Proceedings of 2022 6th International Conference on Green Technology and Sust...
Burak cacan thesis 900008137
1. EFFECT OF TENSILE STRESS ON
CATHODIC DISBONDMNET OF
COATINGS
NAME: BURAK CACAN
SUPERVISORS: PROFFESOR MIKE TAN, FARI MAHDAVI
MAY 31, 2015
2. Acknowledges
My special appreciation to my supervisors Professor Mike Tan and Fari Mahdavi, for their full
support and communications during the project. I am also very grateful for answering my
questions and encourage me every step of the project. I also, would like to thank for Dr. Aman
Maung Than Oo during project a advises and A/Prof Tim Hilditch for project B. Also, I am
grateful for the support of chemical engineer and my industry supporter Erol Dag (corrosion
engineer) to answer all my questions and helping with analysing experiment results. I owe my
parents and sibling and my friend Jazeem for their support and encouragements in every stage, my
father Muhittin cacan, my mother Meral cacan and my sister Merve Cacan.
3. Table of Contents
Acknowledges .........................................................................................................................................1
Table of Figures.......................................................................................................................................4
Abstract...................................................................................................................................................5
1. Introduction ........................................................................................................................................6
2. Project Definition................................................................................................................................7
2.1 Project Objectives...................................................................................................................7
2.2 Project Benefits.............................................................................................................................8
2.3 Key Research Questions................................................................................................................8
3. Literature Review................................................................................................................................9
3.1 Pipeline Corrosion...................................................................................................................9
3.1.1 Pipeline coating......................................................................................................................9
3.1.2 Cathodic Protection .............................................................................................................11
3.1.3 Cathodic disbondment.........................................................................................................12
3.2 Mechanical stress of coatings.....................................................................................................14
2.2.1 Coating thickness ..........................................................................................................14
3.2.2 Stress In coatings .................................................................................................................16
3.2.3 Elongation of Coatings .........................................................................................................17
2.3 Applying Tensile Stress on Pipeline Coating .........................................................................18
3.3.1 Three-Point Bending Test.....................................................................................................18
3.3.2 Mandrel Testing............................................................................................................19
4. Background Search ...........................................................................................................................22
5. Methodology.....................................................................................................................................24
5.1 Metal specimen...........................................................................................................................24
5.1.1 Metal Specimen Issues.........................................................................................................25
5.2 Coating application .....................................................................................................................27
5.2.1 Coating application issues....................................................................................................28
5.3 The Instron 100 KN Mandrel Test...............................................................................................29
5.3.1 Mandrel Design....................................................................................................................29
5.3.2 Mandrel Test Steps ..............................................................................................................31
5.4 Cathodic disbondment Test........................................................................................................35
5.4.1 Cathodic disbondment test issues.......................................................................................41
6. Results...............................................................................................................................................41
6.1 Strain calculations.......................................................................................................................41
5. Table of Figures
Figure 1- Steps of coating Application Process.....................................................................................10
Figure 2-– Cathodic Disbondment ........................................................................................................12
Figure 3- before (1) and after (2) cathodic disbondment test..............................................................13
Figure 4- Material property of coating – Green area demonstrates Elastic energy.............................15
Figure 5- Stress vs. Strain % limit shown..............................................................................................17
Figure 6- Three point Bend Test 3.3.1.1 Three Point Bend Testing Issues.........................................18
Figure 7- Strain %..................................................................................................................................20
Figure 8- Nomenclature........................................................................................................................20
Figure 9- Mandrel Shoe Diameter.........................................................................................................21
Figure 10- Mandrel Diameter vs. Strain %............................................................................................22
Figure 11- Delamination area vs. Exposure time..................................................................................23
Figure 12- Mechanical and chemical properties of 5L X65...................................................................24
Figure 13-1) Steel bars before and after sanding 2) Guillotine machine..............................................25
Figure 14-Mandrel shoe dimension limitations....................................................................................25
Figure 15-Milling machine ....................................................................................................................26
Figure 16-Coating applicator machine..................................................................................................27
Figure 17-Coating thicknesses ..............................................................................................................27
Figure 18-Coating applicator issues - 1) Metal specimen not flat enough 2) coating applicator
sensitivity ..............................................................................................................................................28
Figure 19-1) Remaining’s of the coating 2) Sanding machine ..............................................................28
Figure 20-Instron 100 KN mandrel test.................................................................................................29
Figure 21-Mandrel test Design Shown..................................................................................................30
Figure 22-Mandrel shoe applying stress on metal sample by holding metal blocks............................31
Figure 23-1) Base and 2) Top Base........................................................................................................31
Figure 24-Hydraulics on /off button .....................................................................................................32
Figure 25-Instron 100 kN manual remote control................................................................................32
Figure 26-Mandrel shoe attached to threaded Rod.............................................................................33
Figure 27-Instron 100 kN control computer.........................................................................................33
Figure 28-Instron 100 kN emergency button........................................................................................34
Figure 29-1. Drilling process and 2. Flat end mill..................................................................................35
Figure 30-Plastic tube ...........................................................................................................................36
Figure 31-1) Tube area and 2) Titanium mesh area..............................................................................36
Figure 32-DC power supply...................................................................................................................37
Figure 33-DC Power Source Circuit System ..........................................................................................38
Figure 34-Plastic Tube Glued To Metal Sample....................................................................................39
Figure 35-Metal sample hole concentric with plastic tube ..................................................................39
Figure 36-Cathodic Disbondment Test .................................................................................................40
Figure 37-. Points touched, where the mandrel experiment stops......................................................43
Figure 38-8 lines set from the plastic tube boundaries........................................................................45
6. Abstract
This project aims to investigate the effect of tensile stress on cathodic disbondment of
coatings by applying pipelines two different loads with mandrel machine, then applied
cathodic disbondment test by Australian cathodic disbondment standards and then the
effects analysed visually.
