Hakeem Babatunde PhD Research Proposal
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Research Proposal
Student:Abdulhakeem Babatunde
Date: 04/11/2012
Subject: PhD Corrosion and Protection (3 years).
Proposed research topic:Stress corrosion cracking of high strength aluminium alloys.
Background
Stress corrosion cracking (SCC) is the cracking induced on an engineering material from the
combined influence of tensile stress and specific corrosive environment [5]. The three causes of SCC
are stress, corrosive environments and susceptible microstructures[6]
. The phenomenon of SCC is the
same for all engineering materials, but for the purpose of this thesis, aluminium alloys are considered.
Corrosion is one of the commonest mechanism of oilfield component failure because of this, stainless
steels are employed to mitigate this problem because of their chromium content which produces a
protective layer, Cr2O3, on the component surface[8], thus inhibiting corrosion.
Carbon steels are also used in the oil industry but require some form of protection this is because it
produces Fe2O3 on the component surface which is not as protective as Cr2O3[8]
. Protective measures
are used to prolong the life span of carbon steels in service, they are coating of the surface, cathodic
protection and the dosing of an inhibitor. Carbon steels are employed in applications which require a
shorter lifetime with reduced capital expenditure.
Steels have been used over the years as oilfield accessories but Aluminium which is a lighter metal
and corrosion resistant has been rarely used for oilfield applications. Aluminium owes its excellent
corrosion resistance to the protective oxide film, Al2O3, which forms on its surface in oxygen
containing environments[8] and it is cheaper than steels, easier to design, machine and gradually
replacing steels in many applications [9].
The oil and gas industry requires significant tubular products such as drill pipes, casings, tubings and
risers which must be designed and manufactured to specific standards[1]
. These products must
accommodate present and future expectations of drilling deeper, long lasting and more efficient wells
that will maximize production [1].
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For years aluminium alloys (AA) have been widely used as construction materials in the aerospace,
automotive, marine and civil industries. However, AA tubulars has not been used in the petroleum
industry to a great extent[1]
.
Aluminium Drill Pipe (ADP) was used in the Former Soviet Union (FSU)
but were found to be proneto SCC because of chloride present in the oilfield environment
[1].
Aluminium alloys are prone to pitting in aggressive chloride environments and once pits are initiated
they undercut the surface and penetrate along the grain boundary consuming a grain and then diffuse
to an adjacent grain leading to diminishing of the structures mechanical integrity, thus increasing
susceptibility to failure[2]
. In SCC environments, cracking will occur along grain boundaries which is
assisted by the presence of aggressive specie[2]
.
The aim of this study is to determine which grade of aluminium will be suitable in oilfield
environments, problems it may encounter and proffer solutions. This will provide considerable
contributions to efforts made to develop the oil and gas sector of any economy by preventing loss of
oilfield infrastructure to corrosion.
Purpose of study
Claims from previous research
[7]
highlight that the major challenge faced by high strength aluminiumalloys in service is stress corrosion cracking which occurs when it is being attacked by aggressive
corrosive species present in the working environment. The aim of this thesis is to explore the
performance of various high strength aluminium alloys i.e. 2xxx (Al-Mg), 7xxx (Al-Zn-Mg) and 5xxx
(Al-Mg) series in corrosive environments typical to the oil and gas industry under load, study the
effect of stress and characterize the microstructures of failed components.
Project outline
1. A state-of-the-art review of literature regarding SCC of aluminium alloys and its performance in
SCC environments and preventive measures.
2. Develop a new method that will be used to analyse the behaviour of aluminium alloys to SCC while
in service.
3. Develop a SCC model for investigation of crack initiation and propagation.
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4.Perform microscopic analysis of the cracked aluminium alloy in SCC environment to identify crack
morphology, orientation and direction.
5. Identify microstructural features which control the initiation and propagation of SCC cracks
6. Measure all stressing directions, particularly the long-transverse direction in sheet and thin plate
products [4].
Methodology
The ASTM G139-05 standard developed for stress corrosion cracking of Aluminium alloys[4]
will be
employed to create a new methodology in carrying out this research. The stress corrosion testing
technique employed will be slow strain test rig as a result of its realistic stress distribution, cracking
pattern, ease of design, less severity and cost effectiveness[10]
.
Several strings of test samples will be placed in simulated oilfield environments containing significant
amount of corrosive saline solution.
Samples in which cracks are evident will be critically accessed to understand the crack initiation and
propagation mechanisms while the alloy which is not affected by stress will be standardized as the
candidate alloy for oilfield application.
