Erosion-corrosion of candidate HVOF aluminium-based marine coatings

This paper describes gravimetric results of HVOF sprayed commercially pure Al and Al/12% Si eutectic alloy coatings obtained under erosion-corrosion conditions. The performance of these coatings are compared to a hot-dipped zinc coating. These coatings have been applied to AISI 1020 carbon steel substrates and tested in a free jet impingement rig with a variety of fluids and slurries to enable the erosion and corrosion contributions to the total erosion-corrosion removal of the surface to be determined. Tests have been conducted at 30° and 90° jet impingement angles at jet velocities of 3.5 m/s.Erosion damage mechanisms are identified and the erosion-corrosion mass loss results discussed in terms of the contributions made by erosion and corrosion. Zinc was found to suffer severely from flow corrosion, while the other targets did not. All target materials under sand erosion lose mass in a way directly proportional to kinetic energy, and inversely related to hardness. Corrosion losses are relatively small while the generation of solid products sometimes results in net weight gains. There are greater mass losses at nominally normal incidence than at nominally oblique incidence, due to particle interference effects. Synergy is found to be both positive and negative and appears to be angle dependent, but is difficult to determine precisely.     

The slurry erosive wear of physically vapour deposited TiN and CrN coatings under controlled corrosion

The erosion-corrosion of mild steel (BS6323), in the presence and absence of physically vapour deposited (PVD) TiN and CrN coatings, was studied, in comparison with that of AISI 304 stainless steel, in an aqueous alkaline slurry solution containing alumina particles. The influence of applied potential and particle velocity on the total erosion-corrosion loss was examined, and the respective corrosion and erosion damage (both contributing to the overall weight loss) then assessed by means of microscopical investigation of the morphology of the damaged surface, and subsequently evaluated quantitatively. The superior erosion-corrosion resistance of both the coatings compared to that of the uncoated mild and stainless steels was shown to be due to their resistance to both wear and corrosion. According to the detailed corrosion mechanisms revealed and different responses to wear, schematic diagrams were proposed to outline the main features of the corrosion-erosion process and the individual roles of erosion and corrosion. Discrete differences, in terms of the respective erosion and corrosion processes, between the TiN and CrN coatings, and between the mild and stainless steels, were also investigated and discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Full factorial investigation on the erosion-corrosion resistance of UNS S31603

The interactions that occur when erosion and corrosion act simultaneously are extremely complex and are often difficult to interpret. These interactions generate either a synergistic or antagonistic material loss effect for a particular material in a certain environment. The level of interaction between impact energy, number of impacts, fluid temperature, material properties, fluid flow and electrochemical properties severely complicates the analysis of erosion-corrosion wear rates. This paper investigates the interaction between the main parameters influencing erosion-corrosion. A combination of statistical analysis and interaction contour plots has been employed to obtain in-depth understanding of the variables influencing erosion-corrosion, namely particle velocity, sand size, sand concentration and fluid temperature. An empirical equation has been derived from test results to describe the relationship between the test parameters. Analysis of the residuals versus predicted erosion-corrosion shows a normalized distribution and thereby confirms the suitability of this model. Velocity was found to have the strongest influence on erosion-corrosion rate followed by sand concentration, temperature and finally sand size, which had the least significant effect. SEM surface features show that the increase in sand concentration causes the surface to be covered with a higher number of impact craters and lips indicating a linear relationship between the two. The SEM micrographs also show that the increase in sand size produces deeper craters and more prominent lips compared to fine sized particles where the particles tend to graze the surface without sufficient kinetic energy to plastically deform the material surface.     

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.     

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.     

Surface degradation of ductile metals in elevated temperature gas-particle streams

The mechanisms and rates of erosion and combined erosion-corrosion of 9Cr-1Mo steel (where the composition is in approximate weight per cent) and type 310 stainless steel at elevated temperatures were investigated to understand better the behavior of piping steels in fluidized bed combustor environments. Tests were performed in a partially inert gas atmosphere to study erosion behavior and in an air atmosphere to study combined erosion-corrosion behavior. It was determined that the erosion rate remained constant or decreased with increasing temperature in nitrogen until a temperature was reached at which the tensile strength started to decrease more rapidly with increasing test temparature. Above this temperature the erosion rate increased rapidly with temperature.In an erosion-corrosion environment, corrosion was the dominant mechanism at all test conditions. At higher temperatures and velocities the material loss mechanism changed from low loss rate chipping of the scale to high loss rate periodic spailing. The continuous scale formed on 9Cr-1Mo steel in air appeared to protect the metal surface, decreasing its loss rate in α = 30° tests compared with that of type 310 stainless steel tested in the same conditions in nitrogen where a continuous scale did not form.     

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”.     

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 occurs 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, 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.     

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。     

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.     

Electro-mechanical interactions during erosion–corrosion

This study aims to understand the physical processes invoked when solid particle erosion occurs in a corrosive media. The literature on wear-corrosion shows some effort has been placed in qualifying the interactions between erosion and corrosion leading to ‘synergistic’, ‘additive’ and ‘antagonistic’ terms. These terms are difficult to quantify experimentally with multiple experiments being required and generate considerable errors often suggesting these interactive terms are negligible. Hence the current work seeks to investigate these interactions, the errors associated with their measurement and gain understanding of the processes involved by careful examination of microstructural and mechanical property changes of surfaces subjected to erosion–corrosion. Cast nickel–aluminium bronze/NaCl solution has been chosen to study as this system has been studied at Southampton for several years. In situ electrochemical, gravimetric and topographical analyses have been evaluated and discussed using microstructural and hardness measurements.     

