CAUSTIC EMBRITTLEMENT

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Caustic Embrittlement ( Caustic Stress Corrosion Cracking) is the phenomena in which the material of a boiler becomes brittle due to the accumulation of CAUSTIC SODA (Sodium Hydroxide).

 Sodium Carbonate is used in softening of water to prevent scaling, due to this – some sodium carbonate maybe left behind in the water. As water evaporates in the boiler, the concentration of sodium carbonate is increases in the boiler. As the concentration of sodium carbonate increases, it undergoes hydrolysis to form Sodium Hydroxide at temperatures of 200 to 250°C.
Na2CO3 + H2O → 2NaOH + CO2
The presence of Sodium Hydroxide makes the water alkaline . This alkaline water enters to minute cracks present in the inner walls of the boiler by capillary action. Inside the cracks, the water evaporates and concentration of sodium hydroxide increases. This sodium hydroxide attacks the surrounding material and dissolves the iron of the boiler as SODIUM FERRATE (Na2Fe2O4) or Rust. This causes Caustic Embrittlement.

CAUSES:

There are many causes of caustic embrittlement, including the combined action of the following three components:
1. Usage of material like carbon steel
2. Alkaline chemicals such as concentrated NAOH
3. Tensile stress ( for example- around the riveted holes)

PREVENTION:

Caustic embrittlement can be prevented through several methods, including:
1. Controlling the temperature
2. Controlling the stress levels(Residual Or Load) and hardness
3. Use of materials that do not crack when used in given environments
4. Avoiding alkali where it necessary
5. Replacing Sodium Carbonates with Sodium Phosphate as softening reagents
6. Adding Lignin, Tannin or Sodium Sulphate that blocks hairline cracks as well as preventing infiltration of sodium hydroxide into the areas.

Note: Tannins and lignins are organic compounds found in plants and trees, particularly in bark, leaves, and seeds.  By volume, 25-30% of pine needles are composed of tannins and lignins, for instance. When these plants decompose in the environment, the hardy tannic and lignic enzymes are among the last to break down (due to bacterial resistance), and they give many water bodies and streams a naturally rusty color.