Erosion Corrosion of Commercially Pure Titanium and Ti-6Al-4V Alloy in Sodium Chloride Solutions with and Without Suspended Solids

Abstract

This paper describes the findings from an experimental study of the performance of commercially pure titanium and the alloy Ti/6Al/4V in high velocity 3.5% NaCl aqueous solution with and without suspended solids. The investigation involved mass loss measurements, electrochemical monitoring, surface profiling and microscopical examination. Submerged jet testing equipment was utilised using 90° impingement. The two materials exhibited excellent durability in solids-free saline water under an extremely high impingement velocity of 71 m/s at ambient temperature (18-20 °C) and 50 °C. In the presence of suspended sand over the range of 500 - 1800 mg/l, however, the durability of the materials was severely compromised under a jet velocity of about 12.6 m/s. The Ti/6Al/4V alloy demonstrated somewhat superior resistance to erosion corrosion than the commercially pure titanium. A major objective of the work, namely the quantification of the proportions of pure mechanical erosion, pure corrosion and the interactive synergy components, was accomplished. An additional feature of the research involved the adoption of an experimental methodology that facilitates the discrimination of damage between the two hydrodynamic zones (directly impinged and surrounding regions) via the use of a segmented specimen arrangement. This procedure demonstrated that, whilst the most severe damage was experienced in the zone under direct impingement from a small diameter nozzle, there was, nevertheless, a significant attack within the surrounding region where the fluid flow was at lower angles.

Introduction

The early use of titanium-base materials was largely in the aerospace industry [11Williams JC, Boyer RR. Opportunities and Issues in the Application of Titanium Alloys for Aerospace Components. Metals. 2020;10(6):705. https://doi.org/10.3390/met10060705] but, over the subsequent decades, these materials have been adopted in a range of sectors including marine [22Li DH, Hu HX, Pan HD, Wang ZB, An WT, Zheng YG. Corrosion of Ti75 alloy in a 3.5 wt% NaCl solution containing different concentrations of Cu2+. Corros Sci. 2025;243:112567. https://doi.org/10.1016/j.corsci.2024.112567], hydrometallurgical operations [33Liu Y, Alfantazi A, Schaller F, Asselin E. Localised instability of titanium during its erosion-corrosion in simulated acidic hydrometallurgical slurries. Corros Sci. 2020;174:108816. https://doi.org/10.1016/j.corsci.2020.108816], automotive [44Jáquez-Muñoz JM, Gaona-Tiburcio C, Cabral-Miramontes J, Nieves-Mendoza D, Maldonado-Bandala E, Olguín-Coca J, López-Léon JD, Flores-De los Rios JP, Almeraya-Calderón F. Electrochemical Noise Analysis of the Corrosion of Titanium Alloys in NaCl and H2SO4 Solutions. Metals. 2021;11:105. https://doi.org/10.3390/met11010105], geothermal [55Brownlie F, Hodgkiess T, Pearson A, Galloway AM. A study on the erosion-corrosion behaviour of engineering materials used in the geothermal industry. Wear. 2021;477:203821. https://doi.org/10.1016/j.wear.2021.203821] and especially medical implants [66Asserghine A, Filotá D, Nag L, Souto RM, Nag G. Do titanium biomaterials get immediately and entirely repassivated? A perspective. npj Mater Degrad. 2022;6:57. https://doi.org/10.1038/s41529-022-00270-0]. These applications have been driven by the attractive range of properties exhibited by Ti-base materials such as low density – which together with alloying for mechanical strength enables high strength: weight ratios to be achieved. The other crucial attribute of titanium is its superb corrosion resistance in many aqueous environments in which the material maintains its excellent resistance to a variety of types of corrosion including pitting resistance and crevice corrosion resistance. These durability aspects have been described in numerous texts – including a recent detailed review [77Prando D, Brenna A, Diamanti MV, Beretta S, Bolzoni F, Ormellese M, Pedeferri M. Corrosion of titanium: Part 1: aggressive environments and main forms of degradation. J Appl Biomater Funct Mater. 2017 Nov 10;15(4):e291-e302. doi: 10.5301/jabfm.5000387. PMID: 29131299.] and are based on the spontaneous development of an extremely tenacious protective (’passive’) surface film of titanium oxide (essentially TiO₂).

There is often a requirement for resistance to flowing water conditions in which titanium components operate. There has been much evidence, gathered over a lengthy period, that demonstrates the ability of the oxide layer to retain its protective capacity up to extremely high fluid velocities in circumstances where the flowing liquid does not contain appreciable burdens of suspended solids. Sources of this information are given in [88De CP. Use of titanium and its alloys in seawater service. High Temp Mater Process. 1993;11:61. https://doi.org/10.1515/HTMP.1993.11.1-4.61-1010Titanium heat exchangers for service in seawater, brine and other natural aqueous environments: the corrosion, erosion, and galvanic corrosion characteristics of titanium in seawater, polluted inland waters and in brines. Titanium Information Bulletin Imperial Metals Industries (Kynoch) Ltd. May 1970.]. When, however, the water does contain significant levels of suspended solids – such as sand in silted rivers - the excellent durability in turbulent conditions is severely compromised and titanium-base materials (in common with many other metallic alloys) become vulnerable to potentially severe deterioration, by solid/liquid, erosion corrosion processes. The control of erosion-corrosion in industrial set-ups is challenging on account of the intricacy of the degradation phenomenon in which the component material loss involves mechanical and electrochemical action together with fairly complex interactions between the two processes. These different contributory factors can be incorporated into the following generally accepted relationship [1111Zhou S, Stack MM, Newman RC. Electrochemical studies of anodic dissolution of mild steel in a carbonate-bicarbonate buffer under erosion corrosion conditions. Corros Sci. 1996;38:1-14. https://doi.org/10.1016/0010-938X(96)00002-9,1212Brownlie F, Hodgkiess T, Galloway AM, Pearson A. Erosion-corrosion mechanisms of engineering steels in different NaCl concentrations. J Bio-tribo-Corrosion. 2021;7:80. https://doi.org/10.1007/s40735-021-00519-2].

T = E + C₀ + ∆C_e + ∆E_c                                                                 (1)

In which T = total material loss,

E = material loss by pure mechanical erosion mechanisms; this is obtained by undertaking tests in which corrosion is eliminated – usually by the application of cathode protection.

C₀ = pure corrosion rate in static solution.

The final two terms represent the roles of interactions between erosion and corrosion, termed synergy in which ∆C_e = the role of erosion processes in accelerating the damage due to corrosion and ∆E_c = the role of corrosion processes in accelerating the damage due to erosion.

∆C_e = corrosion rate in erosion-corrosion condition minus C₀.

