Platinum Metals Rev., 1996, 40, (2), 54
Ruthenium Enhanced Titanium Alloys
Minor Ruthenium Additions Produce Cost Effective Corrosion Resistant Commercial Titanium Alloys
Several new, more highly corrosion resistant titanium alloys containing a nominal 0.1 weight per cent of ruthenium have been developed and evaluated for industrial service in corrosive environments. These improved ruthenium-enhanced α, α-β and β titanium alloys are lower in cost than the corresponding palladium-containing titanium alloys, and offer essentially the same corrosion performance in dilute reducing acids and hot brine environments. The titanium-0.1 ruthenium binary alloys can be cost effectively substituted for traditional titanium-palladium alloys and should represent a more attractive alternative to nickel-chromium-molybdenum alloys in hot, acidic brine applications. The corrosion database that has been established for the higher strength ruthenium-enhanced α-β and β titanium alloys in high temperature sweet and sour brines provides the basis for their selection for applications in the chemical process, oil/gas production, offshore and geothermal energy industries.
Traditionally, the palladium-containing titanium alloys, ASTM (American Society for Testing and Materials) Grades 7 and 11 titanium (titanium-0.15 weight per cent palladium, Ti-0.15Pd) have been the most corrosion resistant titanium alloys commercially available. These titanium-palladium, Ti-Pd, alloys were selected when other common industrial titanium alloys, such as the unalloyed grades, exhibited susceptibility to crevice and pitting corrosion in more aggressive chemical service. Severe service environments include chlorine-saturated brines, wet halogens, acidic metal chloride solutions (such as FeCl3, ZnCl2, AlCl3) and hydrolysable, concentrated brines (such as MgCl2, CaCl2) at temperatures exceeding ~ 80°C. The Ti-Pd alloys are also corrosion resistant over a much wider range of temperatures and/or acid concentrations in hot dilute inorganic and organic reducing acids (1).
Despite their dramatically enhanced corrosion performance, the utilisation of Grades 7 and 11 titanium alloys has been severely limited over the past thirty years due to their high relative cost. As is shown in Table I, the cost of Ti-Pd alloy is almost twice that of unalloyed titanium and is similar to that of common nickel-chromium-molybdenum, Ni-Cr-Mo, alloys on a dimensional (density-normalised) basis. The higher cost of the titanium alloy results solely from its palladium content, based on a nominal addition of 0.15 to 0.18 weight per cent (at a price taken in November 1995 of $144/troy ounce for palladium powder).
Leaner Palladium-Titanium Alloys
Over the past five years titanium alloy producers have critically re-evaluated the minimum palladium content required in the alloy. Following a closer examination of the original corrosion data established by Stern and Wissenberg in the development of the Ti-Pd alloy (2, 3), it was recognised that significant savings could be achieved by reducing the nominal palladium content. Stern’s profiles of corrosion rates in boiling hydrochloric acid, see Figure 1, clearly suggest that the beneficial effect due to palladium is optimised very quickly at low levels, so that only minimal improvements in corrosion occur for alloys containing above ~ 0.03 weight per cent palladium (2, 3). This behaviour was confirmed in more recent hydrochloric acid corrosion rate profiles devel¬oped by Kitayama, Shida and colleagues (4, 5), and by the author, as shown in Figure 2 (6). As expected, dramatic improvements in alloy crevice corrosion resistance in hot chloride and other halide-rich aqueous media are also achieved at these lower palladium levels, see Figure 3 (5, 6).
Based on these studies, several new lean-palladium alloys, which are described in Table II, have been incorporated into ASTM product specifications. These alloys are allowed to contain 0.04 to 0.08 weight per cent palladium, with the nominal amount being 0.05 per cent. The resulting reductions in cost of Ti-Pd alloy mill products are significant, and are shown in Table I.
For applications where higher strength alloys are required, similar additions of palladium can be made to α-β or β titanium alloys to produce the cost effective alloys outlined in Table II. The corrosion performance of these higher strength palladium-enhanced alloys is documented elsewhere (6, 7).
