T. Monetta

Protective properties of epoxy-based organic coatings on mild steel

The protective properties of epoxy-based organic coatings on mild steel substrates have been studied using the electrochemical impedance spectroscopy (EIS) technique. The effects of coating composition, film thickness and curing temperature on the lifetime of the epoxy coating were investigated. The results obtained indicate that the film composition dramatically affects the rate of corrosion of the metallic substrate. Coatings with low and high content of curing agent exhibit early failure when exposed to an air-saturated 0.5 M aqueous solution of NaCl. Extended lifetimes were observed for samples with a content of the curing agent equal to 8.5% by weight. An increase in lifetime was also observed on increasing the film thickness and the curing temperature. The failure mechanism seems to occur in two steps: the first step is related to water uptake in the film while the second step is related to Cl− ion diffusion through the coating. The results have been interpreted in terms of a model in which the dielectric properties (capacitance and resistance) of the coating depend strongly on the coating composition and are affected by both the water and salt uptake into the film.

A study of corrosion initiation on polyimide coatings

Abstract The protective properties of thin polyimide (PI) films on aluminum and iron metallic substrates were studied using a.c. and d.c. electrochemical techniques. The effects of storage time, film thickness, and the synergistic effect of temperature (80°C) and relative humidity (85%) on the lifetime of the PI were investigated. The results obtained can be summarized as follows: (i) early failure was observed for the PI/AI samples tested directly after curing, while extended lifetime was observed for samples stored in a desiccator for 2 months before testing, (ii) early failure was also observed for the PI/Fe samples even after storage in a desiccator before testing, (iii) the lifetime increases by increasing the film thickness, and (iv) temperature and relative humidity (RH) dramatically affect failure. A mathematical model is presented to relate coating failure to salt uptake by the PI. By applying this model to the experimental findings, an estimate of the heterogeneities or defects in the PIs is given.

Monitoring Degradation of Single and Multilayer Organic Coatings I. Absorption and Transport of Water: Theoretical Analysis and Methods

This paper presents a mathematical model describing the change in the capacitance of both single and multilayer organic coatings exposed to a wet environment. The indication is that the capacitance technique can successfully be employed to monitor, in situ, the absorption of water through single-layer organic coatings for values of equilibrium water uptake to a maximum of 5% by volume. In addition, this model indicates a relationship for d(ln C)/d{radical}t, where C is the coating capacitance and t is the time of exposure to the environment, that can be used to rank different single-layer coatings. Conversely, no quantitative correlation between mass uptake and coating capacitance was found for a multilayer coating system, even for low values of water uptake. Although, at face value, these findings suggest that this technique may be inappropriate for in situ monitoring of water absorption in such complex systems, a comparison between gravimetric and capacitance data indicates that useful qualitative information may still be generated by the use of the latter technique.

THE EFFECT OF FILM THICKNESS ON OXYGEN SORPTION AND TRANSPORT IN DRY AND WATER-SATURATED KAPTON POLYIMIDE

Abstract Sorption and transport of oxygen in dry and water saturated Kapton® polyimide films of different thickness were investigated at 25°C with pure oxygen using: (i) a manometric permeation apparatus (gasmembranegas configuration), (ii) a high pressure gas sorption equipment, and (iii) an electrochemical technique based on the oxygen Clark electrode (liquid-membrane-liquid configuration), respectively. A sharp increase of permeability, solubility, and diffusivety coefficient as thickness increased in the range 13–50 μm was observed for the oxygen transport through both dry and water-saturated Kapton. The variation of the oxygen parameters in the range of thicknesses investigated were small in the case of dry polyimide compared to the case of the water-saturated polymer. An interpretation is presented in which the oxygen transport parameters in both dry and water-saturated conditions are analyzed in terms of ‘dual-sorption—dual-mobility’ model and related to the different polymer morphologies. such as crystallinity level, crystallinity orientation and excess free volume content, characterizing samples of different thickness.

