The Galvanic Table

The table presented here is a composite version derived from several sources. The first thing to be said about it – and all similar tables, come to that – is that the various positions of the metals in the table can be a subject for debate. Even when the actual differential voltages are given, the various relative positions are different in every table seen. It seems that there is no such thing as a standard table of nobility. For instance, in the table here, Monel (a fairly inert nickel-copper alloy, used among other things for shackle mousing wire), is placed lower than one might think it could be; titanium and silver are sometimes seen the other way around; and so on. It does not seem possible to make an absolute statement of the various positions relative to each other.

Certainly, the positions of the stainless steels are likely to be the most fluid, whichever table is consulted: despite a high position, they are not now regarded as safe for long-term underwater use. This, despite the fact that 99% of propshafts are now made from SS316.


Metals and Nobility

Current flows from an anode to a cathode. The anode is wasted away. A metal higher in the table is more noble; a lower one less noble, or base. If there is a physical (metallic) connection between two metal items, and they are in contact via an electrolyte such as sea water (in marine applications meaning a liquid that conducts electricity easily), a circuit is made (a continuous loop) and current will flow easily from the lower-placed metal to the higher.

As an illustration, in the process of electroplating, a metal has a coating of another metal placed on top by the application of an electric current while immersed in an electrolyte. The metal to be plated onto is connected as the cathode, and the metal to be wasted away and collect on top of the other, is the anode. You can do this easily yourself with a 9-volt battery and some salt water. Alternatively, an item of metal can be cleaned (or 'electropolished') by connecting it as the anode briefly – surface material will be removed and collect on the cathode, leaving a bright surface on the anode.

Active and passive relate to the position of the metal item in a galvanic couple: whether the item is acting as anode or cathode. Graphite is not a metal, but is included because it is often encountered in the marine environment. For instance, the table shows clearly that you should never use a graphite-loaded grease underwater, perhaps in the stern gland, since this would be asking for trouble.

In some conditions it may not be necessary for a clear loop circuit to exist: corrosion will occur when two metals of different positions in the table are simply connected by seawater. The closer they are physically, and the further apart in the table, the faster the rate of corrosion. Also, a single metallic object can corrode, due to galvanic cells developing within its own body. This can occur in noble metals that are alloys, and in some cases may mean that they are more subject to corrosion than baser metals. Whether this is due to faulty manufacture or other factors is not clear; for instance, it is has been stated that both 316 stainless steel and corten steel can in some circumstances be less noble than mild steel underwater.

The active-state stainless steels have been omitted; insert them relative to SS 316 active. Stainless grade 312 and 317 are even more corrosion-resistant forms of stainless steel than 316, but much harder to find. SS312 is very corrosion-resistant, but usually only available  as welding rods. It is used as the base for dissimilar metal welding rods and wire, and is also known as 29/9 in the UK, due to its composition of 29% chromium, 9% nickel. This shows that a 312 welding rod could be successfully used as a securing pin underwater; though an anode must protect the other metals.


Table of Nobility in Freshwater

How does the Table change in fresh water? A trick question - it's always the same. The Galvanic Series doesn't change when moving between salt and fresh water. The metals always have the same relationship, it's just the speed of corrosion that varies in electrolytes of differing strength - ie more salty or less salty.


Brass and Bronze Propellers

'Manganese bronze' is not a bronze at all, it is a type of brass. Brasses are alloys of copper and zinc, bronzes are copper alloys that don't contain zinc. Since 'manganese bronze' is 60-39 copper-zinc plus small amounts of other metals including manganese, it is a brass. It is a high-tensile brass suitable for (cheap) propellers. Obviously, it is even more vulnerable to dezincification (electrolytic corrosion that removes the zinc and destroys the metal) than many other brasses, whereas a propeller made from any of the correct materials cannot suffer from this. Despite this, the majority of propellers are made from it, since it is a reasonably strong, cheap, material that is easily cast and machined.

