Investigation of Materials for Use in Exhaust Gas Condensate

Investigation of Materials forUse in Exhaust Gas Condensate

Environment with Focus onEGR Systems

Andreas Olofsson

August 3, 2012Master’s Thesis in Energy Engineering, 30 credits

Supervisor at UmU: Britta SethsonSupervisor at Scania: Baohua Zhu

Examiner: Robert Eklund

Umea UniversityDepartment of Applied Physics and Electronics

SE-901 87 UMEASWEDEN

Abstract

EGR (Exhaust Gas Recirculation) is a method for reducing NOx emissions for heavy-dutydiesel engines. EGR works by introducing part of the exhaust gases back to the enginecylinders. Exhaust gases consists mainly of CO2, NOx, SO2 and H2O. As the temper-ature decreases, these gases form a corrosive condensate. The EGR components whichare exposed to the condensate environment must therefore be of corrosion resistant mate-rials. The objective of this Master’s Thesis is to investigate suitable materials for use inexhaust condensate environment. The goal is to evaluate the pitting corrosion resistance foreight different commercial stainless steels and two commercial aluminium alloys in exhaustgas condensate environment. Furthermore, nitriding surface treatments on one martensiticstainless steel and anodising treatments on one aluminium alloy, were also included in thisstudy.

Five different exhaust gas condensates with different concentrations of sulphuric acid,nitric acid and chloride were chosen to perform electrochemical measurements. Two pHvalues 2.5 and 1.5; three chloride concentrations, 32 ppm, 200 ppm, 3300 ppm were in-cluded in the environmental parameters. The testing temperature was 60oC, since it isthe temperature which can still be expected to produce substantial amount of exhaust gascondensate in the EGR system. The electrochemical method used, was anodic polarisationmeasurements. This is a useful method to evaluate the pitting resistance for stainless steelsin chloride containing solutions.

The results show that the two aluminium alloys and the martensitic stainless steel weresubjected to both general and pitting corrosion in a normal condensate solution at pH 2.5.The anodised film on the aluminium surface was not stable in condensate environmentswith low pH value. After twelve hours of exposure to a condensate at pH 2.5 at 60oC, theprotective effect of the film became negligible.

The austenitic, ferritic and duplex stainless steels show, however good resistance againstboth corrosion types. Increasing condensate acidity from pH 2.5 to 1.5 could not be observedto increase risk of pitting corrosion for the austenitic, ferric and duplex steel stainless steels.

High concentrations of sulphuric acid, low pH value, but low chloride content (200 ppm)do not increase the risk for pitting corrosion for austenitic steels 1.4404 and 1.4301, du-plex 2304 and ferritic 1.4521. However, chloride concentration of 3300 ppm, significantlyincreased risk of pitting corrosion, especially for the austenitic stainless steels. Duplex stain-less steel show better pitting resistance in high chloride environments, in addition to thegood general corrosion resistance in low pH value environments.

There is no difference in corrosion resistance between the nitride coated 1.4112 steel andthe steel without coatings. No differences can be observed between the plasma and gasnitrided samples. Further investigation in less corrosive environment is recommended, sinceanodic polarisation is not a suitable method to study general corrosion behavior.

The pitting corrosion resistance in condensates with high chloride concentrations at60oC follows the sequence 1.4301<1.4521<1.4404<duplex 2304<duplex LDX2404<duplex2205. Clearly, duplex stainless steels have better pitting corrosion resistance in low pH

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environment when chloride concentration is increased. Considering the operating conditionsof the EGR components, the element prices, it is probably more beneficial to consider theduplex stainless steels for use in the EGR system.

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Kartlaggning av material for anvandning iavgaskondensatmiljo med fokus pa EGR teknik

Sammanfattning

EGR (Exhaust Gas Recirculation) ar en teknik som anvands for dieselmotorer, foratt mota de hart satta utslappskraven for kvaveoxider. EGR fungerar genom att en delav avgaserna aterfors till cylindrarna. Avgaserna gor sa att den maximala forbrannings-temperaturen sanks, vilket kraftigt reducerar bildandet av kvaveoxider. Dieselavgaser in-nehaller framst CO2, NOx, SO2 och H2O och innan avgaserna aterfors till cylindrarna kylsde ner. Detta leder till att det bildas ett korrosivt kondensat, eftersom de amnen som finnsi avgaserna bland annat kan bilda svavel- och salpetersyra.

Pa grund av detta korrosiva kondensat maste material i EGR systemet vara korrosions-bestandigt och materialkraven forvantas oka i takt med utslappskraven. I framtiden kandet innebara att mer korrosionsbestandigt material maste anvandas.

Detta examensarbete undersoker gropfratningsmotstandet for atta olika rostfria stal ochfor tva aluminiumlegeringar i syntetiskt avgaskondensat. Dessutom inkluderades tva ytbe-handlingar; anodisering av en aluminiumlegering samt nitrering av ett martensitiskt rostfrittstal. Malet ar att kartlagga vilka material som ar lampliga att anvanda i avgaskondensat-miljo.

Fem olika syntetiska avgaskondensat med olika halter av svavelsyra, salpetersyra ochklorider, valdes ut for elektrokemisk matning. Tva olika pH-nivaer, 2.5, 1.5, inkluderadessamt tre olika halter, 32 ppm, 200 ppm, 3300 ppm, av klorider. Testtemperaturen valdestill 60oC, eftersom det ar den temperaturen som fortfarande kan forvantas producera be-tydlig mangd avgaskondensat i EGR systemet. Anodisk polarisation ansags som den mestlampade elektrokemiska metoden for att na examensarbetets mal.

Erhallna resultat visar att de tva aluminiumlegeringarna och det martensitiska rostfriastalet ar utsatta for generell korrosion och gropfratning vid pH 2.5. Medan de austenitiska,ferritiska och duplexa rostfria stalen uppvisar bra korrosionsbestandighet mot bada dessakorrosionstyper. Att sanka pH fran 2.5 till 1.5 kunde inte ses oka risken for gropfratning forde austenitiska, ferritiska och duplex rostfria stalen.

En kloridhalt av 0.33 vikt% okade markant risken for gropfratning, speciellt for deaustenitiska rostfria stalen. Vid pH 1.5 och hoga halter av klorider klarar sig duplexstalenbast.

Anodisering av aluminium ses initialt ge ett forbattrat korrosionsskydd vid pH 2.5, mendet observerades att det anodiserande oxidskiktet utsattes for kontinuerlig upplosning. Efter12 timmar var oxidskiktet sa kraftigt upplost att dess skyddande effekt var forsumbar. Alu-minium kan darfor inte rekommenderas till anvandning i EGR kondensat miljo.

Nitrering av det martensitiska stalet kunde inte ses paverka materialens korrosionsegen-skaper. Men anodisk polarisation ar inte en lamplig metod att anvanda for att studeragenerell korrosion, darfor rekommenderas fortsatta undersokningar vid antingen en hogrepH-niva eller med en annan lamplig metod.

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Gropfratningsmotstandet for de testade rostfria stalen i kondensat med hoga kloridhaltervid 60oC foljer sekvensen 1.4301<1.4521<1.4404<duplex 2304<duplex LDX2404<duplex2205. Det ar tydligt att duplex stalen har battre motstand mot gropfratning vid lagapH-nivaer och hoga kloridhalter. Med tanke pa driftforhallandena i EGR-systemet och palegeringskostnader ar det lampligt att overvaga duplexa stal for anvandning i EGR-systemet.

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Acknowledgment

This Master’s Thesis was performed at Scania CV AB, Materials Technology, UTMC. Iwould like to thank UTMC for an encouraging and enjoyable working environment.

I want to thank Outokumpu for providing me with the necessary materials, thanks alsoto Bodycote for providing me with the surface treatments.

Also thanks to my supervisor at UmU Britta Sethson for your input and guidance.

Above all i want to thank my supervisor at Scania Baohua Zhu for your invaluable help,support and encouragement throughout the work of this Master’s Thesis.

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Contents

1 Introduction 1

1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.3 Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.4 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Diesel Exhaust Gas Condensation 5

2.1 Condensation of Sulphuric Acid . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.2 Condensation of Nitric Acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.3 Condensation of Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.4 Effect of Fuel-sulfur Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.5 Exhaust Gas Condensate from Biodiesel . . . . . . . . . . . . . . . . . . . . . 7

2.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3 Stainless Steels and Aluminium 9

3.1 Stainless Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.1.1 Austenitic Stainless Steels . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.1.2 Ferritic Stainless Steels . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.1.3 Duplex Stainless Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.1.4 Martensitic Stainless Steels . . . . . . . . . . . . . . . . . . . . . . . . 11

3.2 Aluminium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4 Corrosion & Electrochemistry 15

4.1 Corrosion of Stainless Steels and Aluminium Alloys . . . . . . . . . . . . . . . 15

4.1.1 General Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4.1.2 Localised Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4.2 Electrochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4.3 Anodic Polarisation Sweep . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.4 Corrosion Protection Techniques . . . . . . . . . . . . . . . . . . . . . . . . . 22

4.4.1 Corrosion Inhibitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

4.4.2 Anodic and Cathodic Protection . . . . . . . . . . . . . . . . . . . . . 22

4.5 Automotive Corrosion Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

4.5.1 Corrosion Study of Different Stainless steels in Synthetic Exhaust GasCondensate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

4.5.2 Corrosion Study for Automotive Mufflers . . . . . . . . . . . . . . . . 23

4.6 Engineers Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

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viii CONTENTS

5 Experimental 275.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275.2 Synthetic Exhaust Gas Condensate . . . . . . . . . . . . . . . . . . . . . . . . 295.3 Sample Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305.4 Potentiodynamic Polarisation Measurements . . . . . . . . . . . . . . . . . . . 30

6 Results 316.1 Condensate 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

6.1.1 Condensate 1 – Austenitic, Ferritic and Duplex Grades . . . . . . . . 316.1.2 Condensate 1 - Grade 4112 & Effect of Nitriding . . . . . . . . . . . . 336.1.3 Condensate 1 – Aluminium Grades & Effect of Anodising . . . . . . . 34

6.2 Condensate 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356.2.1 Condensate 2a – Austenitic, Ferritic and Duplex Grades . . . . . . . . 356.2.2 Condensate 2b – Austenitic, Ferritic and Duplex Grades . . . . . . . . 37

6.3 Condensate 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396.3.1 Condensate 3a – Austenitic, Ferritic and Duplex Grades . . . . . . . . 396.3.2 Condensate 3b – Austenitic, Ferritic and Duplex Grades . . . . . . . . 39

7 Discussion and Summary 437.1 Pitting Corrosion in the EGR System . . . . . . . . . . . . . . . . . . . . . . 437.2 Material Choice for EGR Components . . . . . . . . . . . . . . . . . . . . . . 44

8 Conclusions 47

9 Recommendations 49

Bibliography 51

Appendix 53

List of Figures

1.1 NOx and PM legislation for Euro 3, Euro 4, Euro 5 and Euro 6. . . . . . . . 1

1.2 Schematic of a HP-EGR system [2]. . . . . . . . . . . . . . . . . . . . . . . . 2

1.3 Schematic of a LP-EGR system [2]. . . . . . . . . . . . . . . . . . . . . . . . . 2

2.1 Formation mechanisms of exhaust gas condensate summarised from literaturereview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.1 Pourbaix diagram for a Al/H2O system at 25oC. . . . . . . . . . . . . . . . . 13

4.1 Schematic of the autocatalytic process of pitting corrosion. . . . . . . . . . . 16

4.2 Schematic of crevice corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

4.3 Synergistic effect of SCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4.4 Simplified illustration of the double layer at a metal aqueous interface. . . . . 20

4.5 Schematic of an anodic polarisation sweep for stainless steel. . . . . . . . . . 21

4.6 Isocorrosion curves, 0.1 mm/year, in sulphuric acid [24]. . . . . . . . . . . . 25

4.7 Isocorrosion curves, 0.1 mm/year, in sulphuric acid containing 2000 ppmchloride ions [24]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

5.1 Experimental Setup: 1) Solartron 1287 potentiostat 2) Ag/AgCl referenceelectrode (3M KCl 3) Platinum plate counter electrode 4) Working Electrode5) Corrosion Cell 6) B.I.A climatic climate chamber 7) Corrware R© SoftwarePackage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

6.1 Anodic polarisation plot for stainless steel grades Duplex 2205, Duplex 2304,1.4301, 1.4404, 1.4509, 1.4521. . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

6.2 Plot from anodic polarisation measurement of nitride coated and uncoated1.4112 martensitic stainless steel in condensate 1. . . . . . . . . . . . . . . . . 33

6.3 Anodic polarisation plot for aluminium grades AC43000KF and AW-3003 incondensate 1. AW-3003 and AC-43000KF-anodised also tested after 12 hourexposure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

6.4 Open circuit potential (OCP) for Al AC-43000KF in condensate 1. . . . . . . 35

6.5 Anodic polarisation plot for stainless stainless steel 2304, 1.4301, 1.4404,1.4521 in Condensate 2a. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

6.6 Anodic polarisation plot for stainless steel grades 2205, LDX2404, 2304,1.4404, 1.4301, 1.4521 in condensate 2b. . . . . . . . . . . . . . . . . . . . . . 37

6.7 Anodic polarisation obtained for stainless steels 2304, 1.4404, 1.4301, 1.4521in condensate 3a. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

