Erosion-corrosion Process Research Articles

Some thoughts on modelling the effects of oxygen and particle concentration on the erosion–corrosion of steels in aqueous slurries

Advances in the understanding of slurry erosion–corrosion has gained momentum in recent years, due to the wide range of laboratory studies carried out in such environments. There are now new insights into the causes of “synergistic” effects in such environments for many particle/target/environment combinations. However, identifying such effects is difficult because in aqueous conditions, the corrosion and erosion variables are not independent of each other. In this work, some new developments on the modelling of slurry erosion–corrosion processes are described for steels. The effects of increases in flow velocity, and oxygen and particle concentration are considered. Theoretical erosion–corrosion maps are constructed showing the differences between the erosion–corrosion mechanisms as a function of these variables.

Electrochemical Studies of Copper Chemical Mechanical Polishing Mechanism: Effects of Oxidizer Concentration

AbstractThe process of copper chemical-mechanical planarization (CMP) can be considered as an erosion corrosion process. Such a process can be efficiently studied by in situ and ex situ electrochemical techniques, such as potentiodynamic scan and electrochemical impedance spectroscopy (EIS). Using a copper disk as the working electrode in an electrochemical cell, slurries with different oxidizer concentrations have been investigated with the aforementioned techniques. Corresponding dissolution tests were also studied and compared. It is shown that changing the oxidizer concentration leads to the formation of surface films with different structure and composition on the copper surface during CMP process. The nature of these films controls the rate of copper corrosion. These results could be used to explain the change of copper removal rate in different oxidizer concentration, as well as to understand the copper CMP mechanism.

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.

Electrochemical assessment of erosion-corrosion of commercially pure titanium and a titanium alloy in slurry impingement

The economic and effective operation of machinery and plant involved in fluids handling is increasingly dependent on the utilization of materials that combine high corrosion resistance and good wear resistance. This paper focuses on erosion-corrosion where mechanical wear is due to impacting solid particles entrained in a liquid flow. The characteristics of commercially pure (CP) titanium (grade 2) and titanium alloy (Ti 5111) in erosion-corrosion conditions are assessed. There is much literature available on the corrosion behaviour of titanium and titanium alloys but little has been reported on corrosion in wear environments. The erosion-corrosion characteristics of each material at 18 and 50°C were assessed using an impinging jet apparatus. The tests were performed in a 3.5% NaCl saline solution. Experiments were conducted to determine the mass loss by combined erosion-corrosion processes before independently determining the electrochemical corrosion mass loss and the mechanical mass loss. It has been shown that under erosion-corrosion conditions titanium and its alloys exhibit active corrosion behaviour, in contrast to passive behaviour when the tenacious oxide film remains intact in static conditions. The synergistic effect between erosion and corrosion is significant and is dependent on the material.

The Effect of Post-Treatment of a High-Velocity Oxy-Fuel Ni-Cr-Mo-Si-B Coating Part 2: Erosion-Corrosion Behavior

In this paper, a study of the erosion-corrosion characteristics of a Ni-Cr-Mo-Si-B coating applied by the high-velocity oxy-fuel (HVOF) process on to an austenitic stainless steel (UNS S31603) substrate are reported. The coatings were studied in the as-sprayed condition, after vacuum sealing with polymer impregnation and after vacuum furnace fusion. The erosion-corrosion characteristics were assessed in an impinging liquid jet of 3.5% NaCl solution at 18 °C at a velocity of 17 m/s at normal incidence in two conditions: (1) free from added solids and (2) containing 800 ppm silica sand. The methodology employed electrochemical control and monitoring to facilitate the identification of the separate and interrelated erosion and corrosion contributions to the erosion-corrosion process. The rates of erosion-corrosion damage were drastically accelerated in the presence of the suspended solids. The application of cathodic protection significantly reduced the deterioration process. The study showed the effect of sealing with polymer impregnation did not significantly alter the erosion-corrosion behavior of the sprayed coating. However, there was a significant improvement in erosion-corrosion durability afforded by the postfusion process. The mechanisms by which the improved performance of vacuum-fused coatings is achieved are discussed.

