Factors Affecting The Anodizing Of An Aluminum Metal Biology

Table of Content

Anodizing is a procedure for bring forthing cosmetic and protective movies on articles made from aluminium and its metal. It is basically a procedure where a thick movie of aluminium oxide is built up on the surface of the aluminium through the usage of a direct current electrical supply. In the bulk of anodising workss in New Zealand it is carried out in an electrolyte bath incorporating sulphuric acid with aluminum sheet cathodes and the work to be anodized attached to the anode ( Figure 1 ) .

When the current is fluxing in the cell the undermentioned sequence of events is believed to happen. Sulfuric acid begins to break up, the H ions traveling to the cathode where they are reduced to hydrogen gas:

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2H+ + 2e- a†’ H2 ( g ) ( 1 )

Simultaneously, negatively charged anions, i.e. hydrated oxide, sulfate and possibly oxide ions move to the anode. The electrical charge in the circuit causes positively charged aluminium ions ( Al3+ ) to be generated in the anode and in bend move toward the cathode. At the anode surface they react with the oxide/hydroxide ions to organize aluminium oxide ( in the instance of the hydrated oxide ion, H ions are released into the solution ) .

There are two types of ions involved in any Electrolyte. The reactions take topographic point and Anions and Cations are formed and transferred to the opposite terminals of an Electrolyte. Anions are positively charged ions and Cations are negatively charged ions in an electrolyte.

In chemical science, an electrolyte is any substance incorporating free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but liquefied electrolytes and solid electrolytes are besides possible. Ion is a atom which is electrically charged either positive or negative ; an atom or molecule or group that has lost or gained one or more negatrons.

An ion is an atom or molecule where the entire figure of negatrons is non equal to the entire figure of protons, giving it a net positive or negative charge. An anode is an electrode through which electric current flows into a polarized electrical device. A cathode is an electrode through which electric current flows out of a polarized electrical device.

Anodizing is a method of electrolytic passing to increase the thickness of the natural oxide bed of the surface of assorted metal parts. This procedure is called “ anodizing ” because the portion which to be treated signifiers an anode electrode of the electrical circuit. Anodizing chiefly increases corrosion opposition and provides better bond for pigment primers and gums than bare metal. Anodizing is even used to forestall galling of threaded constituents and to do dielectric movies for electrolytic capacitances. Anodic movies are most normally applied to protect aluminium metals, although procedures besides exist for Ti, Zn, Mg, Nb, and Ta. This procedure is non a utile intervention for Fe or C steel because these metals exfoliate when oxidized ; i.e. the Fe oxide, flakes off, invariably exposing the underlying metal to corrosion.

Anodizing alterations the texture of surface and besides changes the crystal construction of the metal near the surface. Thick coatings are usually porous, so a waterproofing procedure is frequently needed to accomplish corrosion opposition. Anodized aluminium surfaces, for illustration, are harder than aluminium but have low to chair wear opposition that can be improved with increasing thickness or by using suited waterproofing substances.

Equations of the anode reactions

Al a†’ Al3+ + 3e- ( 2 )

2Al3+ + 3O2- a†’ Al2O3 ( 3 )

2Al3+ + 3OH- a†’ Al2O3 + 3H+ ( 4 )

For which the overall procedure is:

2Al + 3H2O a†’ Al2O3 + 6H+ + 6e- ( 5 )

The sulphate ions besides play some portion as the oxide coating contains 12 – 15 % sulfate ions. It is suggested that the sulphate ions facilitate the motion of H ions cut downing the cell electromotive forces required.


Fresh aluminium reacts readily with O to bring forth aluminium oxide. Once formed the oxide remains steadfastly bonded to the surface organizing an impenetrable bed. Consequently, farther reaction ceases. The movie is really thin ( 0.01m ) , and despite its doggedness it can be removed by scratch and chemical corrosion. In such cases the aluminium is capable to have on or the surface will tag or go pitted at the site of corrosion.

Anodizing green goodss much thicker coatings ( 12 – 25 m ) which, if decently sealed, can widen the life of the surface appreciably. Recent research in New Zealand has shown that roughness of the surface can be reduced by up to 90 % with a 12 m coating, and by up to 93 % with a 25 m coating.

