STEAM BOILERS 131 



Corrosion often violently attacks the staybolts and rivet heads. 



Pitting or Honeycombing. Pitting or honeycombing is 

 readily perceived. The plates are indented in spots with 

 holes and cavities from 3*5 to \ in. deep. The appearance of a 

 pitted plate is shown in Fig. 1. On the first appearance of 

 pitting, the surface so affected should be thoroughly cleaned 

 and a good coating of thick paint made of red lead and boiled 

 linseed oil should be applied. This treatment should be given 

 from time to time to insure protection to the metal. 



Grooving. Grooving, which means the formation of a dis- 

 tinct groove, is generally caused by the buckling action of the 

 plates when under pressure. Thus, the ordinary longitudinal 

 lap joint of a boiler slightly distorts the shell from a truly 

 cylindrical form, and the steam pressure tends to bend the 

 plates at the joint. This bending action is liable to start a 

 small crack along the lap, which, being acted on by corrosive 

 agents in the water, soon deepens into a groove, as shown in 

 Fig. 2. 



External Corrosion. External corrosion frequently attacks 

 stationary boilers, particularly those set in brickwork. The 

 causes of external corrosion are dampness, exposure to weather, 

 leakage from joints, moisture arising from the waste pipes or 

 blow-off, etc. External corrosion should be prevented by 

 keeping the boiler shell free from moisture, and the stoppage of 

 all leaks as soon as they appear. 



Leakage of rivets and the calking edges of seams may be 

 caused by the delivery of the cold feedwater on to the hot 

 plates; another cause is the practice of emptying the boiler 

 when hot and then filling it with cold water. The leakage in 

 both cases is due to the sudden contraction of the plates. 



In horizontal water-tube boilers of the inclined-tube type, 

 external corrosion principally attacks the ends of the tubes, 

 especially the back ends, close up to the headers into which 

 they are expanded. In the course of time the tubes will leak 

 around the expanded portion in the headers. 



If leaks are attended to as soon as they occur, no corrosion 

 will take place, as the gases of combustion are harmless unless 

 acting in conjunction with water or dampness, or unless the 

 coal is rich in sulphur. Should, however, the ends of several 



