Waste Water Engineering Multiple Choice Questions on “Corrosion Control in Water Conduits”.
1. Corrosion is the deterioration of materials by chemical interaction with their environment.
a) True
b) False
Answer: a
Clarification: Corrosion is the deterioration of materials by chemical interaction with their environment. The term corrosion is sometimes also applied to the degradation of plastics, concrete and wood, but generally refers to metals.
2. Which is the most widely used material in water conduits?
a) Steel
b) Aluminium
c) Copper
d) Nickel
Answer: a
Clarification: The most widely used metal is iron (usually as steel). The term corrosion is sometimes also applied to the degradation of plastics, concrete and wood, but generally refers to metals.
3. A loss in mechanical strength is due to ______
a) Chemicals
b) Reduction of metal thickness
c) Water flow
d) Water velocity
Answer: b
Clarification: Reduction of metal thickness leading to loss of mechanical strength and structural failure or breakdown. When the metal is lost in localised zones so as to give a crack like structure, very considerable weakening may result from quite a small amount of metal loss.
4. The corroding piece of metal is described as a “mixed electrode”.
a) True
b) False
Answer: a
Clarification: The corroding piece of metal is described as a “mixed electrode” since simultaneous anodic and cathodic reactions are proceeding on its surface.
5. What is pH below which pH increases?
a) 7
b) 5
c) 4
d) 3
Answer: c
Clarification: In the range of 4-10 corrosion rate is fairly independent of pH, but it increases rapidly when the pH falls below 4.
6. Increase in __________ concentration usually gives an increase in corrosion rate.
a) Carbon
b) Oxygen
c) Nitrogen
d) Iron
Answer: b
Clarification: Increase in oxygen concentration usually gives an increase in corrosion rate. Corrosion under the deposits can lead to the possibility of pitting (severe attack in small spots).
7. Stainless steel is protected by _______
a) Iron oxide
b) Nitrous oxide
c) Chromium sesquioxide
d) Carbon monoxide
Answer: c
Clarification: Alloying the metal to produce a more corrosion resistant alloy, e.g. stainless steel, in which ordinary steel is alloyed with chromium and nickel. Stainless steel is protected by an invisibly thin, naturally formed film of chromium sesquioxide Cr2O3.
8. What is pH range for oxygen removal?
a) 3.5-4.5
b) 6.5-8.5
c) 10-12
d) 12-13.5
Answer: b
Clarification: By the removal of oxygen from water systems in the pH range 6.5-8.5 one of the components required for corrosion would be absent.
9. ______ are classified as dangerous inhibitors.
a) Cathodic inhibitors
b) Anodic inhibitors
c) Voltage
d) Current flow
Answer: b
Clarification: If an anodic inhibitor is not present at a concentration level sufficient to block off all the anodic sites, localised attack such as pitting corrosion can become a serious problem.
10. Which ions are used as cathodic inhibitors?
a) Ferric irons
b) Aluminium ions
c) Zinc ions
d) Nickel ions
Answer: c
Clarification: There are other cathodic reactions and additives that suppress these reactions called cathodic inhibitors. They function by reducing the available area for the cathodic reaction. This is often achieved by precipitating an insoluble species onto the cathodic sites. Zinc ions are used as cathodic inhibitors because of the precipitation of Zn(OH)2 at cathodic sites as a consequence of the localised high pH.
11. What is the reduced potential required to prevent the corrosion of steel?
a) 350 mV
b) 250 mV
c) 150 mV
d) 50 mV
Answer: a
Clarification: This process called cathodic protection and can only be applied if there is a suitable conducting medium such as earth or water through which a 9 [CORROSION&PROTECTION/BM] current can flow.
12. Galvanizing is the process of applying _______ over steel.
a) Carbon
b) Aluminium
c) Zinc
d) Nickel
Answer: c
Clarification: The process of applying zinc over steel is called “galvanizing” and has been used to protect iron and steel from rusting for over two hundred years.