Contribution to the Study of the Relation between Microstructure and Electrochemical Behavior of Iron-Based FeCoC Ternary Alloys

Hindawi Publishing Corporation Journal of Analytical Methods in Chemistry Volume 2012, Article ID 798043, 4 pages doi:10.1155/2012/798043 Research Article Contribution to the Study of the Relation between Microstructure and Electrochemical Behavior of Iron-Based FeCoC Ternary Alloys Farida Benhalla-Haddad,1 Sif Eddine Amara,1 Abdelkader Benchettara,1 Kamel Taibi,2 and Rafika Kesri1 1 Laboratory of Electrochemistry, Corrosion, Metallurgy and Inorganic Chemistry, Faculty of Chemistry, University of Science and Technology Houari Boumediene, P.O. Box 32, El-Alia, Bab Ezzouar, Algiers 16111, Algeria 2 Laboratory of Materials Science and Engineering, University of Science and Technology Houari Boumediene, P.O. Box 32, El-Alia, Bab Ezzouar, Algiers 16111, Algeria Correspondence should be addressed to Farida Benhalla-Haddad, Received 21 November 2011; Accepted 6 December 2011 Academic Editor: Christophe A. Marquette Copyright © 2012 Farida Benhalla-Haddad et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. This work deals with the relation between microstructure and electrochemical behavior of four iron-based FeCoC ternary alloys. First, the arc-melted studied alloys were characterized using differential thermal analyses and scanning electron microscopy. The established solidification sequences of these alloys show the presence of two primary crystallization phases (δ(Fe) and graphite) as well as two univariante lines : peritectic L+δ(Fe) ↔ γ(Fe) and eutectic L ↔ γ(Fe)+Cgraphite . The ternary alloys were thereafter studied in nondeaerated solution of 10−3 M NaHCO3 + 10−3 M Na2 SO4 , at 25◦ C, by means of the potentiodynamic technique. The results indicate that the corrosion resistance of the FeCoC alloys depends on the carbon amount and the morphology of the phases present in the studied alloys. 1. Introduction 2. Experiment Cobalt is one of the first transition series of elements. It lays between Fe and Ni and close to Cu in the periodic table. In nature, it shows a strong spatial association with these metals. Cobalt is a critical metal and it has many strategic and irreplaceable industrial uses (supperalloys, magnets, corrosionand wear-resistant alloys, high-speed steels, cemented carbides, diamond tool, etc.) [2–4]. Since cobalt shows great application potential, it has been widely studied. This work is an academic study. It deals with the relation between the microstructure and electrochemical behavior of four iron-based FeCoC ternary alloys. The solidification behavior of these alloys was studied in an earlier work [1]. This latter leads to the liquidus surface projection plot. In this paper, we undertake a study on electrochemical behavior of these alloys in nondeaerated solution of 10−3 M NaHCO3 + 10−3 M Na2 SO4 , at 25◦ C. The studied alloys were arc melted in an argon gas atmosphere from pure elements (iron at 99.98 pct and cobalt at 99.5 pct from Aldrich Chemical Co.) and graphite. The solidliquid and the solid-solid transformation temperatures were followed by a DTA-Netzsch 404S differential thermal analysis (cooling rate of 10 K/min) under argon atmosphere. The observation of the phases was performed using an optical microscope (ZEISSICM405) and a scanning electron microscope (SEM-JEOL). The electrochemical tests were conducted using a VoltaLAB PGZ301 potentiostat. The corrosive medium consisted of neutral aqueous solution containing 10−3 M NaHCO3 and 10−3 M Na2 SO4 . The polarisation curves are plotted in potentiodynamic mode. Potential was scanned from −0.8 V/ SCE to +1 V/SCE in the direction of the increasing potentials at a scanning rate of 1 mV/s. Before each polarisation, the 2 Journal of Analytical Methods in Chemistry Mass % Co (1499◦ C) p2 20 2 15 1 Co8 10 Co7 Co6 Co5 5 δ(Fe) Co3 Co1 Co4 Co2 Cgrapite γ(Fe) Fe ◦ e1 (1543◦ C) p1 (1153 C) 5 (1493◦ C) 10 Figure 3: Optical micrograph (×200) showing the matrix (1) and pearlite (2). Mass % C 1 2 Figure 1: Liquidus surface projection of the Fe-Co-C system in the iron-rich corner (metastable system) [1] showing the studied alloys (encircled). −1 −2 (b) log i (A/cm2 ) −3 (a) (d) −4 (c) −5 Figure 4: Co8 optical micrograph (×200) showing the matrix (1) and pearlite (2). −6 −7 −8 −1000 −750 −500 −250 (a) (b) Co2 Co3 0 250 E (mV) (c) (d) 500 750 1000 1250 Co6 Co8 Figure 2: Potentiodynamic polarisation curves of Co2, Co3, Co6, and Co8 alloys in nondeaerated solution NaHCO3 10−3 M + Na2 SO4 10−3 M, at 25◦ C. working electrodes were immersed in the test solution for 45 min. The electrochemical experiments were carried out at 25◦ C with agitation in presence of oxygen. 3. Results and Discussion In an earlier study [1], the compilation of the differential thermal analysis results in relation to the observed microstructures as well as the analysis of different phases allows us to establish the solidification paths of the studied alloys. Thus, the primary crystallization phases and the univariant reactions have been identified. The obtained results are summarized in Table 1. The proposed liquidus surface projection of Fe-Co-C system in the iron-rich corner, presented in Figure 1, shows, for the studied alloys, the presence of two primary crystallization phases (δ(Fe) and graphite) as well as two univariante lines: eutectic L ↔ γ(Fe) + Cgraphite and peritectic L + δ(Fe) ↔ γ(Fe). The studied alloys considered in this work are also shown in Figure 1 (encircled). Potentiodynamic polarisation curves of the studied alloys in nondeaerated solution containing 10−3 M NaHCO3 and 10−3 M Na2 SO4 at 25◦ C are presented in Figure 2. The corresponding electrochemical parameters are given in Table 2. We gathered in Table 3 corrosion current densities (icor ) of the ternary FeCoC alloys with, respectively, the Fe/C ratio for each alloy. The results obtained for these alloys show that the corrosion current densities increase with the diminution of the Fe/C ratio. Co6 and Co8 steels have a better corrosion resistance than Co3 and Co2 cast iron. This would be allotted to more important carbon content in cast iron. The Co8 alloy corrosion current density is slightly lower than that of Co6. For these two alloys, the effect of carbon Journal of Analytical Methods in Chemistry 3 Table 1: Compositions, transformation temperatures, and solidification sequences of FeCoC studied alloys. (∗ Temperature not detected by our differential thermal analysis apparatus limited to temperature lower than 1550◦ C). Alloy Fe Co2 Compositions (wt. %) Co 90.96 4.84 C Temperatures/(◦ C) Solidification sequences ∗ L ↔ Cgraphite L ↔ γ(Fe) L ↔ γ(Fe) + Cgraphite Pearlite L ↔ Cgraphite L ↔ γ(Fe) L ↔ γ(Fe) + Cgraphite Pearlite L ↔ δ(Fe) L + δ(Fe) ↔ γ(Fe) γ(Fe) ↔ α(Fe) Pearlite L ↔ δ(Fe) L + δ(Fe) ↔ γ(Fe) γ(Fe) ↔ α(Fe) Pearlite 1163 1150 753 4.20 ∗ Co3 89 (...truncated)