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Sealing: Enhance Anodic Coatings' Performance

by Ling Hao, Ph.D.

©2001 METALAST International Inc. All Rights Reserved. "METALAST" and the METALAST Logo are registered trademarks of METALAST International Inc.

May not be republished or redistributed without the written consent of METALAST International, Inc.® http://www.metalast.com

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Abstract

This article briefly describes sealing purposes, sealing mechanisms, traditional and emerging seals, and classification of different sealing processes. A general guideline is presented to select an adequate sealing process for a specific application. A number of standard methods for evaluation of sealing performance and the problems in sealing are discussed in the article. A number of important publications addressing the sealing topic are listed for readers who are interested in more details about the advances in sealing over the decades.

What is sealing?

It is well established that the anodic coatings formed on aluminum in sulfuric acid, phosphoric acid, or oxalic acid electrolyte consist of two portions, a very thin non-porous barrier oxide layer and a relatively thick porous layer. The highly porous oxide films contain a huge number of tiny cells, each with a central pore, as shown in Figure 1(a). It is understandable that the porous structure, with a very high specific surface area, has a strong tendency to absorb water and other aggressive agents from the surrounding environment, leading to structural and physical damages of the anodized aluminum. Without further treatment, the anti-corrosion performance of anodized aluminum depends on the physical and chemical properties of the barrier layer of the resultant anodic coating. In order to protect the anodized aluminum, a wide range of post treatment processes have been developed and utilized in practice over the decades. Sealing is a post treatment of anodizing to fill and close or plug the micro pores of anodic coatings by means of chemical conversion, physical absorption, or mechanical impregnation that takes place within the pores, as shown in Figure 1(b). As a result, sealing is able to either block the channels through which corrosive media attack the aluminum substrate or provide the anodic coatings with corrosion inhibitors entrapped in the micro pores, resulting in a substantial enhancement of the corrosion resistance of the anodic coatings.

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What are the functions of sealing?

The initial and principal purpose of sealing is to further improve the corrosion resistance of anodized aluminum. Later research revealed that sealing with solutions

containing transition metal salts are also capable of preventing the leaching of organic dyes absorbed into the micro pores of anodic coatings and enhancing the light fastness of the dyed anodic oxide. Hydrothermal sealing also increases the stain resistance of the oxide. In some electronic and semiconductor applications, sealing is employed to enhance the dielectric strength of the resultant anodic coatings on aluminum industries.

Hydrothermal Seals

The mechanism of sealing anodic coatings is quite complicated and varies with each particular process. The understanding of sealing is essentially based on the investigation of the hydrothermal process carried out in hot water or steam at temperatures above 95 °C. It is generally accepted that in a hydrothermal sealing process the anhydrous oxide (Al2 O3 ) in an anodic coating is partially hydrated to form boehmite-like crystals (AlO(OH)). The

basic reaction of hydrothermal sealing may be expressed by the following transformation taking place at temperatures above 80 °C: Al2 O3 (anodic coating) + H2 O 2AlO(OH) (boehmite) (1) Because boehmite (3.44 g/cm3 ) has a larger volume than aluminum oxide (3.97 g/cm3 ) and two moles of boehmite can be formed from one mole of aluminum oxide, the micro pores are eventually blocked and closed by the expansion of the cell walls of the anodic coating. Theoretically, the conversion of each mole of aluminum oxide to boehmite leads to a 35.8% increase in volume. Figure 2 provides a general description of a dynamic hydrothermal sealing process. It has become very popular to add hydrolysable salts and organic additives to water, which improves the sealing performance and efficiency, saves energy, and minimizes the formation of smut on the surface of anodic coatings. A typical example is the nickel acetate

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seal. It is believed that the nickel ions play a catalytic role in hydrating aluminum oxide to boehmite through the co-precipitation of nickel hydroxide (Ni(OH)2 ): Ni2+ + 2OH - Ni(OH) 2 (2)

