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Pitting corrosion is one of the most dangerous forms of corrosion, leading to sudden catastrophic failures in engineering systems. Once initiated, pit propagation rates are largely dependent on the magnitude of the supporting cathodic current available from the oxygen reduction reaction (ORR) occurring on the external passive film. Here the ORR kinetics on conventional and high Cr content FeCr alloys made by arc-melter were investigated by experimental and computational methods. Density functional theory calculations suggested that the overpotentials for the ORR on FeCr oxides are in the order Cr2O3 > Fe3O4 > Fe2O3 > FeCr2O4, which was experimentally confirmed on the FeCr alloys by rotating disc electrode experiments in both O2 saturated 0.1 M NaOH and 3.5 wt% NaCl. Koutecky-Levich analysis demonstrated that ORR follows a 4-electron pathway on all surfaces investigated. Moreover, the investigation of the Pourbaix diagrams of FeCr revealed that at potentials where pitting corrosion initiates the main constituents of the passive films should be a mixture of Fe2O3 and Cr2O3, rather than FeCr2O4, which is proved to be a good ORR catalyst. The results support the postulation that one role Cr additions play in the prevention of pitting corrosion is to suppress the ORR reaction.

Nanocrystalline and amorphous nickel-tungsten alloy with relatively high tungsten contents of up to 34 at.% were produced by electrodeposition. The Hall-Petch relationship and breakdown was studied in this alloy, with grain sizes ranging from 25 nm to fully amorphous. In this higher tungsten content alloy, evidence of solid-solution strengthening was found, which was previously absent in alloys with

The Scanning Vibrating Electrode Technique is a valuable method for investigating localized corrosion, which has the potential to provide further insights if used in conjunction with simulation. This paper demonstrates, by combining experimental data with finite element simulations, that variation in the height of the probe to the electrode surface will cause an imbalance in the measured anodic and cathodic currents, with IR drop being a main determining factor. In addition, the simulations obtain the actual current density at/across the electrode’s surface, whereas the electrolyte current density is obtained by SVET. The galvanic corrosion at scratched and cut-edge zinc-based coated steel in a saline solution are used as examples.

Motivated by the primary benefit of reduced material cost, the thickness of crystalline silicon solar cells has been continuously reduced. Laboratory and industrial studies have explored ultrathin crystalline silicon solar cells below 50 μ m with ambitious endeavors toward thicknesses of only a few micrometers. Ultrathin crystalline silicon solar cells require compatible small-scale surface textures to enhance the optical absorption. For this purpose, a novel submicron periodic nanostructure—periodic upright nanopyramids (PuNPs)—is fabricated by an integrated process of laser interference lithography and anisotropic etching of silicon in an alkaline solution. By simulation and measurements, we demonstrate that PuNPs are able to reduce front surface reflectance more effectively than conventional micron-scale pyramid textures and previously investigated periodic inverted nanopyramids (PiNPs). With a silicon nitride antireflection coating, we predict that PuNPs reduce the front surface reflectance to below 1% at an angle of incidence of 8°, which is comparable to black silicon. The superior antireflective property of PuNPs contributes to an absorbed photocurrent density of 40.8 mA/cm2 for a 40 μ m silicon absorber layer, which is 0.7 mA/cm2 higher than PiNPs, 0.8 mA/cm2 higher than inverted pyramids and 1 mA/cm2 higher than upright pyramids.

Sensitized AA5083-H2 aluminum alloy was exposed to chloride-laden thin-film electrolyte at ambient temperature (20%–85% relative humidity) and the local Volta potential measured, in-situ and in real-time, using the Scanning Kelvin Probe Force Microscopy, with the intention to elucidate the earliest stage of localized corrosion. Positive Volta potentials vs alloy matrix were measured for magnesium silicides in ambient air, which, however, underwent a severe nobility loss during corrosion, causing their nobility to invert to active potentials (negative) relative to the alloy matrix. The reason for the nobility inversion was explained by the preferential dissolution of Mg2+, which resulted in an electropositive surface. Aluminides, both with and without silicon, were seen to form the main cathodes at all exposure conditions. The local alloy matrix next to closely-separated aluminides were seen to adopt the Volta potential of the neighbor aluminides, which, hence, resulted in local corrosion protection. The phenomenon of nobility adoption introduced in this work raises questions regarding the anode-to-cathode ratio, which was observed to change during corrosion, and the resulting impact to localized micro-galvanic corrosion. This work further demonstrates that it is necessary to measure the Volta potential during corrosion to reflect the true relationship between the Volta potential and corrosion potential or breakdown potential.

