Corrosion and its Control - Lecture 9
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Questions and Answers

What is the primary mechanism behind galvanic corrosion, and what role do anode and cathode play in this process?

Galvanic corrosion occurs when dissimilar metals are in contact, where the more anodic metal corrodes preferentially while the more cathodic metal is protected.

Explain how differential aeration corrosion occurs and its relationship with oxygen concentration.

Differential aeration corrosion arises from variations in oxygen concentration on a metal surface, where areas with lower oxygen levels act as anodes and corrode faster than areas with higher concentrations.

Identify and describe two factors that affect the rate of corrosion in metals.

Factors affecting corrosion rate include the presence of electrolytes, which enhance conductivity, and environmental conditions like temperature and humidity that accelerate electrochemical reactions.

Define pitting corrosion and discuss its characteristic feature.

<p>Pitting corrosion is localized corrosion that leads to the formation of small pits or holes in a metal surface, often progressing downward and can cause significant structural damage.</p> Signup and view all the answers

What is intergranular corrosion and where does it typically occur?

<p>Intergranular corrosion occurs along the grain boundaries of metals, especially in areas where special phases or impurities are present, making those regions more vulnerable to attack.</p> Signup and view all the answers

What are the three conditions necessary for galvanic corrosion to occur?

<p>Electrochemically dissimilar metals must be present, metals must be in electrical contact, and they must be exposed to an electrolyte.</p> Signup and view all the answers

How does the position of a metal in the galvanic series influence its role as an anode or cathode?

<p>A metal above the galvanic series acts as the anode and gets oxidized, while a metal below it acts as the cathode and gets reduced.</p> Signup and view all the answers

What reaction occurs at the anode when iron is involved in a galvanic cell?

<p>The reaction is: Fe → Fe2+ + 2e.</p> Signup and view all the answers

In a galvanic cell with iron and copper, which metal undergoes corrosion and why?

<p>Iron undergoes corrosion because it acts as the anode due to its lower reduction potential compared to copper.</p> Signup and view all the answers

What is the effect of the reduction potential difference on the rate of galvanic corrosion?

<p>The greater the reduction potential difference, the higher the rate of corrosion.</p> Signup and view all the answers

What happens to metals with lower reduction potentials in a galvanic cell?

<p>They act as anodes, get oxidized, and consequently suffer from corrosion.</p> Signup and view all the answers

Explain how a corrosive conducting medium influences galvanic corrosion.

<p>It allows for ion movement between the anode and cathode, facilitating the electrochemical reactions necessary for corrosion.</p> Signup and view all the answers

Can galvanic corrosion take place without water, and if so, how?

<p>Yes, galvanic corrosion can occur in the presence of other electrolytes even if water is absent.</p> Signup and view all the answers

What is the main difference between dry corrosion and wet corrosion?

<p>Dry corrosion occurs through direct chemical reactions with dry gases, while wet corrosion involves electrochemical reactions in an aqueous medium.</p> Signup and view all the answers

Explain the anodic reaction occurring in the electrochemical process of corrosion.

<p>The anodic reaction involves oxidation, where a metal (M) loses electrons, e.g., Fe → Fe2+ + 2e-.</p> Signup and view all the answers

How does the presence of moisture influence wet corrosion?

<p>Moisture facilitates electrochemical reactions by providing a conductive medium that supports the transfer of electrons.</p> Signup and view all the answers

What are the common reactions that occur at the cathodic region in corrosion?

<p>The cathodic reactions can either involve the liberation of hydrogen in acidic conditions or the absorption of oxygen in the presence of oxygen.</p> Signup and view all the answers

What role does the Galvanic series play in predicting corrosion behavior?

<p>The Galvanic series provides a more accurate assessment of corrosion potential by ranking metals based on their behavior in actual corrosive environments.</p> Signup and view all the answers

Describe the overall reaction that leads to the formation of rust.

<p>The overall reaction is 2Fe2+ + 4OH- → 2Fe(OH)2, followed by 2Fe(OH)2 + O2 + (n-2)H2O → Fe2O3·nH2O.</p> Signup and view all the answers

What are some primary factors that affect the rate of corrosion?

