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Corrosion–erosion behavior of TiN-coated stainless steels in aqueous slurries
Wear 258 (2005) 684–692
Corrosion–erosion behavior of TiN-coated stainless steels in aqueous slurries Diana L´opeza , Carlos S´anchezb , Alejandro Toroa,∗ a
Tribology and Surfaces Group, Materials Engineering School, National University of Colombia, Medell´ın, Colombia b Chemistry and Petroleum Engineering School, National University of Colombia, Medell´ın, Colombia Received 22 December 2003 Available online 18 October 2004
Abstract The corrosion–erosion resistance of TiN-coated AISI 304 and AISI 420 stainless steels in aqueous slurries was studied. TiN films with a thickness of 0.6 m were obtained by using the pulsed-arc plasma-assisted physical vapour deposition technique. The corrosion–erosion experiments were performed in a test machine in which the impingement velocity, impact angle, concentration of solids and pH of the solution were controlled. Polarization curves were simultaneously obtained to relate the electrochemical effects to the erosive wear mechanisms. The slurry used consists of quartz particles suspended in a mixture of sulphuric acid solution and 3.5% NaCl, with a pH value of 0.2. Measurement of critical and passive current densities showed that the behavior of coated materials differed according to substrate, but in a general way increasing impact velocity and changing from normal to grazing incidence led to a reduction in resistance to corrosion–erosion and liquid impingement corrosion. Surface analysis by SEM revealed formation of cracks in the coating and plastic deformation in both the substrate and the coating, especially when the mean impact velocity exceeded a critical value between 6.9 and 8.6 ms−1 . Additionally, intergranular corrosion was observed in some specimens. © 2004 Elsevier B.V. All rights reserved. Keywords: PVD coatings; Stainless steels; Corrosion–erosion; Electrochemical tests
1. Introduction Austenitic stainless steels are used in many components where corrosion resistance is crucial, such as slurry handling in food and chemical industries. Nevertheless, under mechanical action of hard particles, those steels show high plastic deformation and wear. If a corrosive solution carries the particles, the surface damage due to corrosion increases as a consequence of synergistic mechanisms between corrosion and erosion [1]. On the other hand, martensitic stainless steel shows better mechanical resistance to erosive particles than austenitic steel, but its corrosion resistance is lower [2].
∗
Corresponding author. Tel.: +57 4 4255339; fax: +57 4 2341002. E-mail address: [email protected] (A. Toro).
0043-1648/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2004.09.015
Titanium nitride (TiN) coatings are used as an alternative method to improve mechanical resistance in austenitic and corrosion resistance in martensitic stainless steels. As a result of high hardness, melting point and good chemical stability, TiN films can enhance the surface properties under wear and corrosion conditions [3,4]. The dependence between film properties and deposition conditions in PVD coatings has been extensively studied, and several authors have shown that substrate-to-film adherence, thickness and porosity are very important features [5–7]. Many studies on corrosion behavior have shown that TiN coatings can improve the corrosion response of a material if microstructural defects like porosity and pinholes are controlled [5]. Lang and Yu [8] reported that commercial, lowcarbon A3 steel coated with TiN showed much higher corrosion resistance in 1N H2 SO4 solution than the uncoated material, with very low critical current and passive current densities.
D. L´opez et al. / Wear 258 (2005) 684–692 Table 1 Chemical composition of the stainless steels used in this investigation (wt.%) Material
AISI 304
AISI 420
Cr C Ni Mn Si
18.6 0.07 8.4 1.0 0.65
13.4 0.34 – 1.0 0.52
Tu et al. [9] studied the slurry erosion behavior of TiNcoated Ti alloys by using distilled water containing 15 wt.% angular SiO2 , with impact velocities ranging from 6.4 to 15.2 ms−1 and normal incidence. They found that TiNcoated samples showed a better erosion resistance for impact velocity lower than 10 ms−1 . After that, the coating was removed from the substrate surface by perforation and spalling. The aim of this work was to study the interaction between corrosion and erosion mechanisms in TiN-coated AISI 304 and AISI 420 stainless steels, submitted to liquid impingement and corrosion–erosion conditions. The effects of impact velocity mean impact angle and particle addition to the flow are considered.
2. Experimental procedure 2.1. Materials Austenitic AISI 304 and martensitic AISI 420 stainless steels cylinders with 9.5 mm in diameter and 5 mm in height were used. The chemical compositions of the steels are shown in Table 1. The AISI 420 samples were austenized at 1373 K for 1 h, oil-quenched and tempered at 473 K for another hour. The microstructure obtained after this procedure was composed of martensite laths, retained austenite and dispersed carbides.
