A Study On The Corrosion Behavior Of Welded 304 Stainless Steel.
UNIVERSITI TEKNIKAL MALAYSIA MELAKA
A STUDY ON THE CORROSION BEHAVIOR OF WELDED 304
STAINLESS STEEL
This report submitted in accordance with requirement of the Universiti Teknikal
Malaysia Melaka (UTeM) for the Bachelor Degree of Manufacturing Engineering
(Engineering Materials)
by
GOO CHAY SHUEN
B050710052
FACULTY OF MANUFACTURING ENGINEERING
2011
DECLARATION
I hereby, declared this report entitle “A Study on the Corrosion Behavior of Welded
304 Stainless Steel” is the results of my own research except as cited in references.
Signature
:
…………………………………………………………………..
Author Name :
…………………………………………………………………..
Date
…………………………………………………………………..
:
APPROVAL
This report is submitted to the faculty of Manufacturing Engineering of UTeM as a
partial fulfillment o the requirements for the degree of Bachelor of Manufacturing
Engineering (Engineering Materials) with Honors. The member of the supervisory
committee is as follow:
(Signature of Supervisor)
………………………………………………..
(Official Stamp of Supervisor)
ABSTRACT
Austenitic stainless steels find important and variety application such as construction
materials in chemical and petrochemical industries, civil engineering, marine,
shipbuilding, oil and gas exploitation, and et cetera. 304 stainless steel is one of most
popular austenitic stainless steel used for bottom hull of ship structure. Nevertheless,
welding of stainless steels alter the stability of the passive layer causes the corrosion
issue arises when it is exposed to the marine environment. This work is aim to
determine the effect of pH and temperature on the corrosion behavior of welded 304
stainless steel. An immersion test and electrochemical test (Tafel Slope) were
employed in order to obtain the corrosion rate of the steel. Two types of samples
were used in this project; unwelded and welded 304 stainless steels. Temperature and
pH of the 3.5% NaCl solution are the parameters that involve in determining the
corrosivity of the welded 304 stainless steels. Electrochemical test results showed
that pitting corrosion in welded samples were more severe in acidic solution (pH3) at
higher temperature (95±5°C) than alkaline solution (pH 12) at room temperature
(25±5°C). Moreover, in immersion test the result indicated that the highest weight
loss rate and corrosion rate was in pH 3 at 95±5°C for both 7 days (0.1460 mm/yr)
and 14 days (0.2812 mm/yr). For instant, the welded samples given the highest
corrosion rate compared to unwelded samples for both immersion test and
electrochemical test. Furthermore, mechanical test for tensile test and hardness test of
weldments decrease when subjected to high temperature (95±5°C) and acidic
solution (pH 3). Visual inspection and optical microstructure (OM) were used to
examine the microstructure features of the test samples before and after corrosion
attacked as well as recognized types of corrosion form on the samples.
i
ABSTRAK
Keluli tahan karat adalah sering digunakan dalam pelbagai bidang industri seperti
dalam bidang kimia dan petrokimia , minyak dan ekploitasi gas, kejuruteraan awan,
marin, serta perkapalan, dan sebagainya. Keluli tahan karat 304 adalah salah satu
jenis austenit keluli tahan karat yang paling popular digunakan di bahagian bawah
struktur kapal. Namun demikian, Kimpalan tahan karat menukar kestabilan dari
lapisan pasif menyebabkan masalah korosi muncul ketika terkena dengan
persekitaran laut. Karya ini bertujuan untuk megetahui pengaruh pH and suhu pada
kelakuan kakisam kimpalan keluli tahan karat 304. Ujian perendaman dan uji
elektrokimia (tafel lereng) telah memperkenalkan untuk mendapatkan kadar kakisan
kepada keluli tahan karat. Sampel yang digunakan dalam projek ini adalah kimpalan
dan tanpa kimpalan keluli tahan karat 304. Perilaku kakisan terhadap kimplam
sample di dalam larutan NaCl pada pH and suhu yang berbeza telah dikaji melalui
teknik ini. Keputusan uji elektrokimia menunjukkan bahawa kakisan bopeng pada
sampel yang dikimpal lebih parah dalam larutan asid (pH 3) pada suhu tinggi
(95±5°C) berbanding dengan larutan alkali (pH 12) pada suhu bilik (25±5°C).
Manakalah, keputusan perendaman menunjukkan bahawa kadar penunrunan berat
sampel dan kadar kakisam yang paling tinggi adalah pada pH 3 dalam 95±5°C untuk
7 (0.1460 mm/tahun) hari dan 14 hari (0.2812 mm/tahun). Selanjut itu, kadar kakisan
untuk sample yang telah dikimpal adalah lebih tinggih daripada sample yang tanpa
dikimpal, fakta ini telah membuktikan dalam ujian perendaman and ujian
elektrokimia. Tamban lagi, keputusan menunjukkan bahawa ujian mekanik bagi
ujian tarik dan ujian kekerasan kepada kimpalan sample menurun apabia terkena
pada suhu tinggi (95±5°C) dalam larutan asid (pH 3). Akhirnya, SEM dan Om
diggunakan untuk mengaji struktur kimpalan sample sebelum dan selepas kakisan
menyerang dan juga menetukan jenis kakisan yang dibentuk pada sample.
ii
DEDICATION
This work is dedicated to my parents, family members, without their patience, caring,
understanding, and support the completion of this work would not have been possible,
and to the memory of my grandparents, who passed on loved of reading and respect
for education.
iii
ACKNOWLEDGEMENT
I would like to thanks and express my gratefulness to my supervisor Mrs Rahmah for
her guidance and support in making this project finish. Her advices, comments,
suggestions and tolerance are so helpful and without her, I would face many
difficulties to finish my Final Year Project.
Last but not least, not forget to express my appreciation to my beloved parent as well
as my beloved friends who kindly assisted and encouraged me to ensure all my
activity of this Final Year project went smoothly according to the plan. And once
again, thanks to everyone who involve in my journey.
