Factors Affecting The Anodizing Of An Aluminum Metal

Factors Affecting The Anodizing Of An Aluminum surfaceAnodizing is a p lower-ranking for producing decorative and protective films on articles made from atomic number 13 and its alloys. It is essentially a bear on where a thick film of atomic number 13 oxide is built up on the appear of the atomic number 13 through the put on of a direct authoritative galvanizing supply. In the majority of anodizing plants in New Zealand it is carried away in an electrolyte lav stomaching sulfuric acid with aluminium sheet cathodes and the work to be anodized attached to the anode (Figure 1).When the real is flowing in the cell the following rank of even upts is believed to occur. sulfuric acid begins to decompose, the total fondness ions moving to the cathode where they jotic payoff 18 reduced to hydrogen feature2H+ + 2e- H2(g) (1)Simultaneously, vetoly supercharged anions, i.e. hydroxide, sulphate and maybe oxide ions move to the anode. The electrical charge in the set causes unequivocally charged aluminium ions (Al3+) to be generated in the anode and in turn move toward the cathode. At the anode surface they respond with the oxide/hydroxide ions to form aluminum oxide (in the case of the hydroxide ion, hydrogen ions argon released into the solution).There be two subjects of ions involved in any Electrolyte. The replys contri only whene place and Anions and Cations ar formed and transferred to the opposite ends of an Electrolyte. Anions are tyrannically charged ions and Cations are negatively charged ions in an electrolyte.In chemistry, an electrolyte is any agency containing free ions that firebrand the substance electrically conductive. The al closely typical electrolyte is an ionic solution, only if jettyten electrolytes and upstanding electrolytes are also possible. Ion is a section which is electrically charged either positive or negative an atom or molecule or group that has lost or gained angiotensin converting enzyme o r more electrons.An ion is an atom or molecule where the total number of electrons is not equal to the total number of protons, giving it a internet positive or negative charge. An anode is an electrode through which electric on-going flows into a polarized electrical device. A cathode is an electrode through which electric flowing flows out of a polarized electrical device.Anodizing is a method of electrolytic passing to increase the oppressiveness of the natural oxide class of the surface of various admixture dowrys. This litigate is called anodizing because the part which to be inured forms an anode electrode of the electrical circuit. Anodizing mainly increases erosion resistance and provides br distributively bond for paint primers and glues than bare surface. Anodizing is even used to prevent scratchy of threaded divisions and to make dielectric films for electrolytic capacitors. Anodic films are or so communally applied to protect aluminum alloys, although but tes also exist for titanium, zinc, magnesium, niobium, and tantalum. This surgery is not a useful word for iron or hundred steel because these admixtures exfoliate when oxidized i.e. the iron oxide, flakes off, constantly exposing the underlying admixturelic element to corrosion.Anodizing changes the texture of surface and also changes the crystal structure of the metal pricy the surface. Thick destinations are normally porous, so a shut process is often needed to achieve corrosion resistance. Anodized aluminum surfaces, for example, are starker than aluminum but earn low to moderate wear resistance that foot be improved with increasing thickness or by applying fitted sealing substances.Equations of the anode reactionsAl Al3+ + 3e- (2)2Al3+ + 3O2- Al2O3 (3)2Al3+ + 3OH- Al2O3 + 3H+ (4)For which the overall process is2Al + 3H2O Al2O3 + 6H+ + 6e- (5)The convert ions also act upon rough part as the oxide coating contains 12 15% sulfate ions. It is suggested that the sulfate ions facilitate the movement of hydrogen ions reducing the cell potentialitys required.THE suppuration OF THE ALUMINIUM OXIDE LAYERFresh aluminum reacts readily with oxygen to conjure aluminum oxide. one time formed the oxide resides firmly bonded to the surface forming an clayey class. Consequently, further reaction ceases. The film is genuinely thin (0.01m), and despite its continuity it atomic number 50 be removed by abrasion and chemic corrosion. In such(prenominal)(prenominal) instances the aluminum is subject to wear or the surface lead mark or become pitted at the site of corrosion.Anodizing hits much(prenominal) thicker coatings (12 25 m) which, if properly sealed, jackpot extend the life of the surface appreciably. young research in New Zealand has manifestn that pitting of the surface open fire be reduced by up to 90% with a 12 m coating, and by up to 93% with a 25 m coating.In the initial stages (i.e. first 60 s) of anodizing the oxide storey f ormed is dense and of even consistency. It provides the outstandingest resistance to wear and corrosion and consequently is called the barricade socio-economic class. The suppuration of this spirit