Experimental Evaluation of High Temperature Corrosion Performance of 75Ni25Cr Coated and Bare 347H SS in Air and Simulated Husk Fired Boiler Environment

Corrosion occurring at elevated temperatures is a serious problem in areas such as gas turbine applications and power plants that use high temperature boilers. In this paper, we carry out experimental investigation for reducing corrosion rate so as to enhance the corrosion and wear characteristics of 347HSS and quantify the effect of 75Ni25Cr on 347HSS in terms of improved corrosion resistance. A coating of 75Ni25Cr was deposited on 347HSS boiler tube by the process of Detonation Gun. Corrosion studies were conducted on bare as well as D-Gun coated 347H stainless steel specimens in air and simulated husk fired boiler atmosphere at 800°C under cyclic conditions. Each cycle consisted one hour of heating in the tubular furnace and 20 min of cooling at room temperature. Fifty such cycles were completed to observe performance of these specimens. The weight change measurements were performed after each cycle to establish the kinetics of corrosion using weighing balance. Scanning Electron Microscopy, X-ray diffraction and X-ray mapping analysis techniques were used to analyse the corrosion products formed on the surface of these substrates.


Introduction
In thermal power plants the efficiency of the boilers may be increased by rising the pressure and temperature of boiler [2]. In past the temperature of boiler was between 400 o C to 600 o C but nowadays the temperature has been increased to 700 o C to 900 o C. However, at such high temperature oxidation of superheater tubes in boiler occurs at very fast rate [1]. These components are made up of austenitic stainless steel. To reduce oxidation rate and sustain longer life of components, austenitic steels having higher chromium content is used such as 347HSS and 304SS [3].The presence of high amount of chromium can help in development of protective oxide scale that tend to reduce oxidation of parent material for longer hours [4]. These alloys undergo very intense oxidation at an elevated temperature, which reduces the life of components significantly [2,3,4]. Hence it is important to provide some kind of external protection to increase the life of the components. There are wide varieties of applications where thermal sprayed coatings have been employed. Among these applications surface treatment of mechanical parts has been used so as to prevent oxidation. [6]. The objective of thermal sprayed coating is to improve surface properties like low porosity, resistance to corrosion and high bond strenght which are dersired for the current study and are best achieved by D-gun process [7]. D-gun spraying process converts the consumable material into molten droplets and then deposits these molten droplets on parent material at supersonic speed to form dense layered coating. This technique is capable of depositing wider range of metallic cermet and ceramic powder on various alloys [7]. The present study focuses on high temperature corrosion behavior of bare 347H SS sample and 347H SS sample coated with 75N i25Cr by D-Gun process in air and simulated boiler environment at 800 o C under cyclic condition.
2. Experimentation 2.1. Substrate and Coating deposition 347HSS having composition 17−19%Cr, 9−13%N i, 0.08%C, 2%M n, 0.75%Si, 0.045%P, 0.030%, S was procured from Cheema Boilers located at Ropar Punjab in the form of sheet. The samples were cut down from the 347HSS sheet into 15 mm x 20 mm. Thickness of both the samples have been reduced to 5 mm. Bare and N i − 25Cr coated 347HSS were used for oxidation studies. D-gun surface coating technique was used for coating steel substrate. Before coating deposition, the substrate were polished using emery papers of 220, 400 and 600 grit size followed by cloth polishing. After that substrates were cleaned with the distilled water and acetone. The samples were coated with N i25Cr coating powder by using D-Gun technique. D-gun technique was available at SVX Powder M Surface, Greater Noida, Uttar Pradesh, India.

Oxidation Test
Alumina boats were used in the experiment to hold the samples. They were heated repeatedly at 1000 o C for two hours cycles until the weight of boat became constant.To monitor the physical dimensions and weight change of the sample, vernier caliper and digital weighing balance were used respectively. To, carryout oxidation study, the specimens were subjected to 800 o C in a tubular furnace for 50 cycles. Every cycle was composed of 1 hour of heating and 20 mins of cooling in furnance and air respectively. After each cycle samples were weighted using digital weighing balance and the weight of these samples were recoreded.

High Temperature Corrosion Test
At the beginning of high temperature corrosion test the salt solution was made by mixing distilled water with 10%KCl, 10%N aCl, 40%N a 2 SO 4 and K 2 SO 4 in china dish.This solution was applied on all six surfaces of the substrate and approximately about 3-4 mg was deposited per cm 2 surface area of the sample. These substrates were then heated at 80 o C for two hours in tubalar furnance in order to evaporate the moisture from the salt that was applied on sample. After that samples were kept in the alumina boats and where subjected to 50 cycles of heating and cooling as was done in oxidation test.

Visual Analysis for Oxidation Test
Visually every sample was observed after each cycle. This type of inspection helps in determing the various changes in colour, luster and amount of spallation that occur during different cycles. It also tends to better understanding of development of cracks if any that have occurred during experimentation.
Macrophotos of bare 347H SS and 75Ni25Cr coated 347H SS after 1 st cycle 5 th cycle and 50 th cycle of oxidation at 800 o C has been shown in Figure 1. In case of bare 347H SS, it was observed that reddish brown scale was formed just after 1 st cycle of oxidation. After 5 th cycle

Visual Analysis of Hot Corrosion Test
The substrates during high temperature corrosion study reviled change in colour as there was formation of oxide scale at the surface of specimens. Macro-photos of bare 347HSS and 75N i25Cr coated 347HSS after 1 st cycle, 15 th cycle and 50 th cycle of hot corrosion test at   formed on bare 347H stainless steel after the completion of 1 st cycle. After 15 th cycle noticeable spallation was observed and the colour of oxide layer changed to light blue with light orange patches on it. Subsequently after 50 th cycle high amount of sputtering of bare 347HSS was observed. In case of 75N i25Cr coated sample showed better resistance to corrosion as there was negligible amount of spallation. From Figure 4 it can be clearly seen that colour of sample changed to greenish blue after 1 st cycle. No colour change after 1 st cycle was observed.

