Corrosion Behavior Of Amorphous Alloys Research Articles

INFLUENCE OF NI AS MINORITY ALLOYING ELEMENT ON THE CORROSION BEHAVIOR OF AMORPHOUS AL-CU-MG ALLOYS IN CHLORIDE SOLUTION

The influence of nickel as minority alloying element on the corrosion behavior of amorphous alloys (Al74Cu16Mg10)99Ni, (Al74Cu16Mg10)98Ni2 and (Al74Cu16Mg10)97Ni3 was investigated. The amorphous alloys were obtained as ribbons by Chill Block Melt Spinning (CBMS). The amorphous structure of the alloys was proven by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The corrosion rate was calculated gravimetric using continuous immersion tests for 360 hours in 3.5% NaCl solution at a temperature of 25oC. The lowest corrosion rate was found in the alloy containing 3 at. % Ni. The chemical composition of the accumulated corrosion products was studied using XRD.The influence of nickel on the local corrosion resistance of the amorphous ribbons of (Al74Cu16Mg10)100-хNiх x = 0, 1, 2, 3% alloys was investigated electrochemically in a solution of 3.5% NaCl at 25°C. Pitting potential (Epitt) and repassivation potential (Erp) were determined. It was found that most resistant to pitting corrosion was the (Al74Cu16Mg10)97Ni3 alloy, which showed the noblest pitting potential (Epitt -0.332 V) and repassivation potential (Erp -0.530 V).All obtained corrosion test results of the nickel-containing amorphous alloys were compared to the base amorphous Al74Cu16Mg10 alloy.

Influence of Remelting Treatment on Corrosion Behavior of Amorphous Alloys

The corrosion behavior of Gd 56 Al 26 Co 18 and Sm 56 Al 26 Co 18 amorphous alloys in 0.01 mol/L NaOH solution has been researched by polarization curves, EIS technique, XRD and SEM. The free volume was also investigated by DSC technique. We find that the corrosion resistance of amorphous alloys in 0.01 mol/L NaOH solution increases after remelting. The corrosion resistance of Gd-based amorphous ribbons is better than that of the Sm-based amorphous ribbons in 0.01 mol/L NaOH solution. In addition, the amount of free volume of remelting amorphous ribbons is less than that of first-time melting amorphous ribbons.

Characterization of Ni-P and Fe-P by X-ray absorption spectroscopy

Due to the lack of long range order and hence the absence of grain boundaries, amorphous alloys have unusual physical properties. For example, they have excellent corrosion resistance. The presence of metalloids such as P and C lead to the formation of amorphous or glassy structure. In a previous study, Lalvani et al. [1] have studied the structure of these alloys by X-ray absorption spectroscopy (XANES). The energy level used in these structures was quite low (less than 2200 eV) which enabled the investigation of P K-edge. Their research indicated both a metalloid-P character in amorphous Ni-P and the transfer of charge from the Ni to the P-p orbitals in this material. They also showed that scratching of the sample leads to a p5+oxidized feature. The Fe-P sample (relative to the NiP) appeared to indicate a stronger transition of metal-P hybradization. The XANES technique is superior to the ones employing X-ray photoelectron microscopy and angleresolved XPS since the latter suffer from a number of deficiencies. They are not able to quantitatively describe the chemical composition of the passive films formed on the surface. In addition, the data obtained on the valency of species present is not clear. One of the major problems in studying the corrosion behavior of amorphous alloys in aqueous solutions is that the passive oxide layers formed are usually very thin (typically a few nanometers), and are formed under the presence of a high electric field [2]. Generally, ex situ surface analysis methods are used for investigation which involve vacuum conditions and beams of electrons or ions in the absence of electric field and electrolyte which could significantly alter the passive layers. Raman spectroscopy on the other hand could be employed in situ, however it has certain limitations as well [3]. XANES probes electron transmissions from core levels (the 1s level for the K edge) to unoccupied levels and is thus sensitive to the valance and coordination of the absorbing atom. In addition, the height of the absorption edge is proportional to the total amount of the absorbing element present in the beam (irrespective of valence) and thus can be used to monitor dissolution of elements from thin films [4]. Fe-P and Ni-P samples were produced by electrodeposition. Electrodeposition was carried out under constant current from acidic solutions on 25 micron thick copper foil. The bath composition is given in Table I. 500 ml of electrolytic solution was used in each case. Significant agitation was provided to the bath during deposition with a magnetic stirrer. The anode and cathode were placed parallel to each other at a distance of 4 cm. The current densities and temperature conditions were: 0.05–0.23 A/cm2 and 50–65 ◦C for Ni-P and 0.02– 0.1 A/cm2 and 40–50 ◦C for Fe-P deposition. The elemental composition of the deposits was found by atomic absorption spectroscopy. Additional experimental details are also provided elsewhere [5, 6]. The EXAFS spectrum (absorption vs. energy) and Fourier transformation (amplitude) of the signal vs. distance (Angstrom) are plotted for both, FeP and Ni-P samples in Figs 1–3. For the Fe-P sample, a peak is observed at the energy level of about 7100 eV which gradually decreases over higher energy levels. The spectrum corresponds to the Fe-K edge. XAS measurements are sensitive both to the atomic site electron states and the near shell coordination number/types. Using one shell data analysis, the bond length for Fe-P sample was estimated to be about 2–3A and the disorder parameter was found to be approximately 0.0066. The Fourier transformation (Fig. 2) shows one single major peak at about 2A which is significant in that it shows that there is no long distance order and only short distance order is possible. Hence, it is concluded that the sample is

Effect of structural relaxation on electrochemical corrosion behaviour of amorphous alloys

Effect of structural relaxation on electrochemical corrosion behaviour of amorphous alloys

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

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

Corrosion behavior of amorphous and crystalline Cu 50Ti 50 and Cu 50Zr 50 alloys

Corrosion rates and anodic polarization curves of amorphous and crystallinecu 50Ti 50 and Cu 50Zr 50 alloys have been examined in various acidic, neutral and alkaline solutions. The amorphous alloys are very stable in acidic and alkaline solutions, but unstable in agressive chloride solutions. The corrosion resistance of these amorphous alloys is higher than that of the crystallized alloys. The high corrosion resistance of amorphous alloys is attributable to the high chemical homogeneity of amorphous alloys without localized crystalline defects such as precipitates, segregates, grain boundaries, etc. Metalloid elements play an important role in the corrosion behavior of amorphous alloys; the addition of phosphorus to amorphous CuTi alloy greatly increases the corrosion resistance, even in 1N HCl.