Review of Various Surface Treatment Techniques on Titanium ...

Vol-23-1&2

1

J. Surface Sci. Technol., Vol 23, No. 1-2, pp. 49-58, 2007 © 2007 Indian Society for Surface Science and Technology, India.

Review of Various Surface Treatment Techniques on Titanium Alloys and Their Protective Effects against Corrosion V. VIJAY a , K . A R J U N a , H . A R U N K U M A R a , A . V I N O T H K U M A R a , P. CHANDRASEKARa* and V. BALUSAMYb a b

Dr. Mahalingam College of Engg. & Tech., Pollachi, 642 003, Tamil Nadu, India. P.S.G. College of Technology, Coimbatore. 641 004, Tamil Nadu, India.

Abstract This paper attempts at a review of the recent developments on the improvement of surface corrosion in titanium alloys and composites by various surface treatment techniques. The surface treatment techniques include different types of laser surface coating, plasma enhanced vapour deposition, high intensity plasma beam coating, cathodic are deposition and reactive magnetron sputtering. Also the effect of laser surface treatment on Ti-TiB composite and its corrosion resistance properties were reviewed. The possibility of laser surface modification of Ti-TiB composite surface for enhancement of corrosion properties was also reviewed and discussed. Keywords : Titanium alloys and composites, corrosion, surface treatment, protective effect.

INTRODUCTION

Titanium alloys are known to have better mechanical properties when compared to titanium. Titanium alloys possess inherent properties of low density, high strength and stiffness, combined with very good creep and corrosion resistance. Titanium alloys have excellent corrosion resistance towards many of the highly corrosive environments, particularly, oxidizing and chloride containing process streams. The excellent corrosion resistant property of titanium alloys result from the formation of highly stable, continuous, adherent and protective oxide films on the metal surfaces. The nature, composition and thickness of the protective surface oxides *Author for Correspondence. e-mail: [email protected]

50

Vijay et al.

that form on these alloys depend on environmental conditions. [1, 2] Titanium alloys and titanium based composites have found application in the areas of industrial, automotive, aerospace and consumer-based applications. Certain specific applications of highly strategic importance include steam turbine blades, automotive springs, bearings, cooking wares, etc. [2] The main objective of this paper is to review and discuss various methods of surface modification treatments done on titanium composites and their significant effects on their anti-corrosion properties. For a comprehensive understanding of the above noted surface modification techniques we start with a brief recapitulation of the possible corrosion mechanisms. CORROSION MECHANISMS IN Ti ALLOYS

Corrosion in Ti alloys takes place by ways, including differing atmospheres and chemical reactants. Some of the most important corrosion mechanisms involved in Ti alloys is discussed in this paper. General Corrosion

General corrosion has been characterized by a relatively uniform attack over the exposed surface of the metal alloy. This type of corrosion takes place rapidly on the titanium surface, when it is in fully passive condition. The corrosion rate, in this case has been found out to be less than 0.04 mm/yr (1.5 mils/yr). The maximum corrosion rate has been reported to be less than 0.13 mm/yr (5 mils/yr). [2] This poses a major threat under reducing acid environments, particularly, when acid concentration and temperature increases. Under the action of strong and hot reducing acids, the oxide film of Ti deteriorates and dissolves and thus the unprotected metal is oxidized to the soluble trivalent ion. Crevice Corrosion

Ti alloys get subjected to localized attack in tight crevices exposed to hot (>70ºC) chloride, bromide, iodide, fluoride or sulphate containing solutions. [2, 3] The mechanism responsible for this type of corrosion has been found to be very close to that of stainless steel. Due to oxygen depletion, a condition similar to reducing acid is developed within the tight crevices. Dissolved oxygen or such oxidizing species in the bulk solution are depleted in the restricted volume of solution in the crevices. Finite surface oxidization in crevices consumes these species faster than diffusion from the bulk solution can take place. As a result, metal potentials

Review of various surface treatment techniques on Titanium Alloys

51

in the crevices become active (-ve) relative to the metal surface exposed to the bulk solution. This creates an electrochemical cell in which the crevices are anodes and corrode the surrounding metal surfaces, which act as cathodes. Because of small, restricted volumes of solution in these crevices, pH levels as low as 1 or even less can develop, thus resulting in rapid, localized and active corrosion within crevices, depending on the resistance of the alloys and temperature. Anodic Pitting Corrosion

