In-vitro corrosion of AZ31 magnesium alloys by using a polydopamine coating

Magnesium alloys are candidates to be used as biodegradable biomaterials for producing medical device. Their use is restricted due to the high degradation rate in physiological media. To contribute to solving this problem, a polydopamine (PDOPA) layer could be used to increase adhesion between the metallic substrate and external organic coating. In this paper, the corrosion behaviour of samples was investigated to determine their performance during the long-term exposure in simulated body fluid. Electrochemical methods including Open Circuit Potential (OCP) and Electrochemical Impedance Spectroscopy (EIS) were used to investigate the corrosion resistance of samples. The results demonstrated a decreasing of the substrate degradation rate when PDOPA was used as interlayer supposing a synergistic effect when it was used together with the organic coating. Keywords: Magnesium alloy, Polydopamine coating, Corrosion resistance, Biodegradabilit

TiO2 Nanotubes on ti dental implant. Part 3: Electrochemical behavior in Hank’s solution of titania nanotubes formed in ethylene glycol

Anodic oxidation is an easy and cheap surface treatment to form nanostructures on the surface of titanium items for improving the interaction between metallic implants and the biological environment. The long-term success of the devices is related to their stability. In this work, titanium nanotubes were formed on a dental screw, made of titanium CP2, through an anodization process using an “organic” solution based on ethylene glycol containing ammonium fluoride and water. Then, the electrochemical stability in the Hank’s solution of these “organic” nanotubes has been investigated for 15 days and compared to that of titanium nanotubes on a similar type of sample grown in an inorganic solution, containing phosphoric and hydrofluoridric acids. Morphological and crystallographic analysis were performed by using scanning electron microscopy (SEM) and X-Ray diffractometry (XRD) tests. Electrochemical measurements were carried out to study the stability of the nanotubes when are in contact with the biological environment. The morphological measurements revealed long nanotubes, small diameters, smooth side walls, and a high density of “organic” nanotubes if compared to the “inorganic” ones. XRD analysis demonstrated the presence of rutile form. An appreciable electrochemical stability has been revealed by Electrochemical Impedance Spectroscopy (EIS) analysis, suggesting that the “organic” nanotubes are more suitable for biomedical devices

The Effect of Graphene on the Protective Properties of Water-Based Epoxy Coatings on Al2024-T3

0.5 and 1% wt. of graphene nanoflakes were added to an anticorrosive additives-free water-based epoxy resin applied to Al2024-T3 samples. Calorimetric (DSC) and adhesion (cross-cut test) tests indicated that the presence of graphene did not affect the polymerization process of the resin or its adhesion to the substrate while it had some effect on its wettability. Electrochemical Impedance Spectroscopy (EIS) results obtained suggested that the addition of a small amount of graphene greatly enhanced the protective properties of the epoxy coating, retarding electrolytes absorption and reducing the total amount of adsorbed water. The latter occurrence suggests that the graphene effect on coating performances is related to both extended diffusion pathway length and graphene/matrix interaction due to the unique properties of graphene

Correlation between electrochemical impedance measurements and corrosion rate of magnesium investigated by real-time hydrogen measurement and optical imaging

The corrosion behaviour of magnesium in chloride-containing aqueous environment was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) performed simultaneously with real-time hydrogen evolution measurements and optical imaging of the corroding surface. The potentiodynamic investigation revealed substantial deviations from linearity in close proximity of the corrosion potential. In particular, differences in the slope of the current/potential curves were observed for small polarizations above or below the corrosion potential. These observations, suggest that the usual method based on the use of the Stern–Geary equation to convert a value of resistance into a value of corrosion current is inadequate. Nonetheless, a very good correlation between values of resistances estimated by EIS and corrosion currents obtained from real-time hydrogen measurement was found. Real-time hydrogen measurement also enabled, for the first time, direct measurement of an ‘apparent’ Stern–Geary coefficient for magnesium. In order to rationalize the complex behaviours experimentally observed, an electrical model for the corroding magnesium surface is presented

TiO2 Nanotubes on Ti Dental Implant. Part 2: EIS Characterization in Hank’s Solution

