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Research on cavitation erosion and wear resistance performance of coatings
Engineering Failure Analysis 55 (2015) 208–223
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Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Research on cavitation erosion and wear resistance performance of coatings Ning Qiu a,⇑, Leqin Wang a, Suhuan Wu b, Dmitriy S. Likhachev a a b
Institute of Process Equipment, Zhejiang University, Hangzhou 310027, China Shanghai BondPoly Engineering Material & Technology Co. Ltd., Shanghai 201601, China
a r t i c l e
i n f o
Article history: Received 10 December 2014 Received in revised form 20 May 2015 Accepted 8 June 2015 Available online 15 June 2015 Keywords: Cavitation erosion Wear resistance Coatings Micro structure Abrasive jet
a b s t r a c t Depending on the nature of the working medium and working conditions, corrosive and cavitation damage shall arise to pump’s components. In industrial applications the corrosion-reducing coatings are sprayed on hydraulic components. But it is questionable whether such products actually do help under wear or cavitation loads or not. Abrasive jet wear tests were carried out to determine the wear resistance of coating materials: polymers and ceramics, cast iron, and steel of various types. The samples were loaded for five hours, and finally the wear depth was measured as a determining indicator of the sample’s wear resistance. Results of investigation on anti-erosion performance of epoxy resin, ceramic and Polyurethane (PU) coatings brushed on alloy steel surface were also presented. Cavitation erosion tests were performed on the ultrasonic rig. The mass loss and surface morphology of the specimens were examined by balance analysis and 3-D laser microscopy, respectively. The investigations showed excellent wear-resisting performance of ceramic coatings, which is better than wear-resistance of stainless steel, cast iron and high chrome alloy steel. But the excellent wear-resisting performance could not guarantee a good erosion-resisting performance. The ceramic coatings’ anti-erosion performances were inferior to that of gray cast iron, and hardly comparable to those of stainless steels. The basic factors that influenced coating’s cavitation erosion endurance were adhesion and thickness of coatings. Analysis of coating’s degradation mechanism showed that PU coatings could withstand longer incubation period thus enhancing the materials’ cavitation erosion resistance. Several practical cases were analyzed, showing some guidance for coatings’ application. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Pressure decrease in fast flowing liquid is the main reason for cavitation phenomenon. The dissolved gas is the source of cavitation nuclei [1]. The dynamic load at the moment when cavitation bubble collapses lasts for a few microseconds or nanoseconds [2]. Alloys are usually used in process equipment and hydraulic machinery. Poor cavitation erosion resistance of impellers’ material poses a serious obstacle for its application in hydraulic machinery [3]. High wear applications cannot be avoided, when hydraulic machinery has to be subjected to heavy static or dynamic impact loads. Materials’ response to static
⇑ Corresponding author. Tel.: +86 13735578065; fax: +86 571 87952406. E-mail address: [email protected] (N. Qiu). http://dx.doi.org/10.1016/j.engfailanal.2015.06.003 1350-6307/Ó 2015 Elsevier Ltd. All rights reserved.
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conditions is different from response to dynamic conditions. Under dynamic impact loads, as during cavitation, materials’ resistance to deformation is lower [4]. Over the last few decades, considerable efforts have been devoted to enhance the erosion resistance of steel by depositing protective coatings on its surface [5–7]. The coatings can be easily formed on the surface of steel, which can exhibit smooth surface and good mechanical properties, such as high hardness, high elastic modulus or high oxidation resistance [8]. Many fluid machinery components are supplied with coatings to prolong their lifespan and to improve their work efficiency. However, when bubble collapses on the coating’s surface, a high temperature may be achieved, which could influence the coatings’ properties, for example makes coatings more ductile [9]. Thousands of pressure pulses act on the surface when bubbles collapse. The coating may undergo local thermal softening, which has an essential influence on the coatings’ deformation. The thermal mismatch between the coating and substrate may also result in adhesion failure [10]. Coatings with weak adhesion could soon be peeled off. If the coatings can avoid adhesive fracture, they protect the substrate from corrosive fluid. Adhesion plays an important role in the incubation period and ensures the protection of the substrate material against mass loss. The outstanding results observed in any given coating can be attributed to the strong adhesive connection. When a coating is very ductile, it is easy to cause the dislocation movement and peeling off of the coating [11]. In addition, cavitation erosion investigations proved that the cavitation erosion resistance of conventional materials (stainless steel, etc.) depends on their mechanical parameters (hardness, Young’s modulus, tensile strength, and fatigue strength) [12–15]. More energy is needed to be absorbed before fracture, if the hardness and Young modulus are high. Delamination may also happen due to the large difference in hardness and Young’s modulus between the substrate and coating. Taking the above statements into consideration, the main aim of this work was to reveal the factors which were responsible for the resistance of coatings. These factors were combination of the properties that were essential in conditions of dynamic impact. And the secondary aim was to find coatings with higher wear-resistance and higher erosion-resistance. 2. Experiment on wear resistance 2.1. Experimental method and materials In order to simulate the wear of FGD (flue gas desulfurization) slurry pump, abrasive jet wear tests were carried out to determine the wear resistance to solid particle attack of several coating materials, cast iron, and other steels. With a view to comparing the materials’ resistance, blast wear experiments were performed under the same impact intensity. And wear problem of most slurry pump increases significantly when the linear velocity is greater than 15 m/s. (Wear is serious for a general slurry pump: the rotating speed is 750 r/min, the impeller diameter is 400 mm, and the linear velocity is 3.14 0.4 750/60 = 15.7 m/s). Solid particles in pumped FGD slurry are often 100–200 grade mesh. Because the large particles are filtered or precipitated in the pre-treatment process. The upper limit of particle concentration of FGD slurry pump is 60%. So the test conditions are as follows: The samples were buffeted by slurry jet of high concentration (60%, 1.3 kg/l) and high speed. The slurry was a mixture of high hardness quartz sand (particle size: 100–200 grade mesh, 0.7–1.5 mm grain’s size) and water, which was sprayed onto the specimens at a velocity up to 18 m/s and an impinging angle of 90°, as shown in Fig. 1. The samples were 3 mm thick. The impact lasted for 5 h, and finally the wear depth was measured as an indicator to determine the sample’s wear resistance. The samples’ materials are shown in Table 1. 2.2. Experimental results After the samples were buffeted by slurry for 5 h, the sample surfaces were worn as shown in Fig. 2. The stainless steel sample (304) was penetrated after 2 h. The wear depth is shown in Fig. 3. As shown in the test results above, the wear resistance of Resto SiC Fine Bead, which is a kind of ceramic coating, is the best and even better than wear-resistant steel Cr30A. Stainless steel 304 is the worst because of its low hardness. The wear resistance of PU coating is comparable to those wear-resistant steels. 3. Cases of wear-resistant coatings’ practical application 3.1. Ceramic coatings Circulating pump transporting flue gas desulfurization (FGD) slurry in a power plant in Shanghai, is one of the main equipment in FGD section. It is responsible for transporting the lime slurry into the absorption tower spraying acidic gas such as SO2 (sulfur dioxide). The transported medium is the lime slurry, which contains solid particles with a concentration of 15– 20%. The main ingredients are CaSO4, CaCO3 and a small amount of high hardness particles, such as SiO2 and Al2O3. Particle size of solid powder is around 300 grade mesh. The liquid is acidic, and pH ranges from 4 to 6, and the temperature ranges from 45 to 60 °C. Concentration of chloride ions (Cl ) is below 20,000 ppm. Particles such as SiO2 and Al2O3 are the ballast
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(a) Schematic diagram of abrasive jet wear test
(b) Sample
(c) Spray chamber
(d) Quartz slurry
Fig. 1. Wear-resistance test apparatus.
