Cleaning the surface prior to conservation. Metal products, which are intended for maintenance after they have left the machining or assembly departments, must be cleaned before an appropriate method of protection is applied. The necessity to clean the metal prior to its protection results from the principles discussed in the chapter on the basics of corrosion. Namely, it was emphasized there, that any impurities present on the surface of the metal constitute the medium, which is the cause of its corrosion. Mechanical impurities cover the metal surface, creating the possibility of local fire. Likewise, air bubbles remaining in a gap filled with oil cause corrosion where it contacts the metal surface.. The salts not removed from the metal promote the formation of a highly corrosive electrolyte film to the metal. Contaminants on the surface block the access of the volatile inhibitor vapors to the metal surface, preventing the agent from exerting a protective effect . Consider what contaminants may be present on the surface of metal products, which are normally subjected to a conservation process and what is their origin. It is also essential, that the physicochemical nature of the pollutants influences the choice of the treatment method. Some products are water soluble, a insoluble in hydrocarbon solvents, e.g.. in gasoline; and some pollutants vice versa. There are such contaminants, which can only be removed by the emulsifying action of the degreasing bath or by an external powerful energy source, e.g.. ultrasound. For the above reasons, the classification of pollutants is carried out, which may be present on metal products intended for maintenance is useful.