Arivarasu M, Manikandan M, Gokulkumar K,Karthikeyan S, Devendranath Ramkumar K and Arivazhagan N
Keywords: Electron beam Welding, Hot corrosion, Scanning electron microscopy
Abstract:
This paper reports on the performance of Electron Beam Welded (EBW) low alloy steel AISI 4140 and stainless steel AISI 304 in air as well as molten salt environments of Na2SO4-60%V2O5 and K2SO4-60% NaCl at 650ï‚°C. The corrosion kinetics has been established by thermo-gravimetric technique during the initial stages. In this work, X-ray diffraction, scanning electron microscopy/energy-dispersive analysis and electron probe micro analysis techniques were used to analyze the corrosion products. It is well observed from the experimental results that the weldments suffered accelerated corrosion in K2SO4-NaCl environment and showed spalling/sputtering of the oxide scale. Furthermore, corrosion resistance of weld interface was found to be lower than that of parent metals in molten salt environment. It is also inferred from the results that NaCl is the one of the main corrosive species in hot corrosion, involving mixtures of K2SO4-NaCl and which is responsible for internal corrosion attack.
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ISSN 1466-8858 Volume 16, Preprint 47 submitted 4 July 2013 Studies on Weldment Corrosion in Different Corrosive Environments Arivarasu M a, Manikandan M a, Gokulkumar K a, Karthikeyan S b, Devendranath Ramkumar K a, Arivazhagan N a,* aSchool of Mechanical and Building Sciences, VIT University, India bCenter for Nano Biotechnology, VIT University, India. *Corresponding author: narivazhagan@vit.ac.in Telephone Number-0416-220-2221, Fax: 0416-224-3092, 224-0411. Keywords: Electron beam Welding; Hot corrosion; Scanning electron microscopy Abstract. This paper reports on the performance of Electron Beam Welded (EBW) low alloy steel AISI 4140 and stainless steel AISI 304 in air as well as molten salt environments of Na2SO4-60%V2O5 and K2SO4-60% NaCl at 650C. The corrosion kinetics has been established by thermo-gravimetric technique during the initial stages. In this work, X-ray diffraction, scanning electron microscopy/energydispersive analysis and electron probe micro analysis techniques were used to analyze the corrosion products. It is well observed from the experimental results that the weldments suffered accelerated corrosion in K2SO4-NaCl environment and showed spalling/sputtering of the oxide scale. Furthermore, corrosion resistance of weld interface was found to be lower than that of parent metals in molten salt environment. It is also inferred from the results that NaCl is the one of the main © 2013 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 47 submitted 4 July 2013 corrosive species in hot corrosion, involving mixtures of K 2SO4-NaCl and which is responsible for internal corrosion attack. 1. Introduction This Dissimilar metals are widely used in critical high service temperature applications. Hence the studies on their weldments have gained importance in recent past. Generally, the combination of low alloy steel and austenitic stainless steel weldments are extensively used for boiler tubing application at elevated temperatures because of its relatively low cost, good weldability and creep resistance [1, 2]. Since these steels exhibit vastly different physical, thermal, mechanical and metallurgical characteristics, they are prone to defects during welding as well as in service environment [3, 4]. Particularly, the failure of dissimilar weldments involving low-alloy steel with austenitic stainless steel has been studied by a number of investigators and factors that contribute to failure have been seldom taken up for research. Advanced welding process like electron beam welding can offer a solution to the problem of sensitization, dilution and cracking of elements in conventional welding. Even though electron beam welding is a comparable alternative, the problem of carbide formation is not yet completely eliminated for the joints when they are exposed to cyclic high temperature service conditions. Usually, low-grade fuels with high concentrations of sulfur, vanadium and sodium