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Contact Corrosion

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CONTACT CORROSION

A copper nail is assembled into a zinc component. Over time, the zinc becomes measurably thinner as the 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 rate. The more noble metal becomes a cathode and its dissolution rate is reduced.

 

In the example, the copper is more noble, with an electrochemical potential of + 0.35 V. Zinc, with an electrochemical potential of – 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).

An experiment that teachers often use to illustrate this principle is as follows:

A piece of iron and a piece of copper are immersed in water.  Both pieces are in contact with each other or connected 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 positively-​charged iron ions (Fe+2) 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. A hydrogen (H2) molecule is formed during the process, which bubbles from the cathode as gas out of the water.

This corrosion may also play a role in materials. For example, this occurs when an iron component is in contact with a copper. If the point of contact is damp, an electrochemical current is induced and electrons move from the less noble iron to the more noble copper. Positively-​charged iron ions dissolve from the metal grid and iron(II) hydroxide is 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 aluminum (Al), magnesium (Mg) or carbon fiber. 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 fiber are fixed to the steel frame of the body with steel bolts. In this case, a thin film of moisture is enough to induce an electrochemical current between the fibers and the bolts. As carbon fibers behave like noble metallic metals, the steel screws would corrode relatively quickly.

The rate at which contact corrosion may result in damage can be slowed, however. To delay the affects of contact corrosion, construction, vehicle, and plant components should be stored in a dry condition. Further options are:

  • Insulation. Screw threads and heads  may be coated with a special epoxy resin to insulate components from the effects of contact corrosion. Provided the resin is durable enough to withstand the forces encountered during assembly, it will not deform or crack when the screw is tightened.
  • Sacrificial anodes. Corrosion can be delayed by pairing the metal that is to be protected with a less noble metal (i.e. with lower electrochemical potential). The corrosive agent will oxidize the less noble metal and is therefore referred to as “sacrificial anode”.

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Technical terms cannot always be avoided. As corrosion experts, we not only want to give you comprehensive advice, we are also interested in making you a corrosion expert yourself.

The variety around the topic of corrosion and corrosion protection is also in our glossary at home: explanations from A as in Adhesion to T as in Thread tolerance. Have fun clicking through!