Synthesis and analysis of Superhydrophobic-based corrosion protection systems for aluminum alloys and their evaluations on friction stir spot-welded metals
The friction stir spot weld (FSSW) could potentially replace rivets as in situ fasteners, but the welding process results in a loss of corrosion resistance in the weld. The cost of corrosion to the U.S. economy in 2013 rose to $1 trillion annually, and the development of more effective corrosion control methods is needed. Of the different types of corrosion, galvanic corrosion is the most common and most destructive. This research investigates the effect of a superhydrophobic coating on the galvanic corrosion mechanism in the FSSW. Dissimilar friction stir spot welds were prepared using AA2024-T3 and AA7075-T6 materials. The development of a superhydrophobic coating was focused on creating a micro-nanostructured hierarchical surface morphology with a low surface energy component. The final coating was composed of a 58.7 wt% mineral spirits. 12.8 wt% silicone (Si) component, 17.8 wt% 74 µm aluminum (Al) particles, and 10.7wt % < 45 nm zinc oxide (ZnO) particles. Testing of the coating included static water contact angle (WCA) testing, which resulted in an average WCA of 158°. Determination of the sliding angle (SA) and WCA hysteresis found a SA of 4.08° with hysteresis of 7.6°. Adhesion of the coating to the substrate was verified by cross-hatch tape testing according to ASTM D3359, which showed less than 5% loss of material. Durability of the coating was tested by exposure to a base solution with a Ph of 9, a 0.5M H2SO4 acid solution, and an acetone solvent. Only the base solution had a significant detrimental effect on the coating WCA, decreasing the WCA from 154° to 109°. Corrosion mitigation of the coating was determined by comparing electrochemical testing of the coated AA2024 specimens, coated AA7075 specimens, and coated friction stir spot welds, with uncoated FSSW and bare aluminum materials. The coating was shown to decrease the current density with a more positive potential, indicating an improvement in corrosion mitigation. The corrosion rates for the coated specimens were significantly lower than those for the bare materials.
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