GB2036807A - Solutions, concentrates and processes for conversion-coating of aluminium - Google Patents

Abstract

Aqueous acidic chromating solutions are here disclosed which as accelerator employ 0.03-3.0 g/l of zinc, together with 0.01-10.0 g/l of hexavalent chromium, 0.01-10.0 g/l of fluoride, 0.02-3.0 g/l of molybdate and sufficient acid to impart a pH less than 7, preferably not greater than 5.5, and desirably in the range 1.0-2.5. Processes for chromating aluminium, especially the high-speed chromating of aluminium coil-stock, using these zinc- accelerated chromating solutions are also disclosed-as too are concentrates for making-up the solutions used in the process.

Description

SPECIFICATION Solutions, concentrates and processes for conversion-coating of aluminium This invention concerns solutions, concentrates and processes for conversion-coating of aluminium.

The term "aluminium" is used herein to mean not only substantially pure aluminium but also alloys thereof wherein aluminium is the predominant ingredient.

It is recognized that for certain purposes it is desirable to apply so-called "conversion" coatings to the surface of aluminium articles (including aluminium sheet and strip), especially for instance so as to enhance its corrosion resistance and to promote the adhesion thereto of subsequently-applied paint or other siccative coating materials. There are indeed many well-known processes and solutions for forming conversion coatings of the chromate/fluoride type upon aluminium (or perhaps other metals).

Typical examples of such processes and solutions, which are often for convenience called "chromating processes" and "chromating solutions", have been described for instance in United States Patents Nos.

2,276,353, 2,471,909, 2,472,864,2,796,370, 2,796,371,2,507,966, 2,839,439,2,843,513, 2,859,144,2,868,679 and 3,009,842.

While some or even perhaps all of these known chromating processes and solutions are excellent in their way, advanting technology creates new problems which they were not designed to overcome.

Thus for instance high-speed lines for coil-coating of aluminium strip have now been introduced into commercial operation which are capable of continuously processing the strip metal at speeds so great that more than 1,000 square feet per minute (more than about 92.9 square metres per minute) of aluminium surface have to be chromate coated. While the increased line speed promotes the efficiency of the operation in other obvious respects, it does however give rise to certain difficulties where chromating solutions are employed, because some finite contact-period must be allowed between the aluminium surface and the chromating solution in order that the conversion coating reaction can take place therebetween.Unless the speed of that reaction can be speeded up, it therefore follows that the faster the line-speed the greater must be the dimensions of the bath of chromating solution through which the strip metal is advanced. There are however distinct limits to the bath dimensions which are acceptable in commercial operations.

In order to speed up the chromate conversion coating step it has already been proposed to add activating agents, usually known as accelerators, to the chromating solutions. The purpose of such accelerators is primarily to improve the coating efficiency of the chromating solution by increasing the coating weight achieved in a given period of time; but in some cases the presence of the accelerator may also promote a better final product.

The activating agent or accelerator which has met with some success is ferricyanide, disclosed for instance in United States Patents Nos. 2,796,370,2,796,371 and 2,988,465. Despite the appreciable commercial use of ferricyanide-activated chromating solutions, they do however give rise to problems.

Thus for instance such ferricyanide-activated chromating solutions are liable to suffer heat degradation at higher operating temperatures; and these solutions are also rather sensitive to changes in acidity.

Perhaps the currently most serious obstacle to the use of ferricyanide-accelerated chromating solutions is however the waste-disposal problem encountered when the spent chromating solutions, which sometimes contain free cyanide moieties, require to be emptied away. While another aluminium chromating process is known which employs a non-ferricyanide accelerator, namely a tungstanate material as disclosed for instance in United States Patent No. 3,391,031,this cannot be regarded as comparable with the use of ferricyanide, if only because it is costly.

There is therefore today an unfulfilled and urgent need to develop chromating solutions and processes which are practical, economical and effective, quickly yielding chromate-coated aluminium surfaces with enhanced corrosion-resistance and paint-bonding characteristics -- and which will achieve these results without the detrimental environmental impact of ferricyanide-accelerated chromating solutions.