Pitting is defined as localized corrosion attack occurring on openly exposed metal surface in the absence of any apparent crevices. Pitting occurs when the potential of metal exceeds the anodic breakdown potential of the metal oxide film in a given environment. When the anodic breakdown potential of the metal is equal to or less than the corrosion potential under a given set of conditions, spontaneous pitting is expected. Because of the presence of protective oxide film, Ti exhibits very high anodic pitting potentials. Thus, in most cases, pitting corrosion is not of major concern for Ti alloys. [2, 3] Hydrogen Damage

Ti alloys are widely used in hydrogen containing environments and under conditions in which galvanic coupler or cathodic charging causes hydrogen to be evolved on metal surfaces. Surface oxide film is a highly effective barrier to hydrogen penetration. In = and >-alloys, excessive hydrogen intake can induce the precipitation of titanium hydride in the =-phase. Suitable conditions are when the metal temperature is above 80ºC, where the diffusion of hydrogen is significant and the pH of the solution is less than 3 or greater than 12. Under these conditions, nascent hydrogen may be formed on the alloy surface resulting in galvanic couple formation or severe continuous abrasion of Ti surface in an aqueous medium. [2] Stress Corrosion Cracking

It is a fracture or cracking phenomenon caused by the combined action of tensile stress, a susceptible alloy and a corrosive environment. It takes place by anodicassisted cracking generally beginning when a corrosion pit is formed. In the presence of tensile stress, the pit will produce a stress concentration that depends on the pit. If the corrosion is not as rapid as to blunt the crack tip, the crack propagates, leading to failure. Once the crack is formed, the balance between crack tip corrosion rate, crack tip environment and the crack tip stress state is critical for the crack to propagate. [2, 3]

52

Vijay et al.

SURFACE TREATMENT TECHNIQUES ON TITANIUM ALLOYS

A variety of surface treatment methods have been developed and implemented to improve the existing mechanical properties of Ti alloys. Most of these surface modification processes have been developed for improving the properties of wear, corrosion, hardness, etc. Some of the most widely followed surface treatment techniques are discussed. Plasma-Enhanced Vapour Deposition Coating

Plasma-Enhanced Vapour Deposition Coating (PECVD) is mainly done on tool steels, to improve the wear resistance and corrosion resistance properties of such metal alloys. This method allows low temperature working and also complex geometric shaped metals can be coated without much complexity [4]. PECVD coatings of different concentrations of TiBN, TiB2 and TiN were coated, elsewhere, on tool steels and their wear and corrosion tests were carried out by suitable methods. The SEM (Scanning Electron Microscope) micrographs reveal that the surface morphologies were very dense and comprised fine structures. Also in TiB2, micro voids were observed, which lead to reduction of micro level hardness. The corrosion tests (polarization tests) revealed improved corrosion resistance (reduction of potential voltage from 800 mV to 250 mV). In this case, not much difference was found in the corrosion resistance improvement, when compared between individual coating compositions. The PECVD coating was also applied in industrial method for the production of rotor poles, and found that hard TiBN coating was very much anti-corrosive (200 mV), compared to other coatings. [5, 6] Plasma Immersion Ion Implantation and Deposition

Recently, much work has been reported on NiTi alloys [7, 8]. TiN coating deposition on titanium nickel alloy and wear and corrosion tests were carried out on the treated composition by Atomic Force Microscopy (AFM) and electrochemical corrosion. Here, primarily, pure titanium strip is deposited on the alloy surface along with nitrogen and argon gases, in the ratio 1:2, being circulated on it, to form even layers of TiN coating. The experimental analysis revealed very large clusters, resulting to pitting corrosion, due to low bias voltage (5 kV). This was not the same as under high bias voltage (15-30 kV). Hence, it was reported that due to increase of negative bias voltage (from 5 to 30 kV), the corrosion current density value (Icorr) significantly decreased, showing a clear improvement in the corrosion resistance property. Considering surface roughness property, coating deposited at 15 kV showed the least root mean square (RMS) value of 4.591 nm for surface roughness, while at 20 kV, corrosion resistance was found to be most satisfactory, with high corrosion potential


53

(Ecorr) of -0.194 . [7] High Intensity Pulsed Plasma Beam Coating

An attempt was made by F.A.Bonillaa, et. al [9], for coating palladium in pure strips of titanium under High Intensity Pulsed Plasma Beam Coating (HIPPB) technique. This process was carried out using two methods, Pulsed Implantation Doping (PID) and Deposition by Pulsed Erosion (DPE). The corrosion behavior of the treated pieces was tested by immersion in 0.1 M H2SO4 solution at 80ºC for upto 100 hours. After corrosion test, the surface alloy modified by DPE using two pulses, revealed acicular phase across the alloyed layer, due to acid immersion. Similarly, the morphology of the surface alloy modified by PID using three pulses revealed wavy textured appearance. The weight loss measurements indicated that PID had more weight loss rate compared to DPE [9]. Hence it is reported that the surface treatment improved the anti-corrosive properties as compared to the untreated alloys (decrease of potential from 780 mV to 450 mV). [9, 10] Excimer Laser Surface Treatment

