Galvanic Series and Metal Corrosion
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Questions and Answers

Which factor is negatively associated with the rate of corrosion?

  • Higher impurity levels
  • Increased temperature
  • Greater specific volume ratio (correct)
  • Smaller anodic area

The purity of a metal has no effect on its tendency to corrode.

False (B)

What is the relationship between humidity and the rate of corrosion?

Higher humidity increases the rate of corrosion.

Corrosive products that are ______ in nature promote continuous corrosion as they expose the underlying metal surface.

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

Match the following factors with their effect on corrosion:

<p>Temperature = Increased corrosion rate Acidic media = More corrosive than neutral Impurities in atmosphere = Increases corrosion risk Physical state of metal = Smaller grain size leads to greater corrosion</p> Signup and view all the answers

What happens to a metal with smaller grain size in terms of corrosion?

<p>It shows greater corrosion. (D)</p> Signup and view all the answers

A pure metal is more likely to corrode than an alloy.

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

How does the presence of suspended particles in the atmosphere influence corrosion?

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

What is the preferred characteristic of the metal selected to minimize corrosion?

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

Using metals that are far apart in the electrochemical series can help reduce corrosion.

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

What should be avoided in the design of structures to minimize impurities related to corrosion?

<p>Sharp corners and recesses</p> Signup and view all the answers

An anodic portion will suffer __________ due to the process of corrosion.

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

Which method is recommended to connect dissimilar metals to prevent corrosion?

<p>Insulating fitting (C)</p> Signup and view all the answers

Match the following design elements to their effectiveness in preventing corrosion:

<p>Poor design = Bolt joints Better design = Smooth bends Best design = Weld joints</p> Signup and view all the answers

Preventing the creation of stagnant pools or damp areas is crucial for corrosion control.

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

What is one method to improve the connection of dissimilar metals?

<p>Use an insulating fitting</p> Signup and view all the answers

Which method of cathodic protection uses a more anodic metal to protect the metallic structure?

<p>Sacrificial anodic protection (A)</p> Signup and view all the answers

Impressed current cathodic protection involves using an external source to neutralize the corrosion current.

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

What is the purpose of a sacrificial anode in cathodic protection?

<p>To corrode instead of the protected metallic structure.</p> Signup and view all the answers

In the galvanizing process, a layer of _____ is applied to iron or steel.

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

Match the following coating methods with their descriptions:

<p>Hot dipping = Coating with a low melting metal Galvanizing = Coating with zinc on iron or steel Tinning = Coating of tin on iron Surface coatings = Applying a protective layer onto the surface</p> Signup and view all the answers

Which of the following constituents of paint is responsible for providing color?

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

The vehicle in paint refers to the solid component that gives color to the paint.

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

Anodic coatings involve coating a base metal with a more _____ metal.

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

What is the primary purpose of metal cladding?

<p>Protect the base metal from corrosion. (C)</p> Signup and view all the answers

Electroplating involves coating a noble metal over a more reactive metal.

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

What is another name for electroless plating?

<p>Immersion plating or displacement plating</p> Signup and view all the answers

Organic coatings, such as ______ and lacquers, protect metal surfaces from corrosion.

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

Which of the following methods involves using a more active metal to protect iron from corrosion?

<p>Sacrificial Anode Protection (D)</p> Signup and view all the answers

Match the method of corrosion protection with its description:

<p>Sacrificial Anode Protection = More active metal prevents corrosion of iron Impressed Current Cathodic Protection = External current neutralizes corrosion current Anodic Coatings = Coated with more active metals Cathodic Coatings = Coated with more noble metals</p> Signup and view all the answers

Organic coatings are exclusively used for decorative purposes and have no protective function.

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

In anodic coatings, metal is coated with a more _____ metal.

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

Flashcards

Grain size and corrosion

Smaller grain sizes in metals lead to higher solubility, resulting in more corrosion.

Purity and corrosion

Pure metals resist corrosion because they lack impurities that create electrochemical cells.

Surface film and corrosion

Metal oxides, like rust, on the surface can either protect or accelerate corrosion depending on their thickness.

Over voltage and corrosion

Higher over voltage means slower corrosion. Think of it like a barrier.

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Anodic area and corrosion

Smaller anodic areas mean faster and more localized corrosion.

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Galvanic Series and corrosion

Metals higher in the galvanic series are more likely to corrode when in contact with metals lower in the series.

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Solubility of corrosion products

Corrosion products that dissolve in the environment can accelerate corrosion.

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Volatility of corrosion products

Corrosion products that evaporate quickly expose the underlying metal, leading to rapid corrosion.

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Nature of Ions

The type of ions present in the environment can influence corrosion rate. For instance, chloride ions are highly aggressive toward many metals.

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Conductance of the Corroding Medium

The conductivity of the environment surrounding the metal plays a crucial role in corrosion. High conductivity allows for easier flow of electrons, accelerating corrosion.

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Oxygen Concentration Cell

Oxygen concentration variations create a difference in electrical potential, making the area with less oxygen anodic (prone to corrosion) and the area with more oxygen cathodic.

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Flow Velocity of Process Stream

The velocity of the flow surrounding the metal can influence corrosion. Higher flow rates tend to reduce corrosion by preventing build-up of corrosive substances.

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Polarization of Electrodes

Polarization refers to the change in electrical potential at the metal surface during corrosion. It impacts the rate and type of corrosion.

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Material Selection

Selecting a metal resistant to corrosion is crucial: Pure metals resist corrosion better than alloys, while noble metals like gold and platinum are highly resistant.

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Dissimilar Metal Contact

Dissimilar metals in contact can lead to corrosion, especially in corrosive environments. The less noble metal (more anodic) is more prone to corrosion.

