Analysis of Local Dissolution Behavior of Intermetallic Particles on Chromate-Treated AA1050 Using Micro-Electrochemical System

Abstract

Because of the high formability and high strength, Al-Fe alloys are used for ships, battery materials, and so on. The mechanical properties of Al-Fe alloys can be improved by controlling the shape and hardness of the intermetallic particles. However, it has been reported that some of the intermetallic particles become pit initiation sites. Because the electrode potential of the intermetallic particles, such as Al3Fe and Al8Fe2Si, is higher than that of Al matrix, oxygen reduction reaction readily occurs on the particles, resulting in the local alkalization. The increase in pH causes the dissolution of Al matrix around the intermetallic particles, and a trench is formed around the particle. A crystallographic pit is known to be initiated in the trench. To prevent the pitting corrosion, surface treatments are often applied. Chromate conversion coatings are employed to protect aluminum alloys in chloride environments. However, the usage is limited because of the toxicity of hexavalent chromium. To develop environmentally friendly coatings, the elucidation of the corrosion inhibition mechanism by chromates is needed. In this study, the local dissolution behaviors of chromate treated Al-Fe alloy was analyzed by in situ observation using micro-electrochemical system. A commercial Al-Fe alloy (AA1050) was used as the specimens. The cross-section of the specimen was polished by Ar+-ion sputtering (6 kV, 5h), and the intermetallic particles were characterized by an optical microscope and a field emission scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectroscopy (EDS). The existence of two types of the intermetallic particles was confirmed in the specimens. These were determined to be Al-Fe or Al-Fe-Si. There was no specific difference in color or morphology between the two particles. For the electrochemical measurements, the specimens were mechanically ground with SiC paper to 4000-grid and polished with a diamond paste (15, 6, 1, and 0.25 µm). Ethanol was used as a lubricant to minimize any water exposure. After the polishing, the specimens were cleaned ultrasonically with ethanol and dried by N2 gas. As a chromate treatment, the specimens were dipped in 10 mM CrO3 solution. Subsequently, the specimen surfaces were rinsed with pure water and dried by N2 gas, and the chromate-treated specimens were stored in ambient air for 24 h. To evaluate the electrochemical properties of the as-polished and chromate-treated specimens, open circuit potentials were measured in naturally aerated 0.1 M NaCl at 298 K. In this research, all potentials refer to the Ag/AgCl (3.33 M KCl) electrode. To elucidate the effect of the chromate treatment time in the CrO3 solution on the corrosion behavior of AA1050, the specimens were immersed in CrO3 solution for 0.06, 0.18 and 3.6 ks, and open circuit potential of each specimen was measured in 0.1 M NaCl (pH 6.0). In the case of the specimen treated for 0.06 ks, the open circuit potential started to increase from -0.81 to -0.63 V after 0.08 ks and became stable at -0.63 V after 1 ks. The open circuit potential of the specimen treated for 3.6 ks decreased to -0.83 V and started to increase to -0.63 V after 0.19 ks. The time before the increase in the open circuit potential in 0.1 M NaCl depended on the chromate treatment time, and it seemed that the most suitable chromate treatment time was 3.6 ks. In the following electrochemical measurements, the chromate-treated specimens which were immersed in the CrO3 solution for 3.6 ks were used. Open circuit potentials of the as-polished and chromate-treated AA1050 were measured for 1 ks in naturally aerated 0.1 M NaCl (pH 6.0). The open circuit potential of the as-polished AA1050 increased from -0.73 to -0.65 V and became stable at -0.65 V. For most of the intermetallic particles on the as-polished specimens, the color was changed after the immersion in naturally aerated 0.1 M NaCl. While trenches were formed around the Al-Fe-Si particles, no trench formation was observed around the Al-Fe particles. In the case of the chromate-treated AA1050, the open circuit potential decreased from -0.8 to -0.83 V in 0.05 ks. After 0.05 ks, the open circuit potential increased and became -0.72 V at 1 ks. Most of the intermetallic particles in the chromate-treated specimen showed no change in the surface color after the immersion. In the chromate-treated specimens, the number of the trenches around the particles was smaller than those on the as-polished specimens. The chromate treatment decreased the open circuit potential of AA1050 in naturally aerated 0.1 M NaCl and reduced the number of the intermetallic particles which acted as trenching sites.