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Thin film corrosion is a serious issue in almost every sector. Thus, simulation of corrosion under thin electrolyte films has always been of high interest as experimental studies are often challenging. Thus far, progress has been made to model the effect of several important factors on thin film corrosion rates. Some of these parameters are electrolyte thickness, electrolyte composition, chemical reactions in the electrolyte, electrode size and change in electrode size, environmental parameters, and corrosion products deposition. However, these parameters are mainly drawn from different studies and have not been modelled concurrently in a single simulation study, making the thin film corrosion model far from being complete yet. Therefore, despite the many efforts so far, thin film corrosion modelers still strive to push the modelling edges further. This paper takes into account some of the highlighted recent advances in thin film corrosion modelling based on the mentioned parameters to provide a perspective on not only how far the field has come, but also how far it still is from a complete thin film corrosion model. Discussions have also been made on future needs and prospects to advance the thin film corrosion models further.

In ferrous alloys, chromia (Cr2O3) and hematite (Fe2O3) could serve as impervious passive layers to protect metals from corrosion. However, their passivating effect becomes questionable when locali...

Volta potential measurements from Scanning Kelvin Probe Force Microscopy (SKPFM) have been used over the years to discuss the nobility of an alloy’s surface and its tendency to localised corrosion. This has led to classical interpretations that may (unfortunately) over-generalise the actual Volta potential behaviour of the surface during ongoing localised corrosion. Consequently, issues on correlating Volta potential measurements to corrosion potentials and mechanisms arise. With the advent of in-situ SKPFM, which allows for the Volta potential measurements during ongoing dynamic surface electrochemical and/or physicochemical reactions, novel observations regarding Volta potential evolution are available. This calls for an updated and re-evaluated interpretation of the Volta potential measurements from SKPFM relating to localised corrosion analysis. The novel interpretation includes an additional layer of information, as compared to classical interpretations; there is greater information on the role of interfacial parameters and time (kinetics). Optimistically, this will bridge the gap of Volta potential measurements and its correlation to corrosion potentials.

The evolution of corrosion products that develop within the scratched region of a 96 wt% zinc-rich paint exposed to continuous salt spray for 1,000 h has been investigated by both electrochemical a...

Economical electro-catalysts for the oxygen reduction reaction (ORR) are highly desirable for a range of advance energy storage technologies. Chromium compounds have been suggested as one possible source of non-precious metal based catalysts for oxygen reduction reaction (ORR), especially chromia (Cr2 O3 ) which is the most stable form of Cr oxide at room temperature. Using density functional theory+U calculations, we investigate the 4-electron ORR on the hydroxylated Cr2 O3 surfaces alloyed with 17 different transition metals. On the one hand, we find that the ORR overpotential is lower when the Cr2 O3 surface alloyed with elements towards the end of both the first and second rows of transition metals. Among these elements, Cd alloyed Cr2 O3 surface is found to promote the ORR the most, but due to its high toxicity and price it loses out to Zn as the recommended alloyant. On the other hand, we find that the ORR overpotential is generally higher and less varied on the Cr2 O3 surface alloyed with the early-to-mid row transition metal elements (e. g. Zr, Ti). As Cr2 O3 is also a major component in the passive film on stainless steels, where a low ORR rate is desirable to reduce the impact of localized corrosion. This implies that alloying with early-to-mid row transition elements could be beneficial to stainless steels. The difference in oxygen reduction activity is attributed to the tendency of forming stable ORR intermediates during the oxygen reduction process.

