J.D. Scantlebury

The application of electrochemical measurements to the study and behaviour of zinc-rich coatings

Abstract Three zinc-rich coatings, two with polyamide cured epoxy binders and one with an ethyl silicate binder applied to grit blasted mild steel were chosen for examination. The two epoxy coatings differed in terms of their particle size distribution, but were similar in all other respects. The silicate system was chosen to be very similar to one of the epoxy systems except for the chemical nature of the binder. Techniques chosen to study these coatings included impedance, potential vs time for a fully coated and partially coated system, exterior exposure under marine conditions and SEM on film sections during exposure. The electrochemical response of each system was found to be highly dependent on the particle size distribution of the zinc rich pigment. Where pigment particles were comparable in size with the film thickness, the electrochemical behaviour could be modelled in terms of simple charge transfer processes. With smaller zinc particles, a diffuse time constant model is more appropriate. In the silicate system, a porous silicate layer is formed above the zinc-rich layer and the electrochemical response is determined by the nature of this porous layer.

The effect of surface/primer treatments on the performance of alkyd coated steel

The wet and dry adhesion, resistance to corrosion and resistance to cathodic delamination of alkyd coated steel pre-treated with a variety of surface/primer treatments were assessed using a variety of techniques. It was found that while some treatments substantially enhanced adhesion, and others substantially improved resistance to corrosion and/or cathodic delamination, the phenomena could not be related. It was possible to enhance measurable wet and dry coating adhesion while decreasing substantially the resistance to corrosion. The reverse was also observed, with some corrosion and delamination restricting treatments substantially reducing the measurable mechanical adhesion strength of the coating.

An electrochemical study of zinc-containing epoxy coatings on mild steel

Abstract A comparative electrochemical study has been carried out between conventional zinc-rich and novel zinc-containing paints. To study the cathodic protection performance of these coatings an assessment method based on a galvanic couple was employed. Current and potential were monitored with immersion time in 3.5% NaCl. The zinc-rich paint offered cathodic protection for about 7 days compared with 40 days for the new paint. However, the steel coupled with the zinc-rich remained corrosion free for a further period. This was probably due to the inhibitive effect of zinc salts deposited at the cathode. Thereafter a reversal in polarity was obtained once rust formed. Thus it is concluded that established cathodic protection criteria cannot be solely used to assess the protection properties of zinc-containing paints. Electrochemical impedance spectroscopy was also used to study the state of the coating with time. Diffusion parameters indicated that the new paints were covered with a lesser amount of corrosion products.

Electrochemical changes in chloride-contaminated reinforced concrete following cathodic polarisation

Abstract The effectiveness of the removal of chloride by electrochemical means from cast-in-sodium chloride-contaminated reinforced concrete has been studied. An externally-derived cathodic current was applied between the reinforcing steel and an inert anode/electrolyte system at the concrete surface. Two applied cathodic-current densities of 1 and 3 A m −2 of concrete surface were investigated. Corrosion-rate results using a potentiostatic polarisation resistance technique indicate that the chloride-extraction treatments successfully removed chloride ions from the reinforcement/concrete interfacial region and repassivated the reinforcing steel.

The influence of light on corrosion phenomena: the behaviour of mild steel in citric acid solution

Abstract Experiments using photopotential, galvanic current, a.c. impedance and harmonic analysis techniques were carried out to observe the effect of visible light on mild steel in a citric acid solution. The photopotential and galvanic current techniques show that the steady state potential and current after illumination of one of the electrodes is negative with respect to the electrode in darkness. Both the impedance and harmonic analysis techniques indicate an increase in the corrosion rate of the illuminated electrode. On coating the mild steel with a vinyl lacquer, the effect of illumination is eliminated.

Poly(2-allyl)phenylene oxide electropolymer films as an adhesion-enhancing treatment for steel

The enhancement of metal-polymer adhesion via thin reactive electropolymer films of poly(2-allyl)phenylene oxide was studied using adhesive joint failure techniques. Thin (0.2-0.4 micron) films were formed on steel blocks using potentiostatic electrooxidation. Two blocks were cemented together to form adhesive joint specimens using an unsaturated polyester/styrene resin system. Adhesive joint specimens were also formed from both untreated blocks and treated blocks where the allyl functionality of the electropolymer was destroyed by heat crosslinking. Failure energies were then determined for joints with variable areas of adhesion using an impact-based technique, and the relationship of adhered area to measured failure energy was obtained for the three systems. The failure interface was examined by scanning electron microscopy. It was found that the poly(2-allyl)phenylene oxide-treated system gave the highest failure energies, followed by the untreated system. The electropolymer film system with pre-crosslinked allyl functionality produced the lowest failure energies. These results were related to the nature of the chemical interactions present at the interfaces, in accordance with the theories of adhesion proposed by Kinloch.

Potential-time measurements on mild steel in etch primer solutions

Abstract Experiments were carried out in a two pack system; polyvinyl butyral, zinc tetroxychromate in one pack, and phosphoric acid in the other with an isopropanol solvent in both packs. This study examines the functions of the phosphoric acid and the chromate containing pigment separately by utilising the feature that the alcohol dispersed components have sufficient conductivity to carry out potential-time measurements in the wet components before and during mixing.

Photopotentials and the corrosion of mild steel in sodium sulphate solution

Abstract Experiments were carried out to observe the effect of light on the photopotential of mild steel in a neutral sodium sulphate solution. It has been shown that the effect is a function of the presence of corrosion products on the specimen. The sign of the photopotential is thought to be related to the semi-conducting properties of the surface layers.

Blistering phenomena on lacquered mild steel

Abstract Concentrically divided mild steel panels were used as substrates underneath artificial paint blisters, and electrical current flow was measured between the concentric regions during immersion in sea water, using a zero resistance ammeter. It was demonstrated that currents do flow between different regions of a steel substrate underneath a paint film, whilst it is corroding. Contamination of the blisters with sodium chloride was found to promote the corrosion processes and the current flow. Different types of corrosion processes were observed in the contaminated blisters, often with frequent movement of anodic and cathodic regions. In one instance, one of the contaminated artificial blisters grew to resemble a standard dome-shaped natural blister, and this was found to have a central anodic region surrounded by an outer cathodic region.

Current flow suppression by polyphenylene-oxide electro-polymer film formation

Abstract This paper examines the phenomena of current flow suppression that can be observed when platinum is anodically polarised in solutions of phenol and several of its derivatives. The formation of polymer films is examined with respect to the mechanism of current flow suppression. The techniques involved in this study are primarily electrochemical in nature, such as potentiodynamic sweeping and a.c. impedance spectroscopy. The results lead to some unusual conclusions, in that for a number of examples the polymer film resistance does not appear to dominate the resistive response.