EFFECT OF NICKEL ON THE OXIDATION KINETICS OF Zn5Al AND Zn22Al ALLOYS

ABSTRACT

The article presents the results of studying the kinetics of oxidation zinc-aluminum alloys Zn5Al and Zn22Al, alloyed with nickel, in various corrosive environments.  

АННОТАЦИЯ

В статье приведены  результаты  исследования  кинетики окисления цинк-алюминиевого сплавов Zn5Al и Zn22Al, легированного никелем, в различных коррозионных средах.

Keywords: Zn5Al, Zn22Al alloys, nickel, thermogravimetric method, oxidation kinetics, activation energy.

Ключевые слова: сплав Zn22Al, никель, термогравиметрический метод, кинетика окисления, энергия активация.

Zinc-based alloys are widely used in manufacturing. Therefore, zinc alloy is used to manufacture small parts used in automobiles, tractor construction, the electrical industry, household appliances, furniture fittings, locks, and other items. The mechanical properties of the ultrafine-grained eutectoid alloy Zn-5% Al and Zn-22% Al have been studied. It was shown that stable equiaxed ultrathin grains are ~0.63 µm in size with a uniform distribution of Zn and Al grains. Various modifications of zinc-aluminum alloys are alloyed with a third component in the literature. In particular, the works [1–4] demonstrated a positive effect of several metals on the corrosion resistance of Zn0.5Al, Zn5Al, and Zn55Al alloys. In this connection, several works are devoted to studying their properties for various applications [5–7]. There are also data on the kinetics of oxidation of zinc and alloys based on it with atmospheric oxygen [8].

This work aimed to study the effect of temperature and chromium dopant on the oxidation kinetics of Zn5Al and Zn22Al alloys. It has been established that alloying zinc-aluminum alloys with nickel (within 0.01–0.05 wt.%) helps reduce the alloy’s corrosion rate by 1.5–3 times. The proposed alloy compositions can be used as an anode coating for corrosion protection of steel products and structures. Alloy samples for this work from zinc of chemically pure grade. (granulated)”, A7 grade aluminum and its master alloys with nickel (2-4% Ni) were obtained under a layer of protective flux in aluminum oxide crucibles in a shaft furnace of the “SshOL” type at 650–750°C. The studies were conducted on a setup [9] consisting of a coal-resistance furnace with an aluminum oxide jacket. With the help of a KM-8 cathetometer, a change in weight was recorded by stretching the spring. Aluminum oxide crucibles with a height of 25-26 mm and a diameter of 18-20 mm were used in the study. Before the experiment, the crucibles were calcined to a constant weight in an oxidizing medium at a temperature of 1000-12000C.

To measure the temperature, a platinum-platinum-rhodium thermocouple (5) was used, which touched the surface of the alloy under study with the hot end. The thermocouple was in an aluminum oxide case. To maintain the set temperature with an accuracy of = 20 With a thyristor, the load of the furnace was adjusted. A PP-63 potentiometer was used to register the temperature. After the experiment, the system was cooled, the reaction surface was determined by weighing the crucible with the contents. Next, the formed oxide film was removed from the sample surface and studied using X-ray phase analysis.

At the end of the experiments, the system was cooled, the crucible with its contents was weighed, and the reaction surface was determined. Then, the resulting oxide film was removed from the surface of the sample and studied by X-ray phase analysis to obtain information about the composition of the phases [10-11]. With increasing temperature, the specific gravity of all samples (g/s) increases with time (t). First, the process of oxidation of alloys proceeds intensively for up to 12 min according to a linear law. Then, as the protective ability of the oxide film develops, the linear dependence becomes a parabola (Fig. 1).

Figure 1. Oxidation kinetic curves of (a) Zn5Al and Zn22Al (a1) alloy, alloyed with 0.01 wt. % nickel (b), (b1) at: 623 (1), 573 (2), and 523 K (3)

The actual oxidation rate of these alloys varies from 3.55 to 2.53 Zn5Al, 3.44 to 2.42 Zn5Al × 10–4 kg/(m2s), and the effective activation energy of the alloys varies in the range of 128.4–173.0 Zn5Al and 151.2–180.0 Zn22Al kJ/mol, at a temperature of 523 L (Table 1).

Table 1.

 Kinetic and energy parameters of the process of oxidation of zinc-aluminum alloys Zn5Al and Zn22Al alloyed with nickel in the solid state

By studying the oxidation products of alloys, particularly the oxide film, which is formed when heated on the surface of samples, it is possible to obtain important information about their oxidation mechanism. Since the diffusion process of its components determines the oxidation process of alloys through the oxide film, this generally changes the kinetics of the process. Oxides of alloying components, which are part of zinc oxides, make it difficult or facilitate the diffusion of zinc ions, thereby slowing down or accelerating the overall oxidation process.

As a result of oxidation of the studied alloys, using the example of the Zn 22 Al alloy containing 0.1% nickel, X-ray phase analysis revealed that the oxides of ZnO, Al₂O₃, NiO and Al2O3∙Ni₂O₃ are formed (Fig. 2).

Figure 2. The stroke of the diffractogram of the oxidation products of the Zn22Al (a) alloy containing 0.1 wt.% nickel

Thus, during the formation of layers of oxides of various compositions, which should be expected during the oxidation of the studied compositions, a vacancy concentration gradient is established at the boundaries of the layers. This gradient creates the possibility of metal diffusion and counter diffusion of oxygen through the oxide. Due to the presence of vacancies, the diffusion of metal ions through the oxide layer to its surface is facilitated, and the diffusion of oxygen into the depth of the oxide layer is carried out by moving its atoms along the interstices of the lattice or along the grain boundaries.

As a result of the study, the parabolic law of oxidation of alloys was established. It was found that alloying Zn5Al and Zn22Al with nickel (0.01–1.0 wt. %) slightly reduced oxidizability. The most promising for an oxidation-resistant protective coating of a carbon steel product is considered to be Zn5Al alloy containing 0.01, 0.05, and 0.1 wt. % nickel.

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