Environmentally Friendly Metal Surface Treatment: Anodizing/Anodic Oxidation

An Environmentally Friendly Metal Surface Treatment: Anodizing/Anodic Oxidation Since the 1930s, anodic oxidation, or anodizing as it is otherwise known, has maintained its position as an electrochemical method for corrosion protection. The surfaces of aluminium alloys are most commonly protected by forming anodic films. Anodic films can also be formed on metals such as titanium, zinc, magnesium, niobium and tantalum. Aluminium alloy parts are anodized (forming an anodic oxidation layer) to greatly increase the thickness of the natural oxide layer in order to enhance corrosion resistance. When an aluminium alloy part surface is exposed to open air, it forms a thin aluminium oxide film that protects it from further oxidation. The anodizing process, generally known as eloksal, increases the thickness of this oxidized surface. This process occurs by immersing the aluminium in an acidic electrolyte bath and passing an electric current through the medium. In an anodizing cell, the workpiece is connected to the positive terminal of a DC power source, making it the anode, while the cathode is connected to the negative terminal of the DC source. To prevent further corrosion, a sealing process is required to close the pores that form in the oxide layer. The oxide layer on anodized aluminium has a highly uniform, porous structure that permits secondary operations such as painting, printing and sealing. The anodizing coating process attracts significant interest in architecture, construction, engineering design and even works of art due to both its high durability properties and aesthetic qualities. There are factors affecting quality in the anodize coating process. In the process, temperature, current and concentration affect the thickness and structure of the oxide layer. Higher layer thicknesses and hardness can be achieved with lower temperatures and higher voltages. Paints, primers and adhesives do not provide good adhesion to bare metal surfaces, whereas anodized metal both increases surface adhesion and improves corrosion and wear resistance. The anodizing coating creates a structure with extremely regular porosity on the surface of the part it is applied to. This layer permits painting, printing and sealing properties. The surface of pure aluminium (unalloyed) exhibits passivity by forming an amorphous aluminium oxide layer 2 to 3 nm thick. This layer provides highly effective protection against corrosion. Thicker oxide layers (5–15 nm thick) form on aluminium alloys, but they exhibit more sensitive behavior toward corrosion. Generally, on aluminium, a nano-porous oxide layer can be created that improves properties such as lubrication and adhesion, permitting additional operations such as coloring (dyeing) and development of surface functions. A thick barrier oxide layer 2–3 nm forms on aluminium. This barrier oxide layer acts as an excellent electrical insulator and protects the surface from further reaction with the environment. The porous aluminium oxide layer is typically grown in dilute sulphuric acid at approximately 10% concentration by weight. Inorganic and organic acids (such as phosphoric acid, chromic acid, oxalic acid) can also be used. In addition, mixtures of organic and inorganic acids can be used. In such baths, a relatively high concentration of aluminium is maintained in the solution. This is extremely important because a large portion of the oxidized aluminium passes into the solution. In the sulphuric acid anodizing process, approximately 60% of the oxidized aluminium is in the layer formed on the surface, with the remainder retained by the solution. By appropriately adjusting the coating process conditions, it is possible to control the pores in the aluminium oxide layer. The pores grow perpendicular to the substrate surface; the pore diameters can be changed by voltage control. The pore diameter can range from 10–300 nm, and the layer thickness can vary up to 300 nm–100 microns. The table below compares two different anodizing processes and their properties. Anodize coatings are not a positive layer on the substrate. They interact with the substrate, with half of the coating layer in the substrate cross-section and the other half outside it. Most problems related to the anodizing process stem from deficiencies in the substrate or anodic film quality, or a combination of both. Therefore, one of the most important skills in troubleshooting is the ability to visualize and record the effect of these factors on the chemical and physical homogeneity on the surface. To do this effectively, the troubleshooter must be knowledgeable about metal production, process procedures, hanging techniques, cleaning and other pre-treatment operations, the anodizing process itself, sealing, rinsing and testing. Some of the important considerations are addressed here in order.

