Editor Note: This is a complicated subject in the marine industry with lots of people speaking and writing about corrosion incorrectly. I will try to ease into the subject here as an introduction to a process that causes millions of dollars worth of damage every year. Look for additional articles dealing with this issue in the future. Please send in any questions regarding marine corrosion that you have and I’ll try to address them in future newsletters.
|Time to replace this anode|
In the recreational marine business, corrosion prevention is serious business. We only get to use our boats 6 months of the year in Chicago, if we are lucky. The worst situation is one that takes our boat out of commission during the season. The second worst situation is if our winter repair invoices are much higher than anticipated. Galvanic corrosion can lead to both of these situations.
The other corrosion causing condition that we see is electrolysis. Electrolysis and galvanic corrosion are sometime difficult to tell apart and the process by which they destroy metals is similar. However, the reasons behind the destruction are different; therefore, the solutions are different. Electrolysis is a reaction between metals that is the result of current from an electrical source. Galvanic corrosion is a reaction between metals that is the result of two dissimilar metals being connected electrically to one another.
|Galvanic series of common|
For galvanic corrosion to occur between two metals a few conditions must be present. One, the metals must be separated on the galvanic series chart. The further they are apart the quicker the more noble metal (top of the list) will destroy the other metal. Two, the metals must be electrically connected. And three, the metals must be immersed in electrolyte. An electrolyte (for the purposes of our discussion) is a liquid that will conduct electricity. The fresh water of the Great Lakes has enough dissolved particles in it to allow it to conduct electricity, although much less so than salt water.
As you can see on the galvanic series list, lots of the metals that find themselves under water on a boat are here…and separated on the list. So what is the solution? One, we can break one of the three requirements. Coating the underwater metals with a non-reactive substance effectively takes them out of the electrolyte. Or, we can electrically isolating them from one another. Two, we can provide a sacrificial piece of metal that is more reactive than the metal we are trying to protect.
The piece of metal that we sacrifice (or the metal lower on the galvanic series list) is called an anode. The other end of the galvanic couple (the metal higher on the list) is called the cathode. Typically, sacrificial anodes are made from zinc or magnesium, since they are the lowest on the list they can work to protect the rest of the under water metals from the ones higher on the list no matter which two become the galvanic couple. Magnesium is very reactive and so isn’t used in saltwater applications. The anode is electrically connected to one of the metals to be protected.
|Galvanic corrosion starting to take hold on this sterndrive|
Some issues to think about that can take the under water metals in and out of a galvanic couple. If you have a raw water cooled engine and the raw water pump is made from bronze, the two dissimilar metals are already in electrolyte with each other (the raw cooling water connects them) so if the pump and the engine block become electrically connected galvanic corrosion will start. They can become connected electrically by any number of seeming unrelated situations including a chaffed wiring harness.
This type of situation can be repeated all over the boat. Copper-based bottom paint on the hull can come in contact with an aluminum stern drive when the coating on the drive is scratched or the bottom paint is applied too close to the drive. Changing from aluminum to stainless steel propellers is a huge change on the galvanic series list which can have dire consequences to an inadequately protected stern drive. I am sure that if you give this a few minutes thought while looking at the galvanic series list you can come up with additional scenarios yourself. Most of the problems that start all of a sudden are the result in a maintenance failure…chaffed wires, deteriorated sacrificial anodes, swapping a bolt made of one metal for another, etc.
The one way of protecting against galvanic corrosion that we haven’t mentioned is active protection. This is a process where a device in the boat emits a very low voltage which counteracts the desire for metals to corrode. MerCruiser uses a system called MerCathode that actively protects the drive. According to MerCruiser it is a solid-state device that operates off of the boat’s 12-volt battery and provides protection by impressing a reverse blocking current that stops the destructive flow of galvanic currents.