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Proper Designing

Proper design can minimize corrosion by preventing the accumulation of corrosive substances and ensuring good drainage. Rounded corners and smooth surfaces reduce the risk of corrosion.

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What is cathodic protection?

A method of protecting metal surfaces from corrosion by making them behave like a cathode, preventing the flow of electrons that cause corrosion.

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What is sacrificial anodic protection?

A type of cathodic protection where a more active metal, like magnesium, is connected to the metal being protected. This causes the active metal to corrode instead, protecting the target.

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What is impressed current cathodic protection?

A method of cathodic protection where an external power source is used to send an electric current to the metal being protected, neutralizing the corrosion-causing current.

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What are surface coatings?

This is a general term for applying coatings to metal surfaces to protect them from corrosion.

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What are anodic coatings?

This method involves coating the base metal with a more active metal, like zinc or aluminum, to create a protective barrier.

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What are cathodic coatings?

These coatings involve applying a more noble metal, like gold or silver, to the base metal, creating a protective barrier.

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What is paint?

A mixture of pigments, driers, and fillers dispersed in a liquid called a vehicle or medium. When applied to a metal surface, it forms a protective film.

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What are common methods of applying metal coatings?

The most common methods include: hot dipping, which involves immersing the metal in a molten metal bath, and galvanizing, which specifically involves coating iron or steel with zinc.

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Metal Cladding

A method of protecting metals from corrosion by coating them with a thin, uniform layer of a more corrosion-resistant metal.

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Electroplating

A process where a more reactive metal is coated with a more noble metal to prevent corrosion. Common examples include tin plating and nickel plating.

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Electroless Plating

A method of coating a metal surface with a noble metal by immersing the object in a bath of a noble metal salt. The noble metal replaces the base metal, creating a protective layer.

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Organic Surface Coatings

Organic coatings, such as paints, varnishes, and enamels, create an inert barrier on the surface of metals to protect against corrosion.

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Sacrificial Anode Protection

A method of preventing corrosion by connecting a more active metal (like magnesium or zinc) to the iron pipe. The active metal corrodes instead of the iron.

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The Impressed Current Cathodic Protection

An impressed current from an external source is applied to the iron object to neutralize the corrosion current.

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Anodic Coatings

A method of protecting metals by coating them with a more active metal like zinc, aluminum, or cadmium. The active metal corrodes first.

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Cathodic Coatings

A method of protecting metals by coating them with a more noble metal, like tin or copper. The noble metal acts as a barrier.

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Study Notes

Galvanic Series

  • A table showing the order of reactivity of metals (or alloys) in a corrosive environment, from most active (anodic) to least active (cathodic).
  • More active metals corrode preferentially when in contact with less active metals.
  • Examples include magnesium (most active) and platinum (least active).
  • The galvanic series helps predict which metal will corrode more quickly in a given environment.

Nature of the Metal

  • Physical state: Corrosion rate depends on the size of the metal's grains; smaller grains lead to higher corrosion rates.
  • Purity: Impurities in a metal create localized electrochemical cells leading to more rapid corrosion. Pure metals are less prone to corrode.
  • Overvoltage: The amount of voltage required to initiate a reaction. Lower overvoltage increases corrosion rate.
  • Surface film: A thin oxide layer forms on most metals. A higher volume ratio of oxide-to-metal indicates slower oxidation corrosion rates.
  • Relative areas of anodic and cathodic parts: If the anodic area is very small compared to the cathodic area, corrosion is accelerated.
  • Position in the galvanic series: The greater the difference in positions within the series, the faster the corrosion of the anodic substance.
  • Passive character of metal: Certain metals display a passive character, where a thin, protective oxide layer forms, thus slowing or stopping corrosion.
  • Solubility of corrosion products: If corrosion products are soluble, corrosion will be faster. If they are insoluble, a barrier is created reducing further corrosion.
  • Volatility of corrosion products: If corrosion products are volatile, corrosion will be more rapid.

Nature of the Corrosive Environment

  • Temperature: Higher temperatures increase corrosion rates due to increased ion diffusion.
  • Humidity: Higher humidity increases corrosion rates by providing more moisture, acting as solvent for corrosive gases.
  • Impurities in the atmosphere: Corrosive gases like CO2, H2S, SO2, and acid fumes increase the acidity and conductivity of the environment, accelerating corrosion.
  • Suspended particles: Chemically active particles like NaCl increase the corrosive nature. Chemically inactive particles (e.g., charcoal) increase corrosion by trapping moisture and gases.
  • pH: Acidic environments are more corrosive than neutral or alkaline environments.
  • Concentration of oxygen: Differential oxygen concentration leads to concentration cells, accelerating corrosion. Higher oxygen concentration leads to faster corrosion
  • Conductance of medium: Higher ion content increases corrosion.
  • Flow velocity: High flow velocities may reduce corrosion, but very high flow or turbulent flow can accelerate it.

Protection from Corrosion (Preventive Measures)

  • Material Selection: Choosing corrosion-resistant materials or using materials with similar electrochemical properties.
  • Proper Designing: Avoid sharp corners and recesses, use smooth bends to prevent accumulation of impurities, avoid dissimilar metals, if inescapable, space them to minimize contact in corrosive environment, create larger anodic surface against cathodic.
  • Corrosion Control: Employing methods to protect metals from corrosion. Examples include material selection, coatings (hot dipping, electroplating, etc.) and cathodic protection.
  • Cathodic Protection: A method to prevent corrosion by forcing the metal to be protected to behave like a cathode, (by connecting to a more anodic metal or by using an impressed current).

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Description

This quiz explores the Galvanic Series and its implications on metal reactivity in corrosive environments. It covers concepts such as the physical state of metals, purity, overvoltage, and surface films that influence corrosion rates. Test your understanding of how different metals behave in relation to one another in terms of corrosion.

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