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Study of erosion behaviour of paint layers for multilayer paint technique applications in slurry erosion
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Wear 264 (2008) 737–743
Short communication
Study of erosion behaviour of paint layers for multilayer paint technique applications in slurry erosion M.N. Noui-Mehidi ∗ , L.J.W. Graham, J. Wu, B.V. Nguyen, S. Smith CSIRO Manufacturing & Materials Technology, P.O. Box 56, Graham Road, Highett, Victoria 3190, Australia Received 11 October 2006; received in revised form 3 April 2007; accepted 4 June 2007 Available online 13 July 2007
Abstract A multilayer paint modelling study was conducted to investigate the erosion properties of different types of paints. The experiments were carried out in a slurry-mixing tank in which 10 painted samples mounted on a shaft were rotated at constant speed for a certain interval of time. The effect of impact angle on the particular type of paint was investigated by orientating the samples consecutively at angles from 0◦ to 90◦ with an increment of 10◦ . In the slurry medium, it was found that most soft paints, such as “Enamel” type have similar erosion maps to ductile materials with a maximum wear rate at around 30◦ . Metallic paints and styrene type paints have shown a slightly higher angle of 40◦ for maximum erosion. In laboratory modelling, erosion studies of ductile materials by multilayer paint techniques are time and cost-effective. © 2007 Elsevier B.V. All rights reserved. Keywords: Erosion modelling; Multilayer paint technique; Slurry erosion
1. Introduction The erosive wear, which occurs when solid particles entrained in a fluid stream impinge on a surface, has been an important and continuing problem in many industries. The first paper reporting erosion effects was written by Reynolds [1] in 1873 and later a further paper by Rayleigh [2] in 1912 discussed erosion in sand blasting. Materials are classified into two large groups: ductile materials, which show relatively soft behaviour when subject to mechanical erosion and brittle materials, which show high resistance to erosion due to the hardness of their surface. These two groups are qualified by characteristic erosion curve with regard to the attack angle of the particles on the material surface [3]. The literature reports that the ductile response to mechanical erosion corresponds to a maximum erosion rate at low impact angles between 20◦ and 40◦ , while in brittle materials the erosion rates increases with increasing impact angles with a maximum being at 90◦ . Several studies have focused on ductile material erosion since the variation of the weight loss with angle of impingement is very
∗
Corresponding author. Tel.: +61 3 9252 6075; fax: +61 3 9252 6252. E-mail address: [email protected] (M.N. Noui-Mehidi).
0043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2007.06.003
similar for materials with widely different thermal and physical properties [4,5]. Finnie [5] proposed an erosion model giving the erosion rate as function of the impact particle velocity to the power n, where n depends on system parameters. Analysis of brittle material erosion reasonably predicted the role of variables, such as particle size, velocity and fracture toughness of the surface when the eroding particle is harder than the surface it is cracking. Humphrey [3] discussed in a review paper the different aspects of material erosion. The eroded material E (where E = mass removed from surface/total mass of particles impinging on a surface) was dependent on the erosion caused by the particle incidence (or impact) angle. Hutchings and Winter [6] presented a study on erosion of ductile metals subject to spherical particle abrasion. They used mild steel as a target and spherical steel particles at velocities up to 256 m s−1 as projectiles. They showed that metal could be removed from the surface by a process involving the shearing of surface material in the direction of motion of the projectile. Wellman and Allen [7] investigated the effects of angle of impact and material properties on the erosion rates of ceramics. They found that the relative hardness of the target and erodent did not provide a clear prediction of erosion resistance although threshold velocity and angle effects were observed in the erosion mechanism. Oka et al. [8] studied the dependence on the impact angle of erosion damage caused by solid particle impact
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on several types of materials. They performed erosion tests on five metallic materials, a ceramic and a plastic material using silica sand particles at angles between 3◦ and 90◦ . They used a compressed air test rig to conduct the erosion study. In their study, they investigated velocities in the range of 50–130 m s−1 and suggested that the erosion was not dependent on the impact velocity in this range but only on the material properties and the impact angle. Omote et al. [9] also used an air blast experimental setup to study the erosion of five types of materials using sand particles impinging at 230 m s−1 . They compared the erosion of polyurethane and titanium alloy. Polyurethane showed a ductile erosion behaviour with a maximum erosion angle at 25◦ , while titanium alloy had a maximum erosion angle at 45◦ . Zu et al. [10] designed a jet impingement slurry erosion test rig for laboratory use. The slurry was recirculated in a loop by the use of a pump, and after impingement, the particles were recollected from a hopper located below the target holder. Slurry pots were also widely used for impact angle effect studies, e.g. refs. [11,12]. Desale et al. [13] developed a pot tester for slurry erosion studies. They carried out experiments in a transparent cylindrical pot in order to determine the minimum speed required for propeller to achieve uniform distribution of solid particles. They found that a 45◦ -pitched four bladed down-pumping propeller gave a better distribution of solids. They have tested specimens mounted on a different shaft inserted from the top of the pot and rotated at a desired speed. They compared their results of erosion of ductile materials using this technique and found their results in agreement with those found in the literature. There are only a few references related to paint and coat modelling for material erosion studies. An early discussion was presented by Fasano [14] who studied the erosion on a rubbercoated impeller. Instead of using the rubber material for his erosion study, he used different paint coatings in order to investigate the erosion rate and mechanism on the pitched blade impeller. He tried different paint for both colour contrast and adherence. The multilayered paint technique was also used to study wear in fluidised bed combustors as reported by Harrison [15]. Parslow et al. [16] also used a multilayer paint technique for study of material erosion. They investigated the effects of different parameters on the erosion of paints, such as the angle of particle impact, the velocity of particles, the time and particles loading in an air blast rig to study the erosion properties of paint. In their study, Parslow et al. [16] reported that the maximum erosion of paint occurred at angles between 45◦ and 60◦ , displaying a mix of both ductile and brittle erosion behaviour, based on the work of Dunn [17]. Later Parslow et al. [18] used the multilayer paint technique to study the erosion of material in a complex geometry and applied it to erosion studies of typical well head geometries used for oil and gas production and discussed ways of reducing the erosion on such devices. Wu et al. [19] investigated the erosion of impellers used in a threephase gas/liquid/solids slurry tank. The multilayer paint method was applied in their study to check modifications of the impeller geometry in order to minimize the impeller wear. Their results were in good agreement with real full-scale wear observed in industrial plants.