Mechanical and electrochemical interactions during liquid–solid impingement on high-alloy stainless steels

The erosion–corrosion behaviour of three high-alloy stainless steels (UNS S31245, UNS S32654 and UNS S32750) under aggressive liquid–solid impingement conditions has been assessed using a combination of electrochemical tests and weight-loss analysis. The material loss occurring under conditions of varying solid loading (900 mg/l), and at a temperature of 18°C, velocity of 17 m/s and impingement angle of 90° has been scrutinised as a function of time. Post-test examination has identified material loss mechanisms and the physical relevance of erosion and corrosion interactions has been illustrated. In this paper it has been shown that the resistance to material loss for the three materials does not follow the expected trend of increasing resistance as hardness increases. Through electrochemical measurements the pure corrosion component of the material can be isolated and it has become clear that corrosion (through direct and indirect effects) plays an important role in the erosion–corrosion process of even high-grade stainless steels due to depassivation–repassivation events corresponding to solid impact in the corrosive medium. Although the pure corrosion process (direct corrosion effect) constitutes only a small part of the total material loss, there is an important synergistic factor defined as the effect of corrosion on erosion which means that corrosion-related processes can be significant in erosion–corrosion.

Some recent advances in the development of theoretical approaches for the construction of erosion–corrosion maps in aqueous conditions

Significant progress has been made in the development of methodologies for the construction of erosion–corrosion maps for erosion by solid particles in aqueous conditions. Mathematical models for solid particle erosion have been combined with those for aqueous corrosion, enabling regime transitions to be identified as a function of the main process parameters. The effects of both erosion and corrosion variables have been identified on the maps. An important issue in such work has been the wide range of variables involved in the erosion–corrosion process. This has been addressed through combining such parameters into dimensionless groups. The results indicate that, although there are some limitations with such an approach, the technique does present some important advantages in dealing with the large number of variables associated with a single erosion–corrosion interaction. This paper describes some new developments on the above work, and indicates how the erosion–corrosion transitions, for pure metals and steels, can be described on a single map. Future directions of the work are also addressed.

Laboratory studies of erosion-corrosion processes under oxidising and oxidising/sulphidising conditions

The development of advanced coal-fired power generation systems is receiving considerable world interest. The successful development of such systems requires that components are manufactured from appropriate materials, that the materials degradation modes are identified and that models exist to predict material performance. Wastage of materials due to erosion-corrosion processes has been identified as a major plant problem both within bubbling and circulating fluidised beds and to a lesser extent within pressurised fluidised beds, unless the pressurised system operates in an oxidising/sulphidising regime, as often found within coal gasification plant. This paper presents a review of a range of laboratory test facilities that have been established to study high temperature erosion-corrosion processes and discusses how each has aimed to simulate in plant conditions, their limitations and successes. By the nature of the available literature much of this review concentrates on erosion-oxidation studies, although recent work under erosion-sulphidation conditions is also considered. The review is split into three sections; simulation of in-bed wastage condition; erosion-corrosion within convective path components, superheaters and economisers; and erosion-corrosion within gas turbine components as part of a coal fired combined cycle plant. In all three systems, low wastage rates are associated with the formation of a mechanically protective surface oxide. The inability to form such oxide glazes, or the failure of the oxide, can lead to very high wastage rates, up to 4 μm h−1 for in-bed components as an example. Thus successful simulation of plant conditions within the laboratory depends on the ability of the laboratory rigs to duplicate the erosive wear processes, and this is discussed. Some stoichastic models capable of predicting wear patterns and wear rates are also reviewed.

Peculiarities of erosion-corrosion processes

Investigations have been carried out on an erosion-corrosion apparatus to investigate the behaviour of corrosion-resistant highalloy iron-base materials containing hard phases. These materials had been optimized for increased wear resistance under complex stress conditions. As expected, in dry erosion tests, very low mass losses were established; there was little difference between the materials. Also, in corrosive environments (NaCl, H 2SO 4), the materials showed good stability; however, the duplex structures were superior to the martensitic ones. The combination of erosion and corrosion produced large differences and, in some cases, markedly reduced resistance to damage. These results confirmed observations that it is impossible to deduce the environmentally influenced mechanical behaviour of an alloy from its behaviour in separate corrosion and erosion experiments. The basic mechanisms underlying these processes have been investigated only tentatively.

