Abstract

 Pitting scan (PS) and electrochemical impedance spectroscopy (EIS) techniques were used to evaluate the corrosion behavior of nanocrystalline (NC) samples and conventional Al 5083 (UNS A95083) in three solutions with different chloride concentrations. The PS obtained in 0.5 N sodium chloride (NaCl) and in 0.5 M sodium sulfate (Na₂SO₄) + 0.1 N NaCl indicated that pitting should occur in these solutions. These results were confirmed by EIS data that were recorded for up to 25 days. For exposure to 0.5 M Na₂SO₄ + 0.05 N NaCl the pitting (E_pit) and protection (E_prot) potentials determined after exposure for 2 h suggested that pitting would not occur in this solution. However, the initial impedance spectra were indicative of pitting. Tests carried out after exposure for 2 days and 3 days showed passive behavior, but starting with exposure for 5 days, pitting was again indicated for all three samples. PS performed at the end of the EIS tests suggested that this result was due to the increase in corrosion potential (E_corr) over the E_prot values recorded at the start of exposure. Evaluation of the samples exposed in the EIS tests showed that the pits were much smaller, but more numerous for Al 5083 after exposure to 0.5 N NaCl. Nitric acid mass loss tests (NAMLT) according to ASTM G 67 indicated that the NC samples were not susceptible to intergranular corrosion. Observation with the SEM showed intergranular corrosion for the conventional alloy, while a large number of roughly spherical holes were found for the two NC samples.