level ceases when the high electrical resistance of the oxide reduces the potential of the applied voltage in the electrolytic cell. The depth of the coating at this stage is most 0.08m. Subsequent growth is very slow and competes with the acid reactionAl2O3 + 6H+ _ 2Al3+(aq) + 3H2Owhich releases Al3+ ions into the solution. neb that the H+ nominate be at high concentration estimable the oxide socio-economic class collect to cardinal of the anode reactions above. See equation (5).At low applied voltages only the barrier grade forms. However, the gradual production of Al3+ ions tends to fine-tune out the underlying metal surface and give a brightening set up to the article. Objects such as wheel trims and bumper bars are general treated in this way. At higher voltages the gr owth of the work continues beyond the barrier stage. Unlike the initial barrier layer this secondary layer, although constitutionally the alike(p), has an return pore-like structure a consequence of the competing anodizing and acid solution processes. Electron photomicrographs communicate the structure of these anodized surfaces to be as shown in Figure 2.The conditions required to dumbfound coatings vary according to the concentration and nature of the electrolyte, the voltage actual assiduousness applied, the alloy being anodized and the temperature of the bath. In the majority of electrolytic plants articles are anodized at a potential of 15 20 V and a live density around 1.6 A d-1 m-2 the electrolyte is 3.5 mol L-1 sulfuric acid hold at temperatures between 20 and 23oC. Under these conditions the quality of the coating is suitable for most applications. At higher electrolyte concentrations and temperatures, and at lower voltages or ongoing densities, the acid solut ion process occurs earlier in the development producing thin, break oxide coatings. Conversely, hard dense coatings are leavend at low temperatures and high current densities. The conditions established in each plant are determined by the type of application.PRE-TREATMENTStep 1 CleaningCorrect and adequate clean of the aluminum object prior to anodizing is essential if the finised work is to experience a uniform and attractive appearance. When aluminum arrives from the rolling, casting or gibbosity mills it may be soiled following wayscarbonic deposits from the surface of forgings and die castingsTraces of oil- d easilyingd lubricantsTraces of polishing compounds or sanding gritsOxide films from heat treatmentCleaning these soils from the surface may prove difficult, especially if the requirements of the work do not allow print of the surface. Most cleansing solutions used in New Zealand operations are detergent based. In addition to the detergent, a wetting agent and a com plexion compound may be used. The complexion compound frequently used is atomic number 11 polyphosphate a component of more soap formulations which prevents ions, such as Fe3+, adhering to the surface of the work. If etch is not a problem, atomic number 11 hydroxide or sodium carbonates (Na2CO3 / NaHCO3) may be added to increase the in force(p)ness of the solution.Step 2 EtchingEtching is most often achieved by the use of a impregnable, 10 % (2.5 mol L-1) sodium hydroxide solution. It gives the surface of the metal a light color in satin finish (through diffuse reflection of the incident light). The vast majority of work is pretreated in this way. In theory the reactions occurring in the etching solution areThe etching reaction2Al + 2NaOH + 2H2O 2NaAlO2 + 4H2 (7)Dissolution of the aluminatesNaAlO2 + H2O NaOH + Al (OH)3 (8)Dehydration of the solid hydroxide2Al(OH)3 Al2O3 + 3H2OThe rate of etching is dependent on the concentration of the sodium hydroxide solution, the tem perature and the concentration of aluminum ions which are released into the solution. When high concentrations of aluminum ions are act the solution loses its effectiveness. Presence of antithetical ions, some of which may be a component in the alloy, can also interfere in the process, do blemishes to appear on the surface of the work.The problem of ion contamination is overcome by employing etching solutions which suppress the action of the Al3+ and different metal ions released. The compositions of these solutions are the correctitude of the companies that develop them, but generally contain sequestering agents which complex metal ions. such solutions do not have an infinite capacity to do this but, due to the carryover of solution by the etched work and periodic replacement by fresh etch solution, the etching batch is maintained in an effective condition (Figure 3).Foaming agents are also a constituent of the etching solution their action is to reduce the pungent mists/fum es that allow from the prompt reactions that occur. It is important to note that the appearance of the end result is determined at this stage. Work which is poorly etched go forth reveal scratches or blemishes no matter how well it is anodized or colored.POST TREATMENTAfter cleaning and anodizing the work is colored and sealed. As all anodized work is sealed, sealing go away be considered first, although