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Galvanic Series

With the galvanic series, metals are grouped according to electrical potential measured to a known reference. This galvanic series (or electro-potential series) determines the nobility of metals and semi-metals. In a situation in which galvanic corrosion will occur with two dissimilar metals, the less noble metal will be depleted and physically "dissolved" in the electrolyte. The nobler of the involved metals will remain intact and will not suffer from the corrosion process.

The rate of corrosion is determined by the electrolyte, the difference in nobility, and the relative areas of the anode and cathode exposed to the electrolyte. The difference in nobility can be measured as a difference in voltage potential: the less noble metal is the one with a lower (that is, more negative) electrode potential than the nobler one, and will function as the anode (electron or anion attractor). The two metal parts and the electrolyte device are functioning as a galvanic cell.

Galvanic Table for Seawater

The following galvanic table lists metals in the order of their relative activity in seawater environment. On top of the list are the more active (anodic) metals and proceeding down are the least active (cathodic) metals of the galvanic series. A galvanic series applies to a particular electrolyte solution, hence for each specific solution, which is expected to be encountered for actual use, a different order or series will ensue. If combined in a galvanic couple, the metal higher in the series (or the smaller voltage potential) represents the anode, and will corrode preferentially in the specific environment.

The chart above shows corrosion potentials in flowing seawater at ambient temperature. The unshaded symbols show the ranges exhibited by stainless steels in acidic water such as may exist in crevices or in stagnant, low velocity or poorly aerated water. Notice the shift in voltage potential due to the change of the electrolytic environment.

As explained above, the more noble materials on the left side tend to be cathodic and will be protected, whereas those on the right are less noble and tend to be anodic. These will be the metals to corrode preferably in a galvanic couple. From the above chart it can be seen that in seawater, zinc-based anodes work well for all kinds of stainless steel (galvanic difference is 0.45V or higher), whereas a pure zinc sacrifice anodes may not work well enough to protect corrosion on aluminium hulls (galvanic difference may be close to zero).

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