The effect of crystallization on the electrochemical corrosion behaviour of amorphous Co-Si-B alloy

Abstract

The problem of stability of Co-based amorphous alloys has stimulated the present study of the effect of thermal treatment on corrosion resistance of CoSi-B amorphous alloys. Thermal treatment, especially in the case of ~nultiphase crystallization, may significantly change the electrochemical and corrosion behaviour of amorphous alloys. In the literature there are some data concerning the effect of thermal treatment on corrosion resistance of Co-based alloys. Thus, it is shown [1, 2] that the dissolution rate of amorphous Co75B25 alloy in acidic 0.5 M Na2SO 4 (pH = 1.8) increases after forming heterogeneous alloy upon crystallization. For Cos0B20 alloy, however, it is found [3] that the development of compositional inhomogeneity in the glassy alloy during crystallization does not lead to an increase in dissolution rate or to instability of the passive state in neutral borate solutions. The structural relaxation upon thermal treatment of Co-based amorphous alloys may lead to enhanced corrosion resistance as a result of formation of a denser amorphous structure and redistribution of the electron density in the alloy as found for Co70FesSilsB10 [4]. However, there is no data for the effect of crystallization on the electrochemical and corrosion behaviour of amorphous Co-Si-B alloys. This is interesting both practically, in view of the various applications of Co-Si-B alloys as magnetic soft materials [5,6], as well as for the theory of condensed unordered matter. The purpose of the present work is to study the effect of crystallization on electrochemical corrosion behaviour of amorphous Co75Si~5B~0 alloy. The amorphous alloy in the shape of ribbon (thickness of 30/xm) was obtained by rapid quenching from the melt. Some samples were heated in evacuated quartz tubes at 500°C for 12h. The thermal stability of the alloy was characterized by differential thermal analysis (DTA) and the crystalline phases were identified by X-ray diffraction analysis (CuK~ radiation). The electrochemical corrosion measurements were carried out using a potentialsweep (tmVs -~) polarization technique and a conventional three-electrodes electrochemical cell with a specially designed electrode holder [7]. As model corrosive media, O, 1N H2SO 4 and O, IN NaOH solutions were used. The electrode potentials were measured with respect to a saturated calomel electrode (SCE). Fig. 1 shows the DTA-curve of the amorphous Co75Si~5B~0 alloy. The two exothermic peaks (at 400