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What is the primary reason pure metals have a natural tendency to revert to their combined state?
Which factor does NOT affect the rate of dry corrosion?
What type of corrosion occurs when metals are exposed to dry oxygen?
At high temperatures, which of the following metals is generally NOT oxidized?
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Which atmospheric gas is involved in the formation of oxides during oxidation corrosion?
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What is the outcome of the electron transfer process when metals interact with oxygen?
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Which of the following is NOT classified as a type of dry corrosion?
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What is the result of the direct chemical action of atmospheric gases on metals?
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What is the primary function of anodic protection in metals?
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Which type of environment is anodic protection especially useful for?
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What device is used to maintain a constant potential for the metal being protected?
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Anodic protection is unsuitable for which of the following materials?
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What is the effect of applying anodic current to the metal surface?
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Which of the following is NOT an advantage of anodic protection?
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What effect does the external power supply have on the protective oxide layer?
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Which applications commonly utilize anodic protection?
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What happens to the base metal when the cathodic coating is ruptured?
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Which of the following metals is commonly coated using the hot dipping method?
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Which of the following methods is NOT a method of applying metallic coatings?
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What is the primary purpose of using flux in the hot dipping method?
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What is the temperature range used during galvanizing?
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Why are galvanized iron vessels unsuitable for storing food?
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Which statement about anodic and cathodic coatings is true?
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What is a common application of galvanized articles?
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What is a disadvantage of cathodic protection?
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Which of the following best describes anodic protection?
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What is the first step in the surface preparation process before coating?
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What is a key characteristic of cathodic protection?
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Which scenario would most likely result in a rapid increase in corrosion when employing anodic protection?
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What is true regarding the operational costs of anodic protection?
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Which of the following steps is NOT involved in surface preparation?
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In which type of environment would cathodic protection be most effective?
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What happens to a metal when the impurity present is less active?
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Which condition leads to increased corrosion of a metal.
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How does temperature affect the rate of atmospheric corrosion?
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What effect does a porous oxide film have on corrosion?
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What is the significance of overvoltage in corrosion?
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Among the listed metals, which is likely to develop resistance due to hydrogen adsorption?
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Why does steel corrode more easily than cast iron?
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In an acidic medium, what occurs when metals higher in the EMF series corrode?
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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
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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.
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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.
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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.
Factors Related to the Nature of the Environment
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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.
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Humidity:
- Humidity promotes electrochemical corrosion, which requires moisture.
- Dry environments reduce corrosion.
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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.
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Advantages:
- Low operating cost.
- Effective in highly corrosive media.
- Reliable protection for complex structures.
- Corrosion rate can be monitored through protection current.
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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.
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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.
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Advantages:
- Protects long structures for extended periods.
- Low cost.
- Provides steady, consistent protection.
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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.
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Surface Preparation:
- Remove old coatings.
- Remove oils, chlorides, acids, and other contaminants.
- Remove oxide layers and corrosion products.
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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.
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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.
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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.
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Applications:
- Galvanized iron (G.I.) sheets, pipes, wires, buckets, tubes, screws, nails.
- Not suitable for storing food due to potential formation of poisonous products.
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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.
