Investigation on effect of Hafnium addition on electrical and isothermal oxidation resistance of Ni-super alloy using for metallic interconnect of SOFC

Ni-Cr-Al-Hf alloys containing 0, 0.1, 0.2 and 0.4 wt.% Hf were produced and their isothermal oxidation behavior and electrical resistance has been investigated in air for 75 h at the temperature of 1000 °C. Microstructures of the oxidized samples were examined using scanning electron microscopy (SEM) and electrical resistance was measured. The samples showed different microstructures, and oxidation behavior depending on their chemical composition. The results indicated that the sample containing 0.4 wt.% Hf had the lowest weight gain, while the weight gain of the sample without Hf was the highest. An external scale of spinel overlying a region of internal oxides precipitates formed on Ni-12Cr-4.5Al. Conversely, an external Al2O3 formed on the samples containing Hf additions. In absence of Hf, Cr2O3 was the major scale that formed on surface. The improvement in the oxidation resistance is believed to be due to the transition between the internal and external oxidation of aluminum, adherent and protective Al2O3 oxide layer on the surface of the Ni-12Cr-4.5Al-0.4Hf alloy. It was found that the scale adhesion can be affected by mechanical keying at the alloy/scale interface resulting from the pegs’ formation during oxidation. Higher electrical resistance of Alloy without Hf is due to higher thickness of oxide scale formation on surface and sample with 0.4 wt.% Hf showed lowest electrical resistance due to thinner scale of Al2O3.