ELECTROCHEMICAL AND MICROSTRUCTURAL STUDY OF THE ANODIC BEHAVIOR OF ZA15 ALLOY ALLOYED WITH CHROMIUM, MANGANESE, AND MOLYBDENUM

ABSTRACT

The anodic behavior of the CA15 alloy (Zn–15%Al) alloyed with transition metals — Cr, Mn, and Mo — in NaCl electrolyte solutions of various concentrations was investigated. Electrochemical studies were carried out using polarization measurements with the determination of free corrosion potentials, corrosion current density, and the rate of anodic dissolution. It was established that the introduction of alloying additions within the concentration range of 0.01–0.5 wt.% promotes a shift of the corrosion potential toward more positive values and a decrease in corrosion current density. The corrosion rate of the alloyed alloys is 1.5–2 times lower compared to the unalloyed CA15 alloy, which indicates an improvement in their protective properties. Phase analysis of the corrosion products revealed the formation of oxide compounds Al₂O₃, ZnO, ZnAl₂O₄, as well as complex oxides Al₂O₃•Cr₂O₃, Mn₂O₃, and MoO₃, forming a more stable and protective surface film.

АННОТАЦИЯ

В работе исследовано анодное поведение сплава ЦА15 (Zn–15%Al), легированного переходными металлами — Cr, Mn и Mo, в электролитической среде NaCl различной концентрации. Электрохимические исследования проводились методом поляризационных измерений с определением потенциалов свободной коррозии, плотности коррозионного тока и скорости анодного растворения. Установлено, что введение легирующих добавок в концентрационном интервале 0,01–0,5 мас.% способствует смещению коррозионного потенциала в положительную область и снижению плотности коррозионного тока. Скорость коррозии легированных сплавов в 1,5–2 раза ниже по сравнению с нелегированным сплавом ЦА15, что свидетельствует о повышении их защитных свойств. Фазовый анализ продуктов коррозии показал образование оксидных соединений Al₂O₃, ZnO, ZnAl₂O₄, а также сложных оксидов Al₂O₃•Cr₂O₃, Mn₂O₃ и MoO₃, формирующих более стабильную и защитную поверхностную плёнку.

Keywords: ZA15 alloy, chromium, manganese, and molybdenum, potentiostatic investigation, neutral medium, NaCl electrolyte, anodic behavior, corrosion and pitting potentials, corrosion rate.

Ключевые слова: сплав ЦА15, хром, марганец и молибден, потенциостатическое исследование, нейтральная среда, электролит NaCl, анодное поведение, потенциалы коррозии и питтингообразования, скорость коррозии. 

Introduction

Due to the combination of high corrosion resistance and technological applicability, zinc–aluminum coatings have found wide application in the automotive, construction, and mechanical engineering industries, where materials are required to provide enhanced protection against atmospheric and service corrosion. Among them, coatings of the Galfan-1 and Galfan-2 types are the most widely used; these are alloys containing 5 and 55 wt.% aluminum, respectively. These coatings exhibit high protective properties and can be applied by the hot-dip galvanizing method, in which products are immersed in a molten metal bath [1–5].

An analysis of the scientific literature and open sources revealed a lack of systematic data on the influence of chromium, manganese, and molybdenum on the anodic behavior of the zinc–aluminum ZA15 alloy. Therefore, in the present study, chromium, manganese, and molybdenum were selected as alloying elements to evaluate their effect on the electrochemical characteristics and corrosion resistance of this alloy in a neutral medium.

Materials and methods

The investigated alloy samples were prepared from high-purity granular zinc (grade KhCh), aluminum of grade A7, and master alloys containing chromium, manganese, and molybdenum (2 wt.% of each element). Melting was carried out in alumina crucibles in a shaft-type electric resistance furnace (ShShOL type) within the temperature range of 670–750 °C. The molten alloys were cast into graphite molds to produce rods with a diameter of 8 mm and a length of 140 mm. The lateral surfaces of the samples were insulated with a mixture of 50% rosin and 50% paraffin, ensuring an identical and well-defined exposed surface area for all experiments.

Potentiostatic investigations of the effect of chromium, manganese, and molybdenum additions on the corrosion behavior of the zinc–aluminum ZA15 alloy were performed in NaCl solutions with concentrations of 0.03, 0.3, and 3 wt.% at a potential sweep rate of 2 mV/s using a PI-50.1.1 potentiostat, following the procedure described in Refs. [6–10]. It was established that alloying with these additional components leads to a reduction in the corrosion rate by a factor of 1.0–1.5.

Results and discussion

The time dependence of the free corrosion potential of the alloys is of considerable importance for assessing the kinetics of surface passivation and the stability of the passive film formed in different electrolytic media. Changes in the open-circuit potential with time reflect the balance between anodic dissolution and the formation of corrosion products on the alloy surface.

The introduction of chromium, manganese, and molybdenum in the concentration range of 0.01–0.5 wt.% causes a noticeable shift of the electrochemical potentials of the zinc–aluminum ZA15 alloy toward more positive values. This shift indicates a reduced tendency toward anodic dissolution and an improvement in the protective properties of the surface layer formed in the NaCl electrolyte. The positive displacement of the corrosion potential is associated with the formation of stable oxide and mixed oxide phases containing Cr, Mn, and Mo, which suppress active corrosion processes.

Within the investigated concentration range, alloying with these transition metals results in a systematic reduction in the corrosion rate of the base ZA15 alloy, as summarized in Table 1. This effect can be attributed to microstructural refinement and improved integrity of the passive film, leading to enhanced resistance to uniform corrosion and delayed initiation of localized corrosion.

