Corrosion Engineering Multiple Choice Questions on “Eight Forms – Crevice Corrosion”.
1. Which of the following is/are the reasons for the cause of crevice corrosion?
a) The stagnant solution in the holes and gasket surfaces
b) The stagnant solution in the lap joints and surface deposits
c) Dissimilar metal contact
d) The stagnant solution in the holes, gasket surfaces, lap joints, and surface deposits
Answer: d
Clarification: Crevice corrosion is a type of corrosion in which dissolution of metal is due to the presence of stagnant solution in the small gaps (crevice) such as holes, gasket surfaces, lap joints, and surface deposits, etc.
2. Which of the following type of corrosion is localized in nature?
a) Crevice corrosion
b) Galvanic corrosion
c) Uniform corrosion
d) Inter-granular corrosion
Answer: a
Clarification: Crevice corrosion is localized corrosion because it occurs only at discrete sites on the metal surface. This occurs due to the presence of a stagnant solution in crevices (small spaces). This crevice corrodes rapidly by cathodically protecting adjacent surfaces.
3. Which of the following is the other name of crevice corrosion?
a) Filiform corrosion
b) Deposit or gasket corrosion
c) Two-metal corrosion
d) Surface corrosion
Answer: b
Clarification: Crevice corrosion occurs due to the presence of a stagnant solution in the crevices. These crevices are formed due to the corrosion deposits or gaps at the metal-gasket interface. Thus, it is also called as deposit or gasket corrosion.
4. Which of the following metal is more susceptible to crevice corrosion?
a) Titanium
b) Hastelloy C
c) Stainless steel-430
d) Titanium and stainless steel
Answer: c
Clarification: Stainless steel-430 is highly prone to crevice corrosion. The reasons are as follows:
1. Large critical current density
2. Wide active-passive transitions
3. Limited passive region
5. Which of the following combination results in crevice corrosion?
a) Metal and absorbent gaskets
b) Continuous weld metals
c) Metal and non-absorbent gaskets
d) Single metal piece
Answer: a
Clarification: Metal and absorbent gasket combination results in the crevice corrosion. Absorbent gaskets absorb water vapor from surrounding and make them stagnant at the metal–gasket interface. This results in crevice corrosion.
6. Which of the following characteristics of the crevice is a must to act as a corrosion site?
a) It should be wide enough to permit liquid entry
b) It should be narrow enough to maintain a stagnant zone
c) It should be wide enough to avoid a stagnant zone
d) It should be wide enough to permit liquid entry and narrow enough to maintain a stagnant zone
Answer: d
Clarification: Crevice is a small volume region formed between the interface of metal and gaskets. It acts as a corrosion site if is wide enough to permit liquid entry and narrow enough to maintain a stagnant zone.
7. What is the primary condition regarding the velocity of an electrolyte to form crevice corrosion?
a) The velocity of the electrolyte should be greater than 10cm/s
b) The velocity of the electrolyte should be less than 10cm/s
c) The velocity of the electrolyte should be equal to 10cm/s
d) The stagnant electrolytic solution should be present
Answer: d
Clarification: Crevice corrosion occurs due to the presence of stagnant electrolytic solution in the crevices. It acts at the corrosion sites and it results in the formation of localized corrosion. An increase in the velocity of the electrolyte decreases the extent of crevice corrosion.
8. The difference in metal ion or oxygen concentration between the crevice and its surroundings is the main reason for crevice corrosion.
a) True
b) False
Answer: a
Clarification: Crevice corrosion is mainly occurred due to the difference in metal ion or oxygen concentration between the crevice and its surroundings. This is due to the unavailability of oxygen in the crevice, this results in the dissolution of metal in a crevice by protecting adjacent surface cathodically.
9. Which of the following chemical reaction that usually occurs in the crevice?
a) M++H2O=MOH
b) M++Cl–=MCl(s)
c) M+Cl–+H2O=MOH(aq)+H+Cl–
d) 4OH–=O2+2H2O+4e–
Answer: c
Clarification: As the metallic ion concentration increases in the crevice, the migration of chloride ions takes place to neutralize that. After that, metal chlorides undergo hydrolysis and results in the formation of insoluble metal hydroxide and free acid.
10. Which of the following ions that accelerate the dissolution rate of metals or alloys in a crevice?
a) H+, OH–
b) H+, Cl–
c) OH–, Cl–
d) OH–, Cl– and H–
Answer: b
Clarification: H+ and Cl– ions accelerate the dissolution rate of metals and alloys in the crevice. These are produced by hydrolysis of metal chlorides. It accelerates corrosion because hydrogen ion decreases pH value (more acidic) and chloride ion attacks the metallic surface in the crevice.
11. Which of the following metal is highly resistant to crevice corrosion?
a) Stainless steel
b) Copper
c) Aluminum
d) Hastelloy C-276
Answer: d
Clarification: Hastelloy C-276 is the best crevice corrosion-resistant metal. The composition of Hastelloy C-276 is 15.5%Cr, 54.7%Ni, 15.5%Mo, 3.8%W, and 0.5%Mn. Whereas the other metals depend on oxide film for their corrosion resistance.
12. As the metal dissolution within the crevice increases, the rate of oxygen reduction on adjacent surfaces increases.
a) True
b) False
Answer: a
Clarification: Metallic dissolution in the crevice cathodically protects the external surface. Electrons transfer from M==>M+n+ne (in crevice) to compensate O2+H2O+4e==>4OH (adjacent surface).
13. What is the incubation period associated with a crevice attack?
a) 1 month – 10 months
b) 1 hour – 10 hours
c) 1 day – 10 days
d) 6 months – 1 year
Answer: d
Clarification: The incubation period is the time required to initiate the crevice corrosion on metal. It usually in the range of 6 months to 1 year. It depends on many factors such as the material used, corrosive medium, temperature, etc.
14. Metals or alloys that depend on oxide films for corrosive resistance are highly susceptible to crevice corrosion.
a) True
b) False
Answer: a
Clarification: Metals or alloys that depend on oxide films for corrosive resistance are highly susceptible to crevice corrosion. These passive films are destroyed by high concentrations of hydrogen and chloride ions, and the metallic dissolution increases rapidly.
15. Which of the following type of corrosion is depicted in the given figure?
a) Galvanic corrosion
b) Uniform corrosion
c) Crevice corrosion
d) Pitting corrosion
Answer: c
Clarification: Crevice corrosion is localized corrosion that occurs due to the formation of crevices (small volume gaps) on various metal or gasket junction. A crevice will act as a corrosion site when it is wide enough to permit liquid entry and narrow enough to maintain a stagnant zone.