Corrosion and Degradation of Materials — Chapter 17 Summary from Callister’s Materials Science and Engineering

Chapter 17 of Materials Science and Engineering by William D. Callister, Jr. and David G. Rethwisch examines the mechanisms, types, and prevention of corrosion and degradation in metals, ceramics, and polymers. Corrosion and material degradation are critical concerns for engineers, with major economic and safety impacts across industries. This summary provides a comprehensive look at electrochemical corrosion, unique degradation processes, and practical prevention strategies.

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Corrosion in Metals: Electrochemical Mechanisms

Corrosion in metals involves electrochemical reactions where metal atoms are oxidized at the anode (losing electrons) and reduction reactions occur at the cathode. This process leads to the gradual breakdown of metallic materials, commonly observed as rusting or tarnishing. The chapter covers oxidation-reduction (redox) half-reactions, with the total rate of oxidation balanced by the rate of reduction.

Types of Corrosion and Their Effects

• Uniform Attack: Even corrosion across a surface, such as the general rusting of steel.
• Galvanic Corrosion: Occurs when two dissimilar metals are electrically connected in an electrolyte, causing the more active metal to corrode.
• Crevice Corrosion: Localized corrosion in areas with limited oxygen, such as under gaskets and fasteners.
• Pitting: Formation of small, deep pits that can lead to rapid failure with little warning.
• Intergranular Corrosion: Attacks grain boundaries, especially in stainless steels.
• Selective Leaching: Preferential removal of one element from an alloy (e.g., dezincification of brass).
• Erosion-Corrosion: Combined mechanical and chemical attack, common in piping systems.
• Stress Corrosion: Cracking due to simultaneous tensile stress and a corrosive environment.
• Hydrogen Embrittlement: Loss of ductility caused by hydrogen absorption into metals.

Factors Affecting Corrosion and Rates

• Electrochemical Reactions: Driven by the electromotive force (EMF) series and the galvanic series, which rank metals by oxidation tendency.
• Corrosion Rate: Measured as corrosion penetration rate (CPR), affected by temperature, electrolyte composition, fluid velocity, and surface area.
• Polarization: Deviation from equilibrium electrode potential, either by reaction rate (activation polarization) or ion diffusion (concentration polarization).
• Passivity: Formation of thin, stable oxide layers that reduce metal reactivity, as in chromium, titanium, and aluminum.

Corrosion Prevention Methods

• Material Selection: Using corrosion-resistant materials or alloys tailored to specific environments.
• Environmental Modification: Altering the chemical environment to reduce corrosiveness (e.g., controlling pH or removing oxygen).
• Design Modifications: Avoiding crevices and promoting even fluid flow to minimize localized corrosion.
• Coatings and Paints: Providing physical barriers to protect underlying metal surfaces.
• Cathodic Protection: Making the protected metal the cathode by supplying electrons, often using a sacrificial anode.

Oxidation and Degradation in Ceramics and Polymers

• Oxidation of Metals: In high-temperature environments, metals form oxide scales. The Pilling-Bedworth ratio predicts the protectiveness of these oxide films.
• Ceramic Degradation: Ceramics are generally resistant to corrosion but can degrade under extreme conditions or high temperatures.
• Polymer Degradation: Involves swelling, dissolution, and bond scission from heat, radiation, or chemicals, resulting in loss of mechanical properties and breakdown of the material.

Glossary of Key Terms

• Activation Polarization: Reaction rate limited by the slowest step in an electrochemical process.
• Anode: Electrode where oxidation (electron loss) occurs.
• Cathode: Electrode where reduction (electron gain) occurs.
• Cathodic Protection: Preventing corrosion by supplying electrons, making the metal the cathode.
• Concentration Polarization: Limitation of corrosion rate by ion diffusion.
• Crevice Corrosion: Localized attack in shielded areas or narrow gaps.
• Electromotive Force (EMF) Series: List of elements ranked by their oxidation potential.
• Pilling-Bedworth Ratio: Ratio of oxide to metal volume, indicating whether the oxide film is protective.
• Sacrificial Anode: A more reactive metal that corrodes in place of the protected metal.
• Scission: Breaking of molecular chains in polymers, leading to degradation.
• Stress Corrosion: Cracking due to combined mechanical stress and corrosion.

Conclusion: Managing Degradation for Material Longevity

Understanding the mechanisms and types of material degradation is crucial for designing durable structures and minimizing maintenance costs. By combining proper material selection, design, and prevention methods, engineers can significantly extend the lifespan of components exposed to corrosive environments. For a clear, accessible walkthrough, watch the podcast above and subscribe to Last Minute Lecture for expertly summarized chapters in materials science.

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