CONVERSION COATING AND PROCESS AND SOLUTION FOR ITS FORMATION

Abstract: A process for forming a conversion coating on the surface of a metal, including: contacting the metal with an acidic solution containing an oxidant in order to initiate growth of a metal oxide cell structure on the metal surface; contacting the metal with water for a period of time sufficient to thicken the oxide and form a metal oxide containing layer of a desired thickness; and treating the metal with one or more rare earth elements in order to impregnate and substantially seal the metal oxide.

Description: CONVERSION COATING AND PROCESS AND SOLUTION FOR ITS FORMATION FIELD OF THE INVENTION This invention relates to a conversion coating for metal surfaces and a process and a solution for forming a conversion coating on metal surfaces. The invention is particularly concerned with a conversion coating on aluminium or aluminium alloy and a process and a solution for the formation of a conversion coating on aluminium or aluminium alloy. BACKGROUND OF THE INVENTION The term "conversion coating" is a well known term of the art and refers to the replacement of native oxide on the surface of a metal by the controlled chemical formation of a film. Oxides or phosphates are common conversion coatings. Conversion coatings are used on metals such as aluminium, steel, zinc, cadmium or magnesium and their alloys, and provide a key for paint adhesion and/or corrosion protection of the substrate metal. Accordingly, conversion coatings find application in such areas as the aerospace, automotive, architectural, can stock, instrument and building industries. Known methods for applying conversion coatings to metal surfaces include treatment with chromate or phosphate solutions, or mixtures thereof. However, in recent years it has been recognised that the hexavalent chromium ion, Cr6 + , is a serious environmental and health hazard. Similarly, phosphate ions are a serious risk, particularly when they find their way into natural waterways and cause algal blooms. Consequently, strict restrictions have been placed on the quantity of these species used in a number of industrial processes and limitations have been placed on their release to the environment. This leads to costly effluent processing. In the search for alternative, less toxic conversion coatings, research has been conducted on conversion coatings based on rare earth compounds. However, there is considerable room for improvement in the adhesion and corrosion protection properties of prior rare earth element based conversion coatings and in the time required to deposit those coatings. Accordingly, it is an object of the present invention to provide a conversion coating for a metal surface and a process for forming a conversion coating on a metal surface which overcome, or at least alleviate, one or more of the disadvantages or deficiencies of the prior art. It is also an object of the present invention to provide an aqueous, rare earth element containing solution for use in providing a conversion coating on a metal surface. SUMMARY OF THE INVENTION According to the present invention, there is provided a process for forming a conversion coating on the surface of a metal, including: contacting the metal with an acidic solution containing an oxidant in order to initiate growth of a metal oxide cell structure on the metal surface; contacting the metal with water for a period of time sufficient to thicken the oxide and form an oxide containing layer of a desired thickness; and treating the metal with rare earth elements in order to impregnate and substantially seal the oxide containing layer. The present invention also provides a conversion coated metal, wherein the conversion coating comprises a metal oxide containing layer which is impregnated with one or more rare earth elements. The present invention further provides an aqueous, rare earth element containing solution for use in providing a conversion coating on a metal, said solution including a sufficient quantity of a rare earth element to impregnate and substantially seal a metal oxide containing layer formed on the surface of said metal. The invention will now be described with particular reference to its use for aluminium or aluminium containing alloys. However, a skilled addressee will understand that the invention is not limited to this use and can be used in relation to other metals, such as zinc. It may be appropriate for the process of the present invention to be preceded by the steps of degreasing and/or cleaning and deoxidising/desmutting the metal surface. The degreasing step, if present, comprises treatment of the metal surface with any suitable degreasing solution to remove any oils or grease (such as lanoline) or plastic coating present on the metal surface. The degreasing step, if present, preferably comprises treating the metal surface with a vapour degreasing agent such as trichloroethane or an aqueous degreasing solution available under the trade name of BRULIN. A degreasing step may be necessary, for example, where the metal has been previously coated with lanoline or other oils or grease or with a plastic coating. Subsequent to the degreasing step, the metal surface preferably undergoes a cleaning step in order to dissolve contaminants and impurities, such as oxides, from the surface of the metal. Preferably, the cleaning step comprises treatment with an alkaline based solution. The alkaline solution is preferably a "non-etch" solution, that is, one for which the rate of etching of material from the metal surface is low. A suitable alkaline cleaning solution is that commercially available under the trade name RIDOLINE 53. The treatment with an alkaline cleaning solution is preferably conducted at an elevated temperature, such as up to 80°C, preferably up to 70°C. Treatment with an alkaline solution often leaves a "smut" on the surface of the metal. As used herein, "smut" is intended to include impurities, oxides and any loosely-bound intermetallic particles which as a result of the alkaline treatment are no longer incorporated into the matrix of the aluminium alloy. It is therefore preferable to treat the metal surface with a "desmutting" or "deoxidizing" solution in order to remove the smut from the metal surface. (Throughout this specification, the terms "desmutting" and "deoxidizing" are used interchangeably). Removal of smut is normally effected by treatment with a desmutting (deoxidizing) solution comprising an acidic solution having effective amounts of appropriate additives. Preferably the desmutting solution also dissolves native oxide from the surface of the metal to leave a homogeneously thin oxide on the metal surface. The desmutting solution may be chromate-based, which due to the presence of Cr6 + ions, presents environmental and health risks. Alternatively the desmutting solution may be one which contains rare earth elements such as the desmutting solution disclosed in co-pending PCT Patent Application No. WO 95/08008, the entire disclosure of which is incorporated herein by reference. Treatment with rare earth containing desmutting solutions lessens the risk to the environment and health and results in improvement in coating time and corrosion performance of subsequently applied conversion coatings. The rare earth element of the desmutting solution preferably should possess more than one higher valence state. By "higher valence state" is meant a valence state above zero valency. Without wishing to be limited to one particular mechanism of smut removal, it is believed that the multiple valence states of the rare earth element imparts a redox function enabling the rare earth element to oxidise surface impurities and result in their removal as ions into solution. Such rare earth elements include cerium, praseodymium, neodymium, samarium, europium, terbium and ytterbium. The preferred rare earth elements are cerium and/or praseodymium and/or a mixture of rare earth elements. Preferably, the rare earth compound is cerium (IV) hydroxide, cerium (IV) sulphate, or