Hydrogen Permeation in Nanostructured Bainitic Steel

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NANOSTRUCTURED bainitic steel is a new class of ultrahigh-strength steel, which has potential application in various industries, including oil and gas structures where improved safety standards are critical. The microstructure of nanostructured bainitic steel comprises a very fine bainitic ferrite and retained austenite layers. The fine bainitic laths along with retained austenite contributes to the excellent mechanical properties of the steel,[1] i.e., tensile strength of ~2.3 GPa, hardness of ~670 HV, and ductility in the range of 5 to 30 pct.[2–5] Our recent work showed that nanostructured bainitic steel exhibits higher corrosion resistance than martensitic steel in chloride-containing solution,[6] but interestingly the nanostructured bainitic steel also showed selective dissolution of the retained austenite phase.[7] However, in the oil industry cathodic protection technique is commonly used for corrosion protection of steels. Hence, the localized dissolution tendency of the nanostructured bainitic steel is not a major concern for potential application in oil pipelines or structures. But the hydrogen evolution that occurs during the cathodic protection technique could be detrimental if the hydrogen diffuses into the steel. Processes such as welding, electroplating, and exposure of steel to H2S sour environment can also be conducive to hydrogen diffusion in the steel.[8–11] Generally, hydrogen in its atomic form can diffuse through the grain boundaries or crystal lattice, and the

OLUWOLE KAZUM, Ph.D. Student, YINGHE HE, Professor, and M. BOBBY KANNAN, Associate Professor, are with the College of Science, Technology and Engineering, James Cook University, Townsville, QLD 4811, Australia. Contact e-mail: bobby.mathan@ jcu.edu.au HOSSEIN BELADI and ILANA TIMOKHINA, Senior Research Fellows, are with the Institute for Frontier Materials, Deakin University, Waurn Ponds, VIC 3216, Australia. Manuscript submitted April 27, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A

hydrogen concentration as low as a few parts per million can lead to hydrogen embrittlement (HE) in materials experiencing tensile stress.[9,10,12–14] It is also well documented in the literature that high hydrogen diffusivity in high-strength structural materials is often responsible for the catastrophic form of failures.[15–19] Adsorption of hydrogen atoms followed by their absorption, through the chemical reactions 1 and 2 shown below, to stress-concentrated regions of materials (e.g., crack sites or voids) would reduce their mechanical properties.[20–22] Hþ þ e ! Hadsorbed

½1

Hadsorbed ! Habsorbed

½2

It is well known that microstructure plays a vital part in the HE susceptibility of high-strength steels.[15,23,24] Grain size, grain boundaries, dislocation density, precipitates, inclusions, interface, and the type and amount of the phases present can influence hydrogen diffusivity.[21,25–27] Hydrogen diffusion through grain boundaries is generally higher than that of grain lattice.[21] However, depending on the grain size, the grain boundaries can faci

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