Anodizing was first used on an industrial scale in 1923 to protect duralumin seaplane parts from corrosion. This first chromic acid-based process was called the Bengough-Stuart process and was documented in the British defense specification DEF STAN 03-24/3. It is still used today despite its former requirements for a complicated voltage cycle which is now known to be unnecessary. Variations of this process soon emerged and the first sulfuric acid anodizing process was patented by Gower and O'Brien in 1927. Sulfuric acid soon became and remains the most common anodizing electrolyte.[1]​.

Oxalic acid anodizing was first patented in Japan in 1923 and was later widely used in Germany, particularly for architectural applications. Anodized aluminum extrusion was a popular architectural material in the 1960s and 1970s, but has since been replaced by cheaper plastics and powder coatings. [2]​ Phosphoric acid processes are the most recent and important development, until now they have only been used as pretreatments for adhesives or organic paints.[1]​ The industry continues to develop a wide variety of proprietary and increasingly complex variations of all these anodizing processes, so the growing trend in military and industrial standards is to classify by coating properties rather than by process chemistry.

Contenido

Las aleaciones de aluminio se anodizan para aumentar la resistencia a la corrosión y permitir el teñido (coloración), una mejor lubricación o una mejor adhesión. Sin embargo, el anodizado no aumenta la resistencia del objeto de aluminio. La capa anódica es aislante.[3]​.

Cuando se expone al aire a temperatura ambiente o a cualquier otro gas que contenga oxígeno, el aluminio puro se autopasiva formando una capa superficial de óxido de aluminio amorfo de 2 a 3 nm de espesor, [4]​ que proporciona una protección muy eficaz contra la corrosión. Las aleaciones de aluminio suelen formar una capa de óxido más gruesa, de 5 a 15 nm de espesor, pero tienden a ser más susceptibles a la corrosión. Las piezas de aleación de aluminio están anodizadas para aumentar en gran medida el espesor de esta capa para resistencia a la corrosión. La resistencia a la corrosión de las aleaciones de aluminio disminuye significativamente por ciertos elementos de aleación o impurezas: cobre, hierro y silicio,[5]​ por lo que las aleaciones de Al de las series 2000, 4000, 6000 y 7000 tienden a ser las más susceptibles.

Aunque el anodizado produce un recubrimiento muy regular y uniforme, las fisuras microscópicas en el recubrimiento pueden provocar corrosión. Además, el recubrimiento es susceptible a la disolución química en presencia de productos químicos de pH alto y bajo, lo que provoca el desprendimiento del recubrimiento y la corrosión del sustrato. Para combatir esto, se han desarrollado diversas técnicas, ya sea para reducir el número de fisuras, para insertar compuestos químicamente más estables en el óxido, o ambas cosas. Por ejemplo, los artículos anodizados con ácido sulfúrico normalmente se sellan, ya sea mediante sellado hidrotérmico o sellado por precipitación, para reducir la porosidad y las vías intersticiales que permiten el intercambio de iones corrosivos entre la superficie y el sustrato. Los sellos precipitantes mejoran la estabilidad química pero son menos efectivos para eliminar las vías de intercambio iónico. Más recientemente, se han desarrollado nuevas técnicas para convertir parcialmente el recubrimiento de óxido amorfo en compuestos microcristalinos más estables que han demostrado una mejora significativa basada en longitudes de enlace más cortas.

Algunas piezas de aeronaves, materiales arquitectónicos y productos de consumo de aluminio están anodizados. El aluminio anodizado se puede encontrar en reproductores de MP3, teléfonos inteligentes, herramientas multiusos, linternas, utensilios de cocina, cámaras, artículos deportivos, armas de fuego, marcos de ventanas, techos "Cubierta (construcción)"), en condensadores electrolíticos y en muchos otros productos, tanto por su resistencia a la corrosión como por su capacidad de retener el tinte. Aunque el anodizado solo tiene una resistencia moderada al desgaste, los poros más profundos pueden retener mejor una película lubricante que una superficie lisa.

Los recubrimientos anodizados tienen una conductividad térmica y un coeficiente de expansión lineal mucho menores que el aluminio. Como resultado, el revestimiento se agrietará por estrés térmico si se expone a temperaturas superiores a 80 °C (353 K). El revestimiento puede agrietarse, pero no se pelará. [6]​ El punto de fusión del óxido de aluminio es de 2050 °C (2323 K), mucho más alto que el del aluminio puro, de 658 °C (931 K). [6]​ Esto y el poder aislante del óxido de aluminio pueden dificultar la soldadura.

