Formation of ultra-thin amorphous conversion films on zinc alloy coatings: Part 1. Composition and reactivity of native oxides on ZnAl (0.05%) coatings

Within the two parts of this contribution a detailed investigation of the nucleation and growth of ultra-thin amorphous conversion coatings on hot dip galvanised steel is reported. The first part deals with the composition and reactivity of the native ultra-thin oxyhydroxide films that are formed on the zinc alloy surface during the hot dip galvanising process due to the enrichment of aluminium at the outer surface of the alloy coating. Complimentary surface analytical techniques such as FT-IR-spectroscopy at grazing incidence and X-ray photo electron spectroscopy, high resolution AFM on selected grains to study the surface topography and cyclovoltammetry as well as quasi stationary current potential curves and Kelvin probe measurements to study surface ion and electron transfer reactions were applied. Changes in the chemical composition, the electronic properties and the morphology of the ultra-thin surface could thereby be analysed. The surface of the ZnAl alloy is composed of an about 3-4 nm thick mixed Zn and Al-oxyhydroxide layer with Zn-oxyhydroxide slightly enriched at the outermost surface. This mixed oxyhydroxide causes to a significant inhibition of electron transfer reactions. During alkaline cleaning the surface is nanoscopically roughened and the mixed oxyhydroxide is converted into an electro-conducting hydroxyl rich pure Zn-oxyhydroxide layer with a thickness of about 4 nm. In the second part of this paper the effect of the inorganic surface layer on the film formation is correlated with these findings.     

Synthesis and characterisation of surface gradient thin conversion films on zinc coated steel

Surface gradient layers on hot-dip galvanised steel were synthesised in order to determine the barrier properties and corrosion resistance of thin amorphous conversion coatings as a function of layer thickness and processing time. For this purpose, a dip coating procedure was established that yields well-defined gradient layers. As a model system for conversion film formation on zinc coated steel, a zirconium based bath chemistry was used. The synthesised zirconium oxyhydroxide gradient films were characterised by localised electrochemical techniques, such as Scanning Kelvin Probe (SKP) and electrochemical impedance spectroscopy using an electrochemical capillary cell. Microscopic infrared reflection absorption spectroscopy (μ-FT-IRRAS) measurements and small-spot X-ray photo electron spectroscopy (XPS) were used as complementary surface analytical techniques. The applied analysing techniques provide a spatial resolution of 100-1600 μm. Thereby, a complete variation of thin film properties, such as thickness, barrier properties, corrosion resistance and chemical composition can be measured as function of the time of film growth on a sample with a length of a few centimetres. This approach allows a precise and accurate determination of structure-to-property relationships of thin conversion films. Moreover, it could be shown that a surface gradient film analysis significantly rationalises experimental time and increases the reliability of the experimental results.     

Electrochemistry of conductive polymers: 41. Effects of self-assembled monolayers of aminothiophenols on polyaniline films

Effects of self-assembled monolayers (SAMs) of a few different aminothiophenols (ATPs) on growth, morphology, and electrical properties of polyaniline have been studied at gold electrodes employing current sensing atomic force microscopy. The 4-aminothiophenol (4-ATP) SAM showed the best capability of shuttling electrons from the gold electrode to the solution species, and the 4-ATP SAM covered electrode not only showed the best kinetics for the film growth but also produced the film of the highest electrical conductivity. The vertical conductivity of the film ranged from 58 S cm⁻¹ for a film prepared at a bare gold electrode to as high as 148 S cm⁻¹ for a film obtained at the 4-ATP SAM covered electrode. The conductivity was shown to depend on the thickness of the films as well as how they were prepared.     

Semiconducting photocathodes for the reduction of dioxygen: Part I. Characterisation of crystalline and amorphous p-Si

As a first step toward the development of photocathodes for use in water-purification reactors, we have investigated the behaviour of p-type Si(1 0 0)-oriented single crystals and glow-discharge a-Si:H layers as photocathodes for the reduction of dissolved dioxygen. The electrode surfaces were prepared by HF rinsing and were characterised by cyclic voltammetry in the dark and under tungsten-halogen lamp illumination. This study has been performed in dilute-buffered fluoride medium, in dilute sulphuric acid, and lastly in three buffered phosphate media at pH 2, 7 and 12. A study using a rotating-disk electrode (RDE) arrangement permits to distinguish the contribution from dioxygen reduction (mass-transport dependent) and that from hydrogen evolution (kinetically limited). Crystalline Si and a-Si:H appear to exhibit rather similar behaviours. The Si surface acquires catalytic properties for dioxygen reduction upon negative pre-polarisation. However, these catalytic properties do not appear usable in practice for efficient operation, which calls for chemical modifications of the silicon surface.     

