D0I:10.13374/j.issm1001-053x.1989.06.027 第11卷第6期 北京科技大学学报 Vo1.11No,6 1989年11月 Journal of University of Science and Technology Beijing Nov,1989 Hot Corrosion of Plasma Sprayed Coatings for Waste Gas Turbine Blades' Liao Xiaoning(廖小宁)，Ye Ruizeng(叶锐曾)，Lu Fanxiu(吕反修)， Sun Jingui(孙金奇)，*率Zhang Naiping(张乃平)*率 ABSTRACT:High temperature corrosion tests in 75%Na2 SO,-25%.NaCl molten salt have been conducted on waspalloy specimens plasma sprayed with Co-Cr-B-Si,Ni-Cr-B-Si+Cra C2,X-40,X-40+TiC and Ni/Al+Al2O3 coatings, test coupons were exposed isothermally at 750C up to 200 h.After the cor- rosion tests,each sample was examined by OM,SEM and EMPA.It was found that X-40,X-40+TiC and Ni/Al+Al2Oa were violently corroded by the molten salt due to cracking and spalling of the coatings,whereas the Co-Cr-B-Si and Ni-Cr-B-Si+CraC2 exhibited superior corrosion resistance,which were thought to be related to the formation of chromium and silicon oxide film.A possible corrosion mechanism had been proposed and it is suggested that the excellent corrosion resistance and high hot-hardness of the two B-Si containing coatings may be of significance for applications in petroleum refining industry as protective coatings of power generating turbine blades which are used under severe corrosi- on/erosion conditions. KEY WORDS:hot corrosion,coating,turbine blade In waste-gas or coal energy conversion systems,exposed components may suffer from the attack of hot-corrosion and erosion,and to extend the lifetime of the components,coatings are widely used to protect the components against hot-corrosion and erosion.Failure analysis of X-40 coating applied to waste-gas turbine blades show that the coating is unable to protect the blades effectively due to its low hot-hardness and low high temperature corrosion resistance. Hot-corrosion has been studied widely ~It can occur by one or more of the following mechanisms suggested in the literature:(1)sulfidation-oxidation, Manuscript Received July 14,1989 ..Dept.of Material Science ,·。Wunan Institute of Metallurgy 551
第 卷第 期 。 年 月 北 京 科 技 大 学 学 报 ’ 、 气 ‘ 。 夕 廖 小 宁 ， 夕 叶 锐 曾 ， 二 吕反 修 ， 。 ‘ 夕 ‘ 孙 金贵 ， 夕 ‘夕 张 乃 平 、 。 肠 一 一 一 一 ， 一 一 一 ， 一 ， 一 ， · ， ， 一 ， 一 ， 一 一 一 一 一 一 ， 皿 扭 五 一 一 沮 丘 。 ， ， 、 、 一 ， 一 ， 父 ， 一 · 往 一 一 一 · 一 一 。 一 ， ， 二 … DOI ：10．13374／j ．issn1001－053x．1989．06．027
(2)formation of volatile compounds beneath the oxide film and (3)fluxing. Good corrosion resistance of coatings and alloys depend on the formation of protective oxide films consisting of Cr2O3 and/or Al2O3 on surface of coatings and alloys.Addition of silicon into coatings and alloys can increase their hot- corrosion and oxidation resistance t4,s1.The objective of this paper is to clarify the corrosion mehanisms involved in various plasma-sprayed coatings degradation. Tests on preoxidised sample were carried out to reveal more accurately the mechanisms of corrosion in certain molten sait.The results show that the main reason for sevcrely corroded X-10,X-40+TiC and Ni/Al+Al,O3 is their low density and being unable to form protective scale in low oxygen pressure environ- ment,whereas the formation of prtective scale on the surface of Co-Cr-B-Si and Ni-Cr-B-Si+Cr,C2 made them highly crrosion resistant,here silicon plays an important rle in increasing the corrosion resistance of the above mentioned coatings.Owing to its higa corrasion resistance and hot-hardaess,Co-Cr-B-Si can be expected to replace X-40 coating for applications on hot components in industrial gas turbines where both hot-corrosion and erosion exisls. 