Table 1 Materials in wear-resistance experiment. Tested materials
Chemical composition (%) C
304 0.042 JM3 2.0 Cr30A 1.6 A49 0.6 Sintering Ceramic Brick
Si
Mn
S
P
Cr
0.4 1.0 0.9 1.0
1.12 0.6 1.6 1.1
0.003 0.03 0.05 0.04
0.034 18.03 0.04 16.0 0.03 30 0.03 28 Al2O3 P 90%
Ni
Mo
Cu
8.01 1.3 2.0 3.0
– 0.3 1.9 2.5
– 0.9 1.5 2.0
Hardness
Thickness
HRC 35 HRC 50 HRC 55 HRC 58 HRA 60
3 mm
Polymer coatings
Organic binder base
Fillers
Hardness
Coating thickness
ARC 897 Belzona 1821 EB 206 Resto SiC Fine Bead Devcon 11470 Loctite 98383 Metaline 785
Epoxy Resin P20%
Al2O3 70% Al2O3 72% Al2O3 45% + SiC 20% Al2O3 10% + SiC 60% Al2O3 65% Al2O3 60% + SiC 5% Pigment
Shore Shore Shore Shore Shore Shore Shore
3 mm
Polyurethane
D 85 D 85 D 80 D 90 D 85 D 85 A 82
by-product of limestone grinding, which don’t participate in the chemical reaction of acid gas absorbing. Thus it will accumulate during recirculation, causing the direct wear on the metal surfaces of pump. The material of the front shroud of slurry circulation pump (Sulzer ZAP701) is the precipitation-hardening duplex stainless steel (ASTM 890 5A), which is coated by high temperature sprayed tungsten carbide 1 mm thick coating. After being used for a certain time, small cracks came off tungsten carbide coating, stripping the stainless steel surface. The unprotected stainless steel worn down quickly by the impact of lime slurry hard particles. The lime slurry continued to hollow the metal base under the hard coating. Consequently the tungsten carbide coating was peeled off because of the loss of base material (stainless steel). After the coating was peeled off, the metal base worn down more quickly. After 12 months operation, the front shroud of slurry circulation pump was seriously worn out, as shown in Fig. 4.
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Stainless steel (304)
High-chromium Cast Iron (JM3)
Ceramic coating (Ebond 206)
Protective Agent (Devcon 11470)
Protective Agent (Loctite 98383)
Ceramic coating (ARC 897)
Ceramic coating (Belzona 1821)
Polyurethane (Metaline 785)
Wear-resistant steel (A49)
Sintering Ceramic
Ceramic coating (Resto SiC Fine Bead)
Wear-resistant steel (Cr30A)
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Fig. 2. Worn specimens after 5 h.
The worn components were repaired and the coatings were lined by Shanghai BondPoly Engineering Material & Technology Co. Ltd. The solution was to repair the eroded surface with Resto products (ceramic coatings). SiC Brushable agent is mainly composed of silicon carbide and polymer. After being gelled under normal temperature, a smooth and bright surface formed. SiC Brushable, can be independently coated on metal surface, providing the anti-corrosion effect. It can also be used as an outer coating after the damaged surface was repaired by SiC Fine Bead or SiC Putty. The repair processes are as follows (see Fig. 5): After the front shroud of slurry circulation pump was repaired with Resto products and returned to service for 10 months, the conditions of coatings was as shown in Fig. 6. The photos of long term operation results were taken by the authors. After the front shroud was used for 10 months, thinner coating layer (about 0.5 mm thick) of SiC Putty and SiC Brushable were worn out, leaving the surface of SiC Fine Bead (consisting of small particles of silicon carbide) exposed. The SiC Fine
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Fig. 3. Wear depths of samples.
Fig. 4. Worn front shroud of slurry circulation pump after 12 months operation.
(a) Resto ceramic coating layers
(b)
(1)
(2)
(3)
(4)
Fig. 5. The repair processes: (1) Brush with Primer ceramic; (2) repair potholes with SiC Fine Bead; (3) flatten the surface with SiC Putty; (4) brush the SiC Brushable outer coating.
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(a) 10 months later
(b) 20 months later
(c) 36 months later
Fig. 6. Conditions of coatings after pump operation.
Bead coating was intact, proving that it had excellent wear-resisting performance. The customer was very satisfied, and evaluated the SiC Fine Bead (ceramic coating) much better than the Loctite products that were used before. After the actual application for 10 months, the maximum pitting depth of pump case area without coating reached 3 mm. More than 95% of the area covered with Resto SiC coating was intact. After 20 months, the maximum depth reached 5 mm, and more than 95% of the covered area was intact. After 36 months, the maximum depth reached 10 mm, and more than 90% of the covered area was undamaged. 3.2. PU coatings Coriaceous urethane rubber coatings can be used to protect the equipment which suffered serious impact, collision or friction wear. The PU coatings can be brushed on the steel surface to be protected, forming the 1.27 mm thick layer. After curing, a smooth, hard (shore hardness 86 A) and flexible surface is formed, and it’s resistant to the impact and wear. After being used for two years, the PU coatings of Warman pump which was used for transporting FGD slurry in a power plant, arose problems of wear and perforation, as shown in Fig. 7. PU coatings can withstand the strong corrosive medium and severe impact, and can serve successfully on easily accessible surfaces. However due to the rubber material’s inherent physical limitation, it soon come up against its duty limits in its use in desulfurization pump of power plant. Because the coating will not adequately adhere to the metal substrate and is easy to be peeled off, especially when the work temperature is high. 4. Study on cavitation erosion resistance of coatings 4.1. Experimental method and materials On the basis of their respective organic binder phases, the coatings can be assigned to two categories: epoxy resin group and PU group. To improve the wear resistance, under the condition of vacuum, epoxy resin (as the matrix) was mixed with 50–80% hard-ceramic particles of alumina oxide, silicon carbide or the like, and this kind of coating was consequently called ceramic coating, as shown in Fig. 8(c). Results of investigation on anti-erosion performance of coatings deposited on alloy steel by means of brushing method are presented next. Cavitation erosion tests were performed on the ultrasonic cavitation test rig to explore the coatings’ cavitation erosion resistance according to ASTM G32-10 test method [16], as shown in Fig. 8. The cavitation intensity was kept constant. Three kinds of coatings were investigated in distilled water under the same cavitation intensity. Epoxy coatings with thicknesses between 100 lm and 400 lm were brushed on alloy steel substrate. The evaluation parameter of cavitation erosion resistance is the cumulative mass loss versus exposure time.
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(a) Warman 700GSL pump
(b) Convex protrusion
(c) Outlet protrusion
(d) Annular region protrusion
Fig. 7. Wear problems of PU coatings on Warman pump.
(b) PU coatings
(c) Ceramic coatings (Resto SiC Fine Bead) (a) Ultrasonic erosion test apparatus
(di) Before cavitation erosion
(dii) 100 μm thick, after 3 h
(diii) 400 μm thick, after 5 h
(d) Photos of Epoxy resin coatings before and after cavitation erosion Fig. 8. Ultrasonic erosion test apparatus and tested coatings.
