WOW! What replies by you fellow brewers that posted above, I can't believe all that was posted by them. First off those tanks are worth gold to someone needing that large of capacity, I would grab them in a heart beat as there is money to be made. Scrapping them your the one who will lose big dollars and a big stupid mistake, don't scrap them.
I say this from having a friend that has a multi million dollar a year tank wash business the past 27 years cleaning out big rig tanks. This from small 500 gallon to 6,700 gallon single tanks to twin 3,000 doubles.
A tractor can come in after hauling epoxy resin products, paint products, sulfur, fertilizers, pesticides or any other product that would kill you. The tanks are cleaned out with a crew that goes inside with a air life support hood then begin cleaning the tanks, then chemically clean and wash outs. Then the next load can be food grade FDA aproved to haul, kind of scary isn't it and legal? This happens thousands of times a day across thje country not counting zero washouts who would know?
At that same cleaning facility another friend parks his tank truck he only hauls vinegar almost daily hence no clean outs after each load. The only problem he has is a few deliveries he first must have a Rabi (? Quaker looking dude) look at the tank then charge $960 and after 5 months get a sticker stating he can haul their "Kosher Certified Vinegar". One food processing plant that "Demands Kosher Tankers Only" ran out of vinegar on a 24/7 shift. They had locals in pickups hauling vingar from rusty 55 gallon drums to the plant a Stanford & Son's operation. Those drums had rust and sludge in them but "that's ok we have filtering systems at the plant". Bottom line even a HAZ-MAT tanker can be cleaned out and the next load food grade certified rated. Those are the tankers with the stainless boxes across the top openings you spot them a mile away on the highway.
If you going this big into brewing I would have those tanks cleaned with the PROPER CLEANING AGENTS and rinsed then your good to go as a food grade or brewing use with a certificate stating so if you have a cleaning facility clean them, this is not cheap as replacing them isn't either. Even etched stainless tanks have passed for food grade as vinegar will etch 304 stainless. I'm talking 200 and 300 grain vinegar that will burn the skin. Sorry fellow above reply brewers unless you have been around the tank cleaning business like I have you have given the OP the wrong information and I find this a big dollar mistake that he could make on bad posted information. This made me sad reading these replies. Just think about the next tanker on the highway and what's in that tank and what was in it three loads ago?
Done ranting, sorry not to offend any Kosher related people just the truth on what goes on daily in the tanker business.