Unit 5: Corrosion - Subjective Questions

Definition: Corrosion is the slow and gradual destruction or deterioration of a metal by chemical or electrochemical reaction with its surrounding environment.

It is essentially the reverse of extraction metallurgy, where metals tend to revert to their more stable combined (oxide/salt) states.

Scope and Significance:

• Economic loss: A large fraction of annual metal production is lost due to corrosion, leading to replacement and maintenance costs.
• Safety hazards: Corroded pipelines, bridges, and reactors can fail suddenly, causing accidents.
• Wastage of resources: Both metal and the energy used to extract it are wasted.
• Contamination: Corrosion products can contaminate food, water, and chemicals.
• Loss of efficiency: Deposits from corrosion reduce heat transfer and clog pipes.

Example: Rusting of iron:

Direct Chemical Corrosion (Dry Corrosion): This type occurs through the direct chemical attack of the environment (mainly gases and vapours) on the metal surface in the absence of moisture or an electrolyte.

Three main types:

1. Oxidation Corrosion (Corrosion by Oxygen):

   • Metals react directly with oxygen to form oxides.
   • Nature of the oxide film determines protection (Pilling-Bedworth rule).

2. Corrosion by Other Gases:

   • Gases like , , , attack metals.
   • Example: dry reacting with silver forms protective ; with tin it forms volatile (non-protective).

3. Liquid Metal Corrosion:

   • Flowing liquid metal attacks solid metal, causing weakening.
   • Common in nuclear reactor coolant systems.

Pilling-Bedworth Rule: If the volume of the oxide formed is greater than the volume of the metal consumed, a protective (non-porous) film results. Otherwise, the film is porous and non-protective.

Pilling-Bedworth Rule: The ratio of the volume of metal oxide formed to the volume of metal consumed determines the protective nature of the oxide film.

Effect of oxide film nature:

• Stable, non-porous film (P.B. ratio ): The oxide layer completely covers the metal surface, acting as a barrier and preventing further corrosion. Example: , .
• Porous film (P.B. ratio ): The oxide layer has pores/cracks through which oxygen can penetrate, allowing continuous corrosion. Example: alkali metals like , .
• Volatile oxide: The film evaporates as soon as it forms, exposing fresh metal. Example: forms volatile .
• Unstable oxide: Decomposes back to metal and oxygen, so no corrosion. Example: , , .

Thus a metal is protected when it forms a thin, stable, tightly adhering, and non-porous oxide layer.

Electrochemical Corrosion: Occurs when metal is in contact with a conducting liquid (electrolyte) or when two dissimilar metals are in contact in the presence of moisture. It involves the formation of separate anodic and cathodic areas with electron flow between them.

General features:

• At Anode (oxidation): Metal dissolves as ions.
  
• At Cathode (reduction): Electrons are consumed.

1. Hydrogen Evolution Mechanism:

• Occurs in acidic environments.
• Anode:
• Cathode:
• All metal surface acts as anode and only small cathode; leads to rapid corrosion.

2. Oxygen Absorption Mechanism:

• Occurs in neutral/basic aerated solutions.
• Anode:
• Cathode:
• and combine to form rust:

The electrons flow from anode to cathode through the metal, and ions migrate through the electrolyte, completing the circuit.

Key point: Dry corrosion follows adsorption/direct chemical laws, while wet corrosion follows electrochemical principles involving distinct anodic and cathodic reactions.

Galvanic Corrosion (Bimetallic Corrosion): Occurs when two dissimilar metals are electrically connected and exposed to an electrolyte. The metal higher in the electrochemical/galvanic series (more active/anodic) undergoes corrosion, while the more noble metal (cathodic) is protected.

Mechanism:

• The more active metal acts as anode and dissolves:
  
• The nobler metal acts as cathode where reduction occurs.

Example: When zinc and copper are connected:

• Zinc (more active) is the anode and corrodes:
• Copper (nobler) is the cathode and is protected.

