Clarifying the Relationship Between Chemical States of P in Fe–P Alloys and Pitting Corrosion Resistance

Fe–0.002 P, Fe–0.05 P, Fe–0.2 P, and Fe–2 P alloys (numbers indicate the mass%) were fabricated, and their pitting potentials, depassivation pH values, and active dissolution rates were measured. The order of pitting potentials was (high) Fe–0.002 P ≥ Fe–0.05 P ≥ Fe–0.2 P ≫ Fe–2 P (low), and that of depassivation pH values was (low) Fe–0.002 P ≤ Fe–0.05 P ≤ Fe–0.2 P ≪ Fe–2 P (high). Both parameters changed significantly between the Fe–0.2 P and Fe–2 P alloys. No evidence of grain boundary segregation of P was observed in the Fe–0.05 P alloy. In the Fe–0.2 P alloy, grain boundary segregation of P was observed, but no pitting occurred at the grain boundaries. In the Fe–2 P alloy, Fe 3 P precipitated at the grain boundaries and in grains, but pitting corrosion occurred in the alloy matrix and not in Fe 3 P. This indicated that P in the solid solution was the main cause of the decrease in pitting corrosion resistance. The P concentration in the surface oxide film on Fe–2 P was higher than that on Fe–0.2 P, and the P in the films was determined to be FePO 4 . The decrease in the pitting resistance with an increasing P concentration was due to FePO 4 .