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HVOF coating of Inconel 625 onto stainless and carbon steel surfaces: corrosion and bond testing
Journal of Materials Processing Technology 155–156 (2004) 2051–2055
HVOF coating of Inconel 625 onto stainless and carbon steel surfaces: corrosion and bond testing A.A. Boudi a , M.S.J. Hashmi b , B.S. Yilbas c,∗ a
School of Mechanical and Manufacturing Engineering, Dublin City University, Ireland School of Mechanical and Manufacturing Engineering, Dublin City University, Ireland Mechanical Engineering Department, King Fahd University of Petroleum and Minerals, P.O. Box 1913, Dhahran 31261, Saudi Arabia b
c
Abstract High velocity oxyfuel (HVOF) coating is extensively used in power industry to improve the corrosion and wear resistance of metallic surfaces. In the present study, HVOF coating of Inconel 625 onto stainless steel and mild steel surfaces is considered. The static electrochemical tests are carried out for different durations. Tensile bonding strength of the coatings before and after the electrochemical tests is measured. Optical microscopy and SEM are carried out to investigate the microstructure of the coating before and after the chemical tests. It is found that the coat has a noticeably layered structure with the layers parallel to the substrate’s surface and segregated by a slightly darker contrast phase presumed to be oxide. The tensile bonding strength of the coating reduces considerably after 3 weeks testing in the aqueous corrosion medium. © 2004 Elsevier B.V. All rights reserved. Keywords: HVOF; Coating; Thermal spray; Corrosion; Bonding strength
1. Introduction Advanced thermal spray coating finds wide applications in industry. This is because of the superior surface properties of the coat; in which case, the surface with resistance to corrosion, wear, and high temperature can be resulted. The high velocity oxyfuel (HVOF) thermal spraying is one of the most active and growing areas in research. In applications such as the deposition of the high temperature resistance alloy coatings, HVOF spraying can produce coating quality comparable to the detonation flow spray process and possible superior to air and vacuum plasma spray process. This is due to the modest heating and very high velocities obtained with HVOF. In HVOF, the process is involved with an internal combustion to rapidly heated and accelerated of powder consumable to high velocities. The high combustion pressure and rapid exposures through a nozzle facilitated the particle velocity to reach supersonic speed before impacting on to a surface. This provides excess impacting force which, in turn, results in mechanical adhesion of powders to the solid ∗ Corresponding author. Fax: +966 3 860 2949. E-mail addresses: [email protected] (A.A. Boudi), [email protected] (M.S.J. Hashmi), [email protected] (B.S. Yilbas).
0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.04.146
surface. In order to achieve a sound adhesion, solid surface should be free from any grease as well as dislodged particles. Moreover, surfaces should be prepared for high roughness to improve anchoring of the powder to the solid surface. Considerable research studies were carried out to investigate the HVOF coating performance of corrosion and wear resistance. Tucker and Ashary [1] studied corrosion and wear resistance of thermally sprayed coated surfaces. They indicated that thermal spray coating had superior corrosion and wear properties as compared to electroplated coating. HVOF spraying of a NiCoCrAly and intermetallic NiAl–TaCr alloy was investigated by Zhao and Lugscheider [2]. They indicated that the oxide scales on the NiCoCrAly and NiAl–TaCr coatings formed at 1100 ◦ C consisted mainly of Al2 O3 . Influence of heat treatment on the abrasive wear behavior of HVOF sprayed WC–Co coatings were investigated by Stewart et al. [3]. They showed that the abrasive wear resistance improved over 35% for samples heat treated at 250 ◦ C. The corrosion behavior of HVOF sprayed stainless steel and nickel alloy coatings in artificial seawater was studied by Sturgeon [4]. He indicated that the nickel alloy coating did not match the corrosion resistance of the same nickel alloy in wrought form. The corrosion resistance of thermal spray coatings of stainless steel was examined by Hofman et al. [5]. They indicated that HVOF coating aged rapidly in a 0.5N H2 SO4 solution, which was due to pore opening and
