Corrosion and Its Types

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What is the primary reason pure metals have a natural tendency to revert to their combined state?

They are in a stable energy state.

They exist in a lower energy state.

They are in a higher energy state. (correct)

They have no tendency to combine.

Which factor does NOT affect the rate of dry corrosion?

Chemical affinity between the metal and gases.

Nature of the corrosion product.

Temperature of the environment.

Presence of moisture in the air. (correct)

What type of corrosion occurs when metals are exposed to dry oxygen?

Chemical corrosion.

Liquid metal corrosion.

Oxidation corrosion. (correct)

Galvanic corrosion.

At high temperatures, which of the following metals is generally NOT oxidized?

Silver. Signup and view all the answers

Which atmospheric gas is involved in the formation of oxides during oxidation corrosion?

Oxygen. Signup and view all the answers

What is the outcome of the electron transfer process when metals interact with oxygen?

Formation of metal ions. Signup and view all the answers

Which of the following is NOT classified as a type of dry corrosion?

Electrolytic corrosion. Signup and view all the answers

What is the result of the direct chemical action of atmospheric gases on metals?

Formation of stable compounds. Signup and view all the answers

What is the primary function of anodic protection in metals?

To form a protective oxide layer on the surface Signup and view all the answers

Which type of environment is anodic protection especially useful for?

Highly corrosive media Signup and view all the answers

What device is used to maintain a constant potential for the metal being protected?

Potentiostat Signup and view all the answers

Anodic protection is unsuitable for which of the following materials?

Carbon steel in extremely acidic conditions Signup and view all the answers

What is the effect of applying anodic current to the metal surface?

Causes dissolution of metal and forms a protective layer Signup and view all the answers

Which of the following is NOT an advantage of anodic protection?

Prevents all forms of corrosion Signup and view all the answers

What effect does the external power supply have on the protective oxide layer?

It can increase or decrease the thickness of the layer Signup and view all the answers

Which applications commonly utilize anodic protection?

Storage tanks in highly acidic environments Signup and view all the answers

What happens to the base metal when the cathodic coating is ruptured?

The base metal becomes anodic and experiences pitting. Signup and view all the answers

Which of the following metals is commonly coated using the hot dipping method?

Zinc Signup and view all the answers

Which of the following methods is NOT a method of applying metallic coatings?

Welding Signup and view all the answers

What is the primary purpose of using flux in the hot dipping method?

To prevent oxidation of the melting metal Signup and view all the answers

What is the temperature range used during galvanizing?

425-450°C Signup and view all the answers

Why are galvanized iron vessels unsuitable for storing food?

They react with food, causing poisonous products. Signup and view all the answers

Which statement about anodic and cathodic coatings is true?

Cathodic coating protects the base metal until it is damaged. Signup and view all the answers

What is a common application of galvanized articles?

Pipes and nails Signup and view all the answers

What is a disadvantage of cathodic protection?

High installation cost and high starting current Signup and view all the answers

Which of the following best describes anodic protection?

It increases the passivity of the base metal. Signup and view all the answers

What is the first step in the surface preparation process before coating?

Remove old coatings Signup and view all the answers

What is a key characteristic of cathodic protection?

It can be achieved by methods like impressed current. Signup and view all the answers

Which scenario would most likely result in a rapid increase in corrosion when employing anodic protection?

The system goes out of control. Signup and view all the answers

What is true regarding the operational costs of anodic protection?

Operational costs are low despite high initial costs. Signup and view all the answers

Which of the following steps is NOT involved in surface preparation?

Apply protective coating Signup and view all the answers

In which type of environment would cathodic protection be most effective?

Moderately corrosive environments Signup and view all the answers

What happens to a metal when the impurity present is less active?

Corrosion occurs but at a slower rate. Signup and view all the answers

Which condition leads to increased corrosion of a metal.

The metal is under stress. Signup and view all the answers

How does temperature affect the rate of atmospheric corrosion?

Corrosion rate increases at higher temperatures. Signup and view all the answers

What effect does a porous oxide film have on corrosion?

It can lead to complete destruction of the metal. Signup and view all the answers

What is the significance of overvoltage in corrosion?

It restores the original EMF position of the metal. Signup and view all the answers

Among the listed metals, which is likely to develop resistance due to hydrogen adsorption?

Zinc Signup and view all the answers

Why does steel corrode more easily than cast iron?

Steel's grain size is smaller, making it more reactive. Signup and view all the answers

In an acidic medium, what occurs when metals higher in the EMF series corrode?

They liberate hydrogen gas, which decreases corrosion rate. Signup and view all the answers

Study Notes

Corrosion

Pure metals are less stable than combined metals because they are in a higher energy state.
Metals tend to revert to their combined state through corrosion, interacting with the environment chemically or electrochemically to form a stable compound.
Types of corrosion are determined by the environment in which the metal is placed.

Dry (Chemical) Corrosion

Occurs in dry environments due to direct chemical reactions between metals and atmospheric gases (O2, CO2, H2S, halogens).
Forms compounds like oxides, halides, sulfates, and sulfides.
Products from dry corrosion can be insoluble, soluble, or liquid.
The rate of dry corrosion depends on:
- Chemical affinity between the metal and atmospheric gases.
- The nature of the corrosion product, which may form a protective layer.
Dry corrosion is classified into:
- Oxidation corrosion (caused by oxygen).
- Corrosion by other gases.
- Liquid metal corrosion.

