Substitute conversion coatings on aluminum for waste minimization

Chromate conversion coatings such as Parker Company`s Alodine coatings are widely used to increase the corrosion resistance of aluminum and aluminum alloys. The primary disadvantage of chromate-based processes is that they use and produce as waste hexavalent chromium (Cr{sup 6+}). We have discovered that the corrosion resistance of Al can be increased by forming an inorganic barrier coating using chemicals that pose a relatively small environmental hazard. These new coatings are formed using a process that is procedurally identical to the basic chromate conversion process. We have prepared new and conventional coatings on 1100 (99.0 Al minimum), 2024-T3 (Al-Cu-Mg) and 7075-T6 (Al-Zn-Mg) commercial sheet stock for accelerated electrochemical testing and coating conductivity testing. Results show that the new coatings offer increased corrosion resistance compared to uncoated Al, but do not yet match the performance of the chromate conversion coatings. The conductivity of these new films on 1100 Al is comparable to that of Alodine coatings; however, the new coatings are more resistive than Alodine coatings on 2024-T3 and 7075-T6.