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N. Qiu et al. / Engineering Failure Analysis 55 (2015) 208–223 Table 2 Material chemical composition and mechanical properties. Tested materials
420 4135 316 Ceramic coating Epoxy resin coating PU coating Tested materials
Chemical composition (%) C
Si
Mn
0.18 0.37 0.033
0.66 0.23 0.38
0.81 0.002 0.62 0.004 1.04 0.007 Epoxy resin (Organic Binder Epoxy resin Polyurethane
P
Cr
0.004 0.003 0.006 Base) + Al2O3
Ni
12.1 0.97 16.3 10% + SiC 60%
0.23 0.008 10.4 (Fillers)
Mo
Cu
0.019 0.17 2.01
0.11 0.012 0.23
Mechanical properties Elongation d (%)
420 4135 316 Ceramic coating Epoxy resin coating PU coating
S
17 15 57.5
Yield point
Reduction of area
rs (MPa)
u (%)
722 973 258
59 58 80
Tensile strength rb (MPa)
Compressive strength (MPa)
Hardness
862 1078 575 26.2 – 24.138
– – – 98.8 123 –
263 (HB) 285 (HB) 155 (HB) Shore D 90 Shore D 90 Shore A 86
The physical properties of the coatings investigated in the cavitation erosion experiments are shown in Table 2. 4.2. Cavitation erosion test results Microscopic study can reveal the micro-undulation of the coating-substrate system. SEM images of epoxy resin coating after cavitation erosion are shown below in Fig. 9. Fig. 9 shows the comparison of SEM images of Epoxy resin coatings and Stainless Steel 1.4539 (DIN) after cavitation erosion. The worn surface of Epoxy resin exposed under the same cavitation intensity shown in Fig. 9(e) possesses deep micro cuts compared to the stainless steel surface shown in Fig. 9(f). The Epoxy resin coating’s surface is seriously damaged even after 30 min, while stainless steel is still in its incubation period after 1 h, which shows no measurable mass loss, and only a few erosion pits can be observed on Stainless Steel surface. The relationship between materials’ cumulative mass loss versus exposure time is established in Fig. 10. For purposes of comparison, the cavitation erosion curves of stainless steels 1.4539 (DIN), 1.4571 (DIN) and gray cast iron JL1040 (DIN) are included. The layer thicknesses of epoxy resin coatings were 100 lm, 200 lm and 400 lm. The erosion rate of thicker coating is in accordance with the so-called ‘‘running-in effect’’, i.e. the initially high wear rate gradually decreases in the course of the test, and eventually reaches a constant value. The thinner coating shows no such behavior, and maintains its high initial rates of wear through the entire erosion process. This means that thinner coating offers less protection. The results indicate, the adhesion and thickness are factors that mainly responsible for a long accumulating period and higher anti-erosion performance. During erosion, more and more micro-particles of coating are torn-off, until the substrate surface is revealed. Further cavitation load led to the separation of large pieces of coating from the parent material. A sudden increase of the mass loss rate is due to the breaking off of the large pieces of coating layer. The same kind of trend is also observed in case of the epoxy resin coating, as seen in Fig. 8(d). The photos of epoxy resin coatings before experiment and after they are damaged by cavitation erosion are displayed. The thinner coating soon comes up against its duty limit, when it is exposed to cavitation damage, and is penetrated completely after 1 h. The performance of thicker coating is better, and it is not peeled off after 5 h, protecting the base material. The Epoxy resin erosion-resistance performance can hardly be comparable to those of stainless steels. The mean erosion depths of coatings are much larger than that of un-coated steels, as shown in Fig. 11. Epoxy powder bonded on the preheated steel surface was heated and melted, and further covers the whole steel surface to form a continuous thermosetting polymers. Because of the uniform size of crystal grains, material internal relative movement hardly happens. Epoxy resin is a good binder has a strong resistance to chemical corrosion. However experimental evidence and consideration of cavitation theory suggest that the released energy associated with bubble collapse result in the generation of localized significantly high temperature on the coating surface. Any heating of the coating would lower the yield stress of the material. Not only will it influence the strength of the internal coating but may also initiate micro-cracking leading to the coating removal. The possibility of plastic response to cavitation impact due to increased surface temperature may inadvertently lead to greater lateral spallation. That erosion occurs by progressive damage process that involves: accumulation of plastic strain, fracture of individual grains and cracking between grains. Fig. 12 to some extent showed how the ceramic coating was formulated. We can see lots of bright particles of ceramic material in the matrix.
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(a) Epoxy resin before cavitation erosion, x400
(c) Epoxy resin (20min), 1.7 mg loss, x100
(e) Epoxy resin (30min), 7.8 mg, x1000
(b) Epoxy resin (10min), 0.4 mg loss, x100
(d) Epoxy resin (30min), 7.8 mg loss, x100
(f) Stainless steel 1.4539 (1h), 0 mg, x1000
Fig. 9. SEM images of epoxy resin coating and stainless steel after cavitation erosion.
Fig. 10. Cumulative Erosion Mass Loss–Time Curve of coatings and steels.
Although ordinary ceramic coating has excellent resistance to corrosion and wear, but it dose not has sufficient strength to bear the repeating impact generated by ultrasonic oscillator. Epoxy resin that was mixed with the ceramic particles can
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(a) 30min, 3.6mg loss, 46 μm depth, x20
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(b) 2.5h, 25.3 mg loss, 157 μm depth, x20
Fig. 11. Surface profile of Epoxy resin coatings after erosion.
reduce the problem that pure ceramic material is brittle. The soft epoxy resin was first to erode away, leaving behind the exposed ceramic particles. Not until the hard particles were vibrated free did the anti-erosion effect fail. It might be expected that the bonding force applied by the ceramic particle would decrease as a result of its shedding. This in turn would result in more material being displaced during impact. The greater degree of deformation would create an increase in the driving forces for lateral cracking during the unloading cycle. And the generated pores in the coating surface may act as stress raisers that facilitate crack initiation. The mechanism by which material is removed from a surface upon cavitation erosion attack can be either ductile or brittle. Fig. 13 illustrates the deformation and loss of PU coating. The higher ductility, coupled with the energy absorption capacity, has made erosion more difficult. As a result, the repeated impact by bubble collapse generate very small pits. However local thermal cycling when bubble collapses may assist the formation and propagation of shear force between the coating and subsurface, and very high levels of shear strain may be induced at the bubble collapse location, thereby reducing the resistance to cavitation impacts. More material loss could thus be a consequence. When the shear strain exceeds the elastic strain limit of the PU coating, the surface was penetrated as seen in Fig. 13. Finally many voids and pits formed. Fig. 14(a) and (b) display the depths of pits in different kinds of steels after cavitation erosion for several hours. After 1 h, the epoxy resin coating with a thickness of 100 lm is penetrated, while the stainless steel 420 (ASTM), only has small pits with 1.431 lm depth. Erosion depth of stainless steel 420 is 6.919 lm after 6 h, and erosion depth of alloy steel 4135 (ASTM) after 10 h is 7.612 lm. Fig. 14(c) and (d) shows the cavitation erosion depths of stainless steel 316 (ASTM) and alloy steel
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(a) 30 min, 23.1 mg loss,150 μm depth, x20
(b) 1 h, 37.9 mg loss, 220 μm depth, x20
Fig. 12. Surface profile of Ceramic coatings after erosion.
4135 (ASTM) after 30 h of erosion. For these kinds of steel, even after 30 h, the erosion depths are only 194.4 lm and 88.8 lm for 316 and 4135, respectively. Cavitation erosion rate is considered to be dependent on the shear strain resulting from the bubble collapse. These will, in general, be greater for brittle coating materials than steels, owing to the much lower hardness presented by the former. Among the various cavitation erosion modes, ductile tearing together with plastic deformation have been claimed to be the main reason for the removal process of stainless steels. Cavitation erosion of stainless steel, which belongs to alloys with low stacking fault energy, begins from forming slip bands, plastic deformation associated with phase transformations. Stainless steel is clearly being eroded due to fatigue after suffering strong plastic deformation and to a lesser extent from particle removal. On the contrary, the coatings display predominantly brittle behavior with the breaking out of spray particles. For stainless steels and alloy steels, better correlations were found between the materials’ hardness, where plastic deformation was a major mechanism. Sometimes the greater the hardness of the steel, the more difficult it is for the bubble collapse to generate pits on exposed surface and the lower is the erosion rate. In this case as the hardness increases the depth of penetration decreases. However a complicated relationship exists between the material’s mechanical properties and the erosion rate.
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2 h, 6.3 mg loss, 34 μm depth, x20 Fig. 13. Surface profile of PU coatings after erosion.
