Comparison of plasma electrolytic oxidation of zirconium alloy in silicate- and aluminate-based electrolytes and wear properties of the resulting coatings
Graphical abstract
Highlights
► PEO processes in silicate and aluminate electrolytes are compared for Zircaloy-2. ► Significant different behaviors are observed for the PEO in the two electrolytes. ► Superior wear resistance was observed for the coating from aluminate electrolyte. ► The coatings from aluminate electrolyte possess much higher t-ZrO2 content. ► The results show that Al2O3 is a more efficient tetragonal ZrO2 stabilizer.
Introduction
Plasma electrolytic oxidation (PEO) is a surface treatment process that has been successfully used to improve the corrosion and wear resistances of aluminium, magnesium and titanium by formation of functional ceramic coatings [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. However, the process is also attracting research interest for the surface treatment of a broader range of materials, for instance iron-based [11] and zirconium-based [12], [13], [14] alloys, although for these materials the knowledge base is much less extensive. Zirconium alloys are commonly used as structural materials for water-cooled nuclear power reactors, where they are subject to several types of corrosion [15], [16], [17], [18], [19]. They are also potential biomaterials. Recent studies have shown that PEO treatments can enhance both the corrosion resistance [12], [13], [14] and the biocompatibility of the alloys [20], [21].
PEO coatings on zirconium alloys are composed mainly of zirconium oxide, which is known to have chemical and thermal stability, mechanical strength and wear resistance that make it attractive as an engineering material [22]. The properties of the coatings are strongly dependent on the zirconia phases that are present, which may be greatly affected by the conditions of PEO, including the electrical regime and the electrolyte composition. Among the wide range of electrolytes that is available for PEO, silicate-, phosphate- and aluminate-based ones are commonly used [4], [7], [12], [13], [14], [23], [24], [25], [26]. In previous papers of the present authors [27], [28], [29], the microstructures and phase compositions of PEO coatings formed on zirconium alloys in silicate electrolyte were studied in detail. The findings revealed coatings containing monoclinic (m) and tetragonal (t) zirconia, with the latter being enhanced by the presence of silicon species that are incorporated into the coating from the electrolyte. However, the research on PEO of zirconium alloys in aluminate-based electrolyte is less extensive, with only limited reports [30], [31]. In this paper, the PEO behaviors of Zircaloy-2 in silicate- and aluminate-based electrolytes, and the wear properties of the resultant coatings, are compared for the first time, the results showing large difference in the kinetics of coating formation and in the performance of the coatings.
Section snippets
Experimental
Zircaloy-2 alloy, in the form of rolled plate, was cut and mounted in resin to provide specimens with a working area of 1 cm × 2 cm. The configuration of the experimental setup is the same as described in a previous paper [29]. The compositions of the electrolytes used for the PEO treatments were 8 g l−1 Na2SiO3·9H2O + 1 g l−1 KOH and 6 g l−1 NaAlO2 + 8 g l−1 Na4P2O7·10H2O + 5 g l−1 KOH. The electrical regime was a pulsed bipolar waveform, using a duty cycle of 20% at 1000 Hz [29], with the positive and negative
Cell potential–time responses and spark appearances during formation of coatings
The cell potential–time responses during the PEO treatment in the silicate and aluminate electrolytes are presented in Fig. 1(a). The cell potentials depicted for each electrolyte are the peak positive and negative values of the waveform. A clear difference is exhibited between the curves for the two electrolytes. In the silicate electrolyte, the positive potential rises quickly as a barrier layer is established on the alloy. It then continues to rise gradually, reaching ∼580 V at the
Conclusions
The PEO processing of Zircaloy-2 under the present pulsed current conditions results in different behaviors with respect to coating kinetics and properties in silicate and aluminate-based electrolytes. In the silicate electrolyte, thick coatings are obtained after 1800 s of treatment. However, in the aluminate-based electrolyte, the coating suffers mechanical breakdown after ∼600 s, which correlates with a changing nature of discharges.
The dissolution of the zirconium during PEO in the
Acknowledgements
This work was supported by the National Science Foundation of China (Project 51071066) and Project of the development of youth teachers of Hunan University.
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