Precipitating Seals

Although the bath temperature is high in both dichromate sealing and silicate sealing, little thermal hydration takes place in the micro pores of anodic coatings as the silicate and chromate anions act as inhibitors to the transformation of aluminum oxide to boehmite. Instead, chemical impregnation within the micro pores dominates the sealing processes. In dichromate sealing either aluminum oxydichromate (AlOHCrO4 ) or

aluminum oxychromate ((AlO)2 CrO4 ) forms in the micro pores, as described in the following reactions: Al2 O3 + 2HCrO4 - + H 2 O 2AlOHCrO4 + 2OH- (pH < 6.0) (3) Al2 O3 + HCrO4 - (AlO)2 CrO4 + OH (pH > 6.0) (4)

In silicate sealing, silicate ions react with aluminum oxide to form aluminum silicate (Al2 OSiO 4 ) in the micro pores of an anodic coating. The reaction may be expressed as follows, Al2 O3 + SiO3 2- + H 2 O Al2 OSiO 4 + 2OH(5)

The micro pores of an anodic coating are not completely filled and closed in either dichromate sealing or silicate sealing. Accordingly, poor results may be anticipated if an acid dissolution test or a dye stain test is used to evaluate the sealing quality. However, dichromate sealing or silicate sealing actually enhances the corrosion resistance of anodic coatings on aluminum, which is ascribed to the role of chromate or silicate in inhibiting the corrosion of aluminum. Cold sealing processes, typically nickel fluoride-based sealing, are gaining popularity and acceptance for less energy consumption, non-condensation problem associated with hydrothermal sealing, and almost smut-free finish. Their performance is not as well proven

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in service as the conventional hydrothermal sealing. Since the cold sealing processes usually operate at room temperature, reaction (1) does not normally take place in the micro pores of an anodic coating. With the catalytic effect of co-precipitation of nickel hydroxide and aluminum fluoride, aluminum oxide is transformed to aluminum hydroxide instead of boehmite at temperatures below 70 °C, as expressed in the following reactions, Ni2+ + 2OH- Ni(OH) 2 (2)

Al2 O3 + 6F- + 3H2 O 2AlF3 + 6OH- (6) Al2 O3 + 3H2 O 2Al(OH)3 (7) As with dichromate and silicate sealing, the cold nickel fluoride sealing is actually an impregnation process that does not completely fill and close the micro pores, despite the approximate 150% increase in volume when Al2 O3 (3.97 g/cm3 ) is transformed to Al(OH)3 (2.42 g/cm3 ) in accordance with reaction (7). This increase is because Al(OH)3 tends to be spongy rather than crystalline in form. Accordingly, the seal performs poorly when evaluated with acid dissolution or dye stain tests. It is recognized that aluminum hydroxide is chemically less stable and more soluble in aqueous solutions than boehmite. Consequently, the anti-corrosion performance of anodized work post treated with cold sealing is possibly inferior to that treated with conventional hydrothermal sealing and other impregnation processes mentioned above. Reaction (7) may last several days because a reaction is not as efficient at low temperature as that at high temperature. Thus, seal performance may improve over time. A subsequent hot water sealing is generally

recommended with a cold sealing process to improve the sealing performance and quality. Many unique seals, which are introduced in the next section, are emerging to replace conventional seals for energy saving and environmental protection. The mechanism of the new seals have not been explored yet. In fact, most of them have not yet been used in production.

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Why are sealing processes classified in different terms?

The sealing processes of anodic coatings are often classified in different ways: according to the composition of the seal solution, by the operating temperature or by the mechanism of the process. Classification in terms of composition According to the classification by seal composition, there are traditional sealing processes, including hot (boiling) deionized water sealing, steam sealing, sodium or potassium dichromate sealing, sodium silicate sealing, nickel acetate sealing, nickel fluoride sealing, and new sealing processes, such as cobalt acetate sealing, trivalent chromium sulfate or acetate sealing, cerium acetate sealing, zirconium acetate sealing,

triethanolamine-based sealing, lithium or magnesium salt-based sealing, potassium permanganate sealing, polymer-based sealing, and oxidizing corrosion inhibitor-based sealing (molybdate, vanadate, tungstate, perborate, etc.). Classification in terms of operational temperature Sealing processes are generally classified by temperature into four categories: high temperature sealing (above 95 °C), mid-temperature sealing (80-95 °C), low temperature sealing (70-80 °C), and ambient temperature sealing (25-35 °C). Steam, hot water, and dichromate seals all work in the high temperature range. The majority of other sealing processes operate in the range of middle temperature, such as silicate sealing, di- or trivalent metal acetate sealing, triethanolamine-based sealing, and oxidizing corrosion inhibitor-based sealing. With special formulation, some metal acetate-based seals may be operated in the low temperature range. Cold sealing is gaining popularity for energy saving and smut preventing. A typical example is nickel fluoride sealing, operated at around 30 °C.

