Principles of naval engineering - Historic Naval Ships Association

FChapter 10- PROPULSION BOILERS600 PSl600«F--500''FWATERSIDEDEPOSIT-500°38.131Figure 10-27. — Effect of waterside deposit onboiler tube.deposits, and products formed as the result ofchemical reactions of the tube metal.The term " waterside corrosion " is used toinclude both localized pitting and general corrosion.Most waterside corrosion is electrochemicalin nature. There are always someslight variations (both chemical and physical)in the surface of any boiler metal. These smallchemical and physical variations in the metalsurface cause slight differences in electricalpotential between one area of a tube and anotherarea. Some areas are anodes (positive terminals)and others are cathodes (negative terminals).Iron from the boiler tube tends to go intosolution more rapidly at the anode areas thanat other points on the boiler tube. Electrolyticaction cannot be completely prevented in anyboiler, but it can be kept to a minimum bymaintaining the boiler water at the proper alkalinityand by keeping the dissolved oxygen contentof the boiler water as low as possible.The presence of dissolved oxygen in theboiler water contributes greatly to the type ofcorrosion in which electrolytic action makespits or holes of the type shown in figure 10-28.A pit of this type actually indicates an anodicarea in which iron from the boiler tube hasgone into solution in the boiler water.General corrosion occurs when conditionsfavor the formation of many small anodes and38.138Figure 10-28.— Localized pit in boiler tubecaused by dissolved oxygen in theboiler water.cathodes on the surface of the boiler metal. Ascorrosion proceeds, the anodes and cathodesconstantly change location. Therefore, there isa general loss of metal over the entire surface.General corrosion may occur if the chloridecontent of the boiler water is too high or if thealkalinity is either too low or too high.The third major problem that results fromboiler water contamination is carryover . Undersome circumstances, very small particles ofmoisture (almost like a fine mist) are carriedover with the steam. Under other circumstances,large gulps or slugs of water are carried over.The term priming is generally used to describethe carryover of large quantities of water. Bothkinds of carryover are dangerous and both cancause severe damage to superheaters, steamlines, turbines, and valves. Whatever moistureor water is carried over with the steam bringswith it the solid matter that is dissolved orsuspended in the water. This solid matter tendsto be deposited on turbine blades and in superheatertubes and valves. Figure 10-29 shows asuperheater tube in which solid matter has beendeposited as a result of carryover. Priming, or263