An Evaluation of the use of ZRC on Aluminum in the Prohesion/QUV Environment
Test (ASTM D5894)
Films of ZRC on aluminum panels have been evaluated against control coupons of aluminum in the Prohesion and Cyclic Prohesion/QUV (ASTM D5894) accelerated corrosion tests.
The Prohesion panels were subjected to 2000 hours of a cyclic exposure to an electrolytic solution containing 0.05% sodium chloride and 0.35% ammonium sulfate at 25 ° Celsius for one hour followed by a dry off period of one hour at 35 ° Celsius. The solution acidity was maintained between pH 5.0 and 5.4. The aluminum control panels exhibited numerous areas of localized oxidation forming blisters of a dry, powdery oxide. The ZRC coated panels, which were scribed to expose bare aluminum, show only the formation of zinc corrosion product and no degradation of the aluminum surface. This data indicates that in this test environment, the zinc in ZRC is anodic to the aluminum, thereby protecting the aluminum from corrosive attack.
The Prohesion/QUV panels were subjected to 1200 total hours of the cyclic exposure described above and 800 hours of cyclic exposure to Ultraviolet Light at 60 ° Celsius for four hours followed by four hours of dark condensation at 50 ° Celsius(ASTM G53). The panels were alternated weekly between the Prohesion exposure and the Prohesion/QUV exposure. This cyclic testing procedure is described at length in ASTM D5894. The aluminum control panels faired much better in this cyclic test than in straight Prohesion. The panels show fewer localized areas of oxidation and still exhibit some of the aluminum's original shininess. The ZRC coated panels exhibit no hint of aluminum degradation, but have formed substantially more zinc corrosion product than in the straight Prohesion test. This data suggests that, in the Prohesion/QUV cyclic test, the zinc in ZRC is still anodic to the aluminum, thereby protecting it from corrosive attack, but that this environment is harsher than the straight Prohesion test leading to faster anode consumption.
In either test, it has been shown that ZRC is effective in protecting aluminum from oxidation.