EP1579030A1 - Process for providing a thin corrosion inhibiting coating on a metallic surface - Google Patents

Abstract

The invention relates to a process for coating metallic surfaces with a phosphating coating by contacting metallic surfaces at a temperature not above 45° C. and at a pH value less than 3.5 with an aqueous acidic alkali metal phosphating solution or dispersion containing: At least one compound of at least one phosphorus containing acid and/or at least one of their derivates like esters and salts in a total content of all kinds of acids and all their derivates like esters and salts together of less than 20 g/L calculated on mole base as orthophosphate, whereby the content of such phosphorus containing compounds/ions is at least 50% by weight in comparison to all such compounds/ions and at least one ion selected from the group consisting of at least one alkali metal ion and ammonium ion, whereby the phosphating coating has a coating composition with a phosphorus content of not more than 8 atomic % as measured by Secondary Neutral Mass Spectroscopy (SNMS) and whereby the phosphating coating has a coating weight in the range from 0.01 to 0.5 g/m<2>.

Description

PROCESS FOR PROVIDING A THIN CORROSION INHIBITING COATING ON A METALLIC SURFACE

FIELD OF THE INVENTION

This invention relates to a process for coating the surface of a metallic coil, part or wire with an aqueous acidic phosphating solution containing predominantly alkali metal ions and/or ammonium ions as well as in many cases phosphate ions. It relates further on to a phosphating solution to be used in this process for generating excellent corrosion inhibiting coatings on metallic surfaces. In some instances such coating may be used for coldforming of the metallic part. Such solutions are called alkali metal phosphating solutions or, if used on iron-rich surfaces, iron phosphating solutions.

The invention is particularly concerned with a coating resp. conversion coating on aluminum, aluminum alloy, iron alloy like steel and stainless steel, magnesium alloy, zinc or zinc alloy as well as with a process, a concentrate and a solution for the formation of a phosphating coating on surfaces of these metallic materials.

Such coating solution is especially suitable for the generation of pretreatment coatings on substrate surfaces which will be coated in a second step with at least one organic film, especially at least one film like a thin electrocoating lacquer layer, a paint layer, a silane-rich layer and/or an adhesive layer. Alternatively, the coating may be used for a treatment like a passivation without being covered with a further coating like a paint layer.

BACKGROUND OF THE INVENTION

Processes for the production of alkali metal phosphating coatings, especially prior to a lacquering, are described in relatively few cases in comparison to zinc phosphating or manganese phosphating where there is a huge number of publications. Fresh solutions for alkali metal phosphating that had not yet been used show typically only a very low or even practically no content of aluminum, iron and zinc. The fresh aqueous acid alkali metal phosphating solutions contain ions of at least one type of alkali metal ions and/or ammonium ions as well as phosphate ions. Because of the pickling effect of such acidic solutions on metallic surfaces, the ions of the dissolved metals like aluminum, iron and zinc as well as traces of other alloy constituents of the pickled metallic materials will be enriched during the ongoing phosphating process in the phosphating solution. Typically, the main phases of the alkali metal phosphating coatings are the corresponding phosphates, oxides and/or hydroxides of the metallic constituents of the metallic base material(s).

Alkali metal phosphating solutions resp. coatings are called iron phosphating solutions resp. coatings if used on iron alloy surfaces like steel. The same corresponds to aluminum and aluminum alloys where such solutions resp. coatings are described as aluminum phosphating solutions resp. coatings. Often surfaces of very different metallic materials may be coated in the same alkali metal phosphating bath at the same time or one after the other whereby the ions of the different metals/alloys of the basic materials will be collected in the bath. Such coatings are - in contrast to coatings of the so called zinc-, zinc-manganese- or manganese- phosphating, mostly or totally amorphous or extraordinarily fine-grained.

Alkali metal phosphatings are described in Werner Rausch: The Phosphating of Metals, ASM International, Finishing Publications Ltd., Teddington, England 1990 (especially pages 94 - 100, 120 - 130) in detail and are called the "non-coating phosphating" or in other publications "amorphous phosphating". The term "non- coating phosphating" is misleading, as there will be coatings generated although such coatings will be significantly thinner than created during e.g. zinc phosphating or zinc-manganese phosphating. The very thin alkali metal phosphating coatings are not, poorly or - if coloured or grey - well visible; the coatings may be only visible by colours caused by physical effects, by their kind of grey appearance and/or by their matte appearance. The alkali metal phosphating solution contains always a certain content of at least one alkali metal like sodium, potassium and/or of ammonium. The alkali metal phosphating coatings are typically - in contrary to the well and coursely crystalline coatings of the so-called "coating-forming phosphatings" - more or less amorphous and show under the scanning electron microscope typically no crystalline grain shapes.

The alkali metal phosphating coatings are mostly poor, nearly free or totally free of manganese and zinc, if there will not be manganese and/or zinc rich surfaces, to be pretreated or treated. They are typically poor, nearly free or totally free of chromium, cobalt, copper, nickel, tin and/or other heavy metals. The phases mainly generated and/or precipitated during iron phosphating, which is performed by contacting iron-rich metallic surfaces with an alkali metal phosphating solution, are iron phosphates, iron oxides and iron hydroxides like e.g. vivianite and/or magnetite. The contents of the ions dissolved from the metallic surface and then carried in the alkali metal phosphating solution, especially of aluminum, chromium, copper, iron, magnesium, tin, titanium, resp. zinc are relatively low as such compounds resp. cations are normally not added to the bath, but are only or nearly only present because of the pickling effect of the aqueous acidic alkali metal phosphating solution to the metallic surfaces of the parts, sheets, strips or wires to be . coated. Such contents will precipitate and generate the coating primarily containing phosphates, oxides and/or hydroxides of the metals' content in the solution further on, there may be traces or even low contents of such ions caused by impurities by pickling the bath containers and connecting tubes as well as by dragging in from earlier steps of the process succession.

A significant difference of the alkali metal phosphating process in comparison to phosphating processes of the "coating-forming phosphating" is further that the cation(s) necessary for the coating formation during alkali metal phosphating is/are always present in a small percentage, mostly or totally dissolved from the surfaces of the metallic base substrates, whereby during e.g. zinc-, zinc-manganese-, zinc- nickel- or zinc-mariganese-nickel-phosphating there will be a relatively high addition of e.g. zinc so that zinc is contained mostly in a content of more than 0.3 g/L resp. often of more than 1 g/L in the phosphating solution. This high zinc content is often caused by addition of zinc compounds of at least 40 %, mostly more than 60 %, often even more than 80 % of the total content to the bath, whereas only the remaining content is mostly generated by the pickling effect to zinciferous surfaces. The coatings generated by zinc-, zinc-manganese-, zinc-nickel- or zinc-manganese- nickel-phosphating show typically the predominantly zinc and/or manganese containing phases hureaulithe, phosphophyllite, scholzite and/or hopeite in significant crystalline shapes.

