STRESS-CORROSION CRACKING OF FE3AL AND FE-16-PERCENT 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-A l alloy (Fe-16% Al-5% Cr-1% Mo [at%]) were investigated using U-bend a nd 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 pittin g potentials, at freely corroding open-circuit potentials (E-corr) and at cathodic hydrogen evolution potentials. For the higher-ill iron al uminide, cracking occurred within 200 h only at the highly negative ca thodic 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 rare duct ilities decreased significantly for both alloys with the onset of pitt ing corrosion (high anodic potentials) or the production of hydrogen ( very negative cathodic potentials). Ductilities of the lower-ill alloy were much higher than those of the iron aluminide at the freely corro ding potentials and hydrogen-evolution cathodic potentials. It was con cluded that the cracking mechanisms were related to anodic dissolution (ie., the effect of pitting corrosion at the high anodic potential an d to HE at the very negative cathodic potentials) and that the lower-A l disordered Fe-Al alloy was more resistant to HE than the Fe3Al-based iron aluminide.