Titania nanotubes are widely studied for their potential applications in several fields. In this paper, the electrochemical characterization of a dental implant, made of commercially pure titanium grade 2, covered by titania nanotubes, when immersed in Hank’s solution, is proposed. Few papers were found in the scientific literature regarding this topic, so a brief review is reported, concerning the use of some equivalent circuits to model experimental data. The analysis of results, obtained by using Electrochemical Impedance Spectroscopy, showed that: (i) a good correlation exists between the variation of Ecorr and the estimated values of the charge transfer resistance for both the bare- and the nanotube-covered samples, (ii) the nanostructured surface seems to possess a more active behaviour, while the effect could be over-estimated due to the real extent of the surface covered by nanotubes, (iii) the analysis of the “n” parameter, used to fit the experimental data, confirms the complex nature of nanostructured layer as well as that the nanotubes are partially filled by compounds containing Ca, P and Mg, when immersed in Hank’s solution. The results obtained in this work give a better understanding of the electrochemical behaviour of the nanotubes layer when immersed in Hank’s solution and could help to design a surface able to improve the implant osseointegration

The contribution of hydrogen evolution processes during corrosion of aluminium and aluminium alloys investigated by potentiodynamic polarisation coupled with real-time hydrogen measurement

Water reduction, which leads to the evolution of hydrogen, is a key cathodic process for corrosion of many metals of technological interest such as magnesium, aluminium, and zinc; and its understanding is critical for design of new alloys or protective treatments. In this work, real-time hydrogen evolution measurement was coupled with potentiodynamic measurements on high-purity aluminium and AA2024-T3 aluminium alloy. The results show that both materials exhibit superfluous hydrogen evolution during anodic polarisation and that the presence of cathodically active alloying elements enhances hydrogen evolution. Furthermore, it was observed for the first time that superfluous hydrogen evolution also occurs during cathodic polarisation. Both the anodic and cathodic behaviours can be rationalised by a model assuming that superfluous hydrogen evolution occurs locally where the naturally formed oxide is disrupted. Specifically, during anodic polarisation, oxide disruption is due to the combined presence of chloride ions and acidification, whereas during cathodic polarisation, such disruption is due to alkalinisation. Furthermore, the presence of cathodically active alloying elements enhances superfluous hydrogen evolution in response to either anodic or cathodic polarisation, and results in 'cathodic activation' of the dissolved regions

No rinse Cr free Surface Treatments on Aluminium Foil for Flexible Packaging: Opportunities, Advantages and Electrochemical Measurements for Performance Evaluation

Surface treatments of Aluminium alloys for laminates used for rigid and semi-rigid packaging sectors are well established since many years. In such applications the use of surface treatments enhances the adhesion of the polymer coating to the metallic substrate in order to increase its corrosion resistance. In flexible packaging, however, such treatments are hardly to be applied due the micrometer thickness of the laminates (normally in the range 6.3÷40 mm) that are, currently, subject to a thermal degreasing for the evaporation of fluids used in the cold rolling process. The reason for the almost no knowledge of surface treatments of rolled aluminium with micrometric thickness arises from the fact that these treatments, up to a decade ago, were made only in immersion baths with processes requiring subsequent rinsing in demineralised water (rinse process). This process, for the complexity and length of installations, is not achievable on thin rolled laminates. In the last years, however, the spread and consolidation of the treatments applied by coater not requiring any rinsing (no rinse surface treatments) opens up opportunities in the field of flexible packaging. Moreover, electrochemical techniques, such as: (i) potentiodynamic polarization measurements (used to assess the degree of protection offered by the pre-treatment), (ii) Electrochemical Impedance Spectroscopy EIS (used on rolled after coating application of polymeric coatings, in order to evaluate the corrosion resistance), (iii) capacity measurements at fixed frequency (which allow us to monitor the kinetics of absorption of water and electrolytes in the protective polymer coating), in addition to innovative methodologies such as cathodic delamination test (test carried out in continuous current through which is possible to obtain information on the adhesion of paint to the metal substrate) allow to measure accurately (and rapidly) the advantages offered by the chemical pre-treatment of the surface and to evaluate the improvements both in terms of corrosion and adhesion resistance