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Protection of cylinders by ion-beam sputter deposition: corrosion of carbon-coated aluminium tubes
Surface and Coatings Technology 158 – 159 (2002) 599–603
Protection of cylinders by ion-beam sputter deposition: corrosion of carbon-coated aluminium tubes O. Lenscha, Th. Krausb, Ch. Sundermanna, B. Endersa, W. Ensingera,* a
Philipps-University Marburg, Department of Chemistry, Materials Science Centre, Hans-Meerwein Strasse, 35032 Marburg, Germany b University of Augsburg, Institute of Physics, Augsburg, Germany
Abstract In application, cylinders and tubes may chemically fail when they are exposed to aggressive media. As for other objects, coating with a protective film may improve the situation. However, coating the inner walls of hollow objects, such as tubes, by means of physical vapour deposition techniques is not feasible, because the material to be deposited has to enter the tube under very flat angles to the surface normal of the walls, depending on the ratio of the inner diameter to the tube length. This problem can be overcome when the source of the material to be deposited is located inside the tube. This is possible when sputter deposition is performed with an ion beam. A sputter target is located inside the tube; energetic ions are accelerated into this tube and impinge onto the target. Thus, material is sputtered from the target onto the inner walls of the tube. With this technique, aluminium tubes were coated inside with thin, amorphous carbon films. The corrosion performance in an aqueous chloridecontaining environment, where aluminium suffers from pitting corrosion, was evaluated by means of electrochemical polarisation measurements. These showed that the films exhibited low microporosity with a good corrosion protection effect when appropriate deposition process parameters were used. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Ion beam sputter deposition; Corrosion; Pitting corrosion; Aluminium; Carbon coatings
1. Introduction The mechanical or chemical properties of inner surfaces of hollow objects, such as cylinders and tubes, can be improved when they are coated with a functional thin film. In principle, physical vapour deposition (PVD) techniques are suitable for this purpose. Using PVD techniques such as electron beam evaporation or sputtering, the material to be deposited has to enter the tube under oblique angles to the wall normal, depending on the ratio of the tube diameter to its length. Transport of the vapour through the narrow tube aperture and along the tube, and deposition of the vapour atoms under very oblique angles may create problems, both with film homogeneity and film properties. In this work a modified ion-beam technique was used to solve this problem, which is shown schematically in Fig. 1. For ion-beam sputter deposition, a sputter target is inserted into a tube. Energetic ions enter the tube and impinge *Corresponding author. E-mail address: [email protected] (W. Ensinger).
onto the target, and sputtered material is deposited onto the inner wall of the tube w1,2x. When the sputter target is moved through the tube, coating can be achieved along the whole inner tube wall. Atoms, and to a minor degree clusters, are sputtered and deposited onto the tube inner walls. As the ions do not impinge perpendicularly, the sputter coefficient is enhanced w3x and an increased deposition rate is achieved. A portion of the ions is scattered from the sputter target. These ions penetrate the growing film and exert typical ion-beam effects, such as ballistic intermixing of the coating and substrate, leading to structural changes. These processes affect the interface between the film and substrate, and the film structure and its properties. In the following, the corrosion properties of aluminium tubes coated with amorphous carbon films are discussed. Two aluminium surface-finishing states prior to deposition are discussed, mirror-like polished and technically rough surfaces. 2. Experimental details Tubes of aluminium of 99.9% purity with an inner diameter of 16 mm and a length of 170 mm were
0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 3 1 7 - 1
600
O. Lensch et al. / Surface and Coatings Technology 158 – 159 (2002) 599–603
3. Results and discussion
Fig. 1. Schematic presentation of tube inner-wall coating: an ion beam impinges onto a conical sputter target, which moves through the tube; atoms from the sputter target are deposited onto the tube inner walls, and a part of the ion beam is reflected from the sputter target and leads to ion beam mixing of the film and substrate.
