Theories of corrosion MCQs With Answer

Understanding theories of corrosion is essential for B.Pharm students involved in pharmaceutical packaging, storage, and stainless-steel equipment maintenance. This collection titled Theories of corrosion MCQs With Answer covers electrochemical and chemical corrosion, Pourbaix diagrams, galvanic and crevice corrosion, passivation, pitting, stress corrosion cracking, and corrosion control strategies including coatings, inhibitors, and cathodic protection. Questions emphasize corrosion mechanisms, thermodynamics and kinetics, measurement techniques, and practical implications for drug stability, container compatibility, and GMP equipment upkeep. Ideal for revision, lab practice, assessments, and interview preparation. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the primary driving force for electrochemical corrosion?

• Mechanical stress concentration on a surface
• Electrochemical potential difference between anodic and cathodic sites
• Thermal expansion mismatch
• Microbial colonization on metal

Correct Answer: Electrochemical potential difference between anodic and cathodic sites

Q2. Which theory explains corrosion as an electrochemical cell process with anodic oxidation and cathodic reduction?

• Oxidation-reduction thermodynamic theory
• Electrochemical theory of corrosion
• Mechanical fracture theory
• Diffusion-limited aggregation theory

Correct Answer: Electrochemical theory of corrosion

Q3. In Pourbaix diagrams, what do the axes typically represent?

• Temperature and pressure
• pH and electrode potential (E)
• Concentration and time
• Stress and strain

Correct Answer: pH and electrode potential (E)

Q4. Which stainless steel element is most responsible for passivation by forming a stable oxide film?

• Nickel
• Chromium
• Manganese
• Carbon

Correct Answer: Chromium

Q5. What is pitting corrosion primarily initiated by?

• Uniform dissolution across the entire surface
• Local breakdown of passive film, often due to chloride ions
• Mechanical abrasion removing surface metal
• High tensile stress alone

Correct Answer: Local breakdown of passive film, often due to chloride ions

Q6. Which ion is most notorious for causing breakdown of passive films and promoting pitting?

• Sulfate (SO4 2-)
• Nitrate (NO3 -)
• Chloride (Cl-)
• Carbonate (CO3 2-)

Correct Answer: Chloride (Cl-)

Q7. Faraday’s law relates mass loss by corrosion to which parameters?

• Applied voltage and capacitance
• Current, time, molar mass, valence and Faraday constant
• Temperature and pH only
• Surface roughness and humidity

Correct Answer: Current, time, molar mass, valence and Faraday constant

Q8. Which measurement technique gives instantaneous corrosion rate by measuring polarization resistance?

• Weight loss coupon test
• Electrochemical impedance spectroscopy (EIS)
• Linear polarization resistance (LPR)
• Salt spray (fog) test

Correct Answer: Linear polarization resistance (LPR)

Q9. What does a negative corrosion potential (more cathodic) indicate for a metal in a given environment?

• Higher tendency to act as an anode
• Lower tendency to corrode than reference
• More noble behavior
• Less likely to be oxidized relative to a more positive potential

Correct Answer: Lower tendency to corrode than reference

Q10. Which reaction commonly acts as the cathodic reaction in aerated neutral aqueous corrosion?

• Hydrogen oxidation to protons
• Oxygen reduction to hydroxide (O2 + 2H2O + 4e- → 4OH-)
• Chloride oxidation
• Metal ion precipitation

Correct Answer: Oxygen reduction to hydroxide (O2 + 2H2O + 4e- → 4OH-)

Q11. What is the effect of increasing temperature on corrosion rate for most metals in aqueous environments?

• Corrosion rate always decreases with temperature
• Corrosion rate is independent of temperature
• Corrosion rate generally increases due to accelerated kinetics
• Corrosion rate oscillates unpredictably

Correct Answer: Corrosion rate generally increases due to accelerated kinetics

Q12. What does a Pourbaix diagram show about a metal at a given pH and potential?

