A copper nail lies in a zinc gutter. The zinc becomes measurably thinner, as zinc slowly dissolves. This is a classic example of contact corrosion. Two metals are bound to one another via an electrolyte such as water or humidity. The less noble metal becomes an anode and dissolves at a faster pace. The more noble metal becomes a cathode. Its dissolution rate falls.
In the example of the guttering the copper is more noble, with an electrochemical potential of + 0.35 V, whilst zinc, with – 0.76 V, is the less noble metal.
The potential difference of 1.11 V induces an electrical current from the anode (zinc) to the cathode (copper).
Teachers often use a second classic experiment to illustrate this principle:
they immerse a piece of iron and a piece of copper into a watery solution, allow both pieces to touch or connect them with a conductive wire.
- Electrons flow through the wire or at the point of contact from the anode, i.e. the iron with an electrochemical potential of – 0.42 V, to the copper cathode.
- In the electrolyte cations move from the anode to the cathode. These are doubly positively-charged iron ions from the metal grid. As a number of water molecules are dissociated – as hydroxide ions (OH-) and hydrogen ions (H+) –, iron ions can react with hydroxide ions and form the low-soluble compound iron(II) hydroxide. In turn, the hydrogen ions flow to the copper cathode, absorb electrons there and are thereby reduced. The hydrogen (H2) molecule is formed, which bubbles from the cathode as gas out of the water.
This corrosion may also play a role in materials. An example: an iron component is touching a copper component. If the point of contact is damp, an electrochemical current is induced: then electrons move from the less noble iron to the more noble copper and doubly positively-charged iron ions dissolve from the metal grid, with iron(II) hydroxide formed (fig. 1d).
Contact corrosion currently poses frequent challenges for designers in the construction of light vehicles or aircraft. Lightweight construction means replacing steel components with equally strong but significantly lighter elements made from aluminium (Al), magnesium (Mg) or carbon fibre. In the process, components of different materials are glued or bolted to one another. In the case of screw connections there is a risk of contact corrosion if, for example, vehicle wings made from carbon fibre are fixed to the steel frame of the body with steel bolts. In this case a mere film of moisture is enough to induce an electrochemical current between fibres and bolts. As carbon fibres behave like relatively noble metallic metals, the steel screws would corrode relatively quickly.
The speed with which contact corrosion can lead to damage can be slowed, however. The most important tip is to store construction, vehicle and plant components in a dry condition. Further options are:
- Insulation. An example: thread and heads of screws and bolts can be coated with a special epoxy resin. This resin is so stable that it does not deform or crack when the screw is tightened. Screws treated in this way are more expensive than normal screws.
- Sacrificial anodes. Corrosion can be delayed by pairing the metal that is to be protected with a less noble metal with lower electrochemical potential. The corrosive agent then oxidises the less noble metal, which is therefore referred to as “sacrificial anode”.