Lean Ruthenium-Titanium Alloys
The on-going pursuit of lower cost industrial titanium alloys at the RMI Titanium Company has led to the development of ruthenium-enhanced titanium alloys. From the standpoint of alloy formulation cost, ruthenium represents the lowest cost platinum group metal addition on a per weight basis. The ruthenium powder price, in November 1995, was approximately $30 per troy ounce, which is a factor of four to five times lower than that of palladium powder. However, profiles of the acid corrosion rates for the titanium-ruthenium, Ti-Ru, binary alloy and for other titanium alloys suggest that at least twice as much ruthenium by weight is required to impart corrosion resistance comparable to that of titanium-0.05 weight per cent palladium, Ti-0.05Pd, see Figure 2 (6). Despite the need to double the weight of the ruthenium addition, the titanium alloy containing the nominal 0.1 weight per cent ruthenium still achieves cost savings of approximately 17 per cent compared with the Ti-0.05Pd (titanium Grade 16) alloy and approximately 40 per cent compared with the classic Ti-0.15Pd (titanium Grade 7) alloy. Comparative alloy costs outlined in Table I for thin plate product suggest that ruthenium-enhanced titanium alloys offer substantial cost savings over the corresponding paUadium-contairiing alloys.
Mechanism of Ruthenium Enhancement
The basic mechanism of ruthenium addition to titanium is considered to be very similar to that of palladium and other platinum group metals, and results from alloy ennoblement. In a similar way to palladium, ruthenium exhibits minimal solubility Gess than 0.1 weight per cent) in the α-titanium phase, which results in a fine, uniform dispersion of noble Ti-Ru precipitates within the alloy (8).
When exposed to reducing acids, these precipitates, and/or ruthenium-enriched surfaces produced by selective dissolution, provide cathodic sites of low hydrogen overvoltage and accelerated hydrogen ion (H3O+) reduction (9, 10). This depolarisation of the hydrogen ion reduction reaction, or “cathode-modification” phenomenon, produces a substantial shift in the corrosion potential of the titanium alloy in acid towards the noble (positive) direction where the protective surface oxide film, TiO2, is stable (1), and full passivity can be achieved. This has been a highly effective and well-known technique for improving the corrosion performance of titanium alloy, due to the well established active-passive behaviour of titanium in reducing acids and its exceptionally high anodic pitting potential in acid solutions.
Ruthenium alloy additions also effectively inhibit titanium crevice corrosion in hot aqueous halide and sulphate environments. This enhanced crevice corrosion resistance results from the same “cathode modification” mechanism discussed above for reducing acids. With time, the solution within a tight metal crevice exposed to hot salt solutions often becomes a more aggressive deaerated reducing acid (1). This explains the dual beneficial effects from the ruthenium addition, both in reducing acid exposure and within crevices. Creviced surfaces are ennobled and local passivity is maintained within acidic crevices. The enhanced crevice resistance of Ti-Ru alloys is essentially equivalent to that of Ti-Pd alloys, as indicated by the guidelines in Figure 3.
Higher Strength Ruthenium-Enhanced Titanium Alloys
Greater strength in titanium is commonly achieved by the addition of alloying elements, such as aluminium and vanadium, to form α-β or ß-phase alloys. With the exception of molybdenum, most common alloying elements, and especially aluminium, diminish the reducing acid- and hot halide crevice-corrosion resistance of titanium alloys, with increasing content (11). The titanium-3 aluminium-2.5 vanadium, Ti-3Al-2.5V, (titanium Grade 9) and titanium-6 aluminium-4 vanadium, Ti-6A1-4V, (titanium Grade 5) alloys are two such common α-β alloys which exhibit attractive medium-to-high strength properties, see Table III, but in certain environments they possess corrosion resistance inferior to that of unalloyed titanium. In fact, the Grade 9 titanium alloy was recently incorporated into the ASME (American Society of Mechanical Engineers) Pressure Vessel Code for use at temperatures up to 315°C, and offers significantly higher design allowables compared with other titanium alloys listed in the Code. Unfortunately, this alloy is susceptible to crevice corrosion in chloride- or other halide-rich service environments at temperatures above ~ 80°C (depending upon pH, etc.), thus severely limiting application and design opportunities. The higher strength titanium Grade 5 alloy also exhibits susceptibility to stress corrosion in brine and aqueous halides which similarly limits its use at increased temperatures.
