Background

Galvanic corrosion may occur when two dissimilar metals are in contact in an electrolyte (this includes most aqueous solutions).

Except for graphite, stainless steel is the noble metal in most galvanic pairs. Hence it is protected at the expense of the other metal when in contact, for example, with iron, steel, aluminium, zinc or cadmium. The solution to this problem lies in using, so far as possible, metals of the same composition for complete assemblies when this condition is encountered. In some cases an insulating lacquer or gasket can be used as a separation between two metals at the point of contact.

The following table shows the galvanic behaviour of stainless steels with other metals when tested in sea water.

Galvanic Corrosion of Metals in Sea Water

If two metals in this list are in contact with sea water, then the metal nearest the top of the list is the one most likely to corrode at the metal junction. The degree of corrosion is increased as the separation of the alloys in the list is increased. Potential differences of less than 100mV are unlikely to cause problems. However, the relative areas of the two dissimilar metals are also important - the higher the ratio of the areas of the noble metal to active metal, the greater will be the corrosion and vice versa.

Table 1. Galvanic series of metals and alloys in seawater.

Active

Magnesium

Magnesium alloys

Zinc

Aluminium

Cadmium

Steel

Cast iron

Chromium iron (active)

18-8 chromium-nickel-iron (active)

Titanium (active)

Lead

Tin

Nickel (active)

Brasses

Copper

Bronzes

Copper-nickel alloys

Silver solder

Nickel (passive)

Chromium-nickel (passive)

18-8 chromium-nickel-iron (passive)

18-8-3 chromium-nickel-molybdenum-iron (passive)

Titanium (passive)

Silver

Graphite

Zirconium

Gold

Platinum

Noble