Steam boilers are paramount for hassle-free process operations in manufacturing and process facilities. It is prudent for the operating personnel to ensure that the steam boilers are not facing any issues that would affect the operational purposes of the process plant. Regular maintenance and checking the machinery parts are important for optimal productivity of the steam boiler and safety of the processing unit. Boiler issues such as corrosion, scaling, water carryover, priming, framing, etc., are inevitable and require prompt action before it causes far-reaching loss and harm. One such issue that can severely impact the functioning of the steam boiler is caustic embrittlement.
What is Caustic Embrittlement?
Caustic embrittlement is a phenomenon that takes place in a steam boiler when the caustic substances gather in the materials of the boiler. It is also characterized by the cracking of riveted mild steel boilerplates. Caustic embrittlement takes place at the temperature range of 200°C to 250°C. It is caused due to the local deposition of concentrated hydroxide.
Caustic embrittlement mainly occurs on the stressed parts of the boiler, including cracks, joints, bends, and rivets. The residual sodium carbonate that is utilized for the softening process goes through hydrolysis. As a result, it forms sodium hydroxide at high pressure and temperature. Caustic embrittlement is also termed stress corrosion cracking.
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 that 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 are 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
Conclusion:
Rakhoh Boilers have emerged as a leading steam boiler manufacturer in Pune since its inception in 1983. With extensive knowledge and expertise in industrial boiler manufacturing and thermal solutions, we have successfully installed 3000+ boilers in over 26 countries worldwide. We have catered to 20 process industries by providing an efficient and reliable range of steam boilers, waste heat recovery systems, thermic fluid heaters, and boiler accessories. As a trusted boiler manufacturer, we provide the best boiler services like steam trap assessment, boiler automation, energy audit, annual maintenance contract, management, and training, etc.
To know more about our products and services, visit www.rakhoh.com