NO803937L - PROCEDURE AND SOLUTION FOR COATING ALUMINUM - Google Patents

Description

The invention relates to a solution, method

and concentrate for coating aluminum and, moreover, aluminum objects coated with it.

The term "aluminium" as used here shall include not only pure aluminium, but also alloys of aluminium, e.g. the aluminum alloys that contain smaller amounts of metals, such as magnesium, manganese, copper or silicon.

It is known to coat aluminum surfaces by treating them with aqueous coating solutions that react with the aluminum so that its surface is transformed into a so-called "conversion coating" which is corrosion-resistant and thus protects the underlying aluminum against corrosive attack. Such conversion coatings will also normally and in the desired manner provide a good base to which later applied overlying coatings containing desiccant, which can be decorative or of a functional type and are formed from such materials as paints, lacquers or printing inks, etc., will adhere firmly and strongly.

Among a number of other more extensive applications

is a special application of certain conversion coating solutions in connection with the coating of aluminum cans. Although, however, there are other methods for coating aluminum where it may be desired that the coating should give a colored appearance, e.g. a yellowish-to-green hue,

for the aluminum surface, the corrosion-resistant coatings to be applied to aluminum cans should be uniformly clear and colorless, so that even after they have been coated the cans will still have the natural clear, shiny appearance of the underlying aluminum and which it is desired to retain for the final product even though parts of the box may be covered with overlying drying coating.

However, it is not sufficient that the coatings

are clear and colorless when applied to the cans, but it is also necessary that they remain so afterwards. More specifically, the coating must be able to resist discoloration when the coated box is exposed to moderately hot water, e.g.

for water with a temperature of 60 - 77° C, during processes

which is usually referred to as "pasteurization" of the cans.

Such "pasteurization" tends to cause an uncoated or insufficiently coated aluminum surface to become black or otherwise discolored, giving the can an unappealing appearance.

It is perhaps somewhat strange that, however, it is also desired that under extreme conditions the coated aluminum cans should be able to discolour as this may be the basis for a simple test which has been developed to confirm the presence of the desired coating. One such commonly used test in the can industry is known as the "muffle test". With the help of such a sample, a can producer can be able to take arbitrary can samples from the production line and get confirmation that the desired clear and colorless coating is in reality present on the cans even if it is not visible.

There are currently already a number of coating solutions available that form uniformly clear and colorless coatings on aluminum surfaces. Perhaps the most widely used of these coating solutions is a solution containing chromic acid, phosphoric acid and hydrofluoric acid. In recent years, however, large parts of the industry have switched over from coating solutions containing hexavalent chromium to coating solutions which do not contain this because the use of hexavalent chromium is generally prone to cause waste disposal problems.

There are, however, chromate-free mixtures which are said to be able to form a coating on aluminium. More recently developed mixtures of this type are exemplified by those described in British published patent application 2,014,617 A and in US patents 4,017,334, 3,964,936 and 4,148,670. In all of these publications, acidic aqueous solutions containing a fluorine-containing compound and varying either a zirconium, titanium or, hafnium containing compound. However, this is not all,

and in US patent 4,148,670 a further essential component in the mixture is thus stated to be phosphate, while a polyhydroxy compound with 6 or fewer carbon atoms is

described as an optional component. In US patent 4,017,334, however, not only phosphate, but also tannin is described as additional essential constituents.

The presence of phosphate in the solution is claimed to contribute to the coating gaining corrosion resistance and the ability to receive paint, and furthermore that the coating can be subjected to the so-called "muffle test" should it be desired to use this to confirm the presence of the coating on an aluminum surface. However, phosphate is not a particularly desirable component as it has been shown that it leads to a reduction in adhesion for certain water-based drying coatings, and it would thus be desirable to be able to produce a coating in which phosphate does not constitute a significant component.

The main object of the invention is to provide an aqueous conversion coating solution which, although it is not necessary that hexavalent chromium or any similar toxic material be used therein, and although the solution may, if desired, be phosphate-free, is nevertheless capable of on an aluminum surface to form a clear and colourless, corrosion-resistant coating which provides an excellent substrate for the adhesion of subsequently applied overlying drying coatings. It should also be noted here that "corrosion resistant" as used herein unless otherwise stated > is intended to indicate not only that the coated surface is resistant to corrosion in the ordinary sense, but also that it is resistant to blackening or other discoloration when exposed to the above-mentioned "pasteurization" or another similar treatment in hot water or boiling water.

According to the invention, a chromate-free, acidic, aqueous coating solution is provided which is capable of forming a clear, colorless and corrosion-resistant coating on an aluminum surface, and the solution is characterized by the fact that it contains

(a) one or more of the metals zirconium, hafnium or titanium from group IVa in a total amount of at least 0.5 x 10 -3 mol/litre, (b) fluoride in an amount which is at least sufficient to form a combination with all the metals from group IVa present, and (c) either an organic polyhydroxy compound with up to 7 carbon atoms and which is present in a concentration of at least 0.025 x 10_3 mol/ liter in a solution that is free of phosphate and boron, or

a surfactant which is present in a concentration of at least 10 parts per million (ppm), or

both the organic polyhydroxy compound and the surfactant.

The acidic, aqueous coating solution according to the invention can be used to treat a shiny, shiny aluminum surface in such a way that its shiny, shiny appearance will not be destroyed, but still so that a uniform colorless and clear coating is formed on the surface that exhibits excellent corrosion resistance. as the t and to which overlying drying coating will adhere.

