Performance evaluation on the weathering resistance of two USAF coating systems (standard 85285 topcoat versus fluorinated APC topcoat) via electrochemical impedance spectroscopy

The independent and/or combined effects of UV exposure, moisture, and temperature on the barrier properties of two typical USAF coating systems were investigated. The coating systems consist of Alodine 1200 chromate conversion coat (CCC), Deft MIO-PRF 23377 epoxy primer, and two different topcoats, the standard 85285 polyurethane and the DEFT APC fluorinated polyurethane. The weathering cycles employed were: UVA at 50 and 70 °C, UVA and moisture condensation at 50, 60, and 70 °C, and xenon arc at 50 °C. The barrier properties of the coatings were evaluated periodically using electrochemical impedance spectroscopy (EIS) technique. Low frequency impedance and the coating resistance resulting from equivalent circuit modeling (ECM) were the parameters monitored as a function of aging time. No indication of degradation has been detected (after 5040 h exposure) on either coating systems subjected to UVA exposure without moisture condensation, regardless of the aging temperature. The combination of UVA and moisture condensation induced a strong detrimental effect on the barrier properties of the APC coating system, but to a much less degree on the barrier properties of the 85285 coating system. Under the combination of UVA and moisture condensation, the temperature effect appeared to be following a descending order of 70 °C → 60 °C → 50 °C: the higher the temperature, the earlier the indication of degradation detected by EIS technique. Data analysis up to date seems to support that APC topcoat offers better weathering resistance than 85285 topcoat initially, however the long term performance proved to be otherwise.