Real propellers are commonly made from aluminium bronze (good), phosphor bronze (better), or nickel aluminium bronze (best) – as you can clearly see from the table. Monel is even better, if you can afford it, and Hastelloy (Mr Niarchos only) even better than that. Silicon bronze is not normally used for props; stainless steel props are normally used for speedboats and powerboats that are mostly lifted out between use. They are the toughest, but not the best material for permanent underwater duty.

Hastelloy, Inconel, and Incolloy are very expensive, very hard, very durable, and virtually inert nickel-chromium-molybdenum-iron-copper alloys. Just the thing if you want the best propeller you can get – for the price of a car. Inconel is used for immersion heater elements.

Red brass is a low-zinc high quality brass. Muntz metal is a brass, comprising 60-40 copper-zinc.

Cupro-nickel, or copper-nickel which is the same thing but tends to be used when the material is supplied in plate form, is used in heat exchangers and so on; an aggressive environment with hot seawater present. In air, or even for underwater use, it is regarded as all but inert. When used for boat hulls, as it has been frequently (much more often than stainless), it is said to be eroded at a rate of 0.25 mm per ten years, or less. It is now used for sheathing ship hulls, as it can be more cost-effective than even TBT antifouling paint. This seems to be going back to the old copper-sheathing days. In this application, it is also available for new-builds as combination or sandwich plate with mild steel: a plate with steel one side, copper-nickel the other. At present we do not have the background to decide whether 70-30 or 90-10 copper-nickel is clearly best; but the high-nickel alloy is certainly more expensive, and harder. The welding rods used are always 70-30, for both types. Welding the combination plate has to be done with 70-30 CN rods one side, and 6013's or whatever (7000 series if you prefer them) on the other. It's tricky if you have to do it from just the one side. Of course, you wouldn't new-build like that, but some types of repairs might require it.

I have seen a yacht hull built from cupro-nickel that stayed afloat for more than fifteen years without hauling, antifouling, or any maintenance. A quick scrub got rid of the slime. Subsequent measurements of the hull thickness showed no erosion whatsoever had taken place, except perhaps at the waterline, although precise measurements had not been taken at the time of build. You can take a look yourself: 'Pretty Penny', at Iron Wharf, Faversham, Kent, UK, which is just down the road from me.

I'd have to say, though, that this material is a lot less visually attractive than even raw alloy. The hull above the waterline, if left unpainted, goes a dark green - like a cathedral roof but darker. I'm not a fan of alloy for boatbuilding, but I reckon it's got the edge in looks - the dull batteleship grey of unpainted alloy topsides is better than that verdigris green. On the other hand, as a hull material, copper-nickel is about a thousand percent better, so I guess I'd put up with it if I had the opportunity!


Anodes

Anodes are made from base (low-value) metals, as you can see from the table. They are sacrificial lumps of metal bolted to the bottom of your boat that are eaten away by corrosion instead of your boat or its important metal fittings such as the sterngear.

In seawater, zinc is used. It gives the best balance of vulnerability vs long life. In other words if you make the anode too vulnerable (low on the scale), it will be eaten away too quickly. Make it too noble (higher on the scale) and it won't do it's job: the boat will be eaten as well.

The sea is not universally salty - some areas are saltier than others, and this can make a difference to the anode requirements as corrosion speed will be different. There are charts that show this salt variation, but in any case your fellow boat owners in any given area will tell you if your location is worse than the norm. What is more likely to happen is that some harbours or marinas can be worse in this department than others. There are a huge range of factors that influence this and it's impossible to calculate - ask your berthing neighbours, they'll know what the score is.

Aluminium anodes are sometimes used, which being slightly higher on the table and therefore closer to the metal being protected, give less voltage differential and survive longer in high-strength electrolyte (extra-salty sea).

In fresh water the opposite situation exists. Here, zinc anodes don't work well as they are too noble, since freshwater is a poor electrolyte (a liquid which acts as a conductor). Instead, magnesium is used, as it is the lowest metal, and works correctly in the weak electrolyte that freshwater is. If magnesium were used at sea, it would fizz away, and dissolve too quickly.

