Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C22/05—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions
  • C23C22/06—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6
  • C23C22/34—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6 containing fluorides or complex fluorides
  • C23C22/37—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6 containing fluorides or complex fluorides containing also hexavalent chromium compounds
  • C23C22/38—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6 containing fluorides or complex fluorides containing also hexavalent chromium compounds containing also phosphates

Definitions

• This invention relates to the art of coating aluminum ings has been the subject of extensive research in the past. It was early discovered that the hexavalent chromium ion was the key to corrosion protective coatings and that simple aqueous hexavalent chromium solutions required modification to include activator ions to enable attack on an aluminum surface and the consequent formation of adherent coatings in a reasonable length of time. U.S. Patent 2,114,151 taught that aqueous hexavalent chromium solutions could be activated satisfactorily with one or more of the halo-gen and the ferricyanide ions.
• aqueous hexavalent chromium containing solutions could be advantageously combined with phosphates to form a combination chromium-phosphate coating having certain improved characteristics, but that such solutions required the presence of the fluoride ion and careful control and maintenance of the proportions of the hexavalent chromium ion, the fluoride ion, the P ion as well as the acidity of the solutions.
• Patent 2,438,877 teaches that adherent green coatings are obtained on aluminum by using an aqueous solution consisting of 0.9-12.5 grams per liter flluorine, 3.75-60 grams/liter CrO 2-285 grams/liter P0 when the ratio of FzCrO is in the range of 0.135-0.405' and the pH of the solution is in the range of 1.6'to 2.2 as measured by the lowest value indicated by a glass electrode pH meter within the first ten minutes of immersion of the electrode in the solution.
• Somewhat later U.S. Patent 2,494,910 taught that satisfactory coatings are formed on aluminum or aluminum alloy surfaces with much less acid solutions than those employed in U.S. Patent 2,438,877, namely in the range of pH of 3.2 to 7.0.
• aqueous solutions consisted of 1.5 to 300 grams/liter P0 ion, 3.5 to 150 grams/liter ClgOq ion, 0.75 to 95 grams/liter F ion and operated satisfactorily at room temperature so long as the ratio of Cr O to F remained in the range of 1:0.375 and 1:5.45 and pH remained in the range of 3.2 to 7.0 as measured colorimetrically.
• P0 ion, the dichromate ion and the fluoride ion may be introduced as the acid or as the alkali metal salt and where the alkali metal salt is used any needed adjustment of acidity to get into the specified range is obtained by the use of a conventional acid or alkali.
• the present invention is based on the discovery that aqueous hexavalent chromium solutions containing phosphate and fluoride ions operate more efficiently and subestantially are non-sludge forming at higher acidities than a pH of 1.6, for example at a pH between about 0.8 and 1.5. Additionally, it has been found that as aluminum is coated in an aqueous solution having a pH in the range of 0.3-1.5 and containing quantities of phosphate, fluoride and dichromate ions satisfactory to produce a coating on aluminum when the solution is a new solution, continued use causes aluminum and trivalent chromium ions to build up in the solution. Moreover, at the fluoride and CrO concentrations specified in- U.S.
• Another object of this invention is to provide improved operating, make-up and replenishing solutions suitable for use in practicing the method of this invention.
• a further object of this invention isto provide a solution which is capable of tolerating greater quantities of aluminum and trivalent chromium ions than heretofore known solutions, and a method forcontrolling and replenishing such solutions so that continuous operation is made feasible without sludging and without the necessity of selectively removing aluminum and/or trivalent chromium ionsfrom the solution.
• Another important object is to provide a process for forming corrosion resisting and paint base coatings on aluminum that is suitable for continuous spray application without encountering deposition of sludge on heating pipes, etc.
• Other objects include the provision of a process which concurrently improves the rate of formation of adherent coatings on aluminum and aluminum alloy surfaces and extends the range of variation of concentration of the coating forming ingredients which can occur without derirnentally affecting the adherence or corrosion resistance quality of the resulting coatings; the provision of a process which effects improved consistency of weight, adhesion and color in the coatings which are produced; and the provision of a process which continuously produces a given weight of integral, adherent coating on aluminum with significantly reduced quanheretofore known processes.
• the method of this invention comprises the steps of contacting the surface of aluminum or its alloys with an aqueous acidic solution containing phosphate ions, fluoride ions and dichromate ions in appropriate proportions, at a temperature and for a time suflicient to cause an insoluble, adherent coating to form on the surface, the solution at all times having a pH in the range of 0.8 to about 1.5.
