How Do Boats Corrode? Part 1: Cathodic protection

Written By Geoff Dargan
Sailboat Boat Zinc Anode

INtroduction

When we use zinc (or any other material - more on that later) anodes, we’re using cathodic protection in order to protect your underwater metals from corrosion - both from corrosion in salt water and from each other.

Most sailors know how to change the zinc anodes on their propeller shaft and propeller. Pop off a couple few bolts, pop clean up the mating surfaces, and put the new ones on. Simple. But do you know what the science is behind how they work? What do “zincs” do, and what do they have to do with stopping corrosion? At the dock or at the yacht club, you’ll hear all kinds of terms thrown around with regards to corrosion: electrolysis, a galvanic reaction, galvanic corrosion, etc. It all seems so complicated, and all these potential gremlins are swirling around ready to bite your boat. How do we keep all of these risks straight? Well, it’s not as complicated as it sounds; these terms are all related! In this knowledge base page, we explore the world of corrosion in the marine environment.

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Resources

While researching for this article, I came across some great online resources (and some not-so-great resources, mostly on forums). For further reading, check these links out:

What is Cathodic Protection?

Cathodic protection is the act of protecting one (or more) metals from corrosion that are immersed in an electrolyte (sea water, or less so, freshwater) from corrosion by introducing a third, more corrodable metal to the mix. Metals corrode in electrolytes by losing electrons (negatively-charged particles) to the electrolyte (sea water), causing the metal to break down. If we want to protect a metal immersed in an electrolyte, we can attach a more corrodable metal to it that will donate its electrons more readily to to the electrolyte. This will sacrifice the more corrodable metal and protect the less corrodable metal. In the realm of corrosion science, the property of the readiness of a metal to corrode is called nobility. A metal - such as zinc, aluminum, and magnesium - that readily corrodes is less noble than a metal that does not readily corrode - such as stainless steel and bronze.

Cathodic protection - visually

This very cool picture shows cathodic protection at work. The nail (iron) and copper wire (copper) are submerged in an electrolyte in a petri dish. Positive ions (the green areas around each side of the corroding nail) and electrons (the pink area around the copper wire twisted around the nail) are diffusing into the electrolyte.

The nail, made of iron, is less noble than the copper wire. The nail is protecting the copper wire from corrosion; the copper wire is still visually shiny while the nail is clearly rusting (corroding). Read on for more detail about what is occurring in this reaction.

The chemistry behind Cathodic Protection

Cathodic protection requires four conditions, all applied at the same time, in order to work. They are:

  • An anode metal (such as the zinc on your propeller shaft)

  • A cathode metal (such as your propeller shaft or propeller)

  • The anode and the cathode electrically connected to each other

    • This electrical contact could be electrical wiring connecting the metals, or it could just be direct, physical contact between the metals

  • The anode and the cathode are BOTH immersed in an electrolyte (such as the zinc attached to your propeller shaft/propeller and immersed seawater)

    • If only parts of the metals are submerged, only the parts that are submerged are the parts that will be affected. However, BOTH metals must be submerged - either fully or partially. If only one metal is submerged, the reaction won’t work. You’ll understand why this works later in this article.

I think that the best way to think about cathodic protection is to start with the understanding that there are two sides of the reaction. On the anode side (the nail in the figure above or the zinc on the prop shaft) there is a oxidation reaction happening. In the presence of the electrolyte (sea water) the nail (or the zinc) is breaking down and releasing both positive ions and electrons. Where the positive ion and electrons go when they are released from the surface of the cathode is important to understand.

In the case of cathodic protection, the positive ions are released into the electrolyte (the green coloring surrounding the head and tip of the nail above). The electrons, because there is a preferred pathway (the electrical connection between the two metals) are transferred to the cathode side. On this side, a reduction reaction is occurring in the presence of the electrolyte. The electrons donated from the anode side are released from the surface of the cathode (the copper wire or the prop shaft/propeller) and react with the ions in the electrolyte in order to form a more stable compound. Since the electrons released from the surface of the cathode are “donated” from the anode, no corrosion takes place on the surface of the cathode. The anode is protecting the cathode from corrosion. Pretty cool.

The next drawing shows a cathodic reaction of zinc and copper in a HCl (hydrochloric acid) electrolyte. While the metals and electrolyte shown in this figure aren’t exactly the same as the reaction that occurs at our bronze and stainless steel metals in seawater, the fundamentals of the process are the same.

In this drawing, zinc and iron plates are both immersed in the same electrolyte and connected electrically together (they could be physically in contact to have the same affect), just like a prop shaft or propeller with a zinc anode attached directly to it. In the oxidation reaction on the left, the zinc corrodes, releasing positive zinc ions (Zn2+) from the surface of the zinc plate (causing the zinc plate to break down) and also releasing electrons (2e-) into the cathode, on the right side, through the metal wire connection. On the surface of the cathode, the electrons react with the electrolyte to form a more stable compound. Since the electrons were “donated” from the anode, the cathode is protected from corrosion, while the anode corrodes.

Geoff Dargan
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How Do Boats Corrode? Part 2: The Galvanic series and galvanic corrosion