It's useful to know the difference between galvanic and electrolytic corrosion but equally useful to know how to protect against them.
Galvanic and electrolytic corrosion – the damaging duo
In discussions on corrosion of metal objects the words ‘galvanic’ and ‘electrolytic’ are frequently misunderstood. So let’s start by defining the two.
• Galvanic corrosion takes place when two dissimilar metals are immersed in an electrolyte and connected by a conductor. This creates a ‘galvanic cell’ ‒ basically a very simple form of battery in which the negative cathode remains intact while the positive anode corrodes away. A stainless steel stanchion in a cast aluminium socket is one example. A bronze prop on a stainless steel shaft another. All it takes is a little seawater to act as an electrolyte to start them fizzing.
• Electrolytic corrosion requires an electric current from an external source ‒ often the result of accidental leakage. In this case the metals can be identical, since the potential difference is arises from the current itself, depending in which direction that current flows. Yet again the anode becomes the unfortunate victim.
Sacrificial anodes: The word ‘anode’ is familiar to most boat owners ‒ usually preceded by the word ‘sacrificial’. The ploy is to introduce a strongly anodic metal which sacrifices itself to protect objects more cathodic (and valuable!) than itself. The chosen sacrifice – zinc usually, or in some cases aluminium – comes in various forms. Some are bolted to the hull and bonded internally to vulnerable components, such as engines, shafts and P-struts, and some are attached directly to the object in need of protection (shafts, trim tabs etc.).
Impressed current: Another method employs deliberate electrolytic intervention. The lowly zinc is replaced by a nobler metal and a small current is introduced to counter the direction of the galvanic flow. Impressed current systems are very popular on large commercial vessels. On pleasure craft they are used chiefly to protect highly vulnerable machinery such as aluminium alloy drive legs and jet drives.
Planning any cathodic protection system should be done intelligently. Isolated components such as bronze seacocks (when secured with bronze bolts or attached to bronze through hull fittings) shouldn’t be connected, because to do so might create galvanic cells where none existed before.
Also, care should be taken not to over-protect wooden hulled boats. Traditional boatbuilding timbers such as oak and mahogany are naturally acidic but localised galvanic action can make them alkaline, destroying the lignin that binds the fibres together (the process is known as delignification). Softening of planking around anodes and seacocks is a very common fault caused by this process.
Anodes will also provide protection against electrolytic attack but the length of time they can hold out against such assaults depends on the strength of the currents involved. Once any anodes have been depleted, the most anodic components that remain will be targeted next.
And it needn’t be your fault. Marinas are notorious breeding grounds for rogue currents, either from neighbouring boats or from the pontoons themselves. Other people’s problems can become yours – unseen and often expensive.
The illustration below shows two yachts, both properly connected to the shore power. In compliance with recommended practice, each has its AC and DC grounds bonded together. Neither is actually drawing power from the pontoon but this doesn’t matter. The earth wire continues to link them together, binding them into a galvanic relationship in which the most anodic boat will suffer damage. Leakage currents will make the situation even worse.
Luckily, it’s possible to block the path for tiny DC currents to escape while still allowing full AC fault currents to run safely to earth – yet few boats fit such protection as standard. There are two ways of achieving this:
• Isolation transformers: Although heavy and relatively expensive, these should be seriously considered for larger vessels with elaborate AC systems and those with conductive hulls – i.e. aluminium, steel or carbon fibre composite. As their name suggest, isolation transformers break all direct connection with the shore supply. The power is transferred by induction from one coil to another. You should still fit a ground plate to maintain the neutral at earth potential but at least there are no polarity problems since isolation transformers don’t care which way round the shore power arrives.
• Galvanic isolator: Lighter and less costly, these are a good choice for simpler systems. A pair of diodes are arranged in parallel. An AC current will pass straight through – in or out – while a low voltage DC current will find one diode barring its way entirely while the voltage drop (typically about 0.7V) across the diode will negate its effect if coming in the opposite direction.