Mandrel test was performed to create deformation on the pipeline coatings to observe cracks
occurred by applied load. Then, compare the results with no load applied coating samples to
understand if tensile stress is an effect of cathodic disbondment test.
This project will help industry gain a better understanding of relationship between tensile
stress and pipeline coatings under cathodic disbondment test.
7. 1. Introduction
Buried pipelines play a crucial role for transportation of oil and natural gas around the world
from on/off shores, producing fields and refineries, storage areas and export points to the
consumers (Kennedy 1984). Worldwide, the oil and gas pipeline length is about 3,500.000 km
which is 17 times around the world according to the 5th Asian Pacific IIW International
congress Sydney, Australia (Hopkins 2007 ). Therefore, pipeline corrosion is one of the biggest
problems in pipeline industry and trillions of dollars spend annually (Matthew J. Lieser 2010).
As a naturel process, corrosion occurs, during metals seek the lowest energy to return to its
original state by a chemical reaction (Chikezie Nwaoha 2013). To control and mitigate
corrosion for underground pipelines there are two major precautions, first of all applying
protective coatings outer surface of the metal and then, supplying cathodic protection
(Javaherdashti 2008). Stress apply to buried pipelines by the load (soil, rocks, water and other
substances) which are under cathodic protection. Underground pipeline coatings expected to
deform by the applied forces.
To understand effects of coating mechanical properties, theoretical and experimental results
compared.
8. 2. Project Definition
This project aimed to understand a relationship between applied loading on coating and
effect on cathodic disbondment. Different levels of stress levels applied on high build epoxy
coated metal samples by mandrel test and then compared to no stress applied high build
epoxy coated metal samples, after cathodic disbondment test executed. The results analysed,
if there is a relationship between cathodic disbondment and tensile stress.
2.1Project Objectives
This project was aimed to understand mechanical stress on coated samples and apply each
samples with cathodic disbondment test, then compare the results with no stress applied
sample.
1. Evaluating of different mechanical stress methods.
2. Applying only one type of coating to compare results with each other
3. Understanding different level of stress and coating thickness effects on cathodic
disbondment.
4. Understanding methods of cathodic disbondment test.
9. 2.2 Project Benefits
This project could open a new search areas for pipeline industry and also lead to new
researches. For the industry this project could benefit to understand different coating
materials and search availability for different coating mechanical stress under cathodic
disbondment test. After addressing and understanding project definition, project results
could benefit the pipeline industry to understand;
1. If tensile stress is an effect of a cathodic disbondment of coatings?
2. Does different levels of loads, have different effects on coatings disbondment?
2.3 Key Research Questions
During the project couple of questions addressed for a better understanding;
1) What is the effect of mechanical stress on coating under cathodic disbondment test?
2) How to define maximum or minimum mechanical stress of coating?
3) Is this test proves that stress is critical for coating performance under cathodic
disbondment test?
10. 3. Literature Review
3.1Pipeline Corrosion
According to International journal of industrial chemistry (IJIC), pipeline corrosion is a
common problem for pipeline industry (Lekan Taofeek Popoola 2013). Corrosion is the
deteriorative and destructive attack of a material because of the reacting with natural
environment (Kermani MB 1997). According to ASM international, pipeline deformations
were caused by 63 % external and 36 % internal corrosion (John A. Beavers and Neil G.
Thompson 2006). Therefore, protecting pipeline external by coating, plays an important role
for buried pipelines.
There are two different methods to slow down external corrosion effect, these are applying
coating and cathodic protection method (X. Chen 20009). Understanding mechanical
properties of the coatings is very crucial to determine coating failures. Therefore, coatings
properties should be classified by their elastic modulus, residual stress and hardness (J.
Malzbender 2002).
3.1.1 Pipeline coating
Coating is a preservative between metal surface and natural environment such as oxygen (O2),
water (H2O) and underground minerals. According to Nace international, coating is “a
composition which consists of liquid, liquefiable, or mastic composition that apply to metal
surface then converted into a solid protective, decorative or functional adherent film”
(International 1997). As seen in figure 1 coating application process shown. Pipeline coatings
should meet the conditions such as satisfied mechanical strength and good aging resistance
11. in corrosive soil environment (Y. Joliff 2013). The aim of the coating application is to increase
service life of pipelines. Also, the coating life expectancy depends on chemical stability of the
component materials and ability of the coating strength against destructive mechanical and
chemical applied to the pipelines. To apply cathodic protection, coating must be applied first,
otherwise the cost of cathodic protection would be very high due to high corrosion without
coating.
