Pack-Boriding of Fe-20Cr-5Al Alloy: Nanostructured Boride Layer Formation, Mechanical Performance, and Paradoxical Passivation Loss via Micro-Galvanic Interactions

This study investigates the microstructural evolution, mechanical performance, and electrochemical corrosion behavior of nanocrystalline boride layers formed on an Fe-20Cr-5Al ferritic alloy by pack boriding at 950 °C for 4 h. X-ray diffraction (XRD) and scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDS) analyses confirmed the formation of a hierarchical boride layer approximately 80–85 μm in thickness, consisting of orthorhombic (Fe,Cr)B and tetragonal (Fe,Cr)2B phases at the surface and (Fe,Cr)23(C,B)6 carboboride phases in the diffusion zone, the latter attributed to the carbon push-ahead mechanism. Rietveld refinement yielded a quantitative phase fraction of 51.9 wt.%. (Fe,Cr)B, 46.1 wt.% Fe2B, and 2.0 wt.% (Fe,Cr)23(C,B)6, with a theoretical boride layer density of 7.40 g cm−3. Williamson–Hall analysis yielded an average crystallite size of 50.7 nm and a microstrain of 1.686 × 10−3, confirming the nanocrystalline character of the boride phases. Mechanical evaluation revealed a ~9-fold increase in surface hardness in Fe20Cr5Al-B relative to Fe20Cr5Al, reaching 1854 HV (18.18 GPa). Tribological testing demonstrated an ~18-fold reduction in wear rate (from 3.29 × 10−4 to 1.82 × 10−5 mm3/m) and a 14.5% reduction in the coefficient of friction (0.76→0.65), confirming the effectiveness of the boride layer as a tribological barrier. However, electrochemical analyses in 5 wt.% H2SO4 revealed a paradoxical deterioration in corrosion resistance: despite a noble shift in Ecorr from −0.459 to −0.295 V, the corrosion rate increased ~4-fold (from 9.67 × 10−3 to 3.83 × 10−2 mm/year), driven by Al-repulsion-induced passive film loss and micro-galvanic cell formation through micro-crack and porosity networks. These findings emphasize that while pack-boriding is highly effective for tribological enhancement of FeCrAl alloys, minimizing boride layer defects is essential to achieve concurrent corrosion protection in acidic environments.