GB1570041A

GB1570041A - Acidic aqueous chemical conversion solutions and processes for forming adherent corrosion-resistant coatings therewith upon aluminium surface

Info

Publication number: GB1570041A
Application number: GB14463/77A
Authority: GB
Original assignee: Union Carbide Agricultural Products Inc
Current assignee: Union Carbide Agricultural Products Inc
Priority date: 1976-04-05
Filing date: 1977-04-05
Publication date: 1980-06-25
Also published as: NO148192C; BR7702121A; AU2394477A; PL197203A1; AU499368B2; CS196209B2; NZ183742A; NL7703702A; NL173543B; NO148192B; DE2715292A1; HU176364B; FR2347459A1; SE441190B; ES457569A1; DK148277A; DE2715292C2; JPS5633468B2; MX145560A; FR2347459B1

Description

(54) IMPROVEMENTS IN OR RELATING TO ACIDIC, AQUEOUS CHEMICAL CONVERSION COATING SOLUTIONS, AND PROCESSES FOR FORMING ADHERENT, CORROSION-RESISTANT COATINGS THEREWITH UPON ALUMINIUM SURFACES (71) We, AMCHEM PRODUCTS, INC., [formerly UCAR CORPORATION] a Corporation organised under the Laws of the Commonwealth of Pennsylvania, United States of America, of Brookside Avenue, Ambler, Pennsylvania, United States of America do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention concerns improvements in or relating to acidic, aqueous chemical conversion coating solutions and processes for forming adherent, corrosion-resistant coatings therewith upon aluminium surfaces.

It should be noted that as used herein the term "aluminium" embraces not only pure aluminium but also alloys in which aluminium is predominant, for example aluminium alloys containing minor amounts of metals such as for example magnesium, manganese, copper and silicon. It is however expected that the coating solutions and processes of the present invention will be used mostly upon aluminium surfaces which have a bright, shiny appearance, such as aluminium strip and above all aluminium cans or can-stock. Perhaps the most popular aluminium alloy currently used in the canning industry is Al Alloy No. 3004, which apart from aluminium itself and unavoidable impurities contains 1.2% manganese and 1.0% magnesium.

Acidic, aqueous coating solutions are already known which are effective in forming chemical conversion coatings upon aluminium surfaces that are corrosion-resistant and thereby protect the surface from degradation due to corrosive attack. Such chemical conversion coatings are also intended to help overlying siccative coatings which are either decorative or functional in nature (such as paints, lacquers, inks, etc.) to adhere tightly and strongly to the underlying aluminium surface.

An important field for aluminium coating today, and indeed one with which the present invention is directly concerned, is in the coating of aluminium cans. The chemical conversion coatings applied to aluminium cans must be not only corrosion-resistant and adherent but also should be uniformly clear and colourless, so that the cans though coated still retain the bright shiny natural appearance of the underlying aluminium normally desired in the final product even though the can may be in parts covered with coloured paints or inks, Besides the above-indicated properties desired in virtually all aluminium coatings a highly-desirable property in coated aluminium cans is their ability to resist discolouration when subjected to moderately hot water, for example within the range of from 140"F to 1700F (60"C to about 77"C). In certain kinds of uses aluminium is treated in this way, the process being referred to in the can industry as "pasteurization" of the cans; but this treatment has a tendency to cause an uncoated or even a coated aluminium surface to blacken or otherwise discolour, making the can unattractive in appearance.

There are aqueous, acidic chemical conversion coating solutions available today which will form uniformly clear colourless coatings on aluminium surfaces that meet the various requirements outlined above. Unfortunately the best of these contain chromic acid, phosphoric acid and hydrofluoric acid; and their use creates waste disposal problems, because of the presence therein of hexavalent chromium, a very toxic material.

There is therefore a need for chemical conversion coating solutions, which do not contain hexavalent chromium, capable of forming tighly-adherent, corrosion-resistant conversion coatings, resistant to pasteurization-discolouration and clear and colourless, providing a good foundation for siccative coatings upon aluminium surfaces.

A review of the state of the art shows, so far as we can determine, that till now colourless coatings could be obtained using aqueous coating solutions which contain hexavalent chromium and/or which form coatings requiring the use of a hexavalent chromium afterrinse treatment; or that using a coating and after-rinse solutions which are free from hexavalent chromium the coatings formed on aluminium surfaces have all been coloured. In passing it may also be noted that many prior disclosures recommend against the presence of phosphate.

We have nowever now found a coating solution which does not require the use of hexavalent chromium or similarly-toxic materials, yet which will form a clear and colourless coating on an aluminium surface which resists blackening or other discolouration even when subjected to boiling water.

According to one aspect of this invention there are provided acidic aqueous chemical conversion coating solutions for use upon aluminium surfaces, having a pH within the range of from 1.5 to 4.0 and comprising at least 10 ppm of zirconium and/or titanium, at least 10 ppm of phosphate (measured as (PO4)3-), and also fluoride in an amount not less than 13 ppm (measured as F-) and at least sufficient to cause available fluoride to be present in the solutions, the solutions being moreover capable of forming a uniformly clear and colourless coating on an aluminium surface which coating will resist blackening when subjected to boiling water for a 2-minute period.

Such solutions can be used to treat a bright shiny aluminium surface in such a way that the bright shiny appearance of the surface is not changed, yet a uniformly colourless coating is formed on the surface which is corrosion-resistant and to which overlying siccative coatings will adhere excellently. Moreover the resultant coatings are able to withstand blackening or other discolouration when subjected to boiling water not merely for the specified minimum period of time, namely at least about 2 minutes, but as will be seen from examples given below when the solutions are formulated to give optimum resistance to blackening or other discolouration the coatings are able to withstand exposure to boiling tap water for as long as 15 minutes, even perhaps up to about 30 minutes.

No other ingredients are necessary in the aqueous, acidic coating solutions of the present invention besides those specified above and their accompanying cations, yet these solutions are capable of effectively forming the aforementioned types of coatings on an aluminium surface in the absence of toxic and other materials which create waste-disposal problems.

Specifically the solutions of the invention can be and normally will be free from for example hexavalent chromium and elements such as manganese, iron, cobalt, nickel, molybdenum and tungsten, as well as other troublesome materials such as ferricyanide and ferrocyanide.

