ZINC PHOSPHATE CONVERSION COATING COMPOSITIONS AND PROCESS

Abstract: A process for forming a zinc phosphate coating on an aluminum substrate is provided to obtain good coverage by the coating. The coating preferably has a columnar or nodular crystal morphology and a coating weight of at least about 1612 mg/m<2>. The aluminum substrate is contacted with a zinc phosphate conversion coating bath which contains: from about 0.4 to 2.5 g/l zinc ion; from about 5 to 26 g/l phosphate ion; from about 0.4 to 1.5 g/l fluoride ion; from about 4 to 400 mg/l ferrous ion; and from about 0.01 to 2 g/l ammonium ion. The zinc phosphate conversion coating is formable on an aluminum substrate in the presence or absence of an accelerator. Also provided is an aqueous zinc phosphate conversion coating concentrate which may be diluted with an aqueous medium in a weight ratio of about 1:10 to 1:100 to yield a zinc phosphate conversion coating bath.

Description: ZINC PHOSPHATE CONVERSION COATING COMPOSITIONS AND PROCESS The present invention relates to zinc phosphate conversion coatings and to a process for forming a zinc phosphate coating on a metal substrate, particulaily an aluminum substrate It has long been known that the formation of a zinc phosphate coating, also known as a zinc phosphate conversion coating, on a metal substrate is beneficial m providing corrosion resistance and also in enhancing the adhesion of paint to the coated metal substrate Zinc phosphate coa ings are useful on a variety of metal subst] ateε including aluminum, steel and substrates which comprise more than one metal, such as automobile bodies or parts, which typically include steel, aluminum, zinc and their alloys The zinc phospnate coatings may be applied to the metal substrate by dipping the metal substrate m the zinc phosphate coating composition, spraying the composition onto the metal substrate, oi using various combinations of dipping and spraying It is important that the coating be applied completely and evenly ovei the surface of the substrate and that the coating application not be time or labo intensive In addition, piopei coating weights and crystal morphology are desirable in order to maximize coiiosion protection The crystal morphology of the zinc phosphate coating is most preferably columnar or nodula , allowing foi a heavie , denser coating on the metal surface to maximize corrosion protection and adhesion of subsequently applied paint coatings such as primers and top coats Zmc phosphate coatings with a crystal morphology that has a platelet structure also can provide acceptable coatings when high coating coverage is achieved On aluminum substrates the various coating compositions often yield coatings with less than complete coverage British Patent No. 2 226 829-A discloses a zinc phosphate conversion coating process wherein ferric iron (or ferrous iron plus an oxidizing agent) is added to control the tree acid level in a zinc phosphate conversion coating composition to produce a zinc phosphate coating on the surface of zinc or aluminum alloy surfaces U S Patent No 4,865,653 discloses a zinc phosphate conversion coating process wherein hydroxylamine if used as an accelerator m a zinc phosphate conversion coating composition to produce a columnar o nodular crystal coating stiucture on the surface of cold-iolled steel Also disclosed is the formation of coatings w th platelet morphologies on aluminum An additional discussion is presented on the use of hydroxylamine and ferrous ion m amounts up to the saturation point of the ferrous ion in the bath to expand the effect lve range of zinc ion in the composition It would be desirable to provide a zinc phosphate coating composition and process for forming a zinc phosphate coating having an appropriate coating weight and more complete coating coverage on an aluminum substrate so as to provide improved corrosion resistance for the coated aluminum substrate Ideally, the zinc phosphate coating would form even in the absence of an accelerator and would have a columnar or nodulai crystal morphology to enhance adhesion of subsequently applied paint to the coated aluminum substrate A furthei object of the invention or at least one aspect of the invention would be a single concentrated treating solution with a reduced water content that can be diluted with water at the location of forming the zinc phosphate coating on substrates SUMMARY OF THE INVENTION In accordance with the present invention, a process for forming a zinc phosphate conversion coating, a concentrate of 5 the zinc phosphate coating composition, a pretreatment bath for the zinc phosphate conversion coating of aluminum, and the coated aluminum substrate are provided. The process yields coated aluminum substrates with a coating weight of at least about 150 milligrams per square foot 0 ( g/ft ) which is equivalent to 1612 milligrams per square meter by contacting the aluminum substrate with the zinc phosphate conversion coating bath which contains: (a) from about 0.4 to 2.5 gram/liter (g/1) zinc ion; (b) from