Arivarasu M, Manikandan M, Gokulkumar K, Karthikeyan S, Devendranath Ramkumar K and Arivazhagan N
Keywords: Welding, Corrosion, Scanning electron microscopy
Abstract:
Hot corrosion of Gas Tungsten Arc Welded (GTAW) AISI 304 and AISI 4140 dissimilar weldment exposed in air as well as molten salt environment of Na2SO4-60%V2O5 and K2SO4-60% NaCl are discussed. Weight gain studies were done for composite specimens containing both weld metal and heat-affected zone. The results indicated that the specimens were more corroded in molten salt environment as compared to air oxidation. Also weld interface of the samples showed more attack than base metals.
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ISSN 1466-8858 Volume 16, Preprint 46 submitted 4 July 2013 Hot corrosion Studies on Gas Tungsten Arc Welded AISI 304 and AISI 4140 Dissimilar Metals 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: Welding; Corrosion; Scanning electron microscopy Abstract Hot corrosion of Gas Tungsten Arc Welded (GTAW) AISI 304 and AISI 4140 dissimilar weldment exposed in air as well as molten salt environment of Na 2SO4-60%V2O5 and K2SO4-60% NaCl are discussed. Weight gain studies were done for composite specimens containing both weld metal and heat-affected zone. The results indicated that the specimens were more corroded in molten salt environment as compared to air oxidation. Also weld interface of the samples showed more attack than base metals. © 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 46 submitted 4 July 2013 1. Introduction Dissimilar metals are widely used in critical high service temperature applications. Hence studies on their weldments have gained importance in recent past. Generally, 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]. In power plant engineering applications it is necessary to join low alloyed ferritic steels to austenitic chromium-nickel-molybdenum stainless steels. Primary boilers and heat exchangers operate at high temperatures with corrosive environmental conditions that make low-alloy steels and austenitic stainless steels the best choice [3].Primary boilers and heat exchangers operate at high temperatures with corrosive environmental conditions that make low-alloy steels and austenitic stainless steels the best choice [4]. The role played by chlorides [5], which enter through ingressed air in marine atmospheres, is also important in deciding the degree of corrosion. The role of NaCl in hot corrosion by Na2SO4 has been discussed in detail [6, 7]. In this article, the effect of air as well as mixture of Na 2SO4-60%V2O5 and K2SO4-60% NaCl on hot corrosion behaviour of GTA welded AISI 304 and AISI 4140 specimens are studied. Studies by the authors involving detailed metallurgical and mechanical properties of GTA welded samples are published elsewhere [4]. 2. Experimentation To facilitate the hot corrosion tests, the samples are cut into rectangular pieces (20 × 15 × 5 mm with weld zone in the middle of the specimens and mirror polished. © 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 46 submitted 4 July 2013 Immediately, a coating of uniform thickness with 3–5 mg/cm2 of salt mixture was applied on the preheated sample (250 °C). On these specimens, cyclic studies were performed in the air as well as molten salt (Na2SO4-60%V2O5 and K2SO4-60% NaCl) 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 spalled scale was also retained during the measurement of the weight change to determine the total rate of corrosion. The samples after corrosion tests are subjected to characterization studies using SEM/EDAX, XRD and EPMA for surface and cross-sectional analysis of the scale. 3. Results and Discussions Metals and alloys undergo oxidation when exposed at elevated temperatures in air which may be may be protective or non-protective. Whereas the metals exposed in molten salt environment could accelerate the corrosion rate due to combined form of oxidation, chloridation and sulphidation. The macrographs for hot corroded samples dictate that the weld interface is more prone to hot corrosion (Fig 1.). Fig 2 shows the 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.The parabolic rate constants Kp for wledment after exposed in air oxidation, Na2SO4-60% V2O5 and K2SO4-60% NaCl were 2.96, 7.72 © 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 and 37.85 ×10 -6 (g2 cm-4 s Volume 16, Preprint 46 -1) submitted 4 July 