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.     

Corrosion and erosion performance of HVOF/TiAlN PVD coatings and candidate materials for high pressure gate valve application

The main objective of this paper is to study the slurry erosion and corrosion behavior of WC10Co4Cr, Armcore ‘M’ Stellite 6 and 12 HVOF coatings, TiAlN PVD coating, selected steels, such as X20Cr13, 17Cr–4Ni pH steel and Ti6Al4V titanium alloy alongwith conventional hard weld deposits of Stellite 6 and 21. The slurry erosion studies were carried out at 60° angle of impingement for the velocities in the range of 15–20 m/s using mineral sand of −40 to +80 mesh. The corrosion studies were carried out as per ASTM B 117-73 for 100 h. During slurry erosion testing, WC10Co4Cr HVOF along with TiAlN PVD coating are found out, to be the best coating materials followed by HVOF coating of Armcore ‘M’ material. However, for corrosion, Ti6Al4V, Stellite 6 and 21 hard weld deposits and 17Cr–4Ni pH steel turned out to be the best materials followed by HVOF coating of Stellite 6 and 12. HVOF coatings of WC10Co4Cr and Armcore ‘M’ materials corroded significantly, however, TiAlN PVD coating corroded very badly even after 24 h of testing.     

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.     

Erosion– and cavitation–corrosion of titanium and its alloys

The economic and effective operation of machinery and plant involved in fluids handling is increasingly dependent on the utilisation of materials that combine high corrosion resistance and good wear resistance. This paper studies two wear–corrosion situations: (1) erosion–corrosion, where the wear is due to impacting solids in a liquid medium and (2) cavitation–corrosion, where the wear is due to impacting liquid micro-jets formed by imploding air bubbles. The characteristics of a commercially pure titanium (CP-Ti) and three alloys in erosion–corrosion and cavitation–corrosion conditions have been studied. The erosion–corrosion characteristics of each material was assessed using an impinging-jet apparatus. The tests were performed at an angle of impingement of 90°C at a particle velocity of 17 m/s and in a saline solution of 3.5% NaCl at 18°C. A series of experiments was conducted to determine the mass loss by combined erosion–corrosion before independently determining the electrochemical corrosion contribution to mass loss. It has been shown that exposure to liquid–solid erosion causes disruption of the passive film on Ti and active corrosion occurs. In contrast, the materials exhibited passive behaviour in static conditions and when exposed to a cavitating liquid only CP-Ti became active. The role of corrosion in these wear–corrosion environments on CP-Ti and Ti-alloys is discussed in this paper.     

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.     

The influence of high shear and sand impingement on preferential weld corrosion of carbon steel pipework in CO2-saturated environments

Preferential weld corrosion (PWC) has proved problematic for the oil and gas industry for a number of years. Although the effect of high flow rates on PWC in inhibited CO₂-saturated solutions has been studied by authors, the consideration of a higher, localised turbulence over the weld material and the implications this has on PWC appears minimal. This work considers this very effect, along with developing an understanding of the threats posed to weld integrity by sand particle presence in the process fluid using a submerged impinging jet (SIJ) apparatus.Experiments were conducted using a commercially available film-forming oilfield corrosion inhibitor which was evaluated in both flow-induced corrosion and erosion–corrosion environments in its ability to control PWC. The SIJ setup allowed control over the individual flow velocities at each nozzle, meaning shear stress could be intensified over the 1% Ni–0.25% Mo weld material to simulate localised turbulence at the sample surface. Galvanic current and mixed potential measurements were performed to ascertain changes in galvanic interactions between the two materials. The work demonstrates that localised turbulence over internal weld beads and the presence of sand within oil and gas systems can influence PWC in certain environments.     

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 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”.     

