Titanium - Introduction to Corrosion
Since titanium metal first became a commercial reality in 1950, corrosion resistance has been an important consideration in its selection as an engineering structural material. Titanium has gained acceptance in many media where its corrosion resistance and engineering properties have provided the corrosion and design engineer with a reliable and economic material. Titanium Alloys Many titanium alloys have been developed for aerospace applications where mechanical properties are the primary consideration. In industrial applications, however, corrosion resistance is the most important property. The commercially pure (c.p.) and alloy grades typically used in industrial service are listed in Table 1. Any data given should be used with caution as a guide for the application of titanium. In many cases, data were obtained in the laboratory. Actual in-plant environments often contain impurities which can exert their own effects. Heat transfer conditions or unanticipated deposited residues can also alter results. Such factors may require in-plant corrosion tests. Titanium alloys commonly used in industry ASTM Grade UNS Number Ultimate Tensile Strength (min) psi Yield Strength (min) 0.2% offset - psi Nominal Composition 1 R50250 35000 25000 C.P. Titanium* 2 R50400 50000 40000 C.P. Titanium* 3 R50550 65000 55000 C.P. Titanium* 4 R50700 80000 70000 C.P. Titanium* 5 R56400 130000 120000 6% Al, 4% V 7 R52400 50000 40000 Grade 2+0.15% Pd 9 R56320 90000 70000 3% Al, 2.5% V 11 R52250 35000 25000 Grade 1+0.15% Pd 12 R53400 70000 50000 0.3% Mo, 0.8% Ni 16 R52402 50000 40000 Grade 2+0.05% Pd 17 R52252 35000 25000 Grade 1+0.05% Pd 18 R56322 90000 70000 Grade 9+0.05% Pd *Commercially Pure (unalloyed) titanium Titanium offers outstanding resistance to a wide variety of environments. In general, grades 7 and 12 extend the usefulness of unalloyed titanium to more severe conditions. Grade 5, on the other hand, has somewhat less resistance than unalloyed titanium, but is still outstanding in many environments compared to other structural metals. New Grades Recently, ASTM incorporated a series of new titanium grades containing 0.05% Pd. (See Table 1) These new grades exhibit nearly identical corrosion resistance to the old 0.15% Pd grades, yet offer considerable cost savings. Generally wherever information is given regarding Grade 7 these new titanium grades, 16, 17 and 18, may be substituted. As always, this information should only be used as a guideline. Corrosion Mechanisms Titanium and its alloys provide excellent resistance to general localised attack under most oxidizing, neutral and inhibited reducing conditions. They also remain passive under mildly reducing conditions, although they may be attacked by strongly reducing or complexing media. Titanium metal’s corrosion resistance is due to a stable, protective, strongly adherent oxide film. This film forms instantly when a fresh surface is exposed to air or moisture. Oxide Film Growth The oxide film formed on titanium at room temperature immediately after a clean surface is exposed to air is 12-16 Angstroms thick. After 70 days it is about 50 Angstroms. It continues to grow slowly reaching a thickness of 80-90 Angstroms in 545 days and 250 Angstroms in four years. The film growth is accelerated under strongly oxidizing conditions, such as heating in air, anodic polarization in an electrolyte or exposure to oxidizing agents such as HNO3, CrO3 etc. The composition of this film varies from TiO2 at the surface to Ti2O3, to TiO at the metal interface. Oxidizing conditions promote the formation of TiO2 so that in such environments the film is primarily TiO2. This film is transparent in its normal thin configuration and not detectable by visual means. A study of the corrosion resistance of titanium is basically a study of the properties of the oxide film. The oxide film on titanium is very stable and is only attacked by a few substances, most notably, hydrofluoric acid. Titanium is capable of healing this film almost instantly in any environment where a trace of moisture or oxygen is present because of its strong affinity for oxygen. Anhydrous conditions in the absence of a source of oxygen should be avoided since the protective film may not be regenerated if damaged.
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