Aug 13, 2007 - Cyclic and Linear Polarization of Yttrium Containing Iron-Based. Amorphous Alloys. S. D. Day, T. Lian, J. C. Farmer and Raul B. Rebak.
UCRL-CONF-233640
Cyclic and Linear Polarization of Yttrium-Containing Iron-Based Amorphous Alloys
Sumner D. Day, Tiangan Lian, Joseph C. Farmer, Raul B. Rebak August 13, 2007
Materials Science and Technology 2007 (MS&T'07) Detroit, MI, United States September 16, 2007 through September 20, 2007
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Cyclic and Linear Polarization of Yttrium Containing Iron-Based Amorphous Alloys S. D. Day, T. Lian, J. C. Farmer and Raul B. Rebak Lawrence Livermore National Laboratory Livermore, California, USA Keywords: Iron Amorphous Alloys, Yttrium, Seawater, Corrosion, Ni-Gd, 304B SS Abstract Iron-based amorphous alloys are produced by rapid solidification from the melt. These alloys may possess unique mechanical and corrosion resistant properties. The chemical composition of the alloy may influence the cooling rate that is necessary for the alloys to be completely vitreous. At the same time, the corrosion resistance of the amorphous alloys may also depend on their chemical composition. This paper examines the anodic behavior of iron-based amorphous alloys containing three different concentrations (1, 3 and 5 atomic %) of yttrium (Y) in several electrolyte solutions. Results from polarization resistance potentiodynamic polarization show that when the alloy contains 5% atomic Y, the corrosion resistance decreases. Introduction Metallic amorphous alloys or metallic glasses have been studied extensively for the last three decades due to their unique characteristics, including superior mechanical properties and corrosion resistance [1]. To produce an amorphous alloy from a liquid state, cooling rates in the order of 106 to 1 degrees Kelvin per second are required, depending on the glass forming ability of the melt [1]. The presence of yttrium (Y) in the melt may favor the formation of amorphous alloys. The amorphous alloys are chemically and structurally homogeneous since they do not contain grain boundaries, dislocations and secondary phases, which are common in the crystalline materials [1]. The corrosion resistance of amorphous alloys depends on the alloy composition [2-4]. Amorphous alloys may be more corrosion resistant than their polycrystalline cousins of equivalent composition. Amorphous alloys are hard and can be used in areas where both resistance to wear and corrosion are simultaneously needed. For example the typical Vickers hardness of the polycrystalline Alloy 22 (N06022) is 250 but the Vickers hardness of an amorphous material is higher than 1000 [5]. The fact that amorphous materials are highly corrosion resistant is generally attributed to the absence of crystalline defects in the alloy; however the actual mechanism of this resistance is still not fully understood [1]. When amorphous alloys partially or fully re-crystallize, they may lose some of their characteristic corrosion resistance. This process is called devitrification [6]. The aim of the current study was to study the effect of different amounts of yttrium in the alloy on the corrosion behavior of melt spun ribbons (MSR) in different electrolyte solutions.
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Experimental Table 1 shows the nominal chemical composition of the studied alloys. There were two polycrystalline engineering alloys (borated stainless steel, type 304B or S30466 and the Ni-Gd alloy or N06464) and three amorphous alloys. The borated stainless steel is a typical austenitic stainless steel with the addition of boron and the Ni-Gd alloy is basically the C-4 alloy with the addition of gadolinium, both intended for nuclear applications. The polycrystalline specimens were cut from thick wrought plates in prismatic form (with an connection similar to that described in ASTM G 5) [7]. The area of the 304B SS and the Ni-Gd specimens was 14.4 cm². The amorphous alloys were small ribbons approximately 20 to 50 mm long, 1 mm wide and 25 µm thick (Figure 1). The test area of the ribbons was approximately 0.4 to 1 cm². The ribbons were prepared by dropping molten metal on a water-cooled copper spinning wheel in an inert atmosphere. The initial metal temperature was 1050°C and the wheel was spinning at 17.4 m/sec. The fast cooling fabrication process made the material amorphous. The ribbon had two sides; the side that contacted the spinning wheel was slightly darker and contained small dent-like features and the side that faced away from the wheel was smoother and highly reflective (shiny). Table 1. Chemical Composition of the Studied Alloys. Alloy 304B SS Ni-Gd
Approximate Composition A – Weight %, B – Atomic % Fe-19Cr-14Ni-1.6B A Ni-16Cr-15Mo-2Gd A
SAM3X1 SAM3X3 SAM3X5
49.1Fe-19.3Cr-3.1Mo-4.1C-2.9W-1.7Mn-2.9Si-15.8B-1.1Y B 49Fe-18.8Cr-3Mo-3.9C-2.8W-1.8Mn-2.2Si-15.5B-3.1Y B 48.5Fe-18.3Cr-2.9Mo-3.9C-2.8W-1.2Mn-2.1Si-15.3B-5Y B
Type of Alloy, Specimen S30466, ASTM A 887 N06464, ASTM B 932 Amorphous Melt Spun Ribbon Amorphous Melt Spun Ribbon Amorphous Melt Spun Ribbon
A three-electrode cell (Figure 1), with a capacity of one liter, was used for all the experiments. Generally, 950 mL of electrolyte solution was used in each test. A saturated silver/silver chloride (Ag/AgCl, pre-filled with 4 M KCl saturated with AgCl) reference electrode was used for measuring the potential of the working electrode. The tests were conducted in four electrolyte solutions. Two of these solutions are concentrated versions of ground water at the Yucca Mountain site in Nevada. These multi-ionic solutions are called simulated concentrated water (SCW) and simulated acidified water (SAW) (Table 2). SCW has an alkaline pH of approximately 8 and is the solution that results by concentrating the ground water by a factor of 1000 (via the evaporation of water). The SAW solution has a pH of approximately 3 and is an acidified version of SCW to simulate the activity of microorganisms and the acidification that may result by the hydrolysis of metallic corrosion products. The bridge in the electrochemical cell between the reference electrode and the Luggin capillary was also filled with the test solution of interest. A water cooled jacket was used to maintain the reference electrode at near room temperature. A platinum (Pt) sheet welded to a Pt wire was used as a counter electrode. The electrochemical cell was heated using a heating mantle. Nitrogen gas was bubbled through the test solution for deaeration. The gas exited the cell though a condenser and a liquid trap to prevent evaporation of the solution and the ingress of air into the test cell. The deaeration was started 24 hour before the electrochemical tests. During this period the evolution
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of the corrosion potential was monitored. The electrochemical polarization measurements were conducted through a commercial potentiostat that was integrated with a desktop computer and the companion software. Table 2. Chemical Composition of the Multi-ionic Solutions (mg/L). K+
Na+
Mg2+
Ca2+
F-
Cl-
NO3-
SO42-
HCO3-
SiO2-
pH
SCW
3,400
40,900
