The Inhibition of Hydrogen Embrittlement in SAE 4340 Steel in an Aqueous Environment with the Rare Earth Compound Lantha
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INTRODUCTION
CHROMATES have long been considered one of the most effective corrosion inhibitors. However, because of significant environmental and health concerns,[1] there has been considerable focus on nontoxic alternatives such as rare earth metal compounds.[2–4] In 1988, Hinton et al.[5] showed that the addition of 100 ppm of cerium chloride to flowing tap water reduced the corrosion rate on mild steel by a factor of 10. In unrelated work, the inhibiting properties of organic salts were also investigated by Mercer,[6] who demonstrated that the alkali metal salts of carboxylic acids such as cinnamates effectively inhibited the corrosion of mild steel. Forsyth and co-workers considered these unrelated findings and synthesized compounds consisting of an organic anion (e.g., carboxylate ion) and a rare earth metal cation that could produce a synergistic inhibiting effect.[7–13] In particular, Blin et al.[7,10] and Behrouzvaziri et al.[13,14] showed using electrochemical studies that lanthanum 4-hydroxy cinnamate (La(4OHcin)3), when present at levels up to 900 ppm in 0.01M NaCl, significantly increased the polarization resistance of mild steel with exposure time compared to the control condition. The inhibitor also prevented rusting and pitting during a 7 day immersion test. Previous studies by Gerrard et al.[15,16] with the highstrength low-alloy tempered martensitic SAE 4340 steel B.R.W. HINTON, Adjunct Professor, is with the Department of Materials Engineering, Monash University, Wellington Road, Clayton, VIC 3800, Australia, and is also with the Institute for Technology, Research and Innovation, Deakin University, Burwood Highway, Burwood, VIC 3125, Australia. Contact e-mail: bruce.hinton@monash. edu M. BEHROUZVAZIRI, Student, and R.K. GUPTA, Research Fellow, are with the Department of Materials Engineering, Monash University. M. FORSYTH, Professor, and M. SETER, Research Fellow, are with Institute for Technology, Research and Innovation, Deakin University. P.G. BUSHELL, Scientist, is with the Department of Defence, Defence Science and Technology Organisation, Maritime Platforms Division, Lorimer St., Fishermans Bend, VIV 3207, Australia. Manuscript submitted May 9, 2011. Article published online March 27, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
showed that this steel is very susceptible to corrosion when immersed in NaCl solutions at pH values between 4 and 9, with pits developing in conjunction with a redorange rust film. It has long been recognized that in aqueous environments under freely corroding conditions, such steels are susceptible to hydrogen embrittlement (HE).[17–19] The role of the environment is to allow cathodic reactions such as the reduction of hydrogen (H) ions or the reduction of water to occur, which results in the generation of H atoms at the steel surface and their subsequent diffusion into the steel.[20] Once in the steel, under load, H will diffuse to local stress raisers such as surface irregularities (scratches, notches), changes of section, corrosion damage including pits, and certai
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