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FIGURE 8.19 General corrosion behavior of commercially pure titanium and Ti-Pd alloys compared to other metals and alloys in oxidizing and reducing acids; with and without chloride ions. Each metal or alloy can generally be used for those environments below its respective solid lines [12].

metal such as 0.2 wt% palladium; Table 8.13 [12]. Similar but smaller benefit can be achieved by small alloying additions of nickel and molybdenum (e.g., 0.3 wt% Mo and 0.8 wt% Ni; ASTM Grade 12 titanium).

Unalloyed titanium is especially useful for applications where essentially no corrosion products can be tolerated in the process fluid. The metal is used extensively in the fabrication of food, drug, and dye processing equipment where even trace amounts of metal ion contamination could adversely affect the quality, color, and/or taste of the product produced.

Titanium, in most environments, is an effective cathode, thus coupling the metal to a less noble metal can result in a high galvanic corrosion current and rapid dissolution of the anodic material; and the titanium may absorb hydrogen.

Titanium and its alloys are susceptible to inhibitor-type concentration-cell corrosion when, for example, oxidizing heavy-metal ions are used to inhibit general corrosion and crevices exist. Concentration-cell corrosion of titanium can be mitigated in some cases by using either ASTM Grades 7 or 12 titanium (see Table 8.13) for fabricating entire components or just in local crevice zones. Palladium and nickel in these alloys, respectively, provide improved passivity (i.e., anodic protection) in the crevices.

Titanium's resistance to chloride-induced pitting attack is a primary reason for using this material (e.g., replacing type 316L stainless steel in petroleum refinery processes). However, under certain conditions, titanium is susceptible to pitting attack and has been reported to pit in the hot 130°C (270°F) brine solutions in salt evaporators. Pitting attack can also be mitigated by using ASTM Grades 7 and 12.

Titanium alloys are susceptible to stress-corrosion cracking (SCC) in a number of environments, including anhydrous methanol containing trace quantities of halides, anhydrous RFNA, and hot chloride-containing salts. Several of the alloys and unalloyed titanium (containing relatively high oxygen contents) are known to crack in ambient temperature seawater if the materials contain preexisting cracks.

The strong influence of microstructure on SCC has been demonstrated in the metastable P-titanium alloy Beta III (Ti-11.5Mo-6Zr-4.5Sn) [35]. This work demonstrated that the alloy was susceptible to SCC when equiaxed P grains and continuous grain boundary a were present, but a worked material in which no such microstructural features were observed was immune to SCC.

Although there have been no known service failures related to hot salt stress-corrosion cracking (HSSCC), HSSCC is a potential limitation to the long duration exposure of highly stressed titanium alloys at temperatures above about 220°C (430°F).

The near immunity of relatively high strength titanium alloys to corrosion fatigue in chloride-containing solutions allows these materials to be used in many hostile environments (e.g., body fluids) where other alloys have failed when subjected to cyclic stresses.

The tenacious passive film that forms naturally on titanium and its alloys provides excellent resistance to erosion corrosion. For turbine-blade applications where the components are impinged by high-velocity water droplets, unalloyed titanium has been shown to have superior resistance compared to conventional blade alloys (e.g., austenitic stainless steels and Monel).

It is known that the fatigue behavior of titanium and its alloys is surface condition sensitive; surface damage by fretting can adversely affect the ability of these materials to withstand cyclic stress. For example, fretting corrosion can reduce the fatigue strength of a titanium alloy, such as Ti-6Al-4V by more than 50%.

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