Understanding mechanical strength of coating is very important, during applying coating on
pipelines. According to Duari and Chaudhuri, high build epoxy has shown better performance
with respect to physical properties as pipeline coatings compare to some other coatings
(B.Duari 2010).
Figure 1- Steps of coating Application Process
12. 3.1.2 Cathodic Protection
Cathodic protection is a second line of defence protection, after applied coating to external
surface against corrosion (Banach 2004). Cathodic protection is a method which supplies
electrons to pipelines, aims to balance electron lose due to corrosive environment. No
resultant overall charge builds up on the metal because of the corrosion if the rate of the
anodic and cathodic reactions are equal. Impressed current system is the common method
for buried pipelines because of its high life service and adjustable output capacity, and lower
cost per ampere of cathodic protection current (James B. Bushman). In Impressed current
system, high voltage AC current converts to low voltage DC current by a rectifier and then,
lowered current electrons which acts cathode to protect the buried coated pipelines by
constantly supplying electrons via auxiliary electrode the anode (W. von Baeckmann 1997).
13. As the coating deforms with time cathodic protection system decrease the coating
performance and increase the rate of pipeline coating deformation in time. Pipeline
deformations results holidays on the coating (Francis 2007). Pipeline without cathodic
protection could result cathodically polarized at certain local spots. However, cathodically
protected pipeline’s metal surface entirely cathodically polarised. (subcommittee 1987)
3.1.3 Cathodic disbondment
Cathodic disbondment is a common coating failure for buried pipelines (W. von Baeckmann
1997). Industrial coatings should resist to cathodic disbondment for long-term protection of
pipelines. According to corrosion science, cathodic disbondment is loss of adhesion or bond
between coating and metal substrate, which occurred due to forming a high level of pH
environment beneath the film as a result of cathodic reaction (R. Naderi 2010).
A hole or defects must be occurred on the coating through to the metal surface for cathodic
delamination. When metal surface exposed to corrosive environment cathodic delamination
Figure 2-– Cathodic Disbondment
14. starts on the coating, then those holes filled with substances such as water (H2O) and oxygen
(O2) and as a result, the coating started to losing up (Schweitzer 2006).
3.1.3.1 Cathodic Disbondment test
Cathodic disbondment test has been used and well known laboratory test for pipeline
coatings. Cathodic disbondment test indicated a good understanding of pipeline coating
performance and also critical for quality control of pipeline coatings (Markuz Betz 2012). The
main parameters of cathodic disbondment test are temperature, test duration, diameter of
the drill, electrolyte composition.
Cathodic disbondment test creates a real life scenario for field situations and the only
difference would be electrolyte. In the laboratory testing 3 % NaCl solution used instead of
soil or sand to create cathodic disbondment. A hole drilled through the coating up to metal
surface and test duration taken place 28 days while the metal surface under 3mA cathodic
protection.
Figure 3- before (1) and after (2) cathodic disbondment test
15. 3.2 Mechanical stress of coatings
Since 70’s and 80’s, coating technique has developed due to improvements of coating
technology (Mike O'Donoghue 2003). Coatings isolate the pipeline surface form the external
substances such as water, soil and air to unable the corrosion. However, over the years
coatings loses protection ability because of losing its physical and chemical formation. Some
examples are; presence of holidays, formation of disbondment, blisters, loss of adhesion and
water permeation (Sankara Papavinasam 2006).
The stresses developed in high build epoxy resin results chemical and physical changes on the
coating. Applied mechanical stress results shrinkage, expansion, thickness change and brake
in chemical bounding. Chemically and physically changes effect coating performance (Grosse
2003).
2.2.1 Coating thickness
Understanding mechanical properties of pipeline coating thickness could give better
understanding of coating deformations and pipeline corrosion. (J. Malzbender 2000).
According to Joliff, Belec, Aragon internal stress could occur in all coating thicknesses, which
depends on each coating type, amount of stress levels and internal stresses could result
coating deformation. However, if the applied stress levels approximately the same, coating
thickness wouldn’t be affective on coating adhesion, to the point where coating structure
totally deformed by losing its plasticity. The deformation of coating related to elastic energy
of the coating. Elastic energy, the potential mechanical energy stored upon deformation (Clive
16. L. Dym 1973). The value of the elastic energy (E) directly proportional to stress ( 𝜎𝜎) applied
multiply by the thickness of the coating (𝜏𝜏). Thus, if the stress remains same increase in
coating thickness also, results as higher the elastic energy.
