Electrochemical investigation of erosion-corrosion using a slurry pot erosion tester

The aim of this paper is to use a modified slurry pot erosion tester to perform in-situ electrochemical measurements during solid particle impingement to investigate the effects of velocity, sand size and sand concentration on a passive metal (UNS S31603). Samples are subjected to a set of erosion-corrosion experiments. The electrochemical response of UNS S31603 to the test parameters is plotted and compared to develop an understanding of the erosion-corrosion process. The current trend with variation of test parameters has been explained by an erosion enhanced corrosion synergistic effect. The current transients associated with depassivation and repassivation during solid particle impingement are observed through electrochemical noise measurements. It was observed that the increase in velocity and sand concentration increased the current levels during erosion-corrosion. However, the increase in sand size had a more complex response. Single particle impact experiments conducted revealed that the peak corrosion current and the repassivation time increased with increase in velocity. A linear trend was seen between the peak current and the kinetic energy. A second-order exponential decay was fitted to the repassivation kinetics of the single particle impact. SEM has been used to develop a mechanistic understanding of erosion-corrosion. The surface scars reveal that the depth of the craters and the length of the lips increase with increase in velocity. Micro-cracks also appear on these lips, believed to be due to corrosive action attacking the roots of these lips.     

Erosion-corrosion interactions and their effect on marine and offshore materials

Increasingly the demands of modern fluid handling systems are for low costs with increased reliability and longevity along with no loss of fluid containment. These cannot be achieved without minimising the material damage caused by the combined surface degradation mechanisms of erosion and corrosion when systems are handling solids or are cavitating. This paper reviews the rationale behind the selection of erosion resistance surfaces for fluid handling equipment and highlights the complexities encountered when these surfaces are exposed to environments which contain sand particles or cavitation in a corrosive medium. The erosion and erosion-corrosion performance of a variety of coatings and bulk surfaces are discussed using volume loss rate versus sand impact energy maps. Synergistic terms are identified using standard deviation ratios of electrochemical current noise. Recent researches on the erosion-corrosion of coatings of aluminium, cermets and nickel aluminium bronze are reviewed as candidates for erosion-corrosion resistant surfaces. Electrochemical techniques designed to monitor the erosion-corrosion mechanisms and coating integrity are used to quantify the synergistic terms present when both erosion and corrosion act simultaneously.     

Investigation of erosion-corrosion mechanisms of UNS S31603 using FIB and TEM

Accelerated wear due to synergy during erosion-corrosion of UNS S31603 is extremely complex. It is this reason that current modelling approaches fail to accurately model the physical mechanisms in this wear process. The objective of this work was to perform FIB and TEM analysis on UNS S31603 to investigate the subsurface deformation mechanisms and microstructural changes in the material during erosion-corrosion. FIB investigation revealed a decrease in grain size at the surface and a change in grain orientation towards the impact direction. Networks of cracks were observed near the surface which is believed to be caused by work hardening of the material which increased the material susceptibility to fatigue cracking. Folding of lips is also proposed as an important mechanism for subsurface wear. The large amount of strain imposed on the material also induced martensitic phase transformation. Fragmented erodent particles and oxide film were found embedded into the material which caused formation stress concentrated regions in the material and contributed to crack initiation. A composite structure is formed consisting silicon oxide sand particles and chromium oxide film along with the martensitic phase transformed metal. The corrosive environment is also believed to have played a significant role in the initiation and propagation of cracks. Crack initiation and propagation due to the mechanical and electrochemical processes enhances the material mass loss as the crack networks coalesce and subsequently cause material spalling. Physical models are developed based on these observations to explain the microstructural changes and synergistic mechanisms.     

