Category: Corrosion Engineering Objective Questions

Corrosion Engineering Interview Questions on “Eight Forms – Galvanic Corrosion – 2”.

1. The reason for the noble nature of titanium is that it cathodically polarizes readily in seawater.
a) False
b) True

Answer: b
Clarification: Titanium is one of the noble metals in the galvanic series and the reason is that it cathodically polarizes readily in seawater. It replaces many materials as best corrosive resistant metal in the seawater applications and high-velocity corrosive solutions.

2. Corrosion is greater near the seashore than in a dry rural atmosphere.
a) True
b) False

Answer: a
Clarification: Corrosion occurs due to the presence of oxygen and moisture (electrolyte). The electrical conductivity of the electrolyte is one of the factors of corrosion. Corrosion increases with an increase in electrical conductivity of the electrolyte. Moisture with salt content relatively has high electrical conductivity, hence corrosion is greater at the seashore.

3. Define the distance effect regarding galvanic corrosion?
a) Corrosion occurs only at the junction of two metals
b) Corrosion occurs only at the edges of two metals
c) Corrosion decreases with increase in distance from the junction
d) Corrosion increases with increase in distance from the junction

Answer: c
Clarification: The galvanic effect is the potential difference formed between two or more metals at its junction. It decreases on moving away from the junction. Hence corrosion decreases with an increase in distance from the junction.

4. Define area effect regarding galvanic corrosion?
a) cathodic area * anodic area
b) anodic area/cathodic area
c) cathodic area/anodic area
d) cathodic area + anodic area

Answer: c
Clarification: It is the ratio of cathodic to anodic areas. An unfavorable area ratio with a large cathodic area and small anodic area results in high current density at the anode. The greater the current density at an anodic area the greater the corrosion rate.

5. Which of the following is/are the preventions of galvanic corrosion?
a) Avoid the unfavorable area effect of a small anode and large cathode
b) Select the close combination of metals in galvanic series
c) Select the far combination of metals in galvanic series
d) Avoid the unfavorable area effect of a small anode and large cathode and select the close combination of metals in galvanic series

Answer: d
Clarification: In industrial applications avoiding a dissimilar metal combination is impossible. But it can be minimized by avoiding the unfavorable area effect, selecting the close combination of metals. Proper material selection and regular inspection are other preventive measures.

6. Which of the following principle is utilized in the concept of cathodic protection?
a) Principle of galvanic corrosion
b) Principle of crevice corrosion
c) Principle of pitting corrosion
d) Principle of selective leaching

Answer: a
Clarification: Cathodic protection is a protection of a base structure metal by making it as a cathode. It can be protected by the use of sacrificial anode. The order of metals in the galvanic series is the basis for cathodic protection.

7. Which of the following is an example of cathodic protection?
a) Electroplating of steel
b) Galvanization of steel
c) Magnesium connected to steel pipes
d) Electroplating of steel, galvanization of steel and magnesium connected to steel

Answer: d
Clarification: Electroplating and galvanization are cathodic protections of steel based on the galvanic effect. If steel under zinc or other metal coating exposed to corrosive atmosphere, zinc corrodes preferentially than steel. Magnesium acts as a sacrificial anode when it is connected to underground steel pipes.

8. Which of the following are the limitations of the EMF series?
a) Available for metals and alloys
b) Limited to pure metals only
c) Measured only at standard conditions
d) Limited to pure metals and measured only at standard conditions

Answer: d
Clarification: EMF series is a list of pure metals arranged according to their electrode potential value measured against SHE. It is measured at standard conditions such as 25°C, 1atm pressure, and 1M concentration of hydrogen ions.

9. Which of the following chemical is responsible for the stains on silver?
a) Hydrogen peroxide
b) Hydrogen sulfide
c) Sulfur dioxide
d) Sulfur trioxide

Answer: b
Clarification: The stains on sliver is usually due to the presence of hydrogen sulfide. It can be removed by the application of galvanic corrosion. It can be done with silver-aluminum galvanic attachment in the presence of baking soda.

Corrosion Engineering Multiple Choice Questions on “ Environments – Atmospheric Corrosion”.

1. Which of the following is/are the classifications of atmospheres regarding atmospheric corrosion?
a) Industrial
b) Marine
c) Rural
d) Industrial, marine and rural
Answer: d
Clarification: Atmospheric corrosion can be defined as the corrosion of metals in surrounding corrosive environment. It can be classified into three types namely industrial, marine and rural atmospheres.

