OXIDATION OF ALLOYS OF THE Zn0.5Al-Sr SYSTEM

ABSTRACT

The article presents the results of a thermogravimetric study of the effect of strontium content (0.01–0.5 wt%) on the kinetics and energy parameters of the oxidation process of Zn0.5Al hard alloy at 523–623 K. It was found that the introduction of strontium additives into the zinc alloy leads to a decrease in the true oxidation rate and an increase in the apparent activation energy. It was determined that, with a kinetic calculation, it is possible to obtain polynomial dependences of the mass increment on time and regression coefficients. It is analyzed that the obtained results allow us to identify the catalytic role of strontium in the oxidation process and changes in the nature of the formation of an oxide film consisting of ZnO, Al₂O₃, SrO, ZnAl₂O₄.

АННОТАЦИЯ

В статье предствлены результаты термогравиметрического изучения влияние содержания стронция (0,01–0,5 мас.%) на кинетику и энергетические параметры процесса окисления твёрдого сплава Zn0.5Al при 523–623 К. Установлено, что введение добавок стронция в цинковом сплаве приводит к снижению истинной скорости окисления и повышению кажущейся энергии активации. Определено, что при кинетическом расчёте можно получить полиномиальные зависимости массоприращения от времени и коэффициенты регрессии. Проанализировано, что полученные результаты позволяет выявить каталитической роли стронция в процессе окисления и изменении характера образования оксидной плёнки, состоящие из ZnO, Al₂O₃, SrO, ZnAl₂O₄. 

Keywords: Zn0.5Al alloy, alloyed, strontium, oxidation.  

Ключевые слова: сплав Zn0.5Al, легирование, стронций, окисления.

Introduction.

Understanding the oxidation kinetics [1-10] and energetics [11-18] of these alloys is crucial for predicting their long-term performance under service conditions [19-22] and for optimizing their composition for specific applications [5-8]. Despite numerous studies of Zn–Al alloys [23-35], the effect of strontium additions (0.01–1.0 wt%) on solid-state oxidation behavior remains poorly understood.

The aim of the work is to establish data on the oxidation of alloys of the Zn0.5Al-Sr system in the solid state.

Materials and methods.

Zinc, aluminum, and aluminum-strontium master alloys (10% Sr) were used to study the oxidation of alloys. The change in mass was recorded using an SM-8 cathetometer by stretching a spring. To maintain a preset temperature with an accuracy of ±2 °C, the furnace load was regulated by a thyristor. An APP-63 potentiometer was used to record the temperature [25]. After the experiment, the system was cooled, and the reactive surface was determined by weighing the crucible with its contents. The oxide film formed on the surface of the sample was then removed, and its structure was studied using X-ray phase analysis [30].

Results and discussion.

Based on thermogravimetric data at temperatures of 523–623 K, oxidation curves were constructed for the Zn0.5Al alloy with varying strontium contents (0.01–0.5 wt.%). Figure 1 shows examples of curves for the Zn0.5Al zinc alloy. The curves represent the dependence of mass gain on time during isothermal oxidation.

To analyze the oxidation of the Zn0.5Al alloy with added strontium, quadratic curves were plotted for mass gain versus time. Figure 2 shows examples of such curves for an alloy containing 0.5% strontium. The quadratic curves indicate an acceleration of the process in the initial stages of oxidation, after which the rate of mass gain gradually decreases. This is due to the formation of a dense protective oxide layer on the alloy surface. The quadratic approximation allows us to determine regression coefficients that demonstrate a high correlation with experimental data (R² > 0.98), which confirms the accuracy of the selected model for describing oxidation kinetics.

Figure 1. Oxidation curves of Zn0.5Al alloy.

Figure 2. Quadratic oxidation curves of the Zn0.5Al0.5Sr.

Temperature dependence: an increase in temperature leads to an increase in the oxidation rate for all samples, which corresponds to the classical thermodynamic arrhenius relationship. Thus, the experimental data show that the additions of strontium to the Zn0.5Al alloy not only accelerates oxidation but also alters the energetic parameters of the process, promoting the formation of a more stable and protective oxide film. Oxidation products formed during the oxidation of Zn0.5Al-Sr alloys show the formation of the following oxides: ZnO, Al₂O₃, ZnAl₂O₄, SrO (Figure 3).

Figure 3. Diffractions oxidation of Zn0.5Al (a) and Zn0.5Al0.5Sr (b) alloys.

Analysis of the results showed that oxidation occurs in several stages: a rapid initial phase of oxide layer formation, followed by a slower film stabilization stage. With increasing strontium content, more uniform growth of the oxide layer is observed, indicating a catalytic effect of manganese on the oxidation process.

Conclusion.

Overall, the results show that strontium additions significantly affect the oxidation process of the Zn0.5Al alloy. Kinetic analysis reveals S-shaped curves with a rapid initial oxidation phase followed by a slowdown, and polynomial approximations accurately describe the mass increase over time (R² > 0.98). Increasing temperature further increases the oxidation rate, consistent with the Arrhenius equation. Strontium additions promote the formation of a more uniform and protective oxide layer and improve the oxidation resistance of the Zn0.5Al alloy.

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