Material Selection

In most cases, galvanic corrosion can be easily avoided if proper attention is given to the selection of materials during design of a system. It is often beneficial for performance and operational reasons for a system to utilize more than one type of metal, but this may introduce a potential galvanic corrosion problem. Therefore, sufficient consideration should be given to material selection with regard to the electrical potential differences of the metals.

Galvanic Series

The potential difference of two metals is qualitatively determined by their relative placement on the Galvanic Series, shown in Table 9. Some metals are listed more than once. This is either because they exhibit different galvanic properties when given different heat treatments, or because they can be in two different states. The metal is in an active state when the metal surface has direct interaction with the environment, and the metal is in a passive state when a noble film has formed on the surface.

This table can be helpful in estimating the likelihood of corrosion of a specified bimetallic system by gauging the distance between the two metals on the galvanic series. To state it simply, avoid using metals that are far apart on the galvanic series. The chart is not useful, however, in predicting the degree or rate of corrosion, since there are several other factors that influence the magnitude of corrosion in a given bimetallic system.

The metal that is higher on the Galvanic Series chart is less reactive and thus acts as the cathode, while the metal appearing lower in the series is more reactive and acts as the anode in the electrochemical cell. For example, if copper was to be electrically coupled with tin and immersed in seawater, then copper would be the anode and would corrode more readily than tin, which would act as the cathode. In environments other than seawater, the metal with the least resistance to corrosion in the surroundings acts as the anode and is then more readily corroded than the other, more noble metal.

Other Material Selection Charts

There have been a number of charts and tables created in order to aid in the material selection process and eliminate the potential for galvanic corrosion. Table 10 lists specific metal and alloy compatibilities with other specific metals and alloys in seawater with respect to galvanic corrosion. This table shows whether a certain combination of metals or alloys is compatible, unfavorable or uncertain. Note that the stainless steels listed in the table are all in the same state (active or passive). Table 11, on the other hand, lists metal and alloy compatibilities with respect to galvanic corrosion in environments other than seawater, such as marine and industrial atmospheres.