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Aging of graphite-based materials results in slower heterogeneous electron transfer (HET) kinetics and limits their conducting and catalytic performance. A thermal treatment protocol reportedly maintained a high HET kinetics of graphite by at least nine weeks. The present report investigates the consequences of such HET increases on graphite stabilities. Raman 2D peak evolution, potential windows, EIS, optical AFM, chronoamperometry and potentiodynamic polarization studies were employed to investigate electrochemical, structural and corrosion stabilities and mechanisms of the pristine and thermally treated graphite as they relate to its HET kinetics. Structure-induced electronic changes are suggested to trigger improved tunneling paths through the interfaces from the bulk layer. This tunneling improves and sustains HET kinetics by orders of magnitude in the treated graphite. Except for the graphite fibers, thermal treatment increased HET kinetics without compromises to electrochemical, structural and corrosion stabilities. In most instances, these properties improved over the pristine graphite. Thermal treatment transforms the pristine graphite into well-ordered lattices. AFM images show visual indifference between the pristine and treated graphite. However, their electronic structures and resultant electrochemistry are different. Despite minimal energy differences between both structures, a large thermodynamic barrier hinders the reversal of the treated to the pristine-like state.

Iron (Fe) and chromium (Cr) powders at various concentrations of Cr (6, 9, 12 and 14 wt%) were mechanically alloyed via high-energy ball milling for 10 h. The milled Fe–Cr alloy powders were analyzed using scanning electron microscopy (SEM) and X-ray diffraction (XRD) for understanding the impact of varying chromium content on the characteristics of the milled powder. The powder samples were then consolidated via spark plasma sintering (SPS) at 1000 °C for 45 min at 80 MPa pressure. The density of the consolidated samples was measured and found to reach a maximum of 98%. Microstructural characterization of the spark plasma sintered samples was performed using SEM and XRD. Electrochemical corrosion tests were performed on spark plasma sintered samples in 0.5 M H2SO4, 0.05 M NaH2PO4 and 1 M NaOH solutions using potentiodynamic polarization, electrochemical impedance spectroscopy and Mott–Schottky analysis. The corrosion behavior of the spark plasma sintered materials was compared with that of a wrought HT-9 steel (12Cr based ferritic–martensitic steel). The work presented here is a comparative study of microstructural evolution and corrosion behavior between the different spark plasma sintered Fe–Cr model alloys and wrought HT-9 steel.

Alloy-22 (UNS N06022), a Ni-Cr-Mo-W based alloy shows excellent corrosion resistance in both oxidizing and reducing environments and is a candidate material for fabrication of outer wall of nuclear waste containers [1–3]. This alloy is considered, as readily weldable [3–6]. In solution-annealed condition, wrought Alloy22 reveals a single-phase austenite structure without any grain boundary secondary phase precipitates [1, 2]. Topologically close packed (TCP) phases such as μ, σ , and P, which are stable at higher temperatures, were observed to be retained at room temperature as terminal solidification constituents during solidification of weld metal [6]. As the chemical composition of these TCP phases is rich in Mo and W, presence of these phases could result in localized depletion of alloying elements in the matrix and make the alloy more susceptible to corrosive attack. Moreover, the weld microstructure revealed segregation of Mo and W at the interdendritic regions [3, 4]. Multipass welding and post weld heat treatment could increase the quantity of secondary phase precipitation. As the corrosion and toughness properties are affected by the presence of TCP phases and segregation of alloying elements [3], it is important to quantify these microstructural changes in comparison with that of solution annealed condition. A rapid nondestructive testing technique such as electrochemical potentiokinetic reactivation (EPR) test could be an appropriate tool for such microstructural evaluation, especially in-situ service conditions. EPR test has been successfully applied for quantifying the degree of sensitization in austenitic and duplex stainless steels [7, 8]. There is no standard EPR test procedure available for detecting intergranular corrosion susceptibility of NiCr-Mo alloys. Recently the present authors reported a test procedure to detect Cr depletion in thermally aged Alloy-22 wrought materials using an electrolyte of 1 M H2SO4 + 0.5 M NaCl + 0.01 M KSCN [9]. However, this test solution could not reactivate the weld metal, as the Cr profile was not affected by the solidification structure and the TCP phases [4]. In this short communication, development of a testing procedure, which could detect the presence of Mo and W depleted regions in Alloy-22 welds, is reported. 38 mm thick Alloy-22 (UNS N06022) plates (chemical composition 0.002% C, 0.14% Mn, 0.009% P, 0.001% S, 0.04% Si, 59.2% Ni, 20.62% Cr, 13.91% Mo, 0.01% Co, 0.01% Cu, 2.68% W, 2.8% Fe and