What is anodizing?

Anodizing is an electrochemical process that coats a metal surface with a decorative, durable, and corrosion -resistant anodic oxide coating . Aluminum is ideally suited for anodizing, although other non-ferrous metals such as magnesium and titanium can also be anodised.

The aluminum oxide structure originates from an aluminum substrate and is composed entirely of aluminum oxide. This aluminum oxide is not applied to the surface like a paint or coating, but entirely with its aluminum substrate . Therefore, it cannot be broken or peeled off. It has a very regular and porous structure that allows secondary operations such as coloring and sealing.

The anodizing process is done by immersing the aluminum in an acid electrolyte bath and passing an electric current through the medium. The cathode is installed inside the anode tank. Aluminum acts as an anode, so oxygen ions are released from the electrolyte to combine with aluminum atoms on the surface of the part being anodized. Therefore, anodizing is an oxidation substance that is highly controlled and promotes a natural phenomenon.

Definitions and anodizing methods

What is anodizing?

Anodizing successfully combines science and nature to create one of the best metallic coatings in the world.

This is an electrochemical process that thickens and solidifies the natural protective oxide. The resulting coating, depending on the process, is the second hardest substance known to man after diamond. The anodic coating is part of the metal, but it has a porous structure that allows secondary injections (such as organic and inorganic coloring, lubricating aids, etc.).

Definitions and anodizing methods

While the chemical anodizing process is the same for all applications, the mechanical methods differ based on two physical types and the shape of the metals used:

• Batch anodizing – involves placing parts on racks and immersing them in a series of curing tanks. Extrusions, bent metal plates or parts, castings, cookware, cosmetic boxes, flashlight bodies, and machined aluminum parts are just a few of the items that are batch anodized.
• Continuous Winding – It involves the continuous unwinding of pre-rolled coils through a series of anodizing, drilling and cleaning tanks, then returning for shipment and fabrication. This method is used for oversized panels, laminates, and less formed products such as lighting fixtures, reflectors, louvers, insulating glass spacers, and continuous roof systems.

Appearance and quality options are improved through the use of special color and pre-processing methods. This makes aluminum look like pewter, stainless steel, brass, polished bronze, or brushed brass, and can also be painted in bright blue, green, red, and many other metallic golds and silvers.

The unique insulating properties of anodized coatings provide many opportunities for electrical applications.

The aluminum surface itself is hardened to a degree that cannot be compared to any other process or material. This coating is 30% thicker than the metal it replaces because the volume of oxide produced is greater than the volume of oxide produced. Metal has been replaced

The resulting anodic coating is porous and allows for relatively easy coating and sealing.

Hard anodizing is a term used to describe the production of anodized coatings with film hardness or wear as the primary property. They are usually as thick as conventional anodizing standards (greater than 25 microns) and produced using special anodizing conditions (very low temperature, high current density, special electrolytes). They are used in the engineering industry for parts that require a highly corrosion-resistant surface, such as pistons, cylinders, and hydraulic components . They are often left unsealed, but they can be impregnated with materials such as waxes or silicone liquids to provide special surface properties.

Anodize a batch and file

The batch and file anodizing process is performed in five carefully controlled, calibrated and quality-tested steps:

1. cleaning. Alkaline and/or acidic cleaners remove grease and dirt from the surface.
2. Pre-treatment
   ◦ Embossing. An attractive matte surface is created with a hot sodium hydroxide solution to remove minor surface imperfections. A thin layer of aluminum is removed to obtain a matte or glossy finish.
   ◦ Polish A near-mirror coating is created with a concentrated mixture of phosphoric and nitric acids that chemically smooth the aluminum surface.
3. Anodizing An anodic film is made and combined with the metal by passing an electric current through an acidic electrolyte bath in which aluminum is immersed. Coating thickness and surface properties are tightly controlled to meet final product specifications.
4. coloring. Coloring is done in one of four ways:
   