about 5 to 26 g/1 phosphate ion; (c) from about 0.4 to 1.5 g/1 fluoride ? ion; (d) from about 4 to 400 milligram/liter (mg/1) ferrous ion; and (e) from about 0.01 to 2 g/1 ammonium ion. The zinc phosphate conversion coating can be formed on the aluminum substrate in the presence or absence of an accelerator. The aqueous zinc phosphate conversion coating concentrate 0 of the present invention contains- (a) from about 10 to 60 g/1 zinc ion; (b) from about 125 to 500 g/1 phosphate ion; (c) from about 2 to 40 g/1 fluoride ion, (d) from about 0.1 to 10 g/1 ferrous ion; and (e) from about 0 2 to 50 g/1 ammonium ion. The concentrate may be diluted with an aqueous medium m a S weight ratio of about 1:10 to 1:100 (concentrate to aqueous medium) to yield a zinc phosphate conversion coating solution, otherwise referred to as a pretreatment bath. The pretreatment bath may be contacted with the aluminum substrate by dipping or spraying usually at an elevated temperature for varying times 0 depending on the application technique and processing equipment The zinc phosphate conversion coating pretreatment bath of the present invention is an aqueous solution comprising: (a) from about 0.4 to 2.5 g/1 zmc ion; (b) from about 5 to 26 g/1 phosphate ion; (c) from about 0.4 to 1 5 g/1 of fluoride ion; (d) from about 4 to 400 mg/1 ferrous ion; and (e) from about 0.01 to 2 g/1 ammonium ion, wherein the source of fluoride ion is either ammonium bifluoride alone or with monofluoride and/or complex fluoride ions . DETAILED DESCRIPTION The zinc phosphate coating process of tnc present invention results in a complete or at least near complete coating of the aluminum substrate where the coating has crystals of zinc-iron phosphate. Such a coating is particularly useful on aluminum substrates in. conjunction with cationically electrodeposited films to prov.de corrosion protection and paint adhesion. It is believed, witnout limiting the invention, that the coating predominan ly has crystal types referred to as phosphophyllite [FeZn . 'PO,,) and hopeite [Zn,(P0 4 ) 2 ] - Thus, the present invention w_ll hereinafter be referred to as "zinc-iron phosphate coating process and composition". The coating may ce used . ith other subsequently applied films, such as epoxie , enamc.s and other paints. The solution of the present invention whicn directly contacts the aluminum substrate is referred ro herein as "bath", which is at least an aqueous dilution of a ooncεntratt, which can be one package or one self-contained solution except for the water of dilution. The term "bath" s not intended as a limitation of the manner of application of the zmc phosphate coating which generally can be applied to the aluminum substrate by various techniques. Nonexclusi v e examples of these application techniques are immersion or dipping, involves placing the substrate into the bath, spraying, intermittent spraying, flow coating, and combined methods sucn as spraying-dipping spraying, spraying-dipping, dipp g- s spraying, and the like Also, during the process of the present invention, after some period of initial operation of coating substrates, further high-quality phosphate coatings are achieved by an addition or a concentrate as a replemshei solution The concentrate 0 replenisher or modified veisionε of the concentrate ιeplenιs^= oveicomes the effects of continued opeiation of the process with the concomitant reduction one or more ion concentrations Such reductions are the result of ions 1) removed from the bath through formation of the coating on the S increasing quantity of metal processed through the bath, and/or 2) deposited oi precipitated into any sludge formation The replenisher allows the continued coating of the substiates while reducing any fluctuations in bath composition The zmc-iion phosphate coating bath of the present 0 invention is aqueous and must be acidic This bath, which contacts the aluminum substrates, generally has a pH of betwem about 2 5 to 5 5 and pieferably between about 3 3 to 4 0 T-- pH, if lower than this range, can be adjusted to thi lange a_ necessary with any suitable basic solution as known to thos^ S skilled in the art, a 5 percent sodium hydroxide solution is suitable The free acid content of the zmc phosphate coatir" bath is usually about 0 3 to 1 .. Ihe free acid and total acm can be measured by any method known to those skilled in t h art One example s measuiement by titration of a 10 0 milliliters (ml) sample with 0 1 Normal sodium hydroxide solution to a bromo phenol blue end point Jhe low levels of the free acid in the bath can be maintained without loss of stability due to the presence of ferrous ion. The zmc-iron phosphate coating bath of the present invention is a "lower zmc" coating bath as understood by those S skilled in the art The term "lower z c" refers to baths wherein the z c ion concentration includes the "low-zinc" levels and generally can be slightly higher than those of traditional low-zinc formulations that are from about 0 4 to around 2 g/1 of zmc ion In terms of