Erosion and cavitation corrosion of titanium and its alloys

Transcription

1 Wear 250 (2001) Erosion and cavitation corrosion of titanium and its alloys A. Neville, B.A.B. McDougall Department of Mechanical and Chemical Engineering, Heriot-Watt University, Edinburgh EH14 4AS, Scotland, UK Abstract The economic and effective operation of machinery and plant involved in fluids handling is increasingly dependent on the utilisation of materials that combine high corrosion resistance and good wear resistance. This paper studies two wear corrosion situations: (1) erosion corrosion, where the wear is due to impacting solids in a liquid medium and (2) cavitation corrosion, where the wear is due to impacting liquid micro-jets formed by imploding air bubbles. The characteristics of a commercially pure titanium (CP-Ti) and three alloys in erosion corrosion and cavitation corrosion conditions have been studied. The erosion corrosion characteristics of each material was assessed using an impinging-jet apparatus. The tests were performed at an angle of impingement of 90 C at a particle velocity of 17 m/s and in a saline solution of 3.5% NaCl at 18 C. A series of experiments was conducted to determine the mass loss by combined erosion corrosion before independently determining the electrochemical corrosion contribution to mass loss. It has been shown that exposure to liquid solid erosion causes disruption of the passive film on Ti and active corrosion occurs. In contrast, the materials exhibited passive behaviour in static conditions and when exposed to a cavitating liquid only CP-Ti became active. The role of corrosion in these wear corrosion environments on CP-Ti and Ti-alloys is discussed in this paper Elsevier Science B.V. All rights reserved. Keywords: Erosion; Cavitation; Corrosion; Titanium; Titanium alloys 1. Introduction Titanium and Ti-alloys are seeing increasing application in industrial sectors outside the military and commercial aerospace sectors for which many of the materials were developed. In a recent review of the titanium industry in USA by Seagle [1], the expansion of Ti use in energy extraction, biomedical and consumer products such as sport equipment was discussed. Titanium and its alloys can arguably provide low cost material options through economic processing. Whereas in the USA and in Europe, Ti and Ti-alloys have primarily been utilised for aerospace components, in Japan the trend is very different [2]. In 1996, it was estimated that 80% of Ti was used on non-aerospace applications such as production of turbine blades, marine structures and consumer goods. As Ti increasingly becomes regarded as a viable alternative to other high grade alloys in industrial components, alloy development work is expanding to provide materials with tailored corrosion resistance and mechanical properties. In particular, small additions of Co and Pd have been found to form a Pd and Co-enriched layer which improves passivation through reduction of the hydrogen overvoltage Corresponding author. Tel.: ; fax: address: a.neville@hw.ac.uk (A. Neville). [2]. There is now a wide range of -, / -, and -Ti-alloys available which include additions of Al, V, Ni, Mo, Ru and other elements. The corrosion behaviour of Ti and Ti-alloys has been widely studied and there is an extensive literature relating to corrosion in acidic media [3,4], in biomedical applications [5,6] and in seawater [7,8]. However, there has been much less attention paid to their resistance to tribo-corrosion where corrosion occurs in association with a mechanical degradation process (e.g. abrasion, erosion, cavitation, etc.). Some work has been reported on assessment of surface engineering options (Cr plating, plasma coatings, etc.) for improving the surface resistance of Ti 6Al 4V to slurry abrasion [9]. Jiang et al. [10] reported the susceptibility of Ti 6Al 4V to degradation under sliding conditions in acid where hydrogen was evolved at the surface through a cathodic reaction. Tu [11] reported results from a study in an aqueous slurry where the surface resistance of TiN as a coating and -Ti were compared. The -Ti substrate was found to have a much higher degradation rate through stress corrosion mechanisms. It has been shown in previous work that on high-grade alloys such as the stainless steels and Ni-based alloys material loss rates can be significantly accentuated when the passivity of the material is breached by a mechanical influence [12]. The material can make a transition into a regime which consists of active and passive sites on one surface which /01/$ see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S (01)