Investigation of zinc chromatation: Part II. Electrochemical impedance techniques

A mechanism to explain the formation of a chromate layer on zinc is proposed. It assumes that a ZnO inner film blocks the zinc surface on which the chromate layer grows. This layer has gel-like properties. The diffusion of the protons across the chromate layer and across the solution is supposed to be the kinetically limiting steps. This model was derived and experimentally tested in terms of impedance. The influences of the immersion time, mass transport, and pH of the chromatation solution were examined. A rather good agreement was found between the predictions of the model and the experimental results.     

Silicone-containing polymer matrices as protective coatings: Properties and applications

The paper addresses the technologically important problem of porous building materials protection, e.g. sandstone, limestone, marble against soiling. Protective coatings applied to the surface of porous building material should be characterized by very efficient water vapour permeability and self-cleaning properties.     

Cement composition and sulfate attack: Part I

Four cements were used to address the effect of tricalcium silicate content of cement on external sulfate attack in sodium sulfate solution. The selected cements had similar fineness and Bogue-calculated tricalcium aluminate content but variable tricalcium silicates. Durability was assessed using linear expansion and compressive strength. Phases associated with deterioration were examined using scanning electron microscopy and X-ray diffraction. Mineralogical phase content of the as-received cements was studied by X-ray diffraction using two methods: internal standard and Rietveld analysis.The results indicate that phase content of cements determined by X-ray mineralogical analysis correlates better with the mortar performance in sulfate environment than Bogue content. Additionally, it was found that in cements containing triclacium aluminate only in the cubic form, the observed deterioration is affected by tricalcium silicate content. Morphological similarities between hydration products of high tricalcium aluminate and high tricalcium silicate cements exposed to sodium sulfate environment were also observed.     

Nucleation and diffusion-controlled growth of electroactive centers: Reduction of protons during cobalt electrodeposition

A theory is presented describing, for the first time, the temporal evolution of the fractional surface area, S(t), of 3D non-interacting nuclei growing at a rate limited by diffusion of electrodepositing ions onto substrates of a different nature. Likewise, an equation has been derived describing the potentiostatic current-time transients arising from the formation and growth of such nuclei with redox reactions occurring simultaneously on their surfaces. An equation is also proposed to describe the current due to redox reactions taking place on the surface of interacting growing nuclei. The latter is used to describe the experimental current transients recorded during nucleation and growth of cobalt at applied potentials where the proton reduction reaction occurs simultaneously with the electrocrystallization process.     

Film formation from monodisperse acrylic lattices: Part 3: Drying and ageing of acrylic latex films

The influence of drying/ageing on the structure and properties of acrylic latex films was investigated using turbidity measurements in combination with gravimetry, scanning electron microscopy (SEM), atomic force microscopy (AFM) and water vapour permeability. Ageing above the minimum film formation temperature (MFFT) leads to marked changes in a dried latex film, whereas, after ageing below the MFFT, changes are hardly found. Above the MFFT there is a continuous change in the film properties with time. This becomes obvious from the decreases in the regenerated interference minimum, water vapour permeability and corrugation height. The influence of ageing on the water absorption of the films is less straight forward. It was expected that films with a more compact structure would absorb less water. This is correct for short drying times only, from 0.5 to 3 h. Ageing of better-dried films, however, yields the opposite result: by increase of the ageing time from 3 to 150 h the water uptake increases. There are various reasons for this increase; they are discussed briefly.     

Electrodeposition of platinum nanoparticles on highly oriented pyrolitic graphite: Part II: Morphological characterization by atomic force microscopy

Platinum nanoparticles were electrochemically deposited onto highly oriented pyrolitic graphite (HOPG) from H₂PtCl₆ solutions and observed by tapping mode atomic force microscopy. Spontaneous Pt deposition, which resulted in a wide particle size distribution, would occur on HOPG at open-circuit potential but could be suppressed by using anodic bias of the substrate before and after deposition. Nanoparticles with a narrow size distribution could be obtained when spontaneous reduction was avoided. Pt nucleated both at step edges and on terraces, with a preference for the former. The density of Pt nanoparticles on HOPG was 10⁹-10¹⁰ cm⁻². Increasing the deposition overpotential or adding HCl as supporting electrolyte resulted in more uniform particles and less aggregation. These findings confirm previous results obtained by our group using only electrochemical methods [G. Lu, G. Zangari, J. Phys. Chem. B 109 (2005) 7998].     

