robert-nicholson

For as long as people have been refining metal, they've understood that most metals corrode when exposed to air and moisture. This is particularly true of iron and steel, which rust easily. Some types of corrosion – such as the oxidation that forms on zinc – adhere tightly to the metal, actually slowing further damage. However, because iron oxides (rust) flakes away, fresh metal is exposed to further corrosion.

Corrosion is an electro-chemical reaction. In the first stage of this reaction, the metal gives up free electronics, or acts as an anode. This allows it to bind with oxygen.

History

Almost two hundred years ago, in 1824, this reaction was documented by a scientist named Sir Humphry Davy. At a meeting of the Royal Society in London, he proposed protecting metal from corrosion by attaching a metal that made a better anode.

The British Royal Navy tested this idea. To protect the copper-clad hulls of their ships, they attached blocks of iron or zinc. These metals surrender electrons more easily than copper, so they corroded and the copper did not. The attached blocks eventually corrode away and need to be replaced; they are sometimes called “sacrificial metal.”

This method was called cathodic protection, because it provided a strong anode (the sacrificial metal), causing the metal being protected to act as a cathode.

Cathodic protection with a sacrificial anode continued to be used for over 100 years. In the 1930s and 1040s, sacrificial iron blocks were used to protect steel pipelines.

In 1837, a French scientist named Stanislaus Sorel patented a process called galvanizing. Sorel coated steel with zinc. Although the zinc itself corroded, it formed a protecting coating on the steel, preventing it from rusting for many years or even decades.

Various other forms of coating have been used to protect metals from corrosion by preventing exposure to moisture. Coatings can be very effective. For example, the finish on cars can protect the underlying steel for decades. However, coatings are two expensive to many applications, or may wear away too quickly.

Impressed Current Cathodic Protection (ICCP)

A new form of cathodic protection, called ICCP, was developed in the 1950s. In this method, a DC electrical current is passed through the anodes. This allows the anodes to release electrons without binding with oxygen, so the metal is not sacrificed.

Advanced ICCP systems are commonly used to protect steel in industrial applications, such as pipelines, tanks, bridges and piers. Large scale projects use multiple “zones” with electronic controllers.

Cathodical Protection Engineering

For help with corrosion engineering and cathodic protection, contact Alisto Engineering Group.