<p>Factors include environmental conditions such as humidity, temperature, and the presence of aggressive ions like chlorides.</p> Signup and view all the answers

How does the electrochemical series differ from the Galvanic series?

<p>The electrochemical series ranks metals based on their reduction potentials, while the Galvanic series reflects real-world corrosion behavior in specific environments.</p> Signup and view all the answers

What is stress corrosion and how does it relate to other forms of corrosion?

<p>Stress corrosion occurs when stress and corrosion factors combine, often leading to cracking or structural failure.</p> Signup and view all the answers

Why is the knowledge of anodic and cathodic behavior important in corrosion prevention?

<p>Understanding anodic and cathodic behavior allows targeted strategies to mitigate corrosion, such as applying protective coatings.</p> Signup and view all the answers

What are erosion-corrosion processes and how do they occur?

<p>Erosion-corrosion is a combined process where mechanical wear and electrochemical attack interact, often found in high-flow environments.</p> Signup and view all the answers

In what way does the presence of certain ions, like chlorides, impact corrosion rates?

<p>Chlorides lower the electrochemical potential, which accelerates corrosion rates in metal structures.</p> Signup and view all the answers

How does the formation of galvanic cells contribute to electrochemical corrosion?

<p>The formation of galvanic cells creates anodic and cathodic regions on the metal surface, leading to localized corrosion.</p> Signup and view all the answers

What are the common methods used to prevent corrosion?

<p>Common methods include coating, cathodic protection, and the use of corrosion inhibitors.</p> Signup and view all the answers

What impact does corrosion have on the economy, specifically regarding GNP?

<p>Annual losses due to corrosion are estimated at 4 to 5% of the Gross National Product (GNP), affecting overall economic productivity.</p> Signup and view all the answers

Study Notes

Lecture No. 9: Corrosion and its Control

  • Students will be able to describe the process of corrosion, identify types of corrosion, and explain galvanic series.

Corrosion Science

  • Corrosion is the destruction of a metal by its reaction with its surroundings.
  • Corrosion is a natural process where metals return to their original mineral state (e.g., oxides, sulphides, carbonates).
  • Noble metals (e.g., gold, platinum) are not susceptible to corrosion under normal atmospheric conditions.
  • Silver tarnishes but does not rust. It reacts with H₂S in food or water to form silver sulfide.

Cause of Corrosion

  • Metals have higher free energies than their compounds.
  • Metals tend to transform into their compounds through corrosion.

Corrosion Science

  • Corrosion is the destruction of metals and alloys by chemical or electrochemical changes in the surrounding environment.
  • Corrosion is a natural process.
  • It represents a return of metals to their more natural state as minerals (oxides).

Why Study Corrosion?

  • Corrosion products contaminate chemical industry products, reducing their usefulness.
  • Corrosion damages buildings, bridges, and structures.
  • Corrosion damages tools and equipment.
  • Annual loss due to corrosion is estimated at 4-5% of the Gross National Product (GNP).
  • Corrosion causes injuries due to structural failure.

Types of Corrosion: Dry Corrosion

  • Dry corrosion (also called chemical corrosion) is the direct attack of metals by dry gases, primarily through chemical reactions.
  • Examples: Chlorine (Cl₂) and sulfur dioxide (SO₂) fumes reacting with steel containers or dry air oxidizing a metal to form an oxide, or carbon dioxide reacting with copper to form copper carbonate (a green scale).

Types of Corrosion: Wet Corrosion

  • Wet corrosion (also called electrochemical corrosion) involves reactions in an aqueous medium.
  • A conducting surface of the metal interacts electrochemically with moisture and atmospheric oxygen.
  • An example is the rusting of iron.

