Efficacy of an external chromia layer in reducing nitridation of high temperature alloys

Six high temperature alloys have been exposed in N2/H2 environments at 900 °C. In order to study the efficacy of a chromia barrier layer against nitrogen ingress, experiments were performed in two environments having the same N2/H2 ratio but slightly different water content, chromia formation being spontaneous in one case only. The samples were evaluated by SEM/STEM/EDX, XRD, gravimetry and GD-OES. The presence of an external chromia scale reduced nitridation of the alloy by 50–95%. Furthermore, in the presence of a continuous alumina layer no nitridation of the alloy was detected.