Electrolysis……. What’s That ?



It’s a term that’s widely misused.    Many times as boaters, we hear (and use) the word electrolysis.  But what is it……. Really ?

In the marine world, we are used to speaking about corrosion and/or decomposition of the underwater metal components of our vessels as suffering from electrolysis.  If we better understand what is happening, then we can help prevent (or reduce) the costly destruction.

Let’s start by understanding the players.    In the case of our family boat,  we have a metal component, submerged in water.  (Sometimes we have a shore power cord in the picture as well, which we will talk about later.)  Also, there are two different types of  corrosion.  They are termed “GALVANIC” and “ELECTROLYTIC”.

Elements of the periodic table can be classified as metallic, or non-metallic, for a variety of characteristics.  Two of those characteristics we are particularly concerned with in this discussion are:

1)  Metals generally have 1-to-3 electrons in their outer shell, where non-metallic have 4 or more.

2)  Metallic elements tend to lose electrons in their outer ring more easily.

We also know that when the common metals we deal with in boating are  submerged into an electrolyte* (water), they will begin losing protons to the solution, leaving behind electrons, which give the submerged metal a negative electrical charge  This negative charge can be measured in millivolts


* An electrolyte is any solution which can conduct electricity.

Now, if  a second dissimilar metal is introduced into the same solution, and there is an electrical connection between the two metals, we will begin seeing a flow of electrons through the electrolyte, whereby one metal will erode more rapidly than the other, as the two metals attempt to “balance” their electrical charges.   This phenomenon is known as “GALVANIC CORROSION”.  It’s worth mentioning, that an electrical connection can be accomplished by mounting (or attaching) two components together, without the use of wire.

Electricity is defined as the “Flow of Electrons”.

All metals have a different molecular structure, thus giving them different “base” electrical potentials.  The pecking order of these charges are listed from least -to- most negatively charged on a “Nobility” scale.  The most negative charge, or least noble, erodes more rapidly.

One method to reduce the amount of  galvanic corrosion is to ensure each component is  electrically connected or “bonded” to the others, and install sacrificial anodes into the bonded network.  The sacrificial anodes, commonly referred to as “zincs” will become the least “noble” metal and will thus sacrifice themselves, and preserve the more valuable components of the boat.

A second method of preserving the valued components is to “impress” a current, or electrical charge onto the bonded network.  There are several manufacturers of these “impressed charge” systems on the market today.  Many of the sterndrive manufacturers protect their drive units with these devices.  In doing so, they are primarily concerned with protection of their own unit, and not the whole boat. Therefore, they wish to remain isolated from the bonding  system, so you sometimes will see a border of un-painted area about an inch wide around the mounting point of the drive, to the hull.  This is done because many bottom paints will conduct electricity, and thus become the bond connection to all other through hulls on the boat.

But, what about that pesky shore power cord I mentioned earlier?  This brings another aspect into the picture, called “ELECTROLYTIC CORROSION”.  This occurs when an outside electrical connection is introduced into the vessel.  A malfunction on our boat, or even a neighbor in the harbor could find an electrical path to earth ground, through the shore power cord’s green wire ground.  This DC (Direct Current) could provide an easy path for electron flow, which is capable of rapidly increasing the galvanic process.

Any boat with a shore power connection will benefit from a galvanic isolator.  The objective is to eliminate the possibility of DC current flow of corrosion from flowing to earth ground, via the green wire of the shore power cord.   Galvanic isolators are installed in the green wire of the shore power connection, and through a network of diodes and capacitors, will eliminate DC flow through the shore power cord.

This has been a ten thousand foot overview of a subject which can (and does) fill volumes of literature.

If you are experiencing a corrosion problem with then look into it.   Help is available… call me.  A few tips for happy boating:

Corrosion surveys can save you money, by ensuring proper protection of the boat.

NEVER SWIM IN A HARBOR.  Stray currents from ANY boat can be very dangerous, and YOU could become the electrical connection to “earth ground”.

And, oh yes:   ELECTROLYSIS is a widely misused term, which technically has no meaning in a marine context.  It refers to the degradation of an electrolyte that occurs as a  result of passing an electrical current through it.


Ron Smith

Principal Surveyor For:

Tri-State Marine Services, Inc.

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