VÉL501M Corrosion Overview

• Instructor: Prof. Sigrún Nanna Karlsdóttir

• Date: 21/08/24

What is Corrosion?

• Definition: Corrosion is the natural oxidation process that returns metals to their lower free energy ionic states (e.g. oxides, chlorides, carbonates).

• Characteristics:

  • Involves deterioration of metals and alloys in corrosive environments.

  • A destructive electrochemical attack exemplified by the rusting of automobiles and other equipment.

Importance of Corrosion

• Economic Impact:

  • Direct Costs: Expenses directly related to repair and replacement of corroded materials.

  • Indirect Costs:

    • Plant downtimes, loss of products due to leaks, and loss of efficiency (e.g., in heat exchangers).

    • Environmental contamination (e.g., drinking water contaminants).

• Safety Issues: Increased risks due to structural failures or product contamination.

Key Questions in Corrosion Studies

• How does corrosion occur?

• Which metals are most likely to corrode?

• What environmental factors affect corrosion rates?

• How can we prevent or control corrosion?

Electrochemical Reactions in Corrosion

• Key Components:

  • Anode: Where oxidation occurs.

  • Cathode: Where reduction occurs.

  • Electrolyte: Conducts ions between anode and cathode.

• Example Equation:

  • M (metal) → M+ + e- (Oxidation at Anode)

  • O2 + 2H2O + 4e- → 4OH- (Reduction at Cathode)

Corrosion Reactions Breakdown

• Total Reactions:

  • Rust Formation: 2Fe + O2 + 2H2O → 2Fe(OH)2 → 2Fe(OH)3 (rust)

  • Conservation: Total oxidation = total reduction, indicating no net accumulation of electrons.

Conditions for Steel Corrosion in Acidic Waters

• Electrochemical Reactions:

  • Process involves anode and cathode reactions, emphasizing the dominance of reactions based on pH.

  • At pH > 5.5, O2 reaction dominates; below that, H2 evolution becomes significant.

Corrosion Forms Overview

1. Uniform Corrosion

• Most common type; uniform metal loss can be quantified (e.g. mm/year).

• Increased acidity (lower pH) accelerates corrosion.

2. Pitting Corrosion

• Localized corrosion leads to pitting (holes); much faster than bulk corrosion.

• Commonly caused by chloride ion presence; particularly hazardous in stagnant conditions.

3. Crevice Corrosion

• Occurs in shielded areas (crevices) and is accelerated by stagnant solutions, hydrogen, and chloride ions.

4. Intergranular Corrosion

• Localized near grain boundaries, leading to metal disintegration, especially in heat-affected zones of welded structures.

5. Galvanic Corrosion

• Caused by potential differences between two different metals in contact with an electrolyte (e.g. iron with bronze).

Environmental Assisted Cracking (EAC)

• General term for failures due to tensile stress combined with corrosive environments.

• Types include Stress Corrosion Cracking (SCC), Hydrogen Embrittlement, and Corrosion Fatigue Cracking.

Conditions for Stress Corrosion Cracking (SCC)

• Requires:

  1. Tensile Stress

  2. Susceptible Material

  3. Corrosive Environment

Pitting and Corrosion Scenarios

• Example: SS 304 stainless steel failure in chloride-rich environments.

Erosion-Corrosion

• Occurs due to flowing streams leading to surface film removal, often exacerbated by high velocities.

• More prone in areas like elbows of piping and turbine blades.

Cavitation and Its Implications

• Results from boiling or bubbles collapsing, commonly on ship propellers.

• Requires preventative measures like cathodic protection and careful design.

Microbiological Corrosion

• Caused by bacteria creating corrosive environments, relevant in systems like cooling.

Selective Corrosion (Dezincification)

• Selective removal of zinc from brass alloys due to prolonged exposure to corrosive environments, such as aerated water high in CO2 or chlorides.