Anne Neville

Modelling the tribo-corrosion interaction in aqueous sliding conditions

In corrosion wear, one of the most commonly observed phenomena is the synergism where both corrosion and wear are significantly increased by the tribo-corrosion interactions, leading to much greater material losses than produced by the sum of losses by either process alone. However, mechanisms for the synergistic effect are generally not well understood and there has been little effort in modelling synergistic processes. In this paper, mechanisms for the interactions between corrosion and sliding wear of metals are proposed and a mathematical model is developed which incorporates the various factors affecting corrosion wear, including experimental and environmental conditions and material properties. Many of the observed phenomena in corrosion wear can be explained. It also provides a good basis for wear map construction and would be especially useful in carrying out dimensional analyses for constructing such maps.

A study of the erosion-corrosion behaviour of engineering steels for marine pumping applications

This paper presents results of a series of electrochemically based tests under liquid and liquid-solid erosion conditions on three generic materials, a C-Mn steel, an austenitic and a duplex stainless steel. Separation of the components of pure erosion, pure corrosion and synergy has been achieved under conditions imposed by an impinging saline flow containing a high burden of suspended solids. Although the pure corrosion component of deterioration on the stainless steels has been shown to be very small, the synergistic processes ensure that corrosion has a significant effect on these martials as well as on the less corrosion resistant C-Mn steel. The results have shown that complex hydrodynamic conditions impose mixed modes of attack since local variations in impingement angle exist.

An assessment of the corrosion behaviour of high-grade alloys in seawater at elevated temperature and under a high velocity impinging flow

Abstract Laboratory experiments were carried out to investigate the effect of elevated temperature (up to 60 °C) and a high velocity impinging flow on the corrosion behaviour of stainless steels and related Ni-base and Co-base alloys. DC-electrochemical techniques were employed in conjunction with microscopy to determine the extent and mechanisms of attack under the severe imposed conditions. Experiments demonstrated the clear effect of increased temperature in facilitating premature breakdown of passivity on all materials and the effect of the high velocity impinging flow was to further shift the passivity breakdown potential to more active values but not necessarily to result in greater depth of attack. In addition, the study has indicated that the relative resistances of the different materials to attack under static, ambient temperature conditions is altered significantly when they are subjected to elevated temperatures and/or high velocity seawater impingement.

Linking electrochemical corrosion behaviour and corrosion mechanisms of thermal spray cermet coatings (WC–CrNi and WC/CrC–CoCr)

Abstract In this experimental study the corrosion characteristics (rates and mechanisms) of two thermally-sprayed cermet (ceramic/metal composite) coatings have been examined in a static saline environment. The coatings have complex microstructures where the ceramic hard-phase constituents are embedded in a metallic matrix. Their electrochemical response during accelerated corrosion DC polarisation tests reflects this complexity. A test protocol involving potentiostatic and potentiodynamic tests has been devised which enables the mechanisms of corrosion occurring on the WC/CrC–CoCr cermet and the WC–CrNi cermet to be compared and contrasted. A ‘critical’ temperature has been defined for the WC–CrNi coating similar to the critical pitting temperature defined for stainless steels. Electrochemical behaviour, which can not be simply described as truly ‘passive’ behavior, is displayed by the WC/CrC–CoCr coating and such a well-defined ‘critical’ temperature is not apparent. The contrasting mechanisms of corrosion on the two coatings, identified during post-test examination, can be correlated to the contrasting electrochemical response.

Examining corrosion effects and corrosion/erosion interactions on metallic materials in aqueous slurries

In this paper aspects of the corrosion behaviour of two materials (Co-base Stellite X-40 and austenitic cast iron BS 3468 S2W) under aggressive slurry erosion conditions are considered. Electrochemical techniques are used to assess the rate of corrosion in impingement conditions. The material degradation has been separated into components of mechanical erosion, electrochemical corrosion and interactive effects. It has been shown that the erosion significantly enhances the corrosion rate of both materials and has the effect of moving Stellite X-40 from a passive to an active corrosion regime. However, in terms of the magnitude of the material loss component the effect of corrosion on erosion (often referred to as synergy) is a much more prominent feature for both materials.