We have found that it is possible to devise ferricyanide-free coating solutions and processes as well as concentrates for use in such solutions and processes, which will enable a chromate conversion coating to be rapidly formed on aluminium surfaces, the resultant coatings imparting a high degree of corrosion-resistance to the metal and also operating to promote paint-receptivity thereon- and that it is moreover possible to achieve these desirable objectives at least with the preferred solutions even in a high-speed aluminium coil-coating operation.

According to this invention there are provided aqueous, acidic coating solutions, for forming chromate conversion coatings upon aluminium (as herein defined) surfaces, which coating solutions comprise: (a) from 0.03 to 3.0 grams of zinc (calculated as Zn++) per litre; (b) from 0.01 to 10.0 grams of hexavalent chromium (calculated as Cr+6) per litre; (c) from 0.01 to 10.0 grams of fluoride (calculated as F-) per litre; and (d) from 0.02 to 3.0 grams of molybdate (calculated as Mo) per litre; and including free acid (H+) in an amount sufficient, having regard to the other solution components, to establish an acidic pH.

The solutions of this invention are preferably those wherein the source of the zinc is zinc oxide (ZnO) and/or the source of the hexavalent chromium is chromium trioxide (cur03) and/or the source of fluoride is fluosilicic acid (H2SiF6) and/or the source of the molybdate is molybdic acid.

The currently most preferred solution of this invention is that which comprises zinc added as ZnO in an amount of substantially 0.38 g/l, hexavalent chromium added as CrO3 in an amount of substantially 2.00 g/l, fluoride added as a 23% aqueous solution of H2SiF6 in an amount of substantially 4.56 g/l, molybdate added as 84% molybdic acid in an amount of substantially 0.475 g/l, and nitric acid (100%) present in an amount of substantially 1.9 g/l.

The zinc acts as an accelerator, thus obviating the need for ferricyanide, and must be present within the above-stated concentration range of from 0.03 g/l to 3.0 g/l in order to achieve a proper coating weight.

The fluoride must also be present within the above-stated concentration range of from 0.01 g/l to 10.0 g/l to achieve the desired results. Many different sources of fluoride ion may be employed. The currently-preferred source is fluosilicic acid or one of its salts, but other complex fluorides can equally be used, such as for example metal fluoborates, fluotitanates, fluostannates or fluozirconates. It is also possible to use simple fluorides, such as hydrofluoric acid and its sodium, potassium or ammonium salts.

The chromating solution of this invention must also contain molybdate within the above-stated concentration range of from 0.02 g/l to 3.0 g/l if it is to be effective for the purposes of this invention.

The molybdate may be added as molybdic acid or salts thereof such as sodium molybdate, ammonium molybdate or others of the numerous forms in which molybdate is available on the market today. It should be noted moreover that the term "molybdic acid" as used herein specifically includes the molybdic acid currently sold commercially as 84% MoO3, which is very suitable for use in this invention.

The chromate, which is a necessary component of the chromating solution, must be in the form of hexavalent chromium Cr+6, which must be present within the above-stated concentration range of from 0.01 g/l to 10.0 g/l. Any source of hexavalent chromium ion may be employed, the most convenient source being usually chromium trioxide, CrO3.

To be operable, the chromating solution must display an acidic pH of less than 7. For speedy chromating it is very desirable, especially bearing in mind that the solution must be maintained in acidic operating condition as aluminium is chromated therewith, that the pH of the solution should be established (and subsequently maintained) at not more than 5.5 The chromating solutions of this invention will preferably have a pH within the range of from 1.0 to 2.5.

While the acid used to adjust and maintain the pH of the solution can theoretically be any acid, it is recommended that certain acids should preferably be avoided. The use of chromic acid may sometimes be impractical since it is a source of chromate; and if it needs to be added (for pH adjustment) in amounts such as to carry the chromate concentration of the solution outside the above-stated range, its use is thereby excluded since excess chromate retards coating formation. The use of hydrochloric, sulphuric and phosphoric acids for pH adjustment is also not recommended, since their presence in the chromating solutions of this invention in substantial amounts tends to have a deleterious effect in other respects.

