Aluminium anodising

Applications of anodized aluminum


Properties of anodic oxide coatings on aluminum are unique among other coatings. Therefore, they are widely used in various aspects of human life.

Base for coloring

This was the first industrial application of anodic coatings after the invention of anodizing aluminum (in chromic acid) in the 1920s.. This was the standard surface treatment for aluminum (duralumin) aircraft parts., and she still spelled out in the standards, for example, in modern British military standard DEF STAN 03-24/3. This combination of an organic coating with chrome anodic coating provides maximum life for the ink layer on the protective coating, and secures the metal even after damage to the paint.

Sulfuric anode coatings with bichromate seal are also used as a protective layer and base for applying organic paints.. This protective coating has a long service life, including, in seawater.

Corrosion protection

Uncolored chromium anodizing coating has high corrosion resistance, including saline environment. It is used for corrosion protection of aluminum aircraft parts, when they can not be painted. Sulfates anodic coating hydrothermal filling widely used for corrosion protection of aluminum structures in marine and industrial atmospheres. In recent decades, the anodic coating, colorless and colored, massively used for interior and exterior building materials and components, including, window, doors, facades of buildings, internal partitions and railings.

Military aluminum structures and parts of machines, especially those, which must withstand a long shelf life and work, including, in the tropics and the marine environment, also often protect anodic coatings.

The fundamental difference between anodized aluminum and non-anodized aluminum is that, that the first has a dense layer of oxide on the surface with a thickness 15-25 m. On the surface of non-anodized aluminum, a natural oxide film with a thickness of about 10 nm, i.e 0, 01 m, which has many flaws. Therefore, anodized aluminum products have a much higher corrosion resistance., than non-anodized.

Color anodized aluminum

With the ability to anodic coatings absorb dyes, receive a wide range of "colored aluminum". This method is called absorption, and it is widely used for a variety of aluminum products – cast, pressed, stamped. A more durable coating color - electrolytic - was prepared in different electrolytes, most - a nickel salt solutions, cobalt and tin. His "range" of colors is much narrower, than the adsorption, but quite diverse.

Fig. 1 – Colouring Anodized Aluminium [1]

Cleanliness and hygiene

at application without aluminum anodizing, for example, for the manufacture of stairs, seats, handrails or guardrails, you can often hear complaints, that aluminum leaves gray marks - "dirty".

Anodizing completely solves this problem and is used, for example, all aluminum parts in trains, buses, trolley buses and trams. they say, aluminum knitting needles gained popularity thanks to anodizing: they stopped getting your fingers dirty. The surface of the anodized product consists of aluminum oxide (Fig. 2), which the, unlike aluminum, does not stain hands. An important role in achieving this property of anodized aluminum is played by good sealing of the pores of the anodized coating..

Fig. 2 – Anodic film on aluminium [1]

Reflectance and emissivity of aluminium

Emissivity, ease, with which the substance radiates its own thermal energy, closely related to reflectivity. The best reflective surface is the worst emitter, и, conversely, the worst reflective surface is the best emitter. Ordinary aluminum reflects about 75% light falling on it and 90% thermal radiation. However, the emissivity of the same aluminum is low (<10% on the emissivity of a black body at the same temperature and in the same environment).

The emissivity of the aluminum surface can be greatly improved by anodic treatment, and therefore this process is used in the manufacture of heat exchangers. for instance. transparent anode coatings increase the emissivity from 35 to 65%, and methods with phosphoric and chromic acids are the most effective in this respect. The black anodized finishes have an even greater effect and elevate it to 95%.

On pic. 3 shows the effect of various surface finishes on the emissivity of aluminum.

ultra-pure aluminum, which has been mechanically polished, chemically brightened with the "Brytal" process and anodized in thickness > 3 micron (μm), will give an overall reflectivity (brightness) of more 84% and specular reflectance (mirror image sharpness) is higher. than 99% (cm. Fig. 4).

Fig. 3 – Reflectivity and Emissivity of Aluminium with Various Surface Treatments [2]

Fig. 4 – Comparison of Reflectivity of Various Metals [2]

Anodized aluminum in light reflectors

Sulfate anodization is used for surface protection reflectors spotlights. The initial small loss in reflectivity is considered acceptable, so this condition will persist for years, while the unprotected aluminum will corrode and permanently reduce the ability to reflect light. Moreover, Anodized aluminum is much easier to clean, than normal unprotected aluminum.

Anodized aluminum heat reflector

Anodizing long been used for heating aluminum reflectors - they can be found in every home. Their surface is easy to clean and they even withstand humidity bathrooms. Efficiency of anodized aluminum as a thermal radiation reflector is provided so, that the anode coating thickness is only about one micron. Teplootrazhatelnyj properties thicker anodic coatings used in the manufacture of cooling radiatorov- "combs" in electronic devices, including, in each computer. To increase the thermal emissivity anode coatings it is often dyed black.

Wear resistance

Anodic coating is much harder, than the base aluminum, so the wear resistance and the "anti-soiled" products increased significantly. Prior to use steel hydrothermal hydration anodizing, widely used its physical oil filling, wax and similar substances. Filling anode coating lubricating oils has been used in those engineering solutions, where on a given surface needs constant lubrication. Widespread use is found in aluminum pistons gasoline and diesel engines. Applied also filling graphite anode coating suspensions. Hard anodizing coating with a thickness from normal 40 to 60 um successfully used in various machine parts, for example, hydraulic and pneumatic cylinders.

Electrical isolation

Although the anodic coating and is a good electrical insulator, danger of local electrical breakdown due to small defects restrict the use of anodized aluminum wires. However, an anodized aluminum strip is used for many years for some types of transformers, when it is important to reduce their weight. Anodic oxide coating is much more resistant to heat, than organic insulating materials, however it is often selected for operation at high temperatures.


  1. TALAT Lecture 5203 – Anodizing of Aluminium
  2. TALAT Lecture 1501 – Aluminium: Physical Properties, Characteristics and Alloys