One of the greatest things about boating is that you can do it in so many different environments. From pristine lakes and long flowing rivers, to busy bays or the beautiful wide-open ocean. No matter where you choose to use your boat, it still needs protection from corrosion, and just like all waterways are not created equal, it is important to remember too, not all anodes are created equal. Today we are going to review the effects of SALTWATER, because it is the most common type of water when talking about boating. Come back next week to learn more about BRACKISH and FRESH WATER boating environments.
As a boater, you have 3 options when it comes to anodes; Zinc, Aluminum and Magnesium. Each one of these has a water type that it works best in to protect your boat.
We learned in week #3, corrosion happens because ions are pulled from a less noble metal (the anode) to a more noble metal (the cathode) in the presence of an electrolyte solution. As you know, saltwater, brackish water and fresh water are all electrolyte solutions, but they each have different conductive properties. Their varied conductivity is because they consist of different ratios of solute (the extra stuff that is dissolved in a water solution). This causes the ions that make up the anode to be pulled off at different rates in the different solutions. That is why there are 3 different metals for the 3 different types of water.
Saltwater has the highest salinity of the 3, which in effect makes it the most active when considering galvanic corrosion. Zinc was the first material used as a sacrificial anode, because of this, zinc has been the “go to” for boaters in saltwater. However, there is plenty of information showing that aluminum anodes can also be very effective in this environment. That’s not to take away from the effectiveness of zinc, we just want you to know that you have options. Here is some valuable information about zinc.
If an active zinc anode is taken to fresh water, the surface of that anode will become covered with the crust of oxide, because of how fresh water solution reacts with zinc. This oxide effectively creates a seal around the anode that stops the transfer of ions, causing the anode to become inactive or useless. In this case, corrosion will find the next lowest metal to eat away at, probably a piece of your boat.
Similarly, when an active zinc anode is exposed to air it stops the flow of ions (passive), again causing a seal. It may not be visible, but once a zinc anode is passive, even if it is placed back in the same water environment, it will not be as effective. This passivity may cause the next most anodic item within your anode system to start to sacrifice itself, and that will most likely be a part of your boat.
