Corrosion Problems in Incinerators and Biomass-Fuel-Fired Boilers
Table 1
A summary of corrosion studies on some alloys and coatings under chlorides and sulphates containing environment.
S. number
Material
Environment
Brief detail
1
Vacuum induction melted cast nickel-base superalloy (wt.%) is C = 0.14, Cr = 9.0, Al = 5.8, Ti = 2.7, W = 10.5, MO = 2.1, Co = 10.1, Nb = 1.1, Ni = balance
100% Na2SO4, 75% Na2SO4 + 25% NaCl, and 60% Na2SO4 + 30% NaVO3, + 10% NaCl tested at 900°C, 925°C, 950°C, and 975°C
Parabolic growth, internal sulfidation; formation of volatile species causing voids and pits at grain boundaries was reported [70]
2
Ni-based superalloy bars, which contain mainly Ni with 6.3 wt.% Al, 6 wt.% Cr, and some Co, Ti, Mo, W, and Ta
Na2SO4 and 75 wt.% Na2SO4-25 wt.% NaCl mixture studied in air at 900°C
Corrosion products were laminar, porous, and spallable. Internal sulfidation of the superalloy was seen [71]
3
Powder metallurgy (PM) Rene95 Ni-based superalloy
25% NaCl + 75% Na2SO4 salts at 650°C, 700°C, and 750°C
The corrosion kinetics followed a square power law at 650°C while following linear ones at 700°C and 750°C [72]
4
310 stainless steel
NaCl/Na2SO4 of varying ratio salts at 750°C
75% NaCl mixtures give severe corrosion with uniform internal attack. Increase of Na2SO4 content leads to intergranular attack [73]
5
Silicon and aluminium with and without cerium were simultaneously codeposited by diffusion into austenitic stainless steel (AISI 316L)
50 wt.% NaCl + 50 wt.% Na2SO4 deposits at 750°C for 120 h at 10 h cycle
Ceria addition improves the corrosion resistance of coating [74]
6
T-91 steel and T-22 steel
75 wt.% Na2SO4 + 25 wt.% NaCl at a temperature of 900°C in a cyclic manner
T-91 steel was found to be more corrosion resistant than T-22 steel [75]
7
HVOF sprayed WC-NiCrFeSiB coating deposited on Ni-based superalloy (Superni 75) and Fe-based superalloy (Superfer 800H)
75 wt.% Na2SO4-25 wt.%; NaCl mixture was studied in air at 800°C
Oxidation resistance is improved by formation of oxides of Ni, Cr, and Co and their spinels on the surface scale and at the boundaries of Ni, W rich splats [76]
8
CM 247 LC
Pure Na2SO4 as well as Na2SO4 and NaCl mixtures of different concentrations at various temperatures
Severely corroded in just 4 h and completely consumed in 70 hr when tested in 90% Na2SO4 + 10% NaCl at 900°C [77]
9
Co, Co-10Cr, and Co-10Cr-5Al alloys
Na2SO4-25% NaCl environment at 1173°C
The mass gain is in the following order: Co-10Cr-5Al < Co < Co-10Cr [78]
10
Ni + CrAlYSiN nanocrystalline composite coatings and NiCrAlYSi reference coatings, K438 alloy
75 wt.% Na2SO4 + 25 wt.% NaCl) environment at 900°C
Both coatings improved the hot corrosion resistance of K438. For the composite coating, the oxide scales were composed of -Al2O3, Cr2O3, and minor NiCr2O4 [79]
11
Mar-M 509
Na2SO4 or Na2SO4 + 1% NaCl, Na2SO4 + 25 wt.% NaCl at 750°C
Na2SO4 + 25% NaCl were very aggressive compared to that of either pure Na2SO4 or Na2SO4 + 1% NaCl. Sulfidation-oxidation mechanism has been proposed [80]
12
Ti-48Al-2Cr-2Nb alloy
75% Na2SO4 and 25% NaCl (mass fraction) at 800°C
Interphase selective corrosion of one phase causes pits at the laminar interphase during hot corrosion. The refinement of these laminations leads to mitigation of this problem [81]
13
Cobalt-base superalloy K40S with and without NiCrAlYSi coating
Na2SO4 and Na2SO4 containing 25 wt.% NaCl salt deposit
Bare alloy suffered from accelerated corrosion with nonprotective and nonadherent scale. NiCrAlYSi coating provides protection forming -Al2O3 scale[82]
14
Nickel-base superalloy GH37
75 wt.% Na2SO4-25 wt.% NaCl at temperature of 700 and 850°C under stresses of 471 and 196 MNm−2
Creep rupture life is reduced in this environment at both the temperatures with reduced period of crack initiation and increases rate of creep propagation [83]
15
Ni-based superalloy (Superni 75) and Fe-based superalloy (Superfer 800H)
Air and molten salt (Na2SO4-25% NaCl) environment at 800°C under cyclic conditions
The formation of scale rich in Cr2O3, NiO, and spinel NiCr2O4 has contributed to better oxidation and hot corrosion resistance of Superni 75 as compared to Superfer 800H [69]
