# Anodizing process

The anodizing method [ elɔksaːl ] (of anodizing , an abbreviation for el ektrolytische Ox idation of Al uminium) is a method of surface treatment for generating an oxidic protective layer on aluminum by anodic oxidation . In contrast to the galvanic coating process, the protective layer is not deposited on the workpiece, but an oxide or hydroxide is formed by converting the topmost metal layer . A 5 to 25 micrometer thin layer is created, which protects the deeper layers from corrosion as long as no gaps are created in this layer, for example due to mechanical damage. The atmospheric oxide layer of aluminum is only a few nanometers.

Different colored anodized layers on aluminum
Colored valves for ventilation on a spacesuit

## The procedure

Schematic representation of anodic oxidation (anodizing)

### Basic principle of anodizing

Initial phase of the conversion of aluminum by anodic oxidation and reaction of oxygen at the interface between aluminum and aqueous electrolyte in the initial phase
Progressive conversion of aluminum by anodic oxidation with the formation of pores, whereby the reaction of oxygen continues at the interface between aluminum and aqueous electrolyte

In contact with oxygen, a thin but dense oxide layer forms on aluminum, which protects the aluminum metal against further oxidation under normal ambient conditions . With a thickness between about 5 nm and 20 nm, the layer is also completely transparent (i.e. invisible) and thus preserves the metallic character of the aluminum. The oxide layer protects the metal against oxidation by oxygen and against corrosion in a pH range between about pH 4 and pH 9 - this is why the metal is relatively resistant under normal ambient conditions. However, components such as salt ( sodium chloride ) or sulfur dioxide lead to signs of corrosion, which is why aluminum is corrosive to attack near the sea or under harsh industrial conditions. Above all, the very thin natural oxide layer is not able to protect aluminum against wear .

The natural oxide layer on aluminum can be strengthened by the electrochemical treatment of anodic oxidation. The aluminum is connected in an electrolyte (e.g. sulfuric acid or oxalic acid ) as an anode in a direct current circuit. Oxygen is generated from the water-containing electrolyte on the aluminum surface. This reacts directly on the surface with the metal (which is converted into reactive aluminum ions by the flow of current ) to form aluminum oxide . So there is a direct conversion of the surface into aluminum oxide. Even complex geometries are given an almost uniformly thick aluminum oxide layer. As a result of the process, the oxide layer has micropores immediately after it has been created. By a final compaction or sealing (engl. Sealing ) the pores can be closed to a compact, largely of aluminum hydroxide to produce existing layer. The anodic oxide layer has a significantly better resistance to corrosion and wear . In addition, the oxide layer produced by anodizing is a good electrical insulator with dielectric strengths of around 900 V with an anodized layer thickness of 30 µm.

### Pretreatment

First the aluminum parts are pretreated , i. H. degreased and pickled . In order to obtain a uniform surface, the pickling is usually first basic (with caustic solutions based on caustic soda ), then acidic (with nitric acid or hydrofluoric acid ). The pickling agents and conditions used depend on the properties of the material, such as the type of admixture of foreign metals in an alloy . Pickling also removes the thin, natural oxide layer . The resulting aluminum-containing acids ( aluminum nitrate solution) and alkalis ( sodium aluminate solution) are secondary raw materials and are returned to the industrial aluminum cycle. In some anodizing systems they are also neutralized and the resulting aluminum hydroxide filter cake is also used as aluminum raw material.

### The anodizing

The actual anodizing takes place after the surface pretreatment . Sometimes the fresh porous layer is then colored with dyes and finally densified , usually simply with hot water or steam. There is also the option of chemical compression, which is usually used in combination with steam compressors.

The anodizing process uses electrolysis . The cathode (negative electrode) breaks down the oxonium (H 3 O + ) contained in the acid into hydrogen and water. The hydrogen is released.

