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.     

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.     

Investigation of micro-scale abrasion–corrosion of WC-based sintered hardmetal and sprayed coating using in situ electrochemical current-noise measurements

The micro-scale wear–corrosion interactions of WC-based sintered hardmetals and sprayed coatings are typically investigated by comparing the wear-rates in corrosive environments with neutral (pH 7) conditions and inferring electrochemical activity. However, for a greater understanding of the wear–corrosion interactions, there is a need to examine the repassivation kinetics during micro-abrasion tests under different pH conditions. This paper details in situ electrochemical current-noise measurements performed using a modified micro-abrasion tester to elucidate these wear–corrosion interactions for pH 7–13 conditions for sintered WC–5.7Co–0.3Cr and sprayed WC–10Co–4Cr specimens. Electrochemical measurements and SEM micrographs of worn surfaces are used to detail the degradation process. Discussion will focus on the wear–corrosion interactions present under neutral and alkaline conditions for sintered and sprayed specimens and the influence of microstructure on the electrochemical activity will be detailed.     

Erosion–corrosion performance of high-Cr cast iron alloys in flowing liquid–solid slurries

In many slurry transportation systems, such as in FGD (Flue Gas Desulphurization) and chemical processing applications, corrosion and erosion are the two main mechanisms of material degradation of the pump wet-end components including pump casing, impeller and liners. The performance of a selected material is mostly dependent upon its relative corrosion and erosion resistance to the service environment. In these cases erosion, corrosion and the related synergistic effects can be very complicated since they are affected by numerous factors including solid and slurry properties, chemical contents, hydraulic conditions and temperatures. In this experimental study, sliding Coriolis erosion testing has been performed with various corrosion factors such as pH value, chlorides content and temperature to evaluate the erosion–corrosion resistance of some high-alloyed white cast irons containing different levels of chromium and other elements. Optical microscope and SEM-EDS have also been used to examine microstructure and surface conditions of tested materials. Results indicated that material loss due to corrosion factors increased as acidity-chlorides and temperature increased. At relatively high corrosion intensity, the white cast irons with higher alloy content (especially chromium) clearly showed improved corrosion resistance and combined erosion–corrosion resistance over those with lower alloy content. Under certain corrosion and hydraulic conditions, particle size is perhaps the single most influential factor on erosion–corrosion rate of the high-Cr cast iron alloys. Relatively large particles are much more effective than small ones at removing both the corroded surface layer and the fresh material, causing substantially higher rate of material loss. Some other related factors have also been addressed.     

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.     

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.     

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.     

Modelling particulate erosion–corrosion regime transitions for Al/Al2O3 and Cu/Al2O3 MMCs in aqueous conditions

Very little research effort has been directed at development of models of erosion–corrosion of composite materials. This is because, in part, the understanding of the erosion–corrosion mechanisms of such materials is poor. In addition, although there has been a significant degree of effort in the development of models for erosion of MMCs, there are still difficulties in applying such models to the laboratory trends on erosion rate.In this paper, the methodology for mapping erosion–corrosion processes in aqueous slurries was extended to particulate composites. An inverse rule of mixtures was used for the construction of the erosion model for the particulate MMCs. The corrosion rate calculation was evaluated with reference to the matrix material.The erosion–corrosion maps for composites showed significant dependency on pH and applied potential. In addition, the corrosion resistance of the matrix material was observed to affect the regime boundaries. Materials maps were generated based on the results to show the optimum composite composition for exposure to the environment.     

Effect of friction cycles on wear particle generation of carbon nitride coating against a spherical diamond

The in-situ observations of wear particle generation of carbon nitride coating on silicon repeatedly sliding against a spherical diamond have been studied in terms of the critical friction cycles and normal loads. An environmental scanning electron microscope (E-SEM), in which a pin-on-disk tribotester was installed, has in-situ provided direct evidence of when and how the wear particle generation do occur during the repeated sliding of carbon nitride coating against a spherical diamond. The in-situ observations of non-conductive carbon nitride coating are therefore available free from surface charging with controllable relative humidity. The repeated sliding tests at a sliding speed of 50 μm/s have been carried out with the purpose of observing the ‘No wear particle generation’ region when varying normal load from 10 to 250 mN. It appears that until 20 friction cycles, the maximum Hertzian contact pressure P_max for ‘No wear particle generation’ can be improved from 1.39 Y to 1.53 Y if silicon is coated by carbon nitride with a thickness of 10 nm, where Y is defined as the yield strength of silicon. The applicable enlargement of the ‘No wear particle generation’ region of carbon nitride coating has therefore been comparatively discussed with the silicon substrate from the view points of the friction coefficient and the specific wear rate. The mode transition maps have also been summarized for the repeated sliding of carbon nitride coating in terms of ‘No wear particle generation’, ‘Wear particle generation by microcutting’ and ‘Wear particle generation by microcutting and microfracturing’ three typical modes.     

