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 O2 Al203 Free Energy of -1675 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

Generally2Al + 6H2O

Al2O3*3H2O + 3H2

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 1857 - 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% H2SO4

Finishing

Porous layers must be sealed Most common way is reaction with hot water

Al2O3 + 3H2O 2AlOOH*H2O 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

http://astro.neutral.org/images/anodise_first_job1.jpg

Oxide Thickness

Reflectors: 2 - 4 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, © 1938

http://electrochem.cwru.edu/ed/encycl/art-a02-anodizing.htm