En los procesos típicos de anodizado de aluminio comercial, el óxido de aluminio se introduce en la superficie y se elimina de ella en cantidades iguales. [7]​ Por lo tanto, el anodizado aumentará las dimensiones de la pieza en cada superficie a la mitad del espesor del óxido. Por ejemplo, un recubrimiento de 2 μm "Micrómetro (unidad de longitud)") de espesor aumentará las dimensiones de la pieza en 1 μm por superficie. Si la pieza está anodizada por todos los lados, entonces todas las dimensiones lineales aumentarán según el espesor del óxido. Las superficies de aluminio anodizado son más duras que las del aluminio, pero tienen una resistencia al desgaste baja a moderada, aunque esto se puede mejorar con el espesor y el sellado.

Process

A deoxidizing solution can be applied to the surface of the aluminum to remove contaminants. Nitric acid is generally used to remove carbon (waste), but is being replaced due to environmental concerns.[8]​[9][10][11]​.

The anodized aluminum layer is created by passing a direct current through an electrolyte solution, where the aluminum object serves as the anode (the positive electrode in an electrolytic cell). The current releases hydrogen at the cathode (the negative electrode) and oxygen at the surface of the aluminum anode, creating a buildup of aluminum oxide. It is also possible to use alternating current and pulsed current, but they are rarely used. The voltage required by various solutions can range from 1 to 300 V DC, although most are in the range of 15 to 21 V. Higher voltages are generally required for thicker coatings formed in sulfuric and organic acid. The anodizing current varies depending on the area of ​​aluminum to be anodized and typically ranges between 30 and 300 A/m².

Aluminum anodizing (eloxal or electrolytic oxidation** of aluminum)[12]​ is usually performed in an acid solution, typically sulfuric acid or chromic acid, which slowly dissolves the aluminum oxide. The action of the acid balances with the oxidation rate to form a coating with nanopores, 10–150 nm in diameter.[13] These pores are what allow the electrolyte solution and current to reach the aluminum substrate and continue to grow the coating to a thickness greater than that produced by self-passivation.[14] These pores allow the dye to be absorbed, however, this must be followed by sealing or the dye will not remain. The dye is usually followed by a clean nickel acetate seal. Because the stain is only superficial, the underlying rust can continue to provide corrosion protection even if minor wear and scratches break through the stained 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 concentrated solutions at lower temperatures with higher voltages and currents. Film thickness can vary from less than 0.5 micrometers "Micrometer (unit of length)") for glossy decorative work to 150 micrometers for architectural applications.

double finish

Anodizing can be done in combination with chromate conversion coating. Each process provides corrosion resistance, and anodizing offers a significant advantage in terms of robustness or resistance to physical wear. The reason for combining the processes may vary, however the significant difference between anodizing and chromate conversion coating is the electrical conductivity of the films produced. Although both are stable compounds, the chromate conversion coating has a much higher electrical conductivity. The applications where this can be useful are varied, however the issue of grounding components as part of a larger system is obvious.

The dual finishing process uses the best that each process has to offer: anodizing with its great wear resistance and chromate conversion coating with its electrical conductivity.

Process steps may typically involve a chromate conversion coating of the entire component, followed by surface masking in areas where the chromate coating must remain intact. Furthermore, the chromate layer dissolves in unprotected areas. The component can then be anodized, applying the anodizing to the unmasked areas. The exact process will vary depending on the service provider, component geometry, and required outcome. Helps protect the aluminum item.

La especificación de anodizado más utilizada en Estados Unidos es la especificación militar estadounidense"), MIL-A-8625, que define tres tipos de anodizado de aluminio. El tipo I es anodizado con ácido crómico, el tipo II es anodizado con ácido sulfúrico y el tipo III es anodizado duro con ácido sulfúrico. Otras especificaciones de anodizado incluyen más MIL-SPEC (por ejemplo, MIL-A-63576), especificaciones de la industria aeroespacial de organizaciones como SAE, ASTM e ISO (por ejemplo, AMS 2469, AMS 2470, AMS 2471, AMS 2472, AMS 2482, ASTM B580, ASTM D3933, ISO 10074 y BS 5599) y especificaciones específicas de corporaciones (como las de Boeing, Lockheed Martin, Airbus y otros grandes contratistas). AMS 2468 está obsoleto. Ninguna de estas especificaciones define un proceso o una química detallados, sino más bien un conjunto de pruebas y medidas de garantía de calidad que debe cumplir el producto anodizado. La norma BS 1615 guía la selección de aleaciones para anodizado. Para el trabajo de defensa británico, se describen procesos detallados de anodizado crómico y sulfúrico en DEF STAN 03-24/3 y DEF STAN 03-25/3 respectivamente. [15]​ [16]​.

Chromic acid (type I)

The oldest anodizing process uses chromic acid. It is widely known as the Bengough-Stuart process but, due to safety regulations regarding air quality control, it is not preferred by suppliers when the additive material associated with Type II does not break tolerances. In North America, it is known as Type I because it is designated by MIL-A-8625, but it is also covered by AMS 2470 and MIL-A-8625 Type IB. In the UK it is normally specified as Def Stan 03/24 and is used in areas prone to coming into contact with propellants etc. There are also Boeing and Airbus standards. Chromic acid produces thinner, 0.5 μm to 18 μm (0.00002" to 0.0007") [17] more opaque films that are softer, ductile, and to some extent self-healing. They are more difficult to stain and can be applied as a pretreatment before painting. The film formation method is different from sulfuric acid in that the voltage increases throughout the process cycle.