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Classification in terms of mechanism Sealing processes can also be classified by sealing mechanism as hydrothermal sealing, physical or chemical impregnation, electrochemical sealing, and corrosion inhibition sealing. Hydrothermal sealing involves the transformation of aluminum oxide to boehmite at temperatures above 80 °C, including steam sealing, hot water sealing, and metal acetate sealing operated in the middle temperature range. Dichromate, silicate, cold nickel fluoride, polymer sealing, and other organic seals belong to the category of chemical or physical impregnation, in which the micro pores of anodic coatings are principally plugged by seal components. In an electrochemical sealing process, organic components may be deposited into micro pores through electrochemical reactions or by the migration of corrosion-inhibiting anions into the micro pores by means of an electric field. Electrophoretic sealing is a typical example in this category. Corrosion inhibition sealing refers to the sealing processes in which the corrosion inhibitors absorb into the micro pores of anodic coatings via the thermal motion and diffusion of substances from the bulk solution to the micro pores. Many organic

corrosion inhibitors and inorganic passivating agents can help prevent corrosion in some aggressive environments. A commercial seal is often formulated with a number of components that play different and synergistic roles in a sealing process and corrosion protection. Different reactions may simultaneously take place within the micro pores of an anodic coating in a sealing process. Therefore, the sealing mechanism is quite complicated in a formulated seal. It is sometimes difficult to clearly define the category of a commercial seal in accordance with the sealing mechanism.

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How to select an adequate sealing process for a specific application.

There is no sealing process available for universal purposes. Each sealing process has its own advantages and disadvantages as it is specially formulated and operated under certain conditions. Detailed discussion on the operation of each sealing process, the function of each component, the effect of each variable on sealing performance, and the control of each sealing bath, is beyond the scope of this article. The readers who are interested in detailed information on this topic may refer to the further readings listed at the end of this article. Selection of an adequate sealing process is subject to many factors, such as particular application, production cost, equipment availability, quality control, and environmental concern. This study provides a general guide to the selection of sealing process to meet certain requirements on the basis of published results. Hydrothermal sealing can provide anodized aluminum with excellent corrosion resistance, but substantially reduces the abrasion resistance of anodic coatings. This indicates that hydrothermal sealing is generally suitable for Type II coatings. The seal quality of steam sealing and hot water sealing is dependent on anodizing parameters and substrate material, while nickel acetate sealing is almost not affected by anodizing parameters and substrate alloy, as exhibited in Table 1. It has long been recognized that commercial nickel acetate sealing is superior to conventional hot water sealing in enhancing the corrosion resistance of anodized aluminum and minimizing the smut formation on the coating surface. Nickel ions incorporated into the oxide also have the function of preventing absorbed dyes in the micro pores of anodic coatings from leaching. To cope with the EPA's regulation on nickel salts, trivalent chromium acetate may be used to replace nickel acetate in the seal formulation. Although light metal acetates have been successfully used to substitute for nickel acetate in sealing clear anodic coatings, light metal ions do not have the ability to prevent the organic dyes entrapped in micro pores from leaching in sealing anodized and colored aluminum. Hydrothermal sealing is usually

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not recommended for hard anodizing. If hot water is used to seal hard anodic coatings, sealing time is suggested not to exceed 30 minutes in order to meet the minimum requirement for abrasion resistance. Increasing immersion time to 120 minutes from 30 minutes does not improve the corrosion resistance and dielectric strength while substantially deteriorating the abrasion resistance of hard anodic coatings. As

demonstrated in Table 2, hot water sealing is very beneficial to enhancing the dielectric strength of anodic coatings. In other words, hot water sealing is favored for the