the zmc ion 0 concentration, levels as high as about 0 25 we qht percent (^ c g/1) are permissible, but platelet morphology may iesu.t ever at zmc ion levels below 0 1 weight percent (1 0 g/1) Foi purposes of allowing a safety factor m controlling the process to obtain the desired morphology, a zmc ion level the S middle of the above stated range should preferably be used, about 0 7 to 2 0 g/1 The source of the zmc divalent cation may be one or more conventional zmc ion sources known m the art, such as zmc z c nitrate, zmc oxide, zmc carbonate, and even zmc 0 phosphate, to the extent of solubility, and tt- like Kith t.c use of the zmc phosphate, the quantitative range of the total acid is maintained by a reduced amount of phosohate ion from the other phosphate sources. The phosphate ion content is usually between about 5 to S 26 g/1, and preferably about 10 to 20 g/1 l i e source of phosphate ion may be any material or compound 'nown t r those skilled in the ait to ionize m aqueous acidic solutions to form anions such as (PO/,) from simple compounds as wtll as condensed phosphoric acids including salts thereof Ionization 0 and neutralization of the phosphate ion sources may be to an degree, consistent with the present invention Nonexclusive examples of such sources include- phosphoric acid, alkali metal phosphates such as monosodium phosphate, monopotassium phosphate, disodium phosphate, divalent metal phosphates and the like, as well as mixtures thereof With the use of the divalent metal phosphates, the total phosphate or total aciα as well as the divalent metal should mvolve control of the othei sources of the phosphate and divalent metal, respectively, to obtain the desired quantities of each m the bath The aqueous acidic zmc-iron phosphate coating bath generally has a weight ratio of zmc ion to phosphate ion measured or calculated as Zn P0 4 of 1 2 to 1 65, piefeiablv about 1 5 to 1 30 The z c-iron phosphate coating bath of the present invention also contains fluoride ions present at about 0 4 to 1 5 g/1, preferably about 0 5 to 1.0 g/1, measured as the fluoride anion, F " The source of fluoride ion may be anv fluoride-conta ing compound including monofluorides , bifluorides, fluonde complexes, and mixtures thereof know to generate fluoride ions Examples include ammonium and alkali metal fluorides, acid fluondes, fluoioboric fluoiosilicic fluorotitanic, and fluorozirconic acids and their ammonium and alkali metal salts, and other inorganic fluorides, nonexclusive examples of which are calcium fluoride, zmc fluoride, zmc aluminum fluoride, titanium fluoride, zirconium fluoride, nickel fluoride, ammonium fluoride, sodium fluoride, potassium fluoride, and hydrofluoric acid, as well as other similar materials known to those skilled m the art The preferred source of fluoride ions may be any water soluble bifluoride compound, preferably potassium bifluoride or more preferably ammonium bifluoride Mixtures of bifluorides may also be used The bifluoiidcs may also be combined with monofluoride and/or complex fluoride ions. When such a combination is used, the monofluorides and/or complex fluoride ions are present in an amount of about 0.4 to 1.0 g/1, measured as F " . Though not intending to be bound by any particular theory, it is believed that the bifluorides improve bath stability. In addition, the use of ammonium bifluoride yields smaller, more densely packed nodular or columnar coating crystals on the aluminum surface. The zinc-iron phosphate coating bath of the present invention also contains iron ions, present as ferrous ions. The ferrous ion content of the zinc-iron phosphate coating bath is typically between about 4 to 400 mg/1 or parts per million "ppm" and is preferably about 4 to 50 ppm. Less than 4 ppm ferrous ion will not allow a complete coating to form on the aluminum surface and greater than 400 ppm generally causes a precipitate to form in the bath. Though not intending to be bound by any theory, it is believed that the ferrous ions present the coating bath become bound as a part of the final coating on the aluminum substrate being coated. The source of the ferrous ion may be any water-soluble ferrous compound, such as ferrous sulfate (FeSθ -7H 2 θ) , which is preferred, ferrous chloride, ferrous nitrate, ferrous citrate, and mixtures thereof. The source of the ferrous ion may alternatively (or additionally) be iron or steel filings added to the bath or iron incidentally present in the bath etched from previously treated steel. Ordinarily, in the course of coating various types of substrates, including aluminum and iron-containing substrates like cold-rolled steel (CRS) , the amount of ferrous ion in the bath from the coating of the iron-containing substrates needs to be supplemented for ferrous ion. With the addition of source of ferrous ion, the production of zmc phosphate coating on aluminum can continue. The ferrous ions are preferably added to the coating bath at room temperature. If added to the bath at higher temperatures (i.e. , standard coating