Understanding Corrosion: Types, Causes, and Prevention
Understanding Corrosion
Corrosion is a more complex process than simple oxidation. It’s a galvanic process occurring between a metal and its environment, leading to deterioration. This deterioration manifests as:
- Ruptured exhaust pipes and mufflers in cars
- Failing domestic water heaters
- Gas leaks and explosions in storage tanks or pipelines
- Breaks in water pipes
- The collapse of bridges and other structures
The Electrochemical Process of Corrosion
Corrosion involves a metal interacting with its surroundings, causing physical and chemical deterioration. This results in a significant decrease in size (thickness), weakening the metal’s ability to withstand mechanical loads. Corrosion is primarily an electrochemical process involving oxidation and reduction reactions. These reactions facilitate an exchange of electrons, creating an electric current flow between an anode and a cathode through a conductive medium, similar to a galvanic cell.
Anode and Cathode
The anode, which corrodes, transfers electrons to the circuit. The cathode receives electrons from the circuit through a cathodic reaction. Ions combine with electrons at the cathode to produce a new product. The anode and cathode must have an electrical connection, usually through physical contact, to enable electron flow and sustain the reaction.
Electrolyte
An electrolyte, typically a liquid, must be in contact with both the anode and cathode. This conductive electrolyte closes the circuit, providing a pathway for metal ions to leave the anode and move towards the cathode to accept electrons.
Anodic and Cathodic Reactions
The anodic reaction involves oxidation, where atoms at the anode ionize and dissolve into the electrolyte solution, releasing electrons. The cathodic reaction involves reduction, the opposite of the anodic reaction, where electro-deposition occurs.
Free Energy and Metal Behavior
The free energy of a metal determines its susceptibility to corrosion:
- Positive Free Energy: The metal is active and prone to corrosion (e.g., iron, aluminum, zinc).
- Positive Free Energy (Passive): The metal appears unattacked despite the positive free energy, indicating a passive state.
- Zero or Negative Free Energy: The metal is indifferent to aggressive agents and resistant to corrosion (e.g., noble metals).
Predicting a metal’s behavior in a specific environment is possible based on thermodynamic principles. A positive free energy suggests a possibility of corrosion. While environments can vary, the atmosphere, with its water (electrolyte) and oxygen (oxidant), presents an inevitably corrosive environment for most industrial metals.
Electrolytes and Electrodes in Corrosion
Any solution, including rain or condensed moisture, can act as a corrosive electrolyte. These electrolytes range from hard water and saltwater to strong acids and alkalis. The electrodes involved in corrosion are termed anodes (where corrosion occurs) and cathodes. These electrodes can be composed of different metals.
Conditions for Corrosion
Corrosion requires the following conditions:
- Presence of an anode and a cathode
- Electrical potential difference between the anode and cathode
- Metallic conductor electrically connecting the anode and cathode
- Immersion of both anode and cathode in an electrically conductive, ionized electrolyte
Types of Corrosion Cells
Oxygen Concentration Cell
Oxygen concentration cells form when oxygen distribution in a solution is uneven. This occurs in areas with low oxygen access, such as under deposits or within gaskets. The area exposed to lower oxygen concentration acts as the anode and corrodes.
Protection Measures
- Sealing surfaces to prevent oxygen concentration differences
- Maintaining clean surfaces to avoid deposit buildup
- Avoiding materials that trap moisture against metal surfaces
Active-Passive Concentration Cell
Metals with a passive protective layer, like stainless steel, can experience corrosion if the layer is compromised. This creates an active-passive cell, where the exposed metal becomes the anode and the passive layer acts as the cathode, leading to rapid pitting corrosion.
Protection Measures
- Frequent cleaning to remove deposits and prevent film breakdown
- Application of protective coatings to shield the metal surface
Passivation
Passivation refers to a metal’s loss of reactivity under specific environmental conditions due to the formation of a protective layer. This layer, often an oxide, inhibits further reactions, making the metal resistant to corrosion. Examples include stainless steel, nickel alloys, titanium, and aluminum alloys.
Theories of Passivation
- Oxide Film Theory: A passive oxide layer acts as a barrier, separating the metal from the environment and slowing down the corrosion rate.
- Adsorption Theory: A layer of adsorbed oxygen displaces water molecules, reducing the rate of anodic dissolution.
Occluded Corrosion Cell
Similar to pitting corrosion, occluded corrosion occurs in confined spaces with limited oxygen access. This type of corrosion is common in crevices, under gaskets, and in other tight spaces.
Filiform Corrosion
Filiform corrosion affects painted or plated surfaces when moisture penetrates the coating. It starts at small defects and spreads, causing the coating to detach. Quick-drying paints and lacquers are particularly susceptible.
Intergranular Corrosion
Intergranular corrosion occurs along grain boundaries, which are more chemically active than the grain itself. This is common in stainless steels and heat-treatable aluminum alloys. The anodic behavior of grain boundaries is due to atomic disruptions, solute segregation, and impurity migration.
Protection Measures
- Using stabilized or passivated stainless steel grades (e.g., 321 or 347)
- Opting for low carbon stainless steels (e.g., 304L or 316L)
Friction Corrosion
Friction corrosion occurs at the interface of contacting surfaces under load and subject to slight vibrations. This type of corrosion is accelerated by the constant rubbing and wear between the surfaces.