The role of lubrication and corrosion in abrasion of materials in aqueous environments

A rubber wheel type test apparatus has been constructed which allows abrasion testing to be conducted in slurry or dry environments in otherwise identical conditions. Abrasion tests of a steel, a sintered tungsten carbide–cobalt hardmetal and an HVOF sprayed nickel chrome–chromium carbide cermet coating have been performed in dry and aqueous slurry conditions, the latter with both neutral and acidic carriers. It has been shown that the aqueous carrier acts as an effective lubricant and thus significantly reduces the abrasion rate over that observed in dry conditions. However, enhancement of corrosion by use of an acid slurry lead to an increase in the rate of material removal over that of the neutral aqueous conditions in all cases. Increases were small for the corrosion resistant cermet coating and moderate for the steel. Significant enhancement of wear was observed for the sintered WC–Co hardmetal where rapid removal of the cobalt binder by the acid resulted in a change in dominant mechanism of carbide removal from attritive wear to pullout.     

Tribocorrosion in implants—assessing high carbon and low carbon Co–Cr–Mo alloys by in situ electrochemical measurements

Tribocorrosion is defined as the chemical–electrochemical–mechanical process leading to degradation of materials in tribological contact immersed in a corrosive environment. Degradation results from the combined action of corrosion and mechanical loading and it is well-known that synergistic effects can accentuate the wear–corrosion rate. While the role of lubrication in reducing wear has been identified, there are still some key unanswered questions in relation to the importance of wear/corrosion interactions. In this study in situ electrochemical measurements have been made to isolate corrosion and corrosion-enhanced wear/tribology damage mechanisms on high carbon CoCrMo and low carbon CoCrMo alloys. Tests are carried out in three different biological solutions: 50% calf bovine serum, Dulbecco's Modified Eagle's Medium (DMEM) and 0.36% NaCl solution at 37 °C with the specific objective being to attempt to isolate the effects of proteins and amino acid species in wear–corrosion. In this paper, a detailed analysis of corrosion/wear interactions is presented, which facilitates discussion of exactly how corrosion and wear processes interact and the role of the lubricating fluid in this respect.     

Investigation of erosion–corrosion processes using electrochemical noise measurements

This paper presents an example-based discussion of erosion–corrosion and flow corrosion processes that have been identified using electrochemical noise measurements. Various single and dual phase corrosion and erosion–corrosion experiments on austenitic stainless steels and various thermally sprayed coatings using jet impingement and pipe flow rigs are discussed. Localised corrosion events, metastable and propagating pitting, passive and general corrosion processes have been identified under various flow conditions of NaCl solutions. Oscillations in the electrochemical potential noise signals have been related to an erosion-enhanced corrosion synergistic effect. Electrochemical noise measurements show responses to electrolyte permeation of the coating, coating erosion penetration and substrate activity under erosion–corrosion conditions.     

Interpretation of electrochemical measurements made during micro-scale abrasion-corrosion

This paper brings together and analyzes recent work based on the interpretation of the electrochemical measurements made on a modified micro-abrasion-corrosion tester used in several research programmes. These programmes investigated the role of abradant size, test solution pH in abrasion-corrosion of biomaterials, the abrasion-corrosion performance of sintered and thermally sprayed tungsten carbide surfaces under downhole drilling environments and the abrasion-corrosion of UNS S32205 duplex stainless steel. Various abrasion tests were conducted under two-body grooving, three-body rolling and mixed grooving-rolling abrasion conditions, with and without abrasives, on cast F75 cobalt-chromium-molybdenum (CoCrMo) alloy in simulated body fluids, 2205 in chloride containing solutions as well as sprayed and sintered tungsten carbide surfaces in simulated downhole fluids. Pre- and post-test inspections based on optical and scanning electron microscopy analysis are used to help interpret the electrochemical response and current noise measurements made in situ during micro-abrasion-corrosion tests. The complex wear and corrosion mechanisms and their dependence on the microstructure and surface composition as a function of the pH, abrasive concentration, size and type are detailed and linked to the electrochemical signals. The electrochemical versus mechanical processes are plotted for different test parameters and this new approach is used to interpret tribo-corrosion test data to give greater insights into different tribo-corrosion systems. Thus new approaches to interpreting in-situ electrochemical responses to surfaces under different abrasive wear rates, different abrasives and liquid environments (pH and NaCl levels) are made. This representation is directly related to the mechano-electrochemical processes on the surface and avoids quantification of numerous synergistic, antagonistic and additive terms associated with repeat experiments.     