• Mechanical properties of the oxide
• Regions of immunity, passivity, and corrosion (active dissolution)
• Temperature-dependence of solubility only
• Microbial growth zones

Correct Answer: Regions of immunity, passivity, and corrosion (active dissolution)

Q13. Galvanic corrosion occurs when two dissimilar metals are electrically connected in an electrolyte. Which metal acts as the anode?

• The more noble (cathodic) metal in the galvanic series
• The less noble (more active) metal in the galvanic series
• The thicker metal regardless of nobility
• The metal with higher thermal conductivity

Correct Answer: The less noble (more active) metal in the galvanic series

Q14. What is critical pitting potential (Epit)?

• Potential below which pitting cannot initiate
• Potential at which passive film forms
• Potential above which stable pit growth occurs
• Potential where hydrogen evolution dominates

Correct Answer: Potential above which stable pit growth occurs

Q15. Which protective strategy supplies electrons to the metal structure to prevent anodic dissolution?

• Anodic protection
• Cathodic protection
• Sacrificial inhibition
• Mechanical coating

Correct Answer: Cathodic protection

Q16. Sacrificial anode cathodic protection typically uses which metal for protecting steel underground?

• Gold
• Zinc or magnesium
• Lead
• Stainless steel

Correct Answer: Zinc or magnesium

Q17. What is passivation?

• Uniform dissolution of metal
• Formation of a thin protective oxide film that reduces corrosion rate
• Mechanical polishing of the surface
• Microbial colonization leading to corrosion

Correct Answer: Formation of a thin protective oxide film that reduces corrosion rate

Q18. Which analytical technique identifies oxide composition and thickness on passive films?

• X-ray photoelectron spectroscopy (XPS)
• Optical microscopy only
• Gravimetric analysis exclusively
• Salt spray chamber

Correct Answer: X-ray photoelectron spectroscopy (XPS)

Q19. What is stress corrosion cracking (SCC)?

• Uniform corrosion occurring at high temperature
• Brittle fracture due to combined tensile stress and corrosive environment
• Corrosion caused solely by microorganisms
• Electroplating defect

Correct Answer: Brittle fracture due to combined tensile stress and corrosive environment

Q20. Hydrogen embrittlement is a corrosion-related problem. How does hydrogen typically enter metals?

• By diffusion following atomic hydrogen generation from cathodic reactions
• Through thermal cracking of the metal lattice
• Via mechanical wear only
• Only through anodic oxidation

Correct Answer: By diffusion following atomic hydrogen generation from cathodic reactions

Q21. Which of the following is an anodic corrosion inhibitor mechanism?

• Blocking cathodic oxygen reduction by adsorbing on the site
• Forming an insoluble complex film on the anode to prevent metal dissolution
• Increasing solution conductivity
• Raising solution temperature

Correct Answer: Forming an insoluble complex film on the anode to prevent metal dissolution

Q22. What is crevice corrosion?

• Corrosion that occurs only at high temperature gas environments
• Localized corrosion inside tight gaps where stagnant solution causes aggressive chemistry
• Uniform corrosion on large exposed surfaces
• Biologically induced uniform corrosion

Correct Answer: Localized corrosion inside tight gaps where stagnant solution causes aggressive chemistry

Q23. Which term describes corrosion measured as mass loss per unit area per time often converted to penetration rate?

• Electrochemical impedance
• Corrosion current density
• Corrosion rate (e.g., mm/year)
• Surface tension

Correct Answer: Corrosion rate (e.g., mm/year)

Q24. Which electrode is commonly used as a reference in laboratory corrosion measurements?

• Standard hydrogen electrode (SHE)
• Silver/silver chloride (Ag/AgCl) or saturated calomel electrode (SCE)
• Platinum counter electrode
• Galvanic anode

Correct Answer: Silver/silver chloride (Ag/AgCl) or saturated calomel electrode (SCE)

Q25. What does electrochemical impedance spectroscopy (EIS) primarily provide for corrosion studies?

• Surface roughness values only
• Frequency-dependent impedance revealing kinetic and film properties
• Total weight loss after 1 year
• Purely metallurgical phase identification

Correct Answer: Frequency-dependent impedance revealing kinetic and film properties

Q26. In corrosion context, what is passivity breakdown often caused by?