In the initial phases ( i.e. first 60 s ) of anodising the oxide bed formed is heavy and of even consistence. It provides the greatest opposition to have on and corrosion and accordingly is called the barrier bed. The growing of this bed ceases when the high electrical opposition of the oxide reduces the potency of the applied electromotive force in the electrolytic cell. The deepness of the coating at this phase is about 0.08m. Subsequent growing is really slow and competes with the acerb reaction:

Al2O3 + 6H+ _ 2Al3+ ( aq ) + 3H2O

which releases Al3+ ions into the solution. Note that the H+ can be at high concentration near the oxide bed due to one of the anode reactions above. See equation ( 5 ) .

At low applied electromotive forces merely the barrier bed signifiers. However, the gradual production of Al3+ ions tends to smooth out the underlying metal surface and give a brightening consequence to the article. Objects such as wheel trims and bumper bars are general treated in this manner. At higher electromotive forces the growing of the bed continues beyond the barrier bed. Unlike the initial barrier bed this secondary bed, although constitutionally the same, has an unfastened pore-like construction ; a effect of the viing anodizing and acerb solution procedures. Electron photomicrographs reveal the construction of these anodized surfaces to be as shown in Figure 2.

The conditions required to bring forth coatings vary harmonizing to the concentration and nature of the electrolyte, the electromotive force – current denseness applied, the metal being anodized and the temperature of the bath. In the bulk of electrolytic workss articles are anodized at a potency of 15 – 20 V and a current denseness about 1.6 A d-1 m-2 ; the electrolyte is 3.5 mol L-1 sulfuric acid maintained at temperatures between 20 and 23oC. Under these conditions the quality of the coating is satisfactory for most applications. At higher electrolyte concentrations and temperatures, and at lower electromotive forces or current densenesss, the acerb solution procedure occurs earlier in the development bring forthing thin, unfastened oxide coatings. Conversely, difficult dense coatings are produced at low temperatures and high current densenesss. The conditions established in each works are determined by the type of application.


Measure 1 – Cleaning

Correct and equal cleansing of the aluminium object prior to anodising is indispensable if the finished work is to hold a unvarying and attractive visual aspect. When aluminium arrives from the peal, projecting or bulge Millss it may be soiled following ways:

aˆ? Carbonaceous sedimentations from the surface of forgings and die castings

aˆ? Traces of oil-based lubricators

aˆ? Traces of smoothing compounds or sanding grits

aˆ? Oxide movies from heat intervention

Cleaning these ‘soils ‘ from the surface may turn out hard, particularly if the demands of the work do non let etching of the surface. Most cleaning solutions used in New Zealand operations are detersive based. In add-on to the detergent, a wetting agent and a skin color compound may be used. The skin color compound often used is sodium polyphosphate – a constituent of many soap preparations – which prevents ions, such as Fe3+ , adhering to the surface of the work. If etching is non a job, Na hydrated oxide or Na carbonates ( Na2CO3 / NaHCO3 ) may be added to increase the effectivity of the solution.

Measure 2 – Etching

Etching is most frequently achieved by the usage of a warm, 10 % ( 2.5 mol L-1 ) Na hydroxide solution. It gives the surface of the metal a light Grey satin coating ( through diffuse contemplation of the incident visible radiation ) . The huge bulk of work is pretreated in this manner. In theory the reactions happening in the etching solution are:

aˆ? The etching reaction:

2Al + 2NaOH + 2H2O a†’ 2NaAlO2 + 4H2 ( 7 )

aˆ? Dissolution of the aluminates:

NaAlO2 + H2O a†’ NaOH + Al ( OH ) 3 ( 8 )

aˆ? Dehydration of the solid hydrated oxide:

2Al ( OH ) 3 a†’ Al2O3 + 3H2O

The rate of etching is dependent on the concentration of the Na hydrated oxide solution, the temperature and the concentration of aluminium ions which are released into the solution. When high concentrations of aluminium ions are present the solution loses its effectivity. Presence of other ions, some of which may be a constituent in the metal, can besides interfere in the procedure, doing defects to look on the surface of the work.

