L. Fedrizzi

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.

Characterization of cellular solids in Ti6Al4V for orthopaedic implant applications: Trabecular titanium

EBM (Electron Beam Melting) technology can be used successfully to obtain cellular solids in metallic biomaterials that can greatly increase osseointegration in arthroprothesis and at the same time maintain good mechanical properties. The investigated structures, called Trabecular Titanium, usually cannot be obtained by traditional machining. Two samples: (A) with a smaller single cell area and, (B) with a bigger single cell area, were produced and studied in this project. They have been completely characterized and compared with the results in similar literature pertinent to Ti6Al4V EBM structures. Relative density was evaluated using different methods, the mean diameter of the open porosities was calculated by Scanning Electron Microscope images; the composition was evaluated using Energy-Dispersive X-Ray Spectroscopy; the microstructure (alpha-beta) was investigated using chemical etching and, the mechanical proprieties were investigated using UMTS. The mean porosity values resulted comparable with spongy bone (63% for A and 72% for B). The mean diameter of the single porosity (650 mum for A and 1400 mum for B) resulted compatible with the osseointegration data from the literature, in particular for sample A. The Vickers micro-hardness tests and the chemical etching demonstrated that the structure is fine, uniform and well distributed. The mechanical test proved that sample (A) was more resistant than sample (B), but sample (B) showed an elastic modulus almost equal to the value of spongy bone. The results of this study suggest that the two Ti6Al4V cellular solids can be used in biomedical applications to promote osseointegration demonstrating that they maybe successfully used in prosthetic implants. Additional implant results will be published in the near future.

Electrochemical study of titanium nitride films obtained by reactive sputtering

Abstract TiN coatings were prepared by reactive sputtering on glass substrates. The production parameters were chosen in order to obtain stoichiometric TiN. The electrochemical behaviour of the samples was investigated using both d.c. and a.c. electrochemical techniques in aqueous solutions: two sulphatic and one chloric. The chemical composition of the surface was analysed by Auger electron spectroscopy before and after the polarization tests. The TiN coating exhibited a very high free corrosion potential and a small anodic current, which indicates a very low reactivity in all the considered solutions. In the anodic range a current peak occurred which corresponds to the formation of an oxidized titanium layer with semiconducting properties.

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.

Corrosion behaviour of electrogalvanized steel in sodium chloride and ammonium sulphate solutions; a study by E.I.S.

Zinc and zinc-nickel (13% Ni) electrodeposits were passivated by dipping in chromate baths and characterized by scanning electron microscopy. The corrosion behaviour was studied using a.c. electrochemical techniques; electrochemical impedance spectroscopy (EIS) measurements were performed at open circuit and under galvanostatic control during the 24 h immersion time. In sodium chloride solution the zinc-nickel electrodeposits show a better corrosion resistance compared to the pure zinc coatings. During the immersion time, a surface nickel enrichment was observed which, together with the zinc corrosion products, acts as a barrier layer reducing the total corrosion rate. In the same solution the passivation treatment improves the corrosion resistance of the electrodeposits; nevertheless, on zinc substrates, the protection exerted by the chromate film is not, always effective during the immersion time. On the contrary the chromate coating on zinc-nickel substrates induces a remarkable and durable improvement of the corrosion resistance reducing the zinc dissolution almost completely. In the ammonium sulphate solution, the corrosion mechanism is significantly influenced by hydrogen reduction on the zinc-nickel surfaces, and by the production of a local surface acidity which is aggressive for the chromate coatings.

Dry and wet corrosion behaviour of AISI 304 stainless steel coated by sol-gel ZrO2CeO2 films

Abstract ZrO 2 CeO 2 coatings were deposited on 304 stainless steel substrates by dip coating in alkoxide solution. The coatings were transparent and continuous at thickness below about 700 nm. The films were used as barriers against oxidation for 304 stainless steel at high temperature. The best performance (no oxidation up to 16 h at 750 °C) was obtained with a coating approximately 1 μm thick. The corrosion resistance of the coated samples in an aggressive electrolyte (15 wt. % HCl solution) was studied by electrochemical impedance spectroscopy, a technique frequently used to test the protective properties of organic and metallic coatings on metals. The protective action of the deposited coatings was discussed as a function of the film thickness and quality.