Surface engineering analysis of detonation-gun sprayed Cr3C2–NiCr coating underhigh-temperature oxidation and oxidation–erosion environments
Manpreet Kaur a,⁎, Harpreet Singh b, Satya Prakash c
a Baba Banda Singh Bahadur Engineering College, Fatehgarh Sahib, Indiab Indian Institute of Technology Ropar, Roopnagar, Indiac Indian Institute of Technology Roorkee, Roorkee, India
Detonation-gun (D-gun) spray technology is a novel coating deposition process which is capable of achievingvery high gas and particle velocities approaching 4–5 times the speed of sound. This process provides thepossibility of producing high hardness coatings with strong adherence. In the present study, this techniquehas been used to deposit Cr3C2–NiCr coating on T22 boiler steel. Investigations on the behaviour of this coatingsubjected to high-temperature oxidation in air and oxidation–erosion in actual boiler environment at 700±10 °C under cyclic conditions have been carried out. The weight change technique was used to establish thekinetics of oxidation. X-ray diffraction (XRD), field emission-scanning electron microscopy/energy-dispersivespectroscopy (FE-SEM/EDS) and EDS elemental mapping techniques were used to analyse the oxidation/oxidation–erosion products. The uncoated boiler steel suffered from a catastrophic degradation in the form ofintense spalling of the scale in both the environments. The Cr3C2–NiCr coating showed good adherence to theboiler steel during the exposures with no tendency for spallation of its oxide scale.
Power station boiler-walls and other utility parts of coal-firedplants are subjected to frequent degradation by erosion–corrosionproblems relevant to the reliability and economics of these in-stallations [1]. The environment of the furnaces is characterised byhigh temperature conditions together with aggressive atmospheres,leading to corrosive deposits adhered to the walls and to erosionprocesses due to the ash particles. During the last few years, thisproblem has been the focus of several research groups, which havegenerated the knowledge for the improvement of the usually usedmaterials, and for the development of new structural materials andcoatings capable of withstanding these aggressive conditions [2,3].
Oxidation–erosion is a combined action of flow and oxidation,leading to an accelerated rate of loss of material. Erosive, high-temperature wear of heat exchanger tubes and other structuralmaterials in coal-fired boilers are recognised as being the main causeof downtime at power-generating plants, which could account for 50–75% of their total arrest time. Maintenance costs for replacing brokentubes in the same installations are also very high, and can beestimated at up to 54% of the total production costs. High-temperature oxidation and erosion by the impact of fly ashes andunburned carbon particles are themain problems to be solved in these
applications. Therefore, the development of wear (usually erosion-wear) and high-temperature oxidation protection systems in indus-trial boilers is a very important topic from both engineering and aneconomic perspective [4].
Electric Power Research Institute (EPRI) “Cost of Corrosion” study[5] states that corrosion damage in boilers is a leading cost in the fossilfuel industry. Analysis of the “North American Electric ReliabilityCouncil-Generic Availability Data System” (NERC data) indicates thatthe coal-fired boilers are among the highest economic risk compo-nents in any power plant. “By far, the greatest numbers of forcedoutages in all types of boilers are caused by tube failures” [6]. Severaltechniques such as ultrasonic, infrared thermography and corrosionsensors have been utilised to monitor corrosion and predict corrosionrates in a boiler.
One possible way to counteract these problems is using thin wearand oxidation resistant coatings with good thermal conductivities,such as flame, plasma sprayed or hypersonic velocity oxygen fuel(HVOF) nickel based or cermet (carbide-metal) alloys [4]. Detonation-gun (D-gun) spray is also a versatile technology, which is capable ofachieving very high gas and particle velocities approaching 4–5 timesthe speed of sound. This process provides the possibility of producinghigh hardness coatings with significant adherence strength [7–9]. TheD-gun process offers highest velocity (800–1200 m s−1) for thesprayed powders that are unattainable by the plasma and HVOFconditions [10–13]. The high active energy makes the powder closelyconjoint the surface, and forms a layer with high strength, highhardness and good wear resistance [13–15]. This technology has been
widely used in many fields, such as aviation, space flight, petroleum,metallurgy, and machinery industry [13,16,17].
Cermet coatings, mainly carbide type coatings such as Cr3C2–NiCr,have shown an outstanding performance in different industrial areas[18]. These coatings are composed of carbide particles reinforcing ametallic matrix, combining the properties of ceramic-carbide typematerials, high hardness and toughness and ductility of metals.Carbide based coatings are widely used in abrasive, erosive andoxidising environments for various applications. However, thesecoatings have not been studied in detail with regard to their use forpower plant boiler tubes. These coatings exhibit high hardness with ahigh volume fraction of carbide being preserved during the spraying,providing different wear behaviours [19]. It is learnt from theliterature that further research is still needed to investigate theperformance of these coatings in some newer and more aggressiveenvironments, which may be simulated in the laboratory or in actualindustrial conditions.
The objective of this work is to study the high-temperatureoxidation and oxidation–erosion behaviour of D-gun spray Cr3C2–NiCr coatings in air and in actual boiler environments at an elevatedtemperature of 700 °C. There is no reported literature on the oxidationand oxidation–erosion behaviour of detonation gun spray (D-gun)deposited Cr3C2–NiCr coating on the given boiler steel substrate.Therefore, the present work has been focused to study the influence ofD-gun sprayed Cr3C2–NiCr coating on oxidation–erosion behaviour ofthe boiler steels. Moreover, the high-temperature boiler studies couldprovide an idea regarding the adhesion between the coatings and thesubstrate steels under thermal shocks [20].