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x LIST OF FIGURES

6.8 Anodic polarisation for stainless steel grades 2205, 2304, LDX2404, 1.4301,1.4404, 1.4521 for condensate 3b. . . . . . . . . . . . . . . . . . . . . . . . . . 40

1 Photo of 4112 grade test samples: 1) Before polarisation. 2) After polarisa-tion. 3) Gas nitrided after polarisation. 4) Plasma nitrided after polarisation. 53

2 Photo of aluminium grade test samples: 1) AC-43000KF before polarisation2) AC-43000KF after polarisation 3) AC-43000KF anodised before polarisa-tion 4) AC-43000KF anodised after polarisation 5) AW-3003 before polarisa-tion 6) AW-3003 after polarisation . . . . . . . . . . . . . . . . . . . . . . . . 53

3 Photo of test samples after polarisation in condensate 2b. Pitts visible byvisual inspection have been marked. . . . . . . . . . . . . . . . . . . . . . . . 54

4 Photo of test samples after polarisation in condensate 3b. Pitts visible byvisual inspection have been marked. . . . . . . . . . . . . . . . . . . . . . . . 54

5 Microscopic examinations of plasma nitride film before anodic polarisation. . 556 Microscopic examination of gas nitride film before anodic polarisation. . . . . 567 Microscopic examinations of plasma nitride film after anodic polarisation in

condensate 1. Marks indicate exposed area. . . . . . . . . . . . . . . . . . . . 578 Microscopic examinations of plasma nitride film after anodic polarisation in

condensate 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 589 Microscopic examination of gas nitride film after anodic polarisation in con-

densate 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5910 Microscopic examination of gas nitride film after anodic polarisation in con-

densate 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6011 Test coupon documentation for duplex steel 2205. . . . . . . . . . . . . . . . . 6112 Test coupon documentation for duplex steel LDX2404. . . . . . . . . . . . . . 6213 Test coupon documentation for duplex steel 2304. . . . . . . . . . . . . . . . . 6314 Test coupon documentation for steel grade 1.4521. . . . . . . . . . . . . . . . 6415 Test coupon documentation for steel grade 1.4509. . . . . . . . . . . . . . . . 65

List of Tables

3.1 Composition ranges for different stainless steels [13]. . . . . . . . . . . . . . . 93.2 Austenitic stainless steels and composition. Source: Outokumpu. . . . . . . . 103.3 Ferritic stainless steels and composition. Source: Outokumpu. . . . . . . . . . 113.4 Duplex stainless steels and composition. Source: Outokumpu. . . . . . . . . . 113.5 Martensitic stainless steels. Source: Outokumpu & Metal Ravne. . . . . . . . 123.6 Classification and description for aluminium alloys. . . . . . . . . . . . . . . . 13

4.1 Test materials and steel composition (wt%) [22]. . . . . . . . . . . . . . . . . 234.2 Test materials and steel composition (wt%) [23]. . . . . . . . . . . . . . . . . 24

5.1 Stainless steels grades selected for electrochemical evaluation. . . . . . . . . . 275.2 Aluminium grades selected for electrochemical evaluation. . . . . . . . . . . . 285.3 Chemical composition of synthetic exhaust gas condensates (C) used for an-

odic polarisation measurements. . . . . . . . . . . . . . . . . . . . . . . . . . . 295.4 Grades and condensates (C) experimentally tested in this study. . . . . . . . 29

6.1 Eb, Ep and Imax obtained from anodic polarisation of austenitic, ferritic andduplex grades in condensate 1. . . . . . . . . . . . . . . . . . . . . . . . . . . 32

6.2 Eb, Ep and Imax obtained from anodic polarisation scan in condensate 2a. . . 366.3 Eb, Ep and Imax obtained from anodic polarisation of austenitic, ferritic and

duplex grades in condensate 2b. . . . . . . . . . . . . . . . . . . . . . . . . . . 386.4 Eb, Ep and Imax for tested austenitic, ferritic and duplex grades in condensate

3b. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

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xii LIST OF TABLES

Chapter 1

Introduction

This chapter provides an initial background to the work of this Master’s Thesis. Also de-scribed here is the thesis objectives and limitations, followed by a brief explanation of thesolution methodology used.

1.1 Background

Emission legislations for heavy-duty diesel trucks have become increasingly stringent the lastdecade, see figure 1.1. Current European emission legislation, Euro 5, will at the beginningof 2013 be replaced by Euro 6 and require a 77 % reduction in nitrogen oxide emissions(NOx) and 50 % reduction in particulate matter (PM) [1]. Two frequently used techniquesfor NOx reduction are EGR (Exhaust Gas Recirculation) and SCR (Selective CatalyticReduction). EGR works by recirculating a part of the exhaust gases back to cylinders. Therecirculated exhaust gases act as an inert combustion gas, which absorbs heat and dilutesoxygen concentrations in the combustion zone. This lowers combustion temperature andas a result significantly reduce NOx emissions. SCR works by injection of an additive,called AddBlue, into exhaust gases. AddBlue absorbs heat from exhaust gases and gaseousammonia (NH3) is produced. NH3 and NOx is then catalytically transformed into H2Oand N2.

Figure 1.1: NOx and PM legislation for Euro 3, Euro 4, Euro 5 and Euro 6.

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2 Chapter 1. Introduction

EGR and SCR have their unique advantages and disadvantages. SCR is efficient at highspeeds and high loads, while EGR has advantages at cold starts and low loads. For Euro 5 ithas generally been sufficient to use one of these NOx reduction techniques. A combinationof EGR and SCR is necessary for fulfillment of the requirements in Euro 6 for most heavy-duty engine manufacturers [2]. Euro emission legislation must be met during the enginesfirst seven years of operation or during its first 70 0000 km, whatever comes first [1].

There are different EGR designs, with their respective advantages and disadvantages.The most notable design difference is usually the location from where exhaust gases areextracted and returned. The design with most commercial success is High Pressure EGR(HP-EGR); exhaust gases is extracted before decompression and reintroduced after the air-to-air (charge-air) cooler, see figure 1.2. Another design with some commercial success isLow Pressure EGR (LP-EGR); exhaust gases is extracted after decompression and reintro-duced to the charge-air before compression, see figure 1.3.

Figure 1.2: Schematic of a HP-EGR system[2].

Figure 1.3: Schematic of a LP-EGR system[2].

LP-EGR has from a fuel and emission perspective more potential than HP-EGR. ForLP-EGR all exhaust gases pass through the turbine, having the potential to increase tur-bocharger performance. LP-EGR is also known as long route EGR (LR-EGR), since routefor exhaust gases are longer compared to HP-EGR. This longer route enhances mixing ofEGR gases and intake-air, resulting in a more homogeneous mixture which improves NOx

reduction [2]. Furthermore, exhaust gases are extracted after the diesel particulate filter(DPF), see figure 1.3. Cleaner exhaust gases are thus reaching the cylinders, which betterpreserve engine durability [2].

– Why is then HP-EGR more common than LP-EGR?

There are some challenges that must be overcome to make LP-EGR more attractive forcommercial use, some are listed below:

1) Even though exhaust gases for LP-EGR pass through the aftertreatment system, acertain amount of particles are still remaining in the exhaust stream, its impact on thecompressor wheel as it turns at high speed. May potentially erode the wheel [2].

2) Charge-air-coolers (CAC) have very narrow cooler-passages, which may be problem-atic if soot accumulates for a prolonged time. Since this may potentially cause high pressurelosses, resulting in decreased engine performance and fuel efficiency [2].

1.2. Objectives 3

3) Most commercial charge-air-coolers are made of aluminium, because of the materialshigh thermal conductivity and low weight. Corrosion resistance of aluminium is known tobe heavily dependent on pH. This may potentially be problematic if a mixture of exhaustgases and charge-air is to be cooled.

Exhaust gases consist mainly of CO2, H2O, NOx and small amounts of SO2 dependingon the diesel quality. When temperature is decreased nitric acid, sulphuric acid and organicacids will be formed. Nitric and sulphuric acid are highly corrosive, while the organic acidsare less corrosive. The concentration of these acids in exhaust gas condensate can varysignificantly, since formation/condensation of acids in the EGR system depend on severalfactors, such as fuel quality, EGR-rate and temperature. The EGR components which areexposed to this condensate environment must therefore be of corrosion resistant materials.

Acidity for condensate collected from a 2.0L passenger car engine fulfilling EU4 has beenfound to be below pH 3.7 [3]. A more acidic condensate can be expected from a heavy-dutyEU5 engine and shortly after engine start-up, since wall temperatures then are below thedew point of water, resulting in increased condensation.

1.2 Objectives

The objective of this Master’s Thesis was to investigate suitable materials for use in ex-haust condensate environment. The goal was to evaluate the pitting corrosion resistance foreight different commercial stainless steels and two commercial aluminium alloys in exhaustgas condensate environment. Furthermore, nitriding surface treatments on one martensiticstainless steel and anodising treatments on one aluminium alloy, were also included in thisstudy

1.3 Limitations

A material survey for the EGR system is comprehensive work; several material factors needsto be thoroughly evaluated before making material decisions, e.g. formability, weldability,heat conductivity, etc. Today, there are many different materials, grades and surface treat-ments to choose from. Restrictions were therefore needed, since time was limited. Focus forthis Master’s Thesis was subsequently set on the wet section of the EGR system, i.e. wherethere is risk of aqueous corrosion. Furthermore, focus was set to primarily consider stainlesssteels, aluminium alloys, two surface treatments and to evaluate their tendency for pittingcorrosion in exhaust gas condensate.

1.4 Methodology

Materials surveys are an interdisciplinary work. A literature review was therefore carriedout in the following topics: Exhaust Gas Condensation, Stainless Steels and AluminiumAlloys, Corrosion and Electrochemistry. These topics differ significantly from each otherand were therefore given separate chapters in this report.

4 Chapter 1. Introduction

In addition to the literature review, electrochemical methods is used to study the pit-ting corrosion of selected materials. Furthermore, it is motivated to investigate which levelsof acidic substances is required to cause pitting corrosion in the EGR system. Corrosionof stainless steels and aluminium is mostly of localised type, due to its ability to becomepassivated (explained in chapter 3).

Anodic polarisation is an electrochemical method for testing iron- and nickel- base alloystendency for pitting corrosion. This method was deemed most suitable, since it providessufficiently fast and valuable data for the work of this Master Thesis. Below is a step-by-stepmethodology for how the work was done.

Step-by-Step Methodology

Step One: Literature Review – Drastically narrow down the large quantitative of com-mercially available grades. Ten grades were selected for electrochemical evaluation insynthetic exhaust gas condensates. Two surface treatments were included.

Step Two: Experimental – Anodic polarisation measurements in different synthetic ex-haust gas condensates. Five different exhaust gas condensates were selected, withdifferent concentrations of corrosive species.

Step Three: Result – Evaluation of results, conclusions, material guidelines and recom-mendations for further work.

Chapter 2

Diesel Exhaust GasCondensation

Condensation of exhaust gases has important implications on the design and material choicesof the EGR system. This chapter will therefore address theories and research associated withthe condensation of exhaust gases.

The mechanisms of acidic formation in the EGR system is divided into the following ar-eas: Condensation of Sulphuric Acid, Condensation of Nitric Acid, Condensation of Water.Formation of nitric acid is normally not addressed as a specific issue, since it is associatedwith the many problems derived from condensation of water, but it will still be addressed inthis study. However, studies regarding condensation of nitric acid was found to be sparse.

2.1 Condensation of Sulphuric Acid

Combustion cause fuel-bound sulfur to oxidize and form sulfur oxides (SOx) and whencooled, sulfur oxides may form sulphuric acid. Formation mechanism of sulphuric acid canbe divided in three steps:

1) Oxidation of fuel sulfur, which takes place during combustion, equation 2.1.1.

S(g) +O2(g)→ SO2(g) (2.1.1)

2) Further oxidation of sulfur may take place, as exhaust gases are cooled, equation 2.1.2.Formation mechanisms of sulfur oxides are complex, and involve several intermediate steps.Thus, overall formation reactions are commonly simplified to [4]:

SO2(g) +1 /2 O2(g)→ SO3(g) (2.1.2)

3) SO3 is considered very reactive. All formed SO3 is therefore thought to almostinstantaneously convert to sulphuric acid if water is present (vapor or liquid), equation 2.1.3[4, 5, 6]:

SO3(g) +H2O(l/g)→ H2SO4(l/g) (2.1.3)

5

6 Chapter 2. Diesel Exhaust Gas Condensation

If liquid water is present, sulphuric acid can also form through dissolution of SO2,equation 2.1.4 - 2.1.5 [5, 7]:

SO2 +H2O → H2SO3 (2.1.4)

H2SO3 +1 /2 O2 → H2SO4 (2.1.5)

Mckinley [6] has developed a model for dew point prediction of sulphuric acid in EGR-coolers. The model predicts condensation of sulphuric acid between 63oC to 156oC, depend-ing on fuel-sulfur level and engine operating conditions. Mckinley’s model shows that thedew point for sulphuric acid is well above the dew point of water. However, required inputto this model is the conversion rate of SO2 to SO3. Mckinley based his calculations on theestimation that 3 to 8 % of the gaseous SO2 will convert to SO3. Reaction times involvedin diesel engines are limited. Therefore, formation of SO3 is expected to be kineticallylimited and thus also formation of vapor-phase sulphuric acid, since formation of SO3 isexpected to be kinetically limited, its conversion rate is difficult to calculate. Studies havebeen conducted to try and experimentally determine the conversion rate, but such studieshave shown it difficult to measure this conversion rate.

