Notes prepared by Ian Duff 17th June 2014
MYSTERIES OF ELECTROLYSIS ON BOATS
What I propose to do is to give sort of prejudiced overview, then take a look at the corrosion characteristics of various materials used in boat systems, then talk about what to do or not to do by way of remedy. I tend to take a very different approach to electrolysis problems to that presented in many books on the subject. My basic approach is to use materials noble enough to look after themselves.
Now it is time to get on with the subject in hand although this may induce a glazing over of the eyes as elementary chemistry lessons are re-visited. Seawater is a very corrosive brew that has a total dissolved salt content of about 35 grams per litre. The dissolved salts are present as ions, the dominant two being chloride & sodium ions followed by sulphate, magnesium, calcium, and potassium ions and so on. If you evaporate away the water from a litre of seawater you will get about 25 grams of sodium chloride (common salt) and about 10 grams of magnesium, calcium & potassium chloride & sulphates etc plus a whole range of compounds from all naturally occurring elements. Seawater is slightly alkaline with a ph centred on about 8.
Boats, as in small boats, are usually constructed of fibreglass or wood or steel or aluminium alloy. Each type of construction has its own associated electrolysis problem – okay the fibreglass or wood doesn’t corrode although it can suffer degradation caused by electrolysis of associated metal components such as fastenings, engines, rudders, shafts, skin fittings, propellers, etc. In very basic terms electrolysis is the electrolytic transfer (usually loss) of metal caused by an electrical current flowing to or from that piece of metal.
The tendency of a metal to dissolve in aqueous solutions is basically a function of the electrode potential of that metal (in chemistry labs it is usually with regard to what is known as a hydrogen electrode). For example if you put sodium, potassium, caesium, or lithium metal into water it will catch fire or explode so these types of alkali or base metals are extreme examples of the opposite of “noble” metals. Very noble metals like gold or platinum show no tendency to dissolve in water but they tend to be a bit pricy to build structural boat parts of, even for Larry Ellison.
Basically the more noble a metal is the less likely it is to want to dissolve in seawater, although there are exceptions in both directions. For example, copper is a fairly noble metal but will erode/corrode away quite quickly when the water flow over or through it has significant velocity.
ALUMINIUM ALLOYS. Aluminium is not a noble metal by any stretch of the imagination but it’s tendency to corrode both in air and in water is markedly reduced by a rugged oxide film on the surface of it. However a copper coin or wire offcuts dropped in a wet salty bilge can sink an alloy boat in just a few days and copper-based antifouling paint on an alloy hull or for that matter a sail or stern drive will cause very severe and rapid corrosion. The aerospace industry did use a lot of duralumin (2000 series) but the 7000 series is now more common. Duralumin contains some copper so is a non-no in marine applications. The 5000 & some 6000 series aluminium alloys are the only ones suitable for marine construction. Their alloying components are small amounts of magnesium and silicon.
BRASSES & BRONZES. Common brass is normally about 63% copper and 37% zinc but there are a whole range of brasses made for differing purposes but almost none of them are suitable as underwater materials. Traditional bronzes were primarily copper and tin although most also contain zinc. Other alloying elements used in bronzes can be arsenic, phosphorus, manganese, silicon, aluminium, iron etc depending on the application. The majority of bronze propellers made in NZ are nominally manganese bronze that to be in accordance with ASME standards should be 57-60% copper, 0.8 -2% iron, 0.5 -1.5% tin, 0.5% manganese (maximum), 0.2% lead, balance zinc i.e. getting up to about 40% zinc. Unfortunately many of these propellers cast in so-called manganese bronze are not true to formula. The usual fault is that zinc content is far too high and copper content too low so that pitted, de-zincified propellers are a common sight in any haul-out yard. Attempting to cure the defective bronze composition problem by the addition of zinc anodes on a propeller shaft is the usual start point for a whole can of worms ranging from pulpy timber syndrome to crevice corrosion on the stainless prop shaft as outcomes. The better solution is to put up with some pitting of the defectively alloyed propeller in the short term and then to order a new one in a much better material such as AB1 (aluminium bronze).
STAINLESS STEELS. Type 316 stainless is the conventional choice for underwater applications like propeller & rudder shafts whereas type 304 is almost universal in rigging & deck components. These 18/8 stainless steels achieve their corrosion resistance in a similar manner to aluminium i.e a dependence on a rugged oxide film. Therefore do not regard stainless steels as wonder materials – for a start they only have strengths about the same as mild steel and they are prone to fatigue failure and an “allergy” to carbon. Equally importantly they must have an oxygen partial pressure presence in order for them not to corrode. For that reason stainless should never be used for keel bolts because air (oxygen partial pressure) is absent in the middle of a keel or deadwood. It is not uncommon for the unseen buried parts of stainless keel bolts to be down to about one third of proper diameter i.e down to one ninth or tenth of the original strength of the bolt.
