Sea water is the water of the world's oceans. Its salt content (meaning the total quantity of all salts dissolved in the water) varies from one geographical zone to another and amounts to between 32 and 36 g/kg sea water. More than 90 % of the salt contained in sea water is sodium chloride. The average water temperatures lie between 4°C and 25°C, rising to 30°C in tropical waters. Salt water's corrosive action on metals is owed to the good conductivity created by the salt content in combination with oxygen. A rise in temperature intensifies the corrosive attack considerably. Relative movements also intensify the corrosion by adding fresh oxygen; on the other hand, oxygen is required to build up protective layers on aluminium, brass, copper and nickel as well as passive films on high-alloy steels. To prevent deposits, e.g. sludge, flow velocities should not be lower than 1.5 m/sec., but excessively high velocities can wear away the protective film and promote corrosion. In coastal waters, especially close to estuaries, the concentration of salt is lower. Brackish water (water with a salinity value > 0.5) is

hardly less corrosive than sea water because it is usually somewhat warmer and the sludge it carries contains sulphate-reducing bacteria that mostly intensify the attack on metals. As a rule, the mechanical seals for sealing sea water and brackish water at temperatures up to 50°C are single mechanical seals, often elastomer bellows mechanical seals, with the face material pairing ceramic/carbon, secondary seals made of nitrile rubber and metal parts made of fully austenitic steels. If the seal is intended for use mainly with brackish water, it is advisable to select wear-resistant materials because brackish water nearly always contains notable amounts of fine and coarse solids. The face material pairings tungsten carbide/tungsten carbide and tungsten carbide/ceramic have proven successful. At temperatures above 50°C (sea water employed as cooling water, sea water desalinization), the materials for the mechanical seal's metallic components must be matched with the materials of the adjoining machine parts (mostly Ni alloys, NiCu allays or bronzes).