The electrolyte film thickness condensation and evaporation is an important parameter for complexity of atmospheric corrosion. Atmospheric corrosion rate of zinc at the west coast of Singapore was measured for one year using an electrical resistance monitoring system. The analysis of the data reveals that significant corrosion rates only occur at specific hours on dry days. The beginning of this period corresponds to falling temperature and rising relative humidity resulting in the formation of a film of moisture on the zinc surface and the end corresponds to a point that this film dries as the temperature increases. This finding allowed the drying rate of the moisture film to be estimated for input into a moving boundary simulation model of the galvanic corrosion in scratched and zinc coating samples. The simulation results showed that the maximum corrosion rate occurs at electrolyte thickness of about 8 μm. Moreover, the simulation suggested that cut-edge is a more harmful defect than scratch, which was confirmed by the appearance of iron corrosion products on atmospheric exposed cut-edge samples whereas scratched samples were not corroded after one week of exposure. Finally, moving boundary simulation allowed to predict the changes to the geometry of the corroding electrodes.

Financial support was provided by the Singapore Institute of Manufacturing Technology (SIMTech) under project No. U12-S-036SU. Y. T. Tan acknowledges the support of a NUS Graduate School (NGS) scholarship for his PhD studies.

Electrochemical impedance spectroscopy is used to study the etching of p-type silicon in ethanolic HF over the potential region where porous silicon forms. The impedance data fits a single equivalent circuit for all HF concentrations. The appearance of a finite length Warburg element at potentials negative of the flatband potential is interpreted as evidence that a submonolayer of oxide/hydroxide islands form in the Tafel region. Although this is contrary to previous studies it is consistent with published spectroscopy data. Low values for the double layer capacitance are attributed to the hydrophobic nature of the Si-H terminated surface, whereas an inductance loop or negative differential capacitance behavior seen at low frequencies is interpreted in terms of a potential-dependent surface roughness through the formation/dissolution of a submonolayer oxide. Interpretation of the resistive components of the circuit are in line with the known mechanism of etching, that is charge transfer changing to mixed control as the potential is made more positive and dissolution becomes limited by mass transport processes. (C) 2014 The Electrochemical Society. All rights reserved.

A DC magnetron is used to sputter highly conductive ITO on the surface of porous silicon to form LEDs that not only emit the usual red-orange light in one bias direction, but stable white light when biased in the opposite direction. The direction of the bias required for white light emission depends on whether p- or n-type silicon is used. The white light is clearly visible to the naked eye and stable for >2 h of continuous operation. The likely source of the white light emission is from the intra- or inter-band transitions of hot carriers generated in an avalanche process.

The understanding of microbial influenced corrosion (MIC) in aerobic mixed biofilms benefits from advanced microscopy and microbial ecology characterization of biofilms. Here, the onset of MIC in stainless steel coupons was studied in both natural and artificial seawater. Rapid selection of biofilm-forming microorganisms from natural seawater was observed for field experiments. Potential ennoblement was observed only in natural seawater. A seawater derived mixed microbial consortium enriched in artificial seawater was used to characterize the effect of several parameters on MIC. The concentration of organic carbon was the major determinant of MIC, while shaking speed and polishing played minor roles. The biofilm was preferentially formed at the grain boundaries. These results outline the need for MIC onset characterization with mixed microbial consortia to predict long-term corrosion behavior of stainless steel in seawater. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Published version

We have used electrochemical impedance to investigate the improvement in photovoltaic performance in aging of ionic liquid dye-sensitized solar cells using a high-absorption coefficient organic dye (2E)-2-cyano-3-(5-(5-((E)-2-(10-(2-ethylhexyl)-10H-phenothiazin-7-yl)vinyl)thiophen-2-yl)thiophen-2-yl)acrylic acid, which is in contrast to N719-based devices. It was found that the enhancement is due to reduced recombination of the photoexcited electrons. The decreased recombination plausibly originates from molecular re-orientation along with cation adsorption, with Fourier transform infrared spectra lending support to the former mechanism. After aging, the photovoltaic device using the organic dye outperforms the counterpart by the ruthenium complex dye and achieves an impressive efficiency of 5.6% under AM 1.5 100 mW/cm2 illumination. Copyright © 2011 John Wiley & Sons, Ltd.

