Updated on October 2nd, 2023
Galvanic corrosion is one of the types of metal corrosion destruction. These processes are not that complicated, and a common understanding will save you from many costly problems.
To start the process of galvanic corrosion, it is only necessary to have two dissimilar metals with different electrical potentials, as well as an important intermediary that will play the role of an electrolyte. And sea water is perfect for this role. Due to the fact that this process creates the greatest problems for water transport, galvanic corrosion is also often called “marine corrosion”.
The situation is a little more complicated than it seems. After all, even two seemingly identical metals can easily contain different alloys. Accordingly, it will be difficult to call them a neutral pair, because one of them will be more active. And the very heterogeneity of metals provokes galvanic corrosion. Therefore, for example, brass (an alloy of copper and zinc) is rarely used in shipbuilding.
On all aluminum boat hulls, transom drives, pods and outboard motor legs, zinc or magnesium anodes below the waterline are mandatory. These metals are the least noble and the most reactive, so they act as a sacrificial element to protect aluminum parts.
Protect from what? Well, from more noble metals. For example, from a propeller, which can be made of steel. And also from steel shafts or trim plates.
The operation of the anode can be seen with the naked eye. More precisely, the consequences of his work. For example, the case of a transom drive or outboard motor after some time in salt water will be covered with a whitish coating – zinc oxide. This is a consequence of the release of electrons from the zinc plates, that is, the process of the flow of electric current. In this case, the ions of the substance are transferred from the anode to the cathode, and we sacrifice one part of the boat in order to save the other. The anode overlay itself will gradually lose in size and therefore requires regular replacement.
However, the uneven and very rapid disappearance of the anode can be a sign of the presence of so-called stray parasitic currents. This is already electrolytic corrosion, but the essence remains the same. Essentially, electricity acts as a catalyst for the above processes.
Stray currents are a common problem with electrified marinas. And it has to do with grounding. A ground cable is a necessary safety measure when connecting a yacht to shore power. But it causes direct currents of low voltage. And it is the direct current that is the most destructive.
A steel yacht and an aluminum boat in the marina next to each other and connected to shore power share a common ground cable, so they form an excellent cathode-anode pair. Not in favor of the aluminum hull, of course.
The steel mooring wall will play the same role. But, being more massive, it will accelerate the corrosion of a small aluminum boat even more, since the anodic and cathodic reactions must be equivalent. An increase or decrease in the cathodic reaction rate will have a corresponding increase or decrease in the anodic reaction rate. Imagine an electrochemical interaction between a massive steel structure and a small aluminum propeller.
To prevent such problems, the simplest galvanic isolation or active cathodic protection is used. There are plenty of ready made solutions on the market if your boat manufacturer hasn’t taken care of it. The simplest of these devices act as a filter, blocking the flow of low voltage direct current while maintaining the integrity of the protective earth circuit.
But it is important to remember that parasitic currents can also occur as a result of a malfunction in the on-board electrical wiring. A short circuit, improper connection of devices, insulation failure – all this creates an internal source of stray currents that can cause damage not only to your boat, but to those standing nearby.
Painting the boat’s aluminum hull, drivetrain and other aluminum parts provides additional protection but requires constant updating. Now you know why copper based antifouling paints should never be used on aluminum boats.
There is a table of activity of metals. In this table, one can easily see which of the pair of metals that form the galvanic cell will be the anode and which will be the cathode.
Magnesium anodes are best left for fresh water. For sea water, zinc alloys or aluminium-based should be installed. The anode must be replaced with a new one as soon as the degree of its destruction reaches 50%. Under no circumstances should paint, grease, etc. be applied to the anodes. It will be almost tantamount to his absence.