Low impact velocity wastage in FBCs : experimental results and comparison between abrasion and erosion theories

The use of technologies related to combustion of coal in fluidized bed combustors (FBCs) present attractive advantages over conventional pulverized coal units. Some of the outstanding characteristics are: excellent heat transfer, low emission of contaminants, good combustion efficiencies and good fuel flexibility. However, FBC units can suffer materials deterioration due to particle interaction of solid particles with the heat transfer tubes immersed on the bed (Hou, 2004, Oka, 2004, Rademarkers et al., 1990). Among other issues, some of the most important factors believed to cause wear problems are: the motion of slowly but relatively coarse particles, particles loaded onto the surface by other particles, erosion by relatively fast-moving particles associated with bubbles, and abrasion by blocks of particles thrown into the surface by bubble collapse. Thus, erosion or abrasion processes can occur by a variety of causes. For the case of particle movement against in-bed surfaces, it has been suggested that there is no difference in the ability to cause degradation between solid particle erosion and low stress three body abrasion, and distinctions between the two forms of wear should not to be made (Levy, 1987)

Erosion-corrosion of carbon steel and 310 stainless steel at low impact velocities : Effect of erodent particle type

Se estudio el comportamiento de un acero al carbono (1020) y un acero inoxidable tipo 310 expuestos en condiciones de erosion-corrosion a bajas velocidades de impacto. Las partículas erosivas empleadas fueron SiO2 y Al2O3 con tamaños de partícula muy similar (≈ 600 μm) y el ambiente oxidante fue aire. Los experimentos fueron efectuados a dos temperaturas (450 C y 600 C) durante 24 horas y el rango de velocidades de impacto fue de 2.5 m/s a 4.5 m/s. En general, los resultados mostraron que las partículas de SiO2 causaron menos daño que las partículas de Al2O3, aunque las diferencias en el deterioro observado fueron más significantes para el acero al carbono, en particular a la temperatura más alta. Para el acero al carbono a 2.5 m/s, la morfología superficial mostro formación de ondulaciones (ripples) las cuales son típicamente asociadas a un proceso de erosión. Un aumento en la velocidad de impacto de las partículas modifica notoriamente la morfología de impacto, provocando fractura de oxido formado. Para el acero 310 a 2.5 m/s, la morfología de impacto mostro formación de ondulaciones al emplear ambos tipos de partícula. Sin embargo a velocidades de impacto por arriba de 2.5 m/s, las superficies de impacto muestran mayor pulido y áreas de corte, las cuales son menos pronunciadas cuando se usan las partículas de SiO2 en comparación con las partículas de Al2O3. Lo anterior se asocia con la forma de partícula erosiva: en el primer caso ocurre más deformación plástica superficial debido a la redondez de las mismas, mientras que en el segundo caso las partículas son más agudas. Se analiza y discuten posibles razones para explicar el deterioro observado en ambos materiales

Effect of Water on the Stress Corrosion Cracking Behavior of API 5L-X52 Steel in E95 Blend

The effect of water content (0.5%, 1%, 2%, 5%, 10% y 20 V%) in E95 blend (5 V% gasoline – 95 V% ethanol) on the stress corrosion cracking (SCC) susceptibility of X-52 carbon steel was investigated. Slow strain rate tests (SSRT) coupled with electrochemical noise measurements (ECN) were carried out using a strain rate of 1 X 10-6 s-1. In general, scanning electron microscopy (SEM) observations on fracture surfaces showed a ductile behavior. However, secondary cracking was only observed for specimens exposed to solution containing up to 2 V% water. ECN gave indication of a likely localized corrosion process occurring at low water concentrations, whereas for water content above 2 V%, a uniform corrosion process seems more likely to occur. In addition, the material response immersed into the various solutions was investigated by using linear polarization resistance (LPR) measurements, weight loss and pH measurements. Reasons to explain the behavior found are discusse

Corrosion behavior of AISI 409Nb stainless steel manufactured by powder metallurgy exposed in H2SO4 and NaCl solutions

Powder metallurgy is an effective method for manufacturing stainless steel parts of high quality and accuracy at low cost. However, the use of sintered stainless steels is limited due to their low density, which deteriorates their corrosion resistance. The aim of this study was to determine the corrosion behavior of AISI 409Nb stainless steel specimens sintered with different contents of boron in a hydrogen atmosphere. Boron was added for promoting the formation of a liquid phase during sintering at 1150 °C, thereby achieving a reduction of porosity and increase in density, which is necessary to improve corrosion resistance. The electrochemical techniques of linear polarization resistance (LPR) and electrochemical noise (EN) were used to determine the corrosion behavior of samples with and without additions of boron after immersion in two solutions, 0.5M H2SO4 and 0.5M NaCl. The corrosion rates and the possible corrosion mechanisms in the sintered samples were determined. The results indicate that the samples with boron additions are more prone to corrosion due to chromium carbide precipitation

The erosion-corrosion performance of mild steel and SA313-T22 steel in low velocity conditions

This paper discusses the erosion-corrosion performance of mild steel and SA313-T22 steel in low velocity conditions. It was presented at the 3rd NACE Latin American Region Corrosion Conference in 1998

