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Equal channel angular pressing (ECAP) was performed on ZK60 alloy and pure Mg in the temperature range 150–250 °C. A significant grain refinement was detected after ECAP, leading to an ultrafine grain size (UFG) and enhanced formability during extrusion process. Comparing to conventional coarse grained samples, fracture elongation of pure Mg and ZK60 alloy were significantly improved by 130% and 100%, respectively, while the tensile strength remained at high level. Extrusion was performed on ECAP processed billets to produce small tubes (with outer/inner diameter of 4/2.5 mm) as precursors for biodegradable stents. Studies on extruded tubes revealed that even after extrusion the microstructure and microhardness of the UFG ZK60 alloy were almost stable. Furthermore, pure Mg tubes showed an additional improvement in terms of grain refining and mechanical properties after extrusion. Electrochemical analyses and microstructural assessments after corrosion tests demonstrated two major influential factors in corrosion behavior of the investigated materials. The presence of Zn and Zr as alloying elements simultaneously increases the nobility by formation of a protective film and increase the local corrosion damage by amplifying the pitting development. ECAP treatment decreases the size of the second phase particles thus improving microstructure homogeneity, thereby decreasing the localized corrosion effects.

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The electrochemical behavior and the reversible hydrogen storage capacity of multiwalled carbon nanotubes ͑MWNTs͒ have been investigated both in alkaline ͑6 M KOH͒ and acid ͑0.3 M H 2 SO 4 ) electrolytic solutions by cyclic voltammetry and constant current charge/discharge measurements. Different purification treatments were performed on as-prepared MWNTs to improve hydrogen uptake. About 0.1 wt % of hydrogen was reversibly stored in MWNTs after nitric acid treatment. Vacuum annealing ͑950°C, 1 h͒ of MWNTs lowered the electrochemical hydrogen uptake. Finally, electrochemical oxidation of MWNTs at high potentials allowed an increase in hydrogen charge capacity to 0.3 wt %.Carbon nanotubes ͑CNTs͒ represent a novel class of materials of great interest due to their properties such as chemical stability, low density, high mechanical strength, and large surface area. A most challenging issue related to CNTs regards hydrogen storage for fuel cell powered vehicles. Nowadays hydrogen is considered a valuable alternative to conventional fossil fuels owing to the world-wide rising concern of environmental pollution. However, hydrogen storage represents an important technological problem to be solved to implement fuel cells in the automotive industry. Most methods for hydrogen storage considered so far, such as liquefaction, compressed gas, or metal hydrides, do not fulfil at the same time the requirements of safety and low cost together with a hydrogen storage capacity of 6.5 wt % and 62 kg m Ϫ3 established by the U.S. Department of Energy ͑DOE͒. 1 The uptake of hydrogen by CNTs may be accomplished through physical charging at high pressures 2-4 or electrochemically from aqueous solutions. The latter method has the remarkable advantage of being feasible at room temperature and pressure even though lower capacities have been recorded in comparison with the former one. [5][6][7][8][9] In this paper, electrochemical behavior in terms of hydrogen storage of multiwalled carbon nanotubes ͑MWNTs͒ in alkaline and acid media is discussed. In addition, three different purification procedures as acid treatment, high temperature heat-treatment, and electrochemical oxidation were performed on MWNTs to enhance hydrogen storage properties. ExperimentalMWNT preparation.-MWNTs were produced from a C 2 H 4 -H 2 mixture on a Fe-Al 2 O 3 catalyst by a fluidized bed chemical vapor deposition ͑CVD͒ process at 650°C. The experimental apparatus utilized for MWNT growth consisted of a quartz tube ͑2.8 cm diam͒ located in an electric furnace. The flow rates of the C 2 H 4 -H 2 gases were adjusted by flow controllers, while the temperature of the furnace was measured by two K-type thermocouples. After deposition, the MWNTs were leached in 37 wt % HCl acid for 6 h at 140°C to remove metal particles, Al 2 O 3 substrate, and amorphous carbon and then rinsed with distilled water. A typical transmission electron microscopy ͑TEM͒, ͑JEOL-2010͒ image of the MWNTs is shown in Fig. 1. 10 The produced MWNTs were homogeneous in morphology with an o.d. of 10-20 n...

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