Silicone Ceramer Nanocoatings for Corrosion Protection of Aluminum Alloys and Galvanized Steel: Study of Coating Degradation in Natural Outdoor Environment

A common problem that industry faces is the premature failure of the organic coatings due to delamination. Organic coatings which have porous structures allow the permeation of moisture and dissolved ionic species to the metal-coating interface that often leads to de-bonding by the accumulation of moisture and corrosion products. It is therefore important to reduce permeability and to enhance the adhesive mechanism at the coating-metal interface. Hybrid ceramers (or Ormosils) are largely being used as barrier coatings for the corrosion protection of metals. Such reactive ceramer possess excellent environmental stability such as resistance to corrosive conditions and UV light degradation. The Hawaii Corrosion Lab has recently developed a novel nanocoating formulation (SiloXel^TM) that contains high amounts of silicone and relatively low amounts of hydrocarbon. The nanocoating formulation induces a unique mechanism to react with the metal substrate to form stable covalent bonds. The SiloXel^TM nanocoating forms an impervious barrier that prevents corrosion. The nanocoating composition was studied using different analytical techniques. The hardening mechanism of the nanocoating formulation was studied using FTIR and Raman spectroscopy, while the adhesive nature was studied using nanoindentation and nanoscratch testing. The coupons of three aluminum alloys (i.e. 2024Al, 6061Al, and 7075Al) were coated with SiloXel^TM and compared to that of another industrially available coating. Hot-dipped and electroplated galvanized steel were also coated with SiloXel^TM. The coated and uncoated coupons were exposed at four Hawaii outdoor test sites (representing marine, rainforest, industrial, and volcanic environments). The coupons were retrieved after exposure periods of 6 months and 12 months.  The surface of the specimens was analyzed using scanning electron microscopy.  Moreover, damage due to corrosion was also assessed with an image analyzing system (Atlas VIEEW). The corrosion results from coated and uncoated coupons were compared.