There are various types of corrosion that occur on boats. All can be cured or prevented.

I see a lot of questions and much talk on boating forums, concerning corrosion,  particularly on metal boats. Many people raising concerns ask about “electrolysis”,See the Wikipedia on Electrolysis, which is not the correct terminology to apply to all types of corrosion. It is better to identify the specific type of corrosion such as galvanic corrosion. See Wikipedia on Galvanic Corrosion

Stray current corrosion is often confused with galvanic corrosion.  In addition there is the ever-present metal corrosion, or simply called rust, in ferrous metals which is also a type of electrochemical corrosion or oxidation..   Some metals that are commonly used in boat building, such as aluminum and stainless steel, encounter their own corrosion related problems.  For the wooden boat owner and builder there is a special kind of corrosion that occurs around metal fasteners, most often screws, embedded in wood, called by some woodburning or fastener corrosion.

Galvanic corrosion is usually what people are talking about when they ask about electrolysis on their boat.  Electrolysis is a term they most likely picked up in elementary school when they were being taught basic science, and electricity.  Electrolysis is actually the opposite action to galvanic corrosion so the distinction is not trivial as some people suggest. 

Electrolysis is the forced introduction of a current in an electrolyte (water) to separate the components.  If this current is introduced into water only, this separates the two  components of water producing Hydrogen and Oxygen. If an electrical current is impressed on two metals in water then one metal will lose electrons and they will be accumulated onto the other metal.   Because the current in the water is accidental or inadvertent, this is called stray current corrosion.  This is a form of electrolysis but to distinguish it from galvanic corrosion or other types of corrosion, a specific name describing it is used.

Galvanic corrosion is an electrochemical reaction that causes electrons to flow from one metal to another metal.  This is a chemical reaction that happens without any impressed current and in fact creates the electrical current.  One of the metals is the anode and the other is the cathode. If you put the two in an electrolyte that conducts current, and connect them with a wire, they act pretty much the same way a battery does.  A current flows in the electrolyte between the two metals, and through the wire.  The atoms from one are “sacrificed” and plated onto the other metal.  This happens when you have dissimilar metals such lead and zinc (a battery) and when aluminum and bronze are close to each other. The aluminum disappears because it it is losing electrons, but this is not electrolysis.

BOATUS: Galvanic Corrosion

For this reason boats with aluminum hulls, stern drives and outboard engines often have a special sacrificial anode bolted to them.  The anode, which is often made of zinc, but it can be other metals such as magnesium, gives itself up before the aluminum does and saves the aluminum. The anode is connected to the dissimilar metals through the bonding system.  Often these are generically referred to as zincs.  This is not correct terminology.  They should always be called anodes.

A link to a good page on anodes. Performance Metals - Sacrificial Anodes -FAQs. This page also has some very good links to information about sacrificial anodes and corrosion.

There are various methods of dealing with galvanic corrosion in addition to using anodes.  The most obvious is to minimize the number of metal fittings in contact with the water, and make them all the same metal.  For instance, a through hull fitting such as a water intake should be naval bronze or silicon bronze.  (A tip: do not use brass.  True brass is very subject to corrosion and will soon be gone.  All the fittings used on boats that are often referred to as brass are really naval bronze or silicon bronze, or some similar alloy that is corrosion resistant.)  Some fittings today are made with very strong plastics.  Plastic is, of course, non-conducting and does not corrode.  However, some people are very opposed to using plastics for fittings below the waterline, due primarily to strength and fatigue issues.  Also it can be difficult to get fittings that are all the same metal, or metals that are very similar in composition to each other.

All of the metal fittings below the waterline should be bonded.  What is bonding?  A wire is attached to all metal fittings that are in contact with the water and this wire is also attached to a common grounding point on the boat.  This puts all metal fittings at the same electrical potential, zero, so no current will flow through the water from one fitting to another.  Rather, if there is a current it will flow through the wire. But under normal conditions there is no current flow in this wire.  But like most things having to do with design of boats, some people believe that this actually promotes corrosion.   I do not believe it does and I recommend bonding.   However, there are some metals that should not be bonded together, and on aluminum boats the hull should not be part of the bonding system, and not connected to any part of the electrical system. See The Connection Between Bonding, Grounding and Lightning Protection

Galvanic corrosion is a slow process and often takes years to manifest itself.