Exposure effects of strong alkaline conditions on the microscale abrasion–corrosion of D-gun sprayed WC–10Co–4Cr coating

The synergistic effects between abrasion and corrosion for detonation gun (D-gun) sprayed WC–10Co–4Cr coatings have been studied for alkaline conditions. Comparisons between exposed and freshly polished coating surfaces in strong NaOH solutions (pH 11) show that significantly lower wear rates were seen for the exposed surface due to a negative wear–corrosion synergy. The negative synergy was seen to increase with increasing sliding distances for both fresh and exposed surfaces. The physical mechanisms of this negative synergy are discussed. The effect of anodic surface treatments on wear–corrosion to elucidate the influence of corrosion on the negative synergy is also reported.     

Slurry abrasion response of surface engineered Ti6Al4VELI

Titanium alloys, by virtue of their outstanding combination of properties, continue to evolve as direct replacements for steels in offshore production systems to meet the challenging offshore application conditions. However, the wider use of titanium and its alloys is frequently retarded by their reputation for poor tribological behaviour. It is because of this simple engineering scenario that comparative tribological behaviour of surface engineered Ti6Al4VELI in abrasive slurry has been evaluated to identify treatments capable of improving its wear behaviour. To best simulate potential application conditions, sliding wear tests were carried out using a block-on-wheel test configuration in abrasive mud slurry. The wear volumes lost from the surface engineered Ti6Al4VELI test blocks ranged from negligibly small for substrates plasma sprayed with either WC–Co, Ni–Cr or Mo, to approximately twice that measured for a steel test block in the cases of untreated, shot peened, electroless Ni plated and anodised Ti6Al4VELI. Hard chrome plated material and thermochemically treated material demonstrated a certain degree of wear, which however, was significantly less than that found for the steel block.     

Electro-mechanical interactions during erosion–corrosion

This study aims to understand the physical processes invoked when solid particle erosion occurs in a corrosive media. The literature on wear-corrosion shows some effort has been placed in qualifying the interactions between erosion and corrosion leading to ‘synergistic’, ‘additive’ and ‘antagonistic’ terms. These terms are difficult to quantify experimentally with multiple experiments being required and generate considerable errors often suggesting these interactive terms are negligible. Hence the current work seeks to investigate these interactions, the errors associated with their measurement and gain understanding of the processes involved by careful examination of microstructural and mechanical property changes of surfaces subjected to erosion–corrosion. Cast nickel–aluminium bronze/NaCl solution has been chosen to study as this system has been studied at Southampton for several years. In situ electrochemical, gravimetric and topographical analyses have been evaluated and discussed using microstructural and hardness measurements.     

Friction and wear behaviour of sintered steels submitted to sliding and abrasion tests

Fe–C–Mo and Fe–C–Cr steels were sintered by PM processes carried out using different values of temperature and pressure, leading to different microstructures and density values. Flat specimens were submitted to tribological tests in order to evaluate their behaviour under both dry sliding and abrasive wear conditions. A flat-on-cylinder tribometer was used for the sliding tests, while a micro-scale ball cratering device was used for the abrasion tests. The dry sliding wear resistance of the PM steels was mainly influenced by the composition and sintering conditions. In this regard, the best behavior was observed for the more hardenable Fe–C–Mo steels with higher Mo content, sintered under conditions giving rise to bainitic microstructures. A determining role was also played by the porosity content and pore shape: reduction in porosity (obtained by increasing the sintering temperature and the compacting pressure), as well as an increase in pore roundness, led to a significant improvement in the resistance to sliding wear. A mild oxidative wear regime were observed for all the sintered steels under relatively low values of the applied load, while an increase of the applied load led to a delamination wear regime. The resistance to abrasive wear was low for all the tested steels, irrespective of composition and sintering cycle.     