2. Experimental test rig The present experimental study was conducted in a test rig consisting of a 390 mm diameter tank with a 1 m height. Four diametrically opposed vertical baffles of 32 mm width were placed in the tank. The tank was divided into 10 compartments separated by horizontal baffles of 39 mm width (Fig. 1(a)). Ten arms of 140 mm length each were mounted on the shaft. Each arm designated for holding a test sample had a tip holder inclined at a specific angle as seen in Fig. 1(b). Sample holders were inclined with respect to the vertical from 0◦ to 90◦ from the bottom to the top of the tank, respectively, with an angle increment of 10◦ . The arms were alternately staggered by 180◦ along the shaft in order to balance the shaft. Each arm was located in the middle vertical height of a compartment between two horizontal baffles. The shaft was rotated by a 1.1 kW motor controlled by a variable speed controller. The slurry was a mixture of tap water and alumina particles with a concentration of 15% (v/v). The slurry was premixed in a separate tank with a mixing impeller and then pumped into the experimental tank with a slurry pump. Once the tank was filled with slurry, the pump was used to recirculate the slurry from the bottom of the tank to the top in a closed loop in order to assure uniform particle concentration distribution within the tank. The pump flow rate was set in order to sufficiently pump the slurry particles at the lowest practical flow rate (of 18 L min−1 ) and to avoid possible end effects on
Fig. 1. (a) Schematic representation of the experimental rig. (b) Shaft equipped with holding arms.
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the mixing. While the pump was operated solid concentration measurements at different axial locations in the tank showed quite uniform concentration distribution within 3–4% (v/v) accuracy.
and quantitative estimation of the amount of material removed by erosion.
2.1. Paint sample preparation
Similar to the tests performed with the samples discussed previously, multiple thin layers of paints of different colours were sprayed onto the test impellers. The thickness of the paint layers was typically estimated to 0.7 mm, measured by the thickness probe. The uniformity of paint thickness over an impeller was checked by traversing the probe across the blades and the disc of the impeller, and was found generally to be within ±0.05 mm. Typically, five layers of paint were applied in the colour sequence of silver, yellow, red, blue and purple (from the blade surface outward). Once the paints dried, the painted impellers were tested in a given mixing condition for a prescribed time. The development of the wear pattern provided an indicative wear rate of a test impeller. A benchmark impeller, which was a scaled-down version of an eight-bladed Rushton turbine (DT8), was used as a “standard”, and the wear pattern of each test impeller was compared with the pattern of the benchmark impeller. This made it possible to assess if an improvement with a new impeller design was achieved, which would be indicated by a reduced visual development of the wear pattern for a given test time. This is a relatively inexpensive and rapid method to study an impeller wear problem and to redesign the impeller for better wear resistance. The erosion study conducted on the impeller was achieved in a standard 390 mm mixing tank equipped with four baffles. The impeller had a diameter of 149 mm and was mounted on a shaft and rotated at a speed of 250 rpm. Judgement was made on wear resistant performance based on the size of the paint wear pattern, and the number of layers of paint worn out during the tests, carried out using the laboratory liquid/solids/gas three-phase system, at a solids loading of approximately 40% (w/w) and an air flow of 22 L min−1 . The time duration of the tests were typically fixed at approximately 24 h.