ammonium cerium (IV) sulphate. The mineral acid is preferably a mixture of sulphuric acid and nitric acid with F " ions. The pH of the rare earth containing desmutting solution is preferably less than 1. In one embodiment of the invention, the desmutting solution is combined with the acidic, oxidant containing solution used in the process of the present invention to give a single solution which acts both to desmut and to initiate growth of aluminium oxide on the metal surface. In another embodiment of the present invention, there is provided a process for forming a conversion coating on the surface of a metal including the steps of: (a) contacting the metal with an acidic solution containing an oxidant in order to initiate growth of a metal oxide cell structure on the metal surface; (b) contacting the metal with water for a period of time sufficient to thicken the oxide and form an oxide containing layer of a desired thickness; (c) contacting the metal with an aqueous, rare earth element containing solution in order to impregnate and substantially seal the oxide containing layer. While the rare earth impregnated aluminium oxide layer formed by the process of the invention has good corrosion properties and provides a good base for any subsequently applied coatings, such as paint, it is preferred that the process of the invention includes a further step comprising treatment with a sealing solution. The rare earth impregnated coating may be sealed by treatment with one of a variety of aqueous or non-aqueous inorganic, organic or mixed organic/inorganic sealing solutions. The sealing solution initially penetrates the semi-porous structure then subsequently forms a surface layer on the rare earth containing coating and may further enhance the corrosion resistance of the rare earth containing coating. Preferably the coating is sealed by an alkali metal silicate solution, such as a potassium silicate solution. Examples of potassium silicate solutions which may be used are those commercially available under the trade names "PQ Kasil #2236" and "PQ Kasil #1". Alternatively, the alkali metal sealing solution may be sodium based, such as a sodium silicate or a sodium orthophosphate, or a mixture thereof. Preferably, each step of the process of the present invention is followed by a water rinsing step. DESCRIPTION OF DRAWINGS The invention will become more readily apparent from the following exemplary description in connection with the accompanying drawings and Examples. FIG. 1 is a photomicrograph of the rare earth impregnated aluminium oxide coating at the completion of step 6 of Example 3. FIG. 2 is another photomicrograph of the rare earth impregnated aluminium oxide coating at the completion of step 6 of Example 3. FIG. 3 is an X-ray photoelectron spectroscopy depth profile of the silicate- sealed coating of Example 3, at the completion of step 7. FIGS. 4 & 5 are photomicrographs of the silicate-sealed coating of Example 3 at the completion of step 7. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The process of the present invention includes the step of initiation of growth on the surface of the aluminium or aluminium alloy of an aluminium oxide, or hydrated aluminium oxide. The term "aluminium oxide" shall be herein used to refer to the compounds aluminium oxide, hydrated aluminium oxide or aluminium hydroxide either singly or in any combination thereof. The surface of the metal is treated with a suitable solution in order to initiate aluminium oxide growth and to form a thin oxide layer on the metal surface. The thin oxide layer may be up to 10 nanometres thick. Oxide growth is initiated by immersing the metal in an acidic solution containing an effective quantity of an oxidant. The acidic solution may be selected from one or more of nitric acid, phosphoric acid and sulphuric acid. Preferably, the acid is nitric acid. If present, the nitric acid preferably has a concentration of up to 1.6M. Suitable oxidants include (metal) perchlorates, (metal) bromates, (metal) persulphates, nitrates, hydrogen peroxide and ammonium cerium (IV) nitrate. Preferably, the oxidant is NaBrθ3. The solution may therefore contain nitric acid plus another acid. Nitric acid provides both an acidic and an oxidant function. The preferred acidic oxidant-containing solution contains nitric acid and NaBrθ3- Without wishing to limit the invention to a particular mechanism of oxidation, it is believed that part of the role of the acidic, oxidant containing solution is to react with and remove intermetallic particles from the metal surface. If a halogen is present in the oxidant (such as in NaBrθ3) it may assist in removal of the oxide on the metal surface. The oxidant may be present in solution up to its solubility limit. However, for most applications the oxidant is present at lower concentrations. A suitable maximum concentration is 10 wt%. The lower limit of oxidant concentration may be 0.01wt%. The pH of the acidic, oxidant-containing solution will vary according to the nature of the oxidant and the other species added to the solution. The pH of the acidic solution is preferably 5.5 or below, such as less than 5.0. In some embodiments, the pH is 4.5 and below and may be less than 4.0. In other embodiments of the invention, the pH is below 3.5. In a preferred embodiment, in the presence of HNO3. the acidic solution has a pH of below 1.0. A pH of less than 0.5 is preferred when the solution contains the oxidant NaBrθ3 and HNO3. The low pH arises from use of HNO3 in the NaBrθ3 solution. However other nitrates may partially or completely replace the HNO3, resulting in variation of solution pH. It is thought that a low pH may be required in order to dissolve oxide formed by the oxidant and thereby set up an equilibrium whereby only a thin oxide layer is formed. However, the exact reason for the need for a thin oxide layer to form is not known. The temperature of the acidic, oxidant-containing solution may be any value up to the boiling point of the solution. The lower limit on solution temperature may be ambient temperature, such as from 10°C to 30°C. In some embodiments of the invention, the temperature of the acidic solution is 20°C. Other embodiments of the process are conducted at a temperature higher than 20°C, such as up to 50°C. Alternatively, the solution temperature may be 25°C or higher, and in some embodiments may be as high as 40°C. The metal surface is treated with the acidic, oxidant-containing solution for a period of time sufficient to initiate growth of aluminium oxide on the metal surface to the desired degree. Treatment time may be up to one hour. However advantageously, it is 30 minutes or less, such as up to 20 minutes. In some embodiments, treatment with the acidic solution is conducted for up to 15 minutes and may be 10 minutes or less. In preferred embodiments, the treatment time is up to 7 minutes. In one preferred embodiment, the acidic, oxidant containing solution comprises a 3% (0.2 molar) metal bromate solution containing 7% (1.1 molar) nitric acid having a pH less than 0.5. Treatment with the acidic solution is conducted at 25-40°C for 7 minutes. It is followed by a rinse in water. The process of the present invention further includes thickening the aluminium oxide layer by treatment with water. A continuous layer comprising a porous cellular structure is formed, typically a columnar structure. The treatment water is preferably distilled and/or deionised. However it may advantageously include particular additives. Preferably, the water has a low Si content, such as less than 0.05 ppm, or is Si free, because high Si content has been found to adversely affect oxide thickening. It is preferred that the pH of the treatment water is around neutral, such as from 4 to 7, since the dissolution rate of the oxide layer is minimal in this range. It is also preferred that the water has low halide concentrations or is halide free. Similarly, low or zero sulphate concentration is preferred. A surfactant may be included in the water, in an effective amount, in