2. Experimental procedure
2.1. Materials
The substrate material selected for the study was a boiler steel,namely, SA213-T22 (T22), being procured from Guru Gobind SinghSuper Thermal Power Plant (GGSSTPP), Ropar, Punjab (India). Thecomposition of the steel is 0.15 C, 0.3–0.6 Mn, 0.03 max P, 0.03 max S,0.5 Si, 1.9–2.60 Cr, 0.87–1.13 Mo and 94.66 Fe. Commercially available75Cr3C2–25(NiCr) coating powder was deposited on this boiler steelby the D-gun spray process. The coating powder has a particle size inthe range of−45+15 μm. The procedure for the deposition of the D-gun coating along with the spray parameters have been reported inthe earlier publication of the authors [21]. The average thickness ofthe coatingwasmeasured from BSEI micrographs. The XRD analysis ofthe coating powder revealed the formation of Cr, CrNi and Cr3C2 asvery strong intensity phases and Ni as a medium intensity phase. Theas-coated samples were characterised by XRD and FE-SEM/EDSanalyses to investigate their surface and cross-sectional microstruc-tures and compositions, the detailed results of the same have beenreported elsewhere [21,22].
2.2. High-temperature oxidation and oxidation-erosion tests
High-temperature oxidation tests were conducted in the labora-tory on the coated and uncoated samples as per the proceduresreported elsewhere [22].
The oxidation–erosion studies were conducted in an actual boilerenvironment. The coated, as well as, uncoated samples were hungthrough soot blower dummy points in the middle zone of the lowtemperature superheater of the Stage-II Boiler of Guru Gobind SinghSuper Thermal Power Plant (GGSSTPP), Ropar, Punjab (India) foroxidation–erosion studies under cyclic conditions. In this zone thetemperature was about 700±10 °C and volumetric flow of flue gaseswas around 700 tonnes/h. Flue gases contain 16% CO2 and 3% O2 byvolume. The gas stream contained the ash particles. The chemicalanalysis of ash and flue gases inside the boiler has been given in
Table 1. The velocity of the gas stream was 13 m/s. As the sampleswere coated from all sides, which were hung with the help ofnichrome wires, the angle of the gas stream impingement at the topedge of the surface was 90° and on the other four edges (two majorflat surfaces as well), the gas stream went parallel and at the bottom,no gas streamwas there. The direction of the gas stream is different atdifferent locations in the boiler, which may vary from vertical tohorizontal, downwards to upwards or inclined depending upon theorientation of boiler/superheater tubes. As per the recommendationof the boiler authorities, this particular location presents compara-tively highly aggressive conditions of flow. This zone was selected,since severe failures due to erosion have been reported by theconcerned power plant. The direction of gas flow is verticallydownwards in this zone. It is pertinent to mention that it is notpossible to comment on the direction of impingement of the particlesof the gas stream, due to which each of the surface is receiving erodingparticles with different impingement angles at various locations. Inthat sense the erosion conditions can be regarded as highlyrandomised with turbulent gas flow. This was even not possible tomonitor in the given environment. The studies were conducted for15 cycles; each cycle consisted of 100 h exposure, followed by 1 hcooling at ambient conditions.
Weight change measurements were taken at the end of each cycleusing an electronic balance with a sensitivity of 1 mg. Any spalledscale was also included at the time of weighing to determine the totalrate of oxidation. Efforts were made to formulate the kinetics of theoxidation/oxidation–erosion. However, in actual boiler environments,weight change data could not be of much use for predicting theoxidation behaviour because of suspected spalling and ash depositionon the samples. Hence extent of oxidation–erosion has also beenmonitored bymeasuring the thickness of the exposed sample after thetotal exposure of 1500 h. After the end of exposure time of 1500 h, thesamples were taken out, cooled, cleaned with soft brush, washed withacetone and dried. The thickness of the eroded–corroded specimenswas recorded carefully with a digital vernier calliper (Mitutoyo, Japanmake) and then converted into mils per year (mpy). The exposedsamples were analysed using FE-SEM/EDS and XRD for surfaceanalysis of their scales as per the details reported elsewhere [21,22].
3. Results
3.1. Microhardness and thickness of the as-sprayed coating
The microhardness values of the Cr3C2–NiCr coating on the givenboiler steel were measured. The microhardness profile along thecross-section of the coating as a function of distance from the coating-substrate interface is plotted [Fig. 1]. The microhardness values for theCr3C2–NiCr coated T22 boiler steel lie in the range of 645–823 Hv,with an average value of 802 Hv for the coating, while the substratesteel has an average microhardness of 196 Hv.
Average thickness of the coatings was measured from the BackScattered Electron Images (BSEI) for the Cr3C2–NiCr coating on T22
1400
1200
1000
800
600
400
D-gun Spray Cr3C2-NiCr Coated T22 steel
200
-160
Distance from interface (μμm)
Mic
roh
ard
nes
s (H
v)
80-120 -80 -40 400
0 120
Fig. 1. Microhardness profile of Detonation-gun spray Cr3C2–NiCr coated T22 boilersteel along the cross-section.
5 mm 5 mm
a b
Fig. 3. Macrographs of (a) uncoated and (b) D-gun spray Cr3C2–NiCr coated T22 boilersteel subjected to cyclic oxidation in air at 700 °C for 50 cycles.
532 M. Kaur et al. / Surface & Coatings Technology 206 (2011) 530–541
steel shown in Fig. 2. In the micrograph, three regions namelysubstrate, coating and epoxy are visible. Average thickness of thecoating was measured as 129 μm.
3.2. Visual observations and weight-change analysis
Photographs of the bare and D-gun spray Cr3C2–NiCr coated T22steel after exposure to the oxidation studies in air at 700 °C for50 cycles are shown in Fig. 3. Colour of the oxide scale formed on thebare T22 boiler steel was dark grey with some brownish spots. Thescale showed intensive spalling from the very first cycle of the study,which increased with the passage of time [Fig. 3(a)]. On the otherhand, the D-gun sprayed Cr3C2–NiCr coated T22 boiler steel wasinitially grey in colour. The colour turned to dark grey after the firstcycle of exposure and with the increase in exposure time, the colourcontinued to become darker tending towards blackish grey. Somebrown spots were observed after the 16th cycle. The oxide scale wasadherent with no tendency for spallation at all. At the end of 50 cycles,a dark grey oxide scale was observed [Fig. 3(b)]. This indicates that thecoatings can sustain the thermal shocks owing to heating and cooling(cyclic oxidation), which serves as an index for good adhesion andanti-spallation capability of the coatings, as per the suggestions ofBurman and Ericsson [20].