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On the other hand, the AISI 304 samples had a microstructure composed of austenite grains and some deformation bands formed as a consequence of cold working. 2.2. TiN films deposition TiN-coated samples were obtained by cathodic arc plasma deposition in a non-commercial PVD reactor. Each sample was ultrasonically cleaned and mounted with its surface 5 mm apart from a titanium cathode. A glow discharge was produced at a vacuum pressure of 1.7 mbar for 10 min to clean the substrate. TiN film deposition was carried out at 300 V in nitrogen-enriched plasma, which produced layers of ca. 200 nm in thickness after a single deposition and ca. 600 nm after five successive discharges. 2.3. Surface properties tests Polarization curves were obtained in a BAS 100b electrochemical analyzer with Ag/AgCl reference electrode and Pt counter electrode. The potential scan rate was 1 mV/s and the starting point was −600 mV for all the tests. Liquid impingement tests were performed in 0.5 M H2 SO4 + 3.5% NaCl solution with a pH value of 0.2, while in corrosion–erosion experiments 30 wt.% SiO2 particles with mean size between 212 and 300 m were added to the solution. All the samples were immersed in the slurry for 10 min before starting the potential scan. The corrosion–erosion and liquid impingement tests were performed in the slurry wear testing machine shown in Fig. 1. For each test, the slurry pot was filled with 700 g of solution and 300 g of SiO2 particles. The solids contents (30 wt.%), solution pH (0.2) and exposed area of the samples (7.13 × 10−5 m2 ) were fixed, while the rotation speed of the UHMWPE impeller and the mean impact angle of the flow over the specimens were controlled. The slurry temperature varied between 25 and 28 ◦ C for all the tests.
Fig. 1. Experimental setup for liquid impingement and corrosion–erosion tests.
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Impact angles of 30◦ and 90◦ and rotation speeds of 2000, 3500 and 5000 rpm were used, which correspond to impact velocities of ca. 4.5, 6.9 and 8.6 ms−1 , respectively. It is worth noticing that the reported impact angles represent mean values, given that in slurry erosion the particles are considerably deviated when they reach the surface, due to boundary layer and viscosity effects associated to the liquid carrier [10]. The mean impact velocity was calculated using a classical rotating disk model [11], with a viscosity value of 0.025 N s m−2 . 2.4. Surface examination and microstructure analysis The microstructure of the specimens was analyzed by optical (OM) and scanning electron microscopy (SEM), by using Olympus PME-Leica GZ6 and JEOL 591OLV microscopes, respectively. The structures of substrate and coated materials were studied by X-ray diffraction (XRD) in an Advance D8 equipment operating at 40 kV, 40 mA and grazing angle configuration. The morphology of surfaces, before and after the polarization tests was analyzed by SEM and atomic force microscopy (AFM). Additionally, some XPS spectra were
obtained from TiN-coated specimens using an Escalab 250 spectrometer.
3. Results and discussion 3.1. TiN film characterization Fig. 2a and b show the XRD spectra of coated AISI 304 and AISI 420 specimens respectively, in which the position of the TiN peaks indicates that the film is stoichiometric, with variations of ca. 2% in the lattice parameters. However, some XPS spectra taken from coated AISI 304 specimens (Fig. 2c and d) revealed a lower nitrogen concentration in the TiN coating. SEM examination and AFM mapping of the surfaces of the coated specimens revealed the formation of abundant droplets in both substrates, as a consequence of the TiN pulsed-arc deposition technique employed. Fig. 3 shows images of a TiN-coated surface of AISI 304 steel, whose aspect is similar to that observed in the coated AISI 420 surfaces.
Fig. 2. (a) XRD spectrum of TiN-coated AISI 304 steel; (b) XRD spectrum of TiN-coated AISI 420 steel; (c) XPS spectrum of TiN-coated AISI 304 steel, and (d) detail of XPS spectrum showing the Ti 2p peaks in the range 450–470 eV.
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Fig. 3. Surface morphology of a TiN-coated AISI 304 specimen. (a) AFM map, (b) SEM image. Droplets formed as a consequence of the deposition technique can be observed.