2 depend on the mechanical influence and the frequency and extent of impacts by sand or other impacting species. In this paper, the corrosion behaviour of commercially pure titanium (CP-Ti) and three Ti-alloys is assessed under liquid solid impingement and under cavitation corrosion conditions. The role of corrosion in the overall degradation of the materials is discussed. A. Neville, B.A.B. McDougall / Wear 250 (2001) Experimental methods and materials For erosion corrosion tests, an impinging liquid solid jet containing 500 mg/l solids was generated using a recirculating rig as shown in Fig. 1a. The jet was directed at right angles onto the sample surface through a 4 mm diameter nozzle and the velocity was maintained constant at 17 m/s. The rig contained a dual nozzle system. The solution used as the recirculating medium was 3.5% NaCl fluid and the solids were SiC sand with a size distribution as in Table 1. The temperature of the liquid was 18 C. Cavitation tests were conducted using a Branson Ultrasonics Sonifier Cell Disrupter Model 450 with an exponentially shaped horn to generated a vibration frequency of 20 khz and amplitude of ±25 m. The distance from the horn tip to the static specimen was kept constant at 1 mm and the immersion depth of the sample in the liquid was 12 mm (Fig. 1b). The temperature of the test fluid (3.5% NaCl) was 18 C. All sample surfaces were prepared by a final grinding with 600 grit paper. Electrochemical analysis was used in conjunction with weight-loss analysis to determine the total material loss and to isolate the contribution due to pure corrosion (C). The corrosion rate was measured in situ using a three-electrode electrochemical cell (Fig. 1c) composed of a Ag/AgCl reference electrode connected by means of a salt bridge and a platinum counter electrode. The dc anodic polarisation tests involved scanning the potential of the working electrode (the specimen under examination) from the free corrosion potential (E corr ) in the more noble (positive) direction at a fixed rate of 25 mv/min. The potential was scanned in the positive direction until a value of 0.5 V was reached. Following all tests, the surface was examined using light and scanning electron microscopy to determine the extent of degradation and to identify the material loss mechanisms. The four materials considered in this study were CP-Ti, Ti 5111, Ti 6Al 4V ELI and Ti 6Al 4V ELI/Ru. Details of the alloy type and composition are given as follows. Fig. 1. (a) Recirculating apparatus used for liquid solid impingement tests, (b) experimental set-up for cavitation tests and (c) three-electrode cell used for electrochemical monitoring. CP-Ti: Grade 2 CP-Ti is used in this study. It is one of four grades and has higher strength at the expense of ductility. It is an unalloyed, -phase metal with maximum 0.3%Fe, 0.03%N, 0.1%C, 0.25%O and 0.015%H. Hardness is 219 HV. Table 1 Sand size distribution for erosion corrosion tests Size ( m) > <106 Mass (%)

3 728 A. Neville, B.A.B. McDougall / Wear 250 (2001) Ti 5111: It is a near -alloy of intermediate strength. It is designated ASTM Grade 32. The nominal composition is %Al, %Sn, %V, %Zr and %Mo. Hardness is 265 HV. Ti 6Al 4V ELI: Itisan / -alloy which has 6%Al and 4%V as the major alloying additions. It is designated ASTM Grade 23. The ELI denotes extra low interstitial and refers to the low oxygen content aimed at keeping the interstitial phase content low to combat stress corrosion cracking. Hardness is 342 HV. Ti 6Al 4V ELI/Ru: It is a variation of the Grade 23 alloy in which 0.1%Ru is added. This is referred to as Grade 29. Ru is added primarily to enhance corrosion resistance. Hardness is 348 HV. 3. Results 3.1. Measurement of total weight loss under liquid solid impingement A series of 8 h tests were conducted under liquid solid impingement conditions with 500 mg/l solids in 3.5% NaCl at 18 C. Measurement of the total weight loss (TWL) was carried out after 1, 2, 4 and 8 h. Fig. 2 shows the time trends for each material where it can be clearly seen that there is a distinction between the behaviour of CP-Ti and the other three materials. No significant difference exists between the other three materials on the basis of TWL measurements under these conditions. The time trend of CP-Ti clearly shows that the TWL rate decreases as a function of time. The other three materials exhibit a similar, but less pronounced, decreasing degradation rate as time progresses Measurement of total weight loss under cavitation corrosion conditions Measurement of weight loss was carried out after 1, 2, 4 and 8 h as in the previous section under erosion corrosion conditions. Fig. 3 shows the time trends for each material. CP-Ti exhibits greater weight losses at each time interval than the other three materials in common with the results under erosion corrosion conditions. There is no significant difference between the other three materials after 1 and 2 h. However, after 4 h, the three materials have diverged and the difference is augmented as the time increases to 8 h. Whereas Ti 6Al 4V ELI/Ru shows an almost linear time trend the other three materials all exhibit increasing material Fig. 2. Total weight loss (TWL) after 1, 2, 4 and 8 h under liquid solid erosion, 500 mg/l solids, 18 C and 3.5% NaCl.