• an aqueous acidic solution containing phosphate ions, fluoride ions and dichromate ions in appropriate proportions, at a temperature and for a time suflicient to cause an insoluble, adherent coating to form on the surface, the solution at all times having a pH in the range of 0.8 to about 1.5.
• impurities including aluminum and trivalent chromium
• Patented Mar. 15, 1966- the usual nozzle plugging and increase therein the process may include the further step or steps of adding ingredients so as to adjust the relative proportions of fluoride and dichromate ions to pre-selected values and maintain the acidity in the stated range.
• the process may include the further step or steps of adding ingredients so as to adjust the relative proportions of fluoride and dichromate ions to pre-selected values and maintain the acidity in the stated range.
• creasing quantities of aluminum in such solutions has a softer, and somewhat less adherentthan other coatings but it still has utility as a base for painti
• the"aluminumidissolves from the surface being'coat'ed in an amount gre'ia't'erthah" that which remains'in the coating and the excess remainsf in the coating solution Similarly.
• Such other cations function in conjunction with increasing quantities" of trivalent chromium in the solution to tend to raise the pH of the solution and'a's the quantity'of such'ionjs, panicul'any the alkali metal ions, sodium, potassium and includihg' am monium, build up in the solutionftli'e effect is additiveand the difiiculty' of maintaining the desiredlac'idit'y is increased.
• the P0 ion is preferably kept within the range of 20 to 80 grams/liter, the CrO ion is kept between about 5 and 30 grams/liter, the aluminum ion is kept between about 1 and 30 grams/liter and the fluoride ion is kept between about 2.5 and 69 grams/liter.
• the pH of the solution should be maintained within the range of 0.8 and 1.5 and preferably within the range and above'the upper limit of 1.5 manifests itself in the formation of undesirable insoluble fluoaluminate precipi fate or sludge. For this reason it is undesirable to make up or replenish the operating solutions with salts containingsuch foreign" cations and it is one of the advau tag'eous features of this invention that.
• the P0 ions, the CrO ions andthe fluofide ions are introduced only in the form of the respective acids to maintain theoperating solutions in eflic'ient coatingv condition in a continuous process, for example phosphoric acid, chromic acid, bydrofluoric acid andfl'uoa'lur'ninic acid, H AlF p
• the P0,, ion content of the solutions of this invention should be in the range of aboutSgrams/litertdabout 150 grams/ liter and is preferably introduced from make-up or replenishing materials asphosphoric acid.
• the dichromate ion, calculated as chromic acid, CrO can satisfactorily vary between about 2.5 and about 62 grams/liter and preferably is introduced in the solutionsin the form of chromic anhydride or chromic acid.
• the sum of the concentration of C'rO and P0 ions is prefer ably above a minimum of about if) grams/liter. When the sumof these ions falls below about 10 grams/liter the pilot the solution tends to numerically exceed 1.5 and the tendency for sludgin'g and: the formation of loose coatings is increased.
• The'fluoride' ion concentration ofthe bath may satisfactorily vary from a minimum of about 2,5 to about[123 grams/liter and is preferably introduced into the solutions in the form of hydrofluoric acid.
• the solutions of this invention will continue to form satisfactory coatings in the presence of small quantities of them and in the presence of anions such as sulfates, nitrates and chlorides, and for this reasons it is possible topartially regulate the desired acidity by introducing a strong mineral acid such as hydrochloric, sulfuric or nitric acid, if desired. It is preferred, however, that the acidity of the solution be derived from phosphoric, c'hromic and hydrofluoric acids, and in this case the major acidity effect is derived from the phosphoric and chromic acids.
• the range of fluctuation in the more acid solutions of this invention is noticeably les's than is characteristic of less acid solutions of otherwise'similar type and generally reaches the lowest value ii li a" few seconds to one minute.
• the electrode upon removal from the solution should be immediately rinsed'in a solution of five normal hydrochloric acid by inserting it in that solution for two m nree minutes and thereafter rinsing the el'ectrode in pure water.
• the electrode is then kept immersed in ure Water until the next reading, and preferably is checked against a standard buffer before use. Under this procedure a glass electrode has been found to give reproducible results having a range of :0.05 pH, whereas failure to counteract the fluoride attack will cause the electrode to give inaccurate and non-reproducible readings' after only a' few uses, sometimes even after only one reading. I w
• the solutions of and the method of this invention are useful in the formation of coatings on pure aluminum and the' alloys thereof including extrusions, cast, wrought and sintered alloys...