2052
A.A. Boudi et al. / Journal of Materials Processing Technology 155–156 (2004) 2051–2055
pore widening. Microstructure evaluation and oxidation behavior of HVOF coating were investigated by Lau and Lavernia [6]. They indicated that the resultant coating exhibited a superior micro hardness despite an increased porosity over that of the conventional coating sprayed with the same parameters. Dobler et al. [7] examined oxidation of stainless steel in high velocity oxyfuel process. They showed that for the oxygen-sensitive materials maintaining a relatively low particle temperature throughout the spray process minimized oxygen pickup by preventing an autocatalytic oxidation process. The model study on WC–Co coating structure and adhesion on a copper substrate during high velocity oxyfuel spraying was carried out by Sobolev et al. [8]. They indicated that substrate-coating thermal interaction leads to partial or complete melting in the substrate interfacial region, and this melted region could then bond readily to the coating. In the light of the above arguments, in the present study, the influence of aqueous corrosion medium on the tensile bonding strength of the high velocity oxyfuel coating of Inconel 625 is investigated. Stainless and mild steel workpieces in standard bars are used to coat the surface. The electrolic solution resembling the seawater is used in the static corrosion tests. SEM and optical microscopy were employed to examine the HVOF coated surfaces before and after the corrosion tests.
2. Experimental The hybrid HVOF system was employed to spray the coating powder. Propane was used in the combustor and powder flow was centered by using axial powder feeding with auxiliary air, which surrounded the particle stream during the spraying. The powder unit was based on the fluidized bed system, which could include closed loop control. The control unit provided precision of operation. Nickel based powder namely NiCrMoNb, equivalent to Inconel 625, was used. The powder had a particle size distribution between 35 and 55 m with a spherical morphology. It had excellent high temperature oxidation and corrosion properties. The chemical composition of the powder is given in Table 1. In order to obtain the thermal spray coating, stainless and mild steel bars with high surface roughness (with grid blasting) were used as a base substrate material. The size of the bar is prepared in accordance with the ASTM standards (C 633-01). The powder was sprayed onto the substrate material to secure good adhesion on to the surface of the bars. The process parameters for HVOF spraying are given in Table 2. Table 1 Composition of powder used during thermal spraying (wt.%) Ni Cr Mo Nb
Balance 21.5 9 3.7
Table 2 Process parameters of thermal spray Oxygen pressure (kPa) Fuel pressure (kPa) Air pressure (kPa) Powder feed rate (m3 /h) Spray rate (kg/h) Spray distance (m)
1034 620 724 0.81 6.35 0.28
Table 3 Tensile test loads Tensile Maximum load (kN) Minimum load (kN)
16 01
Table 4 Properties of adhesive used in the experiment Master bond grade Viscosity, RT, cps Color code Storage stability, RT Cure schedule time (min)/temperature (F) Service temperature range, F Tensile strength (psi)
EP 15 90000–100000 Tan 3 months (minimum) 60–90 @ 300–350 −60 to 250 >12000
The static electrochemical test was conducted in 0.1N H2 SO4 + 0.05N NaCl solution bath. The workpieces were degreased in benzene and washed with de-ionized water prior to the electrochemical tests. Bonding tensile tests were carried out using INSTRON 300 instrument. The test conditions are given in Table 3. SEM microscopy was performed on coating sheet surfaces before and after the electrochemical tests using JEOL JDX-3530. A single component epoxy adhesive was select to be used mainly for testing adhesive or cohesive strength of flame-sprayed coatings as per ASTM specification C 633-01. The adhesive is made by Master Bond Inc. This adhesive has the properties given in Table 4. ASTM has established a standard test method and instructions for testing adhesion or cohesion strength of thermal spray coatings. This standard has been designated as (C 633-01). The test method covers the determination of the degree of adhesion (bond strength) of a coating to a substrate or the cohesion strength of the coating in a tension normal to the surface. It is adapted particularly for testing coatings applied by thermal spray process.