3.2. Effect of impact velocity and impact angle on surface properties Under normal impact the increase in rotation speed led to a reduction in uniform corrosion resistance of coated AISI 304 steel, as can be concluded from critical current density
measurements for liquid impingement and corrosion–erosion conditions (Fig. 4a). On the other hand, for grazing impact the polarization curves revealed a reduction in critical and passive current densities when rotation speed increased (Fig. 4b). Since this differential behavior with the mean impact angle was also observed in the uncoated steel (Fig. 4c and d), it is
Fig. 4. Effect of impact velocity on critical and passive current densities under corrosion–erosion conditions. (a) Normal impact, TiN-coated AISI 304; (b) grazing impact, TiN-coated AISI 304; (c) normal impact, AISI 304 and (d) grazing impact, AISI 304.
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Fig. 5. Effect of impact velocity on current density of TiN-coated AISI 420 specimens submitted to corrosion–erosion under grazing impact.
suggested that many of the features observed in the polarization curves for coated specimens are related to the mechanical and chemical properties of the substrate, as it was stated by Bromark et al. for TiN-coated tool steels [12]. Increasing impact velocity led to a reduction in the resistance to corrosion–erosion and liquid impingement of the TiN-coated AISI 420 specimens. Nevertheless, this effect was verified only by a slight increment in current density in polarization curves (Fig. 5). Additionally, when the parti-
Fig. 6. Effect of impact angle on current density of TiN-coated AISI 420 specimens submitted to corrosion–erosion.
cles reached the surface at grazing angles the measured passive current density was higher than under normal impact, as can be seen in polarization curves of Fig. 6 for the specific case of 5000 rpm rotation speed (8.6 ms−1 mean impact velocity). The pitting potential was not significantly affected when the rotation speed changed for all the materials tested. Balance between the beneficial effect of agitation and the harmful effect of the solid particles impact on the kinetics of the passive layer formation could explain this result.
Fig. 7. Wear marks after corrosion–erosion of AISI 304 and AISI 420 steels under normal (a) and (b) and grazing (c) and (d) impact. Slurry composed of (0.5 M H2 SO4 + 3.5% NaCl) solution + 30 wt.% quartz particles; 6.9 ms−1 mean impact velocity.
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Fig. 8. Intergranular and pitting corrosion in TiN-coated AISI 420 steel samples submitted to corrosion–erosion under normal impact, (a) 4.5 ms−1 mean impact velocity; (b) 8.6 ms−1 mean impact velocity.
3.3. Mass removal mechanisms The TiN-coated steels showed differential response as a function of particle impact velocity and impact angle. Under corrosion–erosion conditions the increase in impact velocity led to brittle fracture of the TiN layer for both impact angles. In some cases, the film was able to replicate the plastic strain of the substrate, but this only happened for the lower impact velocities (rotation speed below 3500 rpm). Increments
in velocity impact induced formation of cracks and the TiN coating was more easily removed, as it was confirmed by surface examination with SEM. In the specimens tested with a rotation speed of 2000 rpm the TiN film deformed together with the substrate, while in the specimens tested at 3500 and 5000 rpm cracks were observed. Fig. 7 shows the aspect of erosion marks after corrosion–erosion tests with 3500 rpm rotation speed (6.9 ms−1 mean impact velocity) and mean impact angle of 90◦ and 30◦ . The wear marks for normal
Fig. 9. Effect of the integrity of the TiN film on corrosion–erosion resistance. (a) Pitting corrosion of the substrate due to porosity in the film, 4.5 ms−1 mean impact velocity; (b) loss of mechanical support of the TiN layer due to pitting of the substrate, 6.9 ms−1 mean impact velocity; (c) preferential pitting on locals where droplets were removed by hard particles, 8.6 ms−1 mean impact velocity.
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Fig. 10. Effect of addition of hard particles to the corrosive solution. TiNcoated AISI 420 steel, 6.9 ms−1 mean impact velocity, 90◦ mean impact angle. The passive current density increases due to plastic deformation and cutting of TiN layer.
incidence are similar to hardness indentations (Fig. 7a and b), while under grazing impact evidences of scratching can be recognized (Fig. 7c and d). Surface examination in TiN-coated AISI 420 specimens revealed an additional failure mechanism, as shown in Fig. 8. Intergranular corrosion occurred as a consequence of chromium depletion in regions close to grain boundaries, which is associated to precipitation of dispersed chromium carbides. Mechanical support of the TiN layer is reduced in those regions and the film is then more vulnerable to erosive attack. 3.4. Effect of TiN film integrity The adherence of the TiN coatings to the substrate was determinant for corrosion–erosion resistance. Strongly-bonded layers showed low passive current density and high pitting potential after potentiodynamic tests. Low adherence led to spalling of the coating and occurrence of anodic current peak in polarization curves. Additionally, in some cases pitting
Fig. 11. Effect of addition of hard particles to the corrosive solution. Uncoated AISI 304 steel, 90◦ mean impact angle. The passive current density is higher for liquid impingement conditions.
