OutboardWorld At MarineMotors Cork www.marinemotorscork.com/
outboardworld
Distributors Of HONDA EVINRUDE MERCURY & JOHNSON outboards
Corrosion
Batteries
Some of you will undoubtedly have taken your batteries ashore
while you cleaned up in the engine bay. Make absolutely sure that any green
verdigris from the batteries is cleaned up from around here as these deposits
are highly acidic and can cause extensive damage in wood and metal hulls. If
the batteries have leaked or spilled, then neutralize the area with warm water
and an alkaline-based cleaner (these are available from all supermarkets and
chandlers).
Electro-Chemical Decay
It is important that this type of corrosion is
correctly identified and is not mistaken for normal battery acid damage. The
clue here is that this kind of decay will extend much further than the bilge
areas and can be identified by localized darkening of the timber and light
green crystals around the copper fastenings. The fastenings themselves may well
be crumbly. If this kind of decay is encountered, then get a qualified
shipwright in before any further damage is caused. In extreme cases it may be
necessary to replace fastenings and planking if it goes unchecked.
Metal Craft
In general, ideal conditions need to exist before
corrosion in metal craft can take place. In the first instance, this is why it
is critical to ensure the metal is fully protected against seawater (It may not
have occurred to you, but seawater is a perfect electrolyte). As far as
possible, stray electrical eddy currents, such as those induced by continuous
welding, are to be avoided. Naturally, it follows that the first step is
complete and total protection. If other conditions are ideal, such as the
amount of oxygen present, correct temperatures and inadequate anodic
protection, then corrosion is likely to commence. Make no mistake, the
onslaught can be rapid, destructive and can dangerously weaken metal structures
in months, if not weeks. Extreme vigilance is needed and the metal boat owner
must familiarize himself with all symptoms of encroaching corrosion and what to
look out for.
Mixed Metals
The greatest potential for corrosion exists if an
electrolyte is present (rainwater can dissolve salt crystals and make a good
'seawater' electrolyte) and two or more unlike metals are close or even in
contact. There then exists the perfect physical and chemical combination and
conditions will again be right for the dreaded corrosion. Here, in this case,
crevice corrosion can take a deadly toll as it works unseen under the layers of
metal. Different amounts of surface oxygen on each metal allow differing rates
of damage to progress. The metal boat owner can do a lot to prevent this
occurring by insulating differing screw/metal combinations, applying sealant
between different layers of metals (or wood) and carefully inspecting the hull
at every available opportunity. Even fish oil and the famous WD40 can play a
part here!
Wooden Craft
When dry, scrape away any dissolved or flaky paint and
soak the area well with preservative, using the correct precautions (mask,
gloves and goggles). Leave for at least a week until the area is dry, then
prime with good quality wood primer and bilge paint. Oh yes, and don't forget
to wash and paint the battery boxes out too, especially if they are wooden.
Aeration Corrosion
It is particularly important to check the area around
the rudder and prop where another form of corrosion can hide. Often, water is
aerated by the prop and oxygen can be sucked into the area creating ideal
conditions for 'smooth surface corrosion' to begin. It's not obvious at first,
but can eat away the prop blades at an alarming rate. This form of corrosion
must not be mistaken with cavitation damage provoked by vibration and extreme
pressures to the prop blade edges. This causes disintegration of the blade
edge, roughness, pitting and erosion of the metal surfaces. In all cases,
corrosion is often prevented by thorough inspection, even with a magnifying
glass, if need be. The prop will often ground, sending sand scouring across the
rear hull, damaging paint as it goes. Stripped paint will spell trouble sooner
or later, so tackle it now if the boat is out of the water. Scrape loose paint,
prime and paint with epoxy-based paints. Use two coats.
Expect The Unexpected
Waterline areas often go by the board when it comes to
haul-out time. Dirt, oil and slime collect here and are often hard to remove.
Under the muck, the corrosion gremlins will be hard at work on all sorts of craft,
timber included. Hull fittings are often the first victims here, due to the
unexpected ingress of water via the bolt heads, or even the osmotic process as
water finds its inevitable way in. Chemical outflows from sink cleaners,
washing up liquids and so on, will cheerfully break down the boat's corrosion
defenses, given time.
Usually the physical wearing of
metal to metal surfaces are down to the daily grind of the boat's life. Dings
and dents are sually highly visible and get a thump with a hammer or a lick of
paint! However, there is one noticeable, dangerous exception here and it must
be looked at with extreme care. A little effort must be expended to ensure all
is well. What I'm talking about is the anchor chain, an object that is often
neglected to a dangerous level. Underwater, unseen, the links are constantly
rubbing metal to metal and these tiny particles fall to the bottom, unnoticed.
Back onboard the often muddy, dirty chain lives happily in its warm, wet and
salty home. Unrequired — often for many months at a time — it rusts away
quietly on top of the already partly worn links. Once the rust has a hold,
there is very little that can be done to rectify the vicious circle of wear and
decay. Remember if your anchor chain has corroded by more than 25% of its link
width, that section must be replaced. Ask a surveyor! If you do nothing else
this winter, drag out the chain and have a really good look — you may get one
hell of a nasty shock! On the other hand, everything may be fine. Let's hope
so, because you won't be able to check it on a dark windy night when your
engine's died and the shore lights are getting nearer.
MarineMotors
Little Island Cork Ireland main ZODIAC dealers