Alkali-induced corrosion Current understanding & Research methods. Juho Lehmusto Chemistry in combustion processes II March 14, 2017

Transcription

1 Alkali-induced corrosion Current understanding & Research methods Juho Lehmusto Chemistry in combustion processes II March 14, 2017

2 Contents 1) Introduction 2) Corrosion chemistry 3) Experimental approach

3 Biomass --A part of future s power production -- Replacement for fossil fuel CO 2 neutrality Availability Regulatory benefits

4

5 HEATING VALUE, MJ/kg PETROLEUM COKE ANTRACITE COAL BITUMINOUS COAL BROWN COAL, LIGNITE PEAT BARK POLYOLEFIN PLASTICS COLORED COLORED OR PRINTED OR PRINTED (PE, PP, PC...) PLASTICS, MIXED CONSUMER CLEAN PLASTICS REF II - III REF PELLETS MIXED REF PLASTICS REF I CHIP- - PLY- - COMMERCIAL & BOARD BOARD WOOD INDUSTRIAL PVC WOOD & & PLASTICS RDF WOOD BIOMASS DEMOLITION PLASTIC RDF WOOD S PAPER PAPER & & WOOD BIOMASS WOOD WOOD OIL SHALE PVC MSW AGRO BIOMASS CHICKEN LITTER COW MANURE 5 BIO & FIBER SLUDGE DEINKING SLUDGE SEWAGE SLUDGE STANDARD DESIGN SOME CHALLENGES 10 MULTIPLE CHALLENGES Courtesy of Amec Foster Wheeler

6 Fossil fuel (Coal) Biomass (Wood) Biomass (Agriculture) Waste O 2 (vol-%) ~ 3% ~ 5% ~ 5% 6-11% H 2 O(vol-%) ~ 10% 20-25% 20-25% 20-25% Alkali chlorides Low Medium High High p(so 2 ) High Low Low Low p(hcl) High Low Medium High Steam temperature (present maximum) 650ºC 550ºC 450ºC 450ºC

7 Superheater corrosion Courtesy of Valmet Technologies

8 Cr-containing steel Ability to form a thin, yet remarkably dense oxide layer. Flue gas, ash particles Tube wall Overheated steam Cr 2 O 3 -layer

9 Ash particles in flue gas Source: Clyde Bergemann GmbH

10 Slagging, fouling, and corrosion Source: Frandsen et al.

11 Source: Froitzheim et al.

12 High-alloyed steel with KCl at 548 C Every image represents 2 minutes

13 What can be done? Lower steam temperature Lower power production efficiency High-quality fuel Increased process costs Higher alloyed materials Increased materials costs

14 What can be done? Better understanding on corrosion chemistry Theoretical approach Field and laboratory studies Complemented reaction mechanisms More pieces to the puzzle Tools for material designers, operators, etc.

15 Contents 1) Introduction 2) Corrosion chemistry 3) Experimental approach

16 Corrosion Corrosion is a natural process, which converts a refined metal to a more stable form, such as its oxide, hydroxide, or sulfide. It is the gradual destruction of materials (usually metals) by chemical reaction with their environment. M Alloy Metal Oxide (MO) M 2+ e - Gas O 2 (g) O 2 + 2e - O 2- Aqueous corrosion Atmospheric corrosion High-temperature corrosion Automotive corrosion Aircraft corrosion Aerospace corrosion Microbiological corrosion Glass corrosion Polymer corrosion M M e - M 2+ + O 2- MO

17 High- temperature corrosion The prediction of reaction products when a certain alloy is exposed to a certain atmosphere at a certain temperature and pressure is desired. Chemical thermodynamics and especially phase equilibria are required. Chemical equilibrium in gas mixtures Chemical equilibria between solids and gases Chemical equilibria involving multiple solids Chemical equilibria of gases containing two reactants Equilibria between alloys and a single oxide Equilibria between alloys and multiple oxides The stability of oxides Diffusion rates Corrosion rates

18 Factors affecting the corrosion rate Temperature and time Water vapor Also i.a. SO 2 and HCl (not addressed here) Steel composition Deposit composition

19 Temperature and time The corrosion rate increases as functions of both time and temperature Non-linear oxidation, roughly following the parabolic rate law Controlled by the diffusion of gaseous species Reactions involving gaseous species are prone to the effect of flow rate

20 Source: Israelsson et al.

21 Water vapor Virtually always relevant in biomass combustion Accelerates corrosion compared to dry conditions Enables thermodynamically favored reactions In theory, the rate of oxidation increases as a function of humidity

