Breakaway Oxidation of TP310S Stainless-Steel Foil Initiated by Cr Depletion of the Entire Specimen in a Simulated Flue-Gas Atmosphere

Abstract

TP310S (25wt.% Cr–20% Ni–Fe) sheet and foil specimens of 0.11–1.20 mm thick were oxidized at 700–1000°C for 500 hr and 1050°C for 242 hr in a 3vol.%O₂–9%CO₂–16%H₂0–N₂ gas atmosphere. Except for specimen edges and corners, significant breakaway oxidation resulted for 0.11 mm thick specimens heated at 1000 and 1050°C. For specimens of more than 0.22 mm thick, protection by a Cr₂O₃ scale occurred and breakaway oxidation did not take place. Above 1000°C, the oxidation resistance of thin-foil specimens 0.11 mm thick was appreciably worse than the “thick” sheet specimens. Electron-probe microanalysis (EPMA) depth-profile measurements of Cr have demonstrated that the breakaway oxidation which occurred on the 0.11 mm-thick specimens was initiated upon complete removal and virtually zero content of soluble Cr in the underlying base metal. For the TP310S steel foil specimens, protection by a Cr₂O₃ scale was effective until the concentration of soluble Cr in the base metal decreased to zero percent. The mechanism of the breakaway oxidation observed for the TP310S steel foils was interpreted as “chemical failure” of the Cr₂O₃ scale, rather than the well-known mechanical failure of protective Cr₂O₃ scales.