Erosion–corrosion degradation mechanisms of Fe–Cr–C and WC–Fe–Cr–C PTA overlays in concentrated slurries

In this investigation the microstructure and erosion–corrosion behaviour of a Fe–Cr–C overlay (FeCrC–matrix) produced by plasma transferred arc welding (PTA) and its metal matrix composite (FeCrC–MMC) were assessed. The FeCrC–MMC was obtained by the addition of 65 wt.% of tungsten carbide (WC). The erosion–corrosion tests (ECTs) were carried out using a submerged impinging jet (SIJ); after the ECTs the surface of the overlays was analysed to identify the damage mechanisms. Two different temperatures (20 and 65 °C) and sand concentrations (10 and 50 g/l) were used in a solution of 1000 ppm of Cl⁻ and a pH value of 8.5; the conditions were chosen to be representative of the recycling water in the tailings line in the oilsands industry. The FeCrC–matrix showed a dendritic structure and a high concentration of carbides in the interdendritic zone. The addition of the WC reinforcing phase promoted the formation of W-rich intermetallic phases, increased the microhardness values of the matrix phase of the FeCrC–MMC overlay and dramatically improved its erosion–corrosion performance as expected. For the FeCrC–matrix overlay the main erosion–corrosion degradation mechanisms were severe plastic deformation and the formation and removal of material flakes due to consecutive impacts. At 65 °C the dendritic zone was severely corroded in the area of low impact frequency. The FeCrC–MMC showed greater attack of the matrix phase compared to the WC grains; at high sand concentration the WC grains were severely fractured and flattened. The anodic polarisation analysis showed active corrosion behaviour of the FeCrC–MMC at both temperatures and sand concentrations; however the temperature dramatically increased the corrosion process of the surface studied under erosion–corrosion conditions. The paper assesses the degradation mechanisms of both FeCrC–matrix and FeCrC–MMC with the aim of understanding what aspects of MMCs must be adapted for optimum erosion–corrosion resistance.     

Erosion–corrosion performance of high-Cr cast iron alloys in flowing liquid–solid slurries

In many slurry transportation systems, such as in FGD (Flue Gas Desulphurization) and chemical processing applications, corrosion and erosion are the two main mechanisms of material degradation of the pump wet-end components including pump casing, impeller and liners. The performance of a selected material is mostly dependent upon its relative corrosion and erosion resistance to the service environment. In these cases erosion, corrosion and the related synergistic effects can be very complicated since they are affected by numerous factors including solid and slurry properties, chemical contents, hydraulic conditions and temperatures. In this experimental study, sliding Coriolis erosion testing has been performed with various corrosion factors such as pH value, chlorides content and temperature to evaluate the erosion–corrosion resistance of some high-alloyed white cast irons containing different levels of chromium and other elements. Optical microscope and SEM-EDS have also been used to examine microstructure and surface conditions of tested materials. Results indicated that material loss due to corrosion factors increased as acidity-chlorides and temperature increased. At relatively high corrosion intensity, the white cast irons with higher alloy content (especially chromium) clearly showed improved corrosion resistance and combined erosion–corrosion resistance over those with lower alloy content. Under certain corrosion and hydraulic conditions, particle size is perhaps the single most influential factor on erosion–corrosion rate of the high-Cr cast iron alloys. Relatively large particles are much more effective than small ones at removing both the corroded surface layer and the fresh material, causing substantially higher rate of material loss. Some other related factors have also been addressed.     

Erosion–corrosion and corrosion properties of DLC coated low temperature gas-nitrided austenitic stainless steel

Low temperature nitriding of stainless steel leads to the formation of a surface zone of so-called expanded austenite, i.e. by dissolution of large amounts of nitrogen in solid solution. In the present work the possibility of using nitrogen expanded austenite “layers” obtained by gaseous nitriding of AISI 316 as substrate for DLC coatings are investigated. Corrosion and erosion–corrosion measurements were carried out on low temperature nitrided stainless steel AISI 316 and on low temperature nitrided stainless steel AISI 316 with a top layer of DLC. The combination of DLC and low temperature nitriding dramatically reduces the amount of erosion–corrosion of stainless steel under impingement of particles in a corrosive medium.     