if coloring is to be done it is carried out prior to sealing. sealSealing is the process in which the pores at the surface of the oxide layer are closed off. It is affected by placing the anodized object in boiling wet for a 15 20 minute period. During that time the water reacts with the aluminum oxide to germinate the mineral Boehmite Al2O3.H2O or AlO.OHAl2O3 + H2O 2AlO.OHBoehmite is a hard, transparent square with a greater volume than the aluminum oxide. As it forms it closes off the openings of the pores.As would be expected, the dur superpower of the anodized surface , especially in regard to chemical corrosion, is greatly influenced by the effectiveness of the sealing. If the duration of the sealing is too short the pores, although constricted, remain open for corrosion agents to be inclose proximity to the aluminum surface. Corrosion studies have shown that anodized aluminum which has been sealed for 15 minutes or more has greatly increased resistance to pitting by chemical corrosion agents such as H+ and Cl-.ColouringColouring involves the assimilation of a coloured blotstuff into the pores of the oxide coating which becomes fixed afterward the sealing process has been completed. Dyestuffs which can bond to the oxide or metal ions in the anodized layer have better colour properties than those that do not.Electrolytic colouring is the most important method of colouring anodised aluminum today. It produces attractive finishes of very great colour and heat fastness and is easy to perform. The anodized work is parryped in a tank containing co loured ions of other metals.Under the influence of jump current the colouring agents deposit rapidly at the very base of the pores and the take is even over the entire surface. Unlike the process of sully absorption, electrolytic colouring is easy to control and gives uniformity of colour from one run to the next. The success of this technique is evident in the widespread use of bronze colored aluminum in joinery and house fittings. more or slight 66% of all bronze tinted aluminum is coloured by this technique.To a much lesser extent coloured inorganic compounds can be used to colour the work. Ammonium ferric oxalate is a very common compound used to impart a goldy colour to the metal. Other colours can be impacted by treating the absorbed ferric ammonium ion oxalate with other compounds for example, potassium ferrocyanide solution give react with the ferrioxalate compound to produce a blue colour. The technique used is to dip the work first off in a solution of the ammoniu m ferrioxalate followed by dipping the work in the potassium ferrocyanide solution. This double dipping technique can be used with other compounds to produce a variety of colours e.g. copper sulfate followed by ammonium sulfide gives green, and lead nitrate followed by potassium chromate gives yellow. prove on Anodizing of aluminumMethod onward the demonstrationLine the wrong of the sides of the 1 dm3 beaker with a double thickness of aluminum foil. Fill the beaker with sulfuric acid. This should be at somewhat 25 C discipline the temperature if necessary. go down up the electrical circuit shown in the figure. Make up the dye solution according to the instructions supplied (i.e. brush aside the contents of the tin in about 600 cm3 of water) and add a few cm3 of glacial ethanoic acid.The demonstrationDe-grease the aluminum level by rubbing with a tissue soaked in dimethyl ketone and and then dip the slick into a beaker full of propanone for short time and allowing drying. Fr om now on, hold the aluminum by the circus tent few cm only.Dip the bottom half of the aluminum take aparts into the sodium hydroxide solution in a beaker. expire it until it begins to effervesce, indicating that the surface layer of oxide has been removed. (This impart take about one minute.) Now remove the strip and dip the cleaned portion of it into the nitric acid for a few seconds to pine away the alkali. Then rinse away the acid with water. Clamp the strip so that the lower, cleaned, section is immersed in the sulfuric acid electrolyte and is in the centre of the piston chamber of aluminum foil which forms the cathode. It must not touch the cathode.Complete the circuit with crocodile clips making the aluminum strip positive and the foil negative. Now ad that the power pack and variable resistor so that current flows which give a current density of 10 -20 mA cm-2 of anode field of honor immersed. For example if the anode has an area of 3 cm x 3 cm immersed, the area bequ eath be 3 x 3 x 2 cm2 = 18 cm2 so the current should be between 180 and 360 mA (0.18 and 0.36 A). blank out to electrolyze for about 30 minutes, keeping an eye on the current and adjusting the rheostat if necessary to keep its value constant. (The current may tend to free fall as the oxide layer thickens.)When the electrolysis is complete, switch off the power and remove the aluminum strip. Rinse the strip in water. It exit not look very different at this stage. Now dip the strip into about two hundred cm3 of the dye solution in a beaker. Make sure that some of the non-anodized part of