July 4, 2019

Electropolishing of Aluminum

Electropolishing Aluminum Content:

The Scheme of Electropolishing

Before the aluminum be proceed by anodizing, the material must treat use electropolishing first, this process will open the surface film above the aluminum part, because in the store or travel after the aluminum be produced will react with free air build an aluminum oxide that cover the surface of aluminum. To open the surface pore can't use just dipped in water but must use electric current. Even if dipped in hot water the pores can be closed.

Electropolishing DC electrict

Electropolishing Definition

Electropolishing is an excellent preparation of metals for electroplating, this is a kind of metal surface treatment before anodizing process or other kind of process. In addition to providing a chemically and physically clean surface, electropolishing removes mechanically surface damage. As a result, subsequent electroplates have the best adhesion possible and a decreased tendency toward developing pits and voids that lower corrosion protection. Metal surfaces that have been machined, ground, picked and rolled, abrasive polished, and buffed, have asperities that are detrimental to uniform and pit-free electroplates, adhesion of the plate, and strength of the interface region of the plate basis metal.

Electropolishing is a process of producing a polished and bright limit on the surface of metals. In this process the metal part or the device to be coated with a layer of suitable metal is immersed, as the anode in a solution having such a composition that when the metal is made the anode, there occur unstable conditions of passivity which manifest themselves by an anomalous change in current strength is adjusted to give a current density, previously determined by observation, which is characteristic for the given metal and the given solution, at which there occurs the transformation of the Matt surface into a bright polished one.

Electropolishing Bath:

The most common used in electropolishing bath are based on sulfuric and phosphoric acids in about 50/50 weight proportion at about 54 to 105 oC for ferrous alloys, 85% phosphoric acid for copper at about 43 to 60 oC; and phosphoric acid with about 5 to 7% chromic anhydride (CrO3) at about 60 oC for brass.

Current density of practical electropolishing range from 5 to 40 A/dm2, depending on the metal being finished, type of bath, and temperature. The time required usually is 2 to 7 min for current densities of 15 to 30 A/dm2 and temperature of 54 to 82 oC.

The solution used for the polishing of aluminum can be divided into two groups. The first of these includes the phosphate, alkaline and fluoboric acid solution. They are characterized by a low rate of dissolution of the metal and are used as a finishing treatment for mechanically polished parts with a view to obtain a very high reflectivity. They are employed for treatment of reflectors where a high coefficient of reflection can’t be obtained by mechanical polishing alone. The fluoboric acid and the phosphate electrolytes are suitable only for treatment of aluminum of high purity. The second group includes solutions containing phosphoric acid, sulfuric and chromic acid. They are characterized by a higher rate of dissolution of the metal than the solutions belonging to the first group, and they enable the smoothing out of microasparities and the production of a bright, but not a mirror finished surface. These solution are used in place of mechanical surface finishing operations for parts of aluminum of different purity grades and for certain wrought aluminum alloys. In addition to the above, solutions of perchloric acid and acetic anhydride have also been recommended for the polishing of aluminum. The use of these electrolytes, however, involves taking certain precautions because of the risk of explosions.

The purity of aluminum is an important factor in electropolishing of the metal. An appreciable increase in reflectivity can achieved only in able amount of admixtures is much less satisfactory. Very often polishing becomes impossible because of the non-uniform dissolution of the surface of the anode. This factor renders difficult the polishing of aluminum alloys, especially those containing silicon. For the treatment of castings of an aluminum alloy containing 5 per cent silicon, an electrolyte comprising 13 per cent hydro-fluoroboric acid and 52 percent glycerin is recommended. The solution is used at 25 0C with a current density of 200 amp./sq.ft., the time of polishing being 10 minutes. During polishing the solution is stirred.

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Aluminum Anodizing

aluminum anodized product
anodized aluminum product
Article Contents:
Aluminum Anodizing Practice
Aluminum Anodizing Steps
Alkaline Solution Treatment
Racking and Rinsing
Sealing and Coloring
Polishing after Anodized
Related Articles

Aluminum Anodizing Practice

Aluminum anodizing is a process to cover aluminum surface with substance below the oxidized aluminum layer on the surface. This layer is very thin and transparent so the substance below this surface can be seen and can be coloring etc. To make aluminum surface have good color usually use textile coloring agent.

The electro deposition of anodized aluminum has been investigated for use in coating steel strip, in electrorefening and in the cladding of uranium. Upto 76-cm diameter parabolic mirrors have been electroformed in 750-liter solutions. Limited commercial applications of aluminum plating have been reported.

Aluminum, because it is much more chemically active than hydrogen, probably cannot be electrodeposited from solution that contain water or any other compound with an acidic hydrogen, for example, acids, alcohols, esters, ammonia, and primary and secondary amines. It can be electrodeposited from inorganic and organic fused salt mixtures and from solutions of aluminum compounds in certain organic solvents, only the National Bureau of Standards hydride process has achieved a modest degree of use and is considered in this chapter.

