Hot corrosion studies on fully austenitic stainless steel in air oxidation and simulated waste heat incinerator environment at 600 °C, 650 °C and 700 °C

The purpose of the current study is to compare the hot corrosion performance of SMO 254 exposed to air oxidation and molten salt with a eutectic mixture of 40 wt% Na2SO4 + 40 wt% K2SO4 + 10 wt% NaCl + 10 wt% KCl at 600 °C, 650 °C and 700 °C under cyclic conditions for 50 h period. Kinetics of corrosion were analyzed by thermogravimetric method. By using SEM/EDS, the hot corroded specimens were investigated for surface scales and cross-sectional elemental analysis by point, line mapping methods. An accelerated rate of corrosion rate was noticed in the specimen exposed to molten salt at 700 °C, and the lowest rate of corrosion rate was observed in the specimen subjected to air oxidation at 600 °C. XRD analysis is done to analyze the corrosion products in the oxide scale.

Sanaa et al [7] studied about the susceptibility of 254 SMO alloys to Crevice Corrosion in NaCl Solution. The author reported that the oxides of Cr, Fe and Mo formed during the crevice corrosion protects\gives resistance to the alloy against the crevice.
Meguid et al [9] investigated the critical crevice potential and the critical protection potential for Alloy 254 using potentiodynamic cyclic anodic polarization technique. The author reported that SMO 254 was resistant to pitting and crevice in 4% NaCl solution till 90°C.
Katiki et al [10] indicated that the when the salts like K 2 SO 4 , Na 2 SO 4 , KCl, and NaCl are combined, their melting point falls well below the actual melting point of the salts, resulting in salt fluxing causing destruction of the protective oxide layer triggering corrosion. The main governing factors which affect high-temperature corrosion are salt mixture composition, coverage of temperature, cyclic heating and cooling, and environmental conditions [11]. Hot corrosion is the main issue faced in the petrochemical sections, industrial waste incinerators and power plant parts such as boilers, internal combustion engines, etc [12]. Author Sidhu T S et al [13] observed that hot corrosion cause reduction in the quantity of material and reduction in the load-bearing capabilities, and finally it turns to catastrophic failure.
It has been found that from a thorough literature survey, the authors have studied the pitting and crevice corrosion on SMO 254. However, though this SMO254 is a candidate material for the high-temperature applications in the waste incinerator and power generation no detailed studies were done on hot corrosion study. The experimental goal of this research work is to investigate the behaviour of SMO 254 in hot corrosion environment in the simulated industrial waste incinerators and the power plant environment. A comparative study on the air oxidation and hot corrosion behaviour in the molten salt eutectic mixture of 40 wt%   Na 2 SO 4 +40 wt% K 2 SO 4 +10 wt% NaCl+10 wt% KCl at 600°C, 650°C and 700°C under cyclic conditions is carried out. To derive the corrosion mechanism by a detailed study using the thermogravimetric analysis, surface and cross-sectional analysis using SEM/EDS was done. Also, an attempt is made to identify the corrosion products using EDS along with the SEM/EDS elemental analysis.

Material and experimental procedure
The material SMO 254 was purchased in the plate form from a reputed supplier. The chemical composition of SMO254 is provided in table 1. The base metal was cut in a dimension of 20 mm length 10 mm width and 6 mm thickness. To maintain the uniform surface finish to avoid the effect of surface roughness in the results the samples were polished. Standard procedure was followed using silicon carbide emery papers in the order of 240, 400, 600, 800, 1000, 1200, 1500 and 2000 grits followed by alumina slurry disc polishing. Finally, specimens were cleaned with warm water and acetone. By using a vernier caliper surface area of the specimens was measured and weight was measured by electronic balance with an accuracy of 0.01 mg. Three specimen were coated with 40 wt% Na 2 SO 4 +40 wt% K 2 SO 4 +10 wt% NaCl +10 wt% KCl salt by using camel brush. The salts were applied to the specimen only during the initial stage. The weight of coated salt varies from 3 mg cm −2 to 5 mg cm −2 . The alumina boats were annealed at a temperature of 1200°C for 24 h. Before the hot corrosion studies, specimens were dried in the alumina boats at 200°C for 2 h to remove moisture content. The experiments were carried out in three different tubular furnaces with the heating system precision of (+2°C to −2°C). Both oxidation and hot corrosion were conducted at three different temperatures 600, 650 and 700 up to 5 cycles. Each cycle involves 10 h heating followed by 20 min cooling at room temperature. After completing each cycle weight of specimens along with the boat was measured accurately to find the weight gain\loss corrosion. Proper visual surveillance was made to understand the spalling, sputtering and colour changes of the oxides.

