A study of zinc-nickel galvanized coating of silicon steels

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Notowidjojo, Budi Dharma
Department of Materials Engineering
thesis / dissertation description
The presence of silicon in steel has caused world-wide concern in the galvanizing industry, as it enhances growth of the alloy layers leading to abnormally thick coatings with consequential degradation of the properties of the galvanized steel.A technique, commercially known as the Technigalva process, which incorporates the addition, to the galvanizing metal, of ~0.1 % nickel by means of a Zn-2%Ni master alloy, has been alluded to in some literature as providing a partial solution to the problem but with doubtful economic viability, due to excessive losses of nickel in the dross. Additionally, this process was invented without any scientific investigation based on constructive logic and detailed information of this process is mostly fragmentary, scattered and confidential.The objectives of this research project were to prepare a constructive review related to the Technigalva process, to obtain a scientific understanding of the problems involved in the process and to provide solutions to these problems based on the logic of existing technology.Literature relevant to the Technigalva process has been reviewed. In particular, a fundamental understanding of the mechanisms involved during the galvanizing process has been examined and the current theoretical models explaining the abnormal growth of the alloy layers during galvanizing of silicon steels are discussed. Additionally, a review of possible solutions through alloying addition, for eliminating the adverse effect of silicon during galvanizing has been made. Finally, the development of the Technigalva technology is discussed in constructive detail.The investigation involved three separate studies.Firstly, the mechanism by which nickel is transferred from the master alloy to the dross which accumulates at the bottom of the vessel was investigated. It was confirmed that the formation of dross in the Technigalva process and the formation of dross in normal zinc galvanizing occurs by the same mechanism and can be attributed to a simple consequence of phase equilibria.Initially, the master alloy, which is added to molten zinc, dissolves completely to produce a concentration of ~0.1 % of nickel in the liquid metal. As hot dipping proceeds, iron accumulates in the liquid metal from various sources until the solubility limit of ~0.03% is reached. Further accumulation of iron results in precipitation of a ternary proeutectic phase and the composition of the precipitated phase depends upon the composition of the liquid phase from which it forms. The nickel which is necessarily incorporated in the precipitated phase is thereby lost from the galvanizing metal and transferred to the dross accumulating at the bottom of the galvanizing vessel.In the second study, the effect of both silicon in steel, and nickel in the liquid galvanizing metal, on the structural development of the alloy layers in the galvanized coating were examined.The formation of particles containing iron and silicon has been suggested in much of the literature to be the rate determining factor which enhances growth of the alloy layers, in particular the ζ phase layer, during the galvanizing of silicon steels. A new technique developed in this study enables these particles to be observed using transmission electron microscopy with superior resolution to that reported in the literature. These particles were positively identified as iron silicide (FeSi) with primitive cubic structure and lattice parameter of 446 pm, and were observed to occur in association with the grain boundaries of the ζ phase. Results of this study support the suggestion made in the literature that the FeSi particles promote nucleation of the ζ phase crystals.Addition of ~0.12% nickel to the liquid galvanizing metal evidently promoted preferential development of the δ phase and suppressed development of the ζ phase layers at all levels of silicon in steel. The effect of nickel is attributed to the formation of particles of silicon in steel. The effect of nickel is attributed to the formation of particles of Γ2 phase which nucleate epitaxially with the ζ phase layer at the zinc/ζ interface and provide a mechanical barrier to growth of the ζ phase layer. It is suggested that the presence of nickel in the liquid metal altered the phase equilibria of the system and resulted in modification of the characteristic behaviour of the FeSi particles.Finally, in the third study, a possible solution to problems of high nickel losses in the dross and of ineffectual action in the processing of high silicon steels (> 0.3%Si) during the Technigalva process through alloying addition was examined. It was found that these problems could be eliminated by a simple addition of 0.025% Al to the liquid galvanizing metal. This method was found to be economically justifiable and operationally feasible. Recommendations to improve the cost effectiveness of the Technigalva process are made and further development of a new master alloy is suggested.