Sulfuric acid (type II and III)

Sulfuric acid is the most commonly used solution to produce an anodized coating. Moderately thick coatings of 1.8 μm to 25 μm (0.00007" to 0.001") [18] are known as Type II in North America, as named in MIL-A-8625, while thicker coatings of 25 μm (0.001") are known as Type III, hard coat, hard anodizing, or engineering anodizing. Very thin coatings similar to produced by chromic anodizing are known as Type IIB. Thick coatings require greater process control,[19]​ and are produced in a refrigerated tank near the freezing point of water at higher voltages than thinner coatings. Hard anodizing can be done between 13 and 150 μm (0.0005" to 0.006") thick. ability to retain lubricants and PTFE coatings and electrical and thermal insulation. Type III sealing will improve corrosion resistance at the cost of reducing abrasion resistance. Sealing will greatly reduce this. Standards for thin sulfuric anodizing (smooth/standard) are given by MIL-A-8625 Types II and IIB, AMS 2471 (undyed) and AMS 2472 (dyed), BS EN ISO. 12373/1 (decorative), BS 3987 (architectural). Standards for thick sulfuric anodizing are given by MIL-A-8625 Type III, AMS 2469, BS ISO 10074, BS EN 2536 and the obsolete AMS 2468 and DEF STAN 03-26/1.

organic acid

Anodizing can produce yellowish integral colors without dyes if carried out in weak acids with high voltages, high current densities and strong cooling. [20] Color tones are restricted to a range that includes pale yellow, gold, dark bronze, brown, gray and black. Some advanced variations can produce a white coating with 80% reflectivity. The color tone produced is sensitive to variations in the metallurgy of the underlying alloy and cannot be reproduced consistently. [21]​.

Anodizing in some organic acids, for example malic acid, can go into a "runaway" situation, where the current drives the acid to attack the aluminum much more aggressively than normal, resulting in huge pits and scars. Additionally, if the current or voltage is too high, "burn-in" may occur; In this case, the power supplies act as if they are almost shorted and large, uneven, amorphous black regions develop.

Integral color anodizing is usually done with organic acids, but the same effect has been produced in laboratories with very dilute sulfuric acid. Integral color anodizing was originally performed with oxalic acid, but sulfonated aromatic compounds containing oxygen, particularly sulfosalicylic acid"), have been more common since the 1960s. [22] Thicknesses of up to 50 μm can be achieved. Organic acid anodizing is called type IC according to MIL-A-8625.

phosphoric acid

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

Borate and tartrate baths

Anodizing can also be performed in borate or tartrate baths in which aluminum oxide is insoluble. In these processes, coating growth stops when the part is completely covered and the thickness is linearly related to the applied voltage.[23]​ These coatings are free of pores, relative to sulfuric and chromic acid processes.[23]​ This type of coating is widely used to manufacture electrolytic capacitors because the thin aluminum films (typically less than 0.5 μm) are at risk of being punctured by acid processes.[24]​.

Electrolytic plasma oxidation

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

Magnesium

Magnesium is primarily anodized as a primer for paint. A thin film (5 μm) is sufficient for this. [25] Thicker coatings of 25 μm and more can provide mild corrosion resistance when sealed with oil, wax, or sodium silicate. [25] Standards for magnesium anodizing are found in AMS 2466, AMS 2478, AMS 2479, and ASTM B893.

Niobium

Niobium is anodized similarly to titanium and a range of attractive colors are formed by interference at different film thicknesses. Again, the thickness of the film depends on the anodizing voltage.[26]​[27]​ Uses include jewelry and commemorative coins.

Tantalum

Tantalum "Tantalum (element)") is anodized in a similar manner to titanium and niobium and a range of attractive colors are formed by interference at different film thicknesses. Again, the film thickness depends on the anodizing voltage and generally ranges from 18 to 23 angstroms per volt depending on the electrolyte and temperature. Uses include tantalum capacitors.

Titanium

An anodized oxide layer has a thickness in the range of 30 newtons·meter (22.1 lb·ft) to several micrometers. [28]​ Standards for titanium anodizing are given by AMS 2487 and AMS 2488.

AMS 2488 Type III titanium anodizing produces a range of different colors without dyes, which is why it is sometimes used in art, costume jewelry, body piercing jewelry, and wedding rings. The color formed depends on the thickness of the oxide (which is determined by the anodizing voltage); It is caused by the interference of light reflecting off the surface of the oxide with light traveling through it and reflecting off the underlying metal surface. AMS 2488 Type II anodizing produces a thicker matte gray finish with greater wear resistance. [29]​.