iv
TABLE OF CONTENT
Abstract
i
Abstrak
ii
Dedication
iii
Acknowledgement
iv
Table of Content
v
List of Tables
viii
List of Figures
ix
List Abbreviations
xiii
List of Symbols & Nomenclature
xv
1
INTRODUCTION
1.1
Background of Study
1
1.2
Problem Statement
2
1.3
Objectives of the Project
4
1.4
Scopes of the Project
4
2
LITERATURE REVIEW
2.1
Corrosion Principle
5
2.2
Corrosion Mechanism
6
2.2.1
Alkaline and Neutral Solutions
6
2.2.2
Acidic Solutions
7
2.3
Weld Zone Corrosion
7
2.4
Types of Corrosion of Stainless Steel
9
2.4.1
Uniform (General) Corosion
9
2.4.2
Galvanic Corrosion
10
2.4.3
Pitting Corrosion
11
2.4.4
Crevice Corrosion
13
2.4.5
Intergranular
14
2.4.6
Stress Corrosion Cracking
15
2.5
Electrochemical Measurement
17
v
2.6
Material Properties
21
2.6.1
Stainless Steel
21
2.6.2
Austenitic Stainless Steel
21
2.6.3
304 Stainless Steel
22
2.6.4
Welding of Austenitic Stainless Steel
24
2.7
Factor Affected Corrosion
24
2.7.1
Time
25
2.7.2
Temperature
25
2.7.3
Acidity and Alkalinity (pH)
25
2.7.4
Chlorine Concentration
26
2.8
Welding Process
27
2.8.1
Tungsten Inert Gas Welding Process
27
3
METHODOLOGY
3.1
Introduction
29
3.2
Sample Preparation
31
3.3
TIG Welding
32
3.4
Sample Preparation Before and After Corrosion Test
33
3.5
Metallographic Examination on Corrosion and Mechanical
35
Test
3.6
Corrosion Test
36
3.6.1
Immersion Test
36
3.6.2
Electrochemical Test
38
3.7
Mechanical Test
40
3.7.1
Tensile Test
40
3.7.2
Vickers Hardness Test
42
4
RESULTS AND DISCUSSIONS
4.1
Composition Analysis
45
4.2
Microstructure Examination of As-Received Samples
46
4.3
Microstructure Examination Before Corrosion Test
47
4.4
Result of Immersion Test
49
4.4.1
Visual Inspection for Immersion Testing
53
vi
4.4.2
Microstructure Analysis after Immersion Test
54
4.4.2.1
Base Metal
54
4.4.2.2
Weld Metal
55
4.4.2.3
Fusion Line
56
4.4.2.4
Heat Affected Zone
56
4.5
Result of Electrochemical Test
57
4.5.1
Polarization Result
58
4.5.2
Visual Inspection for Electrochemical Testing
61
4.5.3
Microstructure Analysis after Electrochemical Test
63
4.5.3.1
Base Metal
63
4.5.3.2
Weld Metal
64
4.5.3.3
Fusion Line
65
4.5.3.4
Heat Affected Zone
66
4.6
Surface Morphology
67
4.7
Result of Tensile Test
69
4.8
Result of Hardness Test
71
5
CONCLUSIONS AND RECOMMENDATIONS
5.1
Conclusions
74
5.2
Recommendations
75
REFERENCES
77
APPENDICES
81
A
81
B
82
C
83
D
84
E
85
F
88
G
91
vii
LIST OF TABLES
2.1
Composition Ranges for 316L Stainless Steel
23
3.1
Effect of pH When Temperature is Fixed:
37
3.2
Effect of Temperature When pH is Fixed
37
3.3
Standard Test Pieces Dimension for Tensile Test
42
4.1
Chemical Composition of Unwelded and Welded 304 Stainless
45
Steel
4.2
Corrosion Rate of Specimens Expressed in mm/yr With Different
50
Temperature and pH for 7 Days
4.3
Corrosion Rate of Specimens Expressed in mm/yr With Different
50
Temperature and pH at 14 Days
4.4
Tafel Extrapolation Parameter at Different pH at Different
59
Temperature of Unwelded Samples
4.5
Tafel Extrapolation Parameter at Different pH at Different
59
Temperature of Welded Samples
4.6
Tensile Stress Result of Welded Samples at Different pH With
69
Same Temperature in NaCl Solution After Test
4.7
Tensile Stress Result of Welded Samples at Different
70
Temperature With Same pH in NaCl Solution After Test
4.8
Result of Hardness Test for Unweld and Weld Samples at
72
Different pH With Same Temperature in NaCl Solution After
Test
4.9
Result of Hardness Test for Unweld and Weld Samples at
Different Temperature With Same pH in NaCl Solution After
Corrosion Test
viii
72
LIST OF FIGURES
2.1
Schematic Diagram of Stress Cracking Corrosion at Welded Zone
9
2.2
Schematic Diagram of Uniform Corrosion
9
2.3
Schematic Diagram of Galvanic Corrosion
10
2.4
Schematic Diagram of Types of Pitting Corrosion Shape
11
2.5
Schematic Diagram of The Fracture Surface of 316L Weld-HAZ
16
Metal Specimens After SCC Test
2.6
Schematic Diagram of The Fracture Surface of 316L Weld Metal
16
Specimens After SCC Test
2.7
Tafel Slope
18
2.8
Linear Polarization Resistance
20
2.9
Schematic Diagram Showing Different Region of the Weldment
24
2.10
Schematic Diagram of TIG Welding Set Up
27
3.1
Process Flow Chart
30
3.2
Dimension of AISI 316L Plate
31
3.3
Dimension of AISI 316L Plate After Cut
31
3.4
TIG Welding Machine
32
3.5
Dimension of AISI 316L After Welded Joint
33
3.6
Schematic Diagram for Soldered Copper Wire With Glass Tube
34
Attached to Backside of The Test Specimens
3.7
Grinding Machine and SiC Paper
35
3.8
Schematic Diagram for Mounting Sample
35
3.9
SEM and OM
36
3.10
Schematic Diagram for Immersion Test
38
3.11
Schematic Diagram for Electrochemical Cell
39
3.12
Universal Testing Machine
40
3.13
Schematic Diagram of Rectangular Tension Specimens Which
41
are Machined from Butt-Welded Plate With The Weld Crossing
in The Midsection of The Specimen
3.14
Schematic Diagram for Tensile Weldment
ix
42
3.15
Schematic Diagram for Vickers Weldment
43
3.16
Picture for Vickers Hardness Machine
43
4.1
Optical Micrographs of As Received 304 Stainless Steel With
46
Magnification: x50: and Magnification: x100
4.2
Picture Showing Cross Section of Welded Samples at Different
47
Regions
4.3
Optical Micrographs of As Received 304 Stainless Steel With
47
Magnification: x20: and Magnification: x50
4.4
Weld Metal at Magnification x20 & x100
48
4.5
Fusion Line at Magnification x20 & x100
48
4.6
HAZ at Magnification x20& HAZ x100
49
4.7
Bar Chart Showing Corrosion Rate of Welded Samples Between
51
7 Days and 14 Days With Different Temperature
4.8
Bar Chart Showing Corrosion Rate of Between 7 Days and 14
51
Days With Different pH
4.9
Bar Chart Showing Corrosion Rate Between 7 Days and 14 Days
52
With Different Temperature and pH
