PSI - Issue 71

G. Narasinga Rao et al. / Procedia Structural Integrity 71 (2025) 317–324

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conditions were subsequently utilized to calculate the Corrosion Rate (CR in mmpy), as summarized in Table 3. In Fig. 6, the PDP curves of both the as-received and heat-treated samples of 17-4 PH SS exhibited similar anodic and passive regions from approximately -0.18 to 0.3 V and from ~ 0.3 V to 0.7 V. The as-received (AR) sample, which did not undergo any additional heat treatment, exhibited the most negative corrosion potential (E corr ) at -0.331 V, indicating the highest tendency towards corrosion. Additionally, the AR sample has the highest corrosion current (i corr ) of 3.334 × 10 − 7 A/cm 2 , which directly correlates with the higher kinetics of electrochemical reactions/corrosion rate of 3.56 x 10 -3 millimeters per year (mmpy). As-received 17-4 PH stainless steel exhibited a higher corrosion rate and lower corrosion resistance in a seawater environment. Except for ST+Aging for 32 h, the rest of the heat treated 17-4 PH stainless steel samples had drastically reduced the corrosion rate to 33 – 53 % of that of as-received sample condition, demonstrating the significant effect of heat treatment. Among the heat-treated conditions, the PDP curve of the ST+A ging 4 h sample exhibited a significant shift towards left-upward (E corr at -0.179 V and i corr at 1.131 × 10 − 7 A/cm 2 ) direction, as shown in Fig.6, which is a testament to the drastic reduction in the corrosion rate of 1.19 x 10 -3 mmpy. As per Table 3, the polarization resistance (R P ) value for ST+Aging 4h was highest representing better corrosion resistance. The results obtained from PDP testing were in agreement with the OCP results, which was due to the coarsening of the Cu-rich precipitates and formation of Cr 23 C 6 precipitates in the material matrix during the peak aging of 17-4PH SS (Bühler, Gerlach et al. 2003).

Table 3: Potentiodynamic polarisation test data of as-received and heat-treated 17-4PH

Sample

OCP (V)

E corr (V)

i corr (x 10 -7 A/cm 2 )

RP (x 10 3 Ω.cm 2 )

Corrosion rate (x 10 -3 mmpy)

AR 3.334 ± 0.1667 112.3 ± 0.561 3.56 ± 0.17 ST+Aging 1h -0.1057 ± 0.0063 -0.273 ± 0.0191 1.726 ± 0.1035 149.9 ± 0.794 1.83 ± 0.19 ST+Aging 4h -0.0428 ± 0.0017 -0.179 ± 0.0071 1.131 ± 0.0452 217.4 ± 0.538 1.19 ± 0.47 ST+Aging 8h -0.0980 ± 0.0068 -0.196 ± 0.0137 1.273 ± 0.0891 173.1 ± 0.768 1.35 ± 0.94 ST+Aging 32h -0.1253 ± 0.0082 -0.233 ± 0.01165 3.649 ± 0.1824 108.6 ± 0.678 3.87 ± 0.34 -0.1959 ± 0.0097 -0.331 ± 0.0165

4. Conclusions ● Heat treatment significantly effects the microhardness, microstructure, and corrosion behaviour of 17-4 PH stainless steel. Solution treatment followed by aging at 480 °C for 1 h increased the microhardness from 364 HV to 421 HV owing to the precipitation of copper phases. ● Extending the aging time beyond 1 h does not significantly improve the hardness, with values remaining at 415 HV for 4 and 8 h of aging. Prolonged aging (32 h) decreased the microhardness to 392 HV owing to the coarsening of Cu-rich precipitates due to overaging. ● XRD analysis revealed a predominantly martensitic phase in all samples, with the emergence of NbCr 2 and Cr 23 C 6 peaks after aging. Optical microscopy revealed the evolution of the microstructure from predominantly martensitic with some δ -ferrite in the as-received condition to a completely martensitic microstructure after aging. ● The corrosion resistance improved with heat treatment, and the sample aged for 4 h exhibited the best corrosion performance (lowest corrosion rate of 1.19 x 10 -3 mmpy and highest polarization resistance of 217.4 x 10 3 Ω ⋅ cm 2 ). Prolonged aging (32 h) reverses the beneficial effects on corrosion resistance, likely due to the coarsening of Cu rich precipitates and the formation of Cr 23 C 6 precipitates. ● The optimum heat treatment for 17-4 PH stainless steel appears to be solution treatment followed by aging at 480 °C for 4h, balancing improved hardness and corrosion resistance. References Abbaschian, R. and R. E. Reed-Hill .2008. Physical Metallurgy Principles, Cengage Learning. Bühler, H. E., L. Gerlach, O. Greven and W. Bleck .2003. The electrochemical reactivation test (ERT) to detect the susceptibility to intergranular corrosion. Corrosion Science 45: 2325-2336. Chaudhary, A., S. K. Baral, G. Tiwari, R. S. Buradagunta and R. Dumpala .2023. Influence of Surface Finish on the Dry Sliding Wear and Electrochemical Corrosion Behaviour of 316L Stainless Steel. Journal of Mines, Metals and Fuels: 1293-1301. Chung, C.-Y. and Y.-C. Tzeng.2019. Effects of aging treatment on the precipitation behavior of ε -Cu phase and mechanical properties of metal injection molding 17-4PH stainless steel. Materials Letters 237: 228-231. Eskandari, H., H. R. Lashgari, L. Ye, M. Eizadjou and H. Wang .2022. Microstructural characterization and mechanical properties of additively manufactured 17 – 4PH stainless steel. Materials Today Communications 30: 103075.