Anodizing

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Anodizing, or anodising, is an electrolytic passivation process used to increase the thickness and density of the natural oxide layer on the surface of metal parts. This process is of no use on carbon steel because rust puffs up and flakes off, constantly exposing new metal to corrosion. But on many other metals, anodizing increases corrosion resistance and wear resistance, and provides better adhesion for paint primers and glues than bare metal. Anodic films can also be used for a number of cosmetic effects, either with thick porous coatings that can absorb dyes or with thin transparent coatings that add interference effects to reflected light. Anodization changes the microscopic texture of the surface and can change the crystal structure of the metal near the surface. Coatings are often porous, thick ones inevitably so, so a sealing process is often used to improve corrosion resistance. The process derives its name from the fact that the part to be treated forms the anode portion of an electrical circuit in this electrolytic process. Anodizing can prevent galling of threaded components. Anodic films are generally much stronger and more adherent than most paints and platings, making them less likely to crack and peel. Anodic films are most commonly applied to protect aluminium alloys, although processes also exist for titanium, zinc, magnesium, and niobium.

[edit] Anodized aluminum

Aluminum is anodized both to increase corrosion resistance and to allow dyeing.

When exposed to the atmosphere, aluminum forms a passive oxide layer which provides moderate protection against corrosion. In its pure form aluminum self-passivates very effectively, but its alloys, especially 6000 series due to the magnesium content, are far more prone to atmospheric corrosion and therefore benefit from the protective quality of anodising. Aluminum alloy parts are therefore anodized to increase the thickness of this layer for corrosion resistance. Most aluminum aircraft parts including major components are anodized. Anodized aluminum can be found in many consumer products like mp3 players, flashlights, cookware, cameras, sporting goods, and many other products both for corrosion resistance and the ability to be dyed.

Anodized coatings have a much lower thermal conductivity and coefficient of linear expansion than aluminum. As a result, they will crack if exposed to temperatures above 80°C, although they will not peel.^[1] Their melting point is 2050°C, much higher than pure aluminum's 658°C.^[1] This can make welding more difficult.

The aluminum oxide coating is grown from and into the surface of the aluminum in about equal amounts, so for example a 2μm thick coating will increase part dimensions by 1μm per surface. In most consumer goods the dye is contained in the pores of the Aluminum oxide layer. Anodized aluminum surfaces have low to moderate wear resistance, although this can be improved with thickness and sealing. If wear and scratches are minor then the remaining oxide will continue to provide corrosion protection even if the dyed layer is removed.

If the anodizing is performed in a solution which has some solubility to aluminum oxide, such as sulphuric acid or chromic acid, cylindrical pores, 10-150 nm in diameter, are formed in the coating.^[1] This allows the coating to continue to grow to a greater thickness, but the pores may also permit corrosion if not sealed. The pores are often filled with dyes and/or corrosion inhibitors before sealing.

The most widely used anodizing specification, MIL-A-8625, defines three types of aluminum anodization. Type I is Chromic Acid Anodization, Type II is Sulphuric Acid Anodization and Type III is sulphuric acid hardcoat anodization. AMS 2469 is very similar to Type III. Other anodizing specifications include ASTM B580, ISO 10074 and BS 5599. AMS 2468 is obsolete.

[edit] Type I and Type II Anodization

Before being treated, the aluminum, if wrought, is 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.

In aluminum anodization, this aluminum oxide layer is made thicker by passing a direct current through a sulphuric acid 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. Anodizing at 12 V DC, a piece of aluminum with an area of 1 square decimeter (about 15.5 square inches) can consume roughly 1 ampere of current. In commercial applications the voltage used is more normally in the region of 15 to 21 V.

Conditions such as acid concentration, solution temperature, and current must be controlled to allow the formation of a consistent oxide layer, which can be many times thicker than would otherwise be formed. This oxide layer increases both the hardness and the corrosion resistance of the aluminum surface. The oxide forms as microscopic hexagonal "pipe" crystals of amorphous alumina, each having a central hexagonal pore (which is also the reason that an anodized part can take on color in the dyeing process). The film thickness can range from under 5 micrometres on bright decorative work to over 25 micrometres for architectural applications.

The older Type I (chromic acid) method produces thinner, 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 sulfuric acid in that the voltage is ramped up through the process cycle.

[edit] Type III Hardcoat Anodizing

Type III anodizing, also called hardcoat anodizing, is a thicker variation of type II which requires more process control.^[1] Hard anodizing is usually between 25 and 150μm. Although hardcoat anodizing 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 have no loose liner: instead, the walls are hardcoated. The greater anodizing thickness also increases the electrical and thermal insulation. 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.)

