High Temperature Erosion-Corrosion behavior of HVAF- & HVOF-Sprayed Fe-based Coatings

Transcription

1 High Temperature Erosion-Corrosion behavior of HVAF- & HVOF-Sprayed Fe-based Coatings Esmaeil Sadeghimeresht a, Sudharshan Raman b, Nicolaie Markocsan a, Shrikant Joshi a a Department of Engineering Science, University West, Trollhättan, Sweden b Nanyang Technological University, Singapore, Singapore

2 Outline Motivation & background Experiments High velocity air-fuel (HVAF) spraying High velocity oxy-fuel (HVOF) spraying Exposures Corrosion Erosion Results and discussion Conclusions

3 Motivation & background Boiler industry T/P Thermal/electrical efficiency CO 2 emission Biomass/waste fuels Possible solutions Corrosive-erosive ashes Cost fluidized bed combustors, coal gasifiers, compressor blades, boiler tubes, steam and gas turbines Environment-wise 1. Additives (Sulfur, etc.) 2. T Efficiency Material-wise 1. Advanced materials Costly & time consuming 2. Coatings How could coatings slow down the corrosion and erosion-corrosion of boiler components?

4 Experiments Substrate Feedstock powders Coating methods 16Mo3; a carbon steel in wt%: 0.01Cr-0.3Mo-0.5Mn-0.3Si-0.15C-Bal. Fe Fe-based powders in wt%: 30Cr-11Ni-3.4B-1.5Si-0.6C-0.1V-Bal. Fe Högänas A.B., Sweden μm for HVAF μm for HVOF High velocity air fuel (HVAF) Uniquecoat M3 TM gun High velocity oxy fuel (HVOF) DJ2600 Hybrid gun Corrosion test Ambient air at 600 C up to 168h With and without KCl Free standing coatings Erosion test ASTM G76 Ducum air-jet erosion tester Al 2 O 3 particles:~50 μm Time: 10 min, 5 g/min, 90 Highly erosive environment ASTM G76 Standard Test Method for Conducting Erosion Tests by Solid Particle Impingement Using Gas Jets

5 Corrosion control in biomass boilers Thermal spray coatings Substrate High Velocity Air Fuel Higher particle inflight temperature Air Plasma Spray High velocity oxy-fuel High velocity air-fuel Higher particle inflight velocity Coating Inflight oxidation Inter-splat cohesion Change in feedstock phase and chemical composition

6 Results & discussion HVOF HVAF Porosity 3.1% Hardness (HV 0.3 ) 571 ± 83 Roughness (R a ) 8.5 ± 0.7 Porosity 1.4% Hardness (HV 0.3 ) 815 ± 43 Roughness (R a ) 5.2 ± 0.4 Matrix: γ-fe, & Ni 200 μm 200 μm Hardneing phase(s): (Fe,Cr) 2 B, Fe 3 C, Cr 7 C 3, VC 10 Oxide(s): SiO μm Interconnected porosity, oxides, unmelted particles, etc.! 200 μm

7 Weight change after corrosion test Weight change (mg/cm 2 ) HVAF coating exposed without KCl HVOF coating exposed without KCl HVAF coating exposed under KCl HVOF coating exposed under KCl Time (h) Reasons for the drop: Vaporization (FeCl 2 ) KCl sintering Spallation Reaction of KCl with water vapor (100ppm) and KOH formation Reasons for the weight increase: Oxide formation

8 1) HVAF corroded for 168h in ambient air Oxide thickness: ~1.5 µm Oxide phases: Fe2O3,(Fe,Cr)3O4 20 µm 2 µm A dense, continuous and thin oxide

9 2) HVAF corroded for 168h in ambient air + KCl Oxide thickness: ~17 µm Oxide phases: K 2 CrO 4,Fe2O3, (Fe,Cr)3O4, FeCl2, CrCl2 Is the Cr-rich oxide protective? How could K 2 CrO 4 form? Formation of chromate and metallic chlorides! Decohesion and loss of thickness Transformation of Cr-rich layers into volatile species through their reactions with chlorides 30 µm 5 µm Continuous, but not thin or dense Non-protective oxide