The effect of additional silicon on the corrosion and erosion-corrosion of low chromium steels

The addition of 1wt.%Si to commercial steel containing 2.5wt.%Cr significantly lowered its erosion-corrosion metal wastage rate at 650 °C. The improved erosion resistance was largely a result of a modified corrosion behavior. Static oxidation studies showed that the normal silicon content 2.5Cr steel developed a duplex, spalling prone, iron-rich scale at 650 °C. However, with the addition of silicon, the scale was almost 50 times thinner, and was chromium-rich instead. Metal wastage was significantly reduced for static and dynamic corrosion as well as for erosion-corrosion. The thin scale that formed on the additional silicon steels cracked and chipped off at a low metal loss rate compared with the thick scale that formed on the normal silicon steels that spalled off at a higher metal loss rate.     

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.     

热浸镀锌和热浸镀铝钢铁件的耐蚀性能比较及应用

钢铁件热浸镀锌和热浸镀铝是目前国内外应用较为广泛的表面技术。两种镀层都能保护钢铁基体免受因接触外界介质而产生的腐蚀,还能给钢制件洁白光亮的外表。大大延长了钢铁件在户外的使用寿命。本文比较了这两种工艺产品的异同之处和各自的应用优势,叙述了这两种镀层在不同介质中耐蚀的优劣情况。     

Erosion of aluminium-based claddings on steel by sand in water

This paper describes the slurry erosion of a range of HVOF deposited aluminium-based claddings on steel by sand in water. Coatings, approximately 300 μm thick, of commercially pure aluminium, eutectic aluminium/silicon alloy (12%) and of a novel composite incorporating alumina in this alloy have been tested, both as sprayed and as ground to remove surface roughness as far as possible. Angular silica sand of mean diameter 235 μm was used at a concentration of 2.5% in tapwater at impingement angles of 90° and 30° and a jet velocity of 27 m/s. Mass loss data and surface structure, as shown by electron microscopy and profilometry, are related to the test conditions, initial surface topography, material hardness and microstructure, especially porosity. They are discussed in terms of the mechanisms of erosion that occur in the different materials, with reference to microcutting and plastic deformation of the surface and to the effects of the alumina inclusions. The consequences of poor flow-out, leading to significant residual porosity of the composite cladding are discussed.     

Preparation and characterization of spark-anodized Al-alloys: Physical, chemical and tribological properties

Aerospace applications and energy saving strategies in general boosted the interest and the research in the field of light weight materials, typically on alloys based on aluminium. Aluminium itself does not provide sufficient mechanical strength for structural parts, but there exists a lot of recently developed alloys containing silicon, copper, magnesium, zinc or manganese in various combinations and compositions exhibiting excellent mechanical properties. These alloys require surface treatments or coatings to withstand corrosive ambient conditions. Among those treatments known as chromate replacements, plasma oxidation processes were used for different applications, especially if the surfaces have to face mechanical load or severe environmental conditions. In this work, specimens of different aluminium alloys have been plasma oxidized by micro-arc treatment in silicate and phosphate solutions. The ceramic coatings were characterized with respect to phase composition, micro-hardness and corrosion stability. In addition, the tribological performance of the coatings was investigated using a ball-on-disc tribometer with reciprocating motion against sintered alumina ball. The typical worn surfaces of the Al substrate and the ceramic coatings were observed by a scanning electron microscope. Applying same wear conditions, the wear rates in different depth of the coatings are nearly similar. However, in a defined depth of the coatings, wear rate gradually decreases with wear duration. During friction process, a-stable transfer layer consisting of oxides was formed on the tribo-contact area of the coatings. The friction coefficient in a steady friction state is in the range of 0.8.     

Erosion of corrosion-resistant surface treatments on alloy steels

Protective coatings of both diffused and sprayed-on types are being used to protect chemical process plant and boiler combustor surfaces from elevated temperature corrosion. An investigation was conducted to determine the small solid particle erosion behavior of such coatings at room and elevated temperatures. It was determined that no single coating performed in a superior manner to the others and that hardness was not a factor in establishing the erosion resistance. The erosion rates decreased as the temperature increased from 20 to 500°C. All the coatings reached steady state erosion rates after 50–150 g of erodent particles had impacted their surfaces.     

Behaviour of ferritic–martensitic steel and aluminium base coatings under demanding elevated temperature conditions

Abstract

Ferritic–martensitic steels are found in many elevated temperature applications due to their excellent strength properties and thermal conductivity. However, their resistance to elevated temperature corrosion, wear and combination of these is typically not at a desired level. One solution is to improve the surface properties by the application of a coating. In this study, aluminium diffusion coatings were deposited on 9Cr–1Mo steel and characterised in terms of microstructure and elevated temperature corrosion and erosion–corrosion behaviours. The two behaviours are then compared to those of an uncoated steel. The results from the tests indicate that aluminising shows great potential under the studied demanding elevated temperature conditions. The benefits and challenges of the deposition and use of aluminised coatings are discussed in this paper.     

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.     

Laser Surface Melting Cr–Ni Coatings, in the Erosive-Corrosive Atmosphere of Boilers

This article summarizes the results of a research on laser-treated Cr–Ni-based coatings under the erosive-corrosive environment of steam boilers containing fly ash and a 3–3.5% volume of free oxygen at 500 and 800 °C. The total erosion-corrosion process has been measured by sample weighing. Eroded layers were examined by scanning electron microscopy with an energy-dispersive spectrometer. Results were compared with erosion results for AISI 304 and two Cr–Ni plasma-sprayed coatings taken from previous papers. The significant superior erosion-corrosion resistance of laser-treated coatings compared to that of the plasma-sprayed coatings is considered to be due to a soft and cohesive coating without pores and the formation of a thin oxidized protective film.