Zinc

Zinc is rarely anodized, but the International Lead and Zinc Research Organization developed a process that is covered by the MIL-A-81801 standard. [30] A solution of ammonium phosphate, chromate, and fluoride with voltages up to 200 V can produce olive green coatings up to 80 μm thick. [30]​ The coatings are hard and resistant to corrosion.

Galvanized or zinc steel can be anodized using direct current at lower voltages (20–30 V) in silicate baths containing varying concentrations of sodium silicate, sodium hydroxide, borax, sodium nitrite, and nickel sulfate "Nickel(II) sulfate"). [31]​.

The most common anodizing processes, for example sulfuric acid on aluminum, produce a porous surface that can readily accept dyes. The number of dye colors is almost infinite; however, the colors produced tend to vary depending on the base alloy. The most common colors in the industry, because they are relatively cheap, are yellow, green, blue, black, orange, purple and red. Although some may prefer lighter colors, in practice it can be difficult to produce them in certain alloys, such as high-silicon casting grades and 2000 series aluminum-copper alloys. Another concern is the “lightfastness” of organic dyes: some colors (reds and blues) are particularly prone to fading. Black dyes and gold produced by inorganic means (ferric ammonium oxalate) are more lightfast. Dyed anodizing is usually sealed to reduce or eliminate dye bleeding. The white color cannot be applied because the size of the molecule is larger than the pore size of the oxide layer. [32]​.

Alternatively, metal (usually tin) can be electrolytically deposited into the pores of the anodic coating to provide more lightfast colors. The colors of metallic tints range from pale champagne to black "Black (color)"). Bronze tones are commonly used for architectural metals. Alternatively, color can be produced integrally on the film. This is done during the anodizing process using organic acids mixed with the sulfuric electrolyte and a pulsed current.

Splatter effects are created by staining the unsealed porous surface with lighter colors and then splashing darker colored dyes onto the surface. Mixtures of water-based and solvent-based dyes can also be applied alternatively, as the colored dyes will resist each other and leave staining effects.

Another interesting coloring method is anodizing interference coloring. The thin oil film lying on the water surface shows a rainbow hue due to the interference between light reflected from the oil-water interface and the oil film surface. Since the thickness of the oil film is not regulated, the resulting rainbow color appears random.

In anodized aluminum coloring, the desired colors are achieved by depositing a metal layer of controllable thickness (usually tin) at the base of the porous structure. This involves reflections on the aluminum substrate and the top metal surface. The resulting color of the interference changes from blue, green and yellow to red as the deposited metal layer thickens. Beyond a specific thickness, optical interference disappears and the color becomes bronze. Interference color anodized aluminum parts have a distinctive quality: their color varies when viewed from different angles. [33] Interference coloring involves a three-step process: anodizing with sulfuric acid, electrochemical modification of the anode pore, and metal (tin) deposition. [34]​.

Sealing is the final step in the anodizing process. Acidic anodizing solutions produce pores in the anodized coating. These pores can absorb dyes and retain lubricants, but they are also a pathway for corrosion. When lubrication properties are not critical, they are usually sealed after dyeing to increase corrosion resistance and dye retention. There are three most common types of sealing.

Anodized aluminum surfaces that are not cleaned regularly are susceptible to panel edge staining, a unique type of surface stain that can affect the structural integrity of the metal.

Anodizing is one of the most environmentally friendly metal finishing processes. With the exception of organic anodizing (also known as integral color), the byproducts contain only trace amounts of heavy metals, halogens or volatile organic compounds. Full color anodizing produces no VOCs, heavy metals or halogens, as all byproducts found in the effluent streams of other processes come from their dyes or coating materials. [36]​ The most common anodizing effluents, aluminum hydroxide and aluminum sulfate, are recycled for the manufacture of alum, baking powder, cosmetics, newsprint, and fertilizers or used in industrial wastewater treatment systems.

Anodizing will raise the surface area as the oxide created takes up more space than the converted base metal. [37]​ This will generally not matter except where there are strict tolerances. In this case, when choosing the machining dimension, the thickness of the anodizing layer must be taken into account. A general practice on engineering drawings is to specify that "dimensions are applied after all surface finishes." This will force the machine shop to take the anodizing thickness into account when performing final machining of the mechanical part prior to anodizing. Also in the case of small "Thread (mechanical)") threaded holes to accept screws, anodizing can cause the screws to bind, so it may be necessary to drill the threaded holes with a tap to restore the original dimensions. Alternatively, special oversized taps can be used to pre-compensate for this growth. In the case of unthreaded holes that accept fixed diameter pins or rods, a slightly larger hole may be appropriate to allow for dimension change. Depending on the alloy and the thickness of the anodized coating, this can have a significantly negative effect on fatigue life. Conversely, anodizing can increase fatigue life by preventing corrosion pitting.

