Sandia National Laboratories is a multi-mission laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. DISCLAIMER This information was prepared as an account of work sponsored by an agency of the U.
A stress-modified, critical-strain model of fracture-initiation toughness has been adapted to the case of hydrogen-affected pearlite shear cracking, which is a critical event in transverse fracture of colddrawn, pearlitic steel wire. This shear cracking occurs via a process of cementite lamellae failure, followed by microvoid nucleation, growth, and linkage to create shear bands that form across pearlite colonies. The key model feature is that the intrinsic resistance to shear-band cracking at a transverse notch or crack is related to the effective fracture strain at the notch root. This fracture strain decreases with the logarithm of the diffusible hydrogen concentration (C H ). Good agreement with experimental transverse fracture-initiation-toughness values was obtained when the sole adjustable parameter of the model, the critical microstructural dimension (l*), was set to the mean dimension of shearable pearlite colonies within this steel. The effect of hydrogen was incorporated through the relationship between local effective plastic strain ( f eff ) and C H , obtained from sharply and bluntly notched tensile specimens analyzed by finite-element analysis (FEA) to define stress and strain fields. No transition in the transverse fracture-initiation morphology was observed with increasing constraint or hydrogen concentration. Instead, shear cracking from transverse notches and precracks was enabled at lower global applied stresses when C H increased. This shear-cracking process is assisted by absorbed and trapped hydrogen, which is rationalized to either reduce the cohesive strength of the Fe/Fe 3 C interface, localize slip in ferrite lamellae so as to more readily enable shearing of Fe 3 C by dislocation pileups, or assist subsequent void growth and link-up. The role of hydrogen at these sites is consistent with the detected hydrogen trapping. Large hydrogen-trap coverages at carbides can be demonstrated using trap-binding-energy analysis when hydrogen-assisted shear cracking is observed at low applied strains.
The issue of safe cathodic protection (CP) limits for prestressing steel in concrete was addressed in regard to concerns over hydrogen embrittlement (HE). The local environment at the steel-concrete interface was found to vary as a function of vertical position within a laboratory-scale marine bridge piling. Embedded pH electrodes indicated the pH within a steel crevice embedded within a concrete piling decreased from 11.5 to 6.5 in the atmospheric zone 30.5 cm (12 in.) above the water line. Hydrogen permeation was detected using embedded sensors at applied potentials (E app ) more positive than the reversible potential for hydrogen production calculated for alkaline pore solutions (pH > 12.6). A safe limit based on the reversible electrode potential (REP) would require knowledge of pH and E app as a function of vertical position, as well as an understanding of their influence on HE. Constant extension rate tensile testing (CERT) was performed on notched prestressing steel tensile specimens at various cathodic polarization levels in: (1) saturated calcium hydroxide (Ca[OH] 2 ), (2) ASTM artificial ocean water, (3) under a mortar cover in artificial ocean water, and (4) in pH 4 and pH 6 Ca 2+ -containing environments simulating ferrous ion hydrolysis on corroding prestressing steel. CERT results were combined with permeation measurements to determine the relationship between steel mobile hydrogen concentration (C H ) and fracture initiation stress ( i ) in each environment over a series of cathodic potentials. A relation-ship of the form 1 = 0 -␣log(C H /C 0 ), where 0 is the fracture initiation stress in the absence of mobile hydrogen and C 0 is the mobile hydrogen concentration at or below which no hydrogen embrittlement is observed, was found independent of environment and pH. The previously reported fixed cracking threshold of -900 mV SCE in Ca(OH) 2 solutions pH adjusted with hydrochloric acid (HCl), irrespective of pH (from 7 to 12.5), was explained. Decreasing pH in these environments produced a roughly constant C H at E app = -900 mV SCE due to the opposing influences on hydrogen uptake of increasing hydrogen overpotential but decreasing availability of a Ca(OH) 2 recombination poison.
While the low cost and strong safety record of lead-acid batteries make them an appealing option compared to lithium-ion technologies for stationary storage, they can be rapidly degraded by the extended periods of high rate, partial state-of-charge operation required in such applications. Degradation occurs primarily through a process called hard sulfation, where large PbSO 4 crystals are formed on the negative battery plates, hindering charge acceptance and reducing battery capacity. Various researchers have found that the addition of some forms of excess carbon to the negative active mass in lead-acid batteries can mitigate hard sulfation, but the mechanism through which this is accomplished is unclear. In this work, the effect of carbon composition and morphology was explored by characterizing four discrete types of carbon additives, then evaluating their effect when added to the negative electrodes within a traditional valve-regulated lead-acid battery design. The cycle life for the carbon modified cells was significantly larger than an unmodified control, with cells containing a mixture of graphitic carbon and carbon black yielding the greatest improvement. The carbons also impacted other electrochemical aspects of the battery (e.g., float current, capacity, etc.) as well as physical characteristics of the negative active mass, such as the specific surface area. Valve-regulated lead-acid (VRLA) batteries are a mature rechargeable energy storage technology. Low initial cost, well-established manufacturing base, proven safety record, and exceptional recycling efficiency make VRLA batteries a popular choice for emerging energy storage needs.1,2 VRLA batteries are employed in stationary storage applications such as: utility ancillary regulation services, wind farm energy smoothing, and solar photovoltaic energy smoothing.3 Stationary applications may require short duration, high-rate, and partial state-of-charge cycling (HRPSoC). 4 Under HRPSoC duty, conventional VRLA cells fail prematurely from irreversible PbSO 4 formation within the negative plates.5 Regular cycling to 100% state-of-charge (SoC) mitigates PbSO 4 crystal formation and growth. However, regularly cycling to 100% SoC is not viable for many stationary storage applications. Large PbSO 4 crystals are not easily reduced back to metallic lead during HRPSoC charging, reducing cycle life. Reduced cycle life of VRLA batteries increases the operating cost, thereby limiting their practicality for stationary applications.VRLA battery HRPSoC cycle life can be increased with carbon modification of the negative active material (NAM).6-10 Adding carbon to the negative plate inhibits PbSO 4 crystal formation and/or limits PbSO 4 crystal growth.11-13 The underlying mechanism responsible for reducing PbSO 4 formation/accumulation is dependent on the size of the PbSO 4 crystallites. Controlling PbSO 4 microstructure has been found to be difficult while maintaining low cost. Other methods exist to limit PbSO 4 crystal size; including utilization of a carbon honeyco...
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