The job of ion taint is overcome by using etching solutions which suppress the action of the Al3+ and other metal ions released. The composings of these solutions are the properness of the companies that develop them, but by and large contain sequestering agents which complex metal ions. Such solutions do non hold an infinite capacity to make this but, due to the ‘carryover ‘ of solution by the engraved work and periodic replacing by fresh etch solution, the etching batch is maintained in an effectual status ( Figure 3 ) .

Foaming agents are besides a component of the etching solution ; their action is to cut down the pungent mists/fumes that consequence from the vigorous reactions that occur. It is of import to observe that the visual aspect of the terminal consequence is determined at this phase. Work which is ill etched will uncover abrasions or blemishes no affair how good it is anodized or colored.


After cleansing and anodising the work is colored and sealed. As all anodized work is sealed, sealing will be considered foremost, although if colouring is to be done it is carried out prior to sealing.


Sealing is the procedure in which the pores at the surface of the oxide bed are closed away. It is affected by puting the anodized object in boiling H2O for a 15 – 20 infinitesimal period. During that clip the H2O reacts with the aluminium oxide to bring forth the mineral Boehmite – Al2O3.H2O or AlO.OH:

Al2O3 + H2O a†’ 2AlO.OH

Boehmite is a difficult, crystalline stuff with a greater volume than the aluminium oxide. As it forms it closes off the gaps of the pores.

As would be expected, the lastingness of the anodized surface, particularly in respect to chemical corrosion, is greatly influenced by the effectivity of the waterproofing. If the continuance of the waterproofing is excessively short the pores, although constricted, remain unfastened for corrosion agents to be in

near propinquity to the aluminium surface. Corrosion surveies have shown that anodized aluminium which has been sealed for 15 proceedingss or more has greatly increased opposition to roughness by chemical corrosion agents such as H+ and Cl- .


Coloring involves the soaking up of a colored dye into the pores of the oxide surfacing which becomes fixed after the waterproofing procedure has been completed. Dyes which can bond to the oxide or metal ions in the anodized bed have better coloring material belongingss than those that do non.

Electrolytic coloring is the most of import method of coloring anodised aluminium today. It produces attractive coatings of really great coloring material and heat speed and is easy to execute. The anodized work is dipped in a armored combat vehicle incorporating colored ions of other metals.

Under the influence of jumping current the coloring agents deposit quickly at the very base of the pores and the return is even over the full surface. Unlike the procedure of dye soaking up, electrolytic coloring is easy to command and gives uniformity of coloring material from one tally to the following. The success of this technique is apparent in the widespread usage of bronze colored aluminium in joinery and house adjustments. Approximately 66 % of all bronze tinted aluminium is coloured by this technique.

To a much lesser extent coloured inorganic compounds can be used to color the work. Ammonium ferrous oxalate is a really common compound used to leave a goldy coloring material to the metal. Other colorss can be impacted by handling the captive ferrous ammonium oxalate with other compounds: for illustration, K ferrocyanide solution will respond with the ferrioxalate compound to bring forth a bluish coloring material. The technique used is to dunk the work foremost in a solution of the ammonium ferrioxalate followed by dunking the work in the K ferrocyanide solution. This dual dipping technique can be used with other compounds to bring forth a assortment of colorss: e.g. Cu sulphate followed by ammonium sulphide gives green, and lead nitrate followed by K chromate gives xanthous.

Experiment on Anodizing of aluminium


Electrochemistry/electrolysis/industrial chemical science.


About 1 H.


Upper secondary.


A strip of aluminium is anodized and the thickened surface coating is dyed.


Voltage Direct Current power – adjustable up to 15 V.

0-100 Ohm variable resistor.

0-1 A ammeter.

0-15 V voltmeter.

Connecting leads and crocodile cartridge holders.

Retort base with foreman and clinch.

Ruler ( 15 centimeter ) .

One 1 dm3 beaker.

Four 250 cm3 beakers.

One 1 dm3 conelike flask.


The measures given are for one presentation.

Aluminum foil about 50 centimeters x 50 centimeter.

Dylon cold fabric dye ( Camilla A 16 ) . This is red ruddy. ( Some other colorss may work every bit good. )

1 dm3 of sulphuric acid ( about 2 mol dm-3 ) .

250 cm3 of azotic acid ( about 1 mol dm-3 ) .

