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For the prediction of actual damage to plant component materials and for making the erosion mechanisms clear, it is important to control and to evaluate the particle impact conditions in a testing facility. A sand blast type erosion test rig, which can achieve the particle impact velocities up to 135 m s−1 and a wide range of impact angles has been constructed. The key factors in particle impact conditions of particle flux, impact velocity and impact angle were examined. The relative distance between particles and particle size was discussed, as the particle flux affected erosion rate of material. A new method was proposed to determine particle velocities in this facility. The theoretical velocity of the particle calculated by the equations of particle motion was compared with the experimental results. The divergence of particles from a geometrical angle was evaluated by measuring surface roughness of the specimens. Although some spread of the particles were observed surrounding the central damage area of the specimen surface, the greatest amount of damage was concentrated in the center. As a result, it was found that particle impact conditions were well controlled in this testing unit.

A sealing treatment has been applied to diminish the effects of the pores and micro-cracks on the corrosion properties of the plasma-sprayed Cr 2 O 3 /NiCr dilayer coatings. This treatment decreased the porosity both in the ceramic and in the NiCr dilayer and effectively improved the corrosion resistance. The sealing treatment involved the use of polymethyl silicon and vinyl methyl silicon. Two compositions of plasma sprayed Cr 2 O 3 /NiCr dilayer coated materials (Cr 2 O 3 /80NiCr/316 and Cr 2 O 3 /50NiCr/316) were studied. Optical microscopy and scanning electron microscopy demonstrated that all of the open pores in Cr 2 O 3 /NiCr dilayer were blocked by the sealant resin and only closed pores were remained. The ceramic and sealing resin formed a compact layer without open pores, which protected the materials beneath against corrosion. Corrosion tests of the sealed and unsealed specimens showed that the corrosion resistance of the Cr 2 O 3 /NiCr dilayer coated materials increased markedly after the sealing treatment. Electrochemical experiments showed that after the sealing treatment the difference in corrosion resistance between the sealed Cr 2 O 3 /80NiCr/316 and Cr 2 O 3 /50NiCr/316 was very small probably because the corrosive environment served to mask the effect of the 80NiCr and 50NiCr on the corrosion properties of the dilayer coated materials. Sealed specimens showed small weight grains due to the absorption of water in the resin.

