What Does Caustic Embrittlement Mean?
Caustic embrittlement is a phenomenon that occurs in boilers where caustic substances accumulate in boiler materials. It also can be described as the cracking of riveted mild steel boiler plates. This occurs at temperatures of 200°-250°C as a result of local deposition of concentrated hydroxide.
Caustic embrittlement focuses on the stressed parts of the boiler, including cracks, bends, rivets and joints. Residual sodium carbonate, which is used for the softening process, undergoes hydrolysis, forming sodium hydroxide at high pressures and temperatures.
Caustic embrittlement is also known as stress corrosion cracking.
Corrosionpedia Explains Caustic Embrittlement
There are many causes of caustic embrittlement, including the combined action of the following three components:
- A susceptible material
- A given chemical species
- Tensile stress
Sodium hydroxide (caustic soda) prevents scaling when added to the boiler water. The presence of alkali in the crevices, found around the rivet heads and other hot spots, combined with fabrication stress around rivet holes, causes cracks in the steel boiler shells and tube plates.
The alkaline water enters the minute holes and cracks by capillarity action on the interior of the boiler. The water then diffuses out of the cracks, leaving behind hydroxide salts that accumulate when more water evaporates. The hydroxide then attacks the surrounding material of the boiler and dissolves iron as sodium ferrite.
This corrosion at high pH levels produces hydrogen, which attacks the crystal structure of iron, making it hard and brittle. This is highly dangerous because the tube can then fail at the boiler's normal operating temperature.
Caustic embrittlement can be prevented through several methods, including:
- Controlling the temperature and potential
- Controlling the stress levels and hardness
- Use of materials that do not crack when used in given environments
- Avoiding alkali where necessary
- Replacing sodium carbonates with sodium sulphates as softening reagents
- Adding lignin, tannin or sodium sulphate that blocks hairline cracks as well as preventing infiltration of sodium hydroxide into the areas