Thus the coating solution of the present invention is free from the kind of chemicals whose presence necessitates special treatment of the effluent solution before it is discharged either to the environment or even to a sewage-disposal plant.

It is surprising that such excellent results can be achieved using the solutions of the invention, especially because it had previously been thought that the introduction of phosphoric acid or phosphates into acidic aqueous zirconium-based or titanium-based coating solutions affected them adversely. That view moreover was very understandable, because both zirconium phosphate and titanium phosphate are highly insoluble in aqueous media after all, a known analytical test for determining the amount of zirconium or titanium in aqueous solution involves introducing phosphate into the solution so as to precipitate zirconium or titanium phospate therefrom.Previous attempts to formulate acidic aqueous solutions containing not just zirconium and fluoride but also phosphate, and to use them to form coatings on aluminium of the type to which this invention relates, have resulted in the formation of zirconium phosphate precipitate, which is highly undesirable in an industrial coating operation, as will be explained below. The present invention however succeeds in formulating the acidic aqueous coating solutions so that they contain substantially no solids of zirconium or titanium phosphate which tend to precipitate.

Zirconium and/or titanium must be present in the acidic aqueous coating solutions of the invention, since coatings with satisfactory properties can only be secured when they contain zirconium and/or titanium directly or indirectly bonded to the aluminium surface. Satisfactory coatings can be formed from coating solutions containing as little as 10 ppm of either zirconium or titanium or a mixture of zirconium and titanium; but depending upon other parameters of the coating process greater amounts of these ingredients may be required.

Zirconium and/or titanium can indeed be used in concentration up to their solubility limits in the acidic aqueous coating solution, which however depend on other parameters, including particularly the acidity of the coating solution and the concentration therein of fluoride and of phosphate. These should be controlled so as to avoid the precipitation of zirconium and titanium phosphate, which is undesirable for several reasons. Precipitation not only depletes the amounts of the ingredients but can also adversely affect the coating operation, both upon the aluminium surface and elsewhere - any type of precipitate can for example clog spray nozzles.Should the amounts of zirconium and/or titanium exceed their solubility limits, and precipitation be encountered, the amount of phosphate should be reduced and/or the pH of the coating solution should be lowered and/or the amount of fluoride should be increased.

Phosphate must be present in the acidic aqueous coating solutions of the invention for several reasons. For one thing, the presence of phosphate in the coating solution makes possible a simple test to confirm formation of the coating. This is vital in an industrial operation, perhaps involving treatment in a relatively short time of vast quantities of aluminium, to form a coating thereon which is not visible to the eye. Without some test, a change in the operating parameters of the coating solution could easily take place quite unnoticed, for instance improper replenishment of the coating solution as a result of mechanical or human failure, which renders it ineffective.

In order to determine whether or not the acidic aqueous coating solutions of this invention are functioning as they should to form chemical conversion coatings upon the aluminium surface it is necessary only to take random samples of the coated aluminium and to subject them to a relatively high temperature for a relatively short period of time for example 1,000"F (about 538"C) for 5 minutes, and to observe the appearance of the sample after this so-called "muffle test". A discolouration or colour change to light golden brown or even to darker shades of brown or purple indicates the presence of zirconium and phosphorus in the coating. If zirconium and phosphorus are not being deposited, the aluminium surface after the muffle test has a dull greyish appearance.Electron probe analysis of properly-coated surfaces treated in accordance with the present invention will also show the presence of both zirconium and phosphorus.

Another reason for the presence of phosphate in the solutions of the invention is that it contributes to the corrosion resistance and adherent properties of the coatings, particularly those coatings formed from coating solutions with a pH below 3.5.

We have found that to secure coatings with satisfactory properties and capable of discolouring in the above-mentioned muffle test it is necessary for the coating solution to contain at least 10 ppm of phosphate; and greater amounts may be required to produce optimum results, depending on other parameters of the coating process, as will be explained below. On the other hand excessive concentrations of phosphate can reduce the corrosion resistance of the coatings formed; and the phosphate concentration should therefore be no greater than 1000 ppm. The phosphate concentration selected should also take into account the guidelines given herein concerning the tendency of phosphate to precipitate with zirconium or titanium.

Fluoride must be present in the coating solutions of the invention so as to form a soluble complex with the zirconium and/or titanium, for example as a fluozirconate or fluotitanate, because this deters or prevents the formation of zirconium and/titanium phosphate precipitate. It is in fact necessary that the coating solution contains "available fluonde" - that is, contains fluoride in an amount beyond that which may be complexed in the coating solution with zirconium and/or titanium and any other metal constituents. Available fluoride is thus uncomplexed fluoride and includes fluoride present as HF and fluoride ion.

When no other metal constituents capable of complexing with fluoride are present in the solution, the minimum amount of fluoride necessary to cause the presence of available fluoride will depend on the amount of zirconium and/or titanium in the solution. For general guidance it may be noted that when there are 10 ppm of zirconium in the solution, the minimum amount of fluoride is 13 ppm; and when there are 10 ppm of titanium in the solution, the minimum amount of fluoride is 25 ppm.

There are other considerations which control the amount of fluoride necessary to cause the presence of available fluoride. The coating solution of the invention dissolves aluminium during use; and consequently there is a build-up in concentration of dissolved aluminium in the solution whenever the aluminium is contacted with the coating solution by immersion or even by spraying or flow coating techniques if excess or unreacted spray solution is recycled.

Fluoride is able to complex with aluminium, and in such cases the solution must contain sufficient available fluoride to complex with the dissolved aluminium as it is formed, especially since the dissolved aluminium has adverse effects on the coating process. The extent to which dissolved aluminium builds up in the coating solution will depend on factors such as the shape of the aluminium surface being treated and the manner in which the surface is contacted with the solution.

The concentration of "available fluoride" in the coating solutions of the invention may be conveniently measured as follows. A constant ionic-strength buffer is first prepared which contains 40.8 g/l of sodium acetate, 28.5 ml/l of glacial acetic acid and 58.0 g/l of sodium chloride in deionized water, and is adjusted to apH within the range of 5.0 to 5.3 with NaOH.

A sample of the coating solution diluted with this constant-strength buffer is then tested with an Orion pH meter (model No. 9409) having an Orion fluoride ion specific electrode (model No. 90-01). The reading thus obtained reveal the "available fluoride" concentration in the coating solution.