4.10
Bar Chart Showing The Comparison Between The Corrosion
53
Rate of Unwelded and Welded Samples With Different
Temperature and pH After Immersion Test
4.11
Picture of Specimens Before Immersion Test
53
4.12
Picture Specimens After Immersion Test in 3.5% NaCl Solution
54
in pH7 at 25±5°C & in pH 3 at 95±5°C
4.13
Optical Micrograph of Base Metal After Immersion Test in
55
Different Conditions With Magnification x20
4.14
Optical Micrograph of Weld Metal After Immersion Test in
55
Different Conditions With Magnification x20
4.15
Optical Micrograph of Fusion Line After Immersion Test in
56
Different Conditions With Magnification x20
4.16
Optical Micrograph of Heat Affected Zone After Immersion Test
in Different Conditions With Magnification x20
x
57
4.17
Bar Chart Showing Corrosion Rate of Unwelded and Welded
Samples
at
pH
3
With
Different
Temperature
60
After
Electrochemical Test
4.18
Bar Chart Showing Corrosion Rate of Unwelded and Welded
60
Samples at Same Temperature With Different pH After
Electrochemical Test
4.19
Bar Chart Showing Corrosion Rate of Unwelded and Welded
61
Samples at Different Temperature and pH After Electrochemical
Test
4.20
Picture of Specimens Before Electrochemical Test
62
4.21
Picture Specimens After Test Electrochemical Test in Acidic
62
Solution (pH3) at25±5°C
4.22
Picture Specimens After Test Electrochemical Test in Neutral
62
Solution (pH7) at25±5°C
4.23
Picture Specimens After Test Electrochemical Test in Alkaline
63
Solution (pH12) at25±5°C
4.24
Picture Specimens After Test Electrochemical Test in Acidic
63
Solution (pH3) at 65±5°C
4.25
Picture Specimens After Test Electrochemical Test in Acidic
63
Solution (pH3) at 95±5°C
4.26
Optical Micrograph of Base Metal After Electrochemical Test in
64
Different Conditions With Magnification x20
4.27
Optical Micrograph of Weld Metal After Electrochemical Test in
65
Different Conditions With Magnification x20
4.28
Optical Micrograph of Fusion Line After Electrochemical Test in
66
Different Conditions With Magnification x20
4.29
Optical Micrograph of Heat Affected Zone After Electrochemical
66
Test in Different Conditions With Magnification x20
4.30
Surface Morphology of As Receive Sample With Magnification
67
x1000
4.31
Surface Morphology of Welded Sample With Magnification x18
xi
67
4.32
Surface Morphology of Base Metal With Magnification x1000
68
4.33
Surface Morphology of Weld Metal With Magnification x1000
68
4.34
Surface Morphology of Heat Affected Zone With Magnification
69
x1000
4.35
Bar Chart Showing Tensile Stress Result of Welded Samples at
70
Different pH With Same Temperature in NaCl Solution
4.36
Bar Chart Showing Tensile Stress Result of Welded Samples at
71
Different Temperature With Same pH in NaCl Solution
4.37
Bar Chart Showing Hardness Value of Unweld and Weld
73
Samples at Different pH With Same Temperature After Corrosion
Test
4.38
Bar Chart Showing Hardness Value of Unweld and Weld
Samples at Different Temperature With Same pH After Corrosion
Test
xii
73
LIST OF ABBREVIATIONS
Ag-AgCl
-
Argentum-Argentum Chloride
AISI
-
American Iron and Steel Institute Specification
ASTM
-
American Society for Testing and Materials
AUX
-
Auxiliary Electrode
B
-
Tafel constant
BM
-
Base Metal
C
-
Carbon
Cl
-
Chlorine Atom
Cr
-
Chromium
DECN
-
Direct-Current Electrode Negative (DCEN)
DECP
-
Direct-Current Electrode Positive
DLEPR
-
Double Loop Electrochemical Potentiokinetic Reactivation
EDX
-
Elemental Diffraction X-ray
EPR
-
Electrochemical Potentiokenetic Reactivation
Etc
-
Et Cetera
F
-
Faraday’s constant
FCC
-
Face-Centered Cubic
Fe
-
Ferrous
FL
-
Fusion Line
FS
-
Fracture Strength
GTAW
-
Gas Tungsten-Arc Welding
H
-
Hydrogen atom
HAZ
-
Heat Affected Zone
H2O
-
Water
HV
-
Vickers Hardness
MIC
-
Microbiologically Influenced Corrosion
Mm
-
Millimetre
Mn
-
Manganese
N
-
Nitrogen Atom
xiii
NaCl
-
Sodium Chloride
Nb
-
Niobium
O
-
Oxygen Atom
OH
-
Hydroxyl Atom
OM
-
Optical Microscope
P
-
Phosphorus
Ppm
-
Part Per Million
PREN
-
Pitting Resistance Equivalent Number
Q
-
Total Charge
RE
-
Reference Electrode
Rp
-
Polarization Resistance
S
-
Sulphur
SCC
-
Stress Corrosion Cracking
SEM
-
Scanning Electron Microscope
Si
-
Silicon
SRB
-
Sulfate Reducing Bacteria
ThO2
-
Thrianite
Ti
-
Titanium
TIG
-
Tungsten Inert Gas
UTS
-
Ultimate Tensile Strength
W
-
Tungsten
WE
-
Working Electrode
WM
-
Weld Metal
YS
-
Yield Strength
xiv
LIST OF SYMBOLS & NOMENCLATURE
-
Exposed specimen area,
-
Atomic Weight
-
Anodic Tafel constant
-
Cathodic Tafel constant
°C
-
Degree Celsius
D
-
Arithmetic Mean of Two Diagonal
Eoc
-
Open Circuit Potential
Ecorr
-
Corrosion potential,V
EW
-
Equivalent Weight
-
The mass fraction of the ith element in the alloy
I
-
Applied Current Density, µA
Io
-
Exchanger Current Density, µA
Icorr
-
Corrosion current, µA
η
-
Overpotential
icorr
-
Corrosion current density, µA/
-
Polarization Resistance
AW
P
Load Force Kgf
-
Density, g/cm3
-
Surface Area
-
Volume, cm3
W
-
Mass of Material removal
W%
-
Weight percentage
Z1
-
Valence of Each Component
SA
xv
CHAPTER 1
INTRODUCTION
1.1
Background of Study
The importance of stainless steel has increased in the last 30 years. This is because of
the development of corrosion and oxidation-resistant material. It is commonly used
in variety of field such as in chemical industry, environment technology, civil
engineering, power and energy generation, domestic equipment, and bottom hull of
ship in marine environment.
From Tablbolt and Talbot (2007), stainless steel can be classified into five group
such as martensitic stainless steels, ferritic stainless steels, austenitic stainless steels,
duplex (ferritic-austenitic) stainless steels and precipitation-hardening stainless steels.
