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Effect of Ni and Co in Zn Bath on the Microstructure and Growth Kinetics of Galvanized Coating of Si-containing Steel

Author: LiFaGuo
Tutor: YinFuCheng
School: Xiangtan University
Course: Materials Science
Keywords: Hot-dip galvanized Silica reactivity Reaction Steel Nickel Cobalt
CLC: TG174.443
Type: Master's thesis
Year: 2009
Downloads: 33
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Abstract


Hot-dip galvanized steel corrosion technology is widely used. Contain higher silicon is used extensively for general hot-dip galvanized steel structure, and seriously affect the quality of the zinc coating, due to the presence of silica reactivity (that is, the the Sandelin effect of), this problem has become the urgent problem of hot-dip galvanizing industry . In this paper, optical microscope, SEM, SEM-EDS, SEM-WDS system to study the organization of the nickel and cobalt Q235 and Q345 steel plating, the impact of the growth kinetics and corrosion resistance. The results show that, When the nickel or cobalt in the zinc bath is added in an amount of more than 0.05 wt%, it is possible to completely suppress the reactivity of silicon steel Q235, play a role in thinning coating. Q345 steel, even with the addition of 0.3 wt% of the amount can only partially inhibit silica reactivity. By scanning electron microscopy analysis of the microstructure of the coating, and found that the the Q235 steel coating formed in the zinc bath containing nickel or cobalt organizations have in common is: intermediate phase was also observed in addition to the two-phase δ, ζ Γ phase; relative to pure zinc the bath, ζ thinning, δ thickened; different points: plus nickel plating layer in the ζ, η phase boundary distribution layer Τ (Zn-Fe-Ni) phase particles, δ thickness exceeding ζ thickness; plus cobalt plating ζ phase layer ζ phase layer, δ the thickness of the thickness of less than ζ ζ phase layer into a dense and cobalt. The Q345 steel the various zinc bath plating organizations, in the middle phase mainly by δ, ζ two-phase form, and distribution of a certain amount of Diffused-Δ; entire mesophase layer vast majority of ζ phase. The reason is suppressed for the silicon content of less than 0.2% of a reactive steel, nickel silicon reactive nickel enriched in the ζ-η phase boundary, forming a Zn-Fe-Ni ternary Τ phase, thereby changing the ζ grains grown up process, also played a role of contact barrier liquid zinc and δ phase silica reactivity is suppressed. Using spectral analysis of the distribution of the cobalt and silicon in the coating layer, and found that after a certain amount of cobalt in the zinc bath, the coating tissue loose ζ cobalt-rich layer into direct contact with the liquid, ζ phase and generated by consuming δ dense ζ phase. Dense ζ phase layer prevents direct contact of the liquid phase and a δ phase. Cobalt-rich ζ phase can accommodate about 0.25 wt% Si, silicon enrichment to avoid the solid-liquid interface, liquid channels disappear, to suppress the generation of reactive silicon-containing silicon steel hot-dip galvanized. 5% NaCl solution immersion tests and electrochemical polarization experimental results show that the addition of nickel or cobalt in the zinc bath, the corrosion resistance of the coating improved. This is mainly due to addition of nickel or cobalt, on the one hand formed in the coating layer of nickel-rich layer or a cobalt-rich, role on coating acts as a barrier, on the other hand advantageous in suppressing the corrosion generated early in the Zn (OH) 2 or of ZnCl2 · 4Zn (OH) 2 hydrolysis become larger ZnO conductivity, to improve the corrosion resistance of coating.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > Metal corrosion protection,metal surface treatment > Corrosion control and protection > Metal surface protection technology > Metal complex layer of protection > Hot dip
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