Corrosion and Its Electrochemical Theory
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Corrosion and Its Electrochemical Theory

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Questions and Answers

Which method involves using a more active metal to protect a less active metal from corrosion?

  • Electrical insulation
  • Anodic protection
  • Sacrificial anodic protection (correct)
  • Impressed current cathodic protection

    • What occurs at the anode during hydrogen evolution corrosion?

  • Oxidation of iron to form $Fe^{2+}$ (correct)
  • Formation of a protective oxide layer
  • Reduction of iron ions
  • Hydrogen ions gain electrons to form hydrogen gas

    • What occurs when an iron rod is dipped in concentrated nitric acid?

  • A protective iron hydroxide coating is formed. (correct)
  • The iron rod becomes magnetized.
  • The iron rapidly oxidizes without forming a coating.
  • The iron dissolves completely without any protective layer.

    • What is a key principle in proper designing to minimize localized corrosion?

    <p>Avoiding dissimilar metals contact</p> Signup and view all the answers

    Which of the following metals can be passivated by the deposition of an oxide layer?

    <p>Aluminum</p> Signup and view all the answers

    What is the role of anodic inhibitors in corrosion prevention?

    <p>They separate the rate of corrosion acting at the anode.</p> Signup and view all the answers

    In impressed current cathodic protection, what is applied to counteract corrosion?

    <p>Direct Current (DC)</p> Signup and view all the answers

    How do cathodic inhibitors function in acidic solutions?

    <p>They slow down the diffusion of $H^+$ ions.</p> Signup and view all the answers

    What happens to hydrogen ions in an acidic electrolyte during hydrogen evolution corrosion?

    <p>They become hydrogen gas.</p> Signup and view all the answers

    What is one example of a type of galvanic corrosion?

    <p>Zn-Cu Corrosion</p> Signup and view all the answers

    What describes the process of passivation in anodic protection?

    <p>Formation of a protective oxide layer.</p> Signup and view all the answers

    What is one of the disadvantages of having sharp corners in metal designs?

    <p>They promote accumulation of solids.</p> Signup and view all the answers

    Which of the following is NOT a method of treating metals to prevent corrosion?

    <p>Oxidation</p> Signup and view all the answers

    What condition must be maintained in the medium to reduce corrosion effectively?

    <p>Keeping the pH neutral or acidic.</p> Signup and view all the answers

    Which of the following metals is commonly used for sacrificial anodic protection?

    <p>Zinc</p> Signup and view all the answers

    Which of the following statements about corrosion inhibitors is true?

    <p>They reduce the rate of corrosion by chemical means.</p> Signup and view all the answers

    What is the primary reaction occurring at the anodic areas during corrosion?

    <p>Oxidation leading to metal dissolution</p> Signup and view all the answers

    Which of the following describes the appearance of rust formed on iron?

    <p>Reddish colored and hydrated</p> Signup and view all the answers

    What is formed at the anode during the concentration cell corrosion of a copper rod?

    <p>$Cu^{2+} + 2e^-$</p> Signup and view all the answers

    In the oxygen absorption corrosion mechanism, what forms after the reaction of $Fe^{2+}$ with $OH^-$?

    <p>Ferrous hydroxide</p> Signup and view all the answers

    What is the correct representation of the oxidation reaction occurring at the anode?

    <p>$Fe ightarrow Fe^{2+} + 2e^-$</p> Signup and view all the answers

    What condition is essential for the occurrence of corrosion?

    <p>Presence of a conducting medium</p> Signup and view all the answers

    In corrosion occurring due to defects in the oxide layer, what role do small cracks play?

    <p>They initiate anodic areas for oxidation.</p> Signup and view all the answers

    What is the outcome when $Fe(OH)_2$ oxidizes in the presence of oxygen?

    <p>Conversion to rust, $Fe_2O_3.3H_2O$</p> Signup and view all the answers

    In a concentration cell corrosion scenario with a metal rod half dipped in an electrolytic solution, which region acts as the anode?

    <p>The less oxygenated area</p> Signup and view all the answers

    What role does water play in the electrochemical corrosion process?

    <p>Facilitates electron flow at the cathode</p> Signup and view all the answers

    In water line corrosion, which part of the metal behaves as the cathode?

