Galvanic corrosion can be defined simply by the effect resulting from the contact of two different metals or alloys in a corrosive conductive environment. The term bimetalllism is also used.
In each solution, it is possible to establish a « galvanic series », i.e. a classification of the different metals and alloys according to this measured potential (see chart below).
During a coupling between two different metals or alloys, an electronic current will be established between them resulting from the short-circuit formed. There is generally an increase in corrosion of the less noble alloy and a decrease or elimination of corrosion of the more noble alloy.
The potential difference indicates the direction of the threat, but not the magnitude. So it is not the sole factor to consider. The electrical conduction of the medium and the temperature are also important factors.
To have galvanic corrosion, three conditions are necessary :
- Metals of a different nature : it is the difference in dissolution potential between the two metals that causes the phenomenon. Expérience shows that a potential of 100µV is required for corrosion to appear.
- The presence of an electrolyte, generally aqueous : the presence of ions in the aqueous medium (eg. Salt water) accelerates the phenomenon. This type of corrosion can also exists in an anhydrous but ionic medium such as liquid ammonia.
- Electrical continuity between the two metals : the phenomenon decreases very quickly by moving the two metals apart. There must be transfer of electric charges to have the phenomenon of corrosion.
Depending on the changes in the electrolyte, we can have inversions in the series of potentials. Zinc, for example, coated with corrosion products, can become more « noble » than iron in some hot waters. Tin can become less « noble » than iron in organic acid solutions (problem of canned food).
For a given current between two different metals, the density of the current and consequently, the speed of dissolution of the less noble metal (anode) will be the higher as this anode will have a small surface.
Some means of combating this corrosion : choose metal couples whose elements are as close as possible in the corresponding galvanic series, avoid an unfavorable surface ratio, avoid as far as possible, by using an seal, an insulator, a cotaing, direct contact of two different metals, etc.
Depending on the uses or operating environments indicated by its clients, Filame can offer choices of materials or choices of surface treatment to avoid or reduce the phenomenon of galvanic corrosion. Filame recommends that its customers always clearly indicate these different éléments when requesting a quote.