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Investigation of Materials for Use in Exhaust Gas Condensate Environment with Focus on EGR systems
Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
2012 (English)Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesisAlternative title
Kartläggning av material för användning i avgaskondensatmiljö med fokus på EGR teknik (Swedish)
Abstract [sv]

EGR (Exhaust Gas Recirculation) ar en teknik som anvands for dieselmotorer, for

att mota de hart satta utslappskraven for kvaveoxider. EGR fungerar genom att en del

av avgaserna aterfors till cylindrarna. Avgaserna gor sa att den maximala forbranningstemperaturen

sanks, vilket kraftigt reducerar bildandet av kvaveoxider. Dieselavgaser inneh

aller framst

CO2, NOx, SO2 och H2O och innan avgasernaaterfors till cylindrarna kyls

de ner. Detta leder till att det bildas ett korrosivt kondensat, eftersom de amnen som nns

i avgaserna bland annat kan bilda svavel- och salpetersyra.

Pa grund av detta korrosiva kondensat maste material i EGR systemet vara korrosionsbest

andigt och materialkraven forvantas oka i takt med utslappskraven. I framtiden kan

det innebara att mer korrosionsbestandigt material maste anvandas.

Detta examensarbete undersoker gropfratningsmotstandet foratta olika rostfria stal och

for tva aluminiumlegeringar i syntetiskt avgaskondensat. Dessutom inkluderades tva ytbehandlingar;

anodisering av en aluminiumlegering samt nitrering av ett martensitiskt rostfritt

stal. Malet ar att kartlagga vilka material som ar lampliga att anvanda i avgaskondensatmilj

o.

Fem olika syntetiska avgaskondensat med olika halter av svavelsyra, salpetersyra och

klorider, valdes ut for elektrokemisk matning. Tva olika pH-nivaer, 2.5, 1.5, inkluderades

samt tre olika halter, 32 ppm, 200 ppm, 3300 ppm, av klorider. Testtemperaturen valdes

till 60

oC, eftersom det ar den temperaturen som fortfarande kan forvantas producera betydlig

mangd avgaskondensat i EGR systemet. Anodisk polarisation ansags som den mest

lampade elektrokemiska metoden for att na examensarbetets mal.

Erhallna resultat visar att de tva aluminiumlegeringarna och det martensitiska rostfria

stalet ar utsatta for generell korrosion och gropfratning vid pH 2.5. Medan de austenitiska,

ferritiska och duplexa rostfria stalen uppvisar bra korrosionsbestandighet mot bada dessa

korrosionstyper. Att sanka pH fran 2.5 till 1.5 kunde inte ses oka risken for gropfratning for

de austenitiska, ferritiska och duplex rostfria stalen.

En kloridhalt av 0.33 vikt% okade markant risken for gropfratning, speciellt for de

austenitiska rostfria stalen. Vid pH 1.5 och hoga halter av klorider klarar sig duplexstalen

bast.

Anodisering av aluminium ses initialt ge ett forbattrat korrosionsskydd vid pH 2.5, men

det observerades att det anodiserande oxidskiktet utsattes for kontinuerlig upplosning. Efter

12 timmar var oxidskiktet sa kraftigt upplost att dess skyddande eekt var forsumbar. Aluminium

kan darfor inte rekommenderas till anvandning i EGR kondensat miljo.

Nitrering av det martensitiska stalet kunde inte ses paverka materialens korrosionsegenskaper.

Men anodisk polarisation ar inte en lamplig metod att anvanda for att studera

generell korrosion, darfor rekommenderas fortsatta undersokningar vid antingen en hogre

pH-niva eller med en annan lamplig metod.

Gropfratningsmotstandet for de testade rostfria stalen i kondensat med hoga kloridhalter

vid 60

oC foljer sekvensen 1.4301<1.4521<1.4404<duplex 2304<duplex LDX2404<duplex

2205. Det ar tydligt att duplex stalen har battre motstand mot gropfratning vid laga

pH-nivaer och hoga kloridhalter. Med tanke pa driftforhallandena i EGR-systemet och pa

legeringskostnader ar det lampligt att overvaga duplexa stal for anvandning i EGR-systemet.

Abstract [en]

EGR (Exhaust Gas Recirculation) is a method for reducing NOx emissions for heavy-duty diesel engines. EGR works by introducing part of the exhaust gases back to the engine cylinders. Exhaust gases consists mainly of CO2, NOx, SO2 and H2O. As the temperature decreases, these gases form a corrosive condensate. The EGR components which are exposed to the condensate environment must therefore be of corrosion resistant materials. The objective of this Master's Thesis is to investigate suitable materials for use in exhaust condensate environment. The goal is to evaluate the pitting corrosion resistance for eight dierent commercial stainless steels and two commercial aluminium alloys in exhaust gas condensate environment. Furthermore, nitriding surface treatments on one martensitic stainless steel and anodising treatments on one aluminium alloy, were also included in this study.

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

The results show that the two aluminium alloys and the martensitic stainless steel were subjected 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 environments with low pH value. After twelve hours of exposure to a condensate at pH 2.5 at 60 oC, the protective eect of the lm became negligible.

The austenitic, ferritic and duplex stainless steels show, however good resistance against both corrosion types. Increasing condensate acidity from pH 2.5 to 1.5 could not be observed to 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, duplex 2304 and ferritic 1.4521. However, chloride concentration of 3300 ppm, signficantly increased risk of pitting corrosion, especially for the austenitic stainless steels. Duplex stainless steel show better pitting resistance in high chloride environments, in addition to the good general corrosion resistance in low pH value environments.

There is no dierence in corrosion resistance between the nitride coated 1.4112 steel and the steel without coatings. No dierences can be observed between the plasma and gas nitrided samples. Further investigation in less corrosive environment is recommended, since anodic polarisation is not a suitable method to study general corrosion behavior.

The pitting corrosion resistance in condensates with high chloride concentrations at 60 oC follows the sequence 1.4301<1.4521<1.4404<duplex 2304<duplex LDX2404<duplex 2205. Clearly, duplex stainless steels have better pitting corrosion resistance in low pH environment when chloride concentration is increased. Considering the operating conditions of the EGR components, the element prices, it is probably more benecial to consider the duplex stainless steels for use in the EGR system.

Place, publisher, year, edition, pages
2012. , 79 p.
Series
EN1207
National Category
Energy Engineering
Identifiers
URN: urn:nbn:se:umu:diva-59070OAI: oai:DiVA.org:umu-59070DiVA: diva2:550799
External cooperation
Scania
Educational program
Master of Science Programme in Energy Engineering
Uppsok
Technology
Supervisors
Examiners
Available from: 2012-09-07 Created: 2012-09-07 Last updated: 2012-09-10Bibliographically approved

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