Corrosion Resistance

The corrosion resistance of titanium is well documented. A stable, substantially inert oxide film provides the material with outstanding resistance to corrosion in a wide range of aggressive media. Whenever fresh titanium is exposed to the atmosphere or to any environment containing oxygen, it immediately acquires a thin tenacious film of oxide. It is the presence of this surface film that confers on the material its excellent corrosion resistance. Provided that sufficient oxygen is present, the film is self healing and re-forms almost at once if mechanically damaged.
Oxidising and Non-Oxidising Environments

Since titanium depends for its passivity on the presence of an oxide film, it follows that it is significantly more resistant to corrosion in oxidising solutions than in non-oxidising media where high rates of attack can occur. Thus the material can be used in all strengths of aqueous nitric acid at temperatures up to the boiling point. Similarly, it is not attacked by wet chlorine gas and by solutions of chlorine compounds such as sodium chlorite and hypochlorite.

There is no evidence of pitting or stress corrosion cracking in aqueous solutions of inorganic metal chlorides. Titanium also has exceptional resistance to sea water even under high velocity conditions or in polluted water. While the material normally has a significant corrosion rate in media such as sulphuric or hydrochloric acids which produce hydrogen on reaction with the metal, the presence of a small amount of oxidising agent in the acid results in the formation of a passive film. Hence, titanium is resistant to attack in mixtures of strong sulphuric and nitric acids, hydrochloric and nitric acids and even in strong hydrochloric acid containing free chlorine. The presence in solution of cupric or ferric ions also reduces the corrosion rate, as does alloying with noble metals or the use of an anodic protection technique.
Formation of Protective Oxide Films

Protective oxide films on titanium are usually formed when the metal has access to water, even though this may only be present in trace quantities or in vapour form. Thus, if titanium is exposed to highly oxidising environments in the complete absence of water, rapid oxidation can occur and a violent, often pyrophoric, reaction results. Examples of this type of behaviour are found in reactions between titanium and dry nitric acid and between titanium and dry chlorine. However, the amount of moisture necessary to prevent attack under these conditions is small and can be as little as 50 ppm.