A drydock inspection of the stainless steel alloy propeller shafts has revealed serious pitting corrosion of the shafts in the region where the shafts pass through the stern tube. The stern tube arrangement consists of a shaft seal and a sea-water flooded stern tube with cutless bearing at the aft (sea) end.
(a) Describe the possible reasons for this corrosion.(6)
(b) Describe the modifications that could be made to the stern tube arrangement to reduce the likelihood of future shaft corrosion.(4)
Possible Reasons for Pitting Corrosion on Propeller Shafts
The discovery of serious pitting corrosion on a stainless steel propeller shaft in the stern tube area is a significant issue. Several factors could contribute to this:
1. Oxygen Depletion Corrosion (Crevice Corrosion):
- The area between the shaft and the stern tube creates a confined space where oxygen levels are depleted.
- Stainless steel relies on oxygen for its passive layer formation.
- Without sufficient oxygen, the protective layer breaks down, leading to pitting corrosion.
2. Electrochemical Corrosion:
- Galvanic corrosion: If dissimilar metals are in contact within the stern tube, a galvanic cell can form, accelerating corrosion of the shaft.
- Stray currents: External electrical currents can accelerate corrosion processes.
3. Abrasive Wear:
- Fine particles suspended in seawater can cause abrasive wear, creating micro-scratches that act as initiation points for pitting corrosion.
4. Cavitation:
- If the propeller or shaft operates at high speeds or under specific hydrodynamic conditions, cavitation can occur, leading to pitting.
5. Material Issues:
- The specific alloy of stainless steel used might not have been suitable for the marine environment.
- Defects in the manufacturing process could have weakened the metal’s resistance to corrosion.
6. Maintenance Issues:
- Inadequate flushing of the stern tube or improper sealing could have contributed to the problem.
To accurately determine the root cause of the corrosion, further investigation, including metallurgical analysis and hydrodynamic assessments, would be necessary.
Modifications to Stern Tube Arrangement to Reduce Shaft Corrosion
To reduce the likelihood of shaft corrosion in the stern tube area, several modifications can be implemented:
Material Selection
- Shaft Material: Consider using a more corrosion-resistant alloy for the propeller shaft, such as duplex stainless steel or titanium.
- Stern Tube Material: Evaluate the suitability of the current material and consider alternatives with higher corrosion resistance.
Design Modifications
- Seal Improvements: Upgrade the shaft seal to a more robust and reliable design, ensuring optimal sealing performance.
- Lubrication System: Implement a lubrication system for the shaft within the stern tube to reduce friction and wear.
- Shaft Coating: Apply a protective coating to the shaft surface to enhance corrosion resistance.
Operational Changes
- Increased Flushing: Increase the frequency and duration of seawater flushing through the stern tube to remove contaminants.
- Monitoring: Implement regular inspections and monitoring of the shaft and stern tube condition.
Cathodic Protection
- Sacrificial Anodes: Install sacrificial anodes in close proximity to the shaft to provide cathodic protection.
- Impressed Current System: For severe corrosion conditions, consider an impressed current system to protect the shaft.
By implementing these modifications, the risk of shaft corrosion can be significantly reduced, extending the lifespan of the component and improving vessel reliability.