A comparative electrodissolution and localized corrosion study of 2024Al in halide media

Electrodissolution and pitting characteristics of 2024Al-T4 rotating disks in different halide solutions have been investigated by impedance spectrosc opy and other electrochemical techniques. In 1 M halide solutions and satur ated AlCl3, the pitting potential (vs. SCE), increased as follows: sat. AlC l3 (-0.75 V) < Cl- (-0.60 V) < Br- (-0.48 V) < I- (-0.30 V). Potentiodynami c measurements from -1.0 to 1.5 V show that above 0.0 V the electrodissolut ion rate increased in the order: sat. AlCl3 < I- < Br- < Cl-; at the end of each experiment the 2024A1-T4 surface was covered with a black film. The a ddition of nitrilotrismethylenetriphosphonic acid (NTMP) to chloride soluti ons had only a small effect on the pitting potential, but alloy dissolution at high anodic potentials was significantly diminished. Anodic potential s tep experiments above the pitting potential (E-np) of 2024A1-T4 in Cl-, Br- and I- for a time duration of 15 min showed that pits were initiated at th e periphery of the disk and contained black films; the number of pits incre ased as the anodic potential increased. In sat. AlCl3, there was extensive pitting and a black film covered the entire disk surface. This black film i s possibly an Al-Cu-oxyhalide mixed salt since analysis indicated the prese nce of Cu(II). Black film was not observed in pits on pure AI in these elec trolytes under similar conditions. The presence of tolyltriazole in 1 M NaC l sharply reduced the size and density of pits on 2024A1-T4. The impedance spectra of 2024A1-T4 in the four electrolytes at anodic potentials above E- np indicate two capacitive loops in the complex plane plots and two maxima at high frequencies in the corresponding phase angle Bode plots when pits, which contained black films, were clearly visible. For pure Al and below E- np for 2024A1-T4, the capacitive loop in the highest frequency region was n ot perceived. Electrical circuit analogs have been utilized to simulate the impedance data and satisfactory fits were obtained. (C) 1999 Elsevier Scie nce Ltd. All rights reserved.