There are however no such problems encountered when nitric acid is used, and this is therefore the recommended acid for pH adjustment of the chromating solutions of this invention.

According to a further aspect of this invention there are also provided processes for forming chromate conversion coatings upon aluminium (as herein defined) surfaces, in which the surface is contacted with an aqueous acidic coating solution which comprises: (a) from 0.03 to 3.0 grams of zinc (calculated as Zn) per litre; (b) from 0.01 to 10.0 grams of hexavalent chromium (calcalculated as Cr+6) per litre; (c) from 0.01 to 10.0 grams of fluoride (calculated as F-) per litre; and (d) from 0.02 to 3.0 grams of molybdate (calculated as Mo)per litre; at a temperature and for a time sufficient to form the desired chromate coating thereon.

The desirable characteristics of the aqueous acidic coating solution employed in the process of this invention are those elsewhere stated herein.

The time and the temperature of contact necessary in the process of this invention to achieve coating are related inversely - thus in general the hotter the temperature of the coating solution the shorter the period of time needed to achieve coating. Bearing that consideration in mind, it will be readily understood that widespread variations in the time and temperature of the treatment are possible, depending on the equipment variations and the method of coating employed. Those skilled in the art should however have no difficulty in ascertaining the proper contact time for their particular workpiece and equipment at any chosen operating temperature.

For generai guidance it may however be added that good results can usually be achieved at a solution temperature of I 200C (about 440C) and although the coating time at this temperature varies with equipment a fairly representative contact period at this temperature would be 10 seconds. For economy the solution temperature utilized can and often will normally be the ambient (room) temperature; but the process can and sometimes will advantageously be operated at elevated temperatures of at least as high as 1600F (about 71 OC).

The aluminium surface is contacted with the chromating solution by any of the available procedures and equipment. It is indeed one of the incidental advantages of this invention that the solutions and processes here used are wholly compatible with currently-available equipment and procedures. There is thus no special limitation on the type of equipment which may be utilized, and the chromating solution may for instance be contacted with the aluminium surface by spraying, dipping or flow-coating (flowing the solution over the surface of the metal) all of which are satisfactory methods of contacting the aluminium surface with the solution. The methods most usually employed, dependent however upon the nature of the aluminium article under treatment, are spraying or dipping (immersion).

The solutions and processes of this invention are capable of forming satisfactory chromate coatings upon any kind of individual aluminium workpieces; but currently it seems that the invention displays itself to best advantage when the process is employed for the continuous chromating of aluminium strip, and above all in a coil-coating operation.

Accordingly it is a preferred feature of this invention to provide, in a process for the continuous coil-coating of aluminium coil-stock, the step of chromate-coating the aluminium strip as it continuously advances by contacting it, preferably by spraying, with the chromating solution elsewhere herein identified.

It is often possible to form a satisfactory chromate coating upon the surface of aluminium (dependent of course upon its surface condition) by the process broadly indicated above and elsewhere in this specification, even though that surface has not been previously cleaned. It is however normally preferred that the process of this invention should include one or more preliminary cleaning stages, so as to ensure uniform results in the final product. Any such preliminary cleaning stages may be undertaken in accordance with conventional precleaning procedures for aluminium surfaces.

Optionally whenever desired the processes of this invention also may include the subsequent step of subjecting the chromated aluminium surfaces to an acidulated after-rinse.Such acidulated after-rinses are most often based upon chromates, and the process may sometimes with advantage include the step of subjecting the chromated aluminium surface to the effect of a conventional chromate after-rinse.

As appears from the detailed experimental results which are set out hereinafter, the coating weights achieved by the processes of this invention tend to vary, depending upon the concentrations of the solution components, especially zinc. While it is usually true that as a general rule an increase in coating weight is desirable this is not always so - since the economics of the situation, or the end-use to which the chromated aluminium is to be put, may sometimes dictate otherwise, as is well known in the art.

The chromating solution employed in the process is often most conveniently prepared by first forming a concentrate, frequently termed a "make-up" solution, which can be readily stored, distributed and marketed, and which thereafter is prior to actual use diluted with water to form the desired chromating solution.