Moreover, the coating solution according to the invention is capable of forming this desired coating type on aluminum even in the absence of materials which are either toxic or lead to waste disposal problems. The coating solutions according to the invention can thus be and are free not only of hexavalent chromium, but also of such elements as manganese, iron, cobalt, nickel, molybdenum and tungsten, as well as of toxic materials such as ferricyanide and ferrocyanide. The absence of such materials in the coating solution makes it unnecessary to subject the effluent to a special treatment before it is released into the environment or transferred to a sewage treatment plant.

Moreover, the coatings formed by means of the coating solutions according to the invention are of particular value if they are to serve as a substrate for overlying drying coatings applied in the form of water-based mixtures. There has recently been a trend within the industry away from the use of coating compositions based on organic solvents, in favor of water-based coating compositions. However, industrial experience has shown that whatever their other advantages are, drying coatings formed from water-based mixtures will not adhere to conversion coatings of the known Zr, Ti or Hf type, as well as drying coatings formed from mixtures based on organic solvents. As a particular example, it can be mentioned that the drying coatings formed from water-based mixtures tend not to adhere as well to underlying conversion coatings formed from the phosphate-containing solutions according to US Patent 4,148,670 as those formed from drying coating mixtures based on organic solvents. It is, however, a very advantageous fact that the conversion coating solutions according to the invention can be used to form coatings on aluminum surfaces and which provide an excellent paint-receiving substrate on which drying coatings formed from water-based mixtures will adhere.

When component (c) is made up of the polyhydroxy compound, according to the invention it has been observed that the corrosion resistance of the coatings which have been formed from the coating solutions may tend to vary depending on the type of water that was used in the preparation of such mixtures. It generally seems that the corrosion resistance of the coatings is better if the coatings are formed from solutions made with hard water than if they are formed from solutions made with soft water. The relatively low sodium concentration in soft water seems to be detrimental to the corrosion resistance of the obtained coatings, or perhaps it can instead be said that the relatively high concentration of calcium in hard water is beneficial for the corrosion resistance of the obtained coatings. However, there is no variation in the coating's corrosion resistance depending on the hardness of the water in the solution when component (c) is either made up of the surface-active agent alone or of a mixture of the surface-active agent and the polyhydroxy compound.

At this point it is appropriate to mention that the term "surfactant" is used here to denote any material which will greatly reduce the surface tension of water when present in a small amount. The presence of e.g. thus, as little as 2 ppm of surfactant dissolved in water can reduce the water's surface tension by over 1/3 of its normal value. The term thus includes all the different common groups of surfactants, i.e. anionic, cationic, non-ionic or amphoteric surfactants. However, it should be noted that according to the invention it is preferred to use a non-ionic surfactant.

The acidic aqueous coating solution must, as mentioned above, contain one or more of the metals titanium, zirconium or naphium from group Via together with a fluoride and with a polyhydroxy compound and/or a surfactant.- Any suitable starting material which is soluble in the solution, can be used for these components.

As starting materials for zirconium, titanium or hafnium, e.g. soluble fluorozirconate, fluorotitanate or fluorohafnate compounds are used, such as fluorozircon, fluorotitanium or fluorohafnium acids, and the corresponding salts, such as ammonium or alkali metal fluorozirconates, -fluorotitanates or -fluorohafnates. Other suitable starting materials include metal fluorides, such as zirconium fluoride

(ZrF4), titanium fluoride (TiF3, TiF4) or hafnium fluoride (HfF4).

The coating solutions can also be prepared from mixtures of soluble compounds, one of which contains the metal or metals from group IVa and the other the fluoride. Examples of such compounds are water-soluble salts comprising nitrates and sulfates of Zr, Ti or Hf (e.g. zirconium nitrate, zirconium sulfate, titanium (IvJ sulfate or hafnium nitrate), or hydrofluoric acid or its water-soluble salts, such as the ammonium or alkali metal salts hence.

It is possible to form satisfactory coatings from solutions containing as little as approx. 0.5 x 10 mol/ liter (mol/l) of Zn, Ti or Hf (equivalent to approx. 0.05 .g/l Zr, approx. 0.02 g/l Ti or approx. 0.09 g/ l Hf). When a mixture of more than one of these metals from group IVa is used, they should together amount to at least 0.5 x 10 -3 mol/l. However, depending on other parameters of the coating process, it may be necessary to use larger amounts of these components in order to produce a satisfactory coating, as explained in more detail below.

There is no upper limit to the concentrations of dissolved zirconium, titanium or hafnium that can be used, except of course from the solubility limits for these in the acidic, aqueous coating solution and which are dependent on other parameters of the coating solution, including in particular the acidity and chloride content of the coating solution and the amounts of the other optional ingredients that may be present. These various parameters should be regulated so that precipitation of zirconium, titanium or hafnium compounds is avoided. Any such precipitation is undesirable for several reasons. Sedimentation depletes the solution of the constituents. If a deposit is deposited on the coated aluminum surface, it can adversely affect the properties of the coating, and an accumulation of any type of deposit is likely to adversely affect the coating method, e.g. in that it can clog the nozzle pieces. If the special case should occur that precipitates form, this can be overcome by lowering the pH of the coating solution and/or by increasing the fluoride concentration.

The minimum concentration of fluoride is the concentration that is necessary for fluoride to be able to combine with all present zirconium, titanium and/or nanium while forming a soluble complex with these, e.g. a fluorozirconate, fluorotitanate or fluorohafnate. The minimum amount of fluoride thus depends on the amount

of zirconium, titanium and/or hafnium in the solution. It has generally been shown that at least approx. 4 moles of fluoride are required per mol Zr,_ Ti and/or Hf to prevent precipitation of these metals. According to the invention, it is preferred to use at least 6 mol of fluoride per moles of Zr, Ti and/or Hf.