250 cm3 of Na hydrated oxide ( about 1.5 mol dm-3 ) .

250 cm3 of acetone ( propanone ) .

Strip of thin aluminium sheet about 12 centimeters x 3 centimeter.

2 cm3 of glacial ethanoic acid ( acetic acid ) .


Before the presentation

Line the interior of the sides of the 1 dm3 beaker with a dual thickness of aluminium foil. Fill the beaker with sulphuric acid. This should be at approximately 25 C – adjust the temperature if necessary. Put up the electrical circuit shown in the figure. Make up the dye solution harmonizing to the instructions supplied ( i.e. fade out the contents of the Sn in approximately 600 cm3 of H2O ) and add a few cm3 of glacial ethanoic acid.

The presentation

De-grease the aluminium strip by rubbing with a tissue soaked in acetone and so dunk the strip into a beaker full of acetone for short clip and leting drying. From now on, keep the aluminium by the top few centimeter merely.

Dip the bottom half of the aluminium strips into the Na hydroxide solution in a beaker. Leave it until it begins to foam, bespeaking that the surface bed of oxide has been removed. ( This will take about one minute. ) Now remove the strip and dunk the cleaned part of it into the azotic acid for a few seconds to neutralize the base. Then rinse away the acid with H2O. Clamp the strip so that the lower, cleaned, subdivision is immersed in the sulphuric acid electrolyte and is in the Centre of the cylinder of aluminium foil which forms the cathode. It must non touch the cathode.

Complete the circuit with crocodile cartridge holders doing the aluminium strip positive and the foil negative. Now adjust the power battalion and rheostat so that current flows which give a current denseness of 10 -20 mA cm-2 of anode country immersed. For illustration if the anode has an country of 3 centimeters x 3 centimeters immersed, the country will be 3 ten 3 ten 2 cm2 = 18 cm2 so the current should be between 180 and 360 ma ( 0.18 and 0.36 A ) .

Leave to electrolyze for approximately 30 proceedingss, maintaining an oculus on the current and seting the variable resistor if necessary to maintain its value invariable. ( The current may be given to drop as the oxide bed thickens. )

When the electrolysis is complete, exchange off the power and take the aluminium strip. Rinse the strip in H2O. It will non look really different at this phase. Now dip the strip into about 200 cm3 of the dye solution in a beaker. Make sure that some of the non-anodized portion of the strip is immersed every bit good as the treated subdivision.

Leave for approximately 15 proceedingss – longer submergence will bring forth a deeper coloring material. Some of those who trialed this presentation left the strip in the dye overnight. Rinse to take any dye which has non been absorbed. Dye will merely be absorbed by the anodized subdivision, which will turn a deep cherry ruddy. If desired, seal the dye by plunging the bleached strip for a few proceedingss in H2O that is already boiling. This will do the coloring material less prone to rubbing off, but will rinse out some of it. Many instructors may prefer to exclude this process.

Ocular tips

Large presentation metres will be easy to see. Long linking leads are utile to forestall the electrolysis armored combat vehicle going lost in a labyrinth of wires.

Teaching tips

It would be wise to fix something to make full in the half hr of electrolysis clip and the 15 proceedingss dyeing clip. The category could be asked to cipher the expected addition in mass of the anode or to discourse the chemical reactions involved. Have a choice of anodised objects such as saucepan palpebras available for the category to look at. The presentation ( No. 18 ) of the responsiveness of aluminum without its normal oxide bed could be shown. Some instructors may prefer to anodize some aluminum before the lesson to hold some pieces ready to demo the category.


Untreated aluminum has a bed of oxide about 10-8 m midst. This explains its evident deficiency of responsiveness. Anodising, invented in 1923, is used commercially to inspissate this bed to 10-5 m to better the metal ‘s corrosion opposition.