are used in oil- and coal fired power generation. During combustion, alkali metal sulfates and V2O5 vapors combine with other ash constituents that deposit onto the component surfaces. The boilers exposed in off-shore industrial rigs undergo hot © 2013 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 47 submitted 4 July 2013 corrosion when the sodium chloride from the ocean breeze mixes with Na 2SO4 from the fuel and deposits on the hot-section of the components. This results in severe corrosion attack by oxidation, sulfidation, chloridation and even hot corrosion [5]. It is to be noted that hot corrosion is the serious problem for the weldments exposed to environment containing mixture of the salt K2SO4 -NaCl and Na2SO4-V2O5. The existence of such corrosive condensation layer on the surface leads to hot corrosion which can considerably reduce the service life of high temperature components [68]. The steel’s performance in oxidizing environments is well established, but weldment behavior in corrosive environments, particularly those containing sulfidizing and chlorides have not been studied extensively. In the present investigation, an attempt has been made to evaluate the hot corrosion behavior of electron beam welded AISI 304 and AISI 4140 metals exposed in air as well as molten salt environment consisting of eutectic mixture of Na2SO460% V2O5 and K2SO4-60% NaCl at 650 °C under cyclic conditions. Thermogravimetric technique was used to establish the kinetics of corrosion. The products of hot corrosion studies have been analyzed using X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM), Energy Dispersive X-Ray studies (EDAX) and Electron Probe Micro Analyzer (EPMA) techniques. 2. Experimentation In this study, experiments were carried out on a Low KV electron beam welding machine. and the welded specimen and its cross sectional macro structure are depicted in Fig 1. Studies by the authors involving detailed metallurgical and © 2013 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 47 submitted 4 July 2013 mechanical properties of electron beam welded samples are published elsewhere [18]. Specimens for hot corrosion tests are fabricated using electron beam welding machine. These specimens are initially having the dimensions of 100×50×6 mm. To facilitate the hot corrosion tests, the samples are cut into rectangular pieces (20 × 15 × 6 mm) with weld zone in the middle of the specimens. Mirror polishing down to 1 μm by alumina on a cloth polishing wheel is carried out before the corrosion run. Immediately, a coating of uniform thickness with 3–5 mg/cm2 of salt mixture was applied with a camel hair brush on the preheated sample (250 °C). Cyclic hot corrosion studies were performed on these specimens by subjecting in the air as well as molten salt (Na2SO4-60% V2O5 and K2SO4-60% NaCl) environments for exactly 50 cycles and the duration of each cycle is for 1 h 20 mins in which heating is for one hour at 650 °C in a silicon carbide tube furnace followed by 20 mins of cooling at room temperature. During the corrosion tests, the weight change measurements were taken at the end of each cycle. The samples after corrosion tests are subjected to characterization studies using SEM/EDAX and XRD analysis. 3. Results The corrosion kinetics of specimens with and without molten salt deposits is depicted in Fig 2 as a plot of weight gain per unit area vs. function of time (number of cycles). These figures indicate that the weight gain kinetics under air oxidation shows a steady-state parabolic rate law, whereas the molten salt environment is a multi stage weight-gain growth rate. XRD analysis has been performed on the weldments before as well as after corrosion run and is shown in Fig 3. Investigations © 2013 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 47 submitted 4 July 2013 showed that the phases such as FeNi and Ni3C, Cr-NiMo, CrNiFe, Ni3C, and Cr-Si-C are observed in the as-welded samples. Fe2O3 has been predominated with lower intensities of NiCr2O4, NiO and FeNi in the air oxidation samples at 650°C. Whereas Fe2O3 and Cr2O3 dominate with greater peak intensities with a lesser intensities of (Cr, Fe)2O3, FeNi and FeS on the hot corroded samples under molten salt environment at 650°C. SEM Surface morphology of dissimilar weldment after hot corrosion treatment by keeping 650ºC is shown in Fig 4-6. EDAX analysis for the weldment after air oxidation as well as molten salt environment [Na 2SO4-60% V2O5 and K2SO4-60% NaCl] shows Fe2O3 to be the predominant phase in the entire region. However Cr2O3 content is higher in the scale of heat affected zone of AISI 304. Moreover some minor constituents of MoO3, MnO and NiO were observed on the weldment as seen in Fig 4-6. 