Returning to our defective brew of manganese bronze cited in the propeller example above, in any haul-out yard you will see the zinc clamp anode on the stainless prop shaft and this mindless ritual is spiritually reinforced by the making of sacrifices – therefore it must be good! Unfortunately it is not good because the presence of the zinc anode produces a hydrogen partial pressure presence that negates the oxygen partial pressure so essential for the corrosion resistance of the stainless shaft. End result, crevice corrosion particularly in rubbing areas such as stern bearings, packing glands etc. Crevice corrosion looks a bit like the sort of tooth decay that your dentist plans his retirement on.
STEEL. Mild steel (mainly iron) is not a noble metal as evidenced by high rates of rusting in air and very high rates of corrosion in a seawater environment. They obviously build ships out of steel and it is a practical material for building larger yachts out of as well. On wooden or GRP vessels it is also sometimes used as a wear skid on the bottom of a wooden keel or for rudder components etc. Because steel wants to corrode rapidly the measures used to slow that process down primarily consist of a very well done paint system comprising careful surface preparation, primer coats, undercoats, top-coat paint above the waterline and antifouling paint underwater. This paint system, with the exception of the antifouling, forms a physical and electrical barrier between the hull and the sea but must be backed up by the use of zinc anodes fitted to the outside of the hull and to rudder systems etc.
On a forty footer this will probably comprise about 6 anodes, and they are essential in addition to the paint system. Anode usage will tend to correlate closely with the proportion of underwater hull that has paint system defects such as the area of keel that cannot be painted because it is sitting on slipway cradle cross-members etc. In other words the primary protection on a steel boat is the paint system and anodes are the back-up. The paint system inside and out, above & below the waterline needs to be done to the highest standards.
CROSSED EARTH ELECTROLYSIS. This type of electrolysis can be quite spectacular and is usually caused by chaotic wiring. Most commonly it will be something electrical sited in a bilge area such as a defective bilge pump or float switch. I have also experienced quite a strong electric shock (just 12 volts) when I swam to a friend’s adjacent boat and grabbed the cap shrouds with the intention of hauling myself aboard. The problem was that a navigation light up the mast had a short from the plus side of the light circuit to the masthead fitting. The plus 12 volts was coming down the shrouds to the chain plates and the return to earth, electrical negative etc was via my wet salty body to the sea via prop shaft etc. I certainly found the cause of his very rusty chain plates!
Another example of this kind of fault is running 12 volt circuits such as radios off half a 24 volt bank of batteries.
MARINA CORROSION. In a marina environment you often see 230 volt cables running to a boat to keep batteries charged, heaters on, electrolysis active protection and security systems alert etc. It is also not uncommon to see those 230 volt cables running through the water rather than being sensibly suspended well clear of the water. Ensure that cables to your boat are not offending in this way and raise the issue with the owners of adjacent boats (or marina staff) if you see this situation with other boats.
Most marinas these days have a 30mA rated RCD protection system to each marina finger so that supply is cut off when leakage exceeds 30mA. However 30 mA of leakage can still cause a lot of corrosion quite quickly. The marina situation is complicated by such things as an aluminium boat being in a berth adjacent to a steel boat so I wouldn’t recommend any DIY approach to avoidance other than good practice regarding mains cables etc.
TO BOND OR NOT TO BOND. Bonding means the joining together of all metal systems that are in the water such as skin fittings etc to those other bits that are not quite so obviously connected such as engines, fuel tanks etc. Bonding is good for reducing static build up on fuel tanks and can be useful in reducing radio interference and in crude theory could be good for electrolysis avoidance. Most books on electrolysis recommend doing this but in most cases it is totally impractical and likely to create severe electrolysis problems rather than solve them. The primary reason it is impractical and dangerous is that there are so many different metals used in the various parts of a boat. You may well have some passively benign bronze skin fittings, stern tube & bearing system etc but unless all the bronze came out of the same pot at the foundry in one pour you are just going to create a can of worms. Add to that that diesel engines are not made of bronze nor was there any chance that such bronze if used would have come out of the same pot as the other underwater bits. Engines are constructed of cast iron, cast aluminium, high strength steels, perhaps copper or admiralty brass heat exchanger components and a bronze seawater pump.
