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» Corrosion Resistance of Titanium
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.
Titanium's already wide range of applications can be expanded by alloying with certain noble elements or by impressed anodic potentials (anodic protection).
Titanium is immune to corrosive attack by saltwater or marine atmospheres. It also exhibits exceptional resistance to a broad range of:
- Natural waters
- Corrosive gases
- Reducing atmospheres
- Passivation with inhibitors
- Organic Media
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
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 metal lyrical stability of the leaner titanium alloys and the similar protective oxide which forms on titanium surfaces despite microstructural differences.
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 service temperatures exceed 155°F. (70°C.).
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 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 FeCI3, NiCI2, or CuCI2, 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.
Service and laboratory-based guide lines, shown in Figure 1, have been developed to aid in proper alloy selection. These guidelines apply to most types of chloride solutions over a wide range of salt concentrations. As indicated in Figure 1, the Grade l2 and 7 titanium alloys offer improved crevice corrosion resistance when solution pH increases or temperature increases.
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.
Titanium alloys exhibit excellent resistance to practically all salt solutions over a wide range of pH and temperatures. Good performance can be expected in sulfates, sulfites, 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 nickelous compounds.
» Fresh Water/Steam
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 and body fluids. Typical contaminants found in natural water streams, such as iron and manganese oxides, sulfides, sulfates, 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. sulfides). Twenty year corrosion rates well below .01 mpy have been measured on titanium exposed beneath the 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 100 ft/sec (0.0003 mm/y). (See Table 1). Abrasion and cavitation resistance of these alloys is outstanding, explaining why titanium provides total reliability in many marine and naval applications. In addition, the fatigue strength and toughness of many titanium alloys are unaffected in seawater and lean titanium alloys are immune to seawater stress corrosion.
Titanium tubing has been used with great success for more than 20 years in seawater-cooled heat exchangers in the chemical, oil refining and desalination industries. The pH-temperature guidelines for crevice corrosion presented in Figure 1 are generally applicable to seawater service as well.
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, or 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.
» 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 (Table 2). 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 reboiler loops.
One user cites an example of a titanium heat exchanger handling 60% HNO3 at 380°F. (193°C.) and 300 psi (2.1 MPa) 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 600°F. (315°C.).
» Red Fuming Nitric Acid
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. Minimum water content and maximum NO2 concentration (NO2/NO ratio) guidelines for avoiding pyrophoric reactions in this particular acid have been developed.
» Reducing Acids
Titanium alloys are generally very resistant to mildly reducing acids, but can display severe limitations in strongly reducing acids. Mildly reducing acids such as sulfurous acid, acetic acid, terepthalic 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, sulfuric, phosphoric, oxalic and sulfamic acids can accelerate general corrosion of titanium depending on acid temperature, concentration and purity. TheTi-Pd alloy offers dramatically improved corrosion resistance under these severe conditions. In fact, Ti-Pd often compares quite favorably to nickel alloys in dilute reducing acids as shown in Table 3.
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.
Common potent inhibitors for titanium in reducing acid media include dissolved oxygen, chlorine, bromine, nitrate, chromate, permanganate, molybdate and cationic metallic ions, such as ferric (Fe+3), cupric (Cu+2), nickelous (Ni+2) and many precious metal ions. Figure 2 shows how the useful corrosion resistance of unalloyed titanium is significantly extended as the ferric ion concentration is increased in very small amounts. It is this potent metal ion inhibition phenomenon which permits titanium to be successfully utilized for equipment handling hot HCI and H2SO4 acid solutions in metallic ore leaching processes.
Although inhibition is possible in most reducing acids, protection of titanium from hydrofluoric acid solutions is extremely difficult to achieve. Hydrofluoric acid will generally cause rapid general corrosion of all titanium alloys, and should, therefore, be avoided.
Since the presence of minute quantities of these common inhibitive species can radically influence titanium's performance in reducing acids, one must consider all details of environment chemistry in the alloy selection process. Back ground process stream species are frequently beneficial for titanium. In addition, intentional addition of inhibitive species to the process stream can be a practical approach for extending titanium's corrosion resistance under marginal conditions.
Titanium is generally highly resistant to alkaline media including solutions of sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide and ammonia hydroxide. In the highly basic sodium or potassium hydroxide solutions, however, useful application of titanium may be limited to temperatures below 176°. (80°C.). 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.
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 on titanium.
Titanium is highly resistant to hydrocarbons, chloro-hydrocarbons, fluorocarbons, ketones, aldehydes, ethers, esters, amines, alcohols and most organic acids. Anhydrous methanol is unique in its ability to cause stress corrosion cracking of titanium alloys. However, addition of more than 1.5% water is sufficient to eliminate this problem.
Titanium equipment has traditionally been used for production of terepthalic 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, sulfamic acid and trichloroacetic acids. Performance in these acids depends on acid concentration, temperature, degree of aeration and possible inhibitors present. The grade 7 and 12 titanium alloys are often preferred materials in these aggressive acids.
