SULFUR DIOXIDE AND HYDROGEN SULFIDE
Titanium is resistant to corrosion by gaseous sulfur dioxide and water saturated with sulfur dioxide (Table 24). Sulfurous acid solutions also have little effect on titanium. Titanium has demonstrated superior performance in wet SO2 scrubber environments of power plant FGD systems.

Titanium is not corroded by moist or dry hydrogen sulfide gas. It is also highly resistant to aqueous solutions containing hydrogen sulfide. The only known detrimental effect is the hydriding problem discussed in the previous section. In galvanic couples with certain metals such as iron, the presence of H2S will promote hydriding. Hydriding, however, does not occur in aqueous solutions containing H2S if unfavorable galvanic couples are avoided. For example, titanium is fully resistant to corrosion and stress cracking in the NACE* test solution which consists of oxygen-free water containing about 3,000 ppm dissolved H2S, 5 percent NaCl, and 0.5 percent acetic acid (pH 3.5). Tensile specimens of TIMETAL 50A, 75A, 50A Pd and Code-12 stressed to 98 percent of yield strength in this environment survived a 30-day room temperature exposure.

In addition, C-ring specimens of these same grades of titanium were subjected to a stress corrosion cracking test as specified in ASTM G38-73 Standard Recommended Practice. Two series of tests were run: one with the specimens stressed to 75% of yield, and the other stressed to 100% of yield. The specimens were exposed in an ASTM synthetic seawater solution saturated with H2S and CO2 at 400 degrees F (204 degrees C). Solution pH was 3.5 and specimens were exposed for 30 days. There were no failures and no evidence of any corrosion.

Titanium is highly resistant to general corrosion and pitting in the sulfide environment to temperatures as high as 500 degrees F (260 degrees C). Sulfide scales do not form on titanium, thereby maintaining good heat transfer.

NITROGEN AND AMMONIA
Titanium reacts with pure nitrogen to form surface films having a gold color above 1,000 degrees F (538 degrees C). Above 1,500 degrees F (816 degrees C), diffusion of the nitride into titanium may cause embrittlement.

Jones et al (1977) have shown that titanium is not corroded by liquid anhydrous ammonia at room temperature.^(40) Low corrosion rates are obtained at 104 degrees F (40 degrees C).^(41) Titanium also resists gaseous ammonia. However, at temperatures above 302 degrees F (150 degrees 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 428 degrees F (220 degrees C) in Table 25 is believed to be associated with hydriding.

Table 25 also contains data which illustrate the resistance of titanium to ammonium hydroxide. Excellent resistance is offered by titanium to concentrated solutions (up to 70% NH4OH) to the boiling point.^(41)

The formation of ammonium chloride scale could result in crevice corrosion of TIMETAL 50A at boiling temperatures, as shown in Table 25. TIMETAL Code-12and 50A Pd are totally resistant under these conditions. This crevice corrosion behavior is similar to that shown in Figures 2 and 4 for sodium chloride.