Environmentally Compliant Corrosion Resistant, & Electrically Conductive Inorganic Coatings for Aluminum Alloys

摘要

The objective of this project was to develop an environmentally compliant conversion coating for use on aerospace aluminum alloys (e.g., AA2024- T3). This conversion coating was to replace the current chromate conversion coating processes in both mode of application (bath or spray applied in the depot) and function (stand alone corrosion protection, adhesion to organic layers, self-healing, and low electrical contact resistance). Hydrotalcite (HT) was developed within this program as a replacement to chromate conversion coatings. HT coatings are formed by exposure of aluminum and its alloys to alkaline lithium salt solutions. The coating chemistry used to form these conversion coatings has many processing variables (e.g., time, temperature, anion, etc.). A Fractional Factorial Design was used to determine that temperature was one the more critical processing variables. The FFD study also determined that HT coatings formed from nitrate-based chemistries had consistently better stand-alone corrosion protection properties. Through the use of additional oxidants within the coating bath, HT coatings with the ability to withstand 168 hours of salt spray could be formed in less than 6 min. HT conversion coatings could also be post-treated (e.g., hydrothermally aged, surfactant) to revert the hydrotalcite to aluminum oxide, or augmented to include high valence-state rare earth cations (e.g., cerium). Hydrothermal aging allowed a procedure to chemically anodize aluminum, while incorporation of cerium into the molecular gallery of the hydrotalcite structure provided a means to develop self-healing characteristics, a highly sought property characteristic of chromate-based coatings. Self- healing was indeed demonstrated by the cerium doped HT coatings. The adhesion of epoxy coatings to the hydrotalcite coating was studied in detail. The Lewis-base nature of HTs makes them intrinsically less able to be wet by the Lewis- base nature of epoxy.