HIGH TEMEPARATURE CORROSION BEHAVIOUR OF THERMALLY SPRAYED NANOCOATINGS

Abstract

Nanotechnology is the new and continuously growing field which has influenced various fields of science, engineering and technology. The effect of nanosized grains on the properties of various engineering materials (metals and ceramics) has been of great interest as the properties of these materials vary dramatically as their grain size changes to nano-range. The nanopowders can be used as feedstock powder during deposition of nanocoatings by different thermal spray techniques. Plasma spraying (air plasma Spray, vacuum plasma spray) or high velocity oxy fuel (HVOF) techniques have been used to deposit the nanopowders, which produce nanostructured coatings. Despite of various advancements in thermal spray techniques, the oldest oxy-acetylene flame spraying or low velocity oxy-fuel (LVOF) process is still used in industries. Flame spraying requires very simple equipment and can be readily performed in the factory or on site. The process is fairly inexpensive and is used for the application of wide variety of materials. There are few studies in literature on the oxidation and hot corrosion behavior of the thermally sprayed nanocrystalline (NC) coatings. The mechanisms of oxidation in nanocrystalline coatings can be explored further. Moreover, the available literature is only based on HVOF sprayed nanocrystalline coatings. Hence, in the present work, LVOF technique has been successfully used to deposit the conventional and nanopowders on the superalloy substrate due to its easy operation and economical factor. The performance of the microcrystalline coatings (MC) and NC NiCrAlY coatings was compared and the difference in oxidation/hot corrosion mechanism was explored. The LVOF sprayed MC coatings were also compared with the HVOF sprayed MC coatings to assess performance of the similar coatings obtained by both of these deposition techniques. The thesis has been divided into 8 chapters, which include detailed discussion of the oxidation and hot corrosion behavior of all types of coatings based on the results of various characterization techniques and in light of the existing literature. Chapter 1 contains a brief introduction to the oxidation and hot corrosion problems existed in the power generation plants and their remedial measures by applying protective coatings on the heat facing components. The role of the nanoparticles in improving structural, mechanical properties and corrosion resistance of the coating has been summarized briefly. Chapter 2 provides a brief overview of the working of the gas turbines, materials developed for turbine applications and various modes of material degradation during operation with main ii emphasis on the oxidation and hot corrosion. The phases, microstructure and the other important details of the NiCrAlY coatings have been briefly discussed. Literature regarding the mechanism of the oxidation and hot corrosion of conventional NiCrAlY coatings has been discussed. Subsequently, the HVOF and LVOF thermal spray processes have been described in detail comparing the characteristics of the coatings obtained. Finally, the development of thermally sprayed nanocrystalline coatings as wear, erosion and corrosion resistant coatings has been reviewed. The available literature on oxidation/hot corrosion of the thermally sprayed nanocrystalline coating has been summarized. Chapter 3 comprises of the experimental techniques and procedures employed for depositing the coating and their characterization, oxidation studies in air, molten salt environments (Na2SO4+60 % V2O5). The specification of the equipments and other analytical instruments used in the present investigation and the techniques employed to analyze the corrosion products are discussed. Superni 76 (HastelloyX) procured from Mishra Dhatu Nigam Ltd., Hyderabad (India) was used as a substrate material to deposit the different types of coatings. NiCrAlY powder (Praxair, US) was selected as feedstock powder to deposit the corresponding nanocrystalline (NC) and microcrystalline (MC) coatings. Three types of coating such as NiCrAlY (MC by HVOF), NiCrAlY (MC by LVOF) and NiCrAlY (NC by LVOF) were developed and investigated in the present study. The mechanical milling was used to produce the NiCrAlY nanopowders. The nanopowder after milling and NC coatings obtained from LVOF process were analyzed by using XRD and TEM for confirming the grain size of coatings. The as-sprayed and oxidized coatings were