High Temperature Corrosion of HVAF-Sprayed NiCrAlY Coating Exposed to Various Corrosive Environments

Transcription

1 High Temperature Corrosion of HVAF-Sprayed NiCrAlY Coating Exposed to Various Corrosive Environments Esmaeil Sadeghimeresht a, Reza Jafari b, Taghi Shahrabi Farahani b, Nicolaie Markocsan a, Shrikant Joshi a a Department of Engineering Science, University West, Trollhättan, Sweden b Department of Material Science and Engineering, Tarbiat Modares University, Tehran, Iran

2 Outline Motivation & background Experiments Results and discussion Ambient air with and without KCl, and KCl-K 2 SO 4 H 2 O with and without KCl HCl with and without KCl Conclusions

3 Motivation & background Boiler industry T/P Thermal/electrical efficiency CO 2 emission Biomass/waste fuels Corrosion Operating cost Possible solutions Environment-wise 1. Additives (Sulfur, etc.) 2. T Efficiency Material-wise 1. Advanced materials Costly & time consuming 2. Coatings

4 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

5 Experiments Substrate Feedstock powders Coating methods 16Mo3; a ferritic low-alloyed steel in wt%: 0.01Cr-0.3Mo-0.5Mn-0.3Si-0.15C-Bal. Fe NiCrAlY powder in wt%: 21Cr-7Al-1Y-Bal. Ni Amperit , 45±22 µm High velocity air fuel (HVAF) Uniquecoat M3 TM gun Corrosion 600 C for 168h Ambient air with and without KCl, and KCl-K 2 SO 4 20%H 2 O-5%O 2 -N 2 with and without KCl 500vppm HCl-5%O 2 -N 2 with and without KCl

6 Results & discussion As-sprayed NiCrAl coating a) NiCrAl coating A wt% Ni 72.4 Cr 19.8 Al 6.8 Y 0.8 O 0.2 NiCrAl coating 16Mo3 substrate 200 µm 30 µm 30 µm 16Mo3 substrate Coating thickness = 240 ± 15 µm Surface roughness (as-sprayed coating), R a = 6.5 ± 0.3 µm Surface roughness (polished coating), R a <0.1 µm Almost a dense, and well adherent coating sprayed by HVAF

7 Phase characterization Intensity (a.u.) NiCrAlY powder As-sprayed NiCrAlY γ-ni(nicr) β-ni(nial) ϴ ( ) No oxidation/phase transformation during HVAF β-phase still present

8 Weight change measurement KCl significantly increased the weight Weight change (mg/cm 2 ) K 2 SO 4 resulted in weight loss Without KCl (highly corrosive to less corrosive): HCl >H 2 O>O 2 With KCl (highly corrosive to less corrosive): HCl>O 2 >H 2 O Weight change measurement is tricky! Ambient air Ambient air-kcl K2SO4 K2SO4-KCl H2O H2O-KCl HCl HCl-KCl

9 Ambient Air Formation of a dense and continuous chromia scale gas alloy O Cr Race between: inward O diffusion outward Cr diffusion In General: N Cr D Cr >>N O D O D (Diff. Koeff.) N(Concentration) High Cr concentration/diffusion external oxidation

10 Ambient air - KCl 1 Ele. Wt% O Al Cl K Cr Ni Y µm 2 50 µm Wt% 1 2 O Al Cl K Cr Ni A flake shape Cr 2 O 3 formed on NiCrAl exposed to KCl Formation of potassium chromate (K 2 CrO 4 )

11 Ambient air - KCl Voids and pores in oxide scale porosity Corrosion affected region A C Ni Cr O Coating/sub. interface 50 µm B Wt%. A B C O Al Cl K Cr Ni Y µm Al Formation of K 2 CrO 4 Internal corrosion attack beneath K 2 CrO 4 Al depletion to form Al 2 O 3 Formation of voids due to vaporization of metallic chlorides Cl K

12 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 NiCl 2 (g)+1/2o 2 = NiO(s)+Cl 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 ) Coating Ni(s)+Cl 2 (g)=nicl 2 (s) or/and Cr(s)+3/2Cl 2 (g)=crcl 3 (s) or/and 2 3 NiCl 2 (s)=nicl 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

13 Ambient air - K 2 SO 4 - KCl Micro pores 10 µm C Wt% A B C O Al S Cl K Cr Ni µm 10 µm -Cr2S3 -Ni 3 S 2 -NiO -Cr 2 O 3 Non-homogeneous corrosion attack and corrosion products No protective chromia scale

14 Ambient air - K 2 SO 4 - KCl A B Ni Cr Al C 100 µm 10 µm Wt% A B C O Al S Cl K Cr Ni Sign of Active oxidation mechanism O Cl K Pits of different sizes growing into the coating Lifetime measurement is difficult Internal oxidation of Al Cl accumulation in voids in corrosion front (B)

15 H 2 O H 2 O - KCl 100 µm 200 nm 100 µm 20 µm Ni Cr Al O O Cr Al Cl K Without KCl, a 200nm thin film formed With KCl: Non-uniform formation of K 2 CrO 4 High corrosion attack beneath K 2 CrO 4

16 HCl HCl - KCl 300 µm 20 µm 300 µm 20 µm Cr Al O Cl Cr Al O A new corrosion mechanism proposed A two-stage corrosion mechanism: 1. Electrochemical mechanism 2. Both electrochemical and chlorine active-corrosion mechanisms

17 Electrochemical mechanism Grain boundary diffusion 1 KCl Cl - (ads) 2 3 Cr 2+ 4 Sum:. /

18 Summary Environment Corrosion products Ambient air Cr 2 O 3 Ambient air + KCl NiCl 2, K 2 CrO 4, NiO, Al 2 O 3, Cr 2 O 3, NiCr 2 O 4 Ambient air + KCl + K 2 SO 4 Cr 2 S 3, Ni 3 S 2, NiO, Cr 2 O 3 H 2 O Ni(Cr,Al) 2 O 4 H 2 O+ KCl K 2 CrO 4, CrCl 3, Ni(Cr,Al) 2 O 4 HCl Cr 2 O 3 HCl + KCl K 2 CrO 4, CrCl 3, Ni(Cr,Al) 2 O 4 The level of protectiveness by chromia is different When K 2 CrO 4 forms, chromia loses the protectiveness formation of volatile metallic chlorides No formation of protective alumina

19 Conclusions No substrate damage in any environment Formation of K 2 CrO 4 depleted the coating in Cr, hence Cr 2 O 3 was not supported Cl penetration through the coating s splat boundaries and oxide s grain boundaries Future work 1. Post-heat treatment to promote formation of a highly protective α-alumina 2. Corrosion tests at initial stages

20 Thank you for the attention