Corrosion Resistance of Injection-Molded 17-4PH Steel in Sodium Chloride Solution

The corrosion resistance of 17-4PH powder injection-molded (PIM) martensitic stainless steel (SS) was evaluated in naturally aerated sodium chloride (NaCl) solution (3 wt%) at 25°C. This resistance was investigated by analyzing the curves of the evolution of open-circuit potential with time (E_ocp vs. time), electrochemical impedance spectroscopy (EIS), and surface observation by scanning electron microscopy (SEM) at increasing times of immersion. The susceptibility to pitting was investigated using cyclic potentiodynamic polarization curves and SEM observation after polarization. Additionally, the susceptibility to intergranular corrosion was evaluated by a modified procedure described in ASTM A 262 Practice A. The results of the 17-4PH PIM steel were compared to that of a 17-4PH steel produced by conventional metallurgy. The results showed that under steady-state conditions the PIM steel presented a behavior typical of passive metals during the whole test period (60 days of immersion). This was indicated by the E_ocp vs. time curves, EIS results, and SEM observation of the surface at increasing periods of immersion. The steel showed a bright metallic surface and no signs of corrosion during the whole period of the test. Cyclic potentiodynamic polarization curves indicated that both 17-4PH steels, conventional and PIM, are susceptible to pitting but the PIM steel showed a slightly increased susceptibility to pitting, which was supported by posttest observation by SEM. In the PIM steel, pits seemed to be related to the porosity that had inner oxide inclusions. The 17-4PH steel, produced by both PIM technology and conventional metallurgy, did not show a susceptibility to intergranular corrosion by ASTM A 262 Practice A.