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Quick review (if you want it)…
All types of water are considered an electrolyte solution. All metals are susceptible to corrosion when placed in an electrolyte solution. Stray current leakage will increase the rate of corrosion. The relationship between dissimilar metals in this environment is relative. The metal with the lower atomic number (check out a periodic table if that is something you didn’t retain from high school) will become the anode in this relationship, and the metal with the higher atomic number will become the cathode. The anode sacrifices itself to keep the cathode intact.
You want to a Martyr sacrificial anode to sacrifice itself instead of an important metal piece of your boat! But which one is best for your boating environment, Aluminum, Zinc or Magnesium? (Check out WEEK #5 to review which anode is best in saltwater). Today we will tell you about brackish and freshwater.
Just in case you don’t know, brackish water is a combination of saltwater and fresh water. This is typically found in bays or harbors that have fresh water rivers running into them, or in tropical coastal areas where there is a lot of rainfall that runs into the ocean. So, let’s talk about brackish water first…
Aluminum responds best in brackish water, because brackish water has a lower salinity than saltwater. Since the salinity (salt content) in brackish water is lower, the metal anode can be “weaker” (have a lower atomic number) and still have the same ability to protect your boat because the water around it is also “weaker” in reference to electrical current.
Aluminum has also become very popular because it is a lighter metal, making it easier to ship, which in turn typically makes it cheaper than its zinc and magnesium counterparts. Another great thing about aluminum is that it is the most environmentally friendly anode of the 3 that can be used to protect your boat.
Magnesium has the lowest atomic number of the 3 options. Following the same thought process as above, that makes it best in fresh water, which has the lowest salinity of the 3 water types. If you were to try to use magnesium anodes on your boat in saltwater instead of zinc or aluminum you would have to change your anodes 3 times as often to protect your boat.
We want you to get the most out of your anodes and to protect your boat in the best way possible, so remember this…
SALTWATER = ZINC or ALUMINUM
BRACKISH WATER = ALUMINUM
FRESH WATER = MAGNESIUM
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.
If you’ve been paying attention, now you understand why metals corrode, but wait, there’s more information that can help you take care of your boat. Anodes can and do erode at different rates. One of the most common reasons for an increased rate of erosion is caused by stray current leakage. This leakage is referred to as electrolytic corrosion. Galvanic Corrosion (see ANODE ACADEMY WEEK #3) and Electrolytic Corrosion are often confused with each other, but it is important to know the difference. Here’s how it works and how you can protect your boat.
Electrolytic Corrosion is driven by an external source of electromotive force (EMF). EMF can also be referred to as stray current leakage. This leakage can come from any external power source like your shore power connection, a battery on your boat or the wiring that controls the lights. Stray current leakage typically occurs with a damaged or worn out wiring system or poor installation of electrical equipment. This lack of grounding causes an increased amount of electrical activity which accelerates the rate of corrosion to an anodic metal.
Here are some examples…
#1- If you have twin inboard motors and the shaft anode on the left/port shaft always seems to erode at a faster rate than the one on the right/starboard, you most likely have a wiring issue internally near the left side. It could be a wire connection that has come loose, worn out, or was never grounded properly in the first place.
#2- If you move to a new marina and notice that your anodes are eroding at a faster rate than they did at the old marina, there may be wiring issues with the shore power, or a boat around you may have wiring issues that cause increased electrical activity around your boat.
The bottom line is this, there are numerous factors that can cause damage to your boat, be sure you are using Martyr sacrificial anodes to protect it.
Come back next week to learn which type of anode you should be using for your boating environment.