While generally-speaking the problem is to avoid having too little available fluoride rather than too much, one should avoid using so great a concentration of available fluoride that the aluminium surface suffers undue etching, which will produce a dull and frosty appearance on the surface, and can adversely affect the corrosion-resistance and adherent properties of the coating. It is difficult to give precise limiting values for the concentrations of available fluoride which will lead to such problems, for these vary dependent upon other parameters of the coating process, including for example the pH of the solution and the time and temperature of contact. In our experience however these problems can be avoided if the available fluoride is present in a concentration no greater than 500 ppm.

The pH of the coating solution must be within the range of from 1.5 to 4.0, since at higher pHs phosphate-precipitation can be a problem. We much prefer to employ a coating solution with a pH in the range of from 2.6 to 3.1.

The pH of the solution may be adjusted using any acid or base as pH adjusters which will not interfere with the coating process, for example perchloric acid or sulphuric acid - though it is recommended when using sulphuric acid that the pH of the coating solution be not less than 2, as below this value sulphuric acid adversely affects the coating operation. The preferred pH adjusters are however nitric acid and ammonium hydroxide.

The acidic aqueous coating solution of the invention can be prepared from a variety of readily-available sources of zirconium and/or titanium, fluoride and phosphate which are soluble therein. Generally-speaking we prefer to supply the zirconium as soluble fluozirconate and/or fluotitanate compounds, such as for example fluozirconic acid and ammonium and alkali metal fluozirconates; and similarly we prefer to supply the titanium as soluble fluotitanate compounds, such as for example fluotitanic acid and ammonium and alkali metal fluotitanates.The solution however can also be prepared from zirconium fluoride (ZrF4) and/or titanium fluorides (TiF3, TiF4); and in addition can be prepared from a mixture of soluble compounds, one of which contains zirconium or titanium and the other of which contains fluoride, such as zirconium nitrate, zirconium sulphate and titanium (IV) sulphate on the one hand and hydrofluoric acid and water-soluble salts thereof (for example, ammonium and alkali metal salts) on the other hand. Zirconium carbonates such as ammonium and alkali metal zirconium carbonates can also be used.

Similarly, the acidic aqueous coating solution of the invention can be made up from a variety of readily-available sources of phosphate. Generally-speaking we prefer to supply the phosphate as ortho-phosphoric acid, but ammonium and alkali metal phosphates can be used, and so too can other forms of phosphoric acid, thus itsmeta-,pyro-, tri-poly- andhypo-forms, as well as salts thereof.

Broadly-speaking the same is true for fluoride - any source of fluoride capable of complexing aluminium can be used which is soluble in the coating solution and which does not contain any constituent that adversely affects the coating process. However, if fluoride is added only as a complex fluoride of titanium or zirconium, then the amount of fluoride deriving in the solution from hydrolysis of such complex fluorides may not be sufficient to complex the dissolved aluminium, thus leading to an absence of available fluoride. At the same time the extent of hydrolysis may be such that uncomplexed zirconium or titanium combines with phosphate to form an undesired precipitate.To avoid this problem the solution should then also incorporate another fluoride source capable of complexing the dissolved aluminium which builds up as the process continues, thus ensuring the continual presence of a certain amount of available fluoride. Examples of fluoride sources capable of complexing aluminium are hydrofluoric acid, salts thereof, and ammonium bifluoride (NH4F.HF) and alkali metal bifluorides. Hydrofluoric acid is a particularly good source of fluoride because it readily provides sufficient fluoride to complex the aluminium yet it is not a source of extraneous cations which may interfere with the coating process.

It will of course be understood that whenever waste-disposal is a problem and also whenever the aluminium articles after coating are to find use in the food or drink industries the coating solutions should of course be free from chromium and other toxic materials such as iron cyanides. Of equal importance even if for different reasons is that the solutions of the invention should be kept free from anything which forms solids which tend to precipitate from the solution.

We have found that the incorporation of fluoboric acid in the coating solution can improve the gloss or hardness of paint films which are subsequently applied to aluminium surfaces coated in accordance with the present invention; and also helps to maintain the stability of the coating solution if formulated from hard water. The calcium and magnesium ions in hard water have an affinity for fluoride; and if they should extract the fluoride from the zirconium and/or titanium fluoride complex, the resultant de-complexed zirconium and/or titanium may tend to combine with the phosphate to form an undesirable precipitate of insoluble zirconium and/or titanium phosphate. It is believed that if fluoboric acid is present, it will act as a buffered source of fluoride for the calcium and magnesium ions in hard water.Too much fluoboric acid can however reduce the corrosion resistance of the coating. Generallyspeaking we therefore prefer to employ solutions including fluoboric acid in a concentration within the range of from 8 to 200 ppm. We have also found that certain polyhydroxy compounds may advantageously be added to the coating solution to form coatings which provide better adhesion for subsequently-applied paint or ink coatings, namely any organic compound soluble in the coating solution which when dissolved yields at least 40 ppm of polyhydroxy compounds having no more than 6 carbon atoms, and of course which does not interfere with the ability of the coating solution to form coatings having the desired corrosion resistance and paint adherence. Examples of such polyhydroxy compounds include gluconic acid, salts of gluconic acid, sorbitol, mannitol, dextrose, ethylene glycol and glycerine.

Particularly preferred such polyhydroxy compounds are gluconic acid and alkali metal and ammonium salts thereof, as well as compounds soluble in the coating solution to yield gluconate and/or gluconic acid, for example stable gluconolactones such as glucono-delta- lactone and glucono-gamma-lactone.

Although higher amounts can be used, it is recommended that the polyhydroxy compound be present in an amount no greater than about 1000 ppm, so the preferred range is from 40 to 400 ppm of the polyhydroxy compound.

The coating solutions of the invention will usually contain: a from 10 to 125 ppm of zirconium (or an equivalent amount of titanium); b from 10 to 1,000 ppm of phosphate; and c) from 10 to 500 pm of available fluoride.

