DAMAGE MECHANISMS BOILER
Water Condensate Corrosion
Boiler Water Condensate Corrosion is general corrosion and pitting in boiler system and condensate return piping. Corrosion in boiler feedwater and condensate return systems is usually the result of dissolved gases, oxygen and carbon dioxide. Corrosion from oxygen tends to be a pitting type damage and can show up anywhere in the system even if only very small quantities break through scavenging treatment. Oxygen is particularly aggressive in equipment such as closed heaters and economizers where there is a rapid water temperature rise. Corrosion in the condensate return system tends to be due to carbon dioxide although some oxygen pitting problems can occur if the oxygen scavenging treatment is not working correctly.
General (Uniform) corrosion is a thinning type of corrosion with uniform loss of metal over the entire surface exposed to the corroding medium. No localized attack or cracking is observed. Increases in the concentration of the corrosive agent may upset the equilibrium conditions under which the general corrosion is occurring. In alkaline or neutral environment, i.e., pH=7 or pH>7, reduction of dissolved oxygen is the predominant cathodic process that causes uniform corrosion.
Microbiologically Induced Corrosion (MIC)
MIC is a form of corrosion caused by living organisms such as bacteria, algae or fungi. It is often associated with the presence of tubercles or slimy organic substances. MIC is usually found in aqueous environments or services where water is always or sometimes present, especially where stagnant or low-flow conditions allow and/or promote the growth of microorganisms. MIC is usually observed as localized pitting under deposits or tubercles that shield the organisms.
Damage is often characterized by cup-shaped pits within pits in carbon steel or subsurface cavities in stainless steel.
Pitting is a form of highly localized corrosion that may penetrate deeply into the metal very quickly. It often occurs at only a few locations, and is in the form of a pit that is a cavity or hole with the surface diameter about the same as or less than the depth. Pitting corrosion generally requires stagnant conditions such as water in a storage tank or trapped in a drain. Pits can be randomly formed and may be small or large in diameter. Pitting often occurs rapidly. Major factors controlling pitting corrosion are liquid condition, temperature, velocity, and material.
Most pitting corrosion is caused by chloride or chlorine-containing ions (hypochlorite). Bromide ions are also corrosive to stainless steels. Oxidizing ions, such as ferric ions, in conjunction with chlorides are very aggressive pitting agents.