This article will comprehensively explain the corrosion resistance of niobium, tantalum and titanium.
Niobio
Niobio is a sister element to tantalum. Niobium is tolerrant to small amounts of FLUORIDE ION. strength at elevated temperatures, with resistance to corrosion and liquid metals.
As with other reactive metals, niobium owes its corrosion resistance to a passive, tenaciously adherent oxide film.
The corrosion properties of niobium are similar to tantalum, however, it is less resistant to more aggressive media.
Highlights
Unlike the other reactive metals, niobium will tolerate the presence of small amounts of fluoride ion. It is one of the few materials that are resistant to aqua regia.
Niobium and tantalum can become embrittled with hydrogen in alkaline service.
The oxidation rate of niobium in air increases with temperature and becomes quite rapid above 500°C. Therefore, when niobium is used at higher temperatures, coatings must be employed to minimize detrimental oxidation.
Applicazioni
Because of its strength at elevated temperatures feature it be used in missiles and aerospace and the chemical industry due to corrosion resistance.
Properties
- Atomic: No. 41
- Atomic: Wt. 92.9064
- Specific Gravity: 8.57
- Melting Point: 2477˚C
- Coefficient of Thermal Expansion: 6.9 X 10-6/°K
- Specific Heat: 0.063 cal/g/°K
- Ultimate Tensile Strength (Room Temperature, Annealed): 30 ksi
- Yield Tensile Strength (Room Temperature, Annealed): 15 ksi
- Poisons Ratio: 0.38
- Modulus of Elasticity: 15 X 106 psi
- Recrystalization Temperature: 800˚C–1200˚C
Niobium is less resistant to aggressive media than tantalum. However, it will tolerate aqua regia and small amounts of fluoride ion in acidic solutions. It is especially resistant under oxidizing conditions. Niobium is resistant to a series of metal melts such as Pb, Cd, Cs, Cu, Ga, Li, Mg. However, the material is not resistant to Al, Be, Ni, Zn and Co.
Niobium is resistant to many metals and chemicals. Niobium’s high level of resistance can be achieved at a low weight and it can be formed in a relatively low temperature. So it can be turned into colorful coin inserts, corrosion-resistant evaporation boats and other diamond growth crucibles.
Tantalio
Tantalio is one of the most corrosion resistant metal material, exhibits resistance to acid attack comparable to glass.
Tantalum is corrosion resistant, due to a tenaciously adherent oxide film akin to the entire family of passive, reactive metals. The metal has gained acceptance for use in electronics, missile technology, the chemical industries and the medical field. Tantalum is immune to attack by many acids and salt solutions. It is used to fabricate heat exchangers, reaction vessels, bayonet heaters, thermo wells, surgical implants and radiation shielding.
Svantaggi
- Tantalum easy to hydrogen embrittlement in alkaline solutions.
- Tantalum should not be used in air at temperatures above about 300°C because of severe oxidation.
Tantalum is one of the most corrosion resistant ductile metals exhibiting resistance to mineral acids except hydrofluoric acid, acid solutions containing fluoride ions, or free sulfur trioxide and is embrittled by alkaline materials. Tantalum is the same as its sister metal niobium and is resistant to many molten metals, including lithium and sodium-potassium.
Properties
- Atomic: No. 73
- Atomic: Wt. 180.9479
- Specific Gravity: 16.654
- Melting Point: 2996˚C
- Coefficient of Thermal Expansion: 6.5 X 10-6/°K
- Specific Heat: 0.033 cal/g/°K
- Ultimate Tensile Strength (Room Temperature, Annealed): 41 ksi
- Yield Tensile Strength (Room Temperature, Annealed): 25 ksi
- Poisons Ratio: 0.35
- Modulus of Elasticity: 27 X 106 psi
- Recrystallization Temperature: 900˚C–1200˚C
Tantalum’s resistance to corrosion by many materials is exemplified in the following Corrosion Resistance Table.
