Corrosion behavior of the bulk glassy (Fe 44.3 Cr 5 Co 5 Mo 12.8 Mn 11.2 C 15.8 B 5.9 ) 98.5 Y 1.5 alloy
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The free corrosion behavior and the anodic passivation ability of the bulk glassy (Fe44.3Cr5Co5Mo12.8Mn11.2C15.8B5.9)98.5Y1.5 alloy were fundamentally analyzed. Electrochemical tests were performed in aqueous solutions with pH values in the interval 0.3–14. Corrosion current densities are below 3 mA/cm2 in the entire studied pH interval; compared with commercial grade steels, for example, X210Cr12 steel, the glassy alloy has lower corrosion rates in acidic electrolytes and higher in basic solutions. The alloy exhibits high dissolution rates upon anodic polarization in acidic environments, but with increasing pH values it tends to form passive layers showing the lowest passive current density at pH 11. The passive layers consist mainly of Fe and Cr oxides. There is a strong variation of the passive layer structure and composition with pH. The protective effect of the grown surface layers is negatively influenced by sulfate ions.
I. INTRODUCTION
Fe-based bulk metallic glasses are regarded as having a high potential for industrial and engineering applications due to the high availability and relatively low cost of the main alloying element Fe and their combination of unique properties, for example, high strength and hardness, good magnetic properties, and high corrosion resistance, which are not achievable by conventional crystalline alloys.1 Recently, their previous size limitation has been greatly overcome by the discovery that minor additions of Y or lanthanides significantly improve their glass formation ability (GFA).2–4 Furthermore, by adding small amounts of Y or lanthanides, low purity raw materials and low vacuum conditions (even air atmosphere) may be used to successfully fabricate some Fe based bulk glassy alloys. This contributes to lowering the cost of these alloys.5 These newly developed alloys were named “bulk amorphous steels” (BASs). However, chemical stability can dramatically limit or enhance the applicability of any material, but this was so far scarcely studied for this new class of alloys. The most prominent of these BASs, (Fe44.3Cr5 Co5Mo12.8Mn11.2C15.8B5.9)98.5Y1.5, was chosen to fundamentally study its corrosion behavior. This glassy alloy with maximum sample thickness of 12 mm has a high thermal stability (Tg = 804 K), a high strength (3 GPa) and hardness (1224 HV), and it is nonferromagnetic at ambient temperatures (Tc = 55 K).4 a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0169 J. Mater. Res., Vol. 24, No. 4, Apr 2009
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In general, amorphous alloys are thought of as having better corrosion behavior than crystalline alloys due to their physical and chemical homogeneity.6 This means, for instance, that amorphous alloys lack the structural defects that could initiate localized corrosion. Another implication would be that more laterally uniform passive films form on glassy alloys; therefore less content of passivating elements would be needed in the composition of the alloy to ach
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