Development of plasma electrolytic oxidation coatings on Ti-42Al-2V-2Nb alloy

Abstract

Ti-Al intermetallics based on their low density (3.9–4.7 g/cm3), high melting point (>1450 °C), and high ignition resistance were selected as the optimal materials for the aerospace applications replacing Ni-based superalloys. The current study aims at enhancing the performance of Ti-42Al-2V-2Nb alloy in the corrosive marine environment by developing plasma electrolytic oxidation (PEO) coatings. Three different electrolytes (5 g/l Na2SiO3 + 2.5 g/l KOH, 5 g/l Na3PO4 + 2.5 g/l KOH and 2.5 g/l Na2SiO3 + 2.5 g/l Na3PO4 + 2.5 g/l KOH) were selected for developing the coatings. PEO coatings were developed at a current density of 170 mA/cm2 over a processing time of 12 min. The phase composition and wettability of the obtained PEO coatings were studied by the X-ray diffraction (XRD) and contact angle goniometric studies. XRD results indicated the formation of the ceramic Al2TiO5 phase. Potentiodynamic polarization (PDP) and electrochemical impedance spectroscopic (EIS) studies were carried out in simulated marine condition using 3.5 wt% NaCl solution to analyze the performance of the samples in the corrosive medium. PDP and EIS results revealed that the PEO coated samples exhibited significant enhancement in corrosion properties compared to the uncoated substrate on the account of ceramic oxide layer formation. PDP studies unveiled the successful shielding of the Ti-42Al-2V-2Nb sample from pitting was obtained by developing the PEO coatings. The sample PEO coated using phosphate electrolyte exhibited superior corrosion resistance (icorr = 1.75 × 10−6 mA/cm2) compared to the other PEO coated samples attributing to the higher thickness (12.7 ± 0.5 µm) and higher contact angle (89°).