Erosion Corrosion Characteristics Research Articles

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.

An electrochemical and microstructural assessment of erosion-corrosion of austenitic cast iron for marine applications

In this paper the corrosion and erosion-corrosion characteristics of an austenitic cast iron (BS 3468 S2W) are examined under an impinging flow free from solid particles. Under static and erosion-corrosion conditions the corrosion process is controlled by activation effects and conforms to Tafel kinetics. The corrosion rate is increased under impingement conditions compared with under static conditions but the trend is complex. Although corrosion plays a part in the degradation of BS 3468 S2W under an impinging jet, it is clear that there is a significant role played by mechanical processes and the effect that corrosion has on the mechanical erosion processes. The pure electrochemical corrosion component of the material loss is reduced as a percentage of the overall weight loss on increasing the impinging jet velocity. Corrosion has been seen to initiate at the boundary between the nodules of graphite and the surrounding Fe-rich matrix and hence a physical interpretation of the effect of corrosion on erosion is identified. Elektrochemische und Gefügebeurteilung von Erosionskorrosion an austenitischem Gusseisen für Meeresanwendungen In dieser Arbeit werden Korrosions- und Erosionskorrosionsmerkmale eines austenitischen Gusseisens (BS 3468 S2W) unter einer aufprallenden Strömung ohne Feststoffpartikel untersucht. Unter statischen und Erosionskorrosionsbedingungen wird der Korrosionsprozess durch Aktivierungseffekte kontrolliert und entspricht der Tafel-Kinetik. Die Korrosionsgeschwindigkeit wird unter Aufprallbedingungen erhöht verglichen mit statischen Bedingungen; der Trend ist allerdings komplex. Auch wenn die Korrosion bei der Schädigung von BS 3468 S2W unter Strahlaufprall eine Rolle spielt, gibt es aber einen signifikanten Anteil durch mechanische Prozesse und durch den Einfluss, den die Korrosion auf die mechanischen Erosionsprozesse ausübt. Mit steigender Strahlaufprallgeschwindigkeit wird die rein elektrochemische Korrosionskomponente des Materialverlustes als Prozentanteil des Gesamtgewichtsverlustes geringer. Die Korrosion beginnt an der Grenzfläche zwischen den Graphitkugeln und der umgebenden Fe-reichen Matrix; dadurch kann eine physikalische Interpretation des Effektes der Korrosion auf die Erosion identifiziert werden.

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.

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 17m/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.

Cavitation erosion–corrosion behaviour of laser surface alloyed AISI 1050 mild steel using NiCrSiB

A Ni-based hardfacing alloy NiCrSiB (Ni–16.5%Cr–15.5%Fe–3.5%Si–3.8%B–1%C) was surface alloyed on AISI 1050 mild steel (Fe–0.2%Cr–0.4%Mn–0.5%C) specimens using a two-step process. NiCrSiB powder was preplaced on the substrate by flame spraying and then remelted by a 2 kW continuous wave Nd:YAG laser to achieve surface alloying. The cavitation erosion–corrosion characteristics of the surface alloyed specimens in 3.5% NaCl solution at 23°C were determined by means of a 20 kHz ultrasonic vibrator at a peak-to-peak amplitude of 30 μm and a potentiostat. At a dilution ratio of 12% (corresponding to a laser scanning speed of 25 mm s −1), the overall cavitation erosion–corrosion resistance of the alloyed surface was 8.9 times that of the as-received AISI 1050 specimen. The corrosion resistance was also improved as reflected by a reduction in the current density of least one order of magnitude as compared with the as-received specimen at the same potential. The improvement in cavitation erosion resistance could be attributed to the superior mechanical properties of the NiCr-alloyed matrix formed and the presence of borides and boro-carbides which increased the hardness. The improvement in corrosion resistance was due to the increase in Cr and Ni content in the alloyed layer. The relative contributions of pure mechanical erosion, electrochemical corrosion, and synergism between erosion and corrosion to the overall cavitation erosion–corrosion were also determined.

