US2515934A - Phosphate coating of metal articles - Google Patents

Description

Patented July 18, 195

2,515,934 PHOSPHATE COATING or METAL ARTICLES 1 Harry Grey Verner', (mini al, and Lindley s.

Wood, Syracuse, N. Y., assignors to Du-Lite Chemical Corporation, Middletowri-,*Conn;, a i corporation of Connecticut v r No Drawing. Application ecem b er 17,1947, p

, Serial No. 792,326

This invention'relates to the production of phosphatefcoatings on metal articles, andrelates more particularly to a phosphatizing liquid and a methodof phosphatizing employing an organic liquid as the vehicle for the phosphoric acidin lieuof the water of the conventional aqueous phosphatiz ing solution.

"The production of phosphate coatings on metal articles is today widely employed for the purpose of protecting the metal against oxidation and'providing a surface properlyconditio'ned for paintffinlshes. In present phosphatizing pr'oc esses employing 'an aqueous phosphatizin'g solution, the metal must be thoroughly clean'before phosphatiz'ing, as any rust or a 'thin film of oil or even a fingerprint may cause an unsatisfactory phosphate coating or may produce, a non-uni,- form' appearance, of the final product. Metal articles to be phosphated are therefore usually thoroughly cleaned to remove all traces of rust, and, to afford protection during theoperations preceding phosphatizing, are usually coated with a of rust preventive oil, called slushing oil, orwith some proprietary oily product adapted to'perform asimilar protective function. When ready" for phosphating, the metal articles are (1) degreased by solvent or vapor "d'egr'easing, this degreasing step employing an oil solvent such as trichlorethylene or perchlorethylene; Followingx degreasing, the metal articles are usually (2) cleanedin a hot, mildly alkaline aqueous cleaning solution, (3) thoroughly rinsed. twice in hot water, and (4) phosphatized by being either sprayed with or dipped in the hot aqueous phosphatizing solution. 'Afterthe' phosphate coating is thus applied to the metal articles, they are (5) again rinsed in hot water, ('6) subjected to a passivation treatment in hot dilute chromic acid solution intended to prevent creeper rust under any accidental break in the final paint (7) again thoroughly rinsed in hot water, and (8) dried in hot air, after which the articles are"(9) painted, usually by spraying with paint and then baking. If the painting step does not immediately follow the drying step, the articles must be kept dry or they may oxidize. The process above described is sometimes abbreviated by omitting the passivation treatment (step 6) and the preceding and subsequent rinses (steps 5 and 7).

The aqueous phosphatizing solution used a process such as just described may comprise about 2% of a mixture offree phosphoric acid, zinc phosphateand an oxidizing-agentsuch as a nitrateo'r a chloratepfrequently termed an ac- 10Clairns. ((1148-4515) celerator; The free phosphoric acid reacts with the metal of the article and with oxygen from the, air or from the oxidizin agent or both to form an adherent coating of insoluble metallic phosphate. This phosphate coating protects the metal somewhat,butits primary function is to serve as a bond-for'the paint finish The zinc phosphate of such an aqueous phosphatizing solution is used topirnprove the grain structure of the phosphate coating, and may act as a catalyst. The'phosphate coating obtained improves as the temperature of the solution is raised, and the solution is therefore used as near the boiling pointfas may, beconven'ieht.

Weh'ave discovered that improved results and great economy