P.L. Bonora

Organic coating capacitance measurement by EIS: ideal and actual trends

The coating capacitance is one of the most frequently studied parameters obtained by EIS measurements for characterising the protective properties of organic coatings, because by analysing this parameter it is possible to measure the water uptake phenomena (diffusion till saturation and further increase) which are very important in barrier coatings. n nThe ideal trend described in many cases is not representative of the actual evolution measured on coatings. Often one phase is missing or a reduction of the coating capacitance is measured. n nIn this work the ideal trend of coating capacitance is compared with some real experimental results, discussing the reasons of disagreement and the limits of the model which seem to be in many cases insufficient and too simple to explain the actual water uptake processes, in particular, when a further increase of the coating capacitance occurs after saturation. The presence of different water uptake mechanisms, the heterogeneous distribution of the water in the coating and dimensional variations make the water uptake phenomenon quite complex.

Corrosion behaviour of stressed magnesium alloys

Abstract Potentiodynamic polarisation and impedance measurements are used to examine the corrosion aspects of some Mg-based alloys, which were previously stressed in order to established the effect of mechanical deformation on surface electrochemical reactions. A first approach was made for the unstressed alloys. The electrochemical tests were carried out in a sodium borate buffer solution.

Corrosion behaviour of fluotitanate pretreated and painted aluminium sheets

Abstract “Chromium free” aluminium conversion treatments usually produce a very thin chemical conversion layer; moreover they do not possess the well known inhibition properties of chromates. Hence, the corrosion protection of the substrate is, in general, less effective than that offered by the thicker chromate layers. However, the aluminium/paint adhesion is noticeably improved by the use of fluotitanate pretreatment baths. Our previous works also showed that the behaviour of fluotitanate pretreatments can be similar to that of chromatized aluminium surfaces when the aluminium is painted with a protective thermoplastic polyester resin. The aim of this work is to study new bath formulations and treatment procedures in order to further improve the effectiveness of this kind of environmentally friendly aluminium pretreatment. Some production parameters such as surface degreasing and pickling, pH and salt concentration of the bath and time of immersion were varied in order to obtain more efficient conversion layers. Aluminium alloy sheets were pretreated in fluotitanate baths and for comparison, in traditional chromate baths or they were only degreased and pickled; these samples were then studied with and without the application of organic coatings by the use of electrochemical impedance spectroscopy. The painted samples were coated with epoxy or vinyl resin types. The coatings were kept sufficiently thin (10–20 μm) in order to speed up undercoating corrosion thus highlighting the pretreatment action. By the use of proper testing solutions and of suitable fitting procedures of the impedance data, it was possible to identify the contribution of the pretreatment to the corrosion resistance of the painted aluminium alloys.

Corrosion protection properties of electroless Nickel/PTFE, Phosphate/MoS2 and Bronze/PTFE coatings applied to improve the wear resistance of carbon steel

The aim of this work is the evaluation of corrosion behaviour of some industrial coatings on carbon steel, utilised to improve tribological behaviour. In particular, composite electroless nickel coatings with polytetrafluoroethylene (PTFE) particles, sintered bronze in PTFE matrix layers and zinc phosphates—with and without MoS2 coatings—were analysed. Salt spray fog exposure and electrochemical tests were carried out. Electroless nickel coatings showed the best protection properties with a good barrier protection, because of the coating structure: an external layer, which contains PTFE particles improving tribological properties, and an inner nickel layer, which protects against corrosion because of barrier effect. PTFE-coated sintered-bronze samples showed limited corrosion resistance due to the high number of defects present. The application of a MoS2 polymer-embedded layer improves the characteristics of phosphate conversion coatings, which, by themselves, cannot assure good protection.

Degradation of epoxy coatings on phosphatized zinc-electroplated steel

Abstract The corrosion behavior of phosphatized zinc (Zn)-electroplated steel coated with epoxy films of different thicknesses was studied using electrochemical impedance spectroscopy (EIS), the br...

Determination of the Reactive Area of Organic Coated Metals Using the Breakpoint Method

Abstract Experimental results and mathematical considerations were used to investigate the physical meaning and the application limits of the breakpoint method to evaluate the reactive area in organic coatings. The physical meaning of the reactive area values of a metal under an organic coating, obtained using the breakpoint method, were reviewed. Experimental evidence indicated the area was related to coating defects and porosity, rather than to the total reactive or delaminated area. Limits in applying the breakpoint method were reviewed using mathematical considerations that showed the method was not accurate when values of the coating capacitance and double-layer capacitance were too close, which was a possibility during the initial time of degradation and also when two time constants were distinguishable.