3. Results and discussions HVOF coating of stainless and carbon steel surfaces is carried out. Inconel 625 powders are used during the thermal spraying. Static electrochemical testing of the workpieces is carried out before tensile testing of the coated surfaces. International standard (C 633-01) is employed for tensile bonding tests.
A.A. Boudi et al. / Journal of Materials Processing Technology 155–156 (2004) 2051–2055
The bond of HVOF coating to a substrate is attributed primarily to mechanical interlocking with asperities on the surface of the substrate material. In order to enhance the bonding strength, roughening of the substrate surface is required. Due to very high particle velocities, some fractured powders on the impacted surface are observed. This partially reduces the bond strength of the coat. Oxidation of coating occurs during the process. The formation of oxides can be categorized into three spatial zones. The first zone starts at the point of particle ejection within the nozzle and extends to the end of the jet case. In this zone, particles are exposed to high temperature flame as well as the combustion products. Second zone refers to the time from when the entrained atmosphere reaches the centerline of the flame until the particles impact upon the substrate material. In the third zone, the time the particles spend on the substrate after impact until the next layer covers them. During this period, the individual splats are still exposed to the oxidizing effects of the flame and may undergo some oxidation. Moreover, the contribution of the different oxidation mechanisms to the oxygen content of a coating depends not only on the spray parameters but also on the spray powder feedstock.
2053
Fig. 1. Coat cross-section.
3.1. Microstructure The porosity of the coat varies within 1.5–4.5% for Inconel 625 coating. Consequently, results indicate that the coating were prepared with relatively low levels of porosity. Optical micrographs of coat cross-section is shown in Fig. 1, while SEM micrograph of coat cross-section is depicted in Fig. 2. The micrographs show that the powder was melted during the spraying process to give a lamella type microstructure with oxide strings aligned parallel to the substrate surface. The coat has a noticeable layered structure with the layers parallel to the substrate surface and segregated by a slightly darker contrast phase presumed to be oxide. Each layer is supposed to represent one pass of the spray over the surface. Moreover, the microstructure
Fig. 2. Coat cross-section micrograph.
obtained for the coat of stainless steel surface has higher amounts of porosity than that corresponding to mild steel. The microstructure obtained appears to consist predominantly of well stacked, partially deformed particles. This indicates that the powder particles were at a lower temperature impact with the substrate, reasonably below the melting point. The smaller powder size range (20–35 m) gave coatings with a slightly less porosity.
Fig. 3. Coat cross-section at the side of the workpiece.
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A.A. Boudi et al. / Journal of Materials Processing Technology 155–156 (2004) 2051–2055
Fig. 4. Tensile bond strength of the adhesive.
Fig. 6. Tensile bond strength of carbon steel.
sistant to the particular corrosive environment. It should be noted that most steel samples are highly susceptible to service corrosion particularly in a chloride containing aqueous media. As a result, the substrate may be corroded beneath the coating causing the coating to blister and spall. In the tests, the surface of the one of the samples suffers from the corrosion effect. It is observed that due to gab between the coat and the substrate surface from the side of the workpiece (as seen from Fig. 3), corrosion solution entered through the gab and resulting crevice corrosion beneath the coating. 3.3. Tensile bonding tests Fig. 5. Tensile bond strength of stainless steel.
3.2. Corrosion behavior Since a microstructure consists of well stacked partially deformed particles, the surface appears more resistant to corrosion. In this case, pit formation at the surface is not observed. Moreover, lamella type microstructure with higher oxide prevents the corrosion solution reaching the base substrate. In the case of porous structure, it is possible that some of the porosity being interconnected. As a result of interconnected porosity, a corrosion media may penetrate the coating and attack the substrate, even through the coating itself is re-
Fig. 4 shows the tensile bond strength of the adhesive used in the experiment. It can be observed that the tensile bonding strength of the adhesive is in the range 45–60 MPa. This is, however, lower than the normal bond strength of the coating as presented in the open literature. Consequently, bonding strength as the coating beyond 40 MPa considered as satisfying the condition for sound bonding. Figs. 5 and 6 show the tensile bonding strength of the coating being tested for different durations in the corrosion environment. In case of stainless steel workpieces, the tensile strength of the coating is very low for 1 and 3 weeks testing durations. This occurs because of the low bonding strength
Fig. 7. Coat cross-section with discontinuous line at the interface.