process temperatures of about 90 °F to 160 U F (32°C to 71°C) ) , the free acid level of the coating bath may need to be lowered by the addition of sodium carbonate, sodium hydroxide, or a buffer. The zinc-iron phosphate coating bath of the present invention also contains ammonium ions, which yield coatings wi h columnar or nodular crystal morphology. The ammonium ion content of the zinc- iron phosphate coating bath, is typically between about 0.01 to 2 g/1 and is preferably about 0.05 to 1 g/1. The source of the ammonium ion may be any water-soluble ammonium compound, such as ammonium bifluoride, which is preferred, ammonium phosphate, and the like. Also, ammonium compounds that are sources for anions to the bath may bo used to contribute to the total quantity of the ammonium ion and these include the following examples: ammonium fluorides; accelerators like: ammonium nitrite, ammonium chlorate, and ammonium nitrate, however, such accelerators may oxidize ferrous ions present m the composition, preventing the formation of desired crystal structures on a coated substrate. The use of the accelerators should be judicious to avoid the oxidation of the ferrous ion. Also, compounds with groups convertible to ammonium groups as are known to those skilled in the art are also suitable. For instance, compounds added to adjust the pH of the bath or concentrate like ammonium hydroxide and/or ammonium acid salts of mono , di- , and triethanolamine can be used. Also compounds, such as an ammonium salt of zirconium hydroxy carbonate, zirconium acetate or zirconium oxalate, can be used as well as, the hydroxylamine source such as a hydroxylamine salt or complex, which frequently exist in a hydrated form and hydroxylamine sulfate ("HAS") , a stable salt of hydroxylamine. HAS is also referred to as hydroxyl-ammonium sulfate. HAS may be represented by the formulae (NH^OH) x H 2 SO„ or (NH 3 OH) . x SO,-,. In addition to the zinc ion, phosphate ion, fluoride ion, ferrous ion, and ammonium ion, the aqueous acidic zinc-iron phosphate bath may contain nitrate ion and various metal ions, such as nickel ion, cobalt ion, calcium ion, manganese ion, tungsten ion, and the like. The nitrate ion tray be present in an amount of about 1 to 10 g/1, preferably between about 2 to 5 g/1. When present, nickel or cobalt ion is generally each in a separate amount of about 0.2 to 1.2 g/1, preferably between about 0.3 to 0.8 g/1. Calcium ion may be present but should not exceed 2.5 g/1, or 2500 ppm, to limit the risk of precipitation. In general, this is the case for any hard water salts that may be present. Manganese ion may be present m an amount of about 0.2 to 1.5 g/1 , preferably between abour 0.7 to 1.2 g/1. Also, tungsten may be present m an amount of about 0.01 to 0.5 g/1, preferably between about 0. :2 to 0.2 g/1. Conventional nitrite and/or chlorate arcelerators, such as sodium nitrite;, ammonium nitrite, sodium chlorate, and ammonium chlorate, may be added to the zinc-.ron phosphate coating bath. However, as mentioned above, their presence is not required but is optional and when used their levels should not exceed 0.5 g/1 nitrite ion or chlorate ion order to avoid oxidation of the ferrous ion to ferric ion. Typically, the level of nitrite ion is between about 0.04 a d 0.2 g/1. Other types of accelerators known to those skilled in the art may also be used in the zinc-iron phosphate coating bath. Typical accelerators include: sodium nitrobenzene sulfonates, particularly m-sodium nitrobenzene sulfonate, ammonium nitrobenzene sulfonates, sodium chlorate, potassium chlorate, ammonium chlorate, and hydroxylammes, oximeε like acetaldehyde oxime, and hydrogen peroxide. These additional accelerators, when used, are present amounts of from about 0.05 to 20 g/1 The amounts of the various ions added to the coating bath may be determined theoretically before preparation thereof oi they may be subsequently measured analytically by techniques known to those skilled m the art and adjusted accordingly The specific amounts of each ion and ratios among ions, within the i anges which have been set forth above, will be determined fo each particular coating operation as i s well known m the art The zmc-iron phosphate coating bath of the present invention can be prepared fresh with the above-mentioned ingredients m the concentrations specified or can be prepared from aqueous concentrates m accordance with the present invention m which the concentration of the various ingredients is considerably higher. Concentrates are advantageous in that they may be prepared beforehand and shipped to the application site where they are diluted with an aqueous medium, such a∑ water, or a zmc phosphatmg composition which has been m us for some time. Concentrates are also a practical way of replacing the active ingredients as a replenishing solution / z c- ron phosphate coating "make up" concentrate of the present invention contains ingredien s in amounts (in qrams pei litei) as shown m the ranges of amounts of Table A. TABLE A long Amounts