Titanium is number 22 on the periodic table. It has a melting point of 1668°C and a density of 4.54 gm/cc. Titanium properties and characteristics which are important to design engineers are excellent.  Titanium is immune to corrosive attack by salt water or marine atmospheres. It also exhibits exceptional resistance to a broad range of acids, alkalis, natural waters and industrial chemicals.

The combination of high strength and row density results in exceptionally favorable strength-to-weight ratios for titanium-based alloys. These ratios for titanium-based alloys are superior to almost all other metals and become important in such diverse applications as deepwell tubestrings in the petroleum industry and surgical implants in the medical field.  

Superior Strength-to-Weight Ratios The densities of titanium-based alloys range between .160 lb/in3 (4.43 gm/cm3) and .175 lb/in3 (4.85 gm/cm3). Yield strengths range from 25,000 psi (172 MPa) commercially pure (CP) Grade 1 to above 200,000 psi (1380 MPa) for heat treated beta alloys.

Titanium is immune to corrosive attack by salt water or marine atmospheres. It also exhibits exceptional resistance to a broad range of acids, alkalis, natural waters and industrial chemicals.  Titanium offers superior resistance to erosion, cavitation or impingement at-tack. Titanium is at least twenty times more erosion resistant than the copper-nickel alloys.

Titanium and titanium alloys have proven to be technically superior, highly reliable and cost-effective in a wide variety of chemical, industrial, marine and aerospace applications. Titanium is utilized in many critical services due to its unique set of properties.

Titanium can exist in two crystal forms.  The first is alpha which has a hexagonal close-packed crystal structure and the second is beta which has a body-centered cubic structure. In unalloyed titanium, the alpha phase is stable at all temperatures up to 1620°F. (880°C.) where it transforms to the beta phase. This temperature is known as the beta transus temperature. The beta phase is stable from 1620°F. (880°C.) to the melting point.

Titanium, discovered by William Gregor in 1791, titanium was first isolated and named after the powerful mythological first sons of the Earth - the Titans, Titanium is most commonly associated with jet engines and airframes, but the most recent media attention has been given to fittings for prosthetic devices and the artificial heart.

To produce titanium, the basic ore, usually rutile (Ti02) is converted to sponge in two distinct steps. First, Ti02, is mixed with coke or tar and charged in a chlorinator. Heat is applied and chlorine gas is passed through the charge. The titanium ore reacts with the chlorine to form TiCl4, titanium tetrachloride, and the oxygen is removed as C0 and C02. The resultant crude TiCl4, produced is a colorless liquid and is purified by continues fractional distillation. It is then reacted with either magnesium or sodium under an inert atmosphere. This results in metallic titanium sponge, and either magnesium or sodium chloride which is reprocessed and recycled.

Melting is the second step. Titanium is converted from sponge to ingot by first blending crushed sponge with the desired alloying elements to insure uniformity of composition, and then pressing into briquettes, which are welded together to form an electrode. The electrode is melted in a consumable electrode vacuum  arc furnace where an arc is struck between the electrode and a layer of titanium in a water-cooled copper crucible. The molten titanium on the outer surface solidifies on contact with the cold wall, forming a shell or skull to contain the molten pool. The ingot is not poured, but solidifies under vacuum in the melting furnace. To insure homogeneity of the final ingot, a second or sometimes a third melting operation is applied.

Titanium and its alloys have proven to be technically superior and cost-effective materials of constrLlction for a wide variety of aerospace, industrial, marine and commercial applications. In North America, approximately 70% of the titanium consumed is utilized for aerospace applications. Due to the expansion of existing applications and the development of new uses, the greatest growth will occur in the industrial, marine and commercial sectors.

In the Industrial & aerospace industry, titanium is currently being utilized in: Gas Turbine Engines, Heat Transfer, DSA-Dimensional Stable Anodes, Desalination, Extraction of Electrowinning of Metals, Medical, Hydrocarbon Pressing, Marine Applications, Chemical Processing, Steam Turbines, Automotive, Airframes, Space Structures, FGD (Flue Gas Desulfurization), Nuclear Waste Storage, etc. 

