Passivation of Metals: Practice MCQs
Based on standard metallurgy, material science, and engineering competitive exam patterns.
Must Read Passivation: Complete Guide
1. What is the primary purpose of the passivation process in stainless steel?
Answer & Explanation
Correct Answer: B
Passivation dissolves embedded free iron and surface contaminants left behind from machining or tooling, allowing a thin, protective chromium oxide ($\text{Cr}_2\text{O}_3$) passive layer to naturally form.
Passivation dissolves embedded free iron and surface contaminants left behind from machining or tooling, allowing a thin, protective chromium oxide ($\text{Cr}_2\text{O}_3$) passive layer to naturally form.
2. Which chemical compound predominantly makes up the passive film on stainless steel?
Answer & Explanation
Correct Answer: B
Chromium reacts with oxygen to form a stable, tightly adherent chromium oxide ($\text{Cr}_2\text{O}_3$) layer, which prevents further oxidation of the underlying metal.
Chromium reacts with oxygen to form a stable, tightly adherent chromium oxide ($\text{Cr}_2\text{O}_3$) layer, which prevents further oxidation of the underlying metal.
3. Which of the following acids are commonly used in industrial chemical passivation treatments for stainless steel?
Answer & Explanation
Correct Answer: B
Nitric acid and citric acid are the standard chemical baths specified by industry standards (like ASTM A967) for passivating stainless steels because they efficiently dissolve free iron without harming the chromium matrix.
Nitric acid and citric acid are the standard chemical baths specified by industry standards (like ASTM A967) for passivating stainless steels because they efficiently dissolve free iron without harming the chromium matrix.
4. What is the electrochemical transition known as when a metal shifts from active corrosion to a passive state?
Answer & Explanation
Correct Answer: C
The active-passive transition occurs in potentiodynamic polarization curves when an increase in potential leads to a sharp drop in corrosion current due to the formation of a protective film.
The active-passive transition occurs in potentiodynamic polarization curves when an increase in potential leads to a sharp drop in corrosion current due to the formation of a protective film.
5. What does the term "Flade Potential" signify in passivation studies?
Answer & Explanation
Correct Answer: B
The Flade potential is the critical electrode potential below which an iron or steel surface is active and above which it becomes passivated.
The Flade potential is the critical electrode potential below which an iron or steel surface is active and above which it becomes passivated.
6. Which ASTM standard is widely referenced for chemical passivation treatments of stainless steel parts?
Answer & Explanation
Correct Answer: B
ASTM A967 (and ASTM A380) specifies the standard requirements and testing methods for chemical passivation treatments for stainless steel parts.
ASTM A967 (and ASTM A380) specifies the standard requirements and testing methods for chemical passivation treatments for stainless steel parts.
7. Why is free iron on a stainless steel surface hazardous if left un-passivated?
Answer & Explanation
Correct Answer: B
Free iron particles left on the surface from tools or machining can oxidize rapidly in moisture, creating localized corrosion cells (rust spots) that breach the passive protection of the stainless steel.
Free iron particles left on the surface from tools or machining can oxidize rapidly in moisture, creating localized corrosion cells (rust spots) that breach the passive protection of the stainless steel.
8. What is the role of oxygen exposure immediately following acid treatment in passivation?
Answer & Explanation
Correct Answer: B
After acid rinsing dissolves surface free iron and exposes chromium-rich substrate, exposure to atmospheric oxygen is essential to rebuild the protective $\text{Cr}_2\text{O}_3$ layer.
After acid rinsing dissolves surface free iron and exposes chromium-rich substrate, exposure to atmospheric oxygen is essential to rebuild the protective $\text{Cr}_2\text{O}_3$ layer.
9. What happens in the "transpassive" region of a metal's polarization curve?
Answer & Explanation
Correct Answer: A
In the transpassive region (at potentials higher than the stable passive window), the protective film either dissolves (e.g., oxidation of $\text{Cr}^{3+}$ to soluble $\text{Cr}^{6+}$) or oxygen evolution occurs, causing current to increase again.
In the transpassive region (at potentials higher than the stable passive window), the protective film either dissolves (e.g., oxidation of $\text{Cr}^{3+}$ to soluble $\text{Cr}^{6+}$) or oxygen evolution occurs, causing current to increase again.
10. Which alloying element in stainless steel significantly enhances resistance to pitting corrosion within the passive film?
Answer & Explanation
Correct Answer: B
Molybdenum (found in grades like 316 stainless steel) concentrates in the passive film and greatly enhances resistance to chloride-induced pitting corrosion.
Molybdenum (found in grades like 316 stainless steel) concentrates in the passive film and greatly enhances resistance to chloride-induced pitting corrosion.
11. Carbon steels generally do not form stable passive layers in normal environments because:
Answer & Explanation
Correct Answer: B
Passivation relies on sufficient chromium content (typically at least 10.5% in stainless steel) to form $\text{Cr}_2\text{O}_3$. Carbon steel lacks this high chromium concentration, forming porous, flaky rust instead of a tight passive film.