${\ displaystyle \ mathrm {6 \ H_ {3} O ^ {+} + 6 \ e ^ {-} \ to 3 \ H_ {2} +6 \ H_ {2} O}}$

At the anode (positive electrode) the oxidation (1) of aluminum to aluminum (III) ions takes place and then the further reaction to aluminum (III) oxide (2). The acid is only used to increase the electrical conductivity and is not involved in the reaction.

1. ${\ displaystyle \ mathrm {2 \ Al \ to 2 \ Al ^ {3 +} + 6 \ e ^ {-}}}$
2. ${\ displaystyle \ mathrm {2 \ Al ^ {3 +} + 9 \ H_ {2} O \ to Al_ {2} O_ {3} +6 \ H_ {3} O ^ {+}}}$

The overall reaction can be described as follows:

${\ displaystyle \ mathrm {2 \ Al + 3H_ {2} O \ to Al_ {2} O_ {3} +3 \ H_ {2}}}$

The electrolytic process is usually by means of direct current (considered below) in sulfuric acid - or oxalic acid - electrolyte performed. However, it can also be carried out with alternating current (e.g. WX process) or with the aid of a combination of both types of current.

Thermographic image of an anodizing test

In the direct current variant, the aluminum is connected as an anode ; this is why the process is also called anodizing. The counter electrode usually consists of a material that is not attacked by the electrolyte. The oxidation of aluminum with the anodizing process can be carried out in three ways:

1. Immersion process in stationary baths: the workpieces are completely or partially immersed in the oxidation bath; the power source is attached to the workpiece and to the counter electrode immersed in the bath.
2. Spraying process: the workpiece and a movable nozzle from which the electrolyte emerges are connected to the power source.
3. Continuous process in resting baths: wires, ribbons, etc. Ä. Are pulled through the oxidation bath; the power source is connected as in 1..

Anodized and then colored aluminum should, if possible, be treated in a two-stage sealing process in order to prevent the dyes from bleeding out. Stage 1 in a nickel acetate bath for 15 minutes at 60 ° C. Level 2 in the hot water bath 3 min./µm layer thickness.

The layers produced can be hard, medium-hard and soft. This depends on the oxidation conditions. The oxidation conditions, the choice of electrolyte and its additives have an influence on the quality and type of the layer. It is thus possible to adapt the layer to the respective technical purpose.

### Osh

During anodizing, employees can be exposed to hazardous substances . As part of the risk assessment , the hazardous substances occurring at the workplace must be determined and appropriate protective measures established. DGUV Information 213-716 of the German Social Accident Insurance specifies the procedure and protective measures for anodizing so that compliance with substances with occupational exposure limits is ensured. The state of the art is documented for materials without AGW.

## The layer

Requirements for achieving technically perfect layers:

• As homogeneous a structure of the metal as possible (irregularities are transferred to the oxide layer)
• No use of unsuitable metallic alloy elements or impurities, as their primary and secondary phases can disrupt the regular structure of the anodized layer.
• Avoidance of non-metallic inclusions, such as B. refractory materials or various oxides, as these can disrupt a regular layer structure.

In order to achieve decoratively flawless layers, more demanding requirements with regard to the alloy composition and the structure must be met.

### Stratification processes

The layer formation processes are influenced by:

• Choice of electrolyte, its concentration and temperature
• Type of current (direct current, alternating current)
• electric current density
• Tension and duration of treatment

Temperature changes also have a strong influence. The duration of the treatment is directly related to the layer thickness. Additives that affect the composition of the oxide layers are sometimes mixed in the electrolyte. However, the basic material of the layer always remains aluminum oxide. Additions to the electrolyte can change the properties of the layer (improve resistance, appearance, and so on). Since the layer consists of aluminum oxide, unlike aluminum, it does not conduct electrical current.

### hardness

By anodizing, hardnesses between 200 and 350 HV can be achieved; if a hard anodizing process is used , which creates thicker layers with a higher degree of brittleness, hardnesses of up to 600 HV can be achieved.