A.A. Boudi et al. / Journal of Materials Processing Technology 155–156 (2004) 2051–2055
of the coating, i.e., as observed from SEM micrographs (Fig. 7), long discontinuity line along the coating–substrate interface occurs. Moreover, it is also possible that the corrosion solution penetrates through the discontinuity line, which results in the crevice corrosion at interface. This, in turn, lowers the bonding strength substantially. In the case of mild steel workpieces, similar situation is observed despite the fact that interfacial discontinuity is not considerable. This may suggest that the porous structure of the coating permits the corrosion solution reaching the substrate surface. This reduces to bonding strength of the coating.
4. Conclusion HVOF coating of stainless and mild steel bars are carried out. Inconel 625 powders are sprayed onto the substrate surface. The static electrochemical tests of the coated workpieces are carried out for different durations and the influence of corrosion medium on the tensile bonding strength of the coating is investigated. It is found that the coat thickness of 250–350 m is resulted and the porosity of the coat is of the order of 1.5–4.5%. The bonding strength of the coating reduces substantially for samples, which are tested electrochemically for 3 weeks. The specific conclusions derived from the present work can be listed as follows: 1. The powder was melted during the spraying process to give a lamella type microstructure with oxide stringers aligned parallel to the substrate surface. 2. The lamella type microstructure with higher oxide prevents the corrosion solution reaching the substrate surface. Moreover, it is also possible that the porous structure could be interconnected permitting the corrosive media to penetrate the coating and attacking the substrate surface. 3. Tensile bonding strength of the coating is considerably low. The possible explanation for this situation is that
2055
the discontinuity line extending almost along the coating and substrate interface.
Acknowledgements Acknowledgements are due to King Fahd University of Petroleum and Minerals and Dublin City University.
References [1] R.C. Tucker, A.A. Asharay, Advanced thermal spray coatings for corrosion and wear resistance, in: A.R. Srivatsa, C.R. Clayton, J. Hirvonen (Eds.), Advances in Coatings Technologies for Corrosion and Wear Resistant Coatings, The Minerals, Metals & Materials Society, 1995. [2] L. Zhao, E. Lugscheider, High velocity oxy-fuel of a NiCoCrAlY and intermetallic NiAl–TaCr alloy, Surf. Coat. Technol. 149 (2002) 231–235. [3] D.A. Stewart, P.H. Shipway, D.G. McCartney, Influence of heat treatment on the abrasive wear behaviour of HVOF sprayed WC–Co coatings, Surf. Coat. Technol. 105 (1998) 13–24. [4] A.J. Sturgeon, High velocity oxy-fuel spraying, Trans. Inst. Met. Finish. 72 (1994) 139–140. [5] R. Hofman, M.P.W. Vreijing, G.M. Ferrari, J.H.W. de Wit, Electrochemical methods for characterization of thermal spray corrosion resistant stainless steel coatings, Mater. Sci. Forum 289–292 (1998) 641–654. [6] L.M. Lau, E.J. Lavernia, Microstructural evolution and oxidation behavior of nanocrystalline 316-stainless steel coatings produced by high-velocity oxygen fuel spraying, Mater. Sci. Eng. A: Struct. Mater.: Prop. Microstruct. Process. A272 (1999) 222– 229. [7] K. Dobler, H. Kreye, R. Schwetzke, Oxidation of stainless steel in the high velocity oxy-fuel process, J. Therm. Spray Technol. 9 (2000) 407–413. [8] V.V. Sobolev, J.M. Guilemany, J.A. Calero, Development of coating structure and adhesion during high velocity oxygen-fuel spraying of WC–Co powder on a copper substrate, J. Therm. Spray Technol. 9 (2000) 100–106.