Erosion–corrosion characteristics of squeeze cast aluminium alloy/SiC composites in water and sodium chloride solutions containing sand

The erosion–corrosion behaviour of a squeeze cast aluminium alloy (Al–Cu) dispersed with either 10 vol.-%SiC particles or SiC fibres was investigated in slurry environments of 3%NaCl (aqueous) + 40 wt-% sand and water + 40 wt-% sand over a range of test durations at a linear speed of 7·06 m s-1. Test results indicated higher weight loss for the SiC composites than for the matrix alloy in 3%NaCl + 40 wt-% sand while the reverse trend was noted when the tests were conducted in water + 40 wt-% sand. Severe attack around the dispersoid/matrix and precipitate/matrix interfaces by the NaCl environment caused loosening of the dispersoid phase followed by the subsequent removal of the dispersoids. This resulted in a higher weight loss for composites than for the base alloy in NaCl containing sand. Corrosion processes appear to dominate in this case. The lower weight loss of the composites compared with the matrix alloy in water + 40% sand could be attributed to the protection provided by the SiC phase to the softer matrix. Erosion appears to be the predominant mechanism of material removal in this case. Furthermore, the SiC fibre composite suffered from more weight loss than that containing SiC particles in both the test environments. Results have been explained on the basis of analysis of eroded–corroded surfaces and subsurface regions.

Influence of experimental parameters on the erosive–corrosive wear of Al–SiC particle composite

The performance of an Al-alloy-based composite reinforced with 10 wt.% SiC particle in erosive–corrosive slurry environment was examined. The slurry consists of equiaxed sand particles of varying proportion (20, 40 and 60 wt.%) of size approximately 100 μm and solution of water, sulphuric acid and hydrochloric acid. The specimens were placed at various angles of inclination (0°, 45° and 90°) with respect to the direction of rotation of the specimens and at different radial distances (35, 55 and 75 mm) in order to assess the effect of these parameters on erosion–corrosion characteristics of composite. It was observed that wear rates increase drastically with increasing angle of inclination, sand concentration in the slurry, and radial distance. However, it was noticed that the wear rate remains unchanged when the specimen tested in a slurry containing more than 40 wt.% sand and the same become significantly less when the specimen is placed at a radial distance less than 55 mm. Scanning electron microscopic (SEM) study confirms that wear of composite is mainly governed by the synergic effect of the two simultaneous processes: (i) erosion, corrosion and abrasion of the matrix by the slurry; and (ii) fracture and removal of the dispersoid due to particle impingement at high speed. This study also revealed that wear rate at any condition increases to a maximum and then is reduced with the distance traversed. This might be due to work hardening of the specimen surface.

Erosion—corrosion characteristics of a 2.25 Cr-1 Mo steel exposed at low particle velocities

The erosion-corrosion characteristics of a 2.25 Cr-1 Mo steel at low particle velocities and elevated temperatures were determined using a nozzle type laboratory erosion tester. The tests were performed with 180–360 μm angular alumina particles at 60° angle of impingement at low particle velocities of 2.6–8.2 m/s and in the temperature interval 20–600°C. The steel was tested both in the as-received condition as well as in two preoxidized conditions. The erosion-corrosion rate of the steel, both in the as-received and in the preoxidized conditions, was found to increase with increasing particle velocity. In contrast, the wastage rates were relatively independent of temperature in the temperature range investigated, the only exception being specimens exposed to the lowest particle velocity (2.6 m/s) at the very highest temperature, i.e. 600°C, which displayed a drastic increase in wastage. Specimens preoxidized at 700°C exhibited a somewhat higher erosion rate compared with non-preoxidized specimens and specimens preoxidized at 500°C. Microscopy revealed four different major wastage mechanisms, i.e. (i) plastic deformation, cracking and micro chipping of surface material of a size corresponding to the area impinged by eroding particles, (ii) chipping of somewhat larger oxide fragments (up to 10–15 μm in diameter), (iii) chipping or spalling of relatively large oxide fragments (up to 30–50 μm in diameter), and (iv) spalling along the steel-oxide interface or within an oxide layer due to cohesive failure, of larger (up to 500 μm in diameter) oxide layer fragments. In the present study extensive spalling was only observed for non-preoxidized specimens exposed to the lowest particle velocity (2.6 m/s) and the two highest specimen temperatures (550°C and 600°C).

