Comparison of influences of NaCl and CaCl2 on the corrosion of 11% and 17% Cr ferritic stainless steels during cyclic corrosion test

During the drying stage of the cyclic corrosion test on ferritic stainless steels in the NaCl environment, the current abruptly increased and then decreased to nearly zero, indicating that pits are initiated as the salt concentration is increased, which are then repassivated when the surface is completely dry. During the wet stage, the current remained high, suggesting that pits mainly propagate during the wet stage. In contrast, in the cyclic corrosion test in the CaCl₂ environment, the current was highest during the drying stage, indicating that the electrolyte is not completely dried and corrosion mainly propagates during the drying stage.     

Absorption characteristics of ammonia-water system in the cylindrical tube absorber

Experimental analysis was performed in a cylindrical tube absorber which is considered to be suitable for the bubble mode. Characteristics such as concentration, temperature, and pressure were measured and they reflected the condition of absorber well. The variation of characteristics was conspicuous near the inlet region of the ammonia gas. The ammonia gas and the solution flowed cocurrently and countercurrently and the results were compared. Presented at the Int’l Symp. on Chem. Eng. (Cheju, Feb. 8–10, 2001) dedicated to Prof. H. S. Chun on the occasion of his retirement from Korea University.     

Mechanism for the degradation of MmNi<Subscript>3.9</Subscript>Co<Subscript>0.6</Subscript>Mn<Subscript>0.3</Subscript>Al<Subscript>0.2</Subscript> electrode and effects of additives on electrode degradation for Ni-MH secondary batteries

Electrode degradation can affect the lifetime and safety of Ni-MH secondary batteries. This study examined the factors responsible for the degradation of metal hydride (MH) electrodes. The charge-discharge characteristics and cycle life of an MmNi_3.9Co_0.6Mn_0.3Al_0.2 (Mm: misch metal) type MH electrode were examined in a cell with a KOH electrolyte. After the charge-discharge cycles, the surface morphology of the electrodes was analyzed to monitor the extent of degradation. Electrochemical impedance spectroscopy provided information on the conductivity of the electrode. X-ray photon spectroscopy (XPS) was used to quantify the degradation of the electrode in terms of its composition. The MH electrodes degraded with cycling. This phenomenon was more prominent at higher C-rates and temperatures. The electrode degradation was attributed to the loss of active material from the current collector by the repeated absorption and desorption of hydrogen and the formation of an Al₂O₃ oxide layer on the electrode surface with cycling. In addition, the effects of the addition of Co nano and Y₂O₃ powder on the degradation of the MmNi_3.9Co_0.6Mn_0.3Al_0.2 electrode were examined. The addition of the Y₂O₃ and Co nano powder significantly improved the performance of the MH electrode by increasing the cycle life and initial activation rate.     

Catalytic characteristics of MnO2 nanostructures for the O2 reduction process

Nanorods with an α type MnO(2) structure and a diameter ranging from 25 to 40 nm, along with tipped needles with a β MnO(2) structure and a diameter of 100 nm were obtained. The 25 nm diameter α MnO(2) nanorods showed the best catalytic activity for dissociation of HO(2)(-) formed during oxygen reduction in a KOH solution. The MnO(2) nanostructures preferably followed a two-electron oxygen reduction mechanism in a LiOH solution. The size of the catalyst also affected the specific capacities of the non-aqueous Li/O(2) batteries fabricated using the MnO(2) based air electrode. The highest specific capacity of 1917 mA h g(-1) was obtained for an α MnO(2) nanorod catalyst having a diameter of 25 nm. The cation present in the MnO(2) nanostructures appears to determine the catalytic activity of MnO(2).     