   Electroplating (two-step method) – After anodizing, the metal is immersed in a bath. Anodizing line containing inorganic metallic salt. A current is applied that precipitates the mineral salt at the base of the pores. The resulting color depends on the metal used and the processing conditions (the range of colors can be increased by over-dyeing with organic dyes). Electrolytic paints can be selected from any member of the AAC. Commonly used metals are tin, cobalt, nickel and copper. This process provides color variety and the most advanced coloring quality. Integrated Painting – This one-step process combines anodizing and coating processes to simultaneously form and paint the oxide cell wall in shades of bronze and black while being more corrosion resistant than the traditional anodizing process. This is the most expensive process because it requires much more electricity. Organic Dyeing – The organic dyeing process produces a wide variety of colours. These colors provide vibrant colors with an intensity unmatched by any other color system on the market. It can also provide excellent weather stability and lightweight stability. Many structures built with these finishes have lasted more than 20 years. The range of colors can be extended by over-electrolyzing organic dyes for a greater variety. In terms of colors and shades, this method is relatively cheap and involves the least initial investment compared to other coloring processes. Interference staining – An additional recently developed staining technique that involves modifying the structure of the pores produced in sulfuric acid. Pore ​​enlargement occurs at the base of the follicle. Mineral deposition in this place produces colors at the speed of light from blue, green, yellow to red. The colors result from optical interference effects rather than light scattering as is the case in the electrolytic base dyeing process. In addition, the development results in a greater range of colors.

   Below is detailed information comparing the two most common coating processes: (Note – these two processes do not produce the same colors; both can be overcoated. Source: Aluminum Anodizers Council Technical Bulletin No. 1-94, issued January 1994.) See below.

5. Seal. This process closes the pores of the anodic layer and makes the surface resistant to stains, corrosion, scratches, and color deterioration. Quality control. During the entire anodizing process, AAC members monitor the process and product quality. Electroplating and painting application on all batches and coils pre-planned and approved. This quality control ensures consistency with final product specifications for film thickness, density, abrasion resistance, abrasion resistance, color uniformity, and ensures fade resistance, reflection, image clarity , insulation properties, adhesion. Seal In many cases, AAC members use statistical process control (SPC) methods to meet strict quality assurance standards.

 Comparison of colored anodizing surfaces for A32 / A42 and A34 / A44 aluminum

to oxide

Anodizing is a natural negative electrolyte oxide used on the surface of metal parts.

This is the anodizing process because the part that is the anode of the electrolysis cell. Anodizing increases corrosion and corrosion resistance and provides better adhesion to the primer and paint adhesive compared to bare metal. Anodic membranes can be used for many cosmetic effects, either with thick, porous layers that can absorb dyes, or with thin, transparent layers that interfere with the effects of light waves.

Anodizing is also used to prevent friction of threaded components and to make insulating films for electrolytic capacitors. Anodic membranes are most commonly used to protect aluminum alloys, although there are processes for titanium, zinc, magnesium, niobium, zirconium, hafnium, and tantalum. Iron or carbon steel is peeled when it is oxidized under the conditions of neutral or alkaline microelectrolysis. For example, iron oxide (actually iron hydroxide or aqueous iron oxide, also known as rust) is formed by anodic etching and a large cathodic surface, these etching allow anions such as sulfate and chloride to accelerate the corrosion of the base metal. Scales or nodules of carbon in iron or steel with a high carbon content (high carbon steel and cast iron) may cause electrolysis and interfere with coatings or coatings. Ferrous metals are usually formed electrolytically in nitric acid or by using red and black nitric acid (II, III) to form solid iron oxide. This oxide remains compatible even when it is applied to the wires and the wires are bent.

The anodizing process changes the microscopic texture of the surface and the crystal structure of the metal near the surface. The sealing process often needs to be corrosion resistant. For example, anodized aluminum surfaces are harder than aluminum, but have low to medium corrosion resistance that can be improved by increasing the thickness or using suitable sealants. Anodizing coatings are typically much stronger and more adhesive than most types of metallic paints and coatings, but they are also more brittle. This makes it less susceptible to cracking and flaking from aging and corrosion, but more susceptible to cracking from thermal stress.

