Engineering aluminium alloys from the 7xxx series are one of the most common materials used for aerospace applications. During service, hydrogen evolution may occur on the native aluminium oxide film, leading to hydrogen-induced degradation. Several well-known concepts in the literature explain various hydrogen embrittlement mechanisms. However, most models do not consider the initial interaction between hydrogen and the native aluminium oxide film, with only very few literature sources assessing the cathodic corrosion principles on aluminium and its alloys using electrochemical impedance spectroscopy (EIS). Pure Al 99.5 wt. % was used in this study to compare defective oxide states under hydrogen evolution with Al 7075-T6. The thesis first explores the Mott-Schottky approach to test the influence of acquisition parameters. Defect donor densities of various modified aluminium surfaces were obtained, revealing semiconduction properties under hydrogen uptake. Results on a global and local scale show n-type semiconduction behaviour in the cathodic regime. Hydrogen diffusion experiments with a portable hydrogen gas detector are also introduced through a Devanathan-Stachurski half-cell set-up. Hydrogen outgassing profiles and trap-affected hydrogen diffusion coefficients were determined with the time-lag method. Results for pure Al 99.5 wt. % agree with those reported in the literature. However, values for Al-7075-T6 indicate shortcomings of the setup as superimposed molecular hydrogen diffusion through semi-crystalline polymers introduces significant errors in the determination of accurate hydrogen diffusion coefficients. The thesis introduces a novel approach to determine surface hydrogen concentrations through laser-induced breakdown spectroscopy (LIBS). System performance characterisation was carried out using a portable system on the hydrogen H-alpha line from the Balmer series to test the influence of argon and helium atmospheres. Internal electrochemical hydrogen charged and external certified hydrogen standards were tested to establish hydrogen calibration routes for Al-7075-T6, iron and titanium samples. Results reveal a promising technique for rapid hydrogen quantification. Significant intensity errors arose from peak broadening, the laser energy fluence distribution and surface-adsorbed water molecules, leading to a hydrogen quantification procedure with high uncertainties with the current analysis procedure.
- Mott-Schottky analysis (M-S)
- EC-Pen
- hLIBS
- Hydrogen Uptake
- Al alloys
- Electrochemical Impedance Spectroscopy (EIS)
Hydrogen Surface Interaction Characteristics on Pure Al 99.5 wt. % and Aluminium alloy Al-7075-T6
Kroll, R. (Author). 1 Aug 2023
Student thesis: Phd