Aluminum, its Corrosion Types, and Anodization Sun Mi Kim MSE 410 Project

Outline Introduction to Aluminum  Properties  Naturally Occurring Oxide Layer Corrosion Corrosion Protection  Overview of different types  Anodization

Electromotive Force Series Aluminum has high tendency for oxidation Be and Mg are the only two structural metals more reactive

If aluminum is so reactive then why is it so widely used? Commercial leader of the non-ferrous metals Aluminum sometimes called “passive” due to its oxide layer.

Oxide Layer Al is considered “naturally passive” Oxide layer (Alumina) is formed spontaneously 2Al + 3/2 O 2  Al  Free Energy of kJ  Forms ~ 5 ms Rate of formation independent of oxygen partial pressure

Closer Look at the Oxide Layer Actually two layers First Layer:  “barrier layer”  Compact amorphous alumina  ~ 4 to 10 nm, temperature is determinant of its thickness

Closer Look at the Oxide Layer Second Layer:  Reaction with environment  Usually hydrated oxide

Corrosion on Aluminum Generally 2Al + 6H 2 O  Al 2 O 3 *3H 2 O + 3H 2 Most common type of corrosion due to weathering: pitting Galvanic corrosion also common but anodization does not prevent it

Anodization of Aluminum The purpose of this process is to increase thickness of oxide layer Idea was first published in observed that oxide film appeared when a sample was placed as an anode in an electrolysis cell Some of first anodization patents dealt with plane wings

The basic anodization scheme Current is passed through electrolyte Anion migrates to anode where oxidation occurs Reaction continues depending on oxidation mechanism

Oxidation reactions Depending on electrolyte, different oxidation reactions occur Formation of barrier growth: use of boric acid as electrolyte  Anodized film almost insoluble in electrolyte  Film still strongly adherent and non-conducting  Film grows until it prevents current from reaching anode  Product is thicker version of layer 1

Oxidation Reactions – Porous Layer Most common electrolytes – chromic, sulfuric, or oxalic acids  reaction product (film) is strongly adherent but also sparingly soluble in electrolyte  Causes formation of pores, which allow current to access the metal  As film gets thicker, electrical resistance increases  Stop when rate of film growth = rate of dissolution

Trends Lower temperature to smaller pore size  Heat increases dissolution of oxide Increasing voltage increases cell size

Trends Con’t Barrier Growth: low electrolyte concentration gives maximum thickness  Combinational process gives ~200 microns at 7% H 2 SO 4

Finishing Porous layers must be sealed Most common way is reaction with hot water  Al 2 O 3 + 3H 2 O  2AlOOH*H 2 O  Outer most oxide becomes hydrous  Less dense change in structure  Swelling causes pores to close

Applications Protective coating against corrosion and abrasion Decorative aspects – protected polished surfaces or even to provide color Base for paints and other organic finishes, which require some degree of porosity and adsorption Base for electrodeposits Nanotechnology

Oxide Thickness Reflectors: μm Decorative use (furniture, etc.): 5 – 8 μm Architectural use: 15 – 25 μm Hard anodising (industry, cookware): 50 – 100 μm

Conclusion Anodization broadens use of Aluminum Do you own any of these?

References Wernick, S.; Pinner, R.; The Surface Treatment and Finishing of Aluminum and Its Alloys. Robert Draper LTD, Teddington, ©1959 Vargel, Christian; Corrosion of Aluminum. Elsevier Ltd, Oxford, © 2004 Davis, J.R. (ed); Corrosion of Aluminum and Aluminum Alloys. ASM International, Materials Park, OH, © 1999 Richards, Joseph; Aluminium: Its Properties, Metallurgy and Alloys. Henry Carey Baird & Co., Philadelphia, © 1895 Anderson, Robert J.; The Metallurgy of Aluminium and Aluminium Alloys. Henry Carey Baird & Co., New York, © 1925 Latimer, Wendell; The Oxidation States of the Elements and their Potentials in Aqueous Solutions. Prentice-Hall, New York, ©