HOT CORROSION PERFORMANCE OF ALLOY AND COMPOSITE COATINGS

International Journal of Advanced Mechatronics and Robotics (IJAMR)Vol. 3, No. 2, July-December 2011; pp. 85-95; © International Science Press, ISSN: 0975-6108

Hot-corrosion Resistance of Alloy andComposite Coatings: A Review

Harkulvinder Singh1*, Sukhpal Singh Chatha1,Hazoor Singh Sidhu1 & Kovid Sharma1

1Yadavindra College of Engineering, Punjabi University Guru Kashi Campus,Talwandi Sabo, Punjab, India-151302, (*Corresponding Author: [email protected])

(E-mail: [email protected]), ([email protected]), ([email protected])

ABSTRACT

Hot corrosion is a serious problem in boilers, gas turbines, internal combustion engines, and industrialwaste incinerators. It consumes the materials at an unpredictably rapid rate. Alloys used at hightemperature should possess good mechanical properties, corrosion and oxidation resistance. Thermalspray technology encompasses a group of coating processes that provide functional surfaces toprotect or improve the performance of a substrate or component. Many types and forms of materialscan be thermal sprayed, Which provide protection from corrosion, wear, and abrasion and for avariety of other applications. The use of protective coatings has been an answer to remedy the lackof high temperature surface stability of metals and alloys in harsh environments. This study is donewith the aim of putting together the performance capabilities and applications of alloy and compositecoatings.

Keywords: Hot corrosion, Alloys, Composites, Coating.

1. INTRODUCTION

Coatings and surface modification technologies allow the engineer to improve theperformance, extend the life, and enhance the appearance of materials used for engineeringcomponents. These technologies have been developed because the interactions ofmanufactured components with other components, liquid, and/or gaseous environmentscan result in component degradation and failure (Davis J. R., 2004). Metals and alloyssometimes experience accelerated oxidation when their surfaces are covered with a thinfilm of fused salt in an oxidizing gas atmosphere at elevated temperatures. This is known ashigh temperature or ‘hot’ corrosion where a porous nonprotective oxide scale is formed atthe surfaces and sulphides in the substrate (Singh H et al, 2007).

Hot corrosion is an accelerated form of oxidation that occurs at higher temperature inthe presence of salt contaminants such as Na

2SO

4, NaCl,V

2O

5 that combine to form molten

deposits, which damage the protective oxide layer (N. eliaz et al, 2002). Hot corrosion is aserious problem in boilers, gas turbines, internal combustion engines, and industrial waste

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86 International Journal of Advanced Mechatronics and Robotics

incinerators. As a consequence the load-carrying abilities of the components are reduced(Sidhu T. S. et al, 2006). In combustion products of fuel oil, sulfur is typically present asNa

2SO

4, which occurs when the metals are heated in the temperature range of 700-900ºC,

in the presence of sulfate deposits. Vanadium as an impurity in fuel oil causes seriouscorrosion problems because of the formation of V

2O

5 (Ismail and Anees, 2004). Protective

coatings on superalloys encounter two types of high temperature corrosion degradation i.eHigh temperature hot corrosion (HTHC) and Low temperature hot corrosion (LTHC). Hightemperature hot corrosion (HTHC) also designated as Type 1, occurs at temperatures in the800 to 950ºC range. It is caused by molten salt deposition on the coating surface. Theprimarily active constituent of this salt is sodium sulphate Na

2SO

4. Low temperature hot

corrosion (LTHC), also known as Type 2, occurs in the 650-750ºC range. The low temperaturehot corrosion mechanism involves acidic fluxing of protective oxides by sulphur trioxide(SO

3) dissolved in molten sulphates (Bala N. et al, 2010).

1.1. Coating

Hot components of gas turbines and energy systems operating in aggressive environmentsare subjected to a number of modes of attacks which include oxidation, sulphidising,carburizing, chlorination, erosion and hot corrosion induced by molten salts. The use ofprotective coatings has been an answer to remedy the lack of high temperature surfacestability of metals and alloys in harsh environments (Bhatia R. et al, 2010). Thermal spraycoatings can be applied to overcome the steam oxidation problem since it alters the surfacewithout affecting the bulk material properties. Thermal spray coating of FeAl, FeCrAl andNiAl powders on 9Cr-1Mo steel was attempted to improve the steam oxidation resistancefor boiler applications (Sundararajan T. et al, 2003).

