OXIDATION OF Zn55Al ALLOY DOPED WITH GALLIUM

ABSTRACT

The article presents the results of a study on the oxidation of Zn55Al zinc alloy doped with gallium, in air environment.

АННОТАЦИЯ

В статье представлены результаты проведённого исследования по окислению сплава Zn55Al, легированного галлием, в воздушной среде.

Keywords: Zn55Al alloy, gallium, thermogravimetric method, activation energy, alloys oxidation.

Ключевые слова: сплав Zn55Al, галлий, термогравиметрический метод, энергия активации, окисление сплавов.

Introduction. Zinc is most commonly used in the electrochemical plating industry. Up to 40% of the zinc reserve of the world is used to protect metal structures from corrosion [1-3]. Zinc coating are anodic and protect steel surfaces electrochemically. Under atmospheric conditions, the zinc surface tarnishes due to the formation of a thin oxide layer that protects the metal from further oxidation [4-6]. The most versatile and widespread are zinc-aluminium coating such as galfan-I and galfan-II (zinc alloys with 5 and 55 wt% aluminium) with high protective properties [7-9], which can be applied by immersion of steel structures into the molten metal to create a coating.

Objective.  The purpose of this work was to study the effect of temperature and gallium dopants on the oxidation kinetics of the Zn55Al alloy in the solid state.

Methods. The interaction of the Zn55Al-Ga alloys with atmospheric oxygen in the temperatures 473-623 K, was investigated by thermogravimetric method [10].  

Results. Figure 1 shows the kinetic curves of changes in the specific mass of samples (g/s, kg/m²) depending on the time of interaction (t, min) with oxygen in the gas phase and on temperature. The oxide membrane formed in the initial stages of the process, apparently, does not possess sufficient protective properties, as evidenced by an increase in the rate of oxidation of the studied alloys. The interaction of the Zn55Al alloy with gallium of various concentrations with oxygen in the gas phase at the studied temperatures differs significantly from the oxidation of the initial Zn55Al alloy. The linear dependence persists for 12-15 min, then, as the oxide membrane is formed, the nature of the oxidative process turns into parabolic and the formation of a protective oxide surface ands by 30 min (Figure 1).

Figure 1. Kinetic curves of the oxidation process of Zn55Al (a) alloy doped with 0.01 wt% (b) gallium at Т = 473 (1), 523 (2) и 623 К (3)

The values of the oxidation rate of the Zn55Al alloy at temperatures of 473, 523 and 623 K are 3.04, 3.32 and 3.73·10⁴ kg · m^{-2 .} s^-1, respectively. The effective activation energy of the oxidation process of the investigated alloys varies from 154.4 to 117.6 kJ/mole. Among doped alloys the Zn55Al alloy with 0.01 wt% gallium has the oxidation rate which corresponds to an activation energy of 134.7 kJ/mole, while the activation energy of the initial alloy is 154.4 kJ/mole (table 1).  

Table 1

Kinetic and energy parameters of the oxidation process of Zn55Al alloy doped with gallium

Generally, oxidation parameters of the oxidation of alloys were evaluated, showned that additions of Ga several increase the oxidizability of the Zn55Al alloy.

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