Electrochemical measurements were performed using a silver/silver chloride reference electrode and a platinum auxiliary electrode. The experimental procedure is described in Ref. [11].

Table 1.

Corrosion–electrochemical characteristics (vs. Ag/AgCl) of the ZA15 alloy alloyed with Cr, Mn, and Mo in a neutral medium

The microstructure of the zinc–aluminum alloy exhibits a characteristic two-phase eutectic structure consisting of the α-Al solid solution and the γ-Zn intermetallic phase. The α-Al phase forms a continuous aluminum-rich matrix, within which the γ-Zn phase, enriched in zinc, is uniformly dispersed. Such phase morphology is typical of ZnAl system alloys and plays a crucial role in determining their mechanical behavior, including strength, ductility, and wear resistance.

Microstructural examination of the ZA15 alloy and its modifications alloyed with 0.5 wt.% chromium, manganese, and molybdenum (Figure 2) demonstrates significant changes in phase morphology, distribution, and structural refinement induced by the alloying elements. The addition of transition metals promotes microstructural modification, which may include grain refinement, alteration of eutectic spacing, and redistribution of intermetallic phases. These structural changes contribute to variations in the physicomechanical properties of the alloy and reflect the influence of alloying on phase stability and interatomic interactions within the ZnAl matrix.

Figure 2. Microstructures (×500) of the zinc–aluminum alloy ZA15 (a) and its modifications alloyed with 0.5 wt.% Cr (b), 0.5 wt.% Mn (c), and 0.5 wt.% Mo (d).

In the base alloy (Figure 2a), the microstructure appears relatively homogeneous with a characteristic grain morphology. The addition of chromium (Figure 2b) results in a noticeable reduction in grain size and improved microstructural uniformity, which can be attributed to the strengthening effect of this transition metal. Similarly, alloying with manganese (Figure 2c) promotes a more uniform grain distribution; however, the effect is less pronounced compared to chromium.

The most significant microstructural modifications are observed in the alloy containing molybdenum (Figure 2d). In this case, pronounced grain refinement and the formation of a denser and more homogeneous structure are evident, which positively affect the mechanical properties of the alloy, such as hardness and strength. These changes can be explained by the high efficiency of molybdenum in inhibiting grain growth and improving phase stability within the alloy.

Thus, the addition of transition metals—particularly molybdenum—contributes to the structural strengthening of the ZA15 alloy, which is consistent with the observed improvement in its mechanical characteristics.

Conclusion

The conducted studies made it possible to establish the regularities governing the influence of alloying elements Cr, Mn, and Mo on the electrochemical activity of the ZA15 alloy in neutral chloride solutions of varying concentrations. It was shown that modification of the alloy composition leads to changes in the kinetics of anodic processes and in the nature of surface oxide layer formation.

Comparative analysis demonstrated that the degree of improvement in corrosion resistance depends on the type of alloying element, with the most pronounced surface stabilization observed in alloys containing molybdenum. This indicates its high efficiency as a modifier of the passive state of the alloy.

The obtained results expand the understanding of the mechanisms of electrochemical stabilization in zinc–aluminum systems and may serve as a scientific basis for the targeted selection of alloying additions in the development of corrosion-resistant materials for structural and protective applications.

References:

1. Obidov Z.R. Thermophysical properties and thermodynamic functions of the beryllium, magnesium and praseodymium alloyed Zn-55Al alloy / Z.R.  Obidov // High Temperature. – 2017.– Vol. 55.– No. 1.– P. 150-153.
2. Ganiev I.N., Aliev D.N., Obidov Z.R. Effect of calcium additions on the anodic behavior of Zn5Al zinc-aluminum coating in NaCl medium // Reports of the Academy of Sciences of the Republic of Tajikistan. – 2008. – Vol. 51. – No. 9. – P. 691–695. (In Russian)
3. Maniram S.G., Singh G.M., Dahiya S., Sharma N.S. Effect of fly ash particles on the mechanical properties of Zn–22%Al alloy produced by stir casting method // IOSR Journal of Mechanical and Civil Engineering. – 2013. – Vol. 10. – No. 2. – P. 39–42. (In Russian)
4. Khakimov I.B., Ganiev I.N., Obidov Z.R., Rakhimov F.A. Effect of manganese additions on the anodic behavior of Zn22Al alloy in neutral medium // Bulletin of the Tajik National University. Series of Natural Sciences. – 2019. – No. 4. – P. 132–135. (In Russian)
5. Rahimov F.A., Obidov Z.R., Amini R.N., Ganiev I.N., Novozhenov V.A., Strucheva N.E. Potentiodynamic study of the anodic behavior of Mn-alloyed Zn5Al alloy // Izvestiya of Altai State University. – 2020. – No. 4 (114). – P. 53–58. DOI: 10.14258/izvasu(2020)4-08 (In Russian)
6. Obidov Z.R. Anodic behavior and oxidation of Zn5Al and Zn55Al alloys doped with strontium // Protection of Metals and Physical Chemistry of Surfaces. – 2012. – Vol. 48. – No. 3. – P. 305–308. (In Russian)
7. Mukhidinova M.M., Rahimov F.A., Obidov Z.R. Improvement of the anodic resistance of Mn-alloyed TsA15 zinc alloy // Universum: Technical Sciences. – 2025. – No. 3–4 (132). – P. 53–56. DOI: 10.32743/UniTech.2025.132.3.19635  (In Russian)