5. Practical applications of coatings for erosion and corrosion protection 5.1. Ceramic coating case study for cavitation erosion resistance As shown in Fig. 15, cavitation erosion problem became serious again after only half a year. Coating’s ability to reduce cavitation erosion was very limited, coating only mitigated some of cavitation erosion, or used to repair the worn out holes. Cavitation erosion occurred by the accumulation of fracture and removal of single splats of ceramic coating. The weak bonding of underlying coating would directly lead to fracture in the repaired region. The cavitation impact damage, which consists of surface spalling like the ones seen in Fig. 8(d), is attributable to the combined effect of crack formation in cavitation region which becomes the source of material degradation. Propagation of the initial cracks occurs during subsequent attack by cavitation cloud collapse, and may lead to the removal of ceramic coating. The coating gradually loses its adhesion, and when pressure fluctuates, the adhered coating would be peeled off. Method of embedding austenitic stainless steel liner can resist cavitation erosion better but it requires complicated technology and the cost is very. The method is suitable for important pump station. Repair method of brushing ceramic composite coating is much simpler. For the local vortex cavitation such as cavitation in static guide vane of vertical pump or cavitation arose at local convex surface, after being coated by ceramic coating, the surface can become smooth. As a result the local cavitation would not appear, and the surface remains intact for a long time. 5.2. Ceramic coatings case study for corrosion resistance The vertical circulation pump of KSB corporation was adopted to circulate the cooling water in a power plant. After being used for a year, components partially exposed to the sea water suffered corrosion damage. It was important to find the best technology to repair the damaged components, restore the device to the original geometry, and ensure the repaired components have excellent anti-corrosion performance. Corrosion status of diffuser is shown in Fig. 16. Paintings on the blade edge peeled off, resulting in the corrosion damage. A certain degree of corrosion appeared on the 11 blades. Painting on the diversion bottom was also peeled off. Corrosion damages appeared at 8 positions. The repair steps were as follows. First of all, undergo the sandblasting process, and then clean the surface with acetone to clear grease and dirt away. The particle granularity used for sand blasting process was about 8–16 mesh. The corundum sand was adopted for blasting hard alloy metal. All steel surfaces to be coated must be blast cleaned to a standard of white metal blast (SSPC-SP5) or near-white metal blast (SSPC-SP10) due to exceptionally severe condition that the diffuser was constant immersed in chemical liquid. After sand blasting process, the pothole depth of the damaged region reached between 2 and 6 mil. Then the damaged components of impeller were repaired with Resto SiC Putty, by filling the potholes and restoring the corroded surface to the original profile. After 4 h, the Resto SiC Putty gelled initially, and then was brushed with the Resto Ceram-Coat Red as a primer coating, and after it gelled (4 h), brushed with Resto Ceram-Coat Blue as an outer coating. After all the process were completed the coatings required 24 h to fully gel (see Fig. 17). Compared with the data before repair, the efficiency and pressure of coated circulating pump was significantly improved. Neglecting the uncertainty generated by the electric current fluctuation, the efficiency test results of circulating pumps
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(a) 420 (1 h) 0.1 mg loss, 1.431 μm depth
(c) 316 (30 h), 71 mg loss, 194.4 μm depth
(b) 4135 (10 h), 5.9 mg loss, 7.612 μm depth
(d) 4135 (30 h), 41.9 mg loss, 88.8 μm depth
Fig. 14. Cavitation erosion depth of stainless steel and alloy steel after several hours.
showed that the efficiency was promoted by 6%, contributed from the smooth surface of anti-corrosion coating, as shown in Table 3. After being used for 3 years, the repaired guide blades were in good condition, proving a satisfying anti-corrosion performance. In addition, after the dust on the surface was wiped, the surface was bright as new, showing that the smooth coating also had a function of preventing scaling. Although coating under cavitation load could not effectively resist the impact and was tapped into holes, the efficiency of the pump was improved (Table 3, increased by 6%). This is because the ceramic surface is smooth, therefore the hydraulic loss is reduced. The pump efficiency, especially for the old pump, returned to its original design efficiency, due to the smoothed surface, which could remedy the hydraulic loss caused by cavitation. So metal material will no longer be the only choice for chemical pump and pipeline accessories.
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Fig. 15. Cavitaion erosion problems arose again after half a year operation for ceramic coatings.
Fig. 16. Corrosion problems of the diffuser after one year operation.
From economic considerations, the client will tend to adopt the method that repairing the old pump by brushing coating. It costs much less than purchasing a new impeller made of stainless steel. At the same time, the coating can also solve the problem of corrosion for the cast steel impeller. In the cavitation erosion tests, PU coating showed slow expansion in the pit area because of the characteristics of elastomer materials. The ceramic coating’s characteristic resulted in a rapid expansion of the erosion area after damage began. However ceramic coating is a proven slurry and corrosion protection material. Ceramic coatings are used for slurry pump liners and impellers where conditions are not suited to PU coating, such as with coarse or sharp edged particles, or on duties having high impeller peripheral velocities or high operating temperatures. Rapid in-situ lining is generally not possible for PU coating. When repairs need to be carried out, quit often epoxy-ceramic products are used due to the ease of application. The cost of PU coating is 2–5 times of the ceramic coating. Because the
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(a) Sand blasting process
(b) Brush coatings
(c) Reassemble after repair
(d) Conditions after 3 years
Fig. 17. Repair processes of corroded vertical circulation pump and conditions after 3 years operation.
Table 3 Efficiency data of repaired vertical circulation pump. Equipment
Uncoated circulating pump 1
Coated circulating pump
Uncoated circulating pump 2
Voltage change (%) Electric current change (%) Efficiency change (%)
+1.5 0.7 +2.2
+8.3 0 +8.3
+2.4 0 +2.4
application process is complicated, skilled workers are required for repairing. Moreover, the final outer surface of the ceramic coating was smoother than the PU coating’s, which can result in a higher efficiency. Of course, replacing PU coating by ceramic coating is a questionable approach and not generally to be recommended especially when cavitation problem is serious. Ceramic coatings are resistant to low frequency of impact and they are the most suitable solution to sand erosion. On the other hand, PU coatings have the ability to absorb high frequency impingements hence being adequate for cavitation, but their toughness is less appropriated to sustain sharp edged solids. Therefore, it is required to develop coatings that are able to resist both kinds of damage. Unfortunately, coatings developed to resist particle impact are weak to withstand cavitation collapse. The procedure of selecting the type of coating must account for the constraints and should take into account which factor should be put into the first place. 6. Conclusions (1) Ceramic coatings, which are mainly composed of silicon carbide and polymer, showed excellent slurry-resisting performance, and the slurry-resistance is better than that of stainless steel 304, cast iron JM3 and high chrome alloy steel Cr30A. (2) The resistance of coating to cavitation erosion varies widely with its thickness, and the thicker coating showed better performance. The coatings’ erosion-resisting performances are inferior to that of gray cast iron JL1040, and hardly comparable to stainless steels. Therefore, excellent wear-resisting performance cannot guarantee any good erosion-resisting performance. (3) A good coating is the one that is able to withstand the load and not flake off, consequently protecting the base material until the next maintenance. Among the investigated coatings, the cavitation erosion rate of PU coatings is lowest of all. Only for very low cavitation exposure can PU coatings offer a certain degree of improvement in comparison with unprotected gray cast iron surfaces. (4) When impeller made of gray cast iron is exposed to slightly corrosive fluids, coatings can be the solution to withstand corrosion, as long as they keep the underlying materials from corrosive fluids. If the load exposure is purely cavitation erosion, coatings make little sense, and all coatings fail rapidly. Because coatings peel down more quickly than the underlying steel.
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(5) The pumping of slightly corrosive fluids frequently leads to erosion–corrosion damage on impellers. Corrosion can further amplify the erosion process. Materials with higher resistance to corrosion can solve ‘‘erosion problems’’ that, in reality, are corrosion problems. At the meantime, the machinery efficiency may be improved because of the smooth coating surface.
Acknowledgments This research was conducted under a project funded by KSB-Stiftung in Germany. The authors are grateful to Prof. Hellmann (KSB A.G.) and Prof. Anja Dwars (Georg Simon Ohm Hochschule, Nürnberg) for their experimental assistants. They are also very grateful to Dr. Alexander Böhm (KSB A.G.) and Prof. Yulin Wu (State Key Laboratory of Hydroscience and Engineering, Tsinghua University) for many fruitful discussions. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Franc JP, Michel JM. Fundamentals of cavitation. Kluwer Academic Publishers; 2004. Reisman GE, Wang YC, Brennen CE. Observations of shock waves in cloud cavitation. J Fluid Mech 1998;120:719–27. Dorji Ugyen, Ghomashchi Reza. Hydro turbine failure mechanisms: an overview. Eng Fail Anal 2014;44:136–47. Laguna-Camacho JR, Lewis R, Vite-Torres M, Méndez-Méndez JV. A study of cavitation erosion on engineering materials. Wear 2013;301:467–76. Krella A, Czyzniewski A. Cavitation erosion resistance of Cr–N coating deposited on stainless steel. Wear 2006;260:1324–32. Cheng FT, Kwok CT, Man HC. Laser surfacing of S31603 stainless steel with engineering ceramics for cavitation erosion resistance. Surf Coat Technol 2001;139:14–24. Cheng Feng, Jiang Shuyun. Cavitation erosion resistance of diamond-like carbon coating on stainless steel. Appl Surf Sci 2014;292:16–26. Oliver WC, Pharr GM. An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. J Mater Res 1992;7:1564–83. Watanabe S, Furukawa A. Theoretical investigations on thermodynamic effect of cavitation. Turbomachinery 2006;36(3):26–33. Hattori Shuji, Taruya Keisuke, Kikuta Kengo, Tomaru Hiroshi. Cavitation erosion of silver plated coatings considering thermodynamic effect. Wear 2013;300:136–42. Thiruvengadam A. A unified theory of cavitation damage. Trans ASME J Basic Eng 1963;85:365–76. Krella A, Czyzniewski A. Influence of the substrate hardness on the cavitation erosion resistance of TiN coating. Wear 2007;263:395–401. Krella A. The influence of TiN coatings properties on cavitation erosion resistance. Surf Coat Technol 2009;204:263–70. Hattori Shuji, Ishikura Ryohei. Revision of cavitation erosion database and analysis of stainless steel data. Wear 2010;268:109–16. Hattori Shuji, Itoh Takamoto. Cavitation erosion resistance of plastics. Wear 2011;271:1103–8. ASTM Designation. Annual book of ASTM standards, G32-10; 2010.