order to lower the surface tension of the solution. By lowering surface tension of the solution in the pores of the aluminium oxide layer as it is forming, the amount of cracking of the layer may be minimised. The surfactant may be cationic, anionic or non-ionic. Inclusion of a surfactant is further beneficial in that by reducing surface tension of the coating solution, it thereby minimises "drag-out" from the solution. "Drag-out" is an excess portion of coating solution which adheres to the metal and is removed from solution with the metal and subsequently lost. Accordingly, there is less waste and costs are minimised by adding surfactant to the coating solution. The surfactant may be present in solution at a concentration up to 0.02weight%, such as up to 0.015weight%. Preferably, the concentration is around 0.01weight% or lower. The lower limit on surfactant concentration may be around 0.001 weight%. However, in some embodiments, the lower concentration limit is 0.005weight% whilst in others it is higher, such as 0.0075weight%. An example of a suitable surfactant is that available under the trade name FLUORAD "FC-135", which is a cationic fluorochemical surfactant. A water treatment solution containing FC-135 has a pH of around 5.5. Other substances which may advantageously be added to the water used for oxide thickening include nitrate compounds such as potassium nitrate (KNO3), cerium (III) sulphate octahydrate (Cβ2(Sθ4)3.8H2θ), solutions of ammonia and its salts, such as NH4OAC, NH4NO3, (NH4)2Cθ3, NH4OH and sodium carbonate Na2Cθ3. The temperature of the water used for oxide thickening may be up to the boiling point of the treatment solution (that is, 100°C for pure water), such as up to 95°C. Preferably, the temperature of treatment is up to 90°C. The lower limit of water temperature may be 70°C. In some embodiments of the invention, the temperature of the water treatment step is between 85 and 90°C. While the temperature of the water treatment solution may be less than 85°C, aluminium oxide growth is slower, and is even slower below 70°C. The metal surface may be treated with the water for up to 60 minutes, such as up to 35 minutes. In some embodiments, the maximum treatment time is 20 minutes. The minimum treatment time may be as low as 2 1/2 minutes. However, in some embodiments, the treatment time is greater than three minutes. Alternatively, the treatment time may be 5 minutes or more. The present invention also includes the step of contacting the metal with rare earth elements in order to impregnate and substantially seal the oxide containing layer. The rare earth element is generally provided in the form of ions in an aqueous solution. The rare earth ion may possess more than one higher valence state. By "higher valence state" is meant a valence state above zero. If the rare earth ion does possess more than one higher valence state, the rare earth ion is added to the solution in a lower valence state. Such rare earth elements include cerium, praseodymium, neodymium, samarium, europium, terbium, thulium, lutetium and ytterbium. Preferably, the rare earth element is cerium and/or praseodymium and/or a mixture of rare earth elements. The exact mechanism of sealing the oxide layer by treatment with rare earth elements is not known. However, while not intending to be limited to a particular mechanism of sealing, it is thought that the rare earth cation acts as a substitutional cation for Al 3+ in the aluminium oxide layer. Thus, when the rare earth element is cerium, it is preferably added as Ce 3+ which, it is believed, may substitute for Al 3+ in the oxide layer. A rare earth solution may be made by dissolving a rare earth salt in water. Examples of suitable rare earth salts include Ce(Nθ3)3.6H2θ, Cβ2(Sθ4)3.8H2θ and Pr(Nθ3)3.6H2θ. Preferably, the rare earth salt is cerium (III) nitrate hexahydrate (Ce(Nθ3)3.6H2θ). A rare earth containing solution may contain up to 100 grams per litre (0.23 molar) of dissolved rare earth salt (expressed as equivalent grams of Ce(N03)3 - 6H2O per litre of solution) such as up to 50 grams per litre (0.12 molar). In some embodiments, the maximum concentration of rare earth salt is 40 grams/litre (0.092 molar). In other embodiments, the maximum concentration is 20 grams/litre (approximately 0.05 molar). Alternatively, the maximum concentration of rare earth salt may be 10 grams/litre (0.023 molar). The minimum amount of rare earth salt per litre of solution may be 0.1 grams (2.3 x 10 -4 molar). However in some embodiments, the concentration is 0.5 g/l and above, such as above 1.0 g/l. In yet further embodiments, the minimum concentration is 5.0 grams/litre. These values of molarity of dissolved rare earth salt correspond to the molarities of the rare earth cation. A rare earth containing solution may further include other additives. One such additive is excess nitrate ions, which may enhance oxidation of aluminium at the interface of the metal and the aluminium oxide phases. The excess nitrate ion may be added in various forms, including KNO3, LiN03 or NH4NO3 or as combinations of these. The concentration of excess nitrate ion may be as high as the saturation limit of the corresponding nitrate salt. However lower concentrations of nitrate are also effective, such as up to 2.0M. A suitable concentration may be up to 1.0M. Fluoride ions may also be added to the rare earth containing solution. They may be added as MgF2 or NaF. If present, the F" ions may be present at a concentration of up to 0.01 M, such as up to 0.005M. Preferably, the fluoride ions are present up to 0.001 M. While the exact role of the F- ion is unknown, it is thought that F- attacks aluminium in the aluminium oxide layer to form a soluble Al 3+ complex. The Al in the oxide layer may then be replaced with the rare earth elements from the solution. The rare earth element may then be present in the oxide layer as an oxide or a fluoride or mixture thereof. The temperature of treatment with the rare earth containing solution of the process may be as high as the boiling point of the solution, such as up to 95°C. Preferably, the maximum temperature is 90°C. The lower temperature limit may be 60°C. However, in some embodiments, the temperature of treatment with the solution is 70°C and higher. Preferably, the temperature is 85°C and above. The rare earth containing solution preferably is acidic to neutral. It may have a pH up to 7, such as less than 5.5. In some embodiments of the process, the pH of the solution is less than 5. The pH may advantageously be above 4, such as 4.1 and above. Accordingly, for those embodiments of the invention, the preferred pH range is from 4 to 5. In further embodiments of the invention, the pH of the solution is 2 and above, such as higher than 3. When Ce(Nθ3)3.6H2θ is the rare earth salt used in the rare earth containing solution, it has been observed that the pH of the solution decreases slightly with increasing concentration of Ce(Nθ3)3.6H2θ. The oxide coated metal surface is treated with the rare earth containing solution for a period of time sufficient to enable effective impregnation of the columnar aluminium oxide layer with the rare earth ions in solution. Treatment time may be up to 60 minutes. However normally treatment is for up to 40 minutes. Preferably, the treatment is carried out for a period of time up to 30 minutes. In some embodiments, effective impregnation is possible after treatment for 10 or more minutes, such as for at least 20 minutes. Where the steps of water treatment and rare earth element treatment occur sequentially, the majority of the thickness of the columnar aluminium oxide layer is produced during the water treatment step. However, there may be additional thickening of this layer during the step of treatment with rare earth elements. In one embodiment of the invention, the treatment of the metal with water and with rare earth elements is effected simultaneously by treating the metal with an aqueous solution containing rare earth ions. Hence the aqueous component of the rare earth solution