The abrasion/erosion and erosion-corrosion characteristics of steels

The metal wastage which occurs on the combustion gas side of in-bed heat exchanger tubes in fluidized bed combustors (FBCs) is an important problem in this method of generating power from coal. The particle flow conditions which occur along the tube surfaces that remove material are the subject of some speculation as to whether three-body abrasion or solid particle impact erosion is causing the degradation. In this paper, evidence is presented that shows that the basic mechanism of particles degrading the surface is essentially the same for both descriptions of the active mechanism. The ability to extrapolate to long-term metal wastage rates in actual in-service FBC exposures from short-term laboratory tests that reach steady state loss rate conditions in the order of 5 h is also discussed. The concept of trading off various test conditions such as test temperature and particle size, shape, and velocity to realize similar amounts of metal wastage from balanced test conditions is presented.

The mechanism and the condition of occurrence in the erosion-corrosion damage of carbon steel by gas-liquid two-phase flow.

The erosion-corrosion characteristics of carbon steel commonly used as piping material subjected to an impinging gas-liquid two-phase flow under various pH. chloride ion concentration and flow rate combinations were investigated. Environmental condition saffected not only the damage rate but also the type of damage. One of these was the enhanced general corrosion observed mainly under high chlorine ion concentration (1220ppm) and another was flow-induced pitting corrosin observed under the pH7-[C1-] 140ppm condition. Each condition of the chemical solution had its own flow effect. For example, in the case of high chlorine ion concentration, high flow rate accelerated corrosion damage by decreasing diffusion control. On the other hand, under low chlorine ion concentration, high-speed flow decreased the corrosion rate by the action of removing nonprotective oxide layer. To indicate the condition of occurrence for enhanced corrosion damage, a two-dimensional chart which had axes of [C1-] concentration and flow rate was constructed for each pH value.

The electrochemical mechanism in the erosion-corrosion damage of carbon steel by gas-liquid two-phase flow.

The erosion-corrosion characteristics of a carbon steel commonly used as a piping material subjected to an impinging gas-liquid two-phase jet were investigated. The localized damage was observed, and the range of the erosion rate under the experimental conditions was beteen 45 mm/year and 87 mm/year. It became clear that the principal mechanism of this damage was the occurrence of pitting corrosion caused by the corrosion potential distribution on a specimen and the rapid growth of pits. The significance of flow-induced potential distribution on a specimen was demonstrated by use of an array of test pieces insulated electrically from each other. This potential distribution induced by the flow distribution on a specimen also gave crucial effects on the location where the erosion-corosion damage occurred. Namely, erosion-corrosion damage took place locally, just at the part of the specimen where the corrosion potential was shifted to more nobler than the pitting potential of the condition of interest. This corrosion potential distribution was caused by the distribution of the oxygen supply rate and deposition and/or removal of oxydizer corrosion product (γ-FeOOH). These were originally caused by the flow distribution. This erosion-corrosion damage was completely inhibited by keeping the pitting potential nobler than the corrosion potential ; for example, by the addition of NaNO2 to the system.

Wear of 12 alloys during laboratory milling of phosphate rock in phosphoric acid waste water

As part of its research on wear of mining and mineral processing equipment, the US Bureau of Mines evaluated the erosion-corrosion characteristics of 12 alloys in phosphoric acid waste water. Tests were conducted on 2- and 5-cm-diam specimens in 12- and 60-cm-diam ball mills by grinding phosphate rock in gypsum pond water with an initial pH of 1.6. For comparison, grinding tests also were conducted in tap water. Static corrosion tests in gypsum pond water also were carried out. For all alloys tested, the wear by erosion-corrosion was greater than the sum of erosion plus static corrosion. A low-alloy, high-carbon steel and a high-manganese, nitrided stainless steel were the most cost-effective materials.