৪৯তম বিসিএস ⎯ ফলিত রসায়ন [৫৪১]

Why খ is correct: Corrosion generally involves chemical or electrochemical reactions between the material and its surroundings, leading to the gradual destruction of the material’s properties. Common examples: rusting of iron, pitting in stainless steel, or galvanic corrosion.

Why ক is wrong: Oxygen is not the only culprit; corrosion can occur in environments without oxygen (e.g., acid corrosion, hydrogen embrittlement).

Why গ is wrong: That describes erosion or abrasion, which is a mechanical process, not chemical.

Why ঘ is wrong: Heat treatment improves material properties; it is not corrosion.

ক) Corrosion weakens structures over time → True consequence.

খ) Corrosion can lead to localized attack, causing stress corrosion cracking → True consequence.

ঘ) Common issue in chemical and oil/gas pipelines → True consequence.

গ) While some metals like aluminum and stainless steel can form passive films naturally (reducing corrosion), corrosion itself is not intended to create protective layers. In fact, corrosion generally removes protective films or bypasses them, accelerating damage.

Direct costs: Replacement of corroded materials, protective coatings, corrosion monitoring equipment.

Indirect costs (often much higher): 
      1. Production losses due to plant shutdowns.
      2. Environmental clean-up after leaks.
      3. Legal liabilities and safety incidents.
      4. Increased insurance premiums.

ক is wrong → Costs are not limited to materials and are difficult to predict.

গ is wrong → Corrosion costs can be comparable to or higher than energy costs in some sectors.

ঘ is wrong → Even with coatings and inhibitors, corrosion is not fully eliminated; maintenance remains costly.

Why Option ক is correct:

**Offshore oil and gas infrastructure is constantly exposed to seawater, high pressures, CO₂, H₂S, and other corrosive agents.

**Corrosion can lead to catastrophic failures, environmental damage, and huge downtime costs, making it the most expensive globally.

Option খ is wrong: Textile industry machinery suffers minor corrosion, mostly localized; costs are relatively low.

Option গ is wrong: Food processing uses stainless steel and protective coatings; corrosion is controlled and less costly.

Option ঘ is wrong: Paper and pulping plants experience chemical corrosion, but total economic impact is far lower than offshore oil and gas.

Pitting corrosion creates small, deep cavities on the metal surface.

Dangerous consequences include:
**Localized stress concentrations: Pits act as initiation points for cracks under cyclic loads, increasing the risk of fatigue failure.
**Difficult detection: Pits may be very small and hidden, so significant damage can occur before it is noticed.
**Rapid penetration: Even with minimal overall metal loss, pitting can compromise structural integrity.

ক is wrong: Pitting is localized, not uniform, so failure is unpredictable.

গ is wrong: Pitting does not improve corrosion resistance; it accelerates damage.

ঘ is wrong: Pitting can occur on critical surfaces, making it highly dangerous and not easily repaired.

Why খ is correct:

**Corrosion, especially pitting, crevice, and stress corrosion, creates localized weak points that can fail suddenly under stress.

**Many industrial disasters, including pipeline bursts, tank failures, and offshore rig collapses, have been traced to corrosion-induced defects.

ক is wrong: Corrosion does not significantly affect thermal conductivity to a degree that threatens safety.

গ is wrong: Surface discoloration is a visual indicator, not the primary safety hazard.

ঘ is wrong: Corrosion reduces strength and ductility, making metals more brittle, not more flexible.

ক is correct: Exposure of steel to air and moisture causes even oxidation over the entire surface, which is characteristic of uniform corrosion.

খ is wrong: Galvanic corrosion occurs locally at the anodic metal when two dissimilar metals are in contact in an electrolyte.

গ is wrong: Stress corrosion cracking is highly localized, following cracks under combined stress and corrosive environment.

ঘ is wrong: Pitting corrosion is localized deep attack, not uniform surface loss.

ক is correct:

**Two dissimilar metals (aluminum and copper) are electrically connected.

**Presence of moisture creates a conductive path, forming a galvanic cell.

**Aluminum (more active) corrodes faster, while stainless steel and copper (less active) are protected.

খ is wrong: Single steel in dry air → only slow uniform oxidation, not galvanic corrosion.

গ is wrong: High-temperature oxidation affects individual metals but does not require two metals, so it’s not galvanic.

ঘ is wrong: Pure iron in a dry warehouse has no conductive path and no dissimilar metal → no galvanic corrosion.