Anodizing was first used on an industrial scale in 1923 to protect duralumin seaplane parts from corrosion. This first chromic acid-based process was called the Bengough-Stuart process and was documented in the British defense specification DEF STAN 03-24/3. It is still used today despite its former requirements for a complicated voltage cycle which is now known to be unnecessary. Variations of this process soon emerged and the first sulfuric acid anodizing process was patented by Gower and O'Brien in 1927. Sulfuric acid soon became and remains the most common anodizing electrolyte.[1]​.

Oxalic acid anodizing was first patented in Japan in 1923 and was later widely used in Germany, particularly for architectural applications. Anodized aluminum extrusion was a popular architectural material in the 1960s and 1970s, but has since been replaced by cheaper plastics and powder coatings. [2]​ Phosphoric acid processes are the most recent and important development, until now they have only been used as pretreatments for adhesives or organic paints.[1]​ The industry continues to develop a wide variety of proprietary and increasingly complex variations of all these anodizing processes, so the growing trend in military and industrial standards is to classify by coating properties rather than by process chemistry.

Contenido

Las aleaciones de aluminio se anodizan para aumentar la resistencia a la corrosión y permitir el teñido (coloración), una mejor lubricación o una mejor adhesión. Sin embargo, el anodizado no aumenta la resistencia del objeto de aluminio. La capa anódica es aislante.[3]​.

Cuando se expone al aire a temperatura ambiente o a cualquier otro gas que contenga oxígeno, el aluminio puro se autopasiva formando una capa superficial de óxido de aluminio amorfo de 2 a 3 nm de espesor, [4]​ que proporciona una protección muy eficaz contra la corrosión. Las aleaciones de aluminio suelen formar una capa de óxido más gruesa, de 5 a 15 nm de espesor, pero tienden a ser más susceptibles a la corrosión. Las piezas de aleación de aluminio están anodizadas para aumentar en gran medida el espesor de esta capa para resistencia a la corrosión. La resistencia a la corrosión de las aleaciones de aluminio disminuye significativamente por ciertos elementos de aleación o impurezas: cobre, hierro y silicio,[5]​ por lo que las aleaciones de Al de las series 2000, 4000, 6000 y 7000 tienden a ser las más susceptibles.

Aunque el anodizado produce un recubrimiento muy regular y uniforme, las fisuras microscópicas en el recubrimiento pueden provocar corrosión. Además, el recubrimiento es susceptible a la disolución química en presencia de productos químicos de pH alto y bajo, lo que provoca el desprendimiento del recubrimiento y la corrosión del sustrato. Para combatir esto, se han desarrollado diversas técnicas, ya sea para reducir el número de fisuras, para insertar compuestos químicamente más estables en el óxido, o ambas cosas. Por ejemplo, los artículos anodizados con ácido sulfúrico normalmente se sellan, ya sea mediante sellado hidrotérmico o sellado por precipitación, para reducir la porosidad y las vías intersticiales que permiten el intercambio de iones corrosivos entre la superficie y el sustrato. Los sellos precipitantes mejoran la estabilidad química pero son menos efectivos para eliminar las vías de intercambio iónico. Más recientemente, se han desarrollado nuevas técnicas para convertir parcialmente el recubrimiento de óxido amorfo en compuestos microcristalinos más estables que han demostrado una mejora significativa basada en longitudes de enlace más cortas.

Algunas piezas de aeronaves, materiales arquitectónicos y productos de consumo de aluminio están anodizados. El aluminio anodizado se puede encontrar en reproductores de MP3, teléfonos inteligentes, herramientas multiusos, linternas, utensilios de cocina, cámaras, artículos deportivos, armas de fuego, marcos de ventanas, techos "Cubierta (construcción)"), en condensadores electrolíticos y en muchos otros productos, tanto por su resistencia a la corrosión como por su capacidad de retener el tinte. Aunque el anodizado solo tiene una resistencia moderada al desgaste, los poros más profundos pueden retener mejor una película lubricante que una superficie lisa.

Los recubrimientos anodizados tienen una conductividad térmica y un coeficiente de expansión lineal mucho menores que el aluminio. Como resultado, el revestimiento se agrietará por estrés térmico si se expone a temperaturas superiores a 80 °C (353 K). El revestimiento puede agrietarse, pero no se pelará. [6]​ El punto de fusión del óxido de aluminio es de 2050 °C (2323 K), mucho más alto que el del aluminio puro, de 658 °C (931 K). [6]​ Esto y el poder aislante del óxido de aluminio pueden dificultar la soldadura.