corrosion took place in the substrate even when the film was unaffected, as can be seen in Fig. 9a. The solution was able to pass through the coating and locally attack the substrate, promoting formation of pits and reducing mechanical support of the coating (Fig. 9b). This failure mechanism can be related to the existence of pinholes and pores in the coating, which are often found in TiN layers deposited by PVD [5,6]. Droplet formation seems to have some influence on corrosion–erosion synergism, since most of the pits in TiN specimens were observed in or near to regions where these droplets were detached from the surface. Droplets are removed from the surface by the hard particles, leaving preferential sites for nucleation and growth of pits (Fig. 9c). 3.5. Effect of particle addition Addition of hard particles led to an increase in passive current density in coated specimens, as can be seen in Fig. 10. This effect is attributed to the mechanical damage caused to the protective TiN film by the SiO2 particles. Nevertheless,
Fig. 12. Plastic deformation at the surface of uncoated AISI 304 specimens submitted to corrosion–erosion under normal impact, 4.5 ms−1 mean impact velocity.
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Fig. 13. AFM maps of the surface of TiN-coated AISI 304 samples. (a) Not tested, droplets are visible; (b) after erosion for 1 h under normal impact, high plastic strain is evident; (c) after corrosion–erosion for 1 h under normal impact, surface morphology is smoother than under erosion, 6.9 ms−1 mean impact velocity.
in uncoated AISI 304 specimens a reduction in current density was observed under corrosion–erosion if compared with liquid impingement conditions (Fig. 11). This can be associated to a possible beneficial effect of plastic deformation on corrosion resistance, which has been reported in steels by several authors [13,14]. Fig. 12 shows SEM images of the surface of an AISI 304 sample with high plastic strain due to particle impact, and in Fig. 13 some AFM maps of the surface of TiN-coated specimens are showed before and after the tests.
4. Conclusions • Synergistic effects between corrosion and erosion were found after polarization tests and surface examination of TiN-coated stainless steel specimens. AISI 420 samples showed profuse intergranular and pitting corrosion, while in AISI 304 specimens the surface suffered intense plastic deformation. • Increasing impact velocity of erosive particles led to an increase in critical current density during polarization tests, which revealed the effect of mechanical action on uniform corrosion. • The pitting potential measured in electrochemical tests was unaffected by changes in the hydrodynamic conditions,
while the passive current density showed a trend to increase with impact velocity. However, in some cases the passive current density decreased when the hard particles were added to the solution. • Adherence between the TiN coating and the steel substrate strongly affected the generalized corrosion resistance of the tested specimens.
Acknowledgements The authors thank the Electrochemistry, Plasma Physics and Metallography Laboratories at National University of Colombia. Financial support provided by National University of Colombia in Medell´ın, Projects 2030100674 and 20301003539, is also acknowledged.
References [1] A. Toro, A. Sinatora, D.K. Tanaka, A.P. Tschiptschin, Corrosion– erosion of nitrogen bearing martensitic stainless steels in seawaterquartz slurry, Wear 251 (2001) 1257–1264. [2] A.V. Levy, P. Yau, Erosion of steels in liquid slurries, Wear 98 (1984) 163–182. [3] P.P. Rogers, I.M. Hutchings, J.A. Little, TiN coatings for protection against combined wear and oxidation, Surf. Eng. 8 (1992) 48–54.
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[4] H.W. Wang, M.M. Stack, Erosion of PVD TiN coatings under simultaneous corrosion in sodium carbonate/bicarbonate buffer slurries containing alumina particles, Surf. Coat. Technol. 106 (1998) 1–7. [5] F.S. Shieu, Y.C. Sung, L.H. Cheng, J. Huang, G. Yu, Control of the corrosion resistance of TiN coated AISI 304 stainless steel, Corr. Sci. 39 (5) (1997) 893–899. [6] G. Hˆakansson, L. Huitman, J.E. Sundgren, J.E. Greene, W.D. Munz, Microstructures of TiN films grown by various physical vapour deposition techniques, Surf. Coat. Technol. 48 (1991) 51–67. [7] D. Sanders, A. Anders, Review of cathodic arc deposition technology at the start of the new millennium, Surf. Coat. Technol. 133/134 (2000) 78–90. [8] F. Lang, Z. Yu, The corrosion resistance and wear resistance of thick TiN coatings deposited by arc ion plating, Surf. Coat. Technol. 145 (2001) 80–87.
