What is the Problem in the Heating Tank?

If you get the bottom hot enough to deform under the weight of the oil in the tank, you should be able to get it hot enough to push it flat again.

Then reinforce the bottom of the tank by putting vertical members inside the tank going from the inside of the bottom to the top of tank and tie those members to the sides of the tank, this will do two things:

First, it stops the bottom from deforming and secondly, the reinforcing vertical member [stiffeners] will act as conduits to transfer the heat thru the tank more uniformly so you may have not have to apply as much heat going forward.

"Most austenitic steels, with chromium contents of at least 18%, can be used at temperatures up to 870°C and Grades 309, 310 and 2111HTR (UNS S30815) even higher. Most martensitic and ferritic steels have lower resistance to oxidation and hence lower useful operating temperatures. An exception to this is the ferritic grade 446 - this has approximately 24% chromium, and can be used to resist scaling at temperatures up to 1100°C."

I would replace tank with stainless or any other high chromium steel that was available...

http://www.azom.com/article.aspx?ArticleID=1175

Have you considered, or do you use ,centrifugal cleaning of your waste oil? What are your throughput figures?

https://usfiltermaxx.com/en/content/9-make-black-diesel

oil is acidic, and waste oil is worse. Bringing it to a higher temperature, it becomes more active.

We established that, I'm rather surprise that on after 30 days that you had over 30% corrosion.

You will have to look at replacing it with a more resistant material.

The other issues, is this heat concentrated, you may also want reconsider the design of the heat exchange area. Because Stainless will also warp.

And not having a sketch of what you have as a reference makes it more difficult.

What would adding a circulator to the tank while heating do for your problem?

No qualified Chemical Engineer would design such a thing.

• Let's see.....11.48m³ of oil at 500 degC in an open tank that is wearing thin and buckling...what are the risks of such a thing.....hmmmm....

Process intensification would yield dividends. A smaller, properly-designed heat exchanger would give rise to a system that can be turned on and off easily, producing the same yield with much lower inventory and corresponding risk.

What do the premises' fire insurance company have to say about this installation, please?