Oxygen is a common contaminant in the boiler water that is particularly corrosive to carbon steel. Many corrosion mechanisms are either directly caused by oxygen or are greatly exacerbated. During operation, oxygen typically enters the water through vacuum leaks in the condensate system or from makeup water that was not deaerated. Proper maintenance can minimize ingress during operation.
Of greater concern is the ingress of oxygen when shutdown or in layup conditions. Layup is probably the most neglected aspect of water chemistry control. Often units are vented and open to atmosphere, allowing the introduction of oxygen. Units may have no capability to sample the water chemistry, inject chemicals, or recirculate fluids. This creates the potential for a highly corrosive environment.
General corrosion, pitting, crevice corrosion, and under-deposit corrosion are common corrosion mechanisms that occur during layup. General corrosion during layup is often responsible for generation of large quantities of iron oxides. A thin layer of corrosion over the huge surface area of carbon steel creates a large volume of oxides. The oxides are transport to and deposited on heat transfer surfaces when the unit returns to operation. The deposits impede heat transfer and provide a crevice where chemical impurities can concentrate, further increasing corrosion.
Pitting requires a stagnant fluid and therefore virtually only occurs when the unit is shut down. Again, oxygen provides a corrosive environment, especially if chlorides are present. Pitting can progress rapid enough to penetrate a tube wall in a few weeks time. For cycling units, a more common and more insidious failure scenario is that a pit will form and grow partially through the tube wall while the unit is shut down and in layup. Upon return to service, the pit acts as a stress riser and a corrosion fatigue crack forms and grows. The corrosion fatigue crack eventually causes failure of the tube.
To avoid layup related tube failures, proper chemistry control must be maintained at all times. It is essential to prevent the simultaneous presence of moisture and oxygen. If the unit is kept in wet layup, oxygen ingress must be prevented. The most effective way to do this is by applying nitrogen overpressure and not opening the systems to atmosphere. Parameters such as pH and conductivity should be monitored and corrected as required. In addition, fluids should be periodically recirculated to prevent pit formation. If the unit is to be drained and opened for maintenance, dehumidified air should be used to prevent the presence of moisture .
 Jackson, P. Moelling, D. Malloy, J. Taylor, M. Tube Failure Diagnostic Guide - Third Edition, 2013. ISBN 0-9719616-3-8