Passivation relies on sufficient chromium content (typically at least 10.5% in stainless steel) to form $\text{Cr}_2\text{O}_3$. Carbon steel lacks this high chromium concentration, forming porous, flaky rust instead of a tight passive film.
12. Which of the following is a key chemical reaction representing the formation of the chromium oxide passive layer?
Answer & Explanation
Correct Answer: B
The fundamental chemical reaction of passivation involves chromium reacting with oxygen to produce chromium(III) oxide ($4\text{Cr} + 3\text{O}_2 \rightarrow 2\text{Cr}_2\text{O}_3$).
The fundamental chemical reaction of passivation involves chromium reacting with oxygen to produce chromium(III) oxide ($4\text{Cr} + 3\text{O}_2 \rightarrow 2\text{Cr}_2\text{O}_3$).
13. What is a major advantage of using citric acid over nitric acid for passivation?
Answer & Explanation
Correct Answer: B
Citric acid is an organic, eco-friendly chelating agent that removes free iron effectively without releasing toxic nitrogen oxide ($\text{NO}_x$) fumes associated with nitric acid.
Citric acid is an organic, eco-friendly chelating agent that removes free iron effectively without releasing toxic nitrogen oxide ($\text{NO}_x$) fumes associated with nitric acid.
14. What occurs during the critical anodic current density peak in an active-passive polarization curve?
Answer & Explanation
Correct Answer: A
Before the passive film forms and stifles corrosion, the metal dissolves rapidly at its peak active dissolution rate, corresponding to the critical anodic current density.
Before the passive film forms and stifles corrosion, the metal dissolves rapidly at its peak active dissolution rate, corresponding to the critical anodic current density.
15. Which non-ferrous metal also relies heavily on passivation via titanium dioxide ($\text{TiO}_2$) formation?
Answer & Explanation
Correct Answer: C
Titanium naturally passivates almost instantaneously in the presence of oxygen by forming a stable, robust titanium dioxide ($\text{TiO}_2$) film that gives it exceptional corrosion resistance.
Titanium naturally passivates almost instantaneously in the presence of oxygen by forming a stable, robust titanium dioxide ($\text{TiO}_2$) film that gives it exceptional corrosion resistance.
16. Why is thorough rinsing required immediately after the acid immersion step during passivation?
Answer & Explanation
Correct Answer: B
Rinsing stops the acid reaction and clears away spent acid and metallic salts, preventing localized etching, spotting, or interference with subsequent oxide formation.
Rinsing stops the acid reaction and clears away spent acid and metallic salts, preventing localized etching, spotting, or interference with subsequent oxide formation.
17. In industrial applications, why is passivation particularly critical for components used in pharmaceutical and food processing equipment?
Answer & Explanation
Correct Answer: B
Food and pharmaceutical safety require strict absence of rust or metallic leeching; passivation prevents corrosion products from contaminating sensitive consumer goods.
Food and pharmaceutical safety require strict absence of rust or metallic leeching; passivation prevents corrosion products from contaminating sensitive consumer goods.
18. What is "flash rusting" in the context of stainless steel passivation?
Answer & Explanation
Correct Answer: B
Flash rusting is a common defect where residual iron or poor water quality leads to light surface oxidation right after passivation, requiring re-cleaning.
Flash rusting is a common defect where residual iron or poor water quality leads to light surface oxidation right after passivation, requiring re-cleaning.
19. How does the passive current density ($i_{pass}$) compare to the active corrosion current density?
Answer & Explanation
Correct Answer: A
Once passivated, the passive current density drops drastically (often by several orders of magnitude compared to active dissolution), reflecting the protective barrier's efficacy.
Once passivated, the passive current density drops drastically (often by several orders of magnitude compared to active dissolution), reflecting the protective barrier's efficacy.
20. Which test is commonly used to inspect the effectiveness of passivation by detecting free iron or iron particles on stainless steel?
Answer & Explanation
Correct Answer: B
The copper sulfate test (or alternative ferricyanide/humidity tests) detects minute quantities of free iron on passivated surfaces by precipitating copper where iron is exposed.
The copper sulfate test (or alternative ferricyanide/humidity tests) detects minute quantities of free iron on passivated surfaces by precipitating copper where iron is exposed.
21. Can mechanical finishing methods like grinding or machining damage an existing passive layer?
Answer & Explanation
Correct Answer: B
Machining, grinding, and welding break down the uniform passive film and embed free iron particles into the surface, which is why passivation is performed as a final finishing step.
Machining, grinding, and welding break down the uniform passive film and embed free iron particles into the surface, which is why passivation is performed as a final finishing step.
22. What role do chelating agents play in modern citric acid passivation formulations?
Answer & Explanation
Correct Answer: A
Chelating agents in citric acid solutions securely bind with dissolved free iron ions, holding them in solution and stopping them from settling back onto the clean stainless steel surface.
Chelating agents in citric acid solutions securely bind with dissolved free iron ions, holding them in solution and stopping them from settling back onto the clean stainless steel surface.