The corrosion behavior of commercially pure titanium (UNS R50400, ASTM GRADE 2) was investigated in presence of aggressive, bromides containing solution reported to cause more severe localized corrosion compared to chlorides. To enhance localized corrosion resistance of the metal, chemical oxidation treatments were performed using NaOH 10 M solution at room temperature and at 60°C. Treatment duration effect on final corrosion resistance of samples was investigated spanning from 1 h to 72 h. After treatment optimization, the best one was compared to anodic oxidation at low potential.
To further increase corrosion resistance, annealing at 400°C and 600°C was performed after chemical oxidation and the resulting samples were tested in bromides containing solution.
Titanium has outstanding corrosion resistance due to a thin, amorphous, non-stoichiometric TiO2 protective layer (max 10 nm thick1) that is formed spontaneously on the surface when this is exposed to an aerated environment. This protective layer is very stable and allows the use of titanium in severe working conditions, such as offshore (up to 260°C), acid environments, aerospace2,3, automotive, high temperature, chemical & food industry, 4-6 marine hydrometallurgical applications and nuclear fuel wastes containment, 7,8 where no other metal can be used.
Nevertheless, commercially pure titanium may suffer different forms of corrosion in very severe environments.9 Generalized corrosion is caused by small quantity of fluorides ions (more than 0.002 M) that combining with titanium forming TiF4, destroying passivity film. Hydrogen embrittlement happens prevalently on a- and a-ß titanium due to their low hydrogen solubility in a-Ti, stress corrosion cracking can also happen in very specific environments, such as anhydrous methanol, nitrogen tetroxide, red- fuming nitric acid or solid cadmium.
However, the most critical forms of corrosion of titanium are due to localized breaking of passive layer and this is favored by the presence of concentrated halides, such hot salty water (above 200°C) or bromide containing species.