The particularly pre erred coating solutions for use in this invention are those with a pH within the range of from 2.6 to 3.1, which contain: Ingredient Approximate Concentration in ppm Zr 45 to 125 PO4 50 to 200 Available fluoride 10 to 200 The preferred source of Zr in the above solution is ammonium fluozirconate, and the preferred source of phosphate is ortho-phosphoric acid. Hydrofluoric acid is preferably used as the source of available fluoride, and nitric acid is used to adjust the pH. The inclusion of from 8 to 200 ppm of fluoboric acid and of from 40 to 400 ppm of a polyhydroxy compound such as preferably gluconic acid is advantageous, for the reasons mentioned previously.

Although generally-speaking we prefer at present to use zirconium-based coating solutions, the titanium-based solutions which we prefer are those which contain: Ingredient Approximate Concentration in ppm Ti 20 to 65 P04 50 to 200 Available Fluoride 10 to 200 The preferred source of titanium is fluotitanic acid. The other preferred ingredients and amounts thereof are as described above for the preferred Zr-containing solution.

There are certain general considerations which should be kept in mind for optimum results.

When operating at a relatively high pH, relatively small amounts of zirconium or titanium and/or phosphate should be used to deter precipitation. When contacting the coating solution and the aluminium surface for a relatively short time, relatively high amounts of zirconium or titanium and phosphate should be used. Similarly, when the temperature of contact between the coating solution and the aluminium surface is relatively low, relatively high amounts of ingredients should be used. In general, the lower the amount of phosphate used in the solution, the higher the amount of zirconium and/or titanium that can be used.

According to another aspect of this invention there are also provided processes for forming uniformly clear and colourless yet tightly-adherent chemical conversion coatings which provide a good foundation for subsequently-applied siccative coatings upon aluminium surfaces, in which the aluminium surface is contacted with an acidic, aqueous chemical conversion coating solution as herein described.

The aluminium surface to be contacted with the coating solution should of course be clean.

Any of the currently-available cleaning compositions such as alkaline or acid cleaning solutions can be used to clean the aluminium surface by means of conventional techniques.

As previously indicated, the process is currently preferred for use upon aluminium can stock having a bright shiny surface.

The aluminium surface may be contacted with the coating solution by any suitable method, for example by spraying or immersion by roll - or flow-coating or misting techniques. For many purposes the coating solution can be applied most economically by spraying. Coating can be carried out either upon individual articles, such as cans, or upon unfinished workpieces, such as aluminium strip, which will subsequently be fabricated into articles.

The temperature at which the process is performed must naturally be such that the reactive ingredients of the solution bond to the aluminium surface, but is not otherwise very critical - it can for instance be carried out at room temperature. However we prefer to contact the coating solution with the aluminium surface at a temperature of at least 800F (about 27"C) yet on the other hand to avoid too high a temperature, since then a dull and frosty appearance may be obtained. This occurs at temperatures dependent upon various of the parameters of the coating operation, including for example the time of contact and the reactivity of the solution which in turn depends on the pH and the concentration of ingredients in the solution.

An upper temperature limit of 1500F (about 66"C) is recommended; and we prefer to contact the solution with the aluminium surface at a temperature within the range of from 80 F to 110 F (about 27 C - 43 C).

Satisfactory coatings can be formed by contacting the coating solution with the aluminium surface for at least 5 seconds, and preferably for at least 15 seconds. The lower the temperature of the coating solution, the longer should be the contact time, whereas the higher the temperature of the solution the shorter the contact time required. Generally-speaking it will be unnecessary to contact the surface with the coating solution for more than one minute.

The acidic aqueous coating solution is capable of forming a very thin and very light-weight coating. The coating weight will vary depending upon the concentration of the various ingredients in the coating solution, and the temperature and time of application. For uses of the type referred to herein, the coating should preferably have a weight of about 2 to about 20 mg/sq. ft., preferably about 5 to about 10 mg/sq. ft. Coatings having such weights can be formed by operating within the conditions described above. Higher coating weights can create problems in the aluminium can-coating industry, because the machinery which applies paint or ink to coated aluminium cans has precise tolerances to accommodate cans having very thin coatings, and cans with relatively thick coatings can foul the machinery.

It is possible in accordance with the present invention to form coatings which are very uniform, thus permitting paint or ink to be applied evenly and with desired coverage to the coated aluminium surface. In the aluminium can industry, paint- and ink-coatings are applied to coated aluminium cans by an automatic roll-coating machine, in which paints and inks are applied to a roller and then to the surface of the coated can as the roller is rotated across the surface of the coated can. If the can has a non-uniform coating, the subsequently-applied ink or paint composition may not cover the desired areas of the can.

After the coating solution has been applied to the aluminium surface, it should be water-rinsed, including a final deionized-water-rinse. Rinsing with water that contains a small amount of dissolved solids may lead to a coating which has poor paint-adhesive properties. It is not necessary to rinse the coated aluminium surface secured in accordance with this invention with any kind of aqueous solution of chromium, such as for example the hexavalent chromium solution conventional in many other chemical conversion coating processes.

After the coated surface has been water-rinsed or otherwise treated as described above, the coating should be dried. This can be done by any practical drying methods, such as for example oven drying or forced circulation of hot ailgpplied and usually air.

After the chemical conversion coating has been applied and usually after it has been coated, water-rinsed and dried, the coated surface can be subjected to sanitary or decorative coating operations, including for example the application of siccative coatings. Sometimes the sanitary-coating is applied after the water-rinse, and then both the chemical conversion coating and the sanitary coating are dried simultaneously.

Where aluminium cans are filled with beer, the cans should first be treated with the coating solution of the present invention and then sanitary and/or decorative coatings should be applied. Thereafter the cans are filled with beer and sealed, after which the beer-filled cans are subjected to pasteurization.

The coating solution of the present invention can be prepared most conveniently by diluting an aqueous concentrate of the ingredients with an appropriate amount of water.

In another aspect of the invention there is therefore provided a concentrate such that when a coating solution comprises about 0.5 to about 10 weight percent of the concentrate, the amounts of ingredients present in the coating solution are: (a) at least about 10 ppm of zirconium or titanium; (b) at least about 10 ppm of phosphate; and (c) fluoride in an amount at least sufficient to cause available fluoride to be present in the solution and the pH of the coating solution is within the range of from 1.5 to 4.0.