polished to a mirror finish. Another set was ground with 600-mesh SiC emery paper as an example of a technically rough surface. The apparatus for coating tubes has been described in detail elsewhere w4x. It consists of a commercial-type ion implanter with a separation magnet and beam-guiding elements. The tubes were brought directly into the beam line. They were rotated by means of an electromotor during coating in order to provide maximum homogeneity. The graphite sputter target was conical with a diameter of 14 mm and a cone angle of 608, so that the angle of ion incidence was 608 to the surface normal of the sputter target. It was pushed through the tube at a maximum speed of 5 mmys by means of an in vacuo stepper motor. The target was bombarded with Arq ions with energy of 40, 80 and 100 keV at currents between 30 and 90 mA. The thickness of the films deposited was determined by Rutherford backscattering spectrometry with 3.4-MeV He2q ions at a scattering angle of 1658. For evaluating the thickness, the spectra were simulated by the RUMP code w5x, assuming a value for the density of amorphous carbon. During the experiment, the thickness was set to be in the range from 75 to 85 nm. The corrosion performance was tested in a 0.6% aerated sodium chloride solution at 25 8C by electrochemical polarisation measurements. The experiments were carried out with a standard three-electrode set-up, with the working electrode, a saturated standard calomel reference electrode (SCE) and a graphite counter-electrode connected to a potentiostat and a PC w6x. The potentiostatic technique was used for obtaining current density vs. potential plots. A constant potential was maintained between the sample and the reference electrode for 30 min. Within this time the current response was recorded. The potential was then increased in 10mV steps and the stationary current density values were finally plotted against the electrode potential.
As mentioned in Section 2, the corrosion behaviour was determined with electrochemical polarisation measurements. Fig. 2 shows a current density vs. potential plot of polished, uncoated aluminium. For more detailed information of the corrosion properties of uncoated aluminium, see for example w7–10x. Briefly, the corrosion behaviour can be described as follows. At large negative potential of approximately y800 mV vs. SCE, aluminium acts as a cathode. The main reaction here is hydrogen generation, together with molecular oxygen reduction from the electrolyte. When the potential is increased to less negative values, the metal becomes anodic above the corrosion potential Ecorr. Here, aluminium is protected by a thin, natural, dense oxide film and corrodes in a passive mode at low currents. When the potential is further increased, a steep increase in the current density is observed at the so-called pitting potential Ep. Pitting corrosion starts at this potential and slightly increasing the electrode potential results in high, local dissolution rates and the formation of deep pits. In general, a shift of the pitting potential in the anodic direction and a reduction of the anodic current density is an indication of reduced pitting susceptibility of the material. In Fig. 3 uncoated aluminium is compared with a sample coated with a carbon film, with deposition conditions of an 80-keV argon ion beam with a 90-mA current on the sputter target. In comparison to the uncoated aluminium, the coated sample shows a higher cathodic current density and a more noble corrosion potential. A clear difference between the free corrosion and pitting potential cannot be made here, as aluminium dissolution through the pitting process occurs at potential values slightly above Ecorr. In contrast to uncoated aluminium, the anodic current density is reduced by
Fig. 2. Semilogarithmic current density vs. electrode potential plot of a polished Al sample; measurement points represent steady-state current density under potentiostatic conditions in 0.6% NaCl solution at 25 8C.
O. Lensch et al. / Surface and Coatings Technology 158 – 159 (2002) 599–603
601
Fig. 3. Semilogarithmic current density vs. electrode potential plot of polished Al samples: uncoated and coated with carbon at a sputtering argon ion energy of 80 keV at an argon ion current of 90 mA.
Fig. 4. Semilogarithmic current density vs. electrode potential plot of polished Al sample: coated with carbon at a sputtering argon ion energy of 80 keV at different argon ion currents.