characterized by the techniques such as optical, XRD, SEM/EDS, and X-ray mapping analysis. The coatings were oxidized in air and molten salt for up to 100 cycles at 900 °C. The corroded samples were analyzed at different intervals up to 200 cycles (in case of HVOF sprayed coatings) and 100 cycles (in case of LVOF sprayed coatings) so as to follow the whole oxidation process. The corroded samples obtained at different intervals of cyclic treatment were subjected to surface and cross sectional analysis in order to substantiate the probable mechanism of oxidation and hot corrosion. Chapter 4 deals with the detailed investigation of the mechanism of the oxidation and hot corrosion mechanism of the HVOF sprayed NiCrAlY (MC) coatings in air and molten salt environment in the laboratory under cyclic condition at 900 oC for 200 cycles. The techniques such as XRD, SEM, EDS and X-ray mapping were used to analyze the as-sprayed coating and the corroded specimens. The NiCrAlY coating exhibits unmelted and partially melted particles iii of the powder on the as sprayed surface and the average roughness was calculated to be of 7.5 Ra. The microhardness of the coating varies as distance from substrate/coating interface increases. There is presence of Al rich oxides particles in the as sprayed coating, which increases oxidation resistance of the coating. NiCrAlY (MC) coating on Superni76 exhibits excellent oxidation resistance to cyclic oxidation up to 200 cycles at 900 °C in air. There was no sign of scale spallation and cracking up to 200 cycles. The mechanism of oxidation of the HVOF sprayed NiCrAlY coatings at 900 °C in air may be divided in to four stages. The first step exists up to 5 cycles which indicates accelerated weight change due to the formation of spinels and some amount of Al2O3. This corresponds to very fast diffusion of Al and its reaction with O. The second step up to 50 cycles indicates decreased outward diffusion of Al and increased inward diffusion of O resulting in formation of slow growing Al2O3. In the third step, these Al2O3 stringers grow into a continuous sub-layer up to 100 cycles. The last step refers to growth of Al2O3 layer due to internally available O formed due to dissolution of the Cr2O3 in the Al2O3 sub-layer. In case of hot corrosion, in presence of Na2SO4+60 % V2O5, the coatings showed spallation of the scale and higher kp value as compared to that of air oxidation. The different steps were observed during hot corrosion process of the NiCrAlY coatings due to the formation of various oxides at the different depth of the oxide scale. During initial cycles (up to 5 cycles), the salt reacts with coatings and forms corrosive compound, Ni3V2O8. Y also reacts with vanadium to form YVO4. In subsequent cycles, Cr2O3 forms and dissolves into the corrosive salts and spallation of scale was observed. Continuous dissolution of Cr2O3 resulted in Cr-depleted region in the coating. The Al started oxidizing at the interface due to migration of O through the porous oxide layer. Hence, Al rich layer was observed at the interface. This oxidation of Al resulted in Al-depleted region just below the interface after 100 cycles. The Al2O3 sub layer provided the desired protection by inhibiting outward migration of the Cr to the surface. The appearance of Al3Y phase after 100 cycles also increased the resistance to hot corrosion. Between 100 - 200 cycles, minimum rate of the corrosion was observed as O migration through the Al2O3 sublayer is the only mechanism by which the scale was growing. Chapter 5 deals with the detailed investigation of oxidation and hot corrosion mechanism of the LVOF sprayed NiCrAlY (MC) coatings in air and molten salt environment in the laboratory under cyclic condition at 900 oC for 100 cycles. The techniques such as XRD, SEM, EDS and X-ray mapping were used to analyze the as-sprayed coating and the corroded specimens. iv The as sprayed coating surface showed the melted and semimelted powder particles, which is characteristics of the thermal sprayed coatings. The as sprayed LVOF coatings exhibit a typical morphology composed of oxidized and unoxidized areas during deposition. The preoxidized areas indicated the co-existence of the Al, Cr and O. It was inferred that the formation of Al2O3-Cr2O3 occurs in which the Cr atoms partially substitutes Al atoms in Al2O3 structure. The coatings possess large variations in value of microhardness at the preoxidized and unoxidized areas of the coatings. The coating porosity was found to be around 