The preferred concentrates are those such that when the coating solution contains about 0.5 to about 10 weight percent of the concentrate, the coating solution comprises: (a) about 45 to about 125 ppm of zirconium, added as a fluozirconate such as sodium or potassium fluozirconate, most preferably ammonium fluozirconate; (B) about 50 to about 200 ppm of phosphate added as H3PO4; (C) about 8 to about 200 ppm of HBF4; (D) about 10 to about 50 ppm of HF; and (E) nitric acid in an amount such that the pH of the coating solution is within the range of from 2.6 to 3.1.

A polyhydroxy compound, preferably gluconic acid, can also be included in these concentrates, in an amount such that the coating solution comprises from 40 to 400 ppm thereof.

In continuous coating operations, it is of course important to keep the solution properly replenished in order to maintain the effectiveness of the coating process. We have found that various of the ingredients are depleted during the formation of the coating. Zirconium and/or phosphorus are incorporated into the coating, available fluoride is consumed by complexing with dissolved aluminium, and hydrogen is consumed as the aluminium surface is oxidized. Ingredients are also depleted by "drag-out" of the solution on the aluminium surface. The rate of depletion of ingredients by drag-out is moreover related to the shape of the surface being coated, and the manner in which the coating solution is contacted with the aluminium surface - there is for example a greater drag-out loss than when spraying cans than when spraying strip.

Replenishment of the coating solution may be accomplished either by individually monitoring the amount of each ingredient in the coating solution and replacmg this as it is depleted, or by adding to the solution an aqueous concentrate of the ingredients in approximately the correct proportions to maintain the ingredients thereof in effective operating amounts.

Where there is likely to be a build-up of aluminium in the coating solution, it is recommended that the replenishing concentrate should contain a relatively high proportion of available fluoride for complexing the dissolved aluminium, Preferred sources of available fluoride are HF or ammonium bifluoride or a mixture thereof.

The following is a recommended aqueous concentrate for replenishing the coating solution: A) about 5 to about 10 g/l of Zr or about 2.5 to about 5 g/l of Ti; B about 5 to about 10 g/l of PO4; and C a material which is a source of about 5 to about 20 g/l of available fluoride, preferably HF or ammonium bifluoride or a mixture thereof.

When utilizing a polyhydroxy compound, it sould be included in the replenishing concentrate in an amount of about 5 to about 20 g/l.

When utilizing fluoboric acid, it should be included in the replenishing concentrate in an amount of about 1 to about 5 g/l.

EXAMPLES Examples below are illustrative of the practice of the present invention. Comparative examples are set forth also.

Unless stated otherwise, the aluminium surfaces treated with the solutions identified in the examples were drawn and ironed aluminum cans which were first degreased, as necessary, in an acidic aqueous cleaner containing sulphuric acid and detergents. Unless stated otherwise, the coating solutions were applied by spraying for about 25 seconds at a temperature of about 110 F. After treatment with the solutions identified in the examples, the aluminum surfaces were rinsed in deionized water and dried in an oven for 2 minutes at about 400"F.

Thereafter, the aluminum surfaces were tested for corrosion resistance by subjecting them to a pasteurization test. This test consisted of immersing the aluminum surface in water having a temperature, as indicated, and for a period of time, as indicated. A cleaned-only aluminum surface, when subjected to the pasteurization test, turns black after a few minutes.

It will be seen from examples set forth below that prior treatment of the aluminum surfaces with coating solutions of the present invention resulted in the provision of coated surfaces which were not blackened or otherwise discolored or which resisted blackening or other discoloration. The results of the tests were rated as follows: 5, perfect, no blackening; 3 +, acceptable; and 0, total failure, severe blackening.

Aluminum surfaces treated with the solutions described in the examples were tested also for paint adhesion. After the treated surface was dried, as described above, a portion of the surface was painted with a white base coat (No. 12W100A white polyester sold by H.C.I.) and the other portion of the surface was painted with an interior vinyl-lacquer (Modified Vinyl Epoxy Lacquer C-5054 sold by Mobil). After the paint was cured, the painted surface was immersed either in boiling water-detergent or water-NaC1 solution. After removing the painted surface from the solution, it was rinsed in water, and the excess water was removed from the surface by wiping. The painted surface was then cross hatched, using a sharp metal object to expose lines of aluminum which showed through the paint or lacquer, and tested for paint adhesion.This test included applying cellophane tape firmly over the cross hatched area and then drawing the tape back against itself with a rapid pulling motion such that the tape was pulled away from the cross hatched area. The results of the test were rated as follows: 10, perfect - that is when the tape did not peel any paint from the surface; 8, acceptable and 0, total failure.

The various compositions of the first 6 examples and the first 7 comparative examples are listed in Table 1 below and include compositions within the scope of the invention and comparative compositions. Table 2 below lists the results of the pasteurization and paint adhesion tests. Solutions in Table 1 were adjusted to a pH of 2.7 by adding concentrated nitric acid or ammonium hydroxide. The surfaces were subjected to either boiling tap water for 15 minutes or hot tap water (160"F) for 45 minutes, as indicated in Table 2.

In some cases, more than one sample of the aluminum surface was treated in the same way.

In such cases a plurality of ratings are set forth in the tables which follow.

Table 1 Ingredients of Solutions and Amounts Thereof in g/l Ex. (NH4)2ZrF6 HBF4 H2SiF6 H2TiF6 HF H3PO4 No.

1 0.240 - - 0.164 0.050 0.294 2 - - - 0.164 0.050 0.294 3 - 0.264 - 0.164 0.050 0.294 4 0.240 0.264 - 0.164 0.050 0.294 5 0.240 0.264 - - - 0.294 6 0.240 0.264 - 0.164 - 0.294 C-1* 0 0 0 0 0 0 C-2 0.240 - - - 0.050 C-3 0.240 0.264 - - 0.050 C-I - 0.264 - - 0.050 0.294 C-S 0.240 - 0.442 - 0.050 C-6 - - 0.442 - 0.050 0.294 C-7 0.240 - - 0.164 0.050 Table 2 Ex. Pasteurization Test Paint Adhesion Test* No. 212 OF 15 min 160 OF 45 min White Base Coat Interior Vinyl 1 3+ 4- 10,9+, 9+, 9 10,10,10,10 2 3 3 10,9+,9+,9 10,10,10,10 3 2 3- 9,9,9,8 10,10,10,10 4 3 3+ 9+, 8+, 8, 8 10, 10, 10, 10 5 4+ 4+ 10, 10, 9+, 9 10, 10, 10, 10 6 4+ 4+ 10, 10, 10, 10 10, 10, 10, 10 C-l* 0 0 0,0,0,0 10,10,9+,8 C-2 0 0 9,8+,8,6 10,10,10,10 C-3 0 0 6, < 5, < 5, 0 10, 10, 10, 10 C-I 0 0 0,0,0,0 10,10,10,9+ C-S 0 0 0,0,0,0 10,9+,9+,9+ C-6 0 0 0, 0, 0, 0 10, 10, 10 C-7 0 0 7,6, < 5, < 5 10, 10, 10,9+ * Cleaned only, no treatment.

