Q1. Well water contained in an AISI 304 SS pipe network for chemical treatment and posterior distribution by aerial water pipes to different locations in a food manufacturing facility was analysed. The AISI 304 SS pipe network had an approximate 300 m length, 154 mm diameter and 2.0 mm wall thickness. The AISI 304 SS pipes were welded using a Tungsten Inert Gas (TIG) welding procedure. In-situ weld quality assurance was tested by pressurized WW, and no pinhole leakage was detected. After the pipe network remained out of service for 6 months at room temperature containing stagnant, pressurized well water, pitting corrosion was observed on the weld joints and the base material by the presence of multiple pinhole leaks. (a) Discuss, with chemical equations if relevant, TWO corrosion events that may have caused the pitting corrosion. (b) What practical steps could have been taken to mitigate for this potential outcome? (c) Identify corrosion inhibition methods to mitigate for the corrosion events identified in Q1(a). Q2. Explain the mechanism of weld decay in some stainless steels, why it might occur and how it can be avoided. (c) The Pourbaix diagram shows the predominant form in which an element will exist under a given set of environmental conditions. Pourbaix diagrams display thermodynamically preferred species. (i) For a given metal what equation is used to construct a Pourbaix diagram? (ii) Three regions are identified in the Pourbaix diagram, what are these regions and what information do they provide? 3 (a) Discuss, in your own words, THREE conditions where corrosion is initiated via electrochemical reactions. (b) A “salt-water drop” experiment was performed on a sheet of clean steel in the laboratory. (i) What is the name of the indicator used? (ii) What did the indicator contain? (iii) Draw a sketch of the results, explaining what colours could be seen and what caused them. (iv) What type of corrosion cell does it demonstrate? 2 (a) Explain why brittle fracture of materials is called “unstable” and ductile fracture is called “stable”. (b) Discuss the effect of crack length on crack propagation and explain why the fracture stress is determined by the largest pre-existing crack length. Assume crack propagation in brittle materials is modelled by the equation below. = 2ES1/2 c a (c) A steel has a ductile-brittle transition temperature of 0\deg C. Briefly explain whether this steel should be used for construction of a pipeline in a North Sea oil field. (d) A tensile load of 30.3 N is applied to a plastic component (with a cross sectional area of 23.2 mm2). Determine the maximum allowable surface crack length to avoid fracture of the component. Assume the component has a Young’s modulus of 3.0 GPa, a surface energy of 0.35 J m-2 and an elastic strain energy of 0.15 J m-2. (e) A 316L steel specimen has a plane strain fracture toughness of 40MPa m1/2 and is subjected to a stress of 750 MPa. Inspection detected that the largest surface crack has a length of 0.5 mm. Show whether this specimen will fracture under such a stress condition if the specimen has a dimensionless parameter Y = 1.25 and the plain strain fracture toughness is defined by the equation below. KIc = Y c a 5 (a) Predict the possibility of galvanic corrosion in sea water for the following coupled pairs of alloys and metals. Using the Galvanic Series Sheet please provide a reason for your answer. (i) Aluminium alloys bolts and 80-20 Copper-nickel, CDA 710 sheet. (ii) Plain carbon steel pipe and manganese bronze sprinkler. (iii) Low alloy steel tube and stainless steel 410 sealing ring. (iv) 4130 Alloy steel subsea crossover and stainless steel 430 API gasket. (v) Graphite gasket and Inconel 625 flange. (b) In your own words explain the mode of action of an anodic and a cathodic inhibitor. A cell, constructed from tin (anode) and a hydrogen electrode (cathode), is immersed in 0.2 M SnCl2 solution. Plot the cell potential as a function of pH (pH: 1, 3, 5, and 7). (Cell Notation: Sn∣Sn2+,Cl- ,H+∣H2 ∣Pt). 13Cr martensitic stainless steel is used in environments at risk of high uniform corrosion rates. Table Q4(b) illustrates the chemical composition of super 13Cr stainless steel. Table Q4(b): Chemical composition of super 13Cr stainless steel (wt.%) Element C Cr Ni Mo Si Mn P S Fe Content 0.027 12.87 5.32 2.2 0.18 0.47 0.022 0.004 Bal. Super 13Cr stainless steel was employed as the drill collar of a bottom hole assembly within a seawater environment and 10% O2 and 90% CO2. As corrosion can occur when conditions are suitable, describe at least TWO corrosion events that may occur. Describe TWO corrosion events that may occur with this material should the environment become sour. The potential of a zinc electrode on which Zn deposits from a 0.1 M ZnSO4 solution is -1.1 V vs. SCE. (a) Determine the zinc deposition voltage on the hydrogen scale. (4)
Q1. Well water contained in an AISI 304 SS pipe network for chemical treatment and posterior distribution by aerial water pipes to different locations in a food manufacturing facility was analysed. The AISI 304 SS pipe network had an approximate 300 m length, 154 mm diameter and 2.0 mm wall thickness. The AISI 304 SS pipes were welded using a Tungsten Inert Gas (TIG) welding procedure. In-situ weld quality assurance was tested by pressurized WW, and no pinhole leakage was detected. After the pipe network remained out of service for 6 months at room temperature containing stagnant, pressurized well water, pitting corrosion was observed on the weld joints and the base material by the presence of multiple pinhole leaks.