4. Discussions The results obtained in air oxidation at 650°C show a better corrosion resistance as compared to molten salt environment. The authors reported the detailed study of the mechanical properties of AISI 4140 and AISI 304 dissimilar metals made by electron beam welding process in their earlier publication [10]. Thermo-gravimetric curve of molten salt environment studies shows the tendency of multi stage weight-gain growth rate (Fig. 2). It could be due to changes in reaction rate which are associated with the formation of a laminated inner-oxide layer made up of fine and coarse grain spinel oxide as suggested by Hurdus et al [11]. The kinetics of hot corrosion with K2SO4-NaCl mixtures shows rapid weight © 2013 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 47 submitted 4 July 2013 gain compared to Na2SO4-V2O5 as well as air oxidation. In a molten salt environment, sulfur was incorporated into scale and leads to a sulfide formation in the alloy substrate. Therefore, as the formation of protective oxide scale was inhibited by the presence of NaCl, chlorides and sulfides tend to form in the alloy substrate as indicated leading to the propagation of hot corrosion as suggested by Charng et al., [12]. The parabolic rate constants Kp for weldment after exposed in air oxidation, Na2SO4-60% V2O5 and K2SO4-60% NaCl were 0.652, 5.917 and 35.511 ×10 -6 (g2 cm-4s-1) respectively. SEM/EDAX analysis of the corroded sample shows, Fe 2O3 in the scales of weldment after the corrosion cycles signifies non-protective conditions in Na2SO460% V2O5 and K2SO4-60% NaCl at 650 ºC (Fig 4-6). The formation of Fe2O3 in the spalled scale has also been reported to be non-protective during their hot corrosion study on Fe3Al-based iron aluminides in Na2SO4 atmosphere [13, 14]. The corrosion rate of the weldment in Na2SO4-60% V2O5 is higher as compared to air oxidation at 650 Cº due to the formation NaVO3 as proposed by Kolta et al., [15]. As NaVO 3 acts like a catalyst and also serves as an oxygen carrier to the base alloy, it tends to oxidize the basic elements rapidly. Furthermore, the increase in magnitude of weight gain is due to the accelerated corrosion in K2SO4-60% NaCl environment which also opined by Kofstad [16]. It is widely agreed that the corrosion rate will be more severe when the temperature is higher than the melting point of salt deposits. It is observed that the corrosion rate in K2SO4-60% NaCl environment is higher in magnitude as compared to Na2SO4-60% V2O5 and air oxidation environments. It is observed that the thickness of scale is more on 4140 side than 304 side after exposed in Na2SO4-60% V2O5 environment. After the hot corrosion treatment, it is © 2013 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 47 submitted 4 July 2013 observed that the effect of hot corrosion is more on 4140 side as evident from the degree of scaling and spalling of scale. It is observed that the scales get detached by means of the formation of new scale within already growing scale. This is usually attributed to the two-way flow of the reactants [17]. At the end of corrosion tests, it is noticed that the weld interface of the dissimilar weldment is more degraded as compared to base metals (Fig 4-6). Corrosion morphology of the weldment exposed in K2SO4-60% NaCl shows that the weld interface is more prone to formation of fragile scale than base metals. This implies that NaCl plays a vital role in hot corrosion [18]. Moreover the melting of K 2SO4-60% NaCl mixture at 650 °C makes the weldments susceptible to accelerated oxidation due to acidic and