D.J. Blackwood

Corrosion protection by multilayered conducting polymer coatings

Multilayered coatings, consisting of combinations of the conducting polymers polyaniline (Pani) and polypyrrole (Ppy), were galvanostatically deposited on to both carbon steel and stainless steel. Potentiodynamic polarisation was used to assess the ability of these copolymers to provide an effective barrier to corrosion in chloride environments. For carbon steel the performance of these multilayered coatings on carbon steel were not sufficiently better than for single Pani coatings to justify their more complicated deposition procedures. However, in the case of stainless steels the new multilayered coatings proved to be significantly better than previously reported single Pani coatings, especially at protecting against pitting corrosion. It was found that the degree of protection was a function of the deposition order of the copolymer, with films consisting of a Pani layer over the top of a Ppy layer yielding the best results. Scanning electronic microscopy observations and adhesion measurements, along with the electrochemical data suggested that the ability of a conducting polymer film to act as electronic and chemical barriers were more important in providing corrosion protection than its ability to act as a physical barrier.

Corrosion behaviour of porous titanium–graphite composites designed for surgical implants

Abstract Porous titanium is a popular surgical implant material. Its low elastic modulus encourages regular bone growth, whilst its porosity enables bone in-growth. Unfortunately, porous titanium has poor friction and wear properties. The addition of graphite, to lower the friction coefficient and titanium carbide to increase wear resistance, could produce a multi-component material that overcomes these disadvantages. However, polarisation measurements revealed that the graphite causes an increase in the titanium’s corrosion rate. Therefore, the extent that graphite can be used in porous titanium biomaterials will be a compromise between the benefits of improved friction properties against the disadvantage of increased corrosion.

Biomaterials: Past successes and future problems

The practice of using metals and alloys to repair or replace human body parts is now well established. This paper will briefly review the successes and, in reality, the remarkably few failures of the traditional materials-mainly titanium alloys, cobalt-chrome alloys, amalgams and stainless steels. The paper will then concentrate on the problems likely to be encountered before new advanced, but not necessarily corrosion resistant, materials can be successfully used for in-vivo purposes. Two of the most important parameters in determining the suitability of a material for biomedical applications are its biocompatibility and corrosion resistance. With regard to these two parameters pure titanium appears to be almost the perfect biomaterial. Unfortunately, many surgical and dental applications require materials with specific mechanical properties, such as high strength or ductility, which pure titanium is unable to provide. Hence materials such as the Ti6A14V alloy and 316L stainless steels are used despite their poorer corrosion resistance, occasionally with undesirable consequences. Furthermore, there is a desire to make use of a number of advanced materials, such as: memory-shaped alloys, porous materials and composites, low precious metal amalgams and rare earth magnets. Unfortunately, nearly all of these materials have inadequate corrosion resistances to be used directly in-vivo without some form of protection. However, it is obvious that most of the traditional techniques for reducing corrosion rates, such as controlling the environment and cathodic protection, cannot be applied to biomaterials, even coatings are of only limited use since many orthopaedic and dental devices are subjected to wearing and abrasion processes. As a result it has traditionally been considered that the only successful method of reducing corrosion within the human body is to fabricate the implants from a corrosion resistant alloy. However, although this approach may eventually be successful in some areas, for example it may be possible to use alloying additions to design a highly corrosion resistant memory-shape alloy; it is highly unlikely that such an approach will be successful with respect to rare earth magnets. One challenge therefore appears to be to design biocompatible coatings that can resist both the chemical and mechanical environments, which are often poorly defined for both orthopaedic and dental applications, without degrading the very property that is required in the advance material. Potential examples of these applications are hydroxyapatite (HA) coatings on porous titanium, titanium or titanium nitride films on Ni-Ti memory-shaped alloys and very high chromium ferritic stainless steel claddings on rare earth magnets.

Controlled intensity emission from patterned porous silicon using focused proton beam irradiation

We have fabricated light emitting porous silicon micropatterns with controlled emission intensity. This has been achieved by direct write irradiation in heavily doped p-type silicon (0.02Ωcm) using a 2MeV proton beam, focused to a spot size of 200nm. After electrochemical etching in hydrofluoric acid, enhanced photoluminescence is observed from the irradiated regions. The intensity of light emission is proportional to the dose of the proton beam, so the PL intensity of the micropattern can be tuned and varied between adjacent regions on a single substrate. This behavior is in contrast to previous ion beam patterning of p-type silicon, as light is preferentially created as opposed to quenched at the irradiated regions.