Ball-cratering abrasion tests of high-Cr white cast irons

The application of a ball-cratering method to test three-body abrasive wear of bulk materials in the presence of large abrasive particles has been investigated. Three high-Cr white cast irons (WCIs) with different material properties were used as wear samples. Abrasive slurries contained two types of abrasive particles, silica sand and crushed quartz. Silica sand and crushed quartz particles have similar chemical composition and hardness but differ in sharpness. Wear rates of WCI samples were determined and the worn surfaces were examined by optical microscopy, SEM and Talysurf profilometry.It was found that the ball-cratering test can differentiate between the wear resistances of materials with similar properties. The wear resistance of WCIs in the presence of silica sand increased with increasing the hardness of the wear sample and decreasing the size of carbides in the microstructure. Smaller silica sand particles caused less wear damage than larger silica sand particles, even though the smaller particles were slightly sharper than the larger ones. When silica sand and quartz particles of the same size were used, the angular quartz particles caused much higher wear than the rounded silica sand particles. Surface morphologies of the wear craters on the WCI samples were examined in an SEM and then compared with the morphologies of the worn surfaces from slurry pumps. It was found that the silica sand particles generated surface morphologies similar to those found in the worn slurry pumps. In these surfaces the matrix was preferentially worn out and hard carbides were protruding. Wear surface morphologies produced by the angular quartz particles were different. They consisted of numerous superimposed indents and the microstructure phases were not distinguishable. This indicates that the type of abrasive particles used in ball-cratering testing significantly affects the test outcomes in terms of wear rates and wear surface morphology.     

Analysis of impact energy factors in ductile materials using single particle impact tests on gas gun

Wear is surface damage that involves progressive material loss due to relative motion between the contacting surfaces. Removal of material by action of impacting particles is known as erosion. Single particle impact tests were conducted using small particles (95-100 μm) and impact velocity 90 ms⁻¹. A new technique has been developed to measure the impact crater using Laser Scanning Confocal Microscope (LSCM). Depth of craters was calculated based on the impact parameters and the material properties and compared with measured values. The variations are discussed with the high strain-rate deformation and energy loss in the material through strain energy and heating.     

Tribological Properties of Friction Pairs in Lubricant Contaminated with Particles under High Temperature

A liquid–solid lubricant with sand particles of different sizes and concentrations is prepared in advance. The viscosity of the lubricant is measured by a capillary viscometer to determine its relationship to the concentration or size of the sand particles. The relationships between friction and concentration or size of the sand particles are also identified with a UMT2 tribometer. Results indicate that the size of sand particles plays an important role in the lubrication performance; when the size of sand particles is 1–5 μm, the friction coefficient of the liquid–solid lubricant is reduced at low concentration and low load. Contaminant concentration greatly influences the tribological behavior of such a lubricant. The failure probability of the part surface decreases with a reduction in particle concentration; moreover, a high temperature aggravates the friction and wear of this surface. The friction coefficient is 0.14 at 200°C, which is well above the friction coefficient at room temperature (0.078), and the wear volume also increases by 30% compared to the normal temperature. When the temperature is 300°C the wear volume is two times that under room temperature.     

砂粒粒径对航空涡轴发动机压气机叶片冲蚀磨损的影响研究

高原、沙漠和沿海等服役环境中不同粒径的砂粒不可避免地对涡轴发动机压气机叶片造成冲蚀磨损,破坏叶片叶型和动力学特性,严重危及涡轴发动机使用寿命和直升机飞行安全。基于Finnie冲蚀磨损理论推导了颗粒对金属表面的磨损率表达式,分析颗粒粒径对材料冲蚀磨损率的影响,以某型涡轴发动机压气机动叶和静叶为研究对象,设计搭建砂粒冲击速度测试装置和钛合金冲蚀磨损实验装置,通过典型砂粒粒径下冲蚀磨损实验获取磨损率表达式中与靶材材料和冲击速度相关的关键参数,结合气固两相流动力学分析开展砂粒粒径对压气机动叶和静叶冲蚀磨损的影响研究。结果表明：砂粒粒径与冲击速度存在内在关联,材料冲蚀磨损率与砂粒冲击速度呈幂函数关系。实验条件下,砂粒粒径由177 μm增至423 μm时,其冲击速度平均降低约17%。压气机动叶和静叶的磨损集中区域不随砂粒粒径的改变而变化,但磨损程度差异明显,其中177 μm砂粒对动叶和静叶造成的最大冲蚀磨损率浓度值相比423μm砂粒分别增加91%和131%。研究结果为涡轴发动机压气机叶片抗磨损设计提供了理论参考。     

Analysis of impact energy factors in ductile materials using single particle impact tests on gas gun

Wear is surface damage that involves progressive material loss due to relative motion between the contacting surfaces. Removal of material by action of impacting particles is known as erosion. Single particle impact tests were conducted using small particles (95–100 μm) and impact velocity 90 ms⁻¹. A new technique has been developed to measure the impact crater using Laser Scanning Confocal Microscope (LSCM). Depth of craters was calculated based on the impact parameters and the material properties and compared with measured values. The variations are discussed with the high strain-rate deformation and energy loss in the material through strain energy and heating.     

