Enigma's prop and shaft and two mildly corroded shaft anodes
So, if we fancy a bit of that there i-lec-trol-i-sis, we need a
liquid that is good at passing electricity (saltwater) and then we need some
electrical current.
Two dis-similar metals in an electrolyte (the saltwater) will generate electrical
current flow, and luckily underneath Enigma’s hull there is a bronze propellor,
a stainless steel prop shaft, and several brass seacocks.
With all the ingredients assembled we are now ready to start the “chemical
decomposition” of 'Enigma's propeller blades, her seacocks, the boat next
doors propeller blades, and if we are really lucky, the seacocks of the ancient
old cruiser that sits three berths down, rotting under a mouldy green canopy.
"But surely" I hear you cry "there wouldn't be enough electricity
genrated to do any real damage"
"Alone in a vast ocean, probably not, but add the stray electricity from
the rest of the surounding boats in the marina; paying particular regard to
the mammoth mother of all power boats over on ‘A’ pontoon, who not
only has several large lumps of dis-similar metals under his hull, but in addition
has conformed to the necessary safety legislation and earth bonded all his metal
work together and strapped this (correctly) to the mains earth of his shore
power supply, from which he continually runs the deep freeze, wine chiller,
cabin heater, boat alarm and dehumidifier, and a bank of four 24 volt batteries
all leaking stray electrical current out through his twin screws, and we begin
to see that there might be a problem developing.
Oh, and dont call me Surely!
Enigma's prop and shaft and two mildly corroded shaft anodes
There is a thing called the galvanic table, which at first sounds vaguely as
if you need to perform some kind of initiation ritual to join. It sounds even
more likely you will need to start practicing a funny handshake when you discover
it is all to do with how noble a metal is, a sort of Burkes peerage for Metallurgists.
Sadly a metals standing within the galvanic table is nothing to do with its
upbringing, but more to do with how easily it is affected by the effects of
electrolysis.
It is of little surprise to find out that in the galvanic table the most noble
metals are gold and silver, and if we made all our boat fittings out of these
then this tosh about electrolysis would be irrelevant, but contrary to popular
belief, most boat owners are simply not that rich. Bronze, copper, and brass
are somewhere in the middle of the league, with different grades of stainless
steels appearing at various places in the table. Obviously someone has to come
at the bottom, and in the metal world the peasants are magnesium, magnesium
alloy, and zinc.
The upshot of all this is that if we want to protect our bronze, brass and stainless
steel, we must strap on a juicy lump of zinc or magnesium, and let the peasants
die first.
These lumps, or sacrificial anodes, come in a variety of sizes and ‘Enigma’
has hers strapped to the prop shaft. The rate of erosion of the anode is affected
by several factors. The salinity of, and minerals in, the water. The different
amounts and different types of metal that are affected, and their relative position
on the Galvanic table. Even the water temperature can affect the speed at which
the corrosion occurs so with such a large degree of unpredictability you really
do need to keep an eye on how your anode is doing and replace it as required.
