Evaluation of Anodized Aluminum Surfaces with Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy (EIS) has been used to characterize the surfaces of anodized Al alloys during exposure to . The EIS‐data show pronounced effects of anodizing procedure (sulfuric acid, chromic acid), sealing procedure (unsealed, hot water, dichromate), and alloy chemistry (Al 2024, Al 6061, Al 7075). Tests conducted after short exposure times (about 2h) can be used as a quality control test, while tests over longer exposure times give information concerning the corrosion resistance of the anodized surface. The high frequency impedance data show a pronounced difference for hot water and dichromate sealed surfaces which suggests different mechanisms of sealing. The corrosion resistance cannot be determined from the high frequency region in all cases, but becomes evident from the changes with exposure time of the low frequency region which is dominated by the properties of the barrier layer. A damage function based on the impedance at 0.1 Hz describes the corrosion behavior. EIS‐data for an anodized metal matrix composite are different from those obtained for the other Al alloys studied here. Models for the anodized Al surfaces discussed so far in the literature explain some of the features of the experimental EIS‐data, but modifications are needed to explain the spectra observed for dichromate sealing.