ঘ is correct:

**Narrow gaps under gaskets or bolts trap seawater, creating oxygen-depleted zones.

**The anodic area inside the crevice corrodes faster due to differential aeration, while the outside remains relatively protected.

**Common in marine, chemical, and industrial piping systems.

খ is wrong: Exposed surfaces experience uniform corrosion, not crevice corrosion.

গ is wrong: Thermal cycling may induce general corrosion or fatigue, but not crevice corrosion unless there are gaps.

ক is wrong: Dry exposure lacks a conductive electrolyte; crevice corrosion does not occur.

Stress corrosion cracking (SCC) occurs when a metal under tensile stress is exposed to a specific corrosive environment.

Key features:
1. Cracks initiate and propagate without significant overall metal loss.
2. Common in boilers, pipelines, and chemical plants, especially under high stress and aggressive chemicals (e.g., chlorides for stainless steel).

ক is wrong: Pitting is localized corrosion due to chemical attack, not directly linked to mechanical stress.

গ is wrong: Galvanic corrosion occurs between dissimilar metals, not due to stress.

ঘ is wrong: Erosion or erosion-corrosion is caused by mechanical wear from moving fluids or particles, not primarily by stress in a corrosive medium.

Intergranular corrosion occurs when the grain boundaries of stainless steel become depleted in chromium.

How it happens:
     1. Overheating during welding or heat treatment can cause chromium carbides to form at grain boundaries.
     2. This leaves adjacent areas less resistant to corrosion, making the steel vulnerable along the grain boundaries.

খ is wrong: Distilled water is relatively non-aggressive and does not cause intergranular attack.

গ is wrong: Dry nitrogen is inert; no corrosion occurs.

ঘ is wrong: Coating with oil prevents corrosion rather than inducing it.

ক is correct:

**High-velocity fluid containing solid particles or turbulence physically wears away the protective oxide layer.

**Once the layer is removed, chemical corrosion accelerates, leading to erosion-corrosion.

**Common in pumps, elbows, and pipelines handling abrasive slurries or chemicals.

খ is wrong: Calm atmospheric exposure → mostly uniform corrosion, not erosion-corrosion.

গ is wrong: No fluid movement → minimal mechanical wear; mostly uniform or galvanic corrosion if in contact with dissimilar metals.

ঘ is wrong: Crevice corrosion may occur under gaskets, but fluid velocity is not the primary factor.

Passive corrosion prevention methods work by isolating the metal from the corrosive environment, without altering the metal’s electrochemical potential.

Protective coatings (paints, plating, polymer layers, or oxide films) physically block moisture, oxygen, and chemicals from reaching the metal surface, making them passive methods.

ক) Cathodic protection → Active method: alters the metal’s electrochemical potential to prevent corrosion.

খ) Sacrificial anode installation → Active method: the anode corrodes instead of the protected metal.

ঘ) Impressed current system → Active method: applies an external current to suppress corrosion.

খ is correct:

**Hot-dip galvanizing forms a zinc layer on steel.

**Zinc serves as a barrier to environmental exposure and as a sacrificial anode, protecting steel even if the coating is scratched.

**Common in bridges, pipelines, and outdoor structures.

ক is wrong: Anodizing is electrochemical oxidation of aluminum, not zinc coating.

গ is wrong: Electroplating deposits metal via electricity, not molten immersion.

ঘ is wrong: Cladding is a metallurgical bond between two metals, not dipping steel in zinc.

ক is correct:

**The steel is shifted to a cathodic potential, preventing its oxidation.

**Sacrificial anodes (zinc, magnesium) corrode instead of the steel.

**Impressed current systems supply external current to maintain cathodic protection over long distances, common in pipelines and storage tanks.

খ is wrong: Making the steel the anode would accelerate corrosion, the opposite of protection.

গ is wrong: Physical coatings are passive protection, not cathodic protection.

ঘ is wrong: Heating steel forms oxide layers but is not an active electrochemical method.

ঘ is correct:
Aluminium naturally forms a thin oxide layer, but anodizing thickens this layer to improve:
    1. Corrosion resistance in aggressive environments.
    2. Wear resistance and hardness for mechanical durability.
    3. Aesthetic appearance via colored anodized coatings.

**Common in aircraft, aerospace components, consumer electronics, and architectural panels.

ক is wrong: Steel protection relies on galvanizing, coatings, or cathodic protection, not anodizing.

খ is wrong: Copper is generally protected by alloying, coatings, or patination, not anodizing.

গ is wrong: Lead does not require anodizing due to its natural corrosion resistance in many environments.

