Alloy Classifications There are three structural types of titanium alloys:

     Titanium is immune to corrosive attack by saltwater or marine atmospheres. It also exhibits exceptional resistance to a broad range of:

     Titanium alloys resist an extensive range of acidic conditions:

     In general, titanium has excellent resistance to oxidizing acids such as nitric and chromic, over a wide range of temperatures and concentrations.

     Titanium alloys are generally very resistant to mildly reducing acids, but can display severe limitations in strongly reducing acids. Mildly reducing acids such as sulphurous acid, acetic acid, terephthalic acid, adipic acid, lactic acid and many organic acids generally represent no problem for titanium over the full concentration range. However, relatively pure, strong reducing acids, such as hydrochloric, hydrobromic, sulphuric, phosphoric, oxalic and sulphamic acids can accelerate general corrosion of titanium depending on acid temperature, concentration and purity. Ti-Pd alloys offer dramatically improved corrosion resistance under these severe conditions. In fact, Ti-Pd alloys often compare quite favorably to nickel alloys in dilute reducing acids. Titanium is rapidly attacked by hydrofluoric acid of even very dilute concentrations. Therefore, titanium is not recommended for use with hydrofluoric acid solutions or in fluoride- containing solutions below pH7. Certain complexing metal ions, e.g. aluminum, may effectively inhibit corrosion in dilute fluoride solutions.

     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 often adversely affected by acid contaminant. Since titanium's own corrosion product (Ti++++) is highly inhibitive, titanium often exhibits superb performance in recycled nitric acid streams such as reboiler loops. One user cites an example of a titanium heat exchanger handling 60% HNO3 at 193°C (380°F) and 20 bar (300 psi) which showed no signs of corrosion after more than two years of operation. Titanium reactors, reboilers, condensers, heaters and thermowells have been used in solutions containing 10 to 70% HNO3 at temperatures from boiling to 315°F (600°C).

     Although titanium has excellent resistance to nitric acid over a wide range of concentrations and temperatures, it should not be used with red fuming nitric acid because of the danger of pyrophoric reactions. More than 1.34% water and less than 6% NO2 concentration (NO2/NO ratio) are guidelines for avoiding pyrophoric reactions.

     Titanium alloys generally exhibit excellent resistance to organic media. Mere traces of moisture and/or air normally present in organic process streams assure the development of a stable protective oxide film of titanium. Titanium is highly resistant to hydrocarbons, chloro- hydrocarbons, fluorocarbons, ketones, aldehydes, ethers, esters, amines, alcohols and most organic acids.  Titanium equipment has traditionally been used for production of terephthalic acid, adipic acid and acetaldehyde. Acetic acid, tartaric acid, stearic acid, lactic acid, tannic acids and many other organic acids represent fairly benign environments for titanium. However, proper titanium alloy selection is necessary for the stronger organic acids such as oxalic acid, formic acid, sulphamic acid and trichloroacetic acids. Performance in these acids depends on acid concentration, temperature, degree of aeration and possible inhibitors present. The Grade 7 and Grade 12 titanium alloys are often preferred materials in these more aggressive acids.

     Titanium is generally highly resistant to alkaline media including solutions of sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide and ammonium hydroxide. In the high basic sodium or potassium hydroxide solutions, however, useful application of titanium may be limited to temperatures below 80°C (176°F). This is due to possible excessive hydrogen uptake and eventual embrittlement of titanium alloys in hot, strongly alkaline media. Titanium often becomes the material of choice for alkaline media containing chlorides and/or oxidizing chloride species. Even at higher temperatures, titanium resists pitting, stress corrosion, or the conventional caustic embrittlement observed on many stainless steel alloys in these situations.

     Anhydrous methanol is unique in its ability to cause stress corrosion cracking of titanium and titanium alloys. Industrial methanol normally contains sufficient water to provide immunity to titanium and for there to be no problem in practical applications.

     In the past the specification of a minimum of 2% has proved adequate to protect commercially pure titanium equipment for all but the most severe conditions. In such conditions, due to temperature and pressure titanium alloys would more than likely be required.

