Corrosion is a complex electrochemical process that degrades metals. It involves oxidation and reduction reactions, influenced by factors like temperature, pH, and electrolyte composition. Understanding these fundamentals is crucial for predicting and controlling corrosion in various applications.

Different types of corrosion, such as uniform, galvanic, crevice, and pitting, pose unique challenges. Passivation, the formation of protective oxide layers, offers a powerful defense against corrosion. Knowing these mechanisms helps in developing effective prevention strategies.

Here are the updated notes with expanded explanations and examples, following the provided guidelines:

Electrochemical Fundamentals of Corrosion

Electrochemistry of corrosion processes

• Corrosion involves electrochemical oxidation and reduction of metals
  • Oxidation (anodic reaction) causes metal atoms to lose electrons and dissolve into solution as ions (iron forming Fe^2+^ ions)
  • Reduction (cathodic reaction) involves electrons from the metal reducing species in solution, such as oxygen or hydrogen ions (oxygen reduction to form hydroxide ions)
• Galvanic cells form when dissimilar metals are in electrical contact and exposed to an electrolyte (zinc and copper in saltwater)
  • Less noble metal acts as the anode and undergoes oxidation (zinc)
  • More noble metal acts as the cathode and is protected from corrosion (copper)
  • Potential difference between the metals drives the corrosion process (zinc-copper cell potential)

Factors affecting corrosion rates

• Temperature influences corrosion rates
  • Higher temperatures increase corrosion rates due to increased kinetic energy of molecules and faster diffusion (corrosion rates doubling every 10℃ rise)
  • Arrhenius equation relates corrosion rate to temperature: $Rate = A \cdot e^{-E_a/RT}$
    • $A$: Pre-exponential factor
    • $E_a$: Activation energy
    • $R$: Gas constant
    • $T$: Absolute temperature
• pH affects corrosion rates
  • Acidic environments (low pH) often lead to higher corrosion rates due to increased hydrogen ion concentration (sulfuric acid corroding steel)
  • Alkaline environments (high pH) can cause passivation, reducing corrosion rates (steel in concrete)
• Electrolyte composition influences corrosion rates
  • Presence of aggressive ions, such as chlorides, can accelerate corrosion by disrupting protective passive films (seawater corroding stainless steel)
  • Dissolved oxygen concentration affects the cathodic reaction rate, impacting overall corrosion rates (aerated vs. deaerated water)

Types and Prevention of Corrosion

Types of corrosion

• Uniform corrosion proceeds evenly over the entire metal surface
  • Occurs when the metal is exposed to a corrosive environment without localized differences in composition or geometry (steel in hydrochloric acid)
• Galvanic corrosion is caused by electrical contact between dissimilar metals in an electrolyte
  • Less noble metal (anode) corrodes preferentially, protecting the more noble metal (cathode) (magnesium sacrificial anode protecting steel)
• Crevice corrosion is localized corrosion occurring in confined spaces or crevices
  • Caused by differences in oxygen concentration or pH between the crevice and bulk solution (corrosion under washers or gaskets)
  • Restricted diffusion in crevices leads to accumulation of aggressive species, accelerating corrosion (stainless steel in chloride-containing crevices)
• Pitting corrosion results in the formation of small, deep pits on the metal surface
  • Initiated by local breakdown of the passive film due to chemical or mechanical factors (chloride-induced pitting of aluminum)
  • Pits can propagate rapidly, causing significant damage to the metal (perforations in stainless steel tanks)

Passivation and corrosion resistance

• Passivation involves the formation of a thin, protective oxide layer on the metal surface
  • Passive film acts as a barrier, separating the metal from the corrosive environment (chromium oxide film on stainless steel)
  • Passive films are typically a few nanometers thick and composed of metal oxides or hydroxides (aluminum oxide film)
• Factors affecting passivation include:
  1. Alloy composition: Certain alloying elements, such as chromium and nickel, enhance passivation (18/8 stainless steel)
  2. Environment: Passivation is favored in oxidizing environments and at higher pH levels (passivation of titanium in nitric acid)
• Passive films significantly reduce corrosion rates by limiting the access of corrosive species to the metal surface (passivated stainless steel in seawater)
  • Breakdown of the passive film can lead to localized corrosion, such as pitting or crevice corrosion (pitting of passivated stainless steel in chloride-containing environments)
  • Maintaining the integrity of the passive film is crucial for long-term corrosion resistance (regular inspection and maintenance of passivated equipment)