The relevant equations are:


Al2O3 ( s ) + 2OH- ( aq ) + 3H2O ( cubic decimeter ) a†’iˆ 2Al ( OH ) 4- ( aq )

Once the oxide is removed:

2Al ( s ) + 2OH- ( aq ) + 6H2O ( cubic decimeter ) a†’2Al ( OH ) 4

– + 3H2 ( g )

Electrolysis at the anode:

2Al ( s ) + 3H2O ( cubic decimeter ) a†’Al2O3 ( s ) + 6H+ ( aq ) + 6e-

Electrolysis at the cathode:

6H+ ( aq ) + 6e- a†’3H2 ( g )

Electrolysis overall:

2Al ( s ) + 3H2O ( cubic decimeter ) a†’Al2O3 ( s ) + 3H2 ( g )

The oxide surfacing develops a positive charge by the reaction:

Al2O3 ( s ) + H2O ( cubic decimeter ) a†’Al2O3H+ ( s ) + OH- ( aq ) .

Therefore it attracts dyes that contain colored anions. These are absorbed in oxide bed which have pores, where they are trapped by heating the oxide to organize an Al2O3.H2O seal.


There are a great many variables in this experiment such as: electrolysis clip, electromotive force, current denseness, concentration of electrolyte, temperature of electrolyte, temperature of dyebath and type of dye. Probes of some of these could organize interesting undertakings. It is possible to mensurate the addition in mass of the anode by rinsing the aluminum strip with acetone and weighing it instantly before and instantly after electrolysis.

The Different of Anodizing And Electroplating

Electroplating is a technique to plate some metal or non metal with metal utilizing electric current. While anodising is plate metal like aluminium by infixing some substance under the oxidised bed of aluminium by utilizing of electric current force. The map and the consequence of both procedure may the same, like have color surface so that can utilize as cosmetic intents.

Not all metal can utilize anodizing procedure but merely certain metal can utilize this method to do endure to corrosion consequence. Aluminum metal can be treated by anodising because after electric current procedure make the aluminum have oxidized bed on the surface and this bed can hold broad pore so that other chemicals like dye or prevented agent to be impregnated to come in this oxidised bed. After certain substance enter into this oxidized bed so by certain method the pore can be closed by farther procedure. Actually by utilizing merely electric current will make anodized bed, but the job this bed have no coloring material and expression bad, to do this more interesting so on anodising procedure utilizing color agent and inserted below the anodized bed.

Electroplating is straight plate other metal into certain metal with the end to do more interesting or do more endure to corrosion or from other outside consequence. Like on difficult chrome plating on prison guard driver, can do this surface hard and non easy to interrupt if non plate by difficult chrome. By nickel plating on other metal will do the surface is endure from reaction consequence because Ni more baronial than the metal below the nickel surface. Nickel plating normally use in tining procedure, or usage in cosmetic accoutrements. Electroplating rely on the home base stick strong, more strong certain plating metal stick to the base metal is better.


Electroplating is plating to certain metal with other metal that is normally more lasting from corrosion or stronger than original metal. The procedure itself usage of electrochemical by which metal is deposited on the beginning metal through the chemical bath.

Normally utilizing electrode pole that is connected to the negative and the other to the positive pole. Electrode on the negative pole is called as anode and electrode that connected to positive charge is called as cathode. Metallic element on the solution signifier will turn to positive ion and on the electrochemical procedure this ion will pull to negative electrode or to anode and will plate anode. So metal that will be plated is placed on the anode place.

Electroplating is done in a plating bath which is normally a non-metallic armored combat vehicle like plastic or glass. The armored combat vehicle is filled by metal solution, which the metal sort will plate the anode. The anode is substrate to be plated which is connected to the negative terminus of the power supply.

As we apply current, positive metal ions from the solution move towards anode with negative charge and sedimentation on anode. As a refilling of these sedimentation ions, the metal from cathode will fade out and goes into the solution and do the ionic possible balance.

In the instance utilizing of baronial metal similar gold as cathode it is non sacrificial, but it is made out of stuffs which do non fade out in an electrolyte such as Ti.

Electrochemical Theory:

Actually electroplating is based on the Faraday ‘s Laws that province as follows:

The weight of a substance formed at an electrode is relative to the sum of current passed through the cell. The weights of different substances produced at an electrode by the same sum of current are relative to their tantamount weights.

Matching mass in an oxidation-reduction reaction is = molar weight of the compound / algebraic alteration in oxidization figure of the atom that is oxidized or reduced.

