Corrosion can be caused by oxygen and carbon dioxide which contained in condensed steam. Corrosion is reforming of metal from its natural original form such iron to be oxide iron, aluminum, etc. Corrosion can be caused by the following below:
- Corrosive gases such as O2, CO2, H2S
- Crust and deposit
- Different metal type
- Low PH
Type of corrosion which is generally found in steam boiler is general corrosion, pitting corrosion, and embrittlement corrosion. The presence of dissolved oxygen and carbon dioxide in boiler water is the major cause of occurrence of general corrosion and pitting corrosion. The solubility of these gases in boiler water will be decrease if temperature rises. Most of oxygen will be separated in the steam room but small amount of residue is left in solution of pockets or under deposit, it can cause corrosion on metals boiler. Therefore it is important to make deoxygenating process on boiler water.
The average amount of corrosion or electrochemical attack would increase if the pH value decreases. Therefore boiler water must be maintained chemically in high pH value. Form of corrosion that is not common but dangerous is embrittlement corrosion or inter crystalline crack which can occur in steel if it is at high pressure and chemical environment that is not appropriate. Caustic embrittlement occurs in the connection plug and extends to the end tube when slit develop in concentrated caustic environment.
Hydrogen embrittlement is another form of inter crystalline crack that occur on water wall tube which is caused by high pressure and certain temperature condition.
Corrosion can be prevented by the following action:
- Eliminate corrosive gases
- Prevent crust and deposit in boiler tube
- Using a substance that can inhibit corrosive event
- Adjust pH and alkalinity