How to achieve uniform thickness of the anodic aluminum oxide film? Leonid M. Lerner AlZi Anodizing Solutions Co.

Transcription

1 How to achieve uniform thickness of the anodic aluminum oxide film? Leonid M. Lerner AlZi Anodizing Solutions Co AAC Conference September 16-18, Pittsburgh, PA, USA. 1

2 How to achieve uniform thickness of the anodic aluminum oxide film? Leonid M. Lerner AlZi Anodizing Solutions Co. DC voltage applied across the tank creates electric field that exists at any point in the electrolyte and in the oxide film. If a positive test charge is placed at that point it would experience an electric force (a test charge can be approximated by a charge of nearly negligible magnitude).the direction of the electric field at a point is the same as the direction of the force experienced by a positive test charge placed at the point. Electric field lines can be used to sketch electric field. The line through a point has the same direction as the electric field. Field lines come out of positive electrodes (anodes) because positive electrodes repel a positive test charge. Field lines come to negative electrodes (cathodes) because a negative electrode attracts the positive charge. Where the field lines are closest together, the electric forces are the largest. The electric intensity E (also called the electric field stress) at a point is equal to the force experienced by a unit test charge placed at that point. The electric intensity between anode and cathode having plate configuration is uniform. The electric intensity is equal to drop of voltage V = V anode V cathode between plates divided by the distance d between plates: E = V/d This picture is valid for electroplating where the tank voltage ordinarily does not exceed 10 VDC. In aluminum anodizing we have more fields with different electric intensity. The process of high-field ionic conduction is central to anodizing process. Oxide anions move inward to react with aluminum at the metal/oxide interface to form oxide. Aluminum cations move outward from the metal to react with water at the oxide/electrolyte interface to form oxide at that surface. At the cathode, the circuit is completed by the reduction of hydrogen ions to hydrogen gas. New oxide deposits at both oxide interfaces. The rate at which the oxide thickens is proportional to the current density (A/ft 2 ). The field in the oxide does not change with oxide thickness, and has only a small dependence on current density and temperature. As the oxide thickens the voltage across the oxide increases proportionally, and at room temperature the thickness/voltage ratio is close to1.2 nm/v. Thickness is very uniform across the surface because everywhere the voltage drop must be the same, but why non-uniform coating happened? 2 Fig.1

3 The Mechanism of Aluminum Anodic Film Formation The formation of aluminum oxide film take place by the migration of Al 3+ ions away from the aluminum towards the electrolyte interface, while simultaneously O 2- ions from the electrolyte move in the opposite directions. Aluminum is anodically polarized aluminum oxide is produced from reaction: 2Al + 3H2O Al2O3 + 6H + 6 e-. This represent the sum of two partial processes: (1) 2Al 2Al e- and (2) 3H2O 6H + + 3O 2- - O 2- ions A _ + _ O 2- ions - Al 3+ ions Al 3+ ions C _ Al2O3 Al 3+ O 2- C _ The transfer of Al 3+ ions across oxide film/solution (electrolyte) interface consists of the dissolution current IAL 3+ and forming current IO 2 which represent transfer of the of the O 2- ion into the oxide. The efficiency of oxide film formation: Ŋoxide = IO 2- / IAL 3+ 3

4 When a current passes through an electrolyte where aluminum is anode, the negatively charged anion migrates to the anode where it is discharged with a loss of one or more electrons. These electrons need somewhere to go, so they will flow to the cathode for hydrogen evolution. The anodizing process needs these cathodes to run. So anodizing consists of two processes an oxidation process (anodic reaction) and a reduction process (cathodic reaction), and both of them are necessary to run the anodizing process but most of the time we actually neglect this second reaction, the cathodic reaction. When anodizing in sulfuric acid the major cathodic reaction is the hydrogen evolution. The position of the cathodes (the blue parts in the drawing) in the anodizing tank is very important. The Anodizing solution has a good throwing power compared to most plating solution. The reason for this is the high electrical resistance of the aluminum oxide film. This high electrical resistance will produce an anodic film on both sides of a sheet of aluminum close to a single cathode. So in theory there shouldn t be any problems placing the cathodes where they fit best in the tank. The film formation starting on the back side as soon as the resistance between the cathode and the near side is equal to the resistance between the cathode and the back side of the plate. Even so you will often find thickness variation on complicated shapes and over large complex loads. The reason for this is often an insufficient agitation in the tank, or cathode placed in areas where there never are any parts to be anodized. Joints between the cathode and aluminum (or protected copper) bar should be as easy as possible to maintain aluminum and the sulfuric acid reacts to form aluminum sulfate which is very voluminous. This corrosion product can force the cathodes away from the aluminum bar with the loss of contact between the two. 4

