F. Deflorian

Electrochemical impedance spectroscopy as a tool for investigating underpaint corrosion

Electrochemical Impedance Spectroscopy (EIS) has been widely utilised in the last few years as a tool, together with surface analysis, to investigate the protecting properties of organic coatings for metals. The possible defects and limitations of coatings are detectable with the use of such experimental techniques. The metal coated system is generally fairly complex and consists of a metal substrate (mild steel, aluminium, zinc, magnesium, stainless steel, etc.), a surface pretreatment (chromatation, phosphatation, or mechanical surface preparations, etc.) and some layers of paint with different chemical and physical properties (adhesion promoters, primers, intermediate layers and top coatings, etc.). All these parameters can influence the electrochemical behaviour measured by EIS, and therefore also the electrical models (equivalent electrical circuits) used to explain the impedance results. The aim of this work is to introduce the approach of electrochemical impedance data interpretation by equivalent electrical circuits on organic coated metals showing some examples relevant to different metal substrates or coating cycles.

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. The 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. In 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.

The use of electrochemical techniques to study the corrosion behaviour of organic coatings on steel pretreated with sol–gel zirconia films

The two main features of a protective organic coating are its adhesion and corrosion protection. In order to improve both, chemical pretreatments have been used. The use of chromates was very popular, but recently they have been highly restricted because of their toxicity, so that chromate-free pretreatments have been developed and tested. An interesting alternative seems to be the deposition on the metallic surface of thin layers of zirconia by the sol–gel process. In this study thin films of amorphous zirconia on low carbon steel sheets have been obtained by the dip-coating technique, using two different complexing reagents. Control of the hydrolysis allowed the formation of ZrO2 films suitable as pretreatments, promoting the adhesion of organic coatings. The behaviour of these samples was compared with steel samples pretreated in conventional phosphatation baths. The adhesion of a polyester organic coating was evaluated by the pull-off technique, by measuring the detachment of cross-scratched samples after salt fog chamber testing, or by swelling the organic coating in methyl pyrrolidone. According to the results, the samples pretreated with zirconia layers showed promising performance, in comparison with commercial chemical treatments (tricationic phosphate and iron phosphate). The organic coating adhesion on zirconia films was found to depend strongly on the process parameters, e.g. concentration of the precursors solutions and chelating agents, which determine the thickness and the amount of organic residuals of the amorphous zirconia. Electrochemical impedance spectroscopy (EIS) was also used in sodium sulphate solutions to evaluate adhesion and the corrosion behaviour of these materials. No barrier properties of the zirconia films were observed. Resistance to delamination was studied by using samples where an artificial defect was made by mechanical tools of different diameter. The information obtained by EIS was in good agreement with the data obtained by salt fog chamber tests.

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.

Water up-take evaluation of new waterborne and high solid epoxy coatings. Part II: electrochemical impedance spectroscopy

Abstract Electrochemical impedance spectroscopy (EIS) was used to elaborate the characteristic of waterborne and high solid epoxy coatings. The clear coats were investigated under long-term immersion condition. The coating capacities and the coating resistances of the two classes of materials revealed important influences of the coating formulations, like internal or external emulsification, degree of crosslinking and type of resin and hardener. The high solid systems showed a higher water up-take for 100% crosslinking compared to 80%, which was effected by an increased number of free amine groups. For waterborne coatings the chemical nature of the hardener and the choice of emulsifier dominated the effects of different degrees of crosslinking. Waterborne coatings had always higher dielectric constants, whereas after a few hours of immersion the resistance of high solids were 1–2 order of magnitude higher.

Wear–corrosion mechanism of hard chromium coatings

Abstract Hard chromium coatings are frequently used for application where corrosion and wear degradation are combined, as an example in earthmovers, in mining transport systems or in moulds for plastics. The aim of this work is to study the mechanisms of tribocorrosion of hard chromium coated steel bars in a sodium chlorides solution under sliding wear. Using a self-made wear–corrosion apparatus, many parameters were monitored to analyse both kinds of damage: the mechanical and the electrochemical one. Different test conditions were considered by changing imposed load on the counterface and rotation speed. It was observed that the change of testing parameters was insignificant on the degradation mechanism: mainly adhesive wear was observed and secondary abrasive wear (at test beginning) and localised corrosion were present. Degradation synergy was also observed both by electrochemical tests and weight loss measurements.

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.