Nitridation Resistance of Several Modern High Temperature Ni- and Co-Based Alloys

High temperature nitridation is a high temperature materials degradation process that can occur in air and in environments containing nitrogen or ammonia. A review by Haynes International showed that nitridation data of commercial alloys in nitrogen or ammonia at temperatures near 871°C and above is limited. Five modern commercial high temperature Ni- and Co-based alloys and one new alumina forming alloy were investigated for their high temperature nitridation resistant behaviors. The commercial alloys included four chromia forming alloys, N06230, R30188, N06625 and N06617, and two alumina forming alloys, N07214 and a recently developed NiCoCrMoAl alloy. High temperature tests were conducted for 1,008h cycled weekly in ultra-high purity nitrogen gas at 871 and 982°C. The results showed that the nitridation response was not the same when comparing the chromia and alumina forming alloys. Chromia or alumina scales formed on these alloys. The protective oxides formed during the tests and the oxides provide protection from degradation in the nitrogen atmosphere. Alumina scales exhibited excellent nitridation resistance in all tests, and pre-oxidation to form an alumina scale was proven to be very effective to improve nitridation resistance. Chromia scales were less protective, particularly at 982°C. The nitridation behaviors of these alloys are analyzed and discussed.