• the process has been found to satisfactorily coat all of the commercially available wrought aluminum alloys which have been tested and specifically ihcludingllllil, 2014', 2017, 2024, 3093, 3004, 3005, 5052,. 5056-, 6061, 7072 and 7075 While the compositions ofthesealloys vary and each is adapted for a particular application, they are all alloys in which aluminum is the principal ingredient.
• While the surfaces to be coated should be free of grease, dirt, oxide films and the like, special cleaning is not required: prior to the'u'se of the process of this invention. Conventional cleaning with mild alkali cleaners and/or acid or caustic treatments, as needed, by employing conventional techniques are satisfactory to prepare the surfaces to be coated.
• the solutions can be satisfactorily applied at temperatures varying from room temperatures to as high as about 180 F. and in general the rate of coating formation increases as the temperature increases. However, as the solutions become more acid and the fluoride concentration increases metal attack is increasedto the point at which the resultant coating approaches a loose powdery condition, so that in general the use of temperatures between about l20 F. and 140 F. are best for the majority of applications. At these temperatures the rate of coating formation is sufficiently fast to produce any desired coating weight in a reasonably short time, for example, five seconds to three minutes, and the use of higher temperatures is less desirable in comparison because of the increase in cost and the relatively minor increase in coating rate formation which accompanies the same.
• the solutions may be satisfactorily applied by spraying the surface to be coated with the solution, by dipping the part in the coating solution or by brushing or flowing the solution over the surface. Because of the substantial non-sludge forming characteristics of the solutions of this invention, application of the solutions by spraying is particularly desirable.
• the use of the solution of this invention to form coatings on aluminum surfaces dissolves more aluminum, during the coating formation, than is contained in the coating and aluminum builds up in the solution. It has been found that in order to maintain the solution in continuous operative coating formation condition, it is important to regulate the content of fluoride in the solution relative to the aluminum which is dissolved in that solution. Unless the fluoride concentration is raised as the aluminum content increases in the bath, the coating ability of the solution decreasesto the point that the desired coating weight is not obtained and for continuous operation to produce coatings of substantially constant weight and color, the relationship of fluoride to aluminum should bemaintained in the proportions set forth below in Table I. Percentages throughout are weight/ volume.
• a fluoride concentration which is about 0.4% above the minimum values given above in Table I represents a good continuous working concentration for the fluoride to produce high quality, adherent, constant color coatings at a rapid rate over the entire range of aluminum concentration in the bath.
• concentration of fluoride set forth above in Table I and in other portions of this specification and in the appended claims refers to fluoride concentration determined by the Willard and Winter method described in Industrial and Engineering Chemistry, Analytical Edition, vol. 5, pp. 7-10, 1933, modified to employ the titration described by Rowley and Churchill in the same journal, vol. 9, pp. 551, 2, 1937.
• the rate of increase in aluminum concentration in' the operating solution is dependent upon the weight of coating which is being formed on the surface treated and that in typical commercial operations aluminum in an amount of approximately 26% to 30% of the weight of the coating which is formed is continuously added to the solution, that is to say, if a Bil-milligram coating is being. formed on the aluminum surface the processing of each square foot of aluminum through the coating solution adds 26% to 30% of 30 milligrams of aluminum to the coating solution, or 7.8-9.0 milligrams of aluminum per square foot of surface processed through the solution. In any given installation continuous operation of the process depletes the coating solution by dragging out a portion of it on the work processed therethrough, and.
• the starting solution can be formulated to include trivalent chromium ions, as well, if desired. It is therefore possible to preliminarily formulate a solution which represents the solution in useduring steady state operating conditions and which thus eliminates all uncertainties regarding the aging period.
• Typical concentrated makeup materials suitable for this purpose may have the following composition:
• v Coatings of this invention having primary utility as a base for paint can be rinsed in a dilute solution of chromic acid, if de sired, for example, a chromic acid solution having a concentration of about 0.02% to 0.10% CrO
• a chromic acid solution having a concentration of about 0.02% to 0.10% CrO
• Such a rinsing with a chromic acid solution has a tendency to yellow the coating and except in applications in which the water available forfinal rinses is impure, the use of such a rinse is usually unnecessary.
• the coatings are normally rinsed in pure water and thereafter may be dried in air or force-air dried at elevated temperature. Satisfactory drying of the coatings by elevated temperature drying can be effected at temperatures of between about 150 F. and 200 F. Temperatures above about 200 F. should be used only for veryshort times since prolonged contact at such temperatures causes the coatings to be loose.