It must also be remembered that during prolonged use of the coating process and. thus in such cases where the coating solution is recycled or a bath of the solution is used continuously, there will be a build-up of the concentration of the aluminum which, due to the solution, dissolves from the surface. Such a buildup of dissolved aluminum can adversely affect the coating process unless the coating solution contains a sufficient amount of fluoride to complex the dissolved aluminum.

Based on practical considerations, it is thus highly desirable that the coating solution, when a process is used on an industrial scale, should contain an excess of fluoride over and above the amount that forms a complex with aluminum and with any other component in the solution that forms a complex with the fluoride . Such an excess of fluoride is referred to herein as "available fluoride", and the manner in which the amount of "available fluoride" is calculated is well known to one skilled in the art. A coating solution containing available fluoride is a solution in which fluoride is available to complex with aluminium.

Although as stated immediately above a certain excess of fluoride or available fluoride is desired, care should be taken to avoid too much of this. It is recommended that the available fluoride concentration not exceed 26.3 x 10 ^ mol/l (or not exceed 500 ppm) to avoid unwanted etching of the aluminum surface as this tends to give the surface a dull or frost-like appearance, and thereby also to avoid adverse effects on the coating's corrosion resistance and ability to receive paint, in addition to avoiding precipitation of calcium or other such ions that are present in the solution.

The fluoride feedstock for use in the coating solution can be any material which is soluble in the solution and which provides a fluoride feedstock capable of complexing with aluminum, provided that the feedstock does not contain anything that adversely affects the coating process. If, however, it is assumed that fluoride is added

■such as a complex fluoride with titanium, zirconium or hafnium, another source material for fluoride should also be added to the solution to serve as a source material for fluoride to be complexed with the aluminum built up during continuous operation. Alternative starting materials for fluoride for this purpose include such materials as HF, salts thereof, NH^F.HF, alkali metal bifluorides, H2SiFg or HBF^. The particularly preferred starting materials for fluoride are HF and HBF^.

As mentioned above, a surface-active material will

in the solution according to the invention preferably consists of a non-ionic surface-active material. Although as little as 10 ppm of surfactant may be sufficient, it is preferred to use the surfactant at a concentration of 20-100 ppm. Even larger amounts, e.g. up to 500 ppm, can be used without this being harmful, but according to the invention it has been shown that usually little or no further improvement is obtained when such higher concentrations are used.

When a polyhydroxy compound is to be present, this can be any water-soluble polyhydroxy compound which does not have more than 7 carbon atoms. The starting material can of course be the polyhydroxy compound as such, but it can also be any compound which is soluble in the coating solution and which, when dissolved in it, gives the desired polyhydroxy compound and is not otherwise harmful to the coating properties of the solution or to the coatings corrosion resistance and adhesion of paints to these. Examples of such compounds include glucosic acid and its salts, sodium glucoheptonate, sorbitol, mannitol, dextrose, ethylene glycol or glycerol. Particularly preferred polyhydroxy compounds are gluconic acid and its alkali metal and ammonium salts. Any compound which is soluble in the coating solution and which gives gluconate and/or gluconic acid can be used. Examples of such compounds are stable gluconolactones, such as glucono-A-lactone or glucono-^-lactone.

It has been found that using the polyhydroxy compound in the coating solution enables the user to perform a simple test to confirm that the coating is present on the aluminum surface even when the solution is free of phosphate. In the case of an industrial process that may include processing very large quantities of aluminum within a relatively short time, it is useful to have a simple test to confirm that the coating solution forms a coating as the coating is not visible to the eye. An unnoticed

in

change in the working parameters of a bath of the coating solution and which makes this not work effectively, can take place due to mechanical failure or human failure. As an example, an incorrect top-up of the coating solution can go unnoticed.

It has been shown that an aluminum surface coated with such a mixture according to the invention, which contains the polyhydroxy compound and no phosphate, changes color from light golden brown to darker shades of brown or purple when it is exposed to a relatively high temperature for a relatively short time, e.g. e.g. 482° C for 5 minutes. This test, which is referred to here as the "muffle test", can be used for arbitrary sampling of treated aluminum surfaces to determine whether the coating solution is deposited on the aluminum surface. If the coating is not deposited, the aluminum surface will have a dull, greyish appearance after the muffle test. It is quite surprising that such surfaces can be subjected to this test with good results, as it has hitherto been assumed that the presence of phosphate was necessary for a positive test to be obtained.

Another advantage which arises from the polyhydroxy compound and which is also obtained by using the surface-active agent is that coatings which have been formed from coating solutions containing this component improve the coating's ability to resist blackening or other discoloration for at least 5 minutes and up to such a long time as

15 minutes when exposed to water at a temperature of

60 - 77° C. As noted above, aluminum cans are occasionally treated in this way when subjected to so-called "pasteurization" methods.

It has also been shown that when the polyhydroxy compound is used, the corrosion resistance and adhesion properties of the coatings are improved, especially when the coatings are formed from a coating solution with a pH below approx. 3.5. It has also been shown that superimposed drying coatings, especially water-based coatings, adhere very well to coatings containing polyhydroxy compounds. Although drying coatings based on organic carriers adhere well to coatings containing phosphates, certain water-based coatings have been found not to adhere nearly as well to such coatings.

Coated aluminum cans which are highly resistant to tarnishing by the action of water and which are capable of discolouration when subjected to the above muffle test have been produced from -3 coating compositions containing as little as 0.025 x 10 mol/l of the polyhydroxy compound. Such coating mixtures preferably contain 0.3 x 10 -3 - 1.75 x 10 -3 mol/l of the polyhydroxy compound. Larger quantities, e.g. up to 2.5 x 10 -3mol/l, can be used, but in general little or no further improvement is obtained by using higher concentrations.

