Stated simply, general corrosion is the reversion of a metal to its ore form. Iron for example, reverts to iron oxide as a result of corrosion. The process of corrosion, however, is a complex electro-chemical reaction. Corrosion may produce general attack over a large metal surface or may result in pinpoint penetration of the metal. Basic corrosion in boilers results primarily from the reaction of oxygen with the metal. Stresses, pH conditions and chemical corrosion have an important influence and produce different forms of attack.
Corrosion may occur in the feedwater system as a result of low pH water and the presence of dissolved oxygen and carbon dioxide. On-line boiler corrosion occurs when boiler water alkalinity is too low or too high. When oxygen-bearing water contacts metal, often during idle periods, corrosion can occur. High temperatures and stresses in the boiler metal tend to accelerate the corrosive mechanisms. In the steam and condensate system, corrosion is generally the result of contamination with carbon dioxide and oxygen. Additional contaminants such as ammonia or sulfur-bearing gases may increase attack on copper alloys in the system.
Corrosion causes difficulty from two respects. The first is deterioration of the metal itself and the second is deposition of the corrosion products in high heat release areas of the boiler. Uniform corrosion of boiler surfaces is seldom of real concern. All boilers experience a small amount of general corrosion. Corrosion takes many insidious forms, however, and deep pits resulting in only a minimal total iron loss may cause penetration and leakage in boiler tubes. Corrosion beneath certain types of boiler deposits can so weaken the metal that tube failure may occur. In steam condensate systems, replacement of lines and equipment due to corrosion can be costly.
With the trend toward higher heat fluxes in today’s modern boilers, corrosion has become an important factor in power plant operation. When iron corrodes, hydrogen gas, which can be measured in the steam, is released. Measuring the amount of hydrogen gas released can detect immediate fluctuations in load, boiler water conditions or fuel changes. This information when interpreted by an experienced, welltrained engineer can indicate if corrosive conditions exist in an operating boiler.
The most common methods for prevention of corrosion include:
· Removing dissolved oxygen from the feedwater
· Maintaining alkaline conditions in the boiler water
· Keeping internal surfaces clean
· Protecting boilers during out-of-service intervals
· Counteracting corrosive gases in steam and condensate systems with chemical treatment
The selection and control of chemicals for preventing corrosion require a thorough understanding of the causes and corrective measures.