[edit] Organic acid anodizing

Anodizing can be carried out in organic acids such as oxalic acid to produce integral colours in the anodized coating without dyes.^[1] The shade of colour produced is sensitive to variations in the metallurgy of the underlying alloy. Thicknesses up to 50μm can be achieved.

[edit] Sealing

Chromic acid and sulfuric acid processes such as Types I, II, and III will produce pores in the anodized coat. These pores can be sealed in a number of ways. Long immersion in boiling-hot deionized water is the simplest, although it is not completely effective and reduces abrasion resistance by 20%.^[1] Teflon, nickel acetate, and hot dichromate seals are common.

[edit] Borate and Tartrate Baths

Anodizing can also be performed in Borate or Tartrate Baths in which aluminum oxide is insoluble. In these processes, the coating growth stops when the part is fully covered, and the thickness is linearly related to the voltage applied.^[1] These coatings are free of pores, relative to the sulfuric and chromic acid processes.^[1]

[edit] Plasma electrolytic oxidation

Plasma electrolytic oxidation is a similar process, but where higher voltages are applied. This causes sparks to occur, and results in more crystalline type coatings.

[edit] Other metals

[edit] Anodized titanium

Anodized titanium is used in a recent generation of dental implants. Anodizing generates a thicker layer of titanium dioxide (>1 µm and up to >2.5 µm compared with much less than 1 µm for un-anodized specimens) and a characteristic surface topography.^{[citation needed]} It has been suggested that both of these parameters improve the performance—longevity, stability—of dental implants, but the technology is still new and there are not yet clear clinical data to support these claims.^{[citation needed]}

Titanium anodic films cannot be made thicker than about 300nm, and are therefore susceptible to mechanical damage.^[2]

Anodizing titanium generates an array of different colors without dyes, for which it is sometimes used in art, costume jewelry and wedding rings.[2][3] The color formed is dependent on the thickness of the oxide (which is determined by the anodising voltage); it is caused by the interference of light reflecting off the oxide surface with light traveling through it and reflecting off the underlying metal surface. Titanium nitride coatings can also be formed, which have a brown or golden color and have the same wear and corrosion benefits as anodization.

[edit] Anodized Magnesium

Magnesium is anodized primarily as a primer for paint. A thin (5μm) film is sufficient for this.^[2] Thicker coatings of 25μm and up can provide mild corrosion resistance when sealed with oil, wax, or sodium silicate.^[2]

[edit] Anodized Zinc

Zinc is rarely anodized, but a process was developed by the International Lead Zinc Research Organization and covered by MIL-A-81801.^[2] A solution of ammonium phosphate, chromate and fluoride with voltages of up to 200V can produce olive green coatings up to 80μm thick.^[2] The coatings are hard and corrosion resistant.

[edit] Anodized niobium

Niobium anodizes in a similar fashion to titanium with a range of attractive colors being formed by interference at different film thicknesses. Again the film thickness is dependent on the anodising voltage. Uses include jewelry and commemorative coins.

[edit] Dyeing

Where appearance is important, the oxide surface can be dyed before the sealing stage, as the dye enters the pores in the oxide surface. The number of dye colors is almost endless; however, the colors produced tend to vary according to the base alloy. Though some may prefer lighter colors, in practice they may be difficult to produce on certain alloys such as high-silicon casting grades and 2000-series (with its high copper content). Another concern is the lightfastness of organic dyestuffs—some colours (reds and blues) are particularly prone to fading. Black dyes and gold produced by inorganic means (ferric ammonium oxalate) are more lightfast.

Alternatively, metal (usually tin) can be electrolytically deposited in the pores of the anodic coating to provide colors that are more lightfast. Metal dye colors range from pale champagne to black. Bronze shades are preferred for architectural use.

Alternatively the color may be produced integral to the film. This is done during the anodizing process using organic acids mixed with the sulfuric electrolyte and a pulsed current.

After dyeing, the surface is usually sealed by using hot water or steam, sometimes mixed with nickel acetate or other anti-bloom agents, to convert the oxide into its hydrated form. This reduces the porosity of the surface as the oxide swells. This also reduces or eliminates dye bleed out and can increase corrosion resistance. Sealing at 20 °C in nickel-cobalt salts, cold sealing, when the pores are closed by impregnation is also popular due to energy savings. Coatings sealed in this method are not suitable for adhesive bonding.

[edit] Mechanical considerations

Anodizing will raise the surface, since the oxide created occupies more space than the base metal converted. This will generally not be of consequence except in the case of small holes threaded to accept screws. Anodizing may cause the screws to bind, thus the threaded holes may need to be chased with a tap to restore the original dimensions. Alternately, special oversize taps may be used to precompensate for this growth. In the case of unthreaded holes that accept fixed diameter pins or rods a slightly oversized hole to allow for the dimension change may be appropriate.