Nitrogen transport through thermally grown chromia scales

Abstract

Nitrogen transport through the chromia scale formed on chromium in N2-O2 atmospheres is studied. In-situ high-energy synchrotron X-ray diffraction (HEXRD) and two-stage thermal analysis results indicated that a scale, free of physical defects, protects the substrate from nitridation. The oxygen activity (pO2) in the nitrogen containing atmosphere played a significant role in scaling kinetics, physical defect formation, and nitrogen transport behavior. Low pO2 atmospheres (pO2 ≈ 10−6 atm) provided slower scaling kinetics, fewer physical defects, and a barrier character against nitrogen transport compared to the high pO2 atmospheres such as air. Two molecular nitrogen transport mechanisms based on “wrinkling” and “micro-cracking-healing” are proposed and discussed.