applications where both corrosion resistance and dielectric strength are preferential concerns, such as paint and adhesive applications. Although sealing is traditionally used to enhance the corrosion resistance of Type II coatings, there has been a growing interest in sealing and dyeing Type III coatings to improve the anti-corrosion performance and appearance of the anodized aluminum without substantially compromising the abrasion resistance. It is indeed a great challenge to choose a good sealing process for Type III anodic coatings. Only dichromate sealing and silicate sealing, among the traditional sealing processes, appear to be able to meet the requirements for the applications of Type III anodized aluminum. Dichromate sealing is especially capable of enhancing paint adhesion and minimizing the loss in fatigue strength of anodized aluminum, apart from improving the corrosion resistance. Unfortunately, hexavalent chromates are harmful to the health of both human and the environment. Dichromate sealing is not suitable for most dyed parts, due to the intrinsic greenish yellow color stemming from the incorporation of chromate ions into the micro pores of anodic coatings. Silicate sealing can substantially enhance the corrosion resistance of Type III anodized aluminum without significantly reducing the abrasion resistance, as can be seen in Table 2. However, it is realized that the silicate solution on an anodizing line poses the tendency to contaminate the anodizing electrolyte. Silicate sealing is not

suitable for applications where the dielectric strength of an anodic coating is an important factor. Cold nickel fluoride sealing can provide Type III anodized aluminum or integral

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color anodized aluminum with a moderate improvement in corrosion resistance and an acceptable abrasion resistance. Virtually no smut and drying marks left on work are additional benefits of cold nickel fluoride sealing. In principle, other impregnation sealing processes in which aluminum oxide does not significantly transform to soft boehmite or aluminum hydroxide may not significantly reduce the abrasion resistance of Type III anodized aluminum while enhancing the corrosion resistance. Electrochemical sealing is a potential candidate. Electrochemical

sealing is particularly effective in enhancing the corrosion resistance of anodized castings and magnesium. Corrosion inhibitor-based seals are also promising in sealing Type III anodized aluminum.

How to evaluate the performance of sealing processes

The methods for evaluating the sealing performance can be divided into three categories: acid dissolution, salt fog spray, and electrochemical measurement. category may include several methods listed as follows: Acid dissolution tests: Dye stain test ((ASTM B136-84(1984); ISO 2143-1981) Phosphoric acid/chromic acid dissolution test (ASTM B680-80(1989); ISO 3210-1984) Acidified sodium sulfite or acetic acid/sodium acetate dissolution test (ISO 2932-1981) Salt spray tests: Standard salt fog spray test (ASTM B117-94(1994); ISO 3768-1976) Acetic acid salt spray test (ASTM B287-74(1980); ISO 3769-1976) Copper-accelerated acetic acid salt spray test (ASTM B368-85(1990); ISO 3770-1976) Electrochemical measurements: Admittance or impedance measurement (ASTM B457-67(1980); ISO 2931-1983) Each

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The dye stain, phosphoric acid/chromic acid dissolution, and standard salt fog spray tests are preferred for sealing quality evaluation in North America, while acidified sodium sulfite or acetic acid/sodium acetate dissolution, acetic acid salt spray, and copper-accelerated acetic acid salt spray tests are popular in Europe. Although the

Kesternicht test (DIN 50018) is sometimes used to evaluate the sealing quality in Germany, the method is rarely employed for sealing quality evaluation in other countries. Boeing specifies the percentage of hydration as a criterion of sealing quality for chromic acid anodizing and sulfuric acid anodizing (Boeing spec BAC 5884). This specification is not used in common practice because hydration has no direct relation with the corrosion resistance of anodized aluminum. The hydration percentage of an anodic coating is not only the function of sealing process but also that of the coating porosity. In other words, the hydration percentage of sealing is low for an anodic coating with low porosity but the sealed parts may actually have excellent corrosion resistance. It was observed by some job shops that the sealed parts surpassed the requirement for corrosion resistance in salt spray testing while failed to meet the requirement for the percentage of hydration. An appropriate method for seal quality evaluation should be chosen in accordance with a specific purpose of the evaluation. Acid dissolution tests are very efficient and simple and can be used for in-situ production quality control but are only suitable for hydrothermal sealing processes. Corrosion testing methods are time-consuming and

require substantial equipment, but provide a better correlation with the actual corrosion situation. Electrochemical measurements can provide fundamental information on the sealing process and quality, but require special electronic instrumentation to perform tests and professional knowledge and skill to analyze the data gathered. As far as

hydrothermal sealing is concerned, electrochemical impedance may correlate well with the corrosion resistance of an anodic coating. However, electrochemical impedance data may not be suitable for quantifying the corrosion resistance performance of anodized aluminum with different sealing treatments.

