The role of surface oxides on annealed high-strength steels in hot-dip galvanizing

Abstract

Oxidized alloying elements on the surface of annealed advanced high-strength steels deteriorate zinc wetting during hot-dip galvanizing. The present work describes reactions taking place during immersion of a steel strip in a zinc bath and gives new insight in changes of the surface oxides during hot-dipping. By using different advanced analysis techniques a direct proof for the occurrence of an aluminothermic reduction of Mn-oxides by Al dissolved in the zinc bath is shown. Furthermore, the morphology – rather than the chemical composition – of the surface oxides is crucial for obtaining a satisfactory galvanizability, mandatory for efficient corrosion prevention.

Introduction

The demand for galvanized advanced high strength steels (AHSSs) is steadily increasing, especially for their use in the automotive industry [1], [2], [3]. Unfortunately, due to the increased amounts of alloying elements, like Mn, Si, Cr, or Al, these steel grades tend to form oxides on the steel surface during recrystallization annealing due to the presence of residual O₂ and H₂O in the annealing atmosphere [4], [5], [6], [7]. These oxides are detrimental to the wetting of the steel surface with liquid zinc in the galvanizing process [8], [9], [10] as well as for zinc adhesion [11], leading to a decreased corrosion protection of the steel substrate. Several reports have been published, describing that the presence of oxides on the steel surface hinders the reactivity of the steel substrate with liquid zinc [12], [13], [14] and that the formation of a defect free inhibition layer (Fe₂Al_5−xZn_x) is essential for obtaining adequate galvanizability [16]. Therefore, an exact understanding of the processes and reactions occurring between the surface oxides and the liquid zinc in the zinc bath is necessary for further improvements of the corrosion protection coatings.

Especially dual-phase steels containing higher amounts of Mn are often discussed with respect to their wettability and inhibition layer formation. Due to the fact that this steel grade shows an adequate galvanizability despite substantial surface oxides being present after annealing under standard conditions (−30 °C dewpoint, N₂-5%H₂ process atmosphere), it is suggested that specific reactions or a removal of the surface oxides occur in the zinc bath. Khondker et al. [17] proposed in this context that an aluminothermic reduction of Mn-oxides by the Al dissolved in the zinc bath at 460 °C proceeds via the reaction:

$$3{\text{MnO}}_{(\text{solid})}+2{\text{Al}}_{(\text{solution})}\to {\text{Al}}_2{\text{O}}_{3(\text{solid})}+3{\text{Mn}}_{(\text{solution})}$$

This aluminothermic reduction is frequently discussed, with e.g. investigations of Kavitha and McDermid [18] indicating that a 90 nm thick Mn-oxide layer can be reduced by the dissolved Al during a dipping time of 4 s. In that work, the resulting Al₂O₃-product as well as an oxygen-deficient MnO_x-phase was found at the MnO–Zn interface by electron energy loss spectroscopy (EELS) analysis in a transmission electron microscope (TEM). According to Huachu Liu et al., beside MnO also Cr₂O₃ or SiO₂ can be reduced by the effective Al in the zinc bath [19]. Staudte et al. [20] reported that the Mn-oxide thickness which can be reduced by Al dissolved in the bath is limited by 20–25 nm. Furthermore, Li et al. [21] state that galvanizability tends to worsen as the Mn-oxide layer becomes dense, not being completely reduced anymore via aluminothermic reduction [21]. Often, the presence of ζ-FeZn₁₃ compounds formed on Fe₂Al_5−xZn_x – crystals is interpreted as an indirect evidence for the occurrence of an aluminothermic reduction of MnO in the zinc bath [18], [22], [23], being formed due to an Al-depletion which results from the consumption of Al according to Eq. (1) [24]. As an example, Alibeigi et al. [23] link the amount of Fe–Zn intermetallics qualitatively to the thickness of the MnO-layer present on the steel surface prior to hot-dipping. The authors also take the wetting force measured via the Wilhelmy plate method as additional parameter being directly related to the Mn-oxide thickness [23]. The same method of wetting force measurement was used by Blumenau et al. [25] to characterize wettability of a high Mn alloyed steel in a Zn–Al–Mg bath. The authors reported increased reduction of Mn-oxides by the Mg dissolved in the Zn bath via the formation of MgO, due to the even higher oxygen affinity of Mg compared to Al. The effective reduction of MnO by Mg was then assumed to lead to an improved galvanizability [25].

In an earlier work of Blumenau et al. [26] an alternative model for the presence of Fe–Zn intermetallics on top of the inhibition layer was presented. As the Fe–Zn – compound is formed due to an Al-depletion at the steel/zinc interface, a reaction has to occur consuming the Al present at this position. In that work, the formation of a Mn–Al intermetallic phase due to the high affinity of Mn to Al was proposed, assuming that Al is not reducing any Mn-oxides, and therefore no aluminothermic reduction is occurring, but Mn is dissolved out of the steel surface into the zinc bath and forming a Mn–Al – compound via the reaction:

$${\text{Mn}}_{\text{Zn}}+6{\text{Al}}_{\text{Zn}}\to {\text{MnAl}}_{6\uparrow }$$

This intermetallic phase formed was proposed to leave the steel/zinc interface as MnAl₆ dross particles. However, such MnAl₆ particles have not been found in the zinc bath [26].

Despite the abundance of work performed on this topic of aluminothermic reduction, it is still not clear if the contribution of this process is significant for obtaining an adequate galvanizability, or if other relevant processes do occur in the zinc bath as well. Therefore, in this present study, three different steel grades with varying amounts of Mn were hot-dip galvanized after recrystallization annealing and are examined in detail with respect to the different phases being present at the steel/zinc interface by using different advanced analysis techniques. The zinc layers were stripped off with different etching techniques, gaining more insight in the reactions taking place at the steel/zinc interface during hot-dipping in the zinc bath. Furthermore, for the first time to the best of our knowledge, a direct evidence for the presence of reaction products resulting from an aluminothermic reduction is obtained, as well as a clear picture regarding the growth mechanisms of the inhibition layer.