Patrik Schmutz

Impedance characterization and modeling of electrodes for biomedical applications

A low electrode-electrolyte impedance interface is critical in the design of electrodes for biomedical applications. To design low-impedance interfaces a complete understanding of the physical processes contributing to the impedance is required. In this work a model describing these physical processes is validated and extended to quantify the effect of organic coatings and incubation time. Electrochemical impedance spectroscopy has been used to electrically characterize the interface for various electrode materials: platinum, platinum black, and titanium nitride; and varying electrode sizes: 1 cm/sup 2/, and 900 /spl mu/m/sup 2/. An equivalent circuit model comprising an interface capacitance, shunted by a charge transfer resistance, in series with the solution resistance has been fitted to the experimental results. Theoretical equations have been used to calculate the interface capacitance impedance and the solution resistance, yielding results that correspond well with the fitted parameter values, thereby confirming the validity of the equations. The effect of incubation time, and two organic cell-adhesion promoting coatings, poly-L-lysine and laminin, on the interface impedance has been quantified using the model. This demonstrates the benefits of using this model in developing a better understanding of the physical processes occurring at the interface in more complex, biomedically relevant situations.

Influence of trace impurities on the in vitro and in vivo degradation of biodegradable Mg-5Zn-0.3Ca alloys

The hydrogen evolution method and animal experiments were deployed to investigate the effect of trace impurity elements on the degradation behavior of high-strength Mg alloys of type ZX50 (Mg-5Zn-0.3Ca). It is shown that trace impurity elements increase the degradation rate, predominantly in the initial period of the tests, and also increase the materials susceptibility to localized corrosion attack. These effects are explained on the basis of the corrosion potential of the intermetallic phases present in the alloys. The Zn-rich phases present in ZX50 are nobler than the Mg matrix, and thus act as cathodic sites. The impurity elements Fe and Mn in the alloy of conventional purity are incorporated in these Zn-rich intermetallic phases and therefore increase their cathodic efficiency. A design rule for circumventing the formation of noble intermetallic particles and thus avoiding galvanically accelerated dissolution of the Mg matrix is proposed.

ICP-MS, SKPFM, XPS, and Microcapillary Investigation of the Local Corrosion Mechanisms of WC – Co Hardmetal

WC-Co hardmetal exhibits high corrosion susceptibility in aqueous solutions, related to complex microscale reaction mechanisms. This paper presents developed methods to characterize the local distribution of surface reactions difficult to assess by conventional electrochemical methods. Laterally resolved electrochemical potential distributions measured using scanning Kelvin probe force microscopy (SKPFM) under controlled humidity identified the more noble nature of WC and the microscale galvanic coupling with the Co areas acting as anodes. Inductively coupled plasma mass spectrometry (ICP-MS) element analysis, carried out using an online flow cell, provided simultaneous, time-resolved detection and quantitative concentration measurement of the dissolved elements. W ions in solution at the open-circuit potential indicated chemical dissolution due to the pH increase on the WC cathodes in addition to electrochemical anodic Co dissolution. Various mechanisms attributed to homogeneous dissolution of microscale phases or dissolution transients related to localized corrosion attack are identified. X-ray photoelectron spectroscopy (XPS) revealed a carbon-rich surface layer on the WC grains supporting a mechanism of selective W dissolution. These different techniques provided information on the microscale reactions on WC-Co surfaces in aqueous solution and allowed construction of a comprehensive model.

In Situ Microtomographically Monitored and Electrochemically Controlled Corrosion Initiation and Propagation in AlMgSi Alloy AA6016

Corrosion susceptibility and the type of corrosion in AlMgSi alloys depend strongly on microstructure. However, it is still unclear what microstructural features trigger corrosion initiation. The direct correlation of second-phase particles with the corrosion propagation path has also never been thoroughly explored. In this work, the role of second-phase particles in corrosion initiation and propagation were studied in alloy AA6016 by using a microelectrochemically controlled microtomography technique. Three topics are discussed in detail, the role of Fe-containing intermetallics, the effect of MgSi particles, and an evidenced type of directed corrosion (exfoliation-like attack): (i) Intergranular corrosion initiates preferentially in Fe-containing intermetallics located at grain boundaries. However, its propagation path is not promoted by these intermetallics. (ii) MgSi particles do not act as transition sites to in-depth corrosion propagation, even if they dissolve. (iii) A type of corrosion called exfoliation-like attack is observed. It is characterized by straight attack into the bulk material. The propagation is found to be independent of grain boundaries and second-phase particles.

The influence of heat treatment and plastic deformation on the bio-degradation of a Mg-Y-RE alloy.