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Research on cavitation erosion and wear resistance performance of coatings Ning Qiu a,⇑, Leqin Wang a, Suhuan Wu b, Dmitriy S. Likhachev a a b
Institute of Process Equipment, Zhejiang University, Hangzhou 310027, China Shanghai BondPoly Engineering Material & Technology Co. Ltd., Shanghai 201601, China
a r t i c l e
i n f o
Article history: Received 10 December 2014 Received in revised form 20 May 2015 Accepted 8 June 2015 Available online 15 June 2015 Keywords: Cavitation erosion Wear resistance Coatings Micro structure Abrasive jet
a b s t r a c t Depending on the nature of the working medium and working conditions, corrosive and cavitation damage shall arise to pump’s components. In industrial applications the corrosion-reducing coatings are sprayed on hydraulic components. But it is questionable whether such products actually do help under wear or cavitation loads or not. Abrasive jet wear tests were carried out to determine the wear resistance of coating materials: polymers and ceramics, cast iron, and steel of various types. The samples were loaded for five hours, and finally the wear depth was measured as a determining indicator of the sample’s wear resistance. Results of investigation on anti-erosion performance of epoxy resin, ceramic and Polyurethane (PU) coatings brushed on alloy steel surface were also presented. Cavitation erosion tests were performed on the ultrasonic rig. The mass loss and surface morphology of the specimens were examined by balance analysis and 3-D laser microscopy, respectively. The investigations showed excellent wear-resisting performance of ceramic coatings, which is better than wear-resistance of stainless steel, cast iron and high chrome alloy steel. But the excellent wear-resisting performance could not guarantee a good erosion-resisting performance. The ceramic coatings’ anti-erosion performances were inferior to that of gray cast iron, and hardly comparable to those of stainless steels. The basic factors that influenced coating’s cavitation erosion endurance were adhesion and thickness of coatings. Analysis of coating’s degradation mechanism showed that PU coatings could withstand longer incubation period thus enhancing the materials’ cavitation erosion resistance. Several practical cases were analyzed, showing some guidance for coatings’ application. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Pressure decrease in fast flowing liquid is the main reason for cavitation phenomenon. The dissolved gas is the source of cavitation nuclei [1]. The dynamic load at the moment when cavitation bubble collapses lasts for a few microseconds or nanoseconds [2]. Alloys are usually used in process equipment and hydraulic machinery. Poor cavitation erosion resistance of impellers’ material poses a serious obstacle for its application in hydraulic machinery [3]. High wear applications cannot be avoided, when hydraulic machinery has to be subjected to heavy static or dynamic impact loads. Materials’ response to static
⇑ Corresponding author. Tel.: +86 13735578065; fax: +86 571 87952406. E-mail address: [email protected] (N. Qiu). http://dx.doi.org/10.1016/j.engfailanal.2015.06.003 1350-6307/Ó 2015 Elsevier Ltd. All rights reserved.
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conditions is different from response to dynamic conditions. Under dynamic impact loads, as during cavitation, materials’ resistance to deformation is lower [4]. Over the last few decades, considerable efforts have been devoted to enhance the erosion resistance of steel by depositing protective coatings on its surface [5–7]. The coatings can be easily formed on the surface of steel, which can exhibit smooth surface and good mechanical properties, such as high hardness, high elastic modulus or high oxidation resistance [8]. Many fluid machinery components are supplied with coatings to prolong their lifespan and to improve their work efficiency. However, when bubble collapses on the coating’s surface, a high temperature may be achieved, which could influence the coatings’ properties, for example makes coatings more ductile [9]. Thousands of pressure pulses act on the surface when bubbles collapse. The coating may undergo local thermal softening, which has an essential influence on the coatings’ deformation. The thermal mismatch between the coating and substrate may also result in adhesion failure [10]. Coatings with weak adhesion could soon be peeled off. If the coatings can avoid adhesive fracture, they protect the substrate from corrosive fluid. Adhesion plays an important role in the incubation period and ensures the protection of the substrate material against mass loss. The outstanding results observed in any given coating can be attributed to the strong adhesive connection. When a coating is very ductile, it is easy to cause the dislocation movement and peeling off of the coating [11]. In addition, cavitation erosion investigations proved that the cavitation erosion resistance of conventional materials (stainless steel, etc.) depends on their mechanical parameters (hardness, Young’s modulus, tensile strength, and fatigue strength) [12–15]. More energy is needed to be absorbed before fracture, if the hardness and Young modulus are high. Delamination may also happen due to the large difference in hardness and Young’s modulus between the substrate and coating. Taking the above statements into consideration, the main aim of this work was to reveal the factors which were responsible for the resistance of coatings. These factors were combination of the properties that were essential in conditions of dynamic impact. And the secondary aim was to find coatings with higher wear-resistance and higher erosion-resistance. 2. Experiment on wear resistance 2.1. Experimental method and materials In order to simulate the wear of FGD (flue gas desulfurization) slurry pump, abrasive jet wear tests were carried out to determine the wear resistance to solid particle attack of several coating materials, cast iron, and other steels. With a view to comparing the materials’ resistance, blast wear experiments were performed under the same impact intensity. And wear problem of most slurry pump increases significantly when the linear velocity is greater than 15 m/s. (Wear is serious for a general slurry pump: the rotating speed is 750 r/min, the impeller diameter is 400 mm, and the linear velocity is 3.14 0.4 750/60 = 15.7 m/s). Solid particles in pumped FGD slurry are often 100–200 grade mesh. Because the large particles are filtered or precipitated in the pre-treatment process. The upper limit of particle concentration of FGD slurry pump is 60%. So the test conditions are as follows: The samples were buffeted by slurry jet of high concentration (60%, 1.3 kg/l) and high speed. The slurry was a mixture of high hardness quartz sand (particle size: 100–200 grade mesh, 0.7–1.5 mm grain’s size) and water, which was sprayed onto the specimens at a velocity up to 18 m/s and an impinging angle of 90°, as shown in Fig. 1. The samples were 3 mm thick. The impact lasted for 5 h, and finally the wear depth was measured as an indicator to determine the sample’s wear resistance. The samples’ materials are shown in Table 1. 2.2. Experimental results After the samples were buffeted by slurry for 5 h, the sample surfaces were worn as shown in Fig. 2. The stainless steel sample (304) was penetrated after 2 h. The wear depth is shown in Fig. 3. As shown in the test results above, the wear resistance of Resto SiC Fine Bead, which is a kind of ceramic coating, is the best and even better than wear-resistant steel Cr30A. Stainless steel 304 is the worst because of its low hardness. The wear resistance of PU coating is comparable to those wear-resistant steels. 3. Cases of wear-resistant coatings’ practical application 3.1. Ceramic coatings Circulating pump transporting flue gas desulfurization (FGD) slurry in a power plant in Shanghai, is one of the main equipment in FGD section. It is responsible for transporting the lime slurry into the absorption tower spraying acidic gas such as SO2 (sulfur dioxide). The transported medium is the lime slurry, which contains solid particles with a concentration of 15– 20%. The main ingredients are CaSO4, CaCO3 and a small amount of high hardness particles, such as SiO2 and Al2O3. Particle size of solid powder is around 300 grade mesh. The liquid is acidic, and pH ranges from 4 to 6, and the temperature ranges from 45 to 60 °C. Concentration of chloride ions (Cl ) is below 20,000 ppm. Particles such as SiO2 and Al2O3 are the ballast
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(a) Schematic diagram of abrasive jet wear test
(b) Sample
(c) Spray chamber
(d) Quartz slurry
Fig. 1. Wear-resistance test apparatus.