2. Which of the following is/are the primary causes of atmospheric corrosion?
a) Moisture
b) Oxygen
c) Moisture, oxygen and sulfur contaminants
d) Sulfur contaminants only
Answer: c
Clarification: The primary causes of atmospheric corrosion is moisture, oxygen and sulfur contaminants. Absence of either moisture or oxygen in at least form will result in no corrosion.

3. Corrosion of steel on the seacoast is 400 to 500 times is more corrosive than in a desert area.
a) True
b) False
Answer: a
Clarification: Corrosion of steel on the seacoast is 400 to 500 times is more corrosive than in a desert area. This due to the contamination of atmosphere with sodium chloride and other impurities at the seacoast.

4. How many times that industrial corrosion is more corrosive than rural atmospheres?
a) 10 to 20
b) 50 to 100
c) 30 to 50
d) 100 to 150
Answer: b
Clarification: Industrial corrosion is 50 to 100 times more corrosive than desert areas. It is due to the presence of Sulphur burned gases such as SO₂, SO₃ and these results in the formation of sulfurous and sulfuric acids in the presence of moisture.

5. Which of the following is/are the reasons for more corrosiveness of industrial atmosphere than rural atmosphere?
a) Formation of sulfurous and sulfuric acid
b) Ejection of corrosive ammoniacal solutions
c) Formation of sulfurous and sulfuric acid and ejection of corrosive ammoniacal solutions
d) High levels of carbon dioxide
Answer: c
Clarification: Formation of sulfurous and sulfuric acid from sulfur burning gases and ejection of corrosive ammoniacal solutions are the reasons of industrial corrosion. Whereas carbon dioxide is not corrosive in nature.

6. Which of the following alloying elements increase resistance of steel to atmospheric corrosion?
a) Copper
b) Nickel
c) Chromium
d) Copper, nickel and chromium
Answer: d
Clarification: Steel is the most widely used metal for various metallic structures in all atmospheres. The resistance of steel can be improved by alloying elements such as copper, nickel and chromium. This is due to formation of stable, thick and adherent protective film.

7. Which of the following metals are widely used for atmospheric applications?
a) Copper and lead
b) Copper, lead, aluminum and galvanized steel
c) Galvanized steel only
d) Galvanized steel and copper
Answer: b
Clarification: Copper, lead, aluminum and galvanized steel are widely used for atmospheric corrosion applications. Improving the corrosion resistance of steel with small alloy additions is now commonly used along with weight-saving applications.

8. Which of the following alloys is also called as weathering steels?
a) Low-alloy steels used for atmospheric applications
b) High-alloy steels used for atmospheric applications
c) Low-alloy copper alloys
d) Low-alloy aluminum alloys
Answer: a
Clarification: Low-alloy steels used for atmospheric applications is also known as weathering steels. It is also used in weight-saving applications and to increase durability of paint coatings.

9. Sheltered corrosion is the type of corrosion where moisture condenses or accumulates and does not dry out for long periods of time.
a) False
b) True
Answer: a
Clarification: Sheltered corrosion is the type of corrosion where moisture condenses or accumulates and does not dry out for long periods of time. Corrosion on inside of an automobile door and inside of a partially filled automobile gasoline tank are the examples of sheltered corrosion.

10. Which of the following phenomenon that results in rapid corrosion on the inside of large storage tanks occurs when moisture-laden salt air is drawn in?
a) Sheltering
b) Breathing
c) Exhalation
d) Crevice
Answer: b
Clarification: Breathing results in rapid corrosion on the inside of large storage tanks occurs when moisture-laden salt air is drawn in. This can be prevented by protective coatings for steels and blanketing with nitrogen or dry air inside the tank.

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.

Corrosion Engineering Multiple Choice Questions on “Materials – Thermoplastics”.

1. Which of the following is/are the classification of plastics based on the thermal behavior?
a) Thermoplastics
b) Thermosetting
c) Neither thermoplastics nor thermosetting
d) Thermoplastics and thermosetting
Answer: d
Clarification: Plastics can be categorized based on various parameters such as monomer units, temperature behavior, and structure. Thermoplastics and Thermosetters are the two types of plastics based on thermal behavior.

2. What is meant by thermoplastic?
a) It softens on heating and returns to original hardness on cooling
b) It hardens on heating and returns to original hardness on cooling
c) It hardens on heating and retains hardness on cooling
d) It softens on heating and retains hardness on cooling
Answer: a
Clarification: Thermoplastics is one of the type of plastics which softens on heating and returns to original hardness on cooling. Fluorocarbons, polyether, and nickel are examples of thermoplastics.