Microbiologically influenced corrosion (MIC) is a major concern in a wide range of industries, with claims that it contributes 20% of the total annual corrosion cost. The focus of this present work is to review critically the most recent proposals for MIC mechanisms, with particular emphasis on whether or not these make sense in terms of their electrochemistry. It is determined that, despite the long history of investigating MIC, we are still a long way from really understanding its fundamental mechanisms, especially in relation to non-sulphate reducing bacterial (SRB) anaerobes. Nevertheless, we do know that both the cathodic polarization theory and direct electron transfer from the metal into the cell are incorrect. Electrically conducting pili also do not appear to play a role in direct electron transfer, although these could still play a role in aiding the mass transport of redox mediators. However, it is not clear if the microorganisms are just altering the local chemistry or if they are participating directly in the electrochemical corrosion process, albeit via the generation of redox mediators. The review finishes with suggestions on what needs to be done to further our understanding of MIC.

The role of biogenic H 2 S in the microbiologically influenced corrosion (MIC) of carbon steel was investigated. Desulfovibrio vulgaris (ATCC 7757), a sulfate reducing bacterium, was tested against C1018 carbon steel in anaerobic vials with three different sizes, each filled with 40 mL of ATCC 1249 culture medium, providing headspace volumes of 10 mL, 85 mL and 160 mL, respectively for H 2 S to escape. Results showed that a larger headspace led to a lower H 2 S concentration in the culture medium, and this increased the sessile cell count and made the iron sulfide film thinner, resulting in increased MIC.

The micro-galvanic interactions between Cu-Fe-Mn-Li-containing aluminides and the alloy matrix of aluminium alloy AA2099 during chloride-induced corrosion were investigated in-situ with real-time monitoring of the local contact potential difference (VCPD) using the scanning Kelvin probe force microscopy (SKPFM) at controlled relative humidities. The aluminides showed noble potentials and were able to ennoble their neighbouring matrix sites when a cluster of aluminides surrounded the matrix. The matrix, hence, adopted a more positive VCPD, towards that of the aluminides. The anode-to-cathode ratio changed throughout the corrosion exposure and was seen to show a dynamic character. Much higher local VCPD activities were recorded during the earliest stages of corrosion, when the Al-Li AA2099 surface was first exposed to high humidities, than in later RH cycles; a phenomenon not seen in other aluminium alloys.

Biofouling induced corrosion of stainless steels grades UNS S31603 and UNS S31254 was investigated at three sites off Singapore for 30 months. No corrosion was observed on grade UNS S31254, while the propensity for corrosion by shellfish of UNS S31603 is ranked as oyster >> barnacles >> green mussels. Oysters caused extensive localised corrosion, penetrating 2 mm thick plates with 12 months with tracks of corrosion several centimetres long. Shallow crevice corrosion was observed under dead barnacles, with an explanation presented for why corrosion is more severe under dead barnacles than live ones. Green mussels did not cause any corrosion.

Investigations in microbiological-influenced corrosion in recent years have suggested a positive correlation in steel corrosion with methanogens. However, many cases reported in the literature were performed in medium containing Na2S·9H2O or l -cysteine, from which the methanogens can produce H2S that in itself is corrosive. Here it is demonstrated that if all sources of H2S are removed, the methanogenic strain Methanosarcina barkeri only cause accelerated corrosion when deprived of traditional energy sources for methanogenesis, such as methanol and H2. Possible mechanisms for this phenomenon of starvation conditions triggering accelerated corrosion are discussed.

Microbially influenced corrosion (MIC), is acknowledged to be the direct cause of catastrophic corrosion failures, with associated damage costs ranging to many billions of US$ annually. In spite of extensive research and numerous publications, fundamental questions relating to MIC remain unanswered. The following review provides an overview of current MIC research and stresses the lack of information related to MIC recognition, prediction and mitigation. The review establishes a link between management decisions and root causes. A holistic, proactive approach to MIC is suggested in which an entire system is considered, monitored and improved.

