Impressed Current Cathodic Protection System
The Aquamatic ® system comprises a number of anodes, reference electrodes and a transformer/rectifier controller power unit. A drawing of a typical installation, which applies to both small and large vessels, is shown in Fig 1. The types and sizes of these components and their positions on the hull are specified according to design parameters, which allow for the size of the vessel and fluctuations in the protection current likely to be encountered during sea-going service.
Whilst these measures protect the hull structure, it is also important to protect the propeller, exposed shafting and the rudder. The propeller and shafting are grounded to the hull structure with a shaft slip ring, making them electrically common with the hull, and the rudder is also grounded with a flexible cable. In this way, electrical continuity is established, ensuring that these components are protected by the Aquamatic system.
The anodes used in an Aquamatic® system may be of a Lead/Silver alloy or a Platinized/Titanium metal, both of which are relatively inert. AII types of anode are provided with a cofferdam, incorporating a double gland assembly approved by the Classification Societies, to ensure a watertight hull penetration for the anode cable.At least two and sometimes four reference electrodes are fitted in the Aquamatic system and these can be surface mounted or recessed. Both types incorporate a block of high purity zinc which, because of its robust construction and stable electrochemical characteristics, is an ideal material for reference purposes.The Controller Power Unit is self-contained in a cabinet situated in the engine room or other convenient space. It includes solid state circuitry with PCB's for easy maintenance. Power supply is from the ship's AC mains, which is transformed and rectified to produce the DC current required for cathodic protection. Full electrical protection is provided on input, whilst output fuses are fitted in each anode circuit. AII components are readily accessible within the drip-proof steel enclosure.The propeller shaft slip-ring assembly, ensures good electrical contact between the propeller and propeller shaft and the ship's hull. This, in turn, inhibits dezincification of bronze propellers, and provides a level of protection to propellers made of other materials. It also protects the shaft bearings from corrosion.
The assembly comprises a silver inlaid copper band, clamped to the shaft, and a brush assembly. High silver content brushes running on the silver strip provide electrical continuity with the hull.
Rudder stock bearings do not give adequate electrical continuity between the rudder and the hull, so in order to provide a bond which will ensure cathodic protection for the rudder, a flexible cable is secured between the rudder stock and the deck head (see figure 3).
Typical Aquamatic® installations are shown in Fig 1. That for 'aft' systems is applicable both for small and large vessels, whereas the forward system applies only to large vessels whose length necessitates the use of a supplementary system near the bow section. For forward systems it is necessary to recess anodes to protect them from mechanical damage and to reduce drag.Once commissioned, an Aquamatic® system operates automatically, requiring no adjustment. However, to confirm that the system is functioning correctly, log sheets are provided, on which daily recordings of operational readings from the control unit are made. The log sheets provide a continuous record of the level of protection and data from which any malfunction of the system and the state of the underwater coating can be assessed.Completed log sheets returned to Marpo are used to evaluate system performance. Possible malfunctions requiring the attention of a service engineer are identified at an early stage. Our group worldwide network of offices and agents ensures that service calls are made quickly and economically.
Although the reliability of the Aquamatic ® system is well proven, we recommend that installations are serviced on a routine basis during dry docking by our trained personnel. Stocks of Aquamatic components are maintained in all Wilson Walton offices to ensure delivery at short notice when required.