A crucial aspect, of attempts to control the extent of erosion-corrosion damage, is to predict, in various environmental conditions and using different constructional materials, the extent of the relative contributions and mechanisms of the above parameters to the overall erosion-corrosion damage. In addition to the obvious value in understanding the fundamental mechanisms of erosion corrosion, knowledge of the extent of the contributory factors can point the way to potential strategies to minimise erosion-corrosion damage. For example, in cases where corrosion factors represent a significant proportion of the overall damage, assessment of potential benefits associated with the application of cathodic protection [55Brownlie F, Hodgkiess T, Pearson A, Galloway AM. A study on the erosion-corrosion behaviour of engineering materials used in the geothermal industry. Wear. 2021;477:203821. https://doi.org/10.1016/j.wear.2021.203821,1313Brownlie F, Giourntas L, Hodgkiess T, Palmeira I, Odutayo O, Galloway AM, Pearson A. Effect of cathodic protection methods on ferrous engineering materials under corrosive wear conditions. Corros Eng Sci Technol. 2020;55(6):480-486. https://doi.org/10.1080/1478422X.2020.1742997] and corrosion inhibitors [1414Barker R, Neville A, Hu A. Evaluating inhibitor performance in CO2-saturated erosion-corrosion environments. Corrosion. 2015;71(1):14-29. https://doi.org/10.5006/1124] may become relevant. The problem, with understanding and controlling erosion corrosion of titanium-base materials, is that detailed information, on the parameters in equation (1), is not readily available from the (rather limited) number of previous research on this topic.

Thus Yang and Swisher [1515Yang J, Swisher JH. Erosion-corrosion behaviour and cathodic protection of alloys in seawater-sand slurries. J Mater Eng Perform. 1993;2(6):843-850. https://link.springer.com/article/10.1007/BF02645684] carried out a limited study of the erosion corrosion of the Ti/6Al/4V alloy in a slurry comprising 3.5% NaCl and 9% sand at 25 °C in equipment that resulted in an impact velocity of 2.4 m/s. The work demonstrated that the material loss of the alloy was increased substantially in such conditions compared to quiescent water. Moreover, the damage was reported to be hardly affected by the application of cathodic protection (implying that there was no involvement of corrosion processes in the overall attack on the alloy) but the choice of cathodic protection parameters was quite rudimentary.

 A dominance of pure erosion on an “α–Ti alloy” in a 3.5% NaCl /10% sand slurry, over a range of jet velocities (4.8 – 12.8 m/s), was reported by Tu []. Electrochemical monitoring was not utilised and, although some aspects of synergism were discussed, no quantification was presented.

Xiulin and co-workers [1717Ji X, Qing Q, Ji C, Cheng J, Zhang YT. Slurry erosion wear resistance and impact-induced phase transformation of titanium alloys. Tribol Lett. 2018;66:64. https://doi.org/10.1007/s11249-018-1015-0] conducted a test programme using slurry jet equipment, in a medium of water containing 15% silica particles at an impact velocity of 15 m/s. Experiments lasted for up to one hour and it was observed that the Ti/6Al/4V alloy experienced substantially greater volume loss than commercially pure titanium. There was no experimental examination of the role of corrosion processes except for the statement that a low-velocity corrosive liquid normally improves the erosion resistance of titanium.

Khayatan, et al. [1818Khayatan N, Ghasemi HM, Abedini M. Synergistic erosion-corrosion behaviour of commercially pure titanium at various impingement angles. Wear. 2017;380-381:154-162. https://dx.doi.org/10.1016/j.wear.2017.03.016] studied the impingement erosion corrosion of commercially pure titanium in 3.5% NaCl solution. They utilised high sand loading (60 g/l) but a low velocity of 4 m/s. Relatively low pure erosion contributions to overall damage were observed with much higher synergy values (75% - 90% of total material loss) – especially the ∆E_c factor. This study utilised an interesting set-up comprising specimens that were impacted over their entire area and has some relation to aspects of the current research which will be assessed in the “Discussion” section. Another project [1919Neville A, MacDougall BAB. Erosion- and cavitation-corrosion of titanium and its alloys. Wear. 2001;250(1-12):726-735. https://doi.org/10.1016/S0043-1648(01)00709-8,2020MacDougall BAB, Neville A. Tribo-corrosion of titanium and its alloys. Mater Perform. Dec 2003:46-50. https://www.researchgate.net/publication/288719343_Tribo-corrosion_of_Ti_and_its_alloys] also reported very low contributions of pure erosion to overall material loss but, rather surprisingly, after slurry jet impingement at a much higher velocity (17 m/s) with sand concentrations of 500 mg/l in 3.5% NaCl at 18 °C. This project found that titanium alloy Ti/6Al/4V experienced improved resistance (by a factor of about two) to erosion corrosion than the commercially pure titanium. Although consideration of synergy was restricted to the ∆E_c parameter in equation (1), this factor was observed to be the dominant contributor (64% of total material loss) for pure titanium but less so (23%) for the Ti/Al/V alloy.

Other workers [2121Aldahash SA, Abdelaal O, Abdelrhman Y. Slurry Erosion-Corrosion Characteristics of As-Built Ti-6Al-4V Manufactured by Selective Laser Melting. Materials (Basel). 2020 Sep 8;13(18):3967. doi: 10.3390/ma13183967. PMID: 32911629; PMCID: PMC7558582.] investigated the erosion corrosion behaviour in a whirling arm test rig, of as-built Ti/Al/V, manufactured by selective laser melting. They measured higher material losses in silica sand slurries of 3.5% NaCl than in tapwater – implying a significant influence of corrosion on damage. There was no explicit consideration of synergy effects.

The research activity reviewed in the preceding sections of this paper has comprised erosion-corrosion in non-acidic saline solutions. Some effort has been devoted to behaviour in acidic slurries. Liu and colleagues [33Liu Y, Alfantazi A, Schaller F, Asselin E. Localised instability of titanium during its erosion-corrosion in simulated acidic hydrometallurgical slurries. Corros Sci. 2020;174:108816. https://doi.org/10.1016/j.corsci.2020.108816] focused on the role of corrosion in the erosion-corrosion situation. They employed a range of electrochemical techniques to study the passive film rupture/re-formation phenomenon associated with an impinging slurry of strongly acidic copper sulphate hydrometallurgical leaching solution. Their findings are relevant to the corrosion aspect of erosion corrosion and are discussed later in this paper in the context of the corrosion monitoring results presented herein. Brownlie, et al. [55Brownlie F, Hodgkiess T, Pearson A, Galloway AM. A study on the erosion-corrosion behaviour of engineering materials used in the geothermal industry. Wear. 2021;477:203821. https://doi.org/10.1016/j.wear.2021.203821] undertook a comparative investigation of erosion-corrosion performances of a range of engineering materials under consideration for use in geothermal plants. The experiments were carried out in an acidified NaCl solution of pH = 4. The behaviour of a Ti/Al/V alloy was, not unexpectedly, positioned in a group of other corrosion-resistant alloys (stainless steels. Ni-Cr-Mo alloy). There was some detailed consideration of synergy mechanisms but focused on the ∆E_c parameter in equation (1).

In summary, the rather limited amount of study of solid/liquid erosion-corrosion of titanium-base materials has provided hardly any assessment of the relative durability of pure titanium and the Ti/Al/V alloy – which is the most widely used titanium alloy in many industries [44Jáquez-Muñoz JM, Gaona-Tiburcio C, Cabral-Miramontes J, Nieves-Mendoza D, Maldonado-Bandala E, Olguín-Coca J, López-Léon JD, Flores-De los Rios JP, Almeraya-Calderón F. Electrochemical Noise Analysis of the Corrosion of Titanium Alloys in NaCl and H2SO4 Solutions. Metals. 2021;11:105. https://doi.org/10.3390/met11010105]. Moreover, the previous works have produced varying indications of the role of pure erosion and have delivered only sparse quantitative information on the crucial synergy processes.