Effects of aging at 475 °C on corrosion properties of tungsten-containing duplex stainless steels

Effects of aging at 475 °C on the corrosion and mechanical properties of Fe–25Cr–7Ni–0.25N–xMo–yW (x=0–3, y=0–6) duplex stainless steels were investigated by an anodic polarization test in HCl solution, a modified double-loop electrochemical potentiodynamic reactivation (DL-EPR) test, and an impact test. Corrosion resistance of the alloys was degraded with aging at 475 °C due to the depletion of Cr around α^′ precipitates where numerous micropits were formed during the anodic polarization test. Especially for over-aged alloys, a second anodic current loop appeared in the passive region during the anodic polarization in 1 M HCl solution. The peak value of the second anodic current loop as well as the ratio of the maximum current in reactivation loop to that in anodic loop (i_r/i_a) determined from the modified DL-EPR test were found to be an effective measure of the precipitation of α^′-phase during the aging. However, the degradation in corrosion resistance and impact toughness of the alloys during the aging was retarded with an increase in the W content of the designed DSS, suggesting that W in duplex stainless steels delays the precipitation rate of α^′-phase due to a slower diffusion rate of W compared with that of Mo in ferrite. Influences of aging on the galvanic corrosion behaviors between austenite and ferrite phases were discussed by atomic force microscopy observation.     

Comparison of commercial and hydriding-combustion-synthesized Mg-hydride

This study employed a fast, simple and cost-effective hydriding combustion synthesis (HCS) to prepare nano/submicron Mg based alloys, which are the most promising solid state hydrogen storage materials owing to their high storage capacity (7.6 wt.%) and highest volumetric density. The microstructural and absorption/desorption kinetic properties of the prepared MgH₂ samples were characterized and compared with commercially available MgH₂ powders. The detailed BET analysis of the HCS prepared samples showed a higher surface area than that of commercial MgH₂, which resulted in better absorption/desorption kinetics. The HCS-prepared MgH₂ powder absorbed 6.2 wt.% H₂ with a rate of 0.31 wt.%/min, whereas it desorbed at a rate of 0.98 wt.%/min. These results highlight the superiority of the HCS method to prepare MgH₂ powders over conventional ingot-metallurgy.     

Effects of pretreatment of LM-Ni3.9Co0.6Mn0.3Al0.2 alloy powders in a KOH/NaBH4 solution on the electrode characteristics and inner pressure of nickel-metal-hydride secondary batteries

The discharge capacities of lanthanum-rich mischmetal (LM)-Ni_3.9Co_0.6Mn_0.3Al_0.2 alloy electrodes are significantly degraded by an increase in the C rate. Nevertheless, the discharge capacity of alloy electrodes pretreated with KOH/NaBH₄ is maintained higher than that of raw alloy electrodes, with the difference in discharge capacities between the raw and pretreated alloy electrodes more prominent at higher C rates. The charge retention of the electrodes decreases with increasing rest time. In particular, the charge retention of the pretreated alloy electrode is lower than that of the raw alloy electrode due to the higher self-discharge rate. The overvoltage for hydrogen evolution of the pretreated alloy electrode is superior to that of the raw alloy electrode, particularly at higher temperatures. This phenomenon indicates that the charge efficiency of the electrode was significantly improved by the surface pretreatment, resulting from its high surface catalytic activity. Repeated charge-discharge increases the inner pressure of the battery. Nevertheless, due to its higher charge efficiency and faster recombination rate, the inner pressure of the battery made using the pretreated alloy electrode is much smaller than that of the battery made using a raw alloy electrode.     

Hydrogen generation using the corrosion of Al-Sn and Al-Si alloys in an alkaline solution

We investigated the effects of adding Sn and Si to Al alloys on the corrosion of the alloys and the generation of hydrogen from an alkaline solution using the alloys. With increasing Sn content of up to 20 wt% in the Al-Sn alloy, the volume fraction of the Sn phase as a cathodic site at grain boundaries increased, and consequently, the hydrogen generation rate from an alkaline solution by the alloy also increased. In addition, the quenched Al-Sn alloys had smaller grain sizes compared to the furnace-cooled alloys, and accordingly, exhibited a slightly higher hydrogen generation rate. A galvanic cell was formed between the Al grain and the Sn phase of the grain boundary, and accordingly, intergranular type corrosion was observed on the Al-Sn alloys. Compared with the Al-Sn alloys, a more uniform type corrosion was observed on the Al-Si alloys because the nobler Si was uniformly distributed in the eutectic region formed between the primary Al grains. The hydrogen generation rate increased with an increasing Si content up to 10 wt% and was greater for the furnace-cooled samples than that for the quenched samples due to the more clearly formed eutectic structure.