Corrosion Is a Serious Issue for Construction and Safety

Corrosion is the deterioration of a metal as a result of chemical reactions between it and the surrounding environment. Both the type of metal and the environmental conditions, particularly gasses that are in contact with the metal, determine the form and rate of deterioration.

Do All Metals Corrode?

All metals can corrode. Some, like pure iron, corrode quickly. Stainless steel, however, which combines iron and other alloys, is slower to corrode and is therefore used more frequently.

All small group of metals, called the Noble Metals, are much less reactive than others. As a result, they corrode rarely. They are, in fact, the only metals that can be found in nature in their pure form. The Noble Metals, not surprisingly, are often very valuable. They include rhodium, palladium, silver, platinum, and gold.

Types of Corrosion

There are many different reasons for metal corrosion. Some can be avoided by adding alloys to a pure metal. Others can be prevented by a careful combination of metals or management of the metal’s environment. Some of the most common types of corrosion are described below.

  1. General Attack Corrosion: This very common form of corrosion attacks the entire surface of a metal structure. It is caused by chemical or electrochemical reactions. While general attack corrosion can cause a metal to fail, it is also a known and predictable issue. As a result, it is possible to plan for and manage general attack corrosion.
  2. Localized Corrosion: This corrosion attacks only portions of a metal structure. There are three types of localized corrosion:
    1. Pitting — the creation of small holes in the surface of a metal.
    2. Crevice corrosion — corrosion that occurs in stagnant locations such as those found under gaskets.
    3. Filiform corrosion — corrosion that occurs when water gets under a coating such as paint.
  3. Galvanic Corrosion: This can occur when two different metals are located together in a liquid electrolyte such as salt water. In essence, one metal’s molecules are drawn toward the other metal, leading to corrosion in only one of the two metals.
  4. Environmental Cracking: When environmental conditions are stressful enough, some metal can begin to crack, fatigue, or become brittle and weakened.

Corrosion Prevention

The World Corrosion Organization estimates the global cost of corrosion to be about US$ 2.5 trillion annually, and that a large portion of this – as much as 25% – could be eliminated by applying simple, well-understood prevention techniques. Corrosion prevention should not, however, be considered solely a financial issue, but also one of health and safety. Corroded bridges, buildings, ships, and other metal structures can and do cause injury and death.

An effective prevention system begins in the design stage with a proper understanding of the environmental conditions and metal properties. Corrosion can be prevented by sealant applications, coatings and cathodic protection systems.

Overview of Cathodic Protection

 Cathodic Protection (CP) is one of the most effective methods for preventing most types of corrosion on a metal surface. In some cases, CP can even stop corrosion damage from occurring. Metals, especially ferrous metals, corrode in the presence of oxygen, water, and other impurities such as sulfur. Without CP, metals act as the anode and easily lose their electrons and thus, the metal becomes oxidized and corroded. CP simply supplies the metal with electrons from an external source, making it a cathode.

Basic Terminology

Oxidation — Loss of electrons

Reduction — Gain of electrons

Anode — Where oxidation reactions take place

Cathode — Where reduction reactions take place

The following are two helpful mnemonics to remember how electrons are transferred in oxidation-reduction (redox) reactions.

  1. OILRIG — Oxidation is Loss, Reduction is Gain
  2. AnOx RedCat — Anode is for Oxidation, Reduction at the Cathode

Types of Cathodic Protection  

  1. Galvanic Cathodic Protection

Galvanic cathodic protection involves protecting a metal surface of a piece of equipment using another metal that is more reactive. The latter metal, usually called the galvanic or sacrificial anode, has a less negative electrochemical potential compared to the metal component being protected. Therefore, the sacrificial anode undergoes oxidation rather than the operating equipment. This technique is illustrated in Figure 1 below for an offshore platform with a steel pipe submerged into seawater. The sacrificial anode is an aluminum anode in this example.

Sometimes, steels are galvanized rather than connected to galvanic anodes. Galvanized steels are steels that are coated with a protective zinc layer. The zinc layer acts to cathodically protect steel against corrosion in most underground and marine environments.

Figure 1. Offshore oil rig using a sacrificial anode.

  1. Impressed Current Cathodic Protection (ICCP)

ICCP is a more economical method of CP when underground pipelines are long or offshore equipment is too large to protect via one or few galvanic anodes. In ICCP, electrons are supplied to the cathodic structure using an external DC power source (also called a rectifier). The steel component is connected to the negative terminal of the power source and the impressed current anodes are connected to the positive terminal of the power source. For simplicity, Figure 2 shows one cathode and one anode connected by a rectifier. In application, multiple anodes are connected to the positive terminal of the power source.

Figure 2. Offshore oil rig using an impressed current.

Industry Applications of Cathodic Protection

Cathodic protection is routinely used to protect equipment operating in aggressive environments. The two most common applications of CP are for buried pipeline systems and vessels as well as offshore platforms. CP is not used to protect equipment in atmospheric conditions or protect components internally.

CP Challenges

Once installed, CP should be monitored and maintained. Furthermore, inadequate CP designs may not maximize the amount of current reaching the protected item. CP designs should consider the environmental conditions and the component to be protected against corrosion. Another critical factor to monitor is stray currents that may interfere with the system. These interfering currents could be due to the environment or neighboring components (especially if new equipment is commissioned). Additionally, the anodes and rectifiers must be maintained in order for CP to be effective and reliable.