The coatings of alkali metal phosphating show significant other properties as such from zinc-rich phosphatings: They have mostly a coating thickness in the range from 0.1 to 0.8 μm resp. only a coating weight in the range from 0.2 to 1.3 g/m². In contrary to the mostly dull grey appearing zinc-rich phosphating coatings, the much thinner alkali metal phosphatings are mostly transparent or show iridescent colours related to their extremely thin thickness. Then they show the colours of "higher orders" and may be e.g. nearly transparent, yellowish, golden, reddish, a bit violet, greenish or often bluish, partly iridescent. Only in the case that the alkali metal phosphating coatings should have a higher coating weight, especially more than 0.7 and perhaps up to about 1.3 g/m², they may show a more matte-grey appearance. Especially aluminum-rich alkali metal phosphatings may occur silvery or silvery- iridescent.

The alkali metal phosphating coatings may be prepared without any later generation of e.g. at least one paint layer and/or another organic paint-like layer. Then this coating process may be called a treatment. If the phosphating coatings should be used for a protection against corrosion for a limited time, then the coatings may be called a passivation. But they can be used under at least one paint layer and/or another organic paint-like layer like a primer, a lacquer, a silane layer, a base coat and/or a topcoat and/or respectively together with an adhesive and may then be called a pretreatment.

In general, alkali metal phosphating coatings are produced prior to painting by contacting the aqueous acidic phosphating solution which contains typically at least one mono- and/or orthophosphate and afterwards by electrocoating the phosphated metallic surfaces and/or often by powder painting e.g. of the parts of the metallic construction that are well accessable from outside like radiators and car bodies.

Typically, today alkali metal phosphating processes are carried out with solutions that contain alkali metal and/or ammonium and at least one type of phosphate, mostly orthophosphate, as well as always at least one accelerator, thereby showing a pH value during operating in the range of 4 to 6. These aqueous acidic solutions are contacted to the metallic surfaces typically at temperatures in the range from 48 to 72 °C. Their typical coating weights are in the range from 0.3 to 1 g/m². The coatings of today are rich in at least one phosphorus compound, show mostly bluish or light grey coloured coatings and often a coating weight in the range from 0.5 to 1.5 g/m². DE-A1-100 06 338 describes a typical process for iron phosphating where there has been added a small amount of copper ions to solutions of a pH value in the range from 3.5 to 6.5 at a temperature in the range from 30 to 70 °C and especially at a pH value of about 4.8 of about 55 °C. DE-A1-1 942 156 teaches an alkali metal phosphating process by using a high pressure spraying method for contacting the metallic surfaces with solutions of a temperature of 60 °C and of a pH value in the range from 3 to 5.5, especially of a pH value of 4. DE-A1-1 914 052 concerns an alkali metal phosphating process by using a rollcoating application with a solution containing 5 to 20 g/L of phosphate ions and 3 to 12.5 g/L of chlorate at a temperature of 54.5 to 60 °C with an extraordinarily unconventional pH value in the range of 1 to 3.5 contacting a coil less than 30 seconds and squeegeeing. EP-B1 -0 968 320 protects a process for an alkali metal phosphating for radiators by using a surfactant rich solution of a pH value in the range from 4 to 6 at a temperature in the range from 35 to 60 °C and especially of at least 50 °C. FR-A-1.155.705 refers to an alkali metal phosphating process by using an ammonium silicon hexafluoride and nitroguanidine containing solution of a pH value in the range from 3 to 6 at a temperature in the range from 50 to 76 °C. GB-A-1 388 435 reports an alkali metal phosphating process by using a free fluoride and chlorate containing solution of a pH value in the range from 3 to 6 at a temperature in the range from 50 to 80 °C, especially used with a pH value in the range from 3.65 to 4.4. US-A-2,665,231 discloses an alkali metal phosphating process by using a fluoride containing solution of a pH value in the range from 3 to 5.8 at a temperature in the range from 60 to 82 °C, especially used with a pH value in the range from 4.25 to 5.5.

It was an object of the invention to propose an alkali metal phosphating process with very stable bath conditions and with excellent coating appearances and coating qualities using at least one accelerator like nitroguanidine. It was further an object to propose a phosphating process with an improved corrosion resistance in comparison to typical alkali metal phosphating processes used today. Further on, it was an object to propose an alkali metal phosphating process that is stable, well suited for industrial application for coils, parts and wires as well as easier and cheaper in comparison to actually used processes.

Astonishingly, it has been observed that it is possible to "phosphate" a metallic surface with an unusually low or even with a zero phosphate content. Even other acids than phosphorus containing acids could be used without a loss of the quality of the coatings' properties. Nevertheless, the term "phosphating" is used here for all kinds of coating processes and coatings independent if they contain phosphorus or not.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a process for coating metallic surfaces with a phosphating coating by contacting metallic surfaces at a temperature not above 45 °C and at a pH value less than 3.5 with an aqueous acidic alkali metal phosphating solution or dispersion containing:

At least one compound of at least one phosphorus containing acid and/or at least one of their derivatives like esters and salts in a total content of all kinds of acids and all their derivatives like esters and salts together of less than 20 g/L calculated on mole base as orthophosphate, whereby the content of such phosphorus containing compounds/ions is at least 50 % by weight in comparison to all such compounds/ions and at least one ion selected from the group consisting of at least one alkali metal ion and ammonium ion, whereby the phosphating coating has a coating composition with a phosphorus content of not more than 8 atomic% as measured by Secondary Neutral Mass Spectroscopy (SNMS) and whereby the phosphating coating has a coating weight in the range from 0.01 to 0.5 g/m².

According to the present invention, there is provided a phosphating coating on a metallic surface prepared by contacting metallic surfaces with an aqueous acidic alkali metal phosphating solution or dispersion having a coating thickness of not more than 0.15 μm and having a good corrosion protection for the protected metallic material. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It has been found that for steel panels treated with an alkali metal phosphating solution based on a conventional composition, dried and later on painted with a polyester paint, show a corrosion inhibiting effect as measured by salt spray (fog) test clearly depending of the pH value of the alkali metal phosphating solution. At a pH value of the solution of about 7, the salt spray (SS) evaluation showed results of about 5, at a pH value of about 5 SS values of about 2.5 and at a pH value of about 2.5 SS (mm creep from scribe) values of about 1.5 or even less. Further details thereto are found in the examples.

Tests were performed to identify the phases of different alkali metal phosphating coatings, but there was no X-ray diffraction result to be able to identify the phases. Therefore, it is believed that the thin coatings are amorphous or nearly amorphous.

Then, the element content of the coatings was analysed by X-ray Photoelectron Spectroscopy (XPS), which may be used successfully as routine measurement method for controlling the different coatings, but which is an insufficient precise measurement method for such coatings to identify the element content dependent from the depth of the coating. Only the upper 8 nm from the surface into the depth could be analysed and therefore there is an influence of surface impurities. The measurement of the content of phosphorus and other elements in the coating was performed by X-ray Photoelectron Spectroscopy with an instrument 5700LSci of Physical Electronics, with an X-ray source of monochromatic aluminum, a power source of 350 Watts, an analysis region of 2 x 0.8 mm, an exit angle of 65 °, a charge correction for C-(C,H) in C 1 s spectra at 284.8 eV and a charge neutralization by electron flood gun.