Electrochemical and Photoelectrochemical Characterization of the Passive Film Formed on AISI 254SMO Super-Austenitic Stainless Steel

The structure and composition of the passive film formed on the super-austenitic AISI 254SMO stainless steel in borate solution were studied using photocurrent spectroscopy coupled with electrochemical measurements. Charge passed calculations suggested that the Cr(VI) phase formed in the transpassive region is insoluble. Evidence for the presence of Ni in the passive film was also found. Photocurrent spectroscopy revealed that an n-type Fe(III) oxide phase dominated at more positive potentials, where Cr(VI) was expected. However, once the Cr(III) phase started to form in the reverse potential scan both Fe(III) and Cr(III) phases appeared together in the passive region between 200 and -300 mV, with both showing n-type behavior. At more negative potentials, cathodic photocurrents were observed, most likely due to the reduction of the Fe 2 O 3 to p-type FeO. The magnitude of the bandgaps of the oxides were found to be independent of the values of the applied potential.

The influence of heat treatment on the corrosion behaviour of amorphous melt-spun binary Mg–18 at.% Ni and Mg–21 at.% Cu alloy

The corrosion behaviour of melt-spun amorphous Mg 82 Ni 18 and Mg 79 Cu 21 ribbons have been investigated using hydrogen evolution testing in 3% (0.51 M) solution and electrochemical techniques in 0.01 M NaCl. Open circuit potential measurements showed that the as-spun alloys were more electrochemically noble than that of pure Mg. Dissolution rate measurements showed that the corrosion rates of the partially crystallised samples were lower than that of the fully amorphous samples. Potentiodynamic polarisation results showed that although there was ennoblement in the corrosion potential E corr , the corrosion current density i corr was higher for the melt-spun alloy than for pure Mg. Comparison of the polarisation responses for the partially crystallised samples in 0.01 M NaCl showed that the passivation current density i p was lower than for the as-spun amorphous sample. With prolonged heat treatment duration, fully crystallised samples exhibit a marked deterioration of corrosion resistance. The corrosion results have been discussed and correlated with the progress of crystallisation processes by means of XRD and TEM.

Effect of heat treatment on the corrosion behaviour of amorphous Mg-18 at% Ni alloy

Abstract The effect of heat treatment on the corrosion resistance of rapidly solidified amorphous Mg 82 Ni 18 ribbons has been studied by hydrogen evolution testing in 3% NaCl solution and potentiodynamic scanning in 0.01 M NaCl solution. The results show that the dissolution rate for the partially crystallised sample obtained after 4 min heat treatment at 160°C, was lower in comparison to the fully crystalline or fully amorphous samples. Comparison of the polarisation responses of the samples showed that, for the partially crystallised samples, passivation current density was lower than that of the fully amorphous sample, although passivation behaviour was weaker. With prolonged heat treatment duration, fully crystallised samples exhibited a marked deterioration of corrosion resistance. The corrosion results were discussed and correlated with the progress of crystallisation processes by means of XRD, DSC, TEM and SEM.

Electrochemical & optical characterisation of passive films on stainless steels

The formation and breakdown of the passive film are mainly controlled by ionic and electronic transport processes; processes that are in turn controlled by the electronic properties of the film. Consequently a comprehensive understanding of mechanisms behind passivity and localised corrosion require a detailed perception of the electronic properties of the passive films together with compositional and structural information. As a step towards this goal the passive film on austenitic stainless steel, AISI 316L, formed in borate solution was characterised by in situ Raman spectroscopy and photocurrent spectroscopy coupled with electrochemical measurements. The composition, structure and semiconductivity of the passive films depended on the potential; Fe rich n-type oxide and a Cr rich p-type oxide dominated at more positive potentials and more negative potentials respectively whilst n-type dual layered film formed at intermediate potentials. Analyses of the bandgap determined for these oxides suggested their structures to be Fe2O3 and a Fe-Cr spinel. This hypothesis was supported by the results of in situ Raman spectroscopy.

Controlled Shift in Emission Wavelength from Patterned Porous Silicon Using Focused Ion Beam Irradiation

Photoluminescence images containing several distinct color emissions, from green to red, have been obtained using high-energy focused ion beam irradiation, in conjunction with metal-aided anodization of 4 Ω cm p-type silicon. The ion irradiation increases the local resistivity in a controlled manner resulting in smaller hole currents flow through the irradiated areas. This causes a controlled redshift of up to 200 nm in the photoluminescence emission, which in terms of the quantum confinement model would correlate to larger nanocrystallites forming in the irradiated region.

Corrosion in Body Fluids

The development of corrosion-resistant alloys means that today, the likelihood of a biomedical device suffering a corrosion-related failure is very small. The important remaining areas of concern are fretting and corrosion fatigue, but even here, advances in TiN coatings and fixation techniques are extremely encouraging. However, device failure is not the only concern, perhaps more worrying is that metallic ion concentrations may reach levels sufficient to harm the patient, possibly even induce cancer. This concern is heightened as younger patients are receiving implants, meaning that required device performance lifetimes are increasing, and with it, the likelihood of corrosion-related failures and health problems instigated by elevated metal levels.