A comparative study of high-temperature erosion wear of plasma-sprayed NiCrBSiFe and WC–NiCrBSiFe coatings under simulated coal-fired boiler conditions

A comparative study was carried out of the behaviour of plasma sprayed NiCrBSiFe and WC–NiCrBSiFe alloys subjected to conditions which simulate a post-combustion gas atmosphere from a coal-fired boiler combustor. The study first evaluates the effects of thermal exposure at high temperatures on the microstructure of the coatings and on the adherence between substrate (austenitic stainless steel) and coatings. The oxidation rates of these coatings in atmospheres with 3–3.5% of free oxygen at 773 and 1073 K were then evaluated. The effect of WC on the low-velocity corrosion–erosion behaviour produced by the impact of fly ashes in the gas stream at high temperatures (773 and 1073 K) was assessed under impact angles of 30° and 90°. Finally, the eroded surfaces were analysed using scanning electron microscopy in order to characterize the ash embedment phenomena and the operating erosive micromechanisms.     

On the construction of micro-abrasion maps for a steel/polymer couple in corrosive environments

There has been significant recent interest in the interactions of micro-abrasion in aqueous conditions, particularly when the environment is corrosive. This is due to the fact that there are many micro-scale-abrasion processes where the presence of the corrosive media may modify the wear on the surface. Furthermore, if the surface dissolves, the mechanical properties of the surface may be modified as a result of the effects of corrosion.In this work, the micro-abrasion of a steel/polymer couple was studied in carbonate/bicarbonate solution. The effects of two variables, applied load and potential, were investigated on the wear rate enabling quantification of the corrosion and wear contributions. Scanning Electron and Atomic Force Microscopy were used to characterize the surfaces following micro-abrasion-corrosion.The results indicated possible micro-abrasion-corrosion mechanisms on the surface. These regimes were suggested on the basis of the ratio of the micro-abrasion to the corrosion rate. In addition, micro-abrasion-corrosion maps were constructed showing the transitions between the micro-abrasion-corrosion regimes as a function of load and applied potential.     

Biotribocorrosion of CoCrMo orthopaedic implant materials—Assessing the formation and effect of the biofilm

Due to the renewed interest in hard-on-hard hip replacement, especially metal-on-metal (MoM) or metal-on-ceramic (MoC) joints, issues relating to their long-term durability need to be addressed. Their effects on the operating environment (human body) and how the body fluid affects the implant materials are the primary concern. For widely used metallic implant materials, such as cobalt–chromium–molybdenum (CoCrMo) alloys, released ions due to electrochemical (corrosion) processes and mechanical-enhanced electrochemical (corrosion-wear/tribocorrosion) processes may cause biological reactions in the human hosts. Proteins are a primary constituent of the synovial fluid in human joints with other organic components such as hyaluronic acid and lubricin, and, although numerous tribological studies in protein-containing fluids have been conducted, there is still a need to fully understand the role of proteins and adsorbed-protein layers in wear, corrosion and tribocorrosion processes in artificial joints.In this study, bovine calf serum was used to simulate the body fluid, and a model solution of 0.36% NaCl solution was employed to isolate the influence of organic species (such as proteins, amino acids etc.). Wrought high carbon cobalt–chromium–molybdenum alloy (HC CoCrMo), Wrought low carbon cobalt–chromium–molybdenum alloy (LC CoCrMo) and stainless steel UNS S31603 (316 L) were included in the study and their corrosion, tribology and tribocorrosion behaviour were assessed by integration of gravimetric analysis and electrochemical measurements. Surface analysis (chemical and topographical) was carried out to fully understand the surface/organic species interactions.The constituents of bovine serum have been shown to have a great influence on the corrosion behaviour of all materials studied here—the mechanism of their action being to accelerate ion release and passive film breakdown in static conditions. In tribological contacts, biofilm can play a role in forming an effective lubricating film that reduces friction. For HC CoCrMo, reactions at the surface in the contact zone form a very complex nanostructured layer which comprises wear debris, biofilm and reaction products and the process also changes the nature of the passive film formation. The film reduces the material loss and hence has a protective nature. Organic species (proteins, etc.) were also shown to enhance corrosion-related damage on all materials.     

Erosion–corrosion of engineering steels—Can it be managed by use of chemicals?

