Aqueous corrosion and environmentally assisted cracking characteristics of iron aluminides

This research focused on characterizations of iron aluminides with respect to aqueous corrosion and environmentally assisted cracking as a function of environment, alloying additions, and thermomechanical processing methods. Electrochemical and immersion tests were used to study the uniform and localized corrosion resistance. The electrochemical results, including cyclic polarization, Tafel extrapolation, and polarization resistance techniques, indicated the beneficial effect of Cr and Mo on both types of corrosion. Immersion tests confirmed the electrochemical results and revealed the time dependence of localized corrosion behavior. The crevice corrosion behavior of iron aluminides in Cl^--containing solutions was investigated by using electrochemical and immersion crevice tests. The difference between crevice and pitting susceptibilities was minimal for the Mo-containing iron aluminide, but significant for the iron aluminides without Mo. X-ray photoelectron spectroscopy (XPS) analyses of surfaces passivated in a mild acid-chloride solution revealed that the passive films of iron aluminides were mainly composed of oxidized aluminum (AI₂O3) which, depending on the alloy, coexisted with chromium oxide/hydroxide (Cr₂O₃/Cr(OH)₃), oxidized molybdenum (MoO₃), and niobium (NbO₅). The XPS results also indicated the role of Mo in improving the localized corrosion resistance of the Cr-containing iron aluminides. Mo facilitated the formation of a high Al content in the passive film and impeded the adsorption of Cl^- on the surface. The susceptibilities to stress corrosion cracking (SCC) and hydrogen embrittlement cracking were also investigated using U-bend and slow-strain-rate tests. U-bend SCC evaluations were conducted on two iron aluminides in acid-chloride (pH=4, 200 ppm Cl^-), thiosulfate, and tetrathionate solutions at the freely-corroding conditions. Cracking failures occurred in the thiosulfate and tetrathionate solutions, but not in the acid-chloride solution. To investigate the effect of applied potential on the cracking behavior, U-bend tests were conducted in the acid-chloride solution at an anodic pitting potential and at cathodic hydrogen-evolution potentials. Cracking occurred only at the highly negative cathodic potentials. These results indicated that the cracking mechanism was related to hydrogen embrittlement. Slow-strain-rate tests were performed in the acid-chloride solution. The effects of applied potential and strain rate on the cracking behavior were investigated. The slow-strain-rate ductility was found to decrease significantly with either: (1) the onset of pitting corrosion (high anodic potential), or (2) the production of hydrogen (very negative cathodic potentials). It was concluded that the cracking mechanism was related to anodic dissolution, i.e., the effects of pitting corrosion, at the high anodic potential and to hydrogen embrittlement at the very negative cathodic potentials. Metallographic examinations by scanning electron microscopy revealed that decreasing Cr content increased the proportion of transgranular cleavage cracking and decreased the proportion of intergranular cracking.