Corrosion Science Overview
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Questions and Answers

What effect do oxidizing agents typically have on corrosion rates?

  • They can either increase or decrease corrosion rates. (correct)
  • They only decrease corrosion rates.
  • They have no effect on corrosion rates.
  • They consistently prevent any form of corrosion.

Which metal is known to corrode rapidly in the presence of air?

  • Aluminum
  • Monel metal (correct)
  • Magnesium
  • Stainless steel

What is considered critical humidity in relation to atmospheric corrosion?

  • The maximum humidity level tolerated by metals
  • The average humidity level in industrial environments
  • The humidity above which the corrosion rate sharply increases (correct)
  • The humidity below which corrosion occurs

Which atmospheric conditions can contribute to electrochemical corrosion?

<p>Presence of moisture and corrosive gases (C)</p> Signup and view all the answers

How do impurities like SO₂ and H₂S in industrial atmospheres affect corrosion?

<p>They enhance the electrical conductivity and acidity of liquids, increasing corrosion. (C)</p> Signup and view all the answers

What role do suspended particles like NaCl play in corrosion?

<p>They absorb water and act as strong electrolytes, enhancing corrosion. (D)</p> Signup and view all the answers

Which type of ions are known to destroy protective surface films on metals?

<p>Chloride ions (C)</p> Signup and view all the answers

What theory is essential for understanding the degradation of metals in various environments?

<p>Theory of corrosion mechanisms (A)</p> Signup and view all the answers

What is the main cause of rotor blade damage in helicopters during the Gulf War?

<p>Desert sand (D)</p> Signup and view all the answers

Which factor is considered the biggest threat to the longevity of defense hardware?

<p>Humidity (A)</p> Signup and view all the answers

Which metal is considered a noble metal that does not corrode easily?

<p>Gold (A)</p> Signup and view all the answers

What happens when two metals with a larger difference in electrode potential are in contact?

<p>Corrosion rates increase (D)</p> Signup and view all the answers

Which of the following pairs is most likely to avoid galvanic corrosion?

<p>Aluminum and aluminum alloy (B)</p> Signup and view all the answers

How do impurities in zinc metal affect its corrosion rate?

<p>Increase corrosion by creating local electrochemical cells (D)</p> Signup and view all the answers

Which condition is NOT required for the optimal storage of defense equipment?

<p>High dust concentration (D)</p> Signup and view all the answers

In the galvanic series, which order is used to arrange metals?

<p>By their oxidation potential (D)</p> Signup and view all the answers

What does a P.B.Ratio of less than 1 indicate about an oxide layer?

<p>The oxide layer is porous and allows for continued corrosion. (B)</p> Signup and view all the answers

Which of the following metals is likely to form a protective oxide layer indicated by a P.B.Ratio of 1 or greater?

<p>Aluminum (Al) (A)</p> Signup and view all the answers

According to the Pilling-Bedworth Rule, what does the P.B.Ratio represent?

<p>The ratio of the volume of the oxide formed to the volume of the metal consumed. (D)</p> Signup and view all the answers

Which of the following statements is true regarding noble metals like Au and Ag?

<p>They form unstable oxides that are non-protective. (C)</p> Signup and view all the answers

What is the implication of an oxide layer being non-porous?

<p>It adheres to the metal surface and prevents further oxidation. (D)</p> Signup and view all the answers

How does grain size affect the rate of corrosion in metals?

<p>Smaller grain size increases corrosion due to greater solubility. (A)</p> Signup and view all the answers

What condition increases the corrosion rate when the ratio of anodic to cathodic regions is considered?

<p>A small anode with a large cathodic area. (C)</p> Signup and view all the answers

Which metals are known to form protective films that resist corrosion?

<p>Titanium, aluminum, and chromium. (B)</p> Signup and view all the answers

How does the solubility of corrosion products influence corrosion rates?

<p>High solubility leads to faster corrosion. (C)</p> Signup and view all the answers

What is the effect of temperature on the rate of corrosion?

<p>Increased temperature accelerates corrosion due to increased conductance. (D)</p> Signup and view all the answers

Which statement about the pH of the medium and corrosion is correct?

<p>Acidic solutions are generally more corrosive than neutral or alkaline solutions. (D)</p> Signup and view all the answers

What role does hydrogen overvoltage play in corrosion susceptibility?

<p>Lower hydrogen overvoltage increases susceptibility to corrosion. (A)</p> Signup and view all the answers

Which effect does the boundary stresses in pure metals have on corrosion?