HVOF coating of Inconel 625 onto stainless and carbon steel surfaces: corrosion and bond testing A.A. Boudi a , M.S.J. Hashmi b , B.S. Yilbas c,∗ a
School of Mechanical and Manufacturing Engineering, Dublin City University, Ireland School of Mechanical and Manufacturing Engineering, Dublin City University, Ireland Mechanical Engineering Department, King Fahd University of Petroleum and Minerals, P.O. Box 1913, Dhahran 31261, Saudi Arabia b
c
Abstract High velocity oxyfuel (HVOF) coating is extensively used in power industry to improve the corrosion and wear resistance of metallic surfaces. In the present study, HVOF coating of Inconel 625 onto stainless steel and mild steel surfaces is considered. The static electrochemical tests are carried out for different durations. Tensile bonding strength of the coatings before and after the electrochemical tests is measured. Optical microscopy and SEM are carried out to investigate the microstructure of the coating before and after the chemical tests. It is found that the coat has a noticeably layered structure with the layers parallel to the substrate’s surface and segregated by a slightly darker contrast phase presumed to be oxide. The tensile bonding strength of the coating reduces considerably after 3 weeks testing in the aqueous corrosion medium. © 2004 Elsevier B.V. All rights reserved. Keywords: HVOF; Coating; Thermal spray; Corrosion; Bonding strength
1. Introduction Advanced thermal spray coating finds wide applications in industry. This is because of the superior surface properties of the coat; in which case, the surface with resistance to corrosion, wear, and high temperature can be resulted. The high velocity oxyfuel (HVOF) thermal spraying is one of the most active and growing areas in research. In applications such as the deposition of the high temperature resistance alloy coatings, HVOF spraying can produce coating quality comparable to the detonation flow spray process and possible superior to air and vacuum plasma spray process. This is due to the modest heating and very high velocities obtained with HVOF. In HVOF, the process is involved with an internal combustion to rapidly heated and accelerated of powder consumable to high velocities. The high combustion pressure and rapid exposures through a nozzle facilitated the particle velocity to reach supersonic speed before impacting on to a surface. This provides excess impacting force which, in turn, results in mechanical adhesion of powders to the solid ∗ Corresponding author. Fax: +966 3 860 2949. E-mail addresses: [email protected] (A.A. Boudi), [email protected] (M.S.J. Hashmi), [email protected] (B.S. Yilbas).
0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.04.146
surface. In order to achieve a sound adhesion, solid surface should be free from any grease as well as dislodged particles. Moreover, surfaces should be prepared for high roughness to improve anchoring of the powder to the solid surface. Considerable research studies were carried out to investigate the HVOF coating performance of corrosion and wear resistance. Tucker and Ashary [1] studied corrosion and wear resistance of thermally sprayed coated surfaces. They indicated that thermal spray coating had superior corrosion and wear properties as compared to electroplated coating. HVOF spraying of a NiCoCrAly and intermetallic NiAl–TaCr alloy was investigated by Zhao and Lugscheider [2]. They indicated that the oxide scales on the NiCoCrAly and NiAl–TaCr coatings formed at 1100 ◦ C consisted mainly of Al2 O3 . Influence of heat treatment on the abrasive wear behavior of HVOF sprayed WC–Co coatings were investigated by Stewart et al. [3]. They showed that the abrasive wear resistance improved over 35% for samples heat treated at 250 ◦ C. The corrosion behavior of HVOF sprayed stainless steel and nickel alloy coatings in artificial seawater was studied by Sturgeon [4]. He indicated that the nickel alloy coating did not match the corrosion resistance of the same nickel alloy in wrought form. The corrosion resistance of thermal spray coatings of stainless steel was examined by Hofman et al. [5]. They indicated that HVOF coating aged rapidly in a 0.5N H2 SO4 solution, which was due to pore opening and
2052
A.A. Boudi et al. / Journal of Materials Processing Technology 155–156 (2004) 2051–2055
