Category: Corrosion Engineering Objective Questions

Corrosion Engineering Multiple Choice Questions on “ Corrosion Prevention – Inhibitors”.

1. An inhibitor is a chemical substance that added in small concentrations to an environment, to decrease the corrosion rate.
a) True
b) False
Answer: a
Clarification: An inhibitor is a chemical substance that added in small concentrations to an environment, to decrease the corrosion rate. It is also regarded as a retarding catalyst.

2. Which of the following is/are the types of inhibitors?
a) Adsorption inhibitors
b) Scavengers
c) Vapor-phase inhibitors
d) Adsorption inhibitors, scavengers, and vapor-phase inhibitors
Answer: d
Clarification: Following are the types of inhibitors:
i. Adsorption inhibitors
ii. Scavengers
iii. Vapor-phase inhibitors
iv. Hydrogen-evolution poisons
v. Oxidizers.

3. Which of the following is an example of adsorption-type inhibitors?
a) Arsenic
b) Sodium sulfite
c) Organic amines
d) Chromates
Answer: c
Clarification: Adsorption-type inhibitors are the type of inhibitors that adsorb on the surface of cathode or anode and alter its surface. Organic amines are examples of adsorption-type inhibitors.

4. Which of the following inhibitors are effective in acid solutions?
a) Oxidizers
b) Hydrogen-evolution poisons
c) Vapor-phase inhibitors
d) Oxidizers and hydrogen-evolution poisons
Answer: b
Clarification: Cathodic reduction of hydrogen ions to a hydrogen atom is the main driving force for the metallic dissolution in many acidic environments. It can be reduced or minimized with hydrogen-evolution poisons.

5. Which of the following is/are the examples of Hydrogen-evolution poisons?
a) Arsenic and Antimony
b) Sodium sulfite and hydrazine
c) Chromates and ferric salts
d) Organic amines
Answer: a
Clarification: Hydrogen-evolution poisons are the inhibitors that retard the rate of evolution of hydrogen gas in an acidic environment. Arsenic and Antimony are examples of hydrogen-evolution poisons.

6. Which of the following is/are the examples of scavengers?
a) Arsenic and Antimony
b) Chromates and ferric salts
c) Sodium sulfite and hydrazine
d) Organic amines
Answer: c
Clarification: Scavengers are the type of inhibitors that removes corrosive reagents from the solution. Sodium sulfite and hydrazine are examples of scavengers. 2Na₂SO₃+O₂==>2Na₂SO₄ is the scavenging action of sodium sulfite.

7. Oxidizers such as chromates, nitrates, and ferric salts inhibit the corrosion of metal and alloys that demonstrate active-passive transition.
a) False
b) True
Answer: b
Clarification: Oxidizers such as chromates, nitrates, and ferric are one type of inhibitors that inhibit the corrosion of metal and alloys that demonstrate active-passive transition. Active-passive transition metals include iron and stainless steel.

8. Which of the following is/are the characteristics of the vapor phase inhibitor?
a) It has a very high vapor pressure
b) Inhibit the atmospheric corrosion without being contact with the metal surface
c) Effective in closed spaces
d) It has very high vapor pressure, inhibit atmosphere corrosion without being contact with the metal surface and very effective in closed spaces
Answer: d
Clarification: Vapor-phase inhibitors are very similar to organic adsorption-type inhibitors. Characteristics of vapor-phase inhibitors are:
i. It has a very high vapor pressure
ii. Inhibit the atmospheric corrosion without being contact with the metal surface
iii. Effective in closed spaces.

9. Which of the following phenomenon in which the inhibiting effect is sometimes greater than that would be achieved by either of the two or substances alone?
a) Galvanic effect
b) Cathodic protection
c) Synergistic effect
d) Inhibiting effect
Answer: c
Clarification: When two or more inhibiting substances are added to a corrosive system, the inhibiting effect is sometimes greater than that achieved by either of the two or more substances alone. This is called a synergistic effect.

10. Which of the following is/are the limitations of inhibitors?
a) It contaminates the environment
b) Inhibitors are toxic
c) Its efficiency decreases with time and temperature
d) It contaminates the environment, toxic in nature, and its efficiency decreases with time and temperature.
Answer: d
Clarification: Limitations of inhibitors:
i. It contaminates the environment
ii. Inhibitors are toxic in nature
iii. Its efficiency decreases with time and temperature
iv. It should not be used directly or indirectly in the preparation of food.

