Corrosion Process – Electrochemical Corrosion of Iron and Steel

Corrosion is the deterioration of a material through chemical or electrochemical interaction with its environment. In engineering systems this can result in material loss, pitting, cracking, leakage, loss of structural strength and eventual component failure.

For iron and steel exposed to moist, aerated environments, corrosion is primarily an electrochemical process. Anodic and cathodic regions develop on the metal surface, allowing oxidation and reduction reactions to occur. The familiar result is the formation of rust.

Why Corrosion Matters in Engineering

Corrosion can affect pipelines, structures, pressure equipment, fasteners, machinery and process equipment. Its consequences extend beyond visible surface deterioration: corrosion can reduce section thickness, initiate cracks, perforate containment systems and impair the operation of mechanical components.

Understanding the corrosion mechanism helps engineers select suitable materials, avoid incompatible material combinations, apply protective systems and design equipment to reduce corrosion throughout its service life.

Basic Electrochemical Corrosion Process

A corrosion cell requires anodic and cathodic reactions, an electrically conductive path through the metal and an environment capable of supporting ionic transport. In atmospheric corrosion of iron and steel, a thin film of moisture on the surface can provide the electrolyte.

Differences in surface condition, composition, oxygen concentration, deposits or other local conditions can establish anodic and cathodic regions. Metal dissolution occurs at the anodic areas while a reduction reaction occurs at the cathodic areas.

Anodic Reaction

At an anodic region on an iron or steel surface, iron is oxidised and enters the electrolyte as ferrous ions. Electrons are released into the metal:

Anodic reaction – metal dissolution

Material loss therefore occurs at the anodic part of the corrosion cell. The released electrons travel through the electrically conductive metal towards cathodic regions.

Cathodic Reaction

For iron or steel exposed to aerated, approximately neutral water or surface moisture, dissolved oxygen can be reduced at the cathodic regions:

Cathodic oxygen-reduction reaction

The particular cathodic reaction depends on the environment. The oxygen-reduction reaction above is characteristic of many ordinary atmospheric and aqueous corrosion conditions and should not be regarded as the only possible cathodic corrosion reaction.

Formation of Rust

The ferrous ions produced at anodic areas and hydroxide ions associated with the cathodic reaction participate in further reactions in the surface moisture. Ferrous hydroxide and other intermediate corrosion products can form and undergo further oxidation.

The familiar corrosion product known as rust is not a single pure compound. It consists principally of hydrated iron oxides and oxyhydroxides, with its composition depending on the environment and corrosion conditions.

Ordinary rust is generally porous and does not form the strongly adherent, continuous protective film required to stop further atmospheric corrosion of carbon steel. Moisture and oxygen can therefore continue to reach the underlying metal.

Corrosion products can also occupy a greater volume than the metal from which they formed. This expansion can disrupt coatings and, where steel is embedded or constrained, can generate significant local forces.

Uniform and Localised Corrosion

Corrosion does not necessarily occur uniformly over an exposed component. The distribution depends on the material, environment and local conditions.

• Uniform corrosion produces relatively even material loss over a substantial part of the exposed surface.
• Localised corrosion concentrates attack in particular areas and can produce severe penetration with relatively little overall material loss.
• Pitting corrosion is a particularly localised form of attack in which small areas penetrate into the material.
• Galvanic corrosion can occur when dissimilar conductive materials are electrically connected in a suitable electrolyte.

For more information on dissimilar-metal combinations see Galvanic and Bi-Metallic Corrosion .

Effect of Dissolved Salts

Dissolved salts can increase the electrical conductivity of surface moisture or an aqueous environment and thereby influence corrosion rates. Chloride-containing environments are particularly important because chloride ions can also contribute to localised attack and breakdown of protective films on susceptible materials.

The actual corrosion rate depends on the complete system, including material, electrolyte composition, temperature, oxygen availability, surface condition and geometry. Salt concentration alone is therefore not a universal measure of corrosion severity.

Industrial and Polluted Atmospheres

Atmospheric contaminants can significantly alter the corrosiveness of surface moisture. Historically, sulphur dioxide has been an important pollutant in industrial atmospheres. Sulphur-containing contaminants can produce acidic species in deposited moisture and increase the corrosion of exposed carbon steel.

The severity of atmospheric corrosion also depends strongly on the time for which the metal surface remains wet, together with temperature, relative humidity, deposited salts and other pollutants.

Conditions Affecting Corrosion

Corrosion behaviour is controlled by the interaction between the material, its environment and the component design. Important factors include:

• Material composition and metallurgical condition
• Presence and composition of moisture or another electrolyte
• Oxygen availability
• Acidity or alkalinity
• Dissolved salts and contaminants
• Temperature
• Flow velocity and fluid conditions
• Deposits, crevices and differential aeration
• Contact with dissimilar materials
• Protective films and surface coatings
• Mechanical stress

Key Points of the Corrosion Process

• Corrosion of metals is commonly an electrochemical process involving oxidation and reduction reactions.
• Metal dissolution and therefore metal loss occur at anodic regions.
• Electrons travel through the metal from anodic regions to sites at which cathodic reduction occurs.
• Moisture can act as the electrolyte during atmospheric corrosion of iron and steel.
• In aerated neutral environments, oxygen reduction is an important cathodic reaction.
• Ordinary rust on carbon steel is generally not sufficiently protective to prevent continued corrosion.
• Salts, pollutants, temperature and local environmental conditions can substantially affect corrosion behaviour.
• Corrosion may be uniform or highly localised, so small total material loss does not necessarily imply a low risk of failure.

Related Corrosion References

• Galvanic and Bi-Metallic Corrosion – corrosion associated with electrically connected dissimilar metals.
• Stress Corrosion Cracking – cracking caused by the interaction of susceptible material, tensile stress and a specific environment.
• Corrosion Protection and Surface Coatings – coatings and surface treatments used to reduce corrosion.
• Corrosion in Piping Systems – corrosion mechanisms and control considerations for piping.
• Corrosion Resistant Materials – material-selection information for corrosive environments.

External Corrosion References

1. Corrosion Doctors – extensive reference information covering corrosion mechanisms and corrosion engineering.
2. Corrosion Source – corrosion information and industry resources.
3. Gordon England – engineering information relating to corrosion, materials and surface engineering.