provides the water required to thicken the oxide on the metal surface and form an oxide layer of the desired thickness and the rare earth ions impregnate the oxide layer. In this embodiment, oxide layer thickening and impregnation with rare earth elements occurs substantially simultaneously. At the completion of the process of the invention, the impregnated oxide coating on the metal surface preferably has a porous, crazed oxide structure. An embodiment of such a structure is shown in Figure 1, relating to Example 3. Figure 2 shows that this coating has a thickness of approximately 1.5μm and has a columnar structure. The sealing step, if present, may comprise treatment of the rare earth impregnated coating with an aqueous or non-aqueous inorganic, organic or mixed inorganic/organic sealing solution. A preferred sealing solution is an inorganic sealing solution. Preferably the sealing solution contains one or more oxidants. In one preferred embodiment, the sealing solution comprises a silicate solution, such as an alkali metal silicate solution. In addition to the sealing solution forming a surface layer on the rare earth impregnated oxide layer, it penetrates and fills the pores of the crazed oxide structure. An X-ray photoelectron spectroscopy depth profile of the sealed, rare earth impregnated oxide coating of Example 3 is given in Figure 3. In the embodiment, after sealing with a silicate solution, the thickness of the coating was less and the structure of the coating had altered from a crazed oxide structure to a smooth surfaced coating with a thickness of less than 1μm. Figures 4 and 5 illustrate these features. Furthermore, the sealed coating has a layered structure comprising a homogeneous, smooth outer layer disposed on the columnar structure of the impregnated aluminium oxide. The depth profile for this embodiment shown in Figure 3 suggests that the outer layer comprises predominantly a silicate phase and the inner, columnar layer, comprises predominantly an aluminosilicate phase. The concentration of the alkali metal silicate may be below 20%, such as below 15%. A preferred upper limit of alkali metal silicate concentration is 3.6% [10%] (approximately 0.012M K2O and 0.04M Siθ2-) The lower concentration limit of the alkali metal silicate may be 0.001%, such as 0.01%. A preferred lower limit of concentration is 0.018% (approx. 2.1 x 10 "5 M K2O/7.4 x 10 "5 M Siθ2-) The temperature of the sealing solution may be as high as 100°C, such as up to 95°C. Preferably the temperature is up to 90°C and is more preferably below 85°C. A suitable temperature is up to 70°C. The lower limit of the temperature is preferably ambient temperature, such as from 10°C to 30°C. The aluminium oxide coating is treated with the sealing solution for a period of time sufficient to produce the desired degree of sealing. A suitable time period may be up to 30 minutes, such as up to 15 minutes. Preferably the treatment lasts for up to 10 minutes. The minimum period of time may be 2 minutes. EXAMPLES The following Examples illustrate, in detail, embodiments of the invention. In all of the Examples, the metal substrate used was a panel of 2024 aluminium alloy having the dimensions 3 inches by 1 inch (7.6 cm by 2.5 cm), with the exception of Examples 1 , 2 and 58 to 61 in which the panels were 3 inches by 4 inches (7.6 cm by 10.2 cm). The 2024 aluminium alloy is part of the 2000 series alloys, which is one of the most difficult to protect against corrosion, particularly in a chloride ion containing environment. Such environments exist, for example, in sea water, or exposure to sea spray and around airport runways and roads, where salt may be applied. In the Examples, corrosion resistance is measured by the amount of time it takes for the metal to develop pitting in neutral salt spray (NSS), according to the American standard salt spray tests described in American Society for Testing of Materials (ASTM) Standard B-117. Time to pitting of 20 hours represents a considerable improvement in the corrosion resistance of 2024 alloy and can be considered as an acceptable indicator for some applications. In other applications, 48 hours in neutral salt spray is normally required, whereas for aerospace applications, 168 hours is normally required. Corrosion resistance is also measured by "TAFEL" values. The TAFEL experiment is an electrochemical method of measuring the rate of corrosion of a coated or uncoated metal. The coated metal is placed in a cell containing 0.5M NaCI together with a saturated KCI calomel reference electrode and a platinum counter electrode. The potential of the coated surface is controlled relative to the reference electrode by a potentiostat which also measures the current. The corrosion rate is calculated from the intercepts of the linear sections of a plot of potential against log 10 (current) (a "Tafel" plot), at the corrosion potential. By measuring Tafel plots over a prolonged period of exposure, an indication of the variation of corrosion rate with time may be obtained. The TAFEL data for each Example includes the time in hours ("h") after preparation of the coating when icon- was measured. A disadvantage of some prior conversion coating processes, as previously stated, is the long coating times that are required to put down the coating. In some cases, a period of between several hours and several days is required. One advantage of the present process is the relatively short coating times, such as under one hour in most embodiments. All conversion coatings described in the Examples were found to have good paint adhesion properties when subsequently tested according to ASTM D-2794. The paint adhesion properties were similar to or better than the properties of alloys coated with chromate conversion coatings. Further, at least the preferred embodiments of the conversion coatings passed adhesion tests such as the scribe tests described in ASTM D-3330 and Boeing Specification Support Standard (BSS) 7225. Moreover, the conversion coatings of the invention often out performed chromate coatings of the prior art in "Impact Resistance Testing", as described in Boeing Material Specification (BMS) 10-11R. In the following Examples, reference is made to potassium silicate sealing solution PQ Kasil #1 and PQ Kasil #2236. PQ Kasil #2236 (Siθ2/K2θ - 3.49) is slightly more alkaline than PQ Kasil #1 (Siθ2/K2θ = 3.92) and has a lower ratio of Si0 2 /K 2 0. EXAMPLES 1 TO 5 A sample of 2024 aluminium alloy was wiped with acetone then conversion coated according to the following process: Step 1 a preliminary degrease in BRULIN for 10 minutes at a temperature of 60 to 70°C. Step 2 : (Examples 1 and 2 only) alkaline clean in RIDOLINE 53 at 60 to 70°C for 5 minutes. Step 3 deoxidise and desmut the metal surface using the deoxidising solutions and conditions given in Table I. Step 4 immerse cleaned metal in a 0.2M NaBrθ3 solution in the presence of 1.1M HNO3 and having a pH of less than 0.5 for 7 minutes at 20°C to initiate growth of aluminium oxide on surface of metal. Step 5 immerse in deionised water for 5 minutes at a temperature of 85 to 90°C to form a thickened, porous oxide layer with a crazed structure. (In Examples 3, 4 and 5, the water includes 0.01wt% of surfactant FC- 135). Step 6 immerse plate in a cerium solution containing 10g/litre (0.023M) of Ce(Nθ3)3.6H2θ, and having a pH between 4 and 5, for 30 minutes at a temperature of 85 to 90°C in order to impregnate the porous oxide layer with Ce ions. Step 7 For Examples 1 , 2 and 4, immerse coated sample in a potassium silicate sealing solution comprising PQ Kasil #1 at a concentration of 2.91 wt% (0.19M) and having a pH between 10.5 to 11 , at 20°C for 2 minutes. For Example 3, immerse coated plate in PQ Kasil # 2236 solution having a concentration of 1.8% (0.006M K2O/O.02M Siθ2) at 20°C for 4 minutes and allow to dry before the final rinse. This step results in a reduction in the overall thickness of the coating and a smooth surfaced coating. There was a 5 minute rinse in water between all the above steps. The