16
Ni3Al containing small additions of Ti, Zr, and B
With and without Na2SO4-NaCl salt deposits at 600–800°C
Accelerates corrosion seen at 600 and 800°C under salt. At 600°C alumina scale is formed and at 800°C NiO-Al2O3 with sulphur compounds [84]
17
Al-gradient NiCoCrAlYSiB coatingon a Ni-base superalloy using arc ion plating (AIP)
Pure Na2SO4 and Na2SO4/NaCl (75 : 25, wt./wt.) salts were performed at 900°C in static air
By partially sacrificing Al2O3 (i.e., Al), the gradient NiCoCrAlYSiB coating specimen behaved excellently in the two kinds of salts [85]
18
Cr13Ni5Si2-based metal silicide alloy
Na2SO4-25 wt.% K2SO4 at 900°C and Na2SO4-25 wt.% NaCl at 850°C
Metal silicide alloy exhibited high hot corrosion resistance in molten Na2SO4-25 wt.% K2SO4 due to the very high Cr content. Addition of NaCl to Na2SO4 accelerated the cracking and spalling of the Cr2O3 oxide scales and promoted the formation of sulphides [86]
19
Fe-28Cr and Fe-28Cr-1Y
0.1 to 1.0% HCl on the oxidation in argon-20% O2 at 600 and 700°C
Formation of FeCl2, CrCl2, and CrO2Cl2 was observed with evaporation during corrosion. 1% Y resulted in marked improvement in corrosion protection by allowing formation of Cr2O3 rich layer [87]
20
Ni-11Cr nanocomposite, Ni-10Cr, and Ni-20Cr alloys
25NaCl + 75 (Na2SO4 + 10K2SO4) in air at 700°C
In case of nanocomposite, fast formation of continuous chromia scale provided the protection, whereas internal sulfidation was only observed in both the alloys [88]
21
Udimet alloy and 310SS
Simulated municipal waste incineration flue gas at 750°C, isothermally for 72 and 120 h, and also under thermal cycling for 120 h
In case of Udimet, the formation of volatile MoO2Cl2 and WO2Cl2 is likely to have caused weight loss and porous oxide. In contrast, the kinetics of 310SS under thermal cycling show linear rate of corrosion [27]
22
Fe, Cr, Ni, the ferritic alloys Fe ± 15Cr and Fe ± 35Cr, and the austenitic alloys: alloy 800, alloy 825, and alloy 600
N2 ± 5 vol% O2He ± 5 vol% O2 + 500–1500 vppm HCl at temperatures between 400 and 700°C using discontinuous exposures
Active oxidation is found to be the main mechanism of corrosion above 500°C [89]
23
Cast Fe-Cr-Ni alloy (Cr-25.5, Ni-13, Mn-0.5, C-0.3, Si-1.2, Ti-0.4, Al-0.05, , and each: <0.01%, and bal. Fe)
(K, Na) Sulfate + 21 wt.% chloride) with the [K]/[Na] ratio on mole basis equal to 1.4
Alkali sulphate deposit increases the corrosion rate by factor of 200 and alkali sulphate chloride mixture increases the rate by about 20000 times as compared to air oxidation [90]
24
Three Fe-30.1Mn-6.93Al-0.86C base alloys with different aluminium and chromium contents
NaCl deposit (2 mgcm−2) 750 to 850°C in air
With 8% Al, air oxidation gives Al2O3. Addition of chromium gives best oxidation resistance at 750°C and 800°C. In hot corrosion with NaCl deposit the addition of chromium reduces the metal loss, but overall resistance was not improved [91]
25
Cr3C2-NiCr cermet coatings deposited on Superni 75, Superni 718, and Superfer 800H
75 wt.% Na2SO4 + 25 wt.% K2SO4 film at 900°C for 100 cycles
Cr3C2-NiCr-coated superalloys showed better hot corrosion resistance than the uncoated superalloys [92]
26
Electroless nickel coating (EN), hot-dip aluminum with added silicon coating (HD), and pack aluminide coatings (PC), on low carbon steel
NaCl (2 mg/cm2) oxidized at 850°C for 1–169 h isothermally
The aluminized coatings (HD and PC) had lower corrosion rates than that of EN coating. Preoxidation is helpful in case of pack cementation [93]
27
Fe-20% Cr alloy
KCl, NaCl, Na2SO4, and K2SO4 pure salts and a mixture of these salts at 800°C
Alkali oxides under oxidizing condition have higher solubility for Fe and Cr containing species as compared to their solubility in alkali sulphates [94]
28
K38G cast alloy by multiarc ion plating and LP-CVD (NiCoCrAlY and diffusion aluminide coating)
75 wt.% Na2SO4 + K2SO4 and 75 wt.% Na2SO4 + NaCl salt mixture at 900°C
Low oxidation rate observed in case of coatings, whereas presence of salts accelerated oxidation rate. NiCoCrAlY coating showed the better hot corrosion [95]
29
IN 788, IN 718C, IN 100, and Ni-30A1 (NiA1)
