Factors Affecting The Anodizing Of An Aluminum Metal

Factors Affecting The Anodizing Of An Aluminum MetalAnodizing is a procedure for producing decorative and protective films on articles do from atomic number 13 and its alloys. It is essentially a work out(p) where a two-ply film of aluminium oxide is built up on the place of the aluminum through the routine of a direct menses galvanising supply. In the majority of anodizing imagets in New Zealand it is carried out in an electrolyte bath containing sulfuric sour with aluminum sheet cathodes and the usage to be anodized affiliated to the anode (Figure 1).When the current is flowing in the cell the following sequence of events is believed to occur. Sulfuric acid begins to decompose, the hydrogen ions moving to the cathode where they be reduced to hydrogen gas2H+ + 2e- H2(g) (1)Simultaneously, negatively charged anions, i.e. hydrated oxide, sulphate and maybe oxide ions move to the anode. The electrical charge in the circuit causes demonstrablely charged aluminum i ons (Al3+) to be generated in the anode and in rick move toward the cathode. At the anode surface they answer 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 types of ions touch in any Electrolyte. The replys take place and Anions and Cations atomic number 18 take and transferred to the opposite completes of an Electrolyte. Anions are positively charged ions and Cations are negatively charged ions in an electrolyte.In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible. Ion is a particle which is electrically charged either positive or negative an atom or molecule or group that has lost or gained mavin or much electrons.An ion is an atom or molecule where the total number of electrons is non equal to the total number 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 condenser passing to increase the thickness of the natural oxide stratum of the surface of various surface parts. This process is called anodizing because the part which to be hard-boiled forms an anode electrode of the electrical circuit. Anodizing mainly increases corrosion resistance and provides better bond for paint primers and glues than bare surface. Anodizing is even used to prevent galling of threaded compvirtuosonts and to make dielectric films for electrolytic capacitors. Anodic films are most commonly applied to protect aluminum alloys, although processes also exist for titanium, zinc, magnesium, niobium, and tantalum. This process is not a useful treatment f or iron out or carbon steel because these coats exfoliate when oxidized i.e. the iron oxide, flakes off, constantly exposing the underlying metal to corrosion.Anodizing changes the texture of surface and also changes the crystal structure of the metal near the surface. Thick coatings are normally porous, so a sealing process is often needed to achieve corrosion resistance. Anodized aluminum surfaces, for example, are harder than aluminum but eat up low to moderate wear resistance that cornerstone be mitigated with increasing thickness or by applying suitable 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 sulfate ions also play some part as the oxide coating contains 12 15% sulfate ions. It is suggested that the sulfate ions serve the movement of hydrogen ions reducing the cell voltages required.THE DEVELOPMENT OF THE ALUMINIUM OXIDE L AYERFresh aluminum reacts readily with oxygen to produce aluminum oxide. Once formed the oxide remains firmly bonded to the surface forming an impenetrable forge. Consequently, further reaction ceases. The film is real thin (0.01m), and despite its tenacity it arsehole be learnd by abrasion and chemical corrosion. In much(prenominal) instances the aluminum is subject to wear or the surface leave behind mark or become pitted at the site of corrosion.Anodizing produces much thicker coatings (12 25 m) which, if properly sealed, tush extend the life of the surface appreciably. Recent research in New Zealand has shown that pitting 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 stages (i.e. first 60 s) of anodizing the oxide bed formed is dense and of even consistency. It provides the greatest resistance to wear and corrosion and consequently is called the barrier stratum. The crop of this layer 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 active 0.08m. Subsequent growth is very slow and competes with the acid reactionAl2O3 + 6H+ _ 2Al3+(aq) + 3H2Owhich releases Al3+ ions into the solution. Note that the H+ can be at high concentration near the oxide layer due to one of the anode reactions above. See equation (5).At low applied voltages only the barrier layer forms. However, the gradual production of Al3+ ions tends to smooth out the underlying metal surface and give a brightening effect to the article. Objects such as wheel trims and bumper bars are general treated in this way. At higher voltages the growth of the layer continues beyond the barrier layer. Unlike the initial barrier layer this secondary layer, although constitutionally the same, has an open pore-like structure a consequence of the competing anodizing and acid solution processes. negatron photomicro graphs reveal the structure of these anodized surfaces to be as shown in Figure 2.The conditions required to produce coatings vary according to the concentration and nature of the electrolyte, the voltage current niggardness 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 current parsimony around 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 voltages or current densities, the acid solution process occurs earlier in the maturatement producing thin, open oxide coatings. Conversely, hard