1 Experimental 1.1 Substrate and Coatings Five coalings were chosen for this study,their compositions are reported in Table l,the compositions of bond coating and substrate alloy (waspalloy)are also shown. Table 1 Chemical composition of coatings and alloys (wt%) Materials Co Cr Ni Al Others X-40 bal24~26 9.5~11.5- W,Si,C X-40+30%TiC bal24~26 9.511.5- W,Si,C,+30%TiC Co-Cr-B-Si bal28~30 8~12 一 W,Si,B,C Ni-Cr-B-Si +306Cr3C: 17~21 bal B,Si,C,+30%CraC2 NiAl+8026Al2O3 80 20 80%A1203 Bond coaling (Ni-Cr-Al) Ni-Cr-Al-Type Substrate (Waspalloy) 12~1518~21 bal 1,2~1,6 Ti,Mo,B,Zr,C X-ray diffraction analysis indicates that X-40 is of two phases structure whi- ch consists of Y-Co and Cr23Ce,while Co-Cr-B-Si is compused of y-Co aud carbides,silicides and borides.A few oxide particles exist iu the interfaces be- tween coaling/bond coating and substrate/bond coating as shown in Fig.1.These 552
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tv)iate:fa:;aa)tly the wea's point concerning corrosion resistance.The hot-hardness is reported in Table 2. 200u 200um Fig.1 Microstrcture of air-plasma sprayed Co-Cr-B-Si (a)and X-40 (b)coatings on waspalloy substrate Bond coatings can be seen in both case. Table 2.Hot-hardness of the coatings,Hv (kg/mm2) X-40 X-40+ Co-Cr-B-Si Ni-Cr-B-Si 30%TiC +30%Cr3C2 873K 305 443 622 471 973K 356 407 563 355 1073K 322 355 476 178 Room Temp. 347 471 824 543 1.2 Corrosion Test All the corrosion tests were carried out at 750C.Samples,20mm in diame- ter and 10mm in length were put into a porcelain crucible containing a mixture of 75%Na2SO,+25%NaCl molten salt.The samples were wholly immersed in the molten salt during the testing period (1,5,10,50,100 h)execept that for 200 h which were half immersed. After completion of the corrosion runs the samples were removed,the surface examined,and then were cut normal to the corroded surface into halves. Corrosion products were removed from one half of the samples for X-ray diffraction phase identification,the remaining half was mounted,sectioned normal to the long axis,and examined metallographically.The element distri- butions were determined by electron probe microanalysis. 553
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2 Results and Discussion Fig.2 shows the appearance of the samples tested in 75%NaaSO+25%NaCl molten salt for 100h at 750C.Violent attack was found to occur in X-40, X-40+TiC and Ni/AI+Al2Os coatings.The corrosion is characterized by the severe spalling of the coatings,which always happened firstly in the corner of b Fig.2 Macroscopic appearance of the samples after immersion in 75%Na2SO,+25%NaCl for 100 h at 1023K. a.X-40,b.X-40+30%TiC,c.Co-Cr-B-Si,d.Ni-Cr-B-Si+30%CraCz e.Ni/A1+80%Al2O3 the sample and then spread to other parts.Gross internal corrosion was found to occur in the substrate of the three coatings.In contrast,macroscopic appea- rance of Co-Cr-B-Si and Ni-Cr-B-Si+30%CraCa exhibited practically no sign of surface and internal corrosion depth of s penetration in coating/substrate systems after 100 h hot corrosion is shown in Table 3. Table 3.Depth of S penetration in coating/substrate systems after100hat750°C Coating X-40X-40+TiC CoCrBSi NiCrBSi+CrsC2 A13O2+Ni/AI Penetration 540 580 125 145 940 (4m) Penet.Depth in substrate 260 230 640 (um) Severe cracking and spalling of coatings after hot corrosion tests. Fig.3 and Fig.4 show respectively the micrographs of X-40 and X-40+Tic coatings tested for 15 h at 750C,where coatings had spalled along the bond coating,a large amount of sulfides had precipitated in the substrate alloy. Sulfides can be seen in both the coating and the substrate alloy,there well also many cracks in the coating normal to the deposition direction (Fig.5). As the corrosion continued,coating became porous and was covered by thick 554