Evaluation of a semi-empirical model in predicting erosion–corrosion

The phenomenon of erosion–corrosion has been studied extensively by various investigators but no accurate model has been developed to predict the interactions between erosion and corrosion. This is mainly attributed to the complexity of the interactions that generate either a synergistic or antagonistic wear effect for a particular material in a certain environment. A semi-empirical model has recently been developed at the University of Southampton which incorporates dynamic Hertzian contact mechanics to model the damage during particle impact and accommodates the effect of erodent deforming the surface leading to an increased corrosion activity. The model was found to have good agreement with erosion–corrosion rates of carbon steel. The aim of this paper is to evaluate the robustness of this semi-empirical model by testing it on a passive metal. UNS S31603 was chosen due to its inherent passivity to corrosion. A slurry pot erosion tester was used as the test rig to perform the experiments. It was found that this passive metal produces high synergistic levels when exposed to erosion–corrosion in 0.3 M HCl with variation in erodent concentrations and flow velocities. SEM and surface profilometry show typical ductile material behaviour with cutting mechanism and deformation mechanism occurring simultaneously. A wear map is presented and it is observed that the increase in velocity and sand concentration causes the material to shift from a corrosion–erosion dominated region to an erosion–corrosion dominated region. This paper will also evaluate the semi-empirical model and discuss its applicability in predicting erosion–corrosion.     

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.     

Erosion– and cavitation–corrosion of titanium and its alloys

The economic and effective operation of machinery and plant involved in fluids handling is increasingly dependent on the utilisation of materials that combine high corrosion resistance and good wear resistance. This paper studies two wear–corrosion situations: (1) erosion–corrosion, where the wear is due to impacting solids in a liquid medium and (2) cavitation–corrosion, where the wear is due to impacting liquid micro-jets formed by imploding air bubbles. The characteristics of a commercially pure titanium (CP-Ti) and three alloys in erosion–corrosion and cavitation–corrosion conditions have been studied. The erosion–corrosion characteristics of each material was assessed using an impinging-jet apparatus. The tests were performed at an angle of impingement of 90°C at a particle velocity of 17 m/s and in a saline solution of 3.5% NaCl at 18°C. A series of experiments was conducted to determine the mass loss by combined erosion–corrosion before independently determining the electrochemical corrosion contribution to mass loss. It has been shown that exposure to liquid–solid erosion causes disruption of the passive film on Ti and active corrosion occurs. In contrast, the materials exhibited passive behaviour in static conditions and when exposed to a cavitating liquid only CP-Ti became active. The role of corrosion in these wear–corrosion environments on CP-Ti and Ti-alloys is discussed in this paper.     

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.     

Effect of impact angle on the slurry erosion–corrosion of 304L stainless steel

This communication describes an investigation of the effect of impact angle on slurry erosion–corrosion of stainless steel using a new slurry erosion rig. With the new apparatus, it is possible to measure the individual erosion events under impact as both electrochemical current/time transients, and mechanical transients through acoustic emission (AE) simultaneously. Each sharp rise observed in the electrochemical current transient under particle impact is accompanied by an AE event. These sharp current rises are attributed to the rupture or removal of the oxide film on the surface by the abrasive particles. The correlation between the current rise and the maximum of the AE event, although scattered, shows an increase with decreasing impact angle. The current transients due to individual erosion events show that on the average, the current rise is greater and the rise time is longer at oblique angles compared with those at normal incidence. The degree of denudation of the metal surface by individual particle impingements, the process which strips the surface of its passivity, increases at more oblique angles. Weight loss measurements demonstrate that the synergistic effect between erosion and corrosion is enhanced by a more oblique angle of impact. The origin of the synergism is discussed.     

CO2 erosion–corrosion of pipeline steel (API X65) in oil and gas conditions—A systematic approach

A systematic study of pipeline steel (API X65) degradation due to erosion–corrosion containing sand in a CO₂ saturated environment has been carried out. This work focuses on the total material loss, corrosion, erosion and their interactions (synergy) as a function of environmental parameters (temperature, flow velocity and sand content) to enable the critical conditions, which move the damage mechanism from a flow-induced corrosion regime to erosion–corrosion regime, to be determined.The experimental results show that the effect of corrosion in enhancing erosion, often referred to as the synergy, is significant and accounts for a high proportion of the deviation of measured material loss from the prediction derived from established CO₂ corrosion models. Ways forward to improve erosion–corrosion prediction are discussed.     

