This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and

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2 Corrosion Science 52 (21) Contents lists available at ScienceDirect Corrosion Science journal homepage: Corrosion behavior of nickel alloyed and austempered ductile irons in 3.5% sodium chloride Cheng-Hsun Hsu a, *, Ming-Li Chen b a Department of Materials Engineering, Tatung University, Taipei 14, Taiwan, ROC b Graduate Student, Department of Materials Engineering, Tatung University, Taipei 14, Taiwan, ROC article info abstract Article history: Received 18 March 21 Accepted 5 May 21 Available online 9 May 21 Keywords: A. Ductile iron B. XRD B. Polarization C. Corrosion inhibition In this study, the nickel alloying and austempering effects on corrosion behavior of ductile irons were investigated. The microstructure of austempered ductile iron (ADI) was analyzed by XRD, and the polarization corrosion tests were conducted using 3.5 wt.% NaCl solution. The results showed Ni-alloyed as-cast has less nodule counts than the unalloyed one; therefore, the former is more corrosion resistant than the latter. For the ADI, the nickel addition increases the retained austenite content, resulting in having better corrosion inhibition than the unalloyed ADI. Comparatively, the order of corrosion resistance in 3.5 wt.% NaCl solution is as follows: 4%Ni ADI > ADI > 4%Ni DI > DI. Ó 21 Elsevier Ltd. All rights reserved. 1. Introduction Since its discovery in 1948, ductile irons have been widely used in various industrial applications such as automotive and machine parts, tubes, and even as nuclear waste containers [1 3]. Ductile iron can acquire various desirable mechanical properties via the addition of alloying element and through heat treatments. For example, the addition of alloying elements such as nickel, molybdenum, and copper could change the microstructure and increase the hardenability of ductile irons [4,5]. Moreover, the ductile irons with enough nickel content (of up to 18%) can obtain full austenitic matrix as Ni-Resist cast irons which are primarily used for their resistance to corrosion, heat, and wear [6], thus nickel is termed as an austenite-stabilizing element [7]. On the other hand, through austempering treatment, ductile iron can be a unique material in which acicular ferrite and retained austenite are both in its microstructure. The austempering treatment allows ductile iron to have the strength and wear resistance comparable to wrought steels while retaining the low cost and design flexibility of cast irons. Austempered ductile iron (ADI) is currently the subject of much R&D, and has applications particular to many heavy machinery and transportation equipments [8 11]. Albeit the facts, most of these literatures are focused on the exploration of mechanical properties, however there is a lack of corrosive information on the material. By comparing the differences in the microstructure * Corresponding author. Tel.: ; fax: address: chhsu@ttu.edu.tw (C.-H. Hsu). and the corrosion resistance, thus this study is interesting to explore both the effects of nickel addition and austempering treatment on the ductile iron. 2. Experimental details 2.1. Material and heat treatment In this study, there were two independent variables designed as: with/without the 4 wt.% nickel alloying element, and with/ without the austempering treatment. Thus, the experimental irons were divided into four groups as follows: (1) unalloyed ductile iron (DI), (2) 4 wt.% nickel alloyed ductile iron (4%Ni DI), (3) austempered ductile iron without nickel alloying (ADI), and (4) austempered ductile iron with 4 wt.% nickel alloying (4%Ni ADI). All of the specimens in the size of 2 mm 2 mm 5 mm were obtained from Y-block ductile iron castings produced by a commercial foundry (Fig. 1). For the alloying process, pure nickel was added to the molten metal in the pouring ladle, and the molten metal was poured into green sand molds after having fully stirred the mixtures. In addition, nodularizer and inoculant were added by 1.1 and.3 wt.%, respectively, in the casting process of the irons. Chemical compositions of the nodularizer, inoculant, and both of the irons (with and without nickel addition) are listed in Table 1. In accordance to previous studies on the methods to obtain ADI [9,1], a single austempering temperature of 3 C was adopted in order to obtain ADI material in this work. The heat treatment was carried out as follows: (1) preheated at 55 C for 15 min, (2) 1-938X/$ - see front matter Ó 21 Elsevier Ltd. All rights reserved. doi:1.116/j.corsci

