Corrosion Mechanism

Corrosion is the degradation of materials through chemical or electrochemical reactions induced by environmental factors, resulting in the loss of physical and mechanical properties. Although commonly associated with metals, many materials including polymers and ceramics are also affected by this process. The fundamental driving force behind corrosion is the tendency of the system to reduce its free energy. Refined metals exhibit a thermodynamic tendency to re-oxidize because they possess higher free energy compared to their natural oxide states.

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Corrosion is typically an electrochemical process in which an anodic region and a cathodic region form. At the anodic site, the metal dissociates into ions while releasing electrons; these electrons participate in a reduction reaction at the cathodic site. This situation can be likened to a short-circuited galvanic cell. The localized potential differences generated in corrosion cells create favorable conditions for the development of localized corrosion types such as pitting and crevice corrosion.

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Some metals, such as aluminum, titanium, and stainless steels, undergo passivation through the spontaneous formation of thin oxide films on their surfaces. This oxide layer slows down metal dissolution and reduces the corrosion rate. However, if this layer is mechanically or chemically disrupted, localized corrosion types may develop.

• Pitting corrosion: Begins with the localized breakdown of the passive film. In areas with limited oxygen diffusion, an acidic environment forms and the pit deepens progressively.
• Crevices corrosion: Occurs in narrow areas with limited liquid volume, particularly where oxygen diffusion is restricted.
• Galvanic corrosion: Occurs when two metals with different electrode potentials come into contact; the metal with the lower potential becomes anodic and dissolves.
• Stress corrosion cracking: Occurs when tensile stress and a corrosive environment act simultaneously, leading to cracking and fracture.
• Hydrogen-induced damage: Diffusion of hydrogen atoms into the metal structure can cause embrittlement and cracking.

The thermodynamic possibility of corrosion does not imply that it will necessarily occur. The rate of occurrence depends on factors such as activation energy, concentration of dissolved gases, ion concentration, and conductivity. Polarization effects—activation, concentration, and ohmic resistance—determine the kinetics of electrode reactions. These effects are visualized using Evans diagrams.

Corrosion rate is commonly measured using weight loss methods, electrochemical polarization tests, impedance spectroscopy, and potentiodynamic curves. Tafel analyses, in particular, provide the ability to evaluate anodic and cathodic reaction tendencies separately.

Corrosion is a multidimensional process shaped by the interaction of materials science, electrochemistry, and environmental conditions. In this process, numerous parameters play decisive roles, including passivation, localized film breakdown, the presence of electroactive ions, and surface morphology. Preventing corrosion is not achievable through material selection alone but requires a comprehensive approach incorporating environmental control, surface treatments, and electrochemical protection systems.