The photographs of the bare and D-gun spray Cr3C2–NiCr coatedT22 steel after exposure in the low temperature superheater of the
Stage-II area of coal fired boiler environments at 700±10 °C for1500 h are shown in Fig. 4. Redish-brown coloured scale appeared onthe surface of bare T22 steel after exposure to very first cycle of thestudy, which continued to become darker towards blackish-brownappearance with the increase in exposure. These colour variationsindicate the probability of onset of oxidation. The scale became veryfragile by the end of the third cycle, and blisters could be seen on thesurfaces of the sample. There were indications of significant spallationof scale after the end of 4th, 9th and 14th cycle, although the minorspallation of the scale was observed invariably throughout theexposure time once it started by the end of the 4th cycle. The oxidescale looked shiny grey and lots of debris with indications of spalledzones observed after the completion of 1500 h of study. The exposedspecimen has been depicted in Fig. 4(a). On the other hand, the D-gunsprayed Cr3C2–NiCr coated T22 boiler steel was observed to be darkgrey after exposure in the actual boiler environment [Fig. 4(b)]. Nospallation was seen and the scale was intact.
Weight change (mg/cm2) versus number of cycles plots for thebare and D-gun sprayed T22 boiler steel after oxidation studies in airenvironment at 700 °C up to 50 cycles are shown in Fig. 5. The weightchange data is usually used to establish the kinetics of the oxidationprocess. A higher weight gain represents higher rates of oxidation.Therefore, the oxidation rates of various materials can be comparedwith the help of weight change data. Based on these facts, it can beinferred from the weight change data that T22 steel has showncomparatively higher rate of oxidation. The overall weight gain in thesteel is enormous (106.99 mg/cm2). On the other hand, D-gun sprayCr3C2–NiCr coated T22 boiler steel was found to be better as thecoating showed no weight change up to 8 cycles and then only a
5 mm
a
5 mm
b
Fig. 4. Macrographs of (a) uncoated and (b) D-gun spray Cr3C2–NiCr coated T22 boilersteel subjected to low temperature superheater of the Stage-II Boiler of GGSSTPP at 700±10 °C after 1500 h.
Uncoated T22 Steel
D-gun spray Cr3C2-NiCr coated T22 steel
Number of Cycles
150
130
110
90
70
Wei
gh
t ch
ang
e/ar
ea (
mg
/m2)
50
30
10
-100 5 10 15 20 25 30 35 40 45 50
Fig. 5. Weight change vs. number of cycles plots for the uncoated and D-gun sprayCr3C2–NiCr coated T22 boiler steel subjected to cyclic oxidation in air at 700 °C for50 cycles.
Fig. 7. Bar charts showing thickness loss values (mpy) for the coated and uncoatedboiler steels subjected to low temperature superheater of the Stage-II Boiler of GGSSTPPat 700±10 °C after 1500 h.
marginal overall weight loss (−3.32 mg/cm2) was observed duringthe remaining cycles of the study.
Weight change (mg/cm2) versus time in hours plots for theuncoated and D-gun coated steel subjected to actual boiler environ-ment are shown in Fig. 6. Both the bare and coated boiler steels haveshown the overall weight loss during the exposure. The bare T22 steelhas shown abrupt variations in its weight change rates by the end of4th, 9th and 14th cycle. At these points, fluctuations in weight changevalues have been indicated. These weight fluctuations may beattributed to the significant spallation of the oxide scale duringthese cycles, as has also been noticed visually. On the other hand, theD-gun spray Cr3C2–NiCr coating showed significantly lesser weightloss. As a matter of fact, only a marginal overall weight loss(−1.066 mg/cm2) was measured.
3.3. Thickness change in actual boiler environment
The extent of oxidation–erosion loss was measured in terms ofmetal layer lost due to scaling after 1500 h exposure [Fig. 7]. Thethickness of metal lost in oxidised–eroded T22 boiler steel is0.046 mm. Based upon these values, the thickness loss rate indicatedby T22 steel is calculated as 10.576 mils per year (mpy).On the other
Fig. 6. Weight change vs. number of cycles plots for the uncoated and D-gun sprayCr3C2–NiCr coated T22 boiler steel subjected to low temperature superheater of theStage-II Boiler of GGSSTPP at 700±10 °C after 1500 h.
hand, the thickness loss value for the D-gun sprayed Cr3C2–NiCrcoating is 0.010 mm and the corresponding thickness loss rate isfound as 2.29 mpy.
3.4. X-ray diffraction (XRD) analysis
XRD patterns for the bare T22 steel in air at 700 °C for 50 cycles areshown in Fig. 8. As is clear from the diffractograms, Cr2O3 and Fe2O3
phases are found to be present in T22 oxidised boiler steel. Further,the diffractograms [Fig. 9] of the surface oxides formed on the Cr3C2–NiCr coated steel indicated the formation of Cr2O3 as a very strongintensity phase. Cr7C3 and Cr2C as strong intensity phases were alsorevealed.
The XRD diffractograms for the uncoated and coated boiler steelsafter exposure to the actual boiler environments for 1500 h are shownin Fig. 10 and Fig. 11 respectively. Fe2O3, Al2O3 and SiO2 are identifiedas the strong intensity phases in T22 steel. Cr2O3 and FeS are formed asmedium intensity phases. Whereas, for the Cr3C2–NiCr coated steel,the formation of Cr7C3, Cr2O3, Cr2C and Cr3C2 as the strong intensityphases was revealed by the XRD analysis. CrNi was observed as themedium intensity phase and Cr3Ni2 as a weak intensity phase.