basic-fluxing mechanism. Many researchers have pointed out that the formation of sodium chromate (Na2CrO4) could result from oxy-chloridation even the temperature is lower than the melting point of salt deposits [19]. As Na2CrO4 is formed, the salt will wet the specimen surface which eventually leads to a mechanism of hot corrosion dominated by molten salt and is further validated by XRD analysis (Fig 3). The analysis of the scale shows predominant Fe 2O3 with low intensities of Cr2O3, Na2CrO4, SO3 and MoO3. This is in confirmation with past studies on the hot corrosion studies in molten salt environment on boiler tube steel [19]. 5. Conclusion From the experimental studies, some of the important conclusions obtained are as follows. © 2013 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 [1] Volume 16, Preprint 47 submitted 4 July 2013 The weight gain of salt coated welded specimen follows a parabolic rate law during hot corrosion. Rate of oxidation was observed to be higher in the earlier cycles of the study in all the aforementioned environments, which may be attributed to the fact that during transient period of oxidation, the scales formed may be providing protection to the underneath metals. [2] In case of salt coated specimens, the surface scale is more porous and spalled out, thereby providing an easy diffusion path for the corrodents. The scale formed due to hot corrosion mainly contains Fe2O3 and Cr2O3, NiCr2O4 and NiFe2O4. [3] The corrosion rates for the investigated electron beam welded dissimilar metals based on the overall weight gains after 50 cycles in all the environments could be arranged in the following order: K2SO4-60% NaCl > Na2SO4-60% V2O5 > Air Reference 1. Hasçalik A. Ünal E. Özdemir N. Journal of Materials Science, 41, 3233–3239 2006. 2. Taktak, Sukru. Materials & Design, 28, 6, pp1836-1843, 2007. 3. Celik S, Ersozlu I Materials & Design, 30, 4, pp970-976, 2009. 4. Singh Raman R.K, Muddle B.C.I. Journal of Pressure Vessels and Piping, 79 , pp585–590, 2002 5. Lai G.Y. American society for metals, Metals Park, 154, 1990 6. Zheng L, Maicang Z, Jianxin D. Materials & Design, 32/4, pp1981-1989,2011 © 2013 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 47 submitted 4 July 2013 7. Johnson D.M, Whittle D.P, Stringer J. Corrosion Science, 15 , 649, 1975 8. Tiwari S.N. PhD Thesis, Met. & Mat. Engg. Deptt, UOR, Roorkee, India, 1997. 9. Wood G.C, Hodgkiss T. Nature, 211, pp1358-1361, 1996 10. Hurdus M.H, Tomlinson L, Tichmarsh J.M. Oxidation of Metals, 34, 5, 1990 11. Tsaur C.-C, Rock J.C, Wang C.-J, Su Y.-H. Materials Chemistry and Physics, 89, pp445–453, 2005 12. Das D, Balasubramaniam R, Mungole M.N, Material Science and Engineering A, 338, 24, 2002 13. Sidhu B.S, Prakash S. Surface and Coating Technology, 166, 89, 2003 14. Kolta G. A, Hewaidy L. F, Felix N. S. Thermochimica Acta, 4, pp151-164, 1972 15. Kofstad P. High temperature corrosion, Elsevier Applied Science, London , 425, 1988 16. Atkinson A. Corrosion Science, 22, 347, 1982 17. Seybolt A.U. Oxidation of Metals, 2, 119, 1970 18. Wang C.J, He T.T. Oxidation of Metals, 58,415, 2002 © 2013 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 47 submitted 4 July 2013 List of Figures Figure.1 Figure.2 © 2013 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 47 submitted 4 July 2013 Figure.3 Figure.4 © 2013 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 47 submitted 4 July 2013 Figure 5 Figure 6 © 2013 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 47 submitted 4 July 2013 Legends of figures Figure.1 Macrograph welded specimen Figure.2 Plots of cumulative weight gain (mg/cm2) as a function of time Figure.3 XRD patterns for hot corroded samples at 650 °C. Figure.4 SEM/EDAX graph shows the EBW weldment of AISI 4140 and AISI 304 exposed at 650 ºC in air oxidation after 50 cycles. Figure.5 SEM/EDAX graph shows the EBW weldment of AISI 4140 and AISI 304 exposed at 650 ºC under K2SO4 + NaCl (60%) after 50 cycles. Figure.6 SEM/EDAX graph shows the EBW weldment of AISI 4140 and AISI 304 exposed at 650 ºC under Na2SO4 + V2O5 (60%) after 50 cycles. © 2013 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work.