Corrosion of tubes and the drums is largely dependent on the water and water chemistry used within the boiler. Some of the more common types of waterside corrosion include caustic corrosion, dilute acid corrosion, oxygen pitting or localized corrosion, and stress corrosion cracking. A significant factor in the degree of waterside corrosion is the amount of corrosion product deposited. Deposits restrict the heat transfer and lead to local overheating, which can cause concentration of contaminants and corrosives. Depending on which contaminants are present in the feedwater during a period of chemical unbalance, different deposition locations, rates, and effects will be experienced.
Caustic corrosion, or caustic gouging, can occur from deposition of feedwater-corrosion products in which sodium hydroxide can concentrate to high pH levels. At high pH levels, the steel’s protective oxide layer is soluble and rapid corrosion can occur. Deposits normally occur where flow is disrupted and in areas of high heat input. When the deposit thickness is great enough to locally concentrate caustic, severe corrosion resulting in irregular thinning or gouging of the tube wall can occur.
Hydrogen damage may occur if the boiler is operated with low-pH water. This may be caused by the ingress of acidic chemicals from the water treatment facility, a leak in a saline-cooling water condenser, contamination from chemical cleaning, or other factors that may lower the boiler feedwater pH to less than seven. Close control over boiler water chemistry and monitoring practices are important factors in preventing hydrogen damage.
Boiler tube failures caused by pitting or localized corrosion often result from oxygen attack on the internal side of the boiler tube. Pitting corrosion of economizer tubing normally results from inadequate oxygen control of the boiler feedwater. For full protection against oxygen pitting during shutdown, the boiler should be kept full of water treated with an oxygen scavenger and blanked or capped with nitrogen.
While stress corrosion cracking is usually associated with boilers in which austenitic tubes are used for superheater and reheater tubing, failures have occurred in ferritic tubes where a desuperheater or attemperator spraying station introduced high levels of caustic concentration. Stress corrosion cracking of B-7 studs may also occur in areas where a leaking gasketed joint may allow caustic concentration.