3.5. FE-SEM/EDS analysis
3.5.1. Air oxidationFE-SEM/EDS analysis of the scale formed after 50 cycles of
oxidation in air at 700 °C on bare T22 steel showed dominance of Fewith small amounts of O, C, Cr and S. The scale looks to have anamorphous upper layer on a blackmatrix. The upper layer has nodularstructure, with irregular sized nodules dispersed in the structure,Fig. 12(a). The oxide scale for the oxidised D-gun sprayed Cr3C2–NiCrcoated T22 boiler steel [Fig. 12(b)] consists of granules, which appearto be interconnected to each other at most of the places. There aresome places where the black matrix is seen. The scale mainly has Crand O as its main constituents.
3.5.2. Oxidation-erosionThe SEM micrograph for the eroded-corroded scale formed on the
uncoated T22 boiler steel after 1500 h exposure in the actual boilerenvironments at 700±10 °C is shown in Fig. 13(a). The micrographindicates a granular scale consisting mainly of Fe and O with smallamounts of Al, Si, Zn and C. The composition of the scale at point 2 isnearly similar to that at 1. The presence of Al and Zn in the oxide scalerepresents the possibility of deposition of some ash particles on the
Inte
nsi
ty (
arb
itra
ry u
nit
s)
Diffraction angle (2θθ)
20 30 40 50 60 70 80 90 100 110 120
Fig. 8. X-ray diffraction patterns for the uncoated T22 boiler steel subjected to cyclic oxidation in air at 700 °C for 50 cycles.
Fig. 9. X-ray diffraction patterns for the D-gun spray Cr3C2–NiCr coated T22 boiler steel subjected to cyclic oxidation in air at 700 °C for 50 cycles.
534 M. Kaur et al. / Surface & Coatings Technology 206 (2011) 530–541
sample surfaces, as these constituents belong to the boiler environ-ment. On the other hand, the oxide scale for the D-gun sprayed Cr3C2–
NiCr coated T22 boiler steel [Fig. 13(b)] consists of uniformlydistributed spherical particles dispersed in the major surface area of
ααβδθ
ρθαβ
αθδ
δα
α
βα
θβαδρ
αρδβθ
θβρ
βρθδ
α- Fe2O3
Inte
nsi
ty (
arb
itra
ry u
nit
s)
Diffraction
20 30 40 50 60 70
Fig. 10. X-ray diffraction patterns for the uncoated T22 boiler steel subjected to low tem
the scale under observation. A significant portion of the oxide scale,corresponding to point 2, is consisting of Cr along with Ni and O andhas a dense appearance. The particles consist mainly of O and Si withsome percentage of Al in them (point 1).
θδρ
δ δ
δρ
δρ
ρ
, β- Al2O3, ρ- Cr2O3, δ-SiO2, θ-FeS
angle (2θ)
80 90 100 110 120
perature superheater of the Stage-II Boiler of GGSSTPP at 700±10 °C after 1500 h.
Fig. 11. X-ray diffraction patterns for the D-gun spray Cr3C2–NiCr coated T22 steel subjected to low temperature superheater of the Stage-II Boiler of GGSSTPP at 700±10 °C after1500 h.
b
50% Cr37% O07% C04% Ni02% Mn
1
58% Cr29% O 05% C05% Ni02% Mn
2
70% Fe12% C10% O04% Cr02% S
1
2
85% Fe07% O05% C01% Cr
a
Fig. 12. Surface scale morphology and EDS analysis for the T22 boiler steel subjected to cyclic oxidation in air at 700 °C for 50 cycles (a) in uncoated condition, and (b) with D-gunspray Cr3C2–NiCr coating.
Fig. 13. Surface scale morphology and EDS analysis for the T22 boiler steel subjected to low temperature superheater of the Stage-II Boiler of GGSSTPP at 700±10 °C after 1500 h(a) in uncoated condition, and (b) with D-gun spray Cr3C2–NiCr coating.
536 M. Kaur et al. / Surface & Coatings Technology 206 (2011) 530–541
3.6. Cross-sectional analysis
3.6.1. Air oxidationOxide scale morphology along with variation of elemental
composition across the cross-section of T22 boiler steel subjected tocyclic oxidation in air at 700 °C after 50 cycles is depicted in Fig. 14(a).The scale formed on the surface of uncoated T22 steel seems to beloosely bounded to the matrix as shown. The scale mainly consists ofiron and carbon along with significant amounts of molybdenum.Some amounts of chromium and manganese are also observed in thescale. Further, the oxide scale for D-gun sprayed Cr3C2–NiCr coatedT22 steel seems to be dense and strongly bonded to the substrate steeleven after the oxidation study for 50 h in the air environment [Fig. 14(b)]. The oxide scale contains mainly chromium along with significantamounts of nickel and carbon. The outermost layer at point 5 containsenhanced concentration of Cr along with C, O and Ni. The substratesteel seems to be unaffected from oxidation as oxygen at point 1 isonly marginal.
3.6.2. Oxidation-erosionA cross-sectional micrograph showing oxide scale morphology for
the eroded-corroded bare T22 steel after 1500 h exposure to theactual environment of hot flue gases of coal fired boiler is shown inFig. 15(a). A thick oxide scale is visible at the location of SEM analysis
on the surface of uncoated T22 steel. The scale consists mainly of ironand oxygen along with significant amounts of carbon and molybde-num. On the other hand, the oxide scale for the D-gun sprayed Cr3C2–NiCr coated T22 steel seems to be dense and adherent and hasretained its continuous contact with the substrate steel even after theoxidation–erosion for 1500 h in the coal-fired boiler [Fig. 15(b)]. Theoxide scale has the richness of chromium along with significantamounts of nickel, carbon and oxygen. This indicates the possibility offormation of oxides and carbides of mainly chromium in the oxidescale. The substrate steel seems to be unaffected from the oxidation.
3.7. EDS elemental mapping analysis
The results of the EDS elemental mapping analysis of the scales aresummarised in Table 2.
