•Increasing exposure in air produced more hydroxylated native oxide films on steel.•The passive film became increasingly more protective with age of native oxide film.•The passive films consist of a ...inner Fe(II)-rich layer and an outer FeOOH layer.•A kinetic model was reasonably optimized on the experimental impedance spectra.
Electrochemical impedance spectroscopy (EIS), open circuit potential (Eoc) measurements and angle-resolved X-ray photoelectron spectroscopy (AR-XPS) were used to investigate the effect of exposure in air on the passivation of carbon steel electrodes in a neutral aerated glutarate solution. The electrochemical measurements showed that for native oxide films aged between 4 h and 120 h the passive films became increasingly more protective. Analysis of the XPS revealed a bilayer structure of the passive film (inner Fe(II)-rich, outer Fe oxyhydroxide), with increasing values of the Fe(III)/Fe(II) intensity ratio, and gave evidence that passivation was favored by progressively hydroxylated native oxide films.
•Zincate treatment on Aluminum has an impact on the quality of the Ni-P deposit.•The speed of deposition impacts composition of Ni/P, higher rate resulted in less P.•Corrosion resistance of Ni-P ...depends on gas bubbled during the electrochemical tests.
AA6061 samples were coated with electroless Ni-P after a pretreatment which consisted of either single, double or triple zincating. It was observed that increasing the number of zincating reduces the amount of zinc on the surface, and this has a direct effect on the initial rate of deposition of the Ni-P coating, which was confirmed through pH measurements. The corrosion performance of uncoated and coated AA6061 was evaluated at open circuit conditions with conventional electrochemical techniques in 0.5 M H2SO4 solution bubbled with H2 or O2 to simulate the anodic and cathodic environments respectively of a Proton Exchange Membrane Fuel Cell (PEMFC). In both environments the corrosion rate was lower in the presence of Ni-P coating compared to uncoated AA6061 and it also decreased while increasing the zincate pretreatment steps. This latter effect was attributed to higher P content and amorphous nature of the coating. Furthermore, it was observed that for all coated samples the corrosion rate was higher in the simulated cathodic environment. Potentiostatic tests were also carried out to further discuss the stability of the coatings and the results showed that the measured current density followed the same trend as the corrosion current density. It decreased with increasing number of zincate steps in both environments. Also, measured currents were lower in the simulated anodic environment, confirming a better stability in the anodic side.
•Applications of rice husk and its derivatives.•Study the available technologies for production of SiC, pure Si, Si3N4, Mg2Si, SiO2 from rice husk.•Explanation of different furnaces for production of ...rice husk ash.•Study the available technologies for production of activated carbon from rice husk.
Recently, much attention has been focused on the utilization of plant biomass to produce engineering materials, encompassing the technological/scientific aspects as well as the economic, environmental and social issues. Amongst the variety of agricultural wastes or biomasses available, rice-husk (RH) occupies a preeminent position, not only in terms of its amount produced worldwide, but also because of its unique chemistry-related features. In this sense, some attempts have been made to produce advanced materials – including SiO2, SiC, Si3N4, elemental Si, Mg2Si and more recently, active carbon –, using RH. The production of those mentioned advanced materials depends largely on the treatments used (physical and/or chemical) and the reactions involved in the process, such as pyrolysis, carbothermal and reduction processes. In this contribution, a critical review on the processing and application of rice husks (RHs) for the production of various silicon-based materials and of active carbon is presented. The review addresses the different processing methods, the effects of various process parameters on the pyrolysis stage, the influence of physical, chemical and thermal treatments, activating conditions and activated carbon consolidation mechanisms. A flow chart with all the possible routes to produce SiO2 was purposely constructed.
The electrochemical and surface properties of mild steel in stagnant aerated solutions containing 5 mM citric acid at pH 5 and pH 9 were investigated by open circuit potential, potentiodynamic ...polarization and electrochemical impedance measurements combined with X-ray photoelectron spectroscopy (XPS) characterization. The results show that the active dissolution behavior of steel in the citric acid solution at pH 5 is well described using a kinetic model consisting of a modified consecutive mechanism, in parallel with a catalytic path involving the divalent intermediate Fe(OH)
2
)
ads
. The surface film is composed of FeO, Fe(OH)
2
, FeOOH and Fe-citrate. Also, the analysis of in-depth distribution of composition indicates a high coverage of the organic layer. In the pH 9 citrate solution, a thin bilayer passive film was formed (with Fe(II) and Fe(III) located mainly in the inner and outer regions, respectively). Its overall composition is Fe
3
O
4
/FeOOH. Electrochemical measurements revealed the low protective ability of this film and this was confirmed by a modest XPS intensity ratio of Fe(III)/Fe(II). Moreover, no clear presence of Fe-citrate was detected in the passive film, and so the poor passivation behavior was correlated to insufficient adsorption of the tri-ionized citrate anion. The importance of solution flow to stimulate the adsorption of citrate anion is emphasized.
