1991 Volume 31 Issue 2 Pages 189-193
The corrosion resistance of titanium exposed in nitric acid vapor is generally inferior to that immersed in nitric acid solution because of an inhibitor effect of elements dissolved by corrosion of titanium in the solution.
A new type corrosion testing apparatus, which would not be influenced by such dissolved elements on the corrosion rate, was designed in order to easily estimate the corrosion rates in a nitric acid condensed zone, a vapor zone and a liquid zone simultaneously. This apparatus was composed of a condenser, an outer vessel, an inner vessel with a siphon pipe, and an Erlenmeyer flask. The solution condensed in the inner vessel was automatically removed to the flask through the siphon pipe and renewed at regular intervals. The nitric acid concentration and the temperature of the above each zone in this apparatus was similar to the data obtained from the previously reported vapor-liquid equilibrium line. As the concentration of dissolved elements in the solution of the condensed zone of this apparatus was sufficiently negligible, the corrosion rate in the condensed zone could be accurately estimated without the influence of dissolved elements from the corroded metal itself.
Using this testing apparatus, the corrosion rate of titanium in the condensed zone was 0.37 mm/year (1.17×10–11 m/s), greater value than those in any other zone and general corrosion occurred. Moreover, the corrosion rate in the liquid zone was less than 0.01 mm/year (3.17×10–13 m/s) and this rate was regarded as the perfect corrosion resistance. That is because of concentrated soluble titanium ions in the solution acted as a corrosion inhibitor.
In the case of 304L stainless steel, the corrosion rate was 4.5 mm/year (1.4×10–10 m/s) in the liquid zone at the highest value, and the intergranular corrosion occurred. On the other hand, the rate in the condensed solution was small to be 0.095 mm/year (3.01×10–12 m/s).