17.6 Corrosion
3 min read•june 25, 2024
is a sneaky destroyer of metals, eating away at structures and objects we rely on daily. It's an electrochemical process where metals lose electrons, like turning to . Understanding this helps us protect our stuff from falling apart.
We've got tricks to fight corrosion, like painting metals, mixing them with other elements, or using . Each method has its strengths and weaknesses, depending on what we're trying to protect and where it's located.
Corrosion and Its Prevention
Electrochemical process of corrosion
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- Involves the of a metal, which is the loss of electrons from a substance
- Common example is iron rusting, where iron () is oxidized to iron(II) ions () at the :
- () is reduced at the :
- Overall reaction:
- further oxidized to form hydrated iron(III) oxide (), known as rust
- Requires four components: (site of where metal ions are released), cathode (site of reduction where electrons are consumed), (solution that allows ion movement, such as water with dissolved salts), and electrical connection (allows electrons to flow from anode to cathode)
Methods of corrosion prevention
- act as a barrier between the metal and the environment
- Examples include paint, plastic, and ceramic coatings
- Prevent exposure to oxygen and moisture, slowing or preventing corrosion
- involves mixing a metal with other elements to improve its corrosion resistance
- Examples include stainless steel (iron alloyed with chromium and nickel) and brass (copper alloyed with zinc)
- Alloying elements can form protective oxide layers or change the metal's reactivity
- connects the metal to be protected (cathode) to a more easily corroded metal (anode)
- Two types: (more reactive metal like zinc or magnesium connected to metal to be protected) and (external power source supplies electrons to metal to be protected, making it a cathode)
- More reactive metal corrodes preferentially, protecting the metal of interest
- Selection of sacrificial anodes often based on the , which ranks metals by their electrochemical potential
Effectiveness of prevention techniques
- Protective coatings suitable for a wide range of applications, from small objects to large structures
- Effectiveness depends on coating material, application method, and environment
- Require regular maintenance and reapplication to maintain protection
- Alloying effective for applications requiring intrinsic corrosion resistance and suitable for harsh environments where coatings may fail
- May be more expensive than using pure metals
- Alloying elements can affect other properties, such as strength and ductility
- effective for large structures (pipelines, storage tanks, ship hulls)
- Sacrificial anodes are simple and require no external power, but need periodic replacement
- Impressed current systems more complex and require constant power supply, but offer adjustable protection
- Less effective in low-conductivity environments (freshwater, dry soil)
Types of Corrosion
- Uniform corrosion: Even degradation across the metal surface
- Localized corrosion: Concentrated in specific areas
- : Formation of small holes or cavities in the metal surface
- : Combination of tensile stress and corrosive environment leading to cracks
- : Formation of a thin protective layer on some metals, reducing further corrosion
Key Terms to Review (23)
Alloying: Alloying is the process of combining two or more metals to create a new material with enhanced properties. This is a common practice in metallurgy and materials science, as alloying can improve the strength, corrosion resistance, and other characteristics of the resulting alloy compared to the individual pure metals.
Anode: The anode is the electrode where oxidation occurs in a galvanic cell. It is typically the negative terminal in such cells.
Anode: The anode is the electrode in an electrochemical cell where oxidation occurs, and electrons are released to flow through an external circuit. It is the negatively charged electrode that attracts positively charged ions and initiates the flow of electrons in a redox reaction.
Cathode: A cathode is an electrode where reduction occurs during an electrochemical reaction. It plays a crucial role in various processes such as galvanic cells, where it attracts cations from the electrolyte, facilitating the flow of electric current. Understanding the function of the cathode is essential for grasping concepts like electrode potentials and energy transformations in electrochemical cells.
Cathodic protection: Cathodic protection is a technique used to prevent the corrosion of metal surfaces by making them the cathode of an electrochemical cell. This is achieved by connecting the metal to a more easily oxidized 'sacrificial' anode.
Cathodic Protection: Cathodic protection is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell. This is accomplished by connecting the metal to be protected to a more easily corroded 'sacrificial metal' to act as the anode, thereby preventing the original metal from corrosion.
