Electrochemical study of Zr–1Nb alloy with oxide coatings formed by plasma electrolytic oxidation

. Plasma electrolytic oxidation (PEO) coatings were formed on Zr–1Nb alloy in electrolyte containing 9 g/L sodium silicate, 5 g/L sodium hypophosphite, and 6 g/L submicron yttrium oxide powder at current densities 20, 30, and 40 A/dm 2 . The coatings surface morphology was studied by scanning electron microscope. The electrochemical behavior of coated and uncoated samples was investigated after 1- and 7-days exposure in 10 % HCl. The samples with PEO coatings formed at 30 A/dm 2 current density had the best corrosion protective properties after 1-day exposure in 10 % HCl. After 7-days exposure in 10 % HCl the samples with PEO coatings formed at 30 and 40 A/dm 2 current densities showed greater corrosion resistance then samples with PEO coatings formed at 20 A/dm 2 current density and uncoated samples.


Introduction
Plasma electrolytic oxidation (PEO) of zirconium and its alloys is the subject of many modern studies. PEO coatings are promising for the protection of reactor structural materials against corrosion, accelerated oxidation at high temperatures, embrittlement and absorption of oxygen and hydrogen [1,2]. Zirconium and its alloys with PEO coatings are also promising in orthopedics and dental prosthetics [3]. Due to the low thermal conductivity the possibilities of PEO coatings application as thermal barrier coatings are also considered [4][5][6].
PEO is electrochemical process using the energy of electrical microdischarges functioning on the surface of the materials being treated [7]. During PEO process electrolyte components can be incorporated into the coatings, forming oxides and various compounds with components of base material. Also, powders of insoluble compounds can be added in the slurry electrolytes to provide certain properties to the coatings: wear resistance, corrosion resistance, heat resistance, etc. For example, the addition of nanoparticles of such oxides as Al 2 O 3 , CeO 2 , and ZrO 2 in PEO electrolytes improves the corrosion resistance of zirconium alloys (up to 10 3 times compared with uncoated alloys) [1]. In present work PEO coatings were formed on Zr-1Nb alloy in electrolyte with yttrium oxide submicron particles addition. Yttrium oxide additives can increase the corrosion protective properties of the PEO coatings, as well as lead to tetragonal and cubic ZrO 2 phases stabilization in oxide layer on zirconium alloys that improve its thermal stability and hardness.

Experimental setup and characterization techniques
PEO coatings were formed on Zr-1Nb alloy in the electrolyte containing 9 g/L sodium silicate, 5 g/L sodium hypophosphite, and 6 g/L submicron yttrium oxide powder. Slurry electrolyte was treated for 3 min using a homogenizer at ultrasonic vibration frequency of 40 kHz to stabilize the suspension. Plasma electrolytic oxidation was carried out for 60 min at AC electrical mode and equal values of anodic and cathodic currents and sum current densities 20, 30, and 40 A/dm 2 .
Investigation of the surface morphology and thickness cross-sections of PEO coatings measurements were carried out using a Quanta 600 scanning electron microscope (SEM). The electrochemical behavior of uncoated and coated samples was investigated in 10 % HCl. Experimental curves were obtained by polarization from the cathodic to the anodic region with the sweep rate of 1 mV/s after 1-and 7-days exposure in 10 % HCl. The studies were carried out in standard threeelectrode cell with silver chloride (Ag/AgCl) reference electrode. Polarization was carried out using a PI-50-1 potentiostat.

Results and discussion
Cross-sections SEM study showed that PEO coatings average thickness formed at current density 20, 30 and 40 A/dm 2 is ~40, ~110 and ~170 μm with accordingly. The surface layer of PEO coatings is characterized by crater-like regions, regions of globular structure (figure 1, a) and incorporated in coating yttrium oxide submicron particles up to 300 nm in size ( figure 1, b). Figure 1b shows that the yttrium oxide particles quite uniformly cover the PEO coating surface, that can decrease open porosity. In addition, yttrium oxide can melt and form a solid solution with zirconia when it enter in areas of micro-discharges functioning, whose temperature reaches several thousand degrees. In this case the stabilization of the high-temperature zirconia phases occurs [8].   Thus, increasing of PEO process current density up to 30 and 40 A/dm 2 leads to higher corrosion protective properties of oxide coatings on Zr-1Nb alloy. It may be due to forming of denser barrier layer forming as a result of higher local temperatures in discharges during PEO process. Increasing of current density also leads to more intensive incorporation of submicron yttrium oxide particles into the PEO coating structure and stabilization of the tetragonal and cubic ZrO 2 phases as was shown in [8]. It was also reported in [1][2] that electrophoretic interaction could be responsible for the migration of yttrium oxide nanoparticles towards the anode during PEO process. Addition of yttrium oxide nanoparticles in PEO electrolyte leads to increasing of the corrosion resistance of coated zirconium alloys by several orders of magnitude [1]. The obtained in present work data suggest the similar effect for submicron particles.

Conclusions
PEO coatings were formed on Zr-1Nb alloy in the electrolyte containing 9 g/L sodium silicate, 5 g/L sodium hypophosphite and 6 g/L submicron yttrium oxide powder at current densities 20, 30 and 40 A/dm 2 . The corresponding average coating thicknesses were ~40, ~110 and ~170 μm. The electrochemical behavior of coated and uncoated samples was investigated after 1-and 7-days exposure in 10 % HCl. The samples with PEO coatings formed at 30 A/dm 2 had the best corrosion protective properties after the 1-day exposure in 10 % HCl. After 7-days exposure in 10 % HCl the samples PEO modified at 30 and 40 A/dm 2 showed greater corrosion resistance that may indicate the presence of denser barrier layers under PEO coatings and more intensive incorporation of submicron yttrium oxide particles into their structure compared with the coatings formed at 20 A/dm 2 .