Boat propellers – some corrode faster than others

In the boat propellers world, the word ‘bronze’ has a reassuring ring to it, does it not? My dictionary describes it as ‘an alloy of copper and tin, known for its durability’. And archaeologists will testify to its indestructible nature, since Bronze Age excavations in Britain and elsewhere continue to yield tools and ornaments that have been under the soil for 3,000 to 5,500 years – emerging in just about as good condition as the day they were made. Yes, there’s no doubt about it, when it comes to longevity, bronze is a considerable star.

So, what the heck happened to the sad looking boat propeller in the photo left? Its owner confirmed that, far from being several millennia old, it has been in service for less than a dozen years. ‘Best quality manganese bronze,’ he assured me glumly. ‘It’s not as if it were brass.’

Now there he was wrong. Or, more accurately, had been misled. It’s well known that brass is an alloy primarily of copper and zinc. Unfortunately, zinc is a highly reactive metal that occupies a very ignoble position on the galvanic scale. It has a very weak atomic bond with most other metals. Immerse a brass fitting in sea water (an electrolyte) and the zinc will be leached from the alloy, leaving a porous and structurally useless residue of copper behind.

The process is known as ‘dezincification’ and is a very good reason why brass should never be used for underwater fittings. Experience tells us that, if we want durability, we should look towards our trusted friend, bronze – manganese bronze being the variety that first springs to mind.

Or should we? What exactly is the composition of manganese bronze – copper and tin? Well, actually no.  It may come as a shock to discover that the type commonly used for boat propellers includes only a trace of tin (0.2%) and a paltry 2.8% of manganese. Even more astonishing is the fact that, apart from the copper, the principal alloying constituent is zinc – up to 28% for marine grades, as much as 40% for others! The truth is that manganese ‘bronze’ has a lot more in common with brass than it has with bronze. It is therefore (and demonstrably) subject to all the threats of dezincification such a classification implies.

Warning signs

Of course, the boat propeller in the photo is an extreme case but all boat owners should keep a weather eye out for trouble. Early signs are pink patches on the surface of the metal. The first signs of more serious structural deterioration usually occur on the edges of the blades – sometime to the point where the metal becomes soft enough to be crumbled away with a thumbnail.

Prevention

The best deterrent is not to use manganese bronze at all. Nearly all folding and feathering props are made of aluminium bronze or nickel aluminium bronze because of its superior strength, but these alloys are inherently more resistant to corrosion. Unfortunately, they’re also more expensive.

If you must use manganese bronze – and sometimes it’s unavoidable – then protection in the form of sacrificial anodes is strongly recommended.