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Anodising, A History Of.


Anodising was first used on an industrial scale in 1923 to protect Duralumin Seaplane parts from corrosion. This early chromic acid process was called the Bengough-Stuart process and was documented in British defence specification DEF STAN 03-24/3. It is still used today despite its legacy requirements for a complicated voltage cycle now known to be unnecessary.

Variations of this process soon evolved, and the first Sulhuric Acid Anodising process was patented by Gower and O'Brien in 1927. Sulhuric Acid soon became, and remains the most common Anodising electrolyte.

Oxalic Acid Anodising was first patented in Japan in 1923 and later widely used in Germany, particularly for architectural applications.

Anodised Aluminium Extrusion was a popular architectural material in the 1960s and 1970s, but has since been displaced by cheaper plastics and powder paint, although latterly there has been a resurgence due to the lower environmental impact compared with plastic

The Phosphoric Acid processes are the most recent major development, so far only used as pretreatments for adhesives or organic paints. A wide variety of proprietary and increasingly complex variations of all these Anodising processes continue to be developed by industry, so the growing trend in military and industrial standards is to classify by coating properties rather than by process chemistry.

Anodised Aluminum alloys are Anodised to increase corrosion resistance, to increase surface hardness, and to allow dyeing (coloring), improved lubrication, or improved adhesion. When exposed to air at room temperature, or any another gas containing oxygen, pure aluminum self-passivates by forming a surface layer of amorphous aluminum oxide 2 to 3 nm thick which provides very effective protection against corrosion.

Aluminum alloys typically form a thicker oxide layer, 5-15 nm thick, but tend to be more susceptible to corrosion. Aluminum alloy parts are Anodised to greatly increase the thickness of this layer for corrosion resistance. The corrosion resistance of aluminium alloys is significantly decreased by certain alloying elements or impurities: copper, iron, and silicon, so 2000, 4000, and 6000-series alloys tend to be most susceptible. Most aluminum aircraft parts, architectural materials, and consumer products are Anodised.

Anodised aluminum can be found on mp3 players, flashlights, cookware, cameras, sporting goods, window frames, roofs, in electrolytic capacitors, and on many other products both for corrosion resistance and the ability to retain dye. Although Anodising only has moderate wear resistance, the deeper pores can better retain a lubricating film than a smooth surface would.

For example, the cylinders of a modern BMW aluminum V8 cylinders have no loose liner: instead, the walls are hard Anodised. This complicates a reboring operation (although not common, given the longevity of modern engines due to improved lubricants), as the hard coating must be restored if the block is rebored. (Earlier liner-free aluminum block designs use specific aluminum alloys, with softer components chemically etched away to expose the harder portions of the mixed crystal structure.)

Anodised coatings have a much lower thermal conductivity and coefficient of linear expansion than aluminum. As a result, the coating will crack from thermal stress if exposed to temperatures above 80 °C. The coating can crack, but it will not peel. The melting point of aluminum oxide is 2050 °C, much higher than pure aluminum's 658 °C. (This can make welding more difficult.) In typical commercial aluminum anodization processes, the aluminum oxide is grown down into the surface and out from the surface by equal amounts. So Anodising will increase the part dimensions on each surface by half of the oxide thickness. For example a coating that is (2 μm) thick, will increase the part dimensions by (1 μm) per surface. If the part is Anodised on all sides, then all linear dimensions will increase by the oxide thickness.

Anodised aluminum surfaces are harder than aluminum but have low to moderate wear resistance, although this can be improved with thickness and sealing.

Process Before being Anodised. wrought alloys are cleaned in either a hot soak cleaner or in a solvent bath and may be etched in sodium hydroxide (normally with added sodium gluconate), ammonium bifluoride or brightened in a mix of acids.

Cast alloys are normally best just cleaned due to the presence of intermetallic substances unless they are a high purity alloy such as LM0.

The Anodised aluminum layer is grown by passing a direct current through an electrolytic solution, with the aluminum object serving as the anode (the positive electrode). The current releases hydrogen at the cathode (the negative electrode) and oxygen at the surface of the aluminum anode, creating a build-up of aluminum oxide. Alternating current and pulsed current is also possible but rarely used. The voltage required by various solutions may range from 1 to 300 V DC, although most fall in the range of 15 to 21 V. Higher voltages are typically required for thicker coatings formed in sulhuric and organic acid. The Anodising current varies with the area of aluminum being Anodised, and typically ranges from 0.3 to 3 amperes of current per square decimeter (20 to 200 mA/in²). Aluminum Anodising is usually performed in an acid solution which slowly dissolves the aluminum oxide.

The acid action is balanced with the oxidation rate to form a coating with microscopic pores, 10-150 nm in diameter. These pores are what allows the electrolyte solution and current to reach the aluminium substrate and continue growing the coating to greater thickness beyond what is produced by autopassivation. However, these same pores will later permit air or water to reach the substrate and initiate corrosion if not sealed. They are often filled with colored dyes and/or corrosion inhibitors before sealing. Because the dye is only superficial, the underlying oxide may continue to provide corrosion protection even if minor wear and scratches may break through the dyed layer. Conditions such as electrolyte concentration, acidity, solution temperature, and current must be controlled to allow the formation of a consistent oxide layer. Harder, thicker films tend to be produced by more dilute solutions at lower temperatures with higher voltages and currents. The film thickness can range from under 0.5 micrometers for bright decorative work up to 150 micrometers for architectural applications.

The most widely used Anodising specification, MIL-A-8625, defines three types of aluminum anodisation. Type I is Chromic Acid Anodisation, Type II is Sulphuric Acid Anodisation and Type III is sulphuric acid hardcoat anodisation. Other Anodising specifications include MIL-A-63576, AMS 2469, AMS 2470, AMS 2471, AMS 2472, AMS 2482, ASTM B580, ASTM D3933, ISO 10074 and BS 5599. AMS 2468 is obsolete. None of these specifications define a detailed process or chemistry, but rather a set of tests and quality assurance measures which the Anodised product must meet. BS 1615 provides guidance in the selection of alloys for Anodising.

For British defence work, a detailed chromic and sulhuric Anodising processes are described by DEF STAN 03-24/3 and DEF STAN 03-25/3 respectively.Chromic acid Anodising The oldest Anodising process uses chromic acid. It is widely known as Type I because it is so designated by the MIL-A-8625 standard, but it is also covered by AMS 2470 and MIL-A-8625 Type IB. Chromic acid produces thinner, (0.00002" to 0.0007" or 0.5 μm to 18 μm) more opaque films that are softer, ductile, and to a degree self-healing. They are harder to dye and may be applied as a pretreatment before painting. The method of film formation is different from using sulhuric acid in that the voltage is ramped up through the process cycle.





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