Kinetics of corrosion
The thermogravimetric charts of the different specimens after air oxidation and hot corrosion is given in figures 1(a) and (b). The images of the specimens after and before air oxidation and hot corrosion are given in figures 2 and 3). It is noted that from the macro images at the end of the first cycle(after 10 h) in the air oxidized samples the colour of specimen turned to reddish-brown at 600 and 650°C. However, in the case of 700°C it turned in to bluish-black. The hot corroded samples turned to black, however without any spalling of oxide scales.
At the end of 20 h spalling was observed in the specimen subjected to hot corrosion at 700°C with the other samples in air oxidization and hot corrosion remained unaffected suggesting that the corrosion rate of the hot corroded sample is accelerating, At the end of the 5th cycle, it is observed that specimen which is exposed at molten salt at 700°C showed a higher rate of corrosion rate than others. The specimen exposed to air oxidation at 600°C suffered the lowest rate of corrosion. The parabolic law of oxidation was followed by all the samples in both environments. The equation which is used to find the parabolic rate (Kp) is given bellows [11].
Where, ΔW is the variation in the substrate weight with respect to the initial weight. 't' is the time of oxidation in seconds. Here the unit area is indicated by symbol 'A' The Kp value of all substrates is given in table 2. Author Nakagawa K et al [14] reported that the eutectic temperature of the molten salt composition is 520°C. From table 2, it is observed that the total weight gain is higher for specimens exposed to molten salt corrosion at 700°C and a comparatively lower amount of weight gain for specimens exposed to air oxidation at 600°C. The samples subjected to air oxidation received lower weight gain than the specimen exposed to the molten salt environment. Also, a higher value of Kp is obtained for the specimen exposed to molten salt corrosion at 700°C, and the lower value of Kp is obtained for the specimen exposed to air oxidation at 600°C.

SEM/EDAX analysis
Surface SEM images of the hot corroded specimens are provided in figures 4(a)-(f). SEM images indicate that formation of the coarse uneven surface. On close observation of the specimen, micropores are noticed on the top Figure 5. (a) Cross -sectional SEM images with EDS results of the specimen exposed to air oxidation at 600°C after 5 cycles. (b): Cross -sectional line mapping images of the specimen exposed to air oxidation at 600°C after 5 cycles.
surface. Also, micro-cracks are observed in figures 4(b) and (f). Volatile chlorides diffuse out the surface through these micro-cracks. Cross-sectional SEM images with point analysis, line mapping are represented in figures 5-10. EDS results imply that the scales mainly include oxides of elements such as Cr, Fe, and Ni. It can be noted that the results produced in the EDS report are closer to that of XRD data. Author Muthu S et al [15] reported that NaCl salt is vigorous at the higher temperature. Hence it can create pores in the surface also Cl in the salt react with metal substrate and form corrosive corrosion products such as ClCo 2 O 4 P, ClCrKO 3 , ClCu, ClCuFe 24 K 6 S, ClCu 2 O 3 , ClK, ClCu 4 . 37 Na 4 and ClNa which are evaporative and porous owe to greater vapour pressure and low melting point. Arivarasu et al mentioned that these metallic chlorides with O 2 create the non-protective oxide layers with porous structure [11]. The chlorine is discharged due to the sulphation of chloride enclosing deposits [16]. Figure 6. (a) Cross-sectional SEM images with EDS results of the specimen exposed to molten salt at 600°C after 5 cycles. (b): Cross sectional line mapping images of the specimen exposed to molten salt at 600°C after 5 cycle. ( ) / / The above Na 2 SO 4 dissociation mechanism can be explained based on the lewis acid-base concept, were initially formed component Na 2 O is a basic part, and SO 3 is acidic. It is clear that SO 3 is in stable condition with oxygen and sulphur. So it can be concluded that when O 2 partial pressure drops, the increment in the partial pressure of S 2 occurs. This result improvement of the partial pressure of S 2 in the molten salt, when the absorption of O 2 has occurred. The S 2 in the Na 2 SO 4 migrate into the base metal and cause degradation of the base metal by increasing the oxidation rate. Which indicates the rapid weight gain the molten salt coated specimen. Cross-sectional SEM Figure 7. (a) Cross -sectional SEM images with EDS results of the specimen exposed to air oxidation at 650°C after 5 cycles. (b): Cross -sectional line mapping images of the specimen exposed to air oxidation at 650°C after 5 cycles.
images with EDS results of the specimen exposed to air oxidation at 600°C after 5 cycles is given in figure 5(a). On analyzing figure 5(a) it can be noted that a higher amount of O 2 in location 1 with weight % of 38.95 and lack of oxygen creation was noted in the location 2, 3 and 4. In location 1 it is observed that lower weight % of Cr (5.01) and higher quantity of Mo with a weight % of 42.42. Cross-sectional line mapping images of the specimen exposed to air oxidation at 600°C after 5 cycles are provided in figure 5(b). Cross-sectional SEM images with EDS results of the specimen exposed to molten salt at 600°C after 5 cycles are given in figure 6(a). While analyzing figure 6(a) it can understand that higher concentration of O 2 is observed in location 1 (weight % of 12.55) and lower weight % of O 2 in location 3 (weight % of 2.61). Cross-sectional line mapping of the specimen exposed to molten salt at 600°C after 5 cycles is provided in figure 6(b). Corrosive nature having Silicon is present in location 3 with 4.61 weight%. Cr and Mo weight % improves in both position 1 and 2. Cross-sectional Figure 8. (a) Cross-sectional SEM images with EDS results of the specimen exposed to molten salt at 650°C after 5 cycles. (b): Crosssectional line mapping images of the specimen exposed to molten salt at 650°C after 5 cycles.
SEM images with EDS results of the specimen exposed to air oxidation at 650°C after 5 cycles is given in figure 7(a), and it can be identified that higher concentration of O 2 at position 1 with a weight % of 27.65 and lower weight of O 2 (3.80) in location 2. Also, it is witnessed that the improved rate of Cr in all positions (1)(2)(3). Nickel quantity is reduced in both positions 1 and 2, also slight increments of concentration in both 3 and 4 locations. Corrosive element sulfur is present in location 3 with a weight % of 2.27. Cross-sectional line mapping of the specimen exposed to air oxidation at 650°C after 5 cycles is given in figure 7(b). Cross-sectional SEM images with EDS results of the specimen exposed to molten salt at 650°C after 5 cycles is provided in figure 8(a) and while analyzing it can be observed that a higher concentration of O 2 in all positions (1-4) with a concentration of (27.58 %, 11.60%, 6.39% and 7.86%). Also, corrosive nature having element Sulphur is present in location 1 and 3 with a concentration of 0.58 % and 3.94%. A higher rate of Cr is observed in all four positions with a concentration of 24%, 27%, 22.81%, and 23.20%. Cross-sectional line mapping of the specimen exposed Figure 9. (a) Cross-sectional SEM images with EDS results of the specimen exposed to air oxidation at 700°C after 5 cycles. (b): Crosssectional line mapping images of the specimen exposed to air oxidation at 700°C after 5 cycles.
to molten salt at 650°C after 5 cycles is given in figure 8(b). Cross-sectional SEM images with EDS results of the specimen exposed to air oxidation at 700°C after 5 cycles is given in figure 9(a) and the EDAX image indicates that higher concentration of O 2 in position 1 with a weight % of 14.81 and reduced quantity of O 2 is observed in the location 4 with a weight % of 1.41. Also, it is noted that a slightly improved rate in the quantity of Ni in location 2, 3 and 4. Cross-sectional line mapping of the specimen exposed to air oxidation at 700°C after 5 cycles is given in figure 9(b). Cross-sectional SEM images with EDS results of the specimen exposed to molten salt at 700°C after 5 cycles is given in figure 10(a). Augmented rate of O 2 is observed in figure 10(a) at location 2 with a weight % of 30.96, and increment in the rate of Mo is observed in location 1 and 2 with a concentration of 9.67% and 28.85 %. Also, corrosive nature having element sodium is observed in location 2 with a weight % of 4.72. Cross-sectional line mapping of the specimen exposed to molten salt at 700°C after 5 cycles is given in figure 10(b).