When the polyhydroxy compound is used in the solutions according to the invention together with the surfactant, it is recommended that at least 40 ppm of the polyhydroxy compound is used. Although higher amounts may be used, it is recommended that the polyhydroxy compound be present in an amount not exceeding 1000 ppm. Preferably, 40 - 400 ppm of the polyhydroxy compound is used.

The coating resolution's arrow can vary within a wide range, e.g. from 1.5 to 5. Improved corrosion resistance which can be attributed to the surface-active agent when this is present in the solution according to the invention, can be observed in particular at a pH of 3.5 - 4.5. Improved corrosion resistance which can be attributed to the polyhydroxy compound when this is present in the solution according to the invention, can be observed in particular at a pH of 3.0 - 5.0, preferably at a pH of 3.0 - 4.0. The pH of the solution can be adjusted by adding suitable amounts of nitric acid or ammonium hydroxide. Although nitric acid and ammonium hydroxide are preferred as pH regulators, any acid or base can be used that will not adversely affect the coating process. As an example, it can be mentioned that perchloric acid or sulfuric acid can be used.

The coating solution according to the invention should be free of chromium, iron cyanides and any materials which in the solution form solids which are prone to precipitation.

Examples of other materials that can possibly be added to the coating solution according to the invention,

are those previously reported to be useful for Zr-, Ti- or Hf- and fluoride-containing compounds. For example, it is described in the above-mentioned US patent 3,964,936 that materials which are starting materials for boron can be used in an amount of 10 - 200 ppm. When a boron compound is added to the acidic, aqueous coating solutions according to the invention which contain a surface-active agent, it is particularly preferred that the boron compound is added in the form of boric acid in the above-mentioned amounts. Boron compounds are not added to the solutions according to the invention which do not contain the surfactant. Tannin is another optional ingredient that can be added to the solution in concentrations of from 25 ppm and up to 10 g/l (see US Patent 4,017,334 and British Patent Application 2,014,617).

When coating mixtures based on organic solvents are used to form the overlying drying coating, the solution according to the invention may possibly contain phosphate in an amount of 10 - 1000 ppm, as described in US Patent 4,148,670, except when the solution contains the polyhydroxy compound and does not contain the surfactant.

Further other materials which can optionally be added to the coating solution according to the invention are various other acids, comprising e.g. glutaric, ascorbic, maleic or salicylic acid. Such acids can be used in an amount of at least 5 ppm, and preferably in an amount of 100 - 500 ppm, to achieve various advantages, including improving the adhesion properties of coatings formed from the solution.

The quantity range for components that can be used

in the mixture according to the invention, is described above. Considerations should be made when special mixtures are to be made for special applications within the above-mentioned areas. When a relatively high pH is used, relatively small amounts of zirconium, titanium and/or hafnium should be used to inhibit precipitation. When the aluminum surface comes into contact with the coating solution for a relatively short time, relatively high amounts of the above-mentioned metals should be used. When, in a similar manner, the contact temperature between the coating solution and the aluminum surface is relatively low, relatively high amounts of components should be used.

A preferred embodiment according to the invention (hereinafter referred to as "preferred embodiment A") has a pH within the range 3.4 - 4 and contain:^.:,

The preferred starting material for Zr for the preferred embodiment A is ammonium fluorozirconate, and the preferred polyhydroxy compound is gluconic acid. Hydrofluoric acid is preferably used as a starting material for available fluoride, and nitric acid is used to regulate the pH.

When hafnium is added to the preferred embodiment A, this is preferably added in an amount of 0.5 x 10 - 1.75 x 10 mol/litre. The preferred starting material for hafnium is HfF4- Other preferred ingredients and amounts thereof that can be used in the preferred Zr-containing solution for the preferred embodiment A are

been described above.

Another preferred embodiment according to the invention (hereinafter referred to as "preferred embodiment B") -3 -3 has a pH of 3.5 - 4.5 and contains 0.75 x 10 - 2 x 10 mol/l zirconium and 10 - 500 ppm surfactant, -3 and it preferably has a pH of 3.7 - 4.3 and contains 1 x 10 1.75 x 10 -3 mol/l zirconium and 20 - 100 pm surfactant, each of the above ore forms contain sufficient fluoride to enter complex formation with all Zr and the dissolved aluminum present in the solution.

The preferred starting material for both Zr and fluoride in the top-up mixture according to the preferred embodiment B is fluorozirconic acid, and nitric acid is preferably used to regulate the pH.

The coating solution according to the invention can be easily prepared by diluting an aqueous concentrate of the components with a suitable amount of water. As regards the preferred embodiment A should e.g. a concentrate be such that when a coating solution contains 0.5 - 10% by weight of the concentrate, the present amounts of the constituents in the coating solution will be: (A) at least

-3

0.5 x 10 mol/litre zirconium and/or hafnium, (B) at least

_3

0.025 x 10 mol/liter polyhydroxy compound and (C) fluoride in an amount at least sufficient to combine with substantially all of the zirconium or hafnium to form a complex therewith, and the pH of the coating solution being 3 - 5. It is even more preferred that the concentrate is such that when the coating solution contains 0.5 - 10% by weight of the concentrate, the coating solution will comprise (A) 0.5 x 10 -3 - 1.75 x 10 -3 mol/ liter of zirconium, added as a fluorozirconate, such as sodium or potassium fluorozirconate, preferably ammonium fluorozirconate, (B) 0.3 x 10<_3>

-3

1.75 x 10 mol/liter po 1 y hyd roxy I or b i.ndcl see l i. I. ::;a I: I: as

-3 -3

gluconic acid, (D) 0.5 x 10 - 2.5 x 10 mol/liter HF and

(E) nitric acid in such a quantity that the pH of the coating solution will lie within the range 3.4 - 4.