A typical and preferred concentrate for use in the process of this invention is one which consists of substantially: (i) 7 grams of ZnO per litre; (ii) 40 grams of CrO3 per litre; (iii) 68 grams of nitric acid per litre; (iv) 90 grams of an approximately 23% aqueous solution of H2SiFs per litre; and (v) 10 grams of an approximately 84% aqueous solution of molybdic acid per litre.

This concentrate can be converted into the ready-for-use chromating solution by dilution thereof with water to form a substantially 5% (by volume) solution.

During operation of the process to form chromate conversion coatings upon aluminium the chromating solution will normally require replenishment to maintain the necessary, above-stated concentrations of the solution components, despite consumption thereof in the chromating reaction and/or any losses due to so-called 'drag-out' of the solution upon the surface of the chromate-coated aluminium. Such replenishment can be simply effected by ascertaining the deficiencies of the various solution components and replacing them either continuously or intermittently in an essentially conventional manner.

The invention of course extends to aluminium articles, and especially aluminium coil-stock, whenever chromate-coated by the processes herein described.

In addition, in accordance with another aspect of this invention, there is also provided an aqueous concentrate for use in the process elsewhere herein described which concentrate consists of substantially (i) 7 grams of ZnO per litre, (ii) 40 grams of CrO3 per litre, (iii) 68 grams of nitric acid per litre, (iv) 90 grams of an approximately 23% aqueous solution of H2SiF , per litre, and (v) 10 grams of an approximately 84% aqueous solution of molybdic acid per litre.

In order that the invention may be well understood it will now be described in more detail, though only for purposes of illustration, with reference to the following Examples, in which all stated percentages are to be understood as meaning percentages by weight unless explicitly otherwise indicated.

EXAMPLE 1 Chromating of Various Aluminium Alloys To check that the chromating solutions and processes of this invention function well upon diverse aluminium alloys, the following procedures were carried out upon a series of different alloys including those identified hereinafter.

Preparation ofa Concentrate Commercially-available materials were combined, to form a concentrate which contained the following ingredients in the amounts specified below: CrO3 40.0 grams/litre ZnO 7.6 grams/litre HN03(380 Be' solution) 68.0 grams/litre H2SiF6 (23% solution) 91.2 grams/litre Molybdic acid (84% MoO3) 9.5 grams/litre Water balance, to make 1 litre Preparation of Chromating Solution The concentrate prepared above was diluted with water to form a 5% (by volume) solution, having a final solution pH of about 1.5.

Chromate Coating Installation A commercial 5-stage aluminium coil-coating line was employed consisting of four successive immersion tanks followed by a final fresh-water spray-rinse. The line speed could be adjusted to vary between as little as 25 feet per minute (0.127 metres per second) and as much as 100 feet per minute (0.508 metres per second).

Cleaning and Coating Procedure The coil-coating line was started, and the coil of aluminium coil-stock (see later) was first cleaned by immersion in both Stages 1 and 2 in a conventional acidic metal cleaning solution, well-known in the art and thus forming no part of this invention. These two cleaning stages were followed by rinsing Stage 3, in which the thus-cleaned aluminium coil-stock was subjected to an immersion water rinse.

The clean, rinsed aluminium coil-stock then proceeded to Stage 4, where it was immersed in the chromating solution prepared as described above, maintained at a pH of about 1.5 and at a temperature of approximately 1 200F (49 C) for various contact-periods of from about 10 to about 30 seconds.

Following chromating in Stage 4 as just described, the aluminium coil-stock was subjected in Stage 5 to a final water spray-rinse, after which the chromated aluminium was dried and after evaluation was painted and then re-evaluated.

The procedures described above were applied to a variety of aluminium coil-stocks of different compositions. Typical such aluminium coil-stocks treated in this manner were those commonly-known in the United States of America as aluminium alloys 3003 (believed to contain 0.6% Si, 0.7% Fe, 0.05-0.2% Cu, 1-1.5% Mn, 0.1% Zn, 0.05% impurities and balance Al), 3105 (believed to contain 0.6% Si, 0.7% Fe, 0.3% Cu, 0.3-0.8% Mn, 0.2-0.8% Mg, 0.2% Cr, 0.4% Zn, 0.1% Ti, and balance Al), 5005 (believed to contain 0.3% Si, 0.7% Fe, 0.2% Cu, 0.2% Mn, 0.5-1.1% Mg, 0.1% Cr, 0.25% Zr and balance Al), 5052 (believed to contain 2.5% Mg, 0.25% Cr and balance Al) as well as so-called "Utility Stock".

