Galvanic & Electro-Chemical Corrosion: The basics

This was a pamphlet Mercury put out a bunch of years ago and it looked just like my physical science book I had in High School. But, the basis of the science involved in corrosion and boats  is covered in a very graphic presentation that almost everyone can understand.


You see the effects of corrosion everywhere. Problems of corrosion are not confined to marine engines and watercraft...they work their havoc on valuable property in practically every area you can think of in everything from the rusting of your car's body metals to the gutters on your house. Corrosion is as expensive as it is ugly on a monstrous scale. At last estimate, corrosion-caused property damage in the United States alone added up to some 10 billion dollars every year.

The nature of corrosion
All corrosion is not alike. The one that most concerns you, the boat owner, is the silent destroyer called Galvanic Corrosion.
What part of your power plant is most vulnerable to corrosion? Answer: The aluminum drive....whether yours is an outboard or a stern drive engine. But don't blame the aluminum. It is by far the only and the best metal for the purpose. Aluminum resists corrosion quite well when used alone.. because a film seals the metal surface with a protective oxide coating. But--combine aluminum with other metals, that must be used in the drive train ..and corrosion becomes a problem.

What happens is this: A natural, electro-chemical process is set in motion when different metals meet under water,termed galvanic coupling. It causes aluminum to become corroded because it's a chemically active metal, while its less active metal partners are actually protected from corrosion by the process. The following pictures tell the story.

Anytime you submerse two pieces of dissimilar metal in brackish or saltwater, you create an electrical potential between them.

Now-- connect those metals together with a conductive material, and an electric current flows The principle is the same as that of a flashlight battery.

The combination of anode and cathode in a flashlight battery is designed to supply electrical power.

That battery effect is no good at all when the pictured anode happens to be the aluminum lower unit on an outboard or stern drive. Galvanic coupling is now at work producing corrosion.

Fortunately, with these facts of life in mind, engine designers have a wide range of metal choices to consider and select from. They range all the way from highly active zinc to highly corrosion-resistant platinum.

So, the crux of the problem is this: When natural current, which is present with dissimilar metals in water, flows between the anode and the cathode. . (the cathode being a component of the interconnected steel of the outboard or stern drive)...the end result is the deterioration of the anode. What you have then is known as galvanic corrosion.

To understand what happens in the electro-chemical battle between the metals, lets divide them into two general classes, those called anodic, or active, and those that resist corrosion, the cathodic, or passive metals. Now, while the anodic metals are gobbled up by corrosive forces, the cathodic metals survive undisturbed.

To demonstrate this principle clearly let's take a look at this graphic experiment.

When you put a strip of aluminum and a strip of steel in a solution of sea water and connect them with a wire, the natural electric current begins to flow. As current flows between these strips, particles of aluminum are released into the sea water. This is galvanic corrosion eating away at the aluminum, while the steel stays intact and bright. Here aluminum is sacrificing itself while saving the steel.

Now, let's turn the tables a bit and use zinc for anode and aluminum for the cathode.

Note how the zinc anode is eaten away while the aluminum cathode stays sound and shiny.

The last experiments you've just seen simulated, are sound reasons why zinc trim tabs are found on all Mercury outboards and MerCruiser stern drives. (* See note Below) They are convincing proof of the effectiveness of this corrosion-fighting method in which the zinc tab sacrifices itself to protect the aluminum drive.

But that's not half the story. We go to much greater lengths to build high resistance to corrosion into your Mercury outboard or MerCruiser stern drive.
Good examples: In every part where its design feasible, we use nothing but the finest stainless steel, which is highly resistant to galvanic corrosion. Like in our drive, prop, and shift shafts. And to isolate metals from direct contact with dissimilar metals, we use nylon and other neutral materials to keep them apart. Corrosion can occur only when dissimilar metals are interconnected Mercury Marine also uses a nearly impervious, multi-step paint protection system that provides a barrier to the flow of galvanic current between anodes and cathodes. This is another basic defense against corrosion.

So you get a lot of peace of mind when your boat is powered by a Mercury outboard or MerCruiser stern drive. There's just one thing to remember--check the zinc trim tab regularly. Remember-- because of its basic principle, the zinc tab is deliberately designed to be eaten away gradually, to protect the life of your aluminum lower unit. Replace this protective tab when necessary and you'll be amply repaid by solid protection from corrosion.

* Note: Today, almost "All" of Mercury Marine anodes are made from an aluminum blend that enhances their corrosion protection. Since the publication of this article, experience showed that under certain circumstances, zinc anodes could glaze over and reduce their corrosion protection ability.

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