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Hot Corrosion Study of High Velocity Oxy-Fuel (HVOF) Sprayed Coatings on Boiler Tube Steel in Actual Coal Fired Boiler.


Hot corrosion has been recognised as a severe problem in boiler tubes of super heater and reheater zone, which may result in premature failure of the tubes. One method to overcome this major problem is the use of thermal spray protective coatings. In past few years, high velocity Oxy-fuel (HVOF) spraying has shown a dramatic influence on the field of thermal sprays. The coating produced is less porous and has higher bond strength than that produced by other methods which is due to the high velocity and high impact of the sprayed powder (Tan et al., 1999). The cost corrosion of metals may rises over $300 billion per year (4.2% GNP) in United States. It is estimated that every year about 40% of total US steel production goes to replace the corroded parts and products. Although these problems of corrosion can not be wholly remedied but corrosion-related costs can be diminished by more than 30% with the development and use of better corrosion resistant coatings (Priyantha et al., 2003). Thermal spraying has been considered successful to apply the protective coatings, without disturbing any other properties of the component (Bala et al., 2009; Tillmann et al., 2000). Mostly all the thermally sprayed coatings include splats and splat boundaries. Due to the porosity present at splat boundaries, the coatings are prone to corrosive attack (Usitalo et al., 2004). HVOF coatings provide low porosity, homogeneous, compact and hard structures with sufficient thickness to stop the advancing of electrolytes (Guilemany et al., 2008). Kawakita et al. (2005) studied that with the addition of molybdenum, a considerable improvement was noticed against corrosion and was most effective on 8% mass molybdenum. Sidhu et al. (2006) studied the role of high velocity oxyfuel (HVOF) coatings in improving hot corrosion resistance of boiler tube steel (ASTM-SA210 GrA1) and investigated that among [Cr.sub.2][C.sub.3]-NiCr, NiCr, Wc-Co and stellite-6 coatings, NiCr showed maximum whereas Wc-Co showed minimum resistance to corrosion. Sidhu et al. (2006a) devised Ni-20Cr wire coating proved effective than [Cr.sub.2][C.sub.3]-NiCr using Ni-based superalloy in environment of molten salt [Na.sub.2][SO.sub.4]-[V.sub.2][O.sub.5] at 900 [degrees]C. Sidhu et al. (2006b) successfully applied Ni-20%Cr wire coatings onto Ni- and Fe- based super-alloys by high velocity oxyfuel process (HVOF) for hot corrosion studies with purpose to characterise the surface and cross-section morphology and confirmed the presence of Ni as main constituent of Ni-20%Cr coating. Sidhu (2006) and Prakash (2006) experimentally investigated that plasma sprayed stellite-6 have higher resistance to erosion-corrosion and T11 coated steel showed the maximum degradation resistance. Sidhu et al. (2006c) investigated that NiCrBSi coatings were beneficial in lowering the corrosion rate in given environment. Sidhu et al. (2006d) used HVOF (high velocity oxy-fuel) process for deposition of NiCrBSi, [Cr.sub.2][C.sub.3]-NiCr, Ni-20Cr and stellite-6 coatings on Fe-based superalloy to study the performance of coatings under cyclic conditions in [Na.sub.2][SO.sub.4]-60%[V.sub.2][O.sub.5] at 900 [degrees]C and noticed that Ni-20Cr coating showed maximum resistance to corrosion. Goyal et al. (2008) studied that Zr[O.sub.2] was capable of reducing high temperature corrosion in all alloys and was most useful in Superni 75 followed by Superco 605 and slightly in Superfer 800H. Rajasekaran et al. (2004) explained that D-gun spraying minimizes the degradation of the feedstock powder due to lower heat and shorter duration involved in the deposition process. Kamal et al. (2009) investigated that hot-corrosion resistance was better for [Cr.sub.3][C.sub.2]-NiCr-coated superalloys as compared to the uncoated superalloys in the presence of 75 wt.% [Na.sub.2][SO.sub.4]+ 25 wt.% [K.sub.2][SO.sub.4] film due to the formation of protective oxides of chromium and nickel. Kaushal et al. (2011) revealed that coating had potential to lower the high temperature corrosion using detonation gun spray method to deposit Ni-20Cr onto ASTM A213 TP347H boiler steel. Manpreet (2011) proved [Cr.sub.3][C.sub.2]-NiCr coating to be highly protective against corrosion. Using detonationgun spray process in which very high particle velocities approaching 4-5 times the speed of sound are achieved. Mishra et al. (2013) reported that aluminium, chromium and yttrium were acting as protective oxides for boiler environmental conditions when the performance of plasma sprayed Ni-22Cr-10Al-1Y coating on different super alloys such as Superni 75, Superni 600, Superni 718 and Superfer 800H was investigated. Kaushal et al. (2014) found Detonation sprayed coating to be most protective among all HVOF, detonation gun and cold sprayed Ni-20Cr coatings on T22 boiler steel tube, he also demonstrated that all three techniques were beneficial in decreasing the corrosion rate of steel. Kumar et al. (2014) noticed that oxidation resistance had been increased by addition of Ce[O.sub.2]in small amount and also found that [Cr.sub.3][C.sub.2]-NiCr coated and [Cr.sub.3][C.sub.2]-NiCr+0.4wt.%CeO2 coated Superni 600 had less parabolic rate constant than that of bare Superni 600.

