What Does Caustic Corrosion Mean?
Caustic corrosion is a type of corrosion that occurs in metal equipment (e.g., boilers) in certain environmental operating conditions that have a high pH water level, resulting in an operating medium that is highly alkalized (i.e., a high concentration of sodium hydroxide (NaOH), also known as caustic soda).
As the liquid evaporates, the salts left behind are deposited on the boiler's internal metallic surface, which can result in metallic stresses during the boiler's operation and lead to cracking or pitting corrosion if unnoticed.
Caustic corrosion is also known as caustic cracking or caustic embrittlement.
Corrosionpedia Explains Caustic Corrosion
There are many causes that lead to caustic corrosion or caustic embrittlement in metallic equipment, including the combined action of the following three components:
- A susceptible material
- A given chemical species
- Tensile stress
A common recurring problem for boilers is caustic corrosion, which is caused by the deposition of caustic soda (sodium hydroxide) on the internal surface. Caustic soda is sometimes added to the boiler feedwater to prevent scaling problems.
In every boiler, there are cervices (small invisible gaps) found around rivet heads and other hot spots where the alkali deposits form over the time and weaken the metal. When boilers operate, the temperature rises and causes fabrication stress around rivet holes, eventually causing cracks in the steel boiler shells and tube plates near the weak portions of the metal. The alkaline water enters the minute holes and cracks by capillary action in the interior of the boiler. The water then diffuses out of the cracks, leaving behind hydroxide salts that accumulate when more water evaporates. These remained salts of sodium hydroxide or caustic soda react and dissolve the protective magnetite layer (Fe3O4), eventually causing further metal loss from inside the boiler shell and finally equipment failure.
Some precautionary measures to control caustic corrosion include:
- Controlling the pH values of boiler feedwater.
- Monitoring the caustic soda (sodium hydroxide) and phosphate levels.
- If equipment is found with large deposits, it needs to be operated in a controlled temperature and pH environment with subsequent removal of the deposits.
- Stress levels on the equipment should be controlled.
- Use of alkali should be avoided wherever possible.
- Use of softening reagents such as sodium sulfates, lignin and tannin should be adopted and sodium carbonates should be stopped. These reagents soften the hard and brittle portions on the inner metal surface, block hairline cracks and prevent infiltration of sodium hydroxide into the areas.