The Metallurgy of Anodizing Aluminum: Connecting Science to Practice

492 Pages·2018·20.85 MB·English

by  Jude Mary Runge (auth.)

Jude Mary Runge
The Metallurgy
of Anodizing
Aluminum
Connecting Science to Practice
The Metallurgy of Anodizing Aluminum
Jude Mary Runge
The Metallurgy of Anodizing
Aluminum
Connecting Science to Practice
Jude Mary Runge
Chicago, IL, USA
ISBN 978-3-319-72175-0 ISBN 978-3-319-72177-4 (eBook)
https://doi.org/10.1007/978-3-319-72177-4
Library of Congress Control Number: 2017960892
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With deepest love and gratitude, I dedicate
this book to my parents.
First to my mother, Dorothy Koop Runge,
whose constant encouragement gave me the
grit to find my passion, to practice it, and to
never let go.
And to my dad, Fred Runge, who showed me
that nonfiction books are interesting,
stimulating, and necessary for answering the
questions of how and why. He gave me
wonder and cultivated my imagination,
which taught me to ask the next important
question: What if?
Preface
For this book, The Metallurgy of Anodizing Aluminum, three goals were set: first, to
make a clear connection between corrosion science and the nucleation and growth
of the anodic oxide; second, to show that the composition and microstructure of the
base metal, as the source of the oxide, are inextricably linked to the process of oxide
growth and development; and third, more than anything else, to show that the qual-
ity of the aluminum substrate defines the quality of the anodic oxide finish. This is
not a book that teaches one how to anodize, but a book that explains what happens
when one anodizes.
However simple the anodizing process may seem, growing the anodic aluminum
oxide is a complex and multidisciplinary process that combines basic chemistry and
electrochemistry with the physical metallurgy of the substrate. Therefore, the pro-
cess of anodizing comprises engineering issues and procedures, not only in the
anodizing plant but also prior to anodizing, during substrate manufacturing. The
main ramification of specifying the anodic oxide as a design feature for an engineer-
ing application is that the component surface must be considered from alloy formu-
lation through all the various casting, deformation, and mechanical finishing
processes required to manufacture it. The interactive factors in manufacturing the
aluminum anode complicate the anodizing process, making some alloys more dif-
ficult to anodize than others.
To date, studies of the anodic oxide have only been undertaken on finished
oxides, under standard temperature and pressure, and as a result, most consider
anodic oxide growth an equilibrium process. The actual dynamic nature of the
growth and development conditions for anodic oxidation is, by definition, a non-
equilibrium process. Due to this misunderstanding, many anodizing phenomena
have not been accurately interpreted, and the anodizing industry has relied on a
substantial body of empirical knowledge.
In this book, much of the empirical knowledge that has sustained the operation
of many anodizing lines is explained by viewing anodic oxidation in terms of corro-
sion science, which elevates anodizing to an engineering process. Metallurgical
concepts bring a new level of understanding to the process, especially in terms of
the functionality and appearance of the anodic aluminum oxide (AAO). By taking a
vii
viii Preface
metallurgical approach to the formation and growth of the anodic oxide, the reader
is invited first, to develop a virtual image of the various possible types of aluminum
and aluminum alloy substrate surfaces that are used in industry, and second, to link
the surface microstructure from which the oxide is grown, by way of the reactions
that occur at the interface, to the and amazing oxide structure. In so doing, an under-
standing of the relationships between the substrate microstructure and the structure
of the AAO can be developed.
In order to develop a complete picture of the various phenomena that occur dur-
ing the dynamic process that leads to the beautiful and uniquely ordered AAO, I felt
compelled to research the history of scientific progress that enables the understand-
ing on the mechanisms that occur during anodic oxidation. A fascinating research of
supporting literature that spans over 200 years, covering significant events over the
last 5000 years, provided the foundation for Chaps. 1 and 2, which present the his-
tory of the science and technology of aluminum and anodizing. The history of alu-
minum and anodizing is presented from their alchemic origins to show the strides in
extractive metallurgy, smelting, alloy development, and even electrochemical coat-
ing processes before electricity was discovered. As the practice of alchemy bifur-
cated between magic and science, the part that evolved into science, gave birth to the
Scientific Revolution at the end of the Renaissance. From this point in time, the
concurrent technological developments in chemistry, electrical power, and metal-
lurgy, together with the related sciences of electrochemistry, thermodynamics,
atomic theory, crystallography, and diffusion continued into the Industrial Revolution,
which culminated with the Aluminum Age that brought us the anodizing industry.
In doing the research to learn and understand the history of aluminum and anod-
izing, I found that aluminum, as with most revolutionary discoveries, has a past
checkered with much drama. From its very beginnings, aluminum has caused
national rivalries, ambition, and strife; indeed, if we accept the suspicion that the
strange metal Pliny the Elder reported as “metal from clay” is aluminum, two men
lost their heads while researching and developing aluminum, one for greed in
ancient Rome (which is most likely a legend) and the other during the most famous
political revolution in France, not for his revolutionary scientific work for decom-
posing elements from their mineral states but for collecting taxes on tobacco (and
that really happened).
