OXIDATION OF Zn22Al ALLOY, DOPED WITH NICKEL

ABSTRACT

The article presents the results of a study on the oxidation of Zn22Al zinc-aluminium alloy doped with nickel, in air environment.

АННОТАЦИЯ

В статье представлены результаты проведённого исследования по окислению цинково-алюминиевого сплава Zn22Al, легированного никелем, в воздушной среде.

Keywords: Zn22Al alloy, nickel, thermogravimetric method, activation energy, alloys oxidation.

Ключевые слова: сплав Zn22Al, никель, термогравиметрический метод, энергия активации, окисление сплавов.

Introduction. Zinc-aluminum alloys are widely used in various fields of technology [1-3]. In this regard, several works [4-6] are devoted to the study of their various properties for various operational purposes. Recently, zinc-aluminum alloys have been used as protective coatings for steel structures, products and structures. The most famous of them are Zn5Al and Zn55Al alloys known under the trademarks Galfan-I, II and Galvalum [7-9].

Objective.  The purpose of this work was to study the effect of temperature and nickel additives on the oxidation kinetics of the Zn22Al alloy in the solid state.

Methods. The interaction of the Zn22Al-Ni alloys with atmospheric oxygen in the temperatures 523-623 K, was investigated by thermogravimetric method [10].  

Results. The values of the true oxidation rate for the base zinc-aluminum alloy Zn22Al are 3.00∙10⁴ and 3.79∙10⁴ kg∙m^-2∙s^-1, and for an alloy containing 0.01 wt% nickel it varies from 2.05∙10⁴ to 2.71∙10⁴ kg∙m^-2∙s^-1, respectively, at temperatures of 523 and 623 K. In this case, the value of the effective activation energy of the Zn22Al alloy is 151.2 kJ/mol, and for an alloyed alloy with 0.01 wt% nickel, this value acquires a value of 188.2 kJ/mol (Table 1).

The high values of the effective activation energy indicate that the oxidation of nickel-doped alloys results in the formation of oxide layers with good properties. The oxidation rate of the base zinc-aluminum alloy Zn22Al somewhat decreases when various amounts of nickel are added to it, its small amounts are especially effective. With an increase in the nickel content in the Zn22Al alloy, the values of the activation energy somewhat decrease, but according to the established value, all additives (0.01÷0.5%) of nickel contribute to an increase in the activation energy of the base Zn22Al alloy (Table 1).

The resistance of the Zn22Al base alloy to oxidation increases significantly when it is alloyed with nickel in amounts of 0.01÷0.1 wt%. High resistance to oxidation is noted for the Zn22Al alloy containing 0.01 wt % nickel. When 0.5 wt% nickel is added to the Zn22Al alloy, a slight decrease in the oxidation rate is also observed (Table 1).

Table 1

Kinetic and energy parameters of the oxidation process of Zn22Al alloy doped with nickel

A change in the composition of the alloy under study during oxidation also affects the values of the effective activation energies. The calculated value of the effective activation energy in terms of the slope angle in the lgK–1/T coordinates for the Zn22Al zinc–aluminum alloy with different nickel contents varies monotonically. Curves (2–5) of alloys alloyed with nickel, compared to curve (1) of unalloyed Zn22Al alloy, are located to the left of it, which indicates an increase in the activation energy of the process (Figure 1).

Figure 1. Isochronous oxidation process (573 K) zinc-aluminum alloy Zn22Al alloyed with nickel

As a result of the oxidation of the studied alloys, using the example of the Zn22Al alloy containing 0.1% nickel, X-ray phase analysis established that the oxides ZnO, Al₂O₃, NiO and Al₂O₃∙Ni₂O₃ are formed (Figure 2).

Figure 2. Strach-roentgenograms of oxidation products zinc-aluminum alloy Zn22Al alloyed with 0.1 wt.% nickel

Generally, as a result of studying the oxidation of chromium-doped zinc-aluminum alloys, it was found that quantitative additions of chromium at a concentration of 0.01-0.1% contribute to a slight decrease in the oxidation rate of the Zn22Al base alloy under isothermal conditions, and, accordingly, an increase in the activation energy of the process.

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