2 FeCl2 + Cl2 a†’ 2 FeCl3

Fe valency is change on the reaction from +2 to +3. However on the reaction will:

Mn + FeSO4 a†’ Fe + MnSO4

Chrome Plating

Chrome plating is a portion of surfacing engineering that usage of electroplating procedure. Electroplating procedure itself can utilize many sort of metal like chrom, Cu, nickel, Ag or gold. The procedure of all metal is similar, that is usage of DC electric current and utilizing of metal solution. But the differences of all the procedure by and large on the solution uses, concentration utilizations, current flow, temperature restriction, plating clip and many other restriction that difference of each metal coating.

Chromium plating is besides different among the consequence intents from the current flow, chromium concentration and other chemical additive that is use on each of Cr plating sort. The Cr plating such as difficult chrome, dull chrom, black chrome and bright chrome. Every consequence end needs different parametric quantity uses that must be followed in order to acquire the consequence as the procedure end. So chromium plating procedure demands rigorous control on the bath concentration, equipment preciseness and on the method uses. Any little substance contain on Cr bath can ensue a different sort of merchandise.

Anodizing aluminium

This is an experiment screening interesting application of electrolysis. The oxide bed on aluminium foil is made thicker by anodising which improves the metal ‘s corrosion opposition. In the procedure, the thickened oxide surface coating is coloured by utilizing dyes.

Lesson organisation

This works good as a category presentation, but there are several undertakings to finish in readying. The anodizing procedure itself takes about 30-40 proceedingss, with nil peculiarly dramatic occurrence, so you will necessitate to be after other activities to make full the clip.

At the start of the experiment, show the pupils the effervescence due to the H evolved from the cylindrical aluminium cathode. A flexi camera connected to a projector could be used here.

During the anodising stage, the theory could be explained with an accent on the applications of the procedure. A aggregation of anodized objects such as saucepan palpebras or athleticss equipment could be available to look at.

A well-disciplined and organized category might be able to transport out this procedure for themselves ( in 2 or 3s ) , but it is strongly recommended that the intervention with Na hydrated oxide solution ( Corrosive ) – prior to the electrolysis – is carried out under rigorous supervising.

Apparatus and chemicals

Eye protection, Low-voltage DC power battalion, adjustable up to 10 Vs, Connecting leads and 4 crocodile cartridge holders Paper cartridge holders, fictile Test-tube holder, wooden, Paper tissues Strip of wood, 15 centimeter long Ruler ( 30 centimeter ) , Beaker ( 1 dm3 ) , Beakers ( 250 cm3 ) , 3Aluminum foil, about 40 centimeters x 15 centimeter, Congo Red dye ( Toxic ) Ethanol ( Highly flammable, Harmful ) Sulfuric acid about 2 mol dm-3 ( Corrosive ) , 1 dm3, Sodium hydrated oxide, about 1.5 mol dm-3 ( Corrosive ) , 250 cm3. Propanone ( propanone ) ( Highly flammable, Irritant )

Technical notes

Congo Red dye ( Toxic ) . Mention to CLEAPSS Hazcard 32. Ethanol ( Highly flammable, Harmful ) . Mention to CLEAPSS Hazcard 40 ( 2007: 40A ) Sulfuric acid about 2 mol dm-3 ( Corrosive ) . Mention to CLEAPSS Hazcard 98 ( 2007: 98A Sodium hydrated oxide, about 1.5 mol dm-3 ( Corrosive ) . Mention to CLEAPSS Hazcard 91.Propanone ( propanone ) ( Highly flammable, Irritant ) .

1 Reasonably thick aluminum foil should be used, but, if unavailable, ordinary kitchen foil works rather good.

2 Propanone is needed to degrease the aluminum foil and it is deserving maintaining a bottle specifically for this intent. The used acetone can be poured back into the bottle and maintain for future usage. This reduces waste disposal demands.

3 The solid Congo Red dye needs to be made up into solution. Use 0.5 g of dye, 50 cm3 of ethyl alcohol and 50 cm3 of H2O and warm to fade out. Dylon cold fabric dye ( Camilla A 16 ) besides gives good consequences. Red fountain-pen ink can be used as an alternate but does non give such good consequences.

4 Alternatively of a power battalion, a battery or series of batteries could be used.


HEALTH & A ; SAFETY: Eye protection must be worn.