• Corrosion Resistance
  
  The protective passive oxide firm on titanium (mainly TiO2) is very stable over a wide range of pH, potential and temperature and is especially favored as the oxidizing character of the environment increases. For this reason, titanium generally resists mildly reducing, neutral and highly oxidizing environments up to reasonably high temperatures. It is only under highly reducing conditions where oxide film breakdown and resultant corrosion may occur.  Another major benefit to the designer is the fact that weldments, heat affected zones and castings of many of the industrial titanium alloys exhibit corrosion resistance equal to their base metal counterparts. This is attributable to the metallurgical stability of the leaner titanium alloys and the similar protective oxide which forms on titanium surfaces despite microstructural differences.

• Chlorine Gas
  
  Titanium is widely used to handle moist or wet chlorine gas, and has earned a reputation for outstanding performance in this service. The strongly oxidizing nature of moist chlorine passivates titanium resulting in low corrosion rates.  Proper titanium alloy selection offers a solution to the possibility of crevice corrosion when wet chlorine service temperatures exceed 155°F. (70°C.).  Chlorine Chemicals and Chlorine Solutions Titanium is fully resistant to solutions of chlorites, hypochlorites, chlorates, perchlorates and chlorine dioxide. It has been used to handle these chemicals in the pulp and paper industry for many years with no evidence of corrosion .
  

• Halogen Compounds
  
  Similar considerations generally apply to other halogens and halides compounds. Special concern should be given to acidic aqueous fluorides and gaseous fluorine environments which can be highly corrosive to titanium alloys.
  

• Resistance to Acids

  Oxidizing Acids In general, titanium has excellent resistance to oxidizing acids, such as nitric and chromic, over a wide range of temperatures and concentrations.  Nitric Acid Titanium is used extensively for handling nitric acid in commercial applications. Titanium exhibits low corrosion rates in nitric acid over a wide range of conditions. At boiling temperatures and above, titanium's corrosion resistance is very sensitive to nitric acid purity. Generally, the higher the contamination and the higher the metallic ion content of the acid, the better titanium will perform. This is in contrast to stainless steels which are often adversely affected by acid contaminants. Since titanium's own corrosion product (Ti+4) is highly inhibitive, titanium often exhibits superb performance in recycled nitric acid streams such as re-boiler loops.
  

• Passivation with Inhibitors
  
  Many industrial acid streams contain normal constituents or contaminants (i.e. up-stream corrosion products) which are oxidizing in nature, thereby passivating titanium alloys in normally aggressive acid media.  Metal ion concentration levels as low as 20-100 ppm can provide extremely effective inhibition.
  

There are three structural types of titanium alloys:

• Alpha alloys are non-heat treatable and are generally very weldable. They have low to medium strength, good notch tough ness, reasonably good ductility and possess excellent mechanical properties at cryogenic temperatures.  The more highly alloyed alpha and near-alpha alloys offer optimum high temperature creep strength and oxidation resistance as well.
  

• Alpha-Beta alloys are heat treatable and most are weldable. Their strength levels are medium to high. Their hot-forming qualities are good, but the high temperature creep strength is not as good as in most alpha alloys.
  

• Beta or near-beta alloys is readily heat treatable, generally weldable, and capable of high strengths and good creep resistance to intermediate temperatures.  Excellent formability can be expected of the beta alloys in the solution treated condition. Beta-type alloys have good combinations of properties in sheet, heavy sections, fasteners and spring applications.

As alloying elements are added to pure titanium, the elements tend to change the temperature at which the phase transformation occurs and the amount of each phase present. Alloy additions to titanium, except tin and zirconium, tend to stabilize either the alpha or the beta phase. Elements called alpha stabilizers stabilize the alpha phase to higher temperatures and beta stabilizers stabilize the beta phase to lower temperatures. 

Titanium alloys exhibit modulus of elasticity values which are approximately 5O% of steel. This low modulus means excellent flexibility which has been the basis for its use in dental fixtures (braces, etc.) and human prosthetic devices (hip joints, bone implants, etc.). Titanium's excellent compatibility provides an additional incentive for titanium's rapidly expanding use in body prosthetics. Other applications include springs, bellows, golf club shafts and tennis racquets. 

Titanium possesses a coefficient of expansion which is significantly less than ferrous alloys. This property also allows titanium to be much more compatible with ceramic or glass materials than most metals, particularly when metal-ceramic/glass seals are involved.

The environmental resistance of titanium depends primarily on a very thin, tenacious and highly protective surface oxide film. Titanium and its alloys develop very stable surface oxides with high integrity, tenacity and good adherence. The surface oxide of titanium will, if scratched or damaged, immediately reheal and restore itself in the presence of air or water.

