It sounds like the damage is done.

Sensitization is the result of the base metal being subjected to the sensitizing temperature range during welding. The carbon combines with the chrome along the grain boundaries thus depleting the HAZ of free chrome that is responsible for corrosion resistance.

You didn't state the tank's use, but SCR is usually associated with an electrolyte containing chlorides. SCR (stress corrosion cracking) is often referred to as IGSCC as well. That is likely to be the source of cracking if it was confined to the heat affected zones (HAZ) along the toes of the welds. You can also get knife line cracking where two welds cross each other. This occurs if the primary weld didn't maintain the high temperatures required to cause sensitization. The combination of the primary weld and the intersecting weld create a situation where the conditions of time and temperature are satisfied and result in localized sensitization only at the intersection where the second weld crosses over the primary weld.

The preventive measure is to use a low carbon grade of stainless, i.e., 304L, with a low carbon stainless filler metal, i.e., 308L. Another measure is to use stabilized grades of stainless, i.e. 321 or 348. These measures must be taken at the design stage of the construction.

Good Luck - Al

Once sensitization has occurred, the only corrective measure I know of is to heat the entire weldment high enough to cause the chromium carbides to dissolve, quench the weldment to prevent the reformation of the chromium carbides. It is unlikely this is a reasonable solution to your problem.