E= 𝜎𝜎 * 𝜏𝜏
So, increase in elastic energy, decreases the coating adherence (Y. Joliff 2013). Also, increase
in elastic energy also increase strain of the coating. As seen in figure 4. Stress, strain, elastic
energy relation shown. Coating elastic energy increases through the deformation point which
is point A as in figure 4. However residual stress seen every stage.
Figure 4- Material property of coating – Green area demonstrates Elastic energy
17. 3.2.2 Stress In coatings
Internal stresses causes most common mechanical deformation of the coating. If the
deformation of the coating reaches, and then go beyond the elastic limit consistent
deformation unavoidable. If the deformation would be consistent, chemical bonds in the
coatings changes are irreversible (Thornton and Hoffman 1989). Therefore, understanding
internal stress of a coating is very important for pipeline industry.
Tensile stress could simply describe as the stress state when load applied to the surface which
could cause stretches on substance internally and externally (Goodno 2008). Tensile stress
could be found as the applied axial force divided by on the force applied area. When tensile
stress pass the elastic limit, internal and external cracking starts in the coating. The cracks
occurred in the coating perpendicular to the tensile stress direction (Anthony J. Perry 1996).
According to Perera, Stress occurred on the structure coating, results coating delamination
and coating adhesion between metal substrate and coating.
Also, applied mechanical stress increase the barrier effect of the coating and water sorption
of the coating decreases by decreasing solubility entropy of coating. Recused water sorption
cause delay the corrosion process (D. Nguyen Dang∗ 2013).Organic coatings do not have
plastic region so, when applied stress pass the plasticity phase it is like to break, however if
the applied stress stays in elasticity phase coating performance increase.
Epoxy strain limit shows a good performance under 7% strain limit. After 7 % (0.7), epoxy
coating started to break and coating could not show good performance after this point (Azo
Materials 2015). As shown in figure 5.
18. Figure 5- Stress vs. Strain % limit shown
3.2.3 Elongation of Coatings
Organic coatings performance could be effected from mechanical changes which could result
cracks on the coating surface. Tensile stress generally causes cracks on the surface of the
coating (Kotnarowska 1991). Cracks occurred on the coating surface creates empty spaces in
organic materials which results holidays on the coating. These cracks depend on the
elongation strength of the coatings. Elongation is the ductility of a material which means the
amount of strain applied before the material failure. Elongation could be found as the
difference between final length and initial length divided by initial length. Also, could easily
explained by displacement percentage. Each coating has elongation limit so if this point
exceed coating could dissolve or break.
19. 2.3 Applying Tensile Stress on Pipeline Coating
Understanding real life effect of pipeline coating is crucial for pipeline corrosion. To test
tensile stress of pipeline coatings there are several laboratory methods which are similar to
real life effects. Some methods are mandrel test, punching test, three point bending test and
stretch tensile stress test. The most suitable test chosen, which is mandrel test.
3.3.1 Three-Point Bending Test
Three point bending test is a laboratory method to apply stress on specimens. In this method
a test specimen with rectangular or flat cross- section could place on top of the two parallel
supporting pins. The main load applied to the middle of the specimen by a third pin.
Figure 6- Three point Bend Test
20. 3.3.1.1 Three Point Bend Testing Issues
Three point bending test had thickness limitations due to stress distribution. This method has
some errors to distribute tensile stress equally to each part of sections of the coating. The
different thickness limitations in the experiment could give wrong results.
3.3.2 Mandrel Testing
Mandrel testing method is a worldwide method for applying tensile stress on coated
specimens. In industry creating laboratory simulations help companies to create real life
conditions. Mandrel test method accepted as the most accurate method to simulate hydro
static testing and stress distribution by bending.
Creating accurate real life effect is very important during manufacturing state for oil and gas
companies. A simple mistake could result million dollars, therefore mandrel test seen as the
most common method for stress distribution and applying different strain levels on coated
metal samples.
3.3.2.1 ASTM D 522
In this experiment Australian standards used during applying mandrel test;
Standard Title
ASTM D 522
Standard Test Method for Mandrel Bend Test of
Attached Organic coatings
21. ASTM D 522 is a standard test method for Mandrel Bend Test for organic coatings, accredited
to ISO 17025 and SAE AS 5505 for the testing and characterization of paint systems. There are
two different methods conical bend test and cylindrical mandrel test. Conical bend test is a
painted test panel clamped into the conical test apparatus and bent over the conical cone
using the rotating bending arm. Cylindrical mandrel test is a painted test panel which is 1800
around a specified diameter steel rod. Also, both methods should be undertake 23 0C and
humidity 50 % relative prior to test.
3.3.2.2 Mandrel Strain
Mandrel test has 10 mandrel shoes to apply different levels of strain to samples. Mandrel test
could apply strain levels in between 0.5 % to 4 % based on shoe diameters and applied time,
as seen in table 1. To calculate applied strain percentage shoe diameter chosen. The thickness
of coating and metal sample thickness estimated as 4.7 mm. Detailed stain % level shown in
Results section.