11. Which of the following inhibitor is used for aluminum in the hydrogen peroxide environment?
a) 2% boron trifluoride
b) Alkali metal nitrates
c) Sodium silicate
d) Benzoic acid
Answer: b
Clarification: Inhibitors are the chemical compounds that decrease the corrosion rate of metal in a specific environment. Alkali metal nitrates are used as inhibitors for the aluminum metal in the hydrogen peroxide environment.

12. Which of the following inhibitor is used for steel in 80% pure sulfuric acid?
a) 2% boron trifluoride
b) Sodium silicate
c) Alkali metal nitrates
d) Benzoic acid
Answer: a
Clarification: Inhibitors are very specific to a particular metal, environment, and the concentration of an environment. 2% boron trifluoride inhibitor is used for steel in 80% pure sulfuric acid.

13. Which of the following inhibitor is used for steel in the water?
a) Sodium silicate
b) Alkali metal nitrates
c) Benzoic acid
d) 2% boron trifluoride
Answer: c
Clarification: Inhibitors are the chemical compounds that decrease the corrosion rate of metal in a specific environment. Benzoic acid is used as an inhibitor for steel in water.

14. Which of the following phenomenon that occurs on metal surface during inhibition?
a) Adsorption only
b) Adsorption and phase layers
c) Phase layers only
d) Neither adsorption nor phase layers
Answer: b
Clarification: Inhibitors are the chemical compounds that decrease the corrosion rate of metal in a specific environment. Inhibition can be caused by both adsorption and phase layers on the metal surface.

Corrosion Engineering Multiple Choice Questions on “Modern Theory Principles – Thermodynamics – 1”.

1. Which of the following is/are the applications of modern electrode kinetic principles?
a) Anodic protection
b) Noble metal alloying
c) Methods to measure corrosion rates rapidly
d) Anodic protection, noble metal alloying and methods to measure corrosion rates rapidly
Answer: d
Clarification: Modern electrode kinetics deals with electrochemical corrosion of aqueous solution and it results in the applications of

Anodic protection

Noble metal alloying

Methods to measure corrosion rates rapidly.

2. What is the direct measure of work capacity or electrical energy available from a system?
a) Change in enthalpy
b) Change in Gibbs free energy
c) Reaction time
d) Concentration of reactants
Answer: b
Clarification: Change is Gibbs free energy is the difference of free energies of the reaction products and reactants. And it is a direct measure of the work capacity or maximum electrical energy available in from a system.

3. If the transition of a system from one state to another state resulted in negative free energy change then it is known as a spontaneous reaction.
a) True
b) False
Answer: a
Clarification: If the transition of a system from one state to another state resulted in negative free energy change then it is known as a spontaneous reaction. It means the free energy of products is less than the free energy of reactants.

4. Which of the following is true, if the transition of a system results in positive Gibbs free energy change?
a) It is a spontaneous reaction
b) Free energy of products is less
c) It is a non-spontaneous reaction
d) Free energy of reactants is more
Answer: c
Clarification: If the transition of a system from one state to another state resulted in positive free energy change then it is known as a non-spontaneous reaction. It means the free energy of reactants is less than the free energy of products. It requires external heat or force to proceeds with the reaction.

5. Which of the following is a spontaneous reaction?
a) Production of ammonia
b) Corrosion of metals
c) Photosynthesis reaction
d) Generation of oxygen from water
Answer: b
Clarification: Spontaneous reaction is a reaction in which the reactants convert into products without any supply of external energy. Oxidation of metal atoms into metal ions due to the instability of metallic atoms is known as the corrosion of metals.

6. The change in Gibbs free energy is a path function and it is independent of the state of the system.
a) False
b) True
Answer: a
Clarification: The change in Gibbs free energy is a state function and it is independent of the path followed by the system to reach that state. It is denoted by ΔG (delta G).

7. Which of the following formula is used to calculate the Gibbs free energy of an electrochemical reaction?
a) ΔG=-nFE⁰_cell
b) ΔG=+nFE⁰_cell
c) ΔG=-nFE_cell
d) ΔG=+nFE_cell
Answer: c
Clarification: ΔG=-nFE_cell where n is the electrons participated in the reaction, F is the faraday constant (96500 coulombs), E_cell is the overall cell potential and E⁰_cell is the standard cell potential.

