Effect of Molybdenum on the Microstructures of As-Cast Fe-B Alloys and Their Corrosion Resistance in Molten Zinc

Abstract

The effect of Mo on the microstructure of as-cast Fe-3.5 B alloys and their corrosion behavior in molten zinc have been investigated. Experimental results show that the as-cast Fe-B alloys with molybdenum addition are mainly composed of α-Fe, Fe2B, FeMo2B2, and metastable Fe3B phases. Corrosion tests show that the Fe-3.5 B alloy with 8.0 wt% added molybdenum has the highest corrosion resistance in molten zinc mainly because the alloy still maintains the reticular structure of boride and improves its thermal stability. When the molybdenum content exceeds 8.0 wt%, the τ-FeMo2B2 + α-Fe eutectic microstructure destroys the reticular structure of the Fe2B phase, leading to reduction in the corrosion resistance of the as-cast Fe-B alloys. Four kinds of corrosion products (δp, δk, ζ, and FeB) were found in the corrosion layers. The corrosion mechanism of Fe-3.5 B alloys with various added molybdenum contents includes the following processes: the preferential corrosion of α-(Fe, Mo), the formation of typical Fe-Zn compounds, the transformation of (Fe, Mo)3B and (Fe, Mo)2B into FeB, and the spalling of borides. The diffusion of molybdenum in the solid matrix cannot occur in the corrosion process. The corrosion depth of the corrosion layer did not follow a parabolic relationship strictly, maybe it caused by the spalling of the corrosion layer under the attack of the liquid zinc. The corrosion process is mainly controlled by the diffusion of liquid zinc atoms.