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
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 : ………..
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)
………..
ABSTRACT
ABSTRAK
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
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.
TABLE OF CONTENT
List of Symbols & Nomenclature xv
1 INTRODUCTION
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
2.6 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
Test
35
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
4.4.2 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
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 Steel
45
4.2 Corrosion Rate of Specimens Expressed in mm/yr With Different Temperature and pH for 7 Days
50
4.3 Corrosion Rate of Specimens Expressed in mm/yr With Different Temperature and pH at 14 Days
50 Same Temperature in NaCl Solution After Test
69
4.7 Tensile Stress Result of Welded Samples at Different Temperature With Same pH in NaCl Solution After Test
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
Metal Specimens After SCC Test
16
2.6 Schematic Diagram of The Fracture Surface of 316L Weld Metal Specimens After SCC Test
16
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 Attached to Backside of The Test Specimens
34
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
are Machined from Butt-Welded Plate With The Weld Crossing in The Midsection of The Specimen
41
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 Magnification: x50: and Magnification: x100
46
4.2 Picture Showing Cross Section of Welded Samples at Different Regions
47
4.3 Optical Micrographs of As Received 304 Stainless Steel With Magnification: x20: and Magnification: x50
47
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 7 Days and 14 Days With Different Temperature
51
4.8 Bar Chart Showing Corrosion Rate of Between 7 Days and 14 Days With Different pH
51
4.9 Bar Chart Showing Corrosion Rate Between 7 Days and 14 Days With Different Temperature and pH
52
4.10 Bar Chart Showing The Comparison Between The Corrosion
Rate of Unwelded and Welded Samples With Different Temperature and pH After Immersion Test
53
4.11 Picture of Specimens Before Immersion Test 53
4.12 Picture Specimens After Immersion Test in 3.5% NaCl Solution
in pH7 at 25±5°C & in pH 3 at 95±5°C
54
4.13 Optical Micrograph of Base Metal After Immersion Test in
Different Conditions With Magnification x20
55
4.14 Optical Micrograph of Weld Metal After Immersion Test in
Different Conditions With Magnification x20
55
4.15 Optical Micrograph of Fusion Line After Immersion Test in
Different Conditions With Magnification x20
56
4.16 Optical Micrograph of Heat Affected Zone After Immersion Test
in Different Conditions With Magnification x20
4.17 Bar Chart Showing Corrosion Rate of Unwelded and Welded
Samples at Different Temperature and pH After Electrochemical Test
61
4.20 Picture of Specimens Before Electrochemical Test 62
4.21 Picture Specimens After Test Electrochemical Test in Acidic
Solution (pH3) at25±5°C
62
4.22 Picture Specimens After Test Electrochemical Test in Neutral
Solution (pH7) at25±5°C
62
4.23 Picture Specimens After Test Electrochemical Test in Alkaline Solution (pH12) at25±5°C
63
4.24 Picture Specimens After Test Electrochemical Test in Acidic
Solution (pH3) at 65±5°C
63
4.25 Picture Specimens After Test Electrochemical Test in Acidic
Solution (pH3) at 95±5°C
63
4.26 Optical Micrograph of Base Metal After Electrochemical Test in
Different Conditions With Magnification x20
64
4.27 Optical Micrograph of Weld Metal After Electrochemical Test in
Different Conditions With Magnification x20
65
4.28 Optical Micrograph of Fusion Line After Electrochemical Test in
Different Conditions With Magnification x20
66
4.29 Optical Micrograph of Heat Affected Zone After Electrochemical
Test in Different Conditions With Magnification x20
66
4.30 Surface Morphology of As Receive Sample With Magnification
x1000
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
x1000
69
4.35 Bar Chart Showing Tensile Stress Result of Welded Samples at
Different pH With Same Temperature in NaCl Solution
70
4.36 Bar Chart Showing Tensile Stress Result of Welded Samples at
Different Temperature With Same pH in NaCl Solution
71
4.37 Bar Chart Showing Hardness Value of Unweld and Weld
Samples at Different pH With Same Temperature After Corrosion Test
73
4.38 Bar Chart Showing Hardness Value of Unweld and Weld
Samples at Different Temperature With Same pH After Corrosion Test
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
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
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
LIST OF SYMBOLS & NOMENCLATURE
- Exposed specimen area,
AW - Atomic Weight - 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
P Load Force Kgf
- Density, g/cm3
SA - Surface Area
- Volume, cm3
W - Mass of Material removal
W% - Weight percentage
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)
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)
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.
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.
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.