Current study was planned to compare the behaviour and effect of 93(WC-[Cr.sub.3][C.sub.2]>7 Ni and 86WC-10Co-4Cr coatings on ASME SA213 T22 and ASME SA213 T91 materials by assessing its surface and sub surface on heating it at 900 [degrees]C under cyclic conditions.

Materials and Methods

Substrate materials. For the present study, two Steel based alloys namely ASME SA213 T22 and ASME SA213 T91 were selected as the substrate materials. These steel alloys were obtained from Guru Nanak Dev Thermal Power Plant, Bathinda, Punjab (India). The samples of both alloy steels were cut with the dimensions of 20 mm X15 mm X 5 mm. Then emery papers of grit sizes 100, 150, 220, 320, 600, 800 and 1000 were used to polish the samples. Alumina powder was used for grit blasting of samples. Following two illustrates the chemical composition for ASME SA213 T22 and ASME SA213 T91 boiler tube steels are illustrated in Table 1-2, respectively.

Coating materials. Coatings selected were of two types namely 93(WC-Cr3C2)-7Ni and 86WC-10Co-4Cr. Further, these coatings were deposited onto two steel based alloys using HVOF spray process. Table 3 shows composition and particle size of coating powder.

HVOF spraying. High velocity oxy-fuel thermal spray method was selected for deposition of coatings on two steel alloys at Metallizing Equipment Co. Pvt. Ltd, Jodhpur, Rajasthan (India). Commercial HVOF (HIPOJET-2100) apparatus functioning with oxygen and liquid petroleum gas (LPG) as input gases, was used. Table 4 shows spray parameters. The coatings have been designated as follows: C1 T22: 93(WC-Cr3C2)-7Ni coated T22 specimen, C1 T91: 93(WC-Cr3C2)-7Ni coated T91 specimen, C2 T22: 86WC-10Co-4Cr coated T22 specimen, C2 T91: 86WC-10Co-4Crcoated T91 specimen.

Studies in coal fired boilers. The uncoated as well as HVOF coated steel alloy specimens were examined in an actual boiler environment in the middle zone of platen superheater of the Stage-II boiler of Guru Nanak Dev Thermal Plant, Bathinda, Punjab (India). Nichrome wire was passed through a predrilled hole of 1 mm among all the samples, then these samples were inserted through the soot blower dummy points at 34.5-m height from the base of the boiler and these specimens were exposed for 10 cycles to combustion environment. Each cycle consisted of 100h heating, followed by1h cooling at ambient conditions. The temperature was measured at regular intervals and the average temperature during the study was observed to be about 900 [degrees]C with variation of[+ or -]10 [degrees]C. The specimens were visually observed for any change on surface and weight of specimens was measured subsequently at the end of each cycle.

The samples were examined by X-ray diffraction (XRD), Scanning Electron Microscopy/Energy Dispersive Spectroscopy (SEM/EDS) after the cyclic studies. The hot corrosion behaviour of bare and HVOF coated ASME SA213 T22 and ASME SA213 T91 in the given environment was assessed by measuring the thermogravimetric data, metal thickness loss corresponding to the corrosion scale development and the depth of internal corrosion attack after 1000 h exposure under cyclic conditions.

Results and Discussion

Visual examination of uncoated and HVOF coated T22 and T91. The macrographs for uncoated steel based alloy and HVOF sprayed 93(WC-Cr3C2)-7Ni and 86WC-10Co-4Cr coatings on T22 and T91 after 1000 h exposure to the superheater zone of the coal fired boiler are depicted in Fig. 1.

Weight change measurements. Weight change measurements of bare, 93(WC-[Cr.sub.3][C.sub.2])-7Ni coated and 86WC-10CO-4Cr coated of both ASME SA213 T22 and ASME SA213 T91 subjected to cyclic oxidation for 10 cycles at 900 [degrees]C are noticed. Weight gained by 86WC-10CO-4Cr coated T22 was minimum and weight gained by bare T22 steel alloys was maximum. Figure 2 shows the graph between weight change per unit area and number of cycles for both the uncoated and HVOF coated (T22, T91) samples subjected to 1000 h cyclic exposure to low temperature superheater zone of coal fired boiler at 900 [degrees]C. Figure 3 shows the column graph for weight gained per unit area of uncoated and coated samples (where C1= 93(WC-[Cr.sub.3][C.sub.2])-7Ni, C2 = 86WC-10CO-4Cr).