Biotribocorrosion—an appraisal of the time dependence of wear and corrosion interactions: I. The role of corrosion

With the increasing interest in metal-on-metal (MoM) joint implants, studies have been focused on their wear and corrosion behaviour. Integrated electrochemical tests have been conducted and are reported in this paper. The corrosion, wear and wear–corrosion behaviour for three materials (high carbon CoCrMo, low carbon CoCrMo and UNS S31603) have been discussed. Corrosion effects on the overall performance for the three materials are analysed. Two distinct regimes have been found for the three materials: (a) the running-in regime and (b) the steady state regime, in line with other research. Even in the steady state regime, 20%–30% of the material degradation can be attributed to corrosion-related damage. High carbon CoCrMo showed excellent corrosion, wear and corrosion–wear resistance and therefore it delivered the best overall performance in terms of a lower wear rate, a lower friction coefficient and a higher resistance to corrosion.

Biotribocorrosion—an appraisal of the time dependence of wear and corrosion interactions: II. Surface analysis

Metal-on-metal joint replacements have been considered an effective alternative to metal-on-polyethylene bearings. Various electrochemical and spectroscopic techniques have been used in studying the corrosion behaviour and surface compositions of these implant materials. X-ray photoelectron spectroscopy is one of the most popular methods of obtaining details regarding the near surface layers of metallic components. This paper is focused on the metal–protein interaction and the change of candidate material (HC CoCrMo, LC CoCrMo and UNS S31603) surfaces under tribological contact in simulated biological fluids. The relationship between wettability and corrosion (specifically tribocorrosion) in a protein-rich environment is also discussed. As a general conclusion, a metal–protein complex was found on the surfaces which grew and developed with the metal oxides (Cr2O3/CoO etc) as sliding progressed. It is believed that wettability, which is affected by the nature of the protein adsorption at the metal interface, can influence material corrosion and tribocorrosion behaviour.

Understanding the role of corrosion in the degradation of metal-on-metal implants.

Abstract In metal-on-metal joints the primary concerns in terms of long-term durability relate to corrosion, wear, and their joint (tribocorrosion) effects. The release of ions through corrosion processes and nanoscale debris from wear processes can seriously affect joint integrity and can lead to an adverse biological reaction by the host. In this paper an integrated study of corrosion-wear interactions in serum, Dulbeccos Modified Eagles Medium and 0.3 per cent NaCl has demonstrated that the biological nature of the fluid affects the total degradation rate and also the level of wear-corrosion interactions. The specific action of proteins in corrosion and tribocorrosion for high-carbon Co-Cr-Mo and low-carbon Co-Cr-Mo alloys is discussed.

Calcareous scales formed by cathodic protection—an assessment of characteristics and kinetics

Abstract An electrochemical technique using the assessment of the rate of oxygen reduction at a rotating disk electrode has been used in conjunction with surface analysis by X-ray photoelectron spectroscopy to study the first layer formation of calcareous deposits under cathodic protection in various solutions. The study has shown that the electrochemical technique is an effective means of monitoring scale formation and that the existing models which predict scale formation by a basal layer of Mg(OH) 2 followed by CaCO 3 are over-simplified. The advantages of using an integrated approach of electrochemical analysis and a surface sensitive analysis technique in characterising the initial scale as a function of the solution composition are presented and discussed and revised models describing the temporal development of calcareous layers in saline solutions are suggested.

Mechanisms of wear on a Co-base alloy in liquid–solid slurries

Abstract The efficient performance of drilling tools in aqueous environments requires materials to be used, which can effectively resist excessive material loss due to the combined effects of erosion and corrosion. In this paper, the interactions between electrochemical corrosion and mechanical erosion have been assessed in liquid–solid slurries on a Co-base material (Stellite X40), which is currently used in the manufacture of oil tools. The material degradation is assessed as a function of aqueous environment and sand loading. Corrosion has been shown to represent an important factor in the degradation of this material even though it essentially exhibits passivity in static saline conditions. The relative importance of corrosion and its synergistic effect when combined with erosion are quantified and the implications in terms of material performance and of potential methods of reducing oil tool degradation are discussed.