Evaluation of the appearance and properties of the various alloys treated in the above fashion indicated that the final product, both before and after painting, was in all ways comparable to similar alloys when treated by conventional ferricyanide-containing chromating solutions and processes.

EXAMPLE 2 Chromating Aluminium with Zinc-Accelerated Solution of the Invention, compared with Unaccelerated and with Ferricyanide-Accelerated Chromating Solutions Six sets of panels, made from 3003 aluminium alloy (see Example 1) and identified as Sets 1-6, were subjected to the following 4-stage treatment sequence: Stage 1 Alkaline pre-cleaning (conventional) Stage 2 Taps water rinse Stage 3 Chromating at about 1 000F (about 380C) with chromating solution identified in Table \ | below - but see Note.

Stage 4 Tap-water rinse [Note: For convenience Table I identifies only the solution components which vary; but the solutions of course also contained appropriate unvarying amounts of the other necessary components].

Another three sets of similar panels were also prepared and subjected to different treatments for purposes of comparison. The panels identified as Sets 7 and 9 were each chromated in the manner described in United States Patent No. 2,988,465 with the same standard conventional ferricyanidecontaining chromating solution, but for different contact periods; and the panels identified as Set 8 were precleaned and rinsed as in Stages 1 and 2 above (for Sets 1-6) but were not chromated.

The weights of the chromate coating thus formed upon the panels in Sets 1-6,7 and 9 were measured, by weighing the panels after chromating, then stripping the panels with a nitric acid solution and reweighing the stripped panels. The weight-difference was attributed to the coating on that panel; and the average coating weight was thus determined for each set of panels. The results for all nine sets of panels are summarized in Table I below, which also (as previously indicated) records the composition of the chromating solution and the contact period employed in each case:- TABLE 1

Chromating Solution Coating Weight Set of Zn++ Cr+@ Free Spray mg/ft g/m Panels Acidity Time No. (see note) 1 - 0.9 g/l 0.9 15 secs. 7.2 2 0.1 g/l 0.9 g/l 0.8 15 secs. 24.3 3 0.2 g/l 0.9 g/l 1.0 15 secs. 31.2 4 0.3 g/l 0.9 g/l 0.9 15 secs. 39.6 5 0.4 g/l 0.9 g/l 0.9 15 secs. 37.8 6 0.5 g/l 0.9 g/l 0.9 15 secs. 27.3 7 - 0.6 g/l - 5 secs. 14.4 8 - not chromate-coated, only cleaned - 0.3 9 - 0.6 g/l - 15 secs. 40.5 [Note.The free acidity of the chromating solution was for convenience recorded in terms of the number of equivalents of base needed to neutralize the acid present determined by titrating a one millilitre sample of the solution with 0.1 N NaOH to a mixed methyl red and bromocresyl green end point].

From the results set out in Table I above it can be seen that the unaccelerated chromating solution (Set 1) yields only very low coating weights in the chosen 1 5-second period; but much higher coating weights are achieved in the same period in the presence of varying amounts of zinc (Sets 2-6), demonstrating therefore that zinc acts as an accelerator in chromating baths comparable in its effect upon coating weight (which can be taken as a rough measure of the likely improvement in paintbonding upon the surface) with that of ferricyanide. In fact, the accelerating effect of zinc at its best (Set 4) is virtually identical with that of ferricyanide (Set 9) over the chosen 1 5-second period.

The various sets of panels chromated (or not) in the manner described with reference to Table I above were then paint-coated and tested to ascertain the durability of the chromated/painted surfaces under standard test conditions designed to simulate actual use. The panels in each of the aforementioned Sets 1-9 were each coated with an acrylic paint conventionally used as a standard for test purposes, namely that known as PPG Duracron 630, manufactured by Pittsburgh Plate Glass. The paint-coated panels were then baked.