Erosivity of particles from operating fluidized bed combustors

The erosion-corrosion (E-C) metal wastage mechanisms and rates that occur in 1018 plain carbon steel used in tubular heat exchangers of fluid bed combustors (FBCs) are discussed. The characteristics of FBC bed material erodent particles such as composition, shape, size and strength were found to have a major effect on the surface degradation mechanisms and rates which occurred. A total of 16 different bed material particles from ten different FBCs were tested. It was determined that when the particles were strong enough not to shatter upon impact, their shape and composition were the most important factors in determining their erosivity. The basic E-C mechanism was the formation of an outer layer of bed material deposit or mixed bed material and iron oxide that was eroded away in a brittle manner by cracking and chipping of a segmented scale. Beneath the outer layer a mixture of varying amounts of iron oxide and bed material occurred, with the iron oxide content increasing as the base metal was approached. The morphology and behavior of the outer layer are closely related to the metal wastage of the tubing steel.     

Ball-cratering abrasion tests with large abrasive particles

The application of a ball-cratering method to test three-body abrasive wear of bulk materials in the presence of large abrasive particles has been investigated. Four types of abrasive particles of different sharpness were used to make slurries: glass beads, silica sand, crushed quartz and alumina. All the particles were sieved to a size of 250–300 μm. Two common industrial materials, mild steel and 27% Cr white cast iron, were used as wear samples. Wear rates of metallic samples were determined and the worn surfaces were examined by optical microscopy, SEM and Talysurf profilometry.It was found that the surface roughness of the ball significantly affects the wear rates and the wear mechanisms of the metallic samples. The surface roughness of the ball steadily increased with testing time and was mainly affected by the angularity of abrasive particles. More angular particles generated higher ball surface roughness. It was found that the gradual increase in the ball surface roughness was responsible for non-linearity of wear rates with sliding time. The increasing depth of the wear craters also contributed to this non-linearity as deeper craters facilitate particle entrainment. Three-body rolling wear dominated when the ball was smooth and the contribution of two-body grooving wear increased with increasing the ball roughness. Softer mild steel samples were more affected by the ball roughness changes than the harder white cast iron samples. Wear surface morphology was also affected by the angularity of particles and by the material properties of wear samples. Particle fracture was found in all four groups of abrasives and the angularity of the particles was slightly altered. Therefore, the ball-cratering test, under the testing conditions used, can be considered as a high-stress abrasion test.     

Numerical simulation of solid particle impacts on Al6061-T6 Part II: Materials removal mechanisms for impact of multiple angular particles

In the accompanying paper (M. Takaffoli, M. Papini, Numerical simulation of solid particle impacts on Al6061-T6 Part I: Three dimensional representation of angular particles), it was demonstrated that realistic 3D models of angular particles could be generated and used with a smoothed particle hydrodynamics model to simulate the damage done to an Al6061-T6 target due to many non-overlapping particle impacts. In this paper, the same methodology was used to simulate overlapping impacts, and thus the material removal mechanisms associated with the solid particle erosion of this material. The evolution of the topography of the blasted surface was simulated, and the surface ripple patterns that typically form during the erosion of aluminum alloys were observed. The predicted volumetric erosion rates at different impact angles were, on average, within 7% of those measured in erosion experiments. An investigation of the simulated trajectory of the impacting particles revealed the cooperative contribution of overlapping impacts to material loss, and solid particle erosion mechanisms such as the micromachining of chips, the ploughing of craters, and the formation, forging and knocking off of crater lips. The results indicate that numerical simulation of the solid particle erosion of ductile metals by realistic angular particles is possible.     

