visual inspection instead of using a diver? Draining a tank gives you a more thorough look, but it costs a lot in lost storage capacity and downtime. A diver inspection is your first line of defense; you can use the report to decide if a full drain-down is necessary.