CHIIITARAUNIVERSITY

Punj ab C ampu s : Chandigarh-P attala National HighwayTehsil Rajpura, Distt .Patiala-l 404A1 GNDIA)Phone No.:+9 1.1762-50708 4, 507086 Fax No.: +9 L.L762-507085

Administrative Office: Saraswati Kendra,

SCO 160-161, Sectot 9 C,Chandigarh 160 009, India'

Tel: +91 .172.4090900

www.chitkara.edu.in

rsBN 978-8 | -92A9 13 -8-9

L

Hot-Corrosion Performance of High-VelocityO*y-Fuel Sprayed Coatings: A Revtew

Harkulvinder SinghI*, Sukhpal Singh Chathaz,Hazoor Singh Sidhu3'Bhai Gurdas Polyechnic college ,Sangrur, Punjab, India-14001, (Email: bgpcsangrur2007@gmail.com )2Yadavindra College of Engineering, Punjabi University Guru Kashi Campus, Talwandi Sabo, Punjab, India-l51302

3YadavindraCoIlegeofEngineerin*,,,,!lili$,ffis,TalwandiSabo,Punjab,India-l5l302(Email: hazoors@vahoo.com), *Corresponding author email: bqpcsanqrur200T@).smail.com

Abstrsct- Hot corrosion is a serious problem in boilers,gas turbines, internal combustion engines, andindustrial waste incinerators. It consumes thematerials at an unpredictably rapid rate. The use ofprotective coatings has been an answer to remedy thelack of high temperature surface stability of metalsand alloys in harsh environments. Coating can bedeposited by electric arc spray, physical vapourdeposition, detonation spraying, llame spray, vacuumplasma spray, low pressure plasma spray, high velocityoxy fuel by sputtering or by evaporation. High-velocityoxy-fuel GfVOF) spraying is a new and rapidlydeveloping technology in combating high-temperaturecorrosion. HVOF coatings have very low porosity, highhardness, high abrasive resistance, good wearresistance with a strong ability to resist high-temperature corrosion resistance. This study is donewith the aim of putting together the performancecapabilities and applications of HVOF process.

KeTwords -Hot corrosion, HVOF, Coatings, Thermalspray

I. INrnooucrroNThe corrosion of materials causes great loss in the

industrial applications, especially under some extremeconditions, the corrosive atmosphere and high temperature( Teng and Dian, 2009). Hot corrosion is an acceleratedform ofoxidation that occurs at higher temperature in thepresence of salt contaminants such as Na2SOa. NaCl, V2O5that combine to form molten deposits, which damage theprotective oxide layer ( N.eliaz et aL,2002). Hot corrosionis a serious problem in boilers, gas turbines, internalcombustion engines, and industrial waste incinerators. Asa consequence the load-carrying abilities of thecomponents are reduced (T.S.Sidhu et al, 2006). Incombustion products of fuel oil, sulfur is typically presentas Na2SOa, which occurs when the metals are heated in thetemperature range of700-900oC, in the presence ofsulfatedeposits. Vanadium as an impurity in fuel oil causesserious corrosion problems because of the formation ofV2O5[smail and Anees, 2004). It is now currentlyaccepted that protective coatings on superalloys encountertwo types of high temperature corrosion degradation i.eHigh temperature hot corrosion (HTHC) and LcA^/

temperature hot corrosion (LTHC). High temperature hotcorrosion (HTHC) also designated as Type l, occurs attemperatures in the 800 to 950oC range. It is caused bymolten r salt deposition on the coating surface. Theprimarily active constituent of this salt is sodium sulphateNa2SOa. Low temperature hot corrosion (LTHC), alsoknown as Type 2 , occurs in the 650-750" C range. Thelow temperature hot corrosion mechanism involves acidicfluxing of protective oxides by sulphur trioxide (SO3)dissolved in molten sulphates (N bala, 2010).

il. NEED oF COATINGSThe development of coating technology stems from, and

is determined by, the progress of knowledge on hightemperature corrosion. Hot components of gas turbinesand energy systems operating in aggressive environmentsare subjected to a number of modes of attacks termed ashigh temperature corrosion, which include oxidation,sulphidising, carburizing, chlorination, erosion and hotcorrosion induced by molten salts. The use of protectivecoatings has been an answer to remedy the lack of hightemperature surface stability of metals and alloys in harshenvironments. Coating provides a way of extending thelimit of use of materials at the upper end of theirperformance capabilities by allowing the mechanicalproperties of substrate materials to be maintained whileprotecting them against the wear and corrosion. Coatingcan be deposited by electric arc spray, physical vapourdeposition, detonation spraylng, flame spray, vacuumplasma spray, low pressure plasma spray, high velocityoxy fuel, by sputtering or by evaporation ( R Bhatia et al,20r0).