Other examples:

• Steel screws in a copper sheet — steel corrodes.
• A steel pipe connected to a copper pipe.

Factors:

• Greater the potential difference between the metals, faster the corrosion.
• Ratio of anodic to cathodic area: a small anode with large cathode causes intense localized attack.

Prevention: Use metals close in the galvanic series, insulate the two metals, or use sacrificial anodes.

Concentration Cell Corrosion: This type of electrochemical corrosion occurs when a metal is exposed to different concentrations of the same electrolyte or different oxygen concentrations at different parts of the same metal surface.

Types:

1. Metal-ion concentration cell:

   • Different concentrations of metal ions in contact with the metal.
   • Region with low metal ion concentration becomes the anode and corrodes.
   • Region with high metal ion concentration becomes the cathode.

2. Oxygen (Differential Aeration) concentration cell:

   • Different oxygen concentrations along the metal surface.
   • The part exposed to less oxygen acts as the anode (corrodes).
   • The part exposed to more oxygen acts as the cathode (protected).

Reactions:

• Anode:
• Cathode:

Principle: A potential difference arises due to concentration gradients, setting up a corrosion cell that drives metal dissolution at the anodic region.

Differential Aeration Corrosion: Arises when different parts of a metal surface are exposed to different concentrations of oxygen (air). A potential difference develops between the poorly aerated and well-aerated regions.

Principle:

• The metal part exposed to lower oxygen concentration becomes the anode and corrodes.
• The metal part exposed to higher oxygen concentration becomes the cathode and is protected.

This contradicts intuition — the less oxygenated area corrodes.

Reactions:

• Anode (less O₂):
• Cathode (more O₂):

Example: A partially immersed iron plate in water.

• The portion below the water (less oxygen) corrodes as anode.
• The portion near the surface (more oxygen) acts as cathode.

Consequence types: Water-line corrosion, pitting corrosion, and crevice corrosion are all forms of differential aeration corrosion.

Water-line Corrosion: A form of differential aeration corrosion observed in water storage tanks, ships, and steel structures partially immersed in water (e.g., ocean-going ships).

Mechanism:

• The water surface is in contact with atmospheric oxygen (well aerated).
• The area just below the water line is poorly aerated.
• Hence a differential aeration cell is set up:
  • Below water line (less O₂) = Anode → corrodes.
  • At/near water line (more O₂) = Cathode → protected.

Reactions:

• Anode:
• Cathode:

Result: A distinct line of intense corrosion appears just below the water line. In ships this leads to formation of brown deposits of rust below the water line.

Prevention:

• Applying anti-corrosive paints.
• Coating the hull with special protective layers.
• Using cathodic protection.

Pitting Corrosion: A localized, accelerated attack resulting in the formation of small pits, cavities, or holes on the metal surface. It is one of the most destructive and insidious forms of corrosion.

Cause: Usually initiated by breakdown of the protective film at a small point due to a scratch, dust particle, water droplet, or non-uniform deposit.

Mechanism (differential aeration):

• A small area under a deposit/droplet is poorly aerated → becomes anode.
• The surrounding large area is well aerated → becomes cathode.
• Small anode + large cathode ratio causes intense localized corrosion.

Reactions:

• Anode (pit):
• Cathode (surroundings):

Autocatalytic nature: Once a pit forms, accumulates and attracts ions, increasing acidity inside the pit and accelerating attack.

Consequences: Deep pits can perforate the metal even though overall metal loss is small, causing sudden failure of pipes and tanks.

Intergranular Corrosion: A localized attack that occurs preferentially along the grain boundaries of a metal or alloy, while the grains themselves remain largely unattacked. This weakens the metal and may cause it to disintegrate along grain boundaries.

Cause:

• Grain boundaries are chemically more reactive due to impurities, precipitates, or depletion of alloying elements.
• Precipitation of second-phase particles at grain boundaries sets up local galvanic cells.