pore widening. Microstructure evaluation and oxidation behavior of HVOF coating were investigated by Lau and Lavernia [6]. They indicated that the resultant coating exhibited a superior micro hardness despite an increased porosity over that of the conventional coating sprayed with the same parameters. Dobler et al. [7] examined oxidation of stainless steel in high velocity oxyfuel process. They showed that for the oxygen-sensitive materials maintaining a relatively low particle temperature throughout the spray process minimized oxygen pickup by preventing an autocatalytic oxidation process. The model study on WC–Co coating structure and adhesion on a copper substrate during high velocity oxyfuel spraying was carried out by Sobolev et al. [8]. They indicated that substrate-coating thermal interaction leads to partial or complete melting in the substrate interfacial region, and this melted region could then bond readily to the coating. In the light of the above arguments, in the present study, the influence of aqueous corrosion medium on the tensile bonding strength of the high velocity oxyfuel coating of Inconel 625 is investigated. Stainless and mild steel workpieces in standard bars are used to coat the surface. The electrolic solution resembling the seawater is used in the static corrosion tests. SEM and optical microscopy were employed to examine the HVOF coated surfaces before and after the corrosion tests.
2. Experimental The hybrid HVOF system was employed to spray the coating powder. Propane was used in the combustor and powder flow was centered by using axial powder feeding with auxiliary air, which surrounded the particle stream during the spraying. The powder unit was based on the fluidized bed system, which could include closed loop control. The control unit provided precision of operation. Nickel based powder namely NiCrMoNb, equivalent to Inconel 625, was used. The powder had a particle size distribution between 35 and 55 m with a spherical morphology. It had excellent high temperature oxidation and corrosion properties. The chemical composition of the powder is given in Table 1. In order to obtain the thermal spray coating, stainless and mild steel bars with high surface roughness (with grid blasting) were used as a base substrate material. The size of the bar is prepared in accordance with the ASTM standards (C 633-01). The powder was sprayed onto the substrate material to secure good adhesion on to the surface of the bars. The process parameters for HVOF spraying are given in Table 2. Table 1 Composition of powder used during thermal spraying (wt.%) Ni Cr Mo Nb
Balance 21.5 9 3.7
Table 2 Process parameters of thermal spray Oxygen pressure (kPa) Fuel pressure (kPa) Air pressure (kPa) Powder feed rate (m3 /h) Spray rate (kg/h) Spray distance (m)
1034 620 724 0.81 6.35 0.28
Table 3 Tensile test loads Tensile Maximum load (kN) Minimum load (kN)
16 01
Table 4 Properties of adhesive used in the experiment Master bond grade Viscosity, RT, cps Color code Storage stability, RT Cure schedule time (min)/temperature (F) Service temperature range, F Tensile strength (psi)
EP 15 90000–100000 Tan 3 months (minimum) 60–90 @ 300–350 −60 to 250 >12000
The static electrochemical test was conducted in 0.1N H2 SO4 + 0.05N NaCl solution bath. The workpieces were degreased in benzene and washed with de-ionized water prior to the electrochemical tests. Bonding tensile tests were carried out using INSTRON 300 instrument. The test conditions are given in Table 3. SEM microscopy was performed on coating sheet surfaces before and after the electrochemical tests using JEOL JDX-3530. A single component epoxy adhesive was select to be used mainly for testing adhesive or cohesive strength of flame-sprayed coatings as per ASTM specification C 633-01. The adhesive is made by Master Bond Inc. This adhesive has the properties given in Table 4. ASTM has established a standard test method and instructions for testing adhesion or cohesion strength of thermal spray coatings. This standard has been designated as (C 633-01). The test method covers the determination of the degree of adhesion (bond strength) of a coating to a substrate or the cohesion strength of the coating in a tension normal to the surface. It is adapted particularly for testing coatings applied by thermal spray process.
