What Does Oxygen Attack Mean?
An oxygen attack is an electrochemical process by which a temperature rise provides enough additional energy to accelerate reactions on metal surfaces, resulting in rapid and severe corrosion.
Oxygen attack is the most common cause of corrosion inside boilers. Dissolved oxygen in feedwater can become very aggressive when heated and reacts with the boiler’s internal surface to form corrosive components on the metal surface. Oxygen attack can cause further damage to:
- Steam drums
- Mud dams
- Boiler headers
- Condensate piping
The entire boiler system is susceptible to oxygen attack. Oxygen attack leads to:
- Failure of critical parts of the boiler system
- Deposition of corrosion products in critical heat exchange areas
- Overall efficiency loss
Oxygen attack may be highly localized or may cover an extensive area.
Corrosionpedia Explains Oxygen Attack
Oxygen attack is a kind of corrosion on a metal surface caused by dissolved oxygen in water. For example, internal boiler corrosion is normally the result of oxygen attack and/or low pH, and is potentially dangerous due to the pressures and temperatures associated with an operating boiler.
Every metal surface of the boiler system is vulnerable to oxygen attack. Oxygen forms localized corrosion areas referred to as pits. Oxygen pits can rapidly "drill" through metal surfaces, leading to metal fatigue and failure. As oxygen corrodes the boiler metal, it dissolves the iron surface. This weakens the metal site, but more importantly, sends dissolved iron into the boiler. This dissolved iron can deposit onto boiler tubes, causing overheating and tube failure.
Three critical factors govern the onset and progress of oxygen attack or corrosion:
- Presence of moisture or water
- Presence of dissolved oxygen
- Unprotected metal surface
The corrosiveness of water increases as temperature and dissolved solids increase, and as pH decreases. Aggressiveness generally increases with an increase in oxygen.
Oxygen control is generally both a mechanical and chemical process. The majority of oxygen in the boiler feedwater is typically reduced to less than 20 parts per billion (ppb) by heating the water to reduce its solubility. A deaerator removes most of the oxygen in feedwater; however, trace amounts are still present and can cause corrosion-related problems. Oxygen scavengers are added to the feedwater, preferably in the storage tank of the feedwater, to remove the trace amount of oxygen that escapes from the deaerator. The most commonly used oxygen scavenger is sodium sulfite. Sodium sulfite is cheap, effective and can be easily measured in water.