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 t o five aluminum alloys was tested by salt spray exposure and electroch emical impedance spectroscopy (EIS), Results were evaluated to determi ne if a relationship existed. Individual salt spray test panels, with an area of 30 in(2) (194 cm(2)), were inspected visually at regular in tervals up to 168 h of exposure. At each inspection interval, panels w ere 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 equivale nt circuit which yielded a coating resistance (R-c), which was used as a figure of merit to assess coating performance. Examination of the d ata showed both tests could be sensitive discriminators of corrosion p rotection, but that EIS was more discriminating in the extremes of coa ting performance, Analysis showed the probability of achieving a passi ng salt spray result increased as R-c increased. In the regimes where both tests were sensitive, regression analysis showed a linear relatio nship 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 t o define the minimum value for which a given coating can be expected t o attain a passing result in a 168-h salt spray test. These values wer e 2 x 10(6) Ohm-cm(2) to 5 x 10(6) Ohm-cm(2) for Al 356 (UNS A13560), Al 2024-T3 (UNS A92024), and Al 6061-T6 (UNS A96061); 1.5 x 10(7) Ohm- cm(2) for Al 7075-T6 (UNS A97075): and 2.3 x 10(6) Ohm-cm(2) for Al 30 03 (UNS A93003).