final coated metal alloy had a two layered coating comprising an outer, relatively smooth homogeneous layer and an inner, columnar structure layer. Table I shows the deoxidising solution used in step 3 for Examples 1 to 4 and the resulting performance in neutral salt spray, expressed as "NSS" - which is the time to pitting of the coated alloy in neutral salt spray. It should be noted that step 5 of Examples 3, 4 and 5 differs from the other Examples in that the water contains 0.01 wt% of the surfactant FC-135. TABLE I : DEOXIDISING SOLUTIONS Ex. Deoxidising Solution T(°C) NSS TAFEI Used in Step 3 (and (hours) 'corr time) of (μA. Step 3 cm -2 ) Amchem #4 (Chromate- RT * 50 based deoxidising (10 min) solution) Turco WO#1 (phosphate RT * 25 based deoxidising (5 min) solution) 0.03M Ce(S04)2 RT * 168 20h C and 0.8 molar H2SO4 (5 min) 140h C (prepared from cerium (IV) hydroxide) 0.02 molar Pr2(Sθ4)3 RT * 70 1h 0.06 and 0.7 molar H2SO4. (5 min) 50h 0.02 (prepared from praseo¬ dymium oxide) 35 grams/litre (0.06 RT * 70 20h 0.01 molar of ammonium cerium (5 min) 90h 0.03 (IV) sulphate in 5.5 wt% (0.5 molar) H2SO4 solution RT = room temperature (20-25°C) Thus, the corrosion resistance of Example 3 far exceeded those of the other Examples. Accordingly, it appears that a cerium based deoxidising solution used in step 3, results in a high corrosion resistance of the subsequently applied conversion coating. Figures 1 and 2 illustrate the impregnated oxide coating of Example 3 prior to treatment with the silicate sealing solution. Figures 3 to 5 relate to Example 3 after the silicate seal step. Moreover, the coated alloy of Example 3 passed adhesion tests described in ASTM D " 3330 and Boeing Specification Support Standard 7225 as well as forward and reverse impact resistance testing in Boeing Material Specification 10-11 R. EXAMPLES 6 and 7 Variations of the temperature of treatment with the deoxidising solution are shown in Table II. All steps are the same as for Example 3, except that the treatment temperature of the rare earth deoxidising solution of step 3 is varied. TABLE II : TEMPERATURE OF DEOXIDISING SOLUTION Example T(°C) NSS (hours) TAFEL 'corr (μA.Cm-2) 6 20 168 20h 0.005 100h 0.05 7 50 60 Oh 0.02 50h 0.007 It is evident that, for the particular conditions of Examples 6 and 7, an increase in temperature of the Ce(S04)2 deoxidising solution in step 3 results in a decrease in corrosion resistance. Example 6 passed adhesion tests described in ASTM D-3330 and Boeing Specification Support Standard (BSS) 7225 and impact resistance tests described in Boeing Material Specification (BMS) 10-11R. EXAMPLES 8 and 9 The temperature of treatment with the deoxidising solution in step 3 of Example 5 was varied according to Table III. TABLE III : TEMPERATURE OF DEOXIDISING SOLUTION Example T(°C) NSS (hours) TAFEL 8 20 70 20h 0.01 90h 0.03 9 40 100 Converse to the trend observed in Examples 6 and 7, Examples 8 and 9 indicate that, for the conditions specified, an increase in temperature of the ammonium cerium (IV) sulphate deoxidising solution results in an increase in corrosion resistance. EXAMPLES 10 and 11 The steps of deoxidising the metal surface and of treating with an acidic solution containing NaBrθ3 for initiation of oxide growth may be combined into a single step. Accordingly, Examples 10 and 11 involve the following steps: Step 1 degrease in BRULIN at 60 to 70°C for 10 minutes. Step 2 treat with a solution comprising 1.5 wt% (0.1 molar) NaBrθ3, 3.4 wt% (0.5 molar) HNO3, 0.5 wt% (0.015 molar) Ce(S04)2 and 3.5 wt% (0.4 molar) H2SO4 for 5 minutes at the temperatures given in Table IV. Step 3 immerse alloy in deionised/distilled water containing 0.01% surfactant FC-135 for 5 minutes at a temperature of 85 to 90°C to form a thickened, porous oxide layer with a crazed structure. Step 4 immerse alloy in a cerium solution containing 10g/litre of Ce(Nθ3)3.6H2θ for 30 minutes at a temperature of 85 to 90°C in order to impregnate the porous oxide layer with Ce ions. Step 5 immerse coated plate in a potassium silicate sealing solution comprising PQ Kasil # 2236 at a concentration of 1.8 wt%, at room temperature for 2 minutes. This results in a reduction in the overall thickness of the coating and a smooth surfaced coating. TABLE IV : TEMPERATURE OF Ce(S0 ) 2 /NaBrθ3 SOLUTION Example T(°C) NSS (hours) TAFEL i corr (μA.cm-2) 10 20 168 Oh 0.04 120h 0.07 11 40 168 Oh 0.03 120h 0.10 Examples 10 and 11 show that, for the particular conditions specified, an increase in temperature of a combined deoxidising/oxide growth initiation solution does not affect corrosion performance. EXAMPLES 12 to 15 A 2024 aluminium alloy plate was cleaned, then conversion coated according to the following process: Step 1 vapour degrease the 2024 alloy plate in trichloroethane vapour for 15 minutes. Step 2 dip in a 35 wt% (5.5 molar) HNO3/O.96 wt% (0.48 molar) HF acid solution at room temperature for 1 minute. Step 3 alkaline clean in 2.5 wt% (0.6 molar) NaOH solution at 35 to 40°C. Step 4 dip in a 35 wt% (5.5 molar) HNO3/O.96 wt% (0.48 molar) HF acid solution at room temperature for 1 minute. Step 5 treat with NaBrθ3 solution, with additional HNO3, under the conditions given in Table V for 7 minutes. Step 6 immerse in water at a temperature of 85 to 90°C for 5 minutes. Step 7 immerse in an aqueous solution of Ce(Nθ3)3.6H2θ at 10 grams/litre (0.023 molar) at a temperature of 85 to 90°C for 30 minutes. Step 8 seal in a potassium silicate solution, PQ Kasil #1 at a concentration of 2.91% (0.19M), at room temperature for 2 minutes. All the above steps have a 5 minute rinse in water after them. TABLE V : CONDITIONS OF TREATMENT WITH NaBr0 3 SOLUTION IN STEP 5 Ex. T(°C) HN0 3 NSS TAFEL Concentration (hours) ϊcorr (μA.Cm- 2 ) 12 20 7% (1.1M) 40 70h 0.02 140h 0.03 13 50 7% (1.1M) 20 Oh 0.20 50h 0.05 14 20 21% (3.3 M) 100 Oh 0.02 50h 0.06 15 20 42% (6.7 M) 20 Oh 1.75 50h 0.50 Examples 12 to 15 illustrate that there is some improvement in corrosion resistance by increasing the HNO3 concentration to 21% in the oxide growth initiation step. However, at concentrations of HNO3 between 21% and 42%, there is a decrease in corrosion resistance. Moreover, Examples 12 and 13 indicate that in the presence of low HNO3 concentrations, an increase in temperature of the NaBrθ3 containing solution results in a decrease in corrosion resistance. EXAMPLES 16. 17 The steps of Examples 12 to 15 are varied by omitting step 4 and replacing step 5 with the step of immersing the plate in 0.1M ammonium eerie nitrate ((NH4)2Ce(N03)6) solution with an addition of 1.1% (0.17 molar) HNO3, for the times listed in Table VI. TABLE VI : TREATMENT TIME WITH (NH )2Ce(N0 3 ) 6 AND HNO3 SOLUTION Example Time (mins.) NSS (hours) TAFEL i corr (μA.cm-2) 16 1 30 Oh 0.20 100h 0.20 120h 0.04 17 40 90h 0.04 120h 0.05 These Examples show that an increase in treatment time with the (NH4)2Ce(Nθ3)β solution results in an improved corrosion resistance. EXAMPLES 18 and 19 Step 6 of Examples 12-15 was varied in Examples 18 and 19 by immersing the plate in H2O at 85 to 90°C for the times listed in Table VII. All other steps are the same as for 12-15, with the exception that in step 5, NaBrθ3 was at 50°C and contains 7% (1.1M) HN03. TABLE VII : TIME OF TREATMENT WITH H 2 0 Example Time (mins.) NSS (hours) TAFEL "corr (μA.cm-2) 18 2.5 40 Oh 0.08 20h 0.02 50h 0.02 19 40 A comparison of Examples 18 and 19 shows that, for these particular conditions, an increase in treatment time with the water and surfactant solution of step 5 results in little effect on corrosion resistance. EXAMPLES 20 to 22 Step 6 of Example 13 was varied by immersing the alloy in H2O with potassium nitrate added at the concentrations given in Table VIM at 85 to 90°C for 5 minutes. All other steps are the same as for Example 13. TABLE VIII : CONCENTRATION OF KNO3 Example KNO3 NSS TAFEL Concentration (hours) icorr (μA.cm ■ 2) (molar) 20 0.05 20 20h 0.01 230h 1.80 21 0.5 20 20h 0.50 230h 0.30 22 1.0 10 Oh 0.40 40h 0.07 Examples 20 to 22 demonstrate that, for the particular conditions of these