Table 1 Materials in wear-resistance experiment. Tested materials
Chemical composition (%) C
304 0.042 JM3 2.0 Cr30A 1.6 A49 0.6 Sintering Ceramic Brick
Si
Mn
S
P
Cr
0.4 1.0 0.9 1.0
1.12 0.6 1.6 1.1
0.003 0.03 0.05 0.04
0.034 18.03 0.04 16.0 0.03 30 0.03 28 Al2O3 P 90%
Ni
Mo
Cu
8.01 1.3 2.0 3.0
– 0.3 1.9 2.5
– 0.9 1.5 2.0
Hardness
Thickness
HRC 35 HRC 50 HRC 55 HRC 58 HRA 60
3 mm
Polymer coatings
Organic binder base
Fillers
Hardness
Coating thickness
ARC 897 Belzona 1821 EB 206 Resto SiC Fine Bead Devcon 11470 Loctite 98383 Metaline 785
Epoxy Resin P20%
Al2O3 70% Al2O3 72% Al2O3 45% + SiC 20% Al2O3 10% + SiC 60% Al2O3 65% Al2O3 60% + SiC 5% Pigment
Shore Shore Shore Shore Shore Shore Shore
3 mm
Polyurethane
D 85 D 85 D 80 D 90 D 85 D 85 A 82
by-product of limestone grinding, which don’t participate in the chemical reaction of acid gas absorbing. Thus it will accumulate during recirculation, causing the direct wear on the metal surfaces of pump. The material of the front shroud of slurry circulation pump (Sulzer ZAP701) is the precipitation-hardening duplex stainless steel (ASTM 890 5A), which is coated by high temperature sprayed tungsten carbide 1 mm thick coating. After being used for a certain time, small cracks came off tungsten carbide coating, stripping the stainless steel surface. The unprotected stainless steel worn down quickly by the impact of lime slurry hard particles. The lime slurry continued to hollow the metal base under the hard coating. Consequently the tungsten carbide coating was peeled off because of the loss of base material (stainless steel). After the coating was peeled off, the metal base worn down more quickly. After 12 months operation, the front shroud of slurry circulation pump was seriously worn out, as shown in Fig. 4.
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Stainless steel (304)
High-chromium Cast Iron (JM3)
Ceramic coating (Ebond 206)
Protective Agent (Devcon 11470)
Protective Agent (Loctite 98383)
Ceramic coating (ARC 897)
Ceramic coating (Belzona 1821)
Polyurethane (Metaline 785)
Wear-resistant steel (A49)
Sintering Ceramic
Ceramic coating (Resto SiC Fine Bead)
Wear-resistant steel (Cr30A)
211
Fig. 2. Worn specimens after 5 h.
The worn components were repaired and the coatings were lined by Shanghai BondPoly Engineering Material & Technology Co. Ltd. The solution was to repair the eroded surface with Resto products (ceramic coatings). SiC Brushable agent is mainly composed of silicon carbide and polymer. After being gelled under normal temperature, a smooth and bright surface formed. SiC Brushable, can be independently coated on metal surface, providing the anti-corrosion effect. It can also be used as an outer coating after the damaged surface was repaired by SiC Fine Bead or SiC Putty. The repair processes are as follows (see Fig. 5): After the front shroud of slurry circulation pump was repaired with Resto products and returned to service for 10 months, the conditions of coatings was as shown in Fig. 6. The photos of long term operation results were taken by the authors. After the front shroud was used for 10 months, thinner coating layer (about 0.5 mm thick) of SiC Putty and SiC Brushable were worn out, leaving the surface of SiC Fine Bead (consisting of small particles of silicon carbide) exposed. The SiC Fine
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Fig. 3. Wear depths of samples.
Fig. 4. Worn front shroud of slurry circulation pump after 12 months operation.
(a) Resto ceramic coating layers
(b)
(1)
(2)
(3)
(4)
Fig. 5. The repair processes: (1) Brush with Primer ceramic; (2) repair potholes with SiC Fine Bead; (3) flatten the surface with SiC Putty; (4) brush the SiC Brushable outer coating.
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(a) 10 months later
(b) 20 months later
(c) 36 months later
Fig. 6. Conditions of coatings after pump operation.
Bead coating was intact, proving that it had excellent wear-resisting performance. The customer was very satisfied, and evaluated the SiC Fine Bead (ceramic coating) much better than the Loctite products that were used before. After the actual application for 10 months, the maximum pitting depth of pump case area without coating reached 3 mm. More than 95% of the area covered with Resto SiC coating was intact. After 20 months, the maximum depth reached 5 mm, and more than 95% of the covered area was intact. After 36 months, the maximum depth reached 10 mm, and more than 90% of the covered area was undamaged. 3.2. PU coatings Coriaceous urethane rubber coatings can be used to protect the equipment which suffered serious impact, collision or friction wear. The PU coatings can be brushed on the steel surface to be protected, forming the 1.27 mm thick layer. After curing, a smooth, hard (shore hardness 86 A) and flexible surface is formed, and it’s resistant to the impact and wear. After being used for two years, the PU coatings of Warman pump which was used for transporting FGD slurry in a power plant, arose problems of wear and perforation, as shown in Fig. 7. PU coatings can withstand the strong corrosive medium and severe impact, and can serve successfully on easily accessible surfaces. However due to the rubber material’s inherent physical limitation, it soon come up against its duty limits in its use in desulfurization pump of power plant. Because the coating will not adequately adhere to the metal substrate and is easy to be peeled off, especially when the work temperature is high. 4. Study on cavitation erosion resistance of coatings 4.1. Experimental method and materials On the basis of their respective organic binder phases, the coatings can be assigned to two categories: epoxy resin group and PU group. To improve the wear resistance, under the condition of vacuum, epoxy resin (as the matrix) was mixed with 50–80% hard-ceramic particles of alumina oxide, silicon carbide or the like, and this kind of coating was consequently called ceramic coating, as shown in Fig. 8(c). Results of investigation on anti-erosion performance of coatings deposited on alloy steel by means of brushing method are presented next. Cavitation erosion tests were performed on the ultrasonic cavitation test rig to explore the coatings’ cavitation erosion resistance according to ASTM G32-10 test method [16], as shown in Fig. 8. The cavitation intensity was kept constant. Three kinds of coatings were investigated in distilled water under the same cavitation intensity. Epoxy coatings with thicknesses between 100 lm and 400 lm were brushed on alloy steel substrate. The evaluation parameter of cavitation erosion resistance is the cumulative mass loss versus exposure time.
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(a) Warman 700GSL pump
(b) Convex protrusion
(c) Outlet protrusion
(d) Annular region protrusion
Fig. 7. Wear problems of PU coatings on Warman pump.
(b) PU coatings
(c) Ceramic coatings (Resto SiC Fine Bead) (a) Ultrasonic erosion test apparatus
(di) Before cavitation erosion
(dii) 100 μm thick, after 3 h
(diii) 400 μm thick, after 5 h
(d) Photos of Epoxy resin coatings before and after cavitation erosion Fig. 8. Ultrasonic erosion test apparatus and tested coatings.