Study of the corrosion behaviour of phosphatized and painted industrial water heaters

Abstract A technology for producing industrial water heaters exists, where the internal protective layer is an organic coating. This technology was largely adopted in the last decades and different methodologies for surface preparation or coating application were proposed and used; as an example bare steel is coated by an epoxy powder resin layer obtained by spray technique after a phosphatization treatment in an iron phosphate bath. However, when defects and organic coating discontinuities are present, the main water heater failure occurs by the detachment of large coating areas. This phenomenon is due to a fast paint delamination caused by a cathodic disbonding induced by the magnesium anode. In order to obtain a better coating adhesion, also under the typical water heater working conditions, a proper preparation of surface steel must be chosen. The aim of this work is the evaluation of different types of surface preparation, including degreasing, sand blasting and the use of iron or zinc phosphate conversion coatings. The samples prepared were coated using epoxy powder resins following the same application procedure. Adhesion was evaluated using classical methods like the pull-off test and the cathodic disbonding test. In addition, electrochemical impedance measurements were carried out on defect free samples and on others having an artificial defect. Coating delamination was studied by following the evolution with time of the electrical parameters measured from the equivalent electrical circuit. Both electrochemical tests and cathodic delamination tests showed that zinc phosphate treatments, and in particular phosphatization baths containing manganese greatly improve the metal–paint adhesion under the very stressing condition typical of the water heaters, whereas chromium passivation of iron phosphates had a very limited action. Electrochemical impedance measurements were shown to be very helpful in evaluating coatings performances, and in particular coating adhesion.

Influence of the photo-oxidative degradation on the water barrier and corrosion protection properties of polyester paints

The barrier properties of a paint are strongly influenced by the presence of defects, by photo-oxidative phenomena and by the adhesive properties of the polymer-metal interface. Impedance spectroscopy is a flexible and efficacious instrument for assessing all these phenomena and is useful in supplying information on the mechanisms of deterioration as well as assessing the physical parameters associated with it. The wide use of paints in external environments that are exposed to the effects of solar radiation and humidity makes the study of the deterioration of organic coatings under these conditions important. The coefficients of humidity diffusion and the values of permeability and saturation for different ultraviolet (u.v.) radiation intervals have been calculated, starting from the impedance data and using appropriate models. The relationship is shown between these values and the effective long-term properties of protection from corrosion of the paints.

Electrochemical studies of zinc-nickel codeposition in chloride baths

Abstract The electrodeposition of Zn-Ni alloys from a chloride bath with different Ni2+/Zn2+ ratios was studied. The bath had the composition 2 mol dm−3 KCl, 1 mol dm−3 NH4Cl, 0.5 mol dm−3 H3BO3, 0.5 mol dm−3 ZnCl2, 0.25–1.25 mol dm−3 NiCl2 · 6H2O. The cathodic polarization during electrodeposition and the alloy composition were greatly influenced by the ratios Ni2+/Zn2+ in the bath and are related to both the pH and temperature of the solutions used for electrodeposition. Under the examined conditions, the electrodeposition of the alloys was of the anomalous type. X-ray diffraction studies revealed that the alloys consisted of the γ phase (Ni5Zn21) or of a mixture of the two phases δ (Ni3Zn22) and γ. Cyclic voltammetry enabled a convenient estimation of phase compositions. Smooth and semi-bright layers with a rather uniform nickel content are obtained using additives containing the phenolic derivative and an amine compound.

Effect of chemical cleaning on the corrosion behaviour of painted aluminium alloys

Abstract Painted aluminium alloys are sometimes affected by filiform corrosion. Such a phenomenon which was intensively studied in the last few years is mainly a cosmetic type of attack. Usually it starts at coating defects or at cut edges following an anodic undermining mechanism. As described in the literature, water permeability and elasticity of the coating, oxygen diffusion paths, and presence of salts are the main controlling parameters. However, the metal coating interface is the critical point; then the substrate chemical composition, as well as the type of chemical cleaning or the type of applied chemical conversion coating proved to be fundamental aspects to improve resistance to filiform corrosion. The aim of this work is to study the filiform corrosion behaviour of aluminium alloys surface cleaned using chemically different products (alkaline, acid, combination of the two) and then treated using environmentally friendly pretreatment baths and chromates baths. The selected aluminium alloy was AA6060 type. After powder painting, aluminium samples were tested following the DIN 65472 standard. At the same time electrochemical tests were carried out following a particular procedure on samples exposed for different times to the environment promoting filiform corrosion. Under the experimental conditions used in our tests, electrochemical impedance spectroscopy was shown to be useful and allowed to obtain more quantitative data in a shorter testing time with respect to traditional testing. It was observed that resistance to filiform corrosion is greatly affected by the chemical composition of the aluminium substrate and that chemical cleaning modifies the surface chemical composition of the alloy so modifying its corrosion behaviour.