Na2SO4, NaCl, and Na2SO4-NaCl mixtures with O2 and O2 + SO2 environments
Na2SO4 with SO2 in the environment gives maximum corrosive rate at 750°C. All the binary and superalloys Al2O3 Cr2O3 forming showed less corrosion when either SO2 or NaCl was absent from the corroding media [96]
30
Low and high alloy steels
NaCl or a fly ash tested at temperature of 500°C and high alloy steels at 600 and 700°C
Formation of porous unprotective scale and active oxidation were noticed to be catalysed by chlorine [97]
31
Boiler tube steel (X20CRMV121 and AISI 347FG)
KCl and/or K2SO4 and real deposits exposed to a synthetic flue gas (6 vol% O2, 12 vol% CO2, 400 ppmv HCl, 60 ppmv SO2, balance N2) in 550°C from 1 week to 5 months were used
Both the alloys suffered minor internal attack with the KCl. Pitting was also observed [98]
95% Na2SO4 5% NaCl and 90% Na2SO4 5% NaCl 5% V2O5 environments at 900°C
MCrAlY exhibits maximum life in both sodium chloride and vanadium containing environments. Presence of trace elements in the coating reduces coating life significantly [99]
33
IMI 834
Na2SO4, 90% Na2SO4-10% NaCl, Na2SO4 + 5% NaCl + 5% V2O5 at 500, 600, and 700°C
Significant weight loss was observed with pitting-type corrosion in the presence of NaCl [100]
34
Nickel aluminide Ni3Al containing up to 8 mass% Cr and up to 0.9 mass% Zr
The chlorination and sulfidation tests were carried out in both oxygen-deficient (HCl/H2 and H2S/H2, resp.) and oxygen-containing (Cl2/O2 and SO2/air, resp.) atmospheres. 750 to 1100°C
Undoped Ni3Al shows excellent resistance in air, in carburizing gases, and in chlorinating/oxidizing atmospheres, whereas in oxygen-deficient chlorinating gas atmospheres and in sulphidising atmospheres its resistance is poor. Zirconium addition is beneficial in case of oxygen-deficient chlorinating environments but increases the oxidation rate in air and in chlorinating [101]
35
Mar-M247 superalloy, aluminized and boroaluminized by pack cementation
Na2SO4-NaCl molten salt, 1000°C
Hot corrosion resistance increased for the specimens containing NiAl. Post heat treatment increased the corrosion resistance of the aluminized layer for Mar-M247; boroaluminized Mar-M247 specimens decrease corrosion resistance due to blocking of outward diffusion of Cr by boron [102]
36
AISI C-1055 (UNS G10550), Inconel wire
In situ test, samples with 2 cm2 of surface were covered with an excess of eutectic salt mixture and tested for 360 h at 400°C. KCl : ZnCl2 salt mixture
Wire and powder HVOF coatings show good properties to protect steel heat exchanger pipes against the erosion produced by the impact of the ashes in the flue gas [103]
37
Fe, Ni, and some model Fe-Ni alloys
Atmospheres of varying chlorine (10−3, 10−5, and 10−7 Pa) and low oxygen potentials (10−11–10−15 Pa) at temperatures of 800 and 1000°C
At 800°C in all three atmospheres the reaction kinetics was linear and the corrosion product was identified as ferrous chloride. At 1000°C under conditions of 10−3 Pa, Ni was found to be inert. A limiting Ni content of 50% was found to confer excellent chlorination resistance to iron [104]
Bioxidant oxygen-chlorine environment ( ) at 1000°C and, for comparison, in oxygen of low fugacity ( Pa) for periods up to 96 h
Increasing levels of chromium gradually suppressed corrosion such that the Fe25Cr alloy exhibited virtually no attack. The principal corrosion product was FeCl2 vapour with small amounts of Cr2O3 scale. Attack of the metal occurred initially via the grain boundaries, leading in the later stages to severe corrosion [105]
This plant had operated for about 7 years without any problems and the coated tubes are expected to have longer life [106]
40
CoNiCrAlYRe alloy
Molten Na2SO4 at 900°C
Advantages of proper preoxidation treatment were suggested, as keeping repairing for Al2O3 scale and inhibiting sulfur penetration [107]
41
Ti-6Al-4V (Ti-31) alloy
In air and 60% V2O5-Na2SO4 and Na2SO4-50% NaCl at 750°C
The degradation of Ti-31 occurs due to the chemical reactions between titania and chloride ions, sulphur, and vanadium present in the environments [108]
42
St35.8 steel, 13CrMo4-5 steel, St35.8 steel with chromium and aluminium diffusion coatings, HVOF (CrC-WCo Cr3C2-NiCr, Ni-55Cr), plasma (TiC-NiCo)