[9] J.P. Tu, L.P. Zhu, H.X. Zhao, Slurry erosion characteristic of TiN coatings on Ti and plasma nitrided Ti alloy substrates, Surf. Coat. Technol. 122 (1999) 176–182. [10] H.M. Clark, K.K. Wong, Impact angle, particle energy and mass loss in erosion by dilute slurries, Wear 186/187 (1995) 454–464. [11] L. Prandtl, Guide a travers de la m´ecanique des fluides, Paris, 1952. [12] M. Bromark, P. Hedenqvist, S. Hogmark, The influence of substrate material on the erosion resistance of TiN coated tool steels, Wear 186/187 (1995) 189–194. [13] X.C. Lu, K. Shi, S.Z. Li, X.X. Jiang, Effects of surface deformation on corrosive wear of stainless steel in sulfuric acid solution, Wear 225–229 (1999) 537–543. [14] E.M. Gutman, G. Solovioff, D. Eliezer, The mechanochemical behavior of type 316L stainless steel, Corr. Sci. 38/7 (1996) 1141– 1145.
Corrosion–erosion behavior of TiN-coated stainless steels in aqueous slurries Diana L´opeza , Carlos S´anchezb , Alejandro Toroa,∗ a
Tribology and Surfaces Group, Materials Engineering School, National University of Colombia, Medell´ın, Colombia b Chemistry and Petroleum Engineering School, National University of Colombia, Medell´ın, Colombia Received 22 December 2003 Available online 18 October 2004
Abstract The corrosion–erosion resistance of TiN-coated AISI 304 and AISI 420 stainless steels in aqueous slurries was studied. TiN films with a thickness of 0.6 m were obtained by using the pulsed-arc plasma-assisted physical vapour deposition technique. The corrosion–erosion experiments were performed in a test machine in which the impingement velocity, impact angle, concentration of solids and pH of the solution were controlled. Polarization curves were simultaneously obtained to relate the electrochemical effects to the erosive wear mechanisms. The slurry used consists of quartz particles suspended in a mixture of sulphuric acid solution and 3.5% NaCl, with a pH value of 0.2. Measurement of critical and passive current densities showed that the behavior of coated materials differed according to substrate, but in a general way increasing impact velocity and changing from normal to grazing incidence led to a reduction in resistance to corrosion–erosion and liquid impingement corrosion. Surface analysis by SEM revealed formation of cracks in the coating and plastic deformation in both the substrate and the coating, especially when the mean impact velocity exceeded a critical value between 6.9 and 8.6 ms−1 . Additionally, intergranular corrosion was observed in some specimens. © 2004 Elsevier B.V. All rights reserved. Keywords: PVD coatings; Stainless steels; Corrosion–erosion; Electrochemical tests
1. Introduction Austenitic stainless steels are used in many components where corrosion resistance is crucial, such as slurry handling in food and chemical industries. Nevertheless, under mechanical action of hard particles, those steels show high plastic deformation and wear. If a corrosive solution carries the particles, the surface damage due to corrosion increases as a consequence of synergistic mechanisms between corrosion and erosion [1]. On the other hand, martensitic stainless steel shows better mechanical resistance to erosive particles than austenitic steel, but its corrosion resistance is lower [2].
∗
Corresponding author. Tel.: +57 4 4255339; fax: +57 4 2341002. E-mail address: [email protected] (A. Toro).
0043-1648/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2004.09.015
Titanium nitride (TiN) coatings are used as an alternative method to improve mechanical resistance in austenitic and corrosion resistance in martensitic stainless steels. As a result of high hardness, melting point and good chemical stability, TiN films can enhance the surface properties under wear and corrosion conditions [3,4]. The dependence between film properties and deposition conditions in PVD coatings has been extensively studied, and several authors have shown that substrate-to-film adherence, thickness and porosity are very important features [5–7]. Many studies on corrosion behavior have shown that TiN coatings can improve the corrosion response of a material if microstructural defects like porosity and pinholes are controlled [5]. Lang and Yu [8] reported that commercial, lowcarbon A3 steel coated with TiN showed much higher corrosion resistance in 1N H2 SO4 solution than the uncoated material, with very low critical current and passive current densities.