Sensitized AA5083-H2 aluminum alloy was exposed to chloride-laden thin-film electrolyte at ambient temperature (20%–85% relative humidity) and the local Volta potential measured, in-situ and in real-time, using the Scanning Kelvin Probe Force Microscopy, with the intention to elucidate the earliest stage of localized corrosion. Positive Volta potentials vs alloy matrix were measured for magnesium silicides in ambient air, which, however, underwent a severe nobility loss during corrosion, causing their nobility to invert to active potentials (negative) relative to the alloy matrix. The reason for the nobility inversion was explained by the preferential dissolution of Mg2+, which resulted in an electropositive surface. Aluminides, both with and without silicon, were seen to form the main cathodes at all exposure conditions. The local alloy matrix next to closely-separated aluminides were seen to adopt the Volta potential of the neighbor aluminides, which, hence, resulted in local corrosion protection. The phenomenon of nobility adoption introduced in this work raises questions regarding the anode-to-cathode ratio, which was observed to change during corrosion, and the resulting impact to localized micro-galvanic corrosion. This work further demonstrates that it is necessary to measure the Volta potential during corrosion to reflect the true relationship between the Volta potential and corrosion potential or breakdown potential.

A perspective is provided on mechanisms for microbially influenced corrosion by sulfate-reducing bacteria and metal-oxidizing bacteria. The authors discuss the confusion introduced by cross-disciplinary discussions of MIC mechanisms and the divergent terminologies used by microbiologists and electrochemists, e.g. “anaerobic” and “direct electron transfer.” Examination of the cathodic depolarization theory for corrosion of carbon steel by sulfate-reducing bacteria suggests that the theory cannot explain the observations. Stainless steels containing

To improve the corrosion resistance of the steel matrix, 1% and 3% Cr and 1% Al are added to ordinary carbon steel for the design of a corrosion-resistant alloying composition. And based on the principle of electrochemical tests, a method for determining the depassivation of Cr and Al alloy steel bars in coral aggregate concrete (CAC) is established. The equivalent circuit model was used based on the electrochemical impedance spectroscopy (EIS) results to characterise the corrosion behaviour of the Cr and Al alloy steel bars and determine their corrosion resistance and the protective effect of the steel bar passivation film. The corrosion resistance of the Cr1 reinforcement to chloride increases by 37.67%, that of the CrAl reinforcement increases by 64.08%, and that of the Cr3 reinforcement increases by 171.82% with respect to that of the HPB 400 bar. The polarisation curves of the alloy steel bars with different Cr and Al contents in CAC were recorded by linear polarisation resistance (LPR), and the time variations of the corrosion potential and corrosion current in the low-alloy steel bars containing Cr and Al were analysed. In addition, the free chloride ion concentration and pH value of the CAC were measured while the Cr and Al alloy steel bars were rusting. The chloride threshold values (% by weight of concrete) of the HPB 400, Cr1, CrAl, and Cr3 steel bars are 0.515%, 0.718%, 0.950%, and 1.084%, and the chloride thresholds (Cl−/OH− ratio) are 0.288, 0.382, 0.502, and 0.693, respectively. Initial-corrosion condition behavior of the Cr and Al alloy steel bars in CAC was obtained. There is considerable scientific value and application significance attainable from the examination of the corrosion mechanism and behavior of corrosion-resistant steel bars in a harsh marine environment.

Grades 2, 5 and Grade 7 were investigated in near anaerobic (

Titanium is being considered as a candidate material for high-level nuclear waste container due to its superior corrosion resistance. However, problems may arise when titanium comes in contact with hydrogen-bearing environments, as it is vulnerable to hydrogen embrittlement. To assess the lifetime of titanium container, hydrogen content and hydrogen permeation efficiency were estimated in infiltrated water through bentonite prepared with simulated groundwater of Beishan, the preselected area in China, by electrochemical and metallurgical analysis. It is concluded that the absorbed hydrogen due to general corrosion will not be sufficient for hydrogen embrittlement to occur in titanium for at least 10,000 years after deep geological disposal. This represents more than 300 and 5,000 half-lives of Cs137 and Cs134 respectively; cesium being the most likely element to escape a repository due to its high solubility in groundwaters.