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Generally speaking, acid dissolution testing methods tend to provide information about the effect of sealing on the chemical resistance or general corrosion resistance of anodic coatings. On the other hand, salt spray testing methods tend to reveal the effect of sealing on the localized corrosion resistance of anodic coatings or the defects present in the sealed anodic coatings. The two categories of testing methods may not quantitatively correlate with each other, and both do not necessarily correlate with the corrosion protection performance in actual corrosive environments. As shown in Table 2, the acid dissolution testing results for the Type III anodized 6061-T6 with dichromate or silicate sealing were very poor, while the same samples demonstrated excellent corrosion resistance in standard salt spray tests. Another example is that anodized castings with nickel acetate sealing can easily pass acid dissolution tests but will severely fail in standard salt spray tests. This suggests that good acid dissolution testing results do not necessarily mean high corrosion resistance in some cases. The apparent acid dissolution testing results not only depend on the chemical properties of a sealed anodic coating but also on the microscopic surface area of the anodic coating and the chemical properties of the substance entrapped in the micro pores during the sealing process. If chemical

resistance is primarily concerned, the acid dissolution testing methods offer more accurate results for corrosion prediction. In contrast, the results from salt spray testing methods correlate better with a real world situation if localized corrosion is predominant. Acetic acid salt spray test and copper-accelerated acetic acid salt spray test are operated under harsh corrosion conditions. Therefore, they may give certain results on "pass" or "fail" in a relatively short time period versus standard salt fog spray test, but it is difficult for them to discriminate the performance of different sealing processes and the effects of various influential factors on seal quality.

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Are there problems with sealing?

There are some problems existing in sealing anodic coatings, as in any production processes. Typical problems include improper sealing, insufficient sealing, over sealing, smutting, crazing, and reduction in abrasion resistance. Improper sealing means that an inadequate sealing process is used for a specific application. Each sealing process has its own unique functions. A correct sealing process should be selected according to the overall requirements of a specific application, such as corrosion resistance, abrasion resistance, dye-leach prevention, dielectric strength, fatigue strength, paint adhesion, and so on. Otherwise, sealing an anodic coating may be unable to achieve the goals desired. Insufficient sealing or over sealing occurs if wrong sealing parameters are used, a sealing process is out of control, or a sealing solution is out of balance. Insufficient sealing leads to poor corrosion resistance and low electrical insulation, while over sealing may result in smutting, crazing, and substantial reduction in abrasion resistance and fatigue strength. Smutting is a very common problem encountered in sealing processes, often in hydrothermal sealing. While the cell walls of an anodic coating transform to boehmite, the same reaction takes place on the coating surface. The latter process leads to the formation of mechanically removable velvety powder, i.e., smut, on the coating surface. Smutting makes the coating appearance unacceptable but has virtually no effect on the properties of the resultant anodic coating. The smutting problem is often associated with high operational temperature and pH, long immersion time, aged sealing solution containing too much dissolved solids and breakdown components of additives, and shortage of antismutting agents and/or surface active agents. Addition of anti-smutting agents to sealing baths is a common practice to minimize the formation of smut on a coating surface. There are numerous commercial anti-smutting agents available in the anodizing market. Antismutting agents can inhibit the formation of boehmite on the coating surface without

