An improved method of accelerated testing reduces the time needed to analyze the abilities of zinc-rich primers to protect steel substrates against corrosion in a seacoast environment. In this method, specimens are placed in racks where they are exposed to the environment. From time to time, the specimens are brought to a laboratory, where they are tested by electrochemical impedance spectroscopy (EIS). The specimens are then returned to the racks for further exposure.
Accelerated testing methods have been sought in this field because when using conventional testing methods, it takes between 1-1/2 and 5 years of exposure to obtain definitive results under natural environmental conditions. Previous attempts at accelerated testing in this field have involved the use of artificial salt sprays and salt fogs to increase rates of corrosion. Unfortunately, the results of experiments conducted under such artificial conditions have been found not to correlate well with results of experiments under natural conditions. In contrast, the improved method drastically reduces (typically to about 8 weeks) the time and thus also the cost of testing, without need for artificial conditions that distort the results.
In the improved method, each specimen from the exposure rack is immersed in a 3.55-percent NaCl solution in the test cell of a commercial EIS apparatus. Impedance measurements are taken in a typical frequency range from 0.01 Hz to 100 kHz. The results of the measurements are digitized and processed to extract parameters of an equivalent-circuit model of the galvanic and barrier mechanisms that have been postulated to explain the corrosion-inhibiting effects of the primers. The results of the impedance measurements are also typically presented as Bode plots (plots of the phase angle of impedance and of the logarithm of the modulus of impedance as functions of the logarithm of frequency) and Nyquist plots (showing the negative of the imaginary component of impedance as a function of the real component of impedance). The figure presents Bode plots for one specimen taken after various exposure times up to 1 year.
In general, the EIS measurements indicate early changes in electrical properties of primers associated with deterioration of the primers at metal/primer interfaces, before macroscopic deterioration becomes apparent. In experiments, it has been found that these properties and their changes are correlated with the long-term performances of the primers. Further research in this field will be devoted to refining the analysis of changes and improving the capability for utilizing the impedance data in their full complexity to predict long-term performances.
This work was done by Louis G. MacDowell, III and Luz M. Calle of Kennedy Space Center. For further information, access the Technical Support Package (TSP)free on-line at www.nasatech.com/tsp under the Materials category.
Inquiries concerning rights for the commercial use of this invention should be addressed to
the Technology Programs and Commercialization Office,
Kennedy Space Center,
Refer to KSC-11850.