the strip is immersed as well as the treated section.Leave for about 15 minutes longer immersion will produce a deeper colour. Some of those who trialed this demonstration left the strip in the dye overnight. Rinse to remove any dye which has not been absorbed. Dye will only be absorbed by the anodized section, which will turn a deep cherry red. If desired, seal the dye by immersing the dyed strip for a few minutes in water that is already boiling. This will make the colour less prone to rubbing off, but will wash out some of it. Many teachers may prefer to vault this procedure.Visual tipsLarge demonstration meters will be easy to see. bulky connecting leads are useful to prevent the electrolysis tank becoming lost in a maze of wires. education tipsIt would be wise to prepare something to receive in the half hour of electrolysis time and the 15 minutes colour time. The class could be asked to calculate the expected increase in peck of the anode or to discuss the chemical reactions involved. Have a selection of anodised objects such as saucepan lids available for the class to look at. The demonstration (No. 18) of the reactivity of aluminium without its normal oxide layer could be shown. Some teachers may prefer to anodise some aluminium before the lesson to have some pieces ready to show the class. systemUntreated aluminium has a layer of oxide about 10-8 m thick. This explains its apparent lack of reactivity. Anodising, invented in 1923, is used commercially to thicken this layer to 10-5 m to improve the metals corrosion resistance.The relevant equations areCleaningAl2O3(s) + 2OH-(aq) + 3H2O(l) 2Al(OH)4-(aq)Once the oxide is removed2Al(s) + 2OH-(aq) + 6H2O(l) 2Al(OH)4 + 3H2(g)Electrolysis at the anode2Al(s) + 3H2O(l) Al2O3(s) + 6H+(aq) + 6e-Electrolysis at the cathode6H+ (aq) + 6e- 3H2(g)Electrolysis overall2Al(s) + 3H2O(l) Al2O3(s) + 3H2(g)The oxide coating develops a positive charge by the reactionAl2O3(s) + H2O(l) Al2O3H+(s) + OH-(aq).Thus it attracts dyes that contain coloured anions. These are absorbed in oxide layer which have pores, where they are trapped by oestrus the oxide to form an Al2O3.H2O seal.ExtensionsThere are a great many variables in this prove such as electrolysis time, voltage, current density, concentration of electrolyte, temperature of electrolyte, temperature of dyebath and type of dye. Investigations of some of these could form raise projects. It is possible to measure the gain in fold of the anode by rinsing the aluminium strip with propanone and slowness it today before and immediately after electrolysis.The Different of Anodizing And Electro coatElectroplating is a technique to plate some metal or non metal with metal use electric current. While anodizing is plate metal like aluminum by inserting some substance under the oxidized layer of aluminum by victimisation of electric current force. The function and the effect of both process may the same, like have color surface so that can use as decorative purposes.Not all metal can use anodizing process but only indisputable metal can use this method to make continue to corrosion effect. Aluminum metal can be treated by anodizing because after electric current process make the aluminium have oxidized layer on the surface and this layer can have wide pore so that other chemicals like dye or prevented agent to be impregnated to draw in th is oxidized layer. After plastered substance enter into this oxidized layer then by certain method the pore can be closed by further process. Actually by using just electric current will create anodized layer, but the problem this layer have no colour and look bad, to make this more interesting then on anodizing process using colour agent and inserted downstairs the anodized layer.Electroplating is directly plate other metal into certain metal with the culture to make more interesting or make more endure to corrosion or from other outside effect. Like on hard chrome plating on screw driver, can make this surface hard and not easy to break if not plate by hard chrome. By plate plating on other metal will make the surface is endure from reaction effect because nickel more noble than the metal below the nickel surface. Nickel plating usually use in canning process, or use in decorative accessories. Electroplating rely on the plate stick strong, more strong certain plating metal st ick to the base metal is better.ElectroplatingElectroplating is plating to certain metal with other metal that is usually more durable from corrosion or stronger than original metal. The process itself use of electrochemical by which metal is deposited on the origin metal through the chemical bath.Usually using 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. Metal on the solution form will turn to positive ion and on the electrochemical process this ion will attract to negative electrode or to anode and will plate anode. So metal that will be plated is placed on the anode position.Electroplating is done in a plating bath which is usually a non-metallic tank like plastic or glass. The tank is filled by metal solution, which the metal pattern will plate the