In a study conducted by M. Mosburger et al. [5] no condensation of sulphuric acid couldbe measured, using either low- or high-sulfur fuels. The likely explanation given, was thatthe coolant temperature (∼87oC) and timescales, do not allow sufficient kinetics-controlledoxidation of SO2 to SO3. Their conclusion was that the main corrosion risk for EGR-coolerswere at operation temperatures below the dew point of water, e.g. during cold-starts andafter engine shutdown.

In a study by A. M. Kreso et al. [4] small condensation of H2SO4 could be measuredin the EGR system at a collector temperature of 65+−4oC. However, their results also showthat condensation through vapor-phase sulphuric acid is limited, and that primary forma-tion mechanisms of sulphuric acid is from dissolution of SO2 in condensed water (Equation2.1.4 - 2.1.5). More fuel-sulfur was found in particulate filters than as vapor-phase sulphuricacid. They recommend engine designers to be more concerned about the warp-up operation,and brief time when EGR-cooler operated below the dew point of water, since the possibilityof forming significant amounts of sulphuric acid is significantly higher when operating nearor below the water dew point.

M. D. Kass et al. [8] evaluated corrosion rate of mild steel in the intake manifold asa function of fuel-sulfur level, EGR fraction, water dewpoint margin and humidity. Forboth tested fuels (15 ppm S, 350 ppm S) it was observed that no significant corrosionoccurred, until onset of water condensation. No significant corrosion was observed for thelow sulfur fuel, even when temperatures where below the dew point of water. However, forthe high-sulfur-fuel, significant increase in corrosion rate was observed when temperature inthe intake manifold was kept below the dew point of water. No clear correlation was foundfor corrosion rate with respect to humidity and EGR fraction.

2.2 Condensation of Nitric Acid

Combustion causes the formation of nitrogen oxides, through a reaction with atmosphericnitrogen and oxygen, equation 2.2.1 - 2.2.2.

N2(g) + 02(g)→ 2 NO(g) (2.2.1)

2.3. Condensation of Water 7

NO(g) + 1/2 O2(g)→ NO2(g) (2.2.2)

Nitrogen dioxides may then later react with water and form nitric acid [9]. However, thedew point of nitric acid is typically below the dew point of water [7]. Therefore it can beexpected that the main formation mechanism of nitric acid in the EGR system is throughdissolution of nitrogen dioxides in condensed water, equation 2.2.3.

2 NO +H20 + 1/2 O2 → 2 HNO3 (2.2.3)

2.3 Condensation of Water

The dew point of water in the EGR system is primarily governed by lambda (air/fuel-ratio),charge pressure and EGR-rate. It is important to take in consideration if EGR gases hasbeen mixed with charge-air or not, since it heavily affects the humidity.

If condensed water is present, formation of acidic elements will significantly increase,since both nitrogen oxides and sulfur dioxides may dissolve in water and form nitric andsulphuric acids.

2.4 Effect of Fuel-sulfur Quality

Diesel fuels can have a wide range of different sulfur levels, e.g. European diesels have amaximum allowed sulfur content of 10 ppm accordingly to EN-590. However, allowed limitscan vary significantly between certain countries, from 10 ppm up to 2000 ppm [10]. Severalstudies have shown that level of sulphuric acid in exhaust gas condensate vary heavilydepending on diesel quality [4, 5, 6, 11].

2.5 Exhaust Gas Condensate from Biodiesel

Biodiesels consists of long chain mono-alkyl esters, rather than long chains of hydro-carbonsas for diesel. Mono-alkyl esters are also known as fatty acid methyl esters (FAME). Physicaland chemical properties for biodiesels are greatly influenced by the feedstock. Most com-mon feedstocks are vegetable oils, such as rapeseed, soy and palm. European biodiesels aremostly produced from rapeseed, while in USA most biodiesels are produced from soybeans.In tropical countries, palm oil is a commonly used biodiesel feedstock, because of its highyield per cultivated land – 5 times higher than rapeseed, and 10 times higher than soybeans[9].

S. Moroz et. al [9] investigated the effect of three different biodiesels (rapeseed, soy, palmoil) with respect to condensate acidity, condensate collection rate, smoke opacity and NOx

emissions. Test engine was a 2.0L direct injected diesel engine for passenger car applicationfulfilling Euro 4. The test engine was equipped with both a HP-EGR and LP-EGR system.A low sulfur diesel acted as reference (7.4 ppm S), while both 100% biodiesels (B100) andfifty/fifty (B50) biodiesel and diesel fuel blends were tested. Condensate were collected fromthe LP-EGR water-cooled charge air cooler (WCAC) at three different load points.

8 Chapter 2. Diesel Exhaust Gas Condensation

All biodiesels showed a significantly lower opacity compared to diesel, between 64 %and 87 % lower, which is expected due to the high oxygen content in biodiesels (∼12 %).Sulfur content in biodiesel is very low, varied between 1.7-2.8 ppm. This very low sulfur con-tent should promote formation of less aggressive condensate, but the higher BSFC (BrakeSpecific Fuel Consumption) of biodiesels is expected to cause more condensation. However,results from this study show that tested biodiesel fuels, produced similar amounts of conden-sate. Analyses of collected condensate, show relatively small difference in acidity betweenbiodiesel and low-sulfur diesel fuels, for all tested fuels and blends pH varied between 3.5and 4.

A study by G. Bourgoin et. al [3], follows the work of S. Moroz, but with focus oncollecting biodiesel condensate from a HP-EGR system. Same type of 2.0L engine andbiodiesel fuels were used as in the previous study. An additional reference fuel was included(490 ppm S). Engine test points selected were similar as for the previous study. Acidityof collected biodiesel condensate varied between pH 2.5-3.2, while pH of reference fuelsvaried between 3.2-3.7. Results in this study show larger differences in acidity for biodieselcondensate compare to low sulfur-diesel, but the differences is relatively small.

2.6 Summary

1) Condensation of sulphuric acid above water dew point is limited.

2) Diesel and biodiesel produce similar amounts of condensates and similar levels of acidity,but studies are only available for EU4 engines for passenger car applications.

Formation mechanisms of exhaust gas condensate in the EGR system are summarisedin the figure below:

Figure 2.1: Formation mechanisms of exhaust gas condensate summarised from literaturereview.

Chapter 3

Stainless Steels and Aluminium

This chapter provides an brief literature review about the many different stainless steels andaluminum grades.

3.1 Stainless Steels

Stainless steels are high-alloyed steels with more than 10.5 % chromium (Cr). Whenchromium is oxidized it will form an thin oxide layer on the metal surface which will protectthe underlying metal from corrosion. With a thin passive film that totally covers the metalsurface the metal is said to be passivated and corrosion is greatly retarded. For passivity tohappen a chromium content of about 10-11 % is required [12, 13, 14]. Stainless steels arecategorised in four groups austenitic, ferritic, duplex, martensitic, depending on the crystalstructure. Table 3.1 lists characteristic composition of these different stainless steels.

Table 3.1: Composition ranges for different stainless steels [13].

Crystal structure Composition (wt%)C Cr Ni Mo Other

Austenite <0.08 16-30 8-35 0-7 N, Cu, Ti, NbFerrite <0.08 12-19 0-5 <5 TiMartensite >0.10 11-14 0-1 - VDuplex <0.05 18-27 4-7 1-4 N,W

3.1.1 Austenitic Stainless Steels

Austenitic stainless steels usually have very good corrosion resistance, combined with goodform- and weldability. They can be distinguished by their high chromium and nickel (Ni)content, see table 3.1. Nickel is a strong austenite former and also enhances the repassiva-tion, especially in reducing environments [13, 14]. Although austenitic stainless steels havea good resistance to most corrosive environments, they have relatively high susceptibility forchloride-induced stress corrosion cracking (SCC) [15]. Molybdenum (Mo) increases the resis-tance against reducing acids and pitting corrosion. Chromium increases corrosion resistancein oxidizing environments. This combination gives Cr-Ni-Mo alloys very good corrosion re-sistance in a broad range of environments [14, 16]. Molybdenum promotes ferritic structure

9

10 Chapter 3. Stainless Steels and Aluminium

and for austenitic stainless steels, this needs to be counterbalanced by addition of austeniteformers such as nickel. Nitrogen (N) is a strong austenitic former and it increases resistanceagainst pitting corrosion [14]. Niobium (Nb) and titanium (Ti) are efficient carbon binders.Carbon can precipitate as detrimental carbides, this increase susceptibility for intergranularcorrosion. Addition of Nb and/or Ti will therefore have a positive effect against intergranu-lar corrosion [13]. Manganese (Mn) in small quantity has a similar effect as N, but in largequantities it can interact with sulfur and form sulfides, which increases corrosion resistance,especially against pitting corrosion [14, 17].

Austenitic stainless steels have usually higher alloy costs compared to other stainlesssteels, due to the high addition of nickel. Table 3.2 lists composition of some commerciallyavailable austenitic stainless steel grades.

Table 3.2: Austenitic stainless steels and composition. Source: Outokumpu.

Steel: EN (ASTM/UNS) C N Cr Ni Mo Other1.4301 (304) 0.04 - 18.1 8.3 - -1.4401(316) 0.04 - 17.2 10.2 2.1 -1.4004 (316L) 0.02 - 17.2 10.1 2.1 -1.4436 (316) 0.04 - 16.9 10.7 2.6 -904L (904L) 0.01 - 20 25 4.3 1.5Cu1.4652 (654SMO) 0.01 0.5 24 22 7.3 3Mn, Cu

3.1.2 Ferritic Stainless Steels

Ferritic stainless steels have high chromium content and low or no nickel content. The fer-ritic structure result in inferior formability and weldability, compared to austenitic stainlesssteels. However, alloying with niobium and titanium increases weldability and toughness[13]. Molybdenum improves corrosion resistance, both general and localised, as for all stain-less steel.

Ferritic stainless steels were initially developed to withstand corrosion and oxidation,while having low susceptibility for SCC [17]. The ferritic structure also gives 30–35% lowerthermal expansion in comparison with austenite. This make ferritic grades suitable for usein high temperature applications and were thermal cycles are frequent [17]. The chromiumoxide film has lower thermal expansion compared to metal bulk. For austenitic stainlesssteels this can become a problem, if exposed to large temperature gradients. Ferritic stain-less steels with their lower thermal expansion are thus less susceptible to this problem.

Silicon gives higher resistance against oxidations. However, it also results in increasedbrittleness. High silicon-ferritic alloys (>14 %) have an exceptional corrosion resistance,e.g. they can withstand any concentration and temperature of sulphuric acid, but the highsilicon content also increases brittleness, as a result high silicon grades must be casted [18].The low nickel content makes ferritic grades in general the least expensive stainless steeltype. Table 3.3 lists composition of some commercially available ferritic grades.

3.1. Stainless Steels 11

Table 3.3: Ferritic stainless steels and composition. Source: Outokumpu.

Steel: EN (ASTM/UNS) C N Cr Ni Mo Other1.4512 (409) 0.02 - 11.5 0.2 - Ti1.4003 (S40977) 0.02 - 11.5 0.5 - -1.4000 (410S) 0.04 - 12.5 - - -1.4016 (430) 0.04 - 16.5 - - -1.4509 (A43932) 0.02 - 18 - - Nb,Ti1.4521 (444) 0.02 - 18 - 2.1 -

3.1.3 Duplex Stainless Steels

Duplex stainless steels have a balanced crystal structure of both ferrite and austenite. Theproportion of ferrite is usually between 40-50% [16]. This combination results in high me-chanical strength; roughly double that of austenitic stainless steels [19]. The increasedstrength makes it possible to reduce section thickness compared to austenitic grades. Du-plex alloys have better ductility and toughness compare to ferritc grades, but lower valuescompared to austenitic grades.

The relative low nickel content of duplex alloys makes them to a cost-effective alternativeto austenitic grades. Nickel is used to improve stability of the austenitic phase, but also toincrease toughness and improve repassivation [19]. Nitrogen also improves the stability ofaustenite phase, but in addition improves strength, weldability and resistance against pittingcorrosion. Molybdenum increases corrosion resistance, especially against pitting corrosion[13, 17, 19].

Duplex stainless steels are generally considered to have better corrosion resistance againstchlorides compared to similar austenitic grades [14]. However, the mixed crystal structure ofdulplex limits the operating temperature range. Outokumpu recommends that the workingtemperature of their duplex steels should be between -40oC and 250-325oC. Usage out-side this interval increases risk of embrittlement [19]. Table 3.4 lists composition of somecommercially available duplex grades.

Table 3.4: Duplex stainless steels and composition. Source: Outokumpu.

Steel: EN (ASTM/UNS) C N Cr Ni Mo Other1.4162 (S32101) 0.03 0.22 21.5 1.5 0.3 5Mn1.4362 (S32304) 0.02 0.1 23 4.8 0.3 -1.4662 (S82441) 0.02 0.1 22 5.7 3.1 -1.4462 (S32205) 0.02 0.17 22 5.7 3.1 -1.4501 (S32760) 0.02 0.27 25.4 - 3.8 W, Cu1.4410 (S32750) 0.02 0.27 25 - 4 Al

3.1.4 Martensitic Stainless Steels

Martensitic stainless steels have great hardness, due to its high alloying with carbon. How-ever, alloying with carbon also increase brittlement and decrease weldability. Futhermore,high carbon content also reduce corrosion resistance, especially against pitting and crevice


corrosion [13].