<p>Boundary atoms become more anodic and more prone to corrosion. (A)</p> Signup and view all the answers

What is the primary focus of the Chemical Theory of Corrosion?

<p>The reaction of atmospheric gases with metals (C)</p> Signup and view all the answers

Which gas is identified as the most common corrosive agent in oxidation corrosion?

<p>Oxygen (O₂) (D)</p> Signup and view all the answers

What does the Pilling-Bedworth Ratio indicate about the oxide layer formed during corrosion?

<p>Its stability and protective nature (A)</p> Signup and view all the answers

Which of the following metals is known to resist oxidation corrosion?

<p>Gold (D)</p> Signup and view all the answers

In oxidation corrosion, what is a common requirement for some metals to react with oxygen?

<p>High temperature (B)</p> Signup and view all the answers

What can happen to an unstable oxide film formed on a metal during oxidation?

<p>It decomposes back into the metal (A)</p> Signup and view all the answers

Which of the following describes the first step in the reaction mechanism of oxidation corrosion?

<p>De-electronation of the metal (B)</p> Signup and view all the answers

What is the main product formed when a metal reacts with oxygen in the context of oxidation corrosion?

<p>Metal oxide (D)</p> Signup and view all the answers

What characterizes corrosion in metals?

<p>Undesired destruction of materials through reactions (A)</p> Signup and view all the answers

Which of the following best explains why metals corrode?

<p>Metals naturally revert to lower energy states (A)</p> Signup and view all the answers

What is extractive metallurgy primarily concerned with?

<p>Extracting metals from ores which requires significant energy (C)</p> Signup and view all the answers

How does the energy level of metals relate to corrosion?

<p>Metals in higher energy states are more prone to revert to stable forms (D)</p> Signup and view all the answers

Which of the following compounds often forms as a result of iron corrosion?

<p>Hydrated iron oxides (B)</p> Signup and view all the answers

What can be inferred about the prevention of corrosion?

<p>Corrosion can only be slowed down, not completely prevented (A)</p> Signup and view all the answers

What happens during the corrosion life cycle of a steel product?

<p>Steel combines with external compounds resulting in degradation (C)</p> Signup and view all the answers

Why is the extraction of metals considered an energy-intensive process?

<p>Extracting metals requires energy to destabilize them from their ores (C)</p> Signup and view all the answers

Flashcards

Extractive Metallurgy

The process of extracting pure metals from their naturally occurring forms, like oxides, carbonates, or sulfides.

Corrosion

The tendency of metals to revert back to their naturally stable combined form, often in the form of oxides, sulfides, or carbonates.

Thermodynamics and Corrosion

Corrosion is a natural process driven by the tendency of metals to return to a lower energy state, which is more stable in their natural environment.

Energy and Corrosion

The greater the energy required to extract a metal from its ore, the more susceptible it is to corrosion.

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Rust Formation

The process where a metal reacts with oxygen and water in the environment, forming hydrated iron oxides, commonly known as rust.

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Preventing Corrosion

The process of corrosion cannot be completely stopped, only slowed down.

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Reversing Corrosion

The process of converting metals from their stable, low-energy states back into pure metals, requiring a significant amount of energy.

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Corrosion and Metallurgy

Corrosion is the reverse process of extractive metallurgy.

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Grain size and corrosion

Smaller grain size increases the surface area exposed to the environment, leading to faster corrosion.

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Stress and corrosion

Stress weakens the metal's structure, making it more susceptible to corrosion.

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Cathodic-anodic ratio and corrosion

A large cathodic area consumes electrons faster, accelerating the anodic reaction and overall corrosion rate.

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Protective films and corrosion

Protective films, like aluminum oxide, slow down corrosion.

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Solubility of corrosion product and corrosion

Soluble corrosion products allow the corrosion process to continue.

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Hydrogen overvoltage and corrosion

Metals with lower hydrogen overvoltage are more susceptible to corrosion due to faster hydrogen evolution.

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Temperature and corrosion

Elevated temperatures increase reaction rates, promoting corrosion.

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pH and corrosion

Acidic solutions are generally more corrosive due to H+ ions that promote oxidation.

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Noble Metals

Metals that resist corrosion due to their high electrode potential. They tend to stay in their pure state.

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Base Metals

Metals that readily corrode because of their low electrode potential. They easily lose electrons and form oxides.

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Galvanic Corrosion

When two different metals are in contact, the one with a lower electrode potential will corrode faster. The greater the difference in potential, the faster the corrosion.

