Corrosion is one of the main causes of reduced reliability in steam generating systems. It is estimated that problems due to boiler system corrosion cost industry billions of pounds per year.

Many corrosion problems occur in the hottest areas of the boiler-the water wall, screen, and superheater tubes. Other common problem areas include de-aerators, feedwater heaters, and economisers.

Methods of corrosion control vary depending upon the type of corrosion encountered. The most common causes of corrosion are dissolved gases (primarily oxygen and carbon dioxide), under-deposit attack, low pH, and attack of areas weakened by mechanical stress, leading to stress and fatigue cracking.

These conditions may be controlled through the following procedures:

• maintenance of proper pH and alkalinity levels.
• control of oxygen and boiler feedwater contamination.
• reduction of mechanical stresses.
• operation within design specifications, especially for temperature and pressure.
• proper precautions during start-up and shutdown.
• effective monitoring and control.

Effective corrosion control monitoring is essential to ensure boiler reliability. A well planned monitoring program should include the following:

• proper sampling and monitoring at critical points in the system.
• completely representative sampling.
• use of correct test procedures.
• checking of test results against established limits.
• a plan of action to be carried out promptly when test results are not within established limits.
• a contingency plan for major upset conditions.
• a quality improvement system and assessment of results based on testing and inspections.