Date of Award
Doctor of Philosophy in Chemical Engineering - (Ph.D.)
Chemical and Materials Engineering
R. P. T. Tomkins
Kamalesh K. Sirkar
David C. Venerus
Aluminum anodizing has been experimented with and studied over the last century because of its ability to form uniform, well ordered cellular coatings on aluminum alloys. Anodizing aerospace alloys has been problematic, due to the alloying elements used to add strength and resistance to stress cracking corrosion. These intermetallic compounds, i.e., copper and zinc, promote oxygen evolution and stress as they accumulate in and on the surface of the forming aluminum oxide. These inclusions lead to increased electrical resistance that forms porous and flawed coating, which can lead to industrial and field failures. The amount of voltage placed on the electrochemical system is shown to alter the current density and has positive and negative effects on coating growth and quality. The magnitude of this variable has been manipulated to control the amount of coating formed and enhance film properties. Changing current density can reduce the amount of overall voltage on the system and reduce the amount of intermetallic contamination in the coating.
In this work, the investigation of the influence of time variation of current density is applied to AA7075-T6 and AA2024-T3 in the course of anodization, on the ramping stage and overall process. These two alloys are chosen because they represent the most used alloys in the aerospace industry and exhibit distinct and unique electrochemical behavior. The experimental work includes monitoring process kinetics and rates of formation of anodic aluminum oxides. Changes in pore structure of anodic coatings, including size, shape and distribution are observed under high scanning electron magnification. Utilizing energy dispersive X-ray spectroscopy and X-ray Diffraction, changes in the alloy intermetallic composition and microstructure are analyzed. Lastly, surface characteristic experiments which measure anodic aluminum oxide compactness, hardness, thickness, and corrosion resistance are conducted.
Utilizing low voltage at the onset of anodizing and gradually raising current density allows for a more aluminum rich oxide and allows time for nonaluminum ions to leave the forming coating or become oxidized. It is expected that using low voltages during the ramp phase of the process will lead to changes in industrial processing as it allows for larger loads to be processed faster with less defective coating.
Totaro, Peter Jr., "Effect of current density ramping on the anodic reaction and morphology of aerospace aluminum alloys" (2022). Dissertations. 1657.