By making this journey through time, I entered another world where science was
truly hands-on and observation-based. By researching copies of the original docu-
ments, from the first publication of the idea that aluminum could exist as an element
by Lavoisier (1784), who knew that the limits of technology failed to provide
enough energy to reduce it, to the trials and failures to successfully reduce it by
Davy, Berzelius and Ørsted, to the time when aluminum was first successfully
reduced and characterized by Wöhler in 1827, a scientific genealogy for the discov-
ery of aluminum and ultimately anodizing developed. During this time, the failure
or success of scientific experiments was communicated and shared like a relayer’s
baton, enabling continued research and development. Original letters between the
scientists and reports from periodicals, books, and scientific meetings from esteemed
scientific organizations such as the Royal Society of London and the Electrochemical
Society are the basis for much of the history presented.
Preface ix
Once aluminum was reduced, the baton was passed to researchers who had the
vision to realize that characterization of the properties of the element was key to
unlocking its potential as an industrial material. The determination by Wöhler and
other colleagues that aluminum has a low density and high strength to weight ratio
indicated the need to develop an industrial process to extract aluminum. Government
interest in new materials, for the purpose of defense, funded the research over 20
years that followed the successful extraction of pure aluminum in 1827. Aluminum
extraction was brought, with some difficulty, to production as an industrial process
by Deville in 1855 and later refined because of improvements in electrical power, by
the Hall-Héroult process in 1886, to the extraction process used today.
In presenting the history of anodizing, another timeline develops that bridges
electricity and physics with chemistry and forms the new science of electrochemis-
try. Along this timeline, impressive names such as Volta, Faraday, Helmholtz,
Ostwald, Nernst, and Tafel are encountered, just to name a few. Prior to the discov-
ery of electricity, in order to identify the unique behavior of the newly discovered
elements, scientists characterized them by way of thermal treatment and exposure
to various acid and alkaline solutions. It was during this time that Schönbein identi-
fied the characteristic we have come to know as passivity, which, with the discovery
of electricity, became the basis for anodizing. It was soon determined that anodic
polarization could produce a change in the corrosion behavior of some metals and
that cathodic polarization could reverse it. Regarding the relationship of the discov-
ery and development of aluminum to that of anodizing, Wöhler, in 1827, determined
that aluminum is conductive, and in 1857 Heinrich Buff characterized aluminum as
a passive metal, showing that impurities in the base material changed the appear-
ance of the oxide that formed, publishing what appears to be the first paper on an
anodic aluminum oxide.
If one considers aluminum to be a rather new metallic element, then one must
consider that anodizing is an even newer process. In order for anodizing to become
an industrial process, the need for functional anodic oxides in engineering designs
had first to be established. Therefore, in deciding on how to pursue the research for
presenting the history of anodizing, it became clear that each step of the path to
anodizing, from the discovery of aluminum to the discovery of its ability to develop
a functional oxide by way of anodic polarization, had to be explored. This, as a mat-
ter of course, led to the development of an industrial scale electrochemical anodic
oxidation operation, or anodizing process. Consequently, the sections in Chap. 2
that regard the history of anodizing are devoted to the various aspects of science and
engineering that are part of the explanations for the reactions that occur during the
anodic oxidation of aluminum, as well as the engineering developments that deter-
mined the applications for the AAO.
As the timeline for sciences of electrochemistry, electrical engineering, and met-
allurgical analysis developed concurrently with the technologies extracting alumi-
num, alloy development, industrial application, and anodizing, I found myself
admiring the people who made and supported these discoveries and therefore made
tremendous strides in basic sciences, but most importantly in the young science and
industry of aluminum metallurgy and even younger science and industry of anod-
x Preface
izing. I hope that by presenting some of the important developments in metallurgy,
chemistry, and electricity as they connect with anodizing aluminum, I am honoring
the memories of the scientists and engineers by sufficiently acknowledging their
accomplishments and by showing that what was learned about aluminum and anod-
izing set the foundation of the industry that we have today.
It was important to me to understand how the anodizing industry developed from
the struggle to determine an accepted theory for the passive behavior of metals, to
characterization experiments with aluminum anodes, to the application in capaci-
tors and rectifiers, which use an entirely different type of oxide, grown with entirely
different types of electrolytes than are commonly used today for different applica-
tions. This required deep dives into old journals and proceedings of the Royal
Society and the Transactions of the American Electrochemical Society and reports
of the Electro-physical Laboratories of the Electro-technical Institute at the
Technical University of Munich, among other periodicals, proceedings, and books.