To address these uncertainties, the objectives of the study reported herein were as follows:-

• To improve the hitherto rather patchy knowledge associated with the comparative erosion corrosion behaviour of pure titanium and the Ti/Al/V alloy. It would be of interest to ascertain, more reliably, to what extent the undoubted benefits obtained, in terms of structural strength by the alloying tactic, are also extended to erosion corrosion durability.
• To undertake a comprehensive study targeted at securing estimates, of the relative values and deterioration mechanisms, of all four parameters that contribute (Equation 1) to overall material loss by erosion-corrosion. Previous work has, at best, only yielded a partial determination of the synergy factors.
• By the use of an experimental methodology that provides estimates of the behaviour (especially in terms of corrosive attack) of the two main zones of a component subjected to slurry impingement:-

o    The region was subjected to direct impingement under a relatively small jet

o    The surrounding areas

In addition to the clear enhancement of the understanding of erosion-corrosion damage, this approach has relevance in exercises designed to yield comparative durability of various engineering materials. In this respect, in some previous studies [55Brownlie F, Hodgkiess T, Pearson A, Galloway AM. A study on the erosion-corrosion behaviour of engineering materials used in the geothermal industry. Wear. 2021;477:203821. https://doi.org/10.1016/j.wear.2021.203821,1212Brownlie F, Hodgkiess T, Galloway AM, Pearson A. Erosion-corrosion mechanisms of engineering steels in different NaCl concentrations. J Bio-tribo-Corrosion. 2021;7:80. https://doi.org/10.1007/s40735-021-00519-2], different comparative material performances in the two zones have been demonstrated than that obtained from the use of the usual single specimen.

The current investigation comprises a detailed investigation of the erosion corrosion behaviour of commercially pure titanium and the Ti/6Al/4V alloy in slurries comprising 3.5% NaCl. The latter was chosen to represent the water constitution in saline environments such as seawater but also other situations (e.g. some mining operations) where process- or natural waters are significantly saline. The suspended solids utilised comprise silica which has been identified [2222Lindgren M, Perolainen J. Slurry pot investigation of the influence of erodent characteristics on the erosion resistance of titanium. Wear. 2014;321:64-69. https://doi.org/10.1016/j.wear.2014.10.005] as a particularly severe erodent.

Methodology

Test materials and specimens

The two materials studied in this work were commercially-pure titanium (hereinafter referred to as, “TiG2”, and a commercially-supplied alloy, titanium-aluminium-vanadium (“Ti/Al/V”). Both were supplied in bar form from which samples of surface areas, 0.28, 5 and 8 cm² (TiG2) and 5 cm² (Ti/Al/V) were cut and machined. The chemical analysis, in wt %, of the Ti/Al/V alloy was as follows (Table 1).

Table 1: Chemical analysis of Ti/Al/V alloy.

To present a consistent surface condition to the experiments, all specimens were investigated in the as-polished condition; this involved successive grinding on 240, 400, 800 and 1200 SiC grit papers and finally one-micron diamond polishing. For mass loss measurement, the polished specimens were enclosed in a tightly fitting holder made of epoxy resin so that they could be removed and weighed after the test exposures. Specimens destined for electrochemical monitoring were mounted in a non-conducting epoxy resin after attachment of an electrical connecting wire to the back face of the sample. After the mounting resin had been set, the above-described polishing procedure was carried out on the exposed face.

A further group of experiments, on TiG2 material, comprised the study of segmented specimens as shown in Figure 1. These comprised a central cylindrical specimen (diameter 6 mm) electrically insulated, via heat shrink coating, from an outer ring with inner and outer diameters, 8 and 30 mm, respectively. Electrical conducting wires were soldered to the back face of both specimens before mounting in the non-conducting epoxy resin and final polishing. This specimen assembly facilitated separate electrochemical monitoring of the central (directly impinged) and outer zones of the material (see Results section).

Figure 1: Assembly of segmented specimen for electrochemical monitoring of separate central (C) and outer (O) zones of TiG2.

Erosion corrosion rigs

The research utilised the submerged jet apparatus schematically shown in Figure 2 for the experiments involving a 3.5% NaCl solution containing a suspension of silica sand particles – see below. The closed loop test rig comprises a high-pressure pump which feeds the saline slurry to a nozzle, 4 mm diameter, at a calculated average velocity of 12.6 m/s at 90° impingement onto the test specimen with a stand-off distance of 5mm maintained throughout the programme. The sand was Congleton HST60 with a size distribution given in Table 2A. Two samples of the slurry were taken throughout each experiment (after 5 minutes and just before the completion) to measure the circulating sand concentrations.

Table 2a: Particle size distribution of silica sand.

Figure 2: Schematic representation of solid-liquid impingement rig: R = reference electrode, A = Platinum Auxiliary electrode, W = Working electrode.

The solid-liquid impingement tests were carried out at a temperature of 18 °C - 20 °C kept constant throughout the test by incorporating a stainless steel cooling coil through which fresh tapwater passed. Test durations varied from 4 – 72 hours and the solid burdens were varied in different experiments between 500 – 1800 mg/l silica sand.

Two basic experimental procedures were utilised.

• Tests under free erosion-corrosion conditions
• Identical tests save for the application of cathodic protection (CP) which involved using a potentiostat to impose a constant electrode potential of -0.8 V (SCE reference) onto the test specimen.

The choice of appropriate CP control potential for titanium is not trivial since using a target potential equal to the equilibrium electrode potential for the reaction, Ti -> Ti²⁺ + 2e^-, about -2V (SCE), is impractical which, for instance, would result in severe mechanical damage of the Ti by hydrogen embrittlement. In this study, the choice of applied CP potential involved back-extrapolation of the anodic polarisation curve for titanium to an electrode potential at which the rate of the anodic reaction is extremely low, about 0.01 μA/cm². The identified electrode potential, -0.80V, is significantly above the values at which (embrittling) hydrogen is injected into the specimen.

For the experiments under liquid impingement (no added solids), a separate rig was utilised with a generally similar set-up as in Figure 2 but incorporating constant filtration of the circulating water to provide a liquid jet free from suspended solids. Experiments were undertaken at 18 °C - 20 °C and 50 °C in a 3.5% NaCl solution again with perpendicular impingement but at a much higher velocity of 71 m/s. Liquid impingement test durations were 12 hours after which the specimens were washed with distilled water, and ultrasonically bathed to remove all corrosion products before air drying and weighing using a mass balance with a precision of 0.1 mg.

Electrochemical monitoring

Anodic polarisation scans were undertaken in both liquid and solid/liquid conditions, using a traditional three-electrode configuration in conjunction with a computer-controlled potentiostat (Sycopel Multistat 2) employing a potential scan rate of 15 mV/minute.

Post-test examination techniques

The modes of attack and mechanisms were assessed using a light-optical microscope, surface profiling (Form Talysurf Series 2 model) and scanning electron microscopy (Leica Cambridge Stereoscan 360).