খ is correct:
Corrosion inhibitors adsorb onto the metal surface and alter anodic or cathodic reactions, slowing metal dissolution.
Types of inhibitors:
   Anodic inhibitors → Form protective oxide layers.
  Cathodic inhibitors → Slow reduction reactions.
  Mixed inhibitors → Affect both reactions.

**Widely used in cooling towers, boilers, and heat exchangers to prevent steel or copper corrosion.

ক is wrong: Increasing fluid velocity can increase erosion-corrosion, not prevent corrosion.

গ is wrong: Removing salts or oxygen may help but is not the mechanism of chemical inhibitors.

ঘ is wrong: Higher conductivity actually accelerates corrosion, opposite to the inhibitor’s goal

Underground steel pipelines are highly susceptible to corrosion due to moist soil, moisture, and electrolytes.

Cathodic protection is the most effective method:
    1. The steel pipeline is made the cathode.
    2. Sacrificial anodes (zinc, magnesium) corrode preferentially, protecting the pipeline.
    3. Can also be implemented with impressed current systems for long pipelines.

ক is wrong: Electro-polishing improves surface smoothness but does not provide long-term underground corrosion protection.

খ is wrong: Oil coating alone cannot protect pipelines buried in soil due to coating damage and moisture ingress.

ঘ is wrong: High-temperature annealing affects mechanical properties, not corrosion protection.

ক is correct:

Rusting starts with iron oxidation at anodic sites.

     Fe atoms lose electrons, forming Fe²⁺ ions, which later react with oxygen and water to form rust (Fe₂O₃·xH₂O).

    Electrons flow through the metal to cathodic sites, enabling reduction reactions.

খ is wrong: This is the cathodic reaction; it occurs at separate sites.

গ is wrong: Reduction of Fe²⁺ back to Fe does not occur in rusting.

ঘ is wrong: Oxidation of Fe²⁺ to Fe³⁺ happens later in the corrosion process, not at the primary anodic site.

In neutral or slightly basic water, the cathodic reaction during iron corrosion is typically the reduction of dissolved oxygen:
O₂ + 2H₂O + 4e^- → 4OH^-

​Process:
 Electrons released at the anodic site (Fe → Fe²⁺ + 2e^-) travel to the cathodic site.
 There, they reduce dissolved oxygen in water to hydroxide ions (OH^-).
 The hydroxide ions later react with Fe²⁺ to form iron hydroxides, which eventually produce rust.

ক is wrong: 2H₂ + O₂ → 2H₂O is not relevant to iron corrosion in neutral water; it’s a general combustion reaction.

গ is wrong: This is the reverse of the correct cathodic reaction.

ঘ is wrong: Fe³⁺ + e^- → Fe²⁺ occurs later in rust formation, not at the cathodic site during initial corrosion.

ঘ is correct:
Electrochemical corrosion requires anodic and cathodic sites on the metal surface.
Differences in electrochemical potential (caused by impurities, alloy variations, or differential aeration) drive electron flow from the anode to the cathode, sustaining corrosion.

​Real-world examples:
    Galvanic corrosion between dissimilar metals.
    Crevice corrosion due to differential oxygen concentration.

ক is wrong: Temperature gradients may accelerate reactions but do not create the electron flow that drives corrosion.

খ is wrong: Pressure variations have negligible effect on corrosion potential.

গ is wrong: Mechanical stress may influence stress corrosion cracking, but it is not the primary electrochemical driving force.

খ is correct:
     Dissolved salts act as electrolytes, increasing solution conductivity.
     This promotes electron flow between anodic and cathodic sites, accelerating corrosion.
     Common in seawater, industrial cooling systems, and de-icing environments.

ক is wrong: Alkaline conditions can slow corrosion by forming protective oxide or hydroxide layers.

গ is wrong: Dry conditions prevent the formation of an electrochemical cell, so corrosion is minimal.

ঘ is wrong: Pure dry air cannot sustain an electrochemical reaction; corrosion requires moisture.

গ is correct:

Rust is mainly hydrated ferric oxide, resulting from the oxidation of Fe²⁺ to Fe³⁺ and subsequent reaction with water.
Moisture and oxygen facilitate the stepwise conversion:

Fe → Fe²⁺ + 2e^-

O₂ + 2H₂O + 4e^- → 4OH^-

Fe²⁺ + O₂/H₂O → Fe³⁺ → Fe₂O₃·xH₂O

The “x” represents variable water content, affecting rust color and texture.

