Promotion of α-Al2O3 formation on an Ni-Al alloy using a Ni-Fe2O3 nano-composite seeding layer

Thin seeding layers of Ni and Ni-Fe2O3 composites were successfully formed on an Ni50Al alloy using the pulse electrodeposition method (PED) from a Watts bath in order to promote α-Al2O3 formation for improving the high temperature oxidation resistance of the Ni50Al alloy. The conditions for the PED and the effects of these seeding layers on Al2O3 scale formation on the Ni50Al alloy were investigated. The coated samples showed thicker oxide scales than bare Ni50Al samples in oxidation tests at 1000 °C in air. At the initial stage of oxidation (1 h), a single layer scale composed of blade-like crystals of metastable θ-Al2O3 was formed. The oxide scales formed on the PED coated samples had a multilayered structure consisting of an inner layer of either a mixture of θ-Al2O3 and α-Al2O3 or of single-phase α-Al2O3, intermediate NiAl2O4 layer and an outer NiO layer. The Ni-seeding layer was found to delay the phase transformation of θ-Al2O3 to α-Al2O3 to 100 h of oxidation. Addition of 3.1% Fe2O3 to the Ni-seeding layer accelerated the transformation of θ-Al2O3 to α-Al2O3 after 9 h of oxidation. Further addition of Fe2O3 nano-powder (5.2%-7.4%) to the Ni-seeding layer successfully suppressed the θ-Al2O3 formation even at 1 h of oxidation. The Al2O3 phase structure and thickness of the oxide scales were found to be considerably affected by these seeding layers via two main factors: (i) formation of NiAl2O4 and (ii) the effect of Fe2O3 to accelerate the transformation of metastable Al2O3 to stable α-Al2O3 or to suppress the formation of metastable Al2O3.