Galvanic and Electrolytic Corrosion – The Metal Eating Menaces
There is often confusion between galvanic corrosion and electrolytic corrosion, with the latter usually getting the blame for havoc wreaked by the former. But the distinction is quite simple: galvanic corrosion describes the interaction between two different metals, whereas electrolytic corrosion is caused by an external source of current – often a leakage somewhere.
Electrolytic corrosion (right) is very common in marinas, where boats can find themselves linked together by the earth (ground) wire of their AC shore supplies. An electrical leak in one boat can cause untold damage in its neighbours.
By contrast, galvanic corrosion (right) is careless of its whereabouts. All it needs is the conjunction between dissimilar metals and an electrolyte – meaning a substance that conducts an electric current – in our case seawater, of which there’s typically plenty about. Unfortunately for boat owners a number of combinations, ripe for galvanic corrosion, abound. For example: bronze props on a stainless steel shafts, stainless fastenings on aluminium fittings (such as stanchion bases), and almost any kind of metallic attachment on carbon fibre spars.
A hierarchy of metals is arranged on what’s known as the ‘galvanic scale’ with the ‘most noble’ – meaning the less likely to be attacked – at the left and the least noble (most vulnerable) at the right. The further apart any two metals are on the galvanic scale, the greater will be the potential difference (in volts) between them. For example titanium and aluminium would make very troubled bedfellows whereas tin and lead could live side by side in reasonable comfort. It’s easy to see why you should never secure aluminium fittings with fastenings made from any of the copper bearing alloys (brass and bronze). So, in that regard, stainless steel is the only obvious choice.
The galvanic scale
*Of course graphite is not a metal but it is a conductor and is becoming increasingly common afloat (think carbon fibre which has a very similar atomic structure).
**Manganese bronze is an alloy containing nearly 40% zinc so it really should be described as ‘brass’.
How to use the galvanic scale
The potential difference (volts) can easily be calculated by reference to the scale. Let's take a fairly extreme example – lead and magnesium. The first is rated at about -0.2V and the second at about -1.4V . Take one from the other and we get the potential difference between them, namely: 1.2V which signals quite a lot of galvanic activity.
This article is adapted from Andrew Simpson's RYA Electrics Handbook which can be bought by clicking the link.