Preferential corrosion of a weldment and/or a heat-affected zone is a widely known phenomenon. High-temperature exposure and welding can significantly affect the microstructure and properties of the weldment and/or heat-affected zone that makes them more susceptible to corrosion. The microstructural changes that occur in weldments or heat-affected zones include carbide precipitation and intermetallic phase formation. Rough welds, especially those associated with mill-scale and slag deposits, may suffer accelerated corrosion. These changes can be minimized by following proper welding procedures and using low carbon versions of stainless steel alloys like 316L. You also mentioned a case of preferential corrosion above the liquid line. Corrosion depends on many factors including the specific nature of the environment so corrosivity can be different for areas in the liquid space, at the interface, and in the vapor space. Sulfuric acid is a tricky substance to deal with as its corrosive properties change dramatically with velocity, concentration, impurities, and temperature. So you may find it beneficial to consult some specific references on the design and construction of sulfuric acid storage tanks and distribution systems. A good place to start is the NACE standard SP0294 "Design, Fabrication, and Inspection of Storage Tank Systems for Concentrated Fresh and Process Sulfuric Acid and Oleum at Ambient Temperatures."
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