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The primary purpose of turbine oil rust inhibitors is to form water-repellent stable films on those exposed iron surfaces within the lubricating system and governor mechanism of steam turbines with which the lubricant comes in contact. Such additives are normally evaluated by ASTM Method D 665, but passing this test does not always assure satisfactory field performance. One reason is that some rust inhibitors, particularly those of the soap type, form loosely bound protective films which prevent adhesion of water only as long as it occurs suspended in an oil phase, but when the oil is absent, such films soon fail. A method to test the hydrolytic stability of rust-protective films is to expose them to the direct action of a continuously agitated water phase.
Another rust test not normally specified for turbine oils is the Naval Research Laboratories static water drop test. A greatly accelerated version of the NRL test is the so-called catechol test, based on the fact that the first corrosion product of iron is colorless, water-soluble ferrous hydroxide which in the presence of catechol is immediately converted to highly colored ferrous catecholate.
When in the D 665 rust test, distilled water is replaced by 0.1 N hydrochloric acid only those additives will stand up which form a closely packed chemically and hydrolytically stable film. The performance of the rust inhibitor is determined by the structure and size of its hydrophobic side chain as well as by the type and strength of the chemical bond formed with the reactive polar group of its molecules. In the complete absence of water, the reaction of rust preventives with iron surfaces is greatly retarded or even inhibited.
Rust inhibitors should also be subjected to other test criteria including their effect on oil oxidation. Even some of the best rust inhibitors will often seriously reduce the oxidation stability of the oil to which they are added because of their tendency to dissolve catalytically active metals such as copper and iron. The ideal rust inhibitor should have no harmful effects on oil life. Other test criteria to be considered are the effect rust inhibitors may have on the demulsibility, foaming and air-release characteristics of the oils; also their hydrolytic stability, chemical purity, and their compatibility with other oil additives.
von Fuchs, George H
Consulting Chemical Engineer, Belmont, Mass.