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CEN810S - Corrosion Engineering 414 - 1st OPP - JUN 2023 |
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nAmlBIA unlVERSITY
OF SCIEnCE Ano TECHn OLOGY
FACULTY OF ENGINEERING AND THE BUILT ENVIRONMENT
DEPARTMENT OF CIVIL, MINING AND PROCESSENGINEERING
QUALIFICATION: BACHELOR OF ENGINEERING IN METALLURGY
QUALIFICATION CODE: 08BMET
LEVEL: 8
COURSECODE: CEN810S
COURSE NAME: Corrosion Engineering 414
SESSION: June 2023
DURATION: 2 HOURS
PAPER:1
MARKS: 70
EXAMINER(S)
FIRST OPPORTUNITY PAPER
Prof D Groot
MODERATOR:
Prof J van der Merwe, University of the Witwatersrand
INSTRUCTIONS
1. Answer all questions.
2. Read all the questions carefully before answering.
3. Marks for each questions are indicated at the end of each question.
4. Please ensure that your writing is legible, neat and presentable.
PERMISSIBLE MATERIALS
1. Examination paper.
2. Scientific calculator, non-programmable
THIS PAPER CONSISTS OF 5 PAGES (Including this front page)
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Question 1
Consider the situation where you are part of a design team, and you are responsible for corrosion issues. The
team is designing a chemical plant, and you are tasked to consider the following. A waste process solution
(temperature about 30 degrees) containing about 3% sodium chloride and saturated in gypsum (calcium
sulfate) has to be pumped from a reactor. Due to cost sensitivity, the team is suggesting hot dip galvanised
mild steel pipes and a centrifugal pump that has a ductile cast iron casing and impeller.
(a) What do you think are the important advantages (excluding cost) and disadvantages of these material
choices for the pipes and pump? Motivate your choices.
(3)
(b) Based on the supplied information, what would you consider as suitable metal choices for the pipes and
pump? Motivate your choices.
(3)
NOTE: See ALLgiven information attached to this paper.
Question 2
Calculate the concentration of zn• 2 ions required to stop the zinc corroding when immersed in a solution
containing 0.1 mol/1 FeCb. State the assumptions you made.
(5)
Question 3
Consider the corrosion of Fe in a hydrogen-saturated solution of HCI at an activity of 0.10 mol/1. The activity
of Fe•2 is 10-6 mol/1. The exchange current density for Fe/Fe+2 is 2x10-10 A/cm 2 with a Tafel slope of 30
mV/dec.
(a) Calculate the exchange equilibrium potentials of the hydrogen and iron redox reactions.
(1)
{b) Calculate the corrosion potential and current.
(5)
Question 4
Micro biologically induced corrosion, MIC, is one of several forms of corrosion.
(a) Describe in detail what is meant by MIC, and how it may be positively identified.
(3)
(b) Give an example of where MIC might occur.
(1)
(c) Discuss how MIC may be controlled.
(2)
Question 5
Corrosion may be controlled by suitable coats or coating systems. Describe the major types of coatings and
coating systems, and briefly discuss how they control corrosion.
(8]
Question 6
Sometimes corrosion occurs at elevated temperatures, where no moisture is directly involved. Briefly discuss
the differences between dry corrosion at elevated temperatures, and hot corrosion, and the results of these,
giving suitable examples of each.
(7)
Question 7
Pitting corrosion is often considered to be identical to that of crevice corrosion because the natures of the
corrosion processes that occur are almost identical. Discuss how these two types of corrosion are different.
(6)
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Question 8
The steel of a ship's hull is normally protected against corrosion by a combination of paint and sacrificial
anodes.
(a) Explain, in terms of the basic aqueous corrosion cell, how the corrosion of unprotected areas can occur.
State the electrochemical reactions that are expected.
(7]
(b) Draw a suitable schematic Evans diagram for the hull corrosion system.
(4]
(c) Is it feasible from a corrosion point of view to make the ship's hull of stainless steel, unpainted, but with
sacrificial anodes made of zinc? Explain your point of view.
(4]
Question 9
It is known that that the presence of stress promotes corrosion.
(a) Briefly discuss the two main types of corrosion promoted by stress. Describe each type, and indicate
what factors affect the time before failure occurs.
[9]
(b) Assume a component that failed by one of these mechanisms was given to you. If you looked at the
failure site on the component, how could you see which of the mechanisms caused the failure?
[2]
Given information
=i io[exp(arizF/RT) - exp((l-a)rizF/RT)
= E E0 - (0.059/n) log ([product]/ [reagent])
rJ =b log ( i/io)
= rJ aa + ba log I
= F 96 485 Coulomb/mo!
L1G= -nFE
aa =-2.303 (RT/(anF)) log io
ba = 2.303 (RT/(anF))
= = = = = E0 for cu+2/Cu 0.337 V; W/H2 0.000 V; Fe+2/Fe -0.440 V; zn+2/Zn -0.763 V; Ag+/Ag 0.799 V
For reaction 2W +
H2
Pt
1 M HCI
10·2
30
0.1 M NaOH
0.7 X 10"3
110
Au
1 MHCI
10·5
50
Ag
0.1 M HCI
0.15
100
100
Fe
0.26 M H2SO4 2 X 10·5
110
Cu
0.1 M HCI
2 X 10"6
120
Zn
0.5 M H2SO4
2 X 10·10
120
Pb
1 M HCI
2 X 10"12
120
For reaction Clz+
2CI"
Pt
lMHCI
5 X 10·3
110
130
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Galvanic series in seawater
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Nickel siln•r
90/10 copper nickel
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Stainless stcd - grade 430
Lead
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Nickel nluminium bronze
Nickel chr·omium alloy 600
Nickel 200
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Stainless sterl - grades 302,304,321 '-~3-'7
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Stainless steel - grades 316 & 317
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Graphite
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