The previous researchers have reported that the corrosion potential of Excimer-lasertreated pure titanium was marginally increased when tested in NaCl solution [11, 12]. Excimer Laser Surface treatment has been performed on dual phase (= + >) Ti-6Al4V alloy to improve the pitting corrosion property of the alloy. KrF Excimer laser surface treatment was performed with two different gas environments, nitrogen and argon [11]. The treated alloys were tested by potentiodynamic polarization method in 2 mol dm3 HCl solution. The specimen treated in nitrogen gas had a corrosion current density (Icorr) 7 times less than critical passive current (Ip), which was taken for untreated alloy. The pitting potentials for untreated, Ar treated and N2 treated specimen were found to be 3.51, 5.56 and 4.48 V respectively, proving that corrosion resistance was better in the specimen treated in argon atmosphere. Cathodic Arc Deposition of TiN / TiAlN

TiN and TiAlN coating was done on Austempered Ductile Iron (ADI) by cathode arc deposition technique and tested for better corrosion property [13]. ADI has very good mechanical properties, except surface hardness and corrosion resistance. Thus coated ADI was tested for corrosion resistance in NaCl and HCl solutions to simulate aggressive aqueous environment of chloride (Cl) ions. Polarization measurement was done and corrosion current (Icorr) was obtained. It was found that, under a lesser target current (50 A compared to 75 A), deposition rate was slow, but the film formation

54

Vijay et al.

had a dense and smooth morphology. It was found that the corrosion resistance of the cathodic arc deposited substrate was better than the untreated ones (reduction of Icorr from 500×10-8 A/cm2 to 2×10-8 A/cm2), irrespective of whether, TiN or TiAlN coating was done. Reactive Magnetron Sputtering of Ti / TiN

In the experiment carried out by R.Huber et al [21], Ti / TiN coating was deposited on 316-L stainless steel substrates by Reactive Magnetron Sputtering method. The corrosion resistance study was carried out by potentiodynamic polarization in an aqueous environment. The micro structural details were corroborated by SEM analysis. It was clearly reported that the reduction of critical current density was drastic (35 nA to 10 nA) between untreated stainless steel and Ti / TiN coated stainless steel samples. Hence it was concluded that the Ti / TiN multilayer coatings provided long term corrosion protection power. Besides these experimental studies, other methods such as High Energy Electron Beam Irradiation, Magnetron Sputtering of targets, etc., have also been studied elsewhere and reported for improvement in corrosion resistance. [14, 20, 22, 23, 24, 25] CORROSION STUDY IN TiB COMPOSITE ALLOY

Titanium boride is an alloy of very high strategic importance in the field of aerospace engineering. This alloy has very good preference over Ti-TiN, Ti-TiC and such alloys, mainly due to the absence of an intermediary phase, as suggested from the Ti-TiB phase diagram. Also this alloy has high thermal stability property, as compared with other similar titanium alloys. This alloy composite is reportedly said to have better surface properties, when treated under various methods. [15, 18] Ti-TiB composite prepared by powder metallurgy technique, by mixing proportionate mixtures of titanium (Ti) and titanium diboride (TiB2) powders or boron powders, followed by the process of hot sintering. Ti + B ® TiB Ti + 2B ® TiB2 Ti + TiB2 ® 2TiB When prepared through this process, three different types of whisker morphologies were evident and reported. 1. Long and needle-shaped TiB whiskers that are isolated and randomly oriented in the Ti matrix (at relatively low volume fractions). [16]


