Corrosion in Tube Bundles

Corrosion in the boiler system is a phenomenon that costs the industry billions of dollars per year and leads to reduced performance from steam generating systems. Also known as stress corrosion cracking, tube corrosion is caused by several different factors. An understanding of the different causes of corrosion will help you find and apply the most effective solution.

Caustic Embrittlement

Caustic embrittlement is characterized by intercrystalline cracking of steel. It’s caused by a combination of mechanical stress, leakage, and the presence of sodium hydroxide — a highly caustic metallic base and alkali salt. The areas of a tube that are most vulnerable to caustic embrittlement are high-stress zones with leakage pathways, such as where the tubes are rolled into sheets, drums, or headers.

Prevention tip:

Add sodium nitrate in proper proportions to boiler water, as recommended by the U.S. Bureau of Mines or water treatment specialists, to mitigate this condition.

Transgranular Stress Corrosion Cracking

This type of cracking occurs along the grain structure of the metal due to cyclic stress — stress that varies over time due to temperature or pressure fluctuations. In boilers, cyclic stress occurs when disparate pockets of hot gases accumulate across the face of the tube sheet. According to Veolia Water Technologies, this type of cracking is the most common form in industrial boilers, and it often results in loose or leaky tubes over time.

Prevention tip:

Always operate the boiler within its engineered temperature and pressure specifications.

Steam Blanketing

Steam blanketing is a phenomenon in which steam and water separate when flowing through a given tube, forming distinct layers rather than a homogenous mixture. This imbalance causes uneven cooling and heating across the tube wall, which over time creates a visible waterline and gradual thinning along the upper portion of the tube. Steam blanketing typically occurs because the velocity of water and steam inside the tube is not strong enough. It is most likely to develop in areas of the tubes with low heat input and inefficient circulation.

Prevention tip:

Ensure proper boiler circulation and maintain sufficient fluid velocity.

Dissolved Oxygen

Oxygen is a well-known corrosive agent for steel, and even trace amounts in boiler feedwater can lead to pitting if it breaches the layer of oxide film that often coats boiler tubes. This will eventually weaken the tube wall and cause premature failure.

Prevention Tip:

Use oxygen scavengers such as catalyzed sodium sulfite or hydrazine, employ deaerators to mechanically strip dissolved oxygen from feedwater, and regularly monitor feedwater quality to maintain ideal chemical balance.

Chelant Corrosion

Chelants are organic compounds used to bind minerals in boiler water and keep them in solution form. However, overuse of chelating agents or poor monitoring can lead to thinning, particularly around stress points like rolled tube ends, threaded members, or baffle edges.

Prevention tip:

Maintain a balanced water treatment program and monitor chelant concentrations to prevent over-treatment.

Copper Deposits

Copper oxides, commonly found in system deposits, can dissolve into cleaning solutions during acid cleaning procedures. If the cleaning is not followed by proper rinsing and passivation, this dissolved copper may replate onto the steel surfaces of tube banks, creating localized anodic corrosion areas. These can lead to rapid pitting and deterioration of the tube material.

Prevention tip:

Use copper-removal solvents after chemical cleaning to prevent copper redeposition.

Conclusion

Corrosion can be induced by several different factors. In addition to the types of corrosion mentioned above, tube bundles can also be affected by hydrogen attacks, acid attacks and caustic attacks. In some cases, corrosion might also simply be a result of boiler design problems. The best practices for removing and preventing damage from corrosion vary, depending on the type of corrosion and specific vulnerabilities of the boiler tube itself. Generally, the best way to keep your boiler tubes free of corrosion, however, is through proper maintenance, control and monitoring.

Use these tips to keep your tube bundle running until old age, and when you finally need a replacement, give our engineers a call at 770-863-0165.

Disclaimer:

TubeBundle.com provides this information for general reference and promotional purposes only. It is not professional advice nor is it a substitute for professional advice, diagnosis, or service. While we strive to provide accurate and up-to-date information, we cannot guarantee that the content is free from errors, omissions, or inaccuracies. Always consult a licensed technician or refer to the official manufacturer specifications and documentation when addressing any issues with your boiler or related equipment.