The electrolyte film thickness condensation and evaporation is an important parameter for complexity of atmospheric corrosion. Atmospheric corrosion rate of zinc at the west coast of Singapore was measured for one year using an electrical resistance monitoring system. The analysis of the data reveals that significant corrosion rates only occur at specific hours on dry days. The beginning of this period corresponds to falling temperature and rising relative humidity resulting in the formation of a film of moisture on the zinc surface and the end corresponds to a point that this film dries as the temperature increases. This finding allowed the drying rate of the moisture film to be estimated for input into a moving boundary simulation model of the galvanic corrosion in scratched and zinc coating samples. The simulation results showed that the maximum corrosion rate occurs at electrolyte thickness of about 8 μm. Moreover, the simulation suggested that cut-edge is a more harmful defect than scratch, which was confirmed by the appearance of iron corrosion products on atmospheric exposed cut-edge samples whereas scratched samples were not corroded after one week of exposure. Finally, moving boundary simulation allowed to predict the changes to the geometry of the corroding electrodes.

The mechanism of corrosion protection of steel substrate by a 96 wt% zinc primer that was damaged by scratch was simulated. The primer was approximated to pure zinc layer due to high percentage of zinc. Results show that while the middle of scratch is protected by sacrificial protection, at the corners of scratch a local cathodic inhibition results in barrier protection due to variations in the dissolved oxygen concentration. Moving boundary simulation allowed to predict the changes to the geometry of the corroding electrodes. Finally, taking into account zinc electrodes consumption, the period of sacrificial protection was predicted by simulations.

Materials of choice for offshore structures and the marine industry have been increasingly favoring materials that offer high strength-to-weight ratios. One of the most promising families of light-weight materials is titanium alloys, but these do have two potential Achilles` heels: (i) the passive film may not form or may be unstable in low oxygen environments, leading to rapid corrosion; and (ii) titanium is a strong hydride former, making it vulnerable to hydrogen embrittlement (cracking) at high temperatures in low oxygen environments. Unfortunately, such environments exist at deep sea well-heads; temperatures can exceed , and oxygen levels can drop below 1 ppm. The present study demonstrates the results of investigations into the corrosion behavior of a range of titanium alloys, including newly developed alloys containing rare earth additions for refined microstructure and added strength, in artificial seawater over the temperature range of to . Tests include potentiodynamic polarization, crevice corrosion, and U-bend stress corrosion cracking.

The critical chloride threshold concentration and the polarizability of the barrier layer/outer layer (as defined by the Point Defect Model) have been determined for AISI 1020 carbon steel in simulated concrete pore solutions of pH 9, 10, 11 and 12.5 with varying chloride and bicarbonate/carbonate concentrations. It was found that bicarbonate/carbonate ions have a beneficial effect on the charge transfer resistance, the critical chloride threshold concentration and the pitting potential. When the critical chloride threshold concentration for pitting is exceeded, the inhibitive effectiveness of bicarbonate/carbonate against pitting corrosion is around one order of magnitude lower compared to hydroxide.

The corrosion performance of commercial hard disk media which was subjected to bi-level surface modification has been reported. The surface treatment was carried out by bombarding the surface of the magnetic media with C+ ions at 350 eV followed by 90 eV using filtered cathodic vacuum arc (FCVA). The energy and embedment depth of the impinging C+ ions were adjusted by applying an optimized bias to the substrate and simulated by a Stopping and Range of Ions in Matter (SRIM) code which predicted the formation of a graded atomically mixed layer at the carbon-media interface. Cross-section transmission electron microscopy (TEM) revealed the formation of a 1.8 nm dense nano-layered carbon overcoat structure on the surface of the media. Despite an ~ 33% reduction in the thickness, the bi-level surface modified disk showed corrosion performance similar to that of a commercially manufactured disk with a thicker carbon overcoat of 2.7 nm. This improvement in the corrosion/oxidation resistance per unit thickness can be attributed to the formation of a dense and highly sp3 bonded carbon layer, as revealed by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. This study demonstrates the effectiveness of the bi-level surface modification technique in forming an ultra-thin yet protective overcoat for future hard disks with high areal densities.