Before the presentation

a ) Cut two pieces of aluminium foil, one 10 centimeter x 3 centimeter ( the anode ) , the other about 30-35 centimeter x 12 centimeter ( the cathode ) . Ensure that when the foil is folded into a cylinder it fits inside the beaker as shown below.

Anodizing aluminum

B ) Work in a fume closet and guarantee that there are no fires near by. Work on a clean surface. Degrease the two pieces of foil by rubbing good with a paper tissue soaked in acetone and so dunk the strips into a beaker of acetone for a few seconds.

degree Celsius ) Remove the strips of aluminium from the acetone and let drying. From this point on, merely keep the aluminium foils at the top borders.

vitamin D ) Arrange the larger piece of aluminium into a cylinder. Repair it in place with fictile paper cartridge holders and so topographic point inside the big beaker as shown in the diagram.

vitamin E ) Set up the strip of wood on the beaker and usage Bluetak to attach two crocodile cartridge holders, one at the border and one in the Centre. Attach the outer cartridge holder to the aluminium cylinder. This is the cathode.

degree Fahrenheit ) Pour some of the cold Na hydrated oxide solution ( Corrosive ) into a 250 cm3 beaker. Keep the smaller piece of aluminium foil with a wooden test-tube holder, and dunk it into the Na hydroxide solution. After a short piece, H gas will be given off quickly. Remove the strip after a few seconds of fizzing, and rinse it in a watercourse of cold running H2O.

g ) Attach the aluminium strip to the cardinal crocodile cartridge holder guaranting that it is arranged vertically ( see diagram ) . This cardinal strip ( the anode ) must non touch the aluminium cylinder.

H ) Carefully make full the beaker with the sulphuric acid from a measurement cylinder up to a degree about 1 centimeters below the top of the aluminium cylinder.

Safety: Remember that H ( Highly flammable ) will be evolved during the electrolysis. Keep all bare fires good off from the experiment ( e.g. when heating the dye solution ) .


a ) Connect up the circuit and utilize a electromotive force of 5-10 V. Electrolysis is happening when bubbling can be seen at the cathode ( H ) . Pass a current for about 20 proceedingss, or longer, if clip licenses.

B ) While the electrolysis is running, heat the dye solution in a beaker to about 70A°C. An electric hot plate is preferred to a Bunsen burner. An extra beaker of boiling H2O will besides be needed.

degree Celsius ) Remove the cardinal aluminium strip ( the anode ) and topographic point it in the hot dye solution. Stir and go forth for about 10-15 proceedingss.

vitamin D ) Transfer the aluminium anode to a beaker of boiling H2O and go forth for another 10 proceedingss. This seals the dye onto the anodized surface of the aluminium and makes the aluminium oxide bed less porous.

vitamin E ) The upper non-anodized part of the strip should be the original metallic grey color whilst the remainder should be coloured ruddy. The aluminium strip can be dried in paper tissue and passed round the category. It should non be possible to rub off the dye off the surface.

Teaching notes

The presentation itself

The instructions may look really elaborate, but experience shows that success depends on acquiring the conditions merely right. You should seek out the experiment before transporting it out as a presentation. It would be utile to hold some sample strips of anodized aluminum to go through unit of ammunition.

The electromotive force will drop during the experiment, since the anode is going progressively coated with aluminum oxide. If a variable resistor and voltmeter are used, the readings can be invariably monitored and accommodations made to maintain the electromotive force about changeless.

A longer submergence in the dye will bring forth a strip with a deeper ruddy coloring material. Leaving the strip in the dye nightlong produces the best consequences.

If clip is short, omit the dye-sealing phase in boiling H2O.

If there is clip, a piece of the cathode could besides be immersed in the dye. It will be found that the dye is non taken up by the metal in the same manner.

This is a good experiment to demo pupils towards the terminal of their survey of electrolysis.

Chemistry points

When a piece of aluminium is exposed to the air, it quickly becomes coated with a protective surface bed of aluminium oxide.

Heating the aluminium in air can do the oxide bed thicker, but anodizing is much more effectual.

The oxide bed can be made to absorb dyes. This is utile in a scope of mundane goods, such as boilers, window frames and some athleticss equipment, all of which need to be able to defy utmost physical conditions.

