The Impact of Hard Anodizing Conditions on the Growth Mechanism, Electrochemical Response and Protective Properties of Porous Alumina Films

  • Jiru Qin

Student thesis: Phd


The primary objective of this thesis is to investigate porous anodic oxide growth under high current density conditions, which are industrially relevant both for the purpose of hard anodic oxide films and for the development of rapid anodizing cycles. The research specifically focuses on exploring the growth mechanisms of anodic films under fast anodizing conditions, commonly referred to as hard anodizing, and assessing the influence of anodizing conditions on both the mechanism of growth, morphology and corrosion resistance of the resulting films. In this work, a new model derived from the field-assisted flow theory is proposed to account for the observations during hard anodizing. Specifically, it is proposed that, under soft anodizing conditions, the homogeneity of the film thickness arises from the fact that the flow of the plasticized barrier layer during anodizing is regulated by the mechanical constraint provided by the rigid cell wall material connecting adjacent pores. Vice versa, under hard anodizing conditions, rapid film growth leads to the cracking of the rigid layer connecting adjacent pores and subsequent loss of the regulation mechanism provided by the cell walls, ultimately resulting in localized oxide growth. The hard anodizing film formation behaviour was further investigated by in-situ EIS measurements. It was found that the estimation of barrier layer thickness by in-situ EIS measurements during hard anodizing is inaccurate due to the formation of an electrolyte composition gradient, which arises due to diffusion limited transport of anions from the pore mouth to the pore base. The pore rearrangement observed during hard anodizing may also contribute to the reduced availability of electrolyte anions at the pore base, and this might impact the film growth mechanism and resulting film morphology. A morphological transition from sponge-like cells to linear cells and increasing barrier layer thickness was observed for the PAA films formed on AA 2024-T3 alloy. The transition occurred due to an increase in current density or a decrease in electrolyte temperature. Corrosion tests and EIS measurements demonstrated that the presence of sponge-like cells in the PAA films enhances the self-sealing properties of the films by increasing the contact area between the porous film and electrolyte and, hence, favouring the hydration processes. This ultimately leads to an improved anticorrosion corrosion performance under immersion testing conditions.
Date of Award31 Dec 2023
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorXiaorong Zhou (Supervisor) & Michele Curioni (Supervisor)


  • Anodic Film
  • Anodizing
  • Hard Anodizing

Cite this