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Galvanic Series

A table that lists metals in descending order of their tendency to corrode. It helps predict how metals will behave in different environments.

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Impurity Effect

Impurities in a metal can act as tiny cathodes, causing the metal to corrode faster at these points. The more impurities, the faster the corrosion.

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Erosion-Corrosion

A type of corrosion caused by a combination of erosion (wear) and corrosion (chemical attack). This is common in harsh environments like deserts.

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Humidity and Corrosion

The most damaging agent for defense equipment. High humidity accelerates corrosion and rust formation.

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Storage of Defense Equipment

The process of storing defense equipment in a controlled environment to minimize corrosion. Key factors include low humidity, limited rainfall, and a dust-free environment.

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How does dissolved oxygen affect corrosion?

Dissolved oxygen in water systems, like boilers and heat exchangers, accelerates corrosion by supporting cathodic reactions.

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What is passivation in corrosion?

Metals like aluminum and magnesium form protective oxide films in oxidizing environments, making them resistant to corrosion.

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What is critical humidity?

The relative humidity above which the rate of atmospheric corrosion of a metal significantly increases.

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How does humidity contribute to corrosion?

Gases like CO₂, O₂, and vapors provide water to the electrolyte, promoting electrochemical corrosion in humid atmospheres.

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How do industrial atmospheres impact corrosion?

Industrial atmospheres contain corrosive gases (O₂, H₂S, SO₂) and fumes (HCl, H₂SO₄) that accelerate corrosion by increasing the acidity and conductivity of the medium.

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How do suspended particles affect corrosion?

Chemically active particles (NaCl, (NH₄)₂SO₄) absorb moisture and act as electrolytes, enhancing corrosion; inactive particles (charcoal) can also increase corrosion by adsorbing sulfur gases and moisture.

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How do ions in the medium affect corrosion?

Anions like silicates form insoluble protective layers, reducing corrosion; chloride ions destroy protective films, increasing corrosion; cations like NH₄⁺ ions accelerate corrosion of metals like iron.

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Why are theories of corrosion important?

Theories of corrosion explain the mechanisms and processes involved in metal degradation in various environments.

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Chemical Corrosion

Corrosion caused by a direct chemical reaction between a metal and atmospheric gases like oxygen, halogens, or sulfur dioxide. A solid film of corrosion product usually forms on the metal's surface.

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Oxidation Corrosion

A type of chemical corrosion where a metal reacts directly with oxygen, forming an oxide layer.

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Pilling-Bedworth Rule

The rule that predicts whether a metal oxide layer is protective or non-protective based on its volume compared to the original metal. If the oxide layer is thicker than the original metal, it's likely protective.

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Stable Oxide Film

A metal oxide layer that prevents further corrosion by acting as a barrier, like aluminum oxide on aluminum.

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Unstable Oxide Film

A metal oxide layer that breaks down easily, allowing further corrosion to occur.

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Pilling-Bedworth Ratio (P.B. Ratio)

The ratio of the volume of metal oxide to the volume of the original metal that determines the protective nature of the oxide layer. A ratio greater than 1 indicates a protective layer.

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Chemical Affinity

A measure of a metal's resistance to forming a protective oxide layer during oxidation corrosion. Metals with higher resistance are less likely to corrode.

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Electrochemical Corrosion

A type of electrochemical corrosion that occurs in the presence of an electrolyte, like water or salt solution.

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What is the Pilling-Bedworth Ratio?

The Pilling-Bedworth Ratio (P.B.Ratio) compares the volume of the oxide formed on a metal's surface to the volume of the metal consumed during oxidation.

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What happens if the P.B.Ratio is greater than or equal to 1?

If the P.B.Ratio is greater than or equal to 1 (P.B.Ratio ≥ 1), the oxide layer formed is non-porous and protective. It creates a barrier, preventing further oxidation.

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What happens if the P.B.Ratio is less than 1?

If the P.B.Ratio is less than 1 (P.B.Ratio < 1), the oxide layer formed is porous and non-protective. It cannot effectively block further corrosion.

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Give examples of metals that form protective oxide layers based on the Pilling-Bedworth Rule.

Aluminum (Al), Iron (Fe), Nickel (Ni), and Chromium (Cr) form protective oxide layers because their P.B.Ratios are greater than or equal to 1.

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Why are noble metals like gold, silver, and platinum less susceptible to corrosion?

