Effect of Boriding Temperature on the Microstructure, Room- and High-Temperature Wear, and Corrosion Behavior of Pack-Borided Compacted Graphite Iron

Compacted graphite iron (CGI) is widely used in thermomechanically demanding engineering applications owing to its high thermal conductivity, vibration-damping capacity, and machinability. However, its surface performance can be limited under the simultaneous effects of elevated temperature, sliding contact, and corrosive environments. In this study, CGI surfaces were subjected to pack boriding using a silica-free B4C–NaBF4 powder mixture at 800, 900, and 1000 °C for 4 h, and the microstructural characteristics of the resulting boride layers, together with their wear behavior at room temperature and 500 °C and their electrochemical corrosion resistance, were systematically investigated. Cross-sectional SEM/EDS analyses revealed the formation of boride layers exhibiting a saw-tooth interface morphology, with thickness increasing from 44 ± 4 µm to 108 ± 9 µm with increasing boriding temperature. XRD results indicated the formation of a dual-phase FeB/Fe2B structure in all borided specimens, with the Fe2B phase dominant at 800 °C and the FeB phase becoming dominant at 1000 °C. Boriding increased the surface hardness from approximately 470–480 HV to a range of 2122–2550 HV. In room-temperature wear tests, specimens B1 and B2 exhibited specific wear rates approximately 13-fold lower than that of the untreated CGI, whereas B3 showed a higher wear loss attributable to the brittle character of the FeB phase. At 500 °C, B1 maintained the most balanced tribological performance, with the lowest volumetric wear loss and coefficient of friction, while microcracking and three-body abrasion effects became more pronounced in B2 and B3. In electrochemical tests conducted in 3.5 wt.% NaCl solution, B1 exhibited more stable open-circuit potential behavior, whereas B3 showed the highest resistance in terms of Tafel kinetics, with the lowest corrosion current density and corrosion rate. Overall, the results demonstrate that the Fe2B-dominant boride layer obtained at 800 °C provides the most balanced performance among hardness, wear, and corrosion behavior, whereas the thick, FeB-dominant layer formed at 1000 °C, despite offering high hardness and favorable corrosion kinetics, may compromise tribological reliability owing to its brittleness, increased surface roughness, and tendency toward microcracking.