date

Anodizing was first used on an industrial scale in 1923 to protect waterjet duralumin from corrosion. This early chromic acid process was called the Bengog-Stewart process and was documented in DEF STAN Defense Specification 03-24/3. It is still used despite its outdated requirement for a complex voltage cycle that is now recognized as unnecessary. Variations of the process soon developed, and the first sulfuric acid anodization process was documented by Gower and O’Brien in 1927. Sulfuric acid quickly became and remains the most common oxidizing electrolyte. ^[1]

Oxalic acid anodizing was first patented in Japan in 1923 and later widely used in Germany, especially for architectural applications. It was replaced with plastic and powder coating. ^[2] Phosphoric acid processes are the most recent major development, so far only used as a pre-treatment of organic adhesives or dyes. ^[1] The industry continues to develop a variety of proprietary and increasingly diverse variations of all of these anodizing processes, so a growing trend in military and industrial standards is to classify based on coating properties rather than process chemistry.

aluminium

Aluminum alloys to increase corrosion resistance, the possibility of dyeing (painting), lubrication or better adhesion. However, anodizing does not increase the strength of the aluminum body. Anodic insulation layer. ^[3]

When exposed to air at room temperature or any other gas containing oxygen, pure aluminum forms a surface layer of amorphous aluminum oxide with a thickness of 2 to 3 nanometers. ^[4] which provides highly effective protection against corrosion. Aluminum alloys usually form a thicker oxide layer, 5-15 nm thick, but they are more susceptible to corrosion. Aluminum alloy parts are oxidized to greatly increase the thickness of this layer to resist corrosion. The corrosion resistance of aluminum alloys is greatly reduced by the alloying elements or impurities: copper, iron and silicon. ^[5] Therefore, the alloys of the 2000, 4000, 6000 and 7000 series alloys are more sensitive.

Although anodizing produces a very neat and uniform coating, microscopic cracks in the coating can lead to corrosion. At the top and bottom, it is subject to decomposition with chemical pH, which leads to peeling of the coating and corrosion of the substrate. To combat this, various techniques have been developed to reduce the number of vacuoles, introduce more stable chemical compounds into the oxide, or both. For example, sulfur oxidized products are usually sealed by either a hydrothermal seal or a precipitation seal to reduce the porosity and interstitial pathways that allow the exchange of corrosive ions between the surface and the substrate. Sediment seals increase chemical stability but are less effective in eliminating ion exchange pathways. Recently, new techniques have been developed to convert a portion of amorphous oxide coatings into more stable microcrystalline compounds, which have shown significant improvement based on shorter bond lengths.

Some aluminum parts for aircraft, architectural materials, and consumer products are anodized. Anodized aluminum can be used in MP3 players, smartphones, multi-tools, light bulbs, cookware, cameras, sporting goods, firearms, window frames, roofing, electrolytic capacitors, and many other products for its corrosion resistance and color retention ability. . Although the anodizing has only moderate wear resistance, the deep pores can retain a better lubrication layer than the smooth surface.

The anodized coating has a much lower thermal conductivity and linear expansion coefficient than aluminum. Thus, if the heat stress coating is exposed to temperatures above 80 °C (353 K), the coating may crack, but not peel off. ^[6] The melting point of aluminum oxide is 2050 ° C (2323 ° K), which is much higher than 658 ° C (931 ° K) for pure aluminium. ^[6] This, in addition to the insulating properties of aluminum oxide, can make welding more difficult.

In typical commercial aluminum anodizing processes, aluminum oxide grows inside and outside the surface in equal amounts. ^[7] Therefore, anodizing increases the dimensions of the part for each surface by half the thickness of the oxide. For example, a coating with a thickness of 2 micrometers increases the dimensions of the part by 1 micrometer per surface. If the part is anodized on all sides, all linear dimensions are increased by the thickness of the oxide. Anodized aluminum surfaces are harder than aluminum, but they have low to medium corrosion resistance, although this can be improved by thickening and stamping.

practical

Desmut

Desmuth’s solution can be applied to the aluminum surface to remove impurities. Nitric acid is commonly used to remove stains, but it is being replaced due to environmental concerns. ^{[8] [9] [10] [11]}

Electrolyzed

The anodized aluminum layer is grown by passing a direct current through an electrolyte solution, and the aluminum body acts as an anode (positive electrode in an electrolyte cell). The current releases hydrogen at the negative electrode (negative electrode) and oxygen at the surface of the aluminum anode, forming aluminum oxide. Alternating current and pulsed current are also possible but rarely used. The voltage requirements for different solutions may vary from 1 to 300 volts DC, although most fall in the range of 15 to 21 volts. High voltages are usually required for thick coatings consisting of sulfuric and organic acids. The anodizing current varies according to the area of ​​the aluminum being anodized and usually ranges between 30 and 300 / m2. ^A be.