22 Source: Jianian et al.

23 The complexity of simplicity Despite the detrimental effect of water vapor, it appears to hinder chloride-induced corrosion A salt-dependent phenomenon; not observed with carbonates Originates most likely from HCl formation: 16 KCl(s,g) + 4 FeCr 2 O 4 (s)+ 9 O 2 (g) + 8 H 2 O(g) 8 K 2 CrO 4 (s) + 2 Fe 2 O 3 (s) + 16 HCl(g)

24 The effect of water vapor on mass gain Source: Pettersson et al.

25 Average oxide thickness (µm) The effect of water vapor on oxide thickness Dry conditions 10 CrMo 304L Alloy 625 Humid conditions < 1 µm < 1 µm Temperature ( o C)

26 Steel composition Classic improvers: Cr and Ni Ni: tendency to form austenite results in a great toughness and high strength at both high and low temperatures. Nickel also improves resistance to oxidation and corrosion. Cr: added to the steel to increase resistance to oxidation through Cr 2 O 3 formation. Others: Mn, Mo, and Nb Mn: an austenite forming element, which improves hot working properties and increases strength, toughness and hardenability Mo: improves resistance to pitting corrosion, especially by chlorides and sulfur chemicals Nb: carbide stabilization, which tends to minimize the occurrence of intergranular corrosion; strengthens steels and alloys for high temperature service

27 Increasing Cr and Ni content Source: Cha et al.

28 Steel composition Newer candidates: Al and reactive elements (RE) Al: Al 2 O 3 forms instead of Cr 2 O 3 ; potassium aluminate (KAlO 2 ) is thermodynamically less favored than K 2 CrO 4. RE (Y, Ce, La, etc.): already very small concentrations improve the corrosion resistance of binary FeCr or NiCr alloys remarkably. 1) Oxide adherence is improved greatly 2) Corrosion kinetics are reduced 3) A lower Cr content is sufficient to stabilize an exclusive Cr 2 O 3 scale 4) Cr 2 O 3 growth mechanism is changed from outward growth by cation (Cr 3+ ) to inward growth by anion (O 2- ) diffusion.

29 Steel composition Thermal Barrier Coatings (TBC) Bond coat: MCrAlY (M=Ni, Fe, or Co) TGO: α-al 2 O 3 Top coat: Y 2 O 3 stabilized ZrO 2 Superior performance at temperatures above 1000 C Vulnerable to cyclic temperature changes and mechanical vibration/erosion

30 Deposit composition Highly complex system with all the components having an influence on one another. T 0, T 15, T 75, and T 100 Temperature gradient

31 Temperature gradient Source: Engblom et al.

32 Temperature gradient Source: Lindberg et al.

33 First melting temperatures Deposit T 0 ( C) Deposit T 0 ( C) KCl 771 K 2 CrO 4 -K 2 Cr 2 O PbCl KCl-PbCl ZnCl K 2 CrO K 2 Cr 2 O ZnCl 2-69KCl 430 KCl-K 2 CrO ZnCl 2-52KCl 250 KCl-K 2 Cr 2 O ZnCl 2-32KCl 230

34 Reaction mechanisms 1) Active oxidation 2) Chromate-dependent corrosion 3) Chloride ion corrosion

35 The starting point KCl KCl Cr 2 O 3 -layer Alloyed steel (Fe, Cr, Ni) KCl(s) + Cr 2 O 3 (s)/fecrni(s)??

36 Active oxidation Air flow Deposit i) KCl, NaCl v) 4CrCl 2 (g) + 3O 2 (g) 2Cr 2 O 3 (s)+ 4Cl 2 (g) Oxide ii) Cl 2 iv) CrCl 2 (g) p(o 2 ) increases p(cl 2 ) increases Steel iii) Cr + Cl 2 CrCl 2 (s,g) Cr

37 Active oxidation First and most cited reaction mechanism Initiation through chromate (CrO 2-4 ) formation Cation (K +, Na + ) plays an active role A cyclic reaction of molecular chlorine Chlorine acting as a catalyst? Driving force for the chlorine diffusion? The molecular sizes of O 2, CrCl x and Cl 2

38 Chromate-dependent corrosion 4Cr(s) + 12MCl(s) 4CrCl 3 (s,g) + 12M(s) 4CrCl 3 (s,g) + 3O 2 (g) 2Cr 2 O 3 (s) + 6Cl 2 (g) 6Cl 2 (g) + 12M(s) MCl(s) 4Cr(s) + 3O 2 (g) 2Cr 2 O 3 (s)

