Although a wide range of mechanisms exist in the field of metallurgy that cause materials to deteriorate and go by the generic name of corrosion, we are going to limit ourselves to focusing on one of these, namely galvanic corrosion.
Galvanic corrosion is a type of deterioration that occurs when two or more metals enter into contact in the presence of an electrolyte. In this type of corrosion the most anodic metal passes electrons on to the most cathodic. The speed of this reaction depends on the reduction potential of the metals involved and on the efficiency of the electrolyte implicated.
This type of corrosion is particularly significant in mechanical fastening applications due to the fact that they invariably involve two or more metals and some type of electrolyte such as water originating from air borne humidity (condensation), rainwater and/or seawater spray in coastal areas. For this reason, the choice of fastener is of vital importance in order to ensure the creation of a corrosion resistant joint that cannot be compromised by a poor selection of the metals involved.
As a general rule, materials with an extremely disparate reduction potential, in other words a very anodic one combined with a very cathodic one, must never be used together.
In order to help our customers make this choice the so-called galvanic series was defined. These consist of an ordered series of materials classified in descending order in accordance with their reduction potential (also known as hydrogen potential), that is to say from the most anodic to the most cathodic, and they are referenced using the presence of a specific electrolyte, usually seawater. An example of a galvanic series in the presence of seawater would be the following:
As a general precaution, in order to prevent or reduce this type of corrosion we shall use materials that are as close as possible to each other in the galvanic series. If this is not possible, or in order to improve the resistance of the joint to corrosion, we shall attempt to break the contact between the two metals by using more cathodic coatings or materials in the fastener, or non-metallic paints or washers that prevent any effective contact between the metals involved. More information can be found in the section entitled “Surface Finishes and Coatings”.