This paper studied the corrosion properties of five kinds of Cr2O3 coated materials: the SUS316L austenitic stainless steels respectively coated with the Cr2O3 layer (Cr2O3/316), Cr2O3 and 80Ni- 20Cr layers (Cr2O3/80NiCr/316), Cr2O3 and 50Ni- 50Cr layers (Cr2O3/50NiCr/316), Cr2O3 + 80Ni- 20Cr functionally gradient materials (Cr2O3 + 80NiCr FGM), and Cr2O3 + 50Ni- 50Cr FGM (Cr2O3 + 50NiCr FGM). All the coatings were made by atmospheric pressure plasma spraying method (APPS). The corrosion resistance of the coated materials was analyzed by immersion tests and electrochemical evaluations. A mechanism of the corrosion failures for these kinds of coated structures was proposed. The different coating structures for improving the adhesion between ceramics and substrates were assessed, and the effect of Cr content in the NiCr coatings on the corrosion property was discussed. The ceramic coatings with the 50Ni- 50Cr intermediate layer possessed a better corrosion resistance than that with 80Ni- 20Cr. The FGM structures appeared to offer weaker resistance to corrosion attack than that with the intermediate layer in general. Under corrosion test conditions, the corrosion-proof abilities of the coated structures were, respectively: the Cr2O3/50NiCr/316 in the best rank; the Cr2O3/316, Cr2O3/80NiCr/316, and Cr2O3 + 50NiCr FGM in the second rank; and the Cr2O3 + 80NiCr FGM in the last rank. This means that applying the 50NiCr intermediate layer under the Cr2O3 ceramic coating can further improve the corrosion resistance of Cr2O3/316. Porosity analysis was used to explain the difference of corrosion resistance between the Cr2O3/50NiCr coated material and the Cr2O3 + 50NiCr FGM. The porosity in each layer of the Cr2O3 + 50NiCr FGM was higher than that in the Cr2O3/50NiCr coating, and as a result the corrosion resistance of Cr2O3/50Ni Cr/316 is better than Cr2O3 + 50NiCr FGM. Korrosionseigenschaften von Werkstoffen beschichtet mit Cr2O3/NiCr und mit Cr2O3 + NiCr funktionellen Gradientenmaterialien Die vorliegende Arbeit untersucht die Korrosionseigenschaften von funf Arten mit Cr2O3 beschichteten Werkstoffen: austenitische nichtrostende Stahle vom Typ 1.4404 bzw. beschichtet mit einer Cr2O3- Schicht (Cr2O3/316), Cr2O3 und 80Ni- 20Cr Schichten (Cr2O3/80NiCr/316), Cr2O3 und 50Ni- 50Cr Schichten (Cr2O3/50NiCr/316), Cr2O3 + 80Ni- 20Cr funktioneller Gradientenmaterialien (Cr2O3 + 80NiCr FGM) und Cr2O3 + 50Ni- 50Cr FGM (Cr2O3 + 50NiCr FGM). Alle diese Beschichtungen wurden durch Plasmaspritzen bei Atmospharendruck hergestellt. Der Korrosionswiderstand der beschichteten Werkstoffe wurde durch Tauchversuche und elektrochemische Untersuchungen analysiert. Fur diese Arten von Beschichtungsaufbau wurde ein Korrosionsschadensmechanismus vorgeschlagen. Die verschiedenen Beschichtungsstrukturen zur Verbesserung der Haftung zwischen Keramik und Substrat wurden beurteilt und der Einfluss des Chromgehaltes in den NiCrBeschichtungen auf die Korrosionseigenschaften diskutiert. Die keramischen Beschichtungen mit 50Ni- 50Cr zeigten eine bessere Korrosionsbestandigkeit als die mit 80Ni- 20Cr. Bei den FGM Strukturen war der Widerstand gegenuber Korrosionsangriffen im allgemeinen schwacher als bei denen mit der Zwischenschicht. Unter Korrosionsversuchsbedingungen ergab sich folgende Reihenfolge bezuglich der Bestandigkeit: Cr2O3/50NiCr/316 an erster Stelle; Cr2O3/316, Cr2O3/80NiCr/316, Cr2O3 + 50NiCr FGM an zweiter Stelle und Cr2O3 + 80NiCr FGM an letzter Stelle. Das bedeutet, dass eine 50NiCr-Zwischenschicht unter der keramischen Cr2O3- Schicht den Korrosionswiderstand von Cr2O3/316 weiter verbessern kann. Die Porositatsanalysen wurden benutzt, um die Unterschiede des Korrosionswiderstandes zwischen den mit Cr2O3/50NiCr beschichteten Werkstoffen und den Cr2O3 + 50NiCr FGM zu erklaren. Die Porositat in jeder Schicht der Cr2O3 + 50NiCr FGM war hoher als in der Cr2O3/50NiCr Beschichtung und als Folge war der Korrosionswiderstand von Cr2O3/50NiCr/316 besser als von Cr2O3 + 50NiCr FGM.

As a result of their great chemical inertness, ceramics and functionally gradient materials (FGM) are being used increasingly as coatings on machine parts. However, studies have shown that...

The NiCr plasma-sprayed deposition, as an intermediate layer, is normally used to improve the adhesion between the metal substrate and the ceramic coating. Defects, such as pores and micro-cracks, generally exist in the ceramic layer, and lead to corrosion beneath the coating. The present work used plasma-sprayed specimens, such as the Cr2O3 ceramic coatings with and without a NiCr intermediate layer and the single NiCr coatings, etc. which were deposited on a SUS316L stainless steel substrate. The results of corrosion tests and electrochemical experiments indicated that the corrosion resistance of the 80NiCr (80Ni–20Cr) coating was weaker than that of the SUS316 substrate, while that of 50NiCr (50Ni–50Cr) was better than that of the substrate. The existence of 80NiCr plasma-sprayed intermediate layer decreased the corrosion resistance of the Cr2O3 ceramic coated specimen, while the 50NiCr increased it. The microstructure analysis demonstrated that both the 80NiCr and 50NiCr plasma-sprayed coatings consisted of the matrix and a second-phase structure. Energy dispersive spectroscopy (EDS), wave dispersive spectroscopy (WDS) and X-ray diffractometry (XRD) analysis showed that the components of the matrix and the second-phase in 50NiCr were different from those in 80NiCr. In addition, this paper preliminarily discusses the corrosion mechanism of 80NiCr and 50NiCr intermediate layers. The different chromium content in the matrix of the 80NiCr and 50NiCr coatings might have caused the difference in the corrosion resistance between them. It can be predicted that the decrease of porosity by means of plasma spraying or sealing treatment of the ceramic coating, as well as by regulating the components of the NiCr intermediate layer, would improve the corrosion resistance of Cr2O3 ceramic coated materials.

