Abstract
Corrosion in process industries often occurs in mixtures of various acids and salts. Such corrosion behavior cannot be predicted simply by extrapolation from the behavior in pure acids. For example, corrosion behavior in wet process phosphoric acid is significantly different from that in pure orthophosphoric acid due to a variety of impurities in the former. Corrosion behavior in sulfuric and hydrofluoric acid mixtures is quite different from that in either of the pure acids. Acid mixtures can be broadly classified as: (1) mixtures of two non-oxidizing strong acids; (2) mixtures with one strong acid containing an aggressive species, such as a halide ion; (3) mixtures in which one of the acids or salts has an inhibitive species, such as a nitrate ion; (4) mixtures in which one component generates a high redox potential; and (5) mixtures of relatively weak organic acids and aggressive salts. Extensive experimental data have been generated on corrosion behavior of Ni-base alloys and stainless steels in various acid mixtures. Additionally, a generalized corrosion model has been constructed as a function of environment speciation and alloying elements. This model combines a thermodynamic framework for predicting speciation in corrosive environments with an electrochemical framework for simulating the anodic and cathodic processes that lead to alloy dissolution. In particular, the electrochemical framework represents general corrosion in the active and passive states including the active-passive transition and effects of both aggressive and inhibitive ions on passive dissolution. The paper presents the available experimental data and provides a modeling framework to predict the observations.