Two novel organic dyes have been synthesized using electron rich phenothiazine as electron donors and oligothiophene vinylene as conjugation spacers. The two dyes (2E)-2-cyano-3-(5-(5-((E)-2-(10-(2 ...

The mechanism of p-type porous silicon formation in dilute hydrofluoric acid has been examined over a wide range of resistivities. The current−voltage curves show a negative potential shift as the resistivity of the silicon is reduced, a phenomenon that is explained in terms of a corresponding shift in the flat-band potential. Furthermore it is found that for all resistivities the flat-band potential is negative of the so called “Jps peak”, and an argument is presented that shows that this observation effectively disproves previous literature that has postulated that the “Jps peak” marks the transition between porous silicon formation and electropolishing. An observed decrease in the magnitude of the Jps peak for the two most heavily doped samples can be accounted for if the rate-determining step in PSi formation becomes the breaking of the Si−Si back-bonds, a process that should be potential dependent.

The formation and breakdown of the passive films on stainless steels are mainly controlled by ionic and electronic transport processes. Both these processes are in part controlled by the electronic properties of the oxide film. Consequently, it is vital to gain a detailed perception of the electronic properties of the passive films together with structural and compositional information for a comprehensive understanding of mechanisms behind passivity and localised corrosion. As a step towards this goal the passive films formed on two main austenitic stainless steels AISI 316L and AISI 304L in borate solution were characterised by in situ Raman spectroscopy and photocurrent spectroscopy coupled with electrochemical measurements. This revealed the formation of an Fe-Cr spinel as the dominant constituent in the passive films with more Cr enrichment in the oxide film on 316L than that of 304L. Bandgap readings and semiconductivities of the two stainless steels suggested that three different applied potential regions existed; 800 mV(SCE) to 300 mV(SCE), 200 mV(SCE) to −300 mV(SCE) and below −500 mV(SCE).

Au/TiO2 composite films were employed in an attempt to improve the photon-electron conversion efficiency of TiO2 film in the visible region, using the surface plasma resonance (SPR) of Au nanoparticles. For investigating the relationship between SPR of Au particle and photocurrent of TiO2 film, a series of Au/TiO2 films with different Au concentrations were synthesized by sol-gel method. Results of studies on the influence of Au particle size on crystallization of TiO2 film, UV-vis absorption and photocurrents generated are discussed. It was shown that SPR performance of the Au nanoparticles was not only related to their size, but also to their distribution in the TiO2 matrix. Even in TiO2 films with large Au particle sizes (100 nm), SPR in visible region was still observed. However, this SPR performance did not contribute to the photon-electron conversion of TiO2 film in the visible region. Contrarily, embedded Au nanoparticles depressed the photocurrent generated by the TiO2 film in UV region. The reason for this decrease is thought to be partly due to the Au simply blocking some of the light and partly because the extent of crystallization of TiO2 decreased with high Au levels.

With the trend of decreasing the thickness of the carbon overcoat on high-density magnetic recordings, the corrosion properties of a hard disk multilayer structure are becoming a hot issue. An ideal overcoat would be uniform across the disk's surface, have low porosity and electrical conductivity as well as being impermeable to water. However, in reality this is not viable and corrosion is inevitable. One possible method of improving the corrosion resistance of carbon overcoats is to introduce carbide bilayers. In this paper the corrosion protective properties of CrC/C and BC/C nanolaminated overcoat structures have been analyzed by an electrochemical technique based on LPR measurements. The results allowed an overcoat's resistance to be correlated to its porosity and a statistical method was developed to allow easy comparison of results. The results from the electrochemical measurements were correlated with a range of surface analysis techniques.

The corrosion behaviour of melt-spun amorphous Mg 82 Ni 18 and Mg 79 Cu 21 ribbons have been investigated using hydrogen evolution testing in 3% (0.51 M) solution and electrochemical techniques in 0.01 M NaCl. Open circuit potential measurements showed that the as-spun alloys were more electrochemically noble than that of pure Mg. Dissolution rate measurements showed that the corrosion rates of the partially crystallised samples were lower than that of the fully amorphous samples. Potentiodynamic polarisation results showed that although there was ennoblement in the corrosion potential E corr , the corrosion current density i corr was higher for the melt-spun alloy than for pure Mg. Comparison of the polarisation responses for the partially crystallised samples in 0.01 M NaCl showed that the passivation current density i p was lower than for the as-spun amorphous sample. With prolonged heat treatment duration, fully crystallised samples exhibit a marked deterioration of corrosion resistance. The corrosion results have been discussed and correlated with the progress of crystallisation processes by means of XRD and TEM.