(a) Discuss, with chemical equations if relevant, TWO corrosion events that may have caused the pitting corrosion.
(b) What practical steps could have been taken to mitigate for this potential outcome?
(c) Identify corrosion inhibition methods to mitigate for the corrosion events identified in Q1(a).
Q2. Explain the
(c) The Pourbaix diagram shows the predominant form in which an element will exist under a given set of environmental conditions. Pourbaix diagrams display
(i) For a given metal what equation is used to construct a Pourbaix
diagram?
(ii) Three regions are identified in the Pourbaix diagram, what are these
regions and what information do they provide?
3 (a) Discuss, in your own words, THREE conditions where corrosion is initiated
via electrochemical reactions.
(b) A “salt-water drop” experiment was performed on a sheet of clean steel in the laboratory.
(i) What is the name of the indicator used?
(ii) What did the indicator contain?
(iii) Draw a sketch of the results, explaining what colours could be seen
and what caused them.
(iv) What type of corrosion cell does it demonstrate?
2 (a) Explain why brittle fracture of materials is called “unstable” and ductile
fracture is called “stable”.
(b) Discuss the effect of crack length on crack propagation and explain why the fracture stress is determined by the largest pre-existing crack length. Assume crack propagation in brittle materials is modelled by the equation
below.
= 2ES1/2 c a
(c) A steel has a ductile-brittle transition temperature of 0\deg C. Briefly explain whether this steel should be used for construction of a pipeline in a North
Sea oil field.
(d) A tensile load of 30.3 N is applied to a plastic component (with a cross sectional area of 23.2 mm2). Determine the maximum allowable surface crack length to avoid fracture of the component. Assume the component has a Young’s modulus of 3.0 GPa, a surface energy of 0.35 J m-2 and an
elastic strain energy of 0.15 J m-2.
(e) A 316L steel specimen has a plane strain fracture toughness of 40MPa m1/2 and is subjected to a stress of 750 MPa. Inspection detected that the largest surface crack has a length of 0.5 mm. Show whether this specimen will fracture under such a stress condition if the specimen has a
dimensionless parameter Y = 1.25 and the plain strain fracture toughness
is defined by the equation below.
KIc = Y c a
5 (a) Predict the possibility of galvanic corrosion in sea water for the following coupled pairs of alloys and metals. Using the Galvanic Series Sheet please provide a reason for your answer.
(i) Aluminium alloys bolts and 80-20 Copper-nickel, CDA 710 sheet.
(ii) Plain carbon steel pipe and manganese bronze sprinkler.
(iii) Low alloy steel tube and stainless steel 410 sealing ring.
(iv) 4130 Alloy steel subsea crossover and stainless steel 430 API gasket.
(v) Graphite gasket and Inconel 625 flange.
(b) In your own words explain the mode of action of an anodic and a cathodic
inhibitor.
A cell, constructed from tin (anode) and a hydrogen electrode (cathode),
is immersed in 0.2 M SnCl2 solution. Plot the cell potential as a function of
pH (pH: 1, 3, 5, and 7). (Cell Notation: Sn∣Sn2+,Cl-
,H+∣H2 ∣Pt).
13Cr martensitic stainless steel is used in environments at risk of high
uniform corrosion rates. Table Q4(b) illustrates the chemical composition
of super 13Cr stainless steel.
Table Q4(b): Chemical composition of super 13Cr stainless steel (wt.%)
Element C Cr Ni Mo Si Mn P S Fe
Content 0.027 12.87 5.32 2.2 0.18 0.47 0.022 0.004 Bal.
Super 13Cr stainless steel was employed as the drill collar of a bottom hole
assembly within a seawater environment and 10% O2 and 90% CO2. As
corrosion can occur when conditions are suitable, describe at least TWO
corrosion events that may occur.
Describe TWO corrosion events that may occur with this material should
the environment become sour.
The potential of a zinc electrode on which Zn deposits from a 0.1 M ZnSO4
solution is -1.1 V vs. SCE.
(a) Determine the zinc deposition voltage on the hydrogen scale. (4)
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