A. Collazo

Characterisation of the barrier properties of different paint systems. Part I. Experimental set-up and ideal Fickian diffusion

The water uptake of a commercial alkyd paint system is studied using the impedance spectroscopy technique. The first part of the paper is devoted to establish the experimental setup to accurately measure capacitance values in the pF range, characteristic of the studied system. It was concluded that electrochemical instrumentation is not adequate to perform this task. The second part of the paper discusses the water uptake of the paint system under two different immersion conditions: deionized water and 5% NaCl solution. This analysis leads to the conclusion that the diffusion process seems to follow the ideal-Fickian behaviour in both systems, the corresponding diffusion coefficients were calculated.

Characterisation of the barrier properties of different paint systems: Part II. Non-ideal diffusion and water uptake kinetics

Abstract Three commercial paint systems, based on water-borne acrylic, chlorinated rubber and alkyd resins, were used to evaluate the water uptake in 5% NaCl solution (wt.%) and partially deionised water. Impedance spectroscopy allows following the coating capacitance evolution and, from this, water uptake versus immersion time can be tracked. Its analysis leads to the conclusion that diffusion process seems to be an ideal Fickian one just in one type of paint. The two others have important deviations from this ideal behaviour. In those conditions, an apparent diffusion coefficient can be estimated. The study of the electrolyte flux with exposition time was analysed and fitted to an expression, which includes four exponential terms with four time constants, associated to different processes. Three time constants behave as exponential decay functions and the fourth one has an exponential growth shape. A physical meaning for the expression is suggested. The values of all the studied parameters indicate that the barrier properties of the water-borne acrylic paint are the worst.

Comparative study between galvanised steel and three duplex systems submitted to a weathering cyclic test

Two types of samples, uncoated galvanised steel (UGS) and galvanised steel coated with chlorinated rubber paint (DS1), alkyd paint (DS2) and water-borne acrylic paint (DS3) were tested in a weathering cyclic chamber. The behaviour evolution of these systems was tracked using electrochemical impedance spectroscopy. For the duplex system, regardless of the type of paint, high impedance values were obtained, even at long exposure time. The UGS samples exhibited lower impedance values. Such behaviours could be interpreted in terms of different nature of the corrosion products generated on zinc surface. Using scanning electron microscopy, energy dispersive X-ray and X-ray diffraction techniques the identification of those compounds was possible. The main corrosion product generated on UGS was hydrozincite, Zn5(CO3)2(OH)6, in the case of the duplex systems, the corrosion product formed was zincite, ZnO. Such difference suggests that the zinc oxide formed under the coating is the responsible of the better duplex system behaviour and so, it leads a better passivation of zinc surface than the obtained with hydrozincite, which is the main corrosion product formed on UGS. In that sense, the coating acts as a selective membrane, avoiding the CO2 uptake and so, hydrozincite formation is inhibited. This behaviour was observed even for coatings with poor barrier properties, such as water-borne acrylic one.

Corrosion behaviour of cermet coatings in artificial seawater

Abstract The electrochemical behaviour of three cermet coatings applied on low alloyed steel has been studied in artificial seawater using Zero Resistance Ammetry techniques. The evolution of corrosion potential and galvanic current with immersion time reveals the existence of two processes involved in coating corrosion. The initial stage involves pore filling by electrolyte and steel corrosion due to oxygen reduction at the cermet coating. The galvanic effect between steel and cermet coating leads to local acidification of pore bottom that decreases galvanic effect and increases steel acid corrosion. EIS measurements on individual components of the steel/coating galvanic couple reveal that the overall impedance spectrum results from the parallel combination of both anodic and cathodic reactions. Steel corrosion defines the shape of the spectrum at the low frequency range.

Electrochemical Impedance Spectroscopy Study of the Corrosion Process on Coated Galvanized Steel in a Salt Spray Fog Chamber

Abstract The behavior of three different paint systems was investigated using the electrochemical impedance spectroscopy (EIS) technique. The coatings, based on chlorinated rubber, alkyd resin, and...

