Pack-Boriding of Fe-20Cr-5Al Alloy: Nanostructured Boride Layer Formation, Mechanical Performance, and Paradoxical
Cengiz Temiz1, Uğur Öztürk2, Seyit Çağlar3
1Department of Electronics and Automation, Alaplı Vocational School, Zonguldak Bülent Ecevit University, Zonguldak 67850, Türkiye.
Abstract:
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.