Mandrel strain equation (Guillaume Michal 2013);
𝜀𝜀 % = 100 * (
𝑇𝑇
𝐷𝐷𝐷𝐷+𝑇𝑇
)
Figure 7- Strain %
Figure 8- Nomenclature
23. Figure 10- Mandrel Diameter vs. Strain %
4. Background Search
Tensile effect on cathodic disbondment hasn’t been proofed experimentally. As discussed in
literature review tensile stress expected to deform mechanically the coating. However, this
deformations must be analysed and proofed. According to Elbasir and Mehta, delamination
rate increase with the applied stress (Mehta 1991). In the experiment that published two
levels of stresses were applied. The applied stresses were 11.1 kg mm-2 and 16.6 kg mm-2
and
metal specimens exposed 3 to 72 hours cathodic disbondment test with potential of -1500 mV current.
In the paper delamination areas found and under high level stress applied specimen resulted higher
24. level of disbondment. Also, low level of stress applied specimen showed better coating quality
compare to no stress applied coating.
Figure 11- Delamination area vs. Exposure time
Paper explained this result as higher stress level exceed plastic phase and low level stress still
stayed in elastic region, however they couldn’t explained why elastic and plastic regions had
an different effect on delamination.
They have admitted that further work is required to done to analyse the effect of deformation
on the structure especially on organic coating and metal surface. Also, they have applied the
stress during the cathodic disbondment test.
25. 5. Methodology
In methodology section, details of the project reflected. This project consist theoretical study
and experimental part. The theoretical study compared to experiment results and discussed.
Every step of the project discussed and advised by the supervisors also, compared with
previous experiments. During the experiment Australian standard AS -3862 were used as
shown below.
5.1 Metal specimen
During the experiment API 5L X65 stainless steel used, X65 is commonly used steel plate for
pipeline industry. Also, API L X65 has high strength, tough and weldable. X65 chemical and
mechanical properties shown in figure 12.
Figure 12- Mechanical and chemical properties of 5L X65
Grade C Si Mn P S
X65 0.04-0.16 0.55 1.00-1.60 0.035 0.035
Chemical Composition
Standard Voltage [V] Temperature C Solution Duration
AS 3862 3 mA 22.5 C 3 w % NaCl 28 days
26. For the experiment, 20 steel bars were cut with the dimensions of 55 mm x 220 mm by a steel
guillotine machine. After all the samples cut, sanding was applied to reduce corrosion on
metal sample surface by basting sanding machine to clean surface grime.
Figure 13-1) Steel bars before and after sanding 2) Guillotine machine
5.1.1 Metal Specimen Issues
Before the metal samples were cut the thickness of the metal samples set 55 cm. However,
human error played a role during the experiment due to mandrel shoe dimension limitations.
As seen in figure 14. Mandrel shoe limitations 5.1 cm
Figure 14-Mandrel shoe dimension limitations
27. Therefore, milling machine were used to decrease sample dimensions by using milling
machine. Milling machine reduce metal sample dimensions with a 100% accuracy. Also,
number of the metal samples not a limitation for milling machine. Metal sample dimensions
set as 5.1 cm.
Figure 15-Milling machine
28. 5.2 Coating application
Secondly, all metal samples were applied high build epoxy by coating applicator with coating
thickness between 254 𝜇𝜇m -357 𝜇𝜇m. Coating applicator was cleaned for coating accuracy.
High build epoxy and hardening were mixed with a ratio of 3:1.
Coating thicknesses shown;
Figure 16-Coating applicator machine
Figure 17-Coating thicknesses
29. 5.2.1 Coating application issues
During coating application a lot of issues faced, high-build epoxy dries in 2-3 minutes after
mixed with hardening. Therefore, epoxy-resin should only prepared for 2-3 metal samples
before coating dries. Also, metal samples weren’t flat enough therefore, keeping the coating
thickness was challenging seen figure18- 1. The other issue was coating applicator scale was
very sensitive, even it was cleaned before applying coating. As seen in figure 18- 2, dried resin
blocked coating applicator. Due to coating thicknesses failures, coating application process
repeated 3 times.
Also, after coating applied some coating remains on the sides of metal samples. Therefore,
sanding machine used to clear the edges. During this process mask were used not to inhale
epoxy.
Figure 18-Coating applicator issues - 1) Metal specimen not flat enough 2) coating applicator sensitivity
Figure 19-1) Remaining’s of the coating 2) Sanding machine
30. 5.3 The Instron 100 KN Mandrel Test
As mentioned in literature review, tensile stress applied by mandrel test to coated specimens.
The test method was used AS/NZS 3862:2002 as seen in appendix A. Eleven mandrels tests
were used to apply different levels of strains between 0.5 % to 4 %. The average strain was
calculated by coating thickness, metal sample thickness and mandrel shoe diameter as seen
in Table 1.
The Instron 100 kN hydraulic compression test machine was used to apply two different stress
levels, which is placed in Ni building. The coated metal samples applied 0.5% and 1% strain
levels to 6 different samples. Also, no stresses were applied to 3 metal samples.