dense coatings are produced at low temperatures and high current densities. The conditions established in each plant are determined by the type of application.PRE- wordStep 1 cleanup blotCorrect and adequate cleaning of the aluminum object front to anodizing is essential if the finished work is to have a uniform and attractive appearance. When aluminum arrives from the rolling, cast or extrusion mills it may be soiled following waysCarbonaceous deposits from the surface of forgings and die castingsTraces of oil-based lubricantsTraces of polishing compounds or sanding gritsOxide films from rut treatmentCleaning these soils from the surface may prove difficult, especially if the requirements of the work do not allow etching of the surface. Most cleaning solutions used in New Zealand operations are detergent based. In addition to the detergent, a wetting agent and a touch compound may be used. The complexion compound frequently used is sodium polyphosphate a component of many soap formulations which prevents ions, such as Fe3+, adhering to the surface of the work. If etching is not a problem, sodium hydroxide or sodium carbonates (Na2CO3 / NaHCO3 ) may be added to increase the dominance of the solution.Step 2 EtchingEtching is most often achieved by the use of a warm, 10 % (2.5 mol L-1) sodium hydroxide solution. It gives the surface of the metal a light grey satin finish (through diffuse reflection of the incident light). The considerable 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) drying up of the solid hydroxide2Al(OH)3 Al2O3 + 3H2OThe rate of etching is dependent on the concentration of the sodium hydroxide solution, the temperature and the concentration of aluminum ions which are released into the solution. When high concentrations of aluminum ions are present the solution loses its effectiveness. Presence of other ions, some of which may be a component in the alloy, can also interfere in the process, causation 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 other metal ions released. The compositions of these solutions are the propriety 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/ feels that result from the vigorous 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 will reveal scratches or blemishes no matter how tumesce it is anodized or disconsolate.POST TREATMENTAfter cleaning and anodizing the work is colored an d sealed. As all anodized work is sealed, sealing will be considered first, although if coloring is to be done it is carried out precedent to sealing.SealingSealing 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 weewee for a 15 20 minute period. During that time the water reacts with the aluminum oxide to produce the mineral Boehmite Al2O3.H2O or AlO.OHAl2O3 + H2O 2AlO.OHBoehmite is a hard, transparent material with a greater volume than the aluminum oxide. As it forms it closes off the openings of the pores.As would be expected, the durability 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 suddenly the pores, although constricted, remain open for corrosion agents to be inclose proximity to the aluminum surface. Corrosion studies have shown that anodized aluminum whic h 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 absorption of a coloured dye into the pores of the oxide coating which becomes fixed after 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 dipped in a tank containing coloured ions of other metals.Under the influence of change 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 dye absorption, electrolytic colouring is easy to control and gives uniformity of colour from one run to the next. The success of this t echnique is evident in the widespread use of bronze colored aluminum in joinery and house fittings. Approximately 66% of all bronze tinted aluminum is coloured by this technique.To a much slighter ending 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 oxalate with other compounds for example, potassium ferrocyanide solution will react with the ferrioxalate compound to produce a blue colour. The technique used is to dip the work firstly in a solution of the ammonium 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.Experiment on Anodizing of aluminumMethodBefor e the materialisationLine the inside 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 almost 25 C adjust the temperature if necessary. Set up the electrical circuit shown in the figure. Make up the dye solution according to the instructions supplied (i.e. dissolve 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 leach by rubbing with a tissue soaked in propanone and then dip the strip into a beaker full of propanone for short time and allowing drying. From now on, hold the aluminum by the pop off few cm only.Dip the bottom half(a) of the aluminum strips into the sodium hydroxide solution in a beaker. give it until it begins to effervesce, indicating that the surface layer of oxide has been removed. (This will take about one minute.) Now remove the strip and dip the cleaned portion of it into the nitric acid for a few seconds to neutralize 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 cylinder of aluminum foil which forms the cathode. It must not touch the cathode.Complete the circuit with crocodile clips qualification the aluminum strip positive and the foil negative. Now adjust the advocate pack and rheostat so that current flows which give a current density of 10 -20 mA cm-2 of anode area immersed. For example if the anode has an area of 3 cm x 3 cm immersed, the area will 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).Leave to electrolyze for about 30 minutes, keeping