10 Active corrosion mechanism Cl 2 (g) Cl 2 (g) 1 Salt deposit: 4KCl(s)+Cr 2 O 3 (s)+5/2o 2 =2K 2 CrO 4 (s)+2cl 2 (g) Oxide scale Cl 2 (g) Cl 2 (g) 4 2FeCl 2 (g)+3/2o 2 = Fe 2 O 3 (s)+2cl 2 (g) or/and 2CrCl 3 (g)+3/2o 2 = Cr 2 O 3 (s)+3cl 2 (g) or/and wherever O 2 is available (high po 2 ) Bulk Fe(s)+Cl 2 (g)=fecl 2 (s) or/and Cr(s)+3/2Cl 2 (g)=crcl 3 (s) or/and 2 3 FeCl 2 (s)=fecl 2 (g) or/and CrCl 3 (s)=crcl 3 (g) or/and wherever O 2 is less available (low po 2 ) At temperature > 400 C / 600

11 3) HVOF corroded for 168h in ambient air Oxide thickness: ~2 µm Oxide phases: Fe2O3, (Fe,Cr)3O4 30 µm 2 µm Dense, continuous and thin oxide, similar to HVAF

12 4) HVOF corroded for 168h in ambient air + KCl Oxide thickness: ~20 µm Oxide phases: K 2 CrO 4,Fe2O3, (Fe,Cr)3O4, FeCl2, CrCl 3 30 µm 5 µm Formation of the metallic chlorides! Continuous, but not thin or dense Non-protective oxide

13 Weight change after erosion test Mass loss (g) HVAF coating exposed under KCl HVOF coating exposed under KCl Oxidation exposure time (h) Reasons for similar behavior: Presence of a thin oxide scale Severe erosion test (long time or high feed rate) Errodants reached the coatings Very short time between successive impacts of the erodent particles does not allow oxidation to come into play!

14 7) HVAF eroded-corroded for 168h in ambient air + KCl Topography Zones 1. Highly porous 2. Cracks 3. Partly affected 4. Small pores 5. No effect pitting Erosion crater 1 2 Oxide Coating Substrate Zone 1 Zone 2 Zone 3 Zone 4 Zone 5 Presence of not well-adherent oxides facilitate the abrasive wear mechanism

15 7) HVAF eroded-corroded for 168h in ambient air + KCl Cross section Zone 1 Affected zone Zone Zone 2 Zone 4 Zone 3 Depth of attack = 56 ± 11 μm

16 8) HVOF eroded-corroded for 168h in ambient air + KCl Topography Zones 1. Highly porous 2. Cracks 3. Partly affected 4. Small pores 5. No effect Oxide Coating Substrate Zone 1 Zone 2 Zone 3 Zone 4 Zone 5 Similar behavior to HVAF

17 8) HVOF eroded-corroded for 168h in ambient air + KCl Cross section Zone 1 Affected zone Zone Zone 2 Zone 4 Zone 3 Depth of attack = 41 ± 9 μm

18 HVAF eroded-corroded for 168h in ambient air + KCl Zone 3 Cross section Topography Erosion crater Oxide Coating Substrate Presence of the (Fe, Cr)-rich oxide Non compact Less porous coatings with small splats have been reported to be favorable for better erosion resistance!

19 Conclusions Fe-based coatings are economically favoured but not highly protective in a harsh corrosive environment The coatings may find application in boiler tubes and other structural materials attacked by slow moving particles Similar behaviour of HVAF and HVOF Low corrosion-erosion resistance due to inherent features of the coatings and formed oxide scales The high Cr content (30 wt%) did not improve as it vaporized in form of CrCl 2 Future works High temperature corrosion-erosion tests Erosion tests in a milder environment with ashes Ni-based coatings As sprayed Oxidized Eroded Eroded oxidized

20 Thank you for the attention! Questions?