Rust and Corrosion

Galvanic Action

Galvanic corrosion or electrochemical corrosion, often misnamed "electrolysis," is one common form of corrosion in marine environments. It occurs when two dissimilar metals are brought into contact in the presence of an electrolyte (moisture). When a galvanic couple forms, a low electric current flows from the metal having the high potential (Anodic) on the galvanic series to the metal having the lower potential (Cathodic). This is known as galvanic action.

The metal in the higher position on the galvanic chart corrodes due to the transfer of electrons (material) from it (the anode) to the cathode metal. 

To minimize galvanic corrosion try to use the same, or similar metals, especially when an electrolyte is present, which in boating it always is present. If dissimilar metals must be coupled together, those that are most widely separated on the chart will generate the highest galvanic effect.  If  dissimilar metals are required then  separate them with a dielectric (insulating) barrier, such as a paint, coating or plastic washer.

Contributing factors

The two major factors affecting the severity of galvanic corrosion are

•  The voltage difference between the two metals on the Galvanic Series, the greater the distance between the two metal on the chart, the greater the effect.  You can find this from the chart below
•  The size of the exposed area of cathodic metal relative to that of the anodic metal, or the C/A ratio

The effect of the C/A ratio, is illustrated in Figure 1 for a rivet in a plate. In both examples aluminum is the anode and stainless steel is the cathode. In couple A, the aluminum rivet is small in comparison to the stainless steel, and the C/A ratio is large. In couple B, the situation is reversed: the stainless steel rivet is small in comparison to the aluminum and the C/A ratio is small.  Corrosion of the aluminum rivet in couple A will be severe, but the corrosion of the large aluminum plate in couple B will be much less, even though the potential difference is the same in each case.

Think of it in these terms, the anode wants to release electrons (dissolve) into the cathode, so a small anode and a big cathode just helps that along.  A grain of salt in a bucket of water will dissolve quickly, but a drop of water in a bucket of salt is not noticeable.

The Galvanic Series, Mil Spec