The painted samples were made of stainless steel sheet of 3 mm thickness. Each sample had a surface of 20 mm × 20 mm and could be mounted on the arm holder with a 6 mm back welded bolt. Before applying the paint, the exposed side of the stainless steel sample was sand blasted before being painted in order to increase the adhesion of the paint film to the metallic surface. The samples were painted in batches of 10 for each type of paint. Paint layers were applied by spray guns operated with compressed air. Once the paint was prepared in the spray gun pot, an even layer of paint was sprayed on the samples aligned on a holding plate. Adequate time depending on the paint type (usually a day) was given to each layer to dry completely before applying the next layer of paint with a different colour using the same technique as previously. In the present study, several types of paints were investigated: Gloss Enamel paint by White Knight recommended for most applications on different types of surfaces, including wood, metal and plastic; Epoxy Enamel pots by White Knight; White Knight Metallic Tones is Enamel-based paint recommended for common use on any type of surface; Standocryl 527 two-pack paint by STANDOX is an Epoxy-based paint which dries hard on metallic surfaces recommended for coating metals mainly in auto refinish industry, it contains polymeric resins and inert inorganic fillers; polyester undercoat paint U-Pol REFACE designed for car body repair. It is based on a styrene monomer formulation as an undercoat for automotive applications made by W. David & Sons Ltd. (Australia). 2.2. Erosion rate measurements Erosion rate measurements were carried out by measuring the thickness of the paint layer before and after test. The measurements were carried out using a Fisher DUALSCOPE-MP20 thickness probe. The measurement probe contains pick-up elements, which generate a magnetic induction or eddy current probe signal. The probe is placed on the test specimen and a signal proportional to the coating thickness is transmitted to the instrument. The measurement signal is processed, displayed and evaluated by the DUALSCOPE. Calibration procedures permitted designation of the origin of the sample surface on the raw stainless steel surface before any paint was applied. The paint thickness measurements were done following a template mesh grid on the sample. Thickness measurements were repeated on the same grid on the sample after the erosion test was finished. The total amount of material removed by erosion was estimated by taking the difference of the thickness measurements on each grid line and adding the differences for the total sample area. This method was accurate for the erosion rate measurements and was consistent with visual observations. The multi-coloured layers associated with the thickness measurements provided qualitative
2.3. Mixing impellers test setup
3. Experimental results All results have confirmed the dependence of erosion on the particle impact angle on each sample. The shaft speed was set to 500 rpm, which assured that a sample was mainly subject to the tangential component of the flow defining the perpendicular direction to the vertical cross-section of the slurry tank. With the present setup, variations of parameters, such as velocity, particle concentration and temperature are eliminated since all samples were subject to the same conditions and only the impact angle was the changing parameter in the investigation. 3.1. Enamel-based paint erosion map Experimental tests were performed on different types of soft Enamel-based paints as described in Section 2.1. The first type of paint studied was the gloss Enamel paint manufactured by White Knight. The test was carried out with four layers of different colours applied to each sample. The colours from bottom to top were: black, white, purple, green. The erosion contours
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Fig. 2. Gloss Enamel paint colour contours erosion. The inclination angles given in degrees are shown on the lower right hand corner of each picture.
are shown in Fig. 2. The results clearly show a maximum erosion rate at an angle of 30◦ . Repeated tests have shown that runs of 3 h were just sufficient to distinguish the impact angle effect on the samples tested. The thickness measurements are given in Fig. 4. The curves display the erosion rate E normalized by the maximum measured erosion rate Emax . The angles are given in degrees. As observed from both paint layer contours and thickness measurements, gloss Enamel paint showed a soft or ductile erosion map and a very fast response to particle erosion. The following test was done by adding particles to the gloss Enamel soft paint in order to investigate possible hardness effects on the erosion. The test run was conducted for a mixture paint of Enamel paint and alumina particles. The amount of particles added to the Enamel paint was 25% (v/v). Only two coloured layers were applied, a black layer on the bottom and a yellow layer on the top. After the samples dried, the painted layers were quite hard to the touch compared to previously test of the Enamel paint. The test was conducted for a total duration of 14 h with two durations of 7 h. The erosion results have also shown a maximum erosion rate occurring at an angle of 30◦ as presented in Fig. 4. It seemed that even though the particles made the mixture harder than the raw paint, the erosion mechanism was mainly working on the medium which was the Enamel paint and provided an erosion map similar to the map of the Enamel paint alone. In the search for harder Enamel-based paints, a test was conducted on an Epoxy–Enamel formulation designated for rust protection and pipe coating applications. The paint is sold in small cans in an already prepared range of colours. A set of samples was
prepared following the same procedures previously discussed. Four layers of sprayed paint were applied to the sample following from bottom to top: white, red, blue and yellow. The test was also conducted for a total duration of 14 h. The thickness measurement results are shown in Fig. 4. It can be clearly seen that maximum erosion occurred at 30◦ . The results showed that this type of paint formulation behaves similarly to the Enamel only-based paint. It was observed that all three studied formulations of Enamelbased paint showed a maximum erosion rate at 30◦ . It was also observed that the angle range between 20◦ and 40◦ shows the largest erosion interval compared to all other inclination angles. The combination of the qualitative erosion study by visual coloured contours and the thickness measurements performed on the eroded samples confirm the ductile nature of soft paints, which are Enamel-based. This type of paint can be used to model the erosion of a large range of ductile materials. In order to compare the erosion data obtained by the multilayer paint modelling with the erosion data of ductile metallic materials, some of the results of the study carried out by Clark and Wong [11] in a slurry pot are presented in Fig. 5. The erosion rate data taken from Clark and Wong [11] for OFHC copper and 1020 steel were normalized by the maximum erosion rate. Fig. 5 shows clearly that the metallic material erosion data and the Enamel paint erosion data present all a maximum erosion rate at an angle of 30◦ . If we take into account that the results in Fig. 5 are normalized by the maximum erosion rate, the lower values obtained for the Enamel paint at angles other than 30◦
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Fig. 3. Metallic paint colour contours erosion. The inclination angles given in degrees are shown on the lower right hand corner of each picture.
compared to the metallic erosion data suggest that the mechanism of erosion works faster on the multilayered paint system than on the real materials. This behaviour consolidates the idea of using the multilayered paint modelling for fast erosion map determination.