One significant contributory factor in the degradation of both pipelines and downhole tubulars in the oil and gas industry is erosion–corrosion. An erosion–corrosion investigation was carried out with three different steels—carbon steel, martensitic stainless steel and superduplex stainless steel. The materials were chosen to represent “active” and “passive” corrosion materials and are the same materials used in completions. Tests were carried out under three different regimes spanning a range of fluid velocities to simulate the severity of the mechanical erosion effect. A commercial corrosion inhibitor was used to investigate the inhibitor ability to reduce damage due to erosion–corrosion. In each of the conditions, pure corrosion and combined erosion–corrosion were studied by electrochemical and gravimetric techniques. The experiments were conducted using a jet impingement rig capable of producing jet velocities up to 20 m/s in a CO₂-saturated environment with sand. Erosion–corrosion mechanisms were determined from microstructural studies by SEM and inhibitor adsorption tests. The paper shows that the inhibitor effectively reduced erosion–corrosion damage for carbon steel; it was only in severe erosion–corrosion conditions that inhibitor has any noticeable effect for martensitic stainless steel and there were no conditions where the inhibitor offered a benefit for the superduplex stainless steel.     

Erosion–corrosion assessment of materials for use in the resources industry

The erosion–corrosion properties of a range of ferrous-based materials that are currently being used or have potential for use in the resources industry have been assessed using a slurry pot erosion–corrosion (SPEC) test rig that has the capability of establishing the separate components of erosion, corrosion and synergy.Testing was performed, at 30 °C, in an aqueous slurry containing 35 wt% AFS 50–70 silica sand and a 3.5 wt% NaCl solution. Erosive action was supplied through high-speed rotation of a rubber-lined impeller.Erosion–corrosion performance of materials evaluated was related to composition/microstructure and hardness. Test data correlated with available service experience.The results showed that the cast Cr white irons with (i) a structure that was essentially a duplex stainless steel containing a distribution of hard carbides and (ii) a near eutectic Cr white iron exhibited the highest erosion–corrosion resistance of the materials tested. The evaluation of the Cr white irons also highlighted the influence of Cr and C levels on the E–C properties of these materials.E–C assessment of selected carbon steels confirmed that the erosion-only rates and synergistic levels showed a general decline with increasing carbon content and hardness. As expected, a low C steel pipe product displayed very mediocre erosion–corrosion behaviour as a consequence of its very low intrinsic corrosion resistance and inferior wear properties. This reflected service experience, however, such products are still being used, due to the comparatively low initial cost.A TiC particle-reinforced AISI 316 stainless steel exhibited an almost 45% improvement in the E–C resistance, when compared with an AISI 316L stainless steel.     

Effects of CeO2 on friction and wear characteristics of Fe–Ni–Cr alloy coatings

The effects of rare earth oxide CeO₂ on the microstructure and wear resistance of thermal sprayed Fe–Ni–Cr alloy coatings were investigated. The powders of Fe–Ni–Cr alloy with the addition of CeO₂ were flame sprayed on to a 1045 carbon steel substrate. The coatings were examined and tested for microstructure feature, compositions, and phase structure. Tribological properties of coatings were tested under reciprocating sliding test. The results were compared with those for coatings of the alloy without CeO₂. The comparison indicated that the addition of rare earth oxide CeO₂ could refine and purify the microstructure of coatings, and increase the microhardness of the coatings. As a result, by CeO₂ addition, the friction coefficient of the coatings was decreased slightly and the wear resistance of the coatings was enhanced significantly.     

Tribocorrosion behaviour of powder metallurgy duplex stainless steels sintered in nitrogen

The combined corrosion–wear of powder metallurgy duplex stainless steels obtained by the mixing of ferritic and austenitic powders in several proportions was studied. All the materials were sintered in a nitrogen atmosphere. Tribological and tribocorrosion behaviour was analyzed using a pin-on-disc. Electrochemical techniques: open circuit potential measurements, potentiodynamic polarization measurements and potentiostatic measurements at passive potential under sliding conditions were used. The obtained results were interpreted and related to microstructural features. The tribocorrion behaviour for powder metallurgy duplex stainless steels was a function of the applied electrochemical potential.     