4. Discussion
Thermally sprayed Cr3C2–NiCr coatings are used in applicationsthat demand protection against surface degradation due to oxidation,wear and corrosion under severe conditions of excessive heat and load[23]. These coatings maintain their highwear and corrosion resistanceup to 1253 K and can be used to improve the performance life ofcomponents working at elevated temperatures [24,25]. Different
1 2 3 4 5
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Epo
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b100
C O Si Cr C O Cr Fe NiMn Fe MoW
eig
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% o
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Point of analysis
90
80
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01 2 3 4 5 6
Point of analysis1 2 3 4 5
Fig. 14. Oxide scale morphology and variation of elemental composition across the cross-section of T22 boiler steel subjected to cyclic oxidation in air at 700 °C for 50 cycles (a) inuncoated condition, and (b) with D-gun spray Cr3C2–NiCr coating.
studies indicate that Cr2O3 (protective oxide) is preferentially formedat high temperatures on the coating surface and so prevents oxidationof thewhole coating. Thermogravimetric studies indicate that at 873 Kthe oxidation of Cr3C2–NiCr starts and at temperatures as high as1073 K is still low [26]. In the current investigation, the Cr3C2–NiCrcoating was successfully deposited by D-gun spray process on T22boiler steel. The as-sprayed steel consisted of a spongy globularstructure [21]. The observed microhardness values are in goodagreement with those reported by Ji et al. [27] for the HVOF sprayedCr3C2–NiCr coating onmild steel plate, Murthy et al. [28] for the D-gunsprayed Cr3C2–NiCr coating on mild steel specimens and Sidhu et al.[29] for the HVOF sprayed Cr3C2–NiCr coating on T22 substrate.
Theweight change data for the D-gun spray Cr3C2–NiCr coated andbare boiler steels in air and actual boiler environment are shown inFig. 5 and 6 respectively. The plots [Fig. 5] indicate higher weight gainvalues (106.99 mg/cm2) attained by the uncoated T22 steel whenexposed to high-temperature oxidation studies in air in comparisonwith its coated counterpart. A severe tendency of cracking andspalling was observed during the course of study in air at 700 °C. Therate of oxidation went on increasing with the progress of theoxidation study. The steel followed the parabolic law of oxidation,in general, with some deviations. The Kp value for the steel showedtransitions from 3.17×10−8 g2 cm−4 s−1 for first four cycles, 10.3×10−8 g2 cm−4 s−1 for sixth to tenth cycle, 5.59×10−8 g2 cm−4 s−1
for tenth to thirty second cycle and 8.34×10−8 g2 cm−4 s−1 for theremaining range of cycles. The small deviations from the parabolic
rate law have also been observed by Levy et al. [30] and Singh et al.[31] during their studies on the oxidation and hot corrosion of someNi-base superalloys at 704 to 1093 °C. They attributed these de-viations to cracking and spalling of the oxide scales. On the otherhand, the same steel when exposed to actual boiler environment hadshown the higher overall weight loss and thickness lost values incomparison to its coated counterpart. These values may be due tospallation and oxidation accompanied by erosion (oxidation–erosion)of the specimen.
The D-gun spray Cr3C2–NiCr coated steel when exposed in air, aswell as, actual boiler showed only a marginal weight loss, indicating agood oxidation and oxidation–erosion resistance. This showed thatthe coating was successful in reducing the oxidation/oxidation–erosion rate of the steel. This observation is consistent with theobservation of Berger et al. [26] according to which the oxidation ofCr3C2–NiCr starts at 873 K and at temperatures as high as 1073 K isstill low. During oxidation studies of D-gun spray Cr3C2–NiCr coatingon D1N12CrMo44 steel, Wang et al. [32], reported a significantlybetter improvement in high-temperature oxidation resistance of thesteel after the application of the coating. They also observed very thickoxide scales with severe spallation on the uncoated specimens bynaked eyes. It was believed that the oxidation resistance of the coatingis attributed to its high Cr content and the dense coating protectedthe substrate from the inward permeation of oxygen and promotedthe selective oxidation of chromium so as to improve the high-temperature oxidation resistance of the steel. When subjected to
1 52 3 4
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C O Si P C O Cr Fe Ni
MnCr Fe Na
S
Mo
Wei
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01 2 3 4 5
Point of analysis
1 2 3 4 5
Fig. 15. Oxide scale morphology and variation of elemental composition across the cross-section of T22 boiler steel subjected to low temperature superheater of the Stage-II Boiler ofGGSSTPP at 700±10 °C after 1500 h (a) in uncoated condition, and (b) with D-gun spray Cr3C2–NiCr coating.
538 M. Kaur et al. / Surface & Coatings Technology 206 (2011) 530–541
actual boiler environments, the degradation rate in terms of thicknesshas reduced to 2.29 mpy from 10.576 mpy after the deposition of thecoating. The lower thickness loss rate in case of Cr3C2–NiCr coatedsteel exposed to the actual boiler environment might be attributed tothe compact and less porous structure of its oxide scale. Fukuda et al.[33] suggested that the melting behaviour of the coating powder is
Table 2Summary of the results of EDS elemental mapping analysis of the scales.
Air Oxidation O
Bare T22 steel D-gun spray Cr3C2–NiCr coated T22 steel Ba
Oxide scale got split-up and consistedmainly of iron and oxygen. The scalealso contained Cr and some amountsof P, S and Mn. Carbon could also beseen in small concentrationsthroughout the scale
The oxidised coating was found to beintact and strongly bonded to thesubstrate steel. It was found to beconsisting mainly of Cr and Ni. A thickouter layer of O could be seen in the oxidescale, which also contained Cr and C.Diffusion of Mn could be seen from thesubstrate steel.
Thunsusespscwchsccosusewin
very important to form poreless and high bond strength coating layer.The more compact and less porous the coating, the higher is itserosion–corrosion resistance [34]. Suegama et al. [35] also found intheir studies that the absence of pores and cracks (micro and macro)is very important when corrosion resistance is required because theelectrolyte penetrates through these defects to reach the substrate.
xidation–Erosion
re T22 steel D-gun spray Cr3C2–NiCr coated T22 steel
e oxide scale appeared to be almostiformly thick and well adherent to thebstrate. Some loose scale could also been in the outer layers due to severeallation of the scale in the boiler. Theale consisted mainly of iron and oxygenith a significant concentration ofromium, especially at the interface ofale and steel matrix. Smallncentrations of manganese, potassium,lphur and phosphorous could also been in the scale. Oxygen and sulphurere found to be higher in concentrationthe inner layers of the oxide scale.