4 A. Neville, B.A.B. McDougall / Wear 250 (2001) Fig. 3. Weight losses after 1, 2, 4 and 8 h under cavitation, 18 C and 3.5% NaCl. loss rates as a function of time in contrast with the response under erosion corrosion conditions Electrochemical monitoring anodic polarisation In the previous section, environmental conditions have been shown to play an important role in the overall degradation of a material in a particular environment (i.e. erosion corrosion or cavitation corrosion). In this work, the effect of wear (cavitation and liquid solid erosion) on corrosion has also been assessed. Anodic polarisation tests have been carried out on samples in static 3.5% NaCl and in situ on samples under erosion or cavitation conditions to determine the corrosion characteristics. In Fig. 4a d, the anodic polarisation plots from the E corr to a potential of 0.5 V are shown for the four materials under static, cavitation and erosion conditions. It can be seen from these anodic polarisation results that each of the materials exhibit passive behaviour in static saline conditions at 18 C. This is characterised by the low currents (<10 A/cm 2 ) recorded in the electrochemical circuit as the potential is scanned away from the free corrosion potential in the positive direction. Fig. 4a and b are plotted as E i curves and c and d are plotted as E log i. Anodic polarisation tests under cavitation conditions in 3.5% NaCl at 18 C showed that the presence of high frequency impinging liquid jets caused by bubble implosion had a pronounced effect on CP-Ti in contrast to the lesser effect on the other materials. CP-Ti exhibits behaviour more akin to active corrosion behaviour under cavitation conditions whereas the three Ti-alloys continue to exhibit passivity albeit with Ti 6Al 4V ELI and Ti 6Al 4V ELI/Ru registering larger currents than in static conditions. As shown in Fig. 4b, there is very little change in the currents measured on Ti 5111 the only change being the slightly less stable current under cavitation conditions. In addition to the larger currents exhibited on Ti 6Al 4V ELI (Fig. 4c) under cavitation conditions, there is also an increased oscillation in the current as the potential is increased. Under liquid solid impingement conditions, there was a drastic change in the corrosion characteristics of all four materials. This is shown clearly for each material in Fig. 4a d, where under liquid solid impingement conditions each material exhibits an initial rapid increase in current density as the potential is scanned away from E corr in the positive direction. Each material exhibits active corrosion behaviour under these conditions but the current did not continue to rise rapidly as expected for truly active

5 730 A. Neville, B.A.B. McDougall / Wear 250 (2001) behaviour. As the potential is pushed to more positive values, the current reaches an almost stable value independent of potential but oscillating by up to 20 A/cm 2. The current oscillations are thought to be caused by the impacting sand and the depassivation/repassivation events occurring over small regions of the material. The anodic polarisation results of each material under static, cavitation and liquid solid impingement conditions were analysed to determine the corrosion current density, i corr, which, through use of Faraday s law, gives a measure of the material loss rate due to corrosion. In order to obtain a value for the corrosion current density, the potential current data is plotted in an E log i form (as shown in Fig. 4c and d) and Tafel extrapolation is used to determine i corr (the corrosion current density at the free corrosion potential). The i corr values resulting from the Tafel extrapolation analysis are presented in Table 2. The table shows that under static conditions very low i corr values are obtained, as expected for CP-Ti and Ti-alloys which exhibit passive behaviour. CP-Ti experiences currents that are at least an order of magnitude higher than those experienced by the alloys under static and cavitation conditions. Under cavitation conditions, the currents are indicative of a material displaying active corrosion behaviour. Under liquid solid impingement conditions, the corrosion currents are considerably larger for each material than those obtained under static or cavitation conditions. These current values are clearly not representative of passive behaviour. Comparison of the measured current at +0.5 V was made as another means of comparing the corrosion characteristics of the four materials. These values are presented in Fig. 4. Anodic polarisation curves under static, cavitation corrosion and erosion corrosion conditions at 18 C in 3.5% NaCl for (a) CP-Ti, (b) Ti 5111, (c) Ti 6Al 4V ELI and (d) Ti 6Al 4V ELI/Ru.

6 A. Neville, B.A.B. McDougall / Wear 250 (2001) Fig. 4. (Continued). Table 3. It can be seen from Table 3 that the current values at +0.5 V reflect the general trends observed in Table 2 for the i corr values with much larger currents measured under liquid solid erosion corrosion conditions than under static and cavitation conditions Visual examination of material loss mechanisms The wear scars formed after 8 h exposure to liquid solid erosion and cavitation conditions are very different on a macro scale. There is a noticeable depth loss of material Table 2 The i corr ( A/cm 2 ) recorded for CP-Ti and Ti-alloys in 3.5% NaCl at 18 C under static, cavitation and erosion corrosion conditions CP-Ti Ti 5111 Ti 6Al 4V ELI Static ( A/cm 2 ) Cavitation mg/l solids Ti 6Al 4V ELI/Ru Table 3 Current values ( A/cm 2 ) at +0.5 V recorded for CP-Ti and Ti-alloys in 3.5% NaCl at 18 C under static, cavitation and erosion corrosion conditions CP-Ti Ti 5111 Ti 6Al 4V ELI Ti 6Al V ELI/Ru Static ( A/cm 2 ) Cavitation mg/l solids