Carbon contributes to the formation of chromium carbides, which cause inhomoge-neous distribution of chromium. This may result in localised areas with significantly lowerchromium content compared to the bulk. These Chromium diluted areas then act as ini-tiation sites for corrosion, referred as intergranular corrosion (IGC). Just as for the otherstainless steels types, corrosion resistance can be increased by increasing chromium andmolybdenum alloying. Sulfur and selenium can be used to increase machinability [18].

New martensitic stainless steels alloyed with N, Ni, Mo in combination with a slightreduction in carbon content results in improved corrosion resistance and toughness [14].However, martensitic stainless steels are generally to be considered as the least corrosion re-sistant stainless steel grade. Martensitic grades are therefore used for applications were highhardness and moderate corrosion resistance is needed e.g. knife blades, turbine blades, watervalves, piston rings. Table 3.5 lists composition of some commercially available martensiticstainless steel grades.

Table 3.5: Martensitic stainless steels. Source: Outokumpu & Metal Ravne.

Steel: EN (ASTM/UNS) C N Cr Ni Mo Other1.4006 (410) 0.12 0.04 12 - - 5Mn1.4005 (416) 0.1 0.04 13 - - -1.4021 (420) 0.2 - 22 13 - 3Mn1.4028 (420) 0.3 - 22 12.5 - -1.4112 (440B) 0.9 - 25.4 18 1.1 Si, 0.1V1.4313 (S41500) 0.03 0.04 12.5 4.1 0.6 W, Cu1.4548 (-) 0.05 0.07 15.5 4.2 - -

3.2 Aluminium

Today, aluminium and aluminium alloys is the preferred engineering material in a widerange of applications, because of its many attractive material properties, e.g. combinationof low density and high strength, high thermal and electrical conductivity, and good corro-sion resistance.

Aluminium gets its corrosion protection, just as stainless steels, from an oxide film,which for aluminium is relative thick compared to stainless steels. Corrosion resistanceof aluminium is strictly dependent on its ability to form Al2O3, since other aluminiumoxides do not passivate the metal. Beyond its passive range, aluminium corrodes in aqueoussolutions, due to the passive films solubility in acids and bases, yielding Al3+ ions in theformer and AlO−

2 in the latter [14], see figure 3.1. Below a certain pH-level (usually belowpH 4) corrosion rate starts to accelerate exponentially [16]. Corrosion is also fast above acertain pH.

3.2. Aluminium 13

Figure 3.1: Pourbaix diagram for a Al/H2O system at 25oC.

Alloying elements used depend on desired material properties. The European classifica-tion system for wrought aluminium alloys (EN AW) categorise aluminium in eight differentgroups. The classification system for cast aluminium alloys (EN AC) is similar. Table 3.6shows the EN AW classification system.

Table 3.6: Classification and description for aluminium alloys.

Series Classification Description [12, 16, 18]1xxx Al > 99 % Pure aluminium, with some impurities, mostly iron and

silica.2xxx Al-Cu alloys Copper considerably increases strength, but also decreases

corrosion resistance and weldability.3xxx Al-Mn alloys Manganese increases strength and formability, while corro-

sions resistance is maintained.4xxx Al-Si alloys Silica lowers the melting point of aluminium and do not

have a general negative effect on corrosion resistance.5xxx Al-Mg alloys Magnesium increases strength while good corrosion resis-

tance is maintained. High contend of magnesium (3-6 %)gives exceptional corrosion resistance against seawater.

6xxx Al-Mg-Si alloys Addition of both magnesium and silica increases strength,but decreases corrosion resistance compared to Al-Mg al-loys.

7xxx Al-Zi alloys Zinc and manganese significantly increases strength andare therefor considered as a high-strength aluminium al-loy. Corrosion resistance is slightly lower than Al-Mg-Sialloys

8xxxx Misc. alloys Miscellaneous alloys, for example aluminium-lithium alloys.

Most relevant for this study are the AW-3xxx- and AC-4xxx grades, since they arefrequently used engineering materials for charge-air-coolers and intake manifolds.


Chapter 4

Corrosion & Electrochemistry

Corrosion in aqueous environment is of electrochemical nature, and the cause for corrosioncan depend on several different factors. Thus, basic knowledge of corrosion and electro-chemistry is vital for understanding why corrosion occurs and how it can be prevented. Thischapter will therefore provide a brief literature review about corrosion and electrochemistry,followed by how corrosion can be studied by electrochemical methods. Furthermore, corro-sion protection techniques and results from earlier corrosion studies in synthetic exhaust gascondensates will be addressed.

4.1 Corrosion of Stainless Steels and Aluminium Alloys

Corrosion of stainless steel and aluminium can be categorised as either general or localised.General corrosion is defined as a corrosion attack, which uniformly thins the metal. Localisedcorrosion is defined to be highly concentrated on local areas, or zones, often resulting information of pits and holes [16]. Localised corrosion can further be divided into: pittingcorrosion, crevice corrosion, stress corrosion, intergranular corrosion, differential aerationcell, depending on initiation origin.

4.1.1 General Corrosion

General corrosion is rarely a problem for stainless steels and aluminium, because it is usuallya slow process, which often can be managed by increasing metal thickness. However, in veryacidic environments formation of the protective oxide layer can become thermodynamicallyunfavorable, leading to loss of passivation and rapid acceleration of uniform corrosion.

For stainless steels addition of molybdenum greatly enhances the ability of passivationin acidic environments. For aluminium this passivation ability can not be as manipulatedas for stainless steels. Studies performed by ASM, shown that below a certain acidic level(usually around pH 4) corrosion rate of aluminium starts to increase exponentially [16].This can become a severe issue, especially for aluminium EGR-coolers, since pH of exhaustgas condensate has been show to be well below pH 4.

Uniform corrosion rate may be summarised to accelerate with increase in temperature,fluid velocity, oxidising power (potential) and concentration (pH) [14]. The corrosion ofaluminium is also severe at high pH, as is clear in the Pourbaix diagram, figure 3.1.

15

16 Chapter 4. Corrosion & Electrochemistry

4.1.2 Localised Corrosion

Pitting Corrosion

Pitting Corrosion is for automotive applications a more severe issue than uniform corrosion,because it is more difficult to detect, predict and design against [16]. Small material pene-tration can be enough to cause severe system failure, such as EGR-cooler leakage. Chlorideions are known to significantly increase susceptibility of pitting corrosion and causing pits tobecome autocatalytic; metal ions, M2+ dissolves in the pit and the increased concentrationof positively charged metal ions will start to attract nearby chloride ions, resulting in theautocatalytic mechanism of pitting corrosion, see figure 4.1. This type of corrosion is verylocalised and can result in very fast and unexpected metal penetration.

Figure 4.1: Schematic of the autocatalytic process of pitting corrosion.

Hydration of dissolved metal ions, Me(OH)n, lowers pH drastically in the pit, H2O →OH− +H+. Susceptibility for pitting corrosion increase with temperature and by presenceof oxidizing agents, such as oxygen. [14]. Fluid velocity is also has an important factor,affecting pitting corrosion. Generally stagnant solutions and low fluid velocities is necessaryfor pitting corrosion to occur [16]. This may especially be an issue for EGR systems, be-cause of the cyclic operation of an automotive engine. The materials in the EGR system willfrequently by exposed by wet and dry cycles, which will frequently replenish condensate informed pits and cavity’s. Thus, special care should be given, when designing EGR systems,so that local zones with possibility of an accumulation of condensate is minimized.

From section 3, it is evident that some alloying elements for stainless steels give betterresistance against pitting corrosions than others. This is well known and several corrosionstudies in the area has lead to the development of the Pitting Resistance Equivalent Num-ber (PREN) = [%Cr+3.3x%Mo+16x%N] [12, 14, 16]. Resistance against pitting and crevicecorrosion is considered to increase with PREN. However, this is not a precise tool, specialconsideration needs to be taken for the EGR system, due to the cyclic nature of operation.

4.1. Corrosion of Stainless Steels and Aluminium Alloys 17

The repassivation effect of nickel should for example be given note, because high nickelcontent is expected to result in faster repassivation.

Duplex stainless steels are considered to have a better resistance against chloride-inducedpitting corrosion. Aluminium have a tendency for pitting in solutions containing chlorides,e.g. aluminium brasses are known to be sensible to pitting in pollutant waters [14].

Crevice Corrosion

Figure 4.2: Schematic of crevice corrosion

Small gaps between joints can trap con-densate, resulting in similar autocat-alytic corrosion mechanisms as for pit-ting corrosion, see figure 4.2. Metalswith high PREN-numbers will thereforealso have good resistance against crevicecorrosion. Crevice corrosion appearsmainly due to reduced oxygen concen-tration. To maintain passivity sufficientoxygen is usually needed. In crevices,oxygen concentrations can become sig-nificantly reduced, which may result inloss of passivity. Thus, resulting in theanodic behavior seen in figure 4.2.

Crevice corrosion is promoted with increase of tightness, depth, chloride and acid concen-trations [14]. The primary prevention measure for crevice corrosion is design [16]. Designstrategy should not be to minimize gaps, since this will promote more aggressive crevicechemistry. Design strategy should instead be to maximize the gap, and to minimize thelength of the gap. The reason is that extremely tight crevices, that are not water tight,exhibit tremendous capillary action [16].

Differential Aeration Cells

Most solutions are in contact with atmospheric oxygen, but situations can arise where pres-ence of oxygen can differ from one local part of the metal to another. This circumstancecan lead to localised attack, and is referred to as differential aeration cells [12, 16]. Thelocal part with the higher oxygen concentration acts as the cathode, while the lower oxygenconcentrated part, acts as the anode. Soot deposits in the EGR system can be expected tocreate such differential aeration cells, since the soot prevent the underlaying metal to comein direct contact with the exhaust gases (oxygen).

Studies in this regarded for EGR systems has not been found, but it is known that dirtand soot deposits reduce oxygen concentration for the underlaying metal. A combinationof elevated levels of corrosive elements in soot and reduced oxygen concentrations, have thepotential to form a very corrosive environment, similar to what can be found in a crevice,i.e. soot can be expected to have the ability to form artificial crevices.


Stress Corrosion Cracking (SCC)

Figure 4.3: Synergistic effect of SCC

Stress corrosion cracking is a term describing thesynergistic interaction between corrosion and me-chanical stress, see figure 4.3. A specific metal al-loy has a critical stress concentration factor, Klc,when stress load reaches past Klc, material fail-ure will occur by cracking. If the metal is exposedto a corrosive solution in combination of stress,then cracking can occur at much lower stress lev-els. The chemical environment needs only to bemildly corrosive for SCC to become a problem.Therefore, SCC can cause sudden and unexpectedmaterial failure. The beneficial alloying elementsagainst SCC are highly dependent on the chemicalenvironment. Thus, it is difficult to give any generalised advice, but the popular 304 and316 austenitic stainless steels has shown to be especially sensitive towards chloride-inducedSCC [12, 16, 17].

The relative merits against SCC for different steels grades, depends on specific solutions,but generally ferritic and duplex grades show much better resistance against SCC thanaustenitic grades. The 2205 duplex grade can for example, basically be considered immuneagainst SCC up to a temperature of 150 oC [17].

For aluminium, SCC occurs for certain alloying grades, often those which have beendeveloped for medium and high strength (Al-Mg, Al-Cu, etc.). SCC has not been observedfor pure aluminium. Furthermore, SCC for casted aluminium is not common, but happenstime to time [12]. However, aluminium should in general, just as austenitic stainless steels,be considered sensitive against SCC in chloride containing environments.

Intergranular Corrosion (IGC)

Metals are composed of crystals, i.e. grains surrounded by grains boundaries. These grainboundaries can have significantly lower chromium alloying content relative to the metal bulk.These chromium diluted areas may therefore act as initiation sites for localised corrosion.This corrosion phenomena is referred as Intergranular Corrosion (IGC). Dilution of alloyingelements is primarily an issue at heat affected zones (HAZ).

HAZ is a area which have had its microstructure altered by welding or other intense heatoperations, i.g. cutting. Steel grades with high carbon contents have increased suscepti-ble for intergranular corrosion, because of carbons ability to form chromium carbides alonggrain boundaries. This results in zones near the grain boundaries with reduced chromiumcontent and thus acting as initiation sites for IGC [16].

Formation of chromium carbides is today often considered as a non-issue, because car-bon content in modern stainless steel grades is generally very low (<0.03 %). Martensiticstainless steels are an exception, since their high hardness properties is due to the relativelyhigh carbon content.

Duplex grades require greater care in welding compared to austenitic grades, because

4.2. Electrochemistry 19

of their mixed microstructure. Too high welding temperatures will cause increased ferriticstructure, which can result in loss of corrosion resistance and thus increased susceptibilityfor IGC. Welding techniques for duplex grades should therefore be selected with minimisedheat dispersion [20].

4.2 Electrochemistry

An electrochemical reaction, is a reaction followed with transport of electrons from oneelectron conducting material to another. These two conducting materials are called elec-trodes. The electron source is referred as the cathode and the electron sink referred as theanode. Oxidation is the loss of electrons and occurs at the anode and reduction is the gainof electrons from the electron source, i.e. cathode, see example below.