more than one order of magnitude and gradually increases with increasing electrode potential. At y260 mV vs. SCE, current density values are in the range for uncoated aluminium. After measurements, the coated sample shows severe pitting and most of the formerly coated area is corroded. It is remarkable to note that the free corrosion potential is considerably shifted in the more noble direction. In comparison to pure aluminium without its native oxide, where Ecorr is approximately y 1600 mV, the native oxide shifts Ecorr by nearly 1 V in anodic direction, to approximately y770 mV. Coating with ion beam-assisted sputtering under the conditions in the present case, the system becomes even more noble, with Ecorr at y560 mV vs. SCE. Therefore, coating aluminium with amorphous carbon can enhance the nobility of the system. It is noteworthy that this finding becomes important when such systems are in electrical contact with more noble materials in mixedcomponent constructions, for example together with stainless steels. The lower the free corrosion potential difference between the two materials in contact, the more the susceptibility to contact corrosion is reduced. As well as the free corrosion potential, the pitting potential also appears at more noble values than for uncoated Al, which indicates an improvement in the corrosion behaviour in sodium chloride solution. Lowering the ion current for sputtering bombardment from 90 to 30 mA and keeping the ion beam energy constant at 80 keV, Fig. 4 shows the current density vs. potential plot results. A dramatic change can be observed here. No stable pitting corrosion in the potential region measured can be observed any more; the carbon-coated aluminium remains mainly passive until y200 mV vs. SCE. After the measurement, the major part of the aluminium surface covered by the amorphous carbon layer deposited with the lower ion-beam current remains visually unaltered. Only a few pits can be observed. In
short, lower ion current on the sputter target leads to an improvement in the pitting corrosion properties of the whole system. In Fig. 5 two example curves are shown for which the ion current was kept constant at 50 mA on the sputter target and the ion energy was varied. The sample prepared with 40 keV shows a much-improved corrosion performance compared to that prepared with 100 keV. It has a more anodic Ecorr value, a lower anodic current density and a more noble pitting potential. Taking the results shown in Figs. 4 and 5 together reveals the fact that covering the surface of aluminium with amorphous carbon by ion beam-assisted sputtering can effectively suppress pitting of the underlying aluminium over a wide electrode potential range if proper deposition conditions are chosen. Lowering either the ion beam current at constant ion energy or the ion energy when keeping the beam current constant, or in other words lowering the overall power input of ion
Fig. 5. Semilogarithmic current density vs. electrode potential plot of polished Al samples: coated with carbon at an argon ion current of 50 mA at different argon ion energy values.
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O. Lensch et al. / Surface and Coatings Technology 158 – 159 (2002) 599–603
Fig. 6. Semilogarithmic current density vs. electrode potential plot of ground Al samples: uncoated and coated with carbon at a sputtering argon ion energy of 80 keV and ion current of 90 mA.
current=ion energy, results in reduced pitting corrosion susceptibility. The reason for this effect is due to either an alteration to the interface between the carbon coating and the aluminium substrate oryand a change in the film porosity. Aluminium carbides might be formed in the aluminiumycarbon interface, which can act as a cathode and therefore enhance the anodic aluminium dissolution. In contrast to the findings on polished aluminium, results for the pitting corrosion properties for coated ground Al samples do not show great differences for amorphous carbon deposited under different conditions. In Fig. 6 the current density vs. potential diagram of a coated ground sample in comparison to uncoated ground aluminium is plotted. The greatest difference here is the higher cathodic current density of the coated ground sample in comparison to uncoated aluminium, together with a more noble free corrosion potential. These facts are not proof of better or worse corrosion behaviour, as the main differences are in the cathodic region, where the materials are mainly inert. In the anodic region the current density values of the coated and uncoated samples are identical within a small error. Therefore, the corrosion behaviour of the coated ground sample changes only slightly compared to the uncoated one. In Fig. 7, a series of inner tube-coated samples of ground aluminium is shown in comparison. Here it is evident that the corrosion behaviour, i.e. the anodic dissolution behaviour, does not depend on the type of the coating. The scanning electron micrograph in Fig. 8 compares the surface of carbon-coated and uncoated ground pure aluminium for the sample prepared with 40-keV Arq and 50 mA. From this picture a difference can be observed. The micrograph shows that the coating on ground Al has a smoother surface on the top parts of scratches, which could be a result of highly energetic ion bombardment and the resultant sputtering. This finding will be a topic of future research.
Fig. 7. Semilogarithmic current density vs. electrode potential plot of ground Al-samples: uncoated and coated with carbon at different sputtering argon ion energy values and argon ion current.
4. Summary Despite being a non-noble metal and unstable from a thermodynamic point of view, aluminium is highly corrosion-resistant, as it is protected by a natural, dense oxide film. However, in the presence of reactive anions, such as halides, it may suffer from pitting corrosion. Tubes of aluminium in contact with halides may develop pits and eventually fail. Ion-beam sputter deposition is a possible method for coating the inside of tubes. A good choice is carbon, as it is stable against pitting and
Fig. 8. Scanning electron micrograph of (a) ground pure aluminium and (b) carbon-coated Al prepared with 40 keV Arq and 50 mA.