The test procedure adopted with the paint-coated and baked test panels was essentially that laid down and described in detail by the American Society for Testing Materials in their Bulletin No. ASTM 'B-287. Briefly stated, this procedure involves first preparing the panels for testing by (a) scribing certain areas of each panel (so as to cut through the paint and the chromate coating down to bare metal) and leaving other areas unscribed, and also (b) taping over certain areas (both scribed and unscribed) of each panel with pressure-sensitive tape while leaving other areas (both scribed and unscribed) untaped -- and finally then subjecting the panels thus prepared to the corrosive atmosphere of a standard acid/salt spray test for periods of either 500 or 1,000 hours, using a 5% solution at a temperature of 959F (350C) in each case.

After testing in this manner, the panels were each visually evaluated; and the corrosion suffered by each panel under test, observed as paint loss, was visually assessed and rated in descriptive and/or numerical terms. The descriptive assessments such as "very slight", "slight", "moderate" and "heavy" refer to paint loss. The numerical ratings are on a scale of from 1 to 10, with the best results (i.e.

virtually no paint loss and therefore so-to-speak "perfect") being represented by 10.

In respect of each panel the evaluation was made separately for (i) the scribed and untaped area, (ii) the unscribed and untaped area, (iii) the scribed and taped area, and (iv) the unscribed and taped area.

As regards scribed areas, the assessment or rating was made in relation to the paint immediately adjacent the scribe mark - this assessment measures the ability of the painted metal to withstand erosion after damage. As regards taped areas, the assessment or rating was made after the pressure sensitive tape had been removed, carrying with it any non-adherent paint - this assessment, compared with that of the untaped areas, gives an indication of the ability of the chromated and painted panel to withstand a corrosive atmosphere.

The results of these tests are summarized in Table II below: TABLE II

500 Hours Acid/Salt Spray 1,000 Hours Acid/Salt Spray Set of Zn content of Un-taped Area Taped Area Un-taped Area Taped Area Panels Chromating No. Solution Scribed Unscribed Scribed Unscribed Scribed Unscribed Scribed Unscribed Very 1 NIL Slight 7 Slight Moderate 3 Moderate Moderate Slight 2 0.1 g/l Very 10 Very Very Slight 3 Slight Moderate Slight Slight Slight 3 0.2 g/l " 10 " " Moderate 3 Moderate " 4 0.3 g/l " 10 " 10 " 3 " Slight 5 0.4 g/l " 10 " 10 " 3 " Moderate 6 0.5 g/l " 10 " 10 " 3 " " 7 - Slight 10 Slight 10 Moderate 3 Moderate Moderate 8 - " 7 " Slight Heavy 2 " " 9 - Very 10 Very 10 Moderate 3 " " Slight Slight From the results set out in Table II above it can be clearly seen that the zinc-accelerated chromating solutions of this invention give results which overall are in every way at least as good as those achieved by the previously-known ferricyanide-accelerated chromating solutions. This however is achieved by the chromating solutions and processes of this invention without the environmental hazards inescapable when using ferricyanide yet employing relatively inexpensive materials and thus without the cost-penalty involved when employing tungstanate.

Claims (22)

1. Aqueous acidic coating solutions, for forming so-called chromate conversion coatings upon aluminium (as hereinbefore defined) surfaces, said solutions comprising: (a) from 0.03 to 3.0 grams of zinc (calculated as Zn++) per litre; (b) from 0.01 to 10.0 grams of hexavalent chromium (calculated as Cr+6) per litre; (c) froim 0.01 to 10.0 grams of fluoride (calculated as F-) per litre; and (d) from 0.02 to 3.0 grams of molybdate calculated as Mo+6) per litre; and including free acid (H+) in an amount sufficient, having regard to the other solution components, to establish an acidic pH.

2. A solution as claimed in claim 1, in which the amount of free acid (H-) is such as to establish a pH of 5.5 or less.

3. A solution as claimed in claim 1 or claim 2 in which the amount of free acid (H+) is such as to establish a pH within the range of from 1.0 to 2.5.