Electrochemical Impedance Spectroscopy as a Tool for Studying Steel Corrosion Inhibition in Simulated Concrete Environments—Red Mud Used as Rebar Corrosion Inhibitor

Red mud is the main by-product of the Bayer process for alumina extraction from bauxite. Rich in iron, aluminum and silicon oxides, it is believed to have corrosion protection properties. Some are studied in the present paper. The corrosion behavior of steel rebars in chlorine containing solutions and mortars is studied and compared with the same systems containing red mud in suspension (case of solutions) or added as powder to the mix (case of mortars). The results in solution show that red mud is able to maintain steel passivity for more than 90 days in highly chlorinated alkaline solutions while in NaOH and Ca(OH)2 solutions having the same pH pit formation is observable at 25 and 55 days, respectively. Additions of red mud to mortar as powder representing 2 % by weight of cement are able to block chloride depassivation. EIS data allow understanding the protecting mechanism and comparing the behavior in mortar and in solution.

Corrosion Protective Properties of Hydrotalcites Doped Hybrid Sol-Gel Coatings on Aluminium Substrates

Hydrotalcite-like compound that belongs to anionic clay family was prepared by classical co-precipitation method using magnesium and aluminium nitrates as precursors. Different weight percentages of hydrotalcite with Mg/Al ratio of 2.5 was added (1, 5 and 10 % wt.) to hybrid sols prepared by copolymerization of 3-Glycydoxypropyltrimethoxysilane (GPTMS) and tetra-n-propoxyzirconium (TPOZ). The aim of this work was to study the effect of the hydrotalcite incorporation to hybrid sol-gel coatings on aluminium alloys as substitution of chromate inhibitors. Hybrid sol-gel coatings were deposited by dip coating method on AA2024-T3 substrates. By Scanning Electron Microscopy (SEM) and mechanic profilometry it was possible to see the distribution of HTCP agglomerates which varies from nanometres to micrometers. The corrosion performance was evaluated by the accelerated test salt fog spray chamber (SSFC) and by Electrochemical Impedance Spectroscopy (EIS). The obtained results show the corrosion resistance improvement at higher weight percentages of HTCP.

Evaluation of environmentally friendly paints over weathering galvanised steel

A comparative study between a high solids paint and a traditional coat was made using electrochemical impedance spectroscopy technique, both paints were applied over weathering galvanised steel. The results indicated that the high solids paint had the better behaviour and thus represents a good environmentally friendly alternative. Previously, the optimum degree of weathering on a galvanised surface was assessed on the basis of the impedance modulus evolution and the roughness profiles. In the last part of this work, the macroscopic behaviour of a new paint system, based on the high solids priming coat and a water-borne resin as a topcoat, was evaluated by submitting it to two different accelerated tests. The influence of weathering degree was also analysed. In both the cases, the increase of this parameter was related to an improvement in behaviour.

Influence of Surface Finishing on Corrosion Behaviour of Mg-Al Hydrotalcite Doped Hybrid Sol-Gel Coatings on Al Alloy in Saline Environment

The behaviour of hybrid sol-gel coatings doped with hydrotalcite-like compound applied on AA2024-T3 alloy was investigated in the present work. The hybrid sol-gel coatings were prepared by copolymerization of 3-Glycydoxypropyltrimethoxysilane (GPTMS) and tetra-n-propoxyzirconium (TPOZ). Three different surface finishing on the aluminium alloy AA-2024-T3, grounded, chemical etching and degreased were studied. A sol-gel film with hydrotalcite 5% w/w was deposited by dip coating procedure. The presence of hydrotalcite particles was confirmed by means of X-ray diffraction technique. The different surface morphologies were characterised using mechanical profilometry. The corrosion resistance properties were studied by means of electrochemical impedance spectroscopy and salt fog spray cabinet. The results show higher barrier properties in grounded aluminium substrates.