Synergistic effect of Cu and Si on hot-dipping galvalume coating

Abstract

The coating microstructures and thicknesses of the iron panels galvanized in galvalume baths containing 0.0, 0.1, 0.5 and 1.0 wt.%Cu for 10 s, 30 s, 60 s, 180 s, 300 s and 600 s have been studied detailedly. The results indicate that Cu can effectively control the Fe–Al reactivity by the synergistic effect with Si. The addition of Cu makes Si be enriched in the reaction region during the hot-dipping. It promotes the formation of the τ₅ phase and hinders the growth of the Fe₂Al₅ phase. The diffusion path model was introduced to studying the effects of Cu and Si in the present study. The addition of 0.5–1.0 wt.%Cu in galvalume bath forms a stable diffusion path, iron substrate/Fe₂Al₅/FeAl₃/τ₅/overlay. The violent reaction between the iron substrate and the Al–Zn liquid is under control by the compact intermetallic layer, and it decreases the thickness of the intermetallic layer.

Introduction

Hot-dipping coating is applied to steel for improved atmospheric corrosion resistance [1]. The corrosion resistance of the 55 wt.%Al–Zn–Si coated steel sheets (galvalume) is superior to that of a single of zinc coated steel sheets and 5 wt.%Al–Zn coated steel sheets [1], [2]. The galvalume processing, which was developed by the Bethlehem Steel Corporation, has been widely applied in the galvanizing industry. The reaction between the steel sheet and the bath is complex and intense in hot-dipping, however, alloy element in the bath would seriously affect the reaction [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. Without any other element in the Al–Zn bath, the violent reaction occurs between the steel sheet and Al, which will bring an adverse influence to the performance of the coating. The hot-dipping bath is always saturated with Fe, as Fe dissolves from the steel sheet. When Si is added into the 55 wt.%Al–Zn hot-dipping bath, the bath essentially becomes a Fe–Al–Zn–Si quaternary system. There are several important ternary intermetallics (such as τ₅ and τ₆) in the Al-rich corner area of the Fe–Al–Si system [13], [14], [15], [16], [17]. The Si suppresses the rapid growth of intermetallic compound by changing the interfacial intermetallic compound from Fe–Al to Fe–Al–Si [18], [19], [20], [21].

The earlier studies show that a small amount of copper added to hot-dipping aluminum bath can reduce the coating thickness [22], a small quantity of Cu in the bath can improve the fluidity of molten zinc [3]. Based on the Al–Fe–Cu ternary phase diagram at temperature relevant to hot-dipping galvalume, several ternary intermetallics (such as ω and Ι) can be observed on the Al-rich corner area [23], [24]. When Cu is added to galvalume bath, the existence of these ternary intermetallics may play the similar role as τ₅ and τ₆ when a small amount of Si exists in galvalume bath. It would achieve the synergistic effect of Cu and Si for controlling the reactivity between Fe and Al. However, it is devoid of theoretical studies and practical applications in this field. Hence, the goal of the present study is to experimentally determine the influence of Cu on the microstructure and thickness of the galvalume coatings.