Preferred Amounts zinc 10 to 60 17 to 50 phosphate 125 to 500 150 to 300 fluoride 2 to 40 10 to 25 ferrous' 0.1 to 10 0.2 to 2 ammonium 0.2 to 50 1 to 25 Optional Ions manganese 5 to 40 14 to 30 nickel 5 to 30 7 to 20 nitrate 25 to 250 50 to 12b Other ions ' up to 250 25 to 100 The ferrous ion may be added direct ly to the concentrate before dilution thereof wrthout precrpitati on occurring. ' Cobalt, calcium and tungsten and other metal ions known to those skilled in the art as well as additional accelerators such as those described previously can also be present . Unless otherwise noted, the sources of the various ions present in the concentrate may be the same as those- used to prepare a bath as noted previously. To prepare a zinc-iron phosphate coating bath accordance with the present invention, the concentrate of the present invention may be diluted with aqueous medium in a weight ratio of about 1 : 10 to 1:100, preferably about 1:20 to 1:50, depending on the aqueous medium used for dilution and the amounts of various ingredients required in the final coating bath. Also, a concentrate may be added to a bath as a replenishing solution as mentioned above in a calculated amount as known co those skilled in the art of adding rep Leni sher solutions to phosphate conversion coating baths. The initial working bath solutions may be formulated with, the component amounts and weight rat ros as have been set forth above. Since the ion components of the acidic aqueous bath are present a narrow range for deposition by chemical reaction on the substrate, the baths after a period of operation may be replenished with one or more concentrates as a replenisher. S The replenisher is added to maintain the amounts and ratios of the necessary components of the bath at the desired operating levels. With use of the bath and make-up concentrate as the replenisher concentrate, the bath can become enriched with some ions which are less prevalent m the coating on the substrate 0 leaving the bath. Usually, the bath is enriched with nickel relative to zmc, and the iron and fluoride ion concentiation will decrease in the bath. With this in mind, it is preferred that when a concentrate is used as a replenisher, a replenisher formulation is modified from the formulation of the make-up s concentrate that was diluted to form the bath. Preferably, replenisher has ion concentrations that are higher than the make-up concentrate by the following factors for the specified ions: zmc around 2.1; manganese around 1.1; phosphate around 1.5; ferrous and fluoride each around 1.5 where all factors re 0 times (multiplied by) those ion concentrations in the make-up concentrate The amount of nickel preferably is 1.8 times less than that of the make-up concentrate. The addition of the replenisher can be done by any method known the art, for instance through quantitative testinq fo_ 5 the concentration of one or more ions that will dec-rease value with the operation of the bath An example is to test for the fluoride on, free acid and/or the total acid and when the values for one or more of these decrease below t he minimum values for the operation of the bath, the replenisher is added 0 With the addition, one or more tested quantities are re urned above the minimum oi the range for the operation of tho bath. The zinc-iron phosphate coating bath is typically used to coat aluminum substrates but can be used to coat other metal substrates including substrates containing more than one metal, such as automobile bodies which besides aluminum contain steel and zinc metal; i.e., galvanized steel. When ferrous metals are present in and/or on the substrates that are treated, an accelerator like the aforementioned should be present the zinc-iron phosphate conversion coating bath. The contact time of the bath with a metal substrate will be within the range of times customary for the particular contacting procedure used. Typically, foi spray contac", t ey. will be from about 0.5 to 3 minutes (30 to ]80 seconds) ; from about 1 to 5 minutes for immersion processes; and about 20 seconds spraying and 2 minutes immersion for a combined spray- immersion process. Typically, in these operations, the bath temperature will be within the range of about 90°F to 160 ϋ F (32°C to 71 "C> and preferably at temperatures ol between about ]2 "F to 13 C °F (49"C to 57°C) . The resulting coating on the substrate is more continuous and uniform with a crystalline structure which is preferably columnar or nodular, as can be observed with a scanning electron microscope by standardized procedures known in the art. The columnar crystalline structure resembles smal. column-shaped crystals, and the nodular morphology resetloles uniformly dispersed small nodular or round shaped crystals. The coating weight, generated can be between about 150 to 400 mg/ft' (1612 to 4300 milligrams per square meterX preferably about 200 to about 350 mg/ft '' (2150 to 3768 mg/m ' i and most preferably 250 to 350 mg/ft ' ' (2690 to 3768 mg/m ' ) These coating weights can be