8. What is the change in Gibb’s free energy of a Cu-Zn system with a cell potential of 1.10V?
a) -11.13 KJ/mol
b) +21.23 KJ/mol
c) +11.13 KJ/mol
d) -21.23 KJ/mol
Answer: d
Clarification: Given, Cell potential = 1.10V
n=number of electrons involved in reaction = 2
F=96500 coulombs
We know, ΔG=-nFE_cell
ΔG=-2*96500*1.10
ΔG=-21.23 KJ/mol

9. Which of the following is/are correct regarding the given figure?

a) It is a reversible cell with unit activity
b) Copper and zinc are cathode and anode respectively
c) It is a reversible cell of Cu-Zn system with standard cell potential of 1.10V, copper and zinc are cathode and anode respectively
d) The standard cell potential is 1.10V
Answer: c
Clarification: Given figure represents a Cu-Zn system with standard (25°C, 1atm) cell potential of 1.10V. Copper and Zinc are cathode and anode respectively. Whereas two half-cells are separated by a porous membrane.

10. What are the cathodic and anodic reactions respectively of a given Cu-Zn system?

a) Cu==>Cu⁺²+2e and Zn⁺²+2e==>Zn
b) Cu⁺²+2e==>Cu and Zn==>Zn⁺²+2e
c) Cu==>Cu⁺²+2e and 2Zn⁺¹+2e==>2Zn
d) Cu⁺²+2e==>Cu and 2Zn==>2Zn⁺¹+2e
Answer: b
Clarification: As the standard hydrogen potential of copper (+0.34V) is relatively higher than standard hydrogen potential of zinc (-0.76V) it acts as cathode and zinc act as anode.
Cathodic reaction is Cu⁺²+2e==>Cu
Anodic reaction is Zn==>Zn⁺²+2e

11. What is the overall standard cell potential of a given Cu-Fe system?

a) +0.78V
b) -0.14V
c) -0.78V
d) +0.14
Answer: a
Clarification: In Cu-Fe system, copper (E⁰=+0.34V) acts as cathode and iron will acts as anode (E⁰=-0.44V). Standard cell potential of a system can be calculated using E⁰_cell=E⁰_cathode–E⁰_anode. Therefore, overall standard cell potential of given Cu-Fe system is 0.34 – (-0.44) = 0.78V.

12. Which of the following metal is used as a solid interface for hydrogen gas in the standard hydrogen electrode?
a) Copper
b) Zinc
c) Gold
d) Platinized platinum
Answer: d
Clarification: Platinized platinum metal is used as the solid interface for hydrogen gas in the standard hydrogen electrode. It is due to the inertness of the metal and ease with which electron transfer occurs on its surface. Platinized platinum is black platinum-coated platinum.

13. What is the Nernst equation for chemical reaction nA + mB ==> pC + qD at temperature T?
a) E=E₀+2.3(RT/nF)log{([A]ⁿ+[B]^m)/([C]^p+[D]^q)}
b) E=2.3(RT/nF)log{([A]ⁿ+[B]^m)/([C]^p+[D]^q)}
c) E=E₀+2.3(RT/nF)log{([n]^A+[m]^B)/([p]^C+ [q]^D)}
d) E=2.3(RT/nF)log{([n]^A+[m]^B)/([p]^C+ [q]^D)}
Answer: a
Clarification: Nernst equation for given chemical reaction nA + mB ==> pC + qD is E=E₀+2.3(RT/nF)log{([A]ⁿ+[B]^m)/([C]^p+[D]^q)} where A and B are reactants, C and D are products and m,n,p and q are the number of moles respectively.

Advanced Corrosion Engineering Questions and Answers on “Corrosion Rate Expression”.

1. What is meant by MPY in corrosion?
a) Mils penetration per year
b) Meter penetration per year
c) Milli meter penetration per year
d) Micro meter penetration per year
Answer: a
Clarification: MPY in corrosion is abbreviated as Mils penetration per year (1 mil = 1/1000 inch). It is usually used in the U.S. measurement system in order to estimate the corrosion rate of pitting corrosion.

2. Which of the following expression is used to calculate MPY?
a) MPY=(543*W)/DAT
b) MPY=(534*W)/DAT
c) MPY=W/DAT
d) MPY=DAT/(534*W)
Answer: b
Clarification: W = weight loss in milligrams, D=Density of a specimen (g/cm³), A=Area of specimen(sq.in), T=time of exposure in hours and 534 is a conversion factor. MPY means mils penetration per year I.e. milli inches penetration per year.

3. MDD in corrosion is abbreviated as ______
a) milligrams/sq. decimeter/decade
b) millimeters/sq. decimeter/day
c) milligrams/sq. decimeter/day
d) microns/decimeter/decade
Answer: c
Clarification: MDD in corrosion known as milligrams/sq.decimeter/day. It is the common unit to measure weight loss of metal per unit area. It is usually used for uniform and galvanic corrosion.

4. MPY is used to measure weight loss per unit area.
a) True
b) False
Answer: b
Clarification: Usually mils per year is used to measure the penetration rate. Penetration is the average depth of metal loss in the specimen. MPY is usually used for pitting corrosion. MDD (milligrams/sq. Decimeter/day) is used to measure the weight loss per unit area.