D. L´opez et al. / Wear 258 (2005) 684–692 Table 1 Chemical composition of the stainless steels used in this investigation (wt.%) Material
AISI 304
AISI 420
Cr C Ni Mn Si
18.6 0.07 8.4 1.0 0.65
13.4 0.34 – 1.0 0.52
Tu et al. [9] studied the slurry erosion behavior of TiNcoated Ti alloys by using distilled water containing 15 wt.% angular SiO2 , with impact velocities ranging from 6.4 to 15.2 ms−1 and normal incidence. They found that TiNcoated samples showed a better erosion resistance for impact velocity lower than 10 ms−1 . After that, the coating was removed from the substrate surface by perforation and spalling. The aim of this work was to study the interaction between corrosion and erosion mechanisms in TiN-coated AISI 304 and AISI 420 stainless steels, submitted to liquid impingement and corrosion–erosion conditions. The effects of impact velocity mean impact angle and particle addition to the flow are considered.
2. Experimental procedure 2.1. Materials Austenitic AISI 304 and martensitic AISI 420 stainless steels cylinders with 9.5 mm in diameter and 5 mm in height were used. The chemical compositions of the steels are shown in Table 1. The AISI 420 samples were austenized at 1373 K for 1 h, oil-quenched and tempered at 473 K for another hour. The microstructure obtained after this procedure was composed of martensite laths, retained austenite and dispersed carbides.
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On the other hand, the AISI 304 samples had a microstructure composed of austenite grains and some deformation bands formed as a consequence of cold working. 2.2. TiN films deposition TiN-coated samples were obtained by cathodic arc plasma deposition in a non-commercial PVD reactor. Each sample was ultrasonically cleaned and mounted with its surface 5 mm apart from a titanium cathode. A glow discharge was produced at a vacuum pressure of 1.7 mbar for 10 min to clean the substrate. TiN film deposition was carried out at 300 V in nitrogen-enriched plasma, which produced layers of ca. 200 nm in thickness after a single deposition and ca. 600 nm after five successive discharges. 2.3. Surface properties tests Polarization curves were obtained in a BAS 100b electrochemical analyzer with Ag/AgCl reference electrode and Pt counter electrode. The potential scan rate was 1 mV/s and the starting point was −600 mV for all the tests. Liquid impingement tests were performed in 0.5 M H2 SO4 + 3.5% NaCl solution with a pH value of 0.2, while in corrosion–erosion experiments 30 wt.% SiO2 particles with mean size between 212 and 300 m were added to the solution. All the samples were immersed in the slurry for 10 min before starting the potential scan. The corrosion–erosion and liquid impingement tests were performed in the slurry wear testing machine shown in Fig. 1. For each test, the slurry pot was filled with 700 g of solution and 300 g of SiO2 particles. The solids contents (30 wt.%), solution pH (0.2) and exposed area of the samples (7.13 × 10−5 m2 ) were fixed, while the rotation speed of the UHMWPE impeller and the mean impact angle of the flow over the specimens were controlled. The slurry temperature varied between 25 and 28 ◦ C for all the tests.
Fig. 1. Experimental setup for liquid impingement and corrosion–erosion tests.
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Impact angles of 30◦ and 90◦ and rotation speeds of 2000, 3500 and 5000 rpm were used, which correspond to impact velocities of ca. 4.5, 6.9 and 8.6 ms−1 , respectively. It is worth noticing that the reported impact angles represent mean values, given that in slurry erosion the particles are considerably deviated when they reach the surface, due to boundary layer and viscosity effects associated to the liquid carrier [10]. The mean impact velocity was calculated using a classical rotating disk model [11], with a viscosity value of 0.025 N s m−2 . 2.4. Surface examination and microstructure analysis The microstructure of the specimens was analyzed by optical (OM) and scanning electron microscopy (SEM), by using Olympus PME-Leica GZ6 and JEOL 591OLV microscopes, respectively. The structures of substrate and coated materials were studied by X-ray diffraction (XRD) in an Advance D8 equipment operating at 40 kV, 40 mA and grazing angle configuration. The morphology of surfaces, before and after the polarization tests was analyzed by SEM and atomic force microscopy (AFM). Additionally, some XPS spectra were
obtained from TiN-coated specimens using an Escalab 250 spectrometer.