En los procesos típicos de anodizado de aluminio comercial, el óxido de aluminio se introduce en la superficie y se elimina de ella en cantidades iguales. [7]​ Por lo tanto, el anodizado aumentará las dimensiones de la pieza en cada superficie a la mitad del espesor del óxido. Por ejemplo, un recubrimiento de 2 μm "Micrómetro (unidad de longitud)") de espesor aumentará las dimensiones de la pieza en 1 μm por superficie. Si la pieza está anodizada por todos los lados, entonces todas las dimensiones lineales aumentarán según el espesor del óxido. Las superficies de aluminio anodizado son más duras que las del aluminio, pero tienen una resistencia al desgaste baja a moderada, aunque esto se puede mejorar con el espesor y el sellado.

Process

A deoxidizing solution can be applied to the surface of the aluminum to remove contaminants. Nitric acid is generally used to remove carbon (waste), but is being replaced due to environmental concerns.[8]​[9][10][11]​.

The anodized aluminum layer is created by passing a direct current through an electrolyte solution, where the aluminum object serves as the anode (the positive electrode in an electrolytic cell). The current releases hydrogen at the cathode (the negative electrode) and oxygen at the surface of the aluminum anode, creating a buildup of aluminum oxide. It is also possible to use alternating current and pulsed current, but they are rarely used. The voltage required by various solutions can range from 1 to 300 V DC, although most are in the range of 15 to 21 V. Higher voltages are generally required for thicker coatings formed in sulfuric and organic acid. The anodizing current varies depending on the area of ​​aluminum to be anodized and typically ranges between 30 and 300 A/m².

Aluminum anodizing (eloxal or electrolytic oxidation** of aluminum)[12]​ is usually performed in an acid solution, typically sulfuric acid or chromic acid, which slowly dissolves the aluminum oxide. The action of the acid balances with the oxidation rate to form a coating with nanopores, 10–150 nm in diameter.[13] These pores are what allow the electrolyte solution and current to reach the aluminum substrate and continue to grow the coating to a thickness greater than that produced by self-passivation.[14] These pores allow the dye to be absorbed, however, this must be followed by sealing or the dye will not remain. The dye is usually followed by a clean nickel acetate seal. Because the stain is only superficial, the underlying rust can continue to provide corrosion protection even if minor wear and scratches break through the stained 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 concentrated solutions at lower temperatures with higher voltages and currents. Film thickness can vary from less than 0.5 micrometers "Micrometer (unit of length)") for glossy decorative work to 150 micrometers for architectural applications.

double finish

Anodizing can be done in combination with chromate conversion coating. Each process provides corrosion resistance, and anodizing offers a significant advantage in terms of robustness or resistance to physical wear. The reason for combining the processes may vary, however the significant difference between anodizing and chromate conversion coating is the electrical conductivity of the films produced. Although both are stable compounds, the chromate conversion coating has a much higher electrical conductivity. The applications where this can be useful are varied, however the issue of grounding components as part of a larger system is obvious.

The dual finishing process uses the best that each process has to offer: anodizing with its great wear resistance and chromate conversion coating with its electrical conductivity.

Process steps may typically involve a chromate conversion coating of the entire component, followed by surface masking in areas where the chromate coating must remain intact. Furthermore, the chromate layer dissolves in unprotected areas. The component can then be anodized, applying the anodizing to the unmasked areas. The exact process will vary depending on the service provider, component geometry, and required outcome. Helps protect the aluminum item.

La especificación de anodizado más utilizada en Estados Unidos es la especificación militar estadounidense"), MIL-A-8625, que define tres tipos de anodizado de aluminio. El tipo I es anodizado con ácido crómico, el tipo II es anodizado con ácido sulfúrico y el tipo III es anodizado duro con ácido sulfúrico. Otras especificaciones de anodizado incluyen más MIL-SPEC (por ejemplo, MIL-A-63576), especificaciones de la industria aeroespacial de organizaciones como SAE, ASTM e ISO (por ejemplo, AMS 2469, AMS 2470, AMS 2471, AMS 2472, AMS 2482, ASTM B580, ASTM D3933, ISO 10074 y BS 5599) y especificaciones específicas de corporaciones (como las de Boeing, Lockheed Martin, Airbus y otros grandes contratistas). AMS 2468 está obsoleto. Ninguna de estas especificaciones define un proceso o una química detallados, sino más bien un conjunto de pruebas y medidas de garantía de calidad que debe cumplir el producto anodizado. La norma BS 1615 guía la selección de aleaciones para anodizado. Para el trabajo de defensa británico, se describen procesos detallados de anodizado crómico y sulfúrico en DEF STAN 03-24/3 y DEF STAN 03-25/3 respectivamente. [15]​ [16]​.

Chromic acid (type I)

The oldest anodizing process uses chromic acid. It is widely known as the Bengough-Stuart process but, due to safety regulations regarding air quality control, it is not preferred by suppliers when the additive material associated with Type II does not break tolerances. In North America, it is known as Type I because it is designated by MIL-A-8625, but it is also covered by AMS 2470 and MIL-A-8625 Type IB. In the UK it is normally specified as Def Stan 03/24 and is used in areas prone to coming into contact with propellants etc. There are also Boeing and Airbus standards. Chromic acid produces thinner, 0.5 μm to 18 μm (0.00002" to 0.0007") [17] more opaque films that are softer, ductile, and to some extent self-healing. They are more difficult to stain and can be applied as a pretreatment before painting. The film formation method is different from sulfuric acid in that the voltage increases throughout the process cycle.