Aluminum Anodizing Steps:

The step to anodized aluminum should be as follows:

1. Electropolishing of Aluminum

Before the aluminum being proceed by anodizing, the material must treat use electropolishing first, this process will open the surface film above the aluminum surface, because in the store or travel after the aluminum be produced will react with free air build an aluminum oxide that cover the surface of aluminum. On this process aluminum oxide film like being peeled up. To open the surface pore can't use just dipped in water but must use electric current. Even if dipped in hot water the pores still closed. To open before aluminum anodized should proceed in electric current on the acid solution.

2. Alkaline Solution Treatment

The next step of aluminum anodizing is treated in alkali solution. The concentration of free alkali in the solution is one of the major factors which determines the program of the polishing process when treating aluminum in an alkaline solution. Although alkali is not one of the constituents used in making up the solution. It is formed in the bath as a result of hydrolysis. As the solution is used, the concentration of free alkali rises as a result of reaction of aluminum with sodium hydrogen phosphate. The optimum of alkali in the solution is 15 – 18 gram per liter.

3. Rinsing

Rinsing is a process that does in between of aluminum anodizing process step, rinsing also do in preparation for plating. The result quality will depend on the rinsing process, if many chemicals or organic substance left on the metal surface will result a bad electroplated or anodized product.

4. Jigging

Jigging is a process to parts which have been polished with polishing pastes are degreased using organic solvent, such as benzine or dichloromethane, and are dried in air until the traces of the solvent is disappeared. They are then mounted on anodizing jigs made of aluminum or clad alloy. The design of the jig requires special considerations. It should ensure a firm contact with both the work and the anode bar and at the same time the area of contact should be as small as possible because this area will remain not anodized. It should be just sufficient to avoid local heating of the metal, which is usually the cause of an attack on the surface of the work during the anodizing process.

5. Aluminum Anodizing

Aluminum anodizing takes into account the composition of the solution, the alloy and the service requirement. To maintain the required temperature cooling arrangements in the case of sulfuric acid anodizing and heating solution is agitated by means of compressed air which is introduced trough a perforated coiled pipe arranged along the bottom of the vat. The cathodes used in anodizing baths, especially those of sulfuric acid, is very great. It is therefore almost unnecessary to use auxiliary cathodes when anodizing of the interior of pipes. If the length of the pipe exceed ten time the diameter, an internal cathode must be used.

6.Racking and Rinsing

Aluminum racks are preferred for plating aluminum alloys. It is recommended that 1100 alloy be used for the spines and 2024 alloys for the contacts. By increasing the cross sectional area of the spines by about 40%, a conductance equal of that of a copper rack is obtained. Where contact marks are not important, regular phosphor-bronze contact may be used. When phosphor-bronze contacts are used, however, the area adjacent to the contact may develop a small blister as a result of galvanic action in the zinc immersion bath. When nitric-hydrofluoric acids etch is used for conditioning, aluminum contacts should be used. Racking and rinsing several time to make sure that the part really clean.

7. Sealing and Coloring

The oxide films obtained after aluminum is anodized have considerable porosity and high adsorption capacity. Sealing of the film increases their resistances to corrosion and improves their electrical insulating properties. Sealing is essential for films that have been produced by anodizing in sulfuric acid, because only after sealing do the film acquire a sufficiently high resistance to corrosion, because surface of aluminum is anodized. The following methods can be used for sealing anodic oxide films:

8. Polishing after Anodized

One of the major applications of anodized aluminum involves the production of finishes resembling bronze, brass, gold, silver and their alloys. In such cases apart from the choice of suitable coloring materials, an important part is played by the conditioning and preparation of the surface of the metal prior to anodizing. A mechanically polished surface become dull after the first four minutes of anodizing in sulfuric acid. The absence of a metallic luster effects the appearance, the dyed film giving the appearance of a plastic surface. To retain the metallic lustrous, it is necessary to produce a passive film capable of withstanding the action of sulfuric acid. This can be achieved by electro polishing. After electro polishing, the surface of the metal retains its luster even after one hour's anodizing. A pretreatment of this nature also helps the production of more intense color.

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Example of Anodizing Products:

Black Anodized Aluminum Bar

Black Anodized Aluminum Bar
Black Anodized Aluminum Bar

Black Anodized Aluminum Sheet:

Black Anodized aluminum Sheet
Black Anodized aluminum Sheet

Black Anodized Aluminum Part:

Black Anodized Aluminum Part
Black Anodized Aluminum Part

Black Anodized Aluminum for Electronic Casing:

Black Anodized Aluminum Electronic Casing
Black Anodized Aluminum Electronic Casing

November 12, 2018

Less People Know Anodizing

If we compare to electroplating, anodizing is less known be general people because there is no people find out metal with already anodized in the store, or people want to anodize of aluminum to make more interested. Beside less people use this service, also less people use pure aluminum metal to make something. Usually they just use the available product from the store that normally already anodized. So that is why this blog about anodizing also less visitor, or only few people want to know about anodizing.