As a further example, it can be stated that in the case of the preferred embodiment B, the concentrate should be such that when a coating solution . comprises 0.5 - 10% by weight of the concentrate, the amounts of component--3 parts in the coating solution shall be: (A) at least 0.5 x 10 mol/liter zirconium, titanium and/or hafnium and (B) fluoride in an amount which is at least sufficient to combine with substantially all of the Zr, Ti or Hf to form a complex therewith, and (C) at least 10 ppm surfactant.

In a continuous coating process, fluoride and the metal are consumed, and these constituents as well as other constituents are further depleted as a result of extraction of the solution onto the aluminum surface. The rate of depletion is related to the shape of the surface being coated, and also to the agent used to apply the coating solution to the surface. In addition to this depletion, a build-up of the concentration of dissolved aluminum occurs, as indicated above. In the case of a continuous coating process, the components should therefore be replenished.

The top-up can be carried out by monitoring each component and by adding a further amount thereof as it is depleted, but replenishment is preferably carried out by adding an aqueous concentrate containing the components to be replenished, in such amounts as will effectively maintain the components in solution in effectively usable amounts. The top-up mixture preferably contains a relatively high proportion of fluoride when it is expected that the aluminum concentration will increase in the coating solution. Preferred starting materials for available fluoride for use in replenishment are HF or ammonium bifluoride or a mixture thereof or HBF^.

Thus, as an example and with reference to the preferred embodiment A, the following is a recommended aqueous concentrate for top-up for the coating solution for the preferred embodiment A: (A) 31 x 10 -3 - 251 x 10 -3 mol/liter zirconium and/ or hafnium,0 (B) 19 x 10 -3 - 148 x 10 -3 mol/liter polyhydroxy compound and (C) a material which is a starting material for 90 x 10_ 3

-3

695 x 10 - mol/litre available fluoride, preferably in the form of HF, ammonium bifluoride or a mixture thereof.

As a further example and with reference to the preferred embodiment B, the following is a recommended aqueous concentrate for replenishing the coating solution for the preferred embodiment B: (A) 0.05 - 0.5 mol/liter zirconium, titanium and/or hafnium and

(B) 0.2 - 10 mol/liter fluoride.

The coating solutions according to the invention should

applied to a clean aluminum surface. Available cleaning agents, such as alkaline or acidic cleaning solutions, can be used to clean the aluminum surface using common methods.

When deep-drawn and dimensionally deep-drawn aluminum cans are coated, it is preferred to treat the cans with a cleaning solution comprising an acidic, aqueous solution of a mixture of HF, H 2 SO 4 and a surfactant, e.g. such solutions as are described in US patents 4,009,115, 4,116,853 and 4,124,407.

The coating solution can be applied to the aluminum surface by any method. The solution can e.g. is applied by spraying it onto the aluminum surface or the aluminum surface can be immersed in the solution which can also be applied by roller or float coating methods or by mist application methods. The solution can be used to coat individual objects, such as e.g. cans, or it can be used to coat aluminum blanks, such as aluminum strips, which are then further processed into end items.

The temperature of the coating solution should be as follows

that the reactive components in the solution will bind to the aluminum surface. When e.g. such a solution as the preferred embodiment A is used, a temperature of at least 43° C is generally necessary to achieve the desired degree of corrosion resistance, and such a coating solution should preferably have a temperature of 54 - 66° C. When a

such solution as the preferred embodiment B is used, a temperature of at least 32° C is generally necessary to achieve the desired degree of corrosion resistance, and temperatures up to 60° C can be used, preferably 43 - 54° C.

If the temperature of the coating solution is too high, problems such as a dull surface and a frost-like surface can occur. The temperature at which this takes place depends on various parameters for the coating process, including e.g. the contact time between the solution and the aluminum surface and the reactivity of the solution which depends on the pH and the concentration of components in the solution. As a further example, it can be mentioned that when such solutions as the preferred embodiment A are used, precipitation of zirconium and/or hafnium oxides at temperatures above 71° C can become a problem if the pH of the coating solution increases to above 4.5.

Desired coatings can be formed by bringing the coating solution and the aluminum surface into contact with each other for at least 5 s, preferably at least 15 s. The lower the temperature of the coating solution, the longer the contact time should be, and the higher the temperature of the solution, the shorter the contact time is necessary. It will generally be unnecessary to keep the surface in contact with the coating solution for longer than 1 minute.

After the coating solution has been applied to the aluminum surface, it should be rinsed with water, including a final rinse with deionized water. Rinsing with water containing a small amount of dissolved solids can result in a coating that does not adhere well to a subsequently applied drying coating. When using the present invention, it is not necessary to rinse the coated surface with an aqueous solution of chromium, e.g. a solution of hexavalent chromium.

After the coated surface has been rinsed with water or otherwise treated as described above, the coating should be dried. This can be done in any practical way, such as e.g. drying in an oven or drying with forced circulation of hot air. Other available drying methods can be used.

After the coating has been applied, sanitary or decorative coatings can be applied, e.g. by applying a drying coating to the coated surface. These coatings are usually applied after the aluminum surface has been coated, rinsed with water and dried. For some applications, the sanitary coating is applied after rinsing with water, and both the coating formed according to the invention and the sanitary coating are dried at the same time.