Finally, the element content of the coatings was analysed by Secondary Neutral Mass Spectroscopy (SNMS) with an INA3 electron gas - SNMS apparatus of Leybold, which is a precise measuring method to identify the element content dependent from the depth of the coating of such thin alkali metal phosphating coatings. The samples were sputtered with Ar ions of 1040 eV energy and at a current density of about 1.2 mA/cm². An area of 5 mm diameter was sputtered and analysed. During the measurement, the atoms of the upper surface layer evaporated and the next atom layers of below were analysed, until the total coating was removed in the sputtered area during the analysis. In 10 seconds of sputtering, about 10 nm of the upper part of the coating were removed. The measurement method could only be calibrated to a certain amount to the composition of the analysed coatings. The results showed a minor dependency of the surface roughness which was considered in the evaluation.

For both analyses, XPS and SNMS, the same four samples of cold rolled steel panels were analysed:

1 ) an only cleaned, but not coated panel,

2) a typical conventional iron phosphating coating according to the state of the art as produced in today practice by first cleaning and then contacting the panel with an iron phosphating solution containing phosphate, sodium and chlorate at a pH value of 4.5 at a temperature of 50 °C generating a coating of about 0.16 to 0.22 μm thickness,

3) a very thin yellow iron phosphating coating according to the invention generated after having cleaned the panel by contacting it with an iron phosphating solution containing phosphate, sodium and 0.2 g/L nitroguanidine at a pH value of 3.0 with a value for total acid of 6 points at a temperature of 37 °C with a coating of about 0.02 to 0.1 μm thickness,

4) a thin bluish iron phosphating coating according to the invention generated after having cleaned the panel by contacting it with an iron phosphating solution containing phosphate, sodium and 0.2 g/L nitroguanidine at a pH value of 3.0 at a temperature of 37 °C with a coating of about 0.06 to 0.12 μm thickness, but the value for total acid had fallen below 3 points. Table 1 : Element content of the samples 1) to 4) as measured by XPS in atomic%:

The results of table 1 show that there is a considerable difference in the composition between the uncoated sample 1 ), the coated sample according to the state of the art 2) and the coated samples 3) and 4) according to the invention.

The figures present the element distribution in atomic% as analysed by Secondary Neutral Mass Spectroscopy (SNMS) depending from the depth of the coating which was analysed from the surface (left) into the massive steel material (medium to right) in nm. Figure 1 for the cleaned, but not coated sample 1 ) shows the impurity effect of the surface region and then the composition of the cold rolled steel material. Figure 2 for sample 2) covered with a typical conventional iron phosphating coating of today indicates via the Fe content the thickness of the iron phosphating coating. As shown in the following figures, the curves of the content of oxygen and phosphorus are - in the logarithmic graph - more or less proportional ("parallel"). There is a level in the upper and middle parts of the coating of about 30 atomic% of Fe, of about 50 atomic% of O and of about 9 atomic% of P. Figures 3 and 4 for the samples 3) and 4) with the coating according to the invention do not show clear content levels. The coating of sample 4) which is some percent thicker than the coating of sample 3), indicates a content of about 50 atomic% of Fe, of about 35 atomic% of O and of about 6 atomic% of P in the upper parts of the coating. Figure 5 represents the results of sample 5) that is comparable with sample 3) but shows higher surface roughness data and therefore higher signal output. Figure 6 represents the results of sample 6) that is comparable with sample 4) but shows higher surface roughness data and therefore higher signal output, too. Figure 7 represents the curves of the P content of the samples 1 ) to 4) in comparison, but now - as linear graph - showing clearly different phosphorus contents dependent from the depth analysed in the coating. Therefore, it is clearly demonstrated that the composition of the conventional iron phosphating coatings is significantly different from the composition of the iron phosphating coatings according to the invention.

The surface roughness of all samples was measured with a white light interferometer NT3300 of Wyko, each coated panel on three areas. For the samples 1 ) to 4), the average data of R_a per panel varied between 0.89 and 1.02 μm, the average data of R₂ per panel varied between 1.11 and 1.22 μm and the average data of R_t per panel varied between 6.17 and 7.25 μm. In comparison to the samples 3) and 4), the samples 5) and 6) had been coated in the same manner and under nearly the same conditions, but they showed surface roughness data nearly twice as high as the samples 3) and 4): The average data of R_a per panel varied at about 1.79 μm, the average data of R_z per panel varied at about 11.7 μm and the average data of R_t per panel varied in the range from 11.4 to 12.1 μm. Sample 3 ) has to be compared with sample 5) for the difference in surface roughness and element content; similarly, sample 4) has to be compared with sample 6). As the rougher surfaces enable a higher amount of neutral parts measured than from more even surfaces, the more even surfaces shall be used for the analytical investigation and evaluation.

Preferably, the P content is less than 8 atomic% in a depth of 0.05 μm below the (original) surface of the alkali metal phosphating coating as analysed by Secondary Neutral Mass Spectroscopy (SNMS) or is less than 6 or even less than 4 atomic% in a depth of 0.1 μm below the surface of the alkali metal phosphating coating or is less than 3 or less than 2 atomic% in a depth of 0.1 μm below the surface of the alkali metal phosphating coating. Preferably, the phosphating coating according to the invention has a thickness of not more or less than 0.15 μm, more preferred of not more than 0.12 μm, much more preferred of not more than 0.10 μm.

The process according to the invention may preferably be characterized in that the temperature of the phosphating solution or dispersion may be during the contacting of the metallic surfaces in the range from 10 to 42 °C or less than 40 °C and more preferred at least 15 °C or up to 38 or up to 35 °C. The pH value may preferably be selected in the range starting from 1.8 resp. reaching up to 3.3, more preferred of at least 2 or up to 3.1 , especially of at least 2.5 or up to 2.9. The coating weight may preferably be selected in the range from 0.03 to 0.4 g/m², more preferred of at least 0.05 or up to 0.36 g/m², most preferred of at least 0.1 or up to 0.32 g/m².

As acids for use in the phosphating solution or dispersion, most organic and inorganic acids as well as their water-soluble and/or water-dispersible derivatives, especially salts and/or esters, may be taken, but hydrochloric acid and chlorides are not recommended as they may cause significant crevice corrosion. There may be even used mixtures a) of acids, b) of at least one acid with at least one of salts and/or with at least one of ethers or c) of at least one of salts and/or of at least one of ethers.

Preferably, at least one acid is used like orthophosphoric acid, diphosphoric acid, monophosphoric acid, at least one of phosphonic acids, e.g. especially at least one with at least one aliphatic and/or aromatic group each, especially at least one of diphosphonic acids, phosphonous acid, phosphorous acid, molybdatophosphoric acid, tungstophosphoric acid and/or at least one of its derivatives like ester(s) and/or salt(s), especially at least one of monoester(s), of diester(s) and/or of triester(s) of a phosphorus containing acid like orthophosphoric acid, more preferred mixed with at least one phosphorus containing acid.