Particle concentration and size effects on the erosion-corrosion of pure metals in aqueous slurries

In previous studies of erosion-corrosion, several different theories have been developed to produce a model which represents the relationship between particle erosion and chemical corrosion. Regimes in the models define how the two mechanisms behave relative to one another, whether it is erosion dominated, corrosion dominated. This paper investigates the effect of particle and target material on the erosion-corrosion mechanisms. The performance of Fe as the target material will be modelled when considering particle concentration and size. A comparison is made between the erosion-corrosion mechanisms of Fe, Ni, Al and Cu under different conditions of particle size and concentration. By producing several maps, the regimes and wastage rates predicted as functions of velocity and applied potential will be discussed.     

Transmission and scanning electron microscopy studies of deformation at erosion impact sites

Scanning and transmission electron microscopy methods have been employed to study topographic features and subsurface damage associated with erosive-particle impact craters in annealed 310 stainless steel surfaces. Angular Al₂O₃ and spherical glass particles approximately 50 μm in diameter were projected at a velocity of 59 m s^?1 to impact the surface at attack angles of 90° and 20°. Under these conditions, material was found to be displaced but not removed from the surface at isolated impact sites. A comparison was made with damage produced at diamond pyramid hardness indentations. Substantial differences were not observed. In general, a high dislocation density zone a few microns wide was found to surround both impact craters and hardness indentations. The width of this zone varied according to the size and shape of the crater and the direction of particle motion. Deformation twinning occurred at some impact sites. The plastic strain associated with impact craters in 310 stainless steel and copper was also determined by a method that is based on an analysis of selected-area electron channelling patterns.     

冲击形式产生的冲蚀、空蚀和腐蚀联合磨损试验研究

舰船等海洋运输装备在海水中行驶时,船体表面受到含沙海水的冲蚀磨损,舰船行驶速度变化引起的空蚀磨损,以及海水的腐蚀磨损,因此舰船表面材料的破坏形式是冲击式的冲蚀、空蚀和腐蚀联合磨损。为了研究联合磨损的影响因素,研制冲蚀、空蚀和腐蚀联合磨损试验台,并实验研究沙粒浓度、沙粒尺寸、含盐浓度以及冲蚀速度对45钢试件的联合磨损影响规律。实验结果表明:冲蚀、空蚀和腐蚀联合磨损程度强于冲蚀和空蚀交互磨损或单纯腐蚀磨损,在较低的沙粒浓度和较低的含盐浓度条件下,金属材料联合磨损质量损失与沙粒浓度、沙粒尺寸、含盐浓度和冲蚀速度均成正比关系。磨损面形貌分析表明:随着沙粒浓度、沙粒尺寸、含盐浓度、冲蚀速度的增加,试件表面的冲蚀沟和空蚀孔的磨痕深度和面积增大,而在人造海水中,试件表面不仅存在冲蚀沟和空蚀孔,还产生了腐蚀坑。     

Elastic emission machining

Numerically controlled ultra-precision machining, which can easily finish the workpiece into an arbitrary shape, utilizing elastic fracture of the order of atomic size, has been realized. Ultra-fine powder particles are employed as a tool. Powder particles and water are mixed, and by utilizing the state of fluid lubrication of the slurry, only the powder particles are made to act on the material surface. Consequently, minute atomic size removal is achieved with no damage. The mechanism of metal removal is considered from the standpoint of the interaction between the metal surface and the surfaces of the powder particles. It is suggested that the removal of the surface atoms of the substrate is carried out through a very similar process to chemical etching.     

Erosion-corrosion regimes: number, nomenclature and justification?

In studies of elevated temperature erosion by solid particles in oxidizing gaseous environments, several regimes of interaction have been identified. These regimes define whether wastage occurs predominantly due to erosion of the alloy substrate, corrosion of the substrate, or a mechanism intermediate between these processes, and various criteria for the transitions between such regimes have been identified by different investigators. In such cases, the possible number and nomenclature of regimes can sometimes be unclear.The purpose of this paper is to review all the available information on the various erosion-corrosion regimes which have been identified. The similarities and the differences between the various approaches to defining regimes are discussed. In addition, the criteria for their identification are evaluated critically. The importance of the identification of such regimes is discussed. The use of such information to establish zones of minimum wastage on erosion-corrosion maps is demonstrated. In addition, the ability of such regimes to identify variations in erosion-corrosion rate with the main erosion-corrosion variables is indicated.     