3. Which of the following thermoplastic is corrosion-resistant to all environments up to 550°F?
a) Polyvinyl chloride
b) Polystyrene
c) Teflon
d) Polyethylene
Answer: c
Clarification: Teflon is one of the fluorocarbons which is regarded as noble metals of plastics. It is corrosion-resistant to practically all environments up to 550°F. The monomer of Teflon is denoted as CF₂=CF₂.

4. Which of the following is the chemical name of nylon (6, 6)?
a) Polyethylene terephthalate
b) Hexamethylene adipamide
c) Polytetrafluoroethylene
d) Polyvinyl fluoride
Answer: b
Clarification: The chemical name of nylon (6.6) is hexamethylene adipamide. It is derived from the monomers of nylon that are hexamethylenediamine and adipic acid.

5. Which of the following thermoplastics is used for making eye lenses?
a) Polytetrafluoroethylene
b) Polyethylene terephthalate
c) Polymethyl methacrylate
d) Polyvinyl chloride
Answer: c
Clarification: Polymethyl methacrylate is the polymer of many methyl methacrylate monomeric units. It is used for making eye lenses as it is transparent to light. It is usually denoted as PMMA.

6. Which of the following is/are the monomers of nylon (6,6)?
a) Hexamethylenediamine
b) Adipic acid
c) Both Hexamethylenediamine and adipic acid
d) Neither Hexamethylenediamine nor adipic acid
Answer: c
Clarification: Hexamethylenediamine and adipic acid are the two monomers of nylon (6,6). The chemical formula of hexamethylenediamine and adipic acid is NH₂-CH₂-CH₂-CH₂-CH₂-CH₂-CH₂-NH₂ and HOOC-CH₂-CH₂-CH₂-COOH respectively.

7. Which of the following is/are the examples of thermoplastics?
a) Polypropylene
b) Polystyrene
c) Polyvinyl chloride
d) Polypropylene, polystyrene and polyvinyl chloride
Answer: d
Clarification: Thermoplastics are the type of plastics that soften on heating and return to their original hardness when cooled. Polypropylene, polystyrene, polyethylene, fluorocarbons, and polyvinyl chloride are examples of thermoplastics.

8. Which of the following thermoplastic monomer is depicted in the given figure?

a) Polystyrene
b) Teflon
c) Polyvinyl chloride
d) Polyurethane
Answer: a
Clarification: Styrene is depicted in the given figure and it is the monomer of polystyrene. It is an amorphous, colorless, rigid, and hard plastics. The monomer of polyvinyl chloride and Teflon is CH₂=CH-Cl and CF₂=CF₂ respectively.

9. Which of the following is the monomer of Polyvinyl chloride (PVC)?
a) CF₂=CF₂
b) CH₂=CH-Cl
c) COOH-CH₂-CH₂-CH₂–CH₂COOH
d) C₆H₅-CH=CH₂
Answer: b
Clarification: Vinyl chloride (CH₂=CH-Cl) is the monomeric unit of polyvinyl chloride. It is used for piping and cable insulation extensively. CF₂=CF₂, COOH-CH₂-CH₂-CH₂–CH₂COOH and C₆H₅-CH=CH₂ are the monomers of Teflon, nylon, and polystyrene respectively.

10. Degradation of plastics is due to swelling, loss of mechanical properties, softening, hardening, and spalling.
a) True
b) False
Answer: a
Clarification: Corrosion of plastics is not the same as that of metals. Corrosion of metals results in the dissolution of metal ions whereas corrosion or degradation of plastics is due to swelling, loss of mechanical properties, softening, hardening, and spalling.

Basic Corrosion Engineering Questions and Answers on “Environments – Sea Water and Fresh Water”.

1. Which of the following is/are true regarding seawater?
a) It contains 3.4% salt
b) It has a pH around 8
c) It is a good electrolyte with high electrical conductivity
d) Seawater is a good electrolyte with high electrical conductivity with 3.4% salt and pH around 8
Answer: d
Clarification: Seawater is a good electrolyte with high electrical conductivity with 3.4% salt and pH around 8. It can cause galvanic and crevice corrosion to various metals.

2. Which of the following is/are the factors that affect corrosion in seawater?
a) Temperature and velocity
b) Oxygen content and biological organisms
c) Temperature, velocity, oxygen content and biological organisms
d) Temperature, velocity and oxygen content
Answer: c
Clarification: The factors that affect the corrosion of materials in seawater are:
i. Temperature
ii. Velocity
iii. Oxygen content
iv. Biological organisms.

3. Which of the following region of the seacoast environment have high corrosion based on the given figure?

a) Quiet seawater
b) Splash region
c) High tide area
d) Mud line
Answer: b
Clarification: Greatest corrosion occurs in the splash region due to alternate wetting and drying and also aeration. We know oxygen and moisture are the primary requirements for corrosion of metals.