5. _____ is/are used to measure the extent of pitting corrosion.
a) MPY
b) MDD
c) Both MPY and MDD
d) MPK
Answer: a
Clarification: Pitting corrosion involves loss of metal at the selected spots on the surface of a metal. It penetrates into various depths based on the concentration of corrosive medium and nature of metal. MPY measures penetration range.

6. Which of the following law is used to derive the corrosion rate expression?
a) Newton’s law
b) Henry’s law
c) Raoult’s law
d) Faraday’s law
Answer: d
Clarification: Faraday’s law is used in order to derive the corrosion rate expression [W/At=I*a_m/n*F]. W/At denotes the corrosion rate, I=current density, a_m=molecular weight, n=number of electrons transferred, F=faraday constant (96500).

7. Which following expression is used to find the corrosion rate from current density of a corroding specimen?
a) W/At = [(I × a_m) / (n × F)]
b) W/At = [(I × a_m) / (n × d × F)]
c) W/At = [534 × (I × a_m) / (n × F)]
d) W/At = [(I × a_m) / (n × F)] and W/At = [(I × a_m) / (d × n × F)]
Answer: d
Clarification: W/At denotes the corrosion rate, I= current density, am=molecular weight, n=number of electrons transferred, F=faraday constant (96500), d=density of a specimen. [(I*a_m) / (n*F)] is used to calculate the MDD whereas the [(I*a_m) / (n*d*F)] is used to calculate the MPY.

8. Rate of oxidation is equals to the rate of reduction in corrosion.
a) True
b) False
Answer: a
Clarification: It is the second statement in the mixed potential theory that the number of electrons generated and consumed are same. Anode produces the same number of electrons that require for the cathode.

9. Iron is corroding at a current density of 1.69*10^-4 amp/cm². What would be the corrosion rate in MDD?
a) 773
b) 723
c) 423
d) 473
Answer: c
Clarification: We know MDD = W/At = (I*a_m) / (n*F). We should convert the given unit values in milligrams/sq. Decimeter/day.
Given, I = 1.69*10^-4 amp/cm²
Atomic weight of iron = a_m = 55.86 g/mol
n = number of electrons = 2 [Fe ==> Fe⁺² +2e^–]
F = faraday’s constant = 96500 coulombs
= (1.96×10^-4coulombs/sec*cm²) × (55.86) ÷ (2×96500 coulombs)
= [(1.96×10^-4)*(60*60*24)*(100)*(55.86 *10³)] / [2*96500]
= 422.53 MDD.

10. Iron is corroding at a current density of 1.69*10^-4 amp/cm². What would be the corrosion rate in MPY?
a) 423
b) 77
c) 473
d) 97
Answer: b
Clarification: We know MDD = W/At = (I*a_m) / (d*n*F). We should convert the given unit values in milli inches per year.
Given, I = 1.69*10^-4 amp/cm²
Atomic weight of iron = a_m = 55.86 g/mol
n = number of electrons = 2 [Fe ==> Fe⁺² +2e^–]
F = faraday’s constant = 96500 coulombs
d=density of iron = 7.875 g/cm³
= [(1.96×10^-4 coulombs/sec*cm²) × (55.86)] ÷ [(7.874 g/cm³×2×96500 coulombs)]
= [(1.96×10^-4coulombs/sec*cm²) × (55.86)] ÷ [(7.874 g/cm²*cm×2×96500 coulombs)]
= [(1.96×10^-4 1/sec) × (55.86)] ÷ [(7.874 g/cm ×2×96500)]
= [1.69×10^-4 × (60*60*24*365) × (55.86) × 10³] / [2 × 96500 × 7.875 × 2.54]
= 77.117 MPY.

11. _____ is used to measure the extent of uniform corrosion.
a) MDD
b) MPY
c) Both MPY and MDD
d) MPE
Answer: a
Clarification: Uniform corrosion is the corrosion that occurs throughout the entire exposed surface of a metal and it is uniform in depth of corrosion layer. Weight loss method I.e. MDD is used to know the extent of uniform corrosion.

12. Corrosion rate of aluminum is _________ when compared with corrosion of mild steel in open oxygenated atmosphere.
a) small
b) medium
c) large
d) very small
Answer: d
Clarification: In oxygenated atmosphere aluminum has high corrosion resistant than the mild steel due to its oxide layer formation. Hence the corrosion rate of aluminum is very small when compared to the corrosion rate of mild steel.