Corrosion: Corrosion is the gradual deterioration of a material, usually a metal, due to chemical or electrochemical reactions with its environment. It is a natural process that can lead to the loss of structural integrity and functionality of materials over time.
Electrolyte: An electrolyte is a substance that, when dissolved in a solvent such as water, dissociates into charged particles called ions. These ions are capable of conducting electricity and are essential for various chemical and physiological processes in the body and in electrochemical devices.
Electrolytes: Electrolytes are substances that dissociate into ions when dissolved in water, allowing the solution to conduct electricity. Common examples include salts, acids, and bases.
Galvanic Series: The galvanic series, also known as the activity series or reactivity series, is a ranking of metals based on their relative reactivity or tendency to lose electrons and form positive ions. This series is crucial in understanding the concept of corrosion, as it helps predict the likelihood and direction of electrochemical reactions between different metals.
Galvanization: Galvanization is the process of applying a protective zinc coating to steel or iron to prevent rusting. It involves immersing the metal in molten zinc to form a corrosion-resistant layer.
Impressed Current: Impressed current is a method of cathodic protection used to prevent the corrosion of metal surfaces, particularly in structures such as pipelines, ships, and offshore platforms. It involves applying a direct current to the metal surface to counteract the natural electrochemical processes that lead to corrosion.
Iron: Iron is a chemical element that is essential for many vital processes in the human body. It is a transition metal that plays a crucial role in various topics in chemistry, including atomic structure, the periodic table, chemical nomenclature, corrosion, periodicity, and the occurrence, preparation, and properties of transition metals and their compounds.
Oxidation: Oxidation is a chemical reaction where an atom, ion, or molecule loses electrons. It often involves gaining oxygen or losing hydrogen.
Oxidation: Oxidation is a fundamental chemical process in which an element or compound loses electrons, resulting in an increase in its oxidation state. This term is central to understanding redox (reduction-oxidation) reactions, the functioning of electrochemical cells, corrosion, and the properties and behavior of oxygen, sulfur, and various organic compounds.
Oxygen: Oxygen is a highly reactive nonmetallic element that is essential for most forms of life. It is the third most abundant element in the universe and the most abundant element on Earth's crust. Oxygen plays a crucial role in various chemical and biological processes, including respiration, combustion, and oxidation-reduction reactions.
Passivation: Passivation is a process in which a metal surface is treated to create a thin, stable, and protective oxide film that prevents further corrosion. This process enhances the metal's resistance to chemical attack and improves its overall durability.
Pitting: Pitting is a type of localized corrosion that results in the formation of small, deep cavities or holes on the surface of a metal. This type of corrosion is often associated with the breakdown of a protective oxide film on the metal, leading to the selective dissolution of the underlying material.
Protective Coatings: Protective coatings are materials applied to the surface of an object or structure to shield it from environmental factors that can cause degradation, such as corrosion, wear, or chemical attack. These coatings act as a barrier to prevent or slow down the deterioration of the underlying material, thereby extending the lifespan and functionality of the protected item.
Rust: Rust is the reddish-brown corrosion that forms on the surface of iron and steel when they are exposed to air and moisture. It is a common problem that affects the durability and appearance of metal objects and structures.
Sacrificial Anode: A sacrificial anode is a metal component used in cathodic protection systems to prevent the corrosion of a more valuable metal structure. It is a type of galvanic corrosion control mechanism that protects the metal from electrochemical deterioration by preferentially corroding itself instead.
Sacrificial anodes: Sacrificial anodes are highly active metals used to prevent a less active material surface from corroding. They work by being intentionally corroded in place of the protected metal.
Stress Corrosion Cracking: Stress corrosion cracking (SCC) is a type of corrosion that occurs when a material is subjected to a tensile stress and a corrosive environment, leading to the formation and propagation of cracks. This phenomenon is particularly problematic in the context of corrosion, as it can lead to the sudden and unexpected failure of materials and structures.