[1]2 7 0 3 , 7 8 1 United States Patent Office 2,703,781 AINODIC TREATMENT OF ALUMINUM SURFACES 5 Frederick H Hesch, Spokane, Wash assignor to Kais--r Alumiiau@; & Chemical Corporati'o'n, Oaldand, Calif., .i corporation of Delaware No Drawing. Application May 25, 1950, 10 Serial No. 164,304 8 Claims. (Cl. 204-29) This invention relates to an electrolyte and method 15 for anodically oxidizing the surfaces of aluminum and aluminum alloys. More particularly, the invention relates to an ii-nproved electrolyte annd method for producing a clear or transparent, substantially non-iridescent anodic oxide film on the surfaces of alumintim 20 and aluminum alloys. Aluminum products are often required to have highly polished surfaces and in many cases a surface of high specular reflectivity must be provided. Several processes may be employed to impart to alutminum and alti- 25 minum alloy surfaces a high lustre of optimum specular reflectivity among which may be mentioned buffin@-, chemical bri.-htening and/or electrobrightening. The latter technique is illustrated -n regard to the electrolytes, current density, temperature and time employed by ref- 30 erence to U. S. Patents Nos. 2,040,617 and 2,040,618 to R. B. Mason et al. and U. S. Patent 2,108,693 to R. B. Mason. The reflecting surface of the aluminum or aluminum alloys subjected to the treatments disclosed in these patents is stated to have a reilectivity 35 of 'roni as high as 80 to over 87%. However, these are total r-flectivity figures including scattered or diffusely reflected light as well as specularly reflected light. More recently chemical baths for brightening al.uminum annd aluminum alloys have been developed 40 whereby a relatively high snecular reflectivity may be imparted, as opposed to a iustrous appearance wherein the surface causes predominantly diffuse reflection of light usually characteristic of the ordinary chemical bright dips. 45 Regard@' ess of the manner in Nvhich the metal surface is - endered specularly reflective, it is usually desirable and ofterl n,-c@-ssary, to fur,,her process the pol:shed or briglitened metal surface in order to preserve the IListrous fipisli. Lacquering has been em- C)o ployed in certain instances, btit the lacquer coating on the polished metal does not provide optimum protection for the manifold conditions presented by the wide variety of kvplications for such reflective surfaces, and, rnoreove r, it is difficult to obtain perinanent adherence 55 of such coat:@ngs especially during subsequent fabrication. Accordingly, the more generally employed method of protecting annd preserving the polished condition of the metal is to form a clear, transpare-.it, anodic illm on the metal surface. 60 it has been know-ii for a number of years that an artificial oxide film cali be formed on alumintim or aiiiminum aroy surfaces by an electrolytic process, kilown as anodizing, in a siiitable -.lectrolyte. There are a number of well-known anodizing processes; but the 65 fundamental principle underlying all is that the ar-ticle to be treated is made the anode (instead of the cathode as in electroplating) in an electrolyte capable of yieldin.a oxygen. When current is passed, an initial oxide film forms on 'Lhe metal surface, and progressive oxida- 70 tion takes p'lace beneath the film thus formed. After passage of the current through the bath for a predetermined time, an inert, durable aluminum oxide coating is obtained. Alt'noijgh the principles of the various anodizi@ig processes are the same, the properties of 75 anodic coatings will vary, depending upon a number of factors amon-. which is the composition of the electrolyte. In the past, the principal electrolytes used for anodizing aluminum and alumiiium alloys have been sulfuric acid (U. S. Patents 1,869,041, 1,869,042, 80 1,869,058 and 1,900,472 cover the concentreition of the Patented Mar. 8, 1955 2 sulfuric acid electrolyte from 5 to 77%) and chromic acid. Other electrolytes, such as oxalic acid, boric acid, and phosphoric acid, have been used, although not as extensively as the former two. The anodizing process most commonly in use which produces a substantially clear anodic film is that wherein sulfuric acid is the electrolyte. Of course, in anodizin.- a highly polished aluminum surface, such as thos-@ obtained by buffing and/or electrobrightening, or chemical brightening, it is imperative that the anodi-- film be non-iridescent and that such oxide film will not markedly impair or decrease the specular reflectivity of the stirface. The disadvantage of the itms produced by the sulfiiric acid electrolyte is that when forined of sufficient thickness to eliminate iridescence, a cloudin@.Y of the polished metal surface takes place during anodizing to give the appeararice of a cloudy grey film which impairs specular reflectivity. Phosphoric acid, as above-mentioned, has been used as an anodizing electrolyte, and produces a clear oxide fflm. However, such films are always relatively thin, and consequeritly iridescent. Prolonged periods of anodizing with phosphoric acid fail to eliminate ir:descence, but do, however cause cloudiness in the Ilm aiid/or on the metal thereby seriously reduci@-ig specuJar reflectivity. Thiis, it may be seen that with such eicetrolytes it is possible to produce anodic coat;i3gs that are non-iridescent or substantially so, but in @,o doinz the epecililar reflectivity of the metal srrface is considerably decreased (sulfuric acid). Coiive.