Chapter 10- PROPULSION BOILERS 



600«F- 



600 PSl 

 -500''F 



WATERSIDE 

 DEPOSIT 



-500° F 



38.131 

 Figure 10-27. — Effect of waterside deposit on 

 boiler tube. 



deposits, and products formed as the result of 

 chemical reactions of the tube metal. 



The term " waterside corrosion " is used to 

 include both localized pitting and general corro- 

 sion. Most waterside corrosion is electro- 

 chemical in nature. There are always some 

 slight variations (both chemical and physical) 

 in the surface of any boiler metal. These small 

 chemical and physical variations in the metal 

 surface cause slight differences in electrical 

 potential between one area of a tube and another 

 area. Some areas are anodes (positive termi- 

 nals) and others are cathodes (negative termi- 

 nals). Iron from the boiler tube tends to go into 

 solution more rapidly at the anode areas than 

 at other points on the boiler tube. Electrolytic 

 action cannot be completely prevented in any 

 boiler, but it can be kept to a minimum by 

 maintaining the boiler water at the proper alka- 

 linity and by keeping the dissolved oxygen con- 

 tent of the boiler water as low as possible. 



The presence of dissolved oxygen in the 

 boiler water contributes greatly to the type of 

 corrosion in which electrolytic action makes 

 pits or holes of the type shown in figure 10-28. 

 A pit of this type actually indicates an anodic 

 area in which iron from the boiler tube has 

 gone into solution in the boiler water. 



General corrosion occurs when conditions 

 favor the formation of many small anodes and 



38.138 

 Figure 10-28.— Localized pit in boiler tube 

 caused by dissolved oxygen in the 

 boiler water. 



cathodes on the surface of the boiler metal. As 

 corrosion proceeds, the anodes and cathodes 

 constantly change location. Therefore, there is 

 a general loss of metal over the entire surface. 

 General corrosion may occur if the chloride 

 content of the boiler water is too high or if the 

 alkalinity is either too low or too high. 



The third major problem that results from 

 boiler water contamination is carryover . Under 

 some circumstances, very small particles of 

 moisture (almost like a fine mist) are carried 

 over with the steam. Under other circumstances, 

 large gulps or slugs of water are carried over. 

 The term priming is generally used to describe 

 the carryover of large quantities of water. Both 

 kinds of carryover are dangerous and both can 

 cause severe damage to superheaters, steam 

 lines, turbines, and valves. Whatever moisture 

 or water is carried over with the steam brings 

 with it the solid matter that is dissolved or 

 suspended in the water. This solid matter tends 

 to be deposited on turbine blades and in super- 

 heater tubes and valves. Figure 10-29 shows a 

 superheater tube in which solid matter has been 

 deposited as a result of carryover. Priming, or 



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