The Behavior of Intermetallic Compounds in Aluminum during Sulfuric Acid Anodizing Part 2: Al-Cu, Al-Mg, Al-Si, Al-Ti, Al-Fe-Si, Al-Zn-Mg 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 second installment of the paper from 1970 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. Howlett1 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, 57 (5), 484-496 (1970)
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. This particular article is part two of two, the second part having been published in 1969. 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 coarse particles of five intermetallic compounds, CuAl2, beta AlMg, Si,* TiAl3, beta AlFeSi and the T (AlZnMg) phase** in high purity based aluminum alloys, during sulfuric acid anodizing under constant d-c potential, has been examined using optical microscopy and electron probe microanalysis. Point analysis was used to identify the constituents and to measure compositions within the matrices and the anodic films while the ultra-slow scan technique was used to determine concentration profiles through the anodic films. Results are compared with those of other workers in this field. As expected, Si was inert as were TiAl3 and beta AlFeSi while CuAl2, beta AlMg, and the T (AlZnMg) phase were anodized faster than the matrix and readily dissolved in the electrolyte. Of the elements in solid solution, silicon was retained in the anodic film, zinc was partly soluble, while copper, iron, and magnesium dissolved in the electrolyte.
In an earlier paper,1 we described the anodic behavior in sulfuric acid, under constant d-c potential, of a few large intermetallic compounds (MnAl6, FeAl3, Mg2Si and CrAl7) in high purity base aluminum alloys. These intermetallic compounds were respectively inert, anodized at the same rate, anodized faster, and readily dissolved in the electrolyte with respect to the matrix during the anodizing process. The anodic behavior of the intermetallic compounds was studied by making extensive use of an electron probe microanalyzer and its scanning backscatter electron and x-ray imaging systems. This tool proved very useful for the identification of intermetallic compounds (confirmed by x-ray diffraction technique) and for the quantitative analysis of intermetallic compounds, matrices and anodic films formed above them. Thus the concentration of the major alloying elements could be determined and, even with an accuracy of ± 5%, it was possible to establish whether or not the elements were retained in the anodic films.
The object of the present work is to describe the anodic behavior of another group of intermetallic compounds. The constituents were selected because of their common occurrence in various commercial aluminum alloys and because these constituents, through their anodizing characteristics, can influence the appearance and corrosion resistance of anodized products. The intermetallic compounds examined were CuAl2, beta AlMg, Si, TiAl3, beta AlFeSi and the T (AlZnMg) phase. Although the anodic behavior of these intermetallic compounds in either sulfuric acid or oxalic acid electrolytes has been reported by various workers,2-8 further evidence was obtained which confirms or questions what has been previously found.
The behavior of alloying elements in solid solution in the aluminum matrix is of importance since this may affect, to some degree, the properties of anodic films. In some instances, a different anodizing response was observed depending upon whether the elements were in solid solution or formed intermetallic compounds.1 Using a technique similar to that of Wood and Brock,8 the distribution of alloying elements across matrix-anodic film was measured by the slow-scan of an electron probe microanalyzer. The behavior of elements in solid solution was then compared with other work where such behavior was determined by the x-ray fluorescence method.9,10
The eight alloys investigated (Table 1) were prepared, as in our earlier work,1 by melting super-purity aluminum (Alcan 99.99%) with the appropriate addition of commercially high purity elements: Cu (99.96%), Si (99.8%), Ti (99.99%), Mg (99.98%), Fe (99.92%), Zn (99.99%). Some of the alloys were cast (Table 1) in a preheated marinite mold, as used previously, so providing a slow cooling rate (1.5 to 4°C/min). However, in other cases, a medium cooling rate (20 to 30°C/min) was necessary because of (1) the too great solubility of alloying element in the matrix which prevented the formation of sufficiently large intermetallic compounds under slow cooling conditions or (2) to minimize peritectic reaction around intermetallic compounds. This was accomplished by using a preheated (400°C) steel mold designed to produce tapered round bar of 2.5 to 3.8 cm by 17.8 cm long (1 to 1.5 in. by 7 in.). The mold was coated with Mica Wash*** to give more uniform cooling conditions.
Table 1 - Casting conditions and spectrographic analyses of super-purity aluminum base alloys.