Most high temperature alloys are iron, nickel or cobalt-base because these elementshave high melting point. Unfortunately, their oxides are not protective enough in thecombustion environment of a fossil-fuel power plant at temperatures above 550°C.Nevertheless, addition of other elements to establish more protective oxides such as Cr

2O

3,

Al2O

3, or SiO

2has improved their corrosion resistance (Calderon J. P. et al, 1997). Overlay

coatings include a family of corrosion resistant alloys specially designed for high temperaturesurface protection. They referred to as M-Cr-Al-Y coatings, where m is alloy base metal(Ni, Co or combination of two). The high resistance of high chromium, Nickle chromiumalloys to high temperature oxidation and corrosion makes them widely used as thermallysprayed coating in fossil fuel-fired boilers, waste incineration boilers and electric furnaces(Parkash S., et al, 2005). Cr and Al are added in Fe and Ni-based superalloys to enhance theoxidation resistance (Chawla V., et al, 2009). Cr

3C

2-NiCr and NiCrBSi coatings obtained

by thermal spraying show good tribological properties in severe working conditions suchas at high temperatures or in aggressive environment applications. These coatings maintaintheir high wear and corrosion resistance up to 1253 K and are used to improve theperformance life of the components working at elevated temperatures (Sidhu T. S. et al,2007). The corrosion resistance of the Ni-based coating founded higher than that of the

Hot-corrosion Resistance of Alloy and Composite Coatings: A Review 87

stainless steel substrate material due to the passive film forming effect of Cr. The hot corrosionbehavior of NiCrBSi coatings resistance imparted by NiCrBSi coatings may be attributedto the formation of oxides of silicon, chromium, nickel and spinels of nickel and chromiumin the molten salt environment at high temperature. (Karagöz M. et al, 2011).

1.2. Thermal Spray Process

Thermal spraying is a group of processes in which metallic materials are deposited in amolten or semi molten conditions to form a coating (Lal et al., 2010). Thermal sprayinggun produced heat using combustion of gases or electric arc. As the material is heated theychange to plastic or semi-molten state and accelerated towards the substrate. A confinedstream of particles strikes on substrate, flattens and forms thin splats that conform andadhere to irregularities of prepared surface and to each other. As the sprayed particlesimpinge upon substrate they cooled down and produce lamellar microstructure as shown inFigure 1 (AWS, 1997).

Figure 1: Basic Principle of Thermal Spray Coating & Coating Microstructure(Steffens H.D and Davis J.R, 1996, 2004).


Alloy and composite Coatings can be deposited by electric arc spray, physical vapourdeposition, detonation spraying, flame spray, vacuum plasma spray, low pressure plasmaspray, high velocity oxy fuel by sputtering or by evaporation. Coatings serve more than onefunctions For high temperature oxidation resistance, a coating should (a) resist oxygen andmetal ion diffusion, (b) have a low vapour pressure at the operating temperature, (c) have amelting point above the operating temperature, (d) have low reactivity with the substrate,and (e) have low reactivity with the high temperature environment (Datta S et al, 2005).

2. STUDIES ON COATINGS

Villafane et al, 1997 observed that when the specimens of SA213 grade T-22, SA213austenitic grade TP347H were coated with actual ash collected from the super heater of apower plant and passed through a simulated flue gas (oxygen and 0.25 v/o SO

2+ 3.6 v/o O

2,

balance N2). The results of x rays diffraction shows the corrosiveness of the deposits varied

with the atmosphere. It is also observed that the TP347H austenitic stainless steel had thebest performance, as expected, and the T22 ferritic steel had the worst when coated withsilicon or chromium.

Calderón F. A. et al, 2011. deposited metallic coating ( NiCrFeNbMoTiAl) on SA213-T22 alloy steel substrates with thermal projection with non-transferred arc plasma(APS) process. These coated test pieces were subject to corrosive attack in saline mixturesof 80wt. %V

2O

5– 20wt. % Na

2SO

4 and 80wt.% V

2O

5 – 20wt. % K

2SO

4 at a temperature of

700ºC. The results of linear polarization resistance (LPR), electrochemical techniques, andelectrochemical impedance spectroscopy (EIS) reveals that corrosive attack became moresevere when exposed to corrosion by molten salts in a mixture composed by 80%V