| SUBSTANCE | REACTION |
| Acetic Acid | 20-392° (68-738°F), all concentrations: No attack |
| Air or Oxygen | At room temperature: practically stableAbove 600°C (1112°F): formation of protective surfaces of Ta oxides |
| Aqueous Ammonia | Practically no attack |
| Aqua Regia | Cold and hot: practically no attack |
| Carbon (Graphite) | At high temperatures: carbide formation |
| Carbon Dioxide | Above 1200°C (2912°F): oxidation |
| Carbon Monoxide | At red heat: reaction (absorption of C and O)In high vacuum above 1400°C: formation of CO |
| Chromic Chloride Acid | 20-100°C (68-212°F), concentrated: no attack |
| Aqueous Caustic | Cold: practically stableHot: noticeable attack |
| Molten Caustic | Stable |
| Chlorine | at 250°C (464°F): beginning attackAbove 450°C (842°F): violent reaction |
| Ferric Chloride | 19°C (66°F) Boiling, 5-30% concentration: no attack |
| Hydrocarbons | Above 800-1000°C (1472-1832°F): carbide formationAbove 1400°C (2552°F): complete carburizing |
| Hydrochloric Acid | Cold and Hot: no attack |
| Hydrofluoric Acid | Strong Attack |
| Hydrofluoric and Nitric Acid | Rapid dissolution |
| Hydrogen | Above 300-400°C (572-752°F): formation of hydrideAbove 1000°C (1832°F): very slight solubility of hydrogenIn high vacuum above 600-700°C (1112-1292°F): evolution of hydrogen |
| Hydrogen Peroxide | Concentrated: good resistance to attack |
| Hydrogen Sulfide | At red heat: sulfide formation |
| Nitric Acid | Cold and Hot: no attack |
| Nitrogen | Up to 150°C (302°F): no attackAbove 800°C (1472°F): nitride formation |
| Oxalic Acid | 20-96°C (68-205°F), saturated: no attack |
| Phosphoric Acid | 85% concentration, 145-210°C (293-410°F): no attack |
| Potassium Hydroxide | 110°C (230°F), 5% concentration: no attack |
| Sodium Hydroxide | 100°C (230°F), 5% concentration: no attack100°C (230°F), 40% concentration: rapid attack |
| Steam | At red heat: rapid oxidation |
| Sulfur Dioxide | Up to 300°C (572°F): stable |
| Sulfuric Acid | Cold and hot: no attack |
| Molten Metals: | |
| Sodium | Up to 1200°C (2192°F): resistant |
| Magnesium | Up to 1150°C (2102°F): resistant |
| Lithium, Potassium, Lead | Up to 1000°C (1832°F): resistant |
| Bismuth | Up to 900°C (1652°F): resistant |
| Mercury | Up to 600°C (1112°F): resistant |
| Zinc | Up to 500°C (932°F): resistant |
| Gallium | Up to 450°C (842°F): resistant |
| Refractory Oxides: | |
| Alumina | Up to 1900°C (3452°F): stable |
| Beryllia | Up to 1900°C (2912°F): stable |
| Magnesia | Up to 1800°C (3272°F): stable |
| Zirconia | Up to 1600°C (2912°F): stable |
| Thoria | Up to 1900°C (3452°F): stable |
Titanio
General Corrosion
Titanium has excellent resistance to corrosion in a wide variety of environments including seawater, salt brines, inorganic salts, bleaches, wet chlorine, alkaline solutions, oxidizing acids, and organic acids. Titanium is incompatible with fluorides, strong reducing acids, very strong caustic solutions, and anhydrous chlorine.
Due to its combustibility, titanium is not suitable for pure oxygen service.
Titanium does not release any toxic ions into aqueous solutions, thus helping to prevent pollution.
Crevice Corrosion
Titanium has excellent resistance to crevice corrosion in salt solutions and generally outperforms stainless steels. Unalloyed CP titanium (grades 1, 2, 3, and 4) typically do not suffer crevice corrosion at temperatures below 80°C (175°F) at any pH.
Palladium alloyed CP titanium (grades 7, 11, 16 and 17) are more resistant and typically do not suffer crevice corrosion at temperatures below 250°C (480°F) at pH greater than 1.