3. Results and discussion 3.1. TiN film characterization Fig. 2a and b show the XRD spectra of coated AISI 304 and AISI 420 specimens respectively, in which the position of the TiN peaks indicates that the film is stoichiometric, with variations of ca. 2% in the lattice parameters. However, some XPS spectra taken from coated AISI 304 specimens (Fig. 2c and d) revealed a lower nitrogen concentration in the TiN coating. SEM examination and AFM mapping of the surfaces of the coated specimens revealed the formation of abundant droplets in both substrates, as a consequence of the TiN pulsed-arc deposition technique employed. Fig. 3 shows images of a TiN-coated surface of AISI 304 steel, whose aspect is similar to that observed in the coated AISI 420 surfaces.
Fig. 2. (a) XRD spectrum of TiN-coated AISI 304 steel; (b) XRD spectrum of TiN-coated AISI 420 steel; (c) XPS spectrum of TiN-coated AISI 304 steel, and (d) detail of XPS spectrum showing the Ti 2p peaks in the range 450–470 eV.
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Fig. 3. Surface morphology of a TiN-coated AISI 304 specimen. (a) AFM map, (b) SEM image. Droplets formed as a consequence of the deposition technique can be observed.
3.2. Effect of impact velocity and impact angle on surface properties Under normal impact the increase in rotation speed led to a reduction in uniform corrosion resistance of coated AISI 304 steel, as can be concluded from critical current density
measurements for liquid impingement and corrosion–erosion conditions (Fig. 4a). On the other hand, for grazing impact the polarization curves revealed a reduction in critical and passive current densities when rotation speed increased (Fig. 4b). Since this differential behavior with the mean impact angle was also observed in the uncoated steel (Fig. 4c and d), it is
Fig. 4. Effect of impact velocity on critical and passive current densities under corrosion–erosion conditions. (a) Normal impact, TiN-coated AISI 304; (b) grazing impact, TiN-coated AISI 304; (c) normal impact, AISI 304 and (d) grazing impact, AISI 304.
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Fig. 5. Effect of impact velocity on current density of TiN-coated AISI 420 specimens submitted to corrosion–erosion under grazing impact.
suggested that many of the features observed in the polarization curves for coated specimens are related to the mechanical and chemical properties of the substrate, as it was stated by Bromark et al. for TiN-coated tool steels [12]. Increasing impact velocity led to a reduction in the resistance to corrosion–erosion and liquid impingement of the TiN-coated AISI 420 specimens. Nevertheless, this effect was verified only by a slight increment in current density in polarization curves (Fig. 5). Additionally, when the parti-
Fig. 6. Effect of impact angle on current density of TiN-coated AISI 420 specimens submitted to corrosion–erosion.
cles reached the surface at grazing angles the measured passive current density was higher than under normal impact, as can be seen in polarization curves of Fig. 6 for the specific case of 5000 rpm rotation speed (8.6 ms−1 mean impact velocity). The pitting potential was not significantly affected when the rotation speed changed for all the materials tested. Balance between the beneficial effect of agitation and the harmful effect of the solid particles impact on the kinetics of the passive layer formation could explain this result.
Fig. 7. Wear marks after corrosion–erosion of AISI 304 and AISI 420 steels under normal (a) and (b) and grazing (c) and (d) impact. Slurry composed of (0.5 M H2 SO4 + 3.5% NaCl) solution + 30 wt.% quartz particles; 6.9 ms−1 mean impact velocity.
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Fig. 8. Intergranular and pitting corrosion in TiN-coated AISI 420 steel samples submitted to corrosion–erosion under normal impact, (a) 4.5 ms−1 mean impact velocity; (b) 8.6 ms−1 mean impact velocity.
3.3. Mass removal mechanisms The TiN-coated steels showed differential response as a function of particle impact velocity and impact angle. Under corrosion–erosion conditions the increase in impact velocity led to brittle fracture of the TiN layer for both impact angles. In some cases, the film was able to replicate the plastic strain of the substrate, but this only happened for the lower impact velocities (rotation speed below 3500 rpm). Increments
in velocity impact induced formation of cracks and the TiN coating was more easily removed, as it was confirmed by surface examination with SEM. In the specimens tested with a rotation speed of 2000 rpm the TiN film deformed together with the substrate, while in the specimens tested at 3500 and 5000 rpm cracks were observed. Fig. 7 shows the aspect of erosion marks after corrosion–erosion tests with 3500 rpm rotation speed (6.9 ms−1 mean impact velocity) and mean impact angle of 90◦ and 30◦ . The wear marks for normal
Fig. 9. Effect of the integrity of the TiN film on corrosion–erosion resistance. (a) Pitting corrosion of the substrate due to porosity in the film, 4.5 ms−1 mean impact velocity; (b) loss of mechanical support of the TiN layer due to pitting of the substrate, 6.9 ms−1 mean impact velocity; (c) preferential pitting on locals where droplets were removed by hard particles, 8.6 ms−1 mean impact velocity.