Preferably, at least one sulfur containing acid and/or at least one of its derivatives like ester(s) and/or salt(s) is used like sulfuric acid, sulfamatic acid, at least one of sulfonic acids like nitrosulfonic acid resp. at least one of their derivatives like ester(s) and/or salt(s).

Preferably, at least one nitrogen containing acid and/or at least one of its derivatives like ester(s) and/or salt(s) is used like nitric acid, at least one acid having at least one nitro and/or at least one amino group resp. at least one of its derivatives like ester(s) and/or salt(s).

Preferably, at least one organic acid and/or at least one of its derivatives like ester(s) and/or salt(s) is used like at least one of aromatic organic acids, hydroxocarboxylic acids, oxo acids, peracids and/or oxocarboxylic acids resp. at least one of its derivatives like ester(s) and/or salt(s) especially like acetic acid, benzoic acid, citric acid, formic acid, gluconic acid, hydroxy acetic acid, lactic acid, malic acid, oxalic acid, succinic acid, tartaric acid and/or its water-soluble and/or water- dispersible derivative(s) like ester(s) and/or salt(s) may be used.

Any acid, derivative of it, acid mixture and/or mixture with at least one of its derivatives like ester(s) and/or salt(s) may be used, especially at least one or any mixture that is able to show a pH value e.g. of about 2.4, of about 2.9, of about 3.4, of about 3.9 and/or of about 4.4 and that is able to generate - at least together with the cations present - a thin coating, but a high amount of hydrochloric acid and of chloride is not favourable to be used because of its very strong corroding effect. Of these acids and derivatives, especially phosphoric acid and dissolved phosphate esters/salts are especially favourable. To accelerate the phosphating process, reducing and/or oxidizing accelerators may be added, but must not be applied. Such accelerator(s) may be favourable to enhance the process, the coating quality and/or to influence the oxidation situation.

In the process according to the invention, the phosphating solution or dispersion contains in many, but not all cases at least one accelerator like such on the base of chlorate, guanidine, of an organic compound with at least one nitro group like nitroguanidine and/or nitrobenzenesulfonic acid and its derivatives, of hydrogen peroxide, hydroxylamine, nitrate and/or of other nitrogen containing accelerators; more preferred are nitroguanidine, nitrobenzenesulfonic acid and/or its derivative(s) like salt(s). All accelerators together show a content in the range from 0.005 to 10 g/L, preferably in the range from 0.01 to 6 g/L, more preferred in the range from 0.02 to 3 g/L, especially preferred of at least 0.03 or up to 1 g/L, most preferred of at least 0.05 or up to 0.7 g/L.

Astonishingly it was observed that it is possible to use the process according to the invention without addition of any accelerator. The quality of the accelerator-free phosphating solutions or dispersions as well as the coating procedure and the quality of the resulting coatings was the same as with a content of at least one accelerator. It may_. only happen that the pickling rate of the accelerator-free solution or dispersion is a bit reduced so that the coating rate is lowered and the contacting time has to be increased by a small amount. In the process according to the invention, an amount of Fe²⁺ ions may be added to the phosphating solution or dispersion, preferably in the range from 0.01 to 1 g/L, more preferred in the range from 0.02 to 0.8 g/L, specifically preferred in the range from 0,03 to 0.5 g/L, most preferred of at least 0.05 or up to 0.3 g/L. The addition may be a dissolved iron phosphate. This addition helps in some cases, especially for nonferrous metal surfaces like such of hot-dip-galvanized (HDG) or electrogalvanized materials (EG), to generate a better corrosion inhibiting performance.

In case of the - especially accelerator-free - coating of steel surfaces, it is favourable to take care that the phosphating solution or dispersion does not contain more than about 0.5, 1 or 1.5 g/L of Fe²⁺ ions, depending on the actual phosphating conditions; the iron content may then be lowered by addition of an oxidizing agent - which may be in some cases an accelerator - and/or by using a cation exchange material, e.g. an adequate resin.

In favourable embodiments, the phosphating solution or dispersion contains free fluoride, preferably in the range from 0.01 to 1 g/L, and/or complex fluoride, especially of aluminum, boron, silicon, titanium and/or zirconium, preferably each in the range from 0.01 to 1 g/L. In such cases, it is more preferred that the content of each of free fluoride resp. of each of the complex fluohde(s) lies in the range from 0.02 to 0.8 g/L, specifically preferred in the range from 0.03 to 0.5 g/L, most preferred of at least 0.05 or up to 0.3 g/L. The content of free fluoride and/or of the at least one complex fluoride enhances the pickling effect, especially on galvanized metallic surfaces as well as on aluminum-rich surfaces as oxide contents may be easier removed from the metallic surface; further on, it improves the performance and the quality of the corrosion inhibition and paint adhesion of the thereof formed coating for all metallic material bases.

In the process according to the invention, an amount of PO₄ ions may be added to the phosphating solution or dispersion preferably in the range from 0.1 to 18 g/L, more preferred in the range from 0.5 to 15 g/L, especially preferred of at least 1 and/or up to 12 g/L, most preferred of at least 2 g/L and/or up to 9 g/L of PO₄ ions. The phosphate content may provide the necessary acidity for the primary pickling effect. It also may help in some cases to remove the excess heavy metal content like an iron content out of the solution, that may predominantly or totally be a result of the pickling. The orthophosphoric acid may be added as acid, as monoacid and/or as poly acid salt of an alkali metal and/or of an ammonium group or in a small amount as an iron phosphate. Instead or partially instead of orthophosphoric acid, its ester(s) and/or its salt(s), a phosphonic acid and/or other phosphorus containing acid and/or at least one of their salts and/or esters may be added to the solution or dispersion, especially at least one water-soluble ester of phosphoric acid.

In the process according to the invention, the phosphating solution or dispersion may contain an amount of SO₄ ions in the range from 0.1 to 10 or 18 g/L, preferably of at least 0.5 and/or up to 15 g/L, more preferred in the range from 1 to 12 g/L, much more preferred of at least 2 g/L and/or up to 9 g/L of SO₄ ions. The sulfate content may provide the necessary acidity for the primary pickling effect. The sulfuric acid may be added as acid or as sulfate of an alkali metal and/or of an ammonium group or in a small amount as an iron sulfate. Especially a mixture of at least one phosphorus containing acid and/or its salt(s) and/or its ester(s) with at least one sulphur containing acid and/or its salt(s) and/or its ester(s) may be added to the solution or dispersion; preferably, the content of such phosphorus containing compounds should be at least 50 % by weight of all such acids, salts and esters.

