Many manufacturers offer epoxy coating inside the tank as an option.
This is true. However, it is unlikely you will see this in any residential or even small industrial receivers. It's an expensive process. Half of the air receivers I inspected at our plant this year were coated and the others not. the coated ones look better, but coatings that are applied like these have to be are prone to failure. Takes without manways are even more difficult to apply to correctly.
Horizontal vessels are inherently more difficult to drain properly. They have to be leveled in a way that allows the low point to be at the drain, and this is often not the case.
There is a lot of speculation, which is about all that can be done. I didn't see much detail in pictures in needed areas. Generally you need a pretty significant amount of corrosion to thin a vessel enough to fail. You can get pin holes/spalling which can affect it. But it's hard to believe that one with little corrosion would zipper.
Over pressurized? Could be. The failure area would have looked this way at just about any air compressor pressure. I struggle to think that the relief valve being small would have been the issue. Usually Mother Nature helps protect you in these cases. The pump keeps running, and the relief keeps it running longer. The longer it runs, the hotter the pump gets and it loses efficiency....which means it can't pump to as high of a pressure. With the fairly large pump on a fairly small vessel, it may have been able to do it, and the extra heat in the compressed air could further weaken the vessel. Maybe. But again this is speculation.
I don't have anything that I can see that supports this, but I would guess manufacturing flaw. The heat affected zone was mentioned earlier. Generally a vessel that is in good condition will fail around a weld if not at the weld. Like getting hit by lightning, your chances of seeing this failure are slim and I would certainly not stay up nights worrying about it. Thank God the damage was limited to things that are not people.
I am a degreed Mechanical Engineer with 20+ years of experience and am working as a Mechanical Integrity engineer for a large company....arranging and evaluating piping and pressure vessel inspections. You don't see pressure vessels come apart maybe ever in your career. That's what you look to avoid. Of course our pressure vessels are several stories high and much higher pressure.....with hydrogen!