Studies of initial oxidation of nickel–chromium alloys: Surface annealing by hydrogen ion bombardment

Nickel‐18% chromium binary alloy and alloy 600 (77% Ni, 16% Cr, 8% Fe) have been oxidized at 600 °C for periods of several minutes under O₂ pressures of <10⁻⁶ to 10⁻⁴ Torr O₂. The polycrystalline surfaces were pretreated by ion bombardment with 5 kV hydrogen ions. The oxide structure developed thereafter is found to be strongly influenced by grain orientation. One major grain orientation produced a chromium‐rich surface oxide, while a second orientation gave rise to a surface oxide with a high nickel concentration. On some grain surfaces, a repetitive pattern of nodule oxide growth was observed with a structural pattern which is peculiar to a specific grain orientation. On surfaces remote from H⁺ bombarded areas, initial surface oxide composition is more governed by local stresses which develop along scratches. It is argued that alloy surface pretreatment with H⁺ anneals local mechanical stresses and allows substrate crystallographic orientation effects to become important.