39 Chromate-dependent corrosion A two-stage reaction Initiation through metal chloride (CrCl x ) formation Chlorine (Cl 2 ) plays an active role Continuation through chromate (CrO 2-4 ) formation Cation (K +, Na +, etc.) plays an active role? Controversial role of chromate formation? Later published carbonate-induced corrosion

40 Corrosion involving chloride ions Source: Pettersson et al.

41 Corrosion involving chloride ions A two-stage reaction Initiation through chromate (CrO 2-4 ) formation Cation (K + ) plays an active role Degradation of the protective oxide Continuation through hematite (Fe 2 O 3 ) formation Anion (Cl - ) plays an active role? Passivity of chromium oxide? Further reactions of potassium? Driving force for anion diffusion

42 What do we know On a general level The effect of various process parameters On a more detailed level The species involved The overall course of the corrosion reaction

43 What do we not know On a general level Universal theory, that The relationship between the process parameters could be applied to More accurate values for the process parameters whatever conditions On a more detailed level with whatever materials The initiation of the corrosion reaction Solid equation for the corrosion reaction For example, the faith of potassium

44 Almost there... KCl KCl Cr 2 O 3 -layer Alloyed steel (Fe, Cr, Ni) KCl(s) + Cr 2 O 3 (s)/fecrni(s)? Fe 2 O 3 (s)

45 Contents 1) Introduction 2) Corrosion chemistry 3) Experimental approach

46 Lab-scale vs. full-scale Laboratory studies Corrosion mechanisms How does it happen? Why does it happen? High-temperature corrosion The effect of corrosion What happens? Field studies

47 Laboratory studies Mimicking the genuine conditions, but focusing on a specific point of interest. Well-controlled conditions Simplified environments well-defined correlations Application of the results CAUTION! Extrapolation of the results

48 Thermogravimetry (TG) The sample is heated up under well-defined conditions Detects mass changes as functions of time and temperature Source: Gallagher and Brown Handbook of thermal analysis and calorimetry: Volume 1 Principle and practice, Elsevier 1998

49 Differential thermal analysis (DTA) Other physical and chemical changes Reactions, melt formation, phase transitions Source: Gallagher and Brown Handbook of thermal analysis and calorimetry: Volume 1 Principle and practice, Elsevier 1998

50 Relative mass (%) Temperature difference ( o C) Cr + KCl; 2 Cmin C ,0 1, TG DTA 1,0 0, Exo Endo Temperature ( o C) 0,0-0,5

51 Relative mass / % Temperature difference / o C Phenomenon without a mass change 130 K 2 CrO 4 in synthetic air 2,0 120 DTA 1,5 1, ,5 100 TG 0,0 90-0,5 80 Exo Endo Temperature / o C -1,0-1,5

52 The initiation of oxidation The mechanism of the very onset of the alkali salt-induced corrosion process is still elusive. How does the degradation of the protective chromium oxide actually start and how does it initially proceed Electrochemical approach not previously used in high-temperature corrosion studies

53 Electrochemical setup Source: Sui et al.

54 Normalized oxidation current [µa] Salt-generated reaction currents 2,0 1,6 1,2 0,8 NaCl KCl K 2 CO 3 0,4 0, Exposure time [min] Source: Sui et al.

55 PET Positron Emission Tomography (PET ): specific specimens/reactants can be marked with isotopes producing strong gamma radiation, which in turn can be identified on-line in 3D as a function of time.

56 Half lives of isotopes 15 O: 2 min 11 C: 11 min 18 F: 120 min 82 Br: 2118 min Some isotopes must be produced close to where to be used Better to, for example, produce CO from 11 C than from 15 O 18 F is very reactive, suitable for many applications

57 Alternative isotopes Among halides KCl and KF turned out to be challenging to begin with Half life and decay type of K 82 Br seemed appropriate High cost of the traditional production process Demand for a large radiation source Neutron-activation of standard suprapur KBr in a cyclotron.

