Shewanella oneidensis MR-1 is a gram-negative facultative anaerobe capable of utilizing a broad range of electron acceptors, including several solid substrates. S. oneidensis MR-1 can reduce Mn(IV) and Fe(III) oxides and can produce current in microbial fuel cells. The mechanisms that are employed by S. oneidensis MR-1 to execute these processes have not yet been fully elucidated. Several different S. oneidensis MR-1 deletion mutants were generated and tested for current production and metal oxide reduction. The results showed that a few key cytochromes play a role in all of the processes but that their degrees of participation in each process are very different. Overall, these data suggest a very complex picture of electron transfer to solid and soluble substrates by S. oneidensis MR-1.
The relationships between microstructure, strength, and ductility of electrodeposited copper (EDCu) has been studied for EDCu in the as-deposited condition. Tensile testing was performed at 25 ° and 370 °C. The EDCu was made from a reagent-grade copper sulfate, 225±10 g/liter, reagent-grade sulfuric acid, 68±5 g/liter, and an electrolyte containing 0.4–0.8 g/liter pentose. The electrolyte temperature was 27 °–32 °C. Anodes were high-purity OFHC copper. Deposition rates were varied from 0.015 to 0.075 mm/h, which is equivalent to current densities of about 1.1–5.5 A/dm2. The EDCu produced under these conditions was very pure (99.995%). Sheets of EDCu approximately 30×22 cm × 0.125–0.15 cm thick were made on a stainless-steel mandrel, separated from the mandrel, and machined into standard, flat-type tensile coupons having a reduced section 0.635 cm in width and 0.125–0.15 cm thick. Overall sample length was about 11.4 cm. The ultimate tensile strength at room temperature of the EDCu changed directly with increased deposition rate. A four- to five-fold increase in deposition rate, 0.015 mm/h up to 0.075 mm/h, corresponding to a four- to five-fold increase in current density, produced an increase of 25% or more in tensile strength (242–310 MN/m2). The 0.2% offset yield strength was increased by 40% (105–147 MN/m2) by this same increase in deposition rate. Microstructure of the deposit decreased in size from a large columnar duplex structure to an extremely fine columnar structure as a result of the increased deposition rate. Elongation over a 0.5-in. (1.27-cm) gauge length decreased from over 50% to about 25%. Tensile strength of the EDCu when tested at 370 °C was 70–90 MN/m2, while yield strength is 35–41 MN/m2, with no assignable effect from the deposition rate. The microstructure shows little or no change after the 370 °C exposure during testing, indicating that increases in as-deposited, room-temperature strength were induced by the deposition process, most likely by internal stresses. However, elongation of the EDCu was consistently higher at 370 °C (15%–20%) for the material deposited at 4–5 A/dm2 than for material deposited at 1–3 A/dm2, which exhibits only 1%–5% elongation. Impurities such as dissolved organics or fine particulates increase strength above that for pure material, with some EDCu having tensile strength as high as 520 MN/m2. Such material was very brittle when tested at 25 ° and 370 °C. Primary usage of the EDCu has been in electroformed rocket-engine combustion chambers and as a barrier to embrittlement of nickel and nickel alloys used in high-pressure gaseous hydrogen environments.
Pitting scan (PS) and electrochemical impedance spectroscopy (EIS) techniques were used to evaluate the corrosion behavior of nanocrystalline (NC) samples and conventional Al 5083 (UNS A95083) in three solutions with different chloride concentrations. The PS obtained in 0.5 N sodium chloride (NaCl) and in 0.5 M sodium sulfate (Na2SO4) + 0.1 N NaCl indicated that pitting should occur in these solutions. These results were confirmed by EIS data that were recorded for up to 25 days. For exposure to 0.5 M Na2SO4 + 0.05 N NaCl the pitting (Epit) and protection (Eprot) potentials determined after exposure for 2 h suggested that pitting would not occur in this solution. However, the initial impedance spectra were indicative of pitting. Tests carried out after exposure for 2 days and 3 days showed passive behavior, but starting with exposure for 5 days, pitting was again indicated for all three samples. PS performed at the end of the EIS tests suggested that this result was due to the increase in corrosion potential (Ecorr) over the Eprot values recorded at the start of exposure. Evaluation of the samples exposed in the EIS tests showed that the pits were much smaller, but more numerous for Al 5083 after exposure to 0.5 N NaCl. Nitric acid mass loss tests (NAMLT) according to ASTM G 67 indicated that the NC samples were not susceptible to intergranular corrosion. Observation with the SEM showed intergranular corrosion for the conventional alloy, while a large number of roughly spherical holes were found for the two NC samples.
A brief discussion is given of recent efforts to use electrochemical techniques for monitoring of atmospheric corrosion phenomena. Problems existing with the interpretation of time-of-wetness measurements are identified, and the finding that electrochemical sensors in their present design and application determine only fractions of the true corrosion rate is discussed. In order to resolve some of these difficulties, a statistically designed experiment is being carried out to evaluate the reproducibility of electrochemical measurements of atmospheric corrosion phenomena and to determine the effects of sensor design on time-of-wetness and cell efficiency. Results are presented for the first phase of this project, in which 15 atmospheric corrosion monitors (ACM) of the copper/steel and 15 ACM's of the steel/steel type, have been fabricated with steel and copper from three different heats. These ACM's have been tested in triplicate runs by exposure to aqueous 1-mM sodium chloride at a relative humidity of 45 percent until the surface had dried out, followed by additional exposure to either a moist air environment at three levels of relative humidity (65, 80 and 95 percent) or a sodium dioxide (SO2) test at three levels of SO2 ≈ 0, 0.2, and 1.1 ppm. During each test, steel plates from each heat were exposed to determine weight loss data. At present, only the drying-out data have been analyzed by statistical methods. It has been found both from electrochemical and weight loss data that the heat of the steel plays an important role, with one heat corroding at a higher rate than the other two which have equal corrosion rates. No differences were found between the five sensors of one heat. Additional factors that influence the measurement are day-to-day variations of the environment in the test chamber and to some extent the position of the ACM's in the test chamber. By comparing the electrochemical and weight loss data, a cell efficiency of about 20 percent was found for copper/steel and about 7 percent for steel/steel. This low cell factor is considered to be due mainly to local cell action on individual plates and to uncompensated ohmic drop in the electrolyte. The copper/steel and steel/steel ACM's are being exposed on the Rockwell International Science Center roof for an aging period of three months, after which another series of laboratory tests will be conducted.
Abstract The corrosion behavior of Al 2024-T3 (UNS A92024) exposed to artificial seawater (AS) and Luria Bertani (LB) medium has been studied using electrochemical impedance spectroscopy (EIS). Tests were performed in sterile media and in the presence of three different biofilm-forming bacteria: a Bacillus subtilis biofilm that was genetically engineered to produce either polyaspartate or polyglutamate, a Bacillus licheniformis bacterial biofilm that naturally produces γ-polyglutamate, and an Escherichia coli biofilm that was genetically engineered to produce polyphosphate. A significant reduction in active pit growth rates and an ennoblement of the corrosion potential (Ecorr) were observed in both media in the presence of these biofilms, which produce corrosion inhibitors. The lowest corrosion rates of Al 2024 exposed to the LB medium were observed in the presence of the B. subtilis bacterial biofilm-producing polyaspartate and the E. coli bacterial biofilm that produced polyphosphate. In the presence of...
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