Corrosion

What is it?

Corrosion, partial or complete wearing away, dissolving, or softening of any substance by chemical or electrochemical reaction with its environment. The term corrosion specifically applies to the gradual action of natural agents, such as air or salt water, on metals.

If metal can corrode around salt water does it make sense to use aluminum boats in a salt water environment?      Yes.      Aluminum boats in general and Duroboats in particular offer great performance, ease of maintenance and economy.  The need to operate in salt water should not deny you those benefits, but you must be smart about it.

  

It is good to be concerned and keep an eye on corrosion.    All metal boats are potential candidates for galvanic corrosion.  We use high quality aluminum, which is corrosion resistant but not corrosion proof. 

Customers should protect against corrosion the best they can and be vigilant in looking for it.  There are many variables.  In some waters you could leave the boat for a significant time without any ill effect. .  In others damage can begin in a short time.   
 
The variables include; salinity of the water, temperature, organic material in the water, electrical activity nearby, presence of other metals, etc.  Commercial harbors can be worse for electric activity than remote harbors.  There is no way to know what your boat will experience without observation.   Protective coatings can help prevent the build up of organic material on your hull.  Organic material can be an aid to corrosion. .  If you use a protective bottom paint make sure it is made for use with aluminum.  It must not be based on a dissimilar metal.  copper is particularly problematic.    Side note:  Copper pennies, copper shell casings, heavy copper wire used as fish clubs have all been known to be starting points for corrosion if left in the bottom of an aluminum boat. 
 
Watch for corrosion near bolt holes, chipped paint, behind peeling paint and any place the aluminum has been scratched or dented. Edges, cracks places that trap salt water are more susceptible than flat surfaces.   Avoid build up of organic material, plant growth, algae, barnacles etc.  Organisms create conditions that make the aluminum more susceptible to corrosion. 
 
Inspect the keel, chine extrusions and corner castings regularly.  If they have been dented or scratched on rocks or beaches they may provide a starting point for electrolysis.
 
Except for sacrificial anodes, don't mount any metal to the hull other than aluminum or high quality stainless steel.     If you use fasteners they should be plastic or stainless, not zinc, brass, copper or iron based.  Inside the boat, don't let any metal sit in the same spot in the bilge for long. Lead sinkers, fish hooks as mentioned earlier copper even a penny sitting in one spot can be a starting point for electrolysis. 
 
Isolate the hull from electrical current. Make sure your battery is grounded to your motor.  .  Don't ground electrical devices to the aluminum hull. 
 
If you are going to moor the boat even with a protective coating it should be inspected often and the hull cleaned often.  Don't let salt residue build up on any surfaces. If you see corrosion arrest it immediately before small damage becomes big damage.

Cathodic protection in the form of sacrificial anodes is required if you will moor your boat in saltwater.

What is that?

Cathodic protection prevents corrosion.

Your boat if not protected is susceptible to electrochemical corrosion.

Galvanic Corrosion--occurs when different metals with different galvanic voltages are electrically connected and wetted by water.  Stray Current Corrosion--occurs when submerged metals are subjected to an electrical current that has strayed from an electrical circuit powered by a battery, generator or dock power. It is the result of an electrical fault.

Metals have different levels of nobility depending on how readily they give up their mass when in a hostile environment.

When corrosion is at work metal actually leaves the least noble metal in any circuit and deposits itself on the most noble metal.  Aluminum is low on the nobility tables so it will give itself up if it is in a salt solution near a more noble metal and there is electrical activity (natural or man made) present.  

Cathodic protection is supplied by the use of sacrificial anodes.  Fortunately for aluminum there are metals that are even less noble.  They include magnesium, zinc and some aluminum, zinc alloys.

These metals can be made in to sacrificial anodes. The anodes when attached directly to an unpainted area of an aluminum hull will protect any part of the hull that is not electrically isolated form the anode.

Which one is best?  for our boats probably an aluminum. zinc allow.    Magnesium is very active in salt water and is sometimes used for fresh water applications.    Zinc works well but can oxidize over when exposed to air.  This means if zincs are not cleaned when a boat has been out of the salt and the boat is returned to the salt the oxidized coating  will protect the zinc and it will not function as a sacrificial anode.

Some aluminum , zinc alloys will function well and not oxidize over so they are better for boats that may be periodically out of the salt water.  

Anodes do not last forever.  Somewhere after 1/3 to 1/2 of the metal in an anode has disappeared it is time to replace it.  Some anodes will have built in warning devices that indicate when replacement is necessary. 

Some questions:

Q.  My outboard has an anode on it. Will that protect my boat?   A. If the zinc is sufficient in size, and the motor is connected electrically to the hull  and the zinc is always in the water it may protect the hull.  When the motor is lifted out, the anode is no longer in the circuit with a hull that remains in the water and there is no protection.   The hull should have its own anode to be safe.

Q.  Is my boat safe from corrosion when it is on the beach?   A. If it is embedded in sand that is often wet there may be some risk.  If saline water collects inside the boat there may be some risk.  An anode mounted outside the boat will not protect metal inside the boat if the anode is not submerged in the water that has collected inside.  A separate anode might be desirable for inside the boat.   Some folks use an anode fish on a wire that can be hung overboard when in the water and left in the bilge if left upright on the beach or a dock.

If you think you may need anodes see your dealer,  read the information at web sites of anode manufacturers. To   protect your boat select anodes correct in size and in composition for your situation.  Inspect the boat and the anodes often.

 

Galvanic corrosion is excluded from our warranty as well as the warranty of most aluminum boats so it pays you to be educated on the subject.  With proper care you should be able to avoid the effects of galvanic corrosion.