IIL HTGH vEl,ocrry oxy FUEL (HVOF) rnocrssHigh-velocity oxy-fuel (HVOF) spraying is a new and

rapidly developing technology in combating high-temperature corrosion. The hypersonic velocity of theflame shortens the time of interaction between the powderand flame, whereas low temperature of flame limits thegrain growth and decomposition of coating. Due to thehigh impact velocity of particles the coatings show a highadhesive strength, high cohesive strength of individual

- splats, uniform microstructure, high density and lowporosity with a strong ability to resist high-temperaturecorrosion resistance (H.Sidhu et al, 2005). This spraying

lnternational Conference on Advances in Materials and Manufacturing Technology-z}Ll82

\system enables metals and alloys with high melting pointup to about 2000'C to be deposited on the target surface.These features are suitable for a deposition of corrosionresistant coatings (T.Sidhu et a1,2005A).In the HVOFprocess the powder/wire material is melted and propelledat a high velocity toward the surface with the use ofoxygen and fuel gas mixtures as shown in fig.I. The mostimportant parameter regarding the coating quality ispowder particle velocity, which

ExpoB*eff

Ols6{dtnoorrtffi

?Bc!s s,S rsme !6a

Fig-l Schematic diagram, of High ffiO

o*, Ur"l process ( V. hotea et

ranges from l00m/s for powder combustion spray to1000m/s for HVOF spraying. HVOF process of thermalspray have particle high speed that produce low porositycoatings , oxides ,better bond strength and hardness ascompared to its other counterparts as shown in fig.2 (GR.heath et al , 2008 ).

Fig-2 Characteristics ofthe HVOF and standard

ir*r.FhllrlPl.sntt,npsd8G fcl lf-0mt&$00

0s0 los08mSrrPlr0chy.turyldrl ffi

Fig-3 Comparison of various processes (Dorftnan M., 2002) plasma-process coatings: (I) hardness,

(ID porcsity (IID oxide content, (tV) bond strength(V) maximum thickness. (T.Sidhu et a1,20058)

Fig-3 shows the velocity and temperature comparison ofHVOF process with its other counterparts.The highresistance of high-chromium, nickel-chromium alloys tohigh-temperature oxidation and corrosion allows them tobe widely used as welded and thermally sprayed coatingsin fossil fuel-fired boilers, waste incineration boilers, and

, electric furnaces. The HVOF process is often used todeposit high-chromium, nickel-chromium coatings ontothe outer surfaces ofvarious parts ofboilers to prevent thepenetration of hot gases, molten ash, and liquids to lessnoble carbon steel boiler tubes (T.Sidhu et al, 20058) .TheHVOF coatings have higher bond strengths than for theplasma-spray coatings by 25%. The better adhesionstrength of the HVOF coatings is attributed to the bettermechanical interlocking of the sprayed droplets with thesubstrate due to the high kinetic energy experienced by theimpinging particles. Hence, the HVOF coatings canperform very well in corrosive environments as comparedto other processes (T.Sidhu et aI,2005C).

IV. HVOF CoATD,TGSCr3C2-NiCr coating was deposited on SAE-347H boiler

steel by HVOF spray process and investigated at 700'Cfor 50 cycles in Na2SOa-Fe2(SOa) molten salt, as well asair environments. The HVOF spray Cr3C2-NiCr coatingwas found to be successful in maintaining its adherence inboth the environments. The surface oxide scales were alsofound to be intact. The formation of chromium rich oxidescale might haye contributed for the better hotcorrosion/oxidation resistance- in the coated steel (M. Kauret at,2009). The Fe-based superalloy Superfer 800H wasused as a subsfrate material and coating alloys Cr3C2-NiCrC alloy powders, namely as-atomized powder with aconventional coarse-grained structure and cryomilledpowder with a nanocrystalline structure, were employedon medium steel with HVAF process. Heat treatment wasconducted at 650 oC in air. Samples were removed after10, 30, 50, 100, 150 and 200 hr. Both the as-sprayedNiCrC coatings possessed a compact microstrucfure whichexhibited a more homogeneous morphology withuniformly distributed fine carbide dispersions and a muchhigher microhardness. The nanoskucture coating exhibitedexcellent thermal stability, whose average grain sizestabilized at about 100 nm after 50 h of exposure at650 "C (K.Tao et al, 2005).

