Corrosion by Mixed Acids and Salts

Mixtures of acids or acids and salts are of great importance to the chemical process industry (CPI) for use in digestion of solids, as a promoter in reactions, as a scale remover, and as a complexant. This article emphasizes the assessment of the performance of Ni-Fe-Cr-Mo alloys in mixed acids and salts in an objective manner. It tabulates the nominal compositions of pertinent Ni-Fe-Cr-Mo corrosion-resistant alloys. The article describes the acid and acid-plus-salt mixtures classified into the following general categories: nonoxidizing acid mixtures (H₂SO₄+H₃PO₄), nonoxidizing acids with halides (H₂SO₄+HCl), oxidizing acid mixtures without halides (H₂SO₄+HNO₃), and oxidizing acid mixtures with halides (HNO₃+HF). It also illustrates the effect of alloying elements on the corrosion rate in the nonoxidizing mixtures and oxidizing acid mixtures.