Coating Development and Screening

Several experiments have been used to validate the in-situ corrosion sensors, compare their results with other measurements, and evaluate different coating systems. The predictive ability of the sensor is illustrated in Figure 4. A variety of different coatings and substrates was exposed to cyclic immersion/drying/humidity conditions that have been correlated with service conditions in the automotive industry. A very good correlation was obtained between the time required for the low-frequency impedance to decrease to 107 W and the amount of corroded area on the specimen after the full exposure, as determined by the ASTM D1654 procedure. The cold-rolled steel specimen lying above the curve experienced corrosion beginning at an unprotected edge and rapidly progressed underneath the paint. The sensor detected the corrosion once it began, but the different corrosion mechanism allowed substrate degradation to occur much more rapidly.

Two examples of evaluation of different coating systems serve to illustrate the screening and evaluation capabilities of the sensor. In the first, a series of primers and appliqués were evaluated in a cyclic test designed to mimic shipboard exposure. Very little visual distinction could be made between the different panels at the end of the five-month test. The difference in the logarithm of the low-frequency impedances at the beginning and end of the test is plotted for the primer/appliqué matrix of Figure 5. The individual measurements are in the interior with the averages for the appliqués and primers in the front and right, respectively. The overall average is at the corner. The data clearly show that Film #1 is superior and that Film #2 gives the worst protection. For the primers, Primer #1 and Primer #2 were the best. Primer #7 is the worst. Based on these results, Film #1 with either Primer #1 or Primer #2 was recommended.

Several topcoat/primer systems were evaluated using another cyclic exposure test (Figure 6). The sensor measurements, which were taken periodically during the exposure, reflect the stability and water resistance of the topcoat. The panels were scribed and creepage or corrosion from the scribe was also measured. The creepage reflects the adhesion and inhibitive capability of the primer. The two parameters are correlated in Figure 6. In this figure, the best coatings would be near the origin - they would exhibit little moisture absorption and little corrosion at the scribe. The data clearly show that the two parameters are only partially related and that the primer and topcoat properties can be independently varied. Several coating systems showed significant water absorption but little spread of corrosion from the scribe. One showed significant corrosion at the scribe although the topcoat was very moisture resistant. Interestingly, the best coatings exhibited the best reproducibility from measurement-to-measurement and from specimen-to-specimen. The poorer coatings were more likely to have significant specimen-to-specimen variation or even changes in a single specimen over the course of taking duplicate measurements.

In-Situ Sensor to Detect Moisture Intrusion and Degradation of Coatings, Composites, and Adhesive Bonds, G.D. Davis, C.M. Dacres, and L.A. Krebs, DACCO SCI Inc.