NiCr and Stellite-6 coatings have been formulated onboiler tube steels namely ASTM-SA-2I0 Grade Al,ASTM-SA213-T-Il and ASTM-SA2l3-T-22 by HVOFtechnique using LPG as fuel gas. The results of Stellite-6coating were better than those of the NiCr coatings for lowvalue of porosity and surface roughness. Microhardnessmeasurement across the cross-section of coating showedthat the Stellite-6 coating has higher hardness as comparedto the NiCr coating, although both coatings have highhardness values compared to the substrate steels(H.S.sidhu et al, 2010). HVOF process was used to depositNi-based hardfacing NiCrFeSiB alloy powder on boiler

International Conference on Advances in Materials and Manufacturing Technology-2}Ll83

rube steels designated as SA210 grade-Al, SA2l3-Tll,and SMI3-T22. Thermocyclic oxidation test wereperformed in static air at 900oC in silicon carbide tubefurnace up to 50 cycles. The microstructure of coatingshas a dense and layered structure with porosity less than0.5%. The superior performance of NiCrFeSiB coatingcan be athibuted to continuous and protective thin oxidescale of amorphous SiOz and Cr2O3 formed on the surfaceof the oxidized coatings (M.R.Ramesh et al, 2010). Thecoatings of 80Ni-20Cr and 50Ni-50Cr are deposited byHVOF process and APS (Air plasma spray) on 9Cr-lMosteel substrate respectively. Steam oxidation test wascarried out at 650oC for 100, 1000 and 3000 hours. HVOFcoatings of both 80Ni-20Cr and 50Ni-50Cr yielded a goodprotection till 750'C by forming Cr oxide as protectivelayer as compared to APS (Sundararajan T et a1,2004).Ni-20%Cr alloy powders gas and water atomized weresprayed on mild steel substrates with Top gun HVOFsystem with a gaseous propylene fuel and Met-Jet IIHVOF system with liquid fuel (kerosene). The resultsobserved that geatest corrosion protection to the steelsubstrate is given by coatings produced from gas atomizedNi-20%Cr powders when sprayed by the liquid fuelledMet Jet II system. Met Jet II spray system producedcoatings with a smaller amount of oxide and less porosity(M.E.Aalamialeagha et al, 2003).

Cr3C2-NiCr, NiCr, WC-Co and Stellite-6 alloy coatingswere sprayed on ASTM SA213-TI I steel specimens usingthe HVOF process, liquid petroleum gas was used as thefuel gas. Hot corrosion testing was done on the specimensafter exposure to molten salt at 900oC under cyclicconditions. NiCr Coating was found to be most protectivefollowed by the Cr3C2-NiCr coating. WC-Co coating wasleast effective to protect the substrate steel. It is concludedthat the formation of Cr2O3, NiO, NiCr2Oa, and CoO in thecoatings may contribute to the development of a betterhot-corrosion resistance (H. S. Sidhu et al, 200 6).

The boiler tube steel, ASTM-SA2l0 grade A1 (GrAl)have been used as substrate and Cr2O3-NiCr, WC-12Coand stellite-6 alloy powder and Ni-20Cr wire coating isdone with HVOF spraylng operating with oxygen andLPG as the fuel gases. Cyclic oxidation was performed inmolten salt (Na2SOa,-60yo V2O5) for 50 cycles, Theresults of XRD, EDAX and EPMA analysis shows theporosity of NiCr coating lies in the range of l-3.5Yo thatprovided highest resistance to hot corrosion (H.S.Sidhu etal, 2006). Low carbon steel ASTM-SA2l0 grade Al(GrAl); lCr-O.SMo steel ASTM-SA2l3- T-l I (Tl l) and2.25Cr-lMo steel ASTM-SA2|3-T-22 (T22) have beenused as substrate and WC-l2YoCo , Cr3C2-25o/otliCrpowder coating were deposited by HVOF thermalspraying process with LPG as fuel gas in the thicknessrange of 350-380pm. It is observed that WC-Co coatingshas slightly higher hardness as compared to the Cr3C2-NiCr coatings and also found lower porosity as compared .to the Cr3C2-NiCr coating that is desired for hot corrosion(H.S.Sidhu et al, 2006).

Ni-20Cr coating was deposit by HVOF on ASTM-A213347H boiler steel specimens and the samples with andwithout coating were exposed to the super heater zone of athermal power plant boiler at a temperature of 973 K(700"C) under cyclic conditions to ascertain their erosion-corrosion (E-C) behavior. Fig.4 shows Schematic diagramillustrating the E-C mode for the HVOF sprayed Ni-20Crcoating after exposure to boiler environment.