Stray current corrosion happens rapidly, sometimes overnight if there is a enough current, usually seen as massive damage to the sacrificed metal.  There may be actual holes in the metal. Stray current corrosion is similar to galvanic corrosion in that it eats away at underwater metal and fittings, but the cause is different.  It is caused by electrical current in the water, introduced by a boat with a badly wired system or by bad wiring on the dock.  

 Normally stray current corrosion is caused by direct current, that is, twelve-volt systems on boats.  But there have been studies done that show that Alternating Current, like you have in your home and may have on your boat, can cause this too.  This is usually from bad wiring on in the dock system, or on boats around you.  DC current is usually very low, in the milliamps range, and you may not be able to feel it tingle even if you stick your hand in the water.  But it is still causing the problem.  AC current can be considerable, and can kill you.  Should you have any part of your body in the water, or even if you touch a metal fitting or the hull of a metal boat, you can receive a life threatening shock. If you are in the water the electrical current will paralyze your muscles and you will drown. This is called Electric Shock Drowning   If you suspect stray electrical currents in the water,  have an electrician check the wiring on the dock or on the boats surrounding the area and find the current leak. Do Not Ignore It!

For AC systems, the best prevention is to make sure your boat is wired properly, in accordance with standards published by the American Boat and Yacht Council (ABYC). If you have an AC system on your boat, you should have at least a galvanic isolator in the AC grounding wire (the third wire or green wire).  A Galvanic Isolator stops low DC currents, less than 1.4 amps from flowing in the grounding wire, but does not stop AC.  This green wire is there for safety and needs to be capable of conducting AC to prevent shock hazards.  Never ever cut the green wire!

The best method though is to have an Isolation Transformer or Polarization Transformer in the shore connection.  This will effectively isolate your boat’s system from the shore side system.  It should also be wired according to ABYC standards.  In Europe and much of the rest of the world you should follow ISO standards. They are very similar to ABYC standards used in the USA and Canada.

Ordinary electrochemical corrosion, rust or oxidation, is also a problem for most ferrous metals, such as steel.  These metals must be cleaned before painting to remove any corrosion or contaminants and then painted with a special primer that treats the metal.  Then they are painted with a marine paint.  If done correctly this treatment will last several years or more with occasional touch up of rust spots.  On a regular basis (sometimes annually) the metal must be cleaned by grinding or sandblasting, re-primed and re-painted.  If done as regular maintenance, steel boats can last many years.  It is not unusual to see steel boats that are fifty or more years old.

Some metals have their own built in protection.  They form an oxide on the surface that excludes contaminants and prevents corrosion.  Marine grade aluminum is one of these. By marine grade I mean 5000 or 6000 series aluminum alloys, such as 5052 or 5058.  Aluminum forms an invisible oxide on the surface.  The aluminum will not corrode unless the oxide is damaged or washed away.  Often aluminum hulls, tanks and other boat parts are not painted because there is no necessity to paint them for protection.  However, if a builder or owner decides to paint aluminum then a special process called etching is required to remove the oxide, because the paint will not adhere well to the metal. 

Aluminum and stainless steel, or any metal, should never be installed where they will always be wet.  Fuel tanks are often mounted on flat surfaces where moisture will collect under the tank.  These tanks will corrode through very rapidly.  Any metal tank should be mounted off of a surface so air can circulate around all sides of the tank.  Metal tanks should never be encased in foam for the same reason. For more on Aluminum see ,Aluminum Tanks and Boats: to Paint or Not to Paint

This is also true of marine grade stainless steels.  All stainless steels are not created equally.  Some are more corrosion resistant than others, but contrary to popular belief, stainless steel will corrode.  The secret is to keep it dry. 

Crevice Corrosion.  Stainless steels are prone to crevice corrosion. Moisture gets into microscopic hairline cracks or pits, where oxygen is excluded, and corrosion sets in. Often these cracks are in welds. However, if kept dry and checked regularly, stainless steel can last for a lifetime. So stainless is best used where it is exposed to the air and has little exposure to moisture.

