Class 4 (4th Engineer) MEP 📅 Aug 2023

Exam Question

Water ballast tanks of dry cargo ships and cargo tanks of oil tankers may be protected from corrosion by sacrificial anodes.

(a) When do these anodes need replacement? (10)

(b) (i) State THREE suitable materials that can be used for anodes (3)

(ii) State any restriction on the use of these materials as anodes in oil tankers. (3)

Reference Answer

### (a) Criteria for Sacrificial Anode Replacement (10 Marks)
Sacrificial anodes require replacement when their capacity to provide effective cathodic protection to the tank structure is diminished. This is determined not merely by a single factor, but through a comprehensive assessment during scheduled surveys, typically aligning with the vessel's Intermediate and Special Survey cycles. The decision to renew is based on the following surveyor-grade criteria:
1. **Percentage of Mass Consumption (Wastage):**
* The primary industry and class society guideline for renewal is when the anode has wasted by **75-80% of its original mass**. While some manufacturers might suggest different figures, this range is a standard for survey purposes.

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