In the process according to the invention, the phosphating solution or dispersion may contain an amount of NO₃ ions in the range from 0.1 to 18 or to 10 g/L, preferably of at least 0.5 and/or up to 15 g/L, more preferred of at least 1 and/or up to 12 g/L, much more preferred of at least 2 g/L and/or up to 9 g/L of NO₃ ions. The nitrate content may provide the necessary acidity for the primary pickling effect. The nitric acid may be added as acid, as nitrate of at least one alkali metal and/or ammonium or in a small amount as an iron nitrate. Especially a mixture of at least one phosphorus containing acid and/or its salt(s) and/or its ester(s) with at least one nitrogen containing acid and/or its salt(s) and/or its ester(s) may be added to the solution or dispersion; preferably, the content of such phosphorus containing compounds should be at least 50 % by weight of all such acids, salts and esters. In the process according to the invention, the phosphating solution or dispersion may contain an amount of groups, ions and compounds together of organic acid(s) and/or of its derivative(s) in the range from 0.1 to 10 or 18 g/L, preferably of at least 0.5 and/or up to 15 g/L, more preferred in the range from 1 to 12 g/L, much more preferred of at least 2 g/L and/or up to 9 g/L of such groups, ions and compounds.

Further on, the phosphating solution or dispersion may contain an amount of nitroguanidine and/or other accelerators on the base of guanidine like acetatoguanidine, aminoguanidine, carbonatoguanidine, melanilinoguanidine, nitratoguanidine and ureidoguanidine in the total range from 0.01 to 5 g/L, preferably in the range from 0.015 to 3 g/L, more preferred in the range from 0.01 to 1.2 g/L, much more preferred of at least 0.02 g/L and/or up to 0.6 g/L of the guanidine compound(s). Nitroguanidine had shown in several instances to give the best results of all accelerators tested. In comparison to the use of aminoguanidine, the addition of nitroguanidine was a small amount more favourable, especially for the corrosion inhibition.

In the process according to the invention, the phosphating solution or dispersion may contain at least one surfactant, especially when cleaning and phosphating is carried out with the same solution or dispersion, then preferably with an amount of all surfactants together in the range from 0.01 to 10 g/L. If using at least one surfactant in the phosphating solution, it is preferred to take care not to generate foam. In some cases, it may be favourable to add a defoamer. This total surfactant content may preferably vary in the range from 0.1 to 7 g/L, more preferred in the range from 0.3 to 5 g/L, much more preferred of at least 0.5 g/L and/or up to 3 g/L of surfactant(s). Especially in one-bath-processes, the cleaning and phosphating may be carried out in the same bath container with the same solution or dispersion, so that in the first time of contacting the metallic components with the phosphating solution or dispersion, the cleaning and pickling effect of the solution or dispersion may prevail, whereas in the further time of the contacting, the coating process with the phosphating coating formation may predominate. In general, nearly all types of surfactants resp. surfactant mixtures are suitable to be added to the phosphating solution or dispersion, especially surfactants resp. surfactant mixtures with low- foaming or non-foaming properties and with a cloud-point in the range from 25 to 40 °C, whereby the surfactant mixtures may be free of further constituents than surfactants.

The phosphating solution is preferably free or nearly free of other heavy metals than those being pickled out of the metallic surface, perhaps with the exception of titanium and/or zirconium, especially in the presence of complex fluoride(s). It is preferably free of chromates, molybdates and tungstates.

In the process according to the invention, the phosphating solution or dispersion may contain at least one solvent like a propylene glycol and/or a glycol ether; further on it may contain at least one biocide, at least one stabilizing agent for a surfactant like a condensed sulfonic salt, at least one stabilizing agent for the accelerator like a fine-particular silicate-, clay- or clay-like material and/or at least one stabilizing agent for the solution or dispersion itself like a biopolymer. A solvent may be preferable for enhancing the cleaning effect of the metallic surface, especially in combination with at least one surfactant. It is favourable to use a guanidine compound in the form of a suspension containing a stabilizing agent, especially the nitroguanidine.

In the process according to the invention, a phosphating coating is generated showing mostly a colourless, faintly coloured, silvery, golden, yellowish, yellowish- brownish, yellowish-reddish and/or bluish colour. If the coating according to the invention is bluish, there seems to be often a phosphorus content of the coating being not as low as typical for such coatings and there are to be found often corrosion inhibition results less than of excellence. This coating may in several cases be less intensively coloured or may show a less brighter and/or even a matter appearance than conventional coatings. This coating may typically have a coating thickness in the range of up to 1 μm, mostly only up to 0.6 μm, often only up to 0.3 μm.

In the process according to the invention, a clean, a cleaned and/or a pickled metallic surface is contacted with the solution resp. dispersion. The metallic surface may be contacted with the solution resp. dispersion by immersing, spraying, steam- phosphating, roll-coating and/or squeegeeing. All application varieties except of steam-phosphating are often used for coil coating. The coated metallic surface is dried after contacting it with the solution resp. dispersion or later on after at least one thereon succeeding rinsing step, preferably by air-drying, oven-drying and/or infrared- drying, especially at temperatures in the range from 20 to 250 °C.

There may be applied at least two coatings one after the other on the metallic surface whereby at least one of them is applied with an alkali metal phosphating solution resp. dispersion and whereby at least one other coating may optionally be applied with a conversion coating solution like a zinc- and/or manganese-rich phosphating.

In the process according to the invention, first an alkali metal phosphating coating is generated on a metallic surface and then a coating selected from the group consisting of a conversion coating like a zinc- and/or manganese-rich phosphating coating, a stearate coating and an organic polymer coating is applied thereon, especially for coldforming.

In the process according to the invention, a metallic surface consisting essentially of metallic materials of aluminum, chromium, titanium and/or zinc as well as at least one alloy containing aluminum, chromium, copper like brass or bronze, iron, magnesium, tin, titanium and/or zinc alloys is covered with a coating of a phosphating solution or dispersion.

The coating prepared with a process according to the invention may be used for the short-term passivation, for the pretreatment prior to at least one succeeding paint layer, layer of any other organic coating and/or adhesive coating, as a lubricant carrier or as one of the lubricating coatings prior to coldforming. The lubricant resp. lubricant carrier may be favourably be used for cans, for machining, for wire drawing and/or for lubricating the moving chains.

The process may be successfully varied by coating the thin phosphating coating according to the invention with a final seal solution resp. dispersion. The results of the salt spray testings show that a second, third and/or fourth coating on the metallic surface generated by contacting the panels phosphated in such way with the final seal solution resp. dispersion enhance the corrosion resistance significantly, although such final seal coatings are very thin. Preferably, such final seal coatings may be generated with a final seal solution/ dispersion containing at least one rare earth element compound like a cerium compound, at least one resin component like acrylic acid and/or at least one silane.

The coating prepared with a process according to the invention may be used for the corrosion inhibition and/or the lubrication of metallic surfaces, especially for use in aerospace industry, automobile industry, rail transportation, shipbuilding, metal forming, metal working like machining and/or grinding, in metallic container and especially can production, coil industry, for metal sheet applications, wire production, appliances, housings, machines and construction of buildings.

EXAMPLES

The following examples illustrate, in detail, embodiments of the invention. The following examples and comparison examples shall help to clarify the invention, but they are not intended to restrict its scope:

Group 1 : Comparison Examples 1 to 6:

First tests were made in which an aqueous acidic solution as standard chlorate and sodium metanitrobenzene sulfonate (SNBS) accelerated alkali metal phosphating concentrate A containing

1.3 % by weight of phosphoric acid,

11.7 % by weight of monosodium phosphate,

1.0 % by weight of SNBS,

10.0 % by weight of sodium chlorate and the rest being deionized water

was compared to another aqueous acidic solution of an alkali metal phosphating concentrate B accelerated only on nitroguanidine containing 1.3 % by weight of phosphoric acid,

11.7 % by weight of monosodium phosphate and the rest being deionized water.