The presence of common oxidizing background or contaminating species often maintain or extend the useful performance limits of titanium in many highly aggressive environments. These inhibitive species include air, oxygen, ferrous alloy corrosion products, other specific metallic ions, and/or other dissolved oxidizing compounds. Titanium's already wide range of application can be expanded by alloying with certain noble elements or by impressed anodic potentials (anodic protection).

Also titanium generally exhibits superior resistance to chlorides and various forms of localized corrosion.  Titanium alloys are considered to be essentially immune to chloride pitting and intergranular attack; and are highly resistant to crevice and stress corrosion.

Titanium is used in chloride salt solutions and other brines over the full concentration range, especially as temperatures increase. Near nil corrosion rates can be expected in brine media over the pH range of 3 to 11. Oxidizing metallic chlorides, such as FeCl3, NiCl2, or CuCl2, extend titanium's passivity to much lower pH levels.

A possible limiting factor of titanium alloy application in aqueous chlorides can be crevice corrosion in metal to metal joints, gasket to metal interfaces or under process stream deposits. Given these potential crevices in  hot chloride containing media, localized corrosion of unalloyed titanium and other alloys may occur depending on pH and temperature.

Once judged to be expensive, titanium, in life cycle costing, is now more often seen to be economical. The key to its cost-effective use is to utilize its unique properties and characteristics in the design rather than to substitute titanium for another metal.  Titanium is the world's fourth most abundant structural metal. It is found in North America, South America, Europe, Africa and Australia in the forms of ilmenite, rutile and other ores. The most widely used means of winning the metal from the ore is the Kroll process which uses magnesium as a reducing agent. Sodium is also used as a reducing agent by some producers.

The Properties and characteristics which are important to design engineers are:

• Excellent Corrosion Resistance
  
  Titanium is immune to corrosive attack by salt water or marine atmospheres. It also exhibits exceptional resistance to a broad range of acids, alkalis, natural waters and industrial chemicals.
  
• Superior Erosion Resistance
  
  Titanium offers superior resistance to erosion, cavitation or impingement at-tack. Titanium is at least twenty times more erosion resistant than the copper-nickel alloys.
  
• High Heat Transfer Efficiency
  
  Under "in service" conditions, the heat transfer properties of titanium approximate those of admiralty brass and copper-nickel. there are several reasons for this:
  (1)Titanium's higher strength permits the use of thinner walled equipment,
  (2) There appear to be unusual and beneficial characteristics in titanium's inherent oxide   film.
  (3) The relative absence of corrosion in media where titanium is generally used leaves the surface bright and smooth for improved lamellar flow.
  (4) Titanium's excellent erosion-corrosion resistance permits significantly higher operating velocities.
  
• Superior Strength-to-Weight Ratios
  
  The densities of titanium-based alloys range between .160 lb/in3 (4.43 gm/cm3) and .175 lb/in3 (4.85 gm/cm3). Yield strengths range from 25,000 psi (172 MPa) commercially pure (CP) Grade 1 to above 200,000 psi (1380 MPa) for heat treated beta alloys.

Titanium and its alloys possess many unique physical properties which make them ideal for equipment design, even when strength or corrosion resistance may not be critical. These unique properties include:

• Low Density

• Low Modulus of Elasticity

• Low Coefficient of Expansion

• High Melting Point

• Non-Magnetic

• Extremely Short Radioactive Half-Life

Titanium's low density, roughly 56% that of steel, means twice as much metal volume per pound. Particularly when combined with alloy strength, this often means smaller and/or lighter components. Although obviously the basis for aerospace applications, positive implications are also apparent for many types of rotating or reciprocating components such as centrifuges and pumps.

The relatively high melting point of titanium has led to consideration of titanium for ballistic armor. The higher melting point tends to reduce susceptibility to armor melting and ignition (burning) during ballistic impact. Good toughness and light weight are additional factors for considering titanium alloys in this application. 

Titanium is virtually non-magnetic, making it ideal for applications where electro-magnetic intencerence must be minimized. Desirable applications include electronic equipment housing and downhole well logging tools.

Titanium has an extremely short half-life, thereby permitting its use in nuclear systems. In contrast to many ferrous alloys, many titanium alloys do not contain a significant amount of alloying elements which may become radioactive.

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