• Protective oxide thickening only
• Local acidification, halide adsorption, or mechanical damage to film
• Complete absence of oxygen
• High surface polish

Correct Answer: Local acidification, halide adsorption, or mechanical damage to film

Q27. Which concept links thermodynamics and kinetics to predict whether corrosion can occur?

• Fourier transform theory
• Gibbs free energy and electrode potential (ΔG and E° via Nernst equation)
• Hooke’s law
• Avogadro’s number only

Correct Answer: Gibbs free energy and electrode potential (ΔG and E° via Nernst equation)

Q28. What is the primary cathodic reaction in acidic solutions leading to corrosion of base metals?

• Oxygen reduction to hydroxide
• Hydrogen ion reduction to hydrogen gas (2H+ + 2e- → H2)
• Chloride oxidation
• Metal oxide formation

Correct Answer: Hydrogen ion reduction to hydrogen gas (2H+ + 2e- → H2)

Q29. Which factor does NOT typically accelerate aqueous corrosion?

• High chloride concentration
• Lower temperatures within physiological range
• Low oxygen availability for passive film repair
• Acidic pH

Correct Answer: Lower temperatures within physiological range

Q30. What is the galvanic series used for?

• To rank polymers by resistance
• To rank metals and alloys by electrochemical nobility in a given environment
• To measure tensile strength of metals
• To list alloys by melting point

Correct Answer: To rank metals and alloys by electrochemical nobility in a given environment

Q31. Which corrosion test is most suitable for long-term estimation of corrosion rate by material loss?

• Electrochemical impedance spectroscopy (EIS)
• Salt spray test
• Weight loss (coupon) immersion test
• Linear polarization resistance (LPR)

Correct Answer: Weight loss (coupon) immersion test

Q32. In the Nernst equation, increasing concentration of oxidized species relative to reduced species shifts the electrode potential in which direction?

• Toward more positive (more noble) potential for the oxidation reaction
• Toward zero potential always
• Toward more negative potential regardless of reaction
• No change

Correct Answer: Toward more positive (more noble) potential for the oxidation reaction

Q33. Which of the following best describes microbiologically influenced corrosion (MIC)?

• Corrosion solely due to galvanic action of dissimilar metals
• Accelerated corrosion caused or influenced by microbial activity such as sulfate-reducing bacteria
• Generalized corrosion in pure water without microbes
• Corrosion only at extremely high temperatures

Correct Answer: Accelerated corrosion caused or influenced by microbial activity such as sulfate-reducing bacteria

Q34. What is the role of chromates and phosphates when used as inhibitors on metal surfaces?

• They are sacrificial anodes
• They form protective conversion films that reduce anodic dissolution
• They increase ionic conductivity of the electrolyte
• They accelerate pitting

Correct Answer: They form protective conversion films that reduce anodic dissolution

Q35. How does tight packaging design influence corrosion of pharmaceutical containers?

• It always prevents corrosion due to reduced oxygen
• It can promote crevice corrosion if moisture and contaminants are trapped
• It increases galvanic coupling always
• It prevents any chemical interaction with drugs

Correct Answer: It can promote crevice corrosion if moisture and contaminants are trapped

Q36. Which parameter obtained from Tafel extrapolation is used to calculate corrosion current density?

• Linear polarization resistance only
• Exchange current density extrapolated from linear regions of anodic and cathodic slopes
• Surface roughness factor
• Optical density

Correct Answer: Exchange current density extrapolated from linear regions of anodic and cathodic slopes

Q37. In pharmaceutical manufacturing, why is corrosion control of stainless steel equipment critical?

• Only aesthetics matter for equipment
• Corrosion can contaminate products, affect sterility, and reduce equipment life
• Corrosion increases electrical conductivity which is desirable
• Stainless steel never corrodes, so control is irrelevant

Correct Answer: Corrosion can contaminate products, affect sterility, and reduce equipment life

Q38. Which of the following best describes uniform corrosion?