This work concerns the optimization of furnace brazing conditions for joining micro-multiport aluminum tubes and fins made with AA4343/AA3003/AA4343 brazing sheet in mini-assemblies mimicking the ...core of an automotive heat exchanger. Taguchi method was used for design of experiment, considering five process parameters with two levels of values. The aim was to maximize the fillet size of the brazed joints, which has an important influence on the thermal integrity and mechanical properties. Fillet length measurements of brazed joints were performed with a metallographic microscope. The statistical analysis allowed to obtain the optimum values of process parameters (peak temperature, residence time, heating rate, microchannel tube type and flux). At a 95% confidence level, the variability of fillet length is most significantly affected by the peak brazing temperature (77%), residence time (15%) and heating rate (7%). The predicted maximum fillet length was (152±11) µm, which was corroborated by confirmation trials. The microstructural analysis of tube−fin joints showed that variations in peak temperature and residence time affect only the size of the eutectic zone of fillet formed, but not the nature or composition of the constituent phases.
Aluminum and its alloys represent a promising alternative as bipolar plates in proton exchange membrane fuel cell (PEMFC) systems. However, to improve the corrosion resistance they are commonly ...coated, and electroless Ni‐P offers great advantages. The effect of multiple zincating process on the corrosion behavior of electroless Ni‐P coatings on AA6061 was investigated. Results of morphological characterization and compositional analysis of the electroless Ni‐P coatings showed that as the number of zincating steps increased, the surface became smoother (with decreasing roughness) and the phosphorous content increased. The results of electrochemical impedance measurements and potentiodynamic polarization curves in 0.5M H2SO4 solution showed that the pretreatment used before coating of aluminum with Ni‐P has a direct effect on corrosion resistance. The corrosion current density decreased with increasing the number of zincating steps. A significant improvement in corrosion resistance was observed on performing the triple zincating.
The presence of deleterious Al4C3 phase has been for long a major challenge in the manufacture of Al matrix composites reinforced with carbide phases. In this contribution, the hydrophilic Al4C3 ...phase has been successfully eliminated from the surface of B4C/Al composites prepared by pressureless infiltration, using a post-processing approach by the hybrid system chemical vapor deposition (HYSY-CVD) route. B4C/Al composites containing intentionally formed Al4C3 were first prepared by pressureless infiltration and subsequently treated by HYSY-CVD, varying temperature (400 and 500°C), time (30 and 60min) and amount of solid precursor Na2SiF6 (10 and 20g). In conformity with reaction thermodynamic predictions using the HSC 6.1® and FactSage® programs and databases, analysis by XRD and SEM confirms the superficial elimination of Al4C3 and the seedless formation of passivating AlF3 and Na5Al3F14. Optimal conditions to maximize the amount of AlF3 deposited were established by ANOVA and the effectiveness of the fluoride phases was evaluated in hardness and corrosion tests, confirming the stability of fluoride phases and the integrity of the composites. Microstructure characterization and hardness tests of specimens after three years substantiate the efficacy of AlF3 and Na5Al3F14 phases. A hydrophobic and impermeable fluoride coating multiphase layer is thus obtained.
Microstructure evolution of B4C/Al composites before and after post-processing. Display omitted
•The hygroscopic and deleterious Al4C3 phase was successfully eliminated by a post-processing approach.•A hydrophobic and impermeable fluoride coating multiphase layer is deposited on the composites.•The stability of fluoride phases and the integrity of the composites are confirmed.•Optimal conditions to maximize the amount of AlF3 deposited were established.
Electrochemical and surface analytical techniques were combined in order to investigate the corrosion inhibition of a carbon steel rotating disc electrode (rde) by N-phosphono-methyl-glycine ...(NPMG)/Zn2+ mixtures with a concentration ratio of 1:1 between NPMG and zinc cations, in chloride solutions. From polarization curves it was shown that the NPMG/Zn2+ mixture retarded both the anodic and cathodic partial reactions of the corrosion process. Inhibition efficiency, estimated from electrochemical impedance measurements, increased from approximately 85 to 95% when inhibitor concentration increased from 10 to 100 mg/l. Analysis of the composition with XPS suggested that the films consisted mainly of hydrous ferric oxides (Fe(OH)3 and FeOOH), with small amounts of Fe-NPMG complex, ZnO and corrosion products. AES depth profiling showed that the most protective surface film (that formed in the solution containing 100 mg/l of inhibitor) was thicker compared to that formed in the solution with low inhibitor concentration. Material: 1018.
The use of recycling aluminum from beverage containers and rice-hull ash (RHA) offers to be an attractive alternative for the economic production of Al/SiCp composites. However, corrosion phenomena ...in the composites represent technological barriers yet to be resolved before they can be exploited to their full potential. A simple methodology involving characterization by XRD, SEM, EDX, FTIR and ICP was designed in order to investigate the causes of the rapid degradation in a humid environment of Al/SiCp composites produced with RHA and aluminum cans. Results reveal that the use of RHA was beneficial to avoid degradation through the formation and subsequent hydration of the Al4C3 phase. However with condensed moisture acting as an electrolyte, localized corrosion took place with aggressive damage manifested by the disintegration of the composite into a powdery mixture. The relevant corrosion mechanism was mainly attributed to microgalvanic coupling between the Mg2Si intermetallic compound and the matrix (although other phases such as SiC, Si, MgAl2O4 could also work as microcathodes).