anode. The anode is substrate to be plated which is connected to the nega tive terminal of the power supply.As we apply current, positive metal ions from the solution move towards anode with negative charge and deposit on anode. As a replenishment of these deposit ions, the metal from cathode will dissolve and goes into the solution and make the ionic potential balance.In the case using of noble metal like gold as cathode it is not sacrificial, but it is made out of materials which do not dissolve in an electrolyte such as titanium.Electrochemical TheoryActually electroplating is based on the Faradays Laws that kingdom as followsThe weight of a substance formed at an electrode is comparative to the amount of current passed through the cell. The weights of different substances produced at an electrode by the same amount of current are proportional to their equivalent weights.Corresponding mass in an oxidation-reduction reaction is = molar weight of the compound / algebraical change in oxidation number of the atom that is oxidized or reduced.2 FeCl2 + Cl2 2 FeCl3Fe valence is change on the reaction from +2 to +3. However on the reaction willMn + FeSO4 Fe + MnSO4Chrome PlatingChrome plating is a part of coating technology that use of electroplating process. Electroplating process itself can use many kind of metal like chrom, copper, nickel, silver or gold. The process of all metal is similar, that is use of DC electric current and using of metal solution. exclusively the differences of all the process generally on the solution uses, concentration uses, current flow, temperature limitation, plating time and many other limitation that difference of each metal coating.Chromium plating is also different among the result purposes from the current flow, chromium concentration and other chemical additive that is use on each of chromium plating kind. The chromium plating such as hard chrome, dull chrom, black chrome and bright chrome. Every result goal needs different parameter uses that must be followed in order to get the result as the process goal. So chromium plating process needs strict control on the bath concentration, equipment precision and on the method uses. Any small substance contain on chromium bath can result a different kind of product.Anodizing aluminumThis is an try out showing interesting application of electrolysis. The oxide layer on aluminum foil is made thicker by anodizing which improves the metals corrosion resistance. In the process, the thickened oxide surface coating is coloured by using dyes.Lesson governmentThis works well as a class demonstration, but there are several tasks to complete in preparation. The anodising process itself takes about 30-40 minutes, with vigor particularly dramatic happening, so you will need to plan other activities to fill the time.At the start of the experiment, show the students the effervescence due to the hydrogen evolved from the cylindrical aluminum cathode. A flexi camera connected to a projector could be used here.During the anodizing phase, the t heory could be explained with an emphasis on the applications of the process. A ingathering of anodized objects such as saucepan lids or sports equipment could be available to look at.A well-disciplined and organised class might be able to carry out this process for themselves (in twos or threes), but it is strongly recommended that the treatment with sodium hydroxide solution (Corrosive) prior to the electrolysis is carried out under strict supervision.Apparatus and chemicals tenderness protection,Low-voltage DC power pack, adjustable up to 10 volts, Connecting leads and 4 crocodile clips study clips, plastic Test-tube holder, woodwinden, Paper tissues Strip of wood, 15 cm long swayer (30 cm), Beaker (1 dm3),Beakers (250 cm3), 3Aluminum foil, virtually 40 cm x 15 cm, congou tearing dye (Toxic) Ethanol (Highly flammable, Harmful) Sulfuric acid approximately 2 mol dm-3 (Corrosive), 1 dm3, sodium hydroxide, approximately 1.5 mol dm-3 (Corrosive), 250 cm3. Propanone (acetone) (H ighly flammable, Irritant)Technical notesCongo cherry dye (Toxic). Refer to CLEAPSS Hazcard 32. Ethanol (Highly flammable, Harmful). Refer to CLEAPSS Hazcard 40 (2007 40A) Sulfuric acid approximately 2 mol dm-3 (Corrosive). Refer to CLEAPSS Hazcard 98 (2007 98A Sodium hydroxide, approximately 1.5 mol dm-3 (Corrosive). Refer to CLEAPSS Hazcard 91.Propanone (acetone) (Highly flammable, Irritant).1 Reasonably thick aluminium foil should be used, but, if unavailable, ordinary kitchen foil works quite well.2 Propanone is needed to degrease the aluminium foil and it is worth keeping a bottle specifically for this purpose. The used propanone can be poured patronize into the bottle and kept for future use. This reduces waste disposal requirements.3 The solid Congo Red dye needs to be made up into solution. Use 0.5 g of dye, 50 cm3 of ethanol and 50 cm3 of water and warm to dissolve. Dylon bleak fabric dye (Camilla A 16) also gives good results. Red fountain-pen ink can be used as an alte rnative but does not give such good results.4 Instead of a power pack, a battery or series of