3. Results and discussions HVOF coating of stainless and carbon steel surfaces is carried out. Inconel 625 powders are used during the thermal spraying. Static electrochemical testing of the workpieces is carried out before tensile testing of the coated surfaces. International standard (C 633-01) is employed for tensile bonding tests.
A.A. Boudi et al. / Journal of Materials Processing Technology 155–156 (2004) 2051–2055
The bond of HVOF coating to a substrate is attributed primarily to mechanical interlocking with asperities on the surface of the substrate material. In order to enhance the bonding strength, roughening of the substrate surface is required. Due to very high particle velocities, some fractured powders on the impacted surface are observed. This partially reduces the bond strength of the coat. Oxidation of coating occurs during the process. The formation of oxides can be categorized into three spatial zones. The first zone starts at the point of particle ejection within the nozzle and extends to the end of the jet case. In this zone, particles are exposed to high temperature flame as well as the combustion products. Second zone refers to the time from when the entrained atmosphere reaches the centerline of the flame until the particles impact upon the substrate material. In the third zone, the time the particles spend on the substrate after impact until the next layer covers them. During this period, the individual splats are still exposed to the oxidizing effects of the flame and may undergo some oxidation. Moreover, the contribution of the different oxidation mechanisms to the oxygen content of a coating depends not only on the spray parameters but also on the spray powder feedstock.
2053
Fig. 1. Coat cross-section.
3.1. Microstructure The porosity of the coat varies within 1.5–4.5% for Inconel 625 coating. Consequently, results indicate that the coating were prepared with relatively low levels of porosity. Optical micrographs of coat cross-section is shown in Fig. 1, while SEM micrograph of coat cross-section is depicted in Fig. 2. The micrographs show that the powder was melted during the spraying process to give a lamella type microstructure with oxide strings aligned parallel to the substrate surface. The coat has a noticeable layered structure with the layers parallel to the substrate surface and segregated by a slightly darker contrast phase presumed to be oxide. Each layer is supposed to represent one pass of the spray over the surface. Moreover, the microstructure
Fig. 2. Coat cross-section micrograph.
obtained for the coat of stainless steel surface has higher amounts of porosity than that corresponding to mild steel. The microstructure obtained appears to consist predominantly of well stacked, partially deformed particles. This indicates that the powder particles were at a lower temperature impact with the substrate, reasonably below the melting point. The smaller powder size range (20–35 m) gave coatings with a slightly less porosity.
Fig. 3. Coat cross-section at the side of the workpiece.
2054
A.A. Boudi et al. / Journal of Materials Processing Technology 155–156 (2004) 2051–2055
Fig. 4. Tensile bond strength of the adhesive.
Fig. 6. Tensile bond strength of carbon steel.
sistant to the particular corrosive environment. It should be noted that most steel samples are highly susceptible to service corrosion particularly in a chloride containing aqueous media. As a result, the substrate may be corroded beneath the coating causing the coating to blister and spall. In the tests, the surface of the one of the samples suffers from the corrosion effect. It is observed that due to gab between the coat and the substrate surface from the side of the workpiece (as seen from Fig. 3), corrosion solution entered through the gab and resulting crevice corrosion beneath the coating. 3.3. Tensile bonding tests Fig. 5. Tensile bond strength of stainless steel.
3.2. Corrosion behavior Since a microstructure consists of well stacked partially deformed particles, the surface appears more resistant to corrosion. In this case, pit formation at the surface is not observed. Moreover, lamella type microstructure with higher oxide prevents the corrosion solution reaching the base substrate. In the case of porous structure, it is possible that some of the porosity being interconnected. As a result of interconnected porosity, a corrosion media may penetrate the coating and attack the substrate, even through the coating itself is re-
Fig. 4 shows the tensile bond strength of the adhesive used in the experiment. It can be observed that the tensile bonding strength of the adhesive is in the range 45–60 MPa. This is, however, lower than the normal bond strength of the coating as presented in the open literature. Consequently, bonding strength as the coating beyond 40 MPa considered as satisfying the condition for sound bonding. Figs. 5 and 6 show the tensile bonding strength of the coating being tested for different durations in the corrosion environment. In case of stainless steel workpieces, the tensile strength of the coating is very low for 1 and 3 weeks testing durations. This occurs because of the low bonding strength
Fig. 7. Coat cross-section with discontinuous line at the interface.