12 [ ] Usually in the process of oxidizing aluminum (aluminum or oxalate oxidation) ^{a solution} uses an acid, usually sulfuric acid or chromic acid, to slowly dissolve the aluminum oxide. ^[6] It is these pores that allow the electrolyte solution to flow into the substrate and increase the thickness of the coating to a thickness greater than that resulting from spontaneous passivation. ^[13] These pores allow the color to absorb, however, this must be accompanied tightly or the color will not remain. The coating is usually followed by a clean nickel acetate seal. Since the coating is only superficial, the base oxide may continue to provide corrosion protection even if minor abrasion breaks the coated layer. ^{[ requires source ]}

Conditions such as electrolyte concentration, acidity, solution temperature, and current must be controlled to allow formation of a stable oxide layer. Harder and thicker films are usually produced by more concentrated solutions at lower temperatures with higher voltages and currents. Film thickness can range from as little as 0.5 µm for light decorative works to 150 µm for architectural applications.

Double achievement

Anodizing can be done with chromate transfer coating. Each process provides corrosion resistance, with anodizing providing a significant advantage in terms of strength or material corrosion resistance. The reason for the combination of processes can be different, however, the major difference between anodizing and chromate transfer coatings is the electrical conductivity of the films produced. Although both compounds are stable, chromate conversion paint has a very high electrical conductivity. The applications, where it may be useful, are diverse, however, the issue of grounding as part of a larger system is a clear one.

The double finishing process uses the best each process has to offer, anodizing with its extreme corrosion resistance and electrically conductive chromate transfer coating. ^[14]

Process steps can usually include chromate transfer coating of the entire component, followed by surface coating in areas where the chromate coating is to remain intact. Moreover, the chromate coating dissolves in unmasked areas. The part can then be anodized and transferred to the unmasked areas. The exact process will vary depending on the service provider, component geometry, and desired outcome. This helps protect aluminum products.

Other commonly used specifications

The most widely used anodizing specification in the United States is the US military specification, MIL-A-8625, which specifies three types of anodizing for aluminum. The first type is a chromic acid oxidizer, the second type is a sulfuric acid oxidizer, and the third type is a sulfuric acid solid oxidizer. Other anodizing specifications include more MIL-SPECs (eg, MIL-A-63576), aerospace industry specifications by organizations such as SAE, ASTM, and ISO (eg, AMS 2469, AMS 2470, AMS 2471, AMS 2472). , AMS 2482, AMS 2482, B580, ASTM D3933, ISO 10074, and BS 5599) and company-specific specifications (such as those for Boeing, Lockheed Martin, Airbus, and other major contractors). AMS 2468 is obsolete. None of these specifications specify an exact process or chemistry, but rather define a set of tests and quality assurance measures that the oxidized product must meet. BS 1615 guides the selection of alloys for anodizing.^[15] [16]

chromic acid (type I)

The oldest chromic acid process. This is widely known as the Bengog-Stewart process, but is not favored by vendors due to safety regulations related to air quality control when additives associated with Type II do not break tolerances. In North America, it is known as Type I because it is specified by MIL-A-8625, but it is also covered by AMS 2470 and MIL-A-8625 Type IB. In the UK it is usually specified as Def Stan 03/24 and is used in areas subject to contact with motors etc. There are also Boeing and Airbus standards. Thinner films of chromic acid, 0.5 µm to 18 µm (0.00002 in to 0.0007 in) ^[17] makes them softer, more flexible, and partially self-healing. It is difficult to dye and can be used as a pre-treatment before dyeing. The method of forming the film using sulfuric acid differs because the voltage increases during the process cycle.