58 Sufficient activity of K 82 Br

59 Scanning Electron Microscopy (SEM) Interactions between the electron beam and atoms Fast with resolution up to 1 nanometer Information about surface topography and composition o Unable to distinguish valence states o Non-optimal depth resolution regarding composition analyses o Poor sensitivity to elements present in low abundances

60 Scanning Electron Microscope

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62 Reference N 2 + H 2 O Air KCl + N 2 KCl + N 2 + H 2 O KCl + N 2 + H 2 18 O KCl + Air KCl+ Air + H 2 O KCl + Air + H 2 18 O

63 X-ray photoelectron spectroscopy (XPS)

64 X-ray photoelectron spectroscopy (XPS) Interactions between X-rays and electrons Kinetic energy and number of electrons are measured Information about elemental composition, chemical or electronic states of elements Surface sensitive (5 nm) o Large analytical area (100 μm 2 ) o Ultra high vacuum required o Time consuming

65 Elemental concentration [%] Elemental concentration [%] X-ray photoelectron spectroscopy (XPS) Synthetic air + KCl Synthetic air + KCl + H 2 O O Fe Cr Ni K O Fe Cr Ni K Sputtered time [min] Sputtered time [min]

66 Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS)

67 Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) Interactions between primary ion beam and ejected secondary electrons Information about elemental, isotopic, or molecular composition of the sample surface Surface sensitive (1-2 nm) o Ultra high vacuum required o Generally, no quantitative results obtained o Very time consuming

68 Surface analysis -- ToF SIMS -- Stainless steel with KCl in synthetic air ( 16 O) + H 2 18 O 18 O 16 O Fe K 18 O- 16 O map K-Fe map

69 High-temperature exposures 1. Pretreatment 2. Tube furnace exposure 3. Sample preparation for SEM/EDXA 4. Analyses

70 Average oxide thickness (µm) The effect of KCl Dry conditions 10 CrMo 304L Alloy 625 Humid conditions < 1 µm < 1 µm Temperature ( o C)

71 10CrMo --K 2 CO 3 -- K O Fe Cr 20 μm 168 h, K 2 CO 3, dry conditions, 600 C

72 304L --K 2 CO h, K 2 CO 3, dry conditions, 600 C local corrosion

73 Alloy K 2 CO 3 -- K O Fe Cr 20 μm Ni 168 h, K 2 CO 3, dry conditions, 600 C

74 Alloy K 2 CO 3 -- Epoxy Oxide Steel Grain boundaries 1 µm 168 h, K 2 CO 3, dry conditions, 600 C

75 High-temperature reaction kinetics Reactions of metal salt vapors in real-time and in-situ at high temperatures A tool to selectively detect trace concentrations of different salt vapors in the gas phase New possibilities to determine the steps of different reactions paths together with their reaction rates. Source: Tampere University of Technology

76 High-temperature reaction kinetics Sample holder with KCl KCl+Cr al K = 0.3 & 1 cm [K] = 30 ppb t 1/e = 30 ms -> [O 2 ] = 310 ppm Source: Tampere University of Technology

77 Deposit formation Source: University of Toronto

78 Test surfaces after 40 minutes 100% rice husk Deposit mass = 10 mg Rice/Eucalyptus 64/36 % Deposit mass = 13 mg 100% eucalyptus bark Deposit mass = 195 mg Source: University of Toronto

79 Field studies The possibility for studies under genuine conditions Complex environment Verification of modelled/lab-scale results CAUTION! Interpretation of the results Conclusions based on the results

80 Full-scale measurements Source: Vainio et al.

81 Corrosion probe A ring of studied steel is located in the probe tip Ring temperature can be adjusted Weight loss Annual tube wall thickness loss (mm/year) Cross section analyses Corrosion and deposit chemistry

82 Corrosion probe Probe Sample rings

83 Gas analysis probe

84 On the corrosion studies The value of both laboratory and field studies The role of new technology Various approaches needed No super-method of super-everything exists! Contribution to the big picture

85 Literature Getting started P. Kofstad, High temperature corrosion, 1988, Elsevier Applied Science, London, England and New York, USA D.J. Young, High temperature oxidation and corrosion of metals 2 nd edition, 2016, Elsevier Science, Amsterdam, Netherlands Relevant theses S. Enestam, Corrosivity of hot flue gases in the fluidized bed combustion of recovered waste wood, 2011, Åbo Akademi D. Bankiewicz, Corrosion behavior of boiler tube materials during combustion of fuels containing Zn and Pb, 2012, Åbo Akademi J. Lehmusto, The role of potassium in the corrosion of superheater materials in boilers firing biomass, 2013, Åbo Akademi H. Wu, Chemistry of potassium halides and their role in corrosion in biomass and waste firing, 2016, Åbo Akademi

86 Thank You for Your attention! Vicza

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