The understanding of microbial influenced corrosion (MIC) in aerobic mixed biofilms benefits from advanced microscopy and microbial ecology characterization of biofilms. Here, the onset of MIC in stainless steel coupons was studied in both natural and artificial seawater. Rapid selection of biofilm-forming microorganisms from natural seawater was observed for field experiments. Potential ennoblement was observed only in natural seawater. A seawater derived mixed microbial consortium enriched in artificial seawater was used to characterize the effect of several parameters on MIC. The concentration of organic carbon was the major determinant of MIC, while shaking speed and polishing played minor roles. The biofilm was preferentially formed at the grain boundaries. These results outline the need for MIC onset characterization with mixed microbial consortia to predict long-term corrosion behavior of stainless steel in seawater. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Published version

AISI 416 is a high sulphur martensitic stainless steel with elliptical shaped sulphide inclusions over 2 μm in length. After laser melting the sulphide inclusions became spherical and reduced in size to below 0.5 μm in diameter. SEM investigations showed that the total number of inclusions increased dramatically and it was calculated that the total volume of sulphur in the stainless steel had not changed during the laser treatment. The pitting potential was increased by over 200 mV with an inverse linear dependency on the average inclusion size. The frequency of metastable pitting events decreased by a factor of 100.

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.

It has long been accepted that manganese sulphide favours pitting on stainless steels. However, there are different standpoints on the most important mechanism for pit initiation; due to dissolution of sulphide inclusions, chromium depletion around the inclusion or mechanical rupture of the passive film by metal chlorides. Analysing the pitting potential and metastable pitting rates on different grades of stainless steels has rationalised the effect of sulphide content on pitting corrosion resistance. In situ atomic force microscopy (AFM) has been used in conjunction with conventional electrochemical techniques for imaging real time pit initiation events.

This study evaluated the corrosion behaviour of two high copper dental amalgam alloys [Dispersalloy (Dentsply-Caulk) and Tytin (Kerr)] in different electrolytes. Amalgam specimens were prepared, coupled to a copper wire, cemented into glass tubes and polished to a 600-grit finish. A corrosion cell was prepared using a carbon counter-electrode, a standard calomel electrode as the reference and amalgam as the working electrode. The alloys were tested in the following mediums at 37 degrees C: (i) artificial saliva based on Fusayama's solution (FS), (ii) artificial saliva with citric acid adjusted to pH 4.0 (FC) and (iii) 1% sodium chloride solution (SC). Corrosion potentials (E(corr)) and corrosion rates (I(corr)) were determined using potentiostatic and impedance spectroscopy methods. Data was subjected to anova/Scheffe's post hoc test at 0.05 significance level. For both alloys, the corrosion potential in FS was significantly greater than in SC. Corrosion potential of Tytin in FS and SC was also significantly greater than in FC. The corrosion rate of Dispersalloy in FC was significantly greater than in FS and SC. For Tytin, corrosion rate in SC was significantly greater than in FS and FC. Although no significant difference in corrosion potential/rate was observed between the alloys when tested in FS, significant differences were observed when electrochemical testing was carried out in FC and SC. The corrosion behaviour of high copper amalgam alloys are both material and environment dependent. Certain food substances may increase the corrosion of high copper amalgams.

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.

In Sweden, high-level radioactive waste will be disposed of in a canister with a copper outer container and a cast iron or carbon steel insert. If the iron insert comes into contact with a...

In Sweden, high-level radioactive waste will be disposed of in a canister with a copper outer container and a cast iron or carbon steel insert. If the iron insert comes into contact with anoxic geological water, anaerobic corrosion will occur. This paper presents a study of the electrochemical aspects of anaerobic corrosion of carbon steel in artificial Swedish granitic groundwaters. The electrochemical measurements confirm that anaerobic corrosion will lead to the generation of hydrogen, but the rate of corrosion will be anodically limited by the formation of a corrosion product film. The corrosion rate was unaffected by hydrogen pressures up to 100 atm.