Figure 20-Instron 100 KN mandrel test
31. Figure 21-Mandrel test Design Shown
5.3.1 Mandrel Design
Mandrel design consist test rig, screw clamps, bottom and top bases, two metal blocks,
threaded bar, mandrel shoes and dimension limitation screw.
32. 5.3.2 Mandrel Test Steps
The metal samples were placed on the test rig which is supported by two metal blocks. The
edges of these metal blocks’ have curved edges. These two metal blocks hold the metal
specimen stable and allows a horizontal movement. Different type of mandrel shoes were
used to apply stresses to deform flat metal samples by simulating bending test as seen in
figure 22.
How the experiment conducted shown as below;
Instron 100 kN machine combines two main base clamps. These clamps hold the mandrel
test rig as seen in figure 22 and mandrel shoe with threaded bar. Mandrel shoe must be
clamped on the top base with threaded rod to stabilize the top shoe. Then, base metal block
clamped with another threaded rod Figure 23.
Figure 22-Mandrel shoe applying stress on metal sample by holding metal blocks
Figure 23-1) Base and 2) Top Base
33. Then, Instron machine hydraulics started up by pressing 1 button. For hydraulics to start up,
the button should be pressed for 5-6 seconds. Hydraulics provides pressure the upper and
bottom bases to move.
When the hydraulics has enough pressure, up and bottom bases were controlled with a
manual control remote. The remote gives fully displacement control of the mandrel shoe and
test rig.
Figure 24-Hydraulics on /off button
Figure 25-Instron 100 kN manual remote control
34. Thirdly, mandrel shoe is inserted to the upper threaded rod and hold up by a bolt to stabilize
the mandrel shoe. If the stress desired to be changed, the mandrel shoe could easily be
changed by removing the bolt and attach a new mandrel shoe.
At the last step 100 kN Instron machine was controlled by a main computer program which is
connected to the machine. The program moves the upward mandrel shoe downwards
automatically as the desired speed and time which is set by the operator. The mandrel shoe
goes downward until metal sample fully bended.
Figure 26-Mandrel shoe attached to threaded Rod
Figure 27-Instron 100 kN control computer
35. Note: The experiment could be stopped any time by the emergency button.
Figure 28-Instron 100 kN emergency button
36. 5.4 Cathodic disbondment Test
Cathodic disbondment test is a laboratory test for pipeline coatings to simulate cathodic
disbondment. During the experiment the coating wouldn’t be delaminate to a large extent
while under cathodic protection and only small coating damage should observe where the
metal surface subjected to solution. Cathodic disbondment test plays an important role for to
test pipeline coating quality (Markus Betz 2012).
During the cathodic disbondment test Australian test standards AS3862. According to
Australian test standards test must be made 28 days at a room temperature under 3mA
protection with a solution of 3% NaCl.
At first, 11 different coated test specimens were drilled a hole with a 6 diameter flat end mill
to start a cathodic disbondment test as mentioned in literature review.
Figure 29-1. Drilling process and 2. Flat end mill
37. Then, 11 plastic tubes were cut with a diameter of 4.4 cm and length of 10 cm to create
cathodic environment during cathodic disbondment test.
Then titanium mesh were cut to put inside the solution while cathodic disbondment test.
Mesh must be inserted to solution during metal specimen protected to complete the circuit.
Titanium mesh area must be equal to plastic tube area which is contacted to coated sample.
𝜋𝜋*r2
Figure 30-Plastic tube
Figure 31-1) Tube area and 2) Titanium mesh area
38. Plastic tube radius was measured as 4.4 cm and width (W) of the titanium mesh set 7.6 cm
length of the titanium mesh set as 2 cm, so as seen in the equation below the areas are equal.
Each plastic tubes were filled 8 cm NaCl solution.
𝜋𝜋*r2
= W*L
𝜋𝜋* (22)2
= 7.6 *2
= 15. 2 cm2
Then, DC power source was made for spreading the current equally for 11 different samples.
DC power source able to change the current as desired and set the current constant during
the cathodic disbondment test. DC power source was built in electrical laboratory and
improvements made by eliminating circuit board. Eliminating circuit board was helped to use
less cables and lighter box.
Figure 32-DC power supply
39. A simple current system used as seen in figure 33.
Figure 33-DC Power Source Circuit System
40. Before the last step conducting cathodic disbondment test, NaCl solution prepared to create
corrosive environment during cathodic disbondment test. According to Australian standards
the corrosive solution 3w % NaCl solution was prepared. Each plastic tubes were filled with 8
cm solutions therefore, 1007.6 L solution was prepared for the experiment.
The last step, the tubes were glued to metal samples by an industry silicon glue.
When sample was glued to metal surface the hole on the metal sample and tube hole set as
concentric. So, cathodic disbondment could easily measure.
Figure 34-Plastic Tube Glued To Metal Sample
Figure 35-Metal sample hole concentric with plastic tube
41. Cathodic Disbondment test set up as seen in the figure 36.
After 28 days, disbonded area observed. Disbonded area is the deformation on the coating
after 28 days of cathodic disbonded test during cathodic protection. To calculate the
disbonded area after the cathodic disbondment test, the disbonded coating were removed
by using a knife.
