The metals most commonly used in commercial and construction applications – including steel and copper – corrode when exposed to oxygen and moisture. That’s a big problem, but especially critical when used in applications such as pipelines and storage tanks, or supporting members in buildings and bridges.

What is Corrosion?

Corrosion is an electro-chemical reaction in which oxygen combines with the metal to form an oxide which is much weaker and less durable than the metal itself. On iron and steel, we call this oxide rust.

The corrosion process actually involves several chemical steps, but it begins with the release of free electrons from the metal. The metal is said to act as an anode.

History

The problem of corrosion has been recognized as long as metals have been used. Serious, scientific efforts to fight corrosion began in the 1800s.

In a paper presented to the Royal Society in London in 1824, Sir Humphry Davy proposed using a sacrificial metal to combat erosion. By attaching metal which made a better anode than the metal being protected, corrosion could be prevented. However, the attached metal – called the sacrificial metal or sacrificial anode – would corrode away and need to be replaced.

This method was tested by the British Navy, which used iron or zinc blocks attached to the hulls of ships to protected the copper-sheathed hulls from corrosion. Later, zinc blocks were used to protect steel-sheathed hulls.

In the 1930s and 1940s, sacrificial anodes made of iron were used to protect steel pipelines.

Coatings

Metals can also be protected by coatings which prevent the metal from coming in contact with moisture or oxygen. The challenge of this method is creating coatings that are durable enough to survive harsh environments and conditions. (The modern paint finishes used on cars are very durable and offer great protection for steel bodies, but they are too expensive for industrial applications.)

Another common technique is galvanizing steel by applying a zinc coating. The zinc coating actually corrodes, but unlike the rust that forms on steel, and quickly flakes away, zinc oxide forms a stable coating on the surface, which slows further corrosion. The zinc coating – found on galvanized nails and similar products – can therefor last for many years.

Impressed Current Cathodic Protection

A more modern method for protecting steel structures, tanks, and pipelines is called impressed current cathodic protection (ICCP). In this method, a DC electrical current provides free electrons, which flow to the steel (which then acts as a cathode rather than an anode). This prevents the steel from releasing electrons, and breaks the first step in the corrosion process.

Modern ICCP systems require minimum current, and easily verifiable monitoring.

Choosing an appropriate method to protect metal from corrosion is a critical component in any large scale construction project. Many engineering firms today provide corrosion protection services for their clients.