INVESTIGATIONS ON HOT CORROSION OF SOME Fe-,Ni- AND Co-BASE SUPERALLOYS IN Na2SO4-1T205 ENVIRONMENT UNDER CYCLIC CONDITIONS

Abstract

In many engineering systems like gas turbines, steam generators etc. materials come in direct contact with corrosive condensed phases which cause hot corrosion. The sources of these phases are the fuel and air necessary for combustion. When residual fuel oils are used, due to the presence of Na, V and S, the occurrence and damage by hot corrosion is likely to increase where more aggressive environments have to be encountered. Superalloys have been developed for such applications in gas turbines and steam boilers. Superfer 800H (alloy A) is an Fe-base alloy containing high Ni and Cr content. Superni 75 (alloy B), Superni 600 (alloy C) and Supemi 718 (alloy D) are Ni-base alloys containing high Cr content with some other alloying elements. Superco 605 (alloy E) is a Co-base alloy containing high Fe, W, Cr and some Ni. All these alloys contain Cr in the range between 15.5 to 21.0%. They have wide applications in steam boilers, gas turbines, heat exchangers, furnace equipments, pump bodies, nuclear parts and pipe lines in chemical/petrochemical industries. The increasing demand of alloys in aggressive environments has stimulated interest in the hot corrosion studies. In the present investigation, hot corrosion studies were carried out on above ION mentioned Fe-, Ni- and Co- base superalloys in different salt/salt-mixture environments. These alloys were studied in air and in c.g environments. The details of these alloys are given in Table 4.1 and those of environments employed in Table 4.2. For convenience these alloys are marked as A, B, C, D and E as shown in Table 4.1. The alloys were cut into specimens of 15 x 20 mm size for hot corrosion in laboratory and in actual studies industrial atmosphere. After polishing the specimens to a mirror finish, coating of Na2SO4, Na2SO4-15%V205 and Na2SO4-60%V205 was applied by aqueous spray or by hair brush. Hot corrosion studies in air were carried out in laboratory for 24 cycles (cycle of 1 hr heating and 20 min. cooling) at 700, 800 and 900°C while the studies in e.g. were made in an industry for 6 cycles (cycle of 24 h heating and 1 h cooling) at 1100°C. The effect of Mg0 addition to the salt mixture (Na2SO4-60%V205) on the hot corrosion of these alloys was studied for 24 cycles at 900°C in air in laboratory. After hot corrosion, (v) the specimens were studied thermogravimetrically to understand corrosion kinetics and by visual observation to characterize the nature of scales. Investigation of the scales included X-ray Diffractometry (XRD), Scanning Elecron Microscopy (SEM), Energy Dispersive X-ray Analysis (EDAX) and Electron Microprobe Analysis (EPMA) to characterize the corrosion products and make an attempt to understand the mechanism of corrosion. The wt. gain values for alloys A, B, C, D and E increased with increase in temperature. The wt. gain occurred in these alloys at a rapid rate during the first two cycles and then the rate of wt. gain decreased. The wt. gain values for these alloys were lower in pure Na2SO4 than in Na2SO4-15%V205. Maximum wt. gain values were obtained in Na2SO4-60%V205. Alloy A was comparatively corroded slightly more than the other alloys in pure Na2SO4 at all temperatures. Alloy D behaved in a very different manner at 900°C in Na2SO4-15%V205 in which finally wt. loss of 6.8 mg/cm2 was observed after an initial increase in wt. gain during the first two cycles. Alloy A also behaved in a different way in Na2SO4-60%V205 at 900°C in which total wt. loss of 23.2 mg/cm2 was observed. The wt. gain plots for all the alloys after exposure in combustion gas at 1100°C show that they obey parabolic rate law. The wt. gain values obtained with Mg0 addition to Na2SO4-60%V205 are lower than those obtained without its addition. This suggests that Mg0 addition is very beneficial in reducing