Drying coatings that include the functional and/or aesthetic coatings that are applied to the coatings formed by using the coating solution according to the invention are of course well known and can be formed from water-based mixtures or mixtures based on organic solvents.

It should be stated as an example that when aluminum cans are to be filled with beer, the cans are treated with the coating solution according to the invention, after which sanitary and/or decorative coatings are applied. The cans are then filled with beer and sealed, after which the beer-filled cans are pasteurized.

It is assumed that the zirconium, titanium or hafnium present in the coating solution according to the invention is in a complex form which is both soluble in the solution and reactive towards the aluminum surface, so that a coating containing such a metal is formed on it, without this" goes beyond the glossy, shiny appearance of the aluminum surface. The solution should therefore be free of components that will enter into a connection with the above-mentioned metals, forming compounds and/or complexes that precipitate from the solution, and/or compounds or complexes that do not react with the aluminum surface or that are reactive towards it, but in such way that the glossy, shiny appearance of aluminum-miniumover latency is changed.

In order to further explain the invention, it will be described below in more detail with the help of the following examples, and the invention will below also be assessed in relation to certain comparative examples which are not covered by the invention.

Unless otherwise stated, the aluminum surfaces that were treated with the solutions described in the examples were deep-drawn and dimension-drawn aluminum cans that were first degreased as needed in an acidic, aqueous cleaning agent containing sulfuric acid, hydrofluoric acid and surface-active agent. Unless otherwise stated,

the coating solutions were applied by spraying for approx.

20 s at the appropriate temperatures. After treatment with the solutions described in the examples, the aluminum surfaces were first rinsed with tap water and then with deionized water, after which they were dried in an oven for 3.5 minutes at approx. 204°C.

The aluminum cans were then tested to determine their corrosion resistance by subjecting them to a water stain resistance test that had been developed to mimic can exposure during commercial pasteurization processes. The test consisted of immersing the cans for 30 minutes in a warm solution of distilled or deionized water containing 0.22 g/l sodium bicarbonate, 0.082 g/l sodium chloride and 2.18 g/l water conditioner (Dubois 915 which has a total alkalinity of 5.8% Na2O and by analysis contains sodium nitrate, -carbonate, triethanolamine and dodecylphenyl-polyethyleneglycol). The solution was maintained at 65.6 + 2.8°C during the test. After the immersion, the cans were rinsed with tap water, dried with a paper towel and then examined to determine staining. An aluminum surface that has just been cleaned will turn black or brown within minutes when subjected to this test. It appears from the examples below that a pre-treatment of aluminum surfaces with the coating solutions according to the invention can result in coated surfaces that resist blackening or other discolouration. The aluminum over rafts were rated according to the following scale: 5 - perfect, identical to a treated but untested surface 4.5 - very slight reduction of the glossy appearance of the surface

4.0 - very slight discoloration

3.5 - slight discoloration but commercially acceptable 3.0 - discoloration considered non-commercial

acceptable

0 - complete failure, characterized by strong blackening.

In some of the examples, the aluminum cans were also examined to determine their paint adhesion after the cans had been treated with the indicated solutions. After the treated surface was dried, as described above, part of the surface was painted with a water-based white primer (No. CE3179-2 white polyester sold by PPG Industries Inc.), and the other part of the surface was painted with a water-based varnish (Purair ® S145-121) which in the examples is denoted by CE3179-2 and S145-121. After the paint had hardened, the painted surface was immersed in boiling water for 15 minutes. After the painted surface was removed from the boiling water, it was cross-scratched with a sharp metal object to expose scratches of aluminum that could be seen through the paint or varnish, and the surface was examined to determine its paint adhesion. This test included non-application of

i.e

a transparent adhesive tape (Scotchr^ transparent tape No. 610) so that it adheres to the cross-scratched area, after which the tape is pulled backwards with a quick pulling motion, so that the tape is pulled away from the cross-scratched area. The results of the test were judged according to the following scale: 10 = perfect as the tape did not pull paint off the surface

8 = acceptable

0 = complete failure.

Examples 1-15 and comparative examples C1-C3

The. the following acidic aqueous concentrate was prepared for use in connection with the first group of examples:

The above mixture was prepared by

mixing an aqueous solution of 23.4 g of 45% by weight fluorozirconic acid with a portion of the water, after which the aqueous ammonia was added to the resulting solution. A white precipitate formed, but this dissolved when nitric acid was added. The clear solution obtained was diluted to 1 liter with deionized water, and this gave a concentrate that was used for the preparation of the treatment solution at a concentration of 2.5 vol% in water comprising 2 parts deionized water and 1 part hard water (in the examples the term "hard water" denotes tap water from the city of Ambler, Pennsylvania, USA, which contains 80 - 100 ppm calcium and

has a conductivity of 400 - 600 ohms ^). To this solution was added, in the amounts indicated in Table 1 below, a non-ionic surfactant (Surfonic LF-17) which is reported to consist of a low-foaming alkyl-polyethoxylated ether. Solutions of varying acidity were prepared by adjusting the pH of the above mixture with suitable amounts of an aqueous solution of 15% (w/v) ammonium carbonate or dilute nitric acid.

The water stain resistance of aluminum cans coated with the mixtures is given in Table I below.

The improved corrosion resistance within the pH range of 3.5 - 4.5 for the mixtures assessed in Table I is clearly evident. Other tests show that for the particular type of mixture assessed in Table I, the resistance to water stains for the coatings formed from mixtures with and without surfactant was approximately the same when the pH of the mixture was approx. 2.5. Further tests showed that for the particular type of mixture rated in Table I, but prepared only with deionized water and containing 20 ppm surfactant, an improved resistance to water stains was obtained when the pH of the mixture was above 3.5, and a significant improvement was obtained at a pH of approx. 4.