Examples, KNO3 may be added to the water treatment of step 6 without adversely affecting corrosion performance. However Example 22 indicates that at a concentration of KNO3 above 0.5 molar, corrosion resistance declines. This value is different however where other parameters of the process have been varied, for instance, when Ce(Nθ3)3.6H2θ concentration in step 7 is varied - see Examples 42 to 44. EXAMPLES 23 AND 24 Examples 23 and 24 contain the same steps as for Examples 20 to 22, with the exception that, instead of KNO3, surfactant is added to the water in step 6. The surfactant is a fluorochemical surfactant commercially available under the trade name FLUORAD FC135. The concentration of FC135 and corrosion performance data are provided in Table IX. TABLE IX : CONCENTRATION OF FC-135 Example FC-135 NSS (hours) TAFEL Concentration i corr (μA.cm-2) 23 0.001 % 25 1h 0.10 20h 0.08 24 0.02% 30 20h 0.09 70h 0.10 160h 0.20 Examples 23 and 24 show that under the specific set of conditions for each Example, increasing the concentration of surfactant in the water of step 6, makes no significant improvement to corrosion performance. EXAMPLE 25 In Example 25, instead of KNO3, cerium (III) sulphate octahydrate (Cβ2(Sθ4)3.8H2θ) is added to the water of step 6 in a concentration of 20 grams/litre (0.028 molar) with all other steps the same as for Examples 20 to 22. TABLE X : ADDITION OF Ce 2 (S0 4 ) 3 .8H2θ Example Cβ2(S04) 3 .8- NSS TAFEL H 2 0 (hours) icorr (μA.Cm-2) Concentration 25 20 grams/litre 20 Oh 0.30 (0.028 molar) 20h 0.05 70h 0.60 Under the particular conditions of this Example when compared with Examples 20 and 21 , there was no apparent change in corrosion performance when KNO3 was substituted with Cβ2(Sθ4)3.8H2θ in step 6. EXAMPLES 26 TO 31 The steps of Example 3 were varied by replacing the water of step 5 with a solution according to the details of Table XI. All other steps are the same as for Example 3 with the exception that surfactant is not added in step 5, unless specified in Table XI. TABLE XI : SOLUTION COMPOSITION IN STEP 4 Example Solution NSS TAFEL (hours) i corr (μA.cm-2) 26 0.1% 10 Oh 0.30 (0.01 M) 80h 0.03 NH4OAC 27 0.1% 20 Oh 0.40 (0.01 M) 70h 0.02 NH4OAC + 0.01% FC-135 28 0.001 M NH4OH 168 Oh 0.05 30h 0.05 29 0.001 M NH4NO3 45 20h 0.02 140h 0.04 30 0.001 M (NH4)2Cθ3 85 20h 0.10 140h 0.02 31 0.001 M Na2Cθ3 40 20h 0.04 100h 0.04 The results show that replacement of the water and surfactant solution of step 5 of Example 3 with the solutions defined in Examples 26, 27, 29, 30 and 31 does not produce as good a corrosion resistant coating as Example 3, but still produces markedly better resistance than the bare metal, under the particular conditions of those Examples. However, when different concentrations of those other solutions are used, different corrosion resistances may result. Example 28 indicates similar corrosion resistance as compared to Example 3, when the water/surfactant solution is replaced with a 0.001 M NH4OH solution. EXAMPLES 32 to 38 The concentration of Ce(Nθ3)3.6H2θ in step 7 was varied according to the values in Table XII. All other steps are the same as for Examples 20 to 22, except that no KNO3 is added to the water in step 6. TABLE XII : Ce(N0 3 ) 3 .6H2θ CONCENTRATION Ex. Ce(N0 3 ) 3 .6- NSS TAFEL H 2 0 (g/l) (hrs) i corr (μA.cm-2) (g/l) (molar) PH 32 0.1 2.3 x 10-4 5.05 40 20h 0.008 90h 0.10 33 0.5 1.15 x 10-3 4.90 60 Oh 0.03 90h 0.08 34 1.0 2.3 x 10- 3 4.75 20 Oh 0.03 90h 0.20 35 5.0 0.012 4.55 50 Oh 0.07 • 20h 0.03 120h 0.10 36 10.0 0.023 4.45 80 Oh 0.06 20h 0.04 100h 0.04 37 20.0 0.046 4.00 50 Oh 0.50 20h 0.10 120h 0.40 38 40.0 0.092 3.75 50 Oh 0.40 120h 0.03 Examples 32 to 38 show that with increasing Ce(Nθ3)3.6H2θ concentration, there is a general increase in corrosion resistance for the particular conditions of these Examples. It should be noted that the pH varies from 5.05 to 3.75. However, it appears that the maximum cost benefit is achieved when the concentration of Ce(Nθ3)3.6H2θ is between 10 g/l and 20 g/l. However, there could be cost benefit in higher concentration when other parameters of the process are varied. EXAMPLES 39 TO 41 Excess nitrate was added to the Ce(Nθ3)3.6H2θ solution according to the following process. Step 1 vapour degrease the 2024 aluminium alloy plate in trichloroethane vapour for 15 minutes. Step 2 treat in NaBrθ3 solution for 7 minutes at 50°C. Step 3 immerse in H2O at a temperature of 85 to 90°C for 5 minutes. Step 4 immerse in Ce(Nθ3)3.6H2θ solution at a concentration of 10 grams/litre (0.023 molar) having 1.0 M excess nitrate as detailed in Table XIII, at a temperature of 85 to 90°C for 30 minutes. Step 5 seal in a potassium silicate solution, PQ Kasil #1 at a concentration of 2.91% (0.19M) having 0.001% anionic fluorochemical surfactant FC-99 added, at room temperature for 2 minutes. The plate is rinsed in water for 5 minutes after each step. TABLE XIII : ADDITION OF EXCESS NITRATE Ex. Nitrate Species PH NSS TAFEL (1.0M) (hours) icorr (μA.Cm-2) 39 KN0 3 4.5-5.0 50 Oh 0.04 50h 0.01 70h 0.01 40 UNO3 4.40 40 20h 0.05 50h 0.03 70h 0.02 90h 0.03 41 NH4NO3 3.75 30 20h 0.02 40h 0.02 70h 0.01 140h 0.02 Examples 39 to 41 illustrate, for the particular conditions of those Examples, an overall increase in corrosion resistance in going from NH4NO3 to UNO3 to KNO3 addition to the rare earth sealing solution. It should be noted that there is a corresponding increase in pH of the rare earth sealing solution. EXAMPLES 42 to 44 Excess nitrate can be added to the Ce(Nθ3)3. 6H2O bath at a concentration of cerium lower than that in Examples 20 to 22. All steps in Examples 42 to 44 are the same as for Examples 20 to 22, except that in step 6, the solution does not contain KNO3 and in step 7, the solution contains 1M KNO3 and the cerium concentrations provided in Table XIV. TABLE XIV : Ce(N03) 3 .6H 2 0 CONCENTRATION Ex. Ce(N0 3 ) 3 .6H 2 0 NSS TAFEL (grams/- (molar) pH (hours) i corr (μA.cm-2 litre) 42 0.1 2.3 x 10-4 5.30 60 Oh 0.02 70h 0.04 120h 0.02 43 0.5 1.15 x 10-3 5.15 60 20h 0.03 90h 0.02 44 1.0 2.3 x 10-3 5.05 40 20h 0.20 90h 0.05 Examples 42 to 44 show that for the particular conditions of Examples 42 to 44, corrosion performance starts to decline at a concentration of Ce(Nθ3)3.6H2θ between 0.5 and 1.0 g/l. EXAMPLE 45 The steps in Example 45 are the same as those for Example 39, except that in step 4, cationic fluorochemical surfactant FC-135 (Fluorad) 0.005wt% is added to the Ce(Nθ3)3.6H2θ solution in the presence of 1M KNO3 and in step 5 the silicate solution was heated to 50°C with immersion for 2 minutes. The approximate pH of the Ce(Nθ3)3.6H2θ solution was 4.75. Accordingly, the change in conditions between Example 39 and Example 45 do not adversely affect corrosion performance. Example NSS (hours) TAFEL i corr (μA.cm-2) 45 50 Oh 0.70 20h 0.04 50h 0.03 70h 0.04 EXAMPLE 46 Step 7 of Example 13 was modified by replacing Ce(Nθ3)3.6H2θ with Cβ2(Sθ4)3.8H2θ at a concentration of 20 grams/litre (0.028 molar). The pH of the rare earth sealing solution was 3.15. All other steps were the same as for Example 13. Accordingly, the change in conditions between Example 13 and Example 46 do not result in an adverse change in corrosion performance. Example NSS (hours) TAFEL icorr (μA.cm-2) 46 20 20h 0.07 70h 0.04 170h 0.07 EXAMPLES 47 AND 48 Step 7 of Example 13 was again modified by adding fluoride ions to the Ce(Nθ3)3-6H2θ bath at the concentrations provided in Table XV, and immersing the plate in the bath for only 15 minutes. All other steps were the same as for Example 13. TABLE XV:FLUORIDE ADDITION TO Ce(N0 3 ) 3 .6H 2 0 SOLUTION Ex. Fluoride pH Concentration NSS TAFEL Species (Molar) (hrs) •corr (μA.cπr 2 ) 47 MgF2 4.10 0.001 5 Oh 0.10 100h 0.02 120h 0.06 48 NaF 4.25 0.002 15 For these specific conditions, addition of the fluoride species to the rare earth sealing solution appears to adversely affect corrosion resistance. EXAMPLES 49 and 50 In step 6 of Example 3, Ce(Nθ3)3.6H2θ was replaced with Pr(Nθ3)3.6H2θ at a concentration of 10 grams/litre (0.02 molar). All other steps were the same