Oxidation Corrosion

Occurs when dry oxygen reacts with a metal surface at low or high temperatures.
Oxidation rates vary with temperature:
- Most metals oxidize slowly at low temperatures.
- At high temperatures, most metals (except silver, gold, platinum) oxidize.
Mechanism of corrosion by oxygen:
- Oxygen is absorbed onto the metal surface even at room temperature due to Vander Waal's forces.
- Absorbed oxygen chemically combines with the metal through electron transfer.
- Metal atoms lose electrons to oxygen atoms, forming metal ions and oxide ions.
- Metal oxide forms from the union of ions, creating a thin layer or film on the metal's surface.

Factors Influencing Dry Corrosion

Nature of the Metal:
- Purity: Impure metals corrode faster than pure metals. Greater impurities lead to faster corrosion with small pits.
- Physical State: Smaller grain sizes in metals and alloys increase the rate of corrosion (e.g., steel corrodes faster than cast iron due to smaller grains).
- Areas under stress are more anodic.
Nature of the Oxide Film:
- Protective films: Non-porous, strongly adhered oxide films prevent both dry and electrochemical corrosion.
- Non-protective films: Porous, loosely adhered films (like those formed on steel) allow corrosion to continue until complete destruction.
Overvoltage:
- Metals higher on the EMF series than hydrogen release hydrogen gas when corroding in acidic mediums.
- Hydrogen bubbles adhere to the metal surface, hindering further corrosion.
- This resistance requires extra voltage (overvoltage) to overcome.
- Metals exhibiting overvoltage include zinc, lead, chromium, and nickel.

Temperature:
- Higher temperatures increase the rate of atmospheric and dry corrosion because both attacking gas and metal become more active.
- Electrochemical corrosion at the anode also increases with higher temperatures due to the Nernst Equation.
Humidity:
- Humidity promotes electrochemical corrosion, which requires moisture.
- Dry environments reduce corrosion.
Presence of Contaminants:
- Impurities, such as salts, acids, and gases, accelerate corrosion by increasing the conductivity of the environment and providing sites for reactions.

Anodic Protection

Principle: Increases the passivity of a metal by applying a current in the anodic direction.
Mechanism: Creates a thin, protective oxide layer on the metal's surface through controlled dissolution.
Advantages:
- Low operating cost.
- Effective in highly corrosive media.
- Reliable protection for complex structures.
- Corrosion rate can be monitored through protection current.
Limitations:
- Applicable only to metals exhibiting active-passive behavior.
- High installation cost.
- Requires high starting current.
- Out of control, corrosion rates increase rapidly.

Cathodic Protection

Principle: Forces the metal to behave like a cathode by reversing the flow of current between dissimilar electrodes.
Methods:
- Sacrificial method: Uses a more reactive metal as a "sacrificial anode" to provide protection.
- Impressed current method: Applies a direct electric current to the metal to be protected.
Advantages:
- Protects long structures for extended periods.
- Low cost.
- Provides steady, consistent protection.
Limitations:
- May not be effective for long-term protection in extremely corrosive environments.

Metallic Coatings

Purpose: Create a barrier between a metal and the environment to prevent corrosion.
Surface Preparation:
- Remove old coatings.
- Remove oils, chlorides, acids, and other contaminants.
- Remove oxide layers and corrosion products.
Types of Metallic Coatings:
- Anodic Coatings: The base metal is less noble than the coating metal. The coated metal is protected as long as the coating remains intact.
- Cathodic Coatings: The base metal is more noble than the coating metal. A ruptured coating leads to rapid corrosion of the base metal.
Methods of Applying Metallic Coatings:
- Hot dipping: Immersing the base metal in a bath of molten coating metal.
- Electroplating: Depositing a metal layer onto the base metal through electrolysis.
- Cementation: Diffusing a coating metal into the base metal through a chemical reaction.
- Cladding: Bonding a sheet of coating metal to the base metal.

Hot Dipping

Process: Dipping the base metal (steel, copper, brass) into a molten bath of low-melting point coating metals (zinc, tin, lead) covered by a molten flux (usually ZnCl2.2NH4Cl).
Purpose: Protects the base metal from corrosion.
Flux: Prevents oxidation of the molten coating metal.
Common Hot Dipping Methods:
- Galvanizing: Coating iron or steel with zinc.
- Tinning (Tin Plating): Coating a metal with tin.

Galvanizing

Process: Cleaning the steel article with dilute sulfuric acid, washing, drying, and immersing in a molten zinc bath (425-450°C) covered with a flux (NH4Cl).
Result: Forms a protective zinc coating that prevents corrosion.
Applications:
- Galvanized iron (G.I.) sheets, pipes, wires, buckets, tubes, screws, nails.
- Not suitable for storing food due to potential formation of poisonous products.

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

This quiz explores the concept of corrosion, focusing on pure and combined metals' stability. It covers the types of corrosion, including dry (chemical) corrosion and oxidation corrosion, as well as the environmental factors influencing these processes. Test your understanding of how metals interact with their surroundings through corrosion.