4. A solution as claimed in any of claims 1 to 3, in which the free acid added to establish the desired acidic pH is nitric acid.

5. A solution as claimed in any of claims 1 to 4, in which the source of zinc is or includes zinc oxide (ZnO).

6. A solution as claimed in any of claims 1 to 5, in which the source of hexavalent chromium is or includes chromium trioxide (cur03).

7. A solution as claimed in any of claims 1 to 6, in which the source of fluoride is or includes fluosilicic acid (H2SiF6).

8. A solution as claimed in any of claims 1 to 7, in which the source of molybdate is or includes molybdic acid.

9. A solution as claimed in any of claims 1 to 8, which comprises: (a) zinc added as ZnO in an amount of substantially 0.38 grams per litre; (b) hexavalent chromium added as CrO3 in an amount of substantially 2.00 grams per litre; (c) fluoride added as a 23% aqueous solution of H2SiF6 in an amount of substantially 4.56 grams per litre; (d) molybdate added as 84% molybdic acid in an amount of substantially 0.475 gram per litre; and (e) nitric acid (100%) present in an amount of substantially 1.9 grams per litre.

10. An aqueous, acidic chromating solution for aluminium, as claimed in any of claims 1 to 9 and substantially as herein described.

11. Processes for forming so-called chromate conversion coatings upon aluminium (as herein defined) surfaces, in which the surface is contacted for a time sufficient to form the desired chromate coating thereon with an aqueous acidic coating solution which comprises: (a) from 0.03 to 3.0 grams of zinc (calculated as Zn++) per litre; (b) from 0.01 to 10.0 grams of hexavalent chromium (calculated as Cr+6) per litre; (c) from 0.01 to 10.0 grams of fluoride (calculated as F-) per litre; and (d) from 0.02 to 3.0 grams of molybdate (calculated as Mo+6) per litre; together with free acid (H+) in an amount sufficient, having regard to the other solution components, to establish an acidic pH.

1 2. A process as claimed in claim 11, in which the solution employsed is one as claimed in any of claims 2 to 10 above.

1 3. A process as claimed in claim 11 or claim 12, which includes the step of chromate-coating aluminium strip by continuously contacting its surface with said aqueous acidic coating solution as the strip is advanced therethrough.

14. A process as claimed in any of claims 11 to 1 3, in which the aluminium surface is supplied from a coil-coating operation.

15. A process as claimed in any of claims 11 to 14, in which the aluminium surface is contacted with the chromating solution by immersion therein.

1 6. A process as claimed in any of claims 11 to 1 4, in which the aluminium surface is contacted with the chromating solution by spraying it therewith.

1 7. A process as claimed in any of claims 11 to 1 6, in which the temperature of the chromating solution lines within the range from ambient up to 1 600F (71 OC).

1 8. A process as claimed in any of claims 11 to 17, in which the chromating solution employed in the process is prepared by first forming a concentrate consisting of substantially: (i) 7 grams of ZnO per litre; (ii) 40 grams of CrO3 per litre; (iii) 68 grams of nitric acid per litre; (iv) 90 grams of an approximately 23% aqueous solution of H2SiF6 per litre; and (v) 10 grams of an approximately 84% aqueous solution of molybdic acid per litre and thereafter diluting said concentrate with water to form the desired chromating solution.

19. A process as claimed in claim 18, in which the concentrate is diluted with wateer to form a substantially 5% (by volume) solution thereof.

20. A process as claimed in any of claims 11 to 1 9 and substantially as herein described.

21. Aluminium articles whose surfaces have been chromated by the process claimed in any of claims 11 to 20.

22. An aqueous concentrate, for use in the process claimed in claim 1 9 or claim 20, which consists of substantially: (i) 7 grams of ZnO per litre; (ii) 40 grams CrO3 per litre; (iii) 68 grams of nitric acid per litre; (iv) 90 grams of an approximately 23% aqueous solution of H2SiF6 per litre; and (v) 10 grams of an approximately 84% aqueous solution-of molybdic acid per litre.