Determination of Aluminum Oxide Thickness on the Annealed Surface of 8000 Series Aluminum Foil by Fourier Transform Infrared Spectroscopy

Aluminum foil produced with prescribed thermomechanical processing route develop oxide film. Alloy chemistry and annealing practices, particularly its duration and exposed temperature, determine the characteristics of the oxide film. The magnitude and characteristics of the oxide film may impair surface features leading to serious problems in some applications, such as coating, printing and in some severe cases failure in formability. Therefore, it is important for the rolling industry to be able to monitor the oxide formation on the foil products and quantify its thickness. Well known methods to measure an oxide thickness that is in the order of nanometer, require meticulous sample preparation techniques, long duration for measurements and sophisticated equipment. However, in this study, a simple and rapid grazing angle attenuated total reflectance infrared (GA-ATR-FTIR) spectroscopic method combined with chemometrics multivariate calibration has been developed for the oxide thickness determination which is validated with x-ray photoelectron spectroscopy (XPS). 3000 and 8000 series aluminum foil materials which were produced by twin roll casting technique were used in this study. Foil samples were annealed at various different temperatures and annealing times in a laboratory scale furnace. Immediately after collecting GA-ATR-FTIR spectra, the 3000 series alloy samples were sent to a laboratory where XPS reference oxide thickness measurements had been performed. Partial Least Squares (PLS) method was used to develop a multivariate calibration model based on FTIR spectra and XPS reference oxide thickness values in order to predict the aluminum oxide thickness. The correlation coefficient of XPS reference oxide thickness values versus grazing angle ATR-FTIR based PLS predicted values was found as 0.9903 the standard error of cross validation (SECV) was found to be 0.29 nm in range of 4.9–14.0 nm for Al₂O₃. In addition, the standard error of prediction (SEP) for the validation set was 0.24 nm with the model generated with three principal components (PCs).