Boiler tube corrosion: How to start and how to prevent it?

Boiler tube corrosion is a common problem in boiler operation and may lead to equipment failure, leakage or even safety accidents. Corrosion types are diverse and the causes are complex, so they need to be analyzed and prevented according to the specific situation. The following are the main corrosion causes, corrosion types and countermeasures.

Common types and causes of boiler tube corrosion:

Corrosion in boiler systems is caused by the interaction of water chemistry, facility environment, operating procedures and system construction materials. Every year, boilers lose billions of dollars due to failures and performance degradation. To find a solution to the problem, you need to find the exact corrosion path and then control and avoid future corrosion problems.

1. Alkali corrosion (caustic embrittlement)

Phenomenon: cracks or intergranular corrosion, mostly appearing in welds or high stress areas.
Cause: High concentration of NaOH (pH>13) works together with stress. Over time, the concentration of sodium carbonate in the boiler increases. Sodium carbonate combines with water evaporation to form sodium hydroxide. This makes the water alkaline, resulting in alkali embrittlement, especially at elbows and joints. It is recommended to add sodium nitrate to boiler water to prevent alkali embrittlement.

2. Oxygen corrosion

Phenomenon: pitting or ulcer-like pits, mostly occurring in feedwater pipes and economizers.

Cause: dissolved oxygen in water is not completely removed, reacting with metal to form Fe₂O₃ (red rust). The oxygen concentration in water is the main cause of severe damage to the water side of boiler pipes, i.e. pitting. Water molecules enter the gas phase at higher temperatures, resulting in more water vapor in the air. Cold water can accommodate more air. A deaerator can be used to heat water with steam to remove oxygen. The most commonly used deoxidizer in low-pressure systems is sodium sulfite (Na2SO3).

Hydroserve Technologies, Inc suggests: "In systems equipped with deaerators, sulfite should be delivered to the deaerator's water storage tank, or to the suction or pressure side of the feedwater pump. In systems without deaerators, sulfite can be delivered almost anywhere in the feedwater system, including the condensate tank."

3. Acidic corrosion (pH is too low)

Phenomenon: uniform thinning, pits or localized ulceration, common in boiler drums and water-cooled walls.
Cause: Low feed water pH (<7) or accumulation of acidic salts (e.g., chlorides, sulfates). The frequency of pitting depends largely on the acidity of the water. Water treatment chemicals called volatile amines are derivatives of ammonia that can neutralize acids or form a protective film. Ideally, the pH should be kept above 9.5. These procedures are usually followed by a deoxidizer to completely remove residual oxygen.

4. Carbon dioxide corrosion

Phenomenon: Uniform thinning, more common in condensate systems.
Cause: CO₂ in water generates carbonic acid (H₂CO₃), which lowers the pH.

5. Under-deposit corrosion

Phenomenon: Local pitting or grooves, occurring in areas covered by scale or deposits.
Cause: Deposits hinder heat transfer, leading to local overheating and concentration of harmful substances.

6. High-temperature sulfur corrosion

Phenomenon: Occurs in high-temperature areas of the furnace (e.g., superheaters), with surface sulfidation.
Cause: The fuel contains high sulfur, generating SO₂/SO₃ that reacts with metals.

7. Electrochemical corrosion (contact of dissimilar metals)

Phenomenon: accelerated corrosion at the contact site.

Cause: the potential difference of different metals forms a primary cell.

Prevention and control measures for boiler tube corrosion:

1. Water quality control

Deoxygenation: use thermal deaerator or chemical deoxygenation (such as sodium sulfite).

Adjust pH value: maintain the pH of feed water at 8.5~9.5 (alkaline environment inhibits acid corrosion).

Control salts: regular sewage discharge to prevent the concentration of harmful ions such as Cl⁻ and SO₄²⁻.

2. Chemical treatment

Add corrosion inhibitors: such as phosphate (anti-scaling) and hydrazine (deoxygenation).
Regular cleaning: acid or alkaline washing to remove sediments (pay attention to corrosion protection).

3. Material optimization

Select corrosion-resistant materials: such as stainless steel (304/316) and aluminized steel pipes.
Anti-corrosion coating: ceramic or alloy coating is used in high temperature areas.

4. Operation and maintenance

Avoid low-load operation: reduce the risk of acid corrosion of condensate water.
Regular inspection: ultrasonic thickness measurement, endoscope detection of corrosion.
Shutdown protection: dry (desiccant) or wet (lye) maintenance.

When the boiler cools for any reason, the steam condenses. This causes a vacuum, which draws air into the system from any possible source. The best way to prevent this from happening is to regularly and thoroughly maintain the entire system.

5. Design improvements

Avoid stagnant areas of water flow: reduce sediment accumulation.
Optimize heat load distribution: prevent local overheating.

Notes:
Safety first: When corrosion is severe, the furnace must be shut down immediately to avoid tube bursts.
Comprehensive prevention and control: A single measure has limited effect and requires multi-dimensional management of water quality, materials, and operation.

By systematically analyzing the type of corrosion and treating it in a targeted manner, the life of the boiler tube can be significantly extended and the operational safety can be improved. If the problem is complex, it is recommended to entrust a professional organization to conduct failure analysis.

Read more: Which material is used for boiler tubes? or How is the corrosion resistance of boiler tubes?