** 0.7% Orvus-K detergent, 212"F, 15-minutes The examples listed in Table 3 below illustrate the effect of varying the phosphate concentration in fluozirconate solutions. The solutions of these examples had a pH of 2.5, except that of Example C-9 which ad a pH of 3.25. The pasteurization test involved immersion of an unpainted aluminium can dome in tap water (160 F) for 45 minutes. Paint adhesion of both white base coat and interior vinyl lacquer was tested by immersion of painted cans in a boiling 1% solution of "Joy" detergent ("Joy" is a Registered Trade Mark) for 15 minutes followed by the standard cross hatching and taping.

Table 3 Ingredients of Solutions and Amounts Thereof in g/l Ex. H3PO4 (NH4)2ZrF6 HF Pasteurization Test Paint Adhesion Test No. White Base Interior Coat Vinyl Lacquer C-8 0 0.24 0.05 0 8, 7, 0, 0 10, 10, 10 7 0.05 0.24 0.05 3 9+, 9+, 9, 9 10, 10, 10, 10 8 0.10 0.24 0.05 4- 9+, 8+, 8, 5 10, 10, 10, 10 9 0.49 0.24 0.05 4- 9+, 9+, 9, 9 10, 10, 10, 10 10 0.98 0.24 0.05 3+ 9+, 9+, 9+, 9+ 10, 10, 10, 10 C-9 0.98 0 0.05 0 0,0 10,7 Example 11 This example illustrates the use of a make-up solution and a replenishing solution in a continuous can coating process in which 400 cans were coated. The make-up concentrate contained 4.8 g/l of ammonium fluozirconate, 3 g/l of phosphoric acid, 0.44 g l of hydrofloric acid, 2.64 g/l of fluoboric acid, 11.4 g/l of nitric acid, and 1.92 g/l of sodium gluconate.This make-up concentrate was then diluted to 2.5% in an aqueous solution. The pH of the solution was 2.70. The replenishing solution was made up of 24 g/l of ammonium fluozirconate, 9.4 g/l of phosphoric acid, 11.7 g/l of hydrofluoric acid, 0.68 g/l of fluoboric acid, 21.4 g/l of nitric acid, and 0.4 g/l of sodium gluconate.

As the aluminum cans were processed, replenishing solution was added, as needed, to maintain the bath at a pH of 2.70 + 0.02. (The pH was checked after every 10 cans were coated. Subsequent calculations showed that an average of about 0.3 ml of replenishing solution per can was used.) The pasteurization test involved immersion in boiling tap water for 15 minutes. Paint adhesion was tested by immersion in boiling water containing 0,7% detergent (Orvus K - "Orvus" is a Registered Trade Mark) for 15 minutes, followed by cross hatching and taping. Test results are shown in Table 4 below.

Table 4 Paint Adhesion Test Number of Cans Pasteurization White Base Interior Processed Test Coat Lacquer 1 3 10, 10 10, 10 50 4+ 10,10 10,10 51 4+ 10, 10 10, 10 100 4+ 10, 8+ 10,10 101 3 8+,7 10,10 150 4+ 9+, 8 10,10 151 4+ 9+,9 10,10 200 4+ 10, 8+ 10,10 201 4+ 10, 8+ 10, 10 250 4+ 10,9 10,10 251 4+ 9+, 9+ 10,10 300 4+ 9,9 10,10 301 4 10, 10 10, 10 350 4+ 10,9+ 10,10 351 4+ 10, 10 10, 10 400 4 10, 10 10, 10 Table 4 shows the results of the pasteurization and the paint adhesion tests.During the coating operation, cans 3, 80, 160, 240, 320 and 399 were subjected to a temperature of about 1,000"F for about 5 minutes. The entire surface of such cans turned a uniform brown color.

It is noted that the solution was dear during the coating of the first 100 cans. Thereafter, the coating solution became faintly hazy and was still faintly hazy when the 400th can was treated. No sludging or precipitation was isolated when the bath was centrifuges at the conclusion of the test run.

The concentrations of zirconium, available fluoride, and phosphate were determined throughout the run. The concentration of zirconium increased from an initial value of 46 ppm to 111 ppm during the run. The concentration of phosphate increased from 70 ppm to 110 ppm during the run. The concentration of available fluoride increased from 85 ppm to about 95 ppm and eventually decreased to 87 ppm.

Examples 12 through 26, as set forth in Tables 5 and 6 below, illustrate the improved ink adhesion obtained by the use of polyhydroxy compounds in the coating solution of the present invention. More specifically, Table 5 illustrates the use of various concentrations of sodium gluconate in a coating solution containing ammonium fluozirconate and/or fluotitanic acid, phosphoric acid, and hydrofluoric acid. Table 6 illustrates the results obtained by the use of polyhydroxy compounds other than sodium gluconate. In these examples, the coating solutions were applied by spraying for 15 seconds at a temperature of 90"F and the thus coated aluminum cans were then coated with a white base polyester ink sold by Acme Printing Ink Co. as M61513 (Schlitz white). Prior to curing the ink, an alkyd-amine overvarnish was applied to the wet ink coating.Curing was effected for 6 minutes at 3750F.

Table 5 (All solutions adjusted to a pH of 2.7 with HNO3) Ex. (NH4)2ZrF6 H2TiF6 H3PO4 HF Sodium Pateurization Adhesion No. (g/l) (g/l) (g/l) (g/l) Gluconate Test 212 F, 1% Joy, 212 F, (g/l) 15 min. 15 min.