Microbiologically Influenced Corrosion (MIC)
Titanium appears to be immune to MIC. They do suffer biofouling, but this can be controlled by chlorination (which does not
impair titanium).
Galvanic Corrosion
Although it is a reactive metal, due to the extreme stability of the passive film which forms on its surface, titanium typically exhibits noble behavior. Thus it functions as the cathode when coupled with other metals. Titanium is not affected by galvanic corrosion, but can accelerate corrosion of other metals.
Stress Corrosion Cracking
CP titanium has excellent resistance to stress corrosion cracking in hot chloride salt solutions.
Erosion Corrosion
CP Titanium exhibits excellent resistance to flow-induced and erosion corrosion at velocities to above 40 m/sec.
Hydrogen Embrittlement
Caratteristiche
Titanium is susceptible to hydrogen embrittlement under some circumstances. This is generally less of a problem for the
low- strength grade 1 and grade 2 titanium alloys than for higher strength titanium alloys. Absorption of hydrogen by
titanium normally occurs when the temperature is above 80°C (175°F), and the titanium is galvanically coupled to an active
metal or an impressed current or the pH is less than 3 or greater than 12.
The excellent corrosion resistance of titanium alloys results from the formation of very stable, continuous, highly adherent, and protective oxide films on metal surfaces. Because titanium metal is highly reactive and has an extremely high affinity for oxygen, these beneficial surface oxide films form spontaneously and instantly when fresh metal surfaces are exposed to air and/or moisture. In fact, a damaged oxide film can generally reheal itself instantaneously if at least traces of oxygen or water are present in the environment. However, anhydrous conditions in the absence of a source of oxygen may result in titanium corrosion, because the protective film may not be regenerated if damaged.
The nature, composition, and thickness of the protective surface oxides that form on titanium alloys depend on environmental conditions. In most aqueous environments, the oxide is typically TiO2, but may consist of mixtures of other titanium oxides, including TiO2, Ti2O3, and TiO. High-temperature oxidation tends to promote the formation of the chemically resistant, highly crystalline form of TiO, known as rutile, whereas lower temperatures often generate the more amorphous form of TiO, anatase, or a mixture of rutile and anatase.
Although these naturally formed films are typically less than 10 nm thick and are invisible to the eye, the TiO; oxide is highly chemically resistant and is attacked by very few substances, including hot, concentrated HCl, H2SO4, NaOH, and (most notably) HF. This thin surface oxide is also a highly effective barrier to hydrogen.
The general corrosion resistance of titanium can be improved or expanded by one or a combination of the following strategies:
- Alloying
- Inhibitor additions to the environment
- Precious metal surface treatments
- Thermal oxidation
- Anodic protection.
Alloying. Perhaps the most effective and preferred means of extending resistance to general corrosion in reducing environments has been by alloying titanium with certain elements. Beneficial alloying elements include precious metals (>0.05 wt% Pd), nickel ( >= 0.5 wt%), and/or molybdenum (>= 4 wt%).
These additions facilitate cathodic depolarization by providing sites of low hydrogen overvoltage, which shifts alloy potential in the noble direction where oxide film passivation is possible. Relatively small concentrations of certain precious metals (of the order of 0.1 wt%) are sufficient to expand significantly the corrosion resistance of titanium in reducing acid media.
These beneficial alloying additions have been incorporated into several commercially available titanium alloys, including the titanium-palladium alloys (grades 7 and 11), Ti-0.3Mo-0.8Ni (grade 12), Ti-3Al-8V-6Cr-4Zr-4Mo, Ti-15Mo-5Zr, and Ti-6Al-2Sn-4Zr-6Mo. These alloys all offer expanded application into hotter and/or stronger HCl, H2SO4, H3PO4, and other reducing acids as compared to unalloyed titanium.
The high-molybdenum alloys offer a unique combination of high strength, low density, and superior corrosion resistance.
Reference
CORROSION RESISTANCE OF NIOBIUM, TANTALUM AND TITANIUM IN SEA WATER AND SULFURIC ACID
Platinized Titanium Anodes, Combining Platinum and Titanium for Outstanding Corrosion Prevention