Sulfuric acid (type II and III)

Sulfuric acid is the most commonly used solution to produce an anodized coating. Moderately thick coatings of 1.8 μm to 25 μm (0.00007" to 0.001") [18] are known as Type II in North America, as named in MIL-A-8625, while thicker coatings of 25 μm (0.001") are known as Type III, hard coat, hard anodizing, or engineering anodizing. Very thin coatings similar to produced by chromic anodizing are known as Type IIB. Thick coatings require greater process control,[19]​ and are produced in a refrigerated tank near the freezing point of water at higher voltages than thinner coatings. Hard anodizing can be done between 13 and 150 μm (0.0005" to 0.006") thick. ability to retain lubricants and PTFE coatings and electrical and thermal insulation. Type III sealing will improve corrosion resistance at the cost of reducing abrasion resistance. Sealing will greatly reduce this. Standards for thin sulfuric anodizing (smooth/standard) are given by MIL-A-8625 Types II and IIB, AMS 2471 (undyed) and AMS 2472 (dyed), BS EN ISO. 12373/1 (decorative), BS 3987 (architectural). Standards for thick sulfuric anodizing are given by MIL-A-8625 Type III, AMS 2469, BS ISO 10074, BS EN 2536 and the obsolete AMS 2468 and DEF STAN 03-26/1.

organic acid

Anodizing can produce yellowish integral colors without dyes if carried out in weak acids with high voltages, high current densities and strong cooling. [20] Color tones are restricted to a range that includes pale yellow, gold, dark bronze, brown, gray and black. Some advanced variations can produce a white coating with 80% reflectivity. The color tone produced is sensitive to variations in the metallurgy of the underlying alloy and cannot be reproduced consistently. [21]​.

Anodizing in some organic acids, for example malic acid, can go into a "runaway" situation, where the current drives the acid to attack the aluminum much more aggressively than normal, resulting in huge pits and scars. Additionally, if the current or voltage is too high, "burn-in" may occur; In this case, the power supplies act as if they are almost shorted and large, uneven, amorphous black regions develop.

Integral color anodizing is usually done with organic acids, but the same effect has been produced in laboratories with very dilute sulfuric acid. Integral color anodizing was originally performed with oxalic acid, but sulfonated aromatic compounds containing oxygen, particularly sulfosalicylic acid"), have been more common since the 1960s. [22] Thicknesses of up to 50 μm can be achieved. Organic acid anodizing is called type IC according to MIL-A-8625.

phosphoric acid

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

Borate and tartrate baths

Anodizing can also be performed in borate or tartrate baths in which aluminum oxide is insoluble. In these processes, coating growth stops when the part is completely covered and the thickness is linearly related to the applied voltage.[23]​ These coatings are free of pores, relative to sulfuric and chromic acid processes.[23]​ This type of coating is widely used to manufacture electrolytic capacitors because the thin aluminum films (typically less than 0.5 μm) are at risk of being punctured by acid processes.[24]​.

Electrolytic plasma oxidation

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

Magnesium

Magnesium is primarily anodized as a primer for paint. A thin film (5 μm) is sufficient for this. [25] Thicker coatings of 25 μm and more can provide mild corrosion resistance when sealed with oil, wax, or sodium silicate. [25] Standards for magnesium anodizing are found in AMS 2466, AMS 2478, AMS 2479, and ASTM B893.

Niobium

Niobium is anodized similarly to titanium and a range of attractive colors are formed by interference at different film thicknesses. Again, the thickness of the film depends on the anodizing voltage.[26]​[27]​ Uses include jewelry and commemorative coins.

Tantalum

Tantalum "Tantalum (element)") is anodized in a similar manner to titanium and niobium and a range of attractive colors are formed by interference at different film thicknesses. Again, the film thickness depends on the anodizing voltage and generally ranges from 18 to 23 angstroms per volt depending on the electrolyte and temperature. Uses include tantalum capacitors.

Titanium

An anodized oxide layer has a thickness in the range of 30 newtons·meter (22.1 lb·ft) to several micrometers. [28]​ Standards for titanium anodizing are given by AMS 2487 and AMS 2488.

AMS 2488 Type III titanium anodizing produces a range of different colors without dyes, which is why it is sometimes used in art, costume jewelry, body piercing jewelry, and wedding rings. The color formed depends on the thickness of the oxide (which is determined by the anodizing voltage); It is caused by the interference of light reflecting off the surface of the oxide with light traveling through it and reflecting off the underlying metal surface. AMS 2488 Type II anodizing produces a thicker matte gray finish with greater wear resistance. [29]​.