results presented in Fig. 4 showed two different ranges of angle at which higher erosion was observed. Fig. 4 shows that the 0◦ and 10◦ samples presented a significant amount of eroded area. The other samples showed maximum erosion at 40◦ where a large area on the sample was completely eroded. This result suggests a different behaviour of this type of formulation although simi-
3.2. Metallic paints and polyester paints A metallic type of paint was investigated. It was oil-based paint with metallic flakes added. A test with purple, green, blue colours, respectively, from bottom to top was conducted. Each paint layer was evenly sprayed on the samples, dried during a day and a second layer was applied the following day. The metallic paint erosion colour contours are shown in Fig. 3 obtained after a run of 3 h. The thickness measurements confirmed that the maximum erosion rate occurred at an angle between 30◦ and 40◦ . Visually it was difficult to decide which of these two angles presented the maximum erosion, but thickness measurements confirmed that 40◦ shows the maximum erosion rate. The most eroded samples were situated between 20◦ and 50◦ as seen in Fig. 4 from the thickness measurements. A polyester coating paint was also studied. This formulation is designed for automotive under-coat applications and is based on a styrene monomer formulation. When dry, although hard to the touch, this paint showed a slight cracking behaviour different from the elastic behaviour of Enamel paint when dry. This formulation, which comes only in one colour, was sprayed evenly on a set of samples with spray gun. The thickness measurement
Fig. 4. Erosion results for all tested paints obtained by paint thickness measurements. E is the erosion rate and Emax is the maximal erosion rate for each type of paint. The angles are given in degrees.
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Fig. 5. Comparison of multilayer paint modelling using Enamel type paints with metallic material wear (copper and steel) data taken from Clark and Wong [11], normalized by the maximum wear Emax .
lar in some aspects to the metallic paint since the existence of a maximum wear at 40◦ was observed. 4. Application of paint modelling In industries, such as the mineral processing industry, high agitation intensity is sometimes used to achieve the required solids suspension and gas to liquid mass transfer rate in threephase systems. However, the high slurry velocities used in a high-intensity agitation vessel may result in particle impingement wear on the impeller blades. The aim of the implementation of this paint method was to observe the wear behaviour with impellers made of a titanium alloy material. Such material is widely used for impellers operating in high-temperature acidic slurries, for example, in pressure leaching/oxidation autoclaves. Titanium alloy shows a mixed ductile/brittle material behaviour with the maximum erosion rate at an impingement angle of 45◦ [16]. The multilayer paint technique, which produces highly visible and accelerated erosion damage, was used to model the wear
of impellers. The previous results showed that the multilayer paint technique is a good model for materials, such as mild steel, titanium and other engineering materials that show similar mixed ductile/brittle erosion characteristics. As previously described in Section 2.3, the method of multilayer paint was implemented to characterise erosion of slurry mixing impellers. The wear patterns developed at the back of impeller DT8 seen in Fig. 6a, after operating in a three-phase slurry in the model test tank was quite severe on the impeller blades. The wear patterns illustrated were found to compare remarkably well with the observed wear patterns on a full-scale Rushton turbine, operating at the same aeration number and similar solids loading as shown in Fig. 6b. The wear pattern is related to two vortices rolling over the blades and impinging onto the back of the blades. These vortices are well known and are referred to as “trailing edge vortices” in literature [20–22]. One notable feature observed in the present study is that it appears that the vortices are not parallel to each other as they develop at the back of the blades before exiting at the impeller tip. This behaviour has not been previously reported in the literature. It appears that as the two vortices move closer at the middle of the blade, a high wear region is produced. This is consistent with the severe wear damage at the middle of the back surface of the full-scale blade (Fig. 6). 5. Conclusions This study has confirmed the ductile character of most soft paints and gave a solid argument for using these types of paint for laboratory modelling of ductile material erosion in slurry flows. The use of a range of paint colours and thickness measurements with a magnetic probe permitted to estimate qualitatively and quantitatively the amount of eroded paint from each studied sample. All Enamel-based paints have shown maximum erosion rate at an impact angle of 30◦ . Metallic paints and styrenebased coating paint have shown a slight shift of the maximum erosion rate to 40◦ . The implementation of such modelling techniques is time and cost-effective and provides highly reliable results.
Fig. 6. Wear on an eight-bladed Rushton turbine. Clockwise rotation when viewed from top. The patterns observed are at the back of the blades: (a) multiple paint wear pattern in laboratory scale and (b) full-scale titanium impeller damage.
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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
O. Reynolds, Philos. Mag. 46 (1873) 337–343. L. Rayleigh, Nature 93 (1912) 188–198. J.A.C. Humphrey, Int. J. Heat Fluid Flow 11 (1990) 170–195. I.M. Hutchings, Chem. Eng. Sci. 42 (1987) 869–878. I. Finnie, Wear 186–187 (1995) 1–10. I.M. Hutchings, R.E. Winter, J. Phys. D: Appl. Phys. 8 (1975) 8–14. R.G. Wellman, C. Allen, Wear 186–187 (1995) 117–122. Y.I. Oka, H. Ohnogi, T. Hosokawa, M. Matsumura, Wear 203–204 (1997) 573–579. T. Omote, H. Morita, A. Hirose, 27th Inter. SAMPE Tech. Conf, October 9-12, 1995. J.B. Zu, I.M. Hutchings, G.T. Burstein, Wear 140 (1990) 331–344. H.Mcl. Clark, K.K. Wong, Wear 186–187 (1995) 454–464. R. Hamzah, D.J. Stephenson, J.E. Strutt, Wear 186–187 (1995) 493–496.