Electrochemical Theory of Corrosion

  • Exposure of a metal surface to atmospheric air initiates gradual electrochemical changes.
  • Formation of galvanic cells (anodic and cathodic regions) results in minute galvanic cells.
  • Oxidation (corrosion) occurs at the anodic region.
  • Example: Fe → Fe²⁺ + 2e⁻
  • Reduction occurs at the cathodic region where electrons flow and cause reduction. Example: O₂ + 2H₂O + 4e⁻ → 4OH⁻
  • Overall process: 2Fe(s) + O₂(aq) + xH₂O(l) → Fe₂O₃·xH₂O(s)

Reactions at Cathodic Region

  • Reactions at the cathode depend on the surrounding medium.

  • a) Liberation of Hydrogen (absence of oxygen):

  • In acidic medium: 2H⁺ + 2e⁻ → H₂

  • In neutral or alkaline medium: 2H₂O + 2e⁻ → 2OH⁻ + H₂

  • b) Absorption of Oxygen (presence of oxygen):

  • In acidic medium: 4H⁺ + O₂ + 4e⁻ → 2H₂O

  • In neutral or alkaline medium: O₂ + 2H₂O + 4e⁻ → 4OH⁻

  • Overall reaction: Fe²⁺ + 2OH⁻ → Fe(OH)₂ 2Fe(OH)₂ + O₂ + nH₂O → Fe₂O₃·nH₂O.

Electrochemical Theory of Corrosion

  • Metal ions combine with hydroxide ions to form metal hydroxide and finally hydrated metal oxide (rust).
  • Over all reaction: 2Fe²⁺ + 4OH⁻ → 2Fe(OH)₂ 2Fe(OH)₂ + O₂ + (n-2)H₂O → Fe₂O₃·nH₂O

Necessity for the Galvanic Series

  • Electrochemical series (standard reduction potential) for pure metals does not accurately predict corrosion in a specific environment.
  • Passivity of metals is not considered in standard reduction potentials.
  • Galvanic series is more accurate because it accounts for alloy corrosion and is built on corrosion studies of metals in sea water (determining nobility of metals and semi-metals).

Galvanic Corrosion

(A table is given correlating the nobility of different metals)

Forms of Corrosion

  • Uniform attack: Oxidation and reduction occur uniformly over the surface (e.g., general rusting).
  • Selective leaching: Preferred corrosion of one element (e.g., zinc from brass).
  • Intergranular corrosion: Corrosion along grain boundaries.
  • Stress corrosion: Occurs in concert with external stresses at crack tips.
  • Erosion-corrosion: Breakdown of the protective layer caused by erosion.
  • Pitting: Localized corrosion that forms pits.
  • Crevice: Corrosion happening between two pieces of the same metal.

Differential Metal Corrosion (Galvanic Corrosion)

  • Galvanic corrosion requires 3 conditions:

  • Differing metals

  • Electrical contact between metals

  • Exposure to an electrolyte (like saltwater)

  • The metal higher in electrochemical potential (less reactive) acts as the cathode.

  • The metal lower in electrochemical potential (more reactive) acts as the anode and corrodes.

  • The more dissimilar the metals (larger the difference in electrode potential), the faster the corrosion.

Types of Corrosion (Differential Metal Corrosion)

  • Differential metal corrosion (galvanic corrosion) is where dissimilar metals are in contact with each other forming galvanic corrosion in a conducting medium.
  • The metal that has a lower reduction potential will corrode.
  • The metal with the higher electrode potential will not corrode.
  • The rate of corrosion depends on how far the two metals are placed on the galvanic series.
  • Greater differences in electrode potential result in faster corrosion rates.

Quiz

  • Can corrosion occur in the absence of water?
  • Why are copper statues green?
  • If copper and zinc are connected, which metal corrodes more?
  • If magnesium and zinc are connected, which metal corrodes more?

Summary

  • Corrosion is metal destruction and loss due to reactions with surroundings.
  • When a metal is exposed to the air, it leads to electrochemical changes and galvanic cell formation.
  • Corrosion has wet and dry types.

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Corrosion and its Control Lecture Notes PDF

Description

This quiz covers Lecture No. 9 on corrosion and its control. Students will explore the various types of corrosion, the processes involved, and understand the galvanic series. Test your knowledge on how metals react with their surroundings and the principles behind corrosion science.

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