This was the alarm that triggered the owner to ask for help. He had recently painted the shafts and props with antifouling, but after just a few weeks the paint came off. It did not necessarily mean it was due to galvanic corrosion – as it turned out there were also some cavitation issues on the props. But the alarm asked for a visit, and upon inspection it became apparent that the cathodic protection system was failing heavily.
A bonding circuit is essentially a system of electrical cables that are connected to all underwater metal components and fittings and eventually to the sacrificial anodes. The idea is to ensure that galvanic corrosion can only occur on the “weakest” metals, i.e. the zinc anodes, protecting from corrosion all other metal parts connected to the circuit: propeller shafts, seacocks, rudders, rudder shafts, trim tabs, p-brackets, propellers etc.
There are two main trends: the US and Australia prefer the bonding system. Here in Europe, it is preferred the all isolated practice: to fit an anode on each component independently, unless the electrical systems on the vessel become so complex that it pays linking all systems and components via a bonding circuit.
Connecting all different metals to a zinc anode is a good way of protecting them from corrosion due to galvanic currents: the anode takes on this responsibility, and it slowly consumes (or sacrifice) and in doing so saves all other metals a lot of trouble.
Although the system is very effective, it is also very sensitive to the quality of the cables and their connections: as soon as the electrical resistance between the anodes and a seacock or other components increases, the protection decreases. In extreme cases, which are not uncommon, the connection to the anodes is lost completely due to age or bad quality of cables.
In a system with bonding, when connection to the anode is lost, who takes the anode’s role? It can be any component, and normally these are the seacocks and propellers, which are lower in the galvanic scale: they will slowly start to protect all other metal parts connected to the bonding circuit, and in doing so they will loose electrons due to galvanic currents and the zinc within the alloy will start to consume, a process known as dezincification. Propellers can corrode very quickly, losing its zinc and debilitating the metal, and eventually they will need replacement.
The main thing is to keep an eye on the cathodic protection installation, being it bonding or individual anodes. Bonding cables that are 20 years old, that are not pre-stagnated, that may have suffered a lot of humidity because they have been laying in partly flooded bilges for a long time, are prone to fail and they need to be replaced. The connection to the anodes is the most critical part, and this has to be inspected regularly and renewed if necessary.
In the case of this vessel, the whole bonding cables and terminals needed replacement: cables were worn, did not conduct anymore and terminals were oxidised. The anodes have been renewed, seacocks replaced where necessary and the electrical conductivity of the system has been checked to ensure the quality of the new installation. The works have been supervised by the author and executed by a professional marine electrician.
Prevention of corrosion is achieved by simply maintaining the cathodic protection system working efficiently. The level of protection from corrosion can be checked both on the dry and on the water, provided proper instrumentation is used. Resistance measurements can be taken between the extremes of the bonding circuit, and should give values lower than 1,0 ohm for the system to be effective (preferably as low as 0,2 ohm). An accurate multimeter is needed for this. If the vessel is on the water, a measure of the voltage between a reference metal (normally an Ag/AgCl cell immersed in water) and any metal components on the vessel can be used to check that the cathodic protection is working effectively and protecting the metals – this is particularly necessary in the case of aluminium vessels, which need an effective system of cathodic protection to prevent the complete hull plating to corrode from the effects of galvanic action.
Other measurements can also be carried out: checks for leaks to earth, current flowing through the 220VAC earth cable or other issues can induce galvanic or stray currents corrosion on metal parts, and damage them very rapidly. The use of a galvanic isolator is often recommendable.
The best practice to prevent corrosion on the metal parts of a boat is to keep an eye on your cathodic protection system, renew the sacrificial anodes at the required intervals and verify their electrical connections. Inspect frequently the cabling and connections of the bonding system, and check all components for corrosion regularly. If still in trouble, calling in an expert can help you diagnose the problem and recommend solutions.
m.rossetti@cormorantmarine.com
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Italia
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