Stress Corrosion Cracking of Fe₃Al and Fe-16% Al Alloys

The stress corrosion cracking (SCC) susceptibilities of an Fe₃Al-based iron aluminide (Fe-28% Al-2% Cr [at%]) and a lower-Al disordered Fe-Al alloy (Fe-16% Al-5% Cr-1% Mo [at%]) were investigated using U-bend and slow strain rate tests (SSRT) in a mild acid-chloride solution (200 ppm Cl⁻ [5.5 x 10⁻³ M sodium chloride, NaCl], pH = 4 [6.3 x 10⁻⁵ M sulfuric acid, H₂SO₄]). U-bend tests were conducted at anodic pitting potentials, at freely corroding open-circuit potentials (E_corr) and at cathodic hydrogen evolution potentials. For the higher-Al iron aluminide, cracking occurred within 200 h only at the highly negative cathodic potentials. These results indicated the cracking mechanism was related to hydrogen embrittlement (HE). For the lower-Al Fe-Al alloy, cracking did not occur at any of the potentials. Slow strain rate ductilities decreased significantly for both alloys with the onset of pitting corrosion (high anodic potentials) or the production of hydrogen (very negative cathodic potentials). Ductilities of the lower-Al alloy were much higher than those of the iron aluminide at the freely corroding potentials and hydrogen-evolution cathodic potentials. It was concluded that the cracking mechanisms were related to anodic dissolution (i.e., the effects of pitting corrosion at the high anodic potential and to HE at the very negative cathodic potentials) and that the lower-Al, disordered Fe-Al alloy was more resistant to HE than the Fe₃Al-based iron aluminide.