Different series of aluminium alloys are tested in 3.5wt % sodium chloride solution by alternate
immersion. However, the concept which uses residual strength as a measure of damage evolution (in
this case environmentally-assisted cracking) can, in principle, be applied to any alloy and
environmental system[4]
.
The exposure periods and conditions will be recorded for the different aluminium samples in order to
ascertain the time of fracture of individual alloys with different exposure conditions [4].
Typical micrograph of stress corrosion cracking of a material with the crack following anintergranular pattern through the grain boundaries
[5].
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The stress intensity factor (K) will be evaluated using the formula, K = , where = applied
stress, a = crack length and Y = geometrical correction factor. The essence of evaluating (K) is to
determine the threshold stress for the occurrence of stress corrosion cracking of the material (KISCC)
which if greater than K will lead to material failure but if lesser than K, no material failure via stress
corrosion cracking[3]
.
American Society of Testing Materials (ASTM) G139 -05[4]
:
i. This test method covers procedures for evaluation of stress corrosion cracking (SCC)resistance by the breaking load test method, a concept which uses residual strength as the
measure of damage evolution (in this case environmentally assisted cracking).
ii. This test method covers specimen type and replication, test environment, stress levels,exposure periods, final strength determination, and statistical analysis of the raw residual
strength data.
iii. The test method was developed for use with heat-treatable aluminium alloys, that is, 2XXXalloys and 7XXX with 1.2 to 3.0 % Cu, and test specimens oriented in the short-transverse
direction relative to grain structure. However, the residual strength measurements and the
statistics used to analyze the data are not specific to heat-treatable aluminium alloys and can
be used for other specimen orientations and different types of materials.
iv. Measure the stress intensity factor for SCC (KISCC) mode 1 because aluminium cracks in aductile manner.
List of potential targets
1. Revise the fundamental rudiments of stress corrosion cracking in oil and gas environmentsand field in order to develop a good understanding of the electrochemical theory and stress
development.
2. Become familiar with practical electrochemistry related to stress corrosion cracking usingstandard equipment and materials.
3. Prepare strings of aluminium alloys using standard metallographic processes.4. Expose different alloys of aluminium into aggressive oilfield environment and subject to load
by means of straining with a stress ring.
5. Characterise the grain orientation and direction by means of scanning electron microscopy.6. Characterise which aluminium alloy will be less prone to stress corrosion cracking in
aggressive oilfield environment.
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List of goals
1. Become conversant with all electrochemical methods used in the oilfield to characterise stresscorrosion cracking.
2. Become proficient with the principles of stress corrosion cracking and be able to apply it tovarious materials not just aluminium.
3. Characterise which aluminium alloy will be suitable for oilfield application.4. Attribute corrosion performance of AA with corrosiveness of oilfield environment.5. Produce works for publication, conferences, and activity reports to the industrial sponsor.6. Develop independent attitude in carrying out research.7. On completion of thesis gain employment at a citadel of learning in the world to disseminate
knowledge generated from thesis for technological advancement and economic boom of the
nation.
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Bibliography
1. Mikhail Y. Gelfgat and Vladamir S. Basovich, Alex Adelman, Aluminium alloy tubulars for the oil
and gas industry (Advances in Drill Pipe), Aquatic company, Russia, 2006.
2. E.L. Colvin, Aluminium Alloys Corrosion, Alcoa Inc., Lafayette, Indiana, USA, 2001.
3. R.A Cottis, Guides to good practice in corrosion control: Stress corrosion cracking, The National
Physical Laboratory, Queens Road, Teddington, Middlesex, England, 2000.
4. American Standard of Testing Materials code G139 -05.
5. Forms of corrosion culled from NACE International, Houston, Texas, USA, 2001.
6. Wei Zhou, Stress corrosion cracking: causes and solutions, Singapore Welding Society Newsletter,
2008.
7. J.R. Pickens, J.R. Gordon and J.A.S Green, The effect of loading mode on the stress corrosion
cracking of aluminium alloy 5083, Metallurgical transactions, 1983.
8. P. Skeldon and N.P. Stevens, Lecture note on kinetics of passive film formation, Corrosion and
Protection centre, The University of Manchester, 2012.
9. I.J Polmear, Light Alloys (Metallurgy of the light metals) second edition, Melbourne, Australia,
1993.
10. N.P. Stevens, Lecture note on stress corrosion cracking testing, Corrosion and Protection centre,
The University of Manchester, 2012.
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Timetable
S/N Task Date
1. Prepare proposal 1/11/2012
2. Complete literature review 15/12/2012
3. Complete fieldwork 15/11/2013
4. Complete analysis 15/11/2014
5. Give presentation 12/11/2015
6. Complete final report 19/10/2015