Results

Liquid impingement tests

In the mass loss tests, with both TiG2 and Ti/Al/V alloy, no measurable mass losses could be detected after 12 hours of liquid impingement at 71 m/s at 19 °C and 50 °C.

As exemplified by the plot in Figure 3 for the Ti/Al/V alloy, the anodic polarisation results, undertaken after a 12-hour exposure at 19 °C, also demonstrated extremely lowcorrosion rates for both materials. Tafel extrapolations [2323Revie RK, Uhlig HH. Corrosion and corrosion control. 4th ed. New Jersey: Wiley; 2008. https://onlinelibrary.wiley.com/doi/book/10.1002/9780470277270,2424Stern M, Geary AL. Electrochemical polarization: I. A theoretical analysis of the shape of polarization curves. J Electrochem Soc. 1957;104:56-63. https://iopscience.iop.org/article/10.1149/1.2428496] yielded a corrosion current density of 0.01 - 0.02 μA/ cm². Assuming oxide corrosion product with Ti⁴⁺ and a specimen surface area of 8 cm², a Faraday’s Law calculation [2323Revie RK, Uhlig HH. Corrosion and corrosion control. 4th ed. New Jersey: Wiley; 2008. https://onlinelibrary.wiley.com/doi/book/10.1002/9780470277270] with 0.01 μA/ cm² yields a mass loss of 0.43 x 10^-6 g/12 hrs, i.e. less than 0.5 μg /12 hrs.

Figure 3: Anodic polarisation of Ti/Al/V alloy after 12 hours of liquid impingement at 19 °C.

After the tests at 19 °C, samples of both materials did not exhibit any detectable effect on their surfaces either visually or under the light microscope. At 50 °C however, the specimen exhibited (Figure 4) coloured rings and shallow pits (Figure 5) in the central (directly impinged) region.

Figure 4: Ti/Al/V alloy after 12 hours of liquid impingement at 50 °C and 71 m/s without electrochemical intervention. (diameter of specimen = 26 mm).

Figure 5: Shallow pits in the centre (impinging region)of Ti/Al/V specimen shown in Figure 4).

Solid/Liquid impingement tests

In the initial phase of the examination of the behaviour of the two materials under solid/liquid erosion-corrosion, experiments were conducted over a range of periods at an impinging velocity of 12.4 m/s and with sand concentrations of 1500 – 1600 mg/l. The test samples exhibited a deep wear scar in the centre, directly beneath the impinging jet. A typical example is shown in the surface profile of Figure 6.

Figure 6: Surface profile of TiG2 after 72 hours of solid/liquid impingement at 12.4 m/s and 1500 – 1600 mg/l suspended solids.

The test results are tabulated in Table 2B and the averages are plotted in Figure 7 and reveal that, at short periods, (4, 8 hrs), there is no clear differentiation between the two materials. After 16 hours – and more clearly 72 hours – it appears, however, that the Ti/Al/V alloy is superior to commercially pure titanium.

Figure 7: Mass loss (upper) and scar depth (lower) for TiG2 and Ti/6/4 after solid/liquid impingement at 12.4 m/s and 1500 – 1600 mg/l solids.

Table 2b: Mass losses and scar depths under solid/liquid impingement.

After the above-described general comparative study of the two materials over a range of exposure times, the next phase of the work focused on a more detailed investigation of the individual contributors to the overall material loss during erosion-corrosion. This included the roles of pure erosion and corrosion and synergistic phenomena. A fixed exposure time of 16 hours was adopted with somewhat more severe test conditions: fluid containing 1700 – 1800 mg/l solid burden impacting at a velocity of 12.6 m/s. The individual test results are presented in Table 3 and, again demonstrate the reduced erosion corrosion damage (in terms of both mass loss and wear scar depth) of the Ti/Al/V alloy compared to commercially pure titanium.

Table 3: Damage parameters for the test materials after 16 hours at 12.6 m/s and 1700 - 1800 mg/l solids.

Table 4 reveals the benefits accruing from the application of cathodic protection during the 16-hour experiments. These are appreciable in terms of mass loss:-

Table 4: Damage parameters for the test materials after 16 hours at 12.6 m/s and 1700 - 1800 mg/l solids with the application of cathodic protection.

TiG2: mass loss_CP /mass loss_{eros corr} = 2.8/4.0 = 70% thus reduction by CP = 30%

Ti/Al/V: mass loss_CP /mass loss_{eros corr} = 2.1/2.85 = 74% thus reduction by CP = 26%

But rather less beneficial on scar depths:-

TiG2: scar depth_CP /scar depth_{eros corr} = 35/45 = 78% thus reduction by CP = 22%

Ti/Al/V: scar depth_CP /scar depth_{eros corr} = 31/35 = 89% thus reduction by CP = 11%

In terms of mechanisms of attack, the numbers in Table 4 represent the contribution of pure mechanical damage to the overall erosion corrosion material loss. The role of corrosion is obtained from the corrosion rates deduced from the electrochemical monitoring exercises that are presented in Figure 8.

Table 4: Damage parameters for the test materials after 16 hours at 12.6 m/s and 1700 - 1800 mg/l solids with the application of cathodic protection.

Figure 8: Comparison of anodic polarisation of TiG2 in static water (left, blue plot) with that undertaken at the termination of 16 hours of solid/liquid erosion-corrosion (right, black plot).

The anodic polarisation monitoring commences at the free corrosion potential (E_corr) and records the increasing current density with positive shifts in electrode potential during the scan. The main features of the plots in Figure 8 are:-

• The most obvious feature of Figure 8 is the substantially higher current densities (indicative of enhanced corrosion rates) during solid/liquid impingement.
• The polarisation plot in impingement conditions, exhibits fluctuating currents around a mean value that is either almost constant or only moderately increasing over a very wide (> 1V) electrode potential range.

It should be pointed out that the “stable” fluctuating currents are of theoretical electrochemical interest only as opposed to the current/potential behaviour in the zone immediately positive to E_corr. It is in this segment of the plot where the “Tafel Extrapolation” procedure - to yield values of the corrosion rate, i_corr, - is undertaken. To facilitate this procedure, the right plot of Figure 8 is expanded, in Figure 9, to show the Tafel procedure. The construction line, drawn through the linear region of the anodic polarisation graph at a distance appropriately distant (about 50mV or more) from E_corr is extrapolated back to E_corr and the intersection point yields an estimate, about 2 μA/cm², of the corrosion rate (i_corr ).

Figure 9: Enlargement of the lower region of anodic polarisation of TiG2 at the termination of 16 hours of solid/liquid erosion-corrosion. Blue construction line illustrating Tafel extrapolation procedure to provide an estimate of icorr.

The analogous anodic polarisation result for the Ti/Al/V alloy, undertaken at the termination of the 16-hour solid/liquid impingement experiment, provided a graph that essentially superimposes the TiG2 plot shown in Figure 8 and therefore has not been included for the sake of clarity.

The components of total material loss – including synergy

It is possible to use some of the data, presented above, to examine the roles and magnitudes of the individual components contributing to overall erosion corrosion material loss. Use is made of the relationship presented in equation (1):-

T = E + C₀ + ∆C_e + ∆E_c

The tests were carried out for 16 hours in 3.5% NaCl containing 1700 – 1800 mg/l solid burden impacting at a velocity of 12.6 m/s. From Tables 3 and 4 and relevant electrochemical monitoring, the total mass loss and the separate components can be summarised as follows.

