A non chromate conversion coating process for corrosion protection of AL2024 aluminium alloys in a marine environment

To provide corrosion protection of Aluminium 2024 and other alloys in a marine environment usually a chromate based conversion coating process is applied. However, chromates are environmentally unfriendly and efforts are underway to replace them. To develop a coating that works in similar manner, an ion with many of the same properties as chromate was required. The titanate ion was chosen for its many similarities. Electrochemical testing indicated that the titanate ion would retard corrosion in the same manner as the chromate ion. Based on successful electrochemical testing a complete conversion coating process was developed. Variables investigated included conversion solution pH and composition, process time, pre-treatment and temperature. Initially 2.5 by 10 cm samples were coated then tested in a salt spray marine environment. Several reached the 1000 hour goal with no corrosion evident. Recently, the process was scaled up to coat 7.5 by 25 cm samples. Again surface pre-treatment, process time and pH were investigated. After 336 hours of exposure, corrosion was absent on the exposed surfaces. The optimum treatment results in a "drop in" replacement for the chromate conversion coating, with only minor, advantageous variation. For example, no cyanide based chemicals are employed and the pH is more neutral at 5 rather then lower used by the chromate, both of these are environmentally friendly in addition to the removal of chromate.