Among all those groups, alloys 316 and 316L are one of the subset of austenitis
stainless steel. Besides 316L stainless steel also called molybdenum-bearing
austenitic stainless steel, which exhibit better corrosion resistance than the
conventional chromium–nickel austenitic stainless steel (304).
Type of 304 stainless steel has lower carbon content of less than 0.08 to avoid
(sensitization) grain boundary chromium carbide precipitated and to provide
corrosion resistance when explore to higher temperature during welding fabrication.
(Atanda et al, 2010)
Recently, 304 stainless steel is considered to be good weldable of the stainless steel
due to their good mechanical properties and corrosion resistance. It is because the
stainless steel is protected by the chromium oxide protective layer on the surface.
(Venu and Rostron, 2009)
1
However, welding induces the degradation or corrosion failure when this layer is
damaged. Other than that, corrosion resistance can be maintained in the welded area
by balanced the composition of alloy to inhibit certain precipitation reaction, or by
increased the nitrogen gas or shielding gas to the weld environment. Besides that,
choosing the proper welding parameter can also maintained the corrosion resistance.
There are two important considerations for weld joints. One is to avoid solidification
cracking, and the other one is to preserve the corrosion resistance of the weld and
heat-affected zones. (Brooks and Lippold, 1993) In order to avoid corrosion, a
tailored combination of steel grade, welding method and filler metal should be
chosen properly. Therefore, tungsten inert gas welding process is one of the best
choices for running the welding process due to it slower arc welding speed, weld can
be made with or without filler, and precise control of welding heat.
1.2
Problem Statement
Normally, corrosion resistance is the significant characteristic of the stainless steel in
various environments due to present the chromium oxide protective layer on the
surface film. The harshness of the environmental factors is mostly dependent on the
chloride concentration content, pH and temperature. Meanwhile, it is aggressive to
cause corrosion failure either pitting or crevice due to the present in acidic chloride
environment and it become severity at low pH and higher temperature. The present
of chloride ions will destroy and breakdown the protective oxide layer consequently
leads failure in stainless steel. (Malik et al, 1992)
Malik and Al-Fozan (1993) had been proved that, in order to overcome this
sophisticated problem, the composition of alloy on this layer should be changes by
substituted appropriate amount of elements. One of the most common stainless steel
often used under marine environments is recognized as austenitic 304 stainless steel.
These types of steel have higher contents of chromium (Cr) which purposely work as
forming the passive film and provide additional protection to resist corrosion attack.
The stainless steel will never corrode uniformly in the marine environment. Basically,
pitting and crevice corrosion is considered as the most common cause of failure of
2
stainless steel in marine crevice. However, Heselmans. (2006) indicated that within
this two corrosion failure, crevice corrosion is said to be induced a major problem in
marine environment than pitting corrosion because of the lower resistivity of the
water.
It has been reported that sulfate reducing bacteria (SRB) can exist in the sea water
environment. When this stainless steel is immersed in the sea water, the potential
problem for this material is occurred due to presence of microorganisms on the
surface for example sulfate-reducing bacterial (SRB) , iron-oxidizing bacterial,
sulfur-oxidizing bacterial, and manganese-oxidizing bacterial. These bacterial can
begin to accelerate the corrosion process by use up the protective layer. (Abraham et
al, 2009).
Furthermore Abraham et al (2009) also indicated that when SRB activity in an
anaerobic conditions it will cause severe attack such as pit propagation. The bacterial
are likely to grow within the pit cavity and produce and acidic chlorine environment.
Once pitting is initiated, the kinetic pitting has strong tendency for it to continue to
grow and propagated. Although true, according to the theory it is well known that
chromium is easily affected by the chlorine attack, therefore the depletion of
chromium in the surface of pitted region led to the loss of passivity. This phenomena
would increase the corrosion rate and leading to corrosion failure. From the research
result its indicated that pitting in the marine environmen is caused by
microbiologically influenced corrosion (MIC) which can led to 316L stainless steel
pipeline system leakage.
Form the reseach result of Heselmans (2006), it is recorded that bioflims are the main
factor to induced corrosion happended in nature seawater and brackish water. Due to
the formation of biofilms and mircobiological active, nature sea water is more
corrosive than properly chlorinated seawater. Natural seawater is saturated with
oxygen and normal aerobic bioflim promotes the cathodic reaction. Thus, pitting will
start at the anodic reaction. While, most of the corrosion problems with stainless steel
can be avoived by used of cathodic protection. This cathodic protection can decrease
the rest potential back in the safe passive area, hence it helps to aviod MIC and
3
localised corrosion. Consequently, its can improved the perfomance of stainless steel
in marine enviroment and brackish water.
1.3
Objectives of the Project
The objectives of this work is to study the effect of temperature and pH on the
corrosion behavior of welded 316L stainless steel by means of electrochemical test
and immersion test to compare the result obtained of these two techniques.
1.4
Scopes of the Project
This project will focus primarily on investigated the corrosion behavior of 316L weld
stainless steel.
The welding method and welding process will be discussed in this project. Basically,
welded joint of austenitic 304 stainless steel is performed by using TIG (Tungsten
Inert Gas) welding process. Mechanical tests at the welded area were carried out
through tensile test and hardness test according to the ASTM standard. Furthermore,
corrosion tests were carried out in order to evaluate the corrosion behavior of 304
stainless steel in various temperature and pH. Therefore, the immersion test (weight
loss) and electrochemical test were utilized to study the influenced of different
factors affecting the corrosion. Both of the corrosion tests were carried out based on
the methods reported in ASTM standard. For metallographic examination OM
(optical microscope) and SEM (scanning electron microscope) is being used in order
to examine the microstructural of the welded area.
4
CHAPTER 2
LITERATURE REVIEW
2.1
Corrosion Principle
Corrosion is a naturally occurring phenomenon. Most metal corrosion occurs via
electrochemical reactions at the interface between the metal and an electrolyte
solution. Roberge (2008) is defined corrosion as a deterioration of the metal due to
reaction with the surrounding aqueous environment. In fact all the corrosion in
metallic materials is an electrochemical process where the anodic and cathodic
reaction is occurring at the same time. The anode is typically thought of as the
negative electrode where oxidation process takes place which give away the electron
to form corrosion. Whereas, the cathode is act as the positive electrode where
reduction process is take place which accepted the electron. When this two reaction
are in equilibrium, the flow of electrons from each reaction is balanced, therefore no
net electron flow occurs.
Bardal (2003) classified corrosion of metallic material into three main groups. First
group is wet corrosion where it is occurred when a liquid is present. This usually
involves aqueous solution or electrolyte and typically undergoes electrochemical
process. Second group is dry corrosion where the corrosive environment is a dry gas.
Dry corrosion also known as chemical corrosion the most popular example is high
temperature corrosion. And the last group is corrosion in other fluids such as fused
salts and molten metals. However, this project is only concentrate on wet corrosion.
