Port Credit Marine Survey
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Society of Accredited Marine Surveyors

American Boat & Yacht Council


Chemical changes in a solution or electrolyte due to the passage of electric current.

As a Certified Corrosion Analyst I'd like to clear up the "ELECTROLYSIS", issue. This term is frequently misused due to lack of understanding of the processes involved in corrosion.

ELECTROLYSIS is actually the opposite action to galvanic corrosion so the distinction is NOT trivial semantics as some people suggest.

ELECTROLYSIS is the forced introduction of an electrical current in an electrolyte (water) to cause a chemical reaction to separate the components of the water. This process produces Hydrogen and Oxygen.

GALVANIC CORROSION on the other hand is an electrochemical reaction that causes electrons to flow from one metal to another metal. 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 between the two metals. The electrons from one are "sacrificed" and plated (somewhat) onto the other metal. This happens when you have dissimilar metals such as aluminum and bronze close to each other and in "electrical" contact. The aluminum disappears.

STRAY CURRENT CORROSION is simply corrosion caused by stray (leaking) current from a bilge
pump or a battery charger among other things.

There are many more types of corrosion however, the two main issues on boats involving electricity are either GALVANIC CORROSION or ELECTROLTYIC COROSION (commonly referred to as "stray current corrosion".

Punch line ........ Do not take advice from or hire a marine electrician who uses the term electrolysis.
1. Galvanic corrosion : Corrosion that occurs at the anode of a galvanic cell.

You may remember high school chemistry class where a battery was created by connecting two dissimilar metals with wire and immersing the whole contraption in salted water thereby activating magnetic fields and starting an electrochemical process causing current to flow.

On a boat with bronze, aluminum, galvanized and stainless steel that are connected with bonding wires or simply touching each other and immersed in the lake...... you accomplish the same thing. The more noble metal is the "cathode", the less noble, the "anode". In this process the less noble metal gives up electrons to the more noble thus weakening the metal, otherwise known as "galvanic corrosion".

The "sacrificial" anodes on your shafts, trim tabs etc. are supposed to sacrifice themselves thereby protecting expensive metal parts. This is why it's important to keep your anodes in good condition and never paint them. let's never refer to anodes as "zincs" as anodes come in three basic materials for different water conditions i.e. Aluminum alloy, and magnesium for fresh and brackish water or zinc for salt water. This topic deserves a little more attention on it's own so take a look at Zincs, Aluminum and Magnesium Anodes.

A vessel suffering from galvanic corrosion is usually the source of it's own problem, although two vessel's linked by shore power grounds can create a galvanic cell between two very close boats.

2. Electrolytic Corrosion (commonly referred to as stray current corrosion) : Corrosion that results from an electrical source causing a metal in contact with an electrolyte (water) to become anodic with respect to some other metal in the same electrolyte.

In simple terms a wire touches something it shouldn't, like a faulty bilge pump float or degraded wiring lying in the bilge sending current into the water, causing one metal to give up electrons and corrode. Again any vessel suffering from this type of corrosion is likely the master of it's own disaster but the culprit could also be a neighboring vessel. This type of corrosion can can eat metals at an alarming rate. I know of one 42' motoryacht that lost both shafts, both rudders and both propellers in a space of less than two weeks.

Complicating this picture somewhat is the fact that DC can be super-imposed on your AC wiring through the common ground on board or the ground in the shore power pedestal we all share on the dock. As all vessels in the marina are connected through shorepower grounds there is potential for widespread damage. Aside from concerns of corrosion there is also potential for electrocution if shorepower cords are allowed to lie in the water let alone the fools that leave their shorepower cord plugged in at the dock while they go out for an afternoon cruise.

Recent tests have shown that AC current from shorepower in the water can also cause corrosion to underwater parts although at a much slower rate than DC. This has been a long argued issue by people who know a lot more about this than me. Ground fault protection systems, galvanic isolators, isolation transformers and impressed current systems are some of the various methods attempting to combat corrosion.

Salt water is generally regarded as a more serious breeding ground for marine corrosion as the salt makes the water more conductive however, polluted fresh water can be even more conductive with the right contaminants.

With our aging fleet of pleasure craft it's likely that at some time, less than expert hands have played with your electrical system. If your vessel is suffering from any electrical faults or unusual corrosion consult with an American Boat and Yacht Council Certified marine electrical technician with specific corrosion control training or give me a call and I will try to set you up with an expert in this field.

Wallace Gouk
Society of Accredited Marine Surveyors, 2000 Ret'd
ABYC® 2001 Ret'd
ABYC® Standards Certified, 2009
ABYC® Certified Corrosion Analyst, 2015
Transport Canada Licensed Master, 2002 Ret'd
Transport Canada Tonnage Measurer, 2004
BoatUS® Approved Marine Surveyor, 2003

A Marine Surveyor for Ontario from Mississauga, Toronto, Hamilton to Oshawa, Whitby, Newcastle, Pickering, Trenton, Kingston, Gananoque and beyond