A.A. Boudi et al. / Journal of Materials Processing Technology 155–156 (2004) 2051–2055
of the coating, i.e., as observed from SEM micrographs (Fig. 7), long discontinuity line along the coating–substrate interface occurs. Moreover, it is also possible that the corrosion solution penetrates through the discontinuity line, which results in the crevice corrosion at interface. This, in turn, lowers the bonding strength substantially. In the case of mild steel workpieces, similar situation is observed despite the fact that interfacial discontinuity is not considerable. This may suggest that the porous structure of the coating permits the corrosion solution reaching the substrate surface. This reduces to bonding strength of the coating.
4. Conclusion HVOF coating of stainless and mild steel bars are carried out. Inconel 625 powders are sprayed onto the substrate surface. The static electrochemical tests of the coated workpieces are carried out for different durations and the influence of corrosion medium on the tensile bonding strength of the coating is investigated. It is found that the coat thickness of 250–350 m is resulted and the porosity of the coat is of the order of 1.5–4.5%. The bonding strength of the coating reduces substantially for samples, which are tested electrochemically for 3 weeks. The specific conclusions derived from the present work can be listed as follows: 1. The powder was melted during the spraying process to give a lamella type microstructure with oxide stringers aligned parallel to the substrate surface. 2. The lamella type microstructure with higher oxide prevents the corrosion solution reaching the substrate surface. Moreover, it is also possible that the porous structure could be interconnected permitting the corrosive media to penetrate the coating and attacking the substrate surface. 3. Tensile bonding strength of the coating is considerably low. The possible explanation for this situation is that
2055
the discontinuity line extending almost along the coating and substrate interface.
Acknowledgements Acknowledgements are due to King Fahd University of Petroleum and Minerals and Dublin City University.
References [1] R.C. Tucker, A.A. Asharay, Advanced thermal spray coatings for corrosion and wear resistance, in: A.R. Srivatsa, C.R. Clayton, J. Hirvonen (Eds.), Advances in Coatings Technologies for Corrosion and Wear Resistant Coatings, The Minerals, Metals & Materials Society, 1995. [2] L. Zhao, E. Lugscheider, High velocity oxy-fuel of a NiCoCrAlY and intermetallic NiAl–TaCr alloy, Surf. Coat. Technol. 149 (2002) 231–235. [3] D.A. Stewart, P.H. Shipway, D.G. McCartney, Influence of heat treatment on the abrasive wear behaviour of HVOF sprayed WC–Co coatings, Surf. Coat. Technol. 105 (1998) 13–24. [4] A.J. Sturgeon, High velocity oxy-fuel spraying, Trans. Inst. Met. Finish. 72 (1994) 139–140. [5] R. Hofman, M.P.W. Vreijing, G.M. Ferrari, J.H.W. de Wit, Electrochemical methods for characterization of thermal spray corrosion resistant stainless steel coatings, Mater. Sci. Forum 289–292 (1998) 641–654. [6] L.M. Lau, E.J. Lavernia, Microstructural evolution and oxidation behavior of nanocrystalline 316-stainless steel coatings produced by high-velocity oxygen fuel spraying, Mater. Sci. Eng. A: Struct. Mater.: Prop. Microstruct. Process. A272 (1999) 222– 229. [7] K. Dobler, H. Kreye, R. Schwetzke, Oxidation of stainless steel in the high velocity oxy-fuel process, J. Therm. Spray Technol. 9 (2000) 407–413. [8] V.V. Sobolev, J.M. Guilemany, J.A. Calero, Development of coating structure and adhesion during high velocity oxygen-fuel spraying of WC–Co powder on a copper substrate, J. Therm. Spray Technol. 9 (2000) 100–106.