• the coatings of this invention, particularly coatings having weights of a range of 200-800 m g/sq. ft. are useful as aids in deformation operations and can be used in conjunction with conventional lubricants, such as aqueous soap solutions, soapborax admixtures, etc.
• the lighter-weight coatings have also been found to be useful as a base for vitreous enamels, particularly coatings having weights in the range of about 10-70 rug/sq. ft. and for this use the coatings require merely to be dried and the vitreous enamels applied by conventional techniques and fired thereover.
• EXAMPLE I a short immersion in a dilute alkaline or alkali bath and water rinsed, were immersed in the solution, one ata" time, at the temperature of 135 F. for one minute and withdrawn. Upon withdrawal from the solution, the panels were squeegeed to remove excess solution and after rinsing and drying the coating weight of the twen-, tieth panel was found to be 250 mg./ sq. ft. and to be decreasing. To prevent decrease in the coating weight the fluoride concentration was then increased to a concentration of 0.26%, to. thus produce an F to CrO ratio of 0.185. Twenty additional similar panels were processed in the solution under the.
• the coating weight of the 40th panel was found to be only 200 mg/sq. ft.
• the bath was again adjusted by adding additional hydrofluoric acid to raise the fluoride concentration to 0.29% and additions were made to re-' plenish the chromic and phosphoric acids in an attempt to maintain the original concentration and thus the bath, after adjustment, had an F to CrO ratio of 0.21.
• 40 additional panels were processed through this solution and inspection of the th panel showed that the coating Weight had been increased back to theoriginal 250 mg./ sq. ft., the coatings having a comparable medium green appearance and being smooth and adherent.
• seseaee By employing a similar procedure of continuing to coat panels in the solution and adjusting the fluoride con centration to maintain the cc :rting weight at approximately the 250 mg./sq. ft. weight, 440 panels were processed equalling 147 sq. ft. through the bath. At this point, an analysis of the solution indicated the aluminum to be 0.18%, F to be 0.65% and Cr+++ to be 0.08%.
• One gallon of an aqueous, acidic solution was prepared to contain 1.4% CrO P0 and 0.38% F and upon determination was found to have a pH of 1.1.
• This solution was heated to 135 F. and a plurality of 4" x 6", #3003 preliminarily cleaned aluminum panels were immersed in the solution, maintained therein for one minute, withdrawn, water rinsed and air dried.
• the coatings which were adherent and medium green in color were stripped from the surface and upon weighing were found to have a weight of 417 ing/sq. ft.
• Suflicient sodium hydroxide was added to raise the pH of the solution to 1.9. Similar aluminum panels immersed in this modified solution were coated in one minute, with a coating having a weight of 544 mg./ sq. ft. but the coating was observed to be dusty and non-ad herent. Additional similar panels were processed through the solution and the coatings formed on the surface of each was observed. It was not until after 44 additional panels had been processed through this solution that the coatings became adherent. On the 45th panel a relatively adherent coating was obtained, generally medium green in color and it was found to have a weight of 341 mg./sq. ft. The solution was analyzed and found to contain 0.024% aluminum and 0.008% trivalent chromium.
• trivalent chromium, 1.4% CrO a ratio of F to CrO of 0.22'andIto have a pH of 1.71.
• the coating formedon'the panels. processed between 40 sq. ft. and. 154.sq'..-.ft. were adherent, medium green in color and averagedabout; 250 mg./sq. ft. in weight. The sludgingcontinued and it is apparent that the aluminum and some fluoride were lost from the solution in the sludge.
• sludge indicated it to be an admixture of sodium fluoaluminate and chromium phosphate having an analysis of 55% F, 11% Al, 1% trivalent chromium and 2.8% P0
• 103 grams of sodium bifluoride had been added to the solution in order to maintain the coating action suflicient to produce anaverage coating weight of 250 mg./sq. ft.
• sufiicient chromic acid and phosphoric acid were added, to maintain the original concentration of 5%.
• EXAMPLE III This example is intended to illustrate the effect on coating weight caused by varying the hydrogen ion concentration of a coating solution comparable to'an aged solution or one which had been in operation for an extended period of time.
• a coating solution was prepared to contain- 0.16% aluminum, 0.7% fluoride, 1.4% CrO and 5% RQ anQ;
• Example II a precleaned panel of, comparable type to thatdescribed in Example I was immersed in the solution at a temperature of- 120 F. for 4; minutes, withdrawn, rinsed, dried, inspected and the coating stripped and weighed and it was found that the coating had a weight of 480 mgjsq. ft. was green in color and adherent.