Water electrolysis under microgravity: Part 1. Experimental technique

Water electrolysis was conducted in both alkaline (25 wt.% KOH, 2 wt.% KOH) and acid (0.1N H₂SO₄) solutions for 8 s under microgravity environment realized in a drop shaft. The gas bubble formation of hydrogen and oxygen on platinum electrodes was observed by CCD camera. In alkaline solutions, a bubble froth layer grew on the electrode surface. Hydrogen bubble size was smaller than that of oxygen. The current density at constant potential decreased continually with time. In spite of the growth of a bubble froth layer on the electrode, the electrolysis never stopped, apparently because fresh electrolyte is supplied to the electrode surface by microconvection induced by the gas bubble evolution. In acid solution, hydrogen gas bubbles frequently coalesced on the cathode surface, yielding a larger average bubble than that of oxygen. The current density did not vary at constant potentials from -0.4 to −0.8 V versus reversible hydrogen electrode (RHE), because the effective electrode surface area was significantly reduced by the larger bubble size compared to alkaline electrolyte. The present experiments indicate that, especially in a microgravity environment, the bubble evolution behavior and the resultant current-potential curves are significantly influenced by the wettability of the electrode in contact with the electrolyte.     

Optimization of cathode catalyst layer for direct methanol fuel cells: Part I. Experimental investigation

The cathode catalyst layer (CL) in direct methanol fuel cells (DMFCs) has been optimized through a balance of ionomer and porosity distributions, both playing important roles in affecting proton conduction and oxygen transport through a thick CL of DMFC. The effects of fabrication procedure, ionomer content, and Pt distribution on the microstructure and performance of a cathode CL under low air flowrate are investigated. Electrochemical methods, including electrochemical impedance, cyclic votammetry and polarization curves, are used in conjunction with surface morphology characterization to correlate electrochemical characteristics with CL microstructure. CLs in the form of catalyst-coated membrane (CCM) have higher cell open circuit voltages (OCVs) and higher limiting current density; while catalyzed-diffusion-media (CDM) CLs display better performance in the moderate current density region. The CL with a composite structure, consisting both CCM and CDM, shows better performance in both kinetic and mass-transport limitation region, due to a suitable ionomer distribution across the CL. This composite cathode is further evaluated in a full DMFC and the cathode performance loss due to methanol crossover is discussed.     

On the mechanism of the anodic protection of aluminium alloy AA5182 by emeraldine base coatings: Evidences of a galvanic coupling

Aluminium AA5182 coupons covered by a polyaniline film in the emeraldine base (EB) form showed increasing corrosion potential and decreasing corrosion current as a function of the thickness of the polymer layer. The cathodic reaction was proved not limited by diffusion of species inside the electrolyte solution and oxygen had no effect on the electrochemical behaviour of the coated samples. An EB coating on indium tin oxide conducting layer appeared slightly electroactive in neutral media. The IR spectra of aluminium coated samples, before and after heating in argon atmosphere, confirmed a redox reaction between the polymer film and the metal. This galvanic coupling can explain the good protective behaviour of emeraldine base against corrosion of aluminium.     

Mechanism of sulfate attack: a fresh look: Part 2. Proposed mechanisms

The first paper in this two-part series [Cem. Concr. Res. 32 (2002) 915] summarized the experimental results from a comprehensive research study on sulfate attack. The current paper utilizes these results to develop models for the mechanism of attack resulting from sodium and magnesium sulfate solutions. Implications of changing the binder constituents or the experimental variables, such as concentration and temperature of the solution on the proposed mechanism, are also discussed. The potential of these mechanistic models for use in service life prediction models has also been identified.According to the proposed mechanism, the attack due to sodium sulfate solution progresses in stages. The expansion of an outer skin of the specimen leads to the formation of cracks in the interior region, which is chemically unaltered. With continued immersion, the surface skin disintegrates, and the sulfate solution is able to react with the hydration products in the cracked interior zone leading to the deposition of attack products in this zone. Now, this zone becomes the expanding zone, leading to further cracking of the interior of the mortar.In the case of magnesium sulfate solution, a layer of brucite (magnesium hydroxide) forms on the surface of the mortar specimen. The penetration of the sulfate solution then occurs by diffusion across this surface layer. As the attack progresses, the formation of attack products such as gypsum and ettringite in the paste under the surface leads to expansion and strength loss. The expansion also causes cracking in the surface brucite layer, and this leaves the mortar susceptible to direct attack by the magnesium sulfate solution. Conditions favorable for the decalcification of calcium silicate hydrate (C-S-H) are thus created, and the ultimate destruction of the mortar occurs as a result of the conversion of C-S-H to the noncementitious magnesium silicate hydrate (M-S-H).     

