ANODIC BEHAVIOR OF NEODYMIUM-DOPED Zn22Al ALLOY IN A CORROSIVE HCl ENVIRONMENT

ABSTRACT

The effect of neodymium alloying additives on the anodic behavior of the binary Zn22Al alloy in various HCl-containing corrosive environments was studied using a potentiostatic method. It was found that stabilization of the anodic corrosion rate during polarization is achieved for low-alloy alloys. Increased anodic resistance of the alloys was observed in lower HCl concentrations. It was found that all neodymium additives (0.01-1.0%) to the Zn22Al alloy demonstrate the potential to improve anodic resistance by reducing the corrosion rate. It has been analyzed that by adding neodymium of varying concentrations to the Zn22Al alloy, electrochemical heterogeneity increases and corrosion resistance is determined by the amount and nature of the alloying element.

АННОТАЦИЯ

Потенциостатическим методом было изучено влияние легирующих добавок неодима на анодное поведение бинарного сплава Zn22Al в различных коррозионных средах HCl. Выявлено, что стабилизация скорости анодной коррозии в ходе поляризации достигается для низколегированных сплавов. Зафиксировано, что повышение анодную стойкость сплавов достигается в пониженной концентрационной среды HCl. Установлено, что все добавки (0,01-1,0%) неодима к сплаву Zn22Al показывают возможность повышения анодную стойкость за счет уменьшение скорость коррозии. Проанализировано, что при добавлении неодима различной концентрации в сплаве Zn22Al электрохимическая неоднородность повышается и коррозионностойкость устанавливается количеством и природой легирующего элемента.

Keywords: Zn22Al alloy, alloyed, neodymium, anodic behavior.

Ключевые слова: сплав Zn22Al, легирование, неодим, анодное поведение.

Introduction.

Significant savings in zinc when using Zn-Al coatings [1–3], associated with the lower density of aluminum [4–10], as well as their higher corrosion resistance under various conditions [11–14], led to the development of an industrial technology for producing steel sheet with a hot Zn-Al coating [15–20].  Trace, alkaline earth, and rare earth metals contribute to the structure and properties of Zn-Al alloys. It is known that small additions can significantly improve the properties of these alloys [21–26].

The practical use of anodic coatings for protecting metal structures from corrosion depends on the structural features of the alloys, the surface condition, the temperature, and the properties of the alloy itself. However, this poses the risk of passivation of the aluminum component of the alloy. In this regard, the possibility of activating this alloy by introducing microadditives of a third component into its composition has been considered [27–35].

The aim of the study is to investigate the effect of neodymium alloying additives on the anodic behavior of Zn22Al alloy in a corrosive HCl environment.

Materials and methods.

Granulated zinc grade HC, aluminum grade A7 and aluminum-neodymium master alloys (10 wt.% Nd) were used as starting materials. Alloys were produced from these metals in corundum crucibles in an electric resistance furnace (SShOL) at temperatures ranging from 700 to 850°C. Rods with a diameter of 8 mm and a length of 140 mm were cast from each melt into a graphite mold.

The anodic behavior of the Zn22Al alloy with Nd alloying additives in 0.001, 0.01, 0.1N HCl environments was studied using the potentiostatical method. The reference electrode was silver chloride, and the auxiliary electrode was platinum. The studies were conducted using a PI-50-1.1 potentiostat and an LCD-4-002 recorder at a potential scan rate of 2 mV/s according to the methods described in the works [14–16].

Results and discussion.

The anodic branches of the potentiodynamic polarization curves for neodymium-containing alloys (0.01–1.0 wt.%) lie below the curves for Zn22Al (curve 1), indicating a decrease in anodic dissolution of the alloys upon alloying. Stabilization of the anodic corrosion rate during polarization is achieved for low-alloy alloys. The dissolution current density from the passive state of the Er-containing alloy is lower than that of the Zn22Al alloy (Figure 1).

Figure 1. Anode curves of Zn22Al-Nd alloys in an acidic environment

The presented results show that the corrosion potentials of the alloys shift toward positive values ​​over time. This potential of the Zn22Al alloy, when alloyed with neodymium, shifts both positively and negatively as the chloride ion concentration in acid solutions increases. The dynamics of changes in corrosion potentials has a positive effect on the parameters of the anodic behavior of alloys in general (Table 1).

Table 1.

Corrosion potentials (E, V) of Zn22Al-Nd alloys

Analysis of the results obtained shows that for both the unalloyed and alloyed alloys, stabilization of the corrosion potential is observed within 26-29 minutes from the start of the process. The greatest potential shift is observed for the alloy containing 0.1% neodymium. The corrosion potential value also changes depending on the HCl environment. As the concentration in an acidic environment increases, a decrease in the anodic resistance of the alloys is observed (Table 1).

Conclusion.

Overall, the presented data on the stabilization of corrosion potential values ​​under changing external conditions demonstrate its higher reproducibility compared to Zn22Al and neodymium-doped alloys, and their potential use as protective coatings for steel. The corrosion products of the alloys serve as a criterion for their protective properties. The repassivation potential is more stable and will depend less on the acid solution composition and the alloy surface condition than the pitting potential. It should be noted that determining corrosion potentials is more important for practical purposes, as it determines the applications of alloys.

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