Oxygen and Air Titanium has excellent resistance to gaseous oxygen and air at temperatures up to 700°F. (370°C.). Above this temperature and below 840°F. (450°C.), titanium may form colored surface oxide films which may thicken slowly with time. Above 1000°F. (545°C.) or so, titanium alloys lack long-term oxidation resistance and will become brittle due to the increased diffusion of oxygen into the metal.
Titanium alloys are totally resistant to all forms of atmospheric corrosion regardless of pollutants present in either marine, rural or industrial locations.
Nitrogen reacts much more slowly with titanium than oxygen. However, above 1400°F. (800°C.), 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 (NH40H) 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 which can only occur when this protective film is disrupted mechanically or broken-down chemically or electro-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.
Excessive absorption of hydrogen in titanium alloys leads to embrittlement if a significant quantity of the brittle titanium hydride phase precipitates in the metal. Generally, hydrogen contents of at least several hundred ppm are required to observe significant embrittlement.
The few cases of hydrogen embrittlement of titanium observed in industrial service have generally been limited to situations involving high temperature, highly alkaline media; titanium coupled to active steel in hot aqueous sulfide streams; and where titanium has experienced severe very prolonged cathodic charging in seawater.
Titanium is highly corrosion resistant to sulfur-bearing gases, resisting sulfide stress corrosion cracking and sulfidation at typical operating temperatures. Sulfur dioxide and hydrogen sulfide, either wet or dry, have no effect on titanium as shown in Table 4. Extremely good performance can be expected in sulfurous acid even at the boiling point. Field exposures in FGD scrubber systems of coal-fired power plants have similarly indicated outstanding performance of titanium (see Table 5).
Wet S03 environments may be a problem for titanium in cases where pure strong, uninhibited sulfuric 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.
Most of the higher strength titanium alloys will exhibit excellent resistance to general corrosion and pitting corrosion in near-neutral environments which are neither highly oxidizing nor highly reducing. The metallurgical condition of the alloy plays a relatively minor role in corrosion performance, since oxide film stability is assured in these situations. When severe crevices exist in hot aqueous chloride media, most high strength titanium alloys will exhibit slightly reduced crevice corrosion resistance compared to unalloyed titanium. The titanium alloys rich in molybdenum, however, exhibit excellent resistance to this form of attack. It is for this reason, along with its favorable strength and low density, the high molybdenum 3AI-8V 6Cr-4Zr-4Mo alloy is a prime candidate for high temperature sour oil well and geothermal brine well production tubulars and other downhole components.
General corrosion resistance of high strength titanium alloys in strongly oxidizing or strongly reducing environments may diminish as aluminum and/or vanadium alloy content increases. Improvements in resistance to hot reducing acids can be achieved by increased alloy molybdenum content. The effects of various alloying elements on titanium alloy corrosion have been studied which indicate that suitable high strength alloys can be selected for a wide range of aggressive environments.
Another major consideration is resistance to stress corrosion cracking (SCC). Although the common industrial grades of titanium are generally immune to chloride SCC, certain high strength alloys may exhibit reduced toughness (Klc) values and/or accelerated crack growth rates in halide environments. Most of these alloys will not exhibit any susceptibilities to SCC in smooth or notched conditions. However, above 480°F. (250°C.), resistance to hot salt SCC should be considered if chlorinated solvents, chloride salts, or other chlorine-containing compounds contact component surfaces.
High strength titanium alloys can successfully avoid SCC-related effects by consideration of alloy chemistry and/or preferred alloy heat treatments and microstructures. Selection of extra low interstitial (ELI) grades or alloys with transformed beta microstructures may offer significantly improved alloy toughness and resistance to SCC.
In summary, although high strength titanium alloys possess corrosion resistance which is generally superior to that of most common engineering alloys, consideration should be given to selecting a titanium alloy with full compatibility to a given environment. With the wide family of titanium alloys commercially available today, optimum high strength titanium alloy selection is almost always possible for a given environment. Technical consultation is available to assist the designer/user in achieving this end.
Titanium's oxide film generally provides super resistance to abrasion, erosion, or erosion-corrosion in high velocity process streams. Some comparative alloy data is presented in Table 1, revealing titanium's superior performance to copper and aluminum alloys in flowing seawater. Even in heavy sand-laden seawater, insignificant metal loss was noted up to 18 ft/sec (5/5 m/sec).
Therefore, in contrast to copper alloys, titanium piping, equipment and heat exchangers can be designed for high flow velocities with little or no detrimental effects from turbulence, impingement or cavitation. This has positive implications relative to optimizing heat transfer efficiency, minimizing equipment wall thicknesses, minimizing tube and piping size and wall requirements, improving equipment reliability and reducing life cycle costs. It is for these reasons that titanium alloys, in either wrought or cast form, have become prime materials for various coastal chemical and power plants and marine/naval applications.
The protective passive oxide film 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.
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 oxidizing, neutral and mildly reducing 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.