Starting from these concentrates, the baths with these solutions were prepared at 3 % by volume for both formulations, this means for the solution A 3.58 % by weight resp. for the solution B 3.30 % by weight of the concentrate. To the solution B, 0.2 g/L nitroguanidine stabilized with a small content of clay-like material was further added. The pH value of both phosphating baths was adjusted to 4.5 resp. to 2.8 with an addition of sodium hydroxide.

Panels of cold rolled steel (CRS) were cleaned with Okemclean^® at 3 % by volume and 54.4 °C for 30 seconds by spraying. The panels were then rinsed and afterwards treated in the phosphating bath A or B for 60 seconds by spraying at various temperatures. This was followed by rinsing and drying with compressed air. The panels were finally painted with a Dupont TGIC polyester powder paint and subjected to a salt spray (fog) test strictly according to ASTM B 117 for 336 hours for the evaluation of the corrosion inhibiting properties strictly according to a ASTM D 1654 rating, with 10 to be the best and 0 the worst.

Table 2: Composition of the coatings of the different groups

Table 3: Coating weight and salt spray test ratings on CRS for a high pH value dependent from the temperature of the coating solutions of differently accelerated alkali metal phosphating systems

The higher the values of the rating, especially after longer testing time, the better the corrosion inhibition results. As the temperature increased in the^' nitroguanidine-accelerated bath B, the coatings became more uniform and changed from grey-brown to blue. Lower temperature treatments and lower coating weights were correlated with a better salt spray performance. The nitroguanidine-accelerated system B showed a better and more homogeneous appearance of the coatings and a better corrosion inhibition than the chlorate-SNBS-accelerated system A. The coating for the panels was homogeneous and went from blue to golden as the temperature increased.

Group 2: Comparison Examples 11 to 42: The same base bath formulations were used for the following tests with the standard chlorate-SNBS-accelerated system A and with the nitroguanidine- accelerated system B as in Group 1. Panels of cold rolled steel (CRS), hot dipped galvanized steel (HDG), electrogalvanized steel (EG) and aluminum alloy AA 6061 were cleaned with Gardoclean^® S 5206, rinsed, treated in the phosphating baths A or B and then rinsed and dried with compressed air. Based on Group 1 , the temperature range covered was changed to lower temperatures used. The panels were finally painted with Morton Corvel Black powder paint and subjected to salt spray (fog) test for 250 hours according to ASTM B 117. The creepage from scribe was measured according to the rating from 0 to 10 according to ASTM D 1654; the higher the SS values, the better are the results.

Table 4: Salt spray test results for different metallic surfaces dependent from the temperature of the coating solutions of differently accelerated alkali metal phosphating systems for a pH value of 4.5

The test results of this test series show that the results are partially better at lower temperatures, but the results depend strongly from the metallic material of the surface contacted. Excellent results could be reached with all panels of the aluminum alloy. Nitroguanidine showed good corrosion inhibition results, whereby it was very astonishing that this could be gained with such a thin coating.

Again, all panels were homogeneous. For the invention, the CRS panels went from grey-brown to blue as temperature increased. The HDG and EG panels showed an etched appearance in all cases, but no colour. The aluminum panels were shiny with no apparently visible coating. For the chlorate samples, the CRS panels went from blue to golden with increasing temperature, the HDG and EG panels had an iridescent appearance and the aluminum panels had a transparent light tan colour.

Group 3: Comparison Examples 43 to 44:

The cold rolled steel panels were treated with aqueous acidic phosphating solutions C containing only a very tiny amount much less of 1 g/L of phosphoric acid only to adapt the pH value of the ready mixed solution to 2.5 resp. 4.5, 0.2 g/L nitroguanidine and 0.2 g/L aminoguanidine bicarbonate. If in all the examples aminoguanidine was added, it was added as bicarbonate, although not always indicated. The panels were cleaned with Gardoclean^® S 5206 and rinsed before the nitro- and the aminoguanidine were added. The panels were contacted with the phosphating solution for the test with a pH value of 2.5 at ambient temperature and for the test with a pH value of 4.5 at 49 °C. This was followed by rinsing and drying the panels with compressed air. The such coated panels had a golden appearance and showed a homogeneous coating. The panels were then painted with a Ferro TGIC polyester powder paint. Finally, the panels were checked for paint adhesion by cross hatch and direct impact. There was significant paint loss when these tests were performed and were not acceptable. The panels were then also subjected to salt spray (fog) testing for 250 hours according to ASTM B 117. The panels had a rating of 0 per ASTM D 1654 for all cases after 250 hours, which is further on a bad result. As the solutions did not contain any alkali metal ions, nor ammonium ions, they were not buffered and lacked a significant content of acid.

Group 4: Comparison Examples 51 to 56:

The comparison examples illustrate the effect of low and very, high pH values of the phosphating solution using 0.2 g/L of nitroguanidine and 0.2 g/L of aminoguanidine carbonate as accelerators and using the base bath solution B of Group 1 containing 3 % by volume of the concentrate containing 1.3 % by weight of phosphoric acid, 11.7 % by weight of monosodium phosphate and the rest being deionized water. The CRS panels were cleaned as in the previous examples. Starting with a very acidic bath, the addition of NaOH resulted in very high pH values. The panels were sprayed with this conversion coating solution for 60 seconds at 48.9 °C.

An unpainted panel from each test was put into a 100 % humidity test chamber for a water fog test according to ASTM D 1735 for 72 hours and thereafter the surface percentage of red rust was rated. The rest of the panels was painted with a Ferro TGIC polyester powder paint and was put into a salt spray test chamber according to ASTM B 117 for 250 hours and for a copper accelerated acetic acid salt spray (fog) test (CASS) test strictly according to General Motors Engineering Standards June 1997 for 72 hours. The salt spray test results and the CASS test results were measured in mm creep from the scribe. The panels from test 3 and 6 did not produce any visible coating and were therefore not further tested. The nitroguanidine bath was not stable above a pH value of about 7.

Table 5: Results of the humidity, salt spray and CASS tests dependent from the pH value of the coating solutions of differently accelerated alkali metal phosphating systems

The corrosion inhibition was only for sample CE 51 of a pH value of 2.8 good and otherwise of medium quality. No coating weights were measured for this group. The coatings in all cases were homogeneous. Both coatings generated with the nitro- and aminoguanidine showed a golden colour at a low pH value and were blue at a high pH.