corrosion rates obtained by Na2SO4-60%V205. All the alloys have shown good corrosion resistance at all temperatures in pure Na2SO4, Na2SO4-15%V205 (except for alloy D at 900°C) and at 700 and 800°C in Na2SO4-60%V205. The average thickness values of the scales, measured with the help of BSE images of the alloys at the cross-section, was lower with pure Na2SO4 than those with Na2SO4-15%V205 while maximum values were obtained with Na2SO4-6,0%V205 coating. The thickness measurement supported the observed wt. gain values to confirm that moderate to severe corrosion was induced by Na2SO4-60%V205 and further Mg0 addition to this salt mixture reduced its corrosiveness. EDAX and XRD analysis of the corrosion products formed on the alloys under different salt environments indicated the prominent phases on alloy A to be of a-Fe203, Cr203, A1203, Ti02, NiO, FeV204 and (Cr,Fe)203. The scales of alloys B, C and D also (vi) contained these phases with an additional phase of Ni(V03)2. The scale of alloy E mainly composed of a-Fe203, Cr203, NiO, FeV204, (Cr,Fe)203, Co304, Co203, CoO, NiCr204, W03 and FeW04. Mg3V208 was present in the scales of the alloys after exposure for 24 cycles at 900°C in Na2SO4-60%V205 + MgO. X-ray mappings of elements such as Fe, Cr, Ni, Co, Na, S, 0, V, W, Mo were also taken. The elemental maps for alloy A revealed Cr depleted zone in the substrate just below the oxide layer and internal oxidation of Ni was observed at 900°C with Na2SO4- 15%V205 and Na2SO4-60%V205 coatings in air, while A1203 and TiO2 were observed at the grain boundaries of the substrate with Na2SO4-60%V205 coating in combustion gas. Sulphur penetration was observed with Na2S 04-60%V205 + Mg0 environment at 900°C at the grain boundaries in the substrates. In alloy B continuous layer of Cr203 scale was formed with different salt coatings. Little internal oxidation of Cr was observed only at 900°C with Na2SO4-60%V205 + Mg0 coating. This alloy showed very good corrosion resistance in nearly all the environments. In alloy C internal oxidation of Al was observed only with Na2SO4-15%V205 coating while continuous layer of Cr203 was observed with other coatings. The alloy D showed good corrosion resistance in pure Na2SO4 due to the formation of a thick continuous Cr203 layer. It was corroded more in Na2SO4-15%V205 which was observed by the irregular surface of the alloy revealed by the BSE image. A1203 and TiO2 formation at the grain boundaries in the substrate was also observed. With Na2SO4-60%V205 coating in c.g. at 1100°C and with Na2SO4-60%V205 + Mg0 coating at 900°C, it has shown good corrosion resistance as no internal oxidation or sulphidation was observed. The alloy E formed a thin continuous layer of Cr03 in pure Na2SO4-15%V205 while with Na2SO4-60%V205 it formed a thick scale of oxides. In actual industrial atmosphere with Na2SO4-60%V205 coating it showed good corrosion resistance. With Na2SO4-60%V205 + Mg0 coating it has shown some corrosion resistance but the presence of internal sulphidation of Ni was also observed in the substrate. These alloys have shown good corrosion resistance against the aggressive environment at 700, 800, 900 and 1100°C under cyclic conditions in laboratory as well as in industrial hot corrosion tests, except with Na2SO4-60%V205 at 900°C, due to the formation of a protective continuous layer of Cr203 on the alloy surface. An attempt has been made to suggest probable corrosion mechanisms, wherever possible with the help of schematic diagrams (Fig. 6.1-6.6) which are mostly supported by the existing literature or is based on the X-ray elemental maps of the cross-section of the corroded specimens. The schematic diagrams drawn for corrosion induced by pure Na2SO4, Na2SO4-15%V205 and Na2SO4-60%V205 in air and in combustion gas as well as for Na2SO4-60%V205 + Mg0 in air, are presented in Fig. 6.1-6.6. They also indicate the direction of diffusion of anions and cations through the scale.