13. ___________ will occur, if current pass from an electrode to electrolyte.
a) Oxidation
b) Reduction
c) Oxidation and reduction
d) Anion formation
Answer: a
Clarification: The direction of current flow is exactly opposite to the movement of electrons. We know electrons starts at the anode and travels towards cathode whereas current will flow from anode to electrolyte. This results in oxidation or corrosion on that electrode.

14. Oxidizing impurities in electrolyte increases the corrosion rate.
a) True
b) False
Answer: a
Clarification: Mixed potential theory states that any electrochemical reaction can be divided into two or more partial reactions (oxidation and reduction reaction). Presence of oxidizing impurities such as Fe⁺³ increases the number of reduction reactions. It correspondingly increases the oxidation rate or corrosion rate.

15. What are the uses of corrosion rate estimation of materials in daily life?
a) To predict the life time of a component
b) To compare the corrosive-resistant of materials
c) To increases the corrosion rate
d) To predict the life time and to compare the corrosive resistant of materials
Answer: d
Clarification: Corrosion rate expression is used to quantify the corrosion process. This helps us to predict the life time of a component and to compare with different materials. This also helps us to change the process variables in order to decrease the corrosion rate.

To practice advanced questions and answers on all areas of Corrosion Engineering,

Corrosion Engineering Multiple Choice Questions on “Eight Forms – Erosion Corrosion”.

1. Erosion corrosion accelerates the rate of attack on a metal because of relative movement between corrosive fluid and the metal surface.
a) True
b) False
Answer: a
Clarification: Erosion corrosion is one of the types of corrosion that accelerates the rate of deterioration or attack on a metal because of the relative movement of corrosive fluid and the metal surface.

2. Which of the following type of directional patterns usually exhibit by erosion-corrosion?
a) Grooves
b) Waves
c) Rounded holes
d) Grooves, waves and rounded holes
Answer: d
Clarification: Erosion corrosion results in wear or abrasion of the surface of the metal. It is characterized in appearance by grooves, gullies, waves, rounded holes, and valleys.

3. Which of the following type of corrosion is depicted in the given figure?

a) Uniform corrosion
b) Erosion corrosion
c) Galvanic corrosion
d) Selective leaching
Answer: b
Clarification: Erosion corrosion is depicted in the given figure. Erosion corrosion accelerates the rate of deterioration or attack on a metal because of the relative movement of corrosive fluid and the metal surface.

4. Erosion corrosion results when the protective oxide films are damaged and the metal or alloy are attacked at a rapid rate.
a) True
b) False
Answer: a
Clarification: As the relative movement of the corrosive solution and the surface of the metal increases the intensity of erosion-corrosion increases. At higher velocities, it results in the damage or worn out of oxide film and it attacks the metal rapidly.

5. Which of the following metals is/are susceptible to erosion-corrosion?
a) Aluminum
b) Lead
c) Stainless steel
d) Aluminum, lead and stainless steel
Answer: d
Clarification: Aluminum, lead, copper, and stainless steel are the metals that are susceptible to erosion-corrosion. Because these metals mainly depend on their oxide film for its corrosive resistance.

6. Which of the following type of equipment is/are highly prone to erosion-corrosion?
a) In piping arrangements at elbows and bends
b) Measuring devices such as orifice and turbine blades
c) In piping arrangements at straight lines
d) In piping arrangements at elbows and bends and measuring devices such as orifice and turbine blades
Answer: d
Clarification: Erosion corrosion can be affected by the relative velocity of the corrosive solution and metal surface and along with impingement velocity at the edges and turnings. In piping arrangements at elbows and bends and measuring devices such as orifice and turbine blades are the equipment that is prone to erosion-corrosion.

7. Which of the following is/are the characteristics of oxide film that exhibits better resistance to erosion-corrosion?
a) Hard
b) Dense
c) Adherent
d) Hard, dense and adherent
Answer: d
Clarification: Erosion corrosion is the type of corrosion that results in mechanical damage of the metal due to the high velocities of corrosive solutions. And a hard, dense, and adherent oxide film can exhibit better resistance to erosion-corrosion.

8. Which of the following characteristics of titanium, makes it resistant to erosion-corrosion?
a) Stability of titanium oxide
b) Instability of titanium oxide
c) The low reactivity of titanium
d) The low reactivity of titanium and instability of titanium oxide
Answer: a
Clarification: Titanium is a reactive metal but is resistant to erosion-corrosion in many environments because of the stability of the titanium oxide (TiO₂) film. It reforms its oxide layer within a fraction of seconds after wearing out.

