A Correlation Between Salt Spray and Electrochemical Impedance Spectroscopy Test Results for Conversion-Coated Aluminum Alloys

The corrosion resistance of 33 inorganic conversion coatings applied to five aluminum alloys was tested by salt spray exposure and electrochemical impedance spectroscopy (EIS). Results were evaluated to determine if a relationship existed. Individual salt spray test panels, with an area of 30 in.² (194 cm²), were inspected visually at regular intervals up to 168 h of exposure. At each inspection interval, panels were assigned a pass rank if < 5 pits were observed, or a fail rank if > 5 pits were observed. EIS data were analyzed using a simple equivalent circuit which yielded a coating resistance (R_c) which was used as a figure of merit to assess coating performance. Examination of the data showed both tests could be sensitive discriminators of corrosion protection, but that EIS was more discriminating in the extremes of coating performance. Analysis showed the probability of achieving a passing salt spray result increased as R_c increased. In the regimes where both tests were sensitive, regression analysis showed a linear relationship existed for each alloy substrate between the log of R_c and the probability of a coating meeting the pass/fail criterion in salt spray. Based upon these relationships, threshold R_c values were proposed to define the minimum value for which a given coating can be expected to attain a passing result in a 168-h salt spray test. These values were 2 x 10⁶ Ω-cm² to 5 x 10⁶ Ω-cm² for Al 356 (UNS A13560), Al 2024-T3 (UNS A92024), and Al 6061-T6 (UNS A96061); 1.5 x 10⁷ Ω-cm² for Al 7075-T6 (UNS A97075); and 2.3 x 10⁶ Ω-cm² for Al 3003 (UNS A93003).