It's no sacrifice

It wasn't a pretty sight on the local slipway. When the prop was finally persuaded to depart the outboard leg shaft it came off in several pieces, because the hub had been eaten away. The owner had saved himself some money over the past two years on the cost of replacement anodes, but was now faced with a hefty bill for new prop, plus the cost of the anodes he should have put on in the first place.

The purpose of sacrificial anodes is exactly that: sacrifice. The metal in the anode wears away, rather than the metal in the critical boat and motor components.

Many boat owners don't understand the reason why metal components corrode, sometimes very quickly, when they're supposedly made for marine use. So why can't boat designers make components that don't corrode?

GALVANIC CORROSION
We're so used to seeing metals in their refined, shiny state that it's easy to forget that they don't exist in nature like that, with the exceptions of gold and some other metals that can be broadly grouped under the collective titles of 'unobtainium' or 'expensivium'.

Most metals exist in the natural world as compounds, such as oxides, chlorides, nitrates and sulphides. If left unprotected, refined metals will undergo a chemical process to eventually return to that natural, unrefined state.

So, Chemistry 101: all matter, including metal, is made up of atoms, which contain electrons. The process speeds up if metals are immersed in an electrolyte: a liquid that contains ions. Ions are atoms, molecules or groups that have lost or gained electrons.

Galvanic corrosion (rust) on boats occurs when dissimilar metals are immersed in water and electron flow occurs.

As for an example of an electrolyte: well, salt water is a beauty.

Commonly-known metals are ranked in terms of 'nobility', with gold being at the top and zinc near the bottom. This nobility ranking refers to the metal's ability to resist galvanic corrosion.

The less noble the metal, the more easily it will corrode. If more than one type of metal is involved in the corrosion process, the least noble metal will corrode first. So, introducing an ignoble metal into the process stops the electron transfer between boat component metals that need to be protected. For example, if stainless-steel and bronze components are immersed in seawater, the less-noble bronze will surrender its electrons to the seawater, corroding away in the process. The stainless steel is protected. But, add a third metal to the process, say, the even less-noble zinc, and the zinc electrons will bleed off before the bronze ones do, so both the bronze and the stainless steel are protected.

As far as we can ascertain, Sir Humphry Davy, the man who pioneered electrochemistry and also proved that diamonds were made of carbon, initiated the use of sacrificial anodes. Sir Humphry fitted zinc plates to the hulls of British Navy vessels in 1824 to prevent corrosion of fastenings and fittings.

Historically, zinc was the first material to be used as a sacrificial anode because it was the simplest and cheapest material available that would do the job.

Magnesium gives the highest voltage protection but has a very short life and needs to be used carefully.

Aluminium, which lasts five times longer than magnesium, also outlasts zinc and is a more active metal; but pure aluminium is useless as an anode material because it immediately forms an oxide layer that insulates it and stops it from working effectively as an anode.

The alloy anode arrived relatively recently, made of aluminium with zinc and indium added to eliminate the formation of the insulating oxide coating.

Aluminium alloy has a greater negative voltage when immersed in water and creates a larger voltage difference between it and the threatened metal, to ensure proper protection.

Although aluminium alloy lasts longer and provides better protection, zinc remains the most common anode metal.

CORRECT ANODE USE
The photograph of the wooden boat with anodes each side (far left) illustrates one of the misconceptions about anode use. You can see that the anode screwed into the plank near the top of the propshaft skeg has corroded very little, but the lower one, mounted on the metal keel, has corroded away entirely.

To be effective, a sacrificial anode must be in metal to metal contact with the item it is intended to protect.

That's why propeller anodes are bolted to the prop casting or clamped to the shaft, and metal rudder anodes are bolted directly to the rudder metal.

A 'fish' type anode that's hung over the side of a vessel will protect an underwater metal component only if it's connected to that metal by a wire.

It's also important that there's bright metal contact between the anode and the metal it's protecting. An anode won't work effectively if it's insulated from the threatened metal by layers of bottom paint.

Another common error is painting anodes: the paint insulates the anode from the necessary sacrificial galvanic action. Anodes must have a bright metal surface, free from any coating.

Most anodes hide underwater, but still need regular inspection for their attachment and integrity. Some are self-indicating, such as sacrificial trim tabs, but most give no warning of their depletion, sometimes until it's too late.

The number of anodes your boat needs to protect its underwater metal parts from galvanic corrosion can be calculated, or can be gauged by trial and error. Changes to through-hull fittings, rudder hinges, engine shafts and propellers or outboards can alter anode requirements of any boat.

The most effective way to determine whether a boat is fully protected is by measuring its cathodic protection voltage or hull potential using a voltmeter, but there are also some simple guidelines for selecting the right anode.

The hull material of a boat determines, in part, which anode material to use. Zinc or aluminium alloy anodes can protect a fibreglass boat that has an inboard engine and a bronze and stainless propshaft and propeller assembly.

The voltage generated by these anodes cannot 'overprotect': cause damage by having excessive anode material, because the maximum voltage generated is the voltage of the anode itself.

Magnesium works well on a fibreglass boat, but only in freshwater.

Aluminium- or wooden-hulled boats can be overprotected by very active magnesium. Steel hulls can also be overprotected and the excessive protection voltage will rapidly lift the paint off the hull.

Sterndrives and outboard motors, because of their very active aluminium assembly, are hard to protect. The anodes for these units used to be zinc, but corrosion problems have caused many engine manufacturers to fit aluminium alternatives. In some cases, using zinc may invalidate an engine's warranty.

Water type is another influential factor when choosing an anode. When used in freshwater, zinc forms a coating of zinc hydroxide that insulates the anode and stops it from working.

Magnesium anodes used in saltwater disappear very quickly and on aluminium sterndrives or outboards can cause hydrogen bubbles to form under the paint on the hull or drive and blow the coating off.

THE ELECTRICAL EFFECT
The galvanic corrosion of metals in contact with seawater can be complicated by the effects of onboard and shore electrical power. Intereference in the process can also be caused by 'bonding' wires that connect metal components of non-metallic hulls, by connections designed to earth radios, and by direct lightning strikes into the water.

Any current leaks into onboard wiring or bonding circuits can quickly negate the relatively weak currents running between sacrificial anodes and metal components.The classic electrical cross-over point is an onboard battery charger that is fed by shorepower.

A sure sign of current leakage or a poor dock earth is rapid depletion of a boat's sacrificial anodes. In extreme cases boats have been found floating in water that carried enough current to electrocute people who dived in.

Obviously, marina owners have a responsibility to ensure that shorepower wiring is in perfect condition, both to ensure effective anode sacrifical corrosion to protect boats, and to avoid dangerous current leakage.

INTERNAL ANODES
Newcomers to the boating scene may not be aware that some boats have internal anodes that protect mechanical components from galvanic corrosion. Seawater cooled engines and heat exchangers have sacrificial anodes, often 'pencil' types that screw into the water-jacketing.

These anodes corrode away and protect expensive engine components from galvanic action, but need to be replaced at specified intervals.

The protection level is similar to that afforded by coolant mixes in motor vehicles, but the chemistry is different.