Corrosion in Materials Science and Engineering

Introduction to Corrosion

  • Corrosion is the interaction of materials with their environment, leading to deterioration of useful properties.

  • Affects mechanical strength, electroconductivity, and the appearance of materials.

Types of Materials and Corrosion

  • Metals: Commonly corroded through dissolution or oxidation.

  • Ceramics: More stable than metals, degrade at high temperatures or strong acids.

  • Polymers: Can degrade through dissolution or molecular bond breakage from UV radiation.

Cost of Corrosion

  • Corrosion prevention accounts for approximately 5% of national income.

  • Occasionally, corrosion processes can be beneficial (e.g., in dry-cell batteries).

Electrochemical Corrosion

  • Two main reactions during corrosion:

    • Oxidation (Anodic reaction): M → Mn+ + ne-

    • Reduction (Cathodic reaction): 2H+ + 2e- → H2

  • Example: Zinc in an acid solution (H+) carries out both reactions simultaneously.

  • Oxidation and reduction maintain electrical neutrality by consuming generated electrons.

Reaction Example with Zinc

  1. Oxidation: Zn → Zn2+ + 2e-

  2. Reduction: 2H+ + 2e- → H2

  3. Combined reaction: Zn + 2H+ → Zn2+ + H2

Oxygen's Role in Corrosion

  • Oxygen can modify reduction reactions based on the environment (neutral, acidic, or basic).

  • Complete corrosion processes combine oxidation and reduction without charge accumulation.

Iron Corrosion Process

  • Example of rust formation involves multi-step oxidation of iron:

    1. Oxidation to Fe2+: Fe + ½O2 + H2O → Fe2+ + 2OH- → Fe(OH)2

    2. Further oxidation to Fe3+: 2Fe(OH)2 + ½O2 + H2O → Fe3+ + 2OH- → Fe(OH)3

Galvanic Cells and Corrosion Potential

  • Galvanic coupling between metals leads to corrosion based on their electropotential: more susceptible metals corrode faster.

  • Standard Electrode Potentials inform about the ease of oxidation:

    • Noble metals like Au and Pt resist oxidation (higher potentials).

    • Alkali metals like Na and K are easily oxidized (lower potentials).

Nernst Equation

  • Relates Gibbs free energy change (ΔG) and electrical energy (ΔV):

    • ΔG = -nFΔV, where n is the number of electrons, and F is Faraday's constant.

  • For non-standard conditions, the Nernst equation predicts cell voltage based on ion concentrations:

    • ΔV = ΔV0 - (RT/nF) ln([M1 n+]/[M2 n+])

Corrosion Rates Calculation

  • Corrosion current (I) can be measured, with density (i) calculated as:

    • i = I/A [A/m2]

  • Corrosion rate can be expressed in various units based on weight loss or thickness loss due to corrosion.

  • Example calculations using thickness loss rates (CPR) consider surface area, density, and weight loss over time.

Common Forms of Corrosion

  • Uniform attack: Occurs evenly across surfaces (e.g., rusting).

  • Galvanic corrosion: Different metals in contact corrode differently.

  • Crevice corrosion: Occurs in confined spaces with stagnant solutions.

  • Pitting corrosion: Localized, causing small holes or pits.

  • Intergranular corrosion: Along grain boundaries, especially in alloys.

  • Erosion-corrosion: Combines mechanical wear with chemical degradation.

  • Stress corrosion: Results from the combination of stress and corrosive environment.

Corrosion Prevention Techniques

  1. Surface coatings (paints, galvanizing) can provide a barrier to prevent corrosion.

  2. Cathodic protection utilizes sacrificial anodes or impressed current to protect metals.

  3. Inhibitors can be added to environments to lessen corrosion rates.

Self-Check Questions

  1. Discuss the primary reactions occurring in metal corrosion versus those in ceramics and polymers.

  2. Describe the effects of environmental factors on corrosion rates such as temperature and solution composition.

  3. Consider the impact of polarization on corrosion measurements and prevention strategies.

Conclusion

  • Understanding corrosion processes and preventive measures is crucial in materials science and engineering to enhance longevity and performance in a variety of applications.