In this study the bio-degradation behavior of a Mg-Y-RE alloy in different heat treatment states with respect to the alloys potential application as biodegradable implant material was investigated by electrochemical impedance spectroscopy in two body-similar fluids. The heat treatments increase the degradation resistance of the alloy and lead to the formation of a thermal oxide layer on the sample surface and to a change in microstructure such as the distribution of yttrium. The varying Y distribution in the alloy does not significantly influence the degradation behavior, and all samples show a similar low polarization resistance. However, samples with a thermal oxide layer, which consists mainly of Y(2)O(3), degrade much more slowly and feature remarkably high polarization resistance. Nevertheless, in some cases localized corrosion attack occurs and drastically impairs performance. Cracks in the oxide layer, intentionally induced by straining of the samples and which in practice could originate from the implantation process, reduce the corrosion resistance. However, these samples perform still better than polished specimens and show a macroscopically homogeneous degradation behavior without localized corrosion. Microscopically, corrosion attacks start at the cracks and undermining of the oxide layer occurs with time. For all the material conditions a remarkable dependence of the degradation rate on the electrolyte is noted.

Flow microcapillary plasma mass spectrometry-based investigation of new Al–Cr–Fe complex metallic alloy passivation

Al-Cr-Fe complex metallic alloys are new intermetallic phases with low surface energy, low friction, and high corrosion resistance down to very low pH values (0-2). Flow microcapillary plasma mass spectrometry under potentiostatic control was used to characterize the dynamic aspect of passivation of an Al-Cr-Fe gamma phase in acidic electrolytes, allowing a better insight on the parameters inducing chemical stability at the oxyhydroxide-solution interface. In sulfuric acid pH 0, low element dissolution rates (in the µg cm(-2) range after 60 min) evidenced the passive state of the Al-Cr-Fe gamma phase with a preferential over-stoichiometric dissolution of Al and Fe cations. Longer air-aging was found to be beneficial for stabilizing the passive film. In chloride-containing electrolytes, ten times higher Al dissolution rates were detected at open-circuit potential (OCP), indicating that the spontaneously formed passive film becomes unstable. However, electrochemical polarization at low passive potentials induces electrical field generated oxide film modification, increasing chemical stability at the oxyhydroxide-solution interface. In the high potential passive region, localized attack is initiated with subsequent active metal dissolution.

The Role of Inclusions in the Corrosion Resistance of Hydrostatically Extruded Steel Products

The corrosion behaviour of 316LVM in NaCl solution steel has been investigated. Large scale polarization measurements and etching tests in V2A solution showed that the hydrostatically extruded (HE) 316LVM steel had a lower corrosion resistance than specimens in the as received condition. Small area measurements were carried out in order to determine the sites that were responsible for the decreased corrosion resistance of the HE 316LVM steel. Tests on cross sections parallel to the HE direction, showed better corrosion resistance than on cross sections perpendicular to the HE direction. These observations indicate, in combination with SEM and AFM measurements, that the inclusions which were deformed during HE, can trigger the onset of corrosion on HE 316LVM steel.

Real space crystallography of a complex metallic alloy: high-angle annular dark-field scanning transmission electron microscopy of o-Al4(Cr,Fe)

High-angle annular dark-field scanning transmission electron microscopy (HAADF STEM) has been performed along the low-index zone axes of the o-Al4(Cr,Fe) complex metallic alloy to obtain a real-space representation of the crystal structure and to elucidate the materials inherent structural disorder. By comparing experiments with multislice STEM simulations, the model previously suggested by X-ray diffraction is further refined to provide a new set of positions and occupancies for the transition metal atoms. Pmnb is suggested as the new space group for the o-Al4(Cr,Fe) phase. A nonperiodic layer-type modulation, averaged out in bulk diffraction methods, is detected, corroborating the need for complementing bulk diffraction analysis with real-space imaging to derive the true crystal structure of Al4(Cr,Fe).

A preliminary quantitative XPS study of the surface films formed on pure magnesium and on magnesium-aluminium intermetallics by exposure to high-purity water

An XPS investigation was carried out on the surface film formed by exposure to high-purity water, on mechanically polished Mg and the two Mg-Al intermetallic compounds: Al3Mg2 and Mg17Al12. The result for mechanically polished pure Mg indicates that a film of MgO covered by a Mg(OH)2 layer, formed by the reaction of MgO with water vapour in the air. On immersion in distilled water, this film was hydrated to a duplex film with an inner MgO layer next to the Mg metal and an external porous layer of hydroxide. For both intermetallics, there was preferential dissolution of magnesium from the mechanically ground surface and also during aqueous immersion. After immersion, there was a 10 nm thick, stable film on the surface; the film composition on Al3Mg2 was whilst that on Mg17Al12 was .

Influence of Composition and Roughness on Localized Corrosion of Al-Mg-Si Alloys Characterized by Microelectrochemistry

The influence of surface roughness on the pitting potential using the electrochemical microcell technique is reported. Two Al-Mg-Si model alloys of varying manganese content were investigated. With potentiodynamic anodic polarization in 0.1 M NaCI solutions and with different sizes of exposed area it is shown, that a critical defect density is present which controls the pitting potential at small polarization potentials. This defect density is related to the crevice distance. With smoother polished surfaces a larger crevice distance is measured. The pitting potential measured on an exposed area smaller than the crevice distance reveals that small effects of the alloying element Mn become visible. For the fine polished surface it is shown that the addition of 0.5 wt% Mn to the model alloy lowers the pitting potential.