Anode problems
If your anodes are being eaten too fast, then:
1. You are moored in a vulnerable place where there are some problems - the site of an old coal wharf perhaps; or a stray electrical current either on your boat or nearby.
2. You don't have enough anodes for your boat size / metal area.
3. (Unlikely but occasionally valid) Your sea area / location requires aluminium anodes as it is more active for one reason or another.

If your boat metalwork is being eaten, then:
1. If it is confined to one area, then there is a local problem: insufficient anodes at that point, or an electrical problem of some kind.
2. If it's more widely dispersed you need more anodes.
3. If you can't tell, because the only metal on the boat is the prop shaft, then add another anode and also check the boat for electrical issues. You are better off getting expert help here as these things can be hard to track down.

If you are going into the canals, then:
1. If you're going into freshwater for less than a month, don't worry about it.
2. If you are going for longer, it might be worth changing to magnesium anodes, especially near the sterngear.
3. An alternative to help with peace of mind is to use a magnesium mooring anode - dangle a chunk of magnesium over the back.

Steel boat anode issues:
1. The best way to attach anodes is to weld stainless steel bolts onto the hull, so that the bolt head is flush onto the hull and the bolt thread projects out perpendicular to the hull. Use dissimilar metal rods for this, 2.5mm 29/9's or 312's are perfect.
2. Clean up well with a wire cup brush on the angle grinder. Paint very well all round with two coats of 2-pack epoxy, including over the weld area - but DO NOT PAINT THE STUD THREAD. They need a good physical / electrical connection to the zinc anodes.
3. Make sure you get a plan for the anode locations for your specific boat. If you can't find that out, look at the similar details for top steel boat designs like van de Stadt. Alternatively, go to MG Duff.
4. In general, you need more than they tell you - especially round the back end. You'll find you need more round the sterngear than is planned for.
5. I've never found that you need a shaft anode, as long as there are sufficient anodes nearby. For a 40-foot steel boat the absolute minimum number of anodes in the region of the sterngear (propshaft, prop, rudder, deck drains, glands etc) is three. Four is a better bet. They can be of different sizes and shapes as appropriate. The single area I've found that needs more is at the bottom of the rudder.

Alloy boat anode issues:
1. If you own an alloy boat you need to be an expert on anodes, since ali boats are highly vulnerable to corrosion. Anode issues, bottom paint and so on are of the utmost importance.
2. Boat management has to be different in order to compensate for this. For example it's said that a copper coin dropped into a wet bilge will eat through the hull eventually; and that a watch battery in a similar situation can eat through the hull within 24 hours. I'd epoxy the bilge, personally.


The worst anchorage

A bad place to anchor – or even worse, have your mooring – is on the site of an old coal wharf. As you can see, graphite is right at the top of the table. It is a form of carbon that conducts electricity well (though not as well as metals). If you are carrying a carbon fibre (graphite) fishing pole, you shouldn't go anywhere near power lines, as many deaths have proved.

Coal is fairly close in composition to graphite, and therefore noble compared to most metals. Because of this, with the tiniest bit of stray current in the area, anything else metallic is likely to corrode away. And that will be your ground tackle, since any other metal around will long since have disappeared.

If by chance you own an aluminium boat, perhaps it would be wise to avoid a coal-fired cabin stove: an aluminium bilge + coal dust + seawater = a nice fizzy mixture...


Conductivity

Although one might think at first that it is, a metal's ability to conduct either electricity or heat is not related to its position in the table. As an example, copper conducts both better than stainless steel or aluminium, which are at opposite ends of the table. Copper is a superior conductor of electricity to stainless, which is why a busbar (a metal bar connecting several electric circuits) is better made from nickel-plated copper, or brass, than stainless.

As any welder will tell you, stainless conducts heat much more slowly than plain steel – heat one end of a mild steel bar while holding the other, and you'll soon have to drop it. Heat the end of a bar of stainless, though, and you can hold it til next Christmas. That's one reason why a thick bar of stainless can be tricky to weld, as it cannot disperse the heat input, and often bends up like a banana.







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