### Layer growth in depth

The layer is finely crystalline, but has innumerable pores . Therefore, the layer is able to "absorb" liquids, which is necessary for the growth of the oxide layer, because for the further growth of the layer the electrolyte must penetrate the inner interface between aluminum oxide and aluminum, where the oxidation of the aluminum takes place. In this way, the layer is very well “crosslinked” with the aluminum via chemical bonds, which is reflected in the high adhesive strength of the layer. The oxide layer therefore grows from the surface into the metal, that is, the interface between aluminum oxide and aluminum lies within the original metal.

### The reason for the porous surface

The barrier layer initially formed insulates. As a result, the resistance of the anode increases. This increases the voltage required for the reaction. This means that the areas of the oxide layer attacked by the electrolyte are broken through. Due to the lack of or only a thin oxide layer, the current density increases in places, which leads to local heating. The oxide layer is increasingly dissolved in the warmer areas, and depressions are formed. The channels enable charge exchange with the electrolyte and are therefore kept open by it. The surrounding oxide layer continues to grow, the channels remain, and a porous structure is formed.

### Electric conductivity

Anodized layers are electrical non-conductors. Anodizing turns the conductive aluminum into a non-conductor. The insulation capacity is strongly dependent on the alloy, the layer thickness and the coating parameters. In the most favorable cases, insulation values ​​of up to almost 1000 volts can be achieved, the value depending on the measurement method used. Measurement methods can be carried out taking into account the ISO 2376 standard. Any scratches and wear and tear on the surface can inhibit the insulation value or destroy it entirely.

Untreated anodized layers are therefore absorbent, which means that the corrosion resistance to liquid agents is reduced. The layers can, however, be sealed by treating them with materials that protect against corrosion or with insulating materials ( wax , enamel varnish , insulating varnish and so on). The most common technical application of the adsorption capacity is the coloring of the anodized layers. These are able to absorb large amounts of dyes from solutions.

### Coloring the oxide layers

Sandalor process in two stages or combined with electrolytic coloring in three stages.

With organic dyes
After anodizing, the aluminum is immersed in a hot dye solution and then rinsed. When dyeing with this process, the dye molecules are mainly deposited in the upper areas of the pores of the anodized layer and form bonds with the oxide layer. The higher the dye concentration in the solution, the more pronounced the dye storage in the pores of the oxide layer.
With inorganic dyes
After anodizing, the aluminum is neutralized, rinsed and colored in color baths with metal salt solutions. The ions of the solution collect in the pores of the anodized layer and become a solid. However, the exact processes have not yet been sufficiently clarified.

#### Electrolytic dyeing

The electrolytic dyeing (colinal process) is carried out with alternating voltage. The electrolyte contains a coloring metal salt ( tin (II) sulfate ). The duration of the electrolysis depends on the desired color depth. The metal ions penetrate deep into the pores of the layer. The pores, which are partially filled with metal, now cause a lightfast coloration through absorption and scattering effects. Many different colors can be achieved. The standard color guide of the European Association of Anodisers (EURAS) for electrolytic coloring and color anodization contains the following colors: C-0 colorless, C-32 light bronze, C-34 dark bronze, C-31 light bronze, C-33 medium bronze, C- 35 black. In addition, the color anodization is given the color codes C-36 light gray, C-37 medium gray, and C-38 dark gray.

The former color fan for dip coloring is used by the Eloxal-Verband e. V. (today: VOA - Association for Surface Refinement of Aluminum eV) no longer published due to lack of demand. The fan had the abbreviations EV1 to EV6 with the following meanings: EV1 natural tone, EV3 gold, EV5 dark bronze, EV2 light nickel silver, EV4 medium bronze, EV6 black.

Colored anodized coatings are very common, and the most diverse possible applications are open, for example cases, dials, identification rings for birds, jewelry or various parts of rifles.