First, the coating hardness checked on the different side of the coating to ensure not to use
too much force to remove the coating.
Before, removing the coating 8 lines set from the boundaries of plastic tube placed. Then,
disbonded area calculated after coating removed.
Figure 36-Cathodic Disbondment Test
42. 5.4.1 Cathodic disbondment test issues
During the experiment due to high amount of evaporation NaCl solution was added every two
days to be sure water level is 8 cm and protection current checked. The other issue corrosion
of titanium mesh. The water colour was also checked every two days.
6. Results
Experimental results were analysed and compared with literature review.
6.1 Strain calculations
The coated metal sample stain levels were calculated by takin consider of metal and coated
metal thickness and mandrel shoe diameter. Thicknesses, shoe diameters and
nomenclature shown in methodology. Calculations were measured according to Guillaume
Michal.
𝜀𝜀 % = 100 * (
𝑇𝑇
𝐷𝐷𝐷𝐷+𝑇𝑇
)
Specimen
Number
Diameter Of the
mandrel shoe
Metal Specimen
Thickness
(mm)
Coating
Thickness
(mm)
Total
Thickness
(mm)
Strain
( 𝜀𝜀 % )
1 1095 4.7 0.402 5.102 0.463775177
2 1095 4.7 0.334 5.034 0.457622219
3 1095 4.7 0.345 5.045 0.458617602
4 N/A 4.7 0.3 5 0
5 545 4.7 0.375 5.075 0.922601463
6 545 4.7 0.263 4.963 0.902424345
7 545 4.7 0.254 4.954 0.900802613
8 N/A 4.7 0.288 4.988 0
9 N/A 4.7 0.276 4.976 0
10 545 4.7 0.266 4.966 0.902964911
Table 2-Strain calculations
43. As mentioned in section 3.2.2 stress in coatings if strain % is higher than 0.7 coating may break.
As seen, sample 5, 6, 7 and 10 have higher strain % than 0.7. This could also increase
disbondment area due to coating break.
Strain % Epoxy coating strain % criteria Meet the Criteria
Sample 1 0.463775
0.70%
Yes
Sample 2 0.457622 Yes
Sample 3 0.458618 Yes
Sample 5 0.922601 No
Sample 7 0.900803 No
Sample 10 0.902965 No
As above shown some coatings likely to deform and not meet the strength criteria.
During the expo presentation, strain levels set as 0.5 % and 1% due to not result any confusion.
Also, as mentioned in the literature review different coating thickness under same load gives
same results.
During the mandrel experiment applied Load compressive extension (mm) versus Load (N)
were shown in Appendix C.
After stain levels were calculated, when the mandrel shoe touched to the metal sample
surface entirely, experiment stopped. In the figure 39, h needs to be 0. As seen in appendix
C, the point in the load vs. time graph where the load decrease suddenly, determine the time
when shoe touches the specimen surface entirely.
44. First of all, coatings stresses found from strain and Young’s modulus (Y) of coating. To find
young’s modulus force (N) divided by area (mm2) is equal to length (mm) of the specimen
divided by extension. All the coating were used identical however, applied stress likely to
change due to load application. So as seen in table 2. All the force and displacement
calculations found from appendix C.
𝐹𝐹
𝐴𝐴
= Y*
∆𝑙𝑙
𝑙𝑙
Y =
𝐹𝐹
𝐴𝐴
*
𝑙𝑙
∆𝑙𝑙
Figure 37-. Points touched, where the mandrel experiment stops
45. Max.
load
Specimen
Area
(mm^2)
Length
(mm)
Displacement
(mm)
Young's
Modulus
1 2500 14000 280 13.1 3.816794
2 3000 14000 280 10.6 5.660377
3 2000 14000 280 12.7 3.149606
5 1150 14000 280 17.2 1.337209
7 1700 14000 280 19.3 1.761658
10 1800 14000 280 19.1 1.884817
By founding young’s modulus, stresses could be calculated. Applied Stress (𝜎𝜎) is equal to
young’s modulus (E) multiply with stain (𝜀𝜀). So, as seen in the table stress is calculated.
𝜎𝜎= Y* 𝜀𝜀
Young's
Modulus
Strain
Stress
(MPa )
1 3.8167939 0.4637752 1.770134263
2 5.660377 0.4576222 2.590314444
3 3.1496063 0.4586176 1.444464888
5 1.3372093 0.9226015 1.233711259
7 1.761658 0.9008026 1.586906158
10 1.8848168 0.9029649 1.701923392
Table 3-Young’s Modulus calculations after load applied.
Table 4-Stress calculations were made after mandrel is applied
46. 6.2 Disbondment Area
Before, removing the coating 8 lines set from the boundaries of plastic tube placed. Then,
disbonded area calculated after coating removed.
Disbonded area were calculated by using the average of the eight evenly – spaced
measurements of disbonded radius. The area calculated weren’t included the area of the
drilled artificial defect.
