EFFECT OF TENSILE STRESS ON CATHODIC DISBONDMNET OF

COATINGS NAME: BURAK CACAN

SUPERVISORS: PROFFESOR MIKE TAN, FARI MAHDAVI

MAY 31, 2015

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.

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

6.2 Disbondment Area ...................................................................................................................... 45

6.3 Elongation Limit .......................................................................................................................... 50

7. Discussion .......................................................................................................................................... 51

7.1 Answering key research Questions? ........................................................................................... 52

7.2 Future Work ................................................................................................................................ 53

References ............................................................................................................................................ 54

Appendix A ............................................................................................................................................ 58

Appendix B ............................................................................................................................................ 60

Appendix C ............................................................................................................................................ 62

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

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.

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.

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.

2.1 Project 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.

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?

3. Literature Review

3.1 Pipeline 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

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

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).

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

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

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

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

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.

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.

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

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

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 * ( 𝑇𝑇𝐷𝐷𝐷𝐷+𝑇𝑇

)

Figure 7- Strain %

Figure 8- Nomenclature

Mandrel shoe diameter calculated % strain level;

Mandrel Number Mandrel Diameter (Dm) Mandrel Strain %

1 1095 0.5

2 728 0.75

3 545 1

4 435 1.25

5 361 1.5

6 309 1.75

7 270 2

8 215 2.5

9 178 3

10 132 4 Table 1- Strain calculations

Figure 9- Mandrel Shoe Diameter

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

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.

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 SX65 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

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

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

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

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

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

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.

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

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

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

Note: The experiment could be stopped any time by the emergency button.

Figure 28-Instron 100 kN emergency button

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

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

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

A simple current system used as seen in figure 33.

Figure 33-DC Power Source Circuit System

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

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

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 * ( 𝑇𝑇𝐷𝐷𝐷𝐷+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

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.

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.

𝐹𝐹𝐴𝐴

= Y* ∆𝑙𝑙𝑙𝑙

Y = 𝐹𝐹𝐴𝐴

* 𝑙𝑙∆𝑙𝑙

Figure 37-. Points touched, where the mandrel experiment stops

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

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

1% Strain applied samples

0.5 % Strain applied samples

Delamination area

Delamination area

No Strain applied samples

Delamination area

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

Dela

mna

tion

Area

% m

m2

Test Duration

Delamination Area

1% 0% 0.50%

Delamination % vs. Test duration

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

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.

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.

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.

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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Appendix A

Appendix B

Appendix C

Sample 1

Sample 2

Sample 3

Sample 5

Sample 7

Sample 10