From Table 3, total mass loss (T) for TiG2 = 4.0 mg and Ti/Al/V = 2.85 mg

The pure erosion (E) values are given by mass losses during the application of cathodic protection; from Table 4, total mass loss for TiG2 = 2.8 mg and Ti/Al/V = 2.1 mg.

The pure corrosion values, C₀, are obtained from the Tafel extrapolations for static conditions and are found to be similar for both TIG2 (see subsequent Figure 15) and Ti/Al/V as 0.1 μA/cm².

Assuming a corrosion product comprising oxide TiO₂ and a specimen surface area of 8 cm², a Faraday’s Law calculation with n = 4 and i_corr = 0.1 μA/ cm² yields a mass loss of 5.76 x 10^-3 mg/ 16 hours.

The corrosion rates in liquid/solid impingement conditions again were found to be similar for the two test materials,
2 μA/cm² (Figure 9). The relevant Faraday calculation yields a mass loss of 0.114 mg/16h.

Thus ∆C_e = 0.11 - C₀ = 0.114 - 0.006 = 0.108 mg.

The various contributions to overall erosion-corrosion damage are summarised in Table 5 which ∆E_c is calculated by ∆E_c = T - (E + C₀ + ∆C_e). The slight discrepancy from 100% is due to rounding procedures in the measurements and calculations.

Table 5: Contributors to overall erosion-corrosion damage.

It is clear that the erosion-corrosion behaviour is erosion-dominated – although there is a significant indirect contribution from corrosion in facilitating the corrosion-induced erosion damage to the extent of over 20%. The total synergy is around 30%. There is not much material dependence on the proportions even though the Ti/Al/V alloy is about 30% more resistant than commercially pure titanium to erosion corrosion damage.

Microscopical examination

The surfaces of tested specimens were examined under the microscope. Similar features were exhibited by the two tested materials and are illustrated, in Figures 10,11, concerning the Ti/Al/.V alloy. As expected, the most intense attack occurred in the directly impinged zone (DIZ) under the impinging jet. Such a region is shown, for a specimen subjected to free erosion-corrosion, in Figure 10a where an irregular, worn surface is evident. There are also local regions comprising microscopic holes presumably formed by the drilling action of the impacting sand particles. Outside the wear scar (OA), Figure 10b, the surface is more uniform but abundant abrasive skid marks are present caused by the sand particles sliding at low angles after spreading from the directly impinged zone.

Figure 9: Enlargement of the lower region of anodic polarisation of TiG2 at the termination of 16 hours of solid/liquid erosion-corrosion. Blue construction line illustrating Tafel extrapolation procedure to provide an estimate of icorr.

Figure 10: Ti/Al/V after 16 hours solid/liquid impingement: (a) centre of specimen, under the impinging jet, (b) outer regions of the specimen.

The microstructures evident, after application of cathodic protection (Figure 11), were virtually identical to those after free erosion corrosion. This indicates that there was no change in damage mechanism and that the influence of erosive attack predominates.

Figure 11: Ti/Al/V after 16 hours solid/liquid impingement with cathodic protection (a) centre of specimen, under the impinging jet, (b) outer regions of specimen.

Comparison of erosion-corrosion in the two hydro-dynamic zones

An important feature of this investigation relates to the relative vulnerabilities of the two main hydrodynamic zones:-

• The region (“DIZ”) directly under the 4-mm impinging jet
• The region (“OA”) surrounding the DIZ

The presence of a relatively deep central wear scar (Figure 6) is a clear demonstration that the “DIZ” region, directly impacted by the impinging jet, is particularly vulnerable to damage. Indeed, it might be speculated that the great majority/ or even total damage occurs in this zone – especially when the metal/alloy readily develops a corrosion-resistant “passive” film on the surface. To throw some light on this matter, the final part of this work comprised a study of TiG2 specimens of different areas: 0.28, 5, and 8 cm² exposed to a range of particle concentrations for 16 hours using a velocity of 12.6 m/s. The results, shown in Figure 12, display higher mass losses experienced as the specimen surface area increases which unambiguously confirms that there is a significant loss of metal outwith the directly impinged zone.

Figure 12: Mass loss versus sand loading for TiG2 after 16 hours of solid/liquid impingement.

In more quantitative terms, if the mass loss of the smallest TiG2 specimen is assumed to represent the mass loss in the central DIZ zone in a larger (say 8 cm²) specimen, it is possible to obtain an approximate estimate of the ratio,

Mass loss_DIZ: Total mass loss

Using data at similar solid loadings in Figure 13. This yields

Mass loss of 0.28 cm² specimen = 1.1 mg

Mass loss of 8 cm² specimen = 1.8 mg

Thus mass loss_DIZ: Total mass loss = 1.1 /1.8 = 61%

Figure 13: Anodic polarisation of TiG2 segmented specimens at the termination of 16 hours of solid/liquid erosion-corrosion. (a) Full polarisation plots (b) Enlargement of lower regions of graphs: Blue construction line illustrating Tafel extrapolation procedure to provide an estimate of icorr (20 μA/cm² ) for the central (DIZ) specimen.

Mass loss in DIZ

A further way of quantifying relative mass losses in the two hydrodynamic zones involves estimating the volume of the central wear scar (Figure 6). Two such examples were selected after tests on TiG2 after exposure to solid/liquid impingement for 16 hours under a velocity of 12.6 m/s. The wear scar volumes were estimated as 0.56 mm² and 0.47 mm² respectively. The calculations of the proportion of material loss within the wear scar to total measured mass loss are summarised in Table 6 which yields such proportions of 68 and 70%. These estimations are quite close to the value of 61% obtained from the data in Figure 13.

Table 6: Mass losses in DIZ for TiG2 (using density of titanium = 4.51 g/cm3). 

Mass losses by corrosion in DIZ and OA

To obtain a comparison of the corrosion rates in the DIZ and outer (OA) regions, polarisation monitoring was undertaken on a segmented specimen (Figure 1) which comprises a central specimen that is electrically insulated from the surrounding zone. The results, of the anodic polarisation scans on the DIZ and OA specimens, are shown in Figure 14a. The most noteworthy feature of these plots is that they demonstrate a considerably higher corrosion rate in the central DIZ zone compared to that in the outer region; anodic currents are more than an order of magnitude higher in the DIZ.

Another aspect shown in Figure 14a, comparable to the data in Figure 8, is the fluctuating currents around an approximate mean value over a very wide (> 1V) electrode potential range.

In Figure 14b, an estimation of the corrosion rate, i_corr, on the central specimen, is obtained. Similar to the procedure described in Figure 9, Tafel extrapolation is undertaken and provides a value of 20 μA/cm² for the corrosion current density. Proceeding to calculate the % contributions to overall material loss in the DIZ, a Faraday’s Law calculation, on the area of 0.28 cm² and i_corr = 20 μA/ cm², yields a mass loss of 0.04 mg/16 hours. Recalling that, for static conditions (C₀), the mass loss = 0.006 mg

Thus ∆C_e = 0.04 - C₀ = 0.04 - 0.006 = 0.034 mg.

