In production, Aluminum, Magnesium, and Titanium materials are relatively expensive metals chosen for their superior strength-to-weight ratios. Therefore, one of the methods to enhance the natural weather resistance of materials without significantly increasing their weight is Anodizing. It creates a protective oxide layer, typically pale gray by default but can be electroplated or dyed in a variety of vibrant colors, including red, green, blue, gold, copper, and black.
Anodizing is a process used to treat the surface of metals. The product is made into an anode and immersed in an electrolyte solution, resulting in the formation of a natural oxide layer on the metal surface. This oxide layer is tough, protective, and self-healing; aluminum oxide is one of the hardest and most renowned non-reactive materials in the world.
There are three main methods, including natural electroplating, hard electroplating, and chromic acid electroplating. Most electroplating processes in architecture, automotive, and general applications are typically carried out in sulfuric acid using natural electroplating or hard electroplating methods. Chromic acid electroplating is a more specialized process.
Natural electroplating creates relatively thick finishes ranging from 5 to 35 micrometers (0.00019-0.0014 in.) and is typically gray in color. These finishes can be dyed in a variety of vibrant colors, such as the Bang & Olufsen Beolab 4000 speakers.
Hard electroplating produces thicker coatings, up to 50 micrometers (0.0020 in.). It is used for applications that require higher durability, as the thicker coating improves resistance to wear and heat.
The electroplating process is employed to protect and enhance metals for both indoor and outdoor use. In fact, most aluminum in industries like automotive, construction, entertainment, and consumer electronics is treated using this method. Famous examples include Maglite flashlights, Apple’s iPod, and the G5 Powermac computer. Other products include carabiner clips and general mountaineering equipment, televisions, phones, household appliances, control panels, picture frames, cosmetic packaging, storefront facades, and various structural products.
Anodizing consists of three main stages
Stage 1: Cleaning and Etching
Stage 2: Anodizing
Stage 3: Sealing
In Stage 1, cleaning and etching tanks prepare the product for the oxidation process. The surface is either etched or brightened in chemical baths. The etching process creates a matte or dull surface and minimizes preparation activities. The brightening (chemical polishing) process produces a highly reflective surface suitable for decorative applications. Alkali and acid baths are used consecutively to neutralize each other
In Stage 2, the anodizing process takes place in an electrolyte solution, typically diluted sulfuric acid. Electric current is passed between the working product (the anode) and the cathode. This causes oxygen to concentrate on the surface of the product, reacting with the underlying metal to form a gas-permeable oxide layer (aluminum forms aluminum oxide). The time in the tank, temperature, and current determine the rate of oxide layer growth. It takes approximately 15 minutes to create 5 microns (0.00019 inches) of anodic film.
The anodizing process adds an anodic film to the metal surface. During this process, a small amount of the base material is consumed (about half the thickness of the anodic film). This affects the surface roughness. Only thin coatings maintain a high-gloss surface and are therefore unsuitable for applications that require high wear resistance.
Color is applied in three main ways. The Anolok™ system, recommended for outdoor applications, is a process that involves electro-coloring. A range of colors is created by precipitating metal salts within the gas-permeable oxide layer before sealing. Colors are produced through light interference. An alternative technique uses tin instead of cobalt. This is common but less UV-resistant compared to the Anolok™ system. The third technique is called ‘dye and seal,’ where colors are created by simply dyeing the anodic film before sealing. This process can yield a wide variety of colors but is less stable and has the poorest UV resistance, so it is typically used for decorative and indoor applications.
In Stage 3, the surface of the gas-permeable oxide layer is sealed in a hot water tank. Sealing the layer imparts durability and weather resistance associated with the anodizing process.
The anodizing process adds thickness to the size of the product, but the etching and cleaning processes typically compensate for this by removing a similar amount of material. However, this depends on the anodizing system and the hardness of the material, so softer materials may erode more quickly. Therefore, consulting with the company performing the anodizing process is crucial for critical-sized parts.
The coloration process does indeed impact the UV resistance of the component. The Anolok™ coloration system, which uses an electro-coloring method to apply cobalt metal salts, ensures that the components can be used in architectural applications for up to 30 years, but the color range is limited to shades of gray, copper, gold, and black.
Changes in the chemical composition of the metal affect the color. This is a concern both for welds and for products containing metal from different batches.
Regarding compatible materials, aluminum, magnesium, and titanium can all be anodized.
In terms of cost, there is no machining cost, but it may be necessary to fabricate clamps to fit the parts into the anodizing tanks. The cycle time is approximately 6 hours.
This process is typically automated, so labor costs are minimal.
The waste generated from the anodizing production process is not hazardous. Companies are closely monitored to ensure they do not pollute wastewater or the surrounding soil. Although acidic chemicals are used in the anodizing process, there are no hazardous byproducts.
The tanks are continuously filtered and reused. Aluminum dissolved in the rinse water is filtered out as aluminum hydroxide, which can be reused or safely treated. The anodized surfaces are non-toxic.