Galvanic Corrosion and Your Boat

According to Wikipedia, "Galvanic corrosion (also called bimetallic corrosion) is an electro-chemical process in which one metal corrodes preferentially when it is in electrical contact with another, in the presence of an electrolyte. A similar galvanic reaction is exploited in primary cells to generate a useful electrical voltage to power portable devices."

What does this mean to boat and yacht owners, you may ask?

Let's break it down like this: 

Galvanic corrosion is an electro-chemical process in which one metal (Bronze Propeller) corrodes preferentially when it is in electrical contact with another, (Stainless Steel Shaft) in the presence of an electrolyte (Salt Water)

This "electro-chemical" process can wreak havoc on your vessels running gear, trim tabs, rudder hardware, bronze thru-hulls and intake grates if left unchecked. So, how do we ensure it isn't left unchecked?

The way you counteract galvanic corrosion is to add a third metal into the circuit, one that is quicker than the other two to give up its electrons. This piece of metal is called a sacrificial anode, and most often it is zinc. In fact, most boaters refer to sacrificial anodes simply as zincs, although sacrificial anodes can be aluminium or magnesium. 

Here are some valuable tips concerning protecting your vessel from galvanic corrosion: 

1) Vessels, depending on their size, running gear and design; require a specific amount of anode protection so, a boat owner should never reduce the overall quantity of zincs on their boat. 

2) Anodes come in different alloy mixtures and each alloy works best in a certain environment. (eg: Magnesium only works in fresh water) so, don't make changes without consulting a professional. 

3. Anodes should be checked regularly and preferably by the same person. This allows for a constant comparison of the anode from one inspection period to another. The accepted standard in the marine industry is to replace anodes once they have reached 50% metal loss however, this can be extremely subjective when viewed by different people. Additionally, water conditions or stray voltage from a marina can cause significant change in an anode from one inspection to another. Having a single person responsible for monitoring the loss will prevent a sudden change from affecting your vessel in a very dramatic way. 

4. Zinc anodes possess one flaw: When used in brackish or fresh water they are prone to developing a calcareous coating, a whitish material that essentially puts a zinc anode to sleep. Many sailors mistakenly perceive these especially “long-lived” zincs as effective. In reality, they’re anything but; even where anodes are still present, they provide no anodic protection at all. An anode that lasts for an inordinately long time probably isn’t working for any number of reasons, calcareous coating or otherwise.

5. Finally, ensure replacement anodes meet stringent military specification standards: A-24779, A-18001 or A-21412 (for aluminum, zinc and magnesium, respectively). These standards ensure the purity of the alloy, an important factor in an anode’s effectiveness, resistance to self-fouling and longevity. Inexpensive anodes often represent false economy and can put your vessel at increased risk for damaging corrosion.