Substrate Surface Issues

The first source of problems may be in the metal itself. One of the most common occurrences is the presence of inclusions, dross or unwanted particles in the ingot or billet. These particles can be spread on the metal surface during rolling or extrusion. In the case of rolled products, oxides may result not from the metal composition but also from oxides rolled on the surface. These regions can cause color and texture differences during the anodizing process, creating scratches or bands on the surface. Among the most common metals for anodizing is the 6063 alloy. The surface color applied to this alloy family is mostly similar. The alloying elements (elemental metals) are finely distributed within the aluminium and do not oxidize during the anodizing step as in inclusions. A dull or grayish anodize layer appearance is a condition that becomes more pronounced with film thickness or the concentration of alloy components. During the etching pre-treatment step, alloys with high concentrations of alloy components can be identified as dark gray spotted. High copper content alloys such as the 2024 series result in a dull gray or golden surface with low wear resistance during anodizing due to dissolution of copper particles.

Metallurgical Properties

The performance of an alloy depends on its metallurgical properties. For architectural applications, quality anodize coating and structural strength must go together. Both properties depend on grain structure in the microstructure. If the structural components (phases, precipitates, etc.) in the alloy are not finely and uniformly distributed, the surface pattern that emerges after etching will have one of two conditions. First, it may be rough and gray in color due to selective dissolution of large, undispersed particles around the aluminium. Or it may exhibit a dull-like surface as a result of slow crystal growth of the aluminium and its components. This may occur as a result of improper homogenization of the billet, poor quenching during extrusion or improper heat treatment applied to the extruded parts. A common problem is hot/cold spots that can form in a random or regular pattern throughout an extrusion. This is usually due to contact between the part and die during extrusion and controls the cooling process. Cooling is typically slower at certain points, and as a result the grain structure is coarser and particles are less dispersed. After etching, this appears as a darker spot coating, and after anodizing the spot becomes more gray and more dull. This is due to large local particle concentrations and cannot be corrected or concealed by the anodizing process. Generally, the higher the temper, the finer the grain structure, the better the anodizing process, and the brighter, smoother and less gray surface obtained.

Transportation and Storage Steps

From the moment parts are subjected to extrusion or sheet is rolled, various processing steps are used. The material is cut to length, stacked on vehicles, heat treated, stored to cool, dismantled, possibly stored again until ready for anodizing, and finally suspended. At each step, unprotected metal is handled by hand (clean or oily, gloved or bare), stacked in contact with wood or cardboard (wet or dry), stored to be exposed to external elements or near interior areas. The anodizing line should be cleaned of naturally occurring airborne contaminants and finally suspended (again by gloved or bare hand). In general, any type of fingerprint or oil trace will appear after anodizing as etching patterns (prominent or unrecognizable). Again, the anodizing coating cannot conceal such defects. When exposed to a humid atmosphere or chemical vapors, the surface may oxidize more at certain points. Cleaning/degreasing alone cannot handle such oxidation, and the result after etching is pitting or a glossy appearance.

Hanging Procedures

The results of poor electrical contact can be easily measured using coating thickness test instruments or by observing the contact point at the end of the part. Burned or gray-appearing areas may indicate overheating. Uneven thickness on a part is an indicator of random hanging contact. (In electrolytic coloring, this is easily seen as color variation or "open ends" on workpieces.)

Pre-Treatment Surface Operations

Oxidation, degreasing, abrading and spot removal are among the main pre-treatment steps that affect finishing quality and appearance. Surface cleaning must be controlled carefully.

Anodizing Process

Bath parameters must be optimized and kept under regular monitoring. Current density, temperature, pH and film thickness are highly influential.

Advantages of Anodize Coatings

1. Anodize coatings are easy to maintain as they can be cleaned with a soft soap and water solution to return to original condition. 2. Color alternatives and bright metallic appearance add aesthetic value to the anodizing process. 3. Low processing and maintenance costs make it a cost-effective process. 4. The good appearance of anodize coatings makes it a practical and popular metal surface finishing method. 5. These coatings do not crack or peel because they are integrated into the substrate, making them more durable. 6. Anodized coatings exhibit color stability when exposed to UV radiation. 7. Anodizing is one of the most environmentally friendly metal finishing methods that does not harm human health, containing no heavy metals or halogens. 8. Anodize coatings are chemically stable and do not decompose. 9. The non-toxic nature of anodize coatings and high heat resistance make it a valuable method for corrosion prevention. 10. Common anodizing wastewater such as Al(OH)3 and Al2(SO4)3 are recycled for the production of various products.  

