Corrosion – Deterioration by Chemical Reaction

Introduction and Definition

Corrosion is the irreversible degradation of a material—most often a metal—due to chemical or electrochemical interactions with its environment. This process is primarily an interfacial reaction, where atoms or ions are transferred between a material (like metal, polymer, or ceramic) and its surroundings, resulting in the material’s transformation or consumption. While corrosion is often associated with rusting iron, it affects a vast range of materials, including non-metals. The consequences are significant: structural failures, safety risks, and economic losses estimated at $2.5–$3 trillion annually worldwide. Modern corrosion management includes predictive modeling, monitoring, and life-cycle analysis to mitigate risks and optimize material selection and maintenance.

Key Concepts and Terms

• Chemical Corrosion: Deterioration via direct chemical reaction, often with acids or oxidizers, without electrical conductivity. For example, sulfuric acid reacting with steel forms iron sulfate and hydrogen.
• Electrochemical Corrosion: The most common type for metals, involving redox reactions in the presence of an electrolyte (like water with dissolved salts). It includes both anodic (metal loss) and cathodic (protection) regions.
• Oxidation: Loss of electrons from metal atoms, forming ions that may combine with oxygen to produce rust or other oxides.
• Reduction: Gain of electrons, usually at the cathode. Oxygen or hydrogen ions in the electrolyte are often reduced.
• Anode: Site of oxidation and material loss—corrosion always happens here.
• Cathode: Site of reduction; protected from corrosion in electrochemical cells.
• Passive Film: A thin, stable oxide layer that forms on metals such as stainless steel and aluminum, protecting against further corrosion. Damage or chemical attack to this film can initiate localized corrosion.

These core concepts are fundamental to understanding how corrosion occurs and how it can be managed or prevented.

Classification of Corrosion Types

Corrosion can take many forms:

Uniform Corrosion

Affects the entire exposed surface at roughly the same rate. Common in unprotected steel exposed to air and moisture, it is predictable and often managed by allowing extra thickness (“corrosion allowance”).

Pitting Corrosion

Highly localized, forming small but deep pits on the surface. Often initiated by breakdown of the passive film in chloride-rich environments (e.g., saltwater). Particularly dangerous because it is hard to detect and can cause failure with little overall material loss.

Crevice Corrosion

Occurs in confined spaces (under gaskets, washers, or overlaps) where stagnant fluid creates aggressive local conditions. It can progress rapidly and is hard to detect, posing risks in assemblies and joints.

Galvanic (Bimetallic) Corrosion

Happens when two dissimilar metals are electrically connected in an electrolyte. The less noble (anodic) metal corrodes preferentially. Severity depends on potential difference, electrolyte conductivity, and area ratio.

Intergranular Corrosion

Targets the grain boundaries in metals, often due to segregation or depletion of protective elements (like chromium in stainless steel). It can cause catastrophic failure without much visible surface damage.

Selective Leaching (Dealloying)

Removes the more reactive element from an alloy (e.g., zinc from brass), leaving a porous, weakened structure.

Erosion Corrosion

Accelerated by mechanical action (fluid flow, impact of particles) that strips away protective films, exposing fresh metal to chemical attack. Common in pumps, pipes, and marine environments.

Stress Corrosion Cracking (SCC)

Cracking caused by the combination of tensile stress and a specific corrosive environment. Can cause rapid, catastrophic failures without warning.

Hydrogen Embrittlement

Absorption and diffusion of atomic hydrogen into metals, especially high-strength steels, leading to sudden, brittle failure.

Exfoliation and Interfacial Corrosion

Exfoliation is a severe form of intergranular corrosion, causing layers of material to lift and delaminate, often seen in rolled or extruded products like aircraft components.

Mechanisms and Science

Corrosion involves redox reactions at the material-environment interface:

• Anodic (Oxidation) Reaction:
  M → Mⁿ⁺ + ne⁻
  (Metal loses electrons and becomes an ion.)

• Cathodic (Reduction) Reaction:

  • Oxygen reduction: O₂ + 2H₂O + 4e⁻ → 4OH⁻
  • Hydrogen ion reduction: 2H⁺ + 2e⁻ → H₂

Electrons released at the anode flow to the cathode, where reduction occurs. Electrolytes (water with dissolved ions) enable ionic conduction and complete the circuit.

Passive films (thin oxide layers) on metals like stainless steel and aluminum can drastically reduce corrosion rates. However, if damaged or exposed to aggressive ions (like chlorides), localized corrosion may start.

Environmental factors such as pH, temperature, oxygen, chloride content, and fluid flow all influence corrosion rates and mechanisms.