Examples 16-39 and Comparative Examples C4-C6

The next group of examples shows the use of mixtures of the type described in examples 1-15 and in comparative examples C1-C3, but which also contained 0.1 g/l gluconic acid which effectively improved the coated surface's resistance to water spots. Mixtures of different acidity and containing the surfactant used for the mixtures according to the first group of examples or another surfactant were evaluated. The second surfactant was a modified low foaming polyethoxylated straight chain

(s)

alcohol (Triton WDF-16) . The particular mixtures that were assessed and the results of the test are reproduced in Table II below.

Example 4 0

Table III below shows the effect of gluconic acid concentration on water stain resistance for coatings applied at temperatures varying from 32.2°C to 65.6°C. Zirconium was present in each solution in

■ form of ammonium fluorozirconate ((NI-^^ZrFg) in a concen-

tion of 1.25 x 10 ^ mol/l, and each solution was adjusted to a pH of 3.8 by the addition of concentrated nitric acid.

Two boxes were used to determine each solution's anti-

resistant to water stains.

Example 41

In the following table IV is also the effect

of the gluconic acid concentration on the resistance to water stains shown, and furthermore on the adhesion of water-based drying coatings, at two different pH and temps. Zirconium was again present in each solution in the form of ammonium fluorozirconate ((NH4)2ZrFg) in a concentration of 1.25 x 10_ 3 mol/l, and the pH of each solution was regulated by the addition of concentrated nitric acid. Two boxes were used to

determine each paint adhesion ("Adhesion"), while each water spot resistance rating ("Resistance") represents the average rating for six cans.

Examples 42 - 45

An aqueous concentrate of the type used to prepare the compositions of Examples 1-15 was diluted with a sufficient amount of water (2 parts deionized water and 1 part hard water) to obtain a coating solution containing 2.5 vol. of the concentrate, to which 20 ppm Surfonic LF-17 was added. Four solutions were thus prepared, 0.1 g/l of one of the following components being added to each solution: glutaric acid, ascorbic acid, maleic acid or salicylic acid. Each of the blends was 3.5, and each was used to treat aluminum cans in the manner described above in connection with the preceding examples. For all boxes treated in this way, water stain resistance ratings were above 3.5, and an improved wood-. adhesion was achieved when the cans were painted with the water-based coatings PPG CE 3179-2 or S145-121 or with the organic solvent-based coatings P1099-7A or P550-G.

Examples 46 - 49

Other mixtures were prepared by introducing 0.5 x 10 -3 mol/l gluconic acid into each of the mixtures according to the above examples 42 - 45. Coatings formed from such mixtures showed improved adhesion over topcoats formed from various water-based resin coating agents.

Example 5 0

Table V below shows the effect of the ammonium fluorozirconate concentration on the water stain resistance of coatings applied at varying temperatures from 32.2°C

to 65.6° C. Gluconic acid was present in each solution at a concentration of 0.5 x 10 -3 mol/l, and each solution was adjusted to a pH of 3.8 by the addition of concentrated nitric acid. Two boxes were used to determine the resistance to water spots obtained using each solution.

Example 51

In Table VI below, the effect of ammonium fluorozirconate concentration on the resistance to water stains is shown, as well as on the adhesion of water-based drying coatings, at three different pH and two different temperatures. Gluconic acid was again present in each solution in a concentration of 0.5 x 10<3>mol/l, and the pH of each solution was regulated by the addition of concentrated nitric acid. Two cans were used to determine each paint adhesion rating, while each water stain resistance rating represents the average of six cans.

Example 52

Table VII below shows the resistance to water stains for coatings formed from a solution of hafnium tetrafluoride, hydrofluoric acid and gluconic acid at varying temperatures from 32.2° C to 65.6° C. The solution contained

-3 -3

1.25 x 10 mol/l hafnium tetrafluoride, 2.5 x 10 mol/l

-3

hydrofluoric acid and 0.5 x 10 mol/l gluconic acid. For comparison, coatings were also formed from a similar solution that was free of gluconic acid. The pH of both solutions was adjusted to 3.8 by adding concentrated nitric acid. Two boxes were used to determine the water stain resistance rating of the solutions.

Examples 53 - 56

In the following tables VIII, IX, X and XI are shown the effect of pH and temperature on the resistance to water stains of coatings formed from a solution of ammonium fluorozirconate and gluconic acid, and also on the adhesion of additional water-based drying coatings to such coatings. The solution used contained 1.25 x 10 -3 mol/l ammonium fluorozirconate and 0.5 - 10 -3 mol/l gluconic acid. For comparison, coatings were also formed from a similar solution that was free of gluconic acid. The pH of both solutions was adjusted to the values given in the tables by adding concentrated nitric acid. Such solutions were then applied at varying temperatures from 32/2° C to 71.1° C. Two boxes were used to determine the rating of each water stain resistance, and one box was used to determine the rating of each paint adhesion at each pH and temperature. The state of each solution (clear or cloudy) at each pH and temperature used was also recorded. It appears from tables X and XI that the presence of gluconic acid is important at a pH of 4.5 and 5.0 in order to be able to maintain a clear solution and prevent precipitation.

Example 57

Table XII below shows how the addition of phosphate to an ammonium fluorozirconate solution adversely affects the adhesion of water-based drying coatings to coatings formed from such solutions. The concentration of phosphate and ammonium fluorozirconate in each of the prepared solutions is given in the table. The phosphate was added in the form of phosphoric acid. The arrow of the solutions was varied as shown in the table, and again concentrated nitric acid was used to regulate the pH. The solutions were applied at a temperature of 54.4° C. Two boxes were used to determine the rating for each paint adhesion for each solution.

