Interdependence of On-Load Corrosion, Creep-Rupture, and Copper Deposit in Augmenting Failure Processes of Boiler Tubes

Abstract

Long-term overheating of boiler tubes in fossil fuel-fired thermal power plants intrinsically is linked with the formation and growth of internal oxide scale at high-heat flux zones, known as hot spots. Preferential and selective deposition of highly aggressive corrodents such as alkalis, chlorides, and some inorganic oxides like cupric oxide/cuprous oxide (CuO/Cu2O) take place at these hot spots. Caustic corrosion develops as the localized concentration of sodium hydroxide (NaOH) at the metal-oxide interface increases either through a wick-boiling process or simply by seepage between cracked oxide layers. At high-pH levels, the magnetite (Fe3O4) layer starts dissolving and results in rapid corrosion. In addition, the rate of corrosion is found to accelerate under sustained loads (hoop stress) and with fluid temperature. Copper corrosion products (typically coming from brass condenser tubes) concentrate at the site of attack and plate out onto the metal surface, aggravating the corrosion process further by galvanic corrosion. The end result is a thickening of preexisting oxide layers and an eventual loss of localized heat-transfer rate. As the metal temperature escalates, the tube creeps much faster and fails in an embrittled manner. In the present paper, the sequence of mechanisms involved in the failure process is discussed, and their interdependence, causing overheating and failure, is described. In addition, the role of copper deposits in expediting the failure process is analyzed critically. A case study is presented to substantiate the proposed mechanisms involved in long-term overheating failures.