Unit5 - Subjective Questions

Definition of Corrosion:
Corrosion is defined as the gradual destruction or deterioration of metals or alloys by the chemical or electrochemical reaction with their surrounding environment. It is essentially reverse metallurgy.

Mechanism of Dry Corrosion (Oxidation Corrosion):

1. Attack: Oxygen attacks the metal surface directly in the absence of moisture.
2. Reaction: The metal forms metal ions () and electrons. Oxygen gains these electrons to form oxide ions ().
   • Overall:
3. Film Formation: An oxide layer forms on the surface. The nature of this layer determines further corrosion:
   • Stable/Non-porous: Stops further corrosion (e.g., Al, Sn, Pb).
   • Unstable: Decomposes back to metal and oxygen (e.g., Ag, Au).
   • Volatile: Evaporates, leaving surface fresh for attack (e.g., Mo).
   • Porous: Allows oxygen diffusion, continuing corrosion (e.g., Fe).

Pilling-Bedworth Rule:
This rule determines whether the oxide layer formed on a metal surface acts as a protective shield or allows further corrosion. It relates the volume of the metal oxide formed to the volume of the metal consumed.

The Ratio:

Significance:

1. If PBR < 1: The oxide layer is porous and non-protective. It cannot cover the metal surface completely, allowing oxygen to penetrate through cracks. (e.g., Alkali and Alkaline earth metals like Li, Na, K, Mg).
2. If PBR 1: The oxide layer is non-porous and protective. It forms a tight barrier that prevents oxygen diffusion, stopping further corrosion. (e.g., Al, Cr, Ni, Cu).

Conclusion: A specific volume ratio greater than unity indicates a protective coating.

Electrochemical (Wet) Corrosion Mechanism:
This occurs when a metal conducts electricity and is in contact with an electrolyte. It involves the formation of anodic and cathodic areas.

1. Evolution of Hydrogen (Acidic Environment):

• Anode: Metal dissolves.
• Cathode: Hydrogen ions from the acidic solution gain electrons.
  
• This type causes displacement of hydrogen gas from acidic solutions.

2. Absorption of Oxygen (Neutral/Alkaline Environment):

• This is the most common mechanism (e.g., rusting of iron).
• Anode:
• Cathode: Dissolved oxygen in water gains electrons in the presence of water.
  
• Product Formation: and combine to form , which further oxidizes to hydrated ferric oxide (Rust).
  

Galvanic Corrosion (Bimetallic Corrosion):
This occurs when two dissimilar metals are electrically connected and exposed to an electrolyte. The metal higher in the electrochemical series (more anodic) corrodes, while the more noble metal (cathodic) is protected.

Mechanism:

1. Anode: The more active metal acts as the anode and undergoes oxidation (corrosion).
2. Cathode: The less active metal acts as the cathode. No corrosion occurs here; electrons are consumed.
3. Electron Flow: Electrons flow from the anodic metal to the cathodic metal through the connection.

Example (Zn-Cu Couple):
If Zinc and Copper are connected in an electrolyte:

• Zinc (): Acts as the anode (More negative potential).
• Copper (): Acts as the cathode.
• Result: Zinc corrodes effectively protecting the copper.

Concentration Cell Corrosion:
This type of corrosion occurs when two parts of the same metal are exposed to an electrolyte with varying concentrations (either of the salt or oxygen). It does not require two different metals.

Mechanism:

1. The part of the metal in contact with the lower concentration of metal ions acts as the Anode (Corrosion occurs).
2. The part in contact with the higher concentration acts as the Cathode (Protected).

Difference from Galvanic Corrosion:

• Materials: Galvanic requires two dissimilar metals; Concentration cell involves only one metal.
• Driving Force: Galvanic is driven by difference in electrode potential of two metals; Concentration cell is driven by difference in concentration of the electrolyte surrounding the metal.

Differential Aeration Corrosion:
This is a specific type of concentration cell corrosion driven by differences in oxygen concentration.

Principle:

• Anode: The part of the metal which is less aerated (less oxygen) becomes anodic and suffers corrosion.
• Cathode: The part of the metal which is more aerated (more oxygen) becomes cathodic and remains protected.
• Current Flow: A difference of potential is created, causing current to flow from the aerated part to the non-aerated part through the metal.

