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Corrosion behavior and passive film properties of nickel-based alloy (Inconel 625) in phosphoric acid were investigated, by means of a variety of electrochemical methods combined with surface analysis techniques. Results showed that Inconel 625 exhibited strong passive behavior in phosphoric acid but their corrosion resistance was deteriorated with increasing concentration of phosphoric acid. The increasing concentration of phosphoric acid also accelerated the corrosion rate. The passive film of Inconel 625 was composed of an inner layer rich in Cr2O3 and an outer layer rich in FeOOH. The initial corrosion of Inconel 625 in phosphoric acid originated from the complex inclusions composed of N, Nb and Ti with a corn rich in Mg and O. At concentrations lower than 1 mol/L, inner layer of the passive film remained intact and only the outer layer was damaged. At concentrations of phosphoric acid higher than 1 mol/L, both the inner and outer layers were destroyed. The insoluble corrosion products Fe phosphate compounds precipitated on the surface of Inconel 625, which accumulated with increasing concentrations of phosphoric acid and became a loose layer covering the surface of Inconel 625 at concentrations of phosphoric acid higher than 1 mol/L.
Zhang et al. (Tue,) studied this question.