In the previous paper (Yabuki and Matsumura, this issue), the critical impact velocity in the erosion of metallic materials by solid particles impact was experimentally determined. In this paper, the critical impact velocity was theoretically derived through analyzing the behavior of the material surface obliquely impacted by a spherical solid particle, where the material is strained tangentially to the surface in the direction of particle movement. As the extent of strain goes over the elastic limit, the impacted solid particle does skid over the surface, which brings about wear (to the surface) by cutting. The threshold strain was theoretically derived as a function of the impact velocity and the rotating velocities as well as the duration of particle surface contact. Those parameters which characterized the impact behavior were derived from the coefficient of friction and the rebounding coefficient, all of which were obtained from the mechanical properties of the target material and particle, and other factors concerning the particle. Consequently, the theoretical value of the critical impact velocity was given solely as a function of the mechanical properties of the target and the particle. The calculated critical impact velocity attained the lowest value at a low impact angle, and a good correlation was found between the critical velocity determined by experiment and that predicted by the theoretical equation.

Abrasive resistances of engineering components encountering the attack of erosive environments during operation can be improved by ceramic coatings on their surfaces with a optimum techniques of plasma spraying. The present paper mainly studied abrasive properties of a series of ceramic coatings with different low pressure plasma spray (LPPS) and combined LPPS/APPS (atmospheric pressure plasma spray) methods, in comparison with that of SUS 329J1 dual phase stainless steel, in the jet-in-slit and the rotating specimen testers. The erosive environments were selected as silica sand slurry and fly ash(x%)+gypsum(y%) slurries, respectively. The test results from the two testers did not always coincide although they did in general. The paper summarizes the possible reasons and the implications of the distinctions. In addition, the impact frequency, velocity and angle of particles on the surfaces of the specimens were measured and calculated, and they were used to explain the wear properties of ceramic coatings. Generally, ceramic coatings, such as Cr2O3 and Al2O3, have much improved wear resistance. But the advantages of their wear properties are decreased with increasing impingement angle. In the fly ash(x%)+gypsum(y%) slurry environments which are usually existed in the desulphurizers of thermal power plants, the most deleterious environment for the pumps is pure fly ash (fly ash(100%)+gypsum(0%) slurry.

Steady-state solid particle erosion of squeeze-cast AC4C Al alloy and 19.5 vol.% Al 18 B 4 O 33 whisker/AC4C Al composites was investigated at room temperature. Experiments were performed with angular silica sand having four velocities (55 m s −1 , 140 m s −1 , 198 m s −1 and 243 m s −1 ) and at impact angle in the range 15° to 90°. The eroded surface and subsurface were observed using scanning electron and optical microscopy. The results indicated that erosion characteristics of the composites was affected by the particle velocity and impact angle. Steady-state rate was found to increase with the addition of whisker to the matrix alloy at high particle velocities or upon increasing impact angle. High erosion rate in the whisker reinforced composites can be attributed to reduced ductility. In addition to plastic deformation, flaking, gouging and microcutting, for the composites, fragmentation and dislodgement of Al 18 B 4 O 33 whisker occurred with impact of particles. At low particle velocity and shallow impact angle, the whisker at the surface can act as protective reinforcement to resist the microcutting and microploughing action. The erosion resistance for the Al 18 B 4 O 33 /AC4C Al composites was improved as a result of whisker addition.

