Tailoring the microstructure of Ni-Hard 4 cast iron for corrosion resistance in acidic and alkaline environments through tempering treatment

This study evaluates the corrosion behavior of Ni-Hard 4 cast iron subjected to different heat treatments, focusing on its performance in both acidic (1 M HCl) and alkaline (1 M NaOH) environments. The microstructure and phase detection of the samples were done through scanning electron microscopy (SEM) and X-ray diffraction test. The corrosion properties were studied using electrochemical impedance spectroscopy and potentiodynamic polarization after 1 h and 24 h of immersion. Microstructural characterization revealed that tempering transformed the as-received austenitic structure into martensitic matrices with distinct carbide distributions: the 400 °C treatment produced stable Cr-rich carbides, while the 500 °C treatment resulted in coarser carbides with reduced micro-galvanic coupling. In HCl, the 400 °C tempered sample exhibited superior resistance, showing the lowest corrosion current density (1.5 mA/cm 2 ) and weight loss, with SEM confirming minimal uniform attack due to its protective microstructure. In NaOH, the 500 °C tempered sample demonstrated optimal long-term performance, maintaining high polarization resistance and minimal weight loss, while SEM revealed a stable passive film formed on its homogenized structure. The as-received sample’s initial advantage in NaOH degraded over time due to the instability of its austenite-derived passive layer. The results establish that 400 °C and 500 °C tempering are optimal for HCl and NaOH environments, respectively, due to micro-structurally-controlled corrosion mechanisms.