O. Lensch et al. / Surface and Coatings Technology 158 – 159 (2002) 599–603
shows low intrinsic microporosity. The inner walls of polished and ground aluminium tubes were coated with thin amorphous carbon films by rare gas ion-beam sputtering of a conical sputter target. Under the conditions given, the temperature remained below 200 8C. Corrosion tests in chloride-containing water showed that, in the case of smooth surfaces, the films may serve for corrosion protection. The effectiveness of corrosion protection strongly depends on the deposition process parameters. In the case of very rough surfaces, the corrosion protection effect is much lower, but coating with carbon under high energetic argon bombardment smoothes the surfaces to a certain extent. Acknowledgments This study was supported by Deutsche Forschungsgemeinschaft.
603
References w1x W. Ensinger, Rev. Sci. Instrum. 67 (1996) 318. w2x W. Ensinger, Nucl. Instrum. Methods Phys. Res. B 127y128 (1997) 775. w3x R. Behrisch (Ed.), Sputtering by Particle Bombardment, Springer, Berlin, 1983. w4x Th. Kraus, R. Kern, B. Stritzker, W. Ensinger, Nucl. Instrum. Methods Phys. Res. B 148 (1999) 912. w5x L.R. Doolittle, Nucl. Instrum. Methods Phys. Res. B 9 (1985) 402. w6x W. Ensinger, Protection against aqueous corrosion by ion implantation and ion beam assisted deposition, in: Y. Pauleau, P.B. Barna (Eds.), Protective Coatings and Thin Films: Synthesis, Characterization and Applications, Kluwer Academic Publishers, Dordrecht, 1997, p. 585. w7x M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, 2nd ed., NACE, Houston, 1974, In French.. w8x Z. Szklarska-Smialowska, Corros. Sci. 41 (1999) 1743. w9x H.H. Uhlig, Corrosion and Corrosion Control, Wiley, New York, 1971. w10x H. Kaesche, The Corrosion of Metals, Springer, Berlin, Heidelberg, NewYork, 1990, In German..
Protection of cylinders by ion-beam sputter deposition: corrosion of carbon-coated aluminium tubes O. Lenscha, Th. Krausb, Ch. Sundermanna, B. Endersa, W. Ensingera,* a
Philipps-University Marburg, Department of Chemistry, Materials Science Centre, Hans-Meerwein Strasse, 35032 Marburg, Germany b University of Augsburg, Institute of Physics, Augsburg, Germany
Abstract In application, cylinders and tubes may chemically fail when they are exposed to aggressive media. As for other objects, coating with a protective film may improve the situation. However, coating the inner walls of hollow objects, such as tubes, by means of physical vapour deposition techniques is not feasible, because the material to be deposited has to enter the tube under very flat angles to the surface normal of the walls, depending on the ratio of the inner diameter to the tube length. This problem can be overcome when the source of the material to be deposited is located inside the tube. This is possible when sputter deposition is performed with an ion beam. A sputter target is located inside the tube; energetic ions are accelerated into this tube and impinge onto the target. Thus, material is sputtered from the target onto the inner walls of the tube. With this technique, aluminium tubes were coated inside with thin, amorphous carbon films. The corrosion performance in an aqueous chloridecontaining environment, where aluminium suffers from pitting corrosion, was evaluated by means of electrochemical polarisation measurements. These showed that the films exhibited low microporosity with a good corrosion protection effect when appropriate deposition process parameters were used. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Ion beam sputter deposition; Corrosion; Pitting corrosion; Aluminium; Carbon coatings
1. Introduction The mechanical or chemical properties of inner surfaces of hollow objects, such as cylinders and tubes, can be improved when they are coated with a functional thin film. In principle, physical vapour deposition (PVD) techniques are suitable for this purpose. Using PVD techniques such as electron beam evaporation or sputtering, the material to be deposited has to enter the tube under oblique angles to the wall normal, depending on the ratio of the tube diameter to its length. Transport of the vapour through the narrow tube aperture and along the tube, and deposition of the vapour atoms under very oblique angles may create problems, both with film homogeneity and film properties. In this work a modified ion-beam technique was used to solve this problem, which is shown schematically in Fig. 1. For ion-beam sputter deposition, a sputter target is inserted into a tube. Energetic ions enter the tube and impinge *Corresponding author. E-mail address: [email protected] (W. Ensinger).