The exposed coating consisted mainly ofCr and Ni. Oxygen was also presentthroughout the scale with somewhatdenser concentration near the coating-substrate interface. Diffusion of Mn couldbe seen from the substrate steel. Someelements like calcium, potassium,phosphorous, silicon and sulphur couldalso be found in small concentrations.
The XRD analysis had revealed the presence of mainly Fe2O3 andCr2O3 in the oxidised scale of T22 steel [Fig. 8]. The surface EDSanalysis also confirmed the presence of an upper layer of Fe2O3. Theformation of Fe2O3 has also been reported by Lai [36], where heobserved an upper layer of Fe2O3 for oxidised iron–chromium alloys.Author further reported that alloys having 2% chromium could onlyform the oxides of chromium along with iron oxide in the innermostlayer. That is probably why only marginal amounts of Cr have beenobserved in the top scale of the steel by the surface SEM/EDS analysisin the current investigation [Fig. 12(a)]. The presence of Cr2O3 asrevealed by the surface scale XRD analysis for T22 steel may perhapsbe due to spalling of top Fe2O3 layer of the scale. Moreover, the XRDanalysis can predict the elements to a comparatively larger depth incomparison with FE-SEM/EDS analysis. The presence of Fe2O3 in thescales of uncoated steel after oxidation experiments indicates thepresence of non-protective conditions as per the suggestions of Das etal. [37]. The presence of chromium in the scale, along with iron oxidefor T22 steel is consistent with the findings of Sadique et al. [38], whoreported that Fe–Cr alloys in oxygen at high temperature form spinel(FeCr2O4) and Cr2O3 on the inner side and Fe2O3 on the outside of thescale. Sidhu et al. [39] and Bala et al. [40] also reported similar phasesduring their oxidation studies on the same boiler tube steel. They alsoobserved spallation in T22 boiler steel during the oxidation studies inair at 900 °C and indicated that the top scale was mainly Fe2O3 for theoxidised steel. Intensive spalling of the scale as observed can beattributed to severe strain developed because of Fe2O3 precipitationfrom the liquid phase and inter diffusion of intermediate layers of ironoxide as has been reported by Sachs et al. [41].
The XRD analysis for the D-gun sprayed Cr3C2–NiCr coating hasindicated the formation of Cr2O3 as a very strong phase and Cr7C3 andCr2C as strong phases [Fig. 9]. Surface and cross-sectional FE-SEM/EDSanalysis further supported the formation of these phases. Therespective EDS elemental mappings also endorse these formations.The oxide particularly Cr2O3 may partially inhibit oxidation of thesubstrate steel by blocking the diffusion of reacting species towardsthe substrate alloys, as has been suggested by Nicoll and Wahl [42],Saxena [43], Stott [44], Tawancy et al. [45], Stroosnijder et al. [46]and Singh et al. [31]. Moreover, the cross-sectional EDS analysis for
Fe, Cr, Mn and Si present in the substrate
Cr, O and S observed in higher concentrations in the inner layers (at interface)
Fe, O and Crthe scale
Some amounts of Mn, P S and K present in the scale
T22
ste
el
Small diffusion of Mn2+
Cr3+
Fe3+
Fig. 16. Schematic diagram showing probable oxidation-erosion mode for the uncoated T2Gobind Singh Super Thermal Power Plant (GGSSTPP), Ropar at 700±10 °C after 1500 h.
the D-gun sprayed T22 steel, showed that the oxygen has notpenetrated into the substrate, which indicates that the substrate isunaffected from the oxidation.
When exposed to the actual boiler environment, the XRD analysishas shown the formation of Fe2O3, Al2O3 and SiO2 as the strong phasesin bare T22 steel. Cr2O3 and FeS are formed as medium intensityphases. Formation of hematite (Fe2O3) as revealed by the X-raydiffractograms might be due to the reaction of iron with oxygen sinceiron is the main constituent of boiler steels. Formation of such type ofoxides has also been analysed by Singh [47] during the failure analysisof superheater tubes caused by fireside corrosion. The formation ofFe2O3 phase has further been supported by the EDS analysis across thecross-section of T22 steel. Further, EDS elemental maps indicate theco-existence of chromium along with iron and oxygen. The formationof oxides of iron and chromium indicates that oxidation of the steel isalso taking place along with erosion. The observed spallation in thiscase may partially be attributed to the erosion by the fly ash particles.The formation of Al2O3 might be due to the deposition of ash on theoxidised–eroded tubes. Levy [48] also observed the presence ofmixture of bedmaterial constituents in the outer scale deposits duringerosion–corrosion of tubing steels in combustion boiler environments.The schematic diagram showing probable oxidation–erosionmode forthe uncoated T22 boiler steel exposed to actual boiler environment at700±10 °C for 1500 h is shown in Fig. 16. The figure shows anoutermost layer containing mainly ash particles getting depositedfrom the boiler environment. Followed by this, chromium along withiron and oxygen are present in the scale. Outward diffusion of iron andchromium from the steel and inward diffusion of oxygen from theenvironment indicates the oxidation of the bare boiler steel. Rapp et al.[49] opined that the particulatematter (e.g. non-combustibles, carbonparticles, spalled corrosion products) often hits the surfaces of alloysduring the corrosion process, so the protective scale is subjected toerosive conditions. When the reaction product is dense and adherentto the alloy, the scale can inhibit the erosion process. However, whenthe reaction product scale is not dense, the erosion conditions cangreatly accelerate the corrosion process.
