Twenty percent sulfuric acid is a reducing acid (in the absence of oxidizing contaminants) and attacks base metals evolving hydrogen. Contaminants can change the reducing nature of 20% sulfuric acid and greatly affect its corrosion characteristics as to whether attack will be general or localized.

Localized forms of corrosion are ion-specific. Aluminum, iron, carbon steel, and 316L stainless steel are all generally unsuitable for 20% sulfuric acid. The "classic" austenitic alloy for 20% sulfuric acid is Alloy 20 which is 20% Cr, 29% Ni, 3% Cu, 2% Mo and the balance Fe. In more recent times, Alloy 20 has been replaced by Alloy 20Cb-3 which is 20% Cr, 35% Ni, 3% Cu, 2% Mo with the balance Fe resulting in better resistance.

The corrosion characteristics of sulfuric acid is a complex subject and further reading is recommended. One good reference is the book published by MTI entitled "Materials Selector for Hazardous Chemicals-Volume 1: Sulfuric Acid."