Group 5: Examples and Comparison Examples 61 to 79:

Cold rolled steel panels were cleaned and rinsed as in the previous examples and comparison examples. The phosphating baths were prepared with varying amounts of the base bath formulations starting from Group 1 and varying the accelerator concentrations of A and B. The conversion coating baths were operated at 26.7 °C and mostly at a pH value 4.5 for 60 second spraying. The last comparison example 79 was the standard chlorate-SNBS-accelerated alkali metal phosphating as outlined in Group 1 , but only this was operated at a pH value of 4.5 and at a temperature of 48.9 °C for 80 seconds of spraying. The salt spray rating was evaluated according to ASTM D 1654 after 500 hours salt spray (fog) test according to ASTM B 117. Table 6: Results of the salt spray test and the coating weight dependent from the accelerator amount of the coating solutions of differently accelerated alkali metal phosphating systems and of the pH value at a temperature of 26.7 °C; * bath without accelerator content

The examples according to the invention, E 72, E 75 and E 78, show significantly better corrosion results than most of the other samples. The coatings were even, of a golden colour at a low pH value and of a blue colour at a high pH value for coatings generated with amino- resp. nitroguanidine. Group 6: Examples and Comparison Examples 81 to 97:

In this group, a so-called multimetal formulation was used. The bath solution contained fluoride to treat cold rolled steel, hot dipped galvanized, electrogalvanized and aluminum. The base solution B of Group 1 was used by with an additional content of free fluoride, whereby the content of all components of this bath were varied at a temperature of 38 °C.

A high number of solutions and tests was performed to generate data for intensive studies with design of experiment evaluation. For these experiments, the metallic surfaces, the content of fluoride (50 - 200 mg/L), the content of added Fe²⁺ (0 - 200 mg/L), the content of phosphate and sodium monophosphate together (1.4 - 7.2 g/L), the content of nitroguanidine as the single accelerator (0.02 - 0.6 g/L) as well as the pH value (2.8 - 4.5) were systematically varied within the mentioned limits, whereby only the examples according to the invention are listed in table 7. In comparison hereto, a chlorate-SNBS-accelerated solution of a pH value of 4.5 was tested at 49 °C with CE 88, CE 91 , CE 94 and CE 97, whereas the other comparison examples belong strictly to the data set as shown for the rest of the examples according to the invention. The number of examples and comparison examples tested was reduced for this overview, so that typical results are represented here. From these experiments, systematical calculations were performed and the regions of excellent resp. good resp. stable behaviour selected.

The panels were painted with a Ferro TGIC polyester powder paint of 38 to 51 μm thickness and were put into a salt spray (SS) test chamber according to ASTM B 117 for 250 hours, whereby the test results were measured in mm creep from the scribe. Further on, adhesion was tested according to ASTMD 3359, whereby 5B means that no flaking did occur in the cross-cut area which is the best possible test result, whereas e.g. 2B means that there is a certain amount of flaking in the crosscut area.

Table 7: Results of the salt spray (fog) test dependent from the chemical composition of the phosphating solutions and from the pH value at a temperature of 32 °C; * bath without accelerator content

Nearly all of the examples according to the invention showed very good corrosion inhibition results resp. for the corrosion-sensitive material HDG even excellent results compared with the results of the comparison examples. The higher the values of the adhesion tests, the better are the results. The coatings were even in all cases. CRS panels according to the invention were grey, HDG panels were very faint golden, EG panels were grey and aluminum panels were of no significant colour. For the control, the CRS panels were golden, EG and HDG panels were transparent and iridescent and aluminum panels were light blue.

Group 7: Examples and Comparison Examples 101 to 111 :

In this group, only cold rolled steel panels were used and different influences were checked, even the influence of the bath temperature. The solutions were free of ,2+ fluoride and added Fe . All the other contents and conditions of the coatings were the same as in Group 6. Further on, the samples CE 109 and CE 110 were coated with Bonderite^® 1000 (CE 109) for having an additional thin chromium final seal covering the phosphate coating resp. Cryscoat^® 547 for having an additional thin non-chromium final seal covering the phosphate coating (CE 110) and the last having no additional final seal (CE 111 ) - each being coated in a typical manner. These coatings may be used as typical industry standards to get a comparison of typical conventional iron phosphating coatings of today.

Table 8: Results of the salt spray (fog) test on three CRS panels each dependent from the chemical composition of the coating solutions, the pH value, the contacting time and the temperature; * bath without accelerator content

The examples according to the invention showed very good corrosion inhibition results compared with the results of the comparison examples. The comparison examples vary with respect to the corrosion inhibition quality depending if there is a further seal or not and especially if this final seal is a chromium containing layer. CE 109 showing such additional chromium containing layer covering the phosphate layer should show the best corrosion inhibition properties. Nevertheless, it is astonishing that the best panels according to the invention were able to reach the excellent corrosion inhibition properties of CE 109 which is the best industry standard material on the base of iron phosphate known in the art which in this case is even covered by a strongly further corrosion inhibiting final rinse layer.

The coatings were even in all cases. The colour changed from golden to blue when either the pH or temperature was increased. The contact time, bath concentration and accelerator concentration did not have any apparent effect on the appearance.

The results of the design of experiments showed clearly a broad region of unusually stable working conditions for an alkali metal phosphating solution below a pH value of 3.5 and astonishingly very constant coating properties. The phosphating results on aluminum alloy 6061 were best at a F^" content of less than 200 ppm and at a Fe²⁺ content less than 120 ppm. On hot dip galvanized steel (HDG), they were best at a F^" content of less than 360 ppm and at a Fe²⁺ content of more than 80 ppm, although the results were - as usual with HDG in such comparisons - worse than for the other metallic materials tested. On electrogalvanized steel (EG), they were best at a very low PO₄ content and at a F^" content of less than 200 ppm. On coldrolled steel (CRS), they were best at a F^" content of less than 250 ppm. During a long throughput study, it was confirmed that these working conditions as well as the coating properties could be maintained nearly without any variation for all the time without changing the bath, but with continuous replenishment.

The appearance of the coatings was at least as good as for comparable good alkali metal phosphating coatings used in the market. As best accelerator during all these studies, nitroguanidine was identified. Further on, the alkali metal phosphating process with the slightly modified working conditions for the solutions according to the invention are well suited for industrial application of coils, parts and wires. The use of the phosphating solution at a significantly lower temperature than today usual for the contacting of metallic surfaces helps to reduce heating costs considerably. The herein proposed phosphating process is easier than the processes used today as it is quite sufficient to control only total and free acid content, but no other parameters of the bath within short time limits, as the bath behaviour is very stable. Finally, this process is not only superior because less heating is needed and therefor is cheaper in comparison to actually used processes as there is a significant lower consumption of all chemical compounds of the solution than usual.