     In order to ensure effective cover for all conditions now being encountered by titanium alloys used in the offshore industry, a revised limit of 5% minimum water content of methanol is recommended.

     Work is in hand to confirm the actual level of water required to provide immunity to stress corrosion cracking in all conditions. Test conducted to date confirm required levels above 2%, but safely below 5% are required. Until this work is deemed to be satisfactorily complete TIG recommends that the 5% limit be used.

     Titanium alloys are highly resistant to wet (aqueous) chlorine, bromine, iodine and other chlorine chemicals because of their strongly oxidizing natures. Titanium's outstanding resistance to aqueous chlorides has been the primary historical incentive for utilizing titanium in industrial service. In many chloride and bromide-containing environments, titanium has cost-effectively replaced stainless steels, copper alloys and other metals which have experienced severe localized corrosion and stress corrosion cracking.

     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 surface temperatures exceed 70°C (158°F). Dry chlorine can cause rapid attack of titanium and may even cause ignition if moisture content is sufficiently low. However, as little as one percent of water is generally sufficient for passivation or repassivation after mechanical damage to titanium in chlorine gas under static conditions at room temperature.

     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.

     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.

     Localized pitting or corrosion, occurring in tight crevices and under scale or other deposits is a controlling factor in the application of unalloyed titanium. Attack will normally not occur on commercially pure titanium or industrial alloys below 70°C(158°F) regardless of solution pH. Seawater and neutral brines above the boiling point will develop localized reducing acidic conditions, and pitting may occur. Enhanced resistance to reducing acid chlorides and crevice corrosion is available from alloy Grades 7, 11 and 12. Attention to design of flanged joints using heavy flanges and high clamping pressure, and to the specification of gaskets (choosing elastic rather than plastic or hard materials) may serve to prevent crevices developing. An alternative strategy is to incorporate a source of nickel, copper, molybdenum or palladium into the gasket.

     Due to recycling of waste fluids and the need for greater equipment reliability and life span, titanium has become the standard material for drum washers, diffusion bleach washers, pumps, piping systems and heat exchangers in the bleaching section of pulp and paper plants. This is particularly true for the equipment developed for chlorine dioxide bleaching systems.


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

     Titanium alloys exhibit excellent resistance to practically all salt solutions over a wide range of pH and temperatures. Good performance can be expected in sulphates, sulphites, borates, phosphates, cyanides, carbonates and bicarbonates. Similar results can be expected with oxidizing anionic salts such as nitrates, molybdates, chromates, permanganates and vanadates; and also with oxidizing cationic salts including ferric, cupric, and nickel compounds.

     Titanium alloys exhibit excellent resistance to practically all salt solutions over a wide range of pH and temperatures. Good performance can be expected in sulphates, sulphites, borates, phosphates, cyanides, carbonates and bicarbonates. Similar results can be expected with oxidizing anionic salts such as nitrates, molybdates, chromates, permanganates and vanadates; and also with oxidizing cationic salts including ferric, cupric, and nickel compounds.

     Titanium alloys are used extensively for applications which entail exposure to fresh and salt water.

     Titanium alloys are highly resistant to water, natural waters and steam to temperatures in excess of 570°F (300°C). Excellent performance can be expected in high purity water, fresh water Titanium is immune to microbiologically influenced corrosion (MIC). Typical contaminants found in natural water streams, such as iron and manganese oxides, sulphides, sulphates, carbonates and chlorides do not compromise titanium's performance. Titanium remains totally unaffected by chlorination treatments used to control biofouling.

     Titanium is fully resistant to natural seawater regardless of chemistry variations and pollution effects (i.e. sulphides). Twenty year corrosion rates well below 0.0003 mm/yr (0.01 mils/yr) have been measured on titanium exposed beneath sea, in marine atmospheres, and in splash or tidal zones. In the sea, titanium alloys are immune to all forms of localized corrosion, and withstand seawater impingement and flow velocities in excess of 30 m/sec (100 ft/sec). Abrasion and cavitation resistance is outstanding, explaining why titanium provides total reliability in many marine and naval applications. In addition, the fatigue strength and toughness of most titanium alloys are unaffected in seawater and many titanium alloys are immune to seawater stress corrosion.

