Vapor Pressures of Volatile Corrosion Inhibitors by the Torsion-Effusion Method.

Abstract

V O L A T I L E corrosion inhibitor is the name applied to compounds which have the ability to vaporize and condense on a metal surface making it less susceptible to corrosion. One of the most important properties of these compounds is vapor pressure. Cyclohexylammonium carbonate, for example, has a high vapor pressure of 0.4 mm. of mercury a t 25 C. and is a very effective corrosion inhibitor. The protective time is very short because of rapid depletion of the inhibitor. There are also inhibitors with low vapor pressures on the order of lo-' mm. of mercury. These compounds, however, cannot vaporize quickly enough before corrosion occurs. Therefore, vapor pressure is important, since it determines the usefulness of the compounds. To utilize volatile corrosion inhibitors it is also important to know the rate of vaporization or the vapor pressure change with temperature. A vapor pressure-temperature curve with 1 / T us. the -log P(mm. of Hg) will ,provide information which can later be related to the effectiveness of the various volatile inhibitors. Most of the information on volatile corrosion inhibitors in the literature does not pertain to their physico-chemical properties, and some of the data are conflicting. For example, the value for the vapor pressure of dicyclohexylammonium nitrite a t 21°C. determined by Rozenfeld ( 4 ) and by Wachter (7) is different by a factor of 10. Therefore, it seemed necessary to find a method which would accurately determine the vapor pressures of these compounds and the@ rates of change with temperature.