2.5 Corrosion processes: galvanic series
A similar concept to the electrochemical series that has been used by engineers for many years is the galvanic series (one example of which is shown in Table 2: here the list should be read down the columns rather than across the rows). It ranks metals and alloys in order of reactivity or electrical potential, just like the electrochemical series. It also has the same properties: the greater the difference in position between two metals or alloys, the greater the likelihood that corrosion will occur. The series differs from the electrochemical series in showing alloys, which are of course of direct practical interest. Closely related alloys such as the brasses and bronzes are grouped together. Again, the most reactive materials are towards the bottom of the list.
However, such lists must be used with caution because they are highly dependent on the actual conditions. Also, the numerical values associated with the electrochemical series can allow more accurate information to be gathered about likely corrosion rates.
Table 2 Galvanic series of some commercial metals and alloys in sea water
|Titanium||‘Active’ stainless steel (unstable oxide film)|
|‘Passive’ stainless steel (stable oxide film)||Cast iron and ‘mild’ steel|
|Bronze (Cu—Sn)||Magnesium and magnesium alloys|
Using first the galvanic series, then checking with the electrochemical series, suggest which pair of alloys below will show the greater tendency to corrode in a marine environment, if the exposed areas of the two components are roughly equal:
- mild steel and bronze
- magnesium alloy and steel.
Reading from Table 2, the separation of steel and bronze is greater than that of steel and magnesium, so one might suggest that corrosion would be greatest for the steel/bronze couple.
However, looking at electrode potentials (Table 1), the standard E0 values are:
Mg/Mg2+: E0 = −2.37 V
Fe/Fe2+: E0 = −0.44 V
Cu/Cu2+: E0 = +0.34 V
The difference between magnesium and iron is much larger than that between iron and copper, so the electrochemical series contradicts the galvanic series in this case. The electrochemical series is a more accurate predictor of corrosion behaviour than the galvanic series alone.