batteries could be used. subroutineHEALTH SAFETY Eye protection must be worn.Before the demonstrationa) Cut two pieces of aluminum foil, one 10 cm x 3 cm (the anode), the other about 30-35 cm x 12 cm (the cathode). Ensure that when the foil is folded into a cylinder it fits inside the beaker as shown below.Anodising aluminiumb) Work in a fume press and ensure that there are no flames close by. Work on a clean surface. Degrease the two pieces of foil by rubbing well with a paper tissue soaked in propanone and then dip the strips into a beaker of propanone for a few seconds.c) Remove the strips of aluminum from the propanone and allow drying. From this point on, only hold the aluminum foils at the top borderlines.d) Arrange the larger piece of aluminum into a cylinder. guarantee it in position with plastic paper clips and then place inside the large beaker as shown in the diagram.e) Set up the strip of wood on the beaker and use Bluetak to attach two crocodile clips, one at the edge and one in the centre. Attach the outer clip to the aluminum cylinder. This is the cathode.f) bombard some of the cold sodium hydroxide solution (Corrosive) into a 250 cm3 beaker. arrest the smaller piece of aluminum foil with a wooden test-tube holder, and dip it into the sodium hydroxide solution. After a short while, hydrogen gas will be given off rapidly. Remove the strip after a few seconds of fizzing, and wash it in a stream of cold running water.g) Attach the aluminum strip to the central crocodile clip ensuring that it is place vertically (see diagram). This central strip (the anode) must not touch the aluminum cylinder.h) Carefully fill the beaker with the sulfuric acid from a measuring cylinder up to a level about 1 cm below the top of the aluminum cylinder.SAFETY Remember that hydrogen (Highly flammable) will be evolved during the electrolysis. Keep all naked flames well a way from the experiment (e.g. when heating the dye solution).Demonstrationa) Connect up the circuit and use a voltage of 5-10 volt. Electrolysis is occurring when bubbling can be seen at the cathode (hydrogen). Pass a current for about 20 minutes, or longer, if time permits.b) While the electrolysis is running, heat the dye solution in a beaker to about 70C. An electric hotplate is preferable to a Bunsen burner. An additional beaker of boiling water will also be needed.c) Remove the central aluminum strip (the anode) and place it in the hot dye solution. Stir and leave for about 10-15 minutes.d) Transfer the aluminum anode to a beaker of boiling water and leave for other 10 minutes. This seals the dye onto the anodized surface of the aluminum and makes the aluminum oxide layer less porous.e) The upper non-anodized portion of the strip should be the original metallic grey colour whilst the rest should be coloured red. The aluminum strip can be dried in paper tissue and passed round the class. It should not be possible to rub off the dye off the surface.Teaching notesThe demonstration itselfThe instructions may seem very detailed, but companionship shows that success depends on getting the conditions just right. You should try out the experiment before carrying it out as a demonstration. It would be useful to have some sample strips of anodized aluminium to pass round.The voltage will spew during the experiment, since the anode is becoming increasingly coated with aluminium oxide. If a rheostat and voltmeter are used, the readings can be constantly monitored and adjustments made to keep the voltage approximately constant.A longer immersion in the dye will produce a strip with a deeper red colour. Leaving the strip in the dye overnight produces the best results.If time is short, omit the dye-sealing stage in boiling water.If there is time, a piece of the cathode could also be immersed in the dye. It will be found that the dye is not taken up by the metal in th e same way.This is a good experiment to show students towards the end of their study of electrolysis.Chemistry pointsWhen a piece of aluminum is exposed to the air, it rapidly becomes coated with a protective surface layer of aluminum oxide.Heating the aluminum in air can make the oxide layer thicker, but anodising is much more effective.The oxide layer can be made to absorb dyes. This is useful in a range of cursory goods, such as kettles, window frames and some sports equipment, all of which need to be able to withstand extreme physical conditions.Untreated aluminium has an oxide layer about 10-8 m thick. This explains aluminiums apparent lack of reactivity in the laboratory. Anodising thickens this layer to about 10-5 m and dramatically improves the metals corrosion resistance.Oxygen is often evolved at the anode during the electrolysis of aqueous solutions. Aluminium is a reactive metal. The oxygen formed reacts immediately with the aluminium. It forms a solid oxide coating on the surface of the metal electrode.Theory for more able studentsFor students working at a higher ability level, some or all of the following equations and explanations could be introducedThe cleaning process with NaOH(1) Al2O3(s) + 2NaOH(aq) + 3H2O(l) 2NaAl