Evaluation of Anodized Aluminum Surfaces with Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy (EIS) has been used to characterize the surfaces of anodized A1 alloys dur-ing exposure to 0.5N NaC1. The EIS-data show pronounced effects of anodizing procedure (sulfuric acid, chromic acid), sealing procedure (unsealed, hot water, dichromate), and alloy chemistry (A12024, A1 6061, A17075). Tests conducted after short exposure t imes (about 2h) can be used as a qual i ty control test, whi le tests over longer exposure t imes give informa-t ion 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 t ime 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 SiC/A1 metal matr ix compos-ite are different from those obtained for the other A1 alloys studied here. Models for the anodized A1 surfaces discusse~i so far in the l iterature xplain some of the features of the exper imental EIS-data, but modif ications are needed to explain the spectra observed for dichromate sealing. The corrosion resistance of anodized A1 alloys prepared for mil itary applications is presently evaluated in a 300h