The commercial application of rare earth magnets has been in part held back by their poor corrosion resistance. As part of e!orts to overcome this problem two types of nickel coatings were electroplated on the surface of Nd 2 Fe 14 B substrates, one from an acidic plating solution and the other from an alkaline solution. The corrosion characteristics of both coatings were evaluated both by electrochemical potentiodynamic polarisation and by physical exposure to high-humidity environments. The results showed that both coatings enhanced the corrosion resistance of substrate. However, both the degree of protection a!orded by the coating and the strength of its adhesion to the substrate strongly depended on the chemical composition of the original plating solution. The coating plated from the acidic solution provided a high degree of protection against both aqueous and atmospheric corrosion, whilst that plated from the alkaline solution behaved only marginally better than the bare substrate. It was postulated that the poor performance of the coatings plated from the alkaline bath could have been due to the spontaneous formation of a passive oxide "lm between the Nd 2 Fe 14 B substrate and the nickel coating. ( 2001 Elsevier Science B.V. All rights reserved.

UV–VIS spectroscopy, electron spin resonance spectroscopy (ESR) and conductivity measurements have been used to characterise the interactions between methanol vapour and polyaniline. The data indicates that the methanol hydrogen bonds to two locations on the emeraldine base such that it is able to form a bridge between the polymer chains causing twisting. This prevents the few remaining polarons from moving beyond a few monomer units, effectively localising them, giving rise to a decrease in conductivity and an increase in absorption at ca. 1.6 eV. However, in the case of the emeraldine salt protonation of the quinoid moieties’ nitrogen restricts the number of potential hydrogen bonding sites, preventing the methanol from forming a bridge between two polymer chains. At the same time, the high conductivity of the emeraldine salt allows the charge that is transfer to the polyaniline, as a result of the hydrogen bonding to the methanol, to be distributed along the conjugated chain in the form of polarons and bipolarons.

The electrochemical behavior of two kinds of Cu–Al–O coatings prepared by CVD at different temperatures was monitored anodically and cathodically in a solution of 0.1M NaCl. The results show that the coating achieved at 700°C has lower resistance against anodic polarization than the coating achieved at 800°C. Alternatively, the former exhibits rather higher susceptibility to hydrogen evolution than the latter. The atomic hydrogen evolves at the surface of the coating and penetrates deeply into the coating resulting in initiation of hydrogen bubbles. With the increase of hydrogen pressure in bubbles, they grow continuously leading to hydrogen-induced cracking ultimately at the moment when the hydrogen pressure reaches a critical value. Essentially, the susceptibility of hydrogen-induced cracking for coatings should be attributed to the temperature that prevailed during CVD. Higher temperature can evidently enhance the resistance against hydrogen-induced cracking.

The hole current density flowing through and around proton-irradiated areas of $p$-type silicon during electrochemical anodization is simulated and studied experimentally using scanning electron microscopy and photoluminescence imaging. It is shown that for certain irradiation geometries the current flow may be either reduced or enhanced in areas adjacent to irradiated lines, resulting in enhanced or reduced rates of porous silicon formation and corresponding changes in photoluminescence intensity and feature height. The current flow to the surface is unaffected by both the beam straggle and the high defect density at the end of ion range, enabling feature dimensions of $\ensuremath{\sim}200\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ to be attained. This study has enabled fabrication of micromachined and patterned porous silicon structures in anodized wafers with accurate control of feature dimensions, layer thickness, and photoluminescence emission wavelength and intensity.

The structure and composition of the passive film formed on the super-austenitic AISI 254SMO stainless steel in borate solution were studied using photocurrent spectroscopy coupled with electrochemical measurements. Charge passed calculations suggested that the Cr(VI) phase formed in the transpassive region is insoluble. Evidence for the presence of Ni in the passive film was also found. Photocurrent spectroscopy revealed that an n-type Fe(III) oxide phase dominated at more positive potentials, where Cr(VI) was expected. However, once the Cr(III) phase started to form in the reverse potential scan both Fe(III) and Cr(III) phases appeared together in the passive region between 200 and -300 mV, with both showing n-type behavior. At more negative potentials, cathodic photocurrents were observed, most likely due to the reduction of the Fe 2 O 3 to p-type FeO. The magnitude of the bandgaps of the oxides were found to be independent of the values of the applied potential.