as for Example 3, except that in Example 50, step 3 comprised pretreatment with a praseodymium containing solution, as for step 3 of Example 4. TABLEXVI : Pr(N0 3 ) 3 .6H 2 0SUBSTITUTEDFORCe(N0 3 ) 3 .6H 2 0 IN STEP6 Ex. Pretreatment Pr(N0 3 ) 3 .6- NSS TAFEL Solution H 2 0 Concen- (hrs) ■ corr centration (μA.cm _ 2) (grams/ (molar) litre) 49 Cerium 10 0.02 60 Oh 0.05 120h 0.06 50 Praseodymium 10 0.02 30 2h 0.07 90h 0.04 These Examples show that, for the specified conditions, pretreatment with a cerium containing deoxidising solution resulted in better corrosion performance than pretreatment with a praseodymium containing deoxidising solution. Moreover, comparison of Example 49 with Example 3 indicates that better corrosion performance results when the cerium containing solution is used in step 6 than when a praseodymium containing solution is used. However, different results may be achieved when other parameters of the process are varied. EXAMPLES 51 to 53 Examples 51 to 53 illustrate varying concentration of silicate in the potassium silicate sealing solution. Steps 1 to 6 are the same as for the corresponding steps of Example 1 , with the exception that in step 3, the deoxidising solution is a rare earth pretreatment solution as described in step 3 of Example 3 and in step 5, the water bath includes 0.01% of the surfactant FC-135. Step 7 comprises sealing with a potassium silicate solution, PQ Kasil #2236 at room temperature for 4 minutes and at the concentrations given in Table XVII. TABLE XVII : SILICATE CONCENTRATIONS Ex. Silicate NSS TAFEL Concentration (hrs.) ■ corr wt% pH (μA.cm _ 2) 51 1.8 10.9 220 Oh 0.04 11 Oh 0.08 52 3.6 11 200 20h 0.02 11 Oh 0.01 53 5.4 11 60 20h 0.006 110h 0.01 Examples 51 to 53 clearly illustrate improved corrosion resistance at silicate concentrations below around 3.6 wt%. For the specific conditions of these Examples, corrosion resistance noticeably decreases between 3.6 and 5.4 wt% of silicate in the silicate sealing solution. This range of silicate concentration may therefore be the maximum cost beneficial silicate concentration. However, there could be cost benefit in higher silicate concentration when other parameters of the process are varied. EXAMPLES 54 to 56 Examples 54 to 56 illustrate varying the temperature of the silicate sealing solution. Step 1 : vapour degrease 2024 alloy in trichloroethane for 15 minutes. Step 2 : treat with a solution of 10 g/l (0.023 molar) Ce(Nθ3)3.6H2θ and 1M KNO3 for 30 minutes at 85 to 90°C. Step 3: seal in 2.91% potassium silicate PQ Kasil #1 for 2 minutes at the temperatures described in Table XVIII. TABLE XVIII : TEMPERATURE OF SILICATE SOLUTION Example T(°C) NSS (hours) TAFEL i co rr(μA.cm ■ 2) 54 30 50 Oh 0.40 20h 0.10 100h 0.06 55 50 50 Oh 0.30 20h 0.09 100h 0.03 56 75 50 Oh 0.40 20h 0.08 100h 0.04 Examples 54 to 56 show that for the particular conditions of those Examples, variation in the temperature of the silicate sealing solution did not affect the corrosion performance. EXAMPLE 57 Instead of potassium silicate, the sealing solution may include sodium silicate. Steps 1 to 4 are the same as for Example 39. Step 5 comprises sealing in sodium silicate solution at 36 grams/ litre (0.3 molar) at 50°C having a pH of approximately 11 for 10 minutes. There was a 5 minute rinse in water between all steps. Comparison of Example 57 with Example 39 shows that, for the particular conditions of these Examples, substitution of potassium silicate solution with sodium silicate solution resulted in a slight decrease in corrosion resistance. However, the result may be different where other variables are varied. TABLE XIX : SODIUM SILICATE SEALING SOLUTION Example NSS (hours) TAFEL ic o rr (μA.cm-2) 57 40 20h 0.04 40h 0.02 70h 0.01 140h 0.02 160h 0.008 EXAMPLES 58 to 61 Other types of sealing solutions are exemplified by Examples 58 to 61. Step 1 : aqueous degrease 2024 alloy in Brulin at 60 to 70°C for 10 minutes. Step 2: immerse in 35 wt% (7.9 molar) HNθ3/0.96wt% (0.48 molar) HF acid solution for one minute at ambient temperature. Step 3: alkaline clean in 2.5% (0.6 molar) NaOH solution at room temperature. Step 4 : treat in NaBrθ3 solution for 7 minutes at room temperature. Step 5 : immerse in H2O at 85 to 90°C for 5 minutes. Step 6: immerse in Ce(Nθ3)3.6H2θ solution having a concentration of 10 grams/litre (0.023 molar) at a temperature of 85 to 90°C for 30 minutes. Step 7: treat with the sealing solutions described in Table XX. There is a 5 minute rinse after each step. TABLE XX : SEALING SOLUTIONS Ex. Sealing Solution pH NSS TAFEL (hrs) ■ corr (μA.cm-2) 58 (CH 3 COO)2Ni.4H2θ (24 g/l; 0.10 molar), MnSθ4 (12 g/l; 0.08- molar), NH4NO3 (30 g/l;-) 5.55 30 Oh 0.01 0.38 molar) 30h 0.07 9 H3BO3 (13.2 g/L;- 0.21 molar), COSO47H2O (6.6 g/l;- 0.02 molar), CH3COONH4 (6.6 g/l;- 6.15 30 Oh 0.02 0.09 molar) 30h 0.40 0 C0SO4.7H2O (6.6 g/l;- 0.02 molar), NH 4 V0 3 (1.3 g/l;- 0.01 molar), H3BO3 (13.2 g/l;- 5.55 80 Oh 0.01 0.21 molar) 30h 0.01 1 H3BO3 (7.9 g/l;- 0.13 molar), Na2B 4 0 7 (7.9 g/l;- 0.02 molar), NaN0 2 (7.9 g/l; 0.11- molar), NH 4 V0 3 (1.3 g/l;- approx 30 Oh 0.02 0.01 molar) 8 20h 0.03 All sealing solutions gave acceptable corrosion resistance. However, under the specific conditions of Examples 58 to 61 , the sealing solution defined in Example 60 gave the best corrosion resistance. EXAMPLES 62 to 64 Variation in the type of oxidant used to initiate aluminium oxide growth is illustrated by the following Examples. Step 1 : alkaline clean the 2024 alloy in Brulin at 60 to 70°C for 10 minutes. Step 2 : deoxidise in a rare earth pretreatment solution prepared from cerium (IV) hydroxide and contains 0.03 molar Ce(Sθ4)2 and 0.8 molar H2SO4. The 2024 is immersed in the rare earth pretreatment solution for 5 minutes at 20°C. Step 3 : immersion in solution containing oxidant in the presence of 7% (1.1M) HNO3 listed in Table XXI below for 7 mins at 20°C. Step 4 : immersion in H2O containing 0.01% surfactant FC-135 at 85-90°C for 5 minutes. Step 5 : immersion in Ce(Nθ3)3.6H2θ at a concentration of 10g/L (0.023 molar) at 85-90°C for 30 minutes and Step 6 : sealed in a 1.8% potassium silicate (PQ Kasil #2236) solution at room temperature for 4 minutes. All steps involve a 5 minute rinse between them. TABLE XXI - OXIDANT TYPES Example Oxidant Oxidant pH NSS (hrs) Type Concentration (Approx) (molar) 62 NaBrθ3 0.2 <1 100 63 KBr0 3 0.2 <1 90 64 KCIO3 0.15 <1 90 Examples 62 to 64 indicate that, at least for the specific conditions of these Examples, use of NaBrθ3 to initiate oxide growth results in better corrosion resistance than use of either KBrθ3 or KCIO3. However, a different result may be achieved when other variables are varied. EXAMPLES 65 and 66 The temperature of the rare earth deoxidising solution was varied according to Examples 65 and 66. Step l alkaline clean the 2024 alloy plate in Brulin at 60 to 70°C for 10 minutes, Step 2 deoxidise in a rare earth pretreatment solution containing 0.1M Ce(Sθ4)2 and 2M H2SO4. The 2024 is immersed in the rare earth pretreatment solution for five minutes at the temperatures shown in Table XXII. Step 3 immersion in NaBrθ3 solution for 7 minutes, at 20°C. Step 4 immersion in H2O containing 0.01% surfactant FC-135 at 85-90°C for 5 minutes. Step 5 immersion in a Ce(Nθ3)3.6H2θ solution at a concentration of 10g/L (0.023 molar) at 85-90°C for 30 minutes and Step 6 sealed in a 1.8% potassium silicate PQ Kasil #2236 solution at room temperature for 4 minutes. All steps involve a 5 minute rinse between them. TABLE XXII - TEMPERATURE OF DEOXIDISING SOLUTION Example Temp (°C) NSS (hrs) 65 20 100 66 40 100 Examples 65 and 66 show that for the conditions specified in these two Examples, variation of the temperature of the deoxidising solution does not affect the corrosion resistance. Finally, it is to be understood that, although the invention has been described with particular reference to the foregoing Examples and accompanying drawings, it will be clear that various modifications and improvements may be made without departing from the spirit and scope thereof.