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[13] G.R. Desale, B.K. Gandhi, S.C. Jain, Wear 259 (2005) 196–202. [14] J.B. Fasano, in: Spring National Meeting, March 29–April 2, Am. Inst. Chem. Eng. (1987). [15] J.S. Harrison, The 1988 Robens Coal Science Lecture, British Coal’s Research, 1988. [16] G.I. Parslow, D.J. Stephenson, J.E. Strutt, S. Tetlow, Wear 212 (1997) 103–109. [17] C.J. Dunn, M.Sc. Thesis, Cranfield Institute of Technology, 1993. [18] G.I. Parslow, D.J. Stephenson, J.E. Strutt, S. Tetlow, Wear 233–235 (1999) 737–745. [19] J. Wu, B. Nguyen, L. Graham, Y. Zhu, T. Kilpatrick, J. Davis, Can. J. Chem. Eng. 83 (2005) 742–835. [20] K. Van’t Riet, J.M. Smith, Chem. Eng. Sci. 30 (1975) 1093–1105. [21] M. Yianneskis, Z. Popiolek, J.H. Whitelaw, J. Fluid Mech. 175 (1987) 537–549. [22] C.M. Stoots, R.V. Calabrese, Am. Inst. Chem. Eng. J. 41 (1995) 1–11.
Wear 264 (2008) 737–743
Short communication
Study of erosion behaviour of paint layers for multilayer paint technique applications in slurry erosion M.N. Noui-Mehidi ∗ , L.J.W. Graham, J. Wu, B.V. Nguyen, S. Smith CSIRO Manufacturing & Materials Technology, P.O. Box 56, Graham Road, Highett, Victoria 3190, Australia Received 11 October 2006; received in revised form 3 April 2007; accepted 4 June 2007 Available online 13 July 2007
Abstract A multilayer paint modelling study was conducted to investigate the erosion properties of different types of paints. The experiments were carried out in a slurry-mixing tank in which 10 painted samples mounted on a shaft were rotated at constant speed for a certain interval of time. The effect of impact angle on the particular type of paint was investigated by orientating the samples consecutively at angles from 0◦ to 90◦ with an increment of 10◦ . In the slurry medium, it was found that most soft paints, such as “Enamel” type have similar erosion maps to ductile materials with a maximum wear rate at around 30◦ . Metallic paints and styrene type paints have shown a slightly higher angle of 40◦ for maximum erosion. In laboratory modelling, erosion studies of ductile materials by multilayer paint techniques are time and cost-effective. © 2007 Elsevier B.V. All rights reserved. Keywords: Erosion modelling; Multilayer paint technique; Slurry erosion
1. Introduction The erosive wear, which occurs when solid particles entrained in a fluid stream impinge on a surface, has been an important and continuing problem in many industries. The first paper reporting erosion effects was written by Reynolds [1] in 1873 and later a further paper by Rayleigh [2] in 1912 discussed erosion in sand blasting. Materials are classified into two large groups: ductile materials, which show relatively soft behaviour when subject to mechanical erosion and brittle materials, which show high resistance to erosion due to the hardness of their surface. These two groups are qualified by characteristic erosion curve with regard to the attack angle of the particles on the material surface [3]. The literature reports that the ductile response to mechanical erosion corresponds to a maximum erosion rate at low impact angles between 20◦ and 40◦ , while in brittle materials the erosion rates increases with increasing impact angles with a maximum being at 90◦ . Several studies have focused on ductile material erosion since the variation of the weight loss with angle of impingement is very
∗
Corresponding author. Tel.: +61 3 9252 6075; fax: +61 3 9252 6252. E-mail address: [email protected] (M.N. Noui-Mehidi).
0043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2007.06.003
similar for materials with widely different thermal and physical properties [4,5]. Finnie [5] proposed an erosion model giving the erosion rate as function of the impact particle velocity to the power n, where n depends on system parameters. Analysis of brittle material erosion reasonably predicted the role of variables, such as particle size, velocity and fracture toughness of the surface when the eroding particle is harder than the surface it is cracking. Humphrey [3] discussed in a review paper the different aspects of material erosion. The eroded material E (where E = mass removed from surface/total mass of particles impinging on a surface) was dependent on the erosion caused by the particle incidence (or impact) angle. Hutchings and Winter [6] presented a study on erosion of ductile metals subject to spherical particle abrasion. They used mild steel as a target and spherical steel particles at velocities up to 256 m s−1 as projectiles. They showed that metal could be removed from the surface by a process involving the shearing of surface material in the direction of motion of the projectile. Wellman and Allen [7] investigated the effects of angle of impact and material properties on the erosion rates of ceramics. They found that the relative hardness of the target and erodent did not provide a clear prediction of erosion resistance although threshold velocity and angle effects were observed in the erosion mechanism. Oka et al. [8] studied the dependence on the impact angle of erosion damage caused by solid particle impact
738
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on several types of materials. They performed erosion tests on five metallic materials, a ceramic and a plastic material using silica sand particles at angles between 3◦ and 90◦ . They used a compressed air test rig to conduct the erosion study. In their study, they investigated velocities in the range of 50–130 m s−1 and suggested that the erosion was not dependent on the impact velocity in this range but only on the material properties and the impact angle. Omote et al. [9] also used an air blast experimental setup to study the erosion of five types of materials using sand particles impinging at 230 m s−1 . They compared the erosion of polyurethane and titanium alloy. Polyurethane showed a ductile erosion behaviour with a maximum erosion angle at 25◦ , while titanium alloy had a maximum erosion angle at 45◦ . Zu et al. [10] designed a jet impingement slurry erosion test rig for laboratory use. The slurry was recirculated in a loop by the use of a pump, and after impingement, the particles were recollected from a hopper located below the target holder. Slurry pots were also widely used for