RED FUMING NITRIC ACID
Although titanium in general has excellent resistance to nitric acid over a wide range of concentrations and temperatures, it should not be used with red fuming nitric acid. A pyrophoric reaction product can be produced resulting in serious accidents. An investigation of these accidents has shown that the pyrophoric reaction is always preceded by a rapid corrosive attack on the titanium.^(24,25) This attack is intergranular and results in a surface residue of finely divided particles of metallic titanium. These are highly pyrophoric and are capable of detonating in the presence of a strong oxidizing agent such as fuming nitric acid.

It has been established that the water content of the solution must be less than 1.34% and the NO2 content greater than 6% for the pyrophoric reaction to develop. This relationship is shown in Figure 6.^(24)

CHROMIC ACID
The data on chromic acid is not as extensive as that on nitric acid. However, the corrosion resistance of titanium to chromic acid appears to be very similar to that observed in nitric acid. This is shown by the data in Table 14 and by service experience.

REDUCING ACIDS
Titanium offers moderate resistance to reducing acids such as hydrochloric, sulfuric, and phosphoric. Corrosion rates increase with increasing acid concentration and temperature. The TIMETAL 50A Pd alloy offers best resistance to these environments, followed by TIMETAL Code-12, unalloyed titanium, and TIMETAL 6-4.

HYDROCHLORIC ACID
Iso-corrosion data illustrate that TIMETAL 50A offers useful corrosion resistance to about 7% hydrochloric acid at room temperature; TIMETAL Code-12 to about 9% HCl; and TIMETAL 50A Pd to about 27% (See Figure 7). This resistance is significantly lowered at near boiling temperatures.

Typical corrosion rate data for TIMETAL 50A, 6-4, Code-12 and 50A Pd in pure HCl solutions are given in Table 15.

Small amounts of certain multi-valent metal ions in solution, such as ferric ion, can effectively inhibit the corrosion of titanium in hydrochloric acid (Figures 8, 9,10). When sufficient ferric ion is present, TIMETAL 50A, Code-12 and 50A Pd show similar corrosion resistance. Other metal ions, such as Cu^-2, Ni^-2, Mo^-6, and Ti^-4, also passivates titanium against attack by hydrochloric acid. Oxidizing agents such as nitric acid, chorine, sodium hypochlorite, or chromate ions, also have been shown to be effective inhibitors. These have allowed titanium to be successfully utilized in many hydrochloric acid applications.

Severe corrosion damage on titanium equipment has resulted from cleaning procedures utilizing pure hydrochloric acid or acid inhibited with amines. If hydrochloric or sulfuric acid is used to clean titanium surfaces, it is recommended that sufficient ferric chloride be added to effectively inhibit corrosion of the titanium.