The dependence on the impact angle of erosion damage caused by solid particle impact was characterized on several kinds of material, as expressed by a trigonometric function of both impact angle and material hardness. Erosion tests were conducted on five metallic materials, a ceramic and a plastic material using silica sand particles (mean particle size 325 μm) at impact angles from 3° to 90°, up to an impact velocity of 130 m s−1. The geometry of the specimen and the nozzle tip were devised in order to obtain erosion damage at shallow impact angles less than 20°. The reproducibility of erosion damage obtained under these conditions was consistent with that of standard erosion tests. The detailed erosion damage at shallow impact angles made the dependence of impact angle clear. Although the amount of erosion damage commonly increased with the increase in impact velocity, it was found that the dependence of the erosion damage of metallic materials on impact angle was independent of the impact velocity within 50–130 m s−1 in terms of the relative erosion damage normalized by that at a normal impact angle. This allowed simplification of the equation for erosion damage curves. It was recognized that the maximum erosion damage and the shape of the damage curves shifted monotonically according to the material hardness, which is considered generally to be correlated with other mechanical properties. As a result, the normalized erosion damage curves for tested materials were basically correlated with a trigonometric function in which constants and exponents depended on the impact conditions, except for the impact velocity, if the properties of the particles were fixed. Moreover, in the case of metallic materials, the values related to the hardness. This simulation probably enables an estimation of erosion damage at a given impact angle from that at the normal impact angle.

Most of the water taps for domestic use are made of copper alloys. Their valve seats occasionally suffer damage owing to corrosion as a result of water quality and seemingly cavitation respectively. The examination of damage mechanism and the development of testing methodology for selecting the durable materials to the attack were requested. In order to understand and solve these problems completely, erosion-corrosion tests were carried out on various copper alloys (five brasses and five bronzes) by using three test methods: (a) the ISO 6509 dezincification test for pure corrosion; (b) the vibratory cavitation test with eccentric stationary specimen for cavitation erosion-corrosion; and (c) the jet-in-slit test for flow induced localized corrosion. The materials tested were ranked in different orders of merit according to the results obtained in three test methods. Among these rankings, the one obtained in the jet-in-slit test has agreed upon with the ranking order that had been determined by the experiences of field engineers who engaged themselves with the troubles under practical conditions. Hence it was concluded that the damage on valve seats has been caused by the flow induced localized corrosion, and the jet-in-slit test was recommended as the reliable test method in selecting durable valve seat materials.

The use of ceramic coatings for pump impellers which circulate the stack gas-cleaning liquid of high acidity containing solid particles of gypsum was examined. In this study, slurry erosion tests were carried out by a jet-in-slit type apparatus using a silica sand-water slurry at normal incidence to the specimen surfaces. The materials tested were SUS 316 stainless steel (substrate), with a coating layer of Al203 and SS 400 steel (substrate) with a coating layer of TiN+Ti functionally gradient material. The former coating was deposited using a low-pressure plasma spraying technique, and the latter by laser thermal spraying. The test results revealed that the durability of coatings to erosion assessed by weight-loss and by eroded depth measurements was inconsistent. Moreover, the erosion-resistance characteristics differed depending on the erosion areas on the specimen surface, even though the parameter of eroded depth was used. Such phenomena may be attributed to the influence of angles of attack by individual slurry particles. The impact angle of each slurry particle varied with the divergence of the jet, thus causing a variation in erosion characteristics along the specimen surface, even though the slurry jet impacted at normal incidence. This paper analyzes the distribution and impact angles of individual slurry particles from the jet nozzle in order to understand the erosion characteristics under various slurry impaction conditions.

Cavitation erosion tests were carried out on five metallic materials which are the standard materials of the International Cavitation Erosion Test Program which is coordinated by Dr. J. Steller, Institute of Fluid-Flow Machinery, the Polish Academy of Science, Gdansk, Poland. Three kinds of testing apparatus were used: a vibratory unit, a vibratory unit with a stationary specimen, and a water tunnel. In the two vibratory tests, the ranking order of the materials according to their durability to cavitation attack showed a complete coincidence with each other. However, the ranking based on the water tunnel test was different from those of the vibratory units. This was because the damage rate of some materials was suddenly accelerated in the middle of the water tunnel test. It was revealed that the remote cause of the sudden increase in erosion rate is the activation of damaged surface in atomic level, which promotes corrosion only when a macro-cell of corrosion is formed between the cavitation damaged area (anode) and the undamaged area (cathode). Thus it was pointed out that some undamaged area is some times necessary on laboratory test specimens when an accurate simulation of the cavitation damage in the filed is demanded.