Also stainless, like aluminum, needs to be marine grade, usually 300 series such as 316L. It is also a common belief that stainless steel is non-magnetic. This is not true of all stainless steels. Be sure you have the correct grade of stainless.

Marine grade 316L stainless is weakly magnetic. Small masses such as the worm screw on stainless steel hose clamps can be determined to be 316L or some other material by use of a magnet because of this weak magnetic attraction. On larger masses of 316 Stainless this may not work. How much a 316L stainless steel fastener or fitting is attracted to a magnet depends on many factors. How it was manufactured, how large a mass it is, the compostion of the alloy, the strength of the magnet, all may affect whether the item shows magnetic properties. However small masses such as a 316L stainless worm screw on hose clamps, when tested with a small magnet, will appear to be non-magnetic. Ordinary steel, or 304 stainless will display a strong attraction to a magnet.

Oregon State University: Understanding and Controlling Crevice Corrosion in Stainless Steel:

Straight Line Marine: Crevice Corrosion On Stainless Steel Propeller Shafts

Passage Maker: Steve D'Antonio: Stainless Steel and Corrosion

New 2/19/2014:

Corrosion influenced by microorganisms:  Yes you read that right.  There are microorganisms that cause corrosion on steel and other metals.  Although rare it does happen particularly to offshore platforms and other metal structures that are submerged for long  periods.   See the following article:

The Danger Of Hungry MicroOrganisms:

Wood boats suffer from a special kind of corrosion called fastener corrosion or woodburning around metal fasteners.  This type of corrosion is caused primarily by water seeps and oxygen starvation around the fastener, and over protecting the boat with anodes. Yes, you can put too many anodes on the boat.  The fasteners in wood boats are generally metal screws but boat nails, rivets, and bolts also suffer from this.  In many old boats the screws are galvanized.  At the point where the screw meets the wood and oxygen is excluded, sodium hydroxide forms, which eats away at the metal and destroys the wood immediately around the fastener.  Sometimes this can be spotted in bare wood by a dark ring around the fastener.  But more often than not you cannot see any outward sign of this type of corrosion.  Marine surveyors will often remove fasteners at random looking for it.  Any fasteners that are corroded must be replaced.  But, you cannot put a new fastener back in the same hole.  The hole must be reamed out and plugged to get rid of the rot.  Then a new hole must be drilled for the new fastener.

USCG NVIC 7-95 Guidance On Inspection, Repair And Maintenance of Wooden Hulls

David Pascoe: Surveying Wooden Hulls

Another way to detect this is to spray white vinegar on the area around the fastener. If it foams, sodium hydroxide is present.  If the fastener is fairly new and there has not been much damage to the wood you can stop the process by twice a day spraying vinegar on it.  Do this for several days until it no longer foams.  But you also need to correct the cause.

Another variation on this type of corrosion occurs in boats with wood hulls and metal interior structure.  This used to be called composite construction, but today that term usually refers to fiberglass boats.  The metal will corrode where the metal meets the wood.  During yard periods when a boat is out of the water, a few planks should be pulled and the metal interior structure checked for this condition. If not, the framing and bulkheads can deteriorate until the hull fails.

The message for boat builders is:  build the boat right. Pick the right materials. Install them correctly. Install electrical systems correctly. Do not overprotect with too many anodes. Follow ABYC standards. If you are not sure consult an ABYC certified corrosion technician.

References: There are thousands of resources available on the Internet.  Here are a few.

Yacht Corrosion Wiki:

To Bond Or Not to Bond:

Types of Marine Corrosion: Don Casey BoatUS.

Corrosion: David Pascoe:

Corrosion Prevention for Metal Boats: Michael Kasten

Corrosion Protect For Metal Fittings

Electro Guard on Boat Corrosion


Metal Corrosion in Boats: Nigel Warren
The Boat Owners Guide to Corrosion: Everett Collier

Published Standards:
(Current versions of theses standards must be purchased from ABYC )
ABYC A-28 Galvanic Isolators
ABYC E-2 Cathodic Protection 
ABYC E-11 AC and DC Electrical Systems on Boats
ABYC Store


ABYC. Marine Corrosion Certification

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