55

2. Colonies of refined and densely packed TiB whiskers (at medium volume fractions). 3. Coarse and elongated TiB particles with a few needle shaped whiskers (at very high volume fractions). Here X-ray Diffraction method is used to determine the volume fractions of TiB whiskers dispersed. In all composites, TiB was predominantly of boride phase. The absence of Ti3B4 phase can be noticed, when examined through XRD tests [15]. In the Ti-TiB composite, >-alloy was preferred, which was achieved by Fe-Mo master alloy powders (> stabilizers). [17, 18] Regarding the corrosion treatment of Ti-TiB composites, the corrosion resistance property of Ti is reported to be better in many studies [18]. In a work done by T.Y.Chen [19], on the corrosion study of boride strengthened nickel and iron alloys, the presence of boride precipitates had a negative effect on the pitting potential and on the resistance to the pitting of microcrystalline alloys. In this case, precipitated borides of MoB4 and MoFe2B4 were reported to provide preferential sites for pit initiation, hence contributing to the detrimental influence on the corrosion resistance. Improvement in corrosion resistance was obtained by the pulsed magnetron sputtering of chromium diboride powder targets on stainless steel and tool steel. This was obtained due to the deposition of hard and corrosion resistant CrB2 coatings. [20] It is reported with 34 vol% of TiB whiskers in > titanium matrix was considered for corrosion test. Surface treatment of the composite was carried out using a 10 kW CO2 laser system. Three laser powers were taken; 3.5 kW, 2.5 kW and 1.5 kW, under a constant traverse speed of 30 mm/s. The effect of laser power alone on corrosion resistance was studied and reported. [18] Corrosion test was done by potentiostatic polarization method. This test revealed an increase in corrosion resistance when compared to untreated composite. The 3.5 kW treated composite had a corrosion resistance of 118 mm/yr compared to the untreated composite surface having 182 mm/yr. It was also observed that there were no appreciable improvement of corrosion resistance in the 1.5 kW and 2.5 kW treated alloys. [18] CONCLUSION

From the above reviews, it is generally seen that surface treatments have contributed to the corrosion resistance improvement of the various titanium alloys in the following ways.

56

Vijay et al.

1. Corrosion resistance in tool steels is highly improved (reduction of potential voltage from 800 mV to 250 mV) by the Plasma Enhanced Vapour Deposition Coating of TiN, TiB2 and TiB with the production of dense and fine structures. 2. Titanium nickel alloy was treated by Plasma immersion and deposition technique with the deposition of TiN and corrosion resistance was reported to be improved by the increase of negative bias voltage (from 5 to 30 kV). 3. Pure Ti strips were coated with palladium coatings by High Intensity Pulsed Plasma Coating method, with corrosion resistance reported to be improved in the Deposition by Pulsed Erosion (DPE) coating than Pulsed Implantation Doping (PID) coating (decrease of corrosion potential from 780 mV to 450 mV). 4. TI-6Al-4V alloy was treated with Excimer Laser surface treatment technique, with pitting corrosion reported to be improved from 3.51 V in the untreated to 5.56 V in the Ar-treated and 4.48 V in the N2-treated specimens. 5. TiN and TiAlN coating was performed on ADI by Cathodic arc deposition technique with improvement reported in the corrosion resistance property (decrease in the corrosion current from 500×10-8 A/cm2 to 2×10-8 A/cm2). 6. Ti / TiN coatings on 316-L stainless steel by reactive magnetron sputtering of powder targets showed the improvement of corrosion resistance properties of the treated stainless steel substrates (reduction of critical current density from 35 nA to 10 nA). Since the surface treatment practices were done by various methods, an optimum surface treatment technique is difficult to identify. Also since, different Ti alloys were studied, the robust alloy pertaining to the various surface characteristics is difficult to identify. Although, not much study has been done in the corrosion resistance of surface treated Ti-TiB composite, this field provides scope for more work. This requires extensive study and experimentation of different laser treatment techniques on Ti-TiB composite. REFERENCES 1. ASM Handbook, Mechanical Testing, Surface Engg. of Ti and Ti Alloys, ASM Int., 8, 840 (1997). 2. ASM Handbook, Corrosion, Specific alloy systems, ASM Int., 13, 671 (1997). 3. ASM Handbook, Properties and selection of non ferrous alloys and specifications, Introduction to Ti and Ti alloys, ASM Int., 2, 587 (1997).