In other words there is a huge range of electrode potentials among the range of materials used for building an engine. Small marine engines these days almost universally have a 12 volt starter motor and alternator, both of which are earthed to ground i.e. engine castings, bell housing, gearbox etc. The practical solution to the realities of normal engines is to connect input and output water, exhaust etc via rubber hoses to minimise current flow between the bulk of the engine and underwater systems. Similarly some sort of electrical isolation such as a Tufnol disc between gearbox and shaft drive is highly desirable for avoiding corrosion problems.
SUMMARY OF ADVICE.
Steel boat. On a steel boat my first choice in anodes would be zinc based ones even though they are more expensive than special aluminium ones. Do not under any circumstances use magnesium anodes for hull protection on steel boats because their electrode potential is excessive and will result in the rapid failure of underwater paint from alkaline action. Their use should be confined to limited commercial application where bare steel is operating in a severe wear environment e.g. the cutters on a suction dredge. If you use aluminium based anodes on a steel boat hull there is a possibility of paint system failure along the same sort of lines as using magnesium based ones due to their electrode potential being a bit high. I am suggesting aluminium based anodes as a second choice and that is because their real voltage potential is usually much lower than the laboratory standard electrode potential. This practical difference is associated with that oxide film that forms rapidly on aluminium. Use only reputable commercial anodes that contain alloying materials that keep the surface of the anode active. For example if you just use a block of straight zinc or aluminium as an anode it will develop a hard crust on it and thus cease being effective. A healthy anode has a sandblasted look to it.
Aluminium stern-drives & sail-drives. Both systems are very convenient to install in a boat but do not expect longevity unless the boat is only used in freshwater lakes, or is trailered and thoroughly flushed with fresh water immediately after use.
Use the horribly expensive genuine-part magnesium based anodes in accordance with the makers instructions. Don’t use any copper based antifouling anywhere near the stern or sail drive unit. Repair any damaged paint on the underwater parts of the drive system. The same advice is applicable to outboards.
Wood & GRP boats. On wooden or GRP boats seek advice if a corrosion pattern with anything changes rapidly. For example you may suddenly notice that pulpit or pushpit metalwork is looking rusty or corroded and perhaps you detect a small tingle if you touch that metal when the navigation lights on that structure are turned on – that could be a crossed earth problem caused by water in the nav light fitting or a pinched wire. It is basically a wiring problem and needs fixing promptly.
I don’t believe in constantly enabled automatic bilge pumps – they will only remedy whatever the bilge leak is until the batteries go flat, then the boat sinks if the leak is significant. They are also a common source of electrolysis trouble. The electrical leak can be a full cross-earth type fault or electrical leakage due to water getting into the pump motor or float switch. Fix whatever is allowing water to accumulate in the boat – this could be just a matter of a small tweak of a stern gland, or pumping grease into it before you leave your boat on the mooring and go home. If you have to put up with a leaky boat then I can only suggest that a float switch energising an alarm and an external strobe light is a better option than an automatic bilge pump.
If all underwater metal looks to be happily passive then leave well alone – above all never contemplate fitting shaft anodes or anodes coupled to the stern bearing if all that underwater metal is nobly looking after itself. If the propeller is showing a bit of pitting but shaft and other underwater metals look OK then the problem is almost certainly that the bronze used to cast the prop is brassy rubbish. Fill the pits with epoxy short term ad get yourself a new propeller ordered in a better material such as AB1 & fit the new prop next year when you are on slip again. If propellor corrosion is suddenly severe after being of no consequence for a number of years then seek advice. Please don’t go fitting those shaft anodes or other anodes to fix the problem – you will simply transfer the problem to other parts of the boat like fastenings, crevice corrosion, soggy wood, lime build ups etc.
My key advice then is to only use properly noble underwater metals on a boat often in conjunction with good quality reinforced plastic components. For example use a quality bronze skin fitting with a good quality plastic ball valve for water intakes and outlets etc. These are quite happy looking after themselves and do not require anodes to be fitted. Ensure that your electrical system is simple, well understood by you, and tidily executed.
I appreciate that much of my advice is contrary to what you may have read in some boating book but the advice I have given is all based on the reality of how boats and systems are constructed tempered with basic electrochemical knowledge and a reasonable amount of experience.
A few oxidation potentials are listed for reference – they are all w.r.t. a hydrogen electrode.
Lithium +3.05 volts (burns or explodes in water)
Sodium 2.71 “ “ “
Magnesium 2.37
Aluminium 1.66 (practical voltage usually regarded as a lot lower)
Zinc 0.76
Iron 0.44
Hydrogen 0.00 (reference)
Copper -0.34 volts
Lead -1.45 volts
Little Ship Club Canterbury 