Fe(s)→ Fe2+(aq) + 2 e− (4.2.1)

1/2 O2 +H2O + 2 e− → 2 OH− (4.2.2)

Reaction 4.2.1 shows the anodic reaction (this is where metal loss through anodic oxi-dation occurs). Reaction 4.2.2 shows the cathodic reaction, in this case oxygen reduction.Reaction 4.2.1 and 4.2.2 are known as redox reactions (oxidation and reduction) and theoverall equation is:

Fe(s) +1 /2 O2 +H2O → Fe2+(aq) + 2 OH− (4.2.3)

In acidic solutions, hydrogen evaluation is the dominating reduction reaction, see equa-tion 4.2.4.

2H+(aq) + 2e− → H2(g) (4.2.4)

An electron does not exist freely in a solution. Therefore oxidation must always beaccompanied with reduction. If a metal, M(s), is immersed in a electrolyte containingMn+(aq) ions, electrochemical reactions will occur until equilibrium is reached, see equation4.2.5.

Mn+(aq) + neM(s) (4.2.5)

These reactions will create a charge on the metal surface, either positive or negative. Anelectric double layer is formed subsequently, since anions and cations with their oppositecharge attracts and bounds electrostatically to the metal surface, see figure 4.4.

The charge separation creates a potential difference between the metal and electrolyte,figure 4.4. This creates a potential difference near the metal surface. This is commonlyreferred to as the electrode potential, E. The charged interface, can because of it is struc-ture be simplified as a capacitor. The significance of the double layer is that it provides abarrier for transfer of electrons. Thus, the double layer act as an energy barrier, that mustbe overcome for electrochemical reactions to occur [14].

Metals with high electrode potentials are therefore more inert, compared to metals withlow electrode potentials. The absolute electrode potential cannot be measured directly, butit is possible to measure a relative value with an reference electrode. Noble metals suchas gold and silver have large positive electrode potentials, while zinc and magnesium have


Figure 4.4: Simplified illustration of the double layer at a metal aqueous interface.

large negative electrode potentials. Zinc and magnesium will because of their lower electrodepotential be more thermodynamically reactive than gold and silver.

Electrode potential gives information about the thermodynamic tendency for an elec-trochemical reaction to occur, but gives no information about reaction rate i.e. corrosionrate. However, accordingly to Faraday’s law the electron transport (current flow) is directlyproportional to the material loss, see equation 4.2.5.

Q =I

T=nFm

M(4.2.6)

where,

Q = Charge (C), I = Current (C/s), t = time (s), F = Faraday’s constant, n = Numberof transfered electrons, m = mass of oxidised metal (g), M = Atomic Weight (g/mol).

Thus, by measuring both potential and current flow, information of both thermodynam-ics and corrosion rate can be obtained. Electrochemical methods such as anodic polarisation,is because of this a powerful tool for studying passivated metals susceptibility for localisedcorrosion.

4.3 Anodic Polarisation Sweep

Anodic polarisation is a method to predict the tendency of an alloy to suffer localisedcorrosion in the form of pitting and crevice corrosion. The method was designed for usewith iron- or nickel- base alloys in chloride environments [14]. This experiment requiresa potentiostat, working electrode (WE), counter electrode (CE), reference electrode (RE)and a corrosion cell. The potentistat provides the necessary potential control for the WEand CE. The reference electrode, RE, provides the means to observe this experiment. Thepotentiodynamic scan is performed at a fixed voltage scan rate (mV/s), during which currentand potential is being measured and registered. Potential is increased until the currentdensity, i, reach a certain value, scan is then reversed, see figure 4.5. The forward scan givesinformation about initiation of pitting, while the reverse scan provides information aboutalloys repassivation behavior.

4.3. Anodic Polarisation Sweep 21

Figure 4.5: Schematic of an anodic polarisation sweep for stainless steel.

Figure 4.5 illustrates which information can be obtained from a anodic polarisationcurve. However, it should be stressed that this is a schematic figure illustrating some of thepossible regions present on a anodic scan. Depending on the metal and environment com-bination, some or all of these features may be present. The polarisation curve is presentedwith potential, E, on one axis and the logarithmic current density, log i, on the other axis.If no polarisation potential is applied, cathodic (reduction of oxygen) and anodic reaction(oxidation of metal) will be in equilibrium. The size of the anodic and cathodic currents willbe identical so that the net current trough the interface is zero. This potential is referredas the corrosion potential Ecorr or mixed potential Emix, see point b). If polarisation scanis performed in negative direction cathodic reactions will dominate, point a), resulting inreduction of water with hydrogen evolution as consequence. However, potential is generallyapplied in the positive (anodic) direction, since this give information about the anodic re-action, i.e. corrosion of the metal.

Metals are usually not fully passivated when the scan is initiated. Therefore, some oxi-dation is needed for passivity to occur, point c). Maximum current density obtained beforepassivation, is referred as icc, and varies between grades. Ease of passivation, increases withdecrease in icc. If potential is increased from icc current density decrease, until reaching themetals passive state, point d). A passive layer has now formed on the metal surface, and itis in this region we want the metal. If the potential is increased, the passive layer begins toperiodically breakdown, see point e). This is referred as transpassive behavior.

If potential is further increased, the passive layer will eventually breakdown, see point f).This results in significant increase of current density and the potential for when this occur,is referred as the breakdown potential, Eb. If the metal has very good corrosion resistance,other reaction can cause this significant increase in current density e.g. oxygen evolution.After reaching a certain potential or current density, scan is then reversed, until the metal


intersects with the anodic scan, point g). This potential is referred as the protection poten-tial, Ep. This potential gives information about how easily the metal will re-passivate. Ifthe loop, referred as hysteresis loop, closes at or above Eb, it indicates very low likeliness forpitting and crevice corrosion [16, 21]. The maximum obtained current density, Imax, willvary between different grades and can therefore also be considered when evaluating metals.

Consequently, when evaluating materials relative susceptibility for pitting corrosion focusis on these three features – Eb, Ep and Imax. In a specific solution and temperature; themetals with highest Eb and Ep are less likely to be susceptibility to localised corrosion.Between these to values Ep should be regarded as more conservative [14]. Meaning, if metalA show higher Eb than metal B, but metal B show higher Ep than metal A. Then metal Bshould be regarded superior.

4.4 Corrosion Protection Techniques

There exist several different corrosion protection techniques, but the easiest way to avoidcorrosion is generally by selecting sufficiently corrosion resistant alloy. However, since thereare other ways of protecting components against corrosion a short review will follow, seebelow.

4.4.1 Corrosion Inhibitors

Inhibitors suppress the electrochemical reactions that take place during corrosion. Corrosioncan only occur if both a cathodic and anodic reaction takes place. Hence, if inhibitorssuccessfully suppress one of these reactions corrosion will be prevented. Inhibitors canbe divided as anodic, cathodic or mixed, depending on which corrosion reactions is beingsuppressed [14]. A mixed inhibitor suppresses both anodic and cathodic reactions. Inhibitorsare frequently used to prevent corrosion for close-looped water recirculation systems, suchas engine coolant systems.

4.4.2 Anodic and Cathodic Protection

In a system with different metals, the metal with lowest electrode potential will corrode(act as anode). Thus, thermodynamic driving force of corrosion can be used as a corrosionprotection technique. By sacrificing the anode, the cathode can be protected. This corro-sion control technique is referred as cathodic protection or galvanic protection. Sacrificialanodes are frequently used to protect ship hulls to seawater corrosion.

Anodic protection works by shifting the electrode potential into the metals passivatedregion, see point d) in figure 4.5. This is a technique used for a limited number of systemswere passivation do not occur naturally. Most frequent use of this technique can be foundin chemical storage tanks. Anodic protection is used to a lesser degree compared to othercorrosion control, since it is only in a few combinations of grades and solutions, this protec-tion technique is advantageous. If anodic protection is used improperly corrosion rate caninstead accelerate [14].

4.5. Automotive Corrosion Studies 23

4.5 Automotive Corrosion Studies

This section will address exhaust gas condensate corrosion studies for automotive applica-tions.

4.5.1 Corrosion Study of Different Stainless steels in Synthetic Ex-haust Gas Condensate

In a study conducted by C. Hoffman et .al.[22] corrosion resistance in synthetic exhaust-gascondensate was investigated for six different stainless steels. Table 4.1, list grades and com-positions for stainless steels included in this study.

Table 4.1: Test materials and steel composition (wt%) [22].

Steel: EN Cr Ni Mo Mn1.4512 11.54 - - -1.4509 17.59 - - -1.4526 16.86 - - 0.9831.4376 18.93 3.33 - 7.771.4301 17.88 9.08 - -1.4404 16.36 11.88 1.84 -

10 ml of their synthetic condensate were comprised of: 11 g acetic buffer solution, 3.3g sodium chloride, 1 g active carbon, which resulted in solution with pH 4. Before testing;samples were cleaned and weighted. The test samples were then immersed in the syntheticcondensate and placed in a climate chamber, which had the initial temperature 85oC and50 % relative humidity. After 12 hours the temperature was lowered to 23oC and relativehumidity was kept at 50 %. After additional 12 hours the samples were cleansed, weightedand new electrolyte was added. This cycle was repeated 48 times. The corrosion resistanceof the materials were then evaluated in two regards – average pit depth and total mass loss.

Steel grade 1.4404 showed best corrosion resistance, while 1.4376 showed lowest massloss, but highest average pit depth. Results from this study indicates that molybdenum hasa strong positive effect on corrosion resistance in exhaust gas condensate. 1.4526 and 1.4376are both alloyed with manganese, but manganese addition in 1.4376 is much higher, see table4.1. This lower addition of manganese resulted in a higher mass loss, but lower average pitdepth. Low alloying with manganese proves to more beneficial than high alloying, sincelower average pit depth is regarded more beneficial than the increased mass loss. 1.4509 hassimilar chromium content as 1.4526, while having no alloying with manganese and resultsshow that that these two grades have similar corrosion resistance. Therefore, the alloyingeffect of manganese is shows to be limited.

4.5.2 Corrosion Study for Automotive Mufflers

Exhaust gas condensate cause severe corrosion inside automotive mufflers. In order tosimulate the corrosion effect of exhaust-gas condensate on mufflers Hirasawa et. al [23]performed cycling immersion tests with synthetic exhaust gas condensate. Table 4.2, listthe grades and their composition of the stainless steels included in this study.


Table 4.2: Test materials and steel composition (wt%) [23].

Steels C Si Mn Cr Cu Mo Ti Nb

Aluminised Steel∗ 0.047 0.12 0.31 0.03 - - - -Type409L 0.012 0.49 0.48 11.2 - - 0.24 -Type439L 0.012 0.09 0.23 17.6 - - 0.26 -Type430J1L 0.016 0.44 0.22 19.3 0.53 - - 0.45Type436L 0.010 0.1 0.29 18.1 - 1.2 0.27 -

∗) Al-plating weight: 80g/m2

In order to simulate the effect of oxidation, caused by the hot exhaust gases, the testsamples were pre-oxidized at 400oC for 5 hours prior to immersion in the synthetic solution.The synthetic exhaust-gas condensate consisted of Cl− (50 ppm), SO2

3− (250 ppm), SO2

4−

(1250 ppm), CO23− (2000 ppm), NH+

4 (2500 ppm) and 50 g/L active carbon. Solution andtest samples where kept in a beaker and test temperature was set to 80oC. The cover ofthe beaker was adjusted so that the solution evaporates completely in 24 hours. After 24hours the beaker was cleansed and refilled with new solution. Test specimens were alsosoft brushed. Corrosions behavior was analysed by maximum corrosion depth and prior tomeasurement the corrosion products were removed by nitric acid or diammonium hydrogencitrate.

Results obtained after 10 cycles show that the maximum corrosion depth for 430J1Land 436L were lowest, while the corrosion resistance of the aluminised steel was very poor.From measured pit depths a pitting index could be derived, accordingly: [%Cr+ 3x%Mo+1.5x%Cu]. Thus, molybdenum was three times as effective as chromium, and copper oneand a half times as effective as chromium.

A long time field test was conducted for comparison reasons. Special mufflers were de-signed so that small test specimens could be built in. Results from the field study showedgood agreement with the relative ranking retrieved from the accelerated tests, but the over-all corrosion rate in the field study was lower than in the accelerated tests. Thus, simulationconditions were too severe. The following two reasons were suggested by the authors: 1)chloride content in the test solution was too high. 2) test specimens in the field study didnot become fully immersed in condensate, because of their placement.

Results from this study show that molybdenum greatly increase corrosion resistance,alloying with copper also shows to be beneficial.

4.6 Engineers Diagram

It is common that steel manufactures provide corrosion data on their different steel grades.These data are usually based on laboratory experiments in clean acid solutions. Resultsfrom these tests are often presented as isocorrosion curves and corrosion tables. The isocor-rosion curve shows how critical pitting temperature, CPT, varies with temperature and acidconcentration. Exhaust gas condensate contains both sulphuric and nitric acid. Therefore,corrosion resistance towards these acids are of interest, see figure 4.6 and 4.7 for example onisocorrosion curves for stainless steels in sulphuric acid. However, exhaust gas condensate

4.6. Engineers Diagram 25

contains several other corrosive elements which may have a synergistic effect. Isocorrosioncurves should therefore be used with caution.

Figure 4.6: Isocorrosion curves, 0.1 mm/year,in sulphuric acid [24].

Figure 4.7: Isocorrosion curves, 0.1 mm/year,in sulphuric acid containing 2000 ppm chlorideions [24].