Mapping erosion-corrosion of carbon steel in oil exploration conditions: Some new approaches to characterizing mechanisms and synergies

Erosion by solid particles in oil/water slurries is a technologically important area. In such conditions, it is necessary to distinguish between the effects of the sand, aqueous environment, and the oil. Erosion-corrosion maps provide a means of identification between erosion-corrosion regimes as a function of erosion and corrosion parameters. However, there has been no work carried out to map the effects of parameters in oil/water slurries. This paper investigates the effect of erosion-corrosion on carbon steel in oil field production and maps the results. Distinctions between “synergistic” and “additive” erosion-corrosion behaviour are superimposed on the maps in the various environments.     

Digraph and matrix method for the performance evaluation of carbide compacting die manufactured by wire EDM

This paper presents a methodology to evaluate the performance of carbide compacting die using graph theoretic approach (GTA). Factors affecting the die performance and their interactions are analysed by developing a mathematical model using digraph and matrix method. Permanent function or die performance index is obtained from the matrix model developed from the digraphs. This permanent function/index value compares and ranks the factors affecting the die performance. It helps in selection of optimum process parameters during die manufacturing. Hence, process output errors such as dimensional inaccuracy, large surface craters, deep recast layers, etc. will be minimised during die manufacturing which helps to achieve better die performance. In present illustration, factors affecting the performance of carbide compacting die are grouped into five main factors namely work material, machine tool, tool electrode, geometry of die and machining operation. GTA methodology reveals that the machine tool has highest index value. Therefore, it is the most influencing factor affecting the die performance. In case of die material low cobalt concentration and small grain size yields good surface finish, while in machine tool low discharge energy (i.e. low values of peak current, pulse-on time, servo voltage and high value of pulse-off time) and high dielectric flow rate yields good surface finish and, hence, favours the good die performance. In case of die geometry, large work piece thickness and small taper angles results in lesser geometrical deviations and hence helps to achieve better die performance.     

Mechanical behavior of erosion-corrosion scales on steels as characterized by single-particle impacts

Samples of 2.25Cr-1Mo (less than 0.5 Si) and 2.5Cr-0.55Mo-1.4Si steels were eroded-corroded at 450 and 650 °C using fluidized bed combustor bed particles at velocities of 10 and 20 m s⁻¹. The steel with higher silicon content showed significantly lower metal loss rates under all conditions.

The samples were subsequently subjected to single-particle impacts using spherical WC particles at velocities around 50 m s⁻. The impact response of the scales could be explained in terms of a combination of substrate hardness and scale morphology effects but could not be consistently related to the superior erosion-corrosion resistance of the steel with higher silicon content. All scales were composed of oxidation product and deposited bed material erodent. Samples eroded-corroded at 450 °C had denser, more mechanically stable scales which could be associated with the generally lower erosion-corrosion rates at this temperature. At 650 °C the scales were more loosely packed, especially at the lower erosion-corrosion velocity, which resulted in apparent ductility by permitting them to densify under impact. Scales were either segmented or continuous in appearance. Thick continuous scales maintained their integrity under the lower velocity conditions of the erosion-corrosion tests, thus leading to low metal losses, but spalled catastrophicaliy under the single impacts. Segmented scales spalled in smaller pieces under single impacts. It is proposed that the segmented scales would exhibit significant failure under low velocity conditions, thus providing less protection to the steels than continuous scales under similar conditions.     

扫描电子显微镜在粒度分析中的应用

扫描电子显微镜(SEM)是利用聚焦极细的电子束作为照明源,以光栅状扫描方式照射到试样表面,并以入射电子与试样相互作用所产生的信息来进行成像的。采用扫描电子显微镜对收集在微孔滤膜上的颗粒进行分析,不仅可以观察微小颗粒的表面形貌,还可以与能谱仪配合进行颗粒粒径及数量的测量与统计,测试准确度高,因而在粒度分析领域具有不可替代的作用。主要介绍扫描电子显微镜在粒度分析中的应用。