2O

5–

20%Na2SO

4decreasing the thickness of the sprayed layer in a larger proportion than when

exposed to 80% V2O

5 – 20% K

2SO

4. Calderon J. P. et al, 1997 used plain carbon steel with

bond coat of Ni20Cr and uncoated 304 stainless steel as substrates and Fe75Si alloy coatingdeposited on bond coat . The substrates were installed in the high-temperature, fireside,corrosion zone of a steam generator for 13 months. Boiler burned heavy fuel oil with highcontents of vanadium. The results of SEM/EDAX shows that the uncoated 304 stainlesssteel were destroyed almost completely by corrosion, whereas the carbon steel coated didnot suffer a significant attack, showing that the performance of the thermal spray coatingwas outstanding and that the coating was not attacked by vanadium salts of the molten slag.Cr

3C

2-NiCr coating was deposited on SAE-347H boiler steel by HVOF spray process and

investigated at 700ºC for 50 cycles in Na2SO

4-Fe

2(SO

4) molten salt, as well as air

environments by Kaur M et al, 2009. The results of HVOF spray Cr3C

2-NiCr coating was

found to be successful in maintaining its adherence in both the environments. The formationof chromium rich oxide scale might have contributed for the better hot corrosion/oxidationresistance in the coated steel.

Parkash S. et al, 2005. deposited NiCrAlY, Ni20Cr, Ni3Al and Stellite metallic coatings

with plasma spray technology on Ni based Superni-718 substrate. NiCrAlY was used asbond coat in all coatings. Hot corrosion studies were performed on 900ÚC under cyclic


conditions. The results of SEM/EDAX and EPMA techniques reveals that NiCrAlY providesbest protection to the base alloy due to oxides of Ni, Al and NiCr2O4 spinal in the scales.Hot corrosion resistance of Ni3Al coating is less as compared to NiCrAlY and Ni-20Cr butit performs better than the satellite-6 coating. Sidhu H. S. et al, 2010. formulated NiCr andStellite-6 coatings on ASTM-SA-210 Grade A1, ASTM-SA213 T-11 and ASTM-SA213-T-22 boiler tube steels by HVOF technique using LPG as fuel gas. These coatings have beenexamined for characterization by metallography, SEM/EDAX and XRD techniques fordescribe the transformations that take place during HVOF spraying. The results of Stellite-6 coating were better than NiCr coatings for low value of porosity and surface roughness.Microhardness of the Stellite-6 coating has higher hardness as compared to the NiCr coating,although both coatings have high hardness values compared to the substrate steels. SidhuB.S. et al, 2005. were used Low carbon steel ASTM-SA210-Grade-A1, ASTMSA213T-11, and 2 ASTM-SA213-T- 22 steels as substrate material and Ni-20Cr-10Al-1Y bondcoat applied on these specimens and Ni-20Cr coating deposited by plasma spraying. Thesamples were exposed to the combustion gases for atotal of 10 cycles, each consisting of a100 h exposure followed by 1 h cooling at ambient conditions. The possible mechanism ofattack for this coating is shown in Figure 2. XRD,SEM/EDAX Analysis shows top-mostscale is rich in nickel, and then a chromium rich layer lying just above the bond coat isfollowed by an intermediate layer where nickel and chromium are coexisting, which confirmsthe formation of spinel. The continuous thin band of chromium lying at the bond coat-substrate interface blocks the transport of species to the substrate, which contributes to theprotection of the base steels.

Figure 2: Mechanism of Attack of Oxidation (Sidhu B.S. et al, 2005).


The uncoated T22 steel showed better resistance to hot corrosion in boiler environmentas compared with T11 and GrA1 bare steels. All the plasma sprayed steels have shownbetter degradation resistance than uncoated steels. Mohsen et al, 2007. InvestigatedNi based inconel 738 material and Amdry962 (Ni-22Cr-10Al-1Y) as bond coat andMetco204YSZ (ZrO

2-8%Y

2O

3) coating that deposited by plasma spray technique on the

substrate. Hot corrosion studies were performed on 1100ÚC under cyclic conditions for22, 24 and 100 hour‘s. The results of SEM/EDAX and EPMA techniques reveal thathorizontal cracks as shown in Figure 3 and spallation of YSZ layer are due to formation ofmonoclinic ZrO

2 and YVO

4 crystals.

Figure 3: SEM Image Showing the Cross-section of the Coating after Hot Corrosion Test(Mohsen et al, 2007).

