New Generation of Anodizing

Written by Jerry L. Patel and Nannaji Saka, Ph.D.

1.0 Introduction The practice of anodizing, or controlled oxidation, of aluminum and aluminum alloys is more than seven decades old. The primary intent of anodizing aluminum and aluminum alloy parts is to protectrepparttar highly reactive surface against corrosion in aqueous environments, such as humid air and sea water. Becauserepparttar 127713 anodic coating can be produced in a variety of colors, painted anodized parts are used in architectural applications. Furthermore, becauserepparttar 127714 anodization process produces a hard ceramic coating, many times harder than that ofrepparttar 127715 substrate from which it is formed, anodic coatings are also used to protect aluminum parts from abrasion, especially sand abrasion.

2.0 Traditional Anodizing Traditional anodizing is an electrochemical oxidation process. The part to be anodized is connected torepparttar 127716 positive terminal of a Direct Current (DC) power source and a nonreactive metal, such as stainless steel, is connected torepparttar 127717 negative terminal. The aluminum part, orrepparttar 127718 anode, andrepparttar 127719 stainless steel cathode are immersed in an electrolytic bath and a DC voltage is applied across them. The potential difference is ofrepparttar 127720 order of 20 -100 V andrepparttar 127721 current densities are 1-10 A/dm2.

The electrolytic baths comprise aqueous solutions of chromic acid, orthophosphoric acid, sulfuric acid, oxalic acid, or combinations thereof. Becauserepparttar 127722 electrolytic baths have appreciable resistivity and becauserepparttar 127723 anodization process itself is exothermicrepparttar 127724 temperature ofrepparttar 127725 electrolytic bath increases greatly during anodizing.

Sincerepparttar 127726 anodizing process is quite sensitive to temperature,repparttar 127727 bath temperature is controlled rather closely by heat exchanger or refrigeration equipment. Today's advanced anodizing technologies include several proprietary hard anodizing processes that employ a wide range of electrolyte compositions, operating conditions and a limited aluminum alloy compositions.

The type and thickness of coating obtained greatly depends onrepparttar 127728 composition ofrepparttar 127729 electrolytic bath, operating conditions and alloy compositions. The military specification MIL-A-8625F, for example, lists at least six types and two classes of electrolytically formed anodic coatings on aluminum and aluminum alloys for non architectural applications.

Despiterepparttar 127730 many decades of experience andrepparttar 127731 expensive equipment employed byrepparttar 127732 traditional anodizing plants,repparttar 127733 acid bath based DC anodizing process has severe limitations.

· Byrepparttar 127734 very nature ofrepparttar 127735 low voltage DC power employed,repparttar 127736 anodic coating is quite porous. Oftenrepparttar 127737 volume percent of pores is as much as 50%. · Because ofrepparttar 127738 low current densities employed, it takes many hours to produce a coating of a few tens of micrometers thick. · The electrolytic baths comprise extremely low pH acidic electrolytes and thusrepparttar 127739 process does not meet many of today's environmental regulations. The expensive equipment, such asrepparttar 127740 electric power supplies and heat exchanger, makesrepparttar 127741 process capital intensive. · The traditional process, for reasons not quite apparent, cannot be used for anodizing aluminum alloys containing high concentrations of Cu and Si. · Thus, many aerospace and automotive parts cannot be satisfactorily anodized, if at all. · The present process, while appropriate for a limited range ofrepparttar 127742 wrought aluminum alloys, cannot be used for anodizing other reactive metals, such as Ti, Zr, Mg, etc., and intermetallic compounds and metal matrix composites. Thus, most ofrepparttar 127743 promising aluminum based advanced alloys and composites cannot be protected byrepparttar 127744 traditional anodizing process. · Above all,repparttar 127745 hardness of evenrepparttar 127746 so called hard anodic coatings is far belowrepparttar 127747 hardness of alpha alumina,repparttar 127748 principal component ofrepparttar 127749 anodic coating. Accordingly,repparttar 127750 full strength potential ofrepparttar 127751 anodic layer cannot be realized byrepparttar 127752 traditional process. · Indeed,repparttar 127753 other potentially beneficial properties of aluminum oxide, such asrepparttar 127754 high thermal and electrical resistivities andrepparttar 127755 high dielectric breakdown strength are not even addressed.