9. Which of the following metal is resistant to seawater, chlorides, and fuming nitric acid?
a) Aluminum
b) Lead
c) Titanium
d) Stainless steel
Answer: d
Clarification: Titanium is highly resistant to seawater, chlorides, and fuming nitric acid. It is also resistant to erosion-corrosion because of its high reactivity and stability of oxide film.

10. Which of the following environmental factor plays an important role in erosion-corrosion?
a) Velocity
b) Corrosive concentration
c) pH of the environment
d) Relative humidity of the atmosphere
Answer: a
Clarification: Erosion corrosion accelerates the rate of attack on a metal because of relative movement between corrosive fluid and the metal surface. It increases as the velocity of the corrosive solution increases.

11. Define critical velocity regarding erosion-corrosion.
a) Velocity at which erosion-corrosion starts
b) Velocity at which erosion-corrosion stops
c) Velocity after which erosion-corrosion increases exponentially
d) Velocity after which erosion-corrosion decreases exponentially
Answer: c
Clarification: Critical velocity is the velocity after which the erosion-corrosion increases exponentially. Case 1 (velocity less than critical velocity):
Erosion corrosion increases linearly with increase with velocity.
Case 2 (velocity greater than critical velocity):
Erosion corrosion increases exponentially with increase with velocity.

12. Which of the following metal shows a decrease in corrosion attack with an increase in velocity in fuming nitric acid?
a) Aluminum
b) Carbon steel
c) Stainless steel 347
d) Copper
Answer: c
Clarification: Under stagnant conditions, stainless steel 347 in nitric acid is attacked autocatalytically because of the formation of nitrous acid as a cathodic reaction product. An increase in velocity sweeps away the corrosive nitrous acid in the environment.

13. Which of the following is/are the preventions of erosion-corrosion?
a) Selection of better resistant materials
b) Alteration of the environment
c) Applying protective coatings
d) Selection of better resistant materials, alteration of the environment and applying protective coatings
Answer: d
Clarification: Methods for minimizing or preventing erosion-corrosion
i. Selection of better resistant materials
ii. Design of equipment
iii. Alteration of environment
iv. Applying protective coatings
v. Cathodic protection of equipment.

14. Which of the following is/are the aspects of design to minimize erosion-corrosion?
a) Increasing pipe diameter
b) Avoiding sharp edges and turnings
c) Readily replaceable impingement plates should be inserted
d) Increasing pipe diameter, avoiding sharp edges and turnings and readily replaceable impingement plates should be inserted
Answer: d
Clarification: Design is an important factor to minimize or combat erosion-corrosion. The design includes increasing pipe diameter, avoiding sharp edges and turnings, readily replaceable impingement plates should be inserted, and maintaining certain angles to minimize impingement area.

15. Which of the following is/are the aspects of alteration of the environment to minimize erosion-corrosion?
a) Deaeration of corrosive solution
b) Decrease the temperature
c) Use of Settling and filtration to remove suspended solids
d) Deaeration of the corrosive solution, decrease the temperature and use of settling and filtration to remove suspended solids
Answer: d
Clarification: Alteration of the environment is also one of the methods to minimize erosion-corrosion. This includes the deaeration of corrosive solution, decrease the temperature and use of settling and filtration to remove suspended solids.

Corrosion Engineering Multiple Choice Questions on “Cathodic and Anodic Protection – 1”.

1. Which of the following is/are the common electrochemical reactions that occur during corrosion?
a) M==>M⁺+ne
b) 2H⁺+2e=>H₂
c) O₂+4H⁺+4e==>H₂O
d) M==>M⁺+ne, 2H⁺+2e=>H₂ and O₂+4H⁺+4e==>H₂O
Answer: d
Clarification: The common electrochemical reactions that occur during corrosion are:

M==>M⁺+ne (metallic reduction or anodic reaction)

2H⁺+2e=>H₂ (hydrogen evolution or cathodic reaction)

O₂+4H⁺+4e==>H₂O (oxygen reduction or cathodic reaction)

2. Cathodic protection is achieved by supplying electrons to the metal structure to be protected.
a) True
b) False
Answer: a
Clarification: Cathodic protection is achieved by supplying electrons to the metal structure to be protected. It can be done by using impressed current in the range of microamperes per unit area of the metallic structure.

3. Which of the following cases in which the metal structure is protected?
a) Current passes from metal to electrolyte
b) Current enters it from the electrolyte
c) Electrical contact with dissimilar metals
d) Connecting the positive terminal of the power supply
Answer: b
Clarification: Metallic structure can be protected when the impressed current is arranged to enter it from the electrolyte. Whereas current passes from metal to electrolyte, electrical contact with dissimilar metals, and connecting the positive terminal of the power supply results in accelerated corrosion.

