SELECTION OF CORROSION INHIBITORS FOR GREASES WITH DIFFERENT COMPOSITIONS

The main approach to the increase in the protective properties of greases, working for long periods of time at high speeds and loads in a wide range of temperatures, is the use of effective corrosion inhibitors (CI). However, the selection of CI for greases is difficult, due to the presence of a structural skeleton, since the formation of adsorption films of CI on the metal surface is based on contact interactions [i]. The active surface of a solid thickening agent complicates significantly the mass transport of the CI to the surface of the metal in the formation of the adsorbed film; this is one of the main causes for the different effectiveness of CI in liquid and structurized lubricants [2, 3]. The surface energy of the structural skeleton and the competing adsorption of the CI molecules and of the dispersion medium on it determine the mechanism of action of surfaceactive substances in greases. This has been confirmed by the results obtained in the SO 2 chamber [4] in the investigation of the protecting efficiency of hydrocarbon (Hc-), soap (LiSt), and silica gel (Si-) greases, based on oils of different polarity, but of similar viscosity: AU (NM) spindle oil, PES-3 (PES) polyethylsiloxane fluid, and pentaerythritol ester (PEE), to which CI of different activity (2 mass %) were added: AYA, KSK, SIM, BTA, KAP-25, and SZhK. It was found (Table i) that the addition of CI to the Hc-grease based on NM leads to a negligible decrease (up to 1 g/m 2) in the loss in mass of the metal plate in the SO 2 chamber. The addition of the same CI to Li- and Si-greases, based on the same oil, leads to significant decreases in the losses in the mass of the plate at analogous testing conditions: by a factor of I0 and 25 respectively. The effectiveness of CI in Hc-greases based on PES is lower by a factor of 2 than in the Si-greases on the same basis. The decrease in the loss of mass of the plate underneath a layer of the Hc-grease is 8-10, under a layer of the Sigrease 15-20 g/m 2. Inhibition of greases based on PEE reduces the loss of mass of the plate: under a layer of Hc-greases by up to 15, under Li-greases by up to 5, and under Si-greases by up to 20 g/m 2. The effectiveness of CI, expressed in the decrease of the loss of mass of the plate under the layer of the inhibited grease, compared with the uninhibited grease during testing in the SO 2 chamber depends on their nature, but is determined mainly by the nature of the thickening agent. In the Hc-greases the effectiveness of CI is the lowest (about i0 g/m2), in Si-greases it is the highest (25 g/m2), and in Li-greases it has an intermediate value (8-15 g/m2). The effectiveness of the CI depends significantly also on the electrophysical properties of the dispersion medium (Fig. i). With increasing polarity of the base of Hc-, Li-, and Si-greases the effectiveness of almost all investigated CI decreases. In the Hc-, Li-, and Si-greases based on the nonpolar hiM the effectiveness of the CI is higher by a factor of 1.3-5 than in the analogous greases based on the polar PEE. The effectiveness of the CI in Hc- and Si-greases based on PES, the dielectric constant of which is 2.4, is lower than on the basis of NM, but higher than on the basis of PEE. Based on the results of the investigations, a relationship exists between the surface properties of the dispersed phase, the electrophysical properties of the dispersion medium, and the effectiveness of CI of different nature in the greases; it depends not as much on the elemental composition of the CI molecules as on their energetic characteristics, which determine the adsorptivity. The adsorptivity of the CI molecules dictates their effective