3 2946 C.-H. Hsu, M.-L. Chen / Corrosion Science 52 (21) Austenitizing (91 o C, 1.5 hrs) Fig. 1. Dimension of the Y-block castings in this experiment. Test specimens were cut and machined from lower portion (gray area) of the Y-block (unit: mm). austenitized at 9 C for 1.5 h; then (3) quenched in a salt bath of 3 C for 3 h, and finally (4) air-cooled to room temperature. The schematic heat-treating process is depicted in Fig. 2. Temperature Quenched in a salt bath Preheating 55 o C, 15mins Time Pearlite Austempering 3 o C, 3hrs Ausferrite R.T Microstructure analysis and corrosion test X-ray diffraction (XRD) patterns were obtained by using a Rigaku D/MAX-3A diffractometer with Cu Ka radiation for analyzing the amount of retained austenite in the ADI. The volume fraction of retained austenite was calculated from the XRD pattern using the following equation [12]: X c ¼ðI c =R c Þ=½ðI c =R c ÞþðI a =R a ÞŠ where I c and I a are the integrated intensity for austenite and ferrite, respectively; and R c and R a are the theoretical relative intensity for austenite and ferrite, respectively. Optical microscopy (OM, model OLYMPUS BH2-UMA) was utilized to observe the material s microstructure. Constituent analytical techniques were adopted and followed. From the metallograph of 1 magnification, the amount of various constituents, including nodule graphite, ferrite, and pearlite, in the microstructure were measured using an image-analyzer with the Optimas/Optimate 6.2 version software. The graphite morphology was rated for nodularity and nodule count in accordance with ASTM standard A 247 [13]. In addition, electron probe microanalyser (EPMA, model JXA-88M) was utilized to examine the dispersive extent of nickel element in the matrix of the alloyed iron. Corrosion behavior of the specimens was evaluated by anodic potentiodynamic polarization tests. A poteniostat/galvanostat (EG&G model 263A, USA) and 3.5 wt.% NaCl solution corrosive media were used to simulate the aggressive aqueous environment containing Cl ions. A standard saturated calomel electrode (SCE) was used as a reference and platinum as a counter or auxiliary electrode. The contact area in all cases was 1 cm 2 and the tests were carried out at ambient temperature. The electrode potential was raised from.9 to.2 V with the scanning rate of 1. mv/s. Corrosion current density values were obtained by Tafel extrapolation method. The percentage inhibition efficiency (w pol %) was calculated using the corrosion current densities to express the relation ð1þ Fig. 2. Schematic diagram of the austempering process in this experiment. w pol % ¼½ðI corr I corr Þ=I corrš1 where I corr and I corr are uninhibited and inhibited corrosion current densities, respectively [14]. 3. Results and discussion 3.1. Microstructure Fig. 3 shows the microstructures of the ductile irons with and without the nickel alloying and before and after the austempering treatments; the four irons are termed DI, 4%Ni DI, ADI, and 4%Ni ADI as mentioned above. From the metallographic observations and comparing the constituent amount in microstructure (Fig. 4), it can be seen that almost all of the irons have adequate nodularity of up to about 9 95%; however, there is a pronounced difference between nodule counts. The unalloyed irons (DI and ADI) have more nodule count (16 nodules/mm 2 ) than the 4 wt.% Ni-alloyed irons (4%Ni DI and 4%Ni ADI; 85 nodules/mm 2 ). This difference implies that the addition of 4 wt.% nickel could decrease the nodule count. In the as-cast microstructure, unalloyed DI presents a typical matrix consisting of mixtures of pearlite and ferrite constituents, as shown in Fig. 3a. Although the 4%Ni DI have about 16 19% graphite content, the pearlite content of it is still more than that of the DI (75% vs. 21%, respectively; Fig. 3b). These results indicate that 4 wt.% nickel appears to accelerate the formation of pearlite even though the nickel element, having similar FCC structure as austenite, is known to act as an austenite stabilizer [7]. The formation of pearlite arises from the influence of small amounts of nickel, which moves the pearlite knee to higher time intervals in solid state transformation. Thus, the eutectoid transformation of ductile cast iron during the cooling of austenite resulted in a significant amount of pearlite structure [5,15]. ð2þ Table 1 Chemical composition of nodularizer, inoculant, and the irons (wt.%). Element C Si Mn P S Mg Ni Ca RE a Fe Nodularizer Bal. Inoculant 74.5 Bal. DI Bal. 4%Ni DI Bal. a RE is rare earth elements.