5 % and average roughness was 8.2 Ra. The coatings showed the steady state weight gain after initial fast increase in weight gain, which indicated that these coatings provided the desired protection against oxidation/hot corrosion. There was no scale spallation or cracking observed in case of air oxidation. In case of hot corrosion, only minor sputtering at the edges was noted. The oxidation and hot corrosion mechanism of LVOF coatings have been studied and explained using the separate role of the oxidized and preoxidized areas. In case of oxidation, in presence of air, the preoxidized areas remained almost unaffected during cyclic oxidation. Spinels and NiO were formed on the unoxidized areas. The combined effect of the oxidation of these two regions (oxidized and preoxidized areas) was capable of protecting the substrate alloy. It was found that the weight gain in case of hot corrosion was comparable to that of the oxidation in air which could be advantage of using LVOF sprayed coatings. Generally, MCrAlY coatings developed by other thermal spray technique showed very high kp values as compared to the oxidation of the corresponding coatings. This difference was again explained and justified by using the various characterization results and the increased hot corrosion resistance of the Al2O3-Cr2O3 system formed during deposition of the coatings. The formation of the NiCr2O4 and Ni (VO3)2 in the scale provides further protection against hot corrosion. The porosity of the coatings showed minimal effect on the air oxidation. In case of hot corrosion, the V penetrated into the pores but did not intensify the corrosive attack. LVOF is a low energy process and economical and it could provide protection against high temperature air oxidation and molten salt environment along with protection against wear and erosion as observed in the present work. Chapter 6 describes the development of the nanocrystalline NiCrAlY coating by using LVOF process. The NiCrAlY powder was mechanically milled and was deposited by using LVOF gun. The LVOF method was modified so as to overcome the disadvantages of poor flowability v of the nanopowder through the spray gun. The NC coatings with approximate thickness of 120 μm were deposited successfully on the superalloy substrate by the modified LVOF process. The as sprayed NiCrAlY (NC) indicates partial oxidation around splat boundaries with partially melted structure throughout coatings. The NC coatings exhibited only γ/γ’ phase in contrast to other studies reported in the literature. The grain size of the coating lies in the range of 40-100 nm as confirmed by TEM analysis. Cyclic oxidation resulted in the formation of very small oxide particles on the as sprayed coating surface. The scale formed on NC coatings showed very good spallation resistance. There was penetration of oxidizing media in the cross section of the coating which perhaps led to filling of the pores. The oxidation resistance of the coatings is attributed to the formation of very fine oxide (spinels and Al2O3) on the surface and Al, Y, oxides around the splats and filling of the pores with the oxides, which may inhibit further migration of O through the scale. The post polishing of the coating surface and subsequent cyclic oxidation led to decrease in extent of spinel formation and the scale consisted of mainly Cr2O3 and Al2O3. The absence of single Al2O3 layer was attributed to the absence of β phase during milling. This coating should have developed an Al2O3 layer at the top on oxidation but this was not seen in present study. It may be ascribed to milling of the powder which resulted in extinction of β (NiAl) phase, whereas in this coating, Ni3Al/Ni was main phase. In case of hot corrosion of NC coatings, the porosity was found to exert more detrimental effect on the hot corrosion resistance as there was penetration of the reacting species into the coatings. However, the coatings showed minor sputtering of the scale but coatings remained intact up to 100 cycles. The inferior hot corrosion resistance as compared to air oxidation resistance has been explained on the basis of the nature of hot corrosion in which the protective oxide layer is dissolved in the molten salt. Chapter7 compares the oxidation and hot corrosion behavior of NC and MC coatings obtained from LVOF process. The role of HVOF and LVOF process on the oxidation/hot corrosion behavior of NiCrAlY (MC) coatings has been also compared. Chapter 8 summaries the conclusions and scope for the future work.