Steel 4404 is the Outokumpu name for the frequently used 1.4404 austenitic steel (17Cr-8Ni-2Mo) and 2304 and 2205 is the Outokumpu name for duplex steel 1.4362 (23Cr-4.8Ni-0.3Mo) and 1.4462 (22Cr-5.7Ni-3.1Mo) respectively. Duplex 2304 show similar corrosionresistance as 1.4404 in most sulphuric acid concentrations, but in low sulphuric acid concen-trations duplex 2304 has superior corrosion resistance compared to 1.4404, see figure 4.6.Duplex 2205 show significantly better corrosion resistance compared to 1.4404 in both thesesolutions, because of its high alloying of chromium and molybdenum. Austenitic stainlesssteels are known to be more sensitive to chlorides compared to duplex grades, and this canbe seen in figure 4.7. These two isocorrosion curves can be summarised to show that du-plex grades are attractive alternatives to austenitic grades regarding corrosion resistance,especially in chloride containing environments.


Chapter 5

Experimental

This chapter presents the materials and the synthetic exhaust gas condensates selected forthe electrochemical measurements, followed by a brief experimental description.

5.1 Materials

Eight different commercial stainless steel grades, as shown in table 5.1, were investigated inthis study. The chemical compositions, pitting resistance equivalent number (PREN) andalloy adjustment factor (AAF) for these alloys are shown in the table. More details of theferritic and duplex grades can be found in appendix figure 14 to 18. For martensitic steelgrade 1.4112, nitriding treatments was in addition performed on the metal surface. Theexperiments were then performed in order to compare the results obtained on martensiticsteel with and without surface treatments.

Table 5.1: Stainless steels grades selected for electrochemical evaluation.

Grade C (%) N (%) Cr (%) Ni (%) Mo (%) Other PREN AAF(EUR)**

Austenitic1.4404 0.02 - 17.2 10.2 2.1 - 24.1 21671.4301 0.04 - 18.1 8.3 - - 18.1 1414Ferritic1.4521 0.018 - 18.2 - 2.02 Ti 25 9541.4509 0.02 - 18.02 - - Nb, Ti 18 642Duplex2205 0.014 0.187 22.4 5.76 3.18 1.5Mn 35 1714LDX2404 0.022 0.271 23.94 3.78 1.59 3.03Mn 33.6 14822304 0.018 0.131 23.51 4.86 039 1.5Mn 25.6 1103Martensitic1.4112* 0.9 - 18 - 1.1 Si, 0.1V 22 -*) Tested plasma and gas nitrided. **) Outokumpu Flat Products, June 2012 [26].

Furthermore, two different commercial aluminium grades EN AW-3003 and EN AC-43000KF were investigated in this study. Table 5.2 shows the chemical compositions ofthese aluminium alloys. For aluminium alloy 43000KF, anodising surface treatment was

27

28 Chapter 5. Experimental

performed according to ISO 7599 [25]. Experiments were performed in order to investigateif the anodising treatments have any positive effects on corrosion resistance in exhaustcondensate environments.

Table 5.2: Aluminium grades selected for electrochemical evaluation.

Grade Si (%) Fe (%) Cu (%) Mn (%) Mg (%) Ni (%) Zn (%) Ti (%)EN AW-3003 0.6 0.7 0.05-0.20 1.0-1.5 - - 0.10 -EN AC-43000 KF* 9.0-11.0 0.55 0.05 0.45 0.20-0.45 0.05 0,10 0.15*) Also tested anodised

Austenitic stainless steel 1.4404 is a common used material for EGR components. Thegeneral corrosion resistance of 1.4404 is good even in environments with low pH value. Thepitting corrosion resistance at room temperature is improved due to molybdenum. However,nickel and molybdenum are expensive alloying elements.

Even though with high nickel and some molybdenum contents, the pitting corrosion re-sistance of steel 1.4404 will be greatly decreased at elevated temperature, with increasedchloride content and under low pH value environment. Sensitivity to chloride induced pit-ting corrosion has been an Achilles heel for austenitic stainless steels.

Whilst, duplex stainless steels consisting of ferritic and austenitic microstructure haveusually better pitting resistance in addition to the good corrosion resistance in low pH valueenvironments. The chloride induced stress corrosion cracking resistance is usually higherfor duplex stainless steels than austenitic stainless steels. Further, duplex and ferritic stain-less steels are also more cost effective, since they contain less nickel compared to austeniticgrades, see table 5.1.

Duplex stainless steels have about twice the tensile strength as regular austenitic stain-less steels, but the toughness and ductility of duplex stainless steels are lower compared toaustenitic stainless steels.

If increased corrosion resistance of austenitic grades are sought, it will most definitelymean higher alloy costs. There is a possibility to maintain corrosion resistance at low mate-rial cost with the use of duplex and ferritic grades. Duplex grade 2304 should provide similarcorrosion properties as the austenitic 1.4404 grade, in addition with better resistance againstchlorides and stress corrosion cracking (SCC). This makes duplex grades such as 2304 to anattractive alternative to the austenitic 1.4404 steel grade.

Use of EGR may cause increased wear on piston rings, resulting in reduced engine lifetime. Wear resistance might be increased by nitriding treatment on the contact surface.However, there are concerns that nitriding will results in decreased corrosion resistance.1.4112 was therefore selected in order to investigate if the nitriding treatment will have anegative effect on corrosion resistance. Two techniques were selected for this purpose: gasand plasma nitriding.

The two aluminium grades selected are typical commercial grades used for constructingcharge-air-coolers. Aluminium has significantly higher thermal conductivity and lower den-sity compared to stainless steel and is therefore a more preferable material to use for cooling

5.2. Synthetic Exhaust Gas Condensate 29

of exhaust gases. It is therefore of interest to compare corrosion resistance of aluminiumand stainless steel. Corrosion resistance of aluminium can be improved by anodising, whichis a technique used to increase the thickness of the passive oxide film.

5.2 Synthetic Exhaust Gas Condensate

Five different synthetic exhaust gas condensates, as shown in Table 5.3, were used for thisstudy. Condensate acidity varies between pH 2.5 and pH 1.5. Chloride concentrations variesbetween 32 ppm, 200 ppm and 3300 ppm. Condensate 1 is the least corrosive condensatewith pH 2.5 and a relatively low chloride concentration. Condensate 2 has a significantlyincreased nitric acid concentration and chloride concentration, and also small concentrationsof organic weak acids.

Table 5.3: Chemical composition of synthetic exhaust gas condensates (C) used for anodicpolarisation measurements.

mg/l C1 C2a C2b C3a C3bH2SO4 110 15 15 2900 2900HNO3 200 2900 2900 15 15Formic Acid - 20 20 20 20Acetic Acid - 20 20 20 20Cl− 32 200 3300 200 3300pH* 2.5 1.5 1.5 1.5 1.5*) Control measured after each batch.

Three different alternatives to condensate 2a were additionally selected (2b, 3a, 3b). Thepurpose was to investigate the parameters that have most influences on pitting corrosionresistance. Test temperature was 60oC,since it is the temperature which can still be expectedto produce substantial amount of exhaust gas condensate in the EGR system. The testmatrix is shown in Table 5.4.

Table 5.4: Grades and condensates (C) experimentally tested in this study.

Grade C1 C2a C2b C3a C3b1.4404 X X X X X1.4301 X X X X X2205 X - X - XLDX2404 - - X - X2304 X X X X X1.4521 X X X X X1.4509 X - - - -1.4112 X - - - -1.4112 Plasma N X - - - -1.4112 Gas N X - - - -AW-3003 X - - - -AC-43000KF X - - - -AC-43000KF Anod X - - - -

30 Chapter 5. Experimental

As seen, not all combinations could be tested, due to limited time.

5.3 Sample Preparation

Test samples were wet abraded by using #120 to #600 grit SiC paper and cleansed af-terwards by deionised water and ethanol. Test samples were then dried with a hairdryerand left exposed to room environment for 12-16 hours before electrochemical measurements.Test samples with surface treatments (4112 and AC 43000KF) were tested as delivered.

A water bath was used to preheat the condensate to 65oC. This higher temperaturewas chosen because of expected heat loss during test preparation. The measurements werethen performed in a climate chamber at 60oC. The reference electrode was preheated in theclimate chamber. A brief description of the electrochemical experiments is given in the nextsection.

5.4 Potentiodynamic Polarisation Measurements

All electrochemical experiments were carried out in a three-electrode cell with Ag/AgCl (3MKCl) as reference electrode and a platinum net as counter electrode. The working electrodewith an immersed area of 1cm2 was mounted into the cell before measurements.

The electrochemical instrument set-up, as shown in figure 5, consists of a Solartron 1287potentiostat, using CorrWare software package. Temperature were kept constant at 60oCwith a B.I.A climate chamber. The scan rate was 0.1667 mV/s, which is the recommendedby ASTM, and scan was programmed to be reversed at a current density of 10 µA/cm2 or2 volt above OCP (open circuit potential). For aluminium grades and stainless steel grade1.4112 the polarisation scan were programmed to be reversed at 0.8V vs reference electrode.

Figure 5.1: Experimental Setup: 1) Solartron 1287 potentiostat 2) Ag/AgCl referenceelectrode (3M KCl 3) Platinum plate counter electrode 4) Working Electrode 5) CorrosionCell 6) B.I.A climatic climate chamber 7) Corrware R© Software Package.

Chapter 6

Results

This chapter presents the results obtained from the electrochemical measurements.

6.1 Condensate 1

Condensate 1 consists of 110 ppm sulphuric acid, 200 ppm nitric acid and 32 ppm Cl−. ThepH value of condensate 1 was 2.5.

6.1.1 Condensate 1 – Austenitic, Ferritic and Duplex Grades

The anodic polarisation curve is a useful method to evaluate the pitting corrosion resistance.Figure 6.1 shows the anodic polarisation curves of different stainless steels in condensate 1at 60oC. A relatively stable corrosion potential (also called open circuit potential, OCP), asindicated in figure 6.1, was obtained after 10 minutes exposure in the condensate. The anodicpolarisation was then started at the corrosion potential with a scan rate of 0.1667 mV/s. Thecurrent density increased with increasing potential, indicating that more corrosion occurred.When the potential was increased to about 0.1 V vs. reference electrode, the material wentinto a passive range. This is because a thin passive film has formed on the metal surfaceconsisting of chromium oxide. Corrosion continues but with a very small current density;the passive current, ip. However, with further increased anodic potential, the passive layerwill breakdown. This potential is called the breakdown potential. The breakdown potential,passive range, passive current density and maximum current density before passivation areparameters to be used to evaluate the pitting corrosion resistance of stainless steels. Astainless steel with good pitting corrosion resistance will have a higher pitting potential,bigger passive range, lower passive current and maximum current.

31

32 Chapter 6. Results

Figure 6.1: Anodic polarisation plot for stainless steel grades Duplex 2205, Duplex 2304,1.4301, 1.4404, 1.4509, 1.4521.

Even though the pH value was only 2.5, the pitting corrosion resistance of all testedaustenitic, ferritic and duplex stainless steels was good. Most notable difference was on themaximum current density before passivation. Plots show that the duplex grades requirelowest current before passivation. This indicates that duplex grades passivate more easilythan austenitic and ferritic grades. The relatively low alloyed 1.4509 ferritic grade requiredhighest current before passivation. For all tested austenitic, ferritic and duplex grades, theanodic polarisation predicts low risk for pitting corrosion in condensate 1, since repassivationpotential was similar to the breakdown potential, as can be observed in table 6.1.

Table 6.1: Eb, Ep and Imax obtained from anodic polarisation of austenitic, ferritic andduplex grades in condensate 1.

Grade Eb (V ) Ep (V ) Imax (µA/cm2)1.4404 ∼0.70 ∼Eb ∼101.4301 ∼0.70 ∼Eb ∼102205 ∼0.70 ∼Eb ∼102304 ∼0.70 ∼Eb ∼101.4521 ∼0.70 ∼Eb ∼101.4509 ∼0.70 ∼Eb ∼10

6.1. Condensate 1 33

6.1.2 Condensate 1 - Grade 4112 & Effect of Nitriding

Figure 6.2 shows the anodic polarisation curves obtained on steel 1.4112, with and withoutnitriding treatment, in condensate 1 at 60oC. Appendix figure 1 shows photos of testedsamples after polarisation scan.

Figure 6.2: Plot from anodic polarisation measurement of nitride coated and uncoated1.4112 martensitic stainless steel in condensate 1.

It was observed that the tested materials was experiencing serious general corrosion andpitting corrosion. No significant passivation can be seen. The current densities are in themagnitude 1000 times higher compared to the austenitic, ferritic and duplex steels at 0.8Vvs reference electrode.

There is no significant differences between the nitrided samples and the steel withoutany surface treatment.

Microscopic examination of nitrided samples were carried out to investigate if the twonitriding films had been affected differently by the anodic polarisation. Appendix figure5 to 10 show nitride film before and after the anodic polarisation. The nitride film forboth test samples is observed to have been damaged by both general and pitting corrosion.Subsequently, no difference between nitriding techniques can be observed. However, it isunclear if similar damage can be expected to occur naturally (un-polarised), since the anodicpolarisation, which these two samples were subjected to, forced the material to undergo largecorrosion currents.


6.1.3 Condensate 1 – Aluminium Grades & Effect of Anodising

Figure 6.3 shows anodic polarisation curves for aluminium samples in condensate 1 at 60oC.It is observed that tested aluminium grades show similar polarisation behavior as that ofthe martensitic stainless steel 1.4112 steel.

