Anodes - how they work...

Essentially, when you submerse metallic items in any kind of water or conductive medium for any period of time, galvanic corrosion will occur due to differences in the potential (charge) on the metals. The amount of corrosion will depend on many factors, including the type of metal, conductivity of the water, surrounding electrical activity, and the bonding and type of metals which are in contact with the water.

To reduce or negate the effect of this corrosion, boat owners fit sacrificial "anodes" to their craft, which effectively concentrate the galvanic corrosion onto these anodes by offering these destructive galvanic currents a more favoured target. When bonded with a metal with less favourable galvanic properties (or a lower cathodic index), the anodes (which have a higher cathodic index) will naturally create a more favourable electrical potential and therefore protect the more valuable metal parts (propellers, sterndrives and the like). This property is even true when considering the application of paint & antifouling, as copper-based antifoul, if applied directly to a metal substrate, may cause similar problems.

It is obvious that the rate of decay of anodes will vary considerably, and even from boat to boat in the same area. It is therefore essential that you check the decay ("wear") on the anodes periodically, and ensure that there is no possibility of them "wearing out" and offering no protection. A yearly inspection is advised, though if problems have been experienced in the past, more frequent checks may be necessary. If checking when the boat is lifted out for annual maintenance,, if any specific anode has more than 60% wear, it is normal to have it replaced, considering that it will probably be as long again before the next opportunity.

It is important to realise that the effectiveness of the sacrificial anode will depend on how well it is bonded to the material it is protecting. This is true in the case of fitting shaft anodes, where the surface area of contact between the anode and the shaft may be small in comparison to the surface area of metal being protected. Be aware also that loose or badly fitted anodes will have a reduced capacity for protection, and that you should never paint over an anode, nor in between the contact surfaces where it is fitted.

Anodes are manufactured from 3 different types of metal - Zinc, Aluminium & Magnesium. These metals are chosen for their cathodic index and also for their other properties that make them suitable. The metal chosen for your particular protection will depend on where the boat is situated and its area of use. As a general guide:

Salt Water - Zinc or Aluminium
Brackish Water - Aluminium
Fresh Water - Magnesium

It is vital that you choose the correct type of anode as if used in an environment outside that stated, the ability of the anode to work will almost certainly be compromised. If you are likely to use your boat in waters other than those for which the current fitted anodic protection is designed for more than a few days (2 weeks maximum), you should consider fitting the correct type of anode or the water s you are visiting.
Zinc and Aluminium anodes, whilst in fresh water, will develop a white oxidisation, which will effectively prevent the anode from operating. Similarly, magnesium anodes are more reactive than Zinc/Aluminium and when used in Salt/Brackish waters will decay far more rapidly and often cause deposits on other protected surfaces.