Formation of oxygen structures by ozonation of carbonaceous materials prepared from cherry stones: II. Kinetic study

In this second part, the kinetics of the ozonation process of a char prepared from cherry stones (CS) is investigated. The char was obtained by heat treatment of CS at 600°C for 2 h in nitrogen. The effects of reaction time, partial pressure of ozone, and mass transport phenomena on the formation of oxygen complexes are studied. The surface chemistry of the samples was examined by FT-IR spectroscopy and the elemental chemical analysis was also determined for some samples. Results showed that the ozonation of the char led to oxygen chemisorption and to carbon gasification. The amount of oxygen complexes formed in the chemisorption stage (i.e., OH groups, CO structures, and ether structures) was found to be very sensitive to the increase in the ozonation time. The type of oxygen complexes was also time dependent. Ozonated products with relatively high concentrations of CO groups and ether structures were prepared by applying high ozone doses, whereas the formation of OH groups was favored at low ozone contents. The particle size did not influence the surface chemistry of the ozonated products. Only when the gas flow rate was lower than 40 l h⁻¹, restrictions to ozone mass transport developed. For kinetics of the char ozonation process, a mechanism based on the Langmuir-Hinselwood adsorption-desorption model was proposed, and the intrinsic reaction rates were calculated as a function of ozonation temperature. The activation energy for the ozonation stage of the char was equal to 41.6 kJ mol⁻¹.     

The liquid-like layer between metal and frozen aqueous electrolytes: An electrochemical approach employing the quartz crystal microbalance

The surface properties of any substance in the solid state differ from its bulk properties. This can give rise to the formation of a liquid-like layer (LLL) at the interface, at temperatures below the melting point. The phenomenon generated interest among both theoreticians and experimentalists for more than 150 years and was studied, employing different techniques. However, electrochemical techniques have not been implemented in studies of this phenomenon.We used, for the first time, the quartz crystal microbalance (QCM) to study the LLL at the ice/gold and the frozen electrolyte/gold interfaces. It was shown that the QCM in contact with ice or with frozen electrolyte at temperatures well below the melting point shows detectable resonance. The parameters of the resonance depend strongly on temperature, composition of the frozen phase, adsorption taking place at the gold surface (including gold surfaces modified by different thiol-derivates) and, in the case of electrolyte, on the potential applied across the interface.Corresponding theoretical models have been developed to understand the kind of information contained in the response of the QCM on the properties of the LLL. Independent data obtained with a device allowing direct optical measurement of the displacement show good agreement with the calculations of thickness of the LLL based on developed models.     

Electrodeposition of CoWP film: III. Effect of pH and temperature

Polarisation behaviour and nucleation mechanism of induced co-deposition of cobalt, tungsten and phosphorus from citrate electrolytes containing cobalt sulphate, sodium tungstate and sodium hypophosphite on copper disc electrode at various electrolyte pH values were studied using cyclic voltammetry and chronoamperometry, respectively. It was found that the onset potentials for reduction and oxidation gradually shifted towards more cathodic direction with increase in pH. Co-deposition of cobalt, tungsten and phosphorus occurred under diffusion control and followed instantaneous type of nucleation mechanism. The highest current efficiency (>70%) was found when the CoWP films were deposited from neutral or slightly acidic solution. Larger variation of film composition, cobalt from 48 to 74 at.%, tungsten from 13 to 38 at.% and phosphorus from 8 to 33 at.%, could be made by changing the pH of the electrolyte between 3 and 8. The films deposited from very acidic solutions contained larger amounts of tungsten and phosphorus and were amorphous; while the films deposited from near neutral solutions were polycrystalline and contained hexagonal cobalt. The surfaces of the amorphous films were smoother than those of the polycrystalline films. Needle-like dendritic crystallites were obtained when the films were deposited from basic solution or at elevated temperature.     

Direct numerical simulation (DNS) modeling of PEFC electrodes: Part II. Random microstructure

The direct numerical simulation (DNS) method, developed for modeling the cathode catalyst layer (CL) of a polymer electrolyte fuel cell (PEFC) in Part I, is further extended wherein the catalyst layer is described as a random three-dimensional porous structure. A random CL microstructure is obtained using a computer-generated random number with specified porosity and pore size as the input structural parameters. Some statistical features of the CL and their dependence on the porosity are identified and demonstrated. The charge and species conservation equations are solved directly on this microscopically complex structure. The results from the DNS calculation are compared with the one-dimensional macrohomogeneous predictions and the Bruggeman factor for transport property correction is evaluated, which can be used as direct input into the macroscopic fuel cell models.     

Physico-chemical properties of low-rank coals: Thermal and demineralisation effects

Five low-rank coals, including leonardites and lignites, have been physico-chemically characterised. The effects of the carbonisation and demineralisation of some coals on mass and physical properties have also been studied. The characterisation was carried out by the chemical analyses of the coals and also by Fourier transform infrared (FT-IR) spectroscopy, gas adsorption (N₂, −196 °C; CO₂, 0 °C), mercury porosimetry, and density measurements. The contents of moisture, volatile matter, oxygen, carboxylic acids, and carbon-carbon double bond-containing structures are higher in leonardites than in lignites. The degree of development of surface area and microporosity is small for all coals. All samples appear to possess constrictions in micropores. The mesopore volume is significantly higher for a number of coals. The carbonisation and demineralisation of the coals produce a marked increase in the open porosity.