S. Mischler

Wear‐Accelerated Corrosion of Passive Metals in Tribocorrosion Systems

A model is presented which describes the effect of mechanical and materials parameters on the wear-assisted corrosion rate of passive metals under sliding wear conditions. The model is based on a consideration of contact between the sliding surfaces at multiple asperities and it takes into account the passivation behavior of the metal. Wear experiments were carried out in a reciprocating pin-on-plate tribometer permitting the control of mechanical and electrochemical conditions. An alumina pin was rubbed on nickel, chromium, stainless steel, and titanium alloy plates, in sulfuric acid or sodium sulfate solution. The relative importance of mechanical and electrochemical metal removal was evaluated while applying an anodic potential. Additional experiments were performed under cathodic polarization. The results show that the proposed model can describe correctly the effect on dissolution rate of different mechanical parameters such as applied normal force, stroke length, frequency, and sliding speed. Qualitative agreement was observed with the predicted effect of the materials parameters hardness and passivation charge, but uncertainties concerning the real value of passivation charge and, in some cases, wear of the alumina pin limit the predictive capability of the model when comparing different materials. The experimental results obtained in this study demonstrate that to understand the mutual interactions between mechanical and electrochemical parameters affecting wear-accelerated corrosion it is necessary to look at the tribocorrosion system as a whole.

Third body effects and material fluxes in tribocorrosion systems involving a sliding contact

Abstract Tribocorrosion is an irreversible transformation of a material resulting from simultaneous physicochemical and mechanical surface interactions taking place in a tribological contact. Tribocorrosion involves numerous synergy effects between mechanical and chemical or electrochemical phenomena. In the present paper a general framework for the interpretation of electrochemically controlled tribocorrosion experiments involving a sliding contact between a passive metal and an inert counter body is proposed. The analysis is based on material fluxes and takes into account the formation, transformation and ejection from the contact of third body particles. The usefulness of the proposed approach is illustrated with examples from the author’s laboratory.

Electrochemical modeling of passivation phenomena in tribocorrosion

Abstract The relationship between repassivation behavior and tribocorrosion is investigated using an iron–chromium alloy (AISI 430) in sulfuric acid as a model system. Tribocorrosion experiments were performed in a reciprocating motion apparatus under electrochemical polarization. A constant potential in the passive region was applied and the current variation during individual stroke periods was measured as a function of applied potential and stroke frequency. Theoretical models were developed taking into account the film growth kinetics and the ohmic drops in the electrolyte between wear scar and reference electrode. The model could simulate the general trends observed in the current transients but the agreement with experimental data was only fair. Several reasons for this are discussed. They include pin surface roughness influencing the real depassivated area, insufficiently known electrochemical conditions in the contact zone and role of third body particles.

Interactive Effects of Albumin and Phosphate Ions on the Corrosion of CoCrMo Implant Alloy

The corrosion behavior of CoCrMo alloy in simulated body fluids has been analyzed by electrochemical techniques and surface analysis. Interaction of albumin and phosphates present in the body fluids on the passive film of the alloy was also investigated. Electrochemical techniques such as potentiodynamic and potentiostatic polarization and electrochemical impedance spectroscopy were employed. Further, ex situ X-ray photoelectron spectroscopy and Auger electron spectroscopy analysis of the passive films were carried out. The study reveals that phosphates and proteins present in simulated body fluid play a significant role in the electrochemical properties of the metal/oxide/electrolyte interface. Surface analysis showed that both species competitively adsorb on the alloy surface. For a given passive potential, the impedance behavior of passive CoCrMo was found to depend on the way passive conditions were established. A simple model has been developed assuming a multilayer structure of the surface including an outer layer where albumin and phosphate ions adsorb, the passive film (inner layer), and the metal. This model is consistent with the obtained electrochemical and surface analysis results.

The role of passive oxide films on the degradation of steel in tribocorrosion systems

Abstract Applying electrochemical polarization during wear experiments permits one to control the surface chemistry (presence or not of a passive oxide film) of a metal rubbing against an insulating body and to determine the relative contribution of mechanical (wear) and chemical (corrosion) mechanisms on the overall metal degradation. The degradation of carbon steel sliding against alumina was investigated in borate solution of pH 8.4 using a reciprocating ball on plate wear test rig equipped with an electrochemical cell. Three different hardness values were produced by application of appropriate heat treatments to the steel samples. It was found that the presence of a passive film decisively influenced the degradation mechanism and led to increased wear compared to the oxide free metal. Two different degradation mechanisms, wear accelerated corrosion and mechanical surface degradation favored by the passive film were identified as being responsible for this behavior and their relative contribution to overall material loss was determined.