an c come out on the current and adjusting the rheostat if necessary to keep its value constant. (The current may tend to drop as the oxide layer thickens.)When the electrolysis is complete, switch off the male monarch and remove the aluminum strip. rinsing the strip in water. It will not look very different at this stage. Now dip the strip into about 200 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 rubicund 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 dribble this procedure.Visual tipsLarge demonstration meters will be easy to see. Long connecting leads are useful to prevent the electrolysis tank becoming lost in a maze of wires.Teaching tipsIt would be advised to prepare somethin g to fill in the half hour of electrolysis time and the 15 minutes dyeing time. The mob could be asked to calculate the expected increase in mass 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. theoryUntreated aluminium has a layer of oxide about 10-8 m thick. This explains its unmistakable 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 heating the oxide to form an Al2O3.H2O seal.ExtensionsThere are a great many variables in this experiment 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 enkindle projects. It is possible to stride the gain in mass of the anode by rinsing the aluminium strip with propanone and weighing it immediately before and immediately after electrolysis.The Different of Anodizing And Electro platingElectroplating is a technique to plate some metal or non metal with metal employ electric current. While anodizing is plate metal like aluminum by inserting some substance under the oxidized layer of aluminum by victimization 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 certain metal can use this method to make endure 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 enter this oxidized layer. After certain substance enter into this oxidized layer then by certain method the pore can be closed by further process. Actually by employ 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 infra the anodized layer.Electroplating is d irectly plate other metal into certain metal with the goal 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 stick to the base metal is better.ElectroplatingElectroplating is plating to certain metal with other metal that is usually more constant 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 credit card or glass. The tank is filled by metal solution, which the metal kind will plate the anode. The anode is substrate to be plated which is connected to the negative 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 plead as followsThe weight of a substance formed at an electrode is proportional to the bar 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 / algebraic 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. But the passings 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 experiment 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 organizationThis 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 nothing 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 theory could be explained with an emphasis on the applications of the process. A collection of anodized objects such as saucepan lids or sports equipment could be available to look at.A well-disciplined and coordinate 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 chemicalsEye security measure,Low-voltage DC power pack, adjustable up to 10 volts, Connecting leads and 4 crocodile clips Paper clips, plastic Test-tube holder, wooden, Paper tissues strip show of wood, 15 cm long Ruler (30 cm), Beaker (1 dm3),Beakers (250 cm3), 3Aluminum foil, approximately 40 cm x 15 cm, congou tea Red 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) (Highly flammable, Irritant)Technical notesCongo Red dye (Toxic). bear on 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 alum inium 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 back 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 cold model dye (Camilla A 16) also gives good results. Red fountain-pen ink can be used as an alternative but does not give such good results.4 Instead of a power pack, a battery or series of batteries could be used.ProcedureHEALTH 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 i n a fume cupboard 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 edges.d) Arrange the larger piece of aluminum into a cylinder. Fix 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 wed 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) Pour some of the cold sodium hydroxide solution (Corrosive) into a 250 cm3 beaker. Hold 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 waterway of cold running water.g) Attach the aluminum strip to the central crocodile clip ensuring that it is arranged 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 away from the experiment (e.g. when heating the dye solution). manifestationa) 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 etna 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. boot and leave for about 10-15 minutes.d) Transfer the aluminum anode to a beaker of boiling water and leave for another 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 experience 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 drop during the experiment, since the a node is becoming increasingly surface 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 the 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 everyday goods, such as kettles, wi ndow 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 antiphonal 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