Electrochemical Properties of Diluted Al-Mg Alloys With Columnar-To-Equiaxed Transition

Abstract

The objective of the present research is to study the corrosion susceptibility of two Al-Mg diluted alloys (Al-0.5wt.%Mg and Al-2wt.%Mg) with different grains structures obtained by directional solidification (columnar, equiaxed and columnar-to-equiaxed transition, CET) in 0.5% NaCl solution, at room temperature. The corrosion resistance is analyzed by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques in both longitudinal and transversal sections of the samples. The columnar grain zone presents higher corrosion resistance than the equiaxed grain zone. In addition, the transversal section shows higher corrosion resistance than the longitudinal section of the samples. Then, the Al-0.5wt.% Mg alloy displays higher corrosion resistance than the Al-2wt.% Mg alloy. The values of the polarization resistance are used as a basic criterion for the evaluation of the corrosion resistance of both alloys. In this way, when the polarization resistance decreases with the increasing in the distance from the base, the grain size, secondary dendritic arm spacings and hardness increases. In addition, when the polarization resistance increases, the critical temperature gradient decreases.

Corrosion Susceptibility and Microhardness of Al-Ni Alloys with Different Grain Structures

“…The physical meaning of the elements that constitute the equivalent electrical circuit of Figure 15 is described below: RΩ represents the resistance of the electrolyte, Rp, is the resistance to polarization due to the load resistance and CPE is the associated constant phase element. The use of a CPE to replace a capacitor is due to the need to represent the non-ideal behavior of the surface due to the lack of uniformity caused by roughness, porosity, roughness or other factors [18,19,35,39]. The parameter values obtained from the EIS fit are shown in Table 3.…”

“…In general, the electrochemical response of the systems will be the result of the combination of the different structures and composition of the alloys. It has been shown that the corrosion behavior of aluminum-based alloys will depend on the nature of the secondary phases, their distribution as a result of the different microstructures, their electrochemical behavior and their interaction with the matrix [12,18,19].…”