Titanium and its alloys exhibit a unique combination of mechanical and physical properties and corrosion resistance which have made them desirable for critical, demanding aerospace, industrial, chemical and energy industry service. Of the primary attributes of these alloys listed in Table 1, titanium's elevated strength-to-density represents the traditional primary incentive for selection and design into aerospace engines and airframe structures and components. Its exceptional corrosion/erosion resistance provides the prime motivation for chemical process, marine and industrial use. Figure 1 reveals the superior structural efficiency of high strength titanium alloys compared to structural steels and aluminum alloys, especially as service temperatures increase. Titanium alloys also offer attractive elevated temperature properties for application in hot gas turbine and auto engine components, where more creep-resistant alloys can be selected for temperatures as high as 600°C (1100°F) [see Figure 2].

The family of titanium alloys offers a wide spectrum of strength and combinations of strength and fracture toughness as shown in Figure 3. This permits optimized alloy selection which can be tailored for a critical component based on whether it is controlled by strength and S-N fatigue, or toughness and crack growth (i.e., critical flaw size) in service. Titanium alloys also exhibit excellent S-N fatigue strength and life in air, which remains relatively unaffected by seawater (Figure 4) and other environments. Most titanium alloys can be processed to provide high fracture toughness with minimal environmental degradation (i.e., good SCC resistance) if required. In fact, the lower strength titanium alloys are generally resistant to stress corrosion cracking and corrosion-fatigue in aqueous chloride media.

For pressure-critical components and vessels for industrial applications, titanium alloys are qualified under numerous design codes and offer attractive design allowables up to 315°C (600°F) as shown in Figure 5. Some common pressure design codes include the ASME Boiler and Pressure Vessel Code (Sections I, III, and VIII), the ANSI (ASME) B31.3 Pressure Code, the BS-5500, CODAP, Stoomwezen and Merkblatt European Codes, and the Australian AS 1210 and Japanese JIS codes.

Figure 1

Figure 2

Figure 3

Figure 4

Corrosion and Erosion Resistance

Titanium alloys exhibit exceptional resistance to a vast range of chemical environments and conditions provided by a thin, invisible but extremely protective surface oxide film. This film, which is primarily TiO₂, is highly tenacious, adherent, and chemically stable, and can spontaneously and instantaneously reheal itself if mechanically damaged if the least traces of oxygen or water (moisture) are present in the environment. This metal protection extends from mildly reducing to severely oxidizing, and from highly acidic to moderately alkaline environmental conditions; even at high temperatures. Titanium is especially known for its elevated resistance to localized attack and stress corrosion in aqueous chlorides (e.g., brines, seawater) and other halides and wet halogens (e.g., wet Cl₂ or Cl₂-sat. brines), and to hot, highly-oxidizing, acidic solutions (e.g., FeCl₃ and nitric acid solutions) where most steels, stainless steels and copper- and nickel-based alloys can experience severe attack. Titanium alloys are also recognized for their superior resistance to erosion, erosion-corrosion, cavitation, and impingement in flowing, turbulent fluids. This exceptional wrought metal corrosion and erosion resistance can be expected in corresponding weldments, heat-affected zones and castings for most titanium alloys, since the same protective oxide surface film is formed.

The useful resistance of titanium alloys is limited in strong, highly-reducing acid media, such as moderately or highly concentrated solutions of HCl, HBr, H₂SO₄, and H₃PO₄, and in HF solutions at all concentrations, particularly as temperature increases. However, the presence of common background or contaminating oxidizing species (e.g., air, oxygen, ferrous alloy metallic corrosion products and other metallic ions and/or oxidizing compounds), even in concentrations as low as 20-100 ppm, can often maintain or dramatically extend the useful performance limits of titanium in dilute-to-moderate strength reducing acid media.

Where enhanced resistance to dilute reducing acids and/or crevice corrosion in hot (75°C) chloride/halide solutions is required, titanium alloys containing minor levels of palladium (Pd), ruthenium (Ru), nickel (Ni), and/or higher molybdenum (>3.5 wt.% Mo) should be considered. Some examples of these more corrosion-resistant titanium alloys include ASTM Grades 7, 11, 12, 16, 17, 18, 19, 20, 26, 27, 28, and 29. These minor alloy additions also inhibit susceptibility to stress corrosion cracking in high strength titanium alloys exposed to hot, sweet or sour brines.