Analyses of plastic strain caused by a dynamic or quasi-static intrusion of a hard steel ball were carried out to compare the plastic strain distributions and to search for triggers of erosion damage to materials around the indentation. Regular square grids with 11.81 line mm −1 were made on the mating surface of a composite block by using a photoengraving method. The two blocks were tightly held in a vice and then indentation tests were conducted at the parting line. Principal shearing strain distributions were obtained for commercially pure aluminium, iron and grey cast iron. In the case of the iron specimen the maximum shearing strain at a dimensionless indentation ratio of d / D = 0.75 was observed below the rim of the dynamic indentation, but below the centre of the quasi-static indentation. The maximum shearing strain shifted from a portion just below the quasi-static indentation into that far below the dynamic indentation for the aluminium specimen. Both the size and form of the elastic-plastic boundary on the cross-sectional surface depended upon the type of the materials and the intrusion processes. Piling-up between two dynamic indentations was simulated to investigate triggers of material removal for the iron specimen. A finite element method simulated not only lip configurations but also plastic strain distributions under the same conditions of quasi-static indentation tests, and produced information about stress distributions around indentations during an intrusion of a projectile.

Cavitation erosion tests were carried out on five metallic materials which are the standard materials of the International Cavitation Erosion Test Program which is coordinated by Dr J. Steller, Institute of Fluid-Flow Machinery, the Polish Academy of Science, Gdansk, Poland. Three kinds of testing apparatus were used: a vibratory unit, a vibratory unit with a stationary specimen, and a water tunnel. In the two vibratory tests, the ranking order of the materials according to their durability to Cavitation attack showed a complete coincidence with each other. However, the ranking based on the water tunnel test was different from those o f the vibratory units. This was because the damage rate of some materials was suddenly accelerated m the middle of water tunnel test. It was revealed that the remote cause of the sudden increase in erosion rate is the activation of dam- aged surface in atomic level, which promotes corrosion only when a macro-cell of corrosion is formed between the cavitation damaged area (anode) and the undamaged area (cathode). Thus it was pointed out that some undamaged area is some times necessary on laboratory test speci- mens when an accurate simulation of the Cavitation damage in the field is demanded.

A model for erosion caused by solid particles was studied and an equation based upon the relationship between dynamic indentation and the hardness of the material was proposed. This equation was established by the relationship between the surface hardness and the erosion damage to the materials. Erosion tests using a gas gun unit were conducted on five materials over a wide range of hardness values (Hv number 9–1100) up to an impact velocity of 150 m s −1 . An increase in the hardness was seen on the fully eroded surfaces obtained in the cases of iron and aluminium, but a decrease was seen in the case of quenched carbon tool steel. No change in the hardness was seen in the cases of the acrylic resin and nylon. The hardness of the eroded surfaces, as opposed to that of the non-eroded surfaces, slightly improved the correlation with erosion damage, but it was not necessarily sound. The softening observed on the eroded surface of the quenched carbon tool steel specimen suggested that heat was generated during the impact of the particles and also that the surface hardness during the course of erosion was different from that both before and after erosion. The temperature increase of an iron leaf sample owing to the impact of a 3.18 mm steel ball or silica sand particles pointed to a transiently high temperature on the impacted surface and the possibility of softening during the course of the erosion process. As a result, the surface hardness of each material estimated with respect to work hardening and softening reasonably correlated with the erosion damage. Also, numerical formulation and the soundness of this equation were discussed through many erosion tests under various conditions.

