Interpretation of EIS Data on the Aluminum Alloy in Binary Salt Hydrate System

摘要

In this study, the corrosion behavior of 6063 alumi num alloy in Na 2SO4- Na2HPO4-H2O system was studied by EIS measurements. As shown in Fig.1, with the developing depth of corrosion pits, the impedance level had shown a sharp decrease with time. A middle frequency inductive loop was found in the Nyquist plo t after 72h immersion, while in the initial (12h) and final (168h) stage of the immersion, the Nyquist plot showed two capacitive loops with absence of inductive behavior. Similar inductive loops had been reported on the EIS data in alkaline media (KOH). I n order to explain the occurrence of inductive behavior in molten salt hydrate systems, six type of different media with pH value equals 3.5, 7 and 9 were prepared, Na 2HPO4 and Na2SO4 were added separately to study influence from different type of anions. Nyquist plots in Fig.2 gave a clear conclusion that the Na 2SO4 media were more sensitive to pH changes, a low frequency inductive loop was found in the acid media showing early stage of pitting corrosion. Inductive loops were found in Na2HPO4 systems under three different pH value media. Considering the hydrolysis reaction of HPO 42-, the interim inductive behavior of impedance data in Na 2SO4-Na2HPO4-H2O system can be explained by the hydrolysis reaction of HPO42-, while the existence of SO42-resulted in a competitive reaction on the sample surface. Fig.1 Morphology of the corroded area and the corresponding Nyquist plot after (a) 12h (b) 72h and (c)168h immersion Fig.2 Nyquist plot of Al-alloy samples in saturate Na2HPO4 and Na2SO4 solutions with different pH values