Therefore, titanium alloys generally offer useful resistance to significantly larger ranges of chemical environments (i.e., pH and redox potential) and temperatures compared to steels, stainless steels and aluminum-, copper- and nickel-based alloys. Table 3 (see page 5) provides an overview of a myriad of chemical environments where titanium alloys have been successfully utilized in the chemical process and energy industries. More detailed corrosion data and application guidelines for utilizing and testing titanium alloys in these and other environments can be found in the reference section in the back of this booklet.

Figure 5

Other Attractive Properties

Titanium's relatively low density, which is 56% that of steel and half that of nickel and copper alloys, means twice as much metal volume per weight and much more attractive mill product costs when viewed against other metals on a dimensional basis. Together with higher strength, this obviously translates into much lighter and/or smaller components for both static and dynamic structures (aerospace engines and airframes, transportable military equipment), and lower stresses for lighter rotating and reciprocating components (e.g., centrifuges, shafts, impellers, agitators, moving engine parts, fans). Reduced component weight and hang-off loads achieved with Ti alloys are also key for hydrocarbon production tubular strings and dynamic offshore risers and Navy ship and submersible structures/components.

Titanium alloys exhibit a low modulus of elasticity which is roughly half that of steels and nickel alloys. This increased elasticity (flexibility) means reduced bending and cyclic stresses in deflection-controlled applications, making it ideal for springs, bellows, body implants, dental fixtures, dynamic offshore risers, drill pipe and various sports equipment. Titanium's inherent nonreactivity (nontoxic, nonallergenic and fully biocompatible) with the body and tissue has driven its wide use in body implants, prosthetic devices and jewelry, and in food processing. Stemming from the unique combination of high strength, low modulus and low density, titanium alloys are intrinsically more resistant to shock and explosion damage (e.g., military applications) than most other engineering materials. These alloys possess coefficients of thermal expansion which are significantly less than those of aluminum, ferrous, nickel and copper alloys. This low expansivity allows for improved interface compatibility with ceramic and glass materials and minimizes warpage and fatigue effects during thermal cycling.

Titanium is essentially nonmagnetic (very slightly paramagnetic) and is ideal where electromagnetic interference must be minimized (e.g., electronic equipment housings, well logging tools). When irradiated, titanium and its isotopes exhibit extremely short radioactive half-lives, and will not remain "hot" for more than several hours. Its rather high melting point is responsible for its good resistance to ignition and burning in air, while its inherent ballistic resistance reduces susceptibility to melting and burning during ballistic impact, making it a choice lightweight armor material for military equipment. Alpha and alpha-beta titanium alloys possess very low ductile-to-brittle transition temperatures and have, therefore, been attractive materials for cryogenic vessels and components.

Heat Transfer Characteristics

Titanium has been a very attractive and well-established heat transfer material in shell/tube, plate/frame, and other types of heat exchangers for process fluid heating or cooling, especially in seawater coolers. Exchanger heat transfer efficiency can be optimized because of the following beneficial attributes of titanium:

• Exceptional resistance to corrosion and fluid erosion
• An extremely thin, conductive oxide surface film
• A hard, smooth, difficult-to-adhere-to surface
• A surface that promotes condensation
• Reasonably good thermal conductivity
• Good strength

Although unalloyed titanium possesses an inherent thermal conductivity below that of copper or aluminum, its conductivity is still approximately 10-20% higher than typical stainless steel alloys. With its good strength and ability to fully withstand corrosion and erosion from flowing, turbulent fluids (i.e., zero corrosion allowance), titanium walls can be thinned down dramatically to minimize heat transfer resistance (and cost). Titanium's smooth, noncorroding, hard-to-adhere to surfaces maintains high cleanliness factors over time. This surface promotes drop-wise condensation from aqueous vapors, thereby enhancing condensation rates in cooler/condensers compared to other metals as indicated in Figure 6. The ability to design and operate with high process or cooling water side flow rates and/or turbulence further enhances overall heat transfer efficiency.

All of these attributes permit titanium heat exchanger size, material requirements and overall initial life cycle costs to be reduced, making titanium heat exchangers more efficient and cost-effective than those designed with other common engineering alloys.