High-temperature oxidation behaviors of plasma electrolytic oxidation coating on hot-dip aluminized HP40Nb alloy

Abstract

In the current investigation, HP40Nb heat-resistant alloy was successfully aluminized. The procedure of aluminizing consisted of pack aluminizing and subsequent hot-dip aluminizing (HDA) process. Plasma electrolytic oxidation (PEO) ceramic coatings were also synthesized on the surface of hot-dip aluminized HP40Nb alloys. PEO coatings were made using aluminate (mixture of 2 g/L KOH, 5 g/L NaAlO2, and deionized water) and silicate (mixture of 2 g/L KOH, 5 g/L Na2SiO3, and deionized water) electrolytes at 440 V for 10 min. To evaluate the high-temperature oxidation behavior of PEO coatings, interrupted isothermal oxidation at 1100 °C for 100 h was employed. The surface and cross-sectional morphologies before and after high-temperature oxidation test, chemical composition, and phase identification of the coatings were characterized by field emission scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. The results revealed that the PEO coating using aluminate electrolyte could increase the oxidation resistance of HP40Nb alloy, more than pack aluminized, hot-dip aluminized samples, and also the PEO coating using silicate electrolyte. The PEO coating using silicate electrolyte contains relatively big surface cracks, which can be responsible for oxide spallation during high-temperature oxidation. The weight gain of samples shows that oxidation resistance of hot-dip aluminized specimen, improved 1.2 times by PEO treatment in aluminate electrolyte. The Kirkendall voids may have a deteriorated effect on the anti-oxidation performance of hot-dip aluminized and PEO coated samples.