5
A STUDY ON THE CORROSION BEHAVIOR OF WELDED 304
STAINLESS STEEL
This report submitted in accordance with requirement of the Universiti Teknikal
Malaysia Melaka (UTeM) for the Bachelor Degree of Manufacturing Engineering
(Engineering Materials)
by
GOO CHAY SHUEN
B050710052
FACULTY OF MANUFACTURING ENGINEERING
2011
DECLARATION
I hereby, declared this report entitle “A Study on the Corrosion Behavior of Welded
304 Stainless Steel” is the results of my own research except as cited in references.
Signature
:
…………………………………………………………………..
Author Name :
…………………………………………………………………..
Date
…………………………………………………………………..
:
APPROVAL
This report is submitted to the faculty of Manufacturing Engineering of UTeM as a
partial fulfillment o the requirements for the degree of Bachelor of Manufacturing
Engineering (Engineering Materials) with Honors. The member of the supervisory
committee is as follow:
(Signature of Supervisor)
………………………………………………..
(Official Stamp of Supervisor)
ABSTRACT
Austenitic stainless steels find important and variety application such as construction
materials in chemical and petrochemical industries, civil engineering, marine,
shipbuilding, oil and gas exploitation, and et cetera. 304 stainless steel is one of most
popular austenitic stainless steel used for bottom hull of ship structure. Nevertheless,
welding of stainless steels alter the stability of the passive layer causes the corrosion
issue arises when it is exposed to the marine environment. This work is aim to
determine the effect of pH and temperature on the corrosion behavior of welded 304
stainless steel. An immersion test and electrochemical test (Tafel Slope) were
employed in order to obtain the corrosion rate of the steel. Two types of samples
were used in this project; unwelded and welded 304 stainless steels. Temperature and
pH of the 3.5% NaCl solution are the parameters that involve in determining the
corrosivity of the welded 304 stainless steels. Electrochemical test results showed
that pitting corrosion in welded samples were more severe in acidic solution (pH3) at
higher temperature (95±5°C) than alkaline solution (pH 12) at room temperature
(25±5°C). Moreover, in immersion test the result indicated that the highest weight
loss rate and corrosion rate was in pH 3 at 95±5°C for both 7 days (0.1460 mm/yr)
and 14 days (0.2812 mm/yr). For instant, the welded samples given the highest
corrosion rate compared to unwelded samples for both immersion test and
electrochemical test. Furthermore, mechanical test for tensile test and hardness test of
weldments decrease when subjected to high temperature (95±5°C) and acidic
solution (pH 3). Visual inspection and optical microstructure (OM) were used to
examine the microstructure features of the test samples before and after corrosion
attacked as well as recognized types of corrosion form on the samples.
i
ABSTRAK
Keluli tahan karat adalah sering digunakan dalam pelbagai bidang industri seperti
dalam bidang kimia dan petrokimia , minyak dan ekploitasi gas, kejuruteraan awan,
marin, serta perkapalan, dan sebagainya. Keluli tahan karat 304 adalah salah satu
jenis austenit keluli tahan karat yang paling popular digunakan di bahagian bawah
struktur kapal. Namun demikian, Kimpalan tahan karat menukar kestabilan dari
lapisan pasif menyebabkan masalah korosi muncul ketika terkena dengan
persekitaran laut. Karya ini bertujuan untuk megetahui pengaruh pH and suhu pada
kelakuan kakisam kimpalan keluli tahan karat 304. Ujian perendaman dan uji
elektrokimia (tafel lereng) telah memperkenalkan untuk mendapatkan kadar kakisan
kepada keluli tahan karat. Sampel yang digunakan dalam projek ini adalah kimpalan
dan tanpa kimpalan keluli tahan karat 304. Perilaku kakisan terhadap kimplam
sample di dalam larutan NaCl pada pH and suhu yang berbeza telah dikaji melalui
teknik ini. Keputusan uji elektrokimia menunjukkan bahawa kakisan bopeng pada
sampel yang dikimpal lebih parah dalam larutan asid (pH 3) pada suhu tinggi
(95±5°C) berbanding dengan larutan alkali (pH 12) pada suhu bilik (25±5°C).
Manakalah, keputusan perendaman menunjukkan bahawa kadar penunrunan berat
sampel dan kadar kakisam yang paling tinggi adalah pada pH 3 dalam 95±5°C untuk
7 (0.1460 mm/tahun) hari dan 14 hari (0.2812 mm/tahun). Selanjut itu, kadar kakisan
untuk sample yang telah dikimpal adalah lebih tinggih daripada sample yang tanpa
dikimpal, fakta ini telah membuktikan dalam ujian perendaman and ujian
elektrokimia. Tamban lagi, keputusan menunjukkan bahawa ujian mekanik bagi
ujian tarik dan ujian kekerasan kepada kimpalan sample menurun apabia terkena
pada suhu tinggi (95±5°C) dalam larutan asid (pH 3). Akhirnya, SEM dan Om
diggunakan untuk mengaji struktur kimpalan sample sebelum dan selepas kakisan
menyerang dan juga menetukan jenis kakisan yang dibentuk pada sample.
ii
DEDICATION
This work is dedicated to my parents, family members, without their patience, caring,
understanding, and support the completion of this work would not have been possible,
and to the memory of my grandparents, who passed on loved of reading and respect
for education.
iii
ACKNOWLEDGEMENT
I would like to thanks and express my gratefulness to my supervisor Mrs Rahmah for
her guidance and support in making this project finish. Her advices, comments,
suggestions and tolerance are so helpful and without her, I would face many
difficulties to finish my Final Year Project.
Last but not least, not forget to express my appreciation to my beloved parent as well
as my beloved friends who kindly assisted and encouraged me to ensure all my
activity of this Final Year project went smoothly according to the plan. And once
again, thanks to everyone who involve in my journey.
