Contact corrosion occurs when two different metals attached to one another come into contact with an electrolyte (a salt dissolved in water), usually rainwater or dirt that has got wet. This results in different electrode potentials in the two metals with respect to the electrolyte. The base metal is transformed into an anode and this leads to the accelerated corrosion of that metal and inhibits the corrosion of the other, more noble metal, which is transformed into a cathode. Contact corrosion is also known as galvanic or bimetallic corrosion.
Every element tends to revert to its basic form. In the case of the metals, this means a return to the ore compound. This is usually the oxide form. Everyone is familiar with the corrosion product of iron: iron oxide, or rust. Similarly, when aluminium corrodes, it becomes aluminium oxide.
Different metals corrode at different speeds. This difference is expressed in the reactivity series in which the electrode potentials are measured to the standard hydrogen electrode H, whereby H has been assigned the potential of 0.
Mg Al Zn Cr Fe Cd Ni Sn Pb - H=0 - Cu - Ag Au
The metals that corrode quickly are known as base metals (Mg to Pb); the metals that corrode very slowly are known as semi-noble metals (Cu) and the metals that do not corrode at all are known as the noble, or precious, metals (Ag and Au).
The position in the reactivity series of metals with respect to one another determines which of the two metals in contact with one another will undergo contact corrosion. When a base metal (such as zinc) is in contact with a more noble metal (such as iron) and they come into contact with a conductive solution such as rainwater, the zinc (anode) will corrode and the iron (cathode) will not.
This rule applies to metals situated at some distance from one another in the reactivity series. They should not be allowed to be in contact with one another in any given structure.
When the metals are close to one another in the series, the situation in practice is often more complex. In that case, not only the composition and the temperature of the electrolyte are important; the size of the contact surface between the metals also plays a role. Contact corrosion becomes more dramatic with an increase in the potential difference and when the surface area of the baser metal is larger than that of the more noble metal. In addition, in real conditions, many metals are covered with a coating of oxide and so the potential difference is different from that stated in the scientific tables.
Contact corrosion table
Architects and builders can usefully draw on the contact corrosion table below for hot-dip galvanised steel parts and products.
Reliability of the combination
surf. zinc < surf. linked metal
* The corrosion rate for blank steel that is linked to zinc is low. A small amount of rust water will, however, spread rapidly over the zinc and result in 'rust stains' that are aesthetically unacceptable. This combination will therefore almost always be rejected.
In certain circumstances, using two different metals in contact with one another is unavoidable. Generally, the careful selection of construction metals and the use of insulation material can prevent contact corrosion.
Preventing contact corrosion
Avoid unequal metal pairings.
Insulate metals where they come into contact by using: PVC, Teflon or nylon underlay rings, strips, bushes and so on.
Cover the contact and the surrounding surface using insulation tape (for example Denso, Densolene).
Apply an insulation coating of varnish or paint on and around the contact surfaces.
Pack in screws, threading and disks where they come into contact with the metals.