Synergism between effects of velocity, temperature, and alloy corrosion resistance in laboratory simulated fluidised bed environments

In studies of the erosion of alloys at elevated temperature, the combined effects of velocity, temperature, and alloy corrosion resistance are not well understood. Wide variations in the effects of velocity have been observed for alloys of different corrosion resistance in various erosion-corrosion environments. There is also some evidence that temperature can affect this relationship. The object of the present work was to undertake a systematic study, of the effects of erodent velocity for two alloys, mild steel and 310 stainless steel, at elevated temperatures (300 and 600 degrees C). The velocity was controlled at values between 1.5 and 4.5 m s(-1). Weight change data and analytical scanning electron microscopy were used to characterise the degradation in the various conditions. The results showed that the ranking order of the erosion-corrosion rates of the two different alloys varied as a function of velocity. The velocity at which the ranking of the two alloys reversed increased with increasing temperature. The reasons for such behaviour are discussed in terms of the dependence of the erosion-corrosion rate on the velocity in the various erosion-corrosion regimes

Relationship between the effects of velocity and alloy corrosion resistance in erosion-corrosion environments at elevated temperatures

Erosion-corrosion by either solid particle or liquid impact in a wide variety of industrial environments which range from coal conversion processes to steam turbines in nuclear power generation. The effects of erosion-corrosion depend on properties of the particle, the target and the nature of the corrosion environment. Various regimes of erosion-corrosion interaction have been identified, ranging from ''erosion-dominated'' (erosion of the substrate) to ''corrosion-dominated'' (erosion of the corrosion product). In studies of erosion-corrosion, the effects of impact velocity are generally not well understood. In some environments in which corrosion occurs, high velocity exponents have been reported have been reported, while, in others, the values are close to 1. In addition, the effects of alloy corrosion resistance in environments of different velocities have been puzzling with differences in the exponents reported, as alloy corrosion resistance is increased. This paper considers the effect of velocity for various erosion-corrosion studies from the literature. The effects of alloy corrosion resistance for such results are evaluated. Some general provisos for the interpretation of the effects of velocity will be made for alloys of different corrosion resistance in erosion-corrosion environments. It is shown that relative erosion-corrosion resistance of alloys in one environment cannot be used arbitrarily to predict resistance in other environments, particularly if parameters such as velocity are varied significantly

The effect of substrate hardness on the erosion-corrosion resistance of materials in low-velocity conditions

Substrate hardness is sometimes used as a guide to the wear resistance of alloys. However, it been documented that there is often much less correlation between particle erosion resistance and hardness. This has been attributed to the high strain rates of erosion impacts compared with those involved in a static hardness test. Under erosion-corrosion conditions, where formation of oxide scales in a gaseous environment can influence markedly the extents of material damage, material hardness may influence the interactions of oxidation and impact damage. The present study has investigated this phenomenon. It has involved testing, in a fluidized-bed erosion-corrosion rig, a range of materials of various hardnesses, including mild steel, 310 stainless steel, a duplex 22% Cr-8% Ni-3% Mo alloy (under two heat-treatment conditions) and two cobalt-based alloys, Ultimet and Castolin 906. The alloys were exposed at temperatures up to 560-degrees-C and at a velocity of 5.8 m s-1 in a fluidized-bed containing 100 mum alumina erodent particles. This paper is concerned primarily with the general trends of erosion resistance as a function of material hardness under erosion-dominated, erosion-corrosion-dominated and corrosion-dominated conditions. At relatively low temperatures, where erosion dominates, there is no obvious correlation between the two parameters. However, at higher temperatures where oxidation plays a more significant role, the rate of erosion generally increases with increasing substrate hardness. Possible reasons for these observations are proposed and discussed in relation to earlier results from the literature

Elevated temperature erosion resistance of Ni-based functionally graded materials

Conference paper details the elevated temperature erosion resistance of Ni-based functionally graded materials

Elevated temperature erosion of range of composite layers of Ni-Cr based functionally graded material

Functionally graded materials can be used in aggressive environments at elevated temperature because they provide the possibility of minimising wastage of materials. Gradation of the volume fraction of hard particles through the layers means that thermal cycling effects are less severe than for many conventional metal-substrate systems. Because such materials may provide resistance to wear and corrosion (by using a corrosion resistant matrix), it is thought that they may be applications to environments at elevated temperatures, in which materials selection involves a compromise between corrosion resistance and high yield strength. The object of the present study was to investigate to erosion resistance of the various layers of a candidate functionally graded material which consisted of WC particles in a Ni-Cr matrix. The performances of the various composite layers were considered separately in order to establish to variation of erosion rates through the graded structure. The effects of temperature, volume fraction of hard particles, and erodent size were investigated in a laboratory simulated fluidised bed erosion rig. Scanning electron microscopy and thickness loss measurements were used to characterise the surfaces following exposure. The results showed that the erosion rate at room temperature was at a minimum at intermediate volume fractions of WC particles. However, this behaviour reversed for erosion with larger particle sizes. Although the thickness losses increased with increasing temperature for all volume fractions of reinforcement particles, a reduction in the thickness loss at the highest temperature studied was observed for exposure to both large and small erodents (600 and 200 mu m alumina). The results are explained in terms of the transition between erosion regimes for the various graded layers of the material