Sacrificial anodes – martyrs in the galvanic protection cause
Sacrificial anodes are often the first and best defence against galvanic corrosion – a subject covered in a related article which you might find helpful to read first. Once you understand the process you will appreciate why the phrase ‘sacrificial anode’ is very appropriate, since it very succinctly describes the way they work.
We know that when two electrodes (one ‘anode’ the other ‘cathode’) are immersed in an electrolyte (in our case seawater) a tiny electrical current will flow from one to the other and the anodic electrode, whatever it happens to be, will slowly waste away. If both electrodes are significant objects – say an aluminium drive leg and it’s stainless steel attachment bolts – then the loss of either one could become hugely expensive. The truth is that the galvanic process doesn’t care a darn how much something costs, it only knows which component is more anodic than the other. Unfortunately, in our example, it’s the intricately engineered drive leg that suffers damage, leaving the cheap-as-chips bolt virtually unscathed.
So, the unenviable job of the sacrificial anode is to be more anodic than anything else in the vicinity. In doing so it sacrifices itself in lieu of something more eminent. Of necessity, the anode material must lie towards the least noble end of the galvanic scale. Zinc is the most common choice but, in some circumstances, aluminium and magnesium are used.
Anodes come in various forms. Some are bolted to the hull (see below) and bonded internally to vulnerable components (engines, rudder stocks and P-struts being notable) and some are attached directly to the object they seek to protect (shafts, trim tabs, propellers etc).
Impressed current protection
Yet 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, where the frequent replacement of anodes is impracticable. On pleasure craft they are used chiefly to protect highly vulnerable machinery such as aluminium alloy encased drive legs and jet drives.
Planning any cathodic protection system should be done thoughtfully and with restraint. Take expert advice if in doubt. 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.
The irony is that over-protection can do more harm than good. If components on your boat aren't apparently suffering any corrrosion problems, leave well alone!
Talking of which…
Care should be taken not to over-protect wooden hulled vessels. 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 – a technique used in the making of wood pulp for paper). 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. Even if you’re scrupulous about guarding your own electrical systems against leakage (a future article), you can still fall victim to the carelessness or misfortune of others. Marinas are notorious breeding grounds for rogue currents, either from neighbouring boats or from the pontoons themselves. Other people’s problems can become your own – unseen and often expensive.
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