The erosion tests with pure SiO2 as erodent. In the E-C tests 0.1 wt.% KCl + SiO2, and KCl (7 g). Oxidising atmosphere 8% oxygen and gas temperature (850°C) and specimen temperature 550°C
The nickel-based HVOF coating with high chromium content showed good resistance against E-C at elevated temperature in presence of chlorine. Carbide containing HVOF coatings cannot resist elevated temperature and oxygen attack in presence of chlorine [109]
43
Ferritic and austenitic boiler steels, five high velocity oxy-fuel (HVOF) coatings, laser-melted HVOF coating, and diffusion chromized steel
Oxidizing atmosphere containing 500 vppm HCl, 20% H2O, 3% O2, and Ar as a balance temp = 550°C for 1000 h
Homogeneous and dense coatings with high chromium content performed well and protected the substrate material. Corrosive species were able to penetrate through some of the HVOF coatings and attack the substrate via interconnected network of voids and oxides at splat boundaries [110]
44
Low alloy ferritic steel and austenitic stainless steel, five high velocity oxy-fuel (HVOF) coatings, a laser cladding, and a diffusion chromized steel
40 wt.% K2SO4, 40 wt.% Na2SO4, 10 wt.% KCl, and 10 wt.% NaCl., oxidizing and in reducing atmospheres at 550°C for 100 h
Corrosion was extremely severe in oxidizing conditions because of active oxidation. In reducing atmosphere corrosion was retarded due to depletion of chlorine in the scales by evaporation of metal chlorides, and formation of a layer rich in chromium, sodium, sulfur, and oxygen adjacent to the metal surface. Chlorine was able to penetrate through the coatings along splat boundaries [54]
45
Ferritic boiler steel, one austenitic boiler steel, five high velocity oxy-fuel (HVOF) coatings, one laser-melted HVOF coating, and one diffusion chromized steel
500 ppm HCl, 600 ppm H2S, 20% H2O, 5% CO, and Ar as a balance at 550°C for 1000 h
Homogeneous and dense coatings with high chromium content performed well and were able to protect the substrate. Some of the HVOF coatings were attacked by corrosive species through interconnected network of voids and oxides at splat boundaries [111]
Argon—5.5% oxygen—0.96% HCl—0.86 SO2 at 900~ under isothermal and thermal cycling conditions
All the alloys showed good resistance under isothermal conditions but degradation under thermal cycling conditions due to failure of the protective scales. Formation of volatile chlorine-containing compounds was observed [113]
48
NiCoCrAlYSi coating and gradient coating
Oxidation tests were conducted in static air at 1000 and 1100°C, respectively. For hot corrosion test: 75 wt.% Na2SO4 + 25 wt.% NaCl at 900°C
The gradient coating has provided better protection against corrosion attack than the normal NiCoCrAlYSi coating due to its advantage of possessing Al rich reservoir. The favourable corrosion resistance should be attributed to the gradient distribution and enrichment of Al [114]
49
316L stainless steel (SS), surface modified with intermetallic coatings. Three different types of intermetallic coating systems containing aluminum, titanium, and titanium/aluminum multilayers
NaCl salt-applied alloys kept in an air furnace at 800°C up to 250 h
Titanium-modified alloys show the best hot-salt oxidation resistance with the formation of an adherent, protective, thin, and continuous oxide layer [115]
50
Platinum-iridium films (Ir = 0; 32; 46; 83; 100 at%) were deposited on the nickel-base single crystal superalloy TMS-82+ through magnetron sputtering
900°C with the Na2SO4 + 10% NaCl salt coatings
The lowest mass gain was observed for the Pt-46Ir aluminide coating, which formed the dense and continuous protective Al2O3 scale on the surface [116]
51
T92, HR3C, and 347HFG steels; nickel-based alloy 625
600, 650, and 700°C
Ferritic alloy (T92) proved to be the poorest performing alloy [117]
52
Superni-75
Actual medical waste incinerator operated at 1100°C
With the growth of a thin Cr2O3 interface layer along the scale/surface boundary, the performance of the alloy improved against the attack by the flue gases in the real service conditions [118]