碳钢奥氏体不锈钢材料在水电工程中应用的局限性

通过对水电站典型应用材料(55钢、1Cr18Ni9Ti、0Cr13Ni5Mo)在冲蚀磨损过程中电化学腐蚀及抗冲蚀磨损性能研究,区分出纯磨损、纯腐蚀、磨损对腐蚀的促进分量及腐蚀对磨损的促进分量等在材料失效过程中各占的比例,考察了试验材料的抗冲蚀磨损特性及其磨损与腐蚀间的交互作用,分析了其失效机制。结果表明:不同的冲蚀速度下,0Cr13Ni5Mo不锈钢的冲蚀磨损失重率最小,55钢最大;纯磨损是材料失去的主要方式:55钢虽然纯磨损量较小,但腐蚀及其磨损与腐蚀的交互作用失去量大,1Cr18Ni9Ti不锈钢虽然纯腐蚀量小,但纯磨损量大,因而都有应用的局限性。     

Slurry Erosion–Corrosion of Carburized AISI 5117 Steel

The erosion–corrosion of carburized and untreated low alloy steel (AISI 5117) has been investigated using slurry whirling-arm test rig. Erosion–corrosion tests were carried out in slurries composed of sand particles and either tap water or 3 % NaCl solution. The tests were carried out with particles concentration of 1 wt% and slurry stream impact velocity of 15 m/s. Silica sand having a nominal size range of 250–355 μm was used as an erodent. It has been shown that the erosion and erosion–corrosion resistance of AISI 5117 low alloy steel can be effectively improved by carburizing for all impact angles. However, the effectiveness of carburizing was the highest for an impact angle of 45°, where the erosion and erosion–corrosion resistance were increased by 60–40 %, respectively, compared with that of the untreated material. The results showed that the treated and untreated specimens behaved as ductile materials under erosion and erosion–corrosion tests, and the maximum mass loss occurred at an impact angle of 45°. SEM analysis showed that the erosion tracks developed on the untreated specimens were wider and deeper than that formed on the carburized specimens for erosion and erosion–corrosion tests.     

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.     

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.     

Erosion–corrosion and synergistic effects in disturbed liquid-particle flow

The present study has been conducted to investigate the interaction between corrosion and erosion processes and to quantify the synergism in realistic flow environments, including sudden pipe constrictions, sudden pipe expansions, and protrusions. Tests were conducted on AISI 1018 carbon steel using 1 wt% sodium chloride (NaCl) solution purged with CO₂ as the corrosive media and silica sand as the erodent.The experiments were designed to understand whether erosion enhances corrosion or corrosion enhances erosion and to evaluate the contribution of the individual processes to the net synergism. It was observed that erosion enhances corrosion and corrosion enhances erosion, with each contributing to significant synergism; however, the dominant process was the effect of corrosion on erosion.     

Erosion, corrosion and erosion–corrosion of EB PVD thermal barrier coatings

Electron beam (EB) physical vapour deposited (PVD) thermal barrier coatings (TBCs) have been used in gas turbine engines for a number of years. The primary mode of failure is attributed to oxidation of the bond coat and growth of the thermally grown oxide (TGO), the alumina scale that forms on the bond coat and to which the ceramic top coat adheres. Once the TGO reaches a critical thickness, the TBC tends to spall and expose the underlying substrate to the hot gases. Erosion is commonly accepted as a secondary failure mechanism, which thins the TBC thus reducing its insulation capability and increasing the TGO growth rate. In severe conditions, erosion can completely remove the TBC over time, again resulting in the exposure of the substrate, typically Ni-based superalloys. Since engine efficiency is related to turbine entry temperature (TET), there is a constant driving force to increase this temperature. With this drive for higher TETs comes corrosion problems for the yttria stabilised zirconia (YSZ) ceramic topcoat. YSZ is susceptible to attack from molten calcium–magnesium–alumina–silicates (CMAS) which degrades the YSZ both chemically and micro-structurally. CMAS has a melting point of around 1240 °C and since it is common in atmospheric dust it is easily deposited onto gas turbine blades. If the CMAS then melts and penetrates into the ceramic, the life of the TBC can be significantly reduced. This paper discusses the various failure mechanisms associated with the erosion, corrosion and erosion–corrosion of EB PVD TBCs. The concept of a dimensionless ratio D/d, where D is the contact footprint diameter and d is the column diameter, as a means of determining the erosion mechanism is introduced and discussed for EB PVD TBCs.     

