Crevice corrosion of titanium

Fig. 7. Temperature—pH limits for crevice corrosion of titanium alloys in naturally aerated sodium chloride-rich brines. The shaded areas indicate regions...

Griess has observed crevice corrosion of titanium in hot concentrated solutions of Cl , SOj I ions, and considers that the formation of acid within the crevice is the major factor in the mechanism. He points out that at room temperature Ti(OH)3 precipitates at pH 3, and Ti(OH)4 at pH 0-7, and that at elevated temperatures and at the high concentrations of Cl ions that prevail within a crevice the activity of hydrogen ions could be even greater than that indicated by the equilibrium pH values at ambient temperatures. Alloys that remain passive in acid solutions of the same pH as that developed within a crevice should be more immune to crevice attack than pure titanium, and this appears to be the case with alloys containing 0-2% Pd, 2% Mo or 2[Pg.169]

Greiss, J. C., Crevice Corrosion of Titanium in Aqueous Salt Solutions Corrosion, 24, 96 (1968)... 

FIGURE 14.13. Crevice corrosion of titanium piate of a brine heat exchanger. (With permission from Denki Kagako Kogyo Co. (1977).)... 

Schutz, R. W., "Understanding and Preventing Crevice Corrosion of Titanium Alloys Parts I and II, Materials Performance, October 1992, pp. 57-62, and November 1992, pp. 54-56. 

The mechanism for crevice corrosion of titanium is similar to that for stainless steels, in which oxygen-depleted reducing add conditions develop within tight crevices (see Fig. 3). Dissolved oxygen or other oxidizing spedes in the bulk solution are depleted in the restricted volume of solution in the crevice. Finite surface oxidation in crevices consumes these spedes faster than diflftision fi x)m the bxilk solution can replenish them (Ref 54). As a result, metal potentials in crevices become active (negative) relative to metal... 

Bromides and Sulfates. Unpublished test results suggest that the pH temperature guidelines for crevice corrosion of titanium in saturated NaCl are applicable in saturated NaBr solutions. However, the rate of crevice attack is measurably lower than that in chlorides at corre-... 

Corrosion of Titanium in Neutral and Alkaline Solutions In water, steam, and seawater, titanium is resistant even at high temperatures [44]. In water with high chloride levels, crevice corrosion could appear if tight crevices are present. Titanium shows very low corrosion rates even in seawater [43],... 

Above 816°C, diffusion of the nitride into titanium may cause embrittlement. Nevertheless, titanium is not corroded by liquid anhydrous ammonia at room temperature. Low corrosion rates are obtained at 40°C. Titanium also resists gaseous ammonia. However, at temperatures above 150°C, ammonia will decompose and form hydrogen and nitrogen. Under these circumstances, titanium could absorb hydrogen and become embrittled. The high corrosion rate experienced by titanium in the ammonia-steam environment at 220°C is believed to be associated with hydriding. The formation of ammonium chloride scale could result in crevice corrosion of Timetal 50A at boiling temperatures. Timetal Code-12 and 50A Pd are totally resistant under these conditions. This crevice corrosion behavior is similar to that shown for sodium chloride. 

Bergman, D. D. and Grauman, J. S., The Detection of Crevice Corrosion in Titanium and Its Alloj Through the Use of Potential Monitoring, Titanium 92—Science and Technology, The Minerals, Metals Materials Society, 1993, pp. 2193-2200. 

Effect of temperature and pH on crevice corrosion of grade 2 unalloyed titanium in saturated brine. (From L.C. Covington and P.A. Schweitzer. 1989. in Corrosion and Corrosion Protection Handbook, 2nd ed., P.A. Sdrweitzer, Ed., NewYork Marcel Etekker.)... 

Source D. Dees, Crevice Corrosion of High-Strength Titanium Al-ioys in Saturated Brine, Industrial AppHcations of Titanium and Zirconium, ASTM STP 830,1984, p 133-142... 

Crevice corrosion on titanium typically generates irregularly shaped pits. Microstructural examination of hand-polished and etched sections of crevices often reveals a surrounding layer of predpi-tated titanium hydride in a alloys. These are a by-product of hydrogen reduction at cathodic sites surrotmding the crevice. 

Chlorides. The susceptibihty of titanium alloys to crevice corrosion in hot, concentrated chloride solutions increases significantly as temperatiires increase and pH decreases. Crevice attack of titanium alloys wdl generally not occur below a temperature of 70 °C (160 °F) regardless of solution pH or chloride concentration, or when solution pH exceeds 10 regardless of temperature. ASTM grade 12 provides crevice corrosion resistance when brine pH falls between 3 and 11 to tem-peratvires as high as 300 °C (570 °F). 

Crevice corrosion occurs mainly (but not exclusively) on passive materials. The most important problem is the crevice corrosion of stainless steels, nickel-base alloys, aluminum alloys, and titanium alloys in aerated chloride environments, particularly in sea or brackish water, but also in environments found in chemical, food, and oil industries. Other cases of crevice corrosion are also known such as the so-called corrosion by differential aeration of carbon steels, which does not require the presence of chloride in the environment. Also mentioned in the literature is the crevice corrosion of steels in concentrated nitric acid and inhibited cooling water and of titanium alloys in hot sulfixric environments. 

For a given crevice geometry, the critical potentials for crevice initiation and repassivation decrease with increasing chloride content (Fig. 10) and increasing temperature (Fig. 11) of the bulk solution. This means that the susceptibility to crevice corrosion of passivated alloys increases with the chloride content and the temperature. For example, titanium alloys become sensitive to crevice corrosion only in hot concentrated chloride solutions around 100/150°C [9,10]. Propagation rates also increase with temperature. 

Titanium is susceptible to pitting and crevice corrosion in aqueous chloride environments. The area of susceptibiUty for several alloys is shown in Figure 7 as a function of temperature and pH. The susceptibiUty depends on pH. The susceptibiUty temperature increases paraboHcaHy from 65°C as pH is increased from 2ero. After the incorporation of noble-metal additions such as in ASTM Grades 7 or 12, crevice corrosion attack is not observed above pH 2 until ca 270°C. Noble alloying elements shift the equiUbrium potential into the passive region where a protective film is formed and maintained. 

In the construction of plants, titanium with 0.2% Pd is mainly used. It can be employed with advantage in nonoxidizing acid media and also has increased resistance to pitting and crevice corrosion because of its more favorable pitting potential [40]. 

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