The Behavior of Intermetallic Compounds in Aluminum during Sulfuric Acid Anodizing, Part 1: Al-Mn, Al-Fe, Al-Mg2Si, Al-Cr Alloys
The 1971 Carl E. Huessner Gold Medal Award was given to J. Cote and co-workers for the Best Paper appearing in Plating or the AES Technical Proceedings in 1970. Actually a two-part paper beginning in 1969, the first installment of the paper is republished here in a series on the AES/AESF/NASF Best Paper Awards. Their work presents very important work on how the anodizing process reacts with the other materials alloyed in with commercial aluminum grades.
J. Cote,1 E.E. Howlett,1 M.J. Wheeler2 and H.J. Lamb2
1Alcan Research and Development, Ltd., Kingston, Ontario, Canada
2Alcan Research and Development, Ltd., Banbury, England, U.K.
Originally published as J. Cote, et al., Plating, 56 (4), 386-394 (1969)
Editor's Note: This paper is part of a series on the AES/AESF/NASF Best Paper Awards. In 1971, J. Cote and co-workers received the Carl E. Huessner Gold Medal Award for Best Paper appearing in Plating in 1970. Although actually published in 1969, this particular article is part one of two, the second part being published in 1970. Although the deliberations of the Awards Committee are lost to history, one can infer that the award was based on the two-part package. A printable PDF version is available by clicking HERE.
The behavior of abnormally large intermetallic compounds in aluminum alloys, during anodic treatment in sulfuric acid under constant potential, was examined using an optical microscope and an electron probe microanalyzer. The intermetallic compounds were classified according to their reactivity during anodizing. In addition, point analysis by the electron probe microanalyzer was used to establish the degree to which alloying elements in the matrix are retained or dissolved during anodizing.
The appearance and properties of anodic films on aluminum alloys are affected by the presence of intermetallic compounds resulting from impurities or deliberate additions of alloying elements. Their effect varies with their type, size, number and distribution. Because of the many and varied applications for anodic films on aluminum and the desire for improved quality, a thorough understanding is required of the behavior of these intermetallic compounds during anodizing.
Valuable work on the behavior during anodizing of several intermetallic compounds was carried out about thirty years ago by Keller and his colleagues.1,2 They grouped the alloying elements into three classifications: those that form solid solutions and have little effect on the anodic film; those that form intermetallic compounds which are either not appreciably dissolved or oxidized by the anodic oxidation treatment, or are readily dissolved or oxidized by the treatment. Fisher and his co-workers3 investigated constituents similar to those studied by Keller, et al., but they used "pure" second phase constituents and elements in addition to commercial alloys. Their work was based on phase dissolution during anodizing in sulfuric and oxalic acids. Recently, Spooner,4 by determining the chemical composition of anodic films formed in sulfuric acid on commercial aluminum alloys, has also tried to establish which alloying elements are inert, or partially or totally dissolved. Brace has also reviewed this field.5
While the above papers provide useful information, the composition and the positive identification of the intermetallic constituents were not always determined by accurate chemical analysis. These are a serious omission, as we found in our work that the presence of impurities in certain intermetallic compounds has a drastic effect upon their anodizing behavior.
Unfortunately, with the conventional metallographic examination or chemical analysis, it is difficult to establish when an intermetallic compound, after being oxidized, has remained undissolved or has been partially dissolved. In the past few years, the advent of electron probe microanalysis has provided a highly potent tool for the study of the behavior of second phase constituents during anodizing. The point analysis facilities, whereby the composition of regions or constituents as small as three microns diameter can be readily determined, and the backscatter and x-ray scanning images are particularly useful for this type of study. The actual phase compositions in or adjacent to the anodized surface can be determined while the scanning images can be used to establish the extent to which phases are attacked and the way in which component elements are distributed after anodizing. Wood and his colleagues6 were the first to report the use of electron probe microanalysis to determine the chemical composition across anodic films. Wood and Brook7 recently substantiated the original studies and extended them to the examination of the anodic behavior in sulfuric acid of a few small constituents, illustrating this behavior with back-scattered electron and x-ray scanning images. Concurrently with our work, Guminski, et al.8 carried out similar work on coatings formed anodically in oxalic acid.
In applying the electron probe microanalyzer to this field, it was considered highly desirable to re-examine the behavior of certain of the compounds which were studied in the earlier work when only optical microscopy was available. In addition, the work has been extended to compounds not previously studied.
This first article describes the sulfuric acid anodizing behavior of four intermetallic compounds, MnAl6, FeAl3, Mg2Si and CrAl7, these being representative of the different types of behavior found with sulfuric acid anodizing. The intermetallic compounds were made by slowly freezing special alloys to obtain particles of a size convenient to study their behavior. In this, as in previously published work, the optical microscope has played a major role but here it has been supported and extended by the electron probe microanalyzer.
The four alloys used (Table 1) were prepared by melting super-purity aluminum (Alcan 99.99%) with the appropriate high purity elements: Mn (99.99%), Fe (99.92%), Si (99.8%), Mg (99.98%), Cr (99.45%). The alloys were cast (750-800°C) in a preheated (425°C) marinite* mold of 3.9 cm (1.5 in.) diameter and 10 cm (4 in.) depth, the walls of which were 5 cm (2 in.) thick. The alloys were cooled slowly (at 1.5 to 4°C/min) to promote the formation of the large intermetallic compounds. The Al-6.4 Mg-3.7 Si alloy was investigated in both the "as cast" and homogenized (16 hr at 535°C) conditions to determine if micro-coring in the "as cast" structure could have an effect on the anodic behavior of Mg2Si.
Table 1 - Spectrographic analyses of super purity aluminum base alloys.