Exam Question
Considering effect of corrosion, describe choice of material for following with reasons;
(a) Sea Water pump body. (4)
(b) Sea Water Pump Impeller. (4)
(c) Sea Water Pump Shaft. (4)
(d) Filter boxes (4)
Class 4 (4th Engineer)
MEP
📅 Jan 2023
Reference Answer
### Fundamental Principles of Material Selection for Seawater Systems
Material selection for components handling seawater is governed by the need to mitigate various forms of corrosion and mechanical failure. Seawater is a highly aggressive electrolyte due to its high chloride content, dissolved oxygen, and potential for microbiological activity. The key considerations are:
1. **Galvanic Corrosion**: Occurs when two dissimilar metals are in electrical contact in an electrolyte. The less noble metal (anode) corrodes preferentially. Materials should be selected to be close together in the galvanic series, or the less noble material should have a much larger surface area than the more noble one (large anode, small cathode).
2. **Erosion-Corrosion**: A combination of mechanical wear and chemical corrosion, accelerated by high-velocity or turbulent fluid flow which continually strips away protective oxide layers. This is critical in pump impellers and casings.
Material selection for components handling seawater is governed by the need to mitigate various forms of corrosion and mechanical failure. Seawater is a highly aggressive electrolyte due to its high chloride content, dissolved oxygen, and potential for microbiological activity. The key considerations are:
1. **Galvanic Corrosion**: Occurs when two dissimilar metals are in electrical contact in an electrolyte. The less noble metal (anode) corrodes preferentially. Materials should be selected to be close together in the galvanic series, or the less noble material should have a much larger surface area than the more noble one (large anode, small cathode).
2. **Erosion-Corrosion**: A combination of mechanical wear and chemical corrosion, accelerated by high-velocity or turbulent fluid flow which continually strips away protective oxide layers. This is critical in pump impellers and casings.
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