Sulfuric acid (type II and III)

Sulfuric acid is the most commonly used solution for the production of oxidized paint. Coatings have an average thickness of 1.8 µm to 25 µm (0.00007 in. to 0.001 in.). ^[17] In North America they are known as Type II, as defined by MIL-A-8625, while coatings thicker than 25 µm (0.001 in) are known as Type III, hard coating, hard anodizing, or engineering anodizing. Very thin coatings similar to those produced with chromium oxide are known as Type IIB. Thicker coatings require more control of the process. ^[6]It is produced in a refrigerated tank near the freezing point. A drop of water has a higher electric potential than thin paint. Hard anodizing can be done with thicknesses ranging from 13 to 150 micrometers (0.0005 to 0.006 in). The thickness of the anodizing increases the corrosion resistance, corrosion resistance, lubricant holding capacity and PTFE, and the third type of electrical and thermal insulation should not be painted or coated to maintain corrosion resistance. Sealing greatly reduces this. Thin sulfur anodizing standards (mild/standard) are given by MIL-A-8625 Types II and IIB, AMS 2471 (unpainted). and AMS 2472 (painted), BS EN ISO 1237 3/1 (decorative), BS 3987 (architectural). Concentrated sulfur anodizing standards are provided by the outdated MIL-A-8625 Type III, AMS 2469, BS ISO 10074, BS EN 2536, AMS 2468, and DEF STAN 03-26/1.

organic acid

If it is performed in weak acids with high voltage, high current density and strong quenching, the anodizing process can produce yellow solids without color. ^[6] Shades of color are limited to a group that includes pale yellow, gold, deep bronze, brown, gray and black. Some advanced mods can create a white coating with a reflectance of 80%. The resulting color shade is sensitive to changes in the metallurgy of the base alloy and cannot be reproduced continuously. ^[2]

Anodization in some organic acids, for example malic acid, can go into a “runaway” state, in which the acid current is directed towards aluminum with greater force than usual, resulting in large voids and causing cuts. Also, if the current or voltage is too high, “burn-in” can occur. In this case, the resources behave as if the black and white areas are almost short, large, uneven, and shapeless.

Solid color anodizing is generally performed using organic acids, but the same effect was produced in laboratories containing very dilute sulfuric acid. Anodizing was initially combined with oxalic acid, but sulphation of oxygen-containing aromatic compounds, especially sulfosalicylic acid, has become more common since the 1960s. ^[2] A thickness of up to 50 micrometers can be achieved. Type IC organic acid anodizing is designated by MIL-A-8625.

phosphoric acid

Anodizing can be done in phosphoric acid, usually as a surface preparation for adhesives. This is described in the ASTM D3933 standard.

The borate and tartrate bath

The anodizing process can also be carried out in borates or tartrate as aluminum oxide is insoluble. In these processes, when the part is completely covered, the growth of the coating stops, and the thickness is linearly related to the applied effort. ^[6] These coatings do not contain pores compared to sulfuric acid and chromic acid processes. ^[6] This type of coating is widely used for the manufacture of electrolytic capacitors because thin layers of aluminum (usually less than 0.5 μm) are at risk of exposure to acidic processes. ^[1]

Plasma electrolytic oxidation

The process of plasma oxidation by electrolysis is a similar process, but where voltages are applied. This causes sparks and more crystal/ceramic coatings.

other minerals

magnesium

Magnesium is treated with an oxide base as a primer. A thin film (5 μm) is sufficient for this purpose. ^[18] Coatings with a thickness of 25 micrometers and above can be used when sealing with oil, wax or sodium silicate. ^[18] Standards for magnesium anodizing are found in AMS 2466, AMS 2478, AMS 2479, and ASTM B893.

We will not be afraid

Niobium is oxidized in a similar way to titanium with a range of attractive colors formed from the overlapping of different film thicknesses. Again, the thickness of the film depends on the anodizing voltage. ^[19] [20] Uses include jewelry and commemorative coins.