Example — Weld Decay in Stainless Steel:

• When stainless steel is heated (e.g., during welding) to 500–800 °C, chromium carbide () precipitates at grain boundaries.
• This depletes chromium in the adjacent regions.
• The chromium-depleted zones become anodic and corrode preferentially.

Conditions favouring it:

• Improper heat treatment / sensitization of alloys.
• Presence of a corrosive electrolyte.

Prevention:

• Use low-carbon stainless steel.
• Add stabilizing elements like Ti or Nb (which form carbides preferentially).
• Proper heat treatment (quenching after solution annealing).

Soil Corrosion: The deterioration of metals (especially underground pipelines, cables, and storage tanks) buried in soil due to electrochemical interactions with the soil environment.

Mechanism: Soil acts as an electrolyte because it contains moisture, dissolved salts, and gases. Differential aeration cells form between areas of soil with different oxygen and moisture content.

Factors affecting soil corrosion:

• Moisture content: More moisture increases conductivity and corrosion.
• Soil porosity/aeration: Differential aeration between well-aerated (sandy) and poorly aerated (clayey) soils promotes corrosion.
• pH of soil: Acidic soils are more corrosive.
• Electrical conductivity: High salt content (low resistivity) accelerates corrosion.
• Presence of dissolved salts: Chlorides and sulphates increase corrosion.
• Microbial activity: Sulphate-reducing bacteria promote anaerobic corrosion.
• Temperature: Higher temperature increases corrosion rate.

Example: A pipeline passing through both sandy and clayey soil corrodes in the clayey (less aerated) region, which acts as the anode.

Prevention: Protective coatings, cathodic protection, and using corrosion-resistant materials.

The rate of corrosion depends on factors relating to both the nature of the metal and the nature of the environment.

A. Nature of the Metal:

• Position in galvanic series: Higher (more active) metals corrode faster.
• Purity of metal: Impurities create local galvanic cells; purer metals resist corrosion better.
• Relative anode-cathode area: Small anode + large cathode → intense corrosion.
• Nature of oxide film: Protective (non-porous) films reduce corrosion (Pilling-Bedworth rule).
• Overvoltage of hydrogen: Higher hydrogen overvoltage reduces corrosion rate.
• Physical state: Smaller grain size, stress, and strain increase corrosion.

B. Nature of the Environment:

• Temperature: Increases reaction rate and diffusion → more corrosion.
• Humidity/moisture: Higher humidity increases corrosion (critical humidity).
• pH: Acidic media (low pH) generally accelerate corrosion.
• Presence of impurities in air: , , , fumes accelerate corrosion.
• Conductivity of medium: Higher conductivity increases electrochemical corrosion.
• Concentration of oxygen: Differential oxygen concentration drives aeration corrosion.
• Flow velocity: Higher velocity of the corrosive medium increases corrosion (erosion-corrosion).

Cathodic Protection: A method of protecting a metal from corrosion by forcing it to behave as a cathode. Since corrosion occurs at the anode, making the entire metal surface a cathode prevents corrosion.

Types:

1. Sacrificial Anodic Protection:

• The metal to be protected is connected to a more active (anodic) metal.
• The more active metal corrodes (sacrifices itself), protecting the base metal.
• Anodes used: Zn, Mg, Al (called sacrificial anodes).
• Applications: Protection of buried pipelines, ship hulls, underground tanks, and water heaters.
• The sacrificial anode is replaced periodically.

2. Impressed Current Cathodic Protection:

• An external DC current is applied in the opposite direction to nullify the corrosion current.
• The metal to be protected is connected to the negative terminal of a DC source; an inert anode (graphite, platinized titanium, scrap iron) is connected to the positive terminal.
• Applications: Protection of large structures like buried pipelines, transmission towers, and marine structures.

Advantage: Impressed current method is suitable for long-term protection of large installations.

Summary: Both convert the protected metal into a cathode, but they differ in the source of protecting current and the type of anode used.

Corrosion can be controlled by several methods:

1. Proper Design and Material Selection:

• Avoid contact of dissimilar metals.
• Avoid sharp corners and crevices where moisture accumulates.
• Use corrosion-resistant materials.