• a second portion of the solution was modified by adding thereto 17.8 ml. of 50% sodium hydroxide per gallon of solution and the resulting solution had a pH of 1.23. Panels processed in this solution for 4 minutes at 120 F. were coated with a coating having aweight of 436 mg'./sq'. ft.
• a third portion of the standard solution was modified to contain 35.6 ml. of 50% sodium hydroxide per gallon of solution to thus produce a solution having a pH of 1.34.
• a panelprocessed in this solution was coated with a coating weight of 244 mg./sq. ft. after four minutes at 120 F.
• a fourth portion of the standard solution was modified to contain 5 3.4 m1. of 50% sodium hydroxide per gallon of solution to thus form a solution having a pH of 1.52.
• a fifth portion of the standard solution was modified to contain 71.2 in].
• a seventh portion of the standard solution was moditied to contain 107 ml. of 50% sodium hydroxide per gallon of solution to thus produce a solution having a pH of 1.90.
• a panel processed through this solution produced a coating of 38 rug/sq. ft. in '4 minutes at 120 F.
• EXAMPLE 1v An aqueous acidic solution was prepared containing 0.5% P 0.4% CrO 0.21% Al, 0.55% -F and 0.05% trivalent chromium. This solution had an F to CrO; ratio of 1.4 and a pH of about 1.46.
• Clean aluminum panels processed through this solution for 4 minutes at 120 F. produced adherent green coatings having an average coating weight of 200 tug/sq. ft., and other panels treated in the same solution for 4 minutes at 135 F. were coated with anadherent green coating having a coating weight of about 280 rug/sq. ft.
• the coatings were adherent green coatings having weights between and mg./sq. ft., and at the end of the processing period the solution contained 0.19% aluminum, 0.06% trivalent chromium, 0.52%
• fluoride 0.25% C103, 1.5% P0 and had a pH of. 1.50 and an F to CrO ratio of 2.0.
• EXAMPLE vr The use of this solution modified to contain less C103 than 3.3% produced coatings which were increasingly dusty as the proportion of C'rO was reduced. While these coatings are inferior to lighter weight more adherent coatings as a base for paint they were found to be advantageous as an aid in deformation operations, of moderate severity when preliminarily treated with an aqueous soap boraxsolution of conventional type.
• EXAMPLE vn An aqueous acidic solution was prepared containing 14.5% P0 0.22% F and 0.4% CrO This solution had an F to CrO ratio of 0.55 and a pH of 1.2.
• Aluminum panels processed in this solution for 4 minutes at 120 F. were coated with an adherent coating having an average coating weight of 30 mg./sq. ft.
• EXAMPLE VIII Operating solutions similar to Example I, maintained at a temperature of 135 F. were used to coat a plurality of types of aluminum panels, which had been preliminarily conventionally cleaned in a dilute alkaline bath, by immersing these panels in a solution for one minute, withdrawing, water rinsing and drying. All of the panels were coated with adherent green to green-gray coatings having weights in the range of 200250 mgJsq. ft.
• the types of aluminum panels treated included aluminum alloys designated by the aluminum association designations numbers: 1100, 2014, 2017, 2024, 3004, 3005, 5052, 5056, 6061, 7072 and 7075.
• I 1 An aqueous acidic solution for coating the surface of a metal from the class consisting of aluminum and alloys thereof in which aluminum is the principal ingredient, which consists essentially of 5-150 grams/liter phosphate ion, 2.5-6 2 grams/liter CrO 1-55 grams/liter aluminum ion, the hydrogen ion and between about 2.5 and 123 grams/liter of fluoride ion, said fluoride ion varying with the aluminum concentration with reference the pH of the solution being between about 0.8 and 1.5, as measured by the lowest value indicated by "glasselectrode pH meter within the first 10 minutes of immersion of the electrode in the solution.
• the pH of the solution being between about 0.8 and 1:5, as measured by the lowest value indicated by glasselectrode pH meter within the first 10 minutes of immersion of the electrode in the solution.
• -Aluminum, Fluoride, -l al the pH of the solution being between about 1.0 and 1.3, as measured by the lowest value indicated by glass-electrode pH meter within the first 10 minutes of immersion of the electrode in the solution.
