OXIDATION OF BARIUM-ALLOYED Zn22Al ALLOY

ABSTRACT

Experimental studies of the oxidized barium-doped Zn22Al alloy were conducted in the temperature range of 523–623 K. The most important characteristics of the alloy oxidation process were identified. Thermogravimetrical analysis demonstrated an increase in the oxidation of the Zn22Al alloy upon alloying with increasing barium concentrations of 0.01–1.0%. It was analyzed that changes in the kinetic and energy characteristics of barium-doped alloys are associated with the heterogeneity of the alloy sample composition and the growth of an oxide film on their surface. It was established that the initial state (heterogeneity, dispersion) and phase composition of the compound samples play a crucial role in the kinetics of oxidation processes.

АННОТАЦИЯ

Экспериментальные исследования окисления легированного барием сплава Zn22Al были проведены в интервале температур 523–623 K. Выявлены наиболее важные характеристики окислительного процесса сплавов. Термогравиметрическим методом обосновано повышение окисляемости сплава Zn22Al при легировании его с добавками 0,01-1,0% бария. Проанализировано, что изменение кинетические и энергетические характеристики легированных барием сплавов связано с гетерогенности структуры состава образцов сплава и ростом формирование оксидной пленки на их поверхности. Установлено, что исходное состояние (гетерогенность, дисперсность) и фазовый состав образцов сплава играют значительную роль в кинетике процессов окисления. 

Keywords: Zn22Al alloy, barium, alloyed, oxidation kinetics.

Ключевые слова: сплав Zn22Al, легирование, барий, кинетики окисления.

Introduction.

One of the key challenges in modern materials science [1–3] is the development of corrosion-resistant materials for operation in aggressive environments [4–10]. Corrosion of marine [11–14], atmospheric [15–20], and underground metal structures causes significant economic losses. Therefore, considerable attention is devoted to the development and application of anodic and sacrificial protection methods [27–35], which are among the most effective approaches to combating chemical and electrochemical corrosion.

The aim of this work is to study the effect of temperature and barium alloying on the oxidation behavior of the Zn22Al alloy.

Materials and methods.

The alloys were prepared using zinc of grade KhCh, aluminum of grade A7, and barium-containing additions (specify grade if needed). Melting was carried out in a shaft furnace of the SShOL type at temperatures of 700–850°C.

The experiments were performed using a setup consisting of a carbon resistance furnace with an alumina lining [25]. Changes in sample mass were recorded by measuring spring elongation with a KM-8 cathetometer. After the experiments, the crucible with the sample was weighed, and the reaction surface area was determined.

Results and discussion.

The obtained kinetic curves indicate the complex nature of the oxidation process. Oxidation kinetics were studied over 1 hour; however, since the curves approach a steady state between 15 and 60 minutes, only the first 30 minutes are presented.

A characteristic feature of the curves is the pronounced power-law region. During the first 15 minutes, the oxidation rate increases sharply due to cracking and the loss of protective properties of the oxide layer. This leads to rapid oxidation and an increase in the specific surface area of the oxidation products (Figure 1).

Figure 1. Oxidation curves of unalloyed Zn22Al and barium-doped alloys

The addition of 0.01–1.0% barium reduces the oxidation rate of the Zn22Al alloy. Alloy composition also affects the activation energy values: oxidation of barium-containing alloys requires lower activation energy. At the same time, reduced activation energy suggests the formation of oxide films with protective properties (Table 1).

Table 1

Oxidation characteristics of Zn22Al-Ba alloys

Analysis of the results shows that the initial state (dispersion) and phase composition of the samples significantly influence oxidation kinetics. Increased dispersion enhances reactivity, which is reflected in higher oxidation onset temperatures and effective activation energies.

The elevated oxidation onset temperature is associated with the formation of a thin oxide film whose thickness is small compared to the sample dimensions. The decrease in protective properties of the oxide film during heating is attributed to the mismatch in thermal expansion coefficients between the alloy and the oxide layer. Higher activation energies in ternary alloys are explained by the formation of chemically stable complex aluminates on their surface.

Conclusion.

The kinetic and diffusion-controlled regimes of alloy oxidation in atmospheric oxygen were established. A slight increase in oxidation was observed for Zn22Al alloys alloyed with 0.01–1.0% barium. Changes in the kinetic and energy characteristics of barium-doped alloys are attributed to structural heterogeneity and the formation and growth of oxide films on their surface.

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