Nitric acid titanium corroded

The sequence of reactions involved in the overall reduction of nitric acid is complex, but direct measurements confirm that the acid has a high oxidation/reduction potential, -940 mV (SHE), a high exchange current density, and a high limiting diffusion current density (Ref 38). The cathodic polarization curves for dilute and concentrated nitric acid in Fig. 5.42 show these thermodynamic and kinetic properties. Their position relative to the anodic curves indicate that all four metals should be passivated by concentrated nitric acid, and this is observed. In fact, iron appears almost inert in concentrated nitric acid with a corrosion rate of about 25 pm/year (1 mpy) (Ref 8). Slight dilution causes a violent iron reaction with corrosion rates >25 x 1()6 pm/year (106 mpy). Nickel also corrodes rapidly in the dilute acid. In contrast, both chromium and titanium are easily passivated in dilute nitric acid and corrode with low corrosion rates. 

Molybdenum tends to be protected by vanadium in aerated 7 1 % hydrochloric acid and it receives a high degree of protection when coupled with copper in this medium. Molybdenum corrodes somewhat faster than normal in 3 1 % nitric acid when coupled with tungsten. It is not affected by contact with titanium in 3-1% nitric acid. It is protected by aluminium and copper in aerated 10% formic acid and by aluminium in air-aerated 9% oxalic acid. In the latter solution, copper had only a slight protective effect when coupled with molybdenum. 

Titanium, like zirconium, relies on a protective passive film for corrosion protection. It has excellent resistance to corrosion in nitric acid, chromic acid, wet chlorine, and hypochlorites. It corrodes quickly in hydrofluoric acid and in hot sulfuric and hydrochloric acids. Titanium is used extensively in sea water applications, primarily as heat-exchanger tubing. 

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