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The use of “preservative” motor oils was a development of the recent war. The need for rust prevention has been recognised for many years, but only in the last decade were efforts concentrated to prevent rusting in an efficient and scientific way. Engine oil specifications have changed during the past ten years to comply with the higher requirements resulting from changes in engine design. The preservative type motor oils, which were developed during the same period, must meet these more rigid specifications and also act as corrosion preventives. Engine Preservative Oils serve the double function of preservation and lubrication. As a preservative, the oil should fully protect steel or any other metal which may be found in an engine assembly, whether the engine is stored or in operation. As a lubricant, it must comply with the exacting requirements set for automotive and aircraft engines. Because of the complexity of aircraft motors, specifications are generally more severe for aircraft than automotive engine preservatives. Illustrations of these specifications are presented for automotive oils, and only reference will be made to aircraft oils. Film forming engine preservatives, as required by the U.S. Bureau of Ships, will not be discussed because of lack of oily constituents.

In the first part of this article in our September issue, DR. SELLEI dealt with the practical side of engine preservative oil evaluation. In this contribution, she discusses the most important papers covering the theoretical considerations. Engine preservative oils, used for the double purpose of preservation and lubrication, are evaluated as rust preventives and also as lubricants. By carrying the accelerated rusting tests, such as the humidity test, beyond specification limits a method was found which shows the merit of the oils tested and rates them according to their potential rust preventive performance. For evaluating the lubricating performance, engine tests are necessary. Preliminary screening with the Underwood machine and the one cylinder Lauson engine is recommended. A brief summary of the theoretical background of corrosion prevention is presented, with reference to correlation between theoretical considerations and humidity protection.

The lubrication process may become interrelated with corrosion phenomena. Lubricants may be the products of a corrosion process, e.g., during boundary lubrication. They may also serve as a medium which protects the metal during the lubrication process from the corrosive attack of a humid and acidic atmosphere. For better understanding of the relation between lubrication and corrosion phenomena, the theory of corrosion is presented.