It became just as important to review the references and citations at the end of per-
tinent articles as it was to read them, to find new original sources for ideas that
translated in more and better detail, the reasons we have accepted for theories of
anodic oxidation. Regardless of the conclusions drawn, all of the research consulted
was tremendously insightful and germane to the process of anodizing.
In every publication that I have reviewed, from Setoh and Miyata (1932), to
Wernick, Pinner, and Sheasby (as early as 1953), to Diggle (1968), to Lee (2014),
references to the work of Adolf GüntherSchulze are made, and very difficult to
locate. In constructing the history of anodizing, I was determined to find his work,
in order to understand its importance. What I found is that over the period of time
from about 1903 to 1910, Adolf GüntherSchulze worked with a simple electro-
chemical cell, characterizing a variety of metal anodes, in a variety of electrolytes.
In 1906, he published his treatise, “Über das Verhalten von Aluminiumanoden” (On
the Behavior of Aluminum Anodes) in Annalen der Physik. In 1907, he published
studies on Tantalum and in 1909 on Niobium. In his last paper directly written on
the subject in 1909, the description of both a passive film and an anodic oxide was
fully developed. This description and explanation of anodic oxide formation, with
only minor modifications, is the same description and explanation considered by
most familiar with anodizing processes today.
By coincidence, in researching GüntherSchulze, I found reference to one of my
own ancestors, a German physicist and mathematician by the name of Iris Runge.
Dr. Runge initially could only attend lectures at the university because women
weren’t allowed to formally study until 1908. She began her career as a teacher and
nevertheless continued her research and studies, ultimately receiving a university
degree and doctorate in 1921. Her dissertation was entitled “Über Diffusion in Feste
Zustände,” “On Diffusion in Solids.” She is the only female researcher that I found
with some connection to anodic oxidation, who, as a colleague of Nernst and
GüntherSchulze, contributed to the sciences of electrochemistry and diffusion.
Social and political issues certainly appear to have impacted how we generally
view the theory of anodic oxidation. Brilliant treatises that focus on anodic alu-
mina growth and structure were written in Germany during the 1930s that exceed
Preface xi
the foundation provided by GüntherSchulze. Under Schumann, Baumann and
Rummel wrote significant papers that identify the columnar structure of the oxide
and suggest its function as a semiconductor. These papers even discuss the nature
of plasma electrolytic oxidation and the effects of pulse anodizing and anodizing
with AC current. In spite of the depth of this research, these papers are given only
a cursory acknowledgement in American and British anodizing publications that
follow the Second World War. Once I realized the wealth of interesting and impor-
tant data and information that were contained in these papers, together with my
husband, these articles were carefully translated so they could be included as part
of the history of anodizing.
Many great comprehensive papers go into more detail about the various theories
for anodizing; that written by Diggle, Downie, and Goulding in 1968 covers in
detail, between 1930 and 1968, many of the theories and concepts for compact
(barrier-type) oxides and porous oxides. The paper by Poinern, Ali, and Fawcett in
2011 provides a review of past processes and updates the concepts to include addi-
tional insights into the use of AAO as templates for nano-technological applica-
tions; that by Lee and Park in 2014 provides a review of many of the classic theories
for anodizing and covers some of the interesting current research. These papers are
recommended for the details that exceed the scope of this book. Of course, the com-
prehensive, 2-volume set—The Surface Treatment and Finishing of Aluminium and
Its Alloys, by Wernick, Pinner, and Sheasby, with six editions from 1956 to 2001—
fills the gap between Diggle’s paper and Poinern’s review. Arthur Brace’s The
Technology of Anodizing Aluminum is a necessary reference book for all interested
in historical yet applicable concepts and practices for anodizing; the most current
version of Brace’s book was published in 2003. A new book about the science of
anodizing is therefore timely and necessary in order to—once and for all, definitely,
and in clear terms—link the reactions that occur during anodizing to the metallurgy
of the substrate. I believe that I have succeeded in fulfilling this need with The
Metallurgy of Anodizing Aluminum.
Chapters 1 and 2 are not meant to be an entirely comprehensive history of alumi-
num and aluminum anodizing. What was desired was to present the interdisciplin-
ary path to aluminum and anodizing science and technology in a coherent,
progressive way in order to complement the following chapters, which focus on the
reactions that occur during anodizing in every part of the electrochemical circuit to
form the highly ordered nanoscale network that is the anodic aluminum oxide,
including its function and application. Without the curiosity, invention, and courage
of our scientific and technological forerunners, the discovery of aluminum, its natu-
ral passivity, the understanding the structure and properties of the aluminum oxide,
and the anodizing process itself would perhaps not exist today. It is hoped that the
background provided leads to continued insight into the science and technology of
anodizing aluminum and its alloys.
Chapter 3 presents a basic overview of industrial anodizing that simplifies the
operation by presenting the anodizing process as an electrical circuit comprised of
a power supply, an electrolyte, a cathode, and the aluminum anode. Images of
anodic oxides produced with different electrolytes by different process parameters