Corrosion in a marine auxiliary boiler occurs mainly on the water/steam side, although corrosion can also occur on the fireside. The important types of corrosion are as follows:

1. Oxygen Corrosion / Pitting

Cause:

Dissolved oxygen may enter the boiler through inadequately deaerated feedwater, air leakage, or during prolonged shutdown or storage.

Symptoms:

It produces localized deep pits, often accompanied by reddish-brown corrosion products. Although the overall metal loss may appear small, oxygen corrosion can cause severe localized penetration.

Consequences:

It can result in rapid tube thinning, boiler-tube perforation or rupture, causing loss of boiler water, forced shutdown and potentially serious safety hazards.

2. Acid Corrosion

Cause:

Occurs when the boiler/feedwater pH becomes too low or when acidic contamination enters the system.

Symptoms:

It generally causes uniform metal wastage, thinning and roughening of internal surfaces, rather than isolated deep pits.

Consequences:

Continued acid attack can reduce the thickness of boiler tubes and drums, resulting in leakage and eventual failure of pressure parts.

3. Caustic Corrosion / Caustic Gouging

Cause:

Caused by excessively high alkalinity, particularly when concentrated caustic soda becomes trapped beneath scale deposits or inside crevices.

Symptoms:

It produces irregular grooves or gouges in the metal and can damage or remove the protective magnetite film. Highly alkaline conditions can also cause caustic stress-corrosion cracking.

Consequences:

It can lead to localized deep metal loss, cracking and eventual boiler-tube failure.

4. Corrosion Under Deposits / Concentration-Cell Corrosion

Cause:

Scale, sludge and other deposits can create stagnant areas where corrosive chemicals become concentrated. Differences in oxygen concentration between the area under the deposit and the surrounding water can also create a concentration cell.

Symptoms:

Localized pitting or grooving may occur beneath deposits. The deposits can conceal the damage, making it difficult to detect during routine inspection.

Consequences:

Deposits reduce heat transfer and can cause local overheating, while also promoting rapid tube wastage and eventual tube failure.

5. Hydrogen / Acid Attack

Cause:

Severe acidic conditions can generate hydrogen, which may enter the steel and cause internal damage.

Symptoms:

There may be little obvious external evidence of the damage, while the metal gradually loses strength and may become brittle.

Consequences:

Hydrogen/acid attack can result in cracking, loss of mechanical strength and potentially sudden failure of boiler pressure parts.

The following measures should be implemented to control corrosion and maintain the efficiency and reliability of the auxiliary boiler:

1. Maintain Correct Boiler-Water Chemistry

Regularly test and control:

• pH and alkalinity
• Phosphate level
• Chloride concentration
• Conductivity/TDS
• Dissolved oxygen
• Chemical-treatment residuals

The required limits should always be maintained according to the boiler manufacturer's instructions and the vessel's water-treatment programme.

Correct control of pH and alkalinity is essential to prevent both acid corrosion and caustic attack.

2. Ensure Effective Deaeration

Dissolved oxygen should be removed through the feedwater/deaerating system, and the correct oxygen-scavenger dosage should be maintained.

This is particularly important because even a small amount of dissolved oxygen can cause severe localized pitting.

3. Apply Correct Chemical Treatment

Use the prescribed boiler-water chemicals, such as:

• Phosphate treatment for hardness control
• Oxygen scavenger
• Alkalinity/pH control chemicals
• Appropriate sludge/dispersant treatment

Chemical dosage should be determined from regular boiler-water analysis rather than by simply adding a fixed quantity.

4. Carry Out Regular Blowdown

Perform bottom and/or surface blowdown, as required, to remove:

• Concentrated dissolved solids
• Sludge
• Precipitated impurities

Blowdown helps control excessive TDS/conductivity and reduces the possibility of corrosive chemicals becoming concentrated beneath deposits.

However, excessive blowdown should be avoided, as it wastes treated water and chemicals.

5. Prevent Contamination

Prevent seawater, cooling water, oil and other contaminants from entering the boiler and feedwater system.

Regularly check condensers, heaters, feedwater systems and condensate returns for leakage.

Particular attention should be given to chloride contamination, as it can seriously disturb boiler-water chemistry and promote corrosion.

6. Keep Heat-Transfer Surfaces Clean

Regularly inspect and clean:

• Water side: remove scale and sludge.
• Fireside: remove soot and other deposits.

Clean heat-transfer surfaces improve boiler efficiency and reduce the possibility of local overheating.

Deposits can also promote localized corrosion and concentration of corrosive chemicals, so keeping the surfaces clean is important for both efficiency and corrosion control.

7. Carry Out Regular Inspection and Testing

During planned maintenance:

• Inspect boiler drums and tubes internally.
• Check for pitting, grooving, cracking and wastage.
• Measure tube and plate thickness where appropriate.
• Inspect burner and furnace surfaces.
• Test safety valves and boiler mountings.
• Examine areas around tube ends and welds.

Early detection of corrosion allows damaged tubes or other components to be repaired or replaced before failure occurs.

8. Proper Lay-Up During Shutdown

A boiler left idle with air and moisture present is particularly vulnerable to oxygen corrosion.

For extended shutdown periods, follow the manufacturer's recommended wet or dry preservation procedure, ensuring that oxygen and moisture are excluded as appropriate.