#### Interference coloring

In contrast to the coloring process mentioned above, the color of the aluminum in the interference coloring process is not produced by embedded foreign ions, but rather by interference within the aluminum oxide layer. Depending on the thickness of the oxide layer and the associated light extinction, different colors (e.g. blue, green, gray or red) can be reproduced.

### Compress

In order to prevent the storage of corrosive substances, the pores must be compressed. The anodized and possibly colored aluminum is compacted in demineralized water by simple boiling. This leads to a reaction between the aluminum oxide and water, aluminum oxide hydroxide ( boehmite ) is formed. Furthermore, the water absorption leads to an increase in volume, so that the pores (more precisely the pore neck) are narrowed and then closed.

## application

Relatively thick anodized layers (20-25 µm) are used primarily as corrosion protection in the construction industry, but also for automobile parts, aircraft parts and household items. Uncolored and colored thinner layers (8–20 µm) are mainly used for decorative purposes (for example for the surfaces of MP3 players or for door fittings ) and for better heat dissipation in heat sinks .

A special case are hard anodized layers, which are much thicker and harder and cannot be colored at will. These are generated in cooled (1–5 ° C) acid baths and higher voltages (up to 120 V) and are found primarily in industrial applications where maximum abrasion resistance and durability (such as in salt water areas) are required.

With both methods it should be noted that not all aluminum alloys are suitable for anodizing; The materials AlMg3 and AlMg5, for example, are easy to anodize, in contrast to AlMg4,5Mn, which is difficult to anodize.

If the surface is applied using the so-called plasma ceramic process (PEO technology), the aluminum, magnesium or titanium parts processed in this way achieve further properties that range from extremely hard and abrasion-resistant to extreme heat resistance and impregnability. The environmentally friendly plasma ceramic is created in the electrolyte, with the metal surface being converted into a dense, atomically adhering ceramic layer in a plasma discharge.

## Partial anodizing

The partial anodizing generates a partial protective oxide layer on aluminum by anodic oxidation .

Anodizing-free areas on aluminum components are required if the entire aluminum component is not to be anodized because, for example, equipotential bonding has to be provided. For applications in the electrotechnical field, parts made of aluminum often have to be provided with a corrosion and wear-resistant oxide layer and at the same time an electrical contact area for earthing has to be created. The components used must be made accessible for electrical contacting , for example when using electrical devices for the purpose of earthing or as EMC protection ( electromagnetic compatibility ) in electronic equipment as well as in vehicles and aircraft.

Due to the special properties of the aluminum oxide layer with regard to electrical insulation, simple contact via an applied contact is not possible. Soldering onto the oxide layer is also not possible due to poor wetting, the oxide layer must be removed beforehand.

In order to ensure the necessary electrical contacting of the aluminum part, the targeted structure of the aluminum oxide layer by anodizing must be dispensed with. On the other hand, the significantly better resistance to corrosion and abrasion and the reduced friction of the remaining surface can and should be guaranteed. This results in two procedures for producing a contact point without an anodized layer: - Removing the anodized layer in the area of ​​the desired contact point - Avoiding the formation of the oxide layer during anodising by covering the desired contact point

### Subsequent removal of the anodized layer

The technology that has hitherto generally been used to create an anodized-free area is the subsequent removal of the oxide at the contact point. The 5 µm to 25 µm thick anodized layer is removed mechanically. This can be done by machining processes such as turning or milling or by thermal processing using a laser . It must be taken into account that the thickness of the coating to be removed can be between about 10 µm and about 50 µm, depending on the requirements for wear and corrosion resistance. This requires a high level of precision when fixing the part to be machined and corresponding adjustment options for the machine tool, both in the machining process and in the use of lasers. Burrs often occur, particularly during machining, which make reworking necessary. Any material removed must also be removed in such a way that the parts are not damaged on the surface (e.g. by scratches). This post-processing represents a high additional effort, both in terms of costs (machinery and equipment investments) and in terms of logistics and work planning, and the additional processing usually leads to a significant increase in defective parts. Especially thin-walled or complex-shaped parts are problematic here.