2. Use of Pure Metals and Alloying:

• Reduce impurities to prevent local cells.
• Add alloying elements (e.g., Cr, Ni) to improve resistance (stainless steel).

3. Cathodic Protection:

• Sacrificial anodic protection (Zn, Mg).
• Impressed current cathodic protection.

4. Anodic Protection:

• Forming a stable passive oxide film by applying anodic current.

5. Use of Inhibitors:

• Anodic inhibitors (chromates, phosphates) reduce anodic reaction.
• Cathodic inhibitors reduce cathodic reaction (e.g., ).

6. Protective Coatings:

• Metallic coatings: galvanizing (Zn), tinning (Sn), electroplating.
• Inorganic coatings: ceramic coatings, anodizing.
• Organic coatings: paints, enamels, varnishes, lacquers.

7. Modifying the Environment:

• Deaeration, dehumidification, and removal of corrosive gases.

These methods can be used individually or in combination for effective protection.

Both are methods of coating iron/steel with a protective metal layer, but they differ in the metal used and the mode of protection.

Key point: Galvanizing offers sacrificial protection, while tinning offers only barrier protection and fails if the coating is damaged.

Ceramic Coating: A protective coating in which a thin layer of ceramic material (inorganic, non-metallic compounds such as oxides, carbides, nitrides, or silicates) is applied to a metal surface to protect it from corrosion, oxidation, and wear at high temperatures.

Types of ceramic coatings:

• Glass/Enamel coatings: Vitreous glass fused onto the metal surface (e.g., enamelled cookware, chemical reactors).
• Oxide coatings: , , used for high-temperature protection.
• Carbide/Nitride coatings: SiC, TiN for wear and corrosion resistance.

Methods of application: Thermal spraying, plasma spraying, chemical vapour deposition (CVD), physical vapour deposition (PVD), and sol-gel techniques.

Advantages:

• Excellent resistance to corrosion and chemical attack.
• High-temperature stability (thermal barrier).
• Good wear and abrasion resistance.
• Electrical insulation.
• Chemically inert and non-toxic.

Applications: Chemical process equipment, gas turbine blades, engine components, cookware, and biomedical implants.

Limitation: Brittle and may crack under thermal or mechanical shock.

Electroplating: An electrochemical process in which a thin layer of a superior/protective metal is deposited on a base metal (or other conducting surface) by passing direct current through an electrolyte containing the coating metal ions.

Objectives:

• To improve corrosion resistance.
• To improve appearance (decorative finish).
• To improve hardness, wear resistance, and surface properties.

Requirements/Setup:

• Cathode: The article to be plated (connected to negative terminal).
• Anode: Usually the coating metal (connected to positive terminal), or an inert anode.
• Electrolyte: A solution of the salt of the coating metal.
• DC power source.

Principle & Reactions (example: copper plating):

• Electrolyte: solution.
• At cathode (article): metal ions get reduced and deposit.
  
• At anode (copper): metal dissolves to replenish ions.
  

Pre-treatment: The article is cleaned, degreased, and pickled (acid treatment) to ensure good adhesion.

Factors affecting quality: Current density, temperature, electrolyte concentration, pH, and additives (brighteners).

Applications: Chromium plating on car parts, gold/silver plating on jewellery, nickel plating, and coating of electronic components.

Comparison of Electroplating and Galvanizing:

Role of Pre-treatment: Proper surface preparation is essential for good adhesion and quality of coating.

• Solvent cleaning/degreasing: Removes oils and grease.
• Alkaline cleaning: Removes remaining dirt and organic matter.
• Pickling (acid treatment): Removes oxide scales and rust.
• Rinsing: Removes residual chemicals.

Without pre-treatment, coatings peel off, become porous, or adhere poorly, leading to premature corrosion. Thus surface preparation ensures a clean, active surface for a strong, uniform, and long-lasting coating.