XRD analysis
By using D8 Advanced Brunker (Germany), XRD analysis was carried out. XRD pattern of the specimens is given in figure 11. The corrosion products formed during air oxidation at 600°C are FeNi, Ni 3 Si, Cr 0.4 Ni 0.6 , Ni 5 P 2, CuNi, Co 0 . 9 Mo 0.1, Mo 0 . 9 Ni 0.91 and Co 0.027 Fe 0.2 . And the corrosion products formed during molten salt hot Figure 10. (a) Cross -sectional SEM images with EDS results of the specimen exposed to molten salt at 700°C after 5 cycles. (b): Cross -sectional line mapping images of the specimen exposed to molten salt at 700°C after 5 cycles. corrosion at 600°C are ClCo 2 O 4 P, ClCrKO 3 , ClCu, ClCuFe 24 K 6 S, ClCu 2 O 3 , ClK, ClCu 4 . 37 Na 4 and ClNa. The corrosion products such as Fe 0 . 8 Mn0.2, Ni 3 Si, MnSi, Co 0.027 Fe0. 2, CrNi 3 , CuNi, Mo. 09 Ni0. 91, and FeNi 3 are produced after air oxidation for 50 h at 650°C. Also, the formation of corrosion products such as ClCuFe 24 K 6 S, ClFe 24 K 6 S 26 , ClK, ClKO 0.1 Na 0.9 , ClNa, Cl 10 MO 2 was observed after molten salt corrosion for 50 h at 650°C. The corrosion products obtained after air oxidation for 50 h at 700°C are FeNi3, Ni3Si Co. 027 Fe 0.2 , MnSi, Co. 85 SiO. 15, and Cr 3 Si. Also, the corrosion products formed after molten salt hot corrosion for 50 h at 700°C are Co. 027 Fe. 2, CrNi 3 , Co. 85 SiO. 15 , MnS, FeNi 3 , Ni 3 Si, CrFe, Cu 2 S, and Fe 0 . 8 Mn 0 . 2 . This data implies that the formation of beneficial corrosion products helped to stay with reduced corrosion rate on the specimen, which was undergone hot corrosion at 600°C while comparing with the specimen exposed at the augmented temperature level. Figure 11. XRD pattern of the specimen after 50 h of cycle (a) specimen after air oxidation at 600°C (b) specimen after molten salt corrosion at 600°C (c) specimen after air oxidation at 650°C (d) specimen after molten salt corrosion at 650°C (e) specimen after air oxidation at 700°C (f) specimen after molten salt corrosion at 700°C.