Investigation On Phase Equilibria Of The Zn-Co-Ni Ternary System And The Effect Of Co And Ni In Zinc Bath On The Galvanized Coating

Hot-dip galvanizing is one of the most effective methods of protecting steel fromcorrosion. Over the years, although advances in galvanizing technology have beenachieved, galvanizing Si-containing steels is still a challenge for galvanizers. Theeffect of silicon, which results in bad coating quality, can be controlled by bathadditions. In the present work, the effect of bath additions Co and Ni on Si-containingsteels galvanized coating and the phase relationships of the Zn-Co-Ni ternary systemhave been investigated.The information of the phase relationships of the Zn-Co-Ni-Fe and Zn-Co-Ni-Siquaternary systems at the galvanizing temperature is important to understand thesynergistic effect of Co and Ni in zinc bath during hot-dip galvanizing, and is helpfulfor designing the new Zn-based alloy. As the part of systematic study, in the presentwork, the phase equilibria of the Zn-Co-Ni system have been determinedexperimentally. It accumulated basic thermodynamic data for the phase relationshipsresearchs of the Zn-Co-Ni-Fe and Zn-Co-Ni-Si quaternary systems. The isothermalsections of the Co-Ni-Zn ternary system at450°C and600°C have been determinedexperimentally using the equilibrated alloys with scanning electron microscopycoupled with energy dispersive X-ray spectroscopy, and X-ray diffraction. Threetri-phase regions have been confirmed at450°C and600°C, respectively. Threecontinuous solid solutions regions, i.e., α-Co/α-Ni, γ-Co₅Zn₂₁/γ-Ni₄Zn₂₂andγ₂-CoZn₁₃/δ-NiZn₈, exist in the system. The maximum solubility of Co in β₁-NiZn isup to16.5at.%. The maximum solubility of Ni in β₁-CoZn and γ₁-CoZn_7.8 is11.8at.%and8.1at.%, respectively. No true ternary compound was found. The X-raypowder diffraction patterns of the Co-Zn intermediate phases β₁and γ have beenobtained experimentally. The results obtained by cell refinement revealed that bothCo-Zn compounds β₁and γ belong to the cell type of cubic, and their latticeparameters are6.3360A and8.9412A, respectively.In the present work, the effect of bath additions Co and Ni on Si-containing steelsgalvanized coating has been investigated. The results showed that: The solubility of Siin the ζ phase increase with Co and Ni dissolving in the ζ phase; adding0.1wt.%Coor0.1wt.%Ni into the Zn bath can completely inhibit the silicon reactivity in thegalvanized coating on the0.1wt.%Si-containing steel. But for the0.3wt.% Si-containing steel, combining additions of0.3wt.%Co and0.1wt.%Ni in bath cannot inhibit the silicon reactivity, it only reduces coating thickness. In view of thesimilarity of the effect of Co and Ni on Si-containing steels galvanized coating, Co isa valid candidate material for controlling silicon reactivity.