Erosion-corrosion Mapping in Dry and in Aqueous Environments: Review of Recent Developments

In studies of erosion-corrosion in “dry” gaseous environments at elevated temperatures, and in wet aqueous environments at room temperatures, there have been recent attempts at mapping such processes. Such maps show the transitions between the erosion-corrosion regimes and thus can be an aid to materials selection in such environments. By plotting the magnitude of the wastage rate on the maps, as a function of the main process parameters, the conditions under which erosion-corrosion damage is minimized are easily identified. The purpose of this paper is to review the approaches to erosion mapping in aqueous and in dry environments. The work carried out on erosion maps for composites and ceramics will also be discussed. The limitations to existing work on the mapping approaches are also be addressed. It is shown how erosion maps can be a potentially powerful tool in minimizing damage due to erosion-corrosion. Further work which needs to be carried out to extend such approaches is discussed. Finally, the use of such maps to construct materials selection maps for both dry and wet erosion-corrosion processes are illustrated.

Depassivation and repassivation of AISI321 stainless steel surface during solid particle impact in 10% H 2SO 4 solution

Erosion-corrosion experiments on AISI321 stainless steel in 10 wt.% H 2SO 4 + 15 wt.% corundum sand were carried out by means of a rotating system with the specimen on the edge of a disc. Dynamic polarization curves during erosion-corrosion were measured at different rotating rates and the weight-loss kinetics was studied at different applied potentials and rotating rates. The straining electrode technique was used to simulate the depassivation and repassivation processes during erosion-corrosion. The experimental results showed that in the whole passive region the apparent passive current density and the weight loss during erosion-corrosion were constant at the fixed rotating rate and increased quickly with the rotating rate in the range of flow rate from 2.5 to 10 m s −1. The dissolution behaviour of the straining electrode at different applied potentials and straining rates was found to correlate with the dynamic polarization current of specimens during erosion corrosion, indicating that the electrochemical corrosion in the erosion-corrosion process was controlled by both the depassivation caused by the solid particle impact and the regrowth of protective film on the fresh specimen surface.

Erosion-corrosion and wear

The basic processes of wear, erosion and erosion-corrosion are reviewed. It is shown that oxide debris can accelerate wear when it is present as particulates, or protect and reduce friction when it is present as a glaze. Erosion-corrosion processes can be understood and organized in a series of regimes that represent a sequence of change in the relative intensities of erosive and corrosive processes. Types of mechanism expected to feature in oxide spallation and in the erosion-corrosion of alloys are described and, finally, the areas that require serious study in order to achieve further understanding of this subject are indicated.

The mechanism of high temperature erosion of coated superalloys

Diffusion and overlay coatings are now widely used for modern gas turbine parts such as nozzle guide vanes and turbine rotor blades. Problems are still encountered, however, when such components operate in environments where significant levels of particulates are present in the hot gas stream. The nature of these particulates depends on the service environment and may include sand-silicates, fly ash, sea salt and pyrolytic carbon. This paper highlights the significant parameters that control the mode of degradation and, in particular, the influence of particle loading, impact energy, temperature, scale morphology and composition on the deformation characteristics of both the surface oxide scale and coating. The erosion degradation mechanisms have been identified and a Monte Carlo model of the erosion-corrosion process developed, which is applicable to typical gas turbine service environments.

The Role of Oxygen Mass Transfer in the Erosion-Corrosion of Slurry Pipelines

Abstract The erosion-corrosion of vertical 38-mm diameter pipes carrying aerated slurry (20 vol% − 30 + 50 mesh silica sand) in 3.13% NaCl solution at 2 to 6 m/s has been measured by weight loss measurements over periods of 100 h. Electrochemical methods have been applied to clarify the mechanism of the process and to determine the corrosion component of the metal. Total wear rates in the range of 6 to 25 mm/y were related to the slurry velocity (v) by wear rate=const vn The value of n was in agreement with oxygen mass transfer controlled corrosion being the dominant mode of metal loss, with n ≈ 1. The absolute values of the wear rate were sufficiently close to those measured by electrochemical methods and those calculated from a mass transfer correlation determined for this system, Sh=0.031 Re0.83 Sc0.33 to conclude that the dominant mode of metal loss is corrosion. The role of erosive action in the erosion-corrosion process is to prevent the formation of a complete rust film that normally stifles corros...

Erosion of 310 Stainless Steel at 975°C in Combustion Gas Atmospheres

Erosion tests were conducted on 310 stainless steel in combustion gas atmospheres at 975°C. SiC particles of 100 mesh (150 μm) size were employed over a velocity range of 15–70 m/s at a 90 deg (perpendicular) angle of impingement. A comparison is made with results obtained at 25°C in air using the same test equipment. Scanning electron microscopy examination of the eroded surfaces revealed important information concerning the nature of the erosion-corrosion process. A multiple component test specimen is described for application in comparing alloys under nearly identical test conditions.