    <p>The metal above the water line</p> Signup and view all the answers

    What is the reaction equation taking place at the anode during pitting corrosion on a metal surface?

    <p>$Fe ightarrow Fe^{2+} + 2e^-$</p> Signup and view all the answers

    Which type of corrosion is characterized by localized spots resulting in pits and holes due to differences in oxygen concentration?

    <p>Pitting corrosion</p> Signup and view all the answers

    How does crevice corrosion occur?

    <p>Due to differences in oxygen levels at crevices</p> Signup and view all the answers

    What is the end product formed from the reaction involving ferrous ions at the anode in water line corrosion?

    <p>Fe(OH)2</p> Signup and view all the answers

    What is a common characteristic of all types of corrosion described?

    <p>They involve an electrochemical process.</p> Signup and view all the answers

    What happens at the anode during the corrosion process involving iron?

    <p>$Fe ightarrow Fe^{2+} + 2e^-$</p> Signup and view all the answers

    Which type of corrosion occurs specifically at grain boundaries due to stress and a corrosive environment?

    <p>Intergranular corrosion</p> Signup and view all the answers

    What is a common result of stress corrosion in materials?

    <p>Sudden and unexpected failure</p> Signup and view all the answers

    Which treatment can effectively reduce surface corrosion on metals?

    <p>Polishing</p> Signup and view all the answers

    What is true about electroplating?

    <p>It deposits a coating using electrolytic solution.</p> Signup and view all the answers

    What type of alloys are particularly susceptible to intergranular corrosion during welding?

    <p>Stainless steel alloys</p> Signup and view all the answers

    What chemical reaction is involved in the caustic embrittlement of mild steel?

    <p>$Na_2CO_3 + H_2O ightarrow 2NaOH + CO_2$</p> Signup and view all the answers

    What is one method to remove strains in metal that can lead to corrosion?

    <p>Heat treatment and annealing</p> Signup and view all the answers

    Study Notes

    Corrosion Defined

    • Corrosion is the gradual decay or eating away of a metal by electrochemical reaction.
    • In the case of Iron, corrosion is known as rusting, and it forms a reddish-brown hydrated layer with the formula Fe2O3.3H2O.

    Electrochemical Theory of Corrosion

    • Occurs when a metal is in contact with, or immersed in, a conducting solution.
    • Requires separated anodic and cathodic areas.
    • Anodic Area: Oxidation occurs, metal is destroyed (dissolving or forming metal oxide). M → M2+ + ne-
    • Cathodic Area: Reduction occurs, dissolved constituents in the conducting medium accept electrons to form ions. 2H2O + 2e- → 2OH- + H2↑

    Conditions and Features of Corrosion

    • Formation of separate anodic and cathodic areas.
    • Presence of a conducting medium to allow current flow between anodic and cathodic areas.
    • Corrosion occurs only at anodic areas.
    • Corrosion products form somewhere between anodic and cathodic areas.
    • Formation of oxide layer on the surface of Iron.

    Mechanism of Electrochemical Theory of Corrosion

    • Oxygen Absorption Corrosion: Occurs with neutral electrolytes, such as in rusting of iron in aqueous NaCl solution.
      • At Anode: Fe → Fe2+ + 2e-
      • At Cathode: 1/2O2 + H2O + 2e- → 2OH-
      • Formation of rust: Fe2+ + 2OH- → Fe(OH)2 (yellowish) which further oxidizes to Fe2O3.3H2O (rust)
    • Hydrogen Evolution Corrosion: Occurs in acidic environments (acidic electrolyte).
      • At Anode: Fe → Fe2+ + 2e-
      • At Cathode: 2H+ + 2e- → H2(g)↑
      • Overall: Fe + 2H+ → Fe2+ + H2(g)↑