Slurry erosion-corrosion tests were conducted on type 304 stainless steel in order to investigate whether the passivating film has any effect of decreasing the damage to the material. Two environments which contrasted with each other were chosen: a slurry consisting of silica sand particles and a 1 % sulfuric acid solution where the passivating film is stable, and a slurry of the same particles and a 5 % hydrochloric acid solution where it is not.As a result, the material was damaged at higher erosion-corrosion rates in the environment where the film was stable than in that in which it was not. Thus, the ability of the film to decrease the damage appeared to be denied. Nevertheless, the anodic polarization curve of the material in the 1 % sulfuric acid solution slurry revealed that the passivating film was mechanically broken by the impact of the particles but it was repaired rapidly enough to inhibit corrosion. Further it was found that in the 5 % hydrochloric acid solution slurry, erosion as well as corrosion was inhibited by chloride ions adsorbed on the surface.Consequently it was concluded that the passivating film has an ability to inhibit erosion-corrosion damage to a certain extent through inhibiting corrosion so long as it is chemically stable in the environment.

Wall thinning at unusually high rates (0.5∼1 mm y−1 ) due to FAC (flow accelerated corrosion) may sometimes occur on the walls of carbon steel piping which transports pure, high-temperature water. FAC generally tends to occur in locations where the solution temperature is 140∼150°C, downstream of pipe fittings and flow-meters. It arises quite accidentally: sometimes occurs and sometimes does not in spite of related parameters such as flow velocity and temperature being identical. In addition, the damage may be drastically reduced, when the material of the pipe tube in which unusual wall thinning had occurred is exchanged with low alloyed steel. The macro-cell corrosion mechanism, which is the principle mechanism for galvanic corrosion as well as for cathodic protection with a sacrificial anode, was modified by taking laboratory test results into consideration, and applied to incident cases to successfully explain the generation mechanism of this unusual pipe wall thinning. It explains the characteristic behavior of FAC described above and the anticorrosive effect of chromium. It was concluded that the unusual wall thinning of carbon steel pipe may be attributed to the formation an active/passive macro-cell on the surface. An active/passive macro-cell occurs due to gaps in the distribution of such passivation affecting parameters as the pipe wall temperature and the dissolved oxygen content, so that the basis of prevention amounts to avoidance to these gaps. Thus, it is recommended that stagnation watershed and localized lowering of temperature should be avoided with pipe lines.Copyright © 2007 by ASME

円管の中心部に置かれた球のまわりのキャビテーションに関し, 衝撃圧の発生機構を明らかにするため高速度カメラ。ピエゾメーターおよびストロボスコープを組合せて衝撃圧発生の瞬間における気泡群挙動を撮影した。得られた写真に基づいて衝撃圧発生機構を論じ, さらにそれに基づいて流速の大きい特性流速域においても球材の受けるキャビテーション損傷が減少する既報の結果の理由を解明した。

内径52.9mmの円管の中心部に置かれた球に生ずるギャビテーション・エロージョンについて, 最大流速15m/sec (Re=1,200,000). 球径9.50～33.00mmの範囲で試験を行なった。高速度写真撮影 (10.000コマ/sec) ならびにストロボスコープによる観察の結果, 球の後部に発生する気泡の挙動に関して3つの流域があること, 鋼球を用いた侵食試験では, これらのうち最も高速の流域でかえって損傷速度が減少することを確め, またその理由をも明らかにした。

The effectiveness of ceramic coatings on pump impellers used in the acidic gypsum-fly ash slurry environments in desulphurisers was assessed by using two kinds of slurry erosion tester. The experimental results from the two testers did not always coincide, although in general they did. The possible reasons and the implications of this distinction between the two testers were discussed, and the following conclusions were reached: (1) Generally, although ceramic coatings, in particular Cr2O3 and Al2O3, have much improved slurry erosion resistance, the resistance of ceramic coatings to slurry-erosion is closely related to the techniques of plasma spraying. At optimum manufacturing conditions, ceramic coatings are about two times as resistant as the base metal (SUS329J1); (2) Vicker's hardness (Hv) is an effective parameter for the assessment of slurry erosion properties of ceramic coatings, as well as bulk ceramics; (3) The most deleterious environment for slurry pumps used in desulphurisers is pure fly ash, therefore, fly ash should be removed as much as possible before desulphurising in thermal power plants.