-sejy, coatings that do not markedly inipair specular reflectivity may be produced, but these show a decided iridescence (phosphoric acid). It is, therefore, a primary object and purpose of this inventioii to provide an im.Droved electrolyte and method for the anodic production of an oxide film oii Lluminiim and aluminum alloys which is substantially clea,.- or transparent and n on-iridescent. A more specific object is to provide an anodizing electrolyte and method to produce on aluniintir@i an,-,I aluminum alloys, having a polished surface of high specular reflectivity, a clear protective anodic film of sufficient thickness to substantially eliminate iridescence, while maintaining the Mgh specular reffectivity of the metal @,urtace at a maximum, that is, with a mi-@iimL@n-i reduction in aloss reflectivity. These and other objects and advantages of the iiivention will be apparent from the following detailed description of theinvention. It has been discovered according to the invention that an anodic film which is clear or substantially transparent and non-iridescent may be produced ou a'@uminiim and aluminuai alloy surfaces by an e',--ctrolytic 'Lrcal,inent in which the metal is the anode, that is, by anodizing, in an electrolyte comprising essentially suli.,Jric anli phosphoric acids in certain suitable proportions. over, -it has been discovered that the sulfui-ic-phosphoric acid electrolyte is particularly suitabie in providing a protective anodic film on aluminum and aluminum ailoy sur.Laces upon which a lustrous finish of hi.-h spec@@,lar reflectiv;ty has been previously been prodticed by any of the known brightening or polishid@ methods. The anodic film thus formed preserves the ITistrous n@nisli from deterioration and is of sufficient thickness to eliminate or substantially eliminate iridescence while producing only a relatively slight loss in the specular reflectivity of the surface. No attempt is here made to explain the improveci result flowing from the fact that the combined phosphoric and sulfuric acids in the electrolyte act to modify the effect of either constitutent Nvhen used alone. The anodizing electrolyte and method of the irver-@tiori may be applied to any aluminum or aliimini-im alloy surface whether previously polished or not to produce a clear, transparent substantially non-iridescent protective Mm. However, it is particularly useful where the metal surface has been polished to a high specular reflectivity by mechanical polishing (buffing), cle ctrobrightening, electropolishing or chemical brightening. The concentrations of the sulfuric acid and phosphoric
[2]3 acid electrolytes may be varied throughout a relatively wide range of percentages by weight. The phosphoric acid constituent predominates in the electrolyte, bein@ present in amounts of frorn about 15 to about 40% by weight of total solution. It is, of course, necessary that the aqueous electrolyte solution contain at least effective amounts of sulfuric acid and ger@erally at least about 2% sulfuric acid is required, while it may advantageously be as high as 10%. Expressed in terms of concentration, beneficial results are obtained with phosphoric acid concentrations varying from about 175 to about 500 ,-rar.is per liter and with sulfuric acid concenttations of from about 30 to slightly over 100 granis per liter. Optimum coatings of oxide film affording very good protection aiid causing a minimum of reduction in specular or -loss reflectivity have been produced with a concentrat on of abotit 45 grains per liter of su]fliric acid and abol-it 408 granis per liter of phosphoric acid, '@hat is, about 3.7% H2SO4 And about 33.6% H3PO4 by weight of total solution. Ttie voltage suitably impressed on the electrolytic cell electrodes may vary from about 5 to 50 volts e. m. f. to produce a current density of from about 5 to about 30 amperes per sqtiare foot. (The current clonsity is here comp,ited by dividing the total current applied by the total area of conductive anode inaterial immersed in the electrolyte.) AlthoLigii direct currejit is preferably employed in the anodizi@-ig process, alternatinctirrent may satisk'actoril3, be used in lieu of directctirrent. The anodizing time may be widelv varied and depends to a large extent on other factors such as electrolyte concentrations, temperattire, and current density. In general, the tinie for aii0dic treatment of a brightened alumin,,im or aluminum alloy stirface is from about one-half minute to i'ifty miriutes to produce a clear non-iridescent film of