αetermined by gravimetric testing by standardized procedures known in the art . It will also be appreciated that certain other εteps may be done both prior to and after the application of the coating by the process of the present invention. For example, the substrate being coated is preferably first cleaned to remove grease, dirt, or other extraneous matter. This is usually done by employing conventional cleaning procedures and materials. These would include, for example, mild or strong alkali cleaners, acidic cleaners, and the like. Such cleaners are generally followed by a water rinse as is known by those skilled m the art. It is preferred to employ a conditioning step following, or as part of, the cleaning step, such as disclosed in U.S. Patent Nos. 2,874,081; 2,884,35] and 3,310,239. The condi¬ tioning step involves application of a condensed titanium phosphate solution to the metal substrate. The conditioning step provides nucleation sites on the surface cf the metal substrate resulting in the formation of a densely packed crystalline coating which enhances performance. After the zmc phosphate conversion coating is formed and water rinsed, it is advantageous to subject the coating to a post-treatment rinse to seal the coating and improve performance. The rinse composition may contain chromium (tπvalent and/or hexavalent) or may be chromium free as is well known to those skilled m the art as shown, for example, in U.S. Patent Nos. 3,450,579, 4,380,406 and 4,457,790, respectively. The invention will be further described by reference to the following examples Unless otherwise indicated, all amounts are parts by weight of the total aqueous acidic zmc phosphating formulation unless the amounts are specifically listed in other units. Examples A to E Examples A to E illustrate the effects of ferrous ion added to a zmc phosphating bath at increasing levels. The bath in Example A contains no ferrous ion; Example B contains 20 mg/1 FeS0 4 ■ 7H 0 (4 mg/1 Fe") ; Example C contains 25o mg/i FeS0„-7H 0 (50 mg/1 Fe") ; Example D contains 2000 mg/1 FeSO., • 7H..0 (400 mg/1 Fe * X , - and Example E contains 500r mg/i FeSO,-7H,0 (1000 mg/1 Fe") . Aqueous acidic zmc phosphate concentrates were prepared from the ingredients listed in Tabl I below, mixed at room temperature : 1onr A_-_E 11. Ions A- E Measured as ppm ferrous sulfate ( eSO, • 7H 0) in bath after dilution of concentrate. ' Morphology was determined by Scanning Electron Microscopy. Crystal type may vary depending on the zinc phosphate coating composition and the substrate. Nodular crystals are indicated as an "N" , platelet crystals as a "P" and columnar crystals as a "C" . This nomenclature also applies to examples; in subsequent tables . Powdery appearance; coating could easily be rubbed off substrate . ' The HN0 , Ni(NO j ) 2 , and KF ingredients were present in separate aqueous solutions in the concentrations indicated by percent by weight of the solution. In the last two columns on the right of Table 1 , there are shown the concentrations for the listed ions in grams per liter in the concentrate from the addition of the parts by weight of the listed ingredient . Each concentrate was diluted with water in a weight ratio of concentrate to water of about 1 to 20 to form the zinc phosphating bath, and, if added, the ferrous sulfate was added to the bath. Aluminum test panels were subjected to the following treatment process in Examples A to E. (a) degreasing: the test panels were first cleaned using an alkaline degreasing agent ( "CIIEMKLEEN 'IVI 163", available from PPG Industries, Inc. , at 1% by weight) which was sprayed onto the metal substrates at 131°F (55°C) for one minute ; (b) rinsing the test panels were then i sed with tap water at room temperature foi 15 to 30 seconds, (c) conditioning the rinsed test panels were then dipped into a surface conditioner ("PPG Pinse Conditioner", s available from PPG Industries Inc , at 0 1% by weight) at loom temperature for one minute, followed by (d) phosphating in which the test panels were d_ppeα into acidic aqueous compositions l/20th of the values given the table above at 120°F to 135°1 (49°C to 57°C) foi two 10 minut es , (e) i smg the coated test panels were then nnseα with tap water at loom tempe) atui e for 15 seconds The results are reported in Table i above under thr heading "Results" IS The data, reported m Table I under the "Results" heading, indicates that when no ferrous ion is present the bath, the coating that is formed is not viable, i e , it f easily rubbed off the substrate Addition of ferrous ion at levels of 20 5000 ppm FeSO,-7H 0 v'lelds a complete and v., able 20 coating Fxample F (Compai at lv ) Example F illustrates the effects of an accelerator add^α to a zmc phosphating bath An aqueous acidic zmc pho c phat e Tl bath was prepared as m Example C ot lable i above, with the addition of a sodium nitrite aceeleiator at 280 mg/1 nit i ltf concentration Aluminum test pant Is wn e subjected to tue s im t reatment process a-- in Examples A to r The results ar r