Bubbles in Paint on DM Storage Tank Wall

Every thing has an ending, only a sausage has two.

It looks like you have had good use of this tank and restoring it, even if the symptoms are caused by KOH, which I don't think is or could be the case, think about a new one. I am not sure what this means:

"was painted 9 Years before i.e Year 2000 with Epoxy paint."

If the tank is 20 years old, I can only repeat:Every thing has an ending, only a sausage has two. I understand that this tank is out doors and subjected to UV light. Have a look if the bubbles are on the "sunny side" more frequent than on any other surface area. If that is the case you have the culprit. It would not surprise me.

To remove the old surface and prepare for a new coat will be tedious work, if done properly. Unless you have a lot of time on your hands or can employ people to do it, think of, and prepare starting anew.

Not much information to go by, Ky.

Dissolved Air/ oxygen in the presence of alkalinity under hot exothermic conditions should have corroded the epoxy.

Or the air bubbles should have occurred during coating stage and now are showing up due to surface corrosion.

Time out!

Interpret this as a DM water - demineralized water - storage tank with a potassium hydroxide breather to prevent carbon dioxide entry to the tank!

Assume that the tank contents is high resistivity deionized water.

Now make your suggestions and observations!

How about a few more questions?

About the 510 M³ storage tank: In-ground or above? Environment (Tropical, Arctic, Desert)? Dimensions and approx volume of DM water? Thickness of steel?

About the coating job in AD2000: Recoat of EXISTING structure, or catalyzed epoxy on new? Was it blasted to near white? Were ALL coatings applied correctly? Are you certain of that?

About these 'bubbles': How many per M²? How large? Inside tank, or outside? As previously asked, predominant on the sun exposure sides? Are they breaking open? If yes, is it apparent it is pit rust? Have they been evident in smaller numbers in the past? Concentrated about 3 meter from bottom... is that about the average level of DM water in tank?

The answer to your question "What could be the reason for bubbles in paint of DM tank wall." is probably Rust Pits... we may be able to help solve why. As ky has already opined, Everything has an ending, only a sausage has two. It may simply be you are only going to get about ten years out of a coating job. With answers to some of these questions, an unbiased opinion might be offered. Certainly not an engineering determination, but free advice.

I'm not sure if he's still listening but a bit more detail would help, Ky.

We had constructed a Above Ground Water Storage Tank, a year ago, and we recently opened it, we too found that there were bubbles in the inside of the tank. The density of the bubbles were around 30 to 40 per m2 and size approx 10 to 15mm. When we wiped out the shell surface, it just appeared to be clean and something like a bubble. But however when we tried to press the bubble there was water inside.

We tried to narrow down the causes for this, but could not conclude anything. We had preblasted the tank before fabrication, spot blasted after fabrication, taken precaution to give enough curing time between coats, but still it has happened.?

Have you experienced similar situations and what were your findings. Pls share.