4. Which of the following regions of the seacoast environment shows the effect of the pitting of metals and alloys?
a) Quiet seawater
b) High tide region
c) Low tide region
d) Splash region
Answer: a
Clarification: Pitting is extremely localized corrosion which results in holes or cavities. Quiet seawater in the region of the seacoast environment that shows the effect of the pitting of metals and alloys. An increase in the velocity of a corrosive solution decreases the pitting tendency of a metal.

5. Corrosion by seawater at greater depth is usually decreased because of the lower temperature.
a) False
b) True
Answer: b
Clarification: Corrosion by seawater at greater depth is usually decreased because of the lower temperature. It decreases by 40°F for one-mile depth.

6. Which of the following impurities contaminates the brackish water due to tidal action in rivers and bays near the ocean?
a) Chlorides
b) Bromides
c) Hydrides
d) Metal oxides
Answer: a
Clarification: Chloride ions are the impurities that contaminate the brackish water due to the tidal action in rivers and bays near the ocean. The source of these chloride ions is sodium chloride in seawater.

7. Which of the following metals have high resistance to crevices in quiet seawater?
a) Hastelloy C
b) Titanium
c) Hastelloy C and Titanium
d) Nickel-copper alloy
Answer: c
Clarification: Hastelloy C and titanium have high resistance to crevices in quiet seawater. Due to this, these metals are used for various corrosive seawater applications. Whereas nickel-copper alloy shows less resistance.

8. Which of the following alloy has high cavitation resistance that usually used for ship propellers and pumps impellers in seawater?
a) Titanium
b) Stellite
c) 17-7 Stainless steel
d) Nickel aluminum bronze
Answer: b
Clarification: Stellite is a cobalt-chromium alloy with high cavitation resistance that usually used for ship propellers and pump impellers in seawater. Composition of Stellite is 27–32% chromium, 4–6% tungsten, 0.9–1.4% carbon, with additions of nickel, iron, silicon, manganese, and molybdenum and cobalt as balance.

9. Which of the following factors that affect the corrosivity in freshwater?
a) Oxygen content
b) Hardness
c) Chloride content
d) Oxygen content, hardness and chloride content
Answer: d
Clarification: Factors that affect the corrosivity in freshwater are:
i. Oxygen content
ii. Hardness
iii. Chloride and sulfur content.

10. Which of the following type of water is more corrosive?
a) Hard water
b) Hard and soft water
c) Soft water
d) High-purity water
Answer: c
Clarification: In hard water, carbonates often deposit on the metal surface and protect it from corrosion. Whereas in soft water protective deposits do not form. Hence, soft water is more corrosive than hard water and high-purity water.

11. Which of the following metals are widely used for handing freshwater?
a) Cast iron
b) Stainless steel
c) Galvanized steel
d) Cast iron, stainless steel, and galvanized steel
Answer: d
Clarification: Cast iron, stainless steel, steel, and galvanized steel are widely used for handling freshwater. Metals such as copper, brass, aluminum, monel, and cupronickels are also used with the factors of temperature, contamination, and longer life.

12. Which of the following corrosion defects that affect the reliability of tubing?
a) Selective weld metal attack
b) Improper pickling and heat treatment
c) Residual stresses, selective weld metal attack, improper pickling, and heat treatment
d) Selective weld metal attack and residual stresses
Answer: c
Clarification: Corrosion defects that affect the reliability of tubing are:
i. Inadequate alloying
ii. Selective weld metal attack
iii. Improper pickling
iv. Residual stresses
v. Improper heat treatment.

13. Corrosion decreases with an increase in the purity of water.
a) True
b) False
Answer: a
Clarification: Corrosion decreases with increasing purity of the water because of less solid and gases and increasing electrical resistance. Resistance is a measure of water purity.

14. Which of the following types of corrosion has been seen in stainless steels in high purity water containing oxygen?
a) Cracking of solution-quenched stainless steel only
b) Intergranular attack and cracking of solution-quenched steel
c) Intergranular attack only
d) Neither intergranular corrosion nor cracking of solution-quenched steel
Answer: b
Clarification: Intergranular corrosion and cracking of solution-quenched stainless steel and alloys have been observed in high-purity water containing oxygen.

15. Which of the following metals/alloys are used for handling high-purity water in nuclear applications?
a) Zirconium and its alloys
b) Stainless steels
c) Inconel and Incoloy
d) Zirconium and its alloys, stainless steels, Inconel and Incoloy
Answer: d
Clarification: Zirconium and its alloys, stainless steels, Inconel and Incoloy are the metals/alloys that are used for handling high-purity water in nuclear applications.