This set of Corrosion Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Eight Forms – Intergranular Corrosion – 1”.
1. Which of the following is/are more reactive in intergranular corrosion?
a) Grain
b) Grain boundary
c) Both grain and grain boundary
d) Neither grain nor grain boundary
View Answer
Explanation: Intergranular corrosion occurs when grain interfaces or grain boundaries are very reactive than the face of grain. Whereas in the uniform attack, grains are more reactive than the grain boundaries.
2. What is Intergranular corrosion?
a) Localized attack at the grain boundaries
b) Localized attack at the grain faces
c) Uniform attack on grains
d) Localized attack at and adjacent to the grain boundaries
View Answer
Explanation: Intergranular corrosion means corrosion in between two or more grains. Intergranular corrosion is a localized attack at and adjacent to the grain boundaries.
3. Which of the following is/are the causes of intergranular corrosion?
a) Impurities at the grain boundaries
b) Enrichment of one of the alloying elements at the grain boundary
c) Depletion of one of the alloying elements at the grain boundary
d) Impurities at the grain boundaries, enrichment, and depletion of one of the alloying elements at the grain boundary
View Answer
Explanation: Intergranular corrosion is the corrosion occurs between two or more grains. It causes due to the presence of a differential concentration of alloying elements at the grains and grain boundaries. And the causes for this are as follows I. Presence of impurities at the grain boundaries
II. Enrichment of one of the alloying elements
III. Depletion of one of the alloying elements
4. Which of the following alloys is/are prone to intergranular corrosion?
a) Iron in aluminum
b) Zinc in brass
c) 18-8 Stainless steel
d) Iron in aluminum, zinc in brass and 18-8 stainless steel
View Answer
Explanation: All the given alloys are prone to intergranular corrosion. Iron in aluminum and zinc in brass are susceptible to this corrosion, due to the segregation of iron and zinc respectively at the grain boundaries. Whereas 18-8 Stainless steel is susceptible due to the depletion of chromium at the grain boundaries.
5. Numerous failures of 18-8 Stainless steel have occurred in the temperature range of 950 to 1450°F
a) True
b) False
View Answer
Explanation: Numerous failures of 18-8 stainless steel have occurred in the temperature range of 950 to 1450°F. When these steels are heated in the range of 950 to 1450°F they become sensitized or susceptible to intergranular corrosion due to the depletion of chromium at the grain boundary.
6. What is the molecular formula of chromium carbide formed due to the depletion of chromium in 18-8 stainless steel?
a) Cr3C
b) Cr5C3
c) CrC
d) Cr23C6
View Answer
Explanation: Intergranular corrosion in 18-8 stainless steel is due to the depletion of chromium content at the grain boundary. This results in the formation of chromium carbide (Cr23C6) at the grain boundary. Chromium depleted zone is highly prone to corrosion and it is known as intergranular corrosion.
7. Which of the following areas is/are corroded in the intergranular corrosion of stainless steel?
a) Grain only
b) Grain boundary only
c) Chromium depleted zone near the grain boundary
d) Both grain and grain boundary
View Answer
Explanation: !8-8 stainless steel is susceptible to intergranular corrosion due to sensitization in the temperature of 950 to 1450°F. At this stage, chromium reacts with carbon near the grain boundary and results in the formation of the chromium depleted zone. This chromium depleted zone is prone to intergranular corrosion.
8. What is the minimum percent of chromium is needed to make stainless steel?
a) 2%
b) 20%
c) 1%
d) 11%
View Answer
Explanation: Stainless steel is a family of iron-based alloys that contain a minimum of 11% chromium. Chromium imparts corrosive resistance to steel. There are 4 types of stainless steel.
They are i. Austenitic stainless steels
ii. Martensitic stainless steels
iii. Ferritic stainless steels
iv. Duplex stainless steels
9. Which of the following characteristics of the impoverished area, results in a rapid corrosion attack?
a) Less corrosion resistance
b) The high content of chromium
c) The large unfavorable area ratio
d) Less corrosion resistance and large unfavorable area ratio
View Answer
Explanation: Rapid corrosion attack in impoverished or chromium depleted zone is due to
i. Less corrosion resistance (As chromium% is decreased)
ii. Two dissimilar metal compositions are in contact (Galvanic effect)
iii. Large unfavorable area ratio is present (Area effect)
10. Which of the following corrosion type is depicted in the given figure?
a) Intergranular corrosion
b) Galvanic corrosion
c) Crevice corrosion
d) Pitting corrosion
View Answer
Explanation: Intergranular corrosion is the corrosion occurs between two or more grains. It causes due to the presence of a differential concentration of alloying elements at the grains and grain boundaries. Stainless steels are highly prone to this intergranular corrosion.
11. What is the structure of chromium carbide that precipitates at the grain boundaries?
a) Acicular structure
b) Leaflike structure
c) Lamellar structure
d) No specific structure
View Answer
Explanation: Mahla and Nielsen of Du Pont, using the electron microscope, have shown that the carbides form on the envelope around the grains in a leaflike structure.
12. Which of the following term is associated with intergranular corrosion in welded structures?
a) Intergranular weld
b) Weld failure
c) Weld decay
d) Weld corrosion
View Answer
Explanation: The intergranular corrosion associated with the welded structures and metals is known as weld decay. The depletion of alloying takes place adjacent to the weld bead as it exposed to high temperature (sensitization temperature).
13. Which of the following is used to visualize the heat flow and temperatures during welding?
a) Weld decay
b) Intergranular corrosion
c) Tablecloth analogy
d) Weld decay and intergranular corrosion
View Answer
Explanation: Tablecloth analogy of heat flow and temperatures is used during welding. To visualize a mountain like a block with the rise and fall of each stripe represents the rise and fall of temperature in a welded plate.
14. Electric arc welding is used more than the gas welding for stainless steel to reduces the chances of weld decay.
a) True
b) False
View Answer
Explanation: Welding time and the peak temperature of welding are the two parameters of weld decay. Electric arc welding produces more intense heat in a shorter time whereas in gas welding it is less intense and requires more time for welding.
15. Which of the following corrosives that induce intergranular corrosion in austenitic stainless steel?
a) Acetic acid
b) Acetic acid+salicylic acid
c) Ammonium nitrate
d) Acetic acid, acetic acid+salicylic acid, ammonium nitrate
View Answer
Explanation: Stainless steel is highly susceptible to intergranular corrosion. These corrosives such as acetic acid, acetic acid+salicylic acid, ammonium nitrate, etc. Induces the intergranular corrosion in austenitic stainless steel.
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