4. Which of the following is/are the types of cathodic protection?
a) External impressed current supply
b) Sacrificial anode
c) External impressed current supply and sacrificial anode
d) Neither external impressed current supply nor sacrificial anode
Answer: c
Clarification: Cathodic protection is achieved by supplying electrons to the metal structure to be protected. Types of cathodic protection are i. External impressed current supply ii. Sacrificial anode.

5. Which of the following terminal of the power supply should be connected to the metal to be protected?
a) Negative terminal
b) Positive terminal
c) Both positive and negative terminals
d) Either positive or negative terminal
Answer: a
Clarification: As the conventional electrical theory states that current flows from (+) to (-), the structure is protected if the negative terminal of the power supply is connected metallic structure.

6. Which of the following method is depicted in the given figure?

a) Anodic protection
b) Impressed current supply
c) Sacrificial anode
d) Anodic and cathodic protection
Answer: b
Clarification: Impressed current supply is a method of cathodic protection that is achieved by supplying electrons to the metal structure to be protected. The given figure depicted is impressed current supply method of an underground tank.

7. Which of the following material is/are used as an impressed-currents anode?
a) Graphite
b) Platinized titanium
c) Silicon-iron
d) Graphite, platinized titanium, and silicon-iron
Answer: d
Clarification: Materials that can be used as an impressed-currents anode are:

Graphite

Platinized titanium

Silicon-iron

Steel.

8. Which of the following materials is/are used as backfill around the anode in impressed currents?
a) Coke breeze
b) Gypsum
c) Bentonite
d) Coke breeze, gypsum, and bentonite
Answer: d
Clarification: Coke breeze, gypsum, and bentonite are the materials that are used as backfill material around the anode. These materials improve the electric contact between the anode and surrounding soil.

9. Which of the following protection method is depicted in the given figure?

a) Impressed-current supply method
b) Anodic protection
c) Sacrificial anode
d) Anodic protection and sacrificial anode
Answer: c
Clarification: Sacrificial anode method is one the type of cathodic protection in which a metal (anode) corrodes preferentially than the metallic structure. Sacrificial anodes are inexpensive and easily available in nature.

10. Which of the metallic structure will require more impressed current density for cathodic protection?
a) Water heaters in hot freshwater
b) Underground pipelines
c) The storage tank of H₂SO₄
d) Reinforcement rods
Answer: c
Clarification: Impressed method current is an external supply of power to provide electrons to metallic structure that needs to be protected. The storage tank of H₂SO₄ requires the highest current density per sq. feet due to the high corrosiveness of acid.

11. Which of the following reference electrode is used for cathodic-protection surveys?
a) Standard hydrogen electrode
b) Copper/copper sulfate reference electrode
c) Calomel electrode
d) Either calomel or standard hydrogen electrode
Answer: b
Clarification: Copper/copper sulfate reference electrode is used for cathodic protection surveys. The potential of a structure is determined with a high-resistance voltmeter. This electrode has the advantages of low cost, good accuracy, and ruggedness.

12. Which of the following is/are the typical sacrificial anodes?
a) Magnesium only
b) Zinc and magnesium
c) Aluminum-tin only
d) Magnesium, Zinc, and aluminum-tin
Answer: d
Clarification: Sacrificial anodes are metals that have a very high negative potential, which provides high current output. Magnesium, zinc, and aluminum-tin are typical examples of sacrificial anodes.

13. Which of the following is an impressed current anode that is efficient in a marine environment?
a) Platinized titanium
b) Silicon-iron
c) Scrap steel
d) Graphite
Answer: a
Clarification: Platinized titanium is an inert anode that is both efficient and expensive in nature. It is finding increased applications in marine environments.

14. Which of the following statements is/are correct about stray current?
a) It usually encountered in cathodic-protection systems
b) It refers to extraneous direct current in the earth
c) Acceleration corrosion will occur at the point where the current enters the soil
d) Stray current refers to extraneous direct current in the earth that usually encountered in cathodic protection systems and accelerated corrosion will occur at the point where current enters the soil
Answers: d
Clarification: Stray current refers to extraneous direct current in the earth that usually encountered in cathodic protection systems and accelerated corrosion will occur at the point where current enters the soil.

15. Which of the following is/are the preventions of stray current effect in an underground channel?
a) Use of bus conductor
b) Rearrangement of anodes
c) Use of bus conductor and rearrangement of anodes
d) Neither use of bus conductor nor rearrangements of anodes
Answer: c
Clarification: Stray current effect caused due to the densely buried underground pipelines in the vicinity of the cathodic protection system. It can be prevented by the use of bus conductors and rearrangements of anodes.