    Control/Prevention of Corrosion

    • Proper Designing: Designing to minimize or localize corrosion. Avoid contact of dissimilar metals in corrosive environments, making anodic area as large as possible when dissimilar metals are in contact, and avoiding sharp corners which can trap corrosive materials.
    • Cathodic Protection: Making the metal act as a cathode to prevent corrosion.
      • Sacrificial Anodic Protection (Galvanic protection): Connecting the metallic structure with a more anodic metal, concentrating corrosion on the more active metal which corrodes slowly. Sacrificial Anode examples: Mg, Zn, Al.
      • Impressed Current (Direct Current) Cathodic Protection (Electrical Protection): Applying current from an external source to neutralize the corrosion current, converting the corroding metal from anode to cathode. Anodic materials include graphite and carbon.
      • Application of Electrical Protection: Used in buried structures like pipelines, tanks, transmission lines, towers, and ships.
    • Anodic Protection: Passivity (passivation) involves forming a protective oxide layer on the metal's surface. Applicable to metals that can be passivated. This is done by dipping an Fe rod in concentrated Nitric acid, which creates a protective Iron hydroxide coating, preventing further corrosion. Other metals also susceptible include Ni, Fe, Al, Cr.
    • Treatment of Metal: Alloying, polishing the surface, electroplating, hot dipping, and cementation.
    • Cementation: Deposition of a metal that forms an alloy with the surface of the metal being protected.
    • Treatment of Medium: Maintaining the pH (acidic or neutral) by adding inhibitors.
    • Corrosion Inhibitors: Chemical substances that reduce the rate of corrosion when added to an electrolyte solution.
      • Anodic Inhibitors (inorganic): Examples: Phosphates, Chromates, Silicates, Borate, molybdate. Separate anode reactions, producing insoluble precipitate on the metal surface that acts as a protective layer.
      • Cathodic Inhibitors (organic): Examples: Substituted Urea, Thiourea, Amines, Metal soaps, Bases. Slow down the diffusion of H+ ions in acidic solutions by getting absorbed on the cathodic metal surface, creating a protective layer.

    Types of Corrosion

    • Galvanic Corrosion: Occurs between dissimilar metals exposed to atmospheric conditions. The metal with higher reduction potential acts as the cathode, the metal with lower reduction potential acts as the anode. Example: Zn-Cu galvanic cell, Zn acts as the anode and corrodes.
    • Concentration Cell Corrosion: Occurs when a metal rod is partially immersed in an electrolyte solution. The less oxygenated area acts as the anode, the more oxygenated area acts as the cathode. Example: Half-dipped Cu rod in electrolyte.
    • Water Line Corrosion (Oxygen Cell Corrosion): Occurs with stagnated water in a steel tank. The area above the water line is more oxygenated, acting as the cathode, the area below the water line is less oxygenated, acting as the anode. Corrosion occurs below the water line.
    • Pitting Corrosion: Occurs when water droplets form on a metal surface. The area under the droplet is less oxygenated, acting as the anode, the area exposed to oxygen acts as the cathode. Localized corrosion in the form of pits or holes occurs at the anode.
    • Crevice Corrosion: Occurs where crevices form between metals in contact with a liquid. The crevice area is less oxygenated, acting as the anode, the exposed areas act as the cathode. Corrosion occurs in the crevices.
    • Intergranular Corrosion: Occurs in alloys, particularly along grain boundaries. During the solidification of molten metal, the grain boundaries can become anodic, prone to corrosion. Example: Welding stainless steel (Fe, Cr, C alloy), carbide precipitates at the grain boundaries, making the Chromium-depleted region at the boundaries anodic and prone to corrosion.
    • Stress Corrosion: Occurs due to a combination of mechanical stress and a corrosive environment. Common in alloys like Zinc and Nickel Brass. Example: Caustic embrittlement of mild steel in alkaline solutions at high temperatures and stress.

    Treatment of Metals

    • Alloying the Metal: Controlling corrosion by forming alloys, such as in Iron, Copper, Nickel.
    • Strains in the Metal: Removing strains through heat treatment and annealing, as strains can create focal points for corrosion reactions.
    • Polishing: Decreases surface corrosion by removing oxygen concentration cells.
    • Electroplating: Depositing a coating metal on the base metal using direct current in an electrolytic solution. The coating metal is the anode, the base metal is the cathode. Example: Tin or zinc plating on steel. Noble metals like Au, Ag, Cr, Ni, Cu, and Sn can be electroplated.

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    Corrosion Complete Notes PDF

    Description

    This quiz explores the fundamental concepts of corrosion, including its definition, the electrochemical theory behind it, and the necessary conditions for corrosion to occur. Learn about the anodic and cathodic areas, and how they contribute to the corrosion process in metals, particularly iron. Test your knowledge on this crucial topic in materials science.

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