Applications of Anodizing

Anodize coatings have made aluminium and its alloys one of the most commonly used materials today. The aesthetic values it imparts to surfaces, environmental safety, appearance and other factors have been influential. This method is widely used in computer hardware manufacturing and in a wide variety of commercial products and systems. 1. Home Appliances: Refrigerators, dryers, coffee makers, stoves, televisions, mp3 players, flashlights, cookware, cameras, microwave ovens, grills, television components, 2. Building Elements: Door handles, window frames, external structural panels, curtain facades, roofing systems, vents, dust covers, lighting fixtures, mailboxes, bathroom accessories, wall switch plates for buildings, ceilings, floors, on escalators, 3. Automobiles: In motor vehicle components such as flooring parts, wheel covers, control panels, panels, 4. Furniture: On tables, beds, files and storage cabinets, 5. Interior Applications: In clocks and electronic products, fire extinguishers, photographic equipment, solar panels, telephones, picture frames, cookware, 6. Aerospace: On external panels and pistons for aircraft, aircraft landing gear components and jet engine control valves, particularly protecting satellites from the harsh environment of space. 7. Leisure Equipment: In camping and fishing equipment, 8. Sports and Recreation Equipment: In sports equipment such as golf carts, boats, 9. Food Industry: In food processing and transport equipment, pans, coolers and grills, 10. Medical Equipment: In surgical component trays, surgical instrument handles, dental components, medical transport and processing equipment, 11. Nanotechnology: The pores in anodized coatings can be used as templates to create structures such as nanowires and nanotubes.  

Related Standards

1. ISO 1463, Metallic and oxide coatings — Measurement of coating thickness — Microscopical method. 2. ISO 2106, Anodizing of aluminium and its alloys — Determination of mass per unit area (surface density) of anodic oxidation coatings — Gravimetric method. 3. ISO 2360, Non-conductive coatings on nonmagnetic electrically conductive base metals — Measurement of coating thickness — Amplitudesensitive eddy-current method. 4. ISO 2376, Anodizing of aluminium and its alloys — Determination of breakdown voltage and withstand voltage. 5. ISO 4516, Metallic and other inorganic coatings — Vickers and Knoop microhardness tests. 6. ISO 6344-1, Coated abrasives — Grain size analysis — Part 1: Grain size distribution test. 7. ISO 7583, Anodizing of aluminium and its alloys — Terms and definitions. 8. ISO 8251, Anodizing of aluminium and its alloys — Measurement of abrasion resistance of anodic oxidation coatings. 9. ISO 9227, Corrosion tests in artificial atmospheres — Salt spray tests.     References • Nisha Mathew, A Short Review on Aluminum Anodizing: An EcoFriendly Metal Finishing Process, Journal for Research Volume 02, Issue 06, 5-9, August 2016. • ASM Handbook, Volume 5: Surface Engineering C.M. Cotell, J.A. Sprague, and F.A. Smidt, Jr., editors, p 482-493. • Wielage B, Alisch G, Lampke T, Nickel D. Anodizing – A Key for Surface Treatment of Aluminium. KEM 2008;384:263–81. • ISO 10074:2021(en):Anodizing of aluminium and its alloys — Specification for hard anodic oxidation coatings on aluminium and its alloys • Richard Mahn, Recognizing and Dealing with General Anodizing Problems Journal of the American Electroplaters and Surface Finishers Society, 1988. • https://www.anodizing.org/Anodizing/definitions.html • https://www.kashima-coat.com/global/aluminum/anodized-aluminum.html • https://macdermidalphaauto.com/lightweighting/aluminum/anodizing • https://sterc.org/files/pf078802.htm   Assoc. Prof. Dr. Ekrem Altuncu TÜYİDER (Association for All Surface Treatments) Member Sakarya University of Applied Sciences Director, Materials and Production Technologies Application and Research Center — SUMAR