4 C.-H. Hsu, M.-L. Chen / Corrosion Science 52 (21) Fig. 3. Microstructure of the irons: (a) DI and (b) 4%Ni DI, (c) ADI, and (d) 4%Ni ADI. a Nodularity (%) Nodularity Nodule count Nodule count (No./mm 2 ) further analyzed with XRD pattern as shown in Fig. 5. It is interesting to note that the 4%Ni ADI obtained more retained austenite aintensity Intensity α (11) ADI b Constituent amount (%) DI Graphite Ferrite Pearlite 4%Ni-DI γ (111) γ (2) α (2) γ (22) α (211) Theta b α (11) 4%Ni-ADI DI 4%Ni-DI Fig. 4. Comparison of the microstructure of the as-cast irons: (a) graphite morphology and (b) constituent amount. The error bars indicate the deviation of data because three measurements were done from each condition of specimens. γ (111) γ (2) α (2) γ (22) α (211) The micrographs (Fig. 3c and d) show that the ADI microstructure, besides nodular graphite, is mainly composed of acicular ferrite and retained austenite. The content of retained austenite was Theta Fig. 5. XRD pattern of the ADIs: (a) ADI and (b) 4%Ni ADI.

5 2948 C.-H. Hsu, M.-L. Chen / Corrosion Science 52 (21) Retained austenite (vol.%) ADI Specimen %Ni-ADI Current density, A cm DI 4%Ni-DI ADI %Ni-ADI Potential, V(SCE) Fig. 6. Comparison of average content of the retained austenite for ADI and 4%Ni ADI. The error bars indicate the deviation of data because three measurements were done from each condition of specimens. Fig. 8. Polarization curves of the irons in 3.5 wt.% NaCl solution. compared to the ADI, as shown in Fig. 6 (22.4 vs vol.%, respectively); however, the ADI has a denser acicular ferritic morphology than the 4%Ni ADI. The 4 wt.% nickel addition had a pronounced effect on the morphology of ADI when the nickel addition slowed down considerably the ADI transformation [16]. In addition, Fig. 7 also shows the EPMA line-scan patterns of the alloyed irons, 4%Ni DI and 4%Ni ADI, for analyzing the dispersion of nickel alloying element. The patterns demonstrate that 4 wt.% nickel could homogeneously dissolve into the matrix of ductile iron Corrosion behavior After polarization tests in 3.5 wt.% NaCl solution at room temperature, the polarization curves (E corr vs. I corr ) of the four irons were obtained as shown in Fig. 8. Various corrosion parameters such as corrosion potential (E corr ), corrosion current density (I corr ), and percentage inhibition efficiency (w pol %) are given in Table 2. The 4%Ni ADI had a significantly higher E corr value,.611 V SCE, as compared to DI (.774 V SCE ), 4%Ni DI (.659 V SCE ), and ADI (.633 V SCE ). Higher E corr value implies more stable electrode potential, which in term, indicates better corrosion resistance behavior. The other measurement, I corr, is commonly utilized as an index to evaluate the kinetics of corrosion resistance. As above mentioned, the I corr is determined by extrapolating the cathodic Tafel lines to the corrosion potential, and the corrosion rate is always proportional to the corrosion current density, i.e., the higher the I corr value, the faster the corrosive rate [17,18]. In this study, corrosion current of these irons is compared in Table 2. The DI corroded far more quickly than other irons due to its higher I corr value; that is, the alloyed and/or austempered ductile irons had an improvement on the corrosion resistance as compared to the DI ( vs lacm 2 ). In particular, the 4%Ni ADI exhibited the best corrosion resistance among these irons (3.3 lacm 2 ), which showed a maximum corrosion inhibition efficiency of 95.6%. This significant phenomenon can be attributed to the combinatory effects of nickel addition and