Journal of Advanced Materials in Engineering (Esteghlal)

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On electropolished steel at low current densities, morphology and texture of electrodeposited zinc were investigated. Zinc coating is consisted of hexagonal crystallites laid on each other to produce packets. These packets are of different sizes and are stacked in different orientations to construct a homogeneous coating on steel substrate. This coating does not have texture, i.e., it has a random texture. With increasing current density, the morphology changes completely as each grain attains a special orientation. In this case, coating has a strong basal plane (0002) along with low angle planes (1013 and 1014). Coating obtained on mechanically polished surfaces consists of individual packets of zinc crystals, which are near each other with different orientations. These coatings have a higher density of basal plane (0002) in comparison to electropolished surfaces. The morphology and texture variations with cathodic polarization and surface preparation of steel are due to their effect on nucleation and growth.

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The addition of ZrO2 particles to the HA coating has received considerable attention because ZrO2 particles increase the bonding strength between HA coating and substrate. In this study, nanostructured hydroxyapatite (HA)/yttria stabilized zirconia (YSZ) coatings were prepared by a sol–gel method. It was found that at 950ºC, the dominant phases were HA and tetragonal (t)-zirconia in 3YSZ, cubic (c)-zirconia in 8 YSZ and t-c-Zirconia in 5YSZ phases with the small amounts of β-tricalcium phosphate (β-TCP) and CaZrO3. The crystallite size of the coating was about ~20-30 nm for tetragonal and cubic zirconia grain size and 40-80 nm for hydroxyapatite grain size. Crack-free and homogeneous HA/YSZ composite coatings were obtained with no observable defects. In vitro evaluation in 0.9% NaCl showed that Ca2+ dissolution rate of composite coatings was lower than that of pure HA coatings. The decrease in electrochemical performance of these coated samples in comparison with the uncoated type 316L St.St could be associated with chloride ion and water penetration into the coating, transport of ions through the coating, and the subsequent electrochemical reactions at the coating–metal interface.

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In this study, effect of potential on composition and depth profiles of passive films formed on 316L stainless steel in 0.05 M sulfuric acid has been examined using X-ray photoelectron spectroscopy (XPS). For passive film formation within the passive region, four potentials -0.2, 0.2, 0.5, and 0.8 VSCE were chosen and films were gown at each potential for 60 min. XPS analysis results showed that atomic concentration of Cr and Fe initially increase (E < 0.5 VSCE) and then decrease with potential. This decrease is due to surface dissolution of the Fe and Cr oxides. For both alloying elements, Ni and Mo, no obvious change in atomic concentration was showed. Results indicated that at higher potentials, before entering transpassive region, oxidation of Cr3+ to Cr6+ is happened.

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In this study, the chemical composition, thickness and tribocorrosion behavior of oxide films prepared on Ti-6Al-4V alloy by anodising treatment in H2SO4/H3PO4 electrolyte at the potentials higher than the dielectric breakdown voltage were evaluated. The thickness measurement of the oxide layers showed a linear increase of thickness by increasing the anodizing voltage. The EDS analysis of oxide films demonstrated precipitation of sulfur and phosphor elements from electrolyte into the oxide layer. Tribocorrosion results indicated that the tribocorrosion behavior of samples was significantly improved by anodising process. Furthermore, the tribocorrosin performance of thesamples anodised at higher voltages was enhanced. SEM and EDS of worn surfaces indicated that the oxide layer on the samples anodised at lower voltages was totally removed, but for the samples anodised at higher voltages, the oxide layer was only locally removed within the wear track. Moreover, measurement of wear volume of the treated samples exhibited lower values on the samples anodised at higher voltages.

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Super-hydrophobic nickel and nickel-cobalt alloy coatings with micro-nano structure were successfully electrodeposited on copper substrates with one and two steps electrodeposition. Surface morphology, wettability and corrosion 

resistance were characterized by scanning electron microscopy, water contact angle measurements, electrochemical impedanc spectroscopy (EIS) and potentiodynamic polarization curves. The results showed that the wettability of the micro-nano Ni and Ni-Co films varied from super-hydrophilicity to super-hydrophobicity by exposure of the surface to air at room temperature. The corrosion results revealed the positive effect of hydrophobicity on corrosion resistance of Ni coating (~10 times) and Ni-Co coating (~100 times) in comparison with their fresh coatings. The results showed that super-hydrophobic nickel coating had higher corrosion resistance than super-hydrophobic nickel-cobalt coating.

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In this study, Ni-Mo nanocrystalline alloys were prepared on steel substrates by electrodeposition method from citrate-ammonia bath by applying current densities 30, 60 and 100 mA/cm². Results indicated that the obtained coatings were uniform and compact. Moreover, molybdenum content in the alloy and current efficiency decreased with increasing electrodeposition current density. X-ray diffraction analyses indicated that all coatings were composed of face-centered cubic solid solution of molybdenum in nickel with grain size of 9-5 nanometer. Moreover, the most intensive plane in X-ray diffragtogram was (111). On the othe hand, roughness measurements indicated that surface roughness escalated with increasing current density. Corrosion behavior study showed decrease in corrosion current density of substrate with applying Ni-Mo alloy coatings. In addition, corrosion current density reduced with increasing molybdenum content of the coating and the minimum amount was related to the coating with 13 atomic percent molybdenum.

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This study has examined the properties of Crofer 22APU stainless steel produced by mechanical alloying for using as interconnect plates in solid oxide fuel cells.This alloy was produced by mixing the source powders and mechanical alloying for 40 hours. For creating a sample with high density, spark-plasma sintering was applied at 1100 °C and 50 Mpa stress for
10 minutes. To achieve the desired properties such as low electrical resistance and high oxidation resistance, a number of samples were coated by manganese-cobalt using electrodiposition technique at current density of 150 mA/cm ² for 40 minutes. Then, considering the properties required for an interconnect plate of solid oxide fuel cell, oxidation resistance and electrical resistance of the coated and uncoated samples were investigated. Oxidation behavior of the coated and uncoated samples, after 100 hours oxidation in air at 800°C did not follow any rule and its curve was a sinus type. The electrical resistance of uncoated samples was in the range of 0.1-0.2 mOhm.cm², but the electrical resistance of the coated samples after 100 h oxidation reached to a less ammount  than that of the corresponding uncoated ones. The alloy produced by mechanical alloying method, compared with commercial ones produced by casting methods, showed similar oxidation behavior after 100 h oxidation, but it had a surface electrical resistance far less than its commercial ones.

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In the present investigation, Mn-Ni binary nano-oxide was deposited by potentiodynamic method on stainless steel at room temperature and the effect of annealing process (at 200 ^oC for 6 h) on microstructure and electrochemical performance of the synthesized pseudocapacitor was studied. The results showed the significant effect of annealing process on increasing the capacitance and decreasing the charge transfer resistance of the electrode. Field Emission Scanning Electron Miscroscopy (FESEM) images depicted interconnected and random nano-flakes in the oxide film microstructure. Moreover, a partially crystallized structure consisting disorder hexagonal birnessite type phase was formed upon annealing in the deposited oxide film with about 10 %at Ni in composition. Based on the galvanostatic charge-discharge plots, the highest specific capacitance (384 F g^-1) and specific energy (53 Wh kg^-1) were found at specific current of 0.1 A g^-1 for the annealed oxide electrode. Finally, cycle life test results at specific current of 10 A g^-1 showed an excellent cyclability and an increase of about 23% in specific capacitance of synthesized pseudocapacitor after 5000 charge-discharge cycles in 1 M Na₂SO₄.

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