Electrochemical Characterization of 17-4 PH Alloys Sintered in Dilute Sulfuric Acid Solution: Role of Titanium Amount and Aging Time

This study examined the impact of the aging time and Ti amount on the corrosion behaviors of powder metallurgy-produced 17-4 PH SS alloys. The powders used as mixtures of pure elements were prepared with addition of four different amounts of Ti (0.5, 1, 1.5, and 2%). The prepared powders were mixed in a turbula-type mixer for 2 h, and then compacted under 800 MPa pressure to produce raw samples of ∅12 × 6 mm dimensions. The generated green compacts were sintered in a vacuum atmosphere with 10^‒6 mbar for 1 h at 1300°C. After being sintered, the samples were taken to a solid solution treatment at 1040°C and then artificially aged for 1, 4, and 8 h at 480°C. After aging, the samples were electrolytically etched in a 25% oxalic acid solution following standard metallographic procedures. The prepared samples were characterized by hardness and density measurements. Microstructure studies were conducted using scanning electron microscopy. Corrosion tests were performed using the potentiodynamic polarization technique on an Iviumstat.XRe potentiostat/galvanostat test instrument. In the corrosion tests, those were performed in a corrosion cell with three electrodes. These three electrodes are the Ag/AgCl reference electrode, a platinum plate, and a counter electrode. The corrosion tests were conducted in 0.1 M H₂SO₄ solution for the produced 17-4 PH stainless steels. The results have indicated that increasing the Ti content in the alloy increases its hardness, but decreases its density. Increasing aging time resulted in an increase in density. The highest hardness was observed in the alloy aged for 4 h among all Ti contents. But as aging time increases more, hardness decreases. According to the corrosion results, the rate of corrosion in the stainless-steel alloy was reduced as the Ti amount increased. The alloy with 2% Ti addition when being non-aged had the lowest corrosion rate, measuring 0.609 mm/year. It was shown that the corrosion rate increased as the aging time increased. For the non-aged alloys, intergranular corrosion mechanism was observed, whereas with increasing aging time, pitting corrosion mechanism was observed.