iv
TABLE OF CONTENT
Abstract
i
Abstrak
ii
Dedication
iii
Acknowledgement
iv
Table of Content
v
List of Tables
viii
List of Figures
ix
List Abbreviations
xiii
List of Symbols & Nomenclature
xv
1
INTRODUCTION
1.1
Background of Study
1
1.2
Problem Statement
2
1.3
Objectives of the Project
4
1.4
Scopes of the Project
4
2
LITERATURE REVIEW
2.1
Corrosion Principle
5
2.2
Corrosion Mechanism
6
2.2.1
Alkaline and Neutral Solutions
6
2.2.2
Acidic Solutions
7
2.3
Weld Zone Corrosion
7
2.4
Types of Corrosion of Stainless Steel
9
2.4.1
Uniform (General) Corosion
9
2.4.2
Galvanic Corrosion
10
2.4.3
Pitting Corrosion
11
2.4.4
Crevice Corrosion
13
2.4.5
Intergranular
14
2.4.6
Stress Corrosion Cracking
15
2.5
Electrochemical Measurement
17
v
2.6
Material Properties
21
2.6.1
Stainless Steel
21
2.6.2
Austenitic Stainless Steel
21
2.6.3
304 Stainless Steel
22
2.6.4
Welding of Austenitic Stainless Steel
24
2.7
Factor Affected Corrosion
24
2.7.1
Time
25
2.7.2
Temperature
25
2.7.3
Acidity and Alkalinity (pH)
25
2.7.4
Chlorine Concentration
26
2.8
Welding Process
27
2.8.1
Tungsten Inert Gas Welding Process
27
3
METHODOLOGY
3.1
Introduction
29
3.2
Sample Preparation
31
3.3
TIG Welding
32
3.4
Sample Preparation Before and After Corrosion Test
33
3.5
Metallographic Examination on Corrosion and Mechanical
35
Test
3.6
Corrosion Test
36
3.6.1
Immersion Test
36
3.6.2
Electrochemical Test
38
3.7
Mechanical Test
40
3.7.1
Tensile Test
40
3.7.2
Vickers Hardness Test
42
4
RESULTS AND DISCUSSIONS
4.1
Composition Analysis
45
4.2
Microstructure Examination of As-Received Samples
46
4.3
Microstructure Examination Before Corrosion Test
47
4.4
Result of Immersion Test
49
4.4.1
Visual Inspection for Immersion Testing
53
vi
4.4.2
Microstructure Analysis after Immersion Test
54
4.4.2.1
Base Metal
54
4.4.2.2
Weld Metal
55
4.4.2.3
Fusion Line
56
4.4.2.4
Heat Affected Zone
56
4.5
Result of Electrochemical Test
57
4.5.1
Polarization Result
58
4.5.2
Visual Inspection for Electrochemical Testing
61
4.5.3
Microstructure Analysis after Electrochemical Test
63
4.5.3.1
Base Metal
63
4.5.3.2
Weld Metal
64
4.5.3.3
Fusion Line
65
4.5.3.4
Heat Affected Zone
66
4.6
Surface Morphology
67
4.7
Result of Tensile Test
69
4.8
Result of Hardness Test
71
5
CONCLUSIONS AND RECOMMENDATIONS
5.1
Conclusions
74
5.2
Recommendations
75
REFERENCES
77
APPENDICES
81
A
81
B
82
C
83
D
84
E
85
F
88
G
91
vii
LIST OF TABLES
2.1
Composition Ranges for 316L Stainless Steel
23
3.1
Effect of pH When Temperature is Fixed:
37
3.2
Effect of Temperature When pH is Fixed
37
3.3
Standard Test Pieces Dimension for Tensile Test
42
4.1
Chemical Composition of Unwelded and Welded 304 Stainless
45
Steel
4.2
Corrosion Rate of Specimens Expressed in mm/yr With Different
50
Temperature and pH for 7 Days
4.3
Corrosion Rate of Specimens Expressed in mm/yr With Different
50
Temperature and pH at 14 Days
4.4
Tafel Extrapolation Parameter at Different pH at Different
59
Temperature of Unwelded Samples
4.5
Tafel Extrapolation Parameter at Different pH at Different
59
Temperature of Welded Samples
4.6
Tensile Stress Result of Welded Samples at Different pH With
69
Same Temperature in NaCl Solution After Test
4.7
Tensile Stress Result of Welded Samples at Different
70
Temperature With Same pH in NaCl Solution After Test
4.8
Result of Hardness Test for Unweld and Weld Samples at
72
Different pH With Same Temperature in NaCl Solution After
Test
4.9
Result of Hardness Test for Unweld and Weld Samples at
Different Temperature With Same pH in NaCl Solution After
Corrosion Test
viii
72
LIST OF FIGURES
2.1
Schematic Diagram of Stress Cracking Corrosion at Welded Zone
9
2.2
Schematic Diagram of Uniform Corrosion
9
2.3
Schematic Diagram of Galvanic Corrosion
10
2.4
Schematic Diagram of Types of Pitting Corrosion Shape
11
2.5
Schematic Diagram of The Fracture Surface of 316L Weld-HAZ
16
Metal Specimens After SCC Test
2.6
Schematic Diagram of The Fracture Surface of 316L Weld Metal
16
Specimens After SCC Test
2.7
Tafel Slope
18
2.8
Linear Polarization Resistance
20
2.9
Schematic Diagram Showing Different Region of the Weldment
24
2.10
Schematic Diagram of TIG Welding Set Up
27
3.1
Process Flow Chart
30
3.2
Dimension of AISI 316L Plate
31
3.3
Dimension of AISI 316L Plate After Cut
31
3.4
TIG Welding Machine
32
3.5
Dimension of AISI 316L After Welded Joint
33
3.6
Schematic Diagram for Soldered Copper Wire With Glass Tube
34
Attached to Backside of The Test Specimens
3.7
Grinding Machine and SiC Paper
35
3.8
Schematic Diagram for Mounting Sample
35
3.9
SEM and OM
36
3.10
Schematic Diagram for Immersion Test
38
3.11
Schematic Diagram for Electrochemical Cell
39
3.12
Universal Testing Machine
40
3.13
Schematic Diagram of Rectangular Tension Specimens Which
41
are Machined from Butt-Welded Plate With The Weld Crossing
in The Midsection of The Specimen
3.14
Schematic Diagram for Tensile Weldment
ix
42
3.15
Schematic Diagram for Vickers Weldment
43
3.16
Picture for Vickers Hardness Machine
43
4.1
Optical Micrographs of As Received 304 Stainless Steel With
46
Magnification: x50: and Magnification: x100
4.2
Picture Showing Cross Section of Welded Samples at Different
47
Regions
4.3
Optical Micrographs of As Received 304 Stainless Steel With
47
Magnification: x20: and Magnification: x50
4.4
Weld Metal at Magnification x20 & x100
48
4.5
Fusion Line at Magnification x20 & x100
48
4.6
HAZ at Magnification x20& HAZ x100
49
4.7
Bar Chart Showing Corrosion Rate of Welded Samples Between
51
7 Days and 14 Days With Different Temperature
4.8
Bar Chart Showing Corrosion Rate of Between 7 Days and 14
51
Days With Different pH
4.9
Bar Chart Showing Corrosion Rate Between 7 Days and 14 Days
52
With Different Temperature and pH
4.10
Bar Chart Showing The Comparison Between The Corrosion
53
Rate of Unwelded and Welded Samples With Different
Temperature and pH After Immersion Test