7 732 A. Neville, B.A.B. McDougall / Wear 250 (2001) Fig. 5. Macro-wear scar on CP-Ti under (a) liquid solid erosion and (b) cavitation; 8 h tests at 18 C in 3.5% NaCl. Fig. 6. (a) Mechanical attack at centre of liquid solid erosion wear scar on Ti 6Al 4V ELI and (b) low angle ploughing damage at outer region of wear scar from liquid solid erosion on Ti 6Al 4V ELI; 8 h tests at 18 C in 3.5% NaCl.

8 A. Neville, B.A.B. McDougall / Wear 250 (2001) under liquid solid erosion compared to cavitation on the macro scale. The wear scar formed under liquid solid erosion conditions takes the form of a circular groove (Fig. 5a) with its greatest depth, a small distance (around 2 mm) from the central stagnation point of the impinging liquid solid water jet. The wear scar formed under cavitation conditions is exactly the size and shape of the vibratory horn that was used during testing. The wear scar takes the form of a uniformly roughened and dulled surface on the macro scale (Fig. 5b). Interesting damage mechanisms are revealed under microscopic examination of the wear scars. Under higher magnification, the central zone of the liquid solid erosion wear scar shows a clear pattern of plastic deformation damage in association with pitting. Little directionality is observed in this region due to the high angle of the mechanical attack (Fig. 6a). Further out from the centre of the wear scar, the outer regions, the damage is composed of low angle directional ploughing and cutting, leading to directional plastic deformation of the surface (Fig. 6b). In contrast to the central wear scar damage under liquid solid erosion conditions, under cavitation conditions, the centre of the wear scar (accounting for the majority of the wear scar) shows severely plastically deformed surface with no directionality evident. The surface has a honeycomb appearance (Fig. 7a). At the edge of the wear Fig. 7. (a) Central wear scar damage under cavitation on CP-Ti and (b) platelets formed under cavitation on Ti 6Al 4V ELI/Ru; 8 h tests at 18 Cin 3.5% NaCl.

9 734 A. Neville, B.A.B. McDougall / Wear 250 (2001) scar, the plastic deformation of the surface decreases and areas of the original surface are evident. The regions at the edge of the wear scar less effected by cavitation damage show the formation of platelets (Fig. 7b), as found by Tu [11] under liquid impingement conditions. These platelets appear to be formed by liquid micro-jets during cavitation but the intensity of cavitation attack on these areas has not been enough to completely plastically deform the surface of the material into the honeycomb structure. The ploughing and cutting by silica sand particles during liquid solid erosion causes the continuous generation of fresh material on the material surface as well as removing the protective passive film of the material temporarily. This ploughing and cutting evidently causes more electrochemical activity during liquid solid erosion than is caused by impinging liquid micro-jets during cavitation since much higher currents are recorded. This may be caused by a difference in impact kinetics. Under cavitation conditions, a single impact cycle takes milliseconds to complete [12] (and only one out of every impacts cause damage) and the area of temporary damage will be less than that under liquid solid erosion with silica sand particles penetrating the passive film while plastically deforming the materials surface, thus leading to more electrochemical activity under liquid solid erosion. 4. Discussion This study has shown that the time trends exhibited by the materials under cavitation conditions are different to those under erosion conditions. The increasing material loss rates exhibited under cavitation conditions are typical of those reported in the literature, where incubation periods for materials are reported as in the work by Kwok et al. [13] and Auret et al. [14] on stainless steels. The decreasing material loss rates exhibited under erosion corrosion conditions is in agreement with other work reported in slurry erosion [15] and may be attributed to surface work hardening which increases the resistance with time and/or decreasing abrasivity of the sand. In this case, very little degradation of the sand was experienced during the 8 h test period. Ti-alloys are normally employed where their corrosion resistance is required. This study has confirmed that even under severe impingement conditions, the alloying additions provide a substantial benefit in resisting degradation when compared with CP-Ti. Benefits are also evident through alloying to resist degradation in cavitation conditions. The ranking of the alloys was different under liquid solid impingement and cavitation as shown below, where > means that the material has greater overall resistance than. Erosion corrosion resistance: Ti 5111 Ti 6Al 4V ELI/Ru > CP-Ti Ti 6Al 4V ELI Cavitation corrosion resistance: Ti 6Al 4V ELI/Ru > Ti 6Al 4V ELI > Ti 5111 > CP-Ti Interestingly, whereas the small addition of 0.1%Ru to Ti 6Al 4V ELI makes little difference under erosion corrosion conditions, it gives a substantial improvement in cavitation conditions. Fig. 8 shows that there is a general trend of increasing resistance to cavitation corrosion as the materials hardness Fig. 8. Vickers hardness vs. total weight loss under erosion corrosion and cavitation corrosion conditions.

