Galvanic corrosion is an
electrochemical process in which one
metal corrodes preferentially when in electrical contact with a different type of metal and both metals are immersed in an
electrolyte. Conversely, a galvanic reaction is exploited in
primary batteries to generate a voltage. A common example is the
carbon-zinc cell where the zinc corrodes preferentially to produce a current. The
lemon battery is another simple example of how dissimilar metals react to produce an electric current.
When two or more different sorts of metal come into contact in the presence of an electrolyte a galvanic couple is set up as different metals have different electrode potentials. The electrolyte provides a means for ion migration whereby metallic ions can move from the anode to the cathode. This leads to the anodic metal corroding more quickly than it otherwise would; the corrosion of the cathodic metal is retarded even to the point of stopping. The presence of electrolyte and a conducting path between the metals may cause corrosion where otherwise neither metal alone would have corroded.
Even a single type of metal may corrode galvanically if the electrolyte varies in composition, forming a concentration cell.
A common example of galvanic corrosion is the rusting of corrugated iron sheet, which becomes widespread when the protective zinc coating is broken and the underlying steel is attacked. The zinc is attacked preferentially because it is less noble, but when consumed, rusting will occur in earnest. With a tin can, the opposite is true because the tin is more noble than the underlying steel, so when the coating is broken, the steel is attacked preferentially.
