Improved corrosion resistance of Ni-modified Fe-Cr-B steel in molten zinc via phase transformation and microstructure control

Abstract This paper presents the microstructure evolution and corrosion resistance of Fe-18wt.%Cr-1wt.%Mo-3.5wt.%B alloy against molten zinc at 520 °C. The objective of this study is to obtain Fe B alloy with superior corrosion resistance through microstructure regulation and to reveal the underlying corrosion mechanism. The results show that the metal matrix of as-cast Fe-Cr-B alloys have undergone a transition from α-(Fe, Cr, Ni) solid solution into γ-(Fe, Cr, Ni) solid solution, while the eutectic boride is further refined with the increasing of Ni addition in the steels. The corrosion resistance of the Fe-Cr-B alloys has significantly been enhanced, with an optimal Ni addition at 10 wt%. Thermodynamic calculations and EPMA results show that Ni mainly exists in the matrix, leading to the transformation of the matrix structure. The improvement of corrosion performance can be attributed to two factors: the refined boride network in Ni-modified Fe-Cr-B alloy act as a barrier for Fe/Zn reaction, and the formation of face-centered cubic (fcc) structure also contributed to improve corrosion resistance owing to a more densely packed structure compared to body-centered cubic (bcc) structure. In the initial stage of corrosion, the de-alloying of Ni occurs first, which result in the transformation from γ-(Fe, Cr, Ni) to α-(Fe, Cr, Ni). Corrosion mechanism for test alloys in molten zinc may be mainly dominated by the formation and propagation of cracks in the borides. The cracks initiated due to the discrepancy in thermal expansion coefficients between Fe Zn phases and the borides.