### Manufacture of a contact point without an anodized layer

Covering processes are one of the introduced possibilities for partial work techniques in coating technology. Various, mostly highly viscous, special resins are available for this. These must be particularly resistant to strongly acidic and strongly alkaline solutions as well as temperature loads of up to 100 ° C. In addition to attaching or detaching the covered area during chemical processing, the application of a defined covering area with smooth edges is time-consuming. In addition, the removal of the covering resin requires a further operation in a suitable dissolving medium, in which the oxide layer can be attacked under certain circumstances, as well as special expenditure (technical and manual type) for thorough cleaning with drying.

#### Partial anodizing or selective anodizing

With regard to the course of the process, it is better to avoid layer formation at the desired contact point. Instead of the adhesive covering resin or covering varnish usually used for this purpose, a flexible covering was developed as part of a funding project, which enables partial anodizing. With this process of selective anodic oxidation, specific areas are left anodized-free.

### Partial anodizing process

The necessary work steps of the technology of partial anodizing / selective anodic oxidation can be compared to the previously usual type of processing.

Partial anodizing (also: selective anodic oxidation) is a coating process for the selective anodizing of aluminum parts, in which the designated areas remain anodized-free. For this purpose, a flexible cover is used, for which a plastic has been modified in such a way that it has a high degree of shape accuracy, elasticity and strength. Thanks to a special frame technology, the cover made of the plastic can be precisely attached to a coating frame. The frame with its special construction ensures that the cover rests positively on the part to be processed and is only deformed so far in the elastic area that the intended contact point is safely protected from the ingress of the anodizing electrolyte and all other chemical process substances. As extensive test series have shown, there is no infiltration of the newly developed covers. This was determined during the project. A delimited oxide-free zone is created. The system of flexible cover and special frame ensures that the parts to be processed are partially manufactured without an anodized layer. The uncovered areas can be provided with anodized layers of any thickness or color. At the same time, deformation or mechanical damage to the parts is avoided. The processing takes place without additional effort and contributes to increasing the energy and material efficiency as well as reducing the logistical effort and the stock in circulation.

### application

Contact surface on an aluminum part

The process developed as part of a ZIM project funded by the BMWi ensures the coating of aluminum parts with the option of electrical contacting without additional post-processing. Using special elastic covers and a new type of frame technology, an area of ​​variable shape is created that enables direct electrical contact, while the remaining surface of the aluminum part can be provided with decorative or functional anodized layers in the classic way.

The technology does not require any special preparatory work before anodizing or when fixing it to the processing frames. Mechanical post-processing is also not necessary. In addition, the previously required and additional transport between the companies for coating and mechanical or thermal removal is no longer necessary.

The process is used on anodized aluminum for electrotechnical applications, for example for lightweight construction in vehicles. Anodizing-free areas that guarantee a protective circuit or very good electrical conductivity without reworking are often necessary for housings for electrical assemblies such as small fuel cells, camera housings, medical devices or outdoor equipment that works with electricity (flashlights).

## literature

• Aluminum headquarters e. V. (Ed.): Aluminum-Merkblatt O4; Anodized aluminum for decorative purposes 14th edition, Düsseldorf, 1987-06 (withdrawn 1991-08).
• Otto-Albrecht Neumüller : Duden - The dictionary of chemical terms. Dudenverlag, 2003.
• TAG (Technological Working Group): Technical and scientific principles of goldsmithing; Part 2: Materials science of precious metal processing. Bielefeld publishing house, 1999.
• TW Jelinek: Surface treatment of aluminum , EG Leuze Verlag, Saulgau 1997, ISBN 3-87480-115-2 .
• Hans Joliet (Ed.): Aluminum: the first 100 years . VDI-Verlag, Düsseldorf 1988, ISBN 3-18-400802-9 .
• Hans Rudolf Christen : Fundamentals of general and inorganic chemistry . 9th edition Salle, Frankfurt am Main 1988, ISBN 3-7935-5394-9 .
• Peter Haupt, Monika Kampling: Color anodizing. In: MNU. 52, No. 6, 1999, pp. 357-363.
• Catrin Kammer: Aluminum Pocket Book - Vol. 1: Fundamentals and materials; 15th edition 1995, Aluminum-Verlag, Düsseldorf.
• Dieter Altenpohl : Aluminum from the inside - the profile of a modern metal; 5th edition 1994, Aluminum-Verlag, Düsseldorf.

