Internal corrosion of boiler tube surfaces is a serious problem in the electric utility industry. Such corrosion usually takes the form of a pinhole leak which at first does not affect the boiler operation or attract the attention of the operator in the control room. The pinhole increases in size until it is large enough to cause the tube to be torn apart.
Heavy deposits of a black oxide frequently are found in a tube at or near the point of failure. Possible explanations for the presence of this oxide are: (1) Iron removed from the piping and equipment in the pre-boiler circuit may be carried back into boiler when the oxygen-free alkaline environment favors its precipitation as black iron oxide (Fe3O4), and (2) Some factors may prevent the formation of the protective film which is needed to prevent bare metal from coming in contact with water. Protective measures based on both premises were put into use.
One method of combatting this type of corrosion involves the maintenance of an optimum concentration of “free caustic” in the boiler water. Another method is to clean the boiler periodically with low strength hydrochloric acid. Many boiler operators found it helpful to make the condensate alkaline in order to protect pre-boiler equipment. Cyclohexylamine, morpholine and ammonia were used in this connection. Although favorable results were reported in many cases there is still some lack of agreement about the effectiveness of such compounds. 7.6.4
