Corrosion Control of Aluminium Alloys Through Novel Anodizing Processes

• Main Author: Banks, Tracey Louise
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2006
• Subjects: Chemistry; Materials science; Organic chemistry
• ISBN: 9798380512565

The corrosion control of99.99 % aluminium and AA 2014-T6 alloy substrates was investigated by forming anodic films in electrolytes containing additions of cerium (IV) sulphate or potassium permanganate. Such additions are claimed to improve the performance of the anodized substrate, whilst assisting the introduction of environmentally compliant anodizing solutions. Initially, barrier films were generated in 0.01 M ammonium pentaborate and porous films were generated in 2.4 M sulphuric acid i.e. without additions. Allowing barrier and porous film growth to be examined, from which the influence of the additions could be determined. The additions to the 0.01 M ammonium pentaborate bath had a significant effect on the voltage-time response, with only limited film growth evident during anodizing. Conversely, for films formed in 2.4 M sulphuric acid containing additions, little effect was evident in the voltage-time behaviour or the resultant porous films. Generally, the morphology of the porous films was similar following anodizing in the presence and absence of the selected additions. Thus, the pores passed perpendicularly across the film thickness from the barrier layer to the film/solution interface, with typical pore diameters within the range 7-14 nm. Considering the films formed on the 99.99 % aluminium substrates at a constant current density of 5 mA cm-2, in the presence and absence of additions, growth efficiencies within the range 60-76 % were revealed. For the AA 2014-T6 alloy substrates, efficiencies of film formation were reduced to 24-28 %. This reduction in efficiency is attributed to the copper present in the alloy, as second phase material and in the matrix. The compositions of the films, determined by Rutherford backscattering spectroscopy, can be expressed as Al2(O0.9543(SO4)0.057±0.001)3, implying the film consists of intimately connected units ofAI2O3 and Al2(SO4)3 in the amorphous film structure. The addition of cerium (TV) sulphate to the solution had no direct effect on the composition of the anodic film. Further, the addition of potassium permanganate did not influence film composition. For films formed on the AA 2014- T6 alloy the composition of the anodic film was Al2(O0.9543(SO4)0.057±0.001)3- Again, the selected additions had little or no influence on film composition. Neutral salt spray testing was employed to determine potential benefits of electrolyte addition on the corrosion resistance of the anodized substrates. Generally, there was no significant improvement from the additions on performance, whether sealed or unsealed. Conversely, sealing had a significant improvement on performance, as expected.