Zinc

Zinc is rarely anodized, but the International Lead and Zinc Research Organization developed a process that is covered by the MIL-A-81801 standard. [30] A solution of ammonium phosphate, chromate, and fluoride with voltages up to 200 V can produce olive green coatings up to 80 μm thick. [30]​ The coatings are hard and resistant to corrosion.

Galvanized or zinc steel can be anodized using direct current at lower voltages (20–30 V) in silicate baths containing varying concentrations of sodium silicate, sodium hydroxide, borax, sodium nitrite, and nickel sulfate "Nickel(II) sulfate"). [31]​.

The most common anodizing processes, for example sulfuric acid on aluminum, produce a porous surface that can readily accept dyes. The number of dye colors is almost infinite; however, the colors produced tend to vary depending on the base alloy. The most common colors in the industry, because they are relatively cheap, are yellow, green, blue, black, orange, purple and red. Although some may prefer lighter colors, in practice it can be difficult to produce them in certain alloys, such as high-silicon casting grades and 2000 series aluminum-copper alloys. Another concern is the “lightfastness” of organic dyes: some colors (reds and blues) are particularly prone to fading. Black dyes and gold produced by inorganic means (ferric ammonium oxalate) are more lightfast. Dyed anodizing is usually sealed to reduce or eliminate dye bleeding. The white color cannot be applied because the size of the molecule is larger than the pore size of the oxide layer. [32]​.

Alternatively, metal (usually tin) can be electrolytically deposited into the pores of the anodic coating to provide more lightfast colors. The colors of metallic tints range from pale champagne to black "Black (color)"). Bronze tones are commonly used for architectural metals. Alternatively, color can be produced integrally on the film. This is done during the anodizing process using organic acids mixed with the sulfuric electrolyte and a pulsed current.

Splatter effects are created by staining the unsealed porous surface with lighter colors and then splashing darker colored dyes onto the surface. Mixtures of water-based and solvent-based dyes can also be applied alternatively, as the colored dyes will resist each other and leave staining effects.

Another interesting coloring method is anodizing interference coloring. The thin oil film lying on the water surface shows a rainbow hue due to the interference between light reflected from the oil-water interface and the oil film surface. Since the thickness of the oil film is not regulated, the resulting rainbow color appears random.

In anodized aluminum coloring, the desired colors are achieved by depositing a metal layer of controllable thickness (usually tin) at the base of the porous structure. This involves reflections on the aluminum substrate and the top metal surface. The resulting color of the interference changes from blue, green and yellow to red as the deposited metal layer thickens. Beyond a specific thickness, optical interference disappears and the color becomes bronze. Interference color anodized aluminum parts have a distinctive quality: their color varies when viewed from different angles. [33] Interference coloring involves a three-step process: anodizing with sulfuric acid, electrochemical modification of the anode pore, and metal (tin) deposition. [34]​.

Sealing is the final step in the anodizing process. Acidic anodizing solutions produce pores in the anodized coating. These pores can absorb dyes and retain lubricants, but they are also a pathway for corrosion. When lubrication properties are not critical, they are usually sealed after dyeing to increase corrosion resistance and dye retention. There are three most common types of sealing.

Anodized aluminum surfaces that are not cleaned regularly are susceptible to panel edge staining, a unique type of surface stain that can affect the structural integrity of the metal.

Anodizing is one of the most environmentally friendly metal finishing processes. With the exception of organic anodizing (also known as integral color), the byproducts contain only trace amounts of heavy metals, halogens or volatile organic compounds. Full color anodizing produces no VOCs, heavy metals or halogens, as all byproducts found in the effluent streams of other processes come from their dyes or coating materials. [36]​ The most common anodizing effluents, aluminum hydroxide and aluminum sulfate, are recycled for the manufacture of alum, baking powder, cosmetics, newsprint, and fertilizers or used in industrial wastewater treatment systems.

Anodizing will raise the surface area as the oxide created takes up more space than the converted base metal. [37]​ This will generally not matter except where there are strict tolerances. In this case, when choosing the machining dimension, the thickness of the anodizing layer must be taken into account. A general practice on engineering drawings is to specify that "dimensions are applied after all surface finishes." This will force the machine shop to take the anodizing thickness into account when performing final machining of the mechanical part prior to anodizing. Also in the case of small "Thread (mechanical)") threaded holes to accept screws, anodizing can cause the screws to bind, so it may be necessary to drill the threaded holes with a tap to restore the original dimensions. Alternatively, special oversized taps can be used to pre-compensate for this growth. In the case of unthreaded holes that accept fixed diameter pins or rods, a slightly larger hole may be appropriate to allow for dimension change. Depending on the alloy and the thickness of the anodized coating, this can have a significantly negative effect on fatigue life. Conversely, anodizing can increase fatigue life by preventing corrosion pitting.