Erosion, corrosion and erosion–corrosion of EB PVD thermal barrier coatings

Electron beam (EB) physical vapour deposited (PVD) thermal barrier coatings (TBCs) have been used in gas turbine engines for a number of years. The primary mode of failure is attributed to oxidation of the bond coat and growth of the thermally grown oxide (TGO), the alumina scale that forms on the bond coat and to which the ceramic top coat adheres. Once the TGO reaches a critical thickness, the TBC tends to spall and expose the underlying substrate to the hot gases. Erosion is commonly accepted as a secondary failure mechanism, which thins the TBC thus reducing its insulation capability and increasing the TGO growth rate. In severe conditions, erosion can completely remove the TBC over time, again resulting in the exposure of the substrate, typically Ni-based superalloys. Since engine efficiency is related to turbine entry temperature (TET), there is a constant driving force to increase this temperature. With this drive for higher TETs comes corrosion problems for the yttria stabilised zirconia (YSZ) ceramic topcoat. YSZ is susceptible to attack from molten calcium–magnesium–alumina–silicates (CMAS) which degrades the YSZ both chemically and micro-structurally. CMAS has a melting point of around 1240 °C and since it is common in atmospheric dust it is easily deposited onto gas turbine blades. If the CMAS then melts and penetrates into the ceramic, the life of the TBC can be significantly reduced. This paper discusses the various failure mechanisms associated with the erosion, corrosion and erosion–corrosion of EB PVD TBCs. The concept of a dimensionless ratio D/d, where D is the contact footprint diameter and d is the column diameter, as a means of determining the erosion mechanism is introduced and discussed for EB PVD TBCs.     

Wear and immersion corrosion of Ni–P electrodeposit in NaCl solution

This paper presents an experimental study on the block-on-cylinder wear behavior of an electroplated Ni–P coating immersed in 5% NaCl solution at different temperatures and polarization potentials. The simply immersion corrosion without normal load was performed first, and then the corrosion effect on the wear behavior including friction coefficient, surface roughness and weight loss were discussed. It is found that the morphology nature of the corrosion film has crucial effect on the wear performance of the coating. Moreover, the bath temperature and the polarization potential greatly influence the formation of the corrosion film.     

In vivo assessment of the corrosion of nickel–titanium orthodontic archwires by using scanning electron microscopy and atomic force microscopy

This study was undertaken to assess in vivo the corrosion in two commercial nickel–titanium (NiTi) orthodontic archwires removed from the oral cavity of patients using fluoride mouthwashes. Five volunteers took part in this study on the corrosion behavior of two brands of NiTi archwires (3M and AO (brand of archwire)) during use of two mouthwashes with neutral sodium fluoride 1.1%, one with acidulated fluoride 1.1%, and one with placebo and a control group. Each patient used one mouthwash in three different periods of time for 1 min a day for 30 days. The archwires were assessed with scanning electron microscopy and atomic force microscopy for qualitative and quantitative analysis. The values obtained with atomic force microscopy (AFM) were submitted to normality test, two‐way analysis of variance, and Tukey's test at a significance level of 5%. The AFM images showed a gradual qualitative increase in the roughness of both types of wire between the treatments: control < placebo < neutral fluoride < acidulated fluoride. The arithmetic average of the roughness and root mean square of the roughness were similar. As for 3M archwires, only the acidulated fluoride group differed statistically from the others. As for AO archwires, the control and placebo groups did not differ from each other, but differed from the other fluoride treatments. The group using neutral fluoride also differed significantly from the acidulated fluoride group. 3M archwires were not affected by daily oral challenges. AO archwires were not affected by daily oral challenges either; their association with fluoride, either neutral or acidulated, increased their roughness.     

Exposure effects of strong alkaline conditions on the microscale abrasion–corrosion of D-gun sprayed WC–10Co–4Cr coating

The synergistic effects between abrasion and corrosion for detonation gun (D-gun) sprayed WC–10Co–4Cr coatings have been studied for alkaline conditions. Comparisons between exposed and freshly polished coating surfaces in strong NaOH solutions (pH 11) show that significantly lower wear rates were seen for the exposed surface due to a negative wear–corrosion synergy. The negative synergy was seen to increase with increasing sliding distances for both fresh and exposed surfaces. The physical mechanisms of this negative synergy are discussed. The effect of anodic surface treatments on wear–corrosion to elucidate the influence of corrosion on the negative synergy is also reported.     

A CFD model of particle concentration effects on erosion–corrosion of Fe in aqueous conditions

A CFD (computational fluid dynamics) model has been developed to evaluate the effects of particle concentration on the erosion–corrosion of the inner surfaces of a circular pipe of 90° bend at room temperatures. The relative intensity of erosion and corrosion around the pipe geometry results in transitions between various erosion–corrosion regimes, for a given inlet particle concentration. The results indicate that the corrosion-dominated regime at the pipe bend is reduced with an increase in particle concentration. Typical results from the model are shown illustrating how this 3D mapping method can be used to model parameters such as particle concentration on the erosion–corrosion regimes over the surface.     