onto the target, and sputtered material is deposited onto the inner wall of the tube w1,2x. When the sputter target is moved through the tube, coating can be achieved along the whole inner tube wall. Atoms, and to a minor degree clusters, are sputtered and deposited onto the tube inner walls. As the ions do not impinge perpendicularly, the sputter coefficient is enhanced w3x and an increased deposition rate is achieved. A portion of the ions is scattered from the sputter target. These ions penetrate the growing film and exert typical ion-beam effects, such as ballistic intermixing of the coating and substrate, leading to structural changes. These processes affect the interface between the film and substrate, and the film structure and its properties. In the following, the corrosion properties of aluminium tubes coated with amorphous carbon films are discussed. Two aluminium surface-finishing states prior to deposition are discussed, mirror-like polished and technically rough surfaces. 2. Experimental details Tubes of aluminium of 99.9% purity with an inner diameter of 16 mm and a length of 170 mm were
0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 3 1 7 - 1
600
O. Lensch et al. / Surface and Coatings Technology 158 – 159 (2002) 599–603
3. Results and discussion
Fig. 1. Schematic presentation of tube inner-wall coating: an ion beam impinges onto a conical sputter target, which moves through the tube; atoms from the sputter target are deposited onto the tube inner walls, and a part of the ion beam is reflected from the sputter target and leads to ion beam mixing of the film and substrate.
polished to a mirror finish. Another set was ground with 600-mesh SiC emery paper as an example of a technically rough surface. The apparatus for coating tubes has been described in detail elsewhere w4x. It consists of a commercial-type ion implanter with a separation magnet and beam-guiding elements. The tubes were brought directly into the beam line. They were rotated by means of an electromotor during coating in order to provide maximum homogeneity. The graphite sputter target was conical with a diameter of 14 mm and a cone angle of 608, so that the angle of ion incidence was 608 to the surface normal of the sputter target. It was pushed through the tube at a maximum speed of 5 mmys by means of an in vacuo stepper motor. The target was bombarded with Arq ions with energy of 40, 80 and 100 keV at currents between 30 and 90 mA. The thickness of the films deposited was determined by Rutherford backscattering spectrometry with 3.4-MeV He2q ions at a scattering angle of 1658. For evaluating the thickness, the spectra were simulated by the RUMP code w5x, assuming a value for the density of amorphous carbon. During the experiment, the thickness was set to be in the range from 75 to 85 nm. The corrosion performance was tested in a 0.6% aerated sodium chloride solution at 25 8C by electrochemical polarisation measurements. The experiments were carried out with a standard three-electrode set-up, with the working electrode, a saturated standard calomel reference electrode (SCE) and a graphite counter-electrode connected to a potentiostat and a PC w6x. The potentiostatic technique was used for obtaining current density vs. potential plots. A constant potential was maintained between the sample and the reference electrode for 30 min. Within this time the current response was recorded. The potential was then increased in 10mV steps and the stationary current density values were finally plotted against the electrode potential.
As mentioned in Section 2, the corrosion behaviour was determined with electrochemical polarisation measurements. Fig. 2 shows a current density vs. potential plot of polished, uncoated aluminium. For more detailed information of the corrosion properties of uncoated aluminium, see for example w7–10x. Briefly, the corrosion behaviour can be described as follows. At large negative potential of approximately y800 mV vs. SCE, aluminium acts as a cathode. The main reaction here is hydrogen generation, together with molecular oxygen reduction from the electrolyte. When the potential is increased to less negative values, the metal becomes anodic above the corrosion potential Ecorr. Here, aluminium is protected by a thin, natural, dense oxide film and corrodes in a passive mode at low currents. When the potential is further increased, a steep increase in the current density is observed at the so-called pitting potential Ep. Pitting corrosion starts at this potential and slightly increasing the electrode potential results in high, local dissolution rates and the formation of deep pits. In general, a shift of the pitting potential in the anodic direction and a reduction of the anodic current density is an indication of reduced pitting susceptibility of the material. In Fig. 3 uncoated aluminium is compared with a sample coated with a carbon film, with deposition conditions of an 80-keV argon ion beam with a 90-mA current on the sputter target. In comparison to the uncoated aluminium, the coated sample shows a higher cathodic current density and a more noble corrosion potential. A clear difference between the free corrosion and pitting potential cannot be made here, as aluminium dissolution through the pitting process occurs at potential values slightly above Ecorr. In contrast to uncoated aluminium, the anodic current density is reduced by
Fig. 2. Semilogarithmic current density vs. electrode potential plot of a polished Al sample; measurement points represent steady-state current density under potentiostatic conditions in 0.6% NaCl solution at 25 8C.