ক is wrong: FeO (ferrous oxide) forms under high-temperature or oxygen-deficient conditions, not typical atmospheric rust.

খ is wrong: Fe₃O₄ (magnetite) is stable under low-oxygen or partially protective conditions, not standard rust.

ঘ is wrong: Fe(OH)₂ is an intermediate product, rapidly oxidized to Fe³⁺ compounds in the presence of air and moisture.

খ is correct:

    Zinc and magnesium are highly anodic in seawater.

   They corrode preferentially, protecting connected metals (like steel) via sacrificial action.

   Widely used in ship hulls, offshore platforms, and pipelines.

ক is wrong: Gold and platinum are noble metals, highly cathodic, and resist corrosion.

গ is wrong: Stainless steel and copper are relatively cathodic; they do not provide sacrificial protection.

ঘ is wrong: Lead and tin are passive in seawater and are not effective sacrificial anodes.

The salt spray (fog) test is a standardized accelerated corrosion test used to evaluate the corrosion resistance of metals and protective coatings.

How it works:
A fine mist of salt solution (usually NaCl) is sprayed over the test sample in a controlled chamber.
The environment accelerates corrosion, simulating long-term exposure to marine or industrial atmospheres.
Used to compare coatings, paints, galvanization, or anodized layers.

ক is wrong: Hardness tests are done using Rockwell, Brinell, or Vickers methods.

খ is wrong: Tensile strength is measured via mechanical testing machines.

গ is wrong: Electrical resistivity requires special electrical measurement techniques, not a salt spray test.

Potentiodynamic polarization test is an electrochemical corrosion testing method:

**Measures current response as the applied potential is varied.

**Produces polarization curves that show:

 1. Corrosion potential (E_corr) – tendency to corrode.
 2. Corrosion current density (I_corr) – rate of corrosion.

**Useful for evaluating material behavior, inhibitors, and coatings in a controlled electrolyte.

ক is wrong: Weight loss method measures mass loss over time, not electrochemical properties.

গ is wrong: Salt immersion (or salt spray) evaluates coating performance or general corrosion, not polarization curves.

ঘ is wrong: Ultrasonic inspection detects structural defects or thinning, not electrochemical behavior.

ক is correct:
• Samples are weighed before and after exposure to corrosive media.
• Mass loss is converted to corrosion rate (e.g., mm/year).
• Suitable for evaluating coatings, inhibitors, metals, or alloys under realistic industrial conditions like cooling water, seawater, or acidic solutions.

খ) is wrong: Surface roughness requires profilometers or microscopic analysis, not weight change.

গ) is wrong: Electrical conductivity is unrelated; corrosion rate is independent of this measurement.

ঘ) is wrong: Yield strength is a mechanical property, not assessed by weight loss.

Electrochemical Impedance Spectroscopy (EIS) is an advanced electrochemical technique used to study corrosion and protective coatings.

Principle:
**Applies a small AC voltage over a range of frequencies.
**Measures the current response to determine the impedance of the metal-electrolyte interface.
**Provides information about:
    1. Corrosion rate
    2. Coating integrity
    3. Charge transfer resistance
    4. Double-layer capacitance

ক is wrong: Hardness is measured mechanically (Rockwell, Brinell).

গ is wrong: Oxide thickness can be inferred indirectly but is not directly measured by EIS.

ঘ is wrong: Metal density is a physical property, unrelated to impedance.

ক is correct:

Radiography penetrates coatings, insulation, or cladding, producing images of internal defects.
Can detect pitting, thinning, and undercoating corrosion without damaging the structure.
Widely used in oil & gas pipelines, offshore structures, and chemical storage tanks.

খ is wrong: Salt spray testing is a destructive accelerated corrosion test, not suitable for in-service inspection.

গ is wrong: Potentiodynamic polarization is an electrochemical lab test, requires exposed metal.

ঘ is wrong: Weight loss testing is destructive, requiring sample removal and cleaning.

গ is correct:

ASTM G48 uses ferric chloride solution at controlled temperature to induce pitting in stainless steels.

Provides quantitative and comparative data on pitting resistance for different alloys.

Widely used in marine, chemical, and food-processing industries to select corrosion-resistant stainless steels.

ক is wrong: Salt spray is a general accelerated corrosion test; it does not reliably produce pitting.

খ is wrong: EIS gives electrochemical properties but does not directly quantify pitting initiation or propagation.

ঘ is wrong: Weight loss measures overall uniform corrosion, not localized attack like pitting.