The capability of ultrafiltrated alkaline and organosolv oil palm (Elaeis guineensis) fronds lignin as green corrosion inhibitor for mild steel in 0.5 M HCl solution

The inhibitive effect of ultrafiltrated oil palm fronds (OPF) lignins on the corrosion of mild steel in 0.5 M HCl solution has been investigated by electrochemical impedance spectroscopy (EIS), potentiodynamic polarization (PP) and weight loss measurement. The presence of smaller lignin fractions reduces remarkably the corrosion rate of mild steel. The highest corrosion inhibition efficiency for all ultrafiltrated lignins were attained at maximum concentration of 500 ppm (IE_P.Soda: 87% > IE_P.Organosolv: 83% > IE_P.Kraft: 81%). The results from this corrosion test clearly reveal that all ultrafiltrated lignins behaved as a mixed-type inhibitor with predominant anodic (organosolv lignin) or cathodic (alkaline lignin) effectiveness. It was deduced that the inhibition process was spontaneous and the inhibitors were mainly physically adsorbed onto the mild steel surface.     

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.     

Investigation of micro-scale abrasion–corrosion of WC-based sintered hardmetal and sprayed coating using in situ electrochemical current-noise measurements

The micro-scale wear–corrosion interactions of WC-based sintered hardmetals and sprayed coatings are typically investigated by comparing the wear-rates in corrosive environments with neutral (pH 7) conditions and inferring electrochemical activity. However, for a greater understanding of the wear–corrosion interactions, there is a need to examine the repassivation kinetics during micro-abrasion tests under different pH conditions. This paper details in situ electrochemical current-noise measurements performed using a modified micro-abrasion tester to elucidate these wear–corrosion interactions for pH 7–13 conditions for sintered WC–5.7Co–0.3Cr and sprayed WC–10Co–4Cr specimens. Electrochemical measurements and SEM micrographs of worn surfaces are used to detail the degradation process. Discussion will focus on the wear–corrosion interactions present under neutral and alkaline conditions for sintered and sprayed specimens and the influence of microstructure on the electrochemical activity will be detailed.     

Electron microscopy of Ag–(Ni–O) core–shell nanowires

This report provides information about an electrodeposition‐based two‐step synthesis methodology for producing core–shell Ag–(Ni–O) nanowires and their detailed structural and compositional characterization using electron microscopy technique. Nanowires were produced by employing anodic alumina templates with a pore diameter of 200 nm. In the first step of the synthesis process, nanocrystalline Ni–O was electrodeposited in a controlled manner such that it heterogeneously nucleated and grew only on the template pore walls without filling the pores from bottom upwards. This alumina template with pore walls coated with Ni–O was then utilized as a template during the electrodeposition of Ag in the second step. Electrodeposited Ag filled the template pores to finally produce Ag–(Ni–O) core–shell nanowires with an overall diameter of 200 nm.     

A transformation anomaly in a Cu–Zn–Al alloy

A comparison is described of the structures of an M9R Cu–Zn–Al alloy as quenched directly into the martensitic state, and as stabilized, with the structure of the material after up-quenching into the β₁ state after which treatment the martensitic memory transformation occurs thermoelastically. Preliminary results on the increased tendency for the stabilized alloy to form f.c.c. stacking sequence regions are given and discussed.     

Synergistic effects of fluid flow and sand particles on erosion–corrosion of aluminum in ethylene glycol–water solutions

The synergistic effects of fluid flow and sand particles on erosion–corrosion of 3003 aluminum (Al) alloy in ethylene glycol–water solution that simulates the automotive engine coolant were studied through a rotating disk electrode by electrochemical measurements. It was found that, in the absence of sand particles, the anodic current density of Al electrode decreases with the increase in electrode rotating speed, which is attributed to the enhanced electrode oxidation accompanying with accelerated oxygen diffusion and reduction. Mass-transport plays an important role in electrode process, as indicated by the presence of diffusive impedance. With the addition of sand particles, the de-stabilization of electrode is predominant and the anodic current density increases with the electrode rotation speed, which is attributed to the enhanced impact damage to electrode by sand particles. The electrode roughening effect due to sand impact is indicated by the presence of inductive loop in impedance plots. The variation of sand particle size does not have apparent effect on polarization behaviour of Al electrode, while an elevated temperature increases significantly the anodic current density of the electrode. There exists a synergism of fluid flow and sand impact on electrochemical corrosion behaviour of Al electrode.