With the trend of decreasing the thickness of the carbon overcoat on high-density magnetic recordings, the corrosion properties of a hard disk multilayer structure are becoming a hot issue. An ideal overcoat would be uniform across the disk's surface, have low porosity and electrical conductivity as well as being impermeable to water. However, in reality this is not viable and corrosion is inevitable. One possible method of improving the corrosion resistance of carbon overcoats is to introduce carbide bilayers. In this paper the corrosion protective properties of CrC/C and BC/C nanolaminated overcoat structures have been analyzed by an electrochemical technique based on LPR measurements. The results allowed an overcoat's resistance to be correlated to its porosity and a statistical method was developed to allow easy comparison of results. The results from the electrochemical measurements were correlated with a range of surface analysis techniques.

Porous titanium has mechanical advantages over its solid counterpart, especially for use in surgical implants, however, its corrosion resistance is not as good. The relative stability of various oxide films on both porous and solid titanium was thus investigates. The point of initial failure of the oxide films on porous titanium was found to be within the depths of the pores. It is also postulated that galvanic coupling to the outer surface, via a mechanism akin to crevice corrosion, may promote the rapid dissolution in the pores. Preliminary, results suggest that thermal oxides offer better corrosion protection to porous titanium than anodic oxides.

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.

The commercial application of rare earth magnets has been in part held back by their poor corrosion resistance. As part of e!orts to overcome this problem two types of nickel coatings were electroplated on the surface of Nd 2 Fe 14 B substrates, one from an acidic plating solution and the other from an alkaline solution. The corrosion characteristics of both coatings were evaluated both by electrochemical potentiodynamic polarisation and by physical exposure to high-humidity environments. The results showed that both coatings enhanced the corrosion resistance of substrate. However, both the degree of protection a!orded by the coating and the strength of its adhesion to the substrate strongly depended on the chemical composition of the original plating solution. The coating plated from the acidic solution provided a high degree of protection against both aqueous and atmospheric corrosion, whilst that plated from the alkaline solution behaved only marginally better than the bare substrate. It was postulated that the poor performance of the coatings plated from the alkaline bath could have been due to the spontaneous formation of a passive oxide "lm between the Nd 2 Fe 14 B substrate and the nickel coating. ( 2001 Elsevier Science B.V. All rights reserved.

Both potentiostatically and galvanostatically controlled electrochemical impedance spectroscopy were used to show that the impedance obtained from passivated titanium electrodes is dominated by the capacitance of the space-charge layer. As a result neither the thickness of the oxide layer nor its rate of thinning can be readily determined from impedance measurements. Furthermore, it appears that the corrosion rates of passive metals determined by EIS may significantly overestimate the true rate of dissolution. No significant differences were observed between the impedance data obtained from the galvanostatic and the potentiostatic measurements.

The electrochemical behavior of two kinds of Cu–Al–O coatings prepared by CVD at different temperatures was monitored anodically and cathodically in a solution of 0.1M NaCl. The results show that the coating achieved at 700°C has lower resistance against anodic polarization than the coating achieved at 800°C. Alternatively, the former exhibits rather higher susceptibility to hydrogen evolution than the latter. The atomic hydrogen evolves at the surface of the coating and penetrates deeply into the coating resulting in initiation of hydrogen bubbles. With the increase of hydrogen pressure in bubbles, they grow continuously leading to hydrogen-induced cracking ultimately at the moment when the hydrogen pressure reaches a critical value. Essentially, the susceptibility of hydrogen-induced cracking for coatings should be attributed to the temperature that prevailed during CVD. Higher temperature can evidently enhance the resistance against hydrogen-induced cracking.

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.

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.