Electrochemical characterisation of multilayer organic coatings

Abstract The protective properties of eight high performance commercial multilayer organic coatings for aeronautical use (based on polyurethane, epoxy and polyurethane-compatible epoxy resins) on anodised 2024-T3 Al alloy were evaluated in neutral aerated 3.5% NaCl aqueous solution using the electrochemical impedance spectroscopy (EIS) technique. The investigation was performed on specimens supplied and prepared from Alenia in accordance with their technical specifications. Results obtained in this paper show that all coating systems exhibit excellent protective properties even after prolonged immersion in the test solution (one year). Although the dielectric properties of all multilayer coatings are quite similar, EIS was proved to be able to discriminate among them, making possible a rank of these protective systems to prevent or to reduce corrosion. Capacitance measurements performed on the same coatings allow their behaviour to be forecasted.

Deposition of gold-containing siloxane thin films

Abstract Thin films containing gold clusters dispersed in a SiO x matrix have been deposited by simultaneous plasma-enhanced chemical vapour deposition of hexamethyldisiloxane-O 2 -Ar mixtures and r.f. sputtering of a gold target. The effect of deposition conditions on film composition and on some physical properties have been investigated. The results show that the films are mainly composed of an inorganic SiO x matrix which contains gold clusters, carbon and hydrogen atoms. By changing the gold content of the film the d.c. electrical conductivity follows the trend reported in the literature for other metal-containing plasma polymers, while the optical absorption in the UV-Vis region shows some differences from published data.

Degradation behaviour of 6013-T6, 2024-T3 alloys and pure aluminium in different aqueous media

The electrochemical and corrosion behaviour of 6013-T6, 2024-T3 aluminium alloys and pure aluminium in 0.6m NaCl and 0.3m Na2SO4 aqueous solutions was investigated using d.c. and a.c. electrochemical techniques. Results show that the 6013-T6 alloy exhibits superior corrosion resistance compared with that exhibited by the 2024-T3 alloy in both environments. These findings were interpreted on the basis of a barrier oxide film model.

Plasma-assisted deposition of tungsten-containing siloxane thin films

Abstract Composite films containing tungsten clusters dispersed in an organosilicon matrix have been deposited by simultaneous plasma-enhanced chemical vapour deposition of hexamethyldisiloxane-O2-Ar mixtures and r.f. sputtering of a tungsten target. The effect of plasma conditions on the deposition process and film composition has been investigated. It is shown that actinometry can conveniently be utilized as an in-situ process parameter.

Transport of water dissolved oxygen in polymers via electrochemical technique

Abstract Transport properties of polymer membranes for dissolved oxygen in a liquid-membrane-liquid configuration were investigated by using an electrochemical technique based on the use of the Clark oxygen electrode. The oxygen permeability and the diffusivity coefficient in a polymer-water system can be determined accurately by appropriate choice of the experimental conditions. Polymers that do not absorb water such as polyethyleneterephtalate (PET) and bioriented polypropylene (OPP) were tested for permeation experiments in both gas-membrane-gas and liquid-membrane-liquid configuration. Data obtained in the liquid-membrane-liquid configuration with the electrochemical technique show excellent agreement with those obtained with the gas-membrane-gas configuration. The permeability of water saturated Kapton® polyimide (PI) for dissolved oxygen was also evaluated and compared with data obtained in the gas-membrane-gas configuration.

Exploring doxorubicin localization in eluting TiO2 nanotube arrays through fluorescence correlation spectroscopy analysis

Drug elution properties of TiO(2) nanotube arrays have been largely investigated by means of solely macroscopic observations. Controversial elution performances have been reported so far and a clear comprehension of these phenomena is still missing as a consequence of a lack of molecular investigation methods. Here we propose a way to discern drug elution properties of nanotubes through the evaluation of drug localization by Fluorescence Correlation Spectroscopy (FCS) analysis. We verified this method upon doxorubicin elution from differently loaded TiO(2) nanotubes. Diverse elution profiles were obtained from nanotubes filled by soaking and wet vacuum impregnation methods. Impregnated nanotubes controlled drug diffusion up to thirty days, while soaked samples completed elution in seven days. FCS analysis of doxorubicin motion in loaded nanotubes clarified that more than 90% of drugs dwell preferentially in inter-nanotube spaces in soaked samples due to decorrelation in a 2D fashion, while a 97% fraction of molecules showed 1D mobility ascribable to displacements along the nanotube vertical axis of wet vacuum impregnated nanotubes. The diverse drug localizations inferred from FCS measurements, together with distinct drug-surface interaction strengths resulting from diverse drug filling techniques, could explain the variability in elution kinetics.