Example:

• Iron rod partially immersed in NaCl: The part below the water line (less oxygen) acts as the anode and corrodes. The part above (more oxygen) acts as the cathode.
• Crevice Corrosion: Accumulation of dirt creates an area of low oxygen underneath, leading to corrosion under the dirt.

Water-line Corrosion:
It is a specific instance of differential aeration corrosion commonly observed in steel water tanks or ships floating in water.

Mechanism:

1. Anode (Corrosion area): The metal area just below the water level is poorly oxygenated compared to the area just above it. This area behaves as the anode and corrodes.
   
2. Cathode (Protected area): The metal area just above the water line (the meniscus) is highly oxygenated (freely exposed to air). This area acts as the cathode.
   

Result: A distinct line of corrosion is formed just below the level of the water meniscus.

Pitting Corrosion:
Pitting is a localized form of corrosion that leads to the creation of small holes or pits in the metal. It occurs when there is a breakdown of the protective surface film (oxide layer) at specific points due to impurities or scratches.

Mechanism:

1. Small Anode/Large Cathode: The small area where the protective layer breaks becomes the anode. The remaining large area with the intact layer acts as the cathode.
2. Autocatalytic Process: The corrosion current is concentrated on the tiny anodic spot. Inside the pit, oxygen is depleted (becoming more anodic), while the surface remains aerated (cathodic).

Why it is dangerous:

• Detection: It is difficult to detect as the pits are often covered by corrosion products.
• Structural Integrity: It causes rapid penetration of the metal cross-section, leading to sudden structural failure without significant weight loss of the overall structure.

Intergranular Corrosion:
This is a localized attack along the grain boundaries of a metal or alloy, while the bulk of the grains remain unaffected.

Mechanism in Stainless Steel:

1. Stainless steel contains Chromium () which provides corrosion resistance via a passive oxide layer.
2. Sensitization: If the steel is heated (e.g., during welding) to temperatures between and , Chromium reacts with Carbon at the grain boundaries to form Chromium Carbide ().
3. Chromium Depletion: The precipitation of chromium carbide depletes the chromium content in the areas adjacent to the grain boundaries (falling below ).
4. Galvanic Cell: The chromium-depleted zone acts as an anode (active), while the grain body acts as a cathode. This leads to rapid corrosion along the boundaries, causing the metal to disintegrate.

Soil Corrosion:
Soil corrosion refers to the deterioration of underground metal structures (pipelines, cables, pilings) due to chemical or electrochemical reactions with the soil environment.

Factors Influencing Soil Corrosion:

1. Acidity (pH): Highly acidic soils cause rapid corrosion of ferrous metals.
2. Moisture Content: Higher moisture increases electrical conductivity of the soil, facilitating electrochemical reactions.
3. Aeration: Soils with different aeration levels (e.g., clay vs. sand) can set up differential aeration cells on long pipelines.
4. Electrical Conductivity: High concentration of soluble salts increases conductivity, enhancing corrosion rates.
5. Micro-organisms: Presence of Sulphate Reducing Bacteria (SRB) in anaerobic conditions can drastically accelerate corrosion (Microbiological Corrosion).

Nature of the Metal - Factors:

1. Position in Galvanic Series: Metals higher in the galvanic series (more anodic, lower reduction potential) corrode faster. The greater the potential difference between two connected metals, the faster the corrosion.
2. Relative Areas of Anode and Cathode: A small anode connected to a large cathode results in severe corrosion (high current density at the anode). Conversely, a large anode and small cathode leads to negligible corrosion.
3. Purity of Metal: Impurities act as tiny cathodes, setting up micro-galvanic cells. Pure metals (e.g., Zn) corrode much slower than impure metals.
4. Physical State: Strained or stressed areas of a metal possess higher energy and tend to become anodic (corrode) relative to unstrained parts.
5. Nature of Surface Film: If the corrosion product (oxide) is stable and non-porous (e.g., Al, Cr), it protects the metal. If porous (e.g., Fe), corrosion continues.

Nature of the Environment - Factors:

1. Temperature: The rate of corrosion generally increases with temperature because chemical reaction rates increase and the viscosity of the medium decreases (enhancing diffusion).
2. Humidity: Moisture acts as the electrolyte necessary for electrochemical corrosion. Critical humidity is the level above which corrosion rate increases sharply.
3. pH of the Medium: Acidic media () generally increase corrosion, especially by hydrogen evolution. Alkaline media may induce passivity (except for amphoteric metals like Al).
4. Presence of Impurities: Gases like , , and in industrial areas dissolve in moisture to form acids, increasing conductivity and acidity.
5. Presence of Suspended Particles: Particles like charcoal can act as cathodes; dust can hold moisture or cause differential aeration.