Tantalus

Tantalum is anodized similarly to titanium and niobium, and a range of attractive colors are formed from overlapping at different film thicknesses. Again, the thickness of the film depends on the anodizing voltage and is typically between 18 and 23 angstroms/V depending on the electrolyte and temperature. Applications include tantalum capacitors.

titanium

The thickness of the oxide layer is 30 nm (1.2 x 1.2 in ^10-6 ) up to several micrometers. ^[21] Standards for titanium anodizing are provided by AMS 2487 and AMS 2488.

AMS 2488 Type III Anodization of titanium produces a variety of colorless colors that are sometimes used in art, fashion, and piercing jewelry, and wedding rings. The color formed depends on the thickness of the oxide (which is determined by the anodizing voltage). This is caused by the interference of the light reflected from the oxide surface and the light that passes through it and is reflected from the underlying metallic surface. Anodizing AMS 2488 Type II produces a thicker gray matte finish with higher corrosion resistance. ^[22]

narrated

Zinc is rarely anodized, but the process is covered by the International Organization for Zinc Research and MIL-A-81801. ^[18] A solution of ammonium phosphate, chromate, and fluoride at a voltage of up to 200 volts can produce an olive green coating up to 80 micrometers thick. ^[18] The coatings are hard and corrosion resistant.

Zinc or galvanized steel can be treated with low voltage anodization (20-30 V) and also with a direct current silicate bath containing various concentrations of sodium silicate, sodium hydroxide, borax, sodium nitrite and nickel sulfate. ^[23]

dyeing

The most common anodizing processes, for example, sulfuric acid on aluminium, produce a porous surface that can readily accept dyes. The number of paint colors is almost endless. However, the colors produced tend to vary depending on the base alloy. The most common colors in the industry are yellow, green, blue, black, orange, purple and red because they are relatively cheap. Although some may prefer lighter colors, in practice they can be produced on certain alloys such as high silicon casting grades and 2000 series aluminum and copper alloys. Another concern is the “light fastness” of organic pigments – some colors (red and blue) are susceptible to Special for fading. Inorganic black and gold paints (ammonium iron oxalate) contain more light. Colored anodizing is usually sealed to reduce or eliminate color bleeding.^[24]

Alternatively, a metal (usually tin) can be electrostatically deposited into the pores of the anode coating to produce faster-than-light colors. Metallic colors range from champagne to black. Bronze is commonly used for architectural metals.

Alternatively, the color can be produced as an integral part of the film. This is done during the anodization process using organic acids mixed with sulfuric current and pulse current.

Spray effects are created by painting a porous surface that is not airtight in lighter colors and then spraying darker colors over the surface. Mixtures of blue paints and solvents can also be used alternately, as colored paints resist each other and leave smeared traces.

Seal

Sealing is the last step in the anodizing process. Acidic oxidation solutions create pores in the oxidized paint. These pores can absorb the paint and retain the lubricant, but it is also a means of corrosion. When the lubricating properties are not critical, it is usually sealed after painting to increase wear resistance and color retention. There are three common types of seal. First, prolonged immersion in hot deionized water or steam (96-100°C / 205-212°F) is the simplest sealing process, although it is not entirely effective and reduces corrosion resistance by up to 20%. ^[6]The oxide reduces to wetting and swelling caused by the porosity of the surface. Second, a medium-temperature sealing process that operates at 160-180 degrees Fahrenheit (60-80 degrees Celsius) in solutions containing organic additives and mineral salts. However, this process is likely to remove the colors. Third, the cold sealing process, in which the pores are sealed by soaking a sealant in a room temperature bath, is more popular because of its energy savings. Casings sealed in this way are not suitable for adhesive bonding. Teflon, nickel acetate, cobalt acetate, and dichromate are used for hot stamping. MIL-A-8625 requires sealing of thin coatings (Types I and II) and is permitted as an option for thick coatings (Type III).

cleaning

Anodized aluminum surfaces that are not cleaned regularly are subject to plate edge staining, which is a unique type of surface staining that can affect the structural integrity of the metal.

