Estudo da resistência à corrosão em ligas Al-3%Cu-xNb (x=0,5, 3 e 5%) solidificadas horizontalmente sob regime transiente de extração de calor

Abstract

Al-Cu and Al-Si casting alloys have been the best choice as materials for the automotive and aerospace industries. In the case of Al-Cu alloys of the duralumin class, they have excellent mechanical properties, however they have lower corrosive resistance than other Aluminum alloys. Niobium plays an important role in the production of metallic alloys due to its high melting point, resistance to acid attack and superconductivity at elevated temperatures. As an innovative feature, in this work, we intend to evaluate the resistance to electrochemical corrosion of Al-3Cu-xNb alloys (x=0,5, 3 and 5%), horizontally solidified under transient heat extraction conditions. To this end, horizontal solidification experiments using a water-cooled device were used to obtain the resulting ingots. Experimental curves for up to 8 metal-located thermocouples were obtained for each alloy in order to determine and evaluate effects of thermal solidification and microstructural parameters, such as solidification velocity (VL), cooling rate (TR), and secondary dendritic spacing (2), respectively, on the electrochemical potential parameters (ECORR) and corrosion current (ICORR), and polarization resistance (RP), samples from two positions 10 and 70mm from the heat transfer interface of the ingots of the investigated alloys were submitted to electrochemical tests. The electrochemical parameters ECORR and ICORR were determined by electrochemical polarization technique, initiated after 30 minutes of immersion in the working electrolyte, and RP was obtained through electrochemical impedance tests in the frequency range from 10 kHz to 10-1 Hz, obeying the stability of the capacitive arcs after two hours of the immersion of the working electrode in HCl 0,2 mol.L-1. It was verified that the experimental values found for ECORR were relatively close for all the investigated alloys and in both analyzed positions, allowing to deduce that they can assume values ranging from -711 to -694 mV, and that the solidification parameters, VL, TR and 2, as well as the Nb content had little influence, but enough to lead to different behavior in the ICORR and RP. Different effects of VL, TR and 2 on ICORR and RP were also observed for investigated alloys, since for the alloy with 0,5wt.%Nb higher VL and TR values, and smaller 2 resulted in lower ICORR values. On the other hand, for alloys with 3 and 5wt.%Nb, coarser microstructures (greater 2) have lower corrosion rates. Microstructural analyzes showed that Nb acts as a protective element against corrosive actions in the dendritic matrix. A comparative analysis with the literature was also conducted.