Figure 14: Anodic polarisation plots of TiG2; (A) in static conditions; (B) outer segmented specimen under solid/liquid erosion corrosion conditions (see also Figure 14).

Erosion mass loss in DIZ

From Tables 3 and 4, scar depths with and without cathodic protection (CP) are 35 and 45 μm respectively. So CP reduces scar depth by 35/45 = 78%.

Assuming small specimen data in Figure 13 relates to DIZ (similar areas),

Total mass loss_DIZ = 1.1 mg. Thus erosive mass loss_DIZ = 1.1 x 0.78 = 0.86 mg

Contributions to overall erosion-corrosion damage

Pure erosion = 0.86/1.1 = 78%

Pure corrosion = 0.006 / 1.1 = 0.5%

Synergy components,

∆C_e = 0.034 /1.1 = 3.1%

∆E_c = T - (E + C₀ + ∆C_e) = 1.1 – (0.86 + 0.006 + 0.034) = 0.20 mg, i.e. 0.20/1.1 = 18%

Discussion

For liquid impingement with no added solids, the findings of this work have been in line with the previous understanding that titanium and its alloys possess excellent resistance to high-velocity water. This finding is based on gravimetric measurements and the extremely low current densities recorded (Figure 3) during anodic polarisation monitoring after a 12-hour liquid impingement exposure. As stated in the “Results” section, no measurable mass loss was detected on the two experimental materials after the 12-hour liquid impingement exposure at 71 m/s and up to 50 °C. Considering the sensitivity of the mass balance as 0.1 mg, an “upper band” of material loss can be calculated as 4.52 x 10^-6 cm thickness loss of the two materials in 12 hours at 71 m/s and 50 °C (equivalent to 0.033 mm/year). This can be compared to previously obtained data in Table 7 and is also in accord with a more recent report [33Liu Y, Alfantazi A, Schaller F, Asselin E. Localised instability of titanium during its erosion-corrosion in simulated acidic hydrometallurgical slurries. Corros Sci. 2020;174:108816. https://doi.org/10.1016/j.corsci.2020.108816] of similar anodic polarisation behaviour of TiG2 in static and solids-free turbulent acidic hydrometallurgical slurry.

Table 7: Previous data for liquid impingement of Ti and Ti-6Al-4V at ambient temperature.

The extremely protective (“passive”) titanium oxide (TiO₂) film that forms spontaneously on the surface of titanium retains its protective properties in highly turbulent conditions in the absence of suspended solids. The film is so thin as to be transparent at ambient temperatures but appears as visible (Figure 4), but still very thin, interference films on the surface after exposure to elevated temperatures (50 °C). It should be noted that comparable interference films have been observed [2525Hussain EAM, Robinson MJ. Erosion corrosion of 2205 duplex stainless steel in flowing seawater containing sand particles. Corros Sci. 2007;49(4):1737-1754. https://doi.org/10.1016/j.corsci.2020.108816] on stainless steels in similar situations.

The excellent resistance of the studied materials is not retained, however, even at the much lower velocity of 12.5 m/s, when the impinging water contains appreciable burdens of finely suspended solids. In the slurry conditions present in this work, the largest contributor to the degree of attack is pure mechanical energy damage. Nevertheless, the appreciable (around 30%) reduction in material loss, that accompanies the application of cathodic protection during slurry impingement, is a testament to the significant role of corrosion processes in the overall damage. This dominance of pure erosion is in slight discord with the findings from a previous study [1919Neville A, MacDougall BAB. Erosion- and cavitation-corrosion of titanium and its alloys. Wear. 2001;250(1-12):726-735. https://doi.org/10.1016/S0043-1648(01)00709-8,2020MacDougall BAB, Neville A. Tribo-corrosion of titanium and its alloys. Mater Perform. Dec 2003:46-50. https://www.researchgate.net/publication/288719343_Tribo-corrosion_of_Ti_and_its_alloys]. In the latter work, pure erosion was also the dominant damage process for the Ti/Al/V alloy, but concerning TiG2, it was observed that synergy (∆Ec) represented the major contributor to the overall erosion-corrosion damage. These findings are testimony to the complexities of the erosion-corrosion phenomenon which involves varying contributions of E and S – depending on the material/environment system under consideration.

Comments on the experimental protocol

The experimental set-up adopted in part of the present project, comprising a relatively small diameter impinging jet onto a single (“composite”) specimen surface of a much larger size, has been employed in most impinging jet investigations. Whilst this arrangement does indeed provide interesting and useful information on erosion-corrosion behaviour, it is subject to the drawback that it yields data that, in effect, represents an “average” of the erosion-corrosion damage within the directly-impinged zone and the surrounding region. As is generally found in this – and other – studies, the extent and mechanisms of erosion-corrosion attacks are quite different in the two regions. This aspect is immediately evident as a result of performing surface profiling which reveals (Figure 6) a deep scar within the directly impinged region. It is unfortunate that in the rare instances where surface profiling is undertaken by other workers [1919Neville A, MacDougall BAB. Erosion- and cavitation-corrosion of titanium and its alloys. Wear. 2001;250(1-12):726-735. https://doi.org/10.1016/S0043-1648(01)00709-8,2020MacDougall BAB, Neville A. Tribo-corrosion of titanium and its alloys. Mater Perform. Dec 2003:46-50. https://www.researchgate.net/publication/288719343_Tribo-corrosion_of_Ti_and_its_alloys], quantification of the associated volume losses appears not to be often done. An improvement in this situation is to focus attention on the two distinct hydrodynamic zones by utilising the experimental approach, introduced by Giourntas, et al. [2626Giourntas L, Hodgkiess T, Galloway A. Enhanced approach of assessing the corrosive wear of engineering materials under impingement. Wear. 2015;338-339:155-163. https://doi.org/10.1016/j.wear.2015.06.004]. This involves quantifying the damage in the two zones and includes the use of segmented specimens to facilitate the separate measurement of the corrosion rates in the two regions. Segmented specimens (Figure 1) yield corrosion rate data that facilitates the individual contributions from C₀, ∆C_e and ∆E_c whereas using cathodic protection provides information on the overall (C₀ + ∆C_e + ∆E_c) corrosion damage.  One possible outcome of this approach [2626Giourntas L, Hodgkiess T, Galloway A. Enhanced approach of assessing the corrosive wear of engineering materials under impingement. Wear. 2015;338-339:155-163. https://doi.org/10.1016/j.wear.2015.06.004] is that it can discriminate differences in erosion-corrosion behaviour in the two hydrodynamic zones induced by such strategies as the comparisons of different materials, the effect of environmental parameters such as temperature, fluid velocities, and solids concentration.

In the current work, comparisons of the data in Tables 5 and 6 indicate, that deductions from the assessment of the two hydrodynamic zones are successful in demonstrating the occurrence of a significant degree of material loss in the outer regions: 30% - 39% of total material loss (Table 6 and Figures 12,13). This feature demonstrates that, even when a component surface is experiencing an almost parallel flow of slurry, an appreciable loss of material can occur. Moreover, the ability to discriminate between the two zones (using segmented specimens) can expose substantial differences in corrosion attacks. In the current study, it has thus revealed:-

• Relatively high corrosion rate in the DIZ; without this data, it might be tempting to assume that damage in this directly impinged zone would be almost completely dominated by erosive action.
• Even though the corrosion rates in the outer area are much less than those in the central zone (Figure 13), it is still of significant extent compared to static conditions (Figure 14) – again displaying corrosion-driven activity in the outer zone.