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adversely affecting the sealing process within the micro pores. Many chemicals have anti-smutting functions if they are used properly. Typical examples are hydroxycarboxylic acids, lignosulphonates, cycloaliphatic or aromatic polycarboxylic acids, naphthalene sulphonic acids, polyacrylic acids, phosphonates, sulphonated phenol, phosphonocarboxylic acids, polyphosphinocarboxylic acids, phosphonic acids, and triazine derivatives, etc. Pitting occurrence in an anodic coating while being sealed is alloy dependent. Both 2000 and 7000 series aluminum materials suffer from pitting in a sealing process more frequently than other aluminum alloys when a titanium rack is used as the fixture. Other factors causing this problem include temperature, sealing time, chloride content in the sealing bath, and anodizing parameters. Substitution of an aluminum rack for a titanium rack can minimize the occurrence of pitting in a sealing process. Lowering temperature, shortening immersion time and eliminating solution contamination also help alleviate the pitting problem. Another common practice in anodizing job shops is to hang a magnesium bar on the titanium rack along with the anodized work while the parts are being sealed. It is necessary to ensure good electric contact between the titanium rack and the magnesium bar. The magnesium bar acts as a sacrificial anode to eliminate the galvanic effect between the titanium rack and the anodized aluminum and provide the anodized aluminum with cathodic protection. The crazing of anodic coatings may happen when the anodic coatings produced at a low temperature and a high current density are rapidly transferred to a sealing solution operated at high temperatures via a cold rinse. Hydrothermal sealing readily brings about the crazing of anodic coatings. It appears that crazing is highly susceptible to substrate material, coating thickness, coating composition, anodizing current density, and temperature. Furthermore, polished surfaces tend to exhibit crazing more readily than etched surfaces. In principle, the crazing of anodic coatings in sealing is mainly caused by the tensile stress built up in the anodic coating, stemming from the considerable difference

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in thermal expansion coefficient between anodic coating and substrate aluminum. Thin Type II coatings do not tend to craze in a sealing process. Crazing is substantially reduced as sealing temperature is lowered. A short warm rinse between a cold rinse and a high temperature sealing treatment can minimize the occurrence of crazing in a subsequent sealing process. The reduction in the abrasion resistance of anodic coatings is chiefly associated with a sealing process. It is well understood that hydrothermal sealing substantially reduces the abrasion resistance of anodic coatings. This is the nature of hydrothermal sealing that transforms hard aluminum oxide to soft hydrated aluminum oxide. Therefore,

hydrothermal sealing is not recommended in the applications where the abrasion resistance of anodic coatings is a necessity. Non-hydrothermal sealing, such as

impregnation or electrochemical deposition, has a minimal impact on the abrasion resistance of anodic coatings. In the case where hydrothermal sealing is the only option, taking into account other factors, the adverse effect on abrasion resistance may be minimized if close-to-lower limit temperature and short sealing time are used.

Further Readings

1. S. Wernick, R. Pinner and P.G. Sheasby, The Surface Treatment and Finishing of Aluminum and Its Alloys, 5th ed., Chapters 9-12, Finishing Publications Ltd., Teddington, England, 1996. 2. A.W. Brace and P.G. Sheasby, The Technology of Anodizing Aluminum, 2nd ed., Chapter 16, Technicopy Limited, England, 1979. 3. A.W. Brace, Anodic Coating Defects: Their Causes and Cure, Chapter 8, Technicopy Books, England, 1992. 4. Charles A. Grubbs and David C. Montgomery, Light Metals Finishing Process Manual, Light Metals Finishing Committee, AESF, Orlando, Florida, 1996. 5. Charles A. Grubbs, "Anodizing of Aluminum", Metal Finishing: Guidebook and Directory Issue, 97(1), 476(1999). 6. J.M. Kape, Finishing Industries, 1(11), 13(1977); 1(12), 38(1977). 7. B. Yaffe, Met. Finish., 88(5), 41(1990).