Some Coriolis slurry erosion test developments

The Coriolis test has developed into a useful way of evaluating the slurry erosion behaviour of materials. Wear mapping has revealed some sensitivity to test conditions so both the tester and test procedures continue to evolve. Experiments with very dilute model slurries comprising a ‘pinch’ of glass beads, or a single glass bead, gave information on bead contact depth and normal component of impingement velocities which confirmed previous theoretical predictions that erodent particles interact with the specimen in a series of low angle impacts. The superior discriminating ability of the Coriolis slurry erosion test is due to the lower interaction intensity of erodent particles with target surfaces compared to that in slurry jet testing.     

Classical, local buckling of tubes having rectangular cross-sections

The stress level for local buckling of a general rectangular elastic tube, loaded in axial compression, has been worked out by Bulson [The Stability of Flat Plates. Chatto and Windus, London (1970)] according to classical bifurcation theory, for a range of values of and . Bulson's results are re-examined with a view to understanding them in simple, physical terms. Computer graphics are used to replot these results in various ways according to different hypothetical schemes of explanation. It is found that most of them can be reproduced to a satisfactory first approximation by considering one set of plates as ‘active’ and the other as ‘passive’; only in a relatively narrow ‘transition region’ does this scheme break down. Finally some simple, rational approximate formulae are presented. The motivation for this study is the thought that non-linear interactive buckling of tubular and other thin-walled members—which is an important practical problem—may be understood more readily if the criterion for classical buckling can be expressed economically.     

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.     

Response of Ti–6Al–4V and Ti–24Al–11Nb alloys to dry sliding wear against hardened steel

Titanium alloys have been of great interest in recent years because of their very attractive combination of high strength, low density and corrosion resistance. Application of these alloys in areas where wear resistance is also of importance calls for thorough investigations of their tribological properties. In this work, Ti–6Al–4V and Ti–24Al–11Nb alloys were subjected to dry sliding wear against hardened-steel counter bodies and their tribological response was investigated. A pin-on-disc type apparatus was used with a normal load of 15–45N and sliding speed of 1.88 ms⁻¹. In the steady state, it was demonstrated that Ti–24Al–11Nb had a substantially higher wear resistance (about 48 times) than that of the Ti–6Al–4V alloy tested under a normal load of 45 N. Severe delamination is found to be responsible for the low wear resistance of Ti-6Al-4V. In the case of Ti–24Al–11Nb, two wear mechanisms have been suggested: delamination with a lower degree of severity and oxidative wear. It is thought that the ability of Ti–24Al–11Nb to form a protective oxide layer during wear results in a much lower wear rate in this alloy.     

Cavitation erosion resistance of Cr–N coating deposited on stainless steel

Results of investigation on cavitation-erosion resistance of Cr–N coating deposited on stainless steel X6CrNiTi18-10 (1H18N9T) by means of the cathodic-arc method are presented. The evaluation of Cr–N coating resistance to cavitation erosion is based on the investigation performed in a cavitation tunnel with a slot cavitator and tap water as a medium. The investigation was performed at variable-cavitation intensity and the estimated cavitation resistance parameters of coatings were the incubation period of damage and the instantaneous erosion rate after exposure of specified duration. It has been confirmed that the incubation period of the Cr–N coating damage is approximately 50% longer than that of the uncoated X6CrNiTi18-10 steel, and the instantaneous erosion rate after exposure of specified duration is comparable in both cases. The scanning microscope analysis indicates that the damage of Cr–N coating is due mainly to its delamination, while the erosion of deeper parts of the coating is of minor importance. The character of the coating and substrate damage in multiple locations indicates that the hard coating microparticles torn-off during the cavitation bubbles implosion hit against the coating and the revealed areas of substrate. As a result, the coating and especially the substrate of relatively low hardness are subject to cavitation erosion and to solid particle erosion with the hard torn-off microparticles of coating. The results of the investigation and the analysis indicate that the factors mainly responsible for a long incubation period and low cavitation erosion rate of the steel substrate/hard coating systems are the gained high hardness of substrate and high level of coating adhesion.     

