The Mystery of Anodes!
By Tom Hale
Every year we put new anodes (zincs) on our boats “to protect the propeller from electrolysis.” So, what exactly are we doing? Why exactly are we putting anodes onto our boats? Few topics in boating are as mystifying and misunderstood, as “electrolysis.” ( In the marine industry the term electrolysis is not used when discussing corrosion. This corrosion process is “galvanic cell corrosion”.) The actual process of galvanic corrosion is explained in many texts. This article is intended merely to explain how it affects your boat. If you are interested, you will be able to collect plenty of technical information on engineering web sites. Be cautious about researching this subject on boat discussion groups as there is much misinformation available there.
Galvanic corrosion is the very same basic chemical reaction as a battery. In a battery you have two poles connected to plates of two different types of lead, sitting in sulfuric acid (an electrolyte). One is positive and one negative. They sit there just in a sulfuric acid electrolyte just fine until they are connected together in an electrical circuit. Then the current will flow from positive (cathode ) to negative (anode). Internally metal ions are flowing from the anodic plate to the cathodic plate. Galvanic cell corrosion requires two different metals directly in contact with each other or tied together with a wire and simultaneously immersed in seawater (an electrolyte). When two different metals are connected in saltwater, a small electrical current will flow between the two metals (just like a battery) . One will lose ions and be corroded. In this simple electrical cell there are two electrical components, the Cathode and the Anode. The anode is the component which gives up ions and is corroded away so that the cathode is protected. When we add an anode to the boat’s galvanic protection system, we are providing a sacrificial metal which will be preferentially destroyed to protect any other metal fittings also in the salt water electrolyte and connected to it.
With a boat there can be a number of different metals underwater and many are bolted together or tied together with a grounding system . You have a shaft(s) and propeller(s) and rudder(s) and perhaps trim tabs. Plus, there may be through hulls that could all tied together in a bonding system. There result is a complicated galvanic cell with several different metal alloys. When a boat is launched these all start the process of galvanic corrosion and whichever piece of metal is the most anodic, will be consumed. When it is gone the process attacks the next most anodic metal. An anode is installed in the system to be the most electrically active metal in terms of corrosion potential. Every year we install new anodes, so that they will be sacrificially destroyed to protect the rest of the underwater metal.
Typically, our goal in providing galvanic anodes to the system is to protect the propeller from its stainless steel shaft. Even a bronze propeller on a bronze shaft will benefit from adding anode anodes. because bronze is an alloy made up of copper, tin, anode, aluminum, zinc and iron. So these different metals are cast together. They share properties and as a result of the alloying, the alloyed metals are stronger together than alone. They share physical properties and they also share intergranular surface boundaries. You cannot have better electrical connections than an alloy. So once immersed into the water the various elements of the alloy begin to trade ions with the weaker elements giving up ions to the stronger. There are anodes and cathodes on the surface of your bronze propeller. Gradually the metal is eaten away, and electrons are conducted from the anode to the cathode. If you have ever seen a bronze propeller with pink spots, this is a propeller where the anodic component (usually zinc) of the bronze alloy has corroded away. It has been removed from the alloy as a result of the galvanic process, leaving bright pink copper behind. This process is call dezincification. But what does it mean to you? It means that connecting anode in close proximity to your propeller is quite important; the anode will be sacrificed instead of the zinc in the propeller alloy.
Here are the common questions about anodes.
- After a season your anodes are almost gone. Is this normal?
- If after a season your anodes are as good as new. Should you reuse them for a second season?
If after a season your anodes are almost gone, they are doing their job. At the end of the season, you hope they are used up. Lost anode material proves your system is working and the anodes are protecting your underwater metals from each other.
If after a season your anodes are as good as new, you might have a problem. If the anodes are as good as new, then they probably are not eclectically connected to the rest of the underwater metal. Remember, without a conductor to the other metal, the anode is not able to protect it. If your anodes are “good as new” after a season, then you probably have a problem.
Always install new anodes every year. After the boat has been stored on the hard, the anode’s electrical contact surfaces will have coating of an oxide which is an electrical insulator. Using anodes for a second season will give you markedly less protection. Always clean the contact surfaces where the anode is attached back to bright metal. A clean electrical connection is essential for the anodes to work for you. Dirty electrical connection means that the anode is not able to provide protection.
Zinc or Aluminum Anodes?
The rate of galvanic corrosion is influenced by salinity. The rate of anode consumption is similarly influenced by salinity. Boats in fresh water or brackish water with very low salinity have much slower corrosion and anode wear than boats stored and used in saltwater, or vice versa. In selecting the anode metal for a galvanic bonding system, the rule of thumb has been, use zinc anodes for salt water and aluminum anodes for fresh water. There are many places where we store our boats in fresh water, but cruise in saltwater which presents an interesting conundrum. So, do we use the anode for saltwater or the anode for fresh?
When zinc anodes on a saltwater boat enter a freshwater environment the electrical driving force drops. The anode will develop a hard, calcareous, coating shown in the picture below. In this case the anode was on the boat in salt water for 5 months and then in fresh water for 6 weeks. This coating insulates the zinc and upon return to the salt water the anode will provide much lower protection. Aluminum anodes on the other hand can make the transition from the salt water regime to fresh water and back again without acquiring this coating and so will continue to provide galvanic protection. Consequently, in the past decade we have begun to see a shift in anode material selection to aluminum anodes for boat which transition from salt to fresh. Now, many boats which are both stored and used only in salt water are using aluminum anodes as well. As a galvanic anode the aluminum anodes provide a very slight favorable shift in galvanic protection compared to zinc, for boats in salt water. The anodes work better, last longer than zinc anodes and they cost less than zinc anodes. ABYC the US boat building standards organization now recognizes both aluminum and zinc as being appropriate for use in the galvanic protection of boats in salt water.
- If you decide to switch to aluminum anodes on your galvanic protection, you must change all the anodes, you cannot mix zinc and aluminum anodes, or the aluminum will be protecting the zinc anodes from corrosion. (this means changing all shaft anodes, rudder anodes, trim tabs, hull anodes and thrusters.)
- The salt water in the engine cooling system is electrically a different electrolyte. Experience shows that while zinc engine anodes also build the calcareous coating the speed of the water flow knocks it off and exposes fresh zinc metal. If you use aluminum alloys for the running gear, you do not need to switch to aluminum anodes in the engine cooling system.
Do not try to save money by getting two seasons out of your anodes. If the boat has been hauled for storage, remove the old anodes. After removing the old anodes (3. picture below) you can see just how dirty and corroded the contact surface is. With the very low voltages (milli-volts) in galvanic protection, you must have an excellent electrical connection. Sandpaper, wire brush or otherwise scrape and clean the base metal till it is shiny. (4 picture below). For added electrical continuity. I use a tiny dab of Koper-Shield, a highly loaded copper grease, which improves the electrical connection. Some use a dielectric grease, but a dielectric grease is an insulating grease not a conductive paste.