Figure 6.3: Anodic polarisation plot for aluminium grades AC43000KF and AW-3003 incondensate 1. AW-3003 and AC-43000KF-anodised also tested after 12 hour exposure.

The general corrosion rate was high. No passivation can be seen, see appendix figure 2.It can be observed that the area which is exposed to the condensate is severely damaged bythe anodic polarisation. These results confirm the general understanding of aluminium, i.e.low pH cause dissolution of the formed aluminium oxide film.

Anodised aluminium show initially relatively high OCP and low Icorr compared to theun-anodised test sample. However, when potential is increased in the anodic direction thecurrent density increases rapidly. When the potential approach 0.8 V vs ref, all specimenshave similar current densities. This shows that not even the anodised film is stable at anodicpolarisation at low pH. To further investigate this, anodic polarisation were preformed aftertwelve hours of exposure to condensate at 60oC. OCP was measured during this twelve hourperiod and result is displayed in figure 6.4.


Figure 6.4: Open circuit potential (OCP) for Al AC-43000KF in condensate 1.

During this entire measuring period, it is observed that OCP moves in the cathodicdirection. This indicates loss of passivity and dissolution of anodised aluminium oxide film.After twelve hours of exposure OCP for the anodised test sample is observed to be close toOCP for unanodised aluminium. The beneficial effect of anodising aluminium is thereforevery limited at low pH condensate environment. Grade AW-3003 was also tested after twelvehours of exposure. The OCP moved slightly in the anodic direction, but did not notablyaffect the anodic polarisation behavior.

6.2 Condensate 2

Condensate 2 is more corrosive than condensate 1, due to lower pH and higher chlorideconcentrations. The aluminium alloys and the martensitic stainless steel did undergo seriousgeneral corrosion in condensate 1. It is therefore not useful to test these materials in morecorrosive condensates, such as condensates 2 and 3. The ferritic steel 1.4509 was expectedto undergo general corrosion in such a low pH condensate and was therefore also excludedfrom further testing, because of shortage of time.

6.2.1 Condensate 2a – Austenitic, Ferritic and Duplex Grades

Condensate 2a contains 15 ppm sulphuric acid, 2900 ppm nitric acid, 20 ppm formic acid,20 ppm acetic acid and 200 ppm Cl−. The chloride concentration is about 6 times higherthan that for condensate 1.

Figure 6.5 shows the anodic polarisation curves for two austenitic, one duplex and oneferritic stainless steel in condensate 2a at 60oC. The breakdown potential, Eb, respassivation


potential, Ep, and maximum breakdown current, Imax is shown in Table 6.2.

Figure 6.5: Anodic polarisation plot for stainless stainless steel 2304, 1.4301, 1.4404, 1.4521in Condensate 2a.

Eb for all tested steels are about 0.75V vs. reference electrode, which is similar to theresults obtained in condensate 1. All hysteresis loops where completed at or above Eb. Theresults indicate a low risk for pitting corrosion of these steels in condensate 2a. Criticalcurrent density, Icc, is similar to what was observed in condensate 1, see table 6.2. However,passive current density, Ip, is slightly higher for all tested grades compared to condensate 1.This is expected since condensate 2a is more acidic than condensate 1. Duplex grades 2205and LDX 2404 were excluded in this test series, since the low alloyed duplex grade 2304showed low likeliness for pitting corrosion in this condensate.

Table 6.2: Eb, Ep and Imax obtained from anodic polarisation scan in condensate 2a.

Grade Eb (V ) Ep (V ) Imax (µA/cm2)1.4404 ∼0.75 ∼Eb ∼101.4301 ∼0.75 ∼Eb ∼102304 ∼0.75 ∼Eb ∼101.4521 ∼0.75 ∼Eb ∼10


6.2.2 Condensate 2b – Austenitic, Ferritic and Duplex Grades

Condensate 2b contains 15 ppm sulphuric acid, 2900 ppm nitric acid, 20 ppm formic acid,20 ppm acetic acid and 3300 ppm Cl-. Hence, chloride concentration is considerably highercompared to condensate 2a (increased from 200 ppm to 3300 ppm). Three duplex stainlesssteels (2205, LDX 2404 and 2304), two austenitic stainless steels (1.4404 and 1.4301) andone ferritic stainless steel (1.4521) were tested in condensate 2b.

Figure 6.6 shows the anodic polarisation curves for all tested steels in condensate 2b.Eb and Ep for austenitc grades 1.4301 and 1.4404 are about 0.73V and 0.3V lower, whichis lower compared to that in condensate 1 and 2a. Whilst Eb and Ep for duplex 2205,LDX2404, 2304 and ferritic 1.4521 are similar to that observed in condensate 1 and 2a, seetable 6.3.

Figure 6.6: Anodic polarisation plot for stainless steel grades 2205, LDX2404, 2304, 1.4404,1.4301, 1.4521 in condensate 2b.

Pits were visually observed for the austenitic grades after polarisation scan, see appendixfigure 3. More pits can be observed for the low alloyed 1.4301 grade. The reduced Eb andEp for 1.4301 and 1.4404 indicates that these two austenitic steels are susceptible to pittingcorrosion in this high chloride concentrated solution. It is unclear if 1.4301 was actuallypassivated, since Eb and Ep is very close to the OCP. Steel 1.4404 shows higher resistanceagainst pitting corrosion compared to 1.4301, which is expected due to its alloying withmolybdenum.


Table 6.3: Eb, Ep and Imax obtained from anodic polarisation of austenitic, ferritic andduplex grades in condensate 2b.

Grade Eb (V ) Ep (V ) Imax (µA/cm2)1.4404 0.45 0.05 1251.4301 0.02 -0.15 9572205 ∼0.75 ∼Eb ∼10LDX2404 ∼0.75 ∼Eb ∼102304 ∼0.75 ∼Eb ∼101.4521 ∼0.75 ∼Eb ∼10

Tendency for pitting corrosion in condensate 2b at 60oC follow the sequence:

1.4301<1.4404<duplex 1.4521∼duplex 2304∼duplex LDX2404∼duplex 2205

Clearly, duplex stainless steels have better pitting corrosion resistance in low pH envi-ronment when chloride concentration is increased.


6.3 Condensate 3

Concentrations of sulphuric and nitric acid were switched in condensate 3, relative to con-densate 2 in order to investigate the effect of increased concentration of sulphuric acid onpitting corrosion resistance. The pH was kept the same as that in condensate 2 (pH 1.5).

6.3.1 Condensate 3a – Austenitic, Ferritic and Duplex Grades

Condensate 3a contains 2900 ppm sulphuric acid, 15 ppm nitric acid, 20 ppm formic acid,20 ppm acetic acid and 200 ppm Cl−.

Two austenitic stainless steels 1.4404 and 1.4301, one duplex stainless steel 2304 andone ferritic stainless steel 1.4521 were tested in condensate 3a. Figure 6.7 shows the anodicpolarisation curves for these tested materials in condensate 3a at 60oC. The results weresimilar to that obtained in condensates 1 and 2a, i.e. predicting low risk for pitting corrosion.

Figure 6.7: Anodic polarisation obtained for stainless steels 2304, 1.4404, 1.4301, 1.4521 incondensate 3a.

Clearly, high concentration of sulphuric acid, low pH value but lower chloride content(200 ppm) do not increase the risk for pitting corrosion for austenitic steels 1.4404 and1.4301, duplex 2304 and ferritic 1.4521.

6.3.2 Condensate 3b – Austenitic, Ferritic and Duplex Grades

Condensate 3b contains 2900 ppm sulphuric acid, 15 ppm nitric acid, 20 ppm formic acid,20 ppm acetic acid and 3300 ppm Cl−. Hence, condensate 3b has considerably higher con-


centrations of chlorides compared to condensate 3a.

Three duplex stainless steels (2205, LDX 2404 and 2304), two austenitic stainless steels(1.4404, 1.4301) and one ferritic stainless steel (1.4521) were tested in condensate 3b. Thepassivation behavior for 1.4301 was again very poor and similar to the behavior in conden-sate 2b. As a result the programmed reverse current density (10µ Amps/cm2) was triggeredshortly after polarsation start. The measurements performed on the steel could thereforenot yield in a usable polarisation curve. This poor passivation behavior was only observedfor 1.4301 and is therefore regarded as inferior to all other tested stainless steels.

Figure 6.8 shows the anodic polarisation curves for the tested materials in condensate 3bat 60oC. The breakdown potential, Eb, repassivation potential, Ep, and maximum break-down current, Imax are shown in Table 6.4.

Figure 6.8: Anodic polarisation for stainless steel grades 2205, 2304, LDX2404, 1.4301,1.4404, 1.4521 for condensate 3b.

As shown in figure 6.8, the Eb and Ep decreased drastically for all tested materials,except for the duplex grade 2205.

Eb and Ep for austenitic 1.4404, duplex 2304 and LDX2404 are similar. 1.4404 hasslightly higher Eb, but slightly lower Ep compared to the two duplex steels. These threegrades show similar pitting corrosion resistance for initiation, but when pitting has beeninitiated, the two duplex steels especially LDX2404 show better tendency for repassivation.

The ferritic grade 1.4521 had a relatively high Eb, but Ep was low and Imax high. Thestochastic nature of Eb makes Ep to a more conservative value. Ep should therefore be given


more weight. Pitting resistance for 1.4521 is therefore only regarded superior to 1.4301.

Table 6.4: Eb, Ep and Imax for tested austenitic, ferritic and duplex grades in condensate3b.

Grade Eb (V ) Ep (V ) Imax (µA/cm2)1.4404 0.35 -0.06 3781.4301 - - -2205 0.75 ∼Eb ∼10LDX2404 0.34 -0.03 882304 0.31 -0.04 2451.4521 0.53 -0.11 620

Tendency for pitting corrosion in condensate 3b at 60oC follows the sequence:

1.4301<1.4521<1.4404∼duplex 2304∼duplex LDX2404<duplex 2205

Clearly again, duplex stainless steels have better pitting corrosion resistance in low pHenvironment when chloride concentration is increased.


Chapter 7

Discussion and Summary

The focus of this Master’s thesis is to investigate the pitting corrosion resistance of materialsin EGR system where exhaust gas condensation may occur. Based on the knowledge fromthe literature review and earlier studies performed at Scania, totally eight different stainlesssteels and two aluminium alloys were included. Furthermore, nitriding surface treatmentson one martensitic stainless steel and anodising surface treatments on one aluminium alloy,were also included in this study.

Totally five condensates with different concentrations of sulphuric acid, nitric acid andchlorides were chosen to perform the electrochemical measurements. Two pH values, 2.5and 1.5, were included. The testing temperature was 60oC since this is the temperaturewhen most exhaust condensation is expected to happen.

Materials in the EGR system are exposed to several changing environmental factors,e.g. material stress, temperature; dry to wet phase and vice versa. During electrochemicalexperiments these environmental factors need to be held constant, since this makes compar-ison possible. Furthermore, composition of exhaust gas condensate is complex, it containsseveral impurities. Active carbon was initially thought to be included in the synthetic ex-haust gas condensate composition, to simulate the effect of soot particles which is present inactual condensate. During initial tries it was observed that the active carbon distributionin the corrosion cell becomes very inhomogeneous, due to its poor solubility in the solution.This inhomogeneous distribution increased experimental uncertainty, decision was thereforemade not to include active carbon in the synthetic condensates.

How differences between experimental condition and operating condition factors willaffect the risk for pitting corrosion is difficult to predict. The term ”predict” is thereforecommonly used for anodic polarisation, since results from anodic polarisation have in somecases been observed to differ from actual service performance.

7.1 Pitting Corrosion in the EGR System

The general corrosion resistance of stainless steels are good in exhaust gas condensate envi-ronments with low pH value. This is why the main EGR components such as EGR-coolersare made of stainless steels. However, the pitting corrosion resistance at elevated temper-ature, with increased chloride content and under low pH value environment will be greatly

43

44 Chapter 7. Discussion and Summary

decreased. Pitting corrosion is localised and difficult to prevent by surface treatment tech-nique. Pitting can result in leakage for EGR components. Pits can also initiate stresscorrosion cracking.

From the results obtained in this study, it can be clearly seen that the pitting corrosionresistance of all tested austenitic stainless steels, duplex stainless steels and ferritic stainlesssteels is good in condensates with low chloride ions (condensate 1, 2a and 3a), even thoughthe pH values were low and sulphuric acid contents high.

Condensate 2a and 3a is a solution containing 200 ppm Cl− at pH 1.5. This is a solutionwhich should be regarded as more corrosive than actual exhaust gas condensate, especiallyif low sulfur diesel fuels are used.

Results from this study indicates that high chloride concentrations are necessary forpitting corrosion to occur. Accumulation of chlorides in localised positions in the EGRsystem is therefore thought to be the root cause if pitting corrosion happens. The questionis where the chlorides comes from? The chloride content in the air in offshore or near seaenvironment may be somewhat high, but still very limited and studies regarding chloridesin the EGR system have not been found.

7.2 Material Choice for EGR Components

Austenitic stainless steel 1.4404 is a commonly used material for EGR components. Eventhough with high nickel and some molybdenum contents, the pitting corrosion resistance ofsteel 1.4404 was greatly decreased at 60oC, with increased chloride content and under lowpH value environment. Sensitivity to chloride induced pitting corrosion is an Achilles heelfor austenitic stainless steels.

