What do corruption and corrosion have in common?

The word corrosion is not directly related to the word corruption, but they have a lot in common. Both words sound unpleasant and both cause economic destruction at the national level. People can reduce and slow down both phenomena, but never eliminate them completely.

Corrosion is a phenomenon associated with metals. The corresponding phenomenon in wood is decay and in plastics embrittlement.

The four basic elements of corrosion

Corrosion is based on the simultaneous effect of four factors:

  1. There must be a metal surface that is the corroding party (called less noble and the anode)
  2. There must be a metal surface that causes the less noble one to corrode (called more noble and the cathode)
  3. There must be electrical contact between the metal surfaces (most often the metal surfaces are attached to each other)
  4. There must be an electrically conductive water contact between the metal surfaces

Metals consist of small grains, and therefore, for example, the surface of steel is a mosaic of small steel micrograins (5-50 micrometres in size). Adjacent grains have slightly different degrees of nobility, which causes an electrical potential difference. When electrically conductive rainwater accumulates on such a surface, a corrosion couple forms between adjacent micrograins. The less noble micrograin begins to corrode and an oxide forms on top of it. In steel, this reaction product is called rust, in copper patina and in zinc white rust. The rust layer on top raises the grain’s degree of nobility, making it in turn more noble than its neighbour, and the adjacent micrograins begin to corrode. This cycle continues as long as the surface is wet or all the metal grains have been completely worn away.

The corrosion of most metals slows down when a rust layer accumulates on them. The speed of corrosion is determined by the compactness of the rust layer and its ability to resist wear caused by the environment, such as acid rain or running water. In some metals, the rust layer may be almost invisible (for example, stainless steel) or highly visible (copper; first dark brown and later light blue-green). With some precious metals, the reaction may practically stop immediately.

Zinc sacrifices itself and provides protection

Corrosion does not occur if one of the four factors listed above is absent. Usually, efforts are made to slow the progress of corrosion by preventing water contact. This can be done either with paint or another insulator. The progress of corrosion can also be slowed by controlled use of a sacrificial protective metal such as zinc. Because zinc remains less noble than steel at all times, it protects the steel through its own slow corrosion.

Experience has shown that combining zinc and paint coatings provides so-called duplex protection, which is more than the sum of paint and zinc protection. For this reason, both the construction and automotive industries very often use galvanized sheet metal beneath a paint coating.

Jorma Kinnunen