This state of affairs is primarily due torepparttar 127756 porosity ofrepparttar 127757 coating produced byrepparttar 127758 traditional acid based electrolytic processes at low power levels, and to certain extentrepparttar 127759 poor bonding betweenrepparttar 127760 aluminum alloy substrate andrepparttar 127761 anodic layer.

3.0 The Microplasmic Process In recent years,repparttar 127762 Microplasmic Corporation, a start up R&D company of Peabody, MA, U.S.A. has developed a unique anodizing technology, calledrepparttar 127763 Microplasmic Process for all types of aluminum alloys. It is an electrochemical micro arc oxidation process for which a US patent is pending. A controlled high voltage AC power is applied torepparttar 127764 aluminum part submerged in an electrolytic bath of proprietary composition. Due torepparttar 127765 high voltage and high current, intense plasma is created by micro arcing atrepparttar 127766 specimen surface and this plasma in turn oxidizesrepparttar 127767 surface ofrepparttar 127768 aluminum specimen. Thusrepparttar 127769 process is called Microplasmic Process. The oxide film is produced by subsurface oxidation and considerably thicker coatings can be produced.


Written by ARTHUR ZULU


Author: Arthur Zulu Contact Author: mailto: Copyright: Copyright © Arthur Zulu 2002 Word Count: 663 Web Address:

Publishing Guidelines: Permission is granted to publish this article electronically or in print as long asrepparttar bylines are included. A courtesy copy of your publication would be appreciated.


By Arthur Zulu

There is a mad competition going on right now at a closed petrol station by a bunch of rascals. Each one is clutching a box of matches. Who will berepparttar 127712 winner ofrepparttar 127713 contest? The first person to strike a match!

Now, suppose you were a spectator atrepparttar 127714 contest. And supposerepparttar 127715 rascals not wanting to be beaten inrepparttar 127716 game, lit their matches simultaneously. What will happen to you, repparttar 127717 rascals andrepparttar 127718 filling station? Everyone will vaporize!

But that’s a small thing -- a picnic in a park, when compared torepparttar 127719 threat ofrepparttar 127720 coming thermal nuclear “Armageddon”.

Nuclear apologists believe strongly that a nuclear holocaust isrepparttar 127721 greatest threat to our survival. So when you talk ofrepparttar 127722 Big Crunch, Asteroids, Space aliens, or a plague wiping out civilization on earth,repparttar 127723 joke is: “Here comesrepparttar 127724 dreamer!”

True,repparttar 127725 idea that man will self-destruct -- destroy himself in an all - out war, is not new. “War”, according to a historian, “is a constant in history”. Right fromrepparttar 127726 ancient times torepparttar 127727 present, it has been an instrument forrepparttar 127728 survival of nations. No wonder it has been said that if you want peace, prepare for war.

But soon, that old adage will change to: “If you want to endrepparttar 127729 world, prepare for war”. Forgetrepparttar 127730 United Nations. Their work does not includerepparttar 127731 prevention of war, norrepparttar 127732 prevention ofrepparttar 127733 world’s destruction.

Look at these quotations and you will find out that this is not a joking matter:

“We are perilously near to a new international anarchy” -- The Washington Post.

“The proliferation of super weapons is nowrepparttar 127734 most dangerous specter facing this planet” -- CRITICAL MASS by Williams E. Burrows and Roberts Windrem.

“There are now more than 50,000 nuclear weapons, … enough to obliterate a million Hiroshimas… There is little question that our global civilization would be destroyed” -- Carl Sagan.

“The outbreak ofrepparttar 127735 war in 1914 isrepparttar 127736 great turning point ofrepparttar 127737 history of humanity.” -- Peter Munch.

Cont'd on page 2 ==> © 2005
Terms of Use