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  • Steven Foy

Galvanic Corrosion and Compatible Metals on Boats

Although the vast majority of modern sailing and power boats are built from glass reinforced plastic (GRP), a huge variety of metal fittings and fixings will always be found aboard. Even the smallest of craft could have aluminium spars, stainless steel rigging, bronze sea cocks, a lead or iron keel and maybe even a titanium propeller.

Being durable, strong and versatile, these metals are well-suited to the harsh marine environment - but only when kept isolated from each other. Some combinations of dissimilar metals, such as stainless steel keel bolts securing an iron keel, are safe to install. Other combinations promote a form of metal decay called galvanic corrosion and should be avoided.



Galvanic Corrosion

Often observed on boats as accelerated wastage of metal fittings, seized fixings, surface powdering or staining, galvanic corrosion occurs when connected dissimilar metals are immersed in an electrolyte (an electrically conductive liquid).

The connection could be direct, a stainless steel screw in an aluminium boom for example, or indirect, through a poorly considered bonding wire or something similar. The electrolyte is of course, sea water.

Metal fittings below the waterline are at obvious risk but puddled water on deck or in the bilges can be equally destructive.

Every metal has its own natural electrical potential which inevitably causes electrical currents to flow when dissimilar metals with different potentials are joined in a completed circuit. These relative electrical potentials are detailed in The Galvanic Series of Metals as shown in the following (abbreviated) table:


Fig 1 The Galvanic Series of Metals


Metals near the top of the list are characterised as noble, those lower down (with more negative potentials) are characterised as base. If dissimilar metals are joined in a cell (see images below), the less noble (more base) metal will suffer from galvanic corrosion while the more noble metal will remain intact.



Fig 2 A Galvanic Cell


This process of ‘protecting’ the more noble metal (cathode) at the expense of the more base metal (anode) is the science behind the now standard practice of attaching sacrificial zinc anodes to boat hulls and electrically bonding them to all the important below waterline metal parts (propellers, shafts, brackets, rudder stocks, sometimes seacocks, keels, exposed ballast etc).

Zinc is a very base metal and readily protects iron, all types of steel, bronze and brass, lead and even aluminium (just about, aluminium is only slightly more noble than zinc).

This method of so called ‘cathodic protection’ is cheap, simple and reliable by design. But beware, by purposefully bonding together the various underwater fittings and launching the boat, a multi-metal galvanic cell has been established. If the zinc anodes are left to erode away to nothing (or if they’re painted or heavily weeded over), then the next most base metal will take over the role of sacrificial anode.

On a typical modern yacht that might mean an expensive bronze propeller starts protecting its stainless steel prop shaft. These processes occur out of sight under the boat and the first you'll know about it might be when the propeller falls off!


Crevice Corrosion and Pitting

Metals used for boat fittings and fixings are rarely pure, they are almost always alloys of different metals, expertly blended to give the finished material enhanced qualities. For example, stainless steels are alloys of iron, chromium, nickel and molybdenum, and bronzes are alloys of copper, tin, phosphor and aluminium.

Galvanic corrosion can occur on the surfaces of these alloys without there being any bonding to another dissimilar metal fitting.

If the corrosion occurs in narrow spaces wetted with water, like in the thread of a fixing or under a washer, it is called crevice corrosion. If it occurs in tiny depressions on a metal surface where droplets of water ‘puddle’, it is called pitting. Pitting is self-worsening, as the pits grow so the area of corrosion widens and deepens.

Stainless steel, which needs a ready flow of oxygen across its surfaces to maintain its ‘stainless’ chromium oxide coating, is particularly vulnerable to crevice corrosion and pitting.


Compatible Metals

On boats, dissimilar metals are often used together to exploit their various positive attributes. For example, cheap yet very strong and commonly available stainless steel fixings are used to attach cleats and the like to aluminium spars, durable bronze bolts are used to attach ‘soft’ lead keels and stainless steel shackles are used to join zinc galvanised mild steel anchors to chain.

If it is not possible to use 'same metal' fittings and fixings together, then metals with similar potentials (i.e. close to each other in the galvanic series) should be used. If even that is not possible (look at the difference in potentials between stainless steel and aluminium) then the two metals should be insulated from each other. This can be practically achieved with nylon washers under nuts, plastic linings in drilled bolt holes or liberally coating fixings with an insulating jointing compound like Duralac.

In general, iron keels should be secured with 316 stainless steel bolts and lead keels should be secured with silicon bronze bolts.

Bronze propellers fitted on 316 stainless steel (or better) shafts are common and this is considered to be a safe combination as long as all parts are protected with a local zinc anode. Beware of crevice corrosion on the shaft under the propeller and where it passes through Cutless bearings and the like.

Also, not all bronzes are truly bronze! Manganese bronze, being an alloy of copper, zinc and manganese is actually a brass. It is often used for propellers but can suffer from dezincification, whereby the zinc in the alloy erodes away, effectively cathodically protecting and preserving the more noble copper. As the prop becomes more ‘coppery’, it starts to get brittle, its edges chip away and eventually it simply crumbles away to nothing.

Apart from the latest reinforced plastic types, almost all below waterline seacocks and skin fittings are cast or turned from bronze, or rather, what appears to be bronze. A huge variety of fittings are available in chandleries, from cheap domestic plumbing parts to heavy-cast high quality tapered plug seacocks. Marine grade bronze fittings are marked with ‘CR’ or ‘DZ’ logos (corrosion resistant/dezincification proof) and should always be used (GRP boats only, different rules apply for metal hulls).

All parts of the skin fitting and its valve assembly must be of the same high quality material, beware of lesser hull plates, bolts, valve balls, handles, clips etc. Many a boat has sunk through a failed skin fitting.


Conclusion

Galvanic corrosion can cause rapid and unexpected failure of critical boat parts, but careful equipment installation, good boat husbandry and adequate cathodic protection can keep it at bay. It is a silent destroyer causing expensive, even catastrophic damage and should always be given due consideration.

Allcraft Marine Surveys Ltd.

2 Silvretta Court, New Street, Lymington, SO41 9BQ, UK

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