Experimental Study Of The 600℃ Isothermal Section Of The Zn-Cr-Fe System

Zinc and zinc-based alloys are used mainly in hot-dip galvanizing to improve the corrosion-resistence of steel. With the development of the automotive industry, high strength steels are used widely. The alloying elements in high strength steels lead to a bad platability. Chromium is frequently added to high strength steels for automotive applications, and stainless steel hardware in galvanizing bath has a high content of chromium. During the galvanizing process of high strength steels, the chromium comes from the steel substrates or galvanizing hardwares will interact with the molten Zn-Al alloy, modifing the Zn-Fe reaction and deteriorating the quality of the coating. The information of phase relations of Zn-Al-Cr-Fe quaternary system is useful to understand the effect of chromium on the galvanizing process.As a series of investigation, the isothermal section of the Zn-Cr-Fe system was determined using the equilibrated alloys with the aid of diffusion couple approach in the present work. The specimens were investigated by means of scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS), electron probe microanalysis (EPMA) and X-ray diffraction (XRD).A ternary phase, P, with an approximate formula of Zn79Cr3Fe18 was found in the experiment. The results of solid-liquid diffusion couples showed that the complete diffusion path in the Fe70Cr30/Zn couple is (α-Fe)→(Γ+σ)→(Γ+ (Cr))→(P + (Cr))→(δ+ (Cr))→(δ+ L)→L. The experiment results indicated that (Cr) is in equilibrium with all phases in the system except (α-Fe) at 600oC. This isothermal section consists of six three-phase equilibrium phase fields:δ+ (Cr) + L,P + (Cr) +δ,Г+ P +δ,Г+ (Cr) + P,σ+ (Cr) +Гand (α-Fe) +σ+Г; twelve two-phase equilibrium phase fields: L + (Cr), L +δ,δ+ (Cr),Г+δ,δ+ P, P + (Cr), P +Г,Г+ (Cr),Г+σ,Г+ (α-Fe),σ+ (α-Fe) andσ+ (Cr); and seven single phase fields: L,δ,P,Г,(α-Fe),σand (Cr). The composition ranges of all intermetallic phases in the system were determined. The solubility of Zn in solid solution (Cr) is about 2.5 at.%, and that of Cr in theδ,Γand liquid zinc is 3.1, 5.4 and 1.4 at.%, respectively.