12 0.120 0 0.098 0.010 0 5, 5 0, 0 13 0.120 0 0.098 0.010 0.04 4+ 8, 8 14 0.120 0 0.098 0.010 0.08 4+ 8, 9+ 15 0.120 0 0.098 0.010 0.195 4+, 4+ 9,9+ 16 0.120 0 0.098 0.010 0.40 4 9+, 9+ 17 0.120 0.041 0.098 0.010 0 4, 4 5, 9 18 0.120 0.041 0.098 0.010 0.195 4,4 9,9+ 19 0.120 0.082 0.098 0.010 0 4-, 3+ 7,9 20 0.120 0.082 0.098 0.010 0.195 3+, 3+ 9+, 9+ Table 6 (All solutions adjusted to a pH of 2.7 with HNO3) Ex. (NH4)2ZrF6 H3PO4 HF Additive Pasturization Adhesion No. (g/l) (g/l) (g/l) (0.1 g/l) 212 F, 15 min. 1% Joy, 212 F, 15 min.

21 0.120 0.098 0.010 None 4+, 4+ 0,5 22 0.120 0.098 0.010 Sorbitol 4+, 4+ 0,9+ 23 0.120 0.098 0.010 Mannitol 4+, 4+ 9,9+ 24 0.120 0.098 0.010 Dextrose 4+, 4+ 5,9 25 0.120 0.098 0.010 Ethylene Glycol 4+, 4+ 7,9 26 0.120 0.098 0.010 Glycerine 4+, 4+ 0,6 The next two groups of examples illustrate the use of prior art coating solutions containing a phosphate, fluoride, and either zirconium or titanium. The examples show the undesirable results that are obtained when utilizing the coating solutions to coat aluminum. Examples C-10, Dull, C-12 and C-13 show the application of coating solutions disclosed respectively in examples 7, 9, 14 and 15 of U. S. Patent No. 3,109, 757 to aluminum cans.

For Examples C-10, C-11, C-12 and C-13, the following concentrate was prepared: Percent by weight ZnO 8.0 H3PO4 (75%) 39.5 Ni (NO3)2-6H20 6.75 HNO3 (380 Be.) 2.87 H2O 42.88 100.00 The above concentrate was diluted with water to a strength of 4% by volume and had added thereto 0.25% (weight/volume) of caustic soda. There were added to the dilute solution a glycerophosphate compound and a complex metal fluoride in the amounts set forth in Table 7 below. The solutions were heated to about 145 F and sprayed on aluminium cans for 30 seconds. In Example C-12, Cu(NO3)2. 6H2O was added in an amount to provide 0.005% copper ion.

Table 7 shows the properties of the coating solution and the coating provided thereby. TABLE 7 Appearance of coated Appearance of Pasteurization Paint Adhesion Example Weight % Weight % Weight % cans before pasteur- Coating Solution test 212 F test 1% Joy, No. glycerophosphate (NH4)2ZrF6 H2TiF6 ization 15 min 212 F, 15 min C-10 0.1 0.05 0 light gray white precipitate 1 9+ C-11 0.4 0.05 0 light gray white precipitate 1 9+ C-12 0.2 0 0.033 dark gray white precipitate 0 9 C-13 0.2 0 0.33 dark gray white precipitate 0 9+ Example C-14 This example shows the use of a coating solution of the type described in Example 8 of U. S.

Patent No. 2,813,81. A concentrate was prepared containing the following ingredients Grams MnCO3 183.4 H3PO4 (75So) 555.3 HNO3 (70%) 22.5 NH4NO3 77.3 NaHF2 6.0 K2TiF6 15.8 H20 573.5 Total 1433.8 In the above formulation, the manganese carbonate and phosphoric acid were reacted to form manganese dihydrogen phosphate. Eight hundred forty grams of the above concentrate were added to 4 liters of water. Then 44.8 g of MnCO3 and 9.2 g of NaHF2 were added. The solution was heated to 2000F and sprayed for 30 seconds on an aluminum can. The coating solution contained substantial amounts of precipitate and formed a gray coating on the can.

Corrosion resistance was tested by immersing the can in water at a temperature of 212"F for 15 minutes. The can turned very dark and was rated 0. White base coat adhesion was tested by immersing the can in 1 % Joy at 212 0F for 15 minutes. The can passed the adhesion test and was rated 9+.

From examples set forth above, it can be seen that the present invention provides a coating solution free of hexavalent chromium and one which is capable of forming on an aluminum surface a colorless and clear coating without modifying the appearance of the aluminum surface. The coated surface resists discoloration even after being subjected to boiling water and has excellent adhesion to overlying siccative coatings. The coating solution can be used in a continuous industrial coating operation to excellent advantage.

WHAT WE CLAIM IS: 1. Acidic aqueous chemical conversion coating solutions for use in forming a uniformly clear and colourless coating upon aluminium surfaces which coating will resist blackening when subjected to boiling water for a 2-minute period, which solutions have a pH of from 1.5 to 4 and comprise at least 10 ppm of zirconium and/or titanium, at least 10 ppm of phosphate (measured as (PO4) 3-), and also fluoride (measured as F-) in an amount not less than 13 ppm and at least sufficient to cause available fluoride (as defined herein) to be present in the solutions.

2. An acidic aqueous coating solution as claimed in claim 1, which contains zirconium supplied as a fluozirconate.

3. An acidic aqueous coating solution as claimed in claim 1 or claim 2, which includes no more than 1,000 ppm of phosphate.

4. An acidic aqueous coating solution as claimed in any of claims 1 to 3, in which the phosphate is supplied as phosphoric acid.

5. An acidic aqueous coating solution as claimed in any of claims 1 to 4, which contains available fluoride in a concentration no greater than 500 ppm.

6. An acidic aqueous coating solution as claimed in any of claims 1 to 5, in which the pH of the solution is within the range of from 2.6 to 3.1.

7. An acidic aqueous coating solution as claimed in any of claims 1 to 6, in which nitric acid is included to impart the desired pH value to the solution.

8. An acidic aqueous coating solution as claimed in any of claims 1 to 7, which additionally includes fluoboric acid in a concentration within the range of from 8 ppm to 200 ppm.

9. An acidic aqueous coating solution as claimed in any of claims 1 to 8, which additionally includes a polyhydroxy organic compound having not more than 6 carbon atoms in a concentration of at least 40 ppm.