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N. Qiu et al. / Engineering Failure Analysis 55 (2015) 208–223 Table 2 Material chemical composition and mechanical properties. Tested materials
420 4135 316 Ceramic coating Epoxy resin coating PU coating Tested materials
Chemical composition (%) C
Si
Mn
0.18 0.37 0.033
0.66 0.23 0.38
0.81 0.002 0.62 0.004 1.04 0.007 Epoxy resin (Organic Binder Epoxy resin Polyurethane
P
Cr
0.004 0.003 0.006 Base) + Al2O3
Ni
12.1 0.97 16.3 10% + SiC 60%
0.23 0.008 10.4 (Fillers)
Mo
Cu
0.019 0.17 2.01
0.11 0.012 0.23
Mechanical properties Elongation d (%)
420 4135 316 Ceramic coating Epoxy resin coating PU coating
S
17 15 57.5
Yield point
Reduction of area
rs (MPa)
u (%)
722 973 258
59 58 80
Tensile strength rb (MPa)
Compressive strength (MPa)
Hardness
862 1078 575 26.2 – 24.138
– – – 98.8 123 –
263 (HB) 285 (HB) 155 (HB) Shore D 90 Shore D 90 Shore A 86
The physical properties of the coatings investigated in the cavitation erosion experiments are shown in Table 2. 4.2. Cavitation erosion test results Microscopic study can reveal the micro-undulation of the coating-substrate system. SEM images of epoxy resin coating after cavitation erosion are shown below in Fig. 9. Fig. 9 shows the comparison of SEM images of Epoxy resin coatings and Stainless Steel 1.4539 (DIN) after cavitation erosion. The worn surface of Epoxy resin exposed under the same cavitation intensity shown in Fig. 9(e) possesses deep micro cuts compared to the stainless steel surface shown in Fig. 9(f). The Epoxy resin coating’s surface is seriously damaged even after 30 min, while stainless steel is still in its incubation period after 1 h, which shows no measurable mass loss, and only a few erosion pits can be observed on Stainless Steel surface. The relationship between materials’ cumulative mass loss versus exposure time is established in Fig. 10. For purposes of comparison, the cavitation erosion curves of stainless steels 1.4539 (DIN), 1.4571 (DIN) and gray cast iron JL1040 (DIN) are included. The layer thicknesses of epoxy resin coatings were 100 lm, 200 lm and 400 lm. The erosion rate of thicker coating is in accordance with the so-called ‘‘running-in effect’’, i.e. the initially high wear rate gradually decreases in the course of the test, and eventually reaches a constant value. The thinner coating shows no such behavior, and maintains its high initial rates of wear through the entire erosion process. This means that thinner coating offers less protection. The results indicate, the adhesion and thickness are factors that mainly responsible for a long accumulating period and higher anti-erosion performance. During erosion, more and more micro-particles of coating are torn-off, until the substrate surface is revealed. Further cavitation load led to the separation of large pieces of coating from the parent material. A sudden increase of the mass loss rate is due to the breaking off of the large pieces of coating layer. The same kind of trend is also observed in case of the epoxy resin coating, as seen in Fig. 8(d). The photos of epoxy resin coatings before experiment and after they are damaged by cavitation erosion are displayed. The thinner coating soon comes up against its duty limit, when it is exposed to cavitation damage, and is penetrated completely after 1 h. The performance of thicker coating is better, and it is not peeled off after 5 h, protecting the base material. The Epoxy resin erosion-resistance performance can hardly be comparable to those of stainless steels. The mean erosion depths of coatings are much larger than that of un-coated steels, as shown in Fig. 11. Epoxy powder bonded on the preheated steel surface was heated and melted, and further covers the whole steel surface to form a continuous thermosetting polymers. Because of the uniform size of crystal grains, material internal relative movement hardly happens. Epoxy resin is a good binder has a strong resistance to chemical corrosion. However experimental evidence and consideration of cavitation theory suggest that the released energy associated with bubble collapse result in the generation of localized significantly high temperature on the coating surface. Any heating of the coating would lower the yield stress of the material. Not only will it influence the strength of the internal coating but may also initiate micro-cracking leading to the coating removal. The possibility of plastic response to cavitation impact due to increased surface temperature may inadvertently lead to greater lateral spallation. That erosion occurs by progressive damage process that involves: accumulation of plastic strain, fracture of individual grains and cracking between grains. Fig. 12 to some extent showed how the ceramic coating was formulated. We can see lots of bright particles of ceramic material in the matrix.
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(a) Epoxy resin before cavitation erosion, x400
(c) Epoxy resin (20min), 1.7 mg loss, x100
(e) Epoxy resin (30min), 7.8 mg, x1000
(b) Epoxy resin (10min), 0.4 mg loss, x100
(d) Epoxy resin (30min), 7.8 mg loss, x100
(f) Stainless steel 1.4539 (1h), 0 mg, x1000
Fig. 9. SEM images of epoxy resin coating and stainless steel after cavitation erosion.
Fig. 10. Cumulative Erosion Mass Loss–Time Curve of coatings and steels.
Although ordinary ceramic coating has excellent resistance to corrosion and wear, but it dose not has sufficient strength to bear the repeating impact generated by ultrasonic oscillator. Epoxy resin that was mixed with the ceramic particles can
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(a) 30min, 3.6mg loss, 46 μm depth, x20
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(b) 2.5h, 25.3 mg loss, 157 μm depth, x20
Fig. 11. Surface profile of Epoxy resin coatings after erosion.
reduce the problem that pure ceramic material is brittle. The soft epoxy resin was first to erode away, leaving behind the exposed ceramic particles. Not until the hard particles were vibrated free did the anti-erosion effect fail. It might be expected that the bonding force applied by the ceramic particle would decrease as a result of its shedding. This in turn would result in more material being displaced during impact. The greater degree of deformation would create an increase in the driving forces for lateral cracking during the unloading cycle. And the generated pores in the coating surface may act as stress raisers that facilitate crack initiation. The mechanism by which material is removed from a surface upon cavitation erosion attack can be either ductile or brittle. Fig. 13 illustrates the deformation and loss of PU coating. The higher ductility, coupled with the energy absorption capacity, has made erosion more difficult. As a result, the repeated impact by bubble collapse generate very small pits. However local thermal cycling when bubble collapses may assist the formation and propagation of shear force between the coating and subsurface, and very high levels of shear strain may be induced at the bubble collapse location, thereby reducing the resistance to cavitation impacts. More material loss could thus be a consequence. When the shear strain exceeds the elastic strain limit of the PU coating, the surface was penetrated as seen in Fig. 13. Finally many voids and pits formed. Fig. 14(a) and (b) display the depths of pits in different kinds of steels after cavitation erosion for several hours. After 1 h, the epoxy resin coating with a thickness of 100 lm is penetrated, while the stainless steel 420 (ASTM), only has small pits with 1.431 lm depth. Erosion depth of stainless steel 420 is 6.919 lm after 6 h, and erosion depth of alloy steel 4135 (ASTM) after 10 h is 7.612 lm. Fig. 14(c) and (d) shows the cavitation erosion depths of stainless steel 316 (ASTM) and alloy steel
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(a) 30 min, 23.1 mg loss,150 μm depth, x20
(b) 1 h, 37.9 mg loss, 220 μm depth, x20
Fig. 12. Surface profile of Ceramic coatings after erosion.
4135 (ASTM) after 30 h of erosion. For these kinds of steel, even after 30 h, the erosion depths are only 194.4 lm and 88.8 lm for 316 and 4135, respectively. Cavitation erosion rate is considered to be dependent on the shear strain resulting from the bubble collapse. These will, in general, be greater for brittle coating materials than steels, owing to the much lower hardness presented by the former. Among the various cavitation erosion modes, ductile tearing together with plastic deformation have been claimed to be the main reason for the removal process of stainless steels. Cavitation erosion of stainless steel, which belongs to alloys with low stacking fault energy, begins from forming slip bands, plastic deformation associated with phase transformations. Stainless steel is clearly being eroded due to fatigue after suffering strong plastic deformation and to a lesser extent from particle removal. On the contrary, the coatings display predominantly brittle behavior with the breaking out of spray particles. For stainless steels and alloy steels, better correlations were found between the materials’ hardness, where plastic deformation was a major mechanism. Sometimes the greater the hardness of the steel, the more difficult it is for the bubble collapse to generate pits on exposed surface and the lower is the erosion rate. In this case as the hardness increases the depth of penetration decreases. However a complicated relationship exists between the material’s mechanical properties and the erosion rate.
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2 h, 6.3 mg loss, 34 μm depth, x20 Fig. 13. Surface profile of PU coatings after erosion.
