SULFURIC ACID
Titanium is resistant to corrosive attack by dilute solutions of pure sulfuric acid at low temperatures. At 32 degrees F (0 degrees C), unalloyed titanium is resistant to concentrations of about 20 percent sulfuric acid. This decreases to about 5 percent acid at room temperature (Figure 11). TIMETAL 50A Pd is resistant to about 45 percent acid at room temperature. In boiling sulfuric acid, unalloyed titanium will show high corrosion rates in solutions with as little as 0.5 percent sulfuric acid. TIMETAL Code-12 has useful resistance up to about 1 percent boiling acid. TIMETAL 50A Pd is useful in boiling sulfuric acid to about 7 percent concentration. The TIMETAL 6-4 alloy has somewhat less resistance than unalloyed titanium.

The presence of certain multi-valent metal ions or oxidizing agents in sulfuric acid inhibit the corrosion of titanium in a manner similar to hydrochloric acid. For instance, cupric and ferric ions inhibit the corrosion of unalloyed titanium in 20 percent sulfuric acid (Table 16). Oxidizing agents, such as nitric acid, chromic acid, and chlorine are also effective inhibitors.

PHOSPHORIC ACID
Unalloyed titanium is resistant to naturally aerated pure solutions of phosphoric acid up to 30 percent concentration at room temperature (Figure 12). This resistance extends to about 10 percent pure acid at 140 degrees F (60 degrees C) and 2 percent acid at 212 degrees F (100 percent C). Boiling solutions significantly accelerate attack.

TIMETAL 50A Pd offers significantly improved resistance. At room temperature, 140 degrees F (60 degrees C), and boiling. TIMETAL 50A Pd will resist concentrations of about 80, 15 and 6 percent, respectively, of the pure phosphoric acid. TIMETAL Code-12 offers somewhat better resistance to phosphoric acid than unalloyed titanium, but not as good as TIMETAL 50A Pd.

The presence of multi-valent metal ions, such as ferric or cupric, or oxidizing species can be used to inhibit titanium corrosion in phosphoric acid.

HYDROFLUORIC ACID
Titanium is rapidly attacked by hydrofluoric acid of even very dilute concentrations. Therefore, titanium is not recommended for use with hydrofluoric acid solutions or in fluoride containing solutions below pH 7. Certain complexing metal ions (i.e., A.^+3, Cr^+6) may effectively inhibit corrosion in dilute fluoride solutions.

SULFUROUS ACID
Corrosion of unalloyed titanium in sulfurous acid is low: 0.02 mpy (0.0005 mm/y) in 6 percent concentration at room temperature. Samples exposed to sulfurous acid (6 percent sulfur dioxide content) 212 degrees F (100 degrees C) showed a corrosion rate of 0.04 mpy (0.001 mm/y).

OTHER INORGANIC ACIDS
Titanium offers excellent resistance to corrosion by several other inorganic acids. It is not significantly attacked by boiling 10 percent solutions of boric or hydriodic acids. At room temperature, low corrosion rates are obtained on exposure to 50 percent hydriodic and 40 percent hydrobromic acid solutions.^(30)

MIXED ACIDS
The addition of nitric acid to hydrochloric or sulfuric acids significantly reduces corrosion rates. Titanium is essentially immune to corrosion by aqua regia (3 parts HCl: 1 part HNO3) at room temperature. TIMETAL 50A, Code-12 and 50A Pd show respectable corrosion rates in boiling aqua regia (Table 17). Corrosion rates in mixed acids will generally rise with increases in the reducing acid component concentration or temperature.