• Localized attack at a crevice
• Even metal loss over exposed surface area
• Pitting at isolated sites only
• Intergranular attack along grain boundaries exclusively

Correct Answer: Even metal loss over exposed surface area

Q39. What is intergranular corrosion?

• Corrosion at high points on a polished surface
• Preferential attack along grain boundaries, often due to segregation or precipitation
• Corrosion only in amorphous metals
• Uniform bulk dissolution

Correct Answer: Preferential attack along grain boundaries, often due to segregation or precipitation

Q40. Which solution property accelerates corrosion by increasing ionic conductivity?

• High purity deionized water
• High concentration of dissolved salts (electrolyte)
• Non-polar organic solvents
• Dry air

Correct Answer: High concentration of dissolved salts (electrolyte)

Q41. What is the main protective mechanism of organic coatings on metal surfaces?

• Create a conductive path for electrons
• Act as a physical barrier to block electrolyte and oxygen access
• Release halide ions to neutralize the solution
• Absorb water to promote passivation

Correct Answer: Act as a physical barrier to block electrolyte and oxygen access

Q42. Which term describes sudden acceleration of corrosion due to local chemistry changes inside a crevice?

• Galvanic corrosion
• Crevice corrosion
• High temperature oxidation
• Intergranular corrosion

Correct Answer: Crevice corrosion

Q43. How is corrosion potential (Ecorr) determined experimentally?

• By measuring open circuit potential relative to a reference electrode
• By measuring mass loss directly
• By calculating from pH only
• By observing color change of the metal

Correct Answer: By measuring open circuit potential relative to a reference electrode

Q44. What does a high polarization resistance (Rp) indicate about a metal’s corrosion rate?

• High Rp indicates a high corrosion rate
• High Rp indicates a low corrosion rate
• Rp is unrelated to corrosion rate
• High Rp indicates imminent pitting only

Correct Answer: High Rp indicates a low corrosion rate

Q45. Which of the following is a common sign of localized pitting corrosion on metal surfaces?

• Uniform gray scale across surface
• Small deep cavities or holes often with raised edges
• Large uniform thinning only at edges
• Change in magnetic permeability

Correct Answer: Small deep cavities or holes often with raised edges

Q46. Which practice reduces galvanic corrosion risk between dissimilar metals?

• Ensure electrical isolation or use similar nobility metals and area ratio control
• Always use the most active metal in contact
• Increase solution temperature only
• Apply mechanical polishing only

Correct Answer: Ensure electrical isolation or use similar nobility metals and area ratio control

Q47. What is the significance of critical pitting temperature (CPT) for stainless steels?

• Temperature below which pitting is inevitable
• Temperature above which pitting is more likely under a given chloride concentration
• Temperature where steel becomes superconductive
• Temperature irrelevant to pitting

Correct Answer: Temperature above which pitting is more likely under a given chloride concentration

Q48. Which is an effective laboratory method to accelerate and study pitting susceptibility?

• Immersion in distilled water at room temperature
• ASTM chloride pitting tests like ASTM G48 with ferric chloride
• Exposing to dry heat only
• Mechanical bending without exposure to electrolyte

Correct Answer: ASTM chloride pitting tests like ASTM G48 with ferric chloride

Q49. What role does oxygen play in the corrosion of iron in neutral aqueous environments?

• Oxygen is inert and plays no role
• Oxygen acts as the electron acceptor (cathodic agent), enabling anodic iron dissolution
• Oxygen converts iron to gold
• Oxygen only causes passivation but never supports corrosion

Correct Answer: Oxygen acts as the electron acceptor (cathodic agent), enabling anodic iron dissolution

Q50. For pharmaceutical containers, which material is generally most inert and least prone to corrosion by aqueous formulations?

• Unlined carbon steel
• Stainless steel (proper grade) or glass, depending on formulation
• Pure aluminum without coating in chloride solutions
• Copper alloys for acidic drugs

Correct Answer: Stainless steel (proper grade) or glass, depending on formulation

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

Mail- Sachin@pharmacyfreak.com