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8. W. Dalla Barba, Aluminum Finishing, 16(3/4), 17(1996). 9. X.A. Ventura, "A Study of the Behavior of the Anodic Layer in Anodizing with Different Types of Sealing", AESF SUR/FIN'99, Cincinnati, Ohio, June 21-24, 1999. 10. F. Mansfeld, C. Chen, C.B. Breslin and D. Dull, J. Electrochem. Soc., 145(8), 2792(1998). 11. Bernard R. Baker, "Sealing of Anodic Oxides ­ A Review of Theory and Practice", Aluminum Finishing Seminar Technical Papers, Vol. 1, The Aluminum Association, St. Louis, Missouri, March 30-April 1, 1982, p. 177-199. 12. H.J. Gohausen and Glenn C. Schoener, "Consideration for the Sealing of Anodized Aluminum to a Smut-free Condition", Aluminum Finishing Seminar Technical Papers, Vol. 1, The Aluminum Association, St. Louis, Missouri, March 30-April 1, 1982, p. 161-176. 13. M. Jozefowiez, "Hard Coat Dyeing, Part 1" & "Hard Coat Dyeing, Part 2", The 7th Annual Symposium of International Hard Anodizing Association, San Diego, California, October 11-12, 1998. 14. J. Rasmussen, "Microhardness and Wear resistance in Type III anodizing", The 7th Annual Symposium of International Hard Anodizing Association, San Diego, California, October 11-12, 1998. 15. T. Westre, B. Rachel Cheng, L. Hao and S. Westre, "Performance Results for Sealed Type III Anodic Oxides", AESF SUR/FIN'2000, Chicago, Illinois, June 26-29, 2000. 16. B. Rachel Cheng and L. Hao, Met. Finish., 98(5), 2000, in print. 17. L. Hao and B. Rachel Cheng, Met. Finish., 98(12), 2000, in print. 18. J.A. González, V. López, E. Otero, and Bautista, J. Electrochem. Soc., 147(3), 984(2000). 19. J. Trolho, "Overview on the Latest Sealing Technologies", The 4th World Congress Aluminium 2000: The Aluspecialists' Meeting, Montichiari (Brescia), Italy, April 12-15, 2000.

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a. Micro pores are open before sealing

b. Micro pores are closed after sealing

Figure 1 SEM photographs of anodic coating surfaces

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Al2 O3 + other salts

H2 O ~100 o C

Aluminum a) Unsealed anodic film

Aluminum b) Formation of gel on pore walls and outside of film

Surface precipitation

H2 O

Anion Intermediate layer

Surface precipitation

Aluminum c) Condensation of gel to form pseudo boehmite

Aluminum d) Recrystallisation to form boehmite starting at the surface, intermediate layer formed by diffusion

Figure 2 Schematic diagram of a dynamic sealing process

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Table 1 Acid dissolution testing results of the Type II anodized aluminum with post sealing treatment Acid dissolution testing results, mg/dm2 Hot water sealing Nickel acetate sealing 2 Anodizing current density, A/dm Anodizing current density, A/dm2 1.1 1100-H14 2024-T3 3003-H14 5005-H34 6061-T6 14 128 15 13 13 1.9 10 90 13 11 11 2.6 10 42 10 11 11 <1 1.1 1.9 2.6

Alloy

7075-T6 67 22 20 Note: 1. Acid dissolution tests were performed in accordance with ASTM B680-80 with a standard deviation of <2 mg/dm2. 2. The anodic coatings were produced under conventional Type II anodizing conditions and the resultant coating thickness was 18 ± µm.

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Table 2 Performance results of sealed Type III anodic coatings on 6061-T6 Post treatment Wear index Acid Salt spray test Dielectric dissolution test strength 2 mg/1000 cycles mg/dm Hours at failure V/µm No post treatment 0.84(0.23) 514(5) 43 28-29 Sodium silicate sealing 0.92(0.20) 120(2) 1578 ~19.0 Sodium dichromate 0.98(0.37) 507(2) 1578 35-36 sealing Nickel fluoride 1.1(0.3) sealing 111(4) 70 29-33 Nickel acetate sealing 1.5(0.4) 1.0(0.2) 495 32-33 Hot deionized water sealing 1.4(0.4) 45(1) 374 37-40 (30 minutes) Hot deionized water sealing 2.0(0.6) 54(2) 327 38-39 (120 minutes) Black dyeing 1.2(0.4) 490(8) 21 34-39

Black dyeing + nickel acetate 1.8(0.4) 3.5(0.5) 23 34-35 sealing Notes: 1. Numbers in parenthesis are the standard deviations. 2. Wear indices were measured in accordance with MIL-A-8625F. 3. Salt spray fog tests were performed in accordance with ASTM B117. Failure was considered occurring in salt spray tests when there were 5 pits on a total sample surface area of 30 square inches 4. Acid dissolution tests were carried out in accordance with ASTM B680-80. 5. Dielectric strength was measured in accordance with ISO 2376 with a single ball electrode. 6. The anodic coatings were produced under conventional Type III anodizing conditions and the resultant coating thickness was 50 ± 3 µm.

Technical Report

19 METALAST International, Inc. Sealing: Enhance Anodic Coatings' Performance

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