Nitrogen transport through thermally grown chromia scales

Highlights: • In-situ high temperature XRD and thermal analysis (TG-DTA, dilatometry) were used. • A chromia scale free of physical defects was a barrier against nitridation. • Formation of physical defects and nitrides depended on pO₂ in the N₂-O₂ atmosphere. • Nitrogen transport through the chromia scale is based on molecular gas transport. • Scale "wrinkling" and "micro-cracking-healing" mechanisms are proposed for N₂ transport. 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.