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An Italian scientist, Luigi Galvani, physician and philosopher, accidentally discovered that under certain circumstances the legs of frogs connected to different metals contract. This discovery then led the physicist Alessandro Volta to invent the electric "battery"... But the basic effect kept the name "galvanic", in honor of the good doctor.
So what is it ?
Various metals immersed in a liquid containing free radicals (called an electrolyte) exchange ions, creating an electric current. One of the metals oxidizes and dissolves, the anode. The other metal receives these ions and forms a layer of complex composition depending on the type of metals present with release of hydrogen. An electrochemical cell is formed.
In this case, galvanic corrosion occurs.
• To obtain galvanic corrosion, 4 conditions are necessary:
An electrolyte in which the ions will circulate. In our case, of course, on the water. It must be able to conduct electricity! It will work much better in salt water than in fresh water. But beware, truly pure water is very rare in nature and natural acidity (pH) plays an equally complex role.
An "A" metal is the anode considered the least "noble".
A metal "C" cathode, which is considered the "noblest".
And current must flow for there to be metal-to-metal contact, or electrical continuity, between the two metals.
How are metals classified?
Metals can be classified according to their "galvanic potential". The potential difference between two metals is called a "galvanic couple". Tables can then be created to predict the risks of corrosion. They are generally based on measurements in salt water (2 to 3% salinity), agitated and at 25°C. The greater the galvanic couple between the two metals, the greater the corrosion. It is accepted that below 200mV the phenomenon is negligible.
"Galvanic pair" table based on metal alliances
How to deal with the problem?
Several methods, from the simplest to the most complicated:
Choose the most compatible metals possible: for a brass screw, a brass nut (0mv) or stainless steel (200mv) is preferable to a galvanized steel nut (750mv).
Cut the circuit avoiding direct contact between the two metals, because the phenomenon decreases very quickly as you move away: use insulating washers.
Eliminate electrolyte/metal contact - very easy with a coat of grease (remember your cars where lead acid battery terminals and brass terminals were greased!). Another very effective solution, a good coat of paint (but without scratching the paint...).
Using a third metal to create what is called a sacrificial anode; this disappears and will protect the other metals. Normally zinc is used, one of the less "noble" metals.
Install a complex battery metering device that cancels corrosion currents.