austempering treatment. Not only nickel addition could reduce the nodule count to mitigate graphitic corrosion [18], but also austempering treatment could obtain the retained austenite content as corrosive inhibitor to increase the corrosion inhibition efficiency (see Table 2). Consequently, the order of these irons in corrosion resistance as arranged from greatest to least is as follows: 4%Ni ADI > ADI > 4%Ni DI > DI. Fig. 9 shows SEM micrographs of the four irons after polarization test. As shown in Fig. 9a and b of the ascasts, the matrix (anode) around the graphite (cathode) was severely corroded to form white FeCl 2 (Fe Cl 1? FeCl 2 ) and graphite peeled off. It was a mixed type of graphitic corrosion and uniform attack. In the case of ADIs (Fig. 9c and d), there was still a little of graphitic corrosion, but uniform attack was Table 2 Electrochemical polarization parameters for various ductile irons in 3.5 wt.% NaCl solution at room temperature. Material E corr (V) I corr (lacm 2 ) Inhibition efficiency (w pol %) Fig. 7. EPMA line-scan pattern of the irons with Ni addition: (a) 4%Ni DI and (b) 4%Ni ADI. DI %Ni DI ADI %Ni ADI

6 C.-H. Hsu, M.-L. Chen / Corrosion Science 52 (21) Fig. 9. Surface appearance of the corroded irons after the polarization tests: (a) DI, (b) 4%Ni DI, (c) ADI, and (d) 4%Ni ADI. not obvious due to the occurrence of retained austenite in the matrix. 4. Conclusions This study explores the various effects of nickel addition and austempering on the microstructure and corrosion resistance of ductile irons. Nickel (4 wt.%) addition to the ductile iron increased the pearlite content but decreased the nodule count in microstructures, thus reducing graphitic corrosion of ductile iron. Similarly, austempering produced retained austenite in ductile iron to raise its corrosion resistance. In particular, the ductile iron undergone both the 4 wt.% nickel addition and the austempering heat treatment had the greatest corrosion inhibition efficiency. The order of corrosion resistance in 3.5 wt.% NaCl solution arranged from the greatest to the least is as follows: 4%Ni ADI > ADI > 4%Ni DI > DI. Acknowledgement The authors express their sincere thanks for the financial support of the National Science Council (Taiwan, ROC) under Contract No. NSC E References [1] C.F. Walton, Iron Casting Handbook, Iron Casting Society, Inc., OH, 1981, pp [2] C.R. Loper Jr., Foundry Manage. Technol. 11 (1994) [3] R. Elliott, Cast Iron Technology, Butterworths, London, 1988, pp [4] S.K. Putatunda, Mater. Sci. Eng. A 315 (21) 7 8. [5] C.H. Hsu, M.L. Chen, C.J. Hu, Mater. Sci. Eng. A 444 (27) [6] ASTM A439: Annual Book of ASTM Standards, vol. 1.2, 1989, pp [7] W.F. Smith, Structure and Properties of Engineering Alloys, McGraw-Hill, Inc., New York, 1983, pp [8] Y.J. Kim, H. Shin, H. Park, J.D. Lim, Mater. Lett. 62 (28) [9] O. Eric, L. Sidjanin, Z. Miskovic, S. Zec, M.T. Jovanovic, Mater. Lett. 58 (24) [1] C.H. Hsu, S.C. Lee, H.P. Feng, Y.H. Shy, Metall. Mater. Trans. A 32A (21) [11] C.H. Hsu, T.L. Chuang, Metall. Mater. Trans. A 32A (21) [12] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley Publishing Company, MA, 1978, pp [13] ASTM A247: Annual Book of ASTM Standards, vol. 1.2, 199, pp [14] K.S. Jacob, G. Parameswaran, Corros. Sci. 52 (21) [15] I. Minkoff, The Physical Metallurgy of Cast Iron, John Wiley & Sons, Inc., New York, 1983, pp [16] D. Krishnaraj, H.N.L. Narasimhan, S. Seshan, AFS Trans. 1 (1992) [17] G. Rocchini, Corros. Sci. 37 (6) (1995) [18] H.H. Uhlig, Corrosion and Corrosion Control, John Wiley & Sons, Inc., New York, 1971, pp