2013 respectively. It is noted that, the hot corrosion in molten salt environment was observed to be more extensive. Moreover, higher corrosion rate is observed during initial hours of study and is mainly attributed to the rapid oxygen pick up by diffusion of oxygen through the molten salt layer and is found to be identical to the results reported by Sidhu and Prakash [8], Tiwari and Prakash [9] during their hot corrosion studies. As revealed by XRD, different phases of various reaction products were formed on the weldments after corrosion cycles. Air oxidation at 650 °C, Fe2O3 has been predominated with small amount of NiCr2O4, NiO and FeNi. Hot corrosion under molten salt environment at 650 °C shows that Fe2O3 and Cr2O3 as the predominant phases and NiCr2O4, (Cr, Fe)2O3, FeNi and FeS are observed with low intensity. 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 [10-12]. 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 [13]. SEM/EDAX analysis of the corroded sample shows, Fe2O3 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). Corrosion morphology of the weldment exposed in K2SO4-60% NaCl shows that the weld interface is more prone © 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 46 submitted 4 July 2013 to formation of fragile scale than base metals. This implies that NaCl plays a vital role in hot corrosion [14-16]. It is observed that, the corrosion rate in K 2SO4-60% NaCl environment is higher in magnitude as compared to Na2SO4-60%V2O5 and air oxidation environments. 4.Conclusions 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 Dissimilar weldment suffered accelerated hot corrosion in the chloride mixed molten salt environment in the form of intense spalling and sputtering of its scale. 5. References [1] Hasçalik A. Ünal E. Özdemir N, Journal of Materials Science, 41, pp3233–3239 2006. [2] Wyatt L.M, Materials of construction for steam power plants, London, 182, 5 ISBN 085334 661. [3] Taktak, Sukru, Materials & Design, 28 , 6, pp1836-1843, 2007 [4] Arivazhagan N, Surendra Singh, Satya Prakash, G. M. Reddy, Materials and Design, 32 ,pp3036–3050, 2011 [5] S. Ahila, S. Ramaktishna Iyer, V.M. Radlmkrishnan and P.B.S.N.V. Prasad. Materials Letters, 16, pp130-133, 1993. © 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 [6] Volume 16, Preprint 46 submitted 4 July 2013 Arivazhagan N, Narayanan S, Surendra Singh, Satya Prakash, G.M.Reddy. Materials and Design, 34, pp459–468, 2012. [7] Arivazhagan N, Surendra Singh, Satya Prakash, G.M.Reddy. Corrosion Engineering Science and Technology, 44, 2009 [8] Sidhu B.S, Prakash S. Surface and Coating Technology , 166, 89 , 2003 [9] Tiwari S.N, Prakash S. Symposium on localised corrosion and environmental cracking, Kalpakkam (India), 33, 1997 [10] Wang C.J, He T.T. Oxidation of Metals, 58, 415, 2002. [11] Hossain M.K, Saunders S.R.J. Oxidation of Metals, 12 , 1, 1978 [12] Hiramatsu N, Uematsu Y, Tanaka T, Kinugasa M. Materials Science and Engineering A, 120, 319 , 1989 [13] Uusitalo M.A, Vuoristo P.M.J, Maantyla T.A. Corrosion Science, 46 , pp1311– 1331, 2004 [14] Seybolt A.U. Oxidation of Metals, 2, 119, 1970 [15] Seybolt A.U. Oxidation of Metals,2,161, 1970 [16] Shinata Y, Fujio T, Kokichi H. Materials Science and Engineering , 87 ,pp399405, 1987 © 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 46 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 46 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 46 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 46 submitted 4 July 2013 Legends for Figures Figure. 1 Macrographs dissimilar TIG Welded AISI 4140 and AISI 304 subjected to cyclic hot corrosion at 650 ºC. (i) Air oxidation, (ii) Na2SO4 + V2O5 (60%) and (iii) K2SO4-60% NaCl after 50 cycles. Figure. 2 Plots of cumulative weight gain (mg/cm2) as a function of time (number of cycles). Figure. 3 X-Ray diffraction patterns for hot corroded dissimilar TIG weldment of AISI 4140 and AISI 304 exposed in air, Na2SO4 + V2O5 (60%) and K2SO4 + NaCl (60%) at 650° C for 50 cycles. Figure 4. SEM/EDAX shows the GTAW weldment of AISI 4140 and AISI 304 exposed in air at 650° C after 50 cycles. Figure 5. SEM/EDAX shows the GTAW weldment of AISI 4140 and AISI 304 exposed in K2SO4 + NaCl (60%) at 650° C after 50 cycles. Figure 6. SEM/EDAX shows the GTAW weldment of AISI 4140 and AISI 304 exposed in Na2SO4 + V2O5 (60%) at 650 ᵒC 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.