5. Practical applications of coatings for erosion and corrosion protection 5.1. Ceramic coating case study for cavitation erosion resistance As shown in Fig. 15, cavitation erosion problem became serious again after only half a year. Coating’s ability to reduce cavitation erosion was very limited, coating only mitigated some of cavitation erosion, or used to repair the worn out holes. Cavitation erosion occurred by the accumulation of fracture and removal of single splats of ceramic coating. The weak bonding of underlying coating would directly lead to fracture in the repaired region. The cavitation impact damage, which consists of surface spalling like the ones seen in Fig. 8(d), is attributable to the combined effect of crack formation in cavitation region which becomes the source of material degradation. Propagation of the initial cracks occurs during subsequent attack by cavitation cloud collapse, and may lead to the removal of ceramic coating. The coating gradually loses its adhesion, and when pressure fluctuates, the adhered coating would be peeled off. Method of embedding austenitic stainless steel liner can resist cavitation erosion better but it requires complicated technology and the cost is very. The method is suitable for important pump station. Repair method of brushing ceramic composite coating is much simpler. For the local vortex cavitation such as cavitation in static guide vane of vertical pump or cavitation arose at local convex surface, after being coated by ceramic coating, the surface can become smooth. As a result the local cavitation would not appear, and the surface remains intact for a long time. 5.2. Ceramic coatings case study for corrosion resistance The vertical circulation pump of KSB corporation was adopted to circulate the cooling water in a power plant. After being used for a year, components partially exposed to the sea water suffered corrosion damage. It was important to find the best technology to repair the damaged components, restore the device to the original geometry, and ensure the repaired components have excellent anti-corrosion performance. Corrosion status of diffuser is shown in Fig. 16. Paintings on the blade edge peeled off, resulting in the corrosion damage. A certain degree of corrosion appeared on the 11 blades. Painting on the diversion bottom was also peeled off. Corrosion damages appeared at 8 positions. The repair steps were as follows. First of all, undergo the sandblasting process, and then clean the surface with acetone to clear grease and dirt away. The particle granularity used for sand blasting process was about 8–16 mesh. The corundum sand was adopted for blasting hard alloy metal. All steel surfaces to be coated must be blast cleaned to a standard of white metal blast (SSPC-SP5) or near-white metal blast (SSPC-SP10) due to exceptionally severe condition that the diffuser was constant immersed in chemical liquid. After sand blasting process, the pothole depth of the damaged region reached between 2 and 6 mil. Then the damaged components of impeller were repaired with Resto SiC Putty, by filling the potholes and restoring the corroded surface to the original profile. After 4 h, the Resto SiC Putty gelled initially, and then was brushed with the Resto Ceram-Coat Red as a primer coating, and after it gelled (4 h), brushed with Resto Ceram-Coat Blue as an outer coating. After all the process were completed the coatings required 24 h to fully gel (see Fig. 17). Compared with the data before repair, the efficiency and pressure of coated circulating pump was significantly improved. Neglecting the uncertainty generated by the electric current fluctuation, the efficiency test results of circulating pumps
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(a) 420 (1 h) 0.1 mg loss, 1.431 μm depth
(c) 316 (30 h), 71 mg loss, 194.4 μm depth
(b) 4135 (10 h), 5.9 mg loss, 7.612 μm depth
(d) 4135 (30 h), 41.9 mg loss, 88.8 μm depth
Fig. 14. Cavitation erosion depth of stainless steel and alloy steel after several hours.
showed that the efficiency was promoted by 6%, contributed from the smooth surface of anti-corrosion coating, as shown in Table 3. After being used for 3 years, the repaired guide blades were in good condition, proving a satisfying anti-corrosion performance. In addition, after the dust on the surface was wiped, the surface was bright as new, showing that the smooth coating also had a function of preventing scaling. Although coating under cavitation load could not effectively resist the impact and was tapped into holes, the efficiency of the pump was improved (Table 3, increased by 6%). This is because the ceramic surface is smooth, therefore the hydraulic loss is reduced. The pump efficiency, especially for the old pump, returned to its original design efficiency, due to the smoothed surface, which could remedy the hydraulic loss caused by cavitation. So metal material will no longer be the only choice for chemical pump and pipeline accessories.
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Fig. 15. Cavitaion erosion problems arose again after half a year operation for ceramic coatings.
Fig. 16. Corrosion problems of the diffuser after one year operation.
From economic considerations, the client will tend to adopt the method that repairing the old pump by brushing coating. It costs much less than purchasing a new impeller made of stainless steel. At the same time, the coating can also solve the problem of corrosion for the cast steel impeller. In the cavitation erosion tests, PU coating showed slow expansion in the pit area because of the characteristics of elastomer materials. The ceramic coating’s characteristic resulted in a rapid expansion of the erosion area after damage began. However ceramic coating is a proven slurry and corrosion protection material. Ceramic coatings are used for slurry pump liners and impellers where conditions are not suited to PU coating, such as with coarse or sharp edged particles, or on duties having high impeller peripheral velocities or high operating temperatures. Rapid in-situ lining is generally not possible for PU coating. When repairs need to be carried out, quit often epoxy-ceramic products are used due to the ease of application. The cost of PU coating is 2–5 times of the ceramic coating. Because the
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(a) Sand blasting process
(b) Brush coatings
(c) Reassemble after repair
(d) Conditions after 3 years
Fig. 17. Repair processes of corroded vertical circulation pump and conditions after 3 years operation.
Table 3 Efficiency data of repaired vertical circulation pump. Equipment
Uncoated circulating pump 1
Coated circulating pump
Uncoated circulating pump 2
Voltage change (%) Electric current change (%) Efficiency change (%)
+1.5 0.7 +2.2
+8.3 0 +8.3
+2.4 0 +2.4
application process is complicated, skilled workers are required for repairing. Moreover, the final outer surface of the ceramic coating was smoother than the PU coating’s, which can result in a higher efficiency. Of course, replacing PU coating by ceramic coating is a questionable approach and not generally to be recommended especially when cavitation problem is serious. Ceramic coatings are resistant to low frequency of impact and they are the most suitable solution to sand erosion. On the other hand, PU coatings have the ability to absorb high frequency impingements hence being adequate for cavitation, but their toughness is less appropriated to sustain sharp edged solids. Therefore, it is required to develop coatings that are able to resist both kinds of damage. Unfortunately, coatings developed to resist particle impact are weak to withstand cavitation collapse. The procedure of selecting the type of coating must account for the constraints and should take into account which factor should be put into the first place. 6. Conclusions (1) Ceramic coatings, which are mainly composed of silicon carbide and polymer, showed excellent slurry-resisting performance, and the slurry-resistance is better than that of stainless steel 304, cast iron JM3 and high chrome alloy steel Cr30A. (2) The resistance of coating to cavitation erosion varies widely with its thickness, and the thicker coating showed better performance. The coatings’ erosion-resisting performances are inferior to that of gray cast iron JL1040, and hardly comparable to stainless steels. Therefore, excellent wear-resisting performance cannot guarantee any good erosion-resisting performance. (3) A good coating is the one that is able to withstand the load and not flake off, consequently protecting the base material until the next maintenance. Among the investigated coatings, the cavitation erosion rate of PU coatings is lowest of all. Only for very low cavitation exposure can PU coatings offer a certain degree of improvement in comparison with unprotected gray cast iron surfaces. (4) When impeller made of gray cast iron is exposed to slightly corrosive fluids, coatings can be the solution to withstand corrosion, as long as they keep the underlying materials from corrosive fluids. If the load exposure is purely cavitation erosion, coatings make little sense, and all coatings fail rapidly. Because coatings peel down more quickly than the underlying steel.
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(5) The pumping of slightly corrosive fluids frequently leads to erosion–corrosion damage on impellers. Corrosion can further amplify the erosion process. Materials with higher resistance to corrosion can solve ‘‘erosion problems’’ that, in reality, are corrosion problems. At the meantime, the machinery efficiency may be improved because of the smooth coating surface.
Acknowledgments This research was conducted under a project funded by KSB-Stiftung in Germany. The authors are grateful to Prof. Hellmann (KSB A.G.) and Prof. Anja Dwars (Georg Simon Ohm Hochschule, Nürnberg) for their experimental assistants. They are also very grateful to Dr. Alexander Böhm (KSB A.G.) and Prof. Yulin Wu (State Key Laboratory of Hydroscience and Engineering, Tsinghua University) for many fruitful discussions. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
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