Microbiologically Influenced Corrosion (MIC): Some bacteria accelerate corrosion by altering local chemistry, especially in pipelines and marine environments.

Prevention and Mitigation Strategies

Effective corrosion prevention relies on several approaches:

• Material Selection: Use corrosion-resistant alloys (stainless steel, titanium, high-nickel alloys, FRP, ceramics) suitable for the environment.
• Alloying: Add elements like chromium and molybdenum to improve resistance.
• Protective Coatings: Apply organic (paint, epoxy), metallic (zinc, nickel), or conversion coatings (chromate, phosphate) to create physical barriers.
• Cathodic Protection: Make the structure the cathode in an electrochemical circuit.
  • Sacrificial Anode Systems: Attach a more reactive metal to corrode instead.
  • Impressed Current Systems: Use an external power source and inert anodes.
• Corrosion Inhibitors: Chemicals added to the environment to reduce corrosion rates by forming films or altering reactions.
• Environmental Control: Remove moisture, control pH, treat water, or use inert gas blanketing.
• Design Considerations: Avoid crevices, sharp corners, and dissimilar metal contact; ensure drainage and access for inspection.
• Inspection and Monitoring: Use visual, ultrasonic, radiographic, or electrochemical methods to detect corrosion early.

A combination of these strategies is often used to maximize service life and minimize costs.

Practical Examples and Use Cases

Construction and Infrastructure

Bridges and buildings are exposed to moisture, pollutants, and salts, all of which accelerate corrosion. Weathering steels, galvanized reinforcements, and robust coatings are common solutions. In reinforced concrete, corrosion of steel rebar (often due to chloride ingress) causes cracking and spalling. Solutions include epoxy-coated or stainless steel rebar and corrosion-inhibiting admixtures.

Industrial Equipment

Pipelines, storage tanks, and process vessels are at risk from both internal and external corrosion (e.g., water, acids, microorganisms). Protective measures include cathodic protection, coatings, and corrosion inhibitors. For aggressive chemicals, linings (rubber, glass, polymers) are also used.

Transportation

Aircraft, trains, and cars face corrosion from moisture, deicing chemicals, and environmental pollutants. The aerospace industry uses aluminum, titanium, and composites, but must manage galvanic corrosion at joints. Automobiles use galvanized steel and advanced coatings, especially in regions using road salt.

Marine Applications

Ships, offshore platforms, and harbor structures are exposed to seawater, oxygen, and biological activity. Corrosion is managed by sacrificial anodes, impressed current systems, high-alloy materials, and robust paint systems. Marine-grade FRP is used for decks and superstructures for its corrosion immunity.

Agricultural and Cooling Towers

FRP panels are common in buildings and cooling towers for their chemical resistance to ammonia and acids, long life, and ease of maintenance, outperforming metal panels in harsh environments.

Glossary of Corrosion and Related Terms

Anode:
Site of oxidation in an electrochemical cell—where metal loss (corrosion) occurs.

Cathode:
Site of reduction—protected from corrosion in the electrochemical process.

Corrosion Allowance:
Extra material thickness designed to be lost to predictable uniform corrosion over the structure’s life.

Corrosion Damage:
Physical deterioration, loss of mechanical properties, or function due to corrosion (includes thinning, pitting, cracking).

Corrosion Inhibitor:
Chemical additive that reduces the corrosion rate by forming a protective film or altering the environment.

Corrosion-Resistant Material:
Material that shows significantly lower corrosion rates due to its composition or a stable passive film.

Dealloying:
Selective removal of one element from an alloy (e.g., zinc from brass), leaving a porous structure.

Electrochemical Cell:
System where corrosion occurs due to simultaneous oxidation and reduction reactions, with electron flow between anode and cathode.

Galvanic Series:
Ranking of metals/alloys by their corrosion potential in a given environment—used to predict galvanic corrosion.

Passivation:
Formation of a stable, protective film (usually oxide) on a metal surface, reducing corrosion rates.

Pitting:
Localized, severe corrosion that creates small, deep holes in the material.

Stress Corrosion Cracking (SCC):
Cracking caused by tensile stress in a specific corrosive environment, leading to sudden and brittle failure.

Uniform Corrosion:
Even material loss across a surface; the most predictable form of corrosion.

Conclusion

Corrosion is a complex, multifaceted process affecting almost every industry and infrastructure system. Understanding its mechanisms, types, and prevention strategies is essential for engineers and asset managers. Through thoughtful design, material selection, protective systems, and regular monitoring, the risks and costs of corrosion can be dramatically reduced, enhancing safety and sustainability for the long term.