Determination of the wear resistance of traditional ceramic materials by means of micro-abrasion technique

Micro-abrasion techniques enable the surface wear of materials to be studied with greater precision than provided by other methods. In addition to their reliability, micro-abrasion techniques allow the wear phenomenon of the top-most layers to be studied while assuring, in the case of thin coatings, that this is not influenced by the substrate.In the present study, micro-abrasion technique (cratering with a steel ball) was used to determine the wear resistance of traditional ceramic materials, as a complementary test to the methodologies on a macroscopic scale that are customarily used for this type of material. In order to adapt the test to these materials, the individual effect of each test condition on wear resistance was isolated, while keeping the other conditions constant. The following variables were studied: diameter and angular velocity of the ball, abrasive suspension feed rate and grain size, sample–ball contact angle and groove in the supporting drive shaft. The values established were validated by performance of the test on materials of a glassy nature.The micro-abrasion test is shown to be a useful method for studying wear performance of ceramic glazes.     

Evaluation of a semi-empirical model in predicting erosion–corrosion

The phenomenon of erosion–corrosion has been studied extensively by various investigators but no accurate model has been developed to predict the interactions between erosion and corrosion. This is mainly attributed to the complexity of the interactions that generate either a synergistic or antagonistic wear effect for a particular material in a certain environment. A semi-empirical model has recently been developed at the University of Southampton which incorporates dynamic Hertzian contact mechanics to model the damage during particle impact and accommodates the effect of erodent deforming the surface leading to an increased corrosion activity. The model was found to have good agreement with erosion–corrosion rates of carbon steel. The aim of this paper is to evaluate the robustness of this semi-empirical model by testing it on a passive metal. UNS S31603 was chosen due to its inherent passivity to corrosion. A slurry pot erosion tester was used as the test rig to perform the experiments. It was found that this passive metal produces high synergistic levels when exposed to erosion–corrosion in 0.3 M HCl with variation in erodent concentrations and flow velocities. SEM and surface profilometry show typical ductile material behaviour with cutting mechanism and deformation mechanism occurring simultaneously. A wear map is presented and it is observed that the increase in velocity and sand concentration causes the material to shift from a corrosion–erosion dominated region to an erosion–corrosion dominated region. This paper will also evaluate the semi-empirical model and discuss its applicability in predicting erosion–corrosion.     

Micro-abrasion resistance of thermochemically treated steels in aqueous solutions: Mechanisms, maps, materials selection

The area of micro-abrasion is an interesting and relatively recent area in tribo-testing methodologies, where small particles of less than 10 μm are employed between interacting surfaces. It is topical for a number of reasons; its direct relation to the mechanisms of the wear process in bio-tribological applications, ease in conducting tests and the good repeatability of the test results. It has widespread applications in conditions used in the space and offshore industries to bio-engineering for artificial joints and implants.There have been many recent studies on the micro-abrasion performance of materials, ranging from work basic metals to nano-structured coatings. However, no significant work is reported on the micro-abrasion resistance of thermochemically treated steels. Hence, this paper looks at the performance of two thermochemically treated steels, Tenifer bath nitride stainless steel (T-SS) and vanadized carbon steel (V-CS) in such conditions with reference to the stainless steel (SS) by varying the applied load and sliding distance.The results indicated that T-SS demonstrates exceptionally poor resistance to micro-abrasion. It was observed that the heat treatment process and properties of the hardened layer (hardness and thickness) are extremely important in determining the micro-abrasion resistance of such steels. Finally, the results were used to develop micro-abrasion mechanism and wastage maps, which can be used to optimize the surface treated materials for micro-abrasion resistance.     

CO2 erosion–corrosion of pipeline steel (API X65) in oil and gas conditions—A systematic approach

A systematic study of pipeline steel (API X65) degradation due to erosion–corrosion containing sand in a CO₂ saturated environment has been carried out. This work focuses on the total material loss, corrosion, erosion and their interactions (synergy) as a function of environmental parameters (temperature, flow velocity and sand content) to enable the critical conditions, which move the damage mechanism from a flow-induced corrosion regime to erosion–corrosion regime, to be determined.The experimental results show that the effect of corrosion in enhancing erosion, often referred to as the synergy, is significant and accounts for a high proportion of the deviation of measured material loss from the prediction derived from established CO₂ corrosion models. Ways forward to improve erosion–corrosion prediction are discussed.