10 A. Neville, B.A.B. McDougall / Wear 250 (2001) Table 4 Weight loss components Material C erosion (%) C cavitation (%) CP-Ti Ti Ti 6Al 4V ELI Ti 6Al 4V ELI/Ru increases. However, the trend is not linear which suggests that there are other factors which affect the performance of CP-Ti and Ti-alloys under these conditions. In particular, the large difference between the performance of Ti 6Al 4V ELI and the Ru containing variation is not expected should hardness of the alloy be the only controlling factor. Under erosion corrosion conditions where there is a large difference in the hardness of Ti 6Al 4V ELI and the Ru containing variation compared with the Ti 5111 alloy, there is little difference in their overall resistance. These inconsistencies in the performance/hardness correlation can be partly attributed to the role played by corrosion processes in the overall degradation process as described in the following section Wear and corrosion components Data obtained from combined wear corrosion testing enables important information relating to the relative influence of the mechanical and electrochemical factors in affecting the mass loss rate under cavitation and liquid solid erosion to be obtained. It is possible to identify how much of the total weight loss due to wear corrosion is due to pure electrochemical corrosion (C). Results are presented in Table 4 for the pure corrosion component (C) of each material under cavitation and liquid solid erosion conditions in 3.5% NaCl at 18 C. As expected from the polarisation results, there is a larger proportion of corrosion contributing to overall deterioration rates on CP-Ti under both cavitation and erosion conditions. The contribution due to corrosion is generally larger under erosion conditions than under cavitation conditions. In comparison with recent work on three high-grade stainless steels, it is clear that the proportion of pure electrochemical corrosion is larger on CP-Ti, Ti 5111 and Ti 6Al 4V ELI under similar conditions [16]. 5. Conclusions This paper has demonstrated that the alloys of Ti, developed primarily to improve corrosion resistance provide increased protection to erosion corrosion and cavitation corrosion when compared with CP-Ti. There is a general trend of increasing resistance to material loss under cavitation conditions as the hardness increases but the relationship is not linear. The large difference between Ti 6Al 4V ELI and the Ru-containing variation is partly attributed to the much lower corrosion rate on Ti 6Al 4V ELI/Ru. Under cavitation conditions, all three alloys remain in a passive corrosion regime yet under erosion corrosion conditions, currents are larger and the materials tend to be active. This is attributed to the different kinetics of impact of solid particles and imploding cavitation bubbles which affect the time for electrochemical charge transfer and the subsequent corrosion rate. References [1] S.R. Seagle, The state of the USA titanium industry in 1995, Mater. Sci. Eng. A213 (1996) 1 7. [2] M. Yamada, An overview on the development of titanium alloys for non-aerospace application in Japan, Mater. Sci. Eng. A213 (1996) [3] S.Y. Yu, C.W. Brodrick, M.P. Ryan, J.R. Scully, Effects of Nb and Zr alloying additions on the activation behaviour of Ti in hydrochloric acid, J. Electrochem. Soc. 146 (12) (1999) [4] Z. Han, H. Zhao, X.F. Chen, H.C. Lin, Corrosion behaviour of Ti 6Al 4V alloy welded by scanning electron beam, Mater. Sci. Eng. A277 (2000) [5] B. Grosgogeat, L. Reclaru, M. Lissac, F. 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Daily, F.G. Hammitt, Cavitation, Eng. Soc. Monographs McGraw-Hill NY (1970) 39. [13] C.T. Kwok, H.C. Man, F.T. Cheng, Cavitation erosion and pitting corrosion of laser melted stainless steels, Surf. Coat. Technol. 99 (1998) [14] J.G. Auret, O.F.R.A. Damm, G.J. Wright, F.P.A. Robinson, Influence of cathodic and anodic currents on cavitation erosion, Corrosion 49 (11) (1993) [15] A. Neville, M. Reyes, T. Hodgkiess, A. Gledhill, Mechanisms of wear on a Co-base alloy in liquid solid slurries, Wear 238 (2000) [16] A. Neville, X. Hu, Mechanical and electrochemical interactions during liquid solid impingement on high-alloy stainless steels, Wear, (1-12) (2001)

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