Repairing Sulphuric-Acid Tanks

I don't know the exact method but, I Would safely remove acid from it clean it and repair it.

1. SEEK PROFESSIONAL GUIDANCE IN PERSON
1. (for general guidance see below)
1. Assuming that your tank is unlined mild steel - see this site NOTE: Make sure as much acid is out of the tank as possible BEFORE you add water. Adding water to acid is not a good idea -
1. It generates heat and the water can boil
2. Puts the concentrated acid into a highly corrosive state
1. Explosive hydrogen is formed when mild steel corrodes


2. Get a hold of qualified NDE inspectors
   
3. Inspect tank as per API 653 - NOTE: special corrosion degradation method called "Hydrogen Grooving"
4. Get a hold of qualified tradesmen
   
5. Repair tank as per API 653

Also - look into getting NACE SP0294-2006:

Design, Fabrication, and Inspection of Tanks for the Storage of Concentrated Sulfuric Acid and Oleum at Ambient Temperatures

http://web.nace.org/Departments/Store/product.aspx?ID=09a920f0-f451-4bd6-bd18-c84cc2caffcc

What are the repairs required?

Inspection and thickness survey is essential to identify thinned areas. In addition to hydrogen grooving look out for horizontal grooving on the walls if rainwater has leaked in and diluted the acid at the wind/water line. If acid leakage has occurred through the base external corrosion will be severe. Do not apply lap patches - cut out and butt weld new material after a detailed thickness scan. Look out for hydrogen blistering of the steel. Make sure the acid inlet is far enough from the wall and/or has a dip pipe to near the base. Fit a sacrificial pad beneath the acid dip pipe. If the inlet is close to the wall and is too short or has broken off check for acid washing and thinning of the wall. Over 25-30 years I have seen all the above (and more).

Why bother - a carbon steel tank will dissolve - do you want to repair a lining?

Unlined carbon steel is the standard material for sulphuric above 90% strength. It is protected by the layer of ferrous sulphate formed on the surface, most of the corrosion mechanisms listed before result from factors which disrupt this passive layer.

You find out something new everyday! Surely then weld it! How did it breakdown in the first place - attack from outside? Needs a good chlor-rubber or vinyl coating!