Group 8: Examples 112 to 119:

For this investigation, cold rolled steel panels supplied by Q Panel were used as the substrates. Table 9 below lists the bath composition for each variation. The examples 117 to 119 were rinsed with a final seal instead of the rinsing with Dl water. CrysCoat UltraSeal is a product of Chemetall Oakite based on water, silane and alcohol. The pretreated panels were painted using TGIC polyester powder paint supplied by Rohm & Haas. Single scribe panels were placed in salt spray testing per ASTM B 117 for 240 hours. The panels were scraped with a metal spatula and the amount of paint loss from the scribe was measured in mm. All variations were subjected to cross hatch adhesion per ASTM D 3359 and direct and reverse impact per ASTM D 2794. In all cases, the rating for the cross hatch adhesion testing was 5B (no loss of adhesion) and for the direct and reverse impact, no cracking or other paint loss was seen up to 160 pounds/inch² (1.84 kilogramm/meter²). The panels were pretreated as written below:

1. Gardoclean S 5206, 3 v/v% b.v., 120 - 125 (49 - 52 °C), 60 second spray

2. Tap water rinse, ambient temperature, 30 second spray

3. Conversion coating, 86 - 92 (30 - 33 °C)

4. Tap water rinse, ambient temperature, 30 second spray

5. Dl water rinse, 10 seconds or final seal, ambient temperature, 30 second immersion

6. Oven dry, 225 °F (107 °C), 5 - 10 minutes

Table 9: Chemical compositions of the examples 112 to 119 in g/L; Cerium nitrate addition calculated as Ce; CCU = CrysCoat UltraSeal; * bath of the final seal

Table 10: Results of the salt spray (fog) test as well as visual appearance of the panels and coating weight of the coatings

The panels showed excellent thin coatings of more or less tan colour and good or even excellent corrosion inhibition. In comparison hereto, the best standard for an iron phosphate coating coated with a chrome final seal reached a salt spray test rating for 240 h of 0.2 mm creep. Within the scope of avoiding poisonous chromium compounds, the results are excellent.

Claims

CLAIMS:

1. Process for coating metallic surfaces with a phosphating coating by contacting metallic surfaces at a temperature not above 45 °C and at a pH value less than 3.5 with an aqueous acidic alkali metal phosphating solution or dispersion containing:

At least one compound of at least one phosphorus containing acid and/or at least one of their derivatives like esters and salts in a total content of all kinds of acids and all their derivatives like esters and salts together of less than 20 g/L calculated on mole base as orthophosphate, whereby the content of such phosphorus containing compounds/ions is at least 50 % by weight in comparison to all such compounds/ions and at least one ion selected from the group consisting of at least one alkali metal ion and ammonium ion, whereby the phosphating coating has a coating composition with a phosphorus content of not more than 8 atomic% as measured by Secondary Neutral Mass Spectroscopy (SNMS) and whereby the phosphating coating has a coating weight in the range from 0.01 to 0.5 g/m²

2. Process according to claim 1 , whereby the phosphating solution or dispersion contains at least one accelerator like such on the base of chlorate, guanidine, of an organic compound with at least one nitro group like nitroguanidine and/or nitrobenzenesulfonic acid and its derivatives, of hydrogen peroxide, hydroxylamine, nitrate, and/or of other nitrogen containing accelerators.

3. Process according to claim 1 or 2, whereby the phosphating solution or dispersion contains an amount of PO₄ ions in the range from 0.1 to 10 g/L.

4. Process according to any of the preceding claims, whereby the phosphating solution or dispersion contains an amount of SO₄ ions in the range from 0.1 to 10 g/L.

5. Process according to any of the preceding claims, whereby the phosphating solution or dispersion contains an amount of NO₃ ions in the range from 0.1 to 10 g/L.

6. Process according to any of the preceding claims, whereby an amount of Fe²⁺ ions is added to the phosphating solution or dispersion, preferably in the range from 0.01 to 1 g/L.

7. Process according to any of the preceding claims, whereby the phosphating solution or dispersion contains free fluoride, preferably in the range from 0.01 to 1 g/L, and/or complex fluoride, especially of aluminum, boron, silicon, titanium and/or zirconium, preferably in the range from 0.01 to 1 g/L;

8. Process according to any of the preceding claims, whereby to the phosphating solution or dispersion contains an amount of nitroguanidine and/or other accelerators on the base of guanidine in the total range from 0.01 to 5 g/L.

9. Process according to any of the preceding claims, whereby the phosphating solution or dispersion contains at least one surfactant, especially when cleaning and phosphating is carried out with the same solution or dispersion, preferably with an amount of all surfactants together in the range from 0.01 to 10 g/L.

10. Process according to any of the preceding claims, whereby the phosphating solution or dispersion contains at least one solvent like a propylene glycol and/or a glycol ether, at least one biocide, at least one stabilizing agent for a surfactant like a condensed sulfonic salt, at least one stabilizing agent for the accelerator like a fine-particular silicate-, clay- or clay-like material and/or at least one stabilizing agent for the solution or dispersion itself like a biopolymer.

11. Process according to any of the preceding claims, whereby a phosphating coating is generated showing a colourless, faint colouring, silvery, yellowish, golden, yellowish-brownish, yellowish-reddish and/or bluish colour.

12. Process according to any of the preceding claims, whereby a clean, a cleaned and/or a pickled metallic surface is contacted with the solution resp. dispersion.

13. Process according to any of the preceding claims, whereby the metallic surface is contacted with the solution resp. dispersion by immersing, spraying, steam-phosphating, roll-coating and/or squeegeeing.

14. Process according to any of the preceding claims, whereby the coated metallic surface is dried after contacting with the solution resp. dispersion or after at least one thereon succeeding rinsing step by air-drying, oven- drying and/or infrared-drying, especially at temperatures in the range from 20 to 250 °C.

15. Process according to any of the preceding claims, whereby there are applied at least two coatings one after the other on the metallic surface whereby at least one of them is applied with a phosphating solution resp. dispersion and whereby at least one other coating may optionally be applied with a conversion coating solution like a zinc- and/or manganese- rich phosphating.

16. Process according to any of the preceding claims, whereby first a alkali metal phosphating coating is generated on a metallic surface and then a coating selected from the group consisting of a conversion coating like a zinc-and/or manganese-rich phosphate coating, a stearate coating and an organic polymer coating is applied thereon, especially for coldforming.

17. Process according to any of the preceding claims, whereby a metallic surface consisting essentially of metallic materials of aluminum, chromium, titanium and/or zinc as well as at least one alloy containing aluminum, chromium, copper, iron, magnesium, tin, titanium and/or zinc alloys is covered with a coating of a phosphating solution or dispersion.

18. Process according to any of the preceding claims, whereby the metallic surface - after coating with the solution of claim 1 - is contacted with a final seal solution or dispersion, especially with a final seal solution/ dispersion containing at least one rare earth element compound, at least one resin component and/or at least one silane.

19. Method of use of the coating prepared with a process according to one of the claims 1 to 18 for the short-term passivation, for the pretreatment prior to at least one succeeding paint layer, layer of any other organic coating and/or adhesive coating, as a lubricant carrier or as one of the lubricating coatings e.g. prior to coldforming.

20. Phosphating coating on a metallic surface prepared by contacting metallic surfaces with an aqueous acidic alkali metal phosphating solution or dispersion having a coating thickness of not more than 0.15 μm and having a good corrosion protection for the protected metallic material.

21. Method of use of the coating prepared with a process according to one of the claims 1 to 18 for the corrosion inhibition and/or the lubrication of metallic surfaces, especially for use in aerospace industry, automobile industry, rail transportation, shipbuilding, metal forming, metal working (machining, grinding), metallic container and especially can production, coil industry, for metal sheet applications, wire production, appliances, housings, machines and construction of buildings.