Erosion-corrosion mapping of Fe in aqueous slurries: some views on a new rationale for defining the erosion-corrosion interaction

In studies of erosion-corrosion of materials in aqueous conditions, there have been various attempts to define regimes of interaction. Such regimes indicate whether erosion or corrosion may dominate the wastage mechanism. However, intermediate regimes in which corrosion and erosion interact with each other may lead to situations where the wastage is far greater than the sum of the processes acting separately.A common method of defining erosion-corrosion interactions has been to distinguish between the regime in which erosion enhances the corrosion rate (the so called “additive effect” because the corrosion contribution can be measured electrochemically and thus added to the erosion contribution to assess the overall wastage rate) and the regime where corrosion enhances erosion (the so-called “synergistic” effect). However, regimes of erosion-corrosion, where corrosion impedes the erosion are also of great importance mechanistically, and usually these are termed as exhibiting “negative synergism”. Defining the conditions in which a transition from “positive” to “negative synergism” occurs, is useful in order to optimize the parameters to minimize the wastage rate.This paper reviews the rationale that has been used to define erosion-corrosion regimes in aqueous conditions as part of a study of the erosion-corrosion of Fe at various pHs. The mathematical definitions are discussed in relation to practical erosion-corrosion problems. In addition, a new “antagonistic” erosion-corrosion regime is proposed to properly define the concept of “negative synergism”.     

Time–frequency analysis of tribological systems—part I: implementation and interpretation

A novel technique adapting the time–frequency analysis has been utilized to characterize stationary and non-stationary signals from tribological interactions. This representation displays time, frequency, and signal magnitude to decipher signals emanating from such interactions. Short-time Fourier transform, Wigner, Coi–Williams, and Zhao–Atlas–Marks distributions are suited to represent stationary and non-stationary signals. Some of the most complex tribological phenomena involve head–disk interactions in magnetic recording systems. Examples drawn from practical head–disk interface tests are analyzed by using the fast Fourier transform algorithm to illustrate the dynamic features of various distributions. Time–frequency representation of output spectrums of laser doppler vibrometer (LDV), strain gage sensor, and acoustic emission (AE) sensor obtained from head–disk experiments giving evidence of stationary and non-stationary behavior are investigated.     

Particle velocity and size effects in laboratory slurry erosion measurements OR… do you know what your particles are doing?

The major factors which determine the erodent particle impact wear process are described. Thus, particle impact velocity, impact angle and impact frequency are dictated by the slurry flow regime about the specimen. The influence of these factors on erosion rates (or on erosion–corrosion rates) can only be understood in terms of a quantitative model for slurry flow and particle impact and an assumption on the nature of the rate-controlling factor governing material loss. For erosion in the absence of corrosion, this latter has been taken to be the rate of dissipation of particle impact energy on the specimen surface. It is emphasised that material loss must be measured by changes in surface profile rather than mass loss, and that the best specimen form for this analysis is a cylinder. The effect of change of particle size on erosion rates is discussed. It is suggested that the application of these experimental and analytical techniques should provide a tool for the quantitative analysis of wastage in conditions of erosion–corrosion.     

High-temperature low cycle fatigue, creep–fatigue and thermomechanical fatigue of steels and their welds

High-temperature low cycle fatigue (LCF) is influenced by various time-dependent processes such as creep, oxidation, phase transformations and dynamic strain ageing (DSA) depending on test conditions of strain rate and temperature. In this paper, the detrimental effects of DSA and oxidation in high-temperature LCF are discussed with reference to extensive studies on 316L(N) stainless steel and modified 9Cr–1Mo steel. DSA has been found to enhance the stress response and reduce ductility. It localizes fatigue deformation, enhances fatigue cracking and reduces fatigue life. High-temperature oxidation accelerates transgranular and intergranular fatigue cracking in modified 9Cr–1Mo steel and during long hold time tests in austenitic stainless steel. In welds, microstructural features such as presence of course grains in the HAZ and formation of brittle phases due to transformation of δ ferrite during testing influence crack initiation and propagation and fatigue life. Thermomechanical fatigue (TMF) studies are suggested as more closer to the actual service conditions. In 316L(N) stainless steel, TMF lives under out-of-phase cycling are found to be lower than those under in-phase conditions in the low-temperature regimes, while the converse holds good when the upper temperature encompassed the creep-dominant regime.