Bala N et al, 2009 examined SA 516 (Grade 70) uncoated and SA 516 (Grade 70) steelsNi-20Cr coated by Cold spray process .Cyclic corrosion was performed in molten salt(Na

2SO

4-60%V

2O

5) at 900ÚC for 50 cycles. The results of XRD,SEM/EDAX shows cold

spray coating of Ni-20Cr alloy powder was found to be useful in developing hot corrosionresistance in SA-516 steel in Na

2SO

4-60%V

2O

5environment at 900ÚC due to Ni and Cr in

its oxide scale, which are reported to be protective oxides. The uncoated steel showedsubstantial spallation of its oxide scale during hot corrosion. The Ni-20Cr coating wasfound to be successful in retaining its continuous surface contact with the substrate steel.Ramesh M.R et al, 2010. examined HVOF process used to deposit NiCrFeSiB alloy powderon boiler tube steels SA210 grade-A1, SA213-T11, and SA213-T22. Thermocyclic oxidationtest were performed in static air at 900ºC in silicon carbide tube furnace up to 50 cycles.The results of SEM/EDAX and EPMA techniques reveal that the microstructure of coatingshas a dense and layered structure with porosity less than 0.5%. The superior performanceof NiCrFeSiB coating can be attributed to continuous and protective thin oxide scale of


amorphous SiO2 and Cr

2O

3 formed on the surface of the oxidized coatings. Sundararajan

T. et al, 2004. Investigated 80Ni-20Cr and 50Ni-50Cr coatings that deposited by HVOF(High Velocity Oxy Fuel) process and APS (Air plasma spray) on 9Cr-1Mo steel substraterespectively. Steam oxidation test was carried out at 650°C for 100, 1000 and 3000 hours.These coatings have been examined by metallography, SEM/EDAX and XRD techniquesthat show HVOF coatings of both 80Ni-20Cr and 50Ni-50Cr yielded a good protectiontill 750°C by forming Cr oxide as protective layer as compared to APS. AalamialeaghaM.E. et al, 2003. Reveals that when Ni-20%Cr alloy gas and water atomized powderssprayed BY Topgun HVOF with a gaseous propylene fuel and Met-Jet II HVOF systemwith liquid fuel (kerosene) on mild steel substrates. The oxide, porosity and the amountof melted material in the coatings were characterised using scanning electron microscopy(SEM) and X-ray diffraction (XRD), whilst the corrosion resistance of the coatings wasevaluated by use of a salt spray chamber and potentiodynamic tests. The results observedthat greatest corrosion protection to the steel substrate is given by coatings producedfrom gas atomized Ni-20%Cr powders when sprayed by the liquid fuelled Met Jet IIHVOF system Sidhu H.S et al, 2006 examined Cr

3C

2-NiCr, NiCr, WC-Co and Stellite-6

alloy coatings that were sprayed on ASTM SA213-T11 steel specimens using the HVOFprocess, liquid petroleum gas was used as the fuel gas. Hot corrosion testing was done onthe specimens after exposure to molten salt at 900°C under cyclic conditions. The testingtechniques shows that NiCr Coating to be most protective followed by the Cr

3C

2-NiCr

coating. WC-Co coating was least effective to protect the substrate steel. In this study itis concluded that the formation of Cr

2O

3, NiO, NiCr

2O

4, and CoO in the coatings may

contribute to the development of a better hot-corrosion resistance. Sidhu H.S et al, 2006Investigated The boiler tube steel, ASTM-SA210 grade A1 as substrate and Cr

2O

3–NiCr,

WC–12Co and stellite-6 alloy powder and Ni–20Cr wire coating deposited by HVOFprocess with oxygen and LPG as the fuel gases. Cyclic oxidation was performed in moltensalt (Na

2SO

4–60% V

2O

5) for 50 cycles, The results of XRD, EDAX and EPMA analysis

shows the porosity of NiCr coating lies in the range of 1–3.5% that provided highestresistance to hot corrosion.

