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aGrade 2, Ti-50A; Grade 7, Ti-0.2Pd; Grade 12, Ti-0.3Mo-0.8Ni. b For a corrosion rate of about 5 mil per year.

aGrade 2, Ti-50A; Grade 7, Ti-0.2Pd; Grade 12, Ti-0.3Mo-0.8Ni. b For a corrosion rate of about 5 mil per year.

reducing conditions, should be avoided. The passive film formed in air may not be adequately stable and may not be regenerated if it is damaged during exposure to these environments.

General (uniform) corrosion rate information for titanium and many of its alloys exposed to a wide variety of environments is available [12]. Broadly, commercial purity titanium is resistant to natural environments, including sea, fresh, brackish, and mine waters; food products; crude oils; body fluids; and waste materials. The outstanding resistance of unalloyed titanium and the Ti-0.2 Pd alloy (ASTM Grades 2 and 7; see Table 8.13) in chloride-containing, aqueous environments is well established. With few exceptions, unalloyed titanium performs well when exposed to oxidizing inorganic acids (e.g., nitric and chromic acid), aqueous ammonia, anhydrous ammonia, molten sulfur, pure hydrocarbons, aqua regia, hydrogen sulfide, wet chlorine, most organic acids, dilute caustic solutions, and chlorine dioxide.

Titanium is not particularly resistant to pure reducing inorganic acids (i.e., those that generate hydrogen during the metal-acid reaction) such as sulfuric, hydrochloric, and phosphoric acids. The metal is dissolved rapidly by hydrofluoric acid. Other environments that should be avoided include fluoride-containing solutions (e.g., ammonium fluoride), hot concentrated caustics, certain organic acids (e.g., oxalic, concentrated citric and trichloroacetic, and nonaerated boiling formic), and powerful oxidizing agents [e.g., anhydrous liquid and gaseous chlorine, liquid and gaseous oxygen, anhydrous red fuming nitric acid (RFNA), anhydrous nitrogen tetroxide, and liquid bromine]. Powerful oxidizers are especially to be avoided because, under certain conditions such as impact, the reaction can be pyrophoric.

Figure 8.19 [12] shows that the use of titanium can be extended into the "reducing acid" region by alloying the metal with small amounts of a noble

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