What is corrosion?

Corrosion is the gradual deterioration of a material—most often metal—when it reacts with its surrounding environment. These chemical or electrochemical reactions cause the material to change and can weaken, damage, or eventually destroy it.

Rust forming on iron or steel is one of the most familiar examples of corrosion, but corrosion can take many forms and affect many different materials. As explained in AMPP's Corrosion Basics, An Introduction, corrosion can occur when materials interact with environmental conditions such as water, oxygen, chemicals, salt, soil, and humidity. 

Corrosion affects the infrastructure, equipment, and systems we rely on every day—from pipelines, bridges, ships, and industrial facilities to water and wastewater systems, vehicles, and buildings. Left unmanaged, it can shorten the life of an asset, increase maintenance and replacement costs, and create risks to safety and the environment.

Corrosion is natural, but it can be controlled.

Understanding why corrosion occurs, what forms it takes, and the conditions that cause it is the first step toward controlling it. Proven methods such as protective coatings, cathodic protection, appropriate materials selection, inspection, and corrosion management can help prevent or reduce corrosion and extend the life of critical assets.

Factors including the type of material, moisture, temperature, soil conditions, saltwater exposure, chemicals, operating environment, and expected service life can all influence how corrosion develops and which prevention or mitigation methods are most effective.

Effective corrosion control begins with understanding the various forms of corrosion, the mechanisms involved, how to detect them, and how and why they occur. 

AMPP brings together the global knowledge, standards, education, and professional expertise used to understand, prevent, and control corrosion.

Continue below to learn how corrosion happens, the different forms corrosion can take, and the methods used to prevent and control it.

What causes corrosion?

All forms of corrosion, except for some types of high-temperature corrosion, occur through the action of the electrochemical cell. The elements that are common to all corrosion cells are:

• an anode where oxidation and metal loss occur,
• a cathode where reduction and protective effects occur,
• metallic and electrolytic paths between the anode and cathode through which electronic and ionic current flows, and
• a potential difference that drives the cell.

Electrochemical cell in a corrosion cell

Electrons flow through a metallic path from sites where anodic reactions are occurring to sites where they allow cathodic reactions to occur. Ions (charged particles) flow through the electrolyte to balance the flow of electrons. Anions (negatively charged ions from cathodic reactions) flow toward the anode and cations (positively charged ions from the anode itself) flow toward the cathode. The anode corrodes and the cathode does not. There is also a voltage, or potential, difference between the anode and cathode.

The driving potential may be the result of differences between the characteristics of dissimilar metals, surface conditions, and the environment, including chemical concentrations. There are specific mechanisms that cause each type of attack, different ways of measuring and predicting them, and various methods that can be used to control corrosion in each of its forms.

Forms of corrosion

Corrosion comes in various forms, and it is rare that a corroding structure or component will suffer from only one form. The combination of metals used in a system and the wide range of environments encountered often cause more than one form of attack. Even a single alloy can suffer corrosion from more than one form depending on its exposure to different environments at different points within a system.

The different forms of corrosion can be categorized into three groups by the way in which it is identified:

Detailed descriptions of the individual Forms of Corrosion

General Attack vs Localized Corrosion

Least Threatening - "General corrosion" or " uniform corrosion" proceeds more or less uniformly over an exposed surface without appreciable localization. This leads to relatively uniform thinning on sheet and plate materials and general thinning on one side or the other (or both) for pipe and tubing. It is recognized by a roughening of the surface and usually by the presence of corrosion products. The mechanism of the attack typically is an electrochemical process that takes place at the surface of the material. Differences in composition or orientation between small areas on the metal surface create anodes and cathodes that facilitate the corrosion process.

Most often caused by misapplying materials in corrosive environments, general corrosion often can be tolerated because the effect of metal loss is relatively easy to assess, and allowances can be made in the initial design. Protective coatings are particularly effective in controlling uniform corrosion. Cathodic protection (CP), an electrochemical technique used for corrosion control, can be used in underground or immersion situations.

Most Encountered - Unlike general attack corrosion, localized corrosion occurs at discrete sites on a metal surface.

Microbiologically Influenced Corrosion

When metals and alloys come in contact with microorganisms, Microbiologically Influenced Corrosion, or MIC, can occur. While the presence of fungi, bacteria, and microalgae can speed the corrosion process and affect its mechanisms, it does not produce a particular form of corrosion.

Assets exposed to sewage; crude oil, hydrocarbon fuels, and process chemicals; seawater, distilled water, and freshwater; and soil can be especially susceptible to MIC. It is therefore a problem for many industries and is made more challenging as determining MIC as the primary cause of the corrosion requires examination from multiple analysis sources.