Phosphoric acid anodizing of aluminium

Summary

Phosphoric acid anodizing of aluminium for adhesive bonding has been studied by high resolution scanning electron microscopy. In the initial stages of anodizing, aluminium oxide protrusions form on the outer surface similar to those formed on etching in the FPL solution. On non-planar surfaces the protrusions form preferentially on ridges and merge laterally to form a continuous deposit along the ridges. When the barrier layer has grown to its voltage determined thickness, the rate of ejection of aluminium ions into solution falls, the growth of protrusions stops, the porous layer develops and dissolution of the protrusions and outer surface of the porous layer takes place as anodizing proceeds.

The spatial arrangement of pores depends on the shape of the surface at the start of anodizing. To maintain pore formation perpendicular to the surface, new pores initiate by branching above concave surfaces and pores terminate above convex surfaces and ridges. Second phase intermetallic particles in aluminium alloys similarly influence pore branching and termination.

Additional information

Notes on contributors

D. J. Arrowsmith

Dr. David Arrowsmith graduated with first class honours in metallurgy at the University of Cambridge where he was also awarded his PhD. His industrial experience includes five years at the British Aluminium Co. Ltd. He was Reader in Industrial Metallurgy at the University of Aston in Birmingham until 1982. He is at present an MOD consultant at the Admiralty Research Establishment, Portland and Hon. Editor of Trans. IMF.