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Fig. 10. Effect of addition of hard particles to the corrosive solution. TiNcoated AISI 420 steel, 6.9 ms−1 mean impact velocity, 90◦ mean impact angle. The passive current density increases due to plastic deformation and cutting of TiN layer.
incidence are similar to hardness indentations (Fig. 7a and b), while under grazing impact evidences of scratching can be recognized (Fig. 7c and d). Surface examination in TiN-coated AISI 420 specimens revealed an additional failure mechanism, as shown in Fig. 8. Intergranular corrosion occurred as a consequence of chromium depletion in regions close to grain boundaries, which is associated to precipitation of dispersed chromium carbides. Mechanical support of the TiN layer is reduced in those regions and the film is then more vulnerable to erosive attack. 3.4. Effect of TiN film integrity The adherence of the TiN coatings to the substrate was determinant for corrosion–erosion resistance. Strongly-bonded layers showed low passive current density and high pitting potential after potentiodynamic tests. Low adherence led to spalling of the coating and occurrence of anodic current peak in polarization curves. Additionally, in some cases pitting
Fig. 11. Effect of addition of hard particles to the corrosive solution. Uncoated AISI 304 steel, 90◦ mean impact angle. The passive current density is higher for liquid impingement conditions.
corrosion took place in the substrate even when the film was unaffected, as can be seen in Fig. 9a. The solution was able to pass through the coating and locally attack the substrate, promoting formation of pits and reducing mechanical support of the coating (Fig. 9b). This failure mechanism can be related to the existence of pinholes and pores in the coating, which are often found in TiN layers deposited by PVD [5,6]. Droplet formation seems to have some influence on corrosion–erosion synergism, since most of the pits in TiN specimens were observed in or near to regions where these droplets were detached from the surface. Droplets are removed from the surface by the hard particles, leaving preferential sites for nucleation and growth of pits (Fig. 9c). 3.5. Effect of particle addition Addition of hard particles led to an increase in passive current density in coated specimens, as can be seen in Fig. 10. This effect is attributed to the mechanical damage caused to the protective TiN film by the SiO2 particles. Nevertheless,
Fig. 12. Plastic deformation at the surface of uncoated AISI 304 specimens submitted to corrosion–erosion under normal impact, 4.5 ms−1 mean impact velocity.
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Fig. 13. AFM maps of the surface of TiN-coated AISI 304 samples. (a) Not tested, droplets are visible; (b) after erosion for 1 h under normal impact, high plastic strain is evident; (c) after corrosion–erosion for 1 h under normal impact, surface morphology is smoother than under erosion, 6.9 ms−1 mean impact velocity.
in uncoated AISI 304 specimens a reduction in current density was observed under corrosion–erosion if compared with liquid impingement conditions (Fig. 11). This can be associated to a possible beneficial effect of plastic deformation on corrosion resistance, which has been reported in steels by several authors [13,14]. Fig. 12 shows SEM images of the surface of an AISI 304 sample with high plastic strain due to particle impact, and in Fig. 13 some AFM maps of the surface of TiN-coated specimens are showed before and after the tests.
4. Conclusions • Synergistic effects between corrosion and erosion were found after polarization tests and surface examination of TiN-coated stainless steel specimens. AISI 420 samples showed profuse intergranular and pitting corrosion, while in AISI 304 specimens the surface suffered intense plastic deformation. • Increasing impact velocity of erosive particles led to an increase in critical current density during polarization tests, which revealed the effect of mechanical action on uniform corrosion. • The pitting potential measured in electrochemical tests was unaffected by changes in the hydrodynamic conditions,
while the passive current density showed a trend to increase with impact velocity. However, in some cases the passive current density decreased when the hard particles were added to the solution. • Adherence between the TiN coating and the steel substrate strongly affected the generalized corrosion resistance of the tested specimens.
Acknowledgements The authors thank the Electrochemistry, Plasma Physics and Metallography Laboratories at National University of Colombia. Financial support provided by National University of Colombia in Medell´ın, Projects 2030100674 and 20301003539, is also acknowledged.
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