5 Major issues to prevent uniform anodic oxide coating Non-uniform coating due to: worn out cathodes (non-uniform distribution of electrical potential) too quick a ramp (may provoke burning and shortening the process time) anode/cathode ratio off (recommended ratio: 3:1 & 5:1. Efficiency of Hungry cathode is very high).the cathode exposed area must be sufficient to cover the total area to be anodized in the anode so the thickness distribution will be more uniform, see Figure 2. If the reference is too small, Fig.2a, the electric field lines distribution will be lower at the edges and denser at the center. Thickness will be thicker in the center compared to the edge. In the contrary case, Fig.2b, with a cathode bigger than the anode, lines will concentrate more on the edges. Edge thickness will be thicker than in the center. Therefore it is better to have something as closer as possible to the area to be anodized. In this way, the field lines are more uniformly distributed in all zones in the Fig.2c. The anode must be separated approximately 3.0 (7.6 cm) apart from the cathode. This will help maintaining a good current line distribution. Anode Anode Anode c c c c c c Fig.2 - Effects on electric filed lines distribution caused by cathode variation in size, a) smaller than the cathode, b) bigger size, and c) almost identical. 5

6 - geometry of cathode (may eliminate thick coating at edges and bottom) too great a load for the tank (maximum load size is determined by current density& rectifier capacity) high current density (low current density can minimize the thickness differences, but increase process time accordingly) 6

7 wrong aluminum alloys for racks (alloy should be not more acceptable for anodizing than the process part) poor part distribution in process tank wrong cross section of racks (Aluminum will carry about 600 to 800 amps per cross-sectional square inch, and also carries 60% more current than titanium different surface finishing on the same part (different oxide thickness on different surface finishing techniques) 7

8 Other obstacles on the surface area: - blind and through holes -sharp corners 8

9 poor metal quality (some casting alloys, and alloys with high contend of silicon, cooper and other non-aluminum material created non-uniform chemical composition on the surface of aluminum part) poor metal storage (corrosion, oxidation, etc.,) poor cleaning with subsequent etching (also leads to: etch relief / patterns, poor anodic coating quality, poor paint adhesion, reduced corrosion resistance) 9

10 current distribution in process tank due to wrong cross section material of racks (the resistance of aluminum or titanium racks depends on its length, and its cross-sectional area, or thickness. The longer the rack is, the greater its resistance. If one rack is twice as long as a rack of identical diameter and material, the longer rack offers twice as much resistance as the shorter one. A thicker rack, however, has less resistance, because a thick rack offers more room for an electric current to pass through than a thin rack does. 10

11 oxide property (e.g. porosity) Aluminum cast porosity Influence of load configuration and geometry of anodize parts (sea shell vs. ball, spiral vs. flat) Uniform anodic oxide coating will grow on ball configuration from the beginning of anodizing process VS. Influence of electrolyte composition and property (conductivity) The more dilute the electrolyte concentration, the less uniform coating during reasonable process time and the higher the breakdown voltage. The highest voltage that is reached in aqueous electrolytes is about 1000 Vdc. VS. The screen effect phenomena is increased with a lower concentration of electrolyte 11

12 screening effect (significant potential drop in the interior of the parallel located plates/parts in process tank) Conclusion: Each of above conditions for non-uniform coating should be considered during any trouble shooting procedure. 12

13 References 1. A. J. Bard and L. Faulkner, Electrochemical Methods (Wiley, New York, 1980) R. S. Alwitt, Anodizing, Electrochemistry Encyclopedia. 4. Rohan Akolkar & Uziel Landau, Yar-Ming Wang & Hong-Hsiang Kuo, Modeling of the Current Distribution in Aluminum Anodization 5. L.M.Lerner, Hard Anodizing of Aerospace Aluminum Alloys. Transactions of the Institute of Metal Finishing, Volume 88, Number1, January 2010, UK. 6. L.M. Lerner, T.P. Cabot, Low Voltage Process for HC on Aluminum, Chinese Surface Finishing Journal, Biographical Sketch Education: MS in Hydro-Technical Engineering. Work experience: for the past 34 years has been with Sanford Process Corporation as an Assistant to the Director of R&D, Project Manager, Engineering Manager, General Manager /Executive Vice President and President. Involved in hard anodizing of aluminum and other metals. Research, development, testing and installation of Low Voltage DC+AC Hard Anodizing Systems around the world. In present time running the International Metal Finishing Consulting Company- AlZi Anodizing Solutions and also still represent the Sanford Process Anodizing Technology. 13