In fact, in the present study, the data presented and calculated in the “Results” section has indicated a good correlation between the detailed erosion corrosion contributions obtained from a composite specimen and those found by a direct focus on the DIZ. This correlation is summarised in

Table 8: Proportions of mass losses for the entire composite specimen and small area (0.28cm2) “DIZ specimen

 for TiG2 after tests for 16 hours in 3.5% NaCl containing 1700 – 1800 mg/l solid burden impacting at a velocity of 12.6 m/s. In contrast, the findings 
from a different investigation on stainless steels [1212Brownlie F, Hodgkiess T, Galloway AM, Pearson A. Erosion-corrosion mechanisms of engineering steels in different NaCl concentrations. J Bio-tribo-Corrosion. 2021;7:80. https://doi.org/10.1007/s40735-021-00519-2] demonstrated that the observations from a single (composite) specimen were generally in accord with the behaviour in the outermost regions but with little, or no, correlation to the damage experienced in the directly– impinged part of the component. In another investigation [55Brownlie F, Hodgkiess T, Pearson A, Galloway AM. A study on the erosion-corrosion behaviour of engineering materials used in the geothermal industry. Wear. 2021;477:203821. https://doi.org/10.1016/j.wear.2021.203821], the behaviour of low-alloy steel compared favourably to corrosion-resistant alloys in the DIZ but was observed to suffer relatively more severe damage in the outer zone.

Mechanisms of damage

Pure erosion: After both free erosion corrosion and the application of cathodic protection, the most intense attack occurred in the Directly Impinged Zone (DIZ) under the impinging jet. The microscopical examination revealed a generally roughened surface with no directionality of damage because of the direct impact of the sand particles which results in plastic deformation of the ductile metal surface. Such repetitive deformation causes surface roughening followed by material removal under continued particle impact. There are also local regions comprising microscopic holes presumably formed by the drilling action of the impacting sand particles.

In the surrounding region, the damage is more directional as particles, spreading from the DIZ, skid along the surface causing low-angle ploughing and abrasive cutting.

A complicating factor in the assessment of erosion damage mechanisms in titanium-base materials is the potential influence of metallurgical transformations during particle impacts. These could involve sub-surface hardening, and phase transformations in the α/β Ti/Al/V alloy [2727Ji X, Qing Q, Ji C, Cheng J, Zhang Y. Slurry erosion wear resistance and impact-induced phase transformation of titanium alloys. Tribol Lett. 2018;66:64. https://doi.org/10.1007/s11249-018-1015-0]. Yet another interesting postulation [2727Ji X, Qing Q, Ji C, Cheng J, Zhang Y. Slurry erosion wear resistance and impact-induced phase transformation of titanium alloys. Tribol Lett. 2018;66:64. https://doi.org/10.1007/s11249-018-1015-0] is that the low elastic modulus of titanium may facilitate the absorption of impact energy from impinging particles.

Pure Corrosion processes: The pure corrosion (C₀) component is of negligible magnitude on account of renowned passive behaviour in quiescent conditions promoted by the stable titanium oxide surface film. It should be recognised, however, that material, such as carbon- and⅘ low-alloy steels, with inherently low corrosion resistance (due to their inability to form protective oxide surface films), exhibit high values of pure corrosion, C₀, attack.

Synergy: Synergy contributions were found to be of noteworthy extent amounting to 20% - 30%. Khayatan and colleagues [1818Khayatan N, Ghasemi HM, Abedini M. Synergistic erosion-corrosion behaviour of commercially pure titanium at various impingement angles. Wear. 2017;380-381:154-162. https://dx.doi.org/10.1016/j.wear.2017.03.016] have reported even more substantive synergies amounting to 75% or more of the total material loss. This was associated with a considerably higher solids content (60 g/l) but lower velocity (4 m/s) than in the current investigation and implies that pure erosion attack is driven more by impingement velocity rather than solids loading. Another interesting feature of Khayatan’s work was the adoption of an experimental arrangement in which the entire specimen was impinged by the jet and thereby comprised a single hydrodynamic zone similar to, including size (5 mm diameter), the DIZ zone in the current study. Such a test set-up has the attraction of providing information on the erosion-corrosion behaviour of a fully impacted component without any complications introduced by interactions with a surrounding region that is not being directly impinged but does beg the question as to what practical scenario it relates to.

As regards the mechanisms of these synergy contributions, it is clear that the erosion-assisted corrosion (∆C_e) component, involves the action of the solid particle bombardment in the mechanical destruction of the protective oxide. Nevertheless, the observation of oscillating currents, during anodic polarisation monitoring, provides evidence of the tenacity of the normally protective oxide film since such fluctuating currents are indicative of periodic de-passivation/re-passivation events under solid/liquid impingement. Such fluctuating currents, during erosion-corrosion, are a common feature of slurry impingement on other “normally corrosion resistant” alloys such as stainless steels [1919Neville A, MacDougall BAB. Erosion- and cavitation-corrosion of titanium and its alloys. Wear. 2001;250(1-12):726-735. https://doi.org/10.1016/S0043-1648(01)00709-8,2626Giourntas L, Hodgkiess T, Galloway A. Enhanced approach of assessing the corrosive wear of engineering materials under impingement. Wear. 2015;338-339:155-163. https://doi.org/10.1016/j.wear.2015.06.004], even at extremely-low 0.05% NaCl [2828Brownlie F, Hodgkiess T, Pearson A, Galloway A. Electrochemical evaluation of the effect of different NaCl concentrations on low alloy- and stainless steels under corrosion and erosion-corrosion conditions. Corros Mater Degrad. 2022;3:101-126. https://doi.org/10.3390/cmd3010006] and have received detailed electrochemical study by Liu, et al. [33Liu Y, Alfantazi A, Schaller F, Asselin E. Localised instability of titanium during its erosion-corrosion in simulated acidic hydrometallurgical slurries. Corros Sci. 2020;174:108816. https://doi.org/10.1016/j.corsci.2020.108816]. Thus, an important factor in this balance between film breakdown and re-passivation is the time intervals involved relative to the impact frequency during solid/liquid exposure. Several such investigations have been made of the current transients associated with the film breakdown/ repair events on titanium-base materials with the following times being recorded: several minutes [66Asserghine A, Filotá D, Nag L, Souto RM, Nag G. Do titanium biomaterials get immediately and entirely repassivated? A perspective. npj Mater Degrad. 2022;6:57. https://doi.org/10.1038/s41529-022-00270-0,2929Wang JL, Liu RL, Majumdar T, Mantri SA, Ravi VA, Banerjee R, Birbilis N. A closer look at the in vitro electrochemical characterisation of titanium alloys for biomedical applications using in-situ methods. Acta Biomater. 2017 May;54:469-478. doi: 10.1016/j.actbio.2017.03.022. Epub 2017 Mar 16. PMID: 28315814.], but 0.05 seconds by Yu Liu, et al. [33Liu Y, Alfantazi A, Schaller F, Asselin E. Localised instability of titanium during its erosion-corrosion in simulated acidic hydrometallurgical slurries. Corros Sci. 2020;174:108816. https://doi.org/10.1016/j.corsci.2020.108816]. Even the shortest time interval (0.05 s) is several orders less than the particle impact frequency in the conditions of the present study. Hence a net oxide-film depassivation process would be expected and this is depicted in the anodic polarisation monitoring exercises undertaken (Figure 14). Nevertheless, although these competing events do not facilitate complete re-passivation, they do reduce the corrosion rates to the extent of limiting the erosion-enhanced corrosion (∆C_e) factor to a few per cent of total material loss.