Effects of Cr3C2 addition on the erosion–corrosion resistance of Ti(C,N)-based cermets in alkaline conditions

Effects of Cr₃C₂ on the erosion–corrosion behavior of Ti(C,N)-based cermets are studied in alkaline conditions. The results indicate that the erosion–corrosion resistance of cermets is improved with proper Cr₃C₂ content. Corrosion performance of cermets is deteriorated by Cr₃C₂ addition in NaOH solution. With the increase of Cr₃C₂, the erosion–corrosion behavior of Ti(C,N)-based cermets is classified to be erosion regime, erosion–corrosion regime, corrosion–erosion regime and corrosion regime. Materials degradation is determined by particles erosion for cermets with low Cr₃C₂ content, while for materials containing more Cr₃C₂ addition, binder corrosion and subsequent erosion are responsible for materials deterioration.     

Cavitation erosion of cobalt-based Stellite® alloys,cemented carbides and surface-treated low alloy steels

A number of Stellite^® alloys, cemented carbides and surface-treated alloy steels have been evaluated for erosion resistance. The ability of the Stellite alloys to withstand erosion is primarily a function of the cobalt-rich solid solution phase while erosion of cemented carbides is controlled predominantly by the binder phase. The nickel-based tungsten carbides are more resistant to erosion than the cobalt-based samples.Investigation of industrial surface treatments has demonstrated that erosion rates of hardened low alloy steels can be improved. For example, a hardened electroless nickel coating on BS 817M40 steel erodes at one-third the rate of uncoated BS 817M40 steel. A Tufftriding treatment, which is a proprietary method of carbonitriding, applied to the same steel caused a similar improvement in performance but only after an initial loss of the compound layer. Hard chrome coating is, in general, less effective than the above treatments in combating cavitation erosion.     

Surface analysis of laser cladded Stellite exposed to self-mated high load dry sliding

Materials from the Stellite family of Co-based alloys are commonly used as low friction, galling resistant materials in high load dry sliding contact applications.In the present investigation, the surface region of a Co-based material (Stellite 21) exposed to self-mated high load dry sliding at room temperature has been analysed in detail.During sliding, an approximately 30 nm thick Co-enriched tribofilm is created. It exhibits low friction properties and a high galling resistance. The transformation from an face-centred-cubic structure to easily sheared hexagonal-closed-packed basal planes in the tribofilm combined with the high load carrying capacity of the underlying deformation hardened zone is suggested to explain the excellent low friction properties and galling resistance of this material.     

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.     

Study of particle erosion damage in Haynes Stellite 6B I: Scanning electron microscopy of eroded surfaces

The erosion behavior of metals and alloys by solid particles entrained in relatively slow moving gases is of current interest as a result of ongoing efforts in coal conversion and the consequent production of dust-laden gases. Haynes Stellite 6B represents a typical alloy used for erosive wear resistance in such situations and also provides an appropriate alloy for the study of the mechanisms of erosion because it comprises essentially large brittle carbide phases in a ductile matrix. A scanning electron microscope study of the surface of Stellite 6B after erosion by alumina particles is described, and the types of erosion damage incurred by the ductile metal matrix and the brittle carbides are characterized. The only mechanism of material loss of the ductile metal for which positive evidence was found was cutting, with the possibility that fracture on a very fine scale may also be involved. The mechanism of material removal from the carbides appeared to be by surface crack interlinkage. Under the conditions studied, corners of the eroding alumina particles were found to break off and to adhere to the alloy or carbide surface; at the highest impact velocity studied an extensive layer of embedded alumina fragments was built up on the alloy surface and probably modified its erosion behavior.