Claims: CLAIMS 1. A process for forming a conversion coating on the surface of a metal, including: contacting the metal with an acidic solution containing an oxidant in order to initiate growth of a metal oxide cell structure on the metal surface; contacting the metal with water for a period of time sufficient to thicken the oxide and form a metal oxide containing layer of a desired thickness; and treating the metal with one or more rare earth elements in order to impregnate and substantially seal the metal oxide. 2. The process of claim 1 , wherein said metal is aluminium or an aluminium containing alloy. 3. The process of claim 1 or claim 2, wherein said oxidant is a metal halate metal persulphate, nitrate, H2O2 or (NH4)2Ce(Nθ3)6- 4 The process of claim 3, wherein said metal halate is selected from NaBrθ3, KBr0 3 and KCIO3. 5. The process of any one of claims 1 to 4 wherein said oxidant is NaBrθ3. 6. The process of any one of claims 1 to 4 wherein said acidic solution contains HNO3. 7. The process of claim 6, wherein the concentration of said HNO3 is up to 1.6 molar. 8. The process of any one of claims 1 to 7 wherein the concentration of said oxidant in said acidic solution is up to 10wt% (0.67M). 9. The process of any one of claims 1 to 7 wherein the concentration of said oxidant in said acidic solution is up to 0.2M. 10. The process of any one of claims 1 to 9, wherein the pH of said acidic solution is less than 1. 11. The process of any one of claims 1 to 9, wherein the pH of said acidic solution is less than 0.5. 12. The process of any one of claims 1 to 11 wherein the temperature of said acidic solution is 50°C or lower. 13. The process of any one of claims 1 to 11 wherein the temperature of said acidic solution is ambient temperature, such as from 10°C to 30°C. 14. The process of any one of claims 1 to 13, further including the step of treating the metal surface with a deoxidising solution. 15. The process of claim 14, wherein said deoxidising solution contains one or more rare earth ions. 16. The process of claim 15, wherein said deoxidising solution comprises cerium (IV) hydroxide, cerium (IV) sulphate or ammonium cerium (IV) sulphate dissolved in a mineral acid solution. 17. The process of claim 16, wherein said deoxidising solution comprises cerium (IV) sulphate dissolved in a sulphuric acid and nitric acid solution. 18. The process of any one of claims 14 to 17, wherein the step of treatment with a deoxidising solution precedes the step of contacting the metal with said acidic solution. 19. The process of any one of claims 14 to 17, wherein the step of deoxidising the metal surface occurs simultaneously with the step of contacting the metal with said acidic solution by contacting said metal surface with a single solution comprising a combination of the acidic oxidant-containing solution and the deoxidising solution. 20. The process of any one of claims 1 to 19, wherein step of contacting the metal with water comprises contact with deionised and/or distilled water. 21. The process of any one of claims 1 to 20, wherein the temperature of said water is from 85 to 90°C. 22. The process of any one of claims 1 to 21 , wherein said water includes a surfactant. 23. The process of any one of claims 1 to 22, wherein said water further includes one or more additives selected from nitrate compounds, such as potassium nitrate, cerium (III) sulphate octahydrate, solutions of ammonia and its salts, such as NH4OAC, NH4Nθ3 t (NH4)2 CO3 and NH4OH, and sodium carbonate. 24. The process of claim 23, wherein said one or more additives includes NH4OH. 25. The process of any one of claims 1 to 24, wherein said rare earth element is provided by an aqueous rare earth ion containing solution comprising a rare earth salt selected from Ce(Nθ3)3-6H2θ, Cβ2(Sθ4)3.8H2θ and Pr(Nθ3)3.6H2θ dissolved in water. 26. The process of claim 25, wherein the oxide thickening step and the rare earth element impregnation step occur substantially simultaneously by treating the metal with an aqueous, rare earth ion containing solution in which the aqueous component provides the water required for thickening the metal oxide layer and the rare earth ions impregnate the metal oxide. 27. The process of claim 25 or claim 26, wherein said rare earth salt comprises Ce(Nθ3)3.6H2θ. 28. The process of any one of claims 25 to 27 wherein the concentration of said rare earth salt in the aqueous, rare earth ion containing solution is up to 50 grams/litre (0.12 molar). 29. The process of any one of claims 25 to 27 wherein the concentration of said rare earth salt in the aqueous, rare earth element containing solution is from 0.1 grams/litre to 40 grams/litre (2.3 x 10" 4 to 0.09 molar). 30. The process of any one of claims 25 to 29, wherein the pH of the aqueous, rare earth ion containing solution is acidic to neutral. 31. The process of any one of claims 25 to 30, wherein the pH of the aqueous, rare earth ion containing solution is from 3 to 5.5. 32. The process of any one of claiims 25 to 30, wherein the pH of the aqueous, rare earth ion containing solution is from 4 to 5. 33. The process of any one of claims 25 to 31 , wherein the temperature of the aqueous, rare earth ion containing solution is between 70°C and 100°C. 34. The process of any one of claims 25 to 31 , wherein the temperature of the aqueous, rare earth ion containing solution is between 85°C and 100°C. 35. The process of any one of claims 25 to 31 , wherein the temperature of the aqueous, rare earth ion containing solution is between 85°C and 90°C. 36. The process of any one of claims 25 to 35, wherein the aqueous, rare earth ion containing solution further includes one or more components selected from nitrate ions, fluoride ions and surfactants. 37. The process of any one of claims 25 to 35, wherein the aqueous, rare earth element containing solution includes additional nitrate ions added as KNO3, LiNθ3 or NH4NO3 or as a combination thereof. 38. The process of claim 36 or claim 37, wherein the concentration of nitrate ions is 2.0 molar or lower. 39. The process of any one of claims 1 to 38 further including the step ofcontacting the metal with a sealing solution to form a surface layer on the rare earth impregnated oxide layer. 40 The process of claim 39, wherein said sealing solution is an inorganic sealing solution. 41. The process of claim 39 or 40, wherein said sealing solution includes an oxidant. 42. The process of any one of claims 39 to 41 , wherein said sealing solution is an alkali metal based solution. 43. The process of claim 42, wherein said sealing solution is a potassium silicate solution. 44. The process of any one of claims 1 to 43, wherein each step is followed by a water rinsing step. 45. A process for forming a conversion coating on a metal surface including the steps of: (a) contacting the metal with an acidic solution containing an oxidant in order to initate growth of a metal oxide cell structure on the metal surface; (b) contacting the metal with water for a period of time sufficient to thicken the metal oxide and form a metal oxide containing layer of a desired thickness; and (c) contacting the metal with an aqueous, rare earth element containing solution in order to impregnate and substantially seal the metal oxide containing layer. 46. A conversion coated metal formed by a process of any one of claims 1 to 45. 47. A conversion coated metal, wherein the conversion coating comprises a metal oxide containing layer which is impregnated with one or more rare earth elements. 48. The conversion coated metal of claim 46 or claim 47, wherein said metal is an aluminium containing metal. 49. The conversion coated metal of claim 47, wherein said metal oxide is aluminium oxide as herein defined. 50. An aqueous, rare earth ion containing solution for use in the process of any one of claims 25 to 45, said solution including a sufficient quantity of a rare earth to impregnate and substantially seal the metal oxide containing layer formed on the surface of the metal. 51. A process for forming a conversion coating on a metal surface according to claim 1 , substantially as herein described with reference to any one of the Examples. 52. A process for forming a conversion coating on a metal surface according to claim 45, substantially as herein described with reference to any one of the Examples. 53. A conversion coated metal according to claim 46, substantially as herein described with reference to any one of the Examples. 54. A conversion coated metal according to claim 47, substantially as herein described with reference to any one of the Examples. 55. An aqueous, rare earth ion containing solution according to claim 49, substantially as herein described with reference to any one of the Examples. 56. A conversion coated metal according to claim 46, substantially as herein described with reference to Figures 1 to 5 of the accompanying drawings. 57. A conversion coated metal according to claim 47, substantially as herein described with reference to Figures 1 to 5 of the accompanying drawings.

Inventors: HENDERSON MARK JULIAN, HINTON BRUCE ROY WILLIAM, HUGHES ANTHONY EWART, NELSON KAREN JOY HAMMON, NUGENT SALLY ANN, TAYLOR RUSSELL JAMES, WILSON LANCE, HENDERSON, MARK, JULIAN, HINTON, BRUCE, ROY, WILLIAM, HUGHES, ANTHONY, EWART, NELSON, KAREN, JOY, HAMMON, NUGENT, SALLY, ANN, TAYLOR, RUSSELL, JAMES, WILSON, LANCE
Applicants: COMMW SCIENT IND RES ORG, HENDERSON MARK JULIAN, HINTON BRUCE ROY WILLIAM, HUGHES ANTHONY EWART, NELSON KAREN JOY HAMMON, NUGENT SALLY ANN, TAYLOR RUSSELL JAMES, WILSON LANCE, COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH OR, HENDERSON, MARK, JULIAN, HINTON, BRUCE, ROY, WILLIAM, HUGHES, ANTHONY, EWART, NELSON, KAREN, JOY, HAMMON, NUGENT, SALLY, ANN, TAYLOR, RUSSELL, JAMES, WILSON, LANCE
Application year: 1995
Main classification: C23C22
IPC codes: C23C22