O. Lensch et al. / Surface and Coatings Technology 158 – 159 (2002) 599–603
601
Fig. 3. Semilogarithmic current density vs. electrode potential plot of polished Al samples: uncoated and coated with carbon at a sputtering argon ion energy of 80 keV at an argon ion current of 90 mA.
Fig. 4. Semilogarithmic current density vs. electrode potential plot of polished Al sample: coated with carbon at a sputtering argon ion energy of 80 keV at different argon ion currents.
more than one order of magnitude and gradually increases with increasing electrode potential. At y260 mV vs. SCE, current density values are in the range for uncoated aluminium. After measurements, the coated sample shows severe pitting and most of the formerly coated area is corroded. It is remarkable to note that the free corrosion potential is considerably shifted in the more noble direction. In comparison to pure aluminium without its native oxide, where Ecorr is approximately y 1600 mV, the native oxide shifts Ecorr by nearly 1 V in anodic direction, to approximately y770 mV. Coating with ion beam-assisted sputtering under the conditions in the present case, the system becomes even more noble, with Ecorr at y560 mV vs. SCE. Therefore, coating aluminium with amorphous carbon can enhance the nobility of the system. It is noteworthy that this finding becomes important when such systems are in electrical contact with more noble materials in mixedcomponent constructions, for example together with stainless steels. The lower the free corrosion potential difference between the two materials in contact, the more the susceptibility to contact corrosion is reduced. As well as the free corrosion potential, the pitting potential also appears at more noble values than for uncoated Al, which indicates an improvement in the corrosion behaviour in sodium chloride solution. Lowering the ion current for sputtering bombardment from 90 to 30 mA and keeping the ion beam energy constant at 80 keV, Fig. 4 shows the current density vs. potential plot results. A dramatic change can be observed here. No stable pitting corrosion in the potential region measured can be observed any more; the carbon-coated aluminium remains mainly passive until y200 mV vs. SCE. After the measurement, the major part of the aluminium surface covered by the amorphous carbon layer deposited with the lower ion-beam current remains visually unaltered. Only a few pits can be observed. In
short, lower ion current on the sputter target leads to an improvement in the pitting corrosion properties of the whole system. In Fig. 5 two example curves are shown for which the ion current was kept constant at 50 mA on the sputter target and the ion energy was varied. The sample prepared with 40 keV shows a much-improved corrosion performance compared to that prepared with 100 keV. It has a more anodic Ecorr value, a lower anodic current density and a more noble pitting potential. Taking the results shown in Figs. 4 and 5 together reveals the fact that covering the surface of aluminium with amorphous carbon by ion beam-assisted sputtering can effectively suppress pitting of the underlying aluminium over a wide electrode potential range if proper deposition conditions are chosen. Lowering either the ion beam current at constant ion energy or the ion energy when keeping the beam current constant, or in other words lowering the overall power input of ion
Fig. 5. Semilogarithmic current density vs. electrode potential plot of polished Al samples: coated with carbon at an argon ion current of 50 mA at different argon ion energy values.