10. An acidic aqueous coating solution as claimed in claim 9, in which the concentration of the polyhydroxy compound is not greater than 1,000 ppm.

11. An acidic aqueous coating solution as claimed in claim 9 or claim 10, in which the polyhydroxy compound is one or a mixture of more than one of the following, namely gluconic acid and salts thereof, sorbitol, mannitol, dextrose, ethylene glycol and glycerine.

12. An acidic aqueous coating solution as claimed in any of claims 1 to 11, which includes from 40 to 400 ppm of gluconic acid.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Example C-14 This example shows the use of a coating solution of the type described in Example 8 of U. S. Patent No. 2,813,81. A concentrate was prepared containing the following ingredients Grams MnCO3 183.4 H3PO4 (75So) 555.3 HNO3 (70%) 22.5 NH4NO3 77.3 NaHF2 6.0 K2TiF6 15.8 H20 573.5 Total 1433.8 In the above formulation, the manganese carbonate and phosphoric acid were reacted to form manganese dihydrogen phosphate. Eight hundred forty grams of the above concentrate were added to 4 liters of water. Then 44.8 g of MnCO3 and 9.2 g of NaHF2 were added. The solution was heated to 2000F and sprayed for 30 seconds on an aluminum can. The coating solution contained substantial amounts of precipitate and formed a gray coating on the can. Corrosion resistance was tested by immersing the can in water at a temperature of 212"F for 15 minutes. The can turned very dark and was rated 0. White base coat adhesion was tested by immersing the can in 1 % Joy at 212 0F for 15 minutes. The can passed the adhesion test and was rated 9+. From examples set forth above, it can be seen that the present invention provides a coating solution free of hexavalent chromium and one which is capable of forming on an aluminum surface a colorless and clear coating without modifying the appearance of the aluminum surface. The coated surface resists discoloration even after being subjected to boiling water and has excellent adhesion to overlying siccative coatings. The coating solution can be used in a continuous industrial coating operation to excellent advantage. WHAT WE CLAIM IS:

1. Acidic aqueous chemical conversion coating solutions for use in forming a uniformly clear and colourless coating upon aluminium surfaces which coating will resist blackening when subjected to boiling water for a 2-minute period, which solutions have a pH of from 1.5 to 4 and comprise at least 10 ppm of zirconium and/or titanium, at least 10 ppm of phosphate (measured as (PO4) 3-), and also fluoride (measured as F-) in an amount not less than 13 ppm and at least sufficient to cause available fluoride (as defined herein) to be present in the solutions.

13. A zirconium-based acidic aqueous coating solution as claimed in any of claims 1 to

12, which comprises: (a) from lo to 125 ppm of zirconium; (b) from 10 to 1,000 ppm of phosphate; and (c) from 10 to 500 ppm of available fluoride.

14. A zirconium-based acidic aqueous coating solution as claimed in any of claims 1 to 13, which has a pH within the range of from 2.6 to 3.1, and comprises: a from 45 to 125 ppm of zirconium; ) from 50 to 200 ppm of phosphate; and from 10 to 200 ppm of available fluoride.

15. A titanium-based acidic aqueous coating solution as claimed in any of claims 1 to 12, which comprises: (a) from 20 to 65 ppm of titanium; (b) from 50 to 200 ppm of phosphate; and (c) from 10 to 200 ppm of available fluoride.

16. Acidic aqueous chemical conversion coating solutions for use upon aluminium surfaces, as claimed in any of the preceding claims and substantially as herein described.

17. Acidic aqueous chemical conversion coating solutions, for use upon aluminium surfaces, substantially as described with reference to any of the Examples given hereinbefore.

18. Processes for forming chemical conversion coatings which are uniformly clear and colourless yet tightly adherent upon an aluminium surface and provide a good foundation for siccative coatings subsequently applied thereto, in which the aluminium surface is contacted with an acidic aqueous chemical conversion coating solution as claimed in any of the preceding claims.

19. A process as claimed in claim 18, applied to aluminium can stock having a bright shiny surface.

20. A process as claimed in claim 18 or claim 19, in which the solution is contacted with the aluminium surface by spraying it thereon.

21. A process as claimed in any of claims 18 to 20, in which the solution is contacted with the aluminium surface at a temperature within the range of from 800to 1100F (about 27"C - 43"C).

22. A process as claimed in any of claims 18 to 21, which includes the step of subjecting the surface, after it has been coated, to so-called "pasteurizing" conditions.

23. Processes for forming chemical conversion coatings upon aluminium surfaces as claimed in any of claims 18 to 22 and substantially as herein described

24. Processes for forming chemical conversion coatings upon aluminium surfaces substantially as herein described with reference to any of the Examples given herein.

25. Aqueous concentrates for use in making-up and/or replenishing acidic aqueous coating solutions as claimed in any of claims 1 to 17, said concentrates being such that when diluted with water to a concentration of from 0.5 to 10 weight percent of the concentrate the resultant dilution has a pH within the range of from 1.5 to 4.0, and substantially consists of: (a) at least 10 ppm of zirconium or titanium or a mixture thereof; (b) at least 10 ppm of phosphate; (c) fluoride in an amount at least sufficient to cause the presence of available fluoride; and (d) water.

26. An aqueous concentrate as claimed in claim 25, in which the resultant dilution has a pH within the range of from 2.6 to 3.1, and substantially consists of: a from 45 to 125 ppm of zirconium; b from 50 to 200 ppm of phosphate; from 8 to 200 ppm of HBF4; from 10 to 50 ppm of HF; e nitric acid in an amount such that the pH of said solution is within the specified range; and (f) water.

27. An aqueous concentrate as claimed in claim 25 or claim 26, which consists of an aqueous solution, substantially free from precipitate, is: (a) from 5 g/l to 10 g/l of zirconium or from 2.5 g/l to 5 g/l of Ti; (b) from 5 g/l to 10 g/l of phosphate; and (c) a material which is a source of from 5 g/l to 20 gll of available fluoride.

28. An aqueous concentrate as claimed in claim 27, in which the available fluoride is supplied as hydrofluoric acid and/or ammonium bifluoride.

29. Aqueous concentrates as claimed in any of claims 25 to 28 and substantially as herein described.