     Titanium tubing has been used with great success for more than forty years in seawater-cooled heat exchangers in the chemical, oil refineries and desalination industries. The pH-temperature guidelines for crevice-corrosion are generally applicable to seawater services as well as NaCl brines.

     When in contact with other metals, titanium alloys are not subject to galvanic corrosion in seawater. However titanium may accelerate attack on active metals such as steel, aluminum and copper alloys. The extent of galvanic corrosion will depend on many factors such as anode to cathode ratio, seawater velocity and seawater chemistry. The most successful strategies eliminate this galvanic couple by using more resistant compatible passive metals with titanium, all-titanium construction, or dielectric (insulating) joints. Other approaches for mitigating galvanic corrosion have also been effective: coatings, linings and cathodic protection.

CORROSIVE GASES

    
Titanium alloys perform well in many aggressive environments:

     Titanium alloys are totally resistant to all forms of atmospheric corrosion regardless of pollutants present in either marine, rural or industrial locations. Titanium has excellent resistance to gaseous oxygen and air at temperatures up to 370°C (700°F). Above this temperature and below 450°C (840°F), titanium forms colored surface oxide films which thicken slowly with time. Above 650°C (1200°F) or so , titanium alloys suffer from lack of long-term oxidation resistance and will become brittle due to the increased diffusion of oxygen in the metal. In oxygen, the combustion is not spontaneous and occurs with oxygen concentration above 35% at pressures over 25 bar (350 psig) when a fresh surface is created.

     Nitrogen reacts much more slowly with titanium than oxygen. However above 800°C (1400°F), excessive diffusion of the nitride may cause metal embrittlement. Titanium is not corroded by liquid anhydrous ammonia at ambient temperatures. Moist or dry ammonia gas, or ammonia water(NH4OH) solutions will not corrode titanium to their boiling-point and above.

     The surface oxide film on titanium acts as a highly effective barrier to hydrogen. Penetration can only occur when this protective film is disrupted mechanically or broken down chemically or ectro-chemically. The presence of moisture effectively maintains the oxide film inhibiting hydrogen absorption up to fairly high temperatures and pressures. On the other hand, pure, anhydrous hydrogen exposures should be avoided particularly as pressures and/or temperatures increase.

     The few cases of hydrogen embrittlement of titanium observed in industrial service have generally been limited to situations involving high temperatures, high alkaline media; titanium coupled to active steel in hot aqueous sulphide streams; and where titanium has experienced severe prolonged cathodic charging in seawater.

     Penetration Diffusion of hydrogen into titanium is very slow at temperatures below 80°C (176°F) except where high residual or applied tensile stresses exist. If the solubility limit of hydrogen in titanium is then exceeded, (100-150 ppm for commercially pure Grade 2), titanium hydride will begin to precipitate. At temperatures not exceeding 80°C (176°F), hydride will normally be restricted to the surface layers of the metal and experience in such cases indicates that this has little or no serious effect on the performance or properties of the metal. Cases of through section hydride formation, leading to embrittlement and cracking or failure under stress are very rare. Hydriding can be avoided by the proper design of equipment and control of operating conditions.

     Titanium is highly corrosion resistant to sulphur-bearing gases, resisting sulphide stress corrosion cracking and sulphidation at typical operating temperatures. Sulphur dioxide and hydrogen sulphide, either wet or dry, have no effect on titanium. Extremely good performance can be expected in sulphurous acid even at the boiling point. Field exposures in FGD scrubber systems of coal-fired power plants have similarly indicated outstanding performance of titanium. Wet SO3 environments may be a problem for titanium in cases where pure, strong, uninhibited sulphuric acid solutions may form, leading to metal attack. In these situations, the background chemistry of the process environment is critical for successful use of titanium.

     Titanium generally resists mildly reducing, neutral and highly oxidizing environments up to reasonably high temperatures. The presence of oxidizing species including air, oxygen and ferrous alloy corrosion products, often extend the performance limits of titanium in many highly aggressive environments. However, under highly reducing conditions the oxide film may breakdown and corrosion may occur.