Brasunas [50] reported that even mildly abrasive conditions mayremove a corrosion film from a surface which is protective of a
Cr2O3 and FeS medium intensity phases revealed in the scale
Loose oxide scale present in the outer layers due to spallation
present in
Co
al-Fired
B
oiler
En
viron
men
t
Fe2O3, Al2O3
and SiO2
phases present in the top scale
2 boiler steel subjected to low temperature superheater of the Stage-II Boiler of Guru
Fe restrictedmainly to the substrate
Cr, Mn, O and Ni present throughout the scale
Cr7C3, Cr2O3, Cr2C and Cr3C2 phases in the top scale+Fly ash deposition
O alongwith Cr present in higher concentrations in the inner layers (at the interface)
T22
ste
el
Co
al- Fired
Bo
iler En
viron
men
t
Mn2+
Cr3+
Fe3+
Fig. 17. Schematic diagram showing probable oxidation-erosion mode for Cr3C2–NiCr coated T22 boiler steel subjected to low temperature superheater of the Stage-II Boiler of GuruGobind Singh Super Thermal Power Plant (GGSSTPP), Ropar at 700±10 °C after 1500 h.
540 M. Kaur et al. / Surface & Coatings Technology 206 (2011) 530–541
substrate, thus exposing a fresh metal to corrode and therebyaccelerate damage. Usually material under tension will corrodesooner and faster than the same material under compressive stress.Generally, stressedmaterials corrode faster than unstressedmaterials,especially if they are loaded near or over their elastic limits. Staubliet al. [51] further suggested that Fe–Cr alloys containing 2–3% Cr arelimited in boiler service to temperatures of 580 to 600 °C.
For the D-gun sprayed Cr3C2–NiCr coated T22 boiler steel, theformation of Cr7C3, Cr2O3, Cr2C and Cr3C2 as the strong phases wasrevealed by the XRD analysis. CrNi was observed as the mediumintensity phase and Cr3Ni2 as a weak intensity phase. Surface andcross-sectional FE-SEM/EDS analysis further supported the formationof the above said phases in the D-gun coated T22 steel. The respectiveEDS elemental mappings also endorse these formations. The surfaceEDS analysis for scales of the coated steel reveals mainly Cr and Owithsome embedment of ash particles. The embedment of ash particles inthe scale of coated tube steels has been further confirmed by the EDSelemental mapping analysis. Further, it could be seen from therespective EDS elemental mapping analysis of the coating that thereare no signs of depletion of basic elements from the substrate steel. Fewhich is the basic element of the substrate steel is restrictedmainly tothe substrate only. This indicates that the coating, in general, has beensuccessful in acting as a reservoir for the formation of protectiveoxides/spinels and consequently may increase the service life of thesubstrate. Continuous, uniform and adherent oxide scale rich in Cr2O3
is formed on the D-gun coated steel without indication of any crack[Fig. 15(b)], which contributes to better oxidation–erosion behaviour.Chromium exhibits higher affinity for oxygen to form Cr2O3 during theearlier stages of hot corrosion. This oxide offers a better protectionagainst oxidation/hot corrosion due to its low growth rate, stronglybounded composition and ability to act as effective barriers againstionic migration [52]. Moreover, the Cr2O3 phase is a harder phase andcan resist erosion as well. The schematic diagram showing probableoxidation–erosion mode of attack for the D-gun spray Cr3C2–NiCrcoated T22 steel exposed to actual boiler environment at 700±10 °Cfor 1500 h is shown in Fig. 17.
Based upon the above discussion and results of the current study, itmay be concluded that the D-gun spray Cr3C2–NiCr coating could beuseful to impart high-temperature oxidation and oxidation–erosionresistance to the given boiler steel.
5. Conclusions
1. Cr3C2–NiCr coating could be deposited on T22 boiler steel by theD-gun spray process and the coatings were found to be promis-ing from the point of view of oxidation and oxidation–erosionresistance.
2. The as-sprayed Cr3C2–NiCr coating was found to have a spongyglobular structure.
3. Bare T22 steel suffered from accelerated oxidation in the form ofintense spalling of the scale during oxidation studies in air with asignificant overall weight gain. During the exposure to the actualboiler environment at 700±10 °C, the steel suffered from higherweight loss in comparison with its coated counterpart.
4. The D-gun spray Cr3C2–NiCr coating was found to be successful inmaintaining its adherence on the steel in both the environments.The surface scales were also found to be intact. The coated boilersteel showed better oxidation–erosion resistance in comparisonwith its uncoated counterpart. The formation of Cr2O3 rich oxidescale might have contributed to the better oxidation–erosionresistance in the coated steel.
Acknowledgements
Harpreet Singh et al. thankfully acknowledge the research grantfrom Department of Science and Technology, New Delhi (India) underSERC FAST Scheme (File No. SR/FTP/ETA-06/06, Dated March 16,2006) to carry out this R & D work, titled “Development of Erosion-Corrosion Resistant Thermal Spray Coatings for Power Plant Boilers.”
References
[1] I. Fagoaga, J.L. Viviente, P. Gavin, J.M. Bronte, J. Garcia, J.A. Tagle, Thin Solid Films317 (1998) 259.
[2] D.B. Meadowcroft, Mater. Sci. Eng. 88 (1987) 313.[3] P. Vuoristo, K. Niemi, A. Makela, T. Mantyla, in: C.C. Berndt, S. Sampath (Eds.),
Thermal Spray Industrial Application, ASM International, Materials Park, OH,1994, p. 121.
[4] V.H. Hidalgo, F.J.B. Varela, C. Menendez, S.P. Martinez, Tribol. Int. 34 (2001) 161.[5] EPRI 1005358, Comparison of Alternate Cooling Technologies for U.S. Power
Plants: Economic, Electric Power Research Institute, Palo Alto, California, USA,2004.
[8] N. Espallargas, J. Berget, J.M. Guilemany, A.V. Benedetti, P.H. Suegama, Surf. Coat.Technol. 202 (2008) 1405.
[9] S. Kamal, R. Jayaganthan, S. Prakash, S. Kumar, J. Alloys Compd. 463 (1–2) (2008)358.