Boilo{ Etl{irufle*t tlwrll 0" C)

Cr30l llprCrdeplodlrler

$i Chrlof

NisdCl rich ifinlrkv*r

S{1ilu voi&

Fig-4 Schematic diagram illustrating the E-C mode for the HVOFsprayed

Ni-20Cr coating after exposure to boiler environment (G.Kaushal et al, 2010). Examination of samples revealedthat there is an outermost layer which contains ashparticles such as Al2O3, SiO2, and Fe2O3 deposited fromthe boiler environment. Due to this, a thin chromia layer,being formed by outward diffusion of the chromium fromthe coating region and inward diffrrsion of O from theenvironment. The outward diffirsion of Cr is evident fromthe presence of a Cr-depleted inner layer just below thethin Cr-rich layer. The coating was found to havesignificant resistance to its oxide scale spallation duringcyclic oxidationexposures; moreover, the coating wasfound to have retained its continuous contact with thesubstrate steel during these thermal cycles. This indicatesthat the coating has good adhesion strength (G. Kaushal etal,20l0).

V. CoNcLUSToNl.Degradation of material in the form of corrosion ,erosion

and wear is a challenge problem faced by the industryinvolving in energy generation systems.

2. Thermal spray coating is an flexible and cost eflectivemethod of improving the life of materials against thedegradation of materials.

3. Among the different thermal spray techniques, Highvelocity oxy fuel process is better with regards tohardness, porosity , adherence strength , corrosionresistance and wear resistance ofthe coatings.

4. Hence HVOF process can be thought of engineeringsolution to enhance surface against wear & corrosiondegradation and other surface phenomena's.t"

.{i1

t7.)l{al

lnternational Conference on Advances in Materials and Manufacturing Technology-z}Ll84

tll

12)

t3l

t4l

t5I

t6l

t7l

t8l

RrpsneNcssAalamialeagha M. E., Harris S.J., Emamighomi M., 2003.lnfluence ofthe HVOF spraying process on the microstructure andcorrosion behavior of Ni-20% Cr coatings. J of materials science.38,4587

-4596.Bala N., Singh H., Prakash S., 2010. Accelerated hot corrosionstudies of cold spray Ni-50Cr coating on boiler steels,J. ofMaterials and Design. 31,244-253.Eliaz N., Shemesh G., latanision R.M.,20O2. Hot corro$ion in gasturbine components. J ofEngineering failure analysis. 9,31-43.Kaushal G., Singh H., Sprakash S., 2010. High-temperatureerosion-corrosion performance of high-velocity oxyfuel sprayedNi-20Cr coating in actual boiler environment. J of metallurgicaland materials transactions a.42Ramesh M.R., Prakash S., Nath S.K, Sapra P.K.,KrishnamurthyN., 2010. Evaluation of thermocyclic oxidationbehavior ofhvof-sprayed NiCrFeSiB coatings on boiler tube steels.J ofthermal spray technology. 20Sidhu T.S., Prakash S., Agrawal R.D., 2005. Performance of high-velocity oxyfuel sprayed coatings on an fe-based superalloy inNazSO+ -607oV2O5 enyironment at 900oC, part ii: hot corrosionbehavior of the coatings, J of materials engineering andperformance. I 5(l ), I 30.Sidhu H S., Sidhu B S., Parkash S., 2006. Characteristicpaxameters of HVOF sprayed NiCr and stellite-6 coatings on theboiler steels using LPG as fuel gas. J of engineering andinformation techn ology. 2, I 33-l 39.Tao K., Zhou X., Cui H., Zhang 1., 2009. Microhardnessvariation in heat-treated conventional and nanostructwes NiCrcoatings prepared by IfVAF spraytng surface & coatingstechnology. 203, I 406-1414.Sidhu T.S., Agrawal R.D., Prakash. S., 2005. Hot corrosion ofsome superalloys and role of high-velocity oxy-fuel spraycoatings. J of surface & coatings technol ogy. 198, 441* 446Sidhu H S., Sidhu B S., Parkash S., 2007. Hot corrosion behaviorof HVOF sprayed coatings on ASTM (SA2l3-Tl1) steel. J ofthermal spray technology volume . 16 (3), 349.Sidhu H S., Sidhu B S., Parkash S., 2006. The role of hvofcoatings in improving hot corrosion resistance of ASTM-SA2I0gral steel in the presence of NazSO+-VzO5 salt deposits. J ofsurface & coatings technology. 200, 5386

- 5394.

Sidhu H S., Sidhu B S., Parkash S., 2006. Mechanical andmicrostructural properties of HVOF sprayed WC{O and Cr;Cz-NiCr coatings on the boiler firbe steels using LPG as the fuel gas. Jof materials processing technology.lT l, 77

-82.Singh H., Puri D ., Prakash S., An overview of Na2SO4 -V2O5induced hot corrosion of Fe- and Ni-based superalloys. J ofmaterial science. 16,27-50.Sudararajan T., Haruyama H., Abe F.,Tsukubal., 2004. Effect ofcoating thickness and the sealant. J of thermal spray. P.no.442.

International Conference on Advances in Materials and Manufacturing Technology-z}L|85

teI

ll0l

llll

u2l

I l3]

u4)