Sacrificial Anodic Protection:
This is a method of Cathodic Protection where the metal structure to be protected is converted into a cathode.

Mechanism:

1. The structure (e.g., a steel ship hull or pipeline) is connected electrically to a more active metal (anode).
2. The active metal has a lower electrode potential (more negative) than the structure.
3. Common sacrificial anodes: Zinc, Magnesium, or Aluminum.

Process:

• The active metal acts as the anode and corrodes (sacrifices itself).
• The steel structure becomes the cathode and electrons flow to it, preventing oxidation of the iron.

Applications: Used for ship hulls, underground pipelines, and water tanks.

Impressed Current Cathodic Protection:
In this method, an external DC power source is used to force electrons into the metal structure to be protected, making it cathodic.

Mechanism:

1. Setup: The structure is connected to the negative terminal of a DC source.
2. Anode: An inert material (like graphite or platinum) or scrap iron is connected to the positive terminal and buried/immersed nearby.
3. Current Flow: Current flows from the external source through the inert anode, through the electrolyte (soil/water), to the structure.
4. Protection: The impressed current nullifies the natural corrosion current, keeping the metal in a cathodic (non-corroding) state.

Advantages: Can protect large structures and long pipelines where sacrificial anodes would be consumed too quickly.

Electroplating:
Electroplating is the process of depositing a thin, uniform layer of a superior metal (coating metal) over a base metal by passing an electric current through an electrolytic solution containing soluble salt of the coating metal.

Process Setup:

1. Cathode: The article to be plated (cleaned base metal).
2. Anode: A rod/plate of the coating metal (or an inert material).
3. Electrolyte: Solution containing ions of the coating metal.

Mechanism (Example: Copper plating on Steel):

• Electrolyte: solution.
• At Anode (Cu): Copper dissolves.
• At Cathode (Steel): Copper ions reduce and deposit.

Applications:

• Decoration (Gold/Silver plating).
• Corrosion protection (Chromium/Nickel plating).

Ceramic Coatings (Inorganic Coatings):
Ceramic coatings involve applying hard, inorganic, non-metallic materials (like oxides, nitrides, carbides, or enamels) onto metal surfaces.

Key Characteristics:

1. Chemical Inertness: Ceramics are chemically stable and do not react with acids, alkalis, or oxygen, providing an excellent barrier.
2. High Temperature Resistance: They protect metals from oxidation at high temperatures where organic coatings (paints) would fail.
3. Hardness: They provide wear and abrasion resistance.

Vitreous Enamel (Porcelain Enamel):

• A glass-like ceramic coating fused to metal.
• Used in household appliances (bathtubs, sinks), chemical reactors, and exhaust pipes.
• Process: A suspension of glass powder (frit) is applied and fired at high heat, melting to form a smooth, impervious layer.

Comparison:

Anodic Coatings:

1. Definition: The coating metal has a lower electrode potential (is more anodic) than the base metal.
2. Example: Zinc on Iron (Galvanizing).
3. Protection Mechanism: It protects mechanically and electrochemically (Sacrificial). Even if the coating is scratched, the coating corrodes instead of the base metal.

Cathodic Coatings:

1. Definition: The coating metal has a higher electrode potential (is more noble/cathodic) than the base metal.
2. Example: Tin or Nickel on Iron.
3. Protection Mechanism: It protects only mechanically (Barrier). If the coating is scratched, the base metal (anode) corrodes rapidly due to the galvanic cell formed with the large cathodic coating.

Control of Corrosion by Design:
Proper structural design can minimize corrosion risks significantly:

1. Avoid Dissimilar Metals: Avoid direct contact between metals with large potential differences. If necessary, use insulating washers.
2. Anode/Cathode Area: If dissimilar metals are used, ensure the anode area is large and the cathode area is small to reduce current density on the anode.
3. Prevent Crevices: Design shapes to be simple and smooth. Avoid sharp corners and crevices where dirt/moisture can accumulate (prevents differential aeration).
4. Drainage: Tanks and containers should be designed for complete drainage to prevent liquid stagnation.
5. Easy Maintenance: Equipment should be accessible for cleaning and painting.
6. Homogeneity: Use the same alloy throughout the structure to prevent local galvanic cells.