1.4404 is alloyed with about 2% of molybdenum and 10% nickel. This gives the steelgood corrosion resistance against most acids, but the molybdenum and nickel addition alsoconsiderably increase alloying cost, see the AAF factor for grades 1.4301 and 1.4404 intable 5.1. Engineers diagrams and corrosion tables from steel manufactures show that du-plex steels are attractive alternatives for austenitic steels and results obtained in this studyclearly supports this.

Duplex steel grades are known to have higher pitting resistance against chlorides andSCC. Duplex steel grades also have higher mechanical strength compared to austenitic steelgrades. This may enable reduction of component thickness, which will save cost and weight.The mixed microstructure of duplex steel grades also results in limitations, most pronouncedare lower formability, weldability and limited operating temperatures (-40 to 250-325oC).

Price for stainless steel is compromised of two factors: AAF and Base Price. All selectedduplex steels grades in this study have lower AAF compared to 1.4404, see 5.1. However,the base price for duplex steel grades are higher compared to austenitic grades, because ofincreased manufacturing complexity and lower manufacturing yield.

The ferritic steel 1.4521 seems to have good resistance against initiation of pitting corro-sion, but once pits have formed repassivation ability compared to the austenitic and duplex

7.2. Material Choice for EGR Components 45

steel grades shows to be inferior. The reason is that ferritic steels are not alloyed with nickel,which is known to increase repassivity for stainless steel grades.

Repassivation ability have a major role on the applicability of stainless steel grades, sinceinitiation of pitting corrosion is a matter of probability and is expected to occur sooner orlater.

Aluminium grades AW-3003 and AC-43000KF are subjected to both general and pittingcorrosion in condensate 1 (pH 2.5) and high current densities were observed during anodicpolarisation. The use of aluminium as engineering material in EGR environment can there-fore not be recommended. Anodising aluminium may increase the corrosion resistance, butthe anodised oxide film was observed to dissolve in low pH environment, and after 12 hoursof exposure at 60oC the protective effect of the anodised film was negligible. It is well knownthat aluminium is not passive in solutions with low pH so these results are not surprising.

Nitriding treatments of the martensitic steel 1.4112 show no effect on corrosion resis-tance in condensate environments. However, since the steel grade was subjected to generalcorrosion anodic polarisation is not a suitable method for studying this issue. Further in-vestigation is therefore recommended.

Field studies are needed before implementing a possible material change. Results ob-tained in this study are what can be expected; lower alloyed grades austenitic and duplexgrades showed to be inferior to the higher alloyed grades of the same type. However, the lowrisk for pitting corrosion in condensate 1, 2a, 3a are somewhat surprising, since this indi-cates that the exhaust gas condensate should not cause pitting corrosion in the EGR system,instead results indicates that elevated levels of corrosive elements need to be reached, beforepitting corrosion should become prominent.

46 Chapter 7. Discussion and Summary

Chapter 8

Conclusions

• Eight different stainless steels and two aluminium alloys were investigated in this studyregarding the pitting corrosion resistance in exhaust gas condensate environment. Fur-thermore, nitriding surface treatments on one martensitic stainless steel and anodisingsurface treatments on one aluminium alloy were also included in this study.

• Five condensates with different concentrations of sulphuric acid, nitric acid and chlorideswere chosen to perform the electrochemical measurements. Two pH values, 2.5 and1.5, were included. The testing temperature was 60oC, since it is the temperaturewhich can still be expected to produce substantial amount of exhaust gas condensatein the EGR system.

• The anodic polarisation curve is a useful method to evaluate the pitting corrosion re-sistance. The results obtained in this study indicates that condensate formed duringnormal EGR operation should not cause pitting corrosion, in an environment with lowchloride content and pH value 1.5 and 2.5. Accumulation of chlorides to critical levelsis therefore predicted to be required to initiate pitting corrosion in the EGR system.

• Decreasing pH value from 2.5 to 1.5 and increasing chloride content from 32 ppm to 200ppm will not increase the risk for pitting corrosion in condensate environments at60oC. This was observed at two austenitic stainless steels (1.4404 and 1.4301), threeduplex stainless steels (duplex 2205, 2304 and LDX2404) and one ferritic stainlesssteel (1.4521).

• Chloride contents of 0.33 wt% in the tested condensates significantly increase the risk forpitting corrosion, especially for austenitic steels. Duplex stainless steels show betterpitting resistance in addition to the good corrosion resistance in low pH value andhigh chloride content environments.

• Aluminium alloys are subjected to general and pitting corrosion in condensate environ-ment at pH 2.5. The anodised film on the surface was not stable in condensateenvironments with low pH value. After twelve hours of exposure to condensate 1 at60oC, the protective effect of the film became negligible.

47

48 Chapter 8. Conclusions

• Martensitic stainless steel 1.4112 was subjected to general and pitting corrosion in con-densate environment at low pH value. There is no difference in corrosion resistancebetween the nitride coated 1.4112 steel and the steel without coatings. No differencescan be seen between the plasma and gas nitrided samples. Further investigation in lesscorrosive environment is needed since the anodic polarisation curves are not suitableto study the general corrosion behavior.

• The pitting corrosion resistance in condensates with high chloride concentrations at 60oCfollows the sequence 1.4301<1.4521<1.4404<duplex 2304<duplex LDX2404<duplex2205. Clearly, duplex stainless steels have better pitting corrosion resistance in lowpH environment when chloride concentration is increased. Considering the operatingconditions of the EGR components, the element prices, it is probably more beneficialto consider the duplex stainless steels for use in the EGR system where temperaturesallows it (<250-325oC).

Chapter 9

Recommendations

The austenitic stainless steels 1.4404 and 1.4301; duplex stainless steels 2205, 2304 and LDX;and ferritic stainless steels 1.4521 show similar pitting corrosion resistance in condensatesat low pH vales. Long term exposure testing in condensates with low chlorides are thereforerecommended in order to investigate if it is possible to choose the cheaper steels in EGRcondensates.

Duplex stainless steels have better pitting corrosion resistance in low pH environmentwhen chloride concentration is increased. It is therefore recommended to do more investiga-tions on how manufacturing cost is affected if components are changed to duplex stainlesssteel. If the cost savings are notable, or if increased robustness is significant, continuousfield studies are needed before changing of material.

It is recommended to further investigate the effect of nitriding surface treatment withlong term exposure testing.

Accumulation of corrosive species in soot deposits are important to make any decision ofmaterial choice. Characterisation of soot deposits is therefore also recommended. It shouldbe investigated if cleaner exhaust gases can be introduced into the EGR system, since thiswill reduce fouling and engine wear. LP-EGR systems are advantageous in this regard, sinceexhaust gases for such systems passes through the after treatment system.

Very limited studies were found regarding corrosiveness of exhaust gas condensates de-rived from use of biodiesel fuels. Future work is therefore recommended and investigationshould include collection and analysis of biodiesel condensate.

49

50 Chapter 9. Recommendations

Bibliography

[1] Euro 6, Emission for heavy duty vehicles: European commission. [Online] [Cited: 0703 2012.],http://ec.europa.eu/enterprise/sectors/automotive/environment/eurovi/

index_en.htm

[2] Magdi K. Khair. Hannu Jaaskelainen, Exhaust Gas Recirculation. [Online] [Cited: 0803 2012.]http://www.dieselnet.com/tech/engine_egr.php

[3] Guillaume Bourgoin, Eva Tomas Jose Lujan and Benjamin Pla, Acidic Condensationin HP EGR Systems Cooled at Low Temperature Using Diesel and Biodiesel Fuels. SAETECHNICAL PAPPER SERIES, Michagan Technological University, 1999

[4] Admir M. Kreso, John H. Johnson, Linda D. Gratz, Susan T. Bagley and David G.Leddy, A Study of the Vapor- and Particle-Phase Sulfur Species in the Heavy-DutyDiesel Engine EGR Cooler. SAE TECHNICAL PAPPER SERIES, Michagan Techno-logical University, 1998

[5] M. Mosburger, J. Fuschetto, D. Assanis & Z. Filipi, Impact of High Sulfur MilitaryJP-8 Fuel on Heavy Duty Diesel Engine EGR Cooler Condensate. SAE International,USA, 2008

[6] Thomas L. Mckinley, Modeling sulphuric Acid Condensation in Diesel Engine EGRCoolers. SAE TECHNICAL PAPPER SERIES, Warrendale, 1997

[7] Huijbregts Corrosion Consultancy, KEMA BV, LATEST ADVANCES IN THEUNDERSTANDING OF ACID DEWPOINT CORROSION: CORROSION ANDSTRESS CORROSION CRACKING IN COMBUSTION GAS CONDENSATES. Anti-Corrosion Methods and Materials, Vol. 51, 3, 2004, pg 173-188.

[8] M. D. Kass, J. F. Thomas, D. Wilson, S. A. Lewis, Sr. Assessment of Corrosivity Associ-ated with Exhaust Gas Recirculation in a Heavy-Duty Diesel Engine. SAE TECHNICALPAPPER SERIES, Oak ridge National Laboratory, 2005

[9] S. Moroz, G. Bourgoin, J. M. Lujan, B. and B. Pla, Acidic condensation in Low PressureEGR systems using Diesel and Biodiesel Fuels. SAE int. J. Fuels Lubr. 2(2):305-312,2009

[10] International Fuel Quality Center, Maximum On-Road Diesel Sulfur Limits. [Online][Cited: 27 03 2012.] http://www.ifqc.org/NM_Top5.aspx

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[11] James W. Girard, Lida D. Grataz, John H. Johnson, Susan T. Bagley and David G.Leddy, A Study of the Character and Deposistion Rates of Sulfur Species in the EGRCooling System of a Heavy-Duty Diesel Engine. SAE TECHNICAL PAPPER SERIES,Michagan Technological University, 1999

[12] L L Shreir, R A Jarman & G TBurstein, Corrosion. Butterworth-Heinemann, Oxford,Volym 1, 3nd Edition, 1994.

[13] Leffler Bela, Stainless – Stainless steels and their properties. [Online] [Cited: 24 022012.]http://www.outokumpu.com/files/Group/HR/Documents/STAINLESS20.pdf

[14] ASM Handbook, Corrosion. ASM International, USA, Volume 13, 1987.

[15] Einar Mattson & Kucera Vladimir, Elektrokemi och Korrosionslara. SWERA Kimab,Stockholm 2009.

[16] ASM Handbook, Corrosion Fundamentals, Testing, and Protection. ASM International,Ohio, Volume 13A, 2003.

[17] Maria Ohman, Literature Review of Stainless steels for Automotive and Bus Applica-tions. Swerea-KIMAB, Stockholm, 2012.

[18] L L Shreir, R A Jarman & G TBurstein, Corrosion. Butterworth-Heinemann, Oxford,Volym 2, 3nd Edition, 1994.

[19] Outukumpu produktblad Materials for winning Ideas – Outokumpu Duplex StainlessSteels. [Online] [Cited: 28 02 2012.]http://www.outokumpu.com/applications/upload/pubs_10583455.pdf

[20] Outokumpu, Corrosions Hanbook. Outokumpu Oyj, 10nd Edition, 2009.

[21] University of South Carolina BASICS OF CORROSION MEASUREMENTS. [Online][Cited: 11 06 2012.]http://www.che.sc.edu/faculty/popov/drbnp/ECHE789b/Corrosion\

%20Measurements.pdf

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[24] Outukumpu produktblad Duplex Stainless Steels. [Online] [Cited: 04 03 2012.]http://www.outokumpu.com/SiteCollectionDocuments/Duplex_Stainless_

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[25] ISO 7599:2010, Anodizing of aluminium and its alloys - General specifications for anodicoxidation coatings on aluminium. International Organization for Standardizationl, 2010

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Flat-Products-AAF-June-2012.pdf

Appendix

Figure 1: Photo of 4112 grade test samples: 1) Before polarisation. 2) After polarisation.3) Gas nitrided after polarisation. 4) Plasma nitrided after polarisation.

Figure 2: Photo of aluminium grade test samples: 1) AC-43000KF before polarisation 2)AC-43000KF after polarisation 3) AC-43000KF anodised before polarisation 4) AC-43000KFanodised after polarisation 5) AW-3003 before polarisation 6) AW-3003 after polarisation

53

54 APPENDIX

Figure 3: Photo of test samples after polarisation in condensate 2b. Pitts visible by visualinspection have been marked.

Figure 4: Photo of test samples after polarisation in condensate 3b. Pitts visible by visualinspection have been marked.

APPENDIX 55

Figure 5: Microscopic examinations of plasma nitride film before anodic polarisation.

56 APPENDIX

Figure 6: Microscopic examination of gas nitride film before anodic polarisation.

APPENDIX 57

Figure 7: Microscopic examinations of plasma nitride film after anodic polarisation in con-densate 1. Marks indicate exposed area.

58 APPENDIX

Figure 8: Microscopic examinations of plasma nitride film after anodic polarisation in con-densate 1.

APPENDIX 59

Figure 9: Microscopic examination of gas nitride film after anodic polarisation in condensate1.

60 APPENDIX

Figure 10: Microscopic examination of gas nitride film after anodic polarisation in conden-sate 1.

APPENDIX 61

Figure 11: Test coupon documentation for duplex steel 2205.

62 APPENDIX

Figure 12: Test coupon documentation for duplex steel LDX2404.

APPENDIX 63

Figure 13: Test coupon documentation for duplex steel 2304.

64 APPENDIX

Figure 14: Test coupon documentation for steel grade 1.4521.

APPENDIX 65

Figure 15: Test coupon documentation for steel grade 1.4509.