Concerning the corrosion-enhanced erosion (∆E_c) factor, the mechanisms in the current project, are less obvious to identify. One possibility is that a corrosive attack might promote wear by causing the surface to become rougher, thus making the protrusions more easily removable by the action of eroding particles especially if local micro-turbulence prevails. Another option is that the metallic crystals may become less coherent on account of the grain boundaries possessing lower resistance to damage [2727Ji X, Qing Q, Ji C, Cheng J, Zhang Y. Slurry erosion wear resistance and impact-induced phase transformation of titanium alloys. Tribol Lett. 2018;66:64. https://doi.org/10.1007/s11249-018-1015-0] – possibly as a result of the occurrence of intergranular corrosion. For the two-phase Ti/Al/V alloy, micro galvanic action between the alpha and beta grains could make them more prone to detachment by impacting particles. Such a mechanism has been discussed [1212Brownlie F, Hodgkiess T, Galloway AM, Pearson A. Erosion-corrosion mechanisms of engineering steels in different NaCl concentrations. J Bio-tribo-Corrosion. 2021;7:80. https://doi.org/10.1007/s40735-021-00519-2] concerning synergy occurrence in dual-phase stainless steel.

Material selection issues

In terms of comparative behaviour of the commercially pure titanium (TIG2) and the Ti/Al/V alloy, the findings (Table 2 and Figure 7) in this work indicate a modest reduction in overall mass loss and wear scar depth of 13% and 17% respectively of the Ti/Al/V alloy compared to TiG2. This was, however, to a much-reduced extent to that reported [1919Neville A, MacDougall BAB. Erosion- and cavitation-corrosion of titanium and its alloys. Wear. 2001;250(1-12):726-735. https://doi.org/10.1016/S0043-1648(01)00709-8,2020MacDougall BAB, Neville A. Tribo-corrosion of titanium and its alloys. Mater Perform. Dec 2003:46-50. https://www.researchgate.net/publication/288719343_Tribo-corrosion_of_Ti_and_its_alloys] from similar environmental conditions except for a lower solids burden than employed in the current research. Taken with the findings [1717Ji X, Qing Q, Ji C, Cheng J, Zhang YT. Slurry erosion wear resistance and impact-induced phase transformation of titanium alloys. Tribol Lett. 2018;66:64. https://doi.org/10.1007/s11249-018-1015-0] of opposite trends (i.e. TiG2 outperforming the alloy) in a slurry containing a considerably higher (15%) burden of solids, this is perhaps indicative of reduced benefits associated with the Ti/Al/v alloy at higher solids concentrations. It is clear, therefore, that the substantial benefits, of alloying to obtain structural-loading capacities without compromising the corrosion resistance in quiescent water, that is provided by the Ti/Al/V alloy, are not replicated in resistance to erosion-corrosion conditions. This feature also does not support the often-claimed assertion that hardness provides enhanced resistance to wear damage. This lack of correlation between conventional material properties and high-angle erosion resistance may be attributed to the high-strain rate conditions which occur during a high-velocity impingement erosion-corrosion experiment [].

It is pertinent to offer a few comments on the comparative erosion-corrosion resistances of titanium-base materials and stainless steels that are often competing materials for engineering components and structures. It would appear [1717Ji X, Qing Q, Ji C, Cheng J, Zhang YT. Slurry erosion wear resistance and impact-induced phase transformation of titanium alloys. Tribol Lett. 2018;66:64. https://doi.org/10.1007/s11249-018-1015-0,2222Lindgren M, Perolainen J. Slurry pot investigation of the influence of erodent characteristics on the erosion resistance of titanium. Wear. 2014;321:64-69. https://doi.org/10.1016/j.wear.2014.10.005,3131Hodgkiess T. The role of advanced materials to combat erosion-corrosion in aqueous environments. Stainless Steel World. 1999 Jul/Aug:39-42. https://www.researchgate.net/profile/Trevor-Hodgkiess] that, dependent upon the actual grade of stainless steel under consideration, there are only modest differences between the erosion-corrosion performances of the two groups of materials. Consequently, the substantially higher corrosion resistance of titanium compared to stainless steel is not replicated in erosion corrosion behaviour.

In summary, except for very low impingement velocities and/or low suspended solids loading, titanium base materials – in common with most metallic materials – do not possess acceptable resistance to erosion corrosion in many industrial situations. Recourse to surface coatings or alternative classes of engineering material – such as ceramic-base options – is likely to be necessary.

Conclusion

The main findings from this experimental investigation of the jet-impingment, erosion-corrosion behaviour, of commercially pure titanium, TiG2, and a Ti/Al/V alloy, in NaCl/sand slurries, are summarised below.

1. The previous knowledge, of the excellent resistance of titanium-base materials in high-velocity water that contains negligible suspended solids, has been confirmed and extended in scope.
2. The vulnerability of these materials where the impinging water contains appreciable burdens of suspended sand (greater than 500 mg/l in this study) has been observed.
3. Although the material loss, in the erosion-corrosion conditions investigated, is dominated by the pure erosive attack, it has been demonstrated that there is also a significant proportion of damage (up to about 30%) associated with corrosion processes. This involvement of corrosion is via a synergy mechanism comprising the role of corrosion in further elevating the damage associated with erosion.
4. The most severe attack occurs in the central, directly impinged zone but the methodology adopted in this project demonstrated that the region outside the central zone was also degraded by around 30-40% of the total component material loss.
5. The adoption of methods to separate the extent of damage in the two main h hydrodynamic zones revealed a much higher rate of corrosive attack in the central,  directly impinged zone than in the surrounding region.
6. Although the margins of improvement were not to a major extent, the Ti/Al/V alloy exhibited reduced erosion-corrosion attack than the commercially pure titanium.
7. The potential versatility of the approach, involving a separate focus on the directly impinged and outer zones including the use of segmented specimens, is discussed. This approach can provide additional sensitivity and thereby avoid somewhat oversimplified conclusions that may arise in some circumstances from the use of a single specimen.

Acknowledgement

Support for DM from the University of Glasgow is gratefully acknowledged.

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Cite this Article

Hodgkiess T, Mantzavinos D. Erosion Corrosion of Commercially Pure Titanium and Ti-6Al-4V Alloy in Sodium Chloride Solutions with and Without Suspended Solids. IgMin Res. January 28, 2025; 3(1): 057-069. IgMin ID: igmin284; DOI:10.61927/igmin284; Available at: igmin.link/p284

• DOI10.61927/igmin284

03 Dec, 2024

Received

27 Jan, 2025

Accepted

28 Jan, 2025

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