X-ray diffraction. X-ray diffraction analysis patterns for both corroded uncoated and HVOF coated samples are shown in Fig. 4. Uncoated specimen after introduction to boiler temperature for 10 cycles indicated the presence of [Fe.sub.2][O.sub.3]. The 93(WC-Cr3C2)-7Ni coated T22 and T91 steel alloys show the existence of a-WC, y-NiCr and [beta]-[Cr.sub.3][C.sub.2] on their surface. In case of 86WC-10CO-4Cr coated T22 and T91 specimen, the surface scale indicated the occurrence of [alpha]-WC, [beta]-[W.sub.2]C.

SEM/EDS. In scanning electron microscopy the pictures show the existence of carbides and oxides. Additionally it has been proved by energy dispersive spectroscopy (EDS) by identifying the carbon and oxygen and also proved by X-ray diffraction (XRD). Scanning electron microscopy (SEM) images of both uncoated as well as coated ASME SA213 T22 and ASME SA213 T91 alloy steel specimens are shown in Fig. 5.


* All the coatings on both the steel alloys were uniform and dense with thickness of coatings was between 200-250 [micro]m.

* HVOF spraying process has been effectively employed for depositing 93(WC-[Cr.sub.3][C.sub.2])-7Ni and 86WC-10CO-4Cr coatings on steel alloys namely ASME SA213 T22 and ASME SA213 T91.

* All the coatings on both steel alloys namely ASME SA213 T22 and ASME SA213 T91 used in present studies have provided resistance to corrosion in coal fired boiler environment in superheater zone when exposed for 10 cycles at 900 [degrees]C and have shown the following order of resistance to corrosion.

86WC-10CO-4Q- coated T22 >93(WC-[Cr.sub.3][C.sub.2])-7Ni coated T91 > 93(WC-[Cr.sub.3][C.sub.2])-7Ni coated T22 > 86 WC-10CO-4G- coated T91.

* Among all uncoated and coated samples, the uncoated samples have shown least resistance to corrosion. But ASME SA213 T22 has shown minimum resistance to corrosion.

* The improved corrosion resistance of 86WC-10CO-4Cr coated on ASME SA213 T22 steel alloys may be due to the formation of thin band of oxides of nickel, chromium and carbides of tungsten.


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Varinder Pal Singh Sidhu (a), Khushdeep Goyal (a*) and Rakesh Goyal (b)

(a) Department of Mechanical Engineering, Punjabi University, Patiala, India

(b) Chitkara University, Rajpura, India

(*) Author for correspondence: E-mail:

(received February 10, 2017; revised May 23, 2018; accepted May 31, 2018)
Table 1. Chemical composition for ASME SA213 T22 boiler tube steel

T22          C          Si   Mn         S      P      Cr

Nominal (%)  0.05-0.15  0.5  0.30-0.60  0.025  0.025  1.90-2.60
Actual (%)   0.09       0.5  0.43       0.025  0.025  2.24

T22          Mo         Fe

Nominal (%)  0.87-1.13  Balance
Actual (%)   0.98       Balance

Table 2. Chemical composition for ASME SA213 T91 boiler tube steel

T91          C          Si         Mn         S     P     Cr

Nominal (%)  0.07-0.14  0.20-0.50  0.30-0.60  0.02  0.02  8.0-9.5
Actual (%)   0.11       0.33       0.41       0.02  0.02  8.8

T91          Mo         Ni   V          Al     Fe

Nominal (%)  0.85-1.05  0.4  0.18-0.25  0.015  Balance
Actual (%)   0.93       0.4  0.21       0.015  Balance

Table 3. Composition and particle size of coating powder (supplied by
MEC Jodhpur)

Coating powder    Composition (wt%)         Particle size

93(WC-Cr3C2)-7Ni  WC(73), Cr3C2(20), Ni(7)  -45+15
86WC-10Co-4Cr     WC(86), Co(10), Cr(4)     -45+15

Table 4. Spray parameters employed during HVOF spray process

Parameters            Values
Oxygen flow rate      250 LPM
Fuel (LPG) flow rate   60 LPM
Air flow rate         900 LPM
Spray distance        200 mm
Fuel pressure           6 kg/[cm.sup.2]
Oxygen pressure         8 kg/[cm.sup.2]
Air pressure            6 kg/[cm.sup.2]
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Title Annotation:Special Paper
Author:Sidhu, Varinder Pal Singh; Goyal, Khushdeep; Goyal, Rakesh
Publication:Pakistan Journal of Scientific and Industrial Research Series A: Physical Sciences
Article Type:Report
Date:Sep 1, 2018
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