Weight Change Analysis
Weight change data in (mg/mm 2 ) 2 versus number of cycles for uncoated and coated 347HSS specimens subjected to oxidation and high temperature corrosion in simulated environment at 800 o C for fifty cycles were plotted and shown in Figure 5 and Figure 6    be seen from the graph that both coated sample and bare sample followed parabolic behaviour. Parabolic rate constant K p was also calculated for both the samples (bare as well as 75N i25Cr coated 347HSS). Values of K p are shown in table 1

Surface analysis
Surface morphology of bare and coated samples after subjected to 50 cycles of oxidation at 800 o C has been shown in Figure 7  appeared on the bare 347HSS specimen. High Cr content was shown by EDS which may be due to the presence of Cr oxide layer which may be protecting the metal form further oxidation. In case of N i25Cr coated sample dense clusters of oxide was formed on the surface of the coating as shown in Figure 7(b) For high temperature corrosion test oxide scale formed on the surface of bare 347H stainless steel and 75N i25Cr coated 347H stainless steel after cyclic oxidation has been shown in Figure 8. Irregular structure and porous oxide scale was found in case of bare347HSS and in case of N i − 25Cr coated 347HSS, platelets like structure was formed. Occurance of O 2 and Cr at the surface of both the specimens in extensive quantities was found by EDS analysis, this might be due to the presence of high quantity of Cr 2 O 3 . The presence of Cr 2 O 3 was also detected by XRD analysis which proves its occurrence.

XRD
X-ray diffraction reviled formation of Cr 2 O 3 and N iCr 2 O 4 for both coated as well as bare sample which were subjected to oxidation test as shown in Figure 9. Cr 2 O 3 was discovered as major phase. Peaks of N a 2 CrO 4 , F e 2 O 3 , and M nO 2 were also identified.
XRD diffractograms for coated and uncoated 347H stainless steel subjected to high temperature corrosion in (40% N a 2 SO 4 , 10% N aCl, 40% K 2 SO 4 , 10 % KCl) environment at 800 o C for 50 cycles are depicted in Figure 10.

Discussion
From visual macrophotos it can be clearly observed that both bare and 75N i25Cr coated 347H stainless steel showed good corrosion resistance after oxidation in air at 800 o C. No spallation or sputtering was observed throughout the 50 th cycle. Occurrence of reddish orange oxide just after the 1 st cycle may be due to formation of iron oxide (F e 3 O 4 ) [10].Weight gain analysis depicted that both the samples have good oxidation resistance which may be due to presence of Cr and N i which forms the protective oxide when combines with oxygen [11]. Weight gain graph of bare 347HSS showed a little increase in weight during first few cycles which can be attributed to the initial development of oxide scale at 800 o C. SEM/EDS study depicted presence of rhombohedral oxide layer on the exterior surface of bare 347HSS. Cr and O 2 where major phases present in this layer. On the other hand in case of N i − 25Cr coated 347HSS presence of thick and dense oxide layer was observed which was composed of chromium, nickel and oxygen. In the literature it was reported that Cr 2 O 3 scale has rhombohedral structure [8,9]. Therefore it could be infered that the Cr 2 O 3 oxide scale was present in both of the specimens. XRD analysis also revealed that the both specimens consist of Cr 2 O 3 as protective oxide layer. The substrates during High temperature corrosion study reviled change in colouration as there was development of oxide scale on the crest of the surface of specimens at a temperature of 800 o C. Orange coloured oxide scale was formed of bare 347H stainless steel after the completion of 1 st cycle which can be due to the occurance of iron oxide layer. After 15 th cycle noticeable spallation was observed and the colour of oxide layer changed to light blue with light orange patches on it. Subsequently after 50 th cycle high amount of sputtering of bare 347HSS was observed. In case of 75N i25Cr coated sample showed better resistance to corrosion as there was negligible amount of spallation. From Figure 4 it could be simply seen that colour of sample changed to greenish blue after 1st cycle. No colour change after 1st cycle was observed. Weight gain analysis showed that 75N i25Cr coated samples provided better protection towards corrosion than bare 347H SS sample as K p value of coated sample was far less than latter one. Also XRD analysis revealed formation of major peaks on coated sample were those that help in reducing rate of corrosion like

Conclusions
• Both bare and N i25Cr coated 347HSS showed good resistance to corrosion in air, whereas in simulated bio-fuel fired boiler environment bare 347HSS underwent excessive spallation • During hot corrosion test, higher weight gain of samples was experienced in the present study in the given environments for both the samples which may be due to the fact that molten salt species may have penetrated into the coating to cause fast corrosion. Nevertheless after formation of oxide scale around the specimens the rate of weight gain decreased. • Porous irregular shaped oxide layer was formed on the bare 347HSS sample. SEM/EDS analysis showed presence F e, O and N i where present in major quantities. XRD analysis revealed F e 2 O 3 as major peaks present in substrate which explains the reason for excessive spallation. • However the study conducted in simulated husk fired boiler environment revealed that 75N i25Cr coated 347HSS sample exhibited superior corrosion performance which is due to the formation of protective Cr 2 O 3 and N iCr 2 O 4 oxide layer as compared to excessive formation of unprotective F e 2 O 3 oxide layer on bare 347HSS sample.