Hello, I wanted to write today to talk about galvanic corrosion. For those of you not familiar with galvanic corrosion it is an electrochemical reaction when one metal corrodes from another when both are immersed in an electrolyte that is conductive (such as sea water). Dissimilar metals and metal alloys have different electrode potentials when two or more of these metals exist in the same electrolyte. When this happens a galvanic couple is created and depending upon the nobility of the metal (which I will talk about further on in this article) one metal will become the anode and another metal will become the cathode (both are electrodes). To explain the difference between an anode and a cathode in simplest terms, an anode is the metal that current flows in to, a cathode is the metal that current flows out from. Do not confuse an anode and cathode with positive or negative charges or voltage. An anode and cathode can both be positively charged. It is the direction of current flow between the two metals that matters. Once the galvanic couple is formed between the two metals, the anode metal will dissolve into the electrolyte (the sea water). During this process the charged ions and atoms are interchanged with electrons at the electrodes and pass through the sea water. This process is known as electrolysis. In a simple way to explain this, electricity passing through the water between two electrodes is electrolysis. The resulting reaction is galvanic corrosion. The rate at which this occurs is dependant on water temperature, the salinity in the water, nearby dissimilar metals in the water, and free flowing current that may exist in the water. This galvanic couple process is how a common household battery works. The result is the electric current produced from the reaction. Given the size of a body of water (such as in a salt water river at a marina) this electrochemical reaction can be measured in millivolts (mV).
Now knowing the basic concept of how galvanic corrosion occurs, imagine all of the underwater dissimilar metals that exist in salt water at a marina near the ocean. Add in warm temperatures, some electic leakage from underground cables or stray current coming from other yachts in close proximity to each other, the stage is set for potential aggressive galvanic corrosion on underwater metal boat fittings. Galvanic corrosion will start at the sacrificial anodes on the vessel. Given the environment, this rate can happen slowly or rapidly. Once the sacrificial anodes have completely dissolved away on the boat, it will attack the more noble metals. Most metals are not pure steel or aluminum. There may be less noble metals that were melted in with the steel or aluminum at the time they were forged such as magnesium or zinc. On marine surveys I sometimes see the early signs of this by observing “pitting” of the steel or aluminum where these less resistive metals have begun to corrode out of the steel or aluminum from galvanic corrosion. Eventually if not caught or corrected, the galvanic corrosion will dissolve all of the less noble metals and attack the more noble metals on the fittings until they are gone. The galvanic corrosion will work its way up the shaft and even into the engine corroding the heat exchangers and coolers mostly from what I see on surveys. Once galvanic corrosion has made its way into the engine (providing the shaft seals don’t leak first and sink the boat), it is pretty much game over for the vessel. Replacing a large diesel or gasoline engine will cost you well over $10,000.00 in the cost of the new engine and the labor to install it.
My advice to boat owners is simply this; Do not assume that your boat is protected from galvanic corrosion no matter how new or recently serviced your boat may have been. Sacrificial anodes on the boat should be checked at least once a month. Other precautions against galvanic corrosion is the installation of a galvanic isolator which provides the continuity of AC electrical current and will block the flow of galvanic corrosion causing currents. Be sure all of your underwater metal fittings are bonded together with at least 8 AWG green colored insulated copper stranded boat wire. This will insure all of your underwater fittings are at the same voltage potential. This will also prevent current from flowing between your underwater metal fittings, which we now know is a bad thing. Many times on surveys I find bonding wires corroded between or at the metal fittings. If you have aluminum underwater fittings, keep in mind that aluminum is very low on the galvanic nobility scale. That being said, if you have an aluminum saildrive, propeller, trim tabs or other fittings, consult the manufacturer for properly protecting these underwater fittings. Most manufacturers will recommend applying an epoxy barrier coat to the aluminum and / or give specific instructions on how to properly protect these fittings through anode placement and bonding. Below is a metal nobility scale for metals in flowing sea water:
I have had some recent findings with advanced corrosion on several surveys in the past few months. It is very important that boat owners understand their vessel no matter how new or old they may be and if these vessels are to remain in the water for any length of time to make sure the galvanic isolator is installed and working, all underwater metal parts are properly bonded together, and if any aluminum or less noble type metals are part of your running gear, that they are properly protected per the manufacturer’s recommendations. These are factors that can sink your vessel if not checked and monitored.
That is all I have to write for now. Be well, enjoy your boating, and fair winds!
Capt. John Banister
Suenos Azules Marine Surveying and Consulting
4521 PGA Boulevard, Suite 461
Palm Beach Gardens, Florida 33418