Cause of Corrosion - Galvanic Corrosion

The common cause of corrosion is galvanic corrosion. The process is relatively simple; if two dissimilar metals are coupled and an electrolyte is introduced, a galvanic cell will be created.

Metals are rated on a galvanic scale. The scale lists metal in relationship to nobility. The less noble or active metals, will lose their properties to that of a more noble or passive metal. 

To understand this in relation to a boat, look at the dissimilar metals commonly found on a yacht mast and what is found in the way of an electrolyte.

The mast constructed from alloy, will have fasteners of stainless steel, these may be rivets securing the spreader sockets and tangs, which may also be stainless steel. The alloy mast is an active or less noble metal than that of the stainless steel.

Galvanic corrosion is often seen at the junction of the two dissimilar metals. Bubbling paint is usually the first indication that corrosion is taking place.

If an insulator is placed between the two metals, or if the join between the two metals is kept dry, then little will happen between the dissimilar metals, they will corrode at their own natural rate.

However a boat floats in an electrolyte and some of this will inevitably end up coating the boat in the way of spray. The sea water will sit over the two metals and act as a pathway for electrons to flow between the two metals. A galvanic cell is then created. The less noble metal, the alloy, will start to waste and this will normally be seen as pitting.

Fortunately the process usually takes a long time to cause any serious damage on a mast, as the electrolyte eventually dries out or is washed off by fresh water. However mast bases are often in a wet state and prone to damage.

If you want to see a dramatic example of galvanic corrosion, fasten a copper bolt through an aluminium plate and immerse it in sea water. The alloy will waste very quickly. What is being demonstrated is the alloy acting as the anode. This process can be slowed by adding a piece of zinc to the alloy. The zinc being less noble will corrode at a greater rate than the alloy thereby adding some protection to the alloy.

There is confusion surrounding the process of galvanic corrosion. An example of this is where the terms electrolysis, or electrolytic action are used to describe corrosion. Electrolysis relates to the chemical breakdown of an electrolyte and electrolytic, relates to corrosion produced by externally generated electrical currents.

Electrolytic corrosion is often referred to as stray current corrosion.
Stray current corrosion is potentially serious corrosion that can cause immense damage to metal and also timber.