impact angle effect studies, e.g. refs. [11,12]. Desale et al. [13] developed a pot tester for slurry erosion studies. They carried out experiments in a transparent cylindrical pot in order to determine the minimum speed required for propeller to achieve uniform distribution of solid particles. They found that a 45◦ -pitched four bladed down-pumping propeller gave a better distribution of solids. They have tested specimens mounted on a different shaft inserted from the top of the pot and rotated at a desired speed. They compared their results of erosion of ductile materials using this technique and found their results in agreement with those found in the literature. There are only a few references related to paint and coat modelling for material erosion studies. An early discussion was presented by Fasano [14] who studied the erosion on a rubbercoated impeller. Instead of using the rubber material for his erosion study, he used different paint coatings in order to investigate the erosion rate and mechanism on the pitched blade impeller. He tried different paint for both colour contrast and adherence. The multilayered paint technique was also used to study wear in fluidised bed combustors as reported by Harrison [15]. Parslow et al. [16] also used a multilayer paint technique for study of material erosion. They investigated the effects of different parameters on the erosion of paints, such as the angle of particle impact, the velocity of particles, the time and particles loading in an air blast rig to study the erosion properties of paint. In their study, Parslow et al. [16] reported that the maximum erosion of paint occurred at angles between 45◦ and 60◦ , displaying a mix of both ductile and brittle erosion behaviour, based on the work of Dunn [17]. Later Parslow et al. [18] used the multilayer paint technique to study the erosion of material in a complex geometry and applied it to erosion studies of typical well head geometries used for oil and gas production and discussed ways of reducing the erosion on such devices. Wu et al. [19] investigated the erosion of impellers used in a threephase gas/liquid/solids slurry tank. The multilayer paint method was applied in their study to check modifications of the impeller geometry in order to minimize the impeller wear. Their results were in good agreement with real full-scale wear observed in industrial plants.
2. Experimental test rig The present experimental study was conducted in a test rig consisting of a 390 mm diameter tank with a 1 m height. Four diametrically opposed vertical baffles of 32 mm width were placed in the tank. The tank was divided into 10 compartments separated by horizontal baffles of 39 mm width (Fig. 1(a)). Ten arms of 140 mm length each were mounted on the shaft. Each arm designated for holding a test sample had a tip holder inclined at a specific angle as seen in Fig. 1(b). Sample holders were inclined with respect to the vertical from 0◦ to 90◦ from the bottom to the top of the tank, respectively, with an angle increment of 10◦ . The arms were alternately staggered by 180◦ along the shaft in order to balance the shaft. Each arm was located in the middle vertical height of a compartment between two horizontal baffles. The shaft was rotated by a 1.1 kW motor controlled by a variable speed controller. The slurry was a mixture of tap water and alumina particles with a concentration of 15% (v/v). The slurry was premixed in a separate tank with a mixing impeller and then pumped into the experimental tank with a slurry pump. Once the tank was filled with slurry, the pump was used to recirculate the slurry from the bottom of the tank to the top in a closed loop in order to assure uniform particle concentration distribution within the tank. The pump flow rate was set in order to sufficiently pump the slurry particles at the lowest practical flow rate (of 18 L min−1 ) and to avoid possible end effects on
Fig. 1. (a) Schematic representation of the experimental rig. (b) Shaft equipped with holding arms.
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the mixing. While the pump was operated solid concentration measurements at different axial locations in the tank showed quite uniform concentration distribution within 3–4% (v/v) accuracy.
and quantitative estimation of the amount of material removed by erosion.
2.1. Paint sample preparation
Similar to the tests performed with the samples discussed previously, multiple thin layers of paints of different colours were sprayed onto the test impellers. The thickness of the paint layers was typically estimated to 0.7 mm, measured by the thickness probe. The uniformity of paint thickness over an impeller was checked by traversing the probe across the blades and the disc of the impeller, and was found generally to be within ±0.05 mm. Typically, five layers of paint were applied in the colour sequence of silver, yellow, red, blue and purple (from the blade surface outward). Once the paints dried, the painted impellers were tested in a given mixing condition for a prescribed time. The development of the wear pattern provided an indicative wear rate of a test impeller. A benchmark impeller, which was a scaled-down version of an eight-bladed Rushton turbine (DT8), was used as a “standard”, and the wear pattern of each test impeller was compared with the pattern of the benchmark impeller. This made it possible to assess if an improvement with a new impeller design was achieved, which would be indicated by a reduced visual development of the wear pattern for a given test time. This is a relatively inexpensive and rapid method to study an impeller wear problem and to redesign the impeller for better wear resistance. The erosion study conducted on the impeller was achieved in a standard 390 mm mixing tank equipped with four baffles. The impeller had a diameter of 149 mm and was mounted on a shaft and rotated at a speed of 250 rpm. Judgement was made on wear resistant performance based on the size of the paint wear pattern, and the number of layers of paint worn out during the tests, carried out using the laboratory liquid/solids/gas three-phase system, at a solids loading of approximately 40% (w/w) and an air flow of 22 L min−1 . The time duration of the tests were typically fixed at approximately 24 h.