• Anodizing anodizing anodizing. In: alutecta.de - aluminum in form, color + function. Alutecta GmbH & Co.KG, accessed on April 29, 2018 (technically detailed description of a manufacturer's anodizing process).
• Hard Anodization vs Plasma Ceramics - Plasma Ceramics Video. Eloxalwerk Ludwigsburg ELB.BIZ, accessed on January 25, 2010 (plasma ceramics - video - difference between plasma ceramics and (hard) anodization).
• Robert S. Alwitt: Anodizing. In: Electrochemistry Encyclopedia. 2002, accessed on January 25, 2010 (English, quite detailed article).
• Partial Anodizing - Whitepaper - Holzapfel Group; Whitepaper with a technically detailed description of a manufacturer's anodizing process, accessed on May 25, 2016

## Individual evidence

1. Robert S. Alwitt: Anodizing. In: Electrochemistry Encyclopedia. 2002, accessed January 25, 2010 .
2. a b Dr. Catrin Kammer: Aluminum Pocket Book - Vol. 1: Fundamentals and materials; 15th edition 1995, Aluminum-Verlag, Düsseldorf; Page 356ff
3. Dieter Altenpohl : Aluminum from the inside - the profile of a modern metal; 5th edition 1994, Aluminum-Verlag, Düsseldorf; Page 260ff
4. TW Jelinek: Surface treatment of aluminum; EG Leuze Verlag, Saulgau, 1997; P. 31
5. ^ Friedrich Ostermann: Application technology aluminum. 2nd edition Springer, Berlin / New York 2007, ISBN 978-3-540-71196-4 , p. 581 ( limited preview in the Google book search).
6. German statutory accident insurance e. V. (DGUV): Electroplating and anodizing - Recommendations for risk assessment by the accident insurance carriers (EGU) according to the Ordinance on Hazardous Substances. Retrieved October 15, 2019 .
7. EUROPEAN ALUMINUM CONGRESS 2009, Norbert William Sucke, Dr. Lothar Löchte, "ALMINOX - The next generation of highly decorative ALUMINUM".
8. ^ Jude Mary Runge: Base Metal Microsctructure for Anodizing Wrought Aluminum Alloys . In: Aluminum Extrusion and Finishing . No. 4 , 2014 ( online ).
9. a b Michael Kolb: Partial anodizing - technological innovation to increase quality and economic efficiency; in: WOMag; WOTech Technical Media, Waldshut, issue 6/2015; Pp. 27–29 (see http://www.wotech-technical-media.de/womag/ausgabe/2015/06/27_kolb_partielles_eloxieren_06j2015/27_kolb_partielles_eloxieren_06j2015.php ).
10. a b c Eloxal partial plus as a process for partial anodizing was developed and implemented by the Holzapfel Group as part of a ZIM development project (ZIM = Central Innovation Program for SMEs), funded by the Federal Ministry for Economic Affairs and Energy on the basis of a resolution by the German Bundestag.
11. a b Aluminum Praxis, Giesel Verlag, Hanover, edition 10/2015, pp. 12-13
12. a b Aluminum Kurier, PSE Redaktionsservice GmbH, Geretsried, edition 10/2015, pp. 8–9