Corrosion resistance of Ni-Zr amorphous alloys in chloride solution [Resistenza alla corrosione di leghe amorfe Ni-Zr in soluzioni di cloruri]

One of the most interesting characteristic of amorphous alloys is the absence of grain boundaries, segregations, dislocations, etc., which enhances the corrosion resistance of some kinds of amorphous alloys [1,2]. However there is a lack of information about the behaviour of these materials with respect to localized corrosion phenomena. The anodic behaviour of a series of Ni-Zr amorphous alloys (Ni33Zr67, Ni36Zr64 and Ni63Zr37) has been studied in chloride containing solutions (NaCl 0.01 M, 0.1 M and 1 M) by means of electrochemical and microstructural investigations. The alloys have been produced in the form of ribbons (1 ÷ 5 · 103 m wide and 3 · 10-5 m thick) by planar flow casting. Electrochemical characterization has been achieved by means of potentiodynamic anodic polarization curves in the above mentioned oxygen-free electrolytes. For comparison purposes, pure crystalline zirconium has been tested. In fig. 1 the anodic polarization curves recorded on the amorphous alloys and on pure Zr are shown. The shape is similar for the three solutions tested. The current density remains low in a potential range depending on the alloys composition and on the chloride concentration, when the Enp (pit nucleation potential) is reached, an abrupt increase of the current density is observed. In figg. 2 - 3 optical and SEM micrograph confirm the occurrence of localized corrosion phenomena on the amorphous alloys. In fig. 4 the dependence of Enp from Cl- concentration may be observed. The pit nucleation potential decreased linearly with the logarithm of Cl- concentration and with the Zr content in the alloy. As a matter of fact Ni33Zr67 and Ni36Zr64, show Enp values lower than the ones of pure Zr and Ni63Zr37. This behaviour is not affected by prepassivation at + 800 mV (SCE) in 0.1 M Na2SO4 solution. In fig. 5 the polarization curves recorded in 0.1 M NaCl solution on 1 h prepassivated samples are shown, while in fig. 6 the effect of prepassivation time on Enp is evidenced for the all samples. Ni63Zr37 improves the stability of its passive state, Enp reaches ≈ 350 mV(SCE) higher than the one of pure Zr, while surprisingly Ni33Zr67 and Ni36Zr64, show lower Enp values neither increased by 72 h prepassivation. In fig. 7 the cyclic voltammograms recorded in 0.1 N Na2SO4 solution give information about the stability of passive state of the alloys in absence of Cl- ions. The highly positive potentials observed for O2 evolution (≈ + 900 mV(SCE)) suggest a good electronic conductivity of the superficial layer formed on the amorphous alloys, extremely different from the insulating film formed on pure Zr. In fig. 8 the induction period of pit nucleation is plotted vs 1/ΔE. In the case of Zr-rich alloys, Ni33Zr67 and Ni36Zr64, the induction period for pitting is very short, suggesting an almost instantaneous contact of Cl- ions with the metal at defects probably generated during film growth.