The high surface area of porous silicon (PSi) makes it attractive for use in chemical and biological sensors. Selectivity, however, will require tailoring its interfacial characteristics with organic molecules. This paper describes in situ functionalizing of PSi during its electrochemical formation with 1-heptyne and 6-heptynoic acid. Fourier transform infrared spectroscopy (FTIR) confirmed the attachment of the organic molecules, which were able to take part in subsequent chemical reactions. However, some disadvantages of in situ functionalization were noted, such as a reduction in the thickness and porosity of the PSi layer, along with incomplete coverage of the PSi; Si-H stretches were still observed on the FTIR spectra. Nevertheless, when the in situ functionalized PSi was treated in boiling 1-decene, all the Si-H groups were converted to S-C bonds, without the 1-decene replacing the molecules attached in the in situ process. Hence, combining in situ and ex situ functionalization, or adding a mixture of organic molecules into the etching solution, may enable different organic molecules with varying chemical functional groups to be incorporated on a single PSi specimen. Finally, the possibility of using PSi functionalized with carbonyl groups as an alcohol sensor was demonstrated by using FTIR to observe reversible shifts in the frequency of the carbonyl stretch on exposure and removal of methanol vapor.

The photocurrent transient effects of passive films grown on austenitic stainless steels in borate solutions have been investigated and explained by extending a mid-bandgap states model that was previously developed for titanium oxide. Tunneling of electrons trapped in mid-bandgap states into the conduction band was proposed as the reason behind the slow relaxation of photocurrent once the light is turned off. In addition, it was found that surface recombination effects started to appear at potentials where Cr(III) is expected to occur in the passive film, supporting earlier suggestions that this species can act as recombination centers.

Photoluminescence images containing several distinct color emissions, from green to red, have been obtained using high-energy focused ion beam irradiation, in conjunction with metal-aided anodization of 4 Ω cm p-type silicon. The ion irradiation increases the local resistivity in a controlled manner resulting in smaller hole currents flow through the irradiated areas. This causes a controlled redshift of up to 200 nm in the photoluminescence emission, which in terms of the quantum confinement model would correlate to larger nanocrystallites forming in the irradiated region.

Data Storage Institute, National Universityof Singapore, Singapore 119260For high density magnetic recording, it is necessary to protect the magnetic layer from wear and corrosion with a thin carbonovercoat. On ultrasmooth disk substrates, magnetron sputtering is still the preferred technique because it can provide protectivecarbon layers as thin as 4 nm. Degradation of the magnetic layer occurs in the atmosphere and during hard disk operation; thus,the carbon-based overcoat functions as an indispensable component in protecting the essential magnetic layer. Moreover, carbonovercoat quality plays a vital role in the strong demand to reduce the flying height between a disk and read/write head, which inturn enhances the density of recording. The impact of sputtering conditions on the quality of deposited amorphous hydrogenatedand nitrogenated carbon was assessed by the electrochemical measurement of the linear polarization resistance. The film’spolarization resistance was correlated to its porosity, and a statistical method was developed to allow easy comparison of resultsin the data analysis. The results obtained suggest that the best overcoats are formed with a substrate voltage bias of 2120 V, avery low deposition pressure of 0.9 mTorr, a dc magnetron sputtering power of 2.8 W/cm

It has been demonstrated that the application of an electric field enabled the so-called polyol method at temperatures lower than the refluxing temperature of 194°C, thereby producing Ni-Co nanostructured magnetic films without wasteful powder precipitation. At the refluxing temperatures there was an inverse relationship between the magnitude of the applied electric field and the thickness of the deposited film. This surprising result has been postulated to be due to a localized increase in the pH close to the electrode surface inhibiting metal deposition. At the lower temperature of 160°C films could only be deposited in the presence of an electric field with only a small amount, of powder being synthesized. Again the compositions of these magnetic films suggest that their deposition involved the electroless mechanism. At the much lower temperature of 100°C, film formation appeared to he due to simple electroplating, without any involvement of the electroless process.