[10] N.Wagner, K. Gnadic, H. Kreye, H. Kronewetter, Surf. Coat. Technol. 22 (1) (1984) 61.[11] C.J. Li, A. Ohmori, Surf. Coat. Technol. 82 (3) (1996) 254.[12] S.Y. Semenov, B.M. Cetegen, Mater. Sci. Eng. A 335 (1–2) (2002) 67.[13] H. Du, H. Weigang, J. Liu, J. Gong, C. Sun, L. Wen, Mater. Sci. Eng. A 408 (2005) 202.[14] K.T. Kembaiyan, K. Keshavan, Wear 186–187 (Part 2) (1995) 487.[15] Y.N. Tyurin, A.D. Pogrebnjak, Surf. Coat. Technol. 111 (2–3) (1999) 269.[16] T.N. Rhys-Jones, Surf. Coat. Technol. 43/44 (1990) 402.[17] E. Kadyrov, V. Kadyrov, J. Therm. Spray Technol. 4 (1995) 280.[18] P. Sahoo, Powd. Meta. Int. (PMI) 25 (2) (1993) 73.[19] T. Sahraoui, N.E. Fenineche, G. Montavon, C. Coddet, Mater. Design 24 (2003) 309.[20] C. Burman, T. Ericsson, in: S.C. Singhal (Ed.), Proc. Sympos., High-Temperature
Protective Coatings, March 7–8, Atlanta, GA, USA, Pub. Metall. Soc of AIME,Warrendale, PA, USA, 1983, p. 51.
[21] M. Kaur, H. Singh, S. Prakash, Surf. Eng. 26 (6) (2010) 428.[22] M. Kaur, H. Singh, S. Prakash, J. Therm. Spray Technol. 18 (4) (2009) 619.[23] M.H. Staia, T. Valente, C. Bartuli, D.B. Lewis, C.P. Constable, Surf. Coat. Technol.
146–147 (2001) 553.[24] D.Y. Kim, M.S. Han, in: C.C. Berndt (Ed.), ASM International, Ohio, USA, 1996,
p. 123.[25] B.Q. Wang, in: T.S. Sudarshan, et al., (Eds.), The Institute of Materials, Paris, 1997,
p. 458.[26] L.M. Berger, P. Vuoristo, T. Mantyla, W. Gruner, in: C. Coddet (Ed.), ASM
International, Material Park, Nice, 1998, p. 75.[27] G. Ji, C. Li, Y. Wang, W. Li, Surf. Coat. Technol. 200 (2006) 6749.[28] J.K.N. Murthy, S. Bysakh, K. Gopinath, B. Venkataraman, Surf. Coat. Technol. 202
(2007) 1.[29] H.S. Sidhu, B.S. Sidhu, S. Prakash, Tribol. Int. 43 (2010) 887.[30] M. Levy, R. Huie, F. Pettit, Corros. 45 (8) (1989) 661.[31] H. Singh, D. Puri, S. Prakash, Proc. International Symposium of Research Scholars
on ‘Mater. Sci. & Eng.,’ Dec., 20–22, IITM, Chennai, India, 2004.
[32] J. Wang, L. Zhang, B. Sun, Y. Zhou, Surf. Coat. Technol. 130 (2000) 69.[33] Y. Fukuda, M. Kumonn, Proc. ‘ITSC'95, High Temperature Society of Japan,’ 22–26
May, 1995, Japan ASM International, Kobe, 1995, p. 107.[34] B.Q. Wang, G.Q. Geng, A.V. Levy, Surf. Coat. Technol. 54–55 (1992) 529.[35] P.H. Suegama, C.S. Fugivara, A.V. Benedetti, J. Fernandez, J. Delgado, J.M.
Guilemany, J. Appl. Electrochem. 32 (2002) 1287.[36] G.Y. Lai, ASM Inter. Book, 1990, p. 15.[37] D. Das, R. Balasubramaniam, M.N. Mungole, J. Mater. Sci. 37 (6) (2002) 1135.[38] S.E. Sadique, A.H. Mollah, M.S. Islam,M.M. Ali, M.H.H. Megat, S. Basri, Oxid. Met. 54
(5–6) (2000) 385.[39] B.S. Sidhu, D. Puri, S. Prakash, J. Mater. Process. Technol. 159 (3) (2005) 347.[40] N. Bala, H. Singh, S. Prakash, Appl. Surf. Sci. 255 (2009) 6862.[41] K. Sachs, Metallurgia (1958) 167 Apr.[42] A.R. Nicoll, G. Wahl, Thin Solid Films 95 (1983) 21.[43] [43]D. Saxena, Effect of Zr and Y Addition on High Temperature Sulphidation
Behaviour of Fe-15Cr-4Al, Ph.D. Thesis, Met. Mat. Engg. Dept., University ofRoorkee, Roorkee, India, 1986.
[44] F.H. Stott, F.I. Wei, C.A. Enahoro, Mater. Corros. 40 (4) (1989) 198.[45] H.M. Tawancy, N.M. Abbas, A. Bennett, Surf. Coat. Technol. 68–69 (1994) 10.[46] M.F. Stroosnijder, R. Mevrel, M.J. Bennet, Mater. High Temp. 12 (1) (1994) 53.[47] B. Singh, Studies on the Role of Coatings in Improving Resistance to Hot Corrosion
and Degradation, Ph.D. Thesis, Met. & Mat. Eng. Dept., Indian Institute ofTechnology Roorkee, Roorkee, 2003.
Society, Pennington, N. J, 1981, p. 159.[50] A. Brasunas, NACE Basic Corrosion Course, Pub. NACE, Houston, Texas, 1977.[51] M. Staubli, K.H. Mayer, T.U. Kern, R.W. Vanstone, R. Hanus, J. Stief, K.H. Schonfeld,
M. Staubli, K.H. Mayer, T.U. Kern, R.W. Vanstone, R. Hanus, J. Stief, K.H. Schonfeld,Paper 1.3, Proc. ‘3 rd EPRI Conference on Advance in Materials Technologies forFossil Power Plants,’ Swansea, , 2001.