• a process for coating the surface of a metal of the class consisting of aluminum and alloys thereof in which aluminum is the principal ingredient which comprises the step of contacting the surface of said metal with an aqueous acidic solution consisting essential of 5-150 grams/liter phosphate ion, 2.5-62 grams/liter CrO l-55 grams/liter aluminum ion, the hydrogen ion and between about 2.5 and 123 grams/liter of fluoride ion, said fluoride ion varying with the aluminum concentration with reference points as follows: s 7
• the pH of the solution being between about 0.8 and 1.5, as measured by the lowest value indicated by glass-electrode pH meter within the first 10 minutes of immersion of the electrode in the solution, and maintaining said solution in contact with said metal surface until an adherent coating is formed thereon.
• a process for coating the surface of a metal of a class consisting of aluminum and alloys thereof in which aluminum is the principal ingredient which comprises the step of contacting the surface of said metal with" an aqueous acidic solution consisting essentially of 20-80 grams/liter phosphate ion, S-3O grams/liter C50,; 1-30 grams/ liter aluminum ion, the hydrogen ion and between about 2.5 and grams/liter of fluoride ion, said fluoride ion varying with the aluminum concentration with reference points as follows:
• the pH of the solution being between about 0.8 and 1.5, as measured by the lowest value indicated by glasselectrode pH meter within the first 10 minutes of immersion of the electrode in the solution, and maintaining said solution in contact with said metal surface until an adherent coating is formed thereon.
• aqueous acidic solution consisting essentially of 20-80 grams/liter phosphate ion, 5-30 grams/liter CrO ,'.1-30 grams/ liter aluminum ion, the hydrogen ion and between about 2.5 and 70 grams/liter of fluoride ion, 'saidfluoride ion varying with the aluminumconcentration with reference points as follows: 5
• aluminum and alloys thereof in which aluminum is the principal ingredient whichcomprises the step of contacting the surface of said metal with anaqueous acidic solution consisting essentially of 5-150 grams/ liter phosphate ion, 2.5-62 grams/liter CrO l-55 grams/liter aluminum ion, the hydrogen ion and between about 2.5 and 123; grams/ liter of fluoride ion, said fluoride ion varying with the aluminum concentration with reference ;points as follows:
• ' 81A method for forming a chromium-phosphate coating on the surface of a metal from the group consisting of aluminum and alloys thereof in' which aluminum is the principal ingredient comprises the step of contacting the surface of said metal with an aqueous acidic solution consisting essentially of -15 0 grams/ liter phosphate ion, 2.5-62 grams/liter C 1-55- grams/liter aluminum ion, the hydrogen ion and between about 2.5 and 123 grams/ liter of fluoride ion, said fluoride ion varying with the aluminum concentration with reference points as follows:
• a composition for use in preparing an aqueous acidic solution for coating aluminum and alloys thereof which. contains as its essential coating producing ingredients, in parts by weight, 70-280 C1 0 345-1370 H P0 (75%), 65-130 A1 0 (30.5%Al) and 480-1715 water.
• the pH of the solution being between about 0.8 and 1.15, as measured by thelowest value indicated by glass electrode pH meter Within the first 10' minutes of immersion of the electrode in the solution, and the ratio of F/CrO is above Fluoride.
• the pH of the solution being between about 0.8 and 1.5, as measured by the lowest value indicated by glasselectrode pH meter within the first 10 minutes of immeralloys thereof in which aluminum is the principal ingre dicnt, which consists essentially of 20-80 grams! liter phosphate ion, 5-20 grams/liter CrO 1-30 grams/liter aluminum ion, the hydrogen ion and between about 2.5 and 70 grams/ liter of fluoride ion, said fluoride ion varying with the aluminum concentration with reference points as follows:
• al al the pH of the solution being between about 1.0 and 1.3, as measured by the lowest value indicated by glass-electrode pH meter within the first 10 minutes of immersion of the electrode in the solution, and the ratio of F/ C1-C is in excess of 0.5.
• the pH of the soltuion being between about 0.8 and 1.5, as measured by the lowest value indicated by glass-electrodepH meter within the first 10 minutes of immersion of the electrode in the solution, the ratio of F/CrO is above 0.405, and maintaining said solution in contact with said metal surface until an adherent coating is f rmed thereon.
• a process for coating the surface of a metal of a' class consisting of aluminum and alloys thereof in which aluminum is the principal ingredient which comprises the step of contacting the surface of said metal with an aqueous acidic solution consisting essentially of 20-80 grams/liter phosphate ion, 5-30 grams/liter Cr0 1-30 grams/liter aluminum ion, the hydrogen ion and between about 2.5 and 70 grams/liter offluoride, ion, said fluoride ion varying with the aluminum concentration with reference points as follows:

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