The disbonded were calculated by the equation:
A= 𝜋𝜋(R2 + 6R)
R= (R1+R2+R3+R4+R5+R6+R7+R8)/8, in millimetres.
A= disbonded area, in square millimetres
R= average radial disbondment beyond the edges of the drilled defect, determined
according to the relationship.
Figure 38-8 lines set from the plastic tube boundaries
47. 1% Strain applied samples
0.5 % Strain applied samples
Delamination area
Delamination area
49. Strain applied on the samples compared to time test time duration
As seen, 1% delamination area is higher than 0% delamination area. Therefore, higher stress
level increase the delamination area, however 0.5 % delamination area decreased the
delamination area.
0
50
100
150
200
250
300
1 2 3 4
DelamnationArea%mm2
Test Duration
Delamination Area
1% 0% 0.50%
Delamination % vs. Test duration
50. Disbonded area shown for all the experimental samples, stress applied and no stress applied.
0.5 % and 1 % strain values compared based on their stress applied and delamination area.
0
50
100
150
200
250
300
350
400
450
500
Sample 1 Sample 2 Sample 3 Sample 5 Sample 7 Sample 10 Sample 4 Sample 8 Sample 9
Disbonded Area
51. 6.3 Elongation Limit
Elongation is equal to changed length ∆l divided by initial length. To find ∆l displacement used
in appendix C. Then elongation divided by the calculated stress applied in section 6.1. At the
end elongation limit were found by dividing stress by elongation %.
Displacement mm Elongation % Stress applied Elongation limit
Sample 1 293.1 1.409134615 1.770134263 1.256185352
Sample 2 290.6 1.397115385 2.590314444 1.85404475
Sample 3 292.7 1.407211538 1.444464888 1.026473169
Sample 5 297.2 1.428846154 1.233711259 0.863431837
Sample 7 299.3 1.438942308 1.586906158 1.102828202
Sample 10 299.1 1.437980769 1.701923392 1.183550871
As seen elongation limit of samples 1, 2, 3 are higher than sample 5,7,10. Therefore, sample
5, 7 and 10 break easily compare to sample 1, 2 and 3. Higher elongation limit increase the
resistance due to breaks and deformations. However, due to sample results elongation did
not change the results a lot.
52. 7. Discussion
The aim of this project was to find, if tensile stress is an effect on cathodic disbondment under
cathodic protection. To understand the results different strain levels applied samples
compared with their disbonded areas, stress levels and compared with other research which
were already tested.
After results analysed and compared with literature review, this experiment proofed that
tensile stress is an effect of cathodic disbondment. The highest delamination area observed
in 1 % and then 0% (no stress applied) applied samples and the least delamination area
observed in 0.5 % applied sample.
According to Mehta (Mehta 1991), tensile effect observed on cathodic disbondment test
under different strain levels which also close to my results too. Also, in Mehta’s results higher
stress level resulted more disbonded area and less stress on coating increased the coating
quality.
Organic coatings do not have plastic region, which explained in literature review section 3.2.1.
If the stress applied on the coating pass elasticity limit, the coating breaks because organic
coating’s plastic region do not exists and coating loses its elasticity.
Lower strain level applied (0.5 %) samples showed better coating performance than no stress
applied coating. When metal substance applied certain load metal surface roughness increase
and as a result, the coating attachment surface area increase. As the coating attachment area
increase, coating touches more area on the metal surface and less disbondment observed
compare to other metal samples.
53. As in literature review in section 3.2.1, different coating thicknesses showed similar coating
performance under same loads, because thickness of the coating in-directly proportional to
elastic energy. If the thickness increase elastic energy decreases. Therefore, under same load
coatings showed same coating performance.
7.1 Answering key research Questions?
1) What is the effect of mechanical stress on coating under cathodic disbondment test?
Mechanical stress definitely effect coating performance by applied strain level. If the applied
strain goes over elastic region of the coated property coating most likely to break, however
if the applied stress could stay in elastic region, then coating performance would increase.
2) How to define maximum or minimum mechanical stress of coating?
The coating which would use must be mechanically analysed before applying stress. The
applied stress must be suitable for the coating’s strain limit and the coating must stay in its’
elastic region.
3) Is this test proves that stress is critical for coating performance under cathodic
disbondment test?
This test proves that to increase coating performance for cathodic disbondment test,
specific stress should be applied. To find the specific stress future work must be done.
54. 7.2 Future Work
I strongly advice that, this project must be continued from where it stopped. As, I have proved
that tensile stress is an effect on cathodic disbondment, with more future work a formula
could be found to decrease disbondment area by applying special stress levels for different
coating types.
If a formula could be found which I strongly believe it could be found, pipeline industry could
find a method to mitigate corrosion of pipelines.
With different type of epoxy coatings different type of stress levels could be applied to
understand the effect. Then, by comparing the results, based on stress levels a formula could
be found from the analyses. 24 test samples could be enough to prove a formula.
Also, different type of tests could be used to see the effects. Mandrel test, stretching test,
punching test and 3 point tests could be used.
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