The deficiencies in the corrosion performances of these high strength titanium alloys can also be effectively reduced by nominal additions of 0.1 weight per cent ruthenium. Corrosion studies performed upon ruthenium-enhanced α-β titanium alloys reveal substantial improvements in their resistance to reducing acids, hot chloride crevice corrosion and stress corrosion cracking (6). Alloy corrosion rate profiles in boiling hydrochloric acid, presented in Figure 4, show the obvious benefit of ruthenium additions. The mechanism of corrosion resistance is again the same “cathode modification” (ennoblement) and oxide film stabilisation phenomenon as discussed previously for the binary Ti-Ru and Ti-Pd alloys.
The dramatic elevation of the threshold temperatures at which crevice corrosion starts in naturally-aerated acidic brines is indicated in Figure 5 for the ruthenium-enhanced α-β alloys. This enhancement has been confirmed via “worst-case” Teflon gasket-to-metal crevice tests in sweet and highly sour concentrated brines and in deaerated hypersaline Saltn Sea geothermal brines down to pH 2 (6). In more aggressive, severely-oxidising (chlorine saturated or FeCl3-rich) acidic brines, the crevice resistance of these higher strength alloys may be restricted to pH values above 3, when temperatures exceed ~ 80°C.
Although the Ti-3A1-2.5V alloy is not generally susceptible to stress corrosion cracking (SCC) in aqueous media, it is known that the Ti-6A1-4V alloy can exhibit halide SCC susceptibility, especially when the aluminum and/or interstitial levels increase (12). This serious limitation can be alleviated during exposure to hot aqueous halide (brine) by ruthenium addition to the ELI (Extra Low Interstitials with a 0.13 per cent oxygen maximum) Ti-6Al-4V alloy base. The SCC test results outlined in Table IV support the selection of these modified α-β titanium alloys for use in either sweet or sour sodium chloride-rich brines at temperatures as high as 330°C. These hot brine test environments are typical of those in Saltn Sea geothermal brine wells in California and in deep sour gas wells in the Gulf of Mexico.
Similar improvements in high temperature corrosion behaviour can be achieved in ß-tita-nium alloys by the addition of ruthenium. Corrosion studies conducted by the author on the Ti-38644 (titanium Grade 19) (Ti Beta-C™) alloy suggest that the mechanism is again “cathode-modification”. Of particular engineering value are the dramatic increases in the threshold temperatures for crevice corrosion and SCC offered by the ruthenium-enhanced Ti-38644 alloy in sweet and sour sodium chloride-rich brines (13), see Figure 6.
Status and Potential Applications for Ramenium-Containing Titanium Alloys
Since the minor addition of 0.1 weight per cent ruthenium to these titanium alloys has no significant influence on their mechanical and physical properties, the new ruthenium-containing alloys are specified with the same minimum tensile properties as the corresponding base alloys. Values for the minimum tensile properties required by ASTM product specifications are listed in Table III. The four new ruthenium-containing α and α-β alloys, with a permitted ruthenium content of 0.08 to 0.14 per cent, have been assigned the ASTM grade numbers indicated in Table II. They have recently been incorporated in appropriate ASTM specifications for sheet, strip and plate (B265), forg-ings (B381), bar and billet (B348), seamless and welded pipe (B337, B861 and B862), fittings (B363), tubing (B338) and wire (B863). ASTM Grades 26, 27 and 28 titanium alloys will soon be submitted for approval and eventual incorporation into the ASME Pressure Vessel Code. The ASME Code design allowables specified for these three alloys should mimic those for titanium Grades 7,11 and 9 alloys, respectively, already in the Code.
Some other possible applications for the titanium-0.1 ruthenium alloys (Grades 26 and 27) in the chemical and process industries are listed in Table V. These alloys offer cost effective, direct replacement of titanium Grade 7 and 11 alloys. The lower cost of these Ti-Ru alloys should also result in increased use of titanium in traditional Ni-Cr-Mo alloy applications which involve dilute acids and/or halide-rich process streams.
Candidate Applications for Titanium-0.1 Ruthenium Alloys
Current candidate applications for the higher strength ruthenium-enhanced titanium alloys are outlined in Table VI. Note that the titanium Grades 28 and 29 alloys are also currently in the final stage of approval for incorporation in the NACE (National Association of Corrosion Engineers) MR-01-75 Standard for use in sour service; allowing these new alloys to be selected for many deep oil/gas wells and offshore production components.
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