4.11
Picture of Specimens Before Immersion Test
53
4.12
Picture Specimens After Immersion Test in 3.5% NaCl Solution
54
in pH7 at 25±5°C & in pH 3 at 95±5°C
4.13
Optical Micrograph of Base Metal After Immersion Test in
55
Different Conditions With Magnification x20
4.14
Optical Micrograph of Weld Metal After Immersion Test in
55
Different Conditions With Magnification x20
4.15
Optical Micrograph of Fusion Line After Immersion Test in
56
Different Conditions With Magnification x20
4.16
Optical Micrograph of Heat Affected Zone After Immersion Test
in Different Conditions With Magnification x20
x
57
4.17
Bar Chart Showing Corrosion Rate of Unwelded and Welded
Samples
at
pH
3
With
Different
Temperature
60
After
Electrochemical Test
4.18
Bar Chart Showing Corrosion Rate of Unwelded and Welded
60
Samples at Same Temperature With Different pH After
Electrochemical Test
4.19
Bar Chart Showing Corrosion Rate of Unwelded and Welded
61
Samples at Different Temperature and pH After Electrochemical
Test
4.20
Picture of Specimens Before Electrochemical Test
62
4.21
Picture Specimens After Test Electrochemical Test in Acidic
62
Solution (pH3) at25±5°C
4.22
Picture Specimens After Test Electrochemical Test in Neutral
62
Solution (pH7) at25±5°C
4.23
Picture Specimens After Test Electrochemical Test in Alkaline
63
Solution (pH12) at25±5°C
4.24
Picture Specimens After Test Electrochemical Test in Acidic
63
Solution (pH3) at 65±5°C
4.25
Picture Specimens After Test Electrochemical Test in Acidic
63
Solution (pH3) at 95±5°C
4.26
Optical Micrograph of Base Metal After Electrochemical Test in
64
Different Conditions With Magnification x20
4.27
Optical Micrograph of Weld Metal After Electrochemical Test in
65
Different Conditions With Magnification x20
4.28
Optical Micrograph of Fusion Line After Electrochemical Test in
66
Different Conditions With Magnification x20
4.29
Optical Micrograph of Heat Affected Zone After Electrochemical
66
Test in Different Conditions With Magnification x20
4.30
Surface Morphology of As Receive Sample With Magnification
67
x1000
4.31
Surface Morphology of Welded Sample With Magnification x18
xi
67
4.32
Surface Morphology of Base Metal With Magnification x1000
68
4.33
Surface Morphology of Weld Metal With Magnification x1000
68
4.34
Surface Morphology of Heat Affected Zone With Magnification
69
x1000
4.35
Bar Chart Showing Tensile Stress Result of Welded Samples at
70
Different pH With Same Temperature in NaCl Solution
4.36
Bar Chart Showing Tensile Stress Result of Welded Samples at
71
Different Temperature With Same pH in NaCl Solution
4.37
Bar Chart Showing Hardness Value of Unweld and Weld
73
Samples at Different pH With Same Temperature After Corrosion
Test
4.38
Bar Chart Showing Hardness Value of Unweld and Weld
Samples at Different Temperature With Same pH After Corrosion
Test
xii
73
LIST OF ABBREVIATIONS
Ag-AgCl
-
Argentum-Argentum Chloride
AISI
-
American Iron and Steel Institute Specification
ASTM
-
American Society for Testing and Materials
AUX
-
Auxiliary Electrode
B
-
Tafel constant
BM
-
Base Metal
C
-
Carbon
Cl
-
Chlorine Atom
Cr
-
Chromium
DECN
-
Direct-Current Electrode Negative (DCEN)
DECP
-
Direct-Current Electrode Positive
DLEPR
-
Double Loop Electrochemical Potentiokinetic Reactivation
EDX
-
Elemental Diffraction X-ray
EPR
-
Electrochemical Potentiokenetic Reactivation
Etc
-
Et Cetera
F
-
Faraday’s constant
FCC
-
Face-Centered Cubic
Fe
-
Ferrous
FL
-
Fusion Line
FS
-
Fracture Strength
GTAW
-
Gas Tungsten-Arc Welding
H
-
Hydrogen atom
HAZ
-
Heat Affected Zone
H2O
-
Water
HV
-
Vickers Hardness
MIC
-
Microbiologically Influenced Corrosion
Mm
-
Millimetre
Mn
-
Manganese
N
-
Nitrogen Atom
xiii
NaCl
-
Sodium Chloride
Nb
-
Niobium
O
-
Oxygen Atom
OH
-
Hydroxyl Atom
OM
-
Optical Microscope
P
-
Phosphorus
Ppm
-
Part Per Million
PREN
-
Pitting Resistance Equivalent Number
Q
-
Total Charge
RE
-
Reference Electrode
Rp
-
Polarization Resistance
S
-
Sulphur
SCC
-
Stress Corrosion Cracking
SEM
-
Scanning Electron Microscope
Si
-
Silicon
SRB
-
Sulfate Reducing Bacteria
ThO2
-
Thrianite
Ti
-
Titanium
TIG
-
Tungsten Inert Gas
UTS
-
Ultimate Tensile Strength
W
-
Tungsten
WE
-
Working Electrode
WM
-
Weld Metal
YS
-
Yield Strength
xiv
LIST OF SYMBOLS & NOMENCLATURE
-
Exposed specimen area,
-
Atomic Weight
-
Anodic Tafel constant
-
Cathodic Tafel constant
°C
-
Degree Celsius
D
-
Arithmetic Mean of Two Diagonal
Eoc
-
Open Circuit Potential
Ecorr
-
Corrosion potential,V
EW
-
Equivalent Weight
-
The mass fraction of the ith element in the alloy
I
-
Applied Current Density, µA
Io
-
Exchanger Current Density, µA
Icorr
-
Corrosion current, µA
η
-
Overpotential
icorr
-
Corrosion current density, µA/
-
Polarization Resistance
AW
P
Load Force Kgf
-
Density, g/cm3
-
Surface Area
-
Volume, cm3
W
-
Mass of Material removal
W%
-
Weight percentage
Z1
-
Valence of Each Component
SA
xv
CHAPTER 1
INTRODUCTION
1.1
Background of Study
The importance of stainless steel has increased in the last 30 years. This is because of
the development of corrosion and oxidation-resistant material. It is commonly used
in variety of field such as in chemical industry, environment technology, civil
engineering, power and energy generation, domestic equipment, and bottom hull of
ship in marine environment.
From Tablbolt and Talbot (2007), stainless steel can be classified into five group
such as martensitic stainless steels, ferritic stainless steels, austenitic stainless steels,
duplex (ferritic-austenitic) stainless steels and precipitation-hardening stainless steels.
Among all those groups, alloys 316 and 316L are one of the subset of austenitis
stainless steel. Besides 316L stainless steel also called molybdenum-bearing
austenitic stainless steel, which exhibit better corrosion resistance than the
conventional chromium–nickel austenitic stainless steel (304).