Corrosion Engineering Objective Questions & Answers on “Modern Theory Principles – Thermodynamics – 2”.

1. Which of the following is a cathode and anode respectively in the Ag-Fe system?
a) Iron (Fe) and Silver (Ag)
b) Silver (Ag) and Iron (Fe)
c) Gold (Ag) and Iron (Fe)
d) Iron (Fe) and Gold (Ag)
Answer: b
Clarification: In the Ag-Fe system, Silver (Ag) (+0.80V) acts as a cathode as the standard hydrogen potential is relatively higher than the Iron (Fe) (-0.44V) and Iron acts as anode due to relatively less standard hydrogen potential.

2. An electrochemical reaction is always a combination of oxidation half-cell and a reduction of half-cell.
a) True
b) False
Answer: a
Clarification: Corrosion is usually an electrochemical process which is a combination of oxidation half-cell and reduction half-cell. Oxidation and reduction reactions are simultaneous in nature and it is also called Redox reactions.

3. What is the cathodic and anodic reactions respectively of Cu-Ag system?
a) Ag==>Ag⁺²+2e and Cu⁺²+2e==>Cu
b) Cu==>Cu⁺²+2e and Ag==>Ag⁺²+2e
c) 2Ag⁺¹+2e==>2Ag and Cu==>Cu⁺²+2e
d) 2Ag==>2Ag⁺¹+2e and Cu⁺²+2e==>Cu
Answer: c
Clarification: Standard hydrogen potential of silver (Ag) is +0.80V and copper is +0.34V.
Cathodic reaction: 2Ag⁺¹+2e==>2Ag
Anodic reaction: Cu==>Cu⁺²+2e

4. Which of the following type of reaction is shown in the given figure?

a) Non-spontaneous reaction
b) Equilibrium reaction
c) Neither spontaneous nor non-spontaneous reaction
d) Spontaneous reaction
Answer: d
Clarification: Spontaneous reaction is a reaction in which the change in Gibbs free energy is negative. It means the free energy of reactants is more than the free energy of products.

5. Which of the following diagrams shows the stability of metals over a range of pH and potential of a system?
a) Phase diagrams
b) Pourbaix diagrams
c) Phase diagrams and Pourbaix diagrams
d) Colling curves
Answer: b
Clarification: Pourbaix diagrams show the stability of metal over a range of pH and potential of a system. It also includes native zone, passive zone, and corrosive zone. It is also called as potential-pH diagrams.

6. What is depicted in the given figure?

a) Phase diagram of the Fe-C system
b) Pourbaix diagram for Fe-O₂ system
c) Pourbaix diagram for Fe-H₂O system
d) Colling curve pure iron
Answer: c
Clarification: Pourbaix diagram is a diagram that shows the stability of species of a metal over a range of pH and potential of a system. Given figures represents the pourbaix diagram of the Fe-H₂O system.

7. Which of the following zone of given Pourbaix diagram indicates the immune zone?

a) Zone 1
b) Zone 2
c) Zone 3
d) Zone 4
Answer: a
Clarification: The Immune zone is the zone of the pourbaix diagram in which the stability of native metal in zero oxidation state is high. It is the zone in which no corrosion or passivation takes place.

8. Which of the following zone(s) of given Pourbaix diagrams indicate a passive zone?

a) Zone 1 and 2
b) Zone 2 and 3
c) Zone 2, 3 and 6
d) Zone 4 and 5
Answer: d
Clarification: Passive zone is the zone in the pourbaix diagram in which the passivation of metal takes place. It is the operating zone in various industries to minimize corrosion and to increase the efficiency of the process.

9. Which of the following is/are the applications of Pourbaix diagrams?
a) Predicting the spontaneous direction of reactions
b) To reduce corrosion attack
c) To reduce corrosion attack, to predict the direction of spontaneous reaction and to estimate the composition of corrosion product
d) To estimate the composition of corrosion product
Answer: c
Clarification: Applications of Pourbaix diagrams:
i. Predicting the spontaneous direction of reactions
ii. To reduce corrosion attack
iii. To estimate the composition of corrosion product.

10. Which of the following is/are the limitations of Pourbaix diagrams?
a) Rate of reaction is can’t be predicted
b) It neglects the impurities of working conditions
c) Stability of metallic species is predicted
d) Rate of reaction is not predicted and it neglects the impurities of working conditions
Answer: d
Clarification: Limitations of Pourbaix diagrams:
i. Rate of reaction can’t be predicted
ii. It neglects the impurities of working conditions
iii. Alloying elements of alloys are also neglected.