602
O. Lensch et al. / Surface and Coatings Technology 158 – 159 (2002) 599–603
Fig. 6. Semilogarithmic current density vs. electrode potential plot of ground Al samples: uncoated and coated with carbon at a sputtering argon ion energy of 80 keV and ion current of 90 mA.
current=ion energy, results in reduced pitting corrosion susceptibility. The reason for this effect is due to either an alteration to the interface between the carbon coating and the aluminium substrate oryand a change in the film porosity. Aluminium carbides might be formed in the aluminiumycarbon interface, which can act as a cathode and therefore enhance the anodic aluminium dissolution. In contrast to the findings on polished aluminium, results for the pitting corrosion properties for coated ground Al samples do not show great differences for amorphous carbon deposited under different conditions. In Fig. 6 the current density vs. potential diagram of a coated ground sample in comparison to uncoated ground aluminium is plotted. The greatest difference here is the higher cathodic current density of the coated ground sample in comparison to uncoated aluminium, together with a more noble free corrosion potential. These facts are not proof of better or worse corrosion behaviour, as the main differences are in the cathodic region, where the materials are mainly inert. In the anodic region the current density values of the coated and uncoated samples are identical within a small error. Therefore, the corrosion behaviour of the coated ground sample changes only slightly compared to the uncoated one. In Fig. 7, a series of inner tube-coated samples of ground aluminium is shown in comparison. Here it is evident that the corrosion behaviour, i.e. the anodic dissolution behaviour, does not depend on the type of the coating. The scanning electron micrograph in Fig. 8 compares the surface of carbon-coated and uncoated ground pure aluminium for the sample prepared with 40-keV Arq and 50 mA. From this picture a difference can be observed. The micrograph shows that the coating on ground Al has a smoother surface on the top parts of scratches, which could be a result of highly energetic ion bombardment and the resultant sputtering. This finding will be a topic of future research.
Fig. 7. Semilogarithmic current density vs. electrode potential plot of ground Al-samples: uncoated and coated with carbon at different sputtering argon ion energy values and argon ion current.
4. Summary Despite being a non-noble metal and unstable from a thermodynamic point of view, aluminium is highly corrosion-resistant, as it is protected by a natural, dense oxide film. However, in the presence of reactive anions, such as halides, it may suffer from pitting corrosion. Tubes of aluminium in contact with halides may develop pits and eventually fail. Ion-beam sputter deposition is a possible method for coating the inside of tubes. A good choice is carbon, as it is stable against pitting and
Fig. 8. Scanning electron micrograph of (a) ground pure aluminium and (b) carbon-coated Al prepared with 40 keV Arq and 50 mA.
O. Lensch et al. / Surface and Coatings Technology 158 – 159 (2002) 599–603
shows low intrinsic microporosity. The inner walls of polished and ground aluminium tubes were coated with thin amorphous carbon films by rare gas ion-beam sputtering of a conical sputter target. Under the conditions given, the temperature remained below 200 8C. Corrosion tests in chloride-containing water showed that, in the case of smooth surfaces, the films may serve for corrosion protection. The effectiveness of corrosion protection strongly depends on the deposition process parameters. In the case of very rough surfaces, the corrosion protection effect is much lower, but coating with carbon under high energetic argon bombardment smoothes the surfaces to a certain extent. Acknowledgments This study was supported by Deutsche Forschungsgemeinschaft.
603
References w1x W. Ensinger, Rev. Sci. Instrum. 67 (1996) 318. w2x W. Ensinger, Nucl. Instrum. Methods Phys. Res. B 127y128 (1997) 775. w3x R. Behrisch (Ed.), Sputtering by Particle Bombardment, Springer, Berlin, 1983. w4x Th. Kraus, R. Kern, B. Stritzker, W. Ensinger, Nucl. Instrum. Methods Phys. Res. B 148 (1999) 912. w5x L.R. Doolittle, Nucl. Instrum. Methods Phys. Res. B 9 (1985) 402. w6x W. Ensinger, Protection against aqueous corrosion by ion implantation and ion beam assisted deposition, in: Y. Pauleau, P.B. Barna (Eds.), Protective Coatings and Thin Films: Synthesis, Characterization and Applications, Kluwer Academic Publishers, Dordrecht, 1997, p. 585. w7x M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, 2nd ed., NACE, Houston, 1974, In French.. w8x Z. Szklarska-Smialowska, Corros. Sci. 41 (1999) 1743. w9x H.H. Uhlig, Corrosion and Corrosion Control, Wiley, New York, 1971. w10x H. Kaesche, The Corrosion of Metals, Springer, Berlin, Heidelberg, NewYork, 1990, In German..