Hull Protection Systems
Pipework Fouling Protection
Anti-fouling systems help avoid problems of blockages in water intakes. The main cause of the problem are barnacles and mussels which are drawn into the intake of vessels as larvae and then attach themselves to the surfaces of pipes where they grow and multiply.
There is also considerable evidence that marine growth accelerates corrosion. Previously the only remedy has been mechanical removal and the replacement of damaged parts. Preventive measures have been used such as chlorination, chemical dosage or an electrolytic system.
The electrolytic system consists of pairs of copper and aluminium or iron anodes which are mounted in the ship's sea chest or strainer, and a control panel, either an LED panel or an analogue digital meter showing the output of each anode.
In operation a dc current is applied to the copper anode which produces ions which are then carried throughout the pipework system by the seawater flow. Although the concentration of copper in solution are said to be extremely small- less than two parts per billion- they create an environment where marine organisms cannot settle or multiply. This also gives continuous protection to valves, condensers engine cooling systems and ancillary equipment.
A second anode is employed to combat corrosion. All metals have layers of protective oxide films which are prone to breakdown by natural means. Seawater which is carrying corrosive agents such as sulphur, can break down these protective films on all metal surfaces. Soft iron anodes are used for protection of alloyed pipework such as yorcalbro ( an aluminium brass).
This involves metering in quantities of an anti- foulant into the sea water boxes. A typical chemical is Ferrous chloride which as a by product coats the pipework with a protective ferrous layer.
Ultrasonics are said to have a two fold effect on anti-fouling: a disturbance action, caused by the high frequency waves, which renders the habitat unacceptable and a mechanical action, which operates on organisms trying to deposit adhesive, by preventing it from solidifying and on already anchored organisms of 4-5mm. A reduction in fouling of as much as 80% is claimed.
A generator produces and then sends electrical impulses at high frequency via a coaxial cable to transducers mounted externally to the sea chests or strainers. Each transducer contains a piezoelectric ceramic crystals, which when excited by the electrical impulses generate the ultrasonic beam. Power levels are said to be low, with an input of around 300-600w for each generator, which can supply four transducers. The main advantages of this system is that it is non-invasive, no parts are in contact with sea water so require replacing, and that no toxic substances are produced.
Chlorine is used as an effective pollution control. However, its application raises difficulties in the form that it is used
Chlorine gas is highly toxic and attacks the mucous membranes in the repiratory tract
Anhydrous liquid chlorine has a very high thermal coefficient of expansion and places high hydrostatic loading on container. Also, it is a vigoorous oxidising agent and can cause instant auto-oxidation of metal surfaces when ignited by a spark. Water in trace amounts can lead to rapid corrosion of the container. The release of a 50 ton tank requires evacuation of a 5 mile radius.
Sodium Hypochlorite is available as a 15% high concentration liquid manufactureed
by chemical industry
It is odourless and requires no special handling. However the economics of use is poor. A sewage plant requireing 6 ton of chlorine per day would require 83 tonnes of sodium hypochlorite (13-15% solution).
Bulk storage is impractical due to the 100 day half life. On site production removes the costs of transportation.
Method of operation
Titanium is used as the cathode material as it is Electrochemically inert at postive voltages less than 9volts. For the anode the titanium is coated with 100micro inches of platinum. This layer is consumed at a rate of 6 mg/ampere per year giving a life expectancy of 3 years.(Note this layer degrades much more rapidly if the unit voltages and currents are not set correctly)
The Anode/Cathode voltage is 7v
Chlorine is generated a the anode along with other elements to form NaOCl ( sodium hyperchlorite). Large quantites of hydrogen are produced which must be safely evacuated.
10pp chlorine in sea water will kill all marine life quickly, 1 PPM will prevent fouling. This may be tested on board.
The total output of chlorine is a function of current rather than flow through the unit, adequate flow is required to ensure cooling and to prevent calcerous deposits.
A typical 1 Kg unit requires a minimum flow of 100 litres per minute. Less than this will mean regualr acide cleaning is required, less than 50Litres per minute will lead to overheating and heavy fouling. Cell damage occurrs at greater than 9v, high voltage alarm/shutdown occurrs at 8 v.
This system is designed to be used in sea water only and not in fresh water.
I have sailed with all these methods and each has proved to be very effective in the control of fouling. The easiest to use by far was the electrolytical unit although the cost of replacement copper anodes ( which are quite some size) is prohibitive.
Like all equipment careful monitoring is essential. I joined one vessel and on walkround with my relief noted that the output from the Chlorinator unit was incorrect. After getting the unit operational, which involved the replacement of cells which has been damaged due to incorrect voltage being applied across them, there followed several weeks of cooler cleaning as the growth in the pipework died off.
Different chemical structure, paint thickness', aeration etc. can lead to one area of the hull becoming more cathodic than another.
Electrons will flow from the anodic area through the hull to the cathodic area. By hydrolysis negatively charged hydroxyl ions will form. At the anode electron depletion leads to positively charged Iron ions. Hydroxyl ions migrated through the water to the anode, here combining with the iron ions to form Fe(OH)2 which combines with dissolved oxygen to form Fe(OH)3 or rust. In this way the anodic area will corrode. To prevent this it would be necessary to make the entire hull cathodic
The anode is insulated from the hull, electrical connection is via cable and ships side gland box. It may be made of lead or Platinised Titanium. With the lead anodes, the hydroxyl ions turn the surface of the lead a rich brown colour (PbO2).
A D.C. voltage is applied to just overcome the natural galvanic voltage. If the current is allowed to become too great then the increased Hydroxyl release causes sponginess and flaking of the paint
The cathodic system should make the hull 200mV more cathodic i.e. 2oomV negatively charged. The system measures this by checking the hull voltage against an insulated reference anode which has a known value of galvanic voltage with the hull material. Typically this may be Zinc which is normally at a voltage 450mV more negative than the hull, or Silver which is 600 mV more positive than the hull. The Cathodic protection system will try to make the potential difference between the hull and the zinc reference anode 250 mV (Zinc anode 250mV more negative than the hull), and the silver anode 600mV(Silver anode 800mV more positive than the hull).