57

4. L. Chaleix and J. Machet, Study of the composition and of the mechanical properties of TiBN films obtained by D.C. magnetron sputtering, Surf. Coat. Technol., 91, 74 (1997). 5. Myung Jin Son, Sung Soo Kang, Eung-Ahn Lee and Kwang Ho Kim, Properties of TiBN coating on the tool steels by PECVD and its applications, J. Mater. Process. Technol., 130-131, 266 (2002) . 6. C. Mitterer, P. H. Mayrhofer, M. Beschliesser, P. Losbichler, P. Warbichler, F. Hofer, P. N. Gibson, W. Gissler, H. Hruby, J. Musil and J. Vleek, Microstructure and properties of nanocomposite TiBN and TiBC coatings, Surf. Coat. Technol., 120121, 405 (1999). 7. Y. Cheng and Y. F. Zheng, Surface characterization and electrochemical studies of biomedical NiTi alloy coated with TiN by PIIID, Mater. Sci. and Engg. : A, 438440, 1146 (2006). 8. S. K. Wu, H. C. Lin and C. Y. Lee, Surf. Coat. Technol., 113, 17 (1999). 9. F. A. Bonillaa, P. Skeldona, G. E. Thompsona, J. Piekoszewskib, A. G. Chmielewskib, B. Sartowskab, J. Stanislawskic, P. Baileyd and T. C. Q. Noakesd, Corrosion resistant Ti-Pd surface alloys produced by high intensity pulsed plasma beams, Surf. Coat. Technol., 200, 4674 (2006). 10. F. A. Bonilla, T. S. Ong, P. Skeldona, G. E. Thompsona, J. Piekoszewski, A. Chmielewski, B. Sartowska and J. Stanislawski, Cor. Sci., 45, 403 (2003). 11. T. M. Yue, J. K. Yu, Z. Mei and H. C. Man, Excimer laser surface treatment of Ti6Al4V alloy for corrosion resistance enhancement, Mater. Lett., 52, 206 (2002). 12. H. Badekas, C. Panagopoulos and S. Economou, J. Mater. Process. Technol., 44, 54 (1994). 13. Cheng-Hsun Hsu., Ming-Li Chen and Kuei-Laing Lai, Corrosion resistance of TiN/ TiAlN-coated ADI by cathodic arc deposition, Mater. Sci. Engg. : A, 421, 182 (2006). 14. Kwangjun Euh, Jongmin Lee and Sunghak Lee, Micro structural Modification and Property Improvement of Boride/Ti-6Al-4V Surface-Alloyed Materials Fabricated by High-Energy Electron-Beam Irradiation, Scripta Mater., 45, 1 (2001). 15. S. S. Sahay, K. S. Ravichandran and R. Atri, Evolution of microstructure and phases in in situ processed TiTiB composites containing high volume fractions of TiB whiskers, J. Mater. Res., 14, 4214 (1999). 16. B. J. Kooi, Y. T. Pei and J. Th. M. De Hosson, The evolution of microstructure in a laser clad TiBTi composite coating, Acta Mater., 51, 831 (2003). 17. K. B. Panda and K. S. Ravi Chandran, Synthesis of Ductile TitaniumTitanium Boride (Ti-TiB) Composites with a Beta-Titanium Matrix: The Nature of TiB Formation and Composite Properties, Metal. Mater. Trans. 34A, 1371 (2003).

58

Vijay et al.

18. P. Chandrasekar and V. Balusamy, Enhancement of hardness and corrosion properties in Ti-TiB composite by CO2 Laser Surface Treatment, Scripta Mater., 2, 64 (2006). 19. T. Y. Chen, Z. Szklarska-Smialowska, The Pitting Corrosion Characteristics of Boride-Strengthened Nickel- and Iron-based microcrystalline alloys, Cor. Sci., 28, 97 (1988). 20. M. Audronis, P. J. Kelly, R. D. Arnell, A. Leyland and A. Matthews, The structure and properties of chromium diboride coatings deposited by pulsed magnetron sputtering of powder targets, Surf. Coat. Technol., 200, 1366 (2005). 21. R. Hubler, A. Cozza, T. L. Marcondes, R. B. Souza and F. F. Fiori, Wear and corrosion protection of 316-L femoral implants by deposition of thin films, Surf. Coat. Technol., 142-144, 1078 (2001). 22. J. Dutta Majumdar, B. Ramesh Chandra, A. K. Nath and I. Manna, In situ dispersion of titanium boride on aluminium by laser composite surfacing for improved wear resistance, Surf. Coat. Technol., 201, 1236 (2006). 23. R. Banerjee, P. C. Collins, A. Genc¸, H. L. Fraser, Direct laser deposition of in situ Ti/6Al 4V/TiB composites, Mater. Sci. Engg. : A, 358, 343 (2003). 24. C. Monticelli, A. Frignani, A. Bellosi, G. Brunoro, G. Trabanelli, The corrosion behavior of titanium diboride in neutral chloride solution, Cor. Sci., 43, 979 (2001). 25. J. L. Ong, L. C. Lucas, G. N. Raikar and J. C. Gregory, Electrochemical corrosion analyses and characterization of surface-modified titanium, App. Surf. Sci., 72, 7 (1993).