Metals like gold (Au), silver (Ag), and platinum (Pt) form unstable oxides with P.B.Ratios less than 1. This means their oxides do not provide good protection against corrosion.

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Study Notes

Corrosion Sciences

  • Corrosion is the undesired destruction of materials by chemical or electrochemical reactions.
  • Almost all metals exist in nature as minerals or ores.
  • Extracting metals from ores uses a considerable amount of energy.
  • Metals tend to revert to their stable, lower-energy combined state.
  • The higher the energy needed to extract a metal, the greater its susceptibility to corrosion.

The Effects of Corrosion

  • Corrosion has direct impacts like damage to car components, grills, and tools.
  • Indirect effects include plant shutdowns, product loss, and contamination.
  • Replacing coolant in car radiators every 12-18 months helps prevent cooling system corrosion.

Consequences of Corrosion

  • Plant shutdowns can disrupt industrial processes and affect consumers.
  • Loss of products due to leaks or contamination.
  • Reduced efficiency in equipment leading to loss of capacity.
  • Contamination of products if corrosion products are mixed in.
  • Nuclear hazards are a serious consequence of corrosion, as contamination can have wide implications.

Departments of Science & Humanities

  • Corrosion is influenced by factors such as metal purity, physical state, anodic/cathodic ratio, the nature of the corrosion product, solubility of corrosion products, hydrogen overvoltage.
  • Purity of metal: Impurities increase corrosion.
  • Physical state: Smaller grain size increases corrosion.
  • Ratio of cathodic/anodic regions: A small anode and large cathode raise corrosion rate.
  • Nature of corrosion product: Insoluble, non-porous films prevent further corrosion; soluble ones accelerate it.
  • Temperature: Higher temperature increases reaction and diffusion rates, accelerating corrosion.
  • pH: Acidic solutions are highly corrosive, while alkaline is less.
  • Oxidizing agents can increase or decrease corrosion rates.

Theories of Corrosion

  • Chemical Theory (Dry Corrosion): Direct reaction of atmospheric gases (O2, Cl2) with metal.
  • Electrochemical Theory (Wet Corrosion): Formation of electrochemical cells.

Types of Electrochemical Corrosion

  • Oxidation: Loss of electrons (anodes)
  • Reduction: Gain of electrons (cathodes)
  • Hydrogen Evolution Corrosion: H+ and metal
  • Oxygen Absorption Corrosion: O2, H2O, and metal

Liquid Metal Corrosion

  • Corrosion of metals by molten metals due to chemical and internal penetration.
  • Example: Mercury dissolving metals to create amalgams.

Factors Affecting Electrode Potential

  • Nature of the metal (galvanic series)
  • Concentration of metal ions in solution
  • Temperature

Measurement of Electrode Potential

  • Using a Standard Hydrogen Electrode (SHE) as a reference potential.
  • Measuring the potential difference between the metal electrode and SHE.
  • The galvanic series is an arrangement of redox couples in order of their standard electrode potentials, in a specific environment (e.g., seawater)

Pilling-Bedworth Ratio

  • A ratio of the volume of oxide formed to the volume of the metal consumed.
  • Pilling-Bedworth Ratio ≥ 1 indicates a protective oxide layer, and ≤1 indicates a non-protective oxide layer.

Protective Coatings

  • Barrier Coatings: prevent corrosion by creating a physical barrier between the metal and the environment.
    • Examples: Painting, electroplating, galvanization, and tinning.
  • Inorganic Coatings: Chemical conversion of the metal surface to create a protective layer.
    • Examples: Phosphate and chromate coatings.
  • Organic Coatings: Include paints, varnishes, and lacquers.
  • Examples: Coatings of paints, varnishes, and lacquers.

Differential Aeration Corrosion (Concentration Cell Corrosion)

  • Occurs when a metal surface is exposed to varying oxygen concentrations.
  • Localized corrosion occurs around areas with less oxygen.

Types of Corrosion

  • Pitting: Localized, deep pits or cavities.
  • Intergranular: Corrosion along grain boundaries.
  • Stress-Corrosion Cracking: Cracking due to tensile stress and a corrosive environment.
  • Waterline: Due to differential aeration at the waterline of storage tanks.

Cathodic Protection

  • Applying a more anodic (reactive) metal to the structure to protect it.
  • An impressed current can be used for greater protection area.
  • Includes Sacrificial Anodes (Mg, Zn, Al) and Impressed Current.

Anodic Protection

  • Corrosion protection for metals that exhibit active-passive behavior.
  • Includes controlling current to avoid complete passivation.

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