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2004m 300m Fig.3 Optical mlcrograph of X-40 Fig.4 Optical micrograph of X- coating corroded for 15h in 40+TiC coating corroded molten salt for 15h in molten salt (1)spalled coatings (2)corroded (1)epoxy resin (2)X-40+Tic (3)crack substrate (3)unaffected substrate (4)corroded substrate (5)unaffected substrate Fig.5 Spalled X-40 coating after testing for 15h in molten sait at 1023K secondary electron image corrosion products,while corrosion of the substrate alloy became more and more violent charaterized by precipitation of sulfides and the formation of thick internal diffusion zone.A typical SEM photograph of a corroded layer in substrate is shown in Fig.6,which was developed near the interface between the internally corroded layer and the unaffected substrate.This Figure also shows that Cr element is uniformly and significantly depleted in the internally corroded layer. Co-Cr-B-Si and Ni-Cr-B-Si+CraC2 were found to be more corrosion resi- stent against the NaaSO.-NaCl molten salt.No spalling was detected during the corrosion period of 200h.Fig-7 shows the morphology and element distribution of Co-Cr-B-Si coating tested for 15h,a thin Cr-rich film had developed on the 555
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surface of the coaling.In addition,a thin scale of sulfide has formed beneath the Cr-rich scale,this layer is probably Si-sulfides because Si is also enriched there.Internal corrosion of substrate has not occured in this sample,it suggests that the surface oxide scale formed on Co-Cr-B-Si coating is very protective, Fig.8 shows the protective scale formed on the Co-Cr-B-Si coating after 100 h corrosion tesi. Bolh pre>xidized X-10 and Co-Cr-B-Si cxhibited good corrosion resistence in molten sait test.Preoxidation was carried out at 800C for 5 h.Only smail crachs were detected in the corner of the sample after wholly immersed in molten sait fr 200 h.Fig.9 shows the X-10 coating and unaffected substrate tested fur 40 h. The protection of c.balt base coatings and alloys against corrosion is usually provided by the formatin of a protective Cr2O3 and/or Al2Oa film.So the rale of formalion of a proteclive oxide film consisting of Cr2O3 will be very imparlant to the c>rrosion resistance of coatings and alloys,besides,the density of coatings is ais)crucial.The coatings in the present study were prepared by air plasma spraying.The por,sity is about 4-66,many defects may exist in the corner area of the samples and in the interface of both top-coating/bond coating and bund coating/substrate. When the samples were put into m,Iten salt,sulfur in the molten salt could penetrate quickly through these defects in the top coating and form sulfides. Fig.10 shows the sulfides formed in X-40 around coating/bond coating interface after only 1 h immersion in the molten salt.Formation of corrosion products results in a large volume expansion,thus internal stress in the coating increases with the cntinuous formation of sulfides and finally cause the coatings (X-40, X-40+TiC and Ni/Al+Al2O3)to spall and detach from the substrate.X-ray diffraction,taken on the corr,ded surface and the insoluble portion of corrosion products brushed from the corrded samples,revealed that CoCr2O,was develo- ped un botlt the surfaces of X-40 and Co-Cr-B-Si coatings while Cr2O3 could als)be f:und in Co-Cr-B-Si sample.Obser vations indicated that black and loose oxide scale w as formed on the surface of X-40 coating (see Fig-3)whereas compa- ratively dence and continuous oxide scale was developed on the surface of Co- Cr-B-Si cating as shown in Iig.7.This explains the phenomena observed in the molten salt tests.Protective scale (dense and continu us)can not form on the surface f X-10 c ating t)prevent the sulfur penctration,conscquently the coa- ting spalled by large volume expansion resulting from the formation of sulfides. On the cntrary,the prolective scalc formed on the surface of Co-Cr-B-Si reduces greatly the penetration of sulfur into the coating and provides the coating very good corrosion resistance in molten salt where the oxygen partial pressure 556