reported m 'lable J I below 30 Exampl e : Morphology Crystal Size, (microns ) The data in Table II indicates that aluminum substrates can be coated with zinc phosphate coating compositions containing ferrous ion with or without a nitrite accelerator. Examples G and H Examples G and H compare the effects of ferrous iron and ferric iron added to a zinc phosphating bath. Aqueous acidic zinc phosphate concentrates were prepared and diluted from the following mixture of ingredients listed in Table III, mixed at room temperature: TABLE III Ingredients G H G Water ,1.94 61.94 (in concentrate) Ingredients G H Ions G H RESULTS : Coating weight, 3768 3229 (mg/m ' ) Crystal Size, 10-20 50-100 (microns) Again, in the last three columns on the right of Table III, there are shown the concentrations for the listed ions in grams per liter in the concentrate from the- addition o *" the parts by weight of the listed ingredient,s . Each of the concentrates listed in Table III was diluted with water in a weight ratio of 4.2 parts concentrate tc 95.8 parts water (1:22.8) . The bath containing ferric ions was cloudy and formed a precipitate of ferric phosphate. Aluminum test panels were subjected to the same treatment process as in Examples A to E . The results are reported in Table III above under the heading "Results" for the appropriate example. The data in Table UJ indicates that higher coating weight can be achieved with baths containing ferrous ions than with baths containing ferric ions. Moreover, the crystal size in the resultant coatings is much smaller for baths containing ferrous ions than for those containing ferric ions. Examples T to K Examples 1 to K compare the effects of various monofIuorides and bifluorides added to a zinc phosphating bath. The bath prepared in Example I contains potassium bifluoride; the bath pi epared in Example J contains ammonium bifluoride:,- and the bath prepared in Example K contains a mixture of potassium fluoride and potassium bifluoride. Aqueous acidic z c phosphate concentrates were prepared from the following mixture of ingredients of Table IV, mixed at room temperature: TABLE IV Ingredients : I J K Ions I sj K Water 61.15 61.94 58.96 (m concentrate) H 3 P0 4 (75%) 22.51 22.51 22.51 PO, 200 200 200 HN0 3 (66%) 1.88 1.88 1.88 NO, 15.2 15.2 15 2 ZnO 2.41 2.41 2 41 Zn 23 5 23.5 23 5 MnO 2.61 2 61 2.61 Mn 24 7 24 7 24 7 Nι(NO,), (43%) 5.93 5.93 5.93 Ni 10 10 10 NO, 21 21 21 KF (40%) 2.85 F 0 0 4 6 KFHF 3.06 2.4 F 18 0 14 FeS0 4 -7H ? 0 0.25 0.25 0.25 FΘ++ 1 0.6 0 6 0 6 NH 4 FHF 2.27 -- F 0 18 0 NH 4 0 9.4 0 Acetaldehyde 0.2 0.2 0.2 Oxime RESULTS : Morphology P N P Crystal Size, 20-50 <10 20-50 (microns) Fe++ indicates ferrous ion. As m Tables I and III in the last four columns on the right of Table IV, there are shown the concentrations for the listed ions in grams per liter in the concentrate from the addition of the parts by weight of the listed ingredients. Each of the aforelisted concentrates was diluted with water in a weight ratio of 4.2 parts concentrate to 95.8 pares water (1:22.8) to form the zinc phosphating bath. Aluminum test panels were subjected to the same treatment process as in Examples A to E. The results are reported in Table IV under the "Results" heading. The data from the Results in Table IV indicates that ammonium bifluoride rather than potassium bifluoride will yield a nodular morphology with smaller crystal sizes . Example L Example L illustrates the effect of introducing ferrous iron to the bath via previous treatment of cold-rolled steel. An aqueous acidic zinc phosphate concentrate was prepared and diluted from the following mixture of ingredients listed in Table V below, mixed at room temperature: Ingredi ents Ions Water HjPO.. (75%) HNO , (66%) ZnO MnO Ni (NO,) , (43%) 93 KF (40%) 2.85 KFHF 2.4 Acetaldehyde Oxime 0.2 /21850 23 Ingredi ents L Ions L Resul t s : Morphology P Crystal Size, 50-100 (microns) Again, in the last two columns on the right of Table V, there are shown the concentrations for the listed ions in grams per liter in the concentrate from the addition of the parts by weight of the listed ingredient. The afoiclisted concentrate was diluted with water m α weight ratio of concentrate to water of about 1 to 22.8 to form 300 ml (milliliters) of the zinc phosphating bath. Three 0.5" x 2" (1.27 cm x 5.08 cm) , where (") indicates "inches", aluminum panels were processed in the bath at 125°F (52°C) for two minutes with no coating being formed. Then two 0.5" x 4" (1.27 cm x 10.16 cm) cold-rolled steel panels were processed in the bath at 125°F (52 ϋ C) for two hours, followed by processing of two more 0.5" x 2" (1.27 cm x 5.08 cm) aluminum panels the bath at 125°F (52°C) for two minutes. The average properties of coatings formed on the last two aluminum panels are reported in Table V above under the "Results" heading. The data in Table V indicates that a ferrous ion can be incorporated into a zinc phosphating bath via processing or cold-rolled steel to yield coatings on subsequently processed aluminum substrates .

