Question

(a) What are the various types of corrosion that can occur in auxiliary boilers on ships? Describe each type, including its causes, symptoms, and potential consequences. (8)

(b) Describe the preventive measures and maintenance practices that can be implemented to mitigate corrosion and ensure the efficiency of the auxiliary boiler. (8)

Answer 1

Part (a)

TYPES OF CORROSION IN AUXILIARY BOILERS

Oxygen (pitting) corrosion: Caused by dissolved oxygen in the feed water. Symptoms: pitting on the water side of the drum, tubes and headers. Consequences: localised thinning, tube failure, leaks.
Caustic (alkaline) corrosion/ caustic embrittlement: Caused by high caustic concentration (high pH) in the boiler water, often under scale/ deposits. Symptoms: cracking (caustic embrittlement) at riveted/ stressed areas, thinning. Consequences: cracking, failure of the drum/ tubes.
Acidic corrosion: Caused by low pH (acidic) boiler water, often from contamination (e.g. sea water, or breakdown of the water treatment). Symptoms: general thinning/ wastage of the water side. Consequences: loss of metal, tube failure.
Galvanic corrosion: Dissimilar metals in contact in the boiler water. Symptoms: preferential corrosion of the more anodic metal. Consequences: localised wastage.
Corrosion fatigue: Caused by cyclic thermal/ mechanical stress combined with a corrosive environment. Symptoms: cracking at stress concentrations. Consequences: tube/ drum failure.
Stress corrosion cracking: Caused by tensile stress in a corrosive environment (e.g. caustic). Symptoms: cracking. Consequences: failure.
Fire-side (external) corrosion: Caused by combustion products (sulphur, acids) condensing on the fire side, especially at low load/ cold surfaces. Symptoms: pitting/ wastage of the tubes and shell on the gas side. Consequences: tube thinning, leaks.
Erosion-corrosion: High-velocity water/ steam eroding the protective film. Symptoms: localised thinning at bends/ inlets. Consequences: tube failure.

Part (b)

PREVENTIVE MEASURES AND MAINTENANCE PRACTICES

Water treatment: Maintain the correct boiler water chemistry (pH, alkalinity, dissolved oxygen, hardness, TDS) using the correct treatment chemicals (oxygen scavengers, alkalinity builders, phosphate/ polymer treatment) and regular testing.
Deaeration: Remove dissolved oxygen from the feed water (deaerator/ oxygen scavenger) to prevent oxygen pitting.
Blowdown: Regular bottom and surface blowdown to remove sludge, scale and dissolved solids.
Feed water quality: Use treated/ distilled feed water; prevent contamination (e.g. sea water, oil).
Corrosion inhibitors: Use the correct inhibitors in the boiler water.
Fire-side maintenance: Keep the fire side clean (remove soot/ deposits), avoid cold surfaces/ condensation, and maintain proper combustion to prevent acid formation.
Regular inspection: Inspect the boiler internally and externally (at survey) for corrosion, pitting, cracking and wastage; measure thickness.
Proper operation: Avoid overloading, maintain correct firing, and avoid rapid temperature changes (thermal shock).
Cathodic protection: Fit sacrificial anodes in the boiler where applicable.
Repair/ renewal: Repair or renew corroded/ wasted tubes and parts promptly.
Record-keeping: Maintain water-treatment and inspection records to track the boiler condition.

✓ Verified Model Answer (Text Solution)

TYPES OF CORROSION IN AUXILIARY BOILERS

PREVENTIVE MEASURES AND MAINTENANCE PRACTICES