To know more about anodizing you can browse through this blog, many articles may useful for your preference.

November 28, 2017

Composition of Alloy and Process Limitation

Composition of aluminum alloy make the process condition of anodizing is conditioning in order can get good performance of anodized alloy. Surface finish, prior processing, temper or heat treatment are influence to the quality of anodic coatings. The limitations imposed by each of these variables on various condition of aluminum alloy are as follows:

Chromic acid process should not use for anodize aluminum casting alloys containing more than 5% of Cu or more than 7.5% of total alloying elements, because this alloy can resulting of excessive pitting, and may burning. Sulfuric acid bath can be used for commercial alloys, while hard anodizing process is usually apply in limited alloys containing less than 5% Cu and 7% Si. Choice of alloys is important when the product should have maximum corrosion or abrassive resistance required. Alloys contain of copper and copper magnesium is able to produce hard, corrosion resistance coating.

Two kind of alloy can be anodized together in the same anodizing bath if the voltage requirement are identical, even simultaneous anodizing of two different alloy is not normal applied. This process can be more difficult for sulfuric acid process than chromic acid process.

Finished anodizing will form an anodic film, and often time formed surface irregularities that are emphasized more by chromic acid bath than by sulfuric acid bath. For this reason, suggest that sulfuric acid bath is use instead of chromic acid bath where optimum corrosion or abrasion resistant surfaces are required. Clad sheet should be handled with careful to prevent mechanical abrasion or exposure of the core material. Scratch on surface of aluminum make this magnifies exposed, with different color of anodized finished.

Anodizing grade must be specified for extruded products so that mill operations are controlled to minimize longitudinal die marks and other surface blemishes. Surface irregularities must be removed from forgings, and surface of forgings must be cleaned by a process that removes trapped and burned-in die lubricants.

Castings can be anodized provided their composition is within the process limits described under alloy composition. Uniform will appearance, however anodizing usually is undesirable for castings.

November 22, 2017

Special Anodizing Process

Special anodizing process is the condition of several anodizing process that is done from several process of by certain company or from proprietary information available to licencess only. Anodizing condition process that is proposed based on licence are Martin Hard Coat (MHC), Aluminate 225 and 226, Alcanodox Hardas, Sandford, Kalcolor and Lasser.

Hard anodizing process uses a sulfuric acid bath containing 10 to 20% of acid, with or without additives. Operating condition temperature of the bath range from 0 to 10 oC, and with current density range between 2 and 3.6 A/dm2. By adding some additives and modifier for power, hard anodizing can be operated at higher temperature than room temperature. But hard anodizing that operate in higher temperature than room temperature may result soft formation and more porous outer layer. The change of character result will reduce wear resistance coating significantly and tends to limit coating thickness. If without use of some additives or modified power, such as superimposed alternating current over direct current or pulsed current, excessive operating temperatures result in dissolution of coating and can burnt and damage the work.

Prefer process condition that are commonly use solution containing 120 to 160 g of sulfuric acid and 12 to 20 g of oxalic acid per 3.8 lt water. Operating temperature at 10 + 1 oC and use current density of 2.5 to 3.6 A/dm2, voltage is increase gradually from zero to 40 or 60 V. Time treatment about 25 minute will result anodizing thickness about 1 mil or 25 micron.

For special condition process that mensioned above will use some parameter limitation as on the table below:

Process NameBathTemperature, oCDuration, minVoltage, voltCurrent Density, A/dm3
Martin Hard Coat (MHC)15% sulfuric acid, 85% water- 4 to 045 (b)20 - 752.7
Aluminate12% sulfuric acid, 1% oxalic acid, water1020, 4010 - 752.8 (b)
AlcanodoxOxalic acid in water2 - 20(a)(a)(a)
Hardas6% Oxalic acid, 94% water4(a)60 DC plus AC overide2
SanfordSulfuric acid with organic additives0 - 15(a)15 - 150 DC1.2 - 1.5
Kalcolor7 - 15 % sulfosalicylic acid, 0.3-4 % sulfuric acid, water18 - 24---------1.5 - 4
Lasser0.75 % oxalic acid, 99.25 % water1 - 7to 20rising A 50 - 500V controlled
a: Lincence only
b: change from last edition

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Electropolishing of Aluminum

Electropolishing Aluminum Content: The Scheme of Electropolishing Electropolishing Definition Electropolishing Bath: Related Articles: ...