The chemical composition of the passive film on Fe24Cr and Fe24Cr11Mo studied by AES, XPS and SIMS

Abstract A systematic study of the composition and of the thickness of passive films formed at 65°C on Fe24Cr and Fe24Cr11Mo model alloys in an acid sulfate electrolyte in the presence and absence of chloride ion at different potentials has been carried out. Non-destructive XPS, AES depth profiling and dynamic SIMS were applied to the study of the average concentration and the depth distribution of cations and anions in the films. The apparent film thickness and the average metal ion concentration were found to be little affected by the presence or absence of chloride ion in the electrolyte. Molybdenum influenced the depth distribution of chloride ion in the films.

Tribocorrosion behaviour of Fe–17Cr stainless steel in acid and alkaline solutions

The effect of applied potential, load and frequency on the tribocorrosion behaviour of a ferritic stainless steel in aqueous solution is investigated under sliding wear conditions using a reciprocating motion tribometer including an alumina pin sliding on a metal plate. The tribometer is equipped with a reference electrode and a counter electrode for electrochemical experiments. The total metal removal rate of the stainless steel is determined from the rate of penetration of the pin and from an analysis of the wear track at the end of a wear experiment while the fraction of electrochemically removed metal is deduced from the current measured during the wear experiments. Two electrolytes are used, 0.5 M H2SO4 and 1 M NaOH. Applied potentials are in the passive potential region except for a few experiments performed under cathodic polarisation. The obtained results demonstrate the interdependence of electrochemical and mechanical mechanisms and the importance of the passivation behaviour of the metal in the electrolyte used. In sulphuric acid the electrochemical metal removal rate depends strongly on the value of the applied potential in the passive region, but not in NaOH because in the latter electrolyte the anodic charge serves only for film formation rather than dissolution. The effect of mechanical parameters on the rate of electrochemical metal removal is in qualitative agreement with a previously developed model. The presence of debris in the contact after test was found to depend on the prevailing electrochemical conditions.

A tribo-electrochemical apparatus for in vitro investigation of fretting-corrosion of metallic implant materials

Abstract A novel apparatus has been developed for the investigation of fretting–corrosion behaviour of metallic materials used in prosthetic implants. Well-defined micro-movements are achieved through a rigid overall structure and the precise alignment of the components. An electrochemical cell is used for controlling the surface chemistry of the metal in contact and for studying the role of anodic oxidation. During an experiment, the most relevant mechanical and electrochemical parameters are continuously monitored using a computer-based data acquisition system. The apparatus permits, thus, in vitro experiments under well-controlled mechanical and electrochemical conditions. The good time resolution and reproducibility of the measurements permit the observation of correlations between different mechanical and electrochemical quantities. Preliminary results show that the prevailing corrosion conditions critically affect the overall fretting behaviour of Ti 6 Al 4 V.

The effect of mechanical and electrochemical parameters on the tribocorrosion rate of stainless steel in sulphuric acid

The tribocorrosion behaviour of a passive AISI430 ferritic stainless steel sliding against alumina in 0.5 M sulphuric acid has been studied using a tribometer that allowed for electrochemical measurements. The effects of applied potential, normal load, sliding velocity, sliding distance and alumina roughness on the friction, wear volume and anodic current were determined. The obtained results are interpreted in terms of a previously proposed theoretical model describing the average current in terms of depassivation rate and repassivation charge, taking into account the role of non-electrochemical parameters such as applied normal force, sliding velocity, metal hardness and contact configuration. Under the experimental conditions of the present study, the depassivation rate was found to govern the measured anodic current. Consideration of how mechanical and operational parameters affect the depassivation rate led to a consistent description of their effect on the tribocorrosion rate in accordance with the proposed model.

Influence of passivity on the tribocorrosion of carbon steel in aqueous solutions

Abstract The degradation of DIN 34CrNiMo6 steel sliding against alumina was investigated in NaOH and in borate buffer solutions of pH 8.4 under electrochemically applied passive potentials. Small amounts of chromate or chloride were added to the borate solutions to vary the nature of the passive film. Film thickness and composition were characterised using surface analysis (Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS)). For comparison, tribocorrosion experiments were carried out at appropriate applied cathodic potentials, where no iron oxidation is thermodynamically possible, and therefore no passive film is formed. Results show the crucial role played by surface oxidation not only with respect to wear accelerated corrosion processes but also on surface mechanical phenomena. Depending on the nature of the solution, passive carbon steel exhibits either subsurface cracking or plastic flow accompanied by wear. In absence of passive film, wear is negligible and large plastic flow of the metal occurs, independent of the nature of the solution.