Electropolishing of p-type silicon has been investigated over a wide range of HF concentrations (0.01-11 wt. %) by potentiodynamic polarization. Oxide dissolution rates were determined from the plateau current densities observed in the electropolishing region during the reverse sweeps; i.e. where the growth and dissolution rates of the anodic oxide film are believed to be equal. Based on the shape of the CV curves the oxide dissolution process was treated as a corrosion process controlled by the dissolution of a salt film, that is its rate controlled by removal of dissolved products away from the surface rather than reactants to the surface as previously proposed. Although a contribution from HF from bulk to surface cannot be completely ruled out, because the removal of the initial dissolution product can be by either mass transport or further chemical reaction with HF species in solution this mechanism is capable of explaining the dependence of the dissolution rate on HF concentration for the whole range investigated.

Aluminum and zinc based sacrificial anodes are routinely used to provide corrosion protection to metals (typically steel) exposed to seawater, for example in steel pipelines and storage tanks. However, the high fouling rates experienced in South East Asia means that both the anodes and the metals to be protected rapidly become coated with macrofoulers, which could potentially prevent the anodes from being effective. The present study, involving exposure tests of up to 18 months, indicates that both aluminum and zinc sacrificial anodes remain effective even after being completely coated with biofouling. Furthermore, it was easier to remove the biofouling on the cathodically protected samples than on their unprotected counterparts, possibly due to the higher local pH produced by cathodic protection at the metal and seawater interface.

When exposed to 0.9% NaCl type 304 stainless steel undergoes severe pitting corrosion within a matter of days. However, a Sherman plate fabricated from type 304 stainless steel remained inside a patient's arm for almost 40 years without any visible indications of corrosion. Given the previous understanding of the pathological environments this was considered quite remarkable. It is proposed that the low dissolved oxygen levels found in human-body fluids makes the long-term in vivo environment much more benign than would be anticipated from in vitro experiments. Furthermore, it is proposed that previous cases of localized pitting corrosion on stainless steel implants most likely arose due to the development of short-term aggressive conditions due to pathological changes in the surrounding tissue as a result of the trauma of the implant procedure. In the present case the Sherman plate was sufficiently small that the surrounding tissue was not aggravated sufficiently to lead to the development of such an environment aggressive. The conclusion that surgical implants are at most risk during the first few weeks of service implies that short-term corrosion protection methods, such as coatings, may be more effective than previously thought.

Data Storage Institute, National Universityof Singapore, Singapore 119260For high density magnetic recording, it is necessary to protect the magnetic layer from wear and corrosion with a thin carbonovercoat. On ultrasmooth disk substrates, magnetron sputtering is still the preferred technique because it can provide protectivecarbon layers as thin as 4 nm. Degradation of the magnetic layer occurs in the atmosphere and during hard disk operation; thus,the carbon-based overcoat functions as an indispensable component in protecting the essential magnetic layer. Moreover, carbonovercoat quality plays a vital role in the strong demand to reduce the flying height between a disk and read/write head, which inturn enhances the density of recording. The impact of sputtering conditions on the quality of deposited amorphous hydrogenatedand nitrogenated carbon was assessed by the electrochemical measurement of the linear polarization resistance. The film’spolarization resistance was correlated to its porosity, and a statistical method was developed to allow easy comparison of resultsin the data analysis. The results obtained suggest that the best overcoats are formed with a substrate voltage bias of 2120 V, avery low deposition pressure of 0.9 mTorr, a dc magnetron sputtering power of 2.8 W/cm

The corrosion of titanium in acidic solution leads to the accumulation of substantial amounts of hydrogen in the metal, modifying the subsequent voltammetric behaviour of the electrode. SIMS has been used to show that hydrogen penetrates into the metal to a depth of several microns during prolonged exposure to acid solutions and the SIMS images obtained suggest that crystalline platelets of titanium hydride are formed in a titanium matrix under these conditions. The hydride is only partially oxidised during subsequent anodisation of the titanium, so that successive passivation/activation cycles lead to the progressive build up of hydride.