Sidhu H.S et al, 2006 used Low carbon steel ASTM-SA210 grade A1, 1Cr–0.5Mosteel ASTM-SA213-(T11) and 2.25Cr–1Mo steel ASTM-SA213-(T22) as substrate andWC–12%Co, Cr

3C

2–25% NiCr powder coating were deposited by HVOF thermal spraying

process with LPG as fuel gas in the thickness range of 350–380µm.Characterization weredone in order to achieve microstructure of coatings. SEM/EDAX and XRD techniquesshow that WC–Co coatings have slightly higher hardness and porosity as compared to theCr

3C

2-NiCr coatings that is desired for hot corrosion. Kamal S. et al, 2010 formulated

NiCrAlY + 0.4 wt.% CeO2 coatings with D-gun process on Nickel- and Iron based

superalloys superni 75, superni 718, and superfer 800H specimens .Hot corrosion studieswere performed in a molten salt (40% Na

2SO

4-60% V

2O

5) for 100 cycles at 900ÚC under

cyclic conditions. The proposed hot corrosion mechanism of the NiCrAlY + 0.4 wt.%CeO2

coated superalloy superfer shown in Figure 4.


The SEM/EDAX, XRD results reveals that bare and coated Fe-based superfer 800Hsuperalloy showed least and highest resistance to the hot corrosion, respectively. D-gun-sprayed NiCrAlY + 0.4 wt.% CeO

2 coating found to be effective in imparting hot corrosion

resistance to superfer 800H in the molten salt environment Pint B. A. et al, 2001 investigatedFe-9Cr-1Mo (ferritic alloy),and an austenitic stainless steel, 304L as substrate material andiron-aluminide coating were deposited by Chemical vapor deposited (CVD). Hightemperature cyclic oxidation testing in air with 10 ± 0.5vol.%H

2O was done in an automated

test rig with a cycle consisting of 1h at 800°C temperature and 10min cooling at roomtemperature. The XRD, SEM/EDAX results show that CVD aluminized coatings haveexcellent resistance to environments containing water vapor but do not appear to havesufficient Al for long-term resistance to low oxygen, high sulfur environments. The highcoefficient of thermal expansion of 304L may be more compatible with Fe

3Al coatings

while the ferritic substrates should avoid the intermetallic phase formation Lasota B. S.et al, 2005 examined composite coatings FeAl- FexAly strengthened by a fine dispersiveAl

2O

3 were thermally sprayed by HVOF process. The cyclic corrosion behavior of coatings

with FeAl intermetallic matrix was investigated in N2 + 9% O

2 + 0,2% HCl +0,08% SO

2

aggressive gases at 600ÚC for exposure times of up to 500 hours. Model of the corrosionmechanism of intermetallic coatings in aggressive gas.

Mixture is shown in Figure 5.

One portion of intermetallic coatings was sealed with an inorganic phosphate seal. Theresults of SEM/EDAX, XRD show that an adherent alumina scale and iron oxide layerswere formed on all studied coatings. A stable Al

2O

3 phase on the surface of the studied

Figure 4: Schematic Diagram Showing Proposed Hot Corrosion Mechanism of the NiCrAlY + 0.4wt.%CeO

2 Coated Superalloy Superfer 800H at 900 ÚC in Na

2SO

4 + 60% V

2O

5 after 100 Cycles

(Kamal S. et al, 2010).


coatings ensure high oxidation resistance. All the results confirm good heat proofness ofHVOF sprayed coatings with an intermetallic FeAl matrix.

3. CONCLUSION

Degradation of metals is a severe problem in the industrial applications. It is not possible forsingle material to have different properties to meet the demand of today‘s industries. So alloyand composite materials is required to provide the necessary mechanical properties andprotective surface layer which immune the substrate surface from hot corrosion, wear anderosion also. The formation of chromium rich oxide scale might have contributed for thebetter hot corrosion/oxidation resistance in the coated steel. NiCrAlY provides best protectionto the base alloy due to oxides of Ni , Al and NiCr

2O

4 spinal in the scales. Alloy and composite

coatings has high hardness and low value of porosity as compared to substrate steel that isdesirable in hot corrosion resistance. In order to improve the corrosion resistance of coatings,post-treatments are often used to eliminate the inherent defects in the ceramic coatings, suchas liquid metal impregnation, Sealing treatment .sealing is to close or fill open structuraldefects connecting to the surface and make the sealants penetrate into the coating as deep aspossible. Sealing treatment is an effective method to improve the corrosion resistance ofporous materials. Therefore high temperature oxidation and hot corrosion behaviour of alloyand composite coatings need to be further investigated to understand the behaviour andperformance of these coatings at higher temperature in different corrosive environment.

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HOT CORROSION PERFORMANCE OF ALLOY AND COMPOSITE COATINGS