Acidic chemistries
Acid attacks metal directly by donating protons that react with the metal surface. The lower the pH, the more aggressive the reaction. Concentrated PAA sits well below pH 1; at use it's still acidic. Acidified sodium chlorite, organic acid blends, and any sulfuric-acid-buffered system live in the same acidic range. That low pH is what wears stainless steel around dip tanks and spray cabinets, eats fasteners and brackets faster than spec, and feeds the cracks in the concrete underneath.