     Many industrial acid streams contain contaminants 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 inhibit corrosion extremely effectively. Potent inhibitors for titanium in reducing acid media are common: dissolved oxygen, chlorine, bromine, nitrate, chromate, permanganate, molybdate and cationic metallic ions, such as ferric (Fe+3), cupric (Cu+2), nickel (Ni+2) and many precious metal ions.

     It is this potent metal ion inhibition which permits titanium to be successfully utilized for equipment handling hot HCl and H2SO4 acid solutions in metallic ore leaching processes.

     Fluorides are frequently present in a variety of chemical plant and industrial processes. The resistance of titanium to many acidic fluoride-bearing environments can be explained by the abundant presence of metal ions, particularly aluminum and iron, in condensates, liquors and sludges. These ions chemically complex the active fluorides and thus render them inert to the titanium. Frequently, fluoride-metal complexes are spontaneously formed early in the process cycle. Aluminum, in particular, is effective in complexing fluorides - even at very low pH.

     Although inhibition is possible in most reducing acids, including those containing fluorides, protection of titanium from solutions of hydrofluoric acid itself is difficult to achieve. Titanium cannot be recommended for plants where conditions permit active, uncomplexed fluorides to persist.

     Titanium develops a thin, tenacious and highly protective surface oxide film. The surface oxide of titanium will, if scratched or damaged, immediately reheal and restore itself in the presence of air or even very small amounts water. The corrosion resistance of titanium depends on a protective TiO2 surface oxide film.

     This substantially inert surface oxide has high integrity and tenacity. The oxide will, if scratched or damaged, immediately restore itself in the presence of air or water. The film is stable over a wide range of pH, electro-potentials and temperature, particularly in neutral and oxidizing environments.

     Titanium alloys are metallurgically stable and the protective oxide forms equally on all titanium surfaces, on wrought products, welds and castings irrespective of composition or micro-structural differences.

     Because of the nature of its oxide film, titanium has superior resistance to erosion, cavitation and impingement attack. Titanium is over twenty times more erosion resistant than the copper-nickel alloys.   Erosion of Unalloyed Titanium in Seawater Containing Suspended Solids.

     Under 'in service' conditions, the heat transfer properties of titanium are similar to those of admiralty brass and copper-nickel. There are several reasons for this :

The higher strength of titanium permits the use of thinner walled equipment.

The oxide film confers unusual characteristics which are beneficial to heat transfer.

The absence of corrosion leaves the surface bright and smooth for improved lamellar flow.

Titanium's excellent erosion-corrosion resistance permits significantly higher operating velocities.

     The combination of high strength and low density results in exceptionally favorable strength-to-weight ratios for titanium-based alloys.

     The densities of titanium-based alloys range between 4.43 gm/cm³ (0.160lb/in³) and 4.85 gm/cm³ (0.175lb/in³). Yield strengths range from 172 MPa (25,000 psi) for commercially pure Grade 1, to above 1380 MPa (200,000 psi) for heat treated beta alloys. These ratios for titanium- based alloys are superior to almost all other metals and are important in such diverse applications as deep well tubestrings in the petroleum industry and surgical implants in the medical field.

     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.

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

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

     Even at very high temperatures titanium is fire resistant. This is important for applications such as petrochemical plant and firewater systems for offshore platforms, where its ability to survive a hydrocarbon fire is an essential factor.

Fire test

     Increasing pressure from insurance underwriters to reduce risk and increase the capabilities of fire protection methods and equipment has brought titanium to the fore in what has been a major re-evaluation of the particular problems posed by offshore platform fire protection. A description of the test in which uninsulated thin wall welded titanium pipes passed the NPD H-class hydrocarbon fire test is given in the article Titanium Fights Fire by David Peacock and Jarl Skauge. Reference is also made to the high level of shock resistance and damage tolerance of titanium which provide the maximum possibility for survival in the event of explosion, fire or other disaster. Fire system manufacturers can offer all titanium sprinkler and deluge systems detectors, nozzles, valves and pipework installed on Frøy/TCP (Elf Petroleum); Sleipner West (Statoil); and Troll B and Brage (Norsk Hydro).

     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.