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SEM Fig.6 X-ray images obtained at the interface between the internally corroded layer and the unaffected substrate,X-40 coating had already spalled and are not shown here SEMI 28KUX688 17191080R3 Fig.7 X-ray images from Co-Cr-B-Si sample tested for 15 h at 1023K is very low.Here silicon is suposed to have beneficial effect of increasing coating's corrosion resistance.Test result indicated that silicon seems to have the effect of promoting the formation of oxide in low oxygen partial pressure environment.The Si-sulfides under oxide scale had avcided the formation of Ni or Cr-sulfides thus eliminated the detrimental effect of Ni-sulfides and the deple- tion of chromium by Cr-sulfides.Large amount of chromium was consumed by Cr-sulfides in X-40 coating during immersion in molten salt as shown in Fig.6. 557
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scale 28K20837 OU PMRIS Fig.8 SEM photograph of Co-Cr-B -Si coating tested for 100 h at 1023K 001 200m 103 Fig.9 Preoxidized X-40 coating Fig.10 Optical micrograph of X-40 tested for 40 h at 1023K coating corroded only Th In molten salt To conclude,whether the protective oxide scale can form is the key to corrosion resistance of coatings in molten salt,while the density is also of vital importance,so excellent corrosion resistance can be expected in preoxidized coa- tings.The protective oxide film formed in preoxidation process provided both X-40 and Co-Cr-B-Si coating with high corrosion resistance against molten salt. Fig-10 shows an example of preoxidized X-40 coating where no severe corrosion can be detected after 40 h immersion in the molten salt. 3 Conclusions The following significant results have been obtained concerning the attack of coatings by NaaSO,-NaCl molten salt: (1)The X-40,X-40+TiC and Ni/Al+Al2Os show poor corrosion resistance in the molten salt test,being unable to develop quickly a protective scale in low oxygen partial pressure environment is the main reason for their poor performance 558
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wnile low density of these coatings is another important factor,especially for Ni/Al+Al2Os coating.Preoxidation and advanced spray technology (eg.LPPS) would greatly increase the corrosion resistance of these coatings in molten salt by forming a protective oxide film before test and decreasing coating's porosity. (2)After spaliation of the coating,the substrate alloy (waspalloy)was attacked violently.This indicates that the Ni-base waspalloy has low corrosion resistance against molten salt. (3)Co-Cr-B-Si and Ni-Cr-B-Si+Cr3Ca (30%)owe their high corrosion resistance in molten salt to the formation of protective scale on the surface of the coatings.The excellent corrosion resistance and high hot-hardness (see Table 2)of the two B-containing coatings may be of significance for applications in petroleum refining industry as protective coatings of power generating turbine blades using waste-gas where both severe corrosion and erosion are encountered. (4)Silicon seems to have effect of promoting the formation of protective oxide scale in low oxygen pressure environment.Formation of Si-sulfides under oxide scale avoided the formation of Cr or Ni-sulfides,thus the depletion of chromium by Cr-sulfides and the detrimental effect of Ni-sulfides can be avoided. REFERENCES 1 Gebel J A,Petit F S,Goward G W.Met.Trans.,1973;(4):261 2 Luthra K L.Low Temperature Hot Corrosion of Cobalt Base Alloys: Part I Morphology of the Reaction Products;Part II Reaction Mechnisms, Metall.Trans.A.Vol.13A 1989;1943 3 Nagarajan V,et al.Corrosion Science,1982;22(5):407 4 Bauer R.Thin Solid Films,1982;95:3 5 Gupta D K,Duvall D S.Superalloys Proc.,1984 559
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