Untreated aluminum has an oxide bed about 10-8 m midst. This explains aluminum ‘s evident deficiency of responsiveness in the research lab. Anodizing thickens this bed to about 10-5 m and dramatically improves the metal ‘s corrosion opposition.

Oxygen is frequently evolved at the anode during the electrolysis of aqueous solutions. Aluminium is a reactive metal. The O formed reacts instantly with the aluminum. It forms a solid oxide surfacing on the surface of the metal electrode.

Theory for more able pupils

For pupils working at a higher ability degree, some or all of the undermentioned equations and accounts could be introduced:

The cleansing procedure with NaOH:

( 1 ) Al2O3 ( s ) + 2NaOH ( aq ) + 3H2O ( cubic decimeter ) a†’ 2NaAl ( OH ) 4 ( aq )

( 2 ) 2Al ( s ) + 2NaOH ( aq ) + 6H2O ( cubic decimeter ) a†’ 2NaAl ( OH ) 4 ( aq ) + 3H2 ( g )

Electron transportation happening at the anode ( + ) :

6OH- ( aq ) + 2Al ( s ) a†’ 3H2O ( cubic decimeter ) + Al2O3 ( s ) + 6e-

Electron transportation happening at the cathode ( – ) :

6H+ ( aq ) + 6e- a†’ 3H2 ( g )

Negative hydrated oxide ions, OH- , are destroyed ( discharged as O ) at the surface of the anode. Therefore H+ ions predominate in the solution instantly around the anode. The solution around the aluminum strip is acidic ( due to H+ ions ) and the oxide surfacing hence develops a positive charge. The oxide surfacing attracts dyes which contain a colored anion. The negatively-charged dye molecules are absorbed in the pores of the sponge-like oxide bed.

The Aluminum Anodizing Procedure

Pre-Treatment: Cleaning is done in a non-etching, alkalic detergent heated to about 145 grades Fahrenheit. This procedure removes accumulated contaminations and visible radiation oils.

Rinsing: Multiple rinses, some utilizing purely de-ionized H2O, follow each procedure measure.

Etching ( Chemical Milling ) : Etching in acerb sodium carbonate ( sodium hydrated oxide ) prepares the aluminium for anodising by chemically taking a thin bed of aluminium. This alkalic bath gives the aluminium surface a matte visual aspect.

Desmutting: Rinse in an acidic solution removes unwanted surface metal component particles non removed by the etching procedure.

Anodizing: Aluminum is immersed in a armored combat vehicle incorporating an electrolyte holding a 15 % sulphuric acerb concentration. Electric current is passed through the electrolyte and the aluminium is made the anode in this electrolytic cell ; the armored combat vehicle is the cathode. Voltage applied across the anode and cathode causes negatively charged anions to migrate to the anode where the O in the anions combines with the aluminium to organize aluminium oxide ( Al2O3 ) .

Coloring: Anodic movies are good suited to a assortment of colourising methods including absorbent dyeing, both organic and inorganic dyestuffs, and electrolytic colouring, both the SandocolorA® and AnolokA® procedures.

Sealing: In all the anodising procedure, the proper waterproofing of the porous oxide coating is perfectly indispensable to the satisfactory public presentation of the coating. The pores must be rendered nonabsorptive to supply maximal opposition to corrosion and discolorations. This is accomplished through a hydrothermal intervention in proprietary chemical baths or by cresting the pores via the precipitation of metal salts in the pore gaps.


This is a brief overview of the chemical science of the anodising procedure. In industry the procedure scan encounter many troubles if attention is non taken to guarantee that solutions are controlled with respect to concentration and temperature. Thorough rinse of the work is carried out after each phase to guarantee that it enters the following procedure in the right province. It besides ensures that taint of solutions from one phase by the preceding phase is kept to a minimal.A further facet of the industry non covered herein is that of quality control. Even in smallplants chemists are employed to invariably look into the conditions of the solutions and do recommendations / accommodations. In add-on, frequent cheques are made on the thickness ofthe movie, its denseness and the color quality.

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Factors Affecting The Anodizing Of An Aluminum Metal Biology. (2017, Jul 17). Retrieved from


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