Environmental effects

Anodizing is one of the environmentally friendly metal finishing processes. With the exception of organic oxidation (known as integral coating), by-products contain only small amounts of heavy metals, halogens, or volatile organic compounds. Integrated color anodizing does not produce any volatile organic compounds, heavy metals or halogens, as all by-products in the effluents of other processes come from paints or their coating materials. ^[25] The most common oxidizing effluent, aluminum hydroxide and sulfate, is recycled to produce alum, baking powder, cosmetics, newspapers, and fertilizers, or used in industrial wastewater treatment.

Mechanical considerations

Anodizing increases the surface area because the oxide formed takes up more surface area than the converted base metal. ^[26]This will generally not have any result, except in cases where it is highly tolerated. In this case, the thickness of the anodizing layer must be taken into account when choosing the processing dimensions. It is a general practice in engineering drawing to specify that “dimensions apply after all surfaces are complete”. This requires the machine shop to consider the anodizing thickness when performing the final processing of the mechanical part before anodizing. Also, in the case of small threaded holes to accept the bolts, anodizing may cause the bolts to loosen, so the threaded holes may be tapped to restore the original dimensions. On the other hand, special large taps can be used to compensate for this growth. For non-threading holes that accept pins or rods of a fixed diameter, it may be appropriate to resize a slightly larger hole. Depending on its alloy and the thickness of the anodized coating, it may have a significant negative impact on the fatigue life. Conversely, anodizing may increase fatigue life by preventing pitting corrosion.

Anodizing is an electrolytic process for producing a thick oxide coating, usually on aluminum and its alloys. The oxide layer is usually 5 to 30 μm thick and is used to provide surface resistance to wear and corrosion or as a decorative layer.

In the electrolysis process

Components to be processed are made in the form of anodes in a dilute acid solution. Oxidation occurs on the surface of the component, resulting in the formation of a cohesive oxide layer that adheres firmly to the underlying metal substrate. Most anodizing processes are performed on aluminum and its alloys. Other materials that can be processed include magnesium oxide and titanium alloys.

Before anodizing, the surface of the aluminum alloy must be pre-treated. This pre-treatment affects the final appearance and properties of the anodised coating. Types of pretreatment can range from mechanical processes such as abrasive polishing to chemical treatments such as chemical polishing or electrical polishing. In addition, any machining, drilling, or welding of the component must be performed prior to anodizing.

Three types of electrolyte solutions are commonly used in the anodizing process. The first is a solution of 10-15% sulfuric acid at a temperature of 25 degrees Celsius. This electrolyte gives a coating formation rate of about 25 micrometers per hour. The second electrolyte solution is a mixture of sulfuric acid and oxalic acid at a temperature of 30 degrees Celsius. This gives a higher coating rate of about 30 μm/h. The third electrolyte is 10% chromic acid at a temperature of 38-42°C, which gives a film formation rate of about 15 µm/hr. These ordinary oxidized coatings are porous and transparent and are usually used with paints for decorative coatings.

“Hard Andes”

Refers to the preparation of a thicker oxide coating, about 25-100 μm, with a higher hardness, typically 500-900 HV, and is used to provide a corrosion-resistant surface for aluminum alloys. This is achieved by using a mixture of sulfuric acid/oxalic acid in higher concentrations and at lower temperatures of about 0-10°C. The paint produced is from gray to black and is non-porous. Not all aluminum alloys can be anodized to create hard anodized coatings. Alloys of the 5xxx and 6xxx series respond well to hard anodizing, while alloys 2xxx and other alloys, including cast alloys with high copper and silicon content, do not. For these alloys that contain higher silicon and copper, the oxidized layer is porous and has a low hardness.

Hard anodizing is often the lowest cost corrosion resistance that can be applied to aluminum alloys and is particularly suitable for low stress corrosion protection. As a result, hard anodized coatings are often used with aluminum components in sliding systems. These coatings are also used to protect aluminum components that are subjected to liquid-assisted corrosion, slurry corrosion, solid-particle corrosion, and liquid corrosion. The anodized coating is also resistant to most chemicals except alkalis. Anodizing is not used in impact corrosion applications due to the brittle nature of the coating.

What is anodizing?