Claims: I CLAIM : 1. A process for forming a zinc phosphate coating having a coating weight of at least about 150 milligrams/foot '' (1612 milligrams/meter ) on an aluminum substrate, comprising contacting said aluminum substrate with a zinc phosphate conversion coating bath which contains: (a) from about 0.4 to 2.5 g/1 zinc ion; (b) from about 5 to 26 g/1 phosphate ion; (c) from about 0.4 to 1.5 g/1 of fluoride ion; (d) from about 4 to 400 mg/1 ferrous ion; and (e) from about 0.0! to 2 g/1 ammonium ion, wherein the source of fluoride ion s ammonium bifluoride and wherein said zinc phosphate coating is formable on the aluminum substrate optionally in the presence of an accelerator . 2. The process of claim J in which the .source of fluoride ion further includes materials selected from the group consisting of monofIuorides , complex fluoride ions, and mixtures thereof . 3. The process of claim 1 in which the zinc phosphate coating has a columnar or nodular crystal morphology. 4. The process of claim 1 in which tiie aluminum substrate is contacted with the zinc phosphate conversion coating bath at a bath temperature of about 90°F to 160°F (32 C 'C to 7i°C) . 5. The process of claim 1 in which the aluminum substrate is contacted with the zinc phosphate conversion coating bath by spraying or immersion. 6. The process of claim 1 in which the source of ferrous ion is selected from the group consisting of ferrous sulfate, ferrous chloride, ferrous nitrate, ferrous citrate, s iron, steel , and mixtures thereof. 7. The process of claim 1 in which the bath contains from about 0.7 to 2.0 g/1 zinc ions ; from about 10 to 20 g/1 phosphate ions; from about 0.5 to 1.0 g/1 fluoride ion,- from 0 about 4 to 30 mg/1 ferrous ion; and from about 0.05 to 1 g/1 ammonium ions . 8. The process of claim 1 m which t he bath further contains an accelerator selected from the group consisting of 5 nitrite ion in an amount of about 0.04 to 0.2 g/1 and oxime present m an amount from about 0.05 to 20 g/1. 9 Tho process of claim 1 in which the bath further contains: at least one ion selected from the group consisting 0 of: nickel ion, cobalt ion, calcium ion, manganese ion, tungsten ion, nitrate ion, and mixtures thereof; wherein when present the ions are in amounts m the range of: about 0.2 to 1.2 g/1 of nickel ion, about 0.2 to 1.2 g/L of cobalt ion 5 up to about 2.5 g/1 of calcium ion; about 0.2 to 1.5 g/1 of manganese ion; about 0.01 to 0.5 g/1 of tungsten ion; and about 0.25 to 10 g/1 of nitrate ion. 0 10. The process of claim 1 in which t lie coating weight it front about 2SO to 400 mg/ft (2690 4307 mg/m") 11 The process of claim 1 wherein the bath has a weight ratio of zmc ion to phosphate ion measured or calculated as Zn P0 4 of 1 2 to 1 6! 12 An aluminum substrate coated in accordance uith the piocess of claim 1 13 An aqueous z c phosphate conv lsiυn coating concentiate which comprises (a) fro about IC to fO α/1 zmr ion, (b) from about 160 to 400 g/1 phosphate on (c) iron about 2 to 40 g/1 fluoride ion (d) from about 0 J to 10 g/1 ferrous ion, and (e) fio about 0 2 to 50 g/1 ammonium ion 14 The concentrate of claim 13 in which the source of ferrous ion is selected from the group consisting of fc ιour sulfate ferrous chloride ferious nitrate ferrou c eiti at ^ iron, steel and mixtures thereof 15 Ihe concentiate of claim 13 whr ch al c o contain it least one ion selected from the group con c i'tιng of nickel ion cobalt ion calcium ion manganese ion t ungster ion nitrate ion and thereof wherein when present the ion aie m amounts in the range of to 30 g/1 of nickel on 5 to 40 g/1 of manganese ion, 25 to 250 g/1 of nitrate ion and up to 250 g/1 of the other ions 16 The concentrate of claim l r in which the imount of the ions m g/1 is the following zmc ion 17 to 50 phosphate ion 150 to 300 fluoride ion 10 to 25 ferrous ion 0.2 to 2 ammonium ion 1 to 25 nickel ion 7 to 20 manganese ion 14 to 30 nitrate ion 25 to 100 17 The concentrate cf claim 15 wherein the concentrate is a replenisher having ion concentrations that are higher than s the make-up concentrate υy around 2.1 times for zmc, around 1.1 times for manganese, around 1.5 times for phosphate; and around 1.5 times for each of ferrous ion and fluoride ion and having an ion concentration for nickel that is around 1.8 times lower than the nickel ion concentration m the make-up 0 concentrate. 18. The concentrate of claim 13 which the source of fluoride ion is selected from the group consisting of potassium bifluoride, ammonium bifluoride, and mixtures thereof. 5 19. The concentrate of claim 18 in which the source of fluoride ion is ammonium bifluoride. 20 The concentrate of claim 18 m which the-' source cf 0 fluoride ion further includes materials selected from tho group consisting of monofIuorides , complex fluoride ion, and mixtures thereof . - 2 ϊ 21. The concentrate of claim 13 that further contains water in an amount of about 1:10 to 1:100 weight ratio with the z c phosphate conversion coating concentrate to form an aqueous acidic zinc phosphate conversion coating bath 22. The concentrate of claim 20 wherein the weight ratio is about 1:20 to 1:50.

Inventors: ROSSIO CHARLES E, ROSSIO, CHARLES, E.
Applicants: PPG INDUSTRIES INC, PPG INDUSTRIES, INC.
Application year: 1996
Main classification: C23C22
IPC codes: C23C22