Type of 304 stainless steel has lower carbon content of less than 0.08 to avoid
(sensitization) grain boundary chromium carbide precipitated and to provide
corrosion resistance when explore to higher temperature during welding fabrication.
(Atanda et al, 2010)
Recently, 304 stainless steel is considered to be good weldable of the stainless steel
due to their good mechanical properties and corrosion resistance. It is because the
stainless steel is protected by the chromium oxide protective layer on the surface.
(Venu and Rostron, 2009)
1
However, welding induces the degradation or corrosion failure when this layer is
damaged. Other than that, corrosion resistance can be maintained in the welded area
by balanced the composition of alloy to inhibit certain precipitation reaction, or by
increased the nitrogen gas or shielding gas to the weld environment. Besides that,
choosing the proper welding parameter can also maintained the corrosion resistance.
There are two important considerations for weld joints. One is to avoid solidification
cracking, and the other one is to preserve the corrosion resistance of the weld and
heat-affected zones. (Brooks and Lippold, 1993) In order to avoid corrosion, a
tailored combination of steel grade, welding method and filler metal should be
chosen properly. Therefore, tungsten inert gas welding process is one of the best
choices for running the welding process due to it slower arc welding speed, weld can
be made with or without filler, and precise control of welding heat.
1.2
Problem Statement
Normally, corrosion resistance is the significant characteristic of the stainless steel in
various environments due to present the chromium oxide protective layer on the
surface film. The harshness of the environmental factors is mostly dependent on the
chloride concentration content, pH and temperature. Meanwhile, it is aggressive to
cause corrosion failure either pitting or crevice due to the present in acidic chloride
environment and it become severity at low pH and higher temperature. The present
of chloride ions will destroy and breakdown the protective oxide layer consequently
leads failure in stainless steel. (Malik et al, 1992)
Malik and Al-Fozan (1993) had been proved that, in order to overcome this
sophisticated problem, the composition of alloy on this layer should be changes by
substituted appropriate amount of elements. One of the most common stainless steel
often used under marine environments is recognized as austenitic 304 stainless steel.
These types of steel have higher contents of chromium (Cr) which purposely work as
forming the passive film and provide additional protection to resist corrosion attack.
The stainless steel will never corrode uniformly in the marine environment. Basically,
pitting and crevice corrosion is considered as the most common cause of failure of
2
stainless steel in marine crevice. However, Heselmans. (2006) indicated that within
this two corrosion failure, crevice corrosion is said to be induced a major problem in
marine environment than pitting corrosion because of the lower resistivity of the
water.
It has been reported that sulfate reducing bacteria (SRB) can exist in the sea water
environment. When this stainless steel is immersed in the sea water, the potential
problem for this material is occurred due to presence of microorganisms on the
surface for example sulfate-reducing bacterial (SRB) , iron-oxidizing bacterial,
sulfur-oxidizing bacterial, and manganese-oxidizing bacterial. These bacterial can
begin to accelerate the corrosion process by use up the protective layer. (Abraham et
al, 2009).
Furthermore Abraham et al (2009) also indicated that when SRB activity in an
anaerobic conditions it will cause severe attack such as pit propagation. The bacterial
are likely to grow within the pit cavity and produce and acidic chlorine environment.
Once pitting is initiated, the kinetic pitting has strong tendency for it to continue to
grow and propagated. Although true, according to the theory it is well known that
chromium is easily affected by the chlorine attack, therefore the depletion of
chromium in the surface of pitted region led to the loss of passivity. This phenomena
would increase the corrosion rate and leading to corrosion failure. From the research
result its indicated that pitting in the marine environmen is caused by
microbiologically influenced corrosion (MIC) which can led to 316L stainless steel
pipeline system leakage.
Form the reseach result of Heselmans (2006), it is recorded that bioflims are the main
factor to induced corrosion happended in nature seawater and brackish water. Due to
the formation of biofilms and mircobiological active, nature sea water is more
corrosive than properly chlorinated seawater. Natural seawater is saturated with
oxygen and normal aerobic bioflim promotes the cathodic reaction. Thus, pitting will
start at the anodic reaction. While, most of the corrosion problems with stainless steel
can be avoived by used of cathodic protection. This cathodic protection can decrease
the rest potential back in the safe passive area, hence it helps to aviod MIC and
3
localised corrosion. Consequently, its can improved the perfomance of stainless steel
in marine enviroment and brackish water.
1.3
Objectives of the Project
The objectives of this work is to study the effect of temperature and pH on the
corrosion behavior of welded 316L stainless steel by means of electrochemical test
and immersion test to compare the result obtained of these two techniques.
1.4
Scopes of the Project
This project will focus primarily on investigated the corrosion behavior of 316L weld
stainless steel.
The welding method and welding process will be discussed in this project. Basically,
welded joint of austenitic 304 stainless steel is performed by using TIG (Tungsten
Inert Gas) welding process. Mechanical tests at the welded area were carried out
through tensile test and hardness test according to the ASTM standard. Furthermore,
corrosion tests were carried out in order to evaluate the corrosion behavior of 304
stainless steel in various temperature and pH. Therefore, the immersion test (weight
loss) and electrochemical test were utilized to study the influenced of different
factors affecting the corrosion. Both of the corrosion tests were carried out based on
the methods reported in ASTM standard. For metallographic examination OM
(optical microscope) and SEM (scanning electron microscope) is being used in order
to examine the microstructural of the welded area.
4
CHAPTER 2
LITERATURE REVIEW
2.1
Corrosion Principle
Corrosion is a naturally occurring phenomenon. Most metal corrosion occurs via
electrochemical reactions at the interface between the metal and an electrolyte
solution. Roberge (2008) is defined corrosion as a deterioration of the metal due to
reaction with the surrounding aqueous environment. In fact all the corrosion in
metallic materials is an electrochemical process where the anodic and cathodic
reaction is occurring at the same time. The anode is typically thought of as the
negative electrode where oxidation process takes place which give away the electron
to form corrosion. Whereas, the cathode is act as the positive electrode where
reduction process is take place which accepted the electron. When this two reaction
are in equilibrium, the flow of electrons from each reaction is balanced, therefore no
net electron flow occurs.
Bardal (2003) classified corrosion of metallic material into three main groups. First
group is wet corrosion where it is occurred when a liquid is present. This usually
involves aqueous solution or electrolyte and typically undergoes electrochemical
process. Second group is dry corrosion where the corrosive environment is a dry gas.
Dry corrosion also known as chemical corrosion the most popular example is high
temperature corrosion. And the last group is corrosion in other fluids such as fused
salts and molten metals. However, this project is only concentrate on wet corrosion.
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