Novel Non Chrome Processes for the Protection of Metal Substrates

For commercial applications, specially formulated conversion coating treatments based on chromates and phosphates have been used extensively to prevent rapid Zn dissolution in aerospace, food processing and automobile industries. Chromate coatings are favored due to their barrier and corrosion resistance properties. Chromium deposition is obtained from hexavalent Cr salt baths, which are subject to stringent regulations. Due to the toxicity of Chromium and its salts, effluent treatment in the plants preparing chromate conversion coatings is a costly and time-consuming problem. Further, the European Union has banned the use of hexavalent chromium passivates in automobiles to facilitate recycling. Hence, there exists a need to develop an environmentally friendly coating that is also capable of preventing Zn dissolution. The alternative trivalent chromium that is widely seen as a replacement is less robust and lacks self healing properties. Also trivalent chromium is a temporary solution and it also toxic. Silica is a white, crystalline solid with a tetrahedral crystalline structure and a melting point of 1710 C. Due to its inert nature, silica is widely used for preventing corrosion in aggressive environments in chemical reactors. Silicadizing is a process that has been explored to protect the surface of metals from corrosion. However, the formation of silica coatings for preventing corrosion on steel surfaces remains a challenge and is being actively explored. As an alternative to chrome passivates, Elisha technologies have come out with process for depositing silica which can be used as a substitute for hex chrome. However, the process developed by Elisha lacks robustness and can not be used as a stand alone coating. At University of South Carolina, efforts are on to develop processes by which a thin uniform silica coating can be deposited. Initially two processes, namely Electrolytic process and Electroless Process were developed and patented in collaboration with Elisha technologies. Recently, novel processes based on electrocatalytic precipitation were developed independently at USC.