The painted samples were made of stainless steel sheet of 3 mm thickness. Each sample had a surface of 20 mm × 20 mm and could be mounted on the arm holder with a 6 mm back welded bolt. Before applying the paint, the exposed side of the stainless steel sample was sand blasted before being painted in order to increase the adhesion of the paint film to the metallic surface. The samples were painted in batches of 10 for each type of paint. Paint layers were applied by spray guns operated with compressed air. Once the paint was prepared in the spray gun pot, an even layer of paint was sprayed on the samples aligned on a holding plate. Adequate time depending on the paint type (usually a day) was given to each layer to dry completely before applying the next layer of paint with a different colour using the same technique as previously. In the present study, several types of paints were investigated: Gloss Enamel paint by White Knight recommended for most applications on different types of surfaces, including wood, metal and plastic; Epoxy Enamel pots by White Knight; White Knight Metallic Tones is Enamel-based paint recommended for common use on any type of surface; Standocryl 527 two-pack paint by STANDOX is an Epoxy-based paint which dries hard on metallic surfaces recommended for coating metals mainly in auto refinish industry, it contains polymeric resins and inert inorganic fillers; polyester undercoat paint U-Pol REFACE designed for car body repair. It is based on a styrene monomer formulation as an undercoat for automotive applications made by W. David & Sons Ltd. (Australia). 2.2. Erosion rate measurements Erosion rate measurements were carried out by measuring the thickness of the paint layer before and after test. The measurements were carried out using a Fisher DUALSCOPE-MP20 thickness probe. The measurement probe contains pick-up elements, which generate a magnetic induction or eddy current probe signal. The probe is placed on the test specimen and a signal proportional to the coating thickness is transmitted to the instrument. The measurement signal is processed, displayed and evaluated by the DUALSCOPE. Calibration procedures permitted designation of the origin of the sample surface on the raw stainless steel surface before any paint was applied. The paint thickness measurements were done following a template mesh grid on the sample. Thickness measurements were repeated on the same grid on the sample after the erosion test was finished. The total amount of material removed by erosion was estimated by taking the difference of the thickness measurements on each grid line and adding the differences for the total sample area. This method was accurate for the erosion rate measurements and was consistent with visual observations. The multi-coloured layers associated with the thickness measurements provided qualitative
2.3. Mixing impellers test setup
3. Experimental results All results have confirmed the dependence of erosion on the particle impact angle on each sample. The shaft speed was set to 500 rpm, which assured that a sample was mainly subject to the tangential component of the flow defining the perpendicular direction to the vertical cross-section of the slurry tank. With the present setup, variations of parameters, such as velocity, particle concentration and temperature are eliminated since all samples were subject to the same conditions and only the impact angle was the changing parameter in the investigation. 3.1. Enamel-based paint erosion map Experimental tests were performed on different types of soft Enamel-based paints as described in Section 2.1. The first type of paint studied was the gloss Enamel paint manufactured by White Knight. The test was carried out with four layers of different colours applied to each sample. The colours from bottom to top were: black, white, purple, green. The erosion contours
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Fig. 2. Gloss Enamel paint colour contours erosion. The inclination angles given in degrees are shown on the lower right hand corner of each picture.
are shown in Fig. 2. The results clearly show a maximum erosion rate at an angle of 30◦ . Repeated tests have shown that runs of 3 h were just sufficient to distinguish the impact angle effect on the samples tested. The thickness measurements are given in Fig. 4. The curves display the erosion rate E normalized by the maximum measured erosion rate Emax . The angles are given in degrees. As observed from both paint layer contours and thickness measurements, gloss Enamel paint showed a soft or ductile erosion map and a very fast response to particle erosion. The following test was done by adding particles to the gloss Enamel soft paint in order to investigate possible hardness effects on the erosion. The test run was conducted for a mixture paint of Enamel paint and alumina particles. The amount of particles added to the Enamel paint was 25% (v/v). Only two coloured layers were applied, a black layer on the bottom and a yellow layer on the top. After the samples dried, the painted layers were quite hard to the touch compared to previously test of the Enamel paint. The test was conducted for a total duration of 14 h with two durations of 7 h. The erosion results have also shown a maximum erosion rate occurring at an angle of 30◦ as presented in Fig. 4. It seemed that even though the particles made the mixture harder than the raw paint, the erosion mechanism was mainly working on the medium which was the Enamel paint and provided an erosion map similar to the map of the Enamel paint alone. In the search for harder Enamel-based paints, a test was conducted on an Epoxy–Enamel formulation designated for rust protection and pipe coating applications. The paint is sold in small cans in an already prepared range of colours. A set of samples was
prepared following the same procedures previously discussed. Four layers of sprayed paint were applied to the sample following from bottom to top: white, red, blue and yellow. The test was also conducted for a total duration of 14 h. The thickness measurement results are shown in Fig. 4. It can be clearly seen that maximum erosion occurred at 30◦ . The results showed that this type of paint formulation behaves similarly to the Enamel only-based paint. It was observed that all three studied formulations of Enamelbased paint showed a maximum erosion rate at 30◦ . It was also observed that the angle range between 20◦ and 40◦ shows the largest erosion interval compared to all other inclination angles. The combination of the qualitative erosion study by visual coloured contours and the thickness measurements performed on the eroded samples confirm the ductile nature of soft paints, which are Enamel-based. This type of paint can be used to model the erosion of a large range of ductile materials. In order to compare the erosion data obtained by the multilayer paint modelling with the erosion data of ductile metallic materials, some of the results of the study carried out by Clark and Wong [11] in a slurry pot are presented in Fig. 5. The erosion rate data taken from Clark and Wong [11] for OFHC copper and 1020 steel were normalized by the maximum erosion rate. Fig. 5 shows clearly that the metallic material erosion data and the Enamel paint erosion data present all a maximum erosion rate at an angle of 30◦ . If we take into account that the results in Fig. 5 are normalized by the maximum erosion rate, the lower values obtained for the Enamel paint at angles other than 30◦
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Fig. 3. Metallic paint colour contours erosion. The inclination angles given in degrees are shown on the lower right hand corner of each picture.