sufficient thickness to atord adequate protection, for example, 0.00005 to .0001 inch. A temperature of from about 60 to about 90' F. is r eco.,ni-nend@ld (about 15 to 35' C.), higher temperatures bein- suitably employed with shorter times of treatment and lower temperatures with longer times. Optimum restilts have been produced in about ten minutes witb a current density of 10 amperes per square foot 4t ali e. n-i. f. ot 25 volts ,ind a temperature of from 75 to 80' F. In -Cneral, all other cotiditions bein.a held constaiit, if the process;n@ time, the ctirrent density Or the teniperature i- increased therc is a tendeticy for a dull cloudy filni to be produced. Conversely decrease of time, cui@rent density or terdperallilre, other factors rem,tin;n,, f@xed, tends to produce a thin filni having a cliaracteristic ir;descence. As - rule, it is desirable to sub;ect the brightened anodized rnetal to any of the conventional scaling treatments, for example.. immersing in water at 80 to 100' C., to redtice the porosity and absorptivity of the oxide film. Tiie invention is further illustrated by the following e:,ample wliich incltides the preliminary bri@_thtening, ard m,hich s noL intended 'to coiistitute a limitation thereof il bein@ u-nderstood, as get fortb above, that the severii @actors involved may be varied Nv:@thin a wide range with beieficial results. Samp'ies of a cominon aluminuni alloy were prepar-. d by first mechaiii-,ally btiffing, after which they were cleaned free of any adhering bufriiig conipound, grease or other adlierin-, foreign material by iinmersion in a mild inhibited alkaline cleaner. The 8amples were then clectrobrightened by being connected as anodes in an aqueo,,ls electrolytic bath of 2.5% by weight fluoboric acid (HBF4) for two minutes at a ctirrelit density of 70 a@npere,, per sqiiare foot and a bath temperattire of 75 to 85' F. The brightened sftmples contained a thin film or sm-Llt oii the suri'ace which ivas romov@@d by a 8ix minute immersion in a removal bath con8i8ting of an aqueous soltition contaiiing z,,bout 5% oi-tho-phosphoric acid and about 2% chromic acid calctilated as CrO@ at a temperatt,.re of about 180 to 212- F. The solution was agitated during immersion. The cleaned smut-free saniples @,vhich had a specular reflectivity of 78% were theti c6nnected as anodes in an aqiieous electfolyte containing 3.1% by weight sulfuric acid iid 33.6% by weight phosphoric acid, the balance 2,703,781 4 beiiig water. A D. C. voltage of 25 volts was impressed on the cell, the current density being 10 amperes per square foot, with the bath at 75-80' F. for a period of ten minutes. The anodized samples were then removed and examined. The oxide film was clear and essentially non-iridescent and the specular reflectivity as measured by a Henry Gardner 60' glossmeter was 65%, as compared with 78% given above for the samples prior to anodizing and subsequent to smut removal after electro10 brightening. The reduction in reflectivity was smaller than samples anodized by the conventional sulfuric acid electrolyte according to the procedures disclosed in U. S. Patents 1,869,041, 1,869,042, 1,869,058 and 1,900,472, above itlei-itioned. The actual absolute value of the 15 specular reflectivity of the treated samples was in most cases at least 65%, while those anodized with sulfuric acid alone were no higher than 50%. It is, of course, essential that all materials to be anodized be clean. Where polishing is accomplished by 2( buffing, a mild alkaline cleaner is recommended to insure a clean surface. In addition, where the article is not to be e'@ectrobrightened, it is advisable in order to sectire maximum specular reflectivity to deoxidize the metal by the use of a (.Ieoxidizing bath sucli as the phosphoric 25 acid-chromic acid smut removal solution, above-mentioned. When the anodizing is preceded by an electrobrightening process which includes a deoxidizing step, only a thorough water rinse is recommended prior to anodizing. 30 The apparatus employed for the process may be constrticted of any material resistant to the acid electrolyte under the conditions employed. Thus, the tanks, cooling or heatin.- equipment and agitators may be lined suitably with rubber, glass, lead, or lead alloy, or carbon. 35 The apparatus preferably is equipped with heating and cooling coils or jackets, such as steam and water coils, so that close control of operating temperatures can be maintained. Agitation is recommended to main40 tain uniform temperature in the bath and consequently of the work and to remove any gas which might foriii and passivate the work. Air or mechanical agitation may be used. The process and electrolyte of the invention are equally 45 oi- more econotiiical than the phosphoric acid process and only moderately more expensive than the sulfuric acid electrolyte while productive of results superior to both. li-i the foregoing specification and in the claims perceiitages by weight are based on total solution weight. 50 What is
