Soil-side Corrosion on Tank Bottom

[clkimbal]

Hi,

We have a tank that has experienced a problem with soil-side corrosion and we believe it is caused by water seeping under the tank bottom edge from rain, snow and or sweating. The dimensions of the tank are 16' in diameter and 14' in height. The concrete foundation is a solid slab and the tank bottom is flat with a 3/4" crown. The tank is constructed of A36 steel and needs to be compliant to the API 650 design standards.It is built to store 50% caustic soda with a temperature range of 100-160 degrees Fahrenheit. Below are my ideas to remedy the soil-side corrosion issues that will be incorporated into the design and or installation of the new tank, please let me know what your thoughts are:

1. Apply a cold tar epoxy to the tank bottom for sealant between the concrete slab and the bottom of the tank?
2. Place an asphalt impregnated fiber board around the outer rim of the tank to elevate it off of the ground from moisture?
3. Elevate the tank by constructing an epoxy coated steel grid that the tank will sit on?
4. Have the tank designed and constructed with a double tank bottom as per API 650?

Please let me know what best practice is in this scenario and what is most cost effective?

Thanks in Advance.

 

[bimr]

We usually slope the tank bottom away from the center. With the temperature that you have, with the bottom sloped, the tank bottom should stay dry and not corrode.

 

[clkimbal]

Yes, we already have a slope on the tank bottom from the center with a drop of 1" every 10' outward. We still have issues of water seeping under the tank from the outside in. This is where I am thinking I need to either elevate the tank or apply a sealant to avoid water contact with the bottom of the tank? It also doesn't help that the concrete pad has a flat surface and does not have a sloped edge for water to runoff.

 

[Duwe6]

"It also doesn't help that the concrete pad has a flat surface and does not have a sloped edge . . . "

Got the exact same problem, about 290 times. Until the concrete is sealed and sloped away from your tank, you will have aggressive corrosion of the underside. Sealing only 'sorta' works. The problem is that the tank is quite a bit larger full & hot than empty & cold. In a few cycles, the caulk is ruined. But it might work if your tank stays hot and doesn't get below 1/4-full or so.

What might help a bunch is to cut a small channel in the concrete all around the tank chime -- think 'moat' about 2" wide and 1/2" deep, sloped away from the tank. With the temperature of your tank, it will dry out the concrete IF most of the rainwater is drained away from the chime area.

 

[bimr]

Some concrete pads also have drainage channels that radiate out from the center of the pad.

API 650 calls out for a drip ring around the base.

Would think you would solve the problem with sloped foundation, drainage channels, coal tar between steel and concrete, and drip ring.

 

[Duwe6]

Minor quibble with bimr's strategy: if a tank sets on a ringwall or pedestal that is only inches larger thab the chime, just sealing the concrete and the tank bottom seems to be satisfactory. Drip-ring is probably unnecessary. I've had a few tanks set up like that; they were years old & kept at 'perfect' CUI temperatures. No metal loss when I gave them API Internal Inspx's.

If I installed a drip-ring, I'd seal-weld it to the chime; otherwise it will wick in & trap water; think flashing on a foof.

 

[chicopee]

I would look at grinding the concrete surface around the circumference of the tank to shed water away from it and then apply a continuous bead of mastic adhesive to seal up the voids between the tank and concrete pad.