Example 58

Table XIII below shows how the addition of phosphate and gluconic acid to ammonium fluorozirconate solutions affects the adhesion of water-based drying coatings to coatings formed from such solutions. The concentration of each of these materials in each of the prepared solutions is given in the table. The phosphate was added in the form of phosphoric acid. The pH of the solutions was varied as shown in the table, and concentrated nitric acid was used to regulate arrowroot. The solutions were applied at the indicated temperatures. Two boxes were used to determine the rating of each paint adhesion for each solution.

Example 59

To demonstrate that aluminum surfaces that have been coated with a coating solution containing gluconic acid, zirconium and fluoride will satisfactorily pass the so-called "muffle test", while aluminum surfaces that have been coated with a similar coating solution that is free of gluconic acid do not pass this test, a number of aluminum cans were coated with solutions of those compositions and applied at the temperatures given in Table XIV below, each solution having a pH of 4.25 after the addition of concentrated nitric acid. The coated cans were then heated at a temperature of 482.2°C for 5 minutes, and the color of the cans was observed. The observed results are reproduced in the table below

XIV.

As a summary, it can be claimed that the present invention provides a means for forming on an aluminum surface a non-chromate-containing coating which is colorless and clear without affecting the appearance of the aluminum surface. The coated surface exhibits improved corrosion resistance, as exemplified by the test results reported above, and it exhibits excellent adhesion for applied drying coatings formed from water-based coating agents or on organic-; solvent-based coating agents.

Claims (12)

1. Method for producing a clear, colourless, corrosion-resistant coating on an aluminum surface, characterized in that the surface is brought into contact with a chromate-free, acidic, aqueous coating solution containing A) one or more of the metals zirconium, hafnium and titanium from group IVA in a total concentration of at least 0.5 x 10 mol/l, B) fluoride in an amount at least sufficient to combine with and form a soluble complex with the entire amount of the Group IVA metal or metals present, and C) an additive' in the form of i) an organic polyhydroxy compound with up to 7 carbon atoms and present in a concentration of at least -3 0.025 x 10 mol/l in a solution that is free of phosphate and boron, and/or ii) a surfactant present in a concentration of at least 10 ppm. 2. Method according to claim 1, characterized in that a number of aluminum surfaces are treated with a conversion coating solution in which the additive (C) consists of a surface-active agent, and that the coating solution is periodically or continuously replaced as needed with an aqueous top-up concentrate in order to maintain the concentration of the components in the coating solution within the specified limits, using a top-up concentrate containing 0.05-0.5 mol/l zirconium, 0.2 -10 mol/l fluoride and 1-100 g/l surfactant. 3. Method according to claim 1, characterized in that a number of aluminum surfaces are treated with a conversion coating solution in which the additive (C) consists of a polyhydroxy compound, and that the coating solution is periodically or continuously replaced as needed with an aqueous top-up concentrate in order to maintain the concentrations of the constituents in the coating solution within the specified limits, using a top-up concentrate containing -3 -3 31 x 10 - 251 x 10 mol/l zirconium, a starting material -3 -3 which provides 90 x 10 - 695 x 10 mol/l uncomplexed -3 -3 available fluoride, and 19 x 10 -148 x 10 mol/l poly-hydroxyf orb compound . 4. Method according to claims 1-3, characterized in that after the conversion coating has been formed on the aluminum surface, an overlying, water-based, drying coating is applied to it. 5. Chromate-free, acidic, aqueous coating solution for use in producing a clear, colorless, corrosion-resistant coating on an aluminum surface, characterized in that it contains A) one or more of the metals zirconium, hafnium and titanium from group IVA in a total concentration of at least 0.5 x 10~ <3> mol/l, (B) fluoride in an amount at least sufficient to combine with and form a soluble complex with the entire amount of the Group IVA metal or metals present, and C) an additive in the form of i) an organic polyhydroxy compound with up to 7 carbon atoms and which is present in a concentration of at least -3 0.025 x 10 mol/l in a solution that is free of phosphate and boron, and/or ii) a surfactant present in a concentration of at least 10 ppm. 6. Solution according to claim 5, characterized in that the metal from group IVA is zirconium or comprises zirconium. 7. Solution according to claim 5 or 6, characterized in that the additive C) is or comprises a surfactant, and that the pH of the solution is 3.5-4.5. 8. Solution according to claims 5-7, characterized in that it also contains at least 10 ppm of a boron compound. 9. Solution according to claims 5-8, characterized in that, in addition to at least 10 ppm surfactant, it also contains at least 0.025 x 10 <-3> mol/l of the polyhydroxy compound. 10. Solution according to claims 5-9, characterized by the fact that it also contains tannin. 11. Aqueous top-up concentrate for use in the production of a clear, colourless, corrosion-resistant coating on an aluminum surface by addition to the top-up step according to claim 2, characterized in that it contains 0.05-0.5 .mol/l zirconium, 0.2-10 mol/fluoride and 1-100 g/l surfactant. 12. Aqueous top-up concentrate for use in the production of a clear, colorless, corrosion-resistant coating on an aluminum surface for addition to the top-up step according to claim 3, characterized in that it contains 31 x 10-3 251 x 10 _3 mol/l zirconium, a starting material that provides 90 x 10 -3 - 695 x 10 -3 mol/l uncomplexed, available -3 -3 fluoride, and 19 x 10 - 148 x 10 mol/l polyhydroxy compound.