Caustic embrittlement: The formation of brittle and in crystalline cracks in the boiler shell is called caustic embrittlement. Caustic embrittlement is the phenomenon in which the material of a boiler becomes brittle due to the accumulation of caustic substances. 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 is a type of corrosion that is caused due to high concentration of sodium hydroxide (NaOH). It is caused by the formation of irregular cracks on the boiler metal, especially at high local bends such as joints and bends.
Caustic embrittlement is the phenomenon of metal becoming brittle due to caustic accumulation at the rivets. The cold working of rivets is the main reason for caustic embrittlement.
Caustic embrittlement can be prevented through: Control of stress level (residual or load) and hardness. Avoid alkalis. Use of materials known not to crack in the specified environment.
Caustic Embrittlement in Steam Boilers:
Caustic embrittlement is a type of corrosion that is caused due to high concentration of sodium hydroxide (NaOH). It is caused by the formation of irregular cracks on the boiler metal, especially at high local bends such as joints and bends.
Caustic embrittlement is a commonly occurring issue taking place when a component is operating in a caustic environment. It is counted as one of the many environmental cracking mechanisms, generally taking place in carbon steel equipment as well as low alloy and austenitic stainless steel equipment.
Caustic environments consist of high concentrations of NaOH and KOH that lead to cracking, usually in welding because of high residual stress. It also affects the base metal with high residual stress.
Caustic embrittlement is an intercrystalline fracture that is caused by the high concentration of caustic soda and the material under stress. The stress corrosion cracks follow the grain or crystal boundaries of the material. Caustic embrittlement is mainly found in the areas of the steam boiler that are highly stressed, such as tube and tube plate connection, riveted head, seam, and boiler mountings.
Causes of Caustic Embrittlement:
Various reasons lead to caustic embrittlement, including the action of the following three components,
- Susceptible materials such as carbon steel
- Particular chemical species like concentrated hydroxide
- Tensile stress around the riveted holes
As the water evaporates in the steam boiler, the concentration of sodium carbonate in the boiler starts increasing. Sodium carbonate is usually used for the softening of lime soda water in high-pressure boilers. If the concentration of sodium carbonate accelerates, it undergoes hydrolysis which leads to the formation of sodium hydroxide.
Scaling is prevented when sodium hydroxide is administered to the boiler water. Cracking in the steam boiler shell and tube plates is caused by the alkali present around the rivet heads, crevices, and hot spots, along with fabrication stress around the rivet holes. As the alkaline water enters the minute holes and cracks on the boiler interior, the water starts dispensing out of the cracks. It results in the accumulation of hydroxide salts left behind as the water evaporates. The hydroxide impacts the material of the boiler and dissolves iron as sodium ferrite.
Corrosion occurring at high pH levels leads to producing hydrogen that affects the crystal structure of iron, making it brittle and hard. It may prove hazardous as the tube can fail at the boiler’s normal operating temperature.
Prevention of Caustic Embrittlement:
Caustic embrittlement can be prevented through several measures such as,
- Managing the temperature and potential
- Managing the Hardness and stress level- residual or load
- Avoiding the use of alkalis
- Utilizing material that does not crack when used in a caustic environment
- Using sodium phosphate instead of sodium bicarbonate as a softening agent for boiler water
- Introducing tannin and lignin in the boiler water assists in blocking the hairline cracks, consequently preventing the accumulation of sodium hydroxide
- Introducing Na2SO4 in boiler water helps in blocking hairline cracks
It is a type of boiler corrosion, caused by highly alkaline water in the boiler.
In the lime-soda process, it is likely that some residual Na2CO3 is still present in the softened water
This Na2CO3 decomposes to give NaOH and CO2 due to which the boiler water becomes “Caustic”.
Na2CO3+ H2O → 2NaOH + CO2↑
This very diluted caustic water flows into the minute hair cracks in the boiler, by capillary action.
On evaporation of water, the dissolved caustic soda increases its concentration inside hair cracks.
This concentrated alkali dissolves iron of the boiler as sodium ferrate.
It causes embrittlement of boiler parts such as bends joints, rivets, etc, due to which the boiler gets fails.
Fe + NaOH → Na2FeO2 + H2
Insoluble Soluble
Caustic embrittlement can be explained by considering the following electrochemical cell
(Anodic site) Conc NaoH || dil NaoH (Cathodic site)
Iron at Joints &bends Iron at the Plane surface the anodic portion undergoes corrosion and gets dissolved.
Prevention Methods:
By using sodium phosphate as softening reagent in the external treatment of boiler water.
By maintaining the pH value of water and neutralization of alkali.
By adding Tannin or lignin or Sodium Sulphate to block the hair cracks thereby preventing the infiltration of caustic soda solution.