compared to the metallic erosion data suggest that the mechanism of erosion works faster on the multilayered paint system than on the real materials. This behaviour consolidates the idea of using the multilayered paint modelling for fast erosion map determination.
results presented in Fig. 4 showed two different ranges of angle at which higher erosion was observed. Fig. 4 shows that the 0◦ and 10◦ samples presented a significant amount of eroded area. The other samples showed maximum erosion at 40◦ where a large area on the sample was completely eroded. This result suggests a different behaviour of this type of formulation although simi-
3.2. Metallic paints and polyester paints A metallic type of paint was investigated. It was oil-based paint with metallic flakes added. A test with purple, green, blue colours, respectively, from bottom to top was conducted. Each paint layer was evenly sprayed on the samples, dried during a day and a second layer was applied the following day. The metallic paint erosion colour contours are shown in Fig. 3 obtained after a run of 3 h. The thickness measurements confirmed that the maximum erosion rate occurred at an angle between 30◦ and 40◦ . Visually it was difficult to decide which of these two angles presented the maximum erosion, but thickness measurements confirmed that 40◦ shows the maximum erosion rate. The most eroded samples were situated between 20◦ and 50◦ as seen in Fig. 4 from the thickness measurements. A polyester coating paint was also studied. This formulation is designed for automotive under-coat applications and is based on a styrene monomer formulation. When dry, although hard to the touch, this paint showed a slight cracking behaviour different from the elastic behaviour of Enamel paint when dry. This formulation, which comes only in one colour, was sprayed evenly on a set of samples with spray gun. The thickness measurement
Fig. 4. Erosion results for all tested paints obtained by paint thickness measurements. E is the erosion rate and Emax is the maximal erosion rate for each type of paint. The angles are given in degrees.
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Fig. 5. Comparison of multilayer paint modelling using Enamel type paints with metallic material wear (copper and steel) data taken from Clark and Wong [11], normalized by the maximum wear Emax .
lar in some aspects to the metallic paint since the existence of a maximum wear at 40◦ was observed. 4. Application of paint modelling In industries, such as the mineral processing industry, high agitation intensity is sometimes used to achieve the required solids suspension and gas to liquid mass transfer rate in threephase systems. However, the high slurry velocities used in a high-intensity agitation vessel may result in particle impingement wear on the impeller blades. The aim of the implementation of this paint method was to observe the wear behaviour with impellers made of a titanium alloy material. Such material is widely used for impellers operating in high-temperature acidic slurries, for example, in pressure leaching/oxidation autoclaves. Titanium alloy shows a mixed ductile/brittle material behaviour with the maximum erosion rate at an impingement angle of 45◦ [16]. The multilayer paint technique, which produces highly visible and accelerated erosion damage, was used to model the wear
of impellers. The previous results showed that the multilayer paint technique is a good model for materials, such as mild steel, titanium and other engineering materials that show similar mixed ductile/brittle erosion characteristics. As previously described in Section 2.3, the method of multilayer paint was implemented to characterise erosion of slurry mixing impellers. The wear patterns developed at the back of impeller DT8 seen in Fig. 6a, after operating in a three-phase slurry in the model test tank was quite severe on the impeller blades. The wear patterns illustrated were found to compare remarkably well with the observed wear patterns on a full-scale Rushton turbine, operating at the same aeration number and similar solids loading as shown in Fig. 6b. The wear pattern is related to two vortices rolling over the blades and impinging onto the back of the blades. These vortices are well known and are referred to as “trailing edge vortices” in literature [20–22]. One notable feature observed in the present study is that it appears that the vortices are not parallel to each other as they develop at the back of the blades before exiting at the impeller tip. This behaviour has not been previously reported in the literature. It appears that as the two vortices move closer at the middle of the blade, a high wear region is produced. This is consistent with the severe wear damage at the middle of the back surface of the full-scale blade (Fig. 6). 5. Conclusions This study has confirmed the ductile character of most soft paints and gave a solid argument for using these types of paint for laboratory modelling of ductile material erosion in slurry flows. The use of a range of paint colours and thickness measurements with a magnetic probe permitted to estimate qualitatively and quantitatively the amount of eroded paint from each studied sample. All Enamel-based paints have shown maximum erosion rate at an impact angle of 30◦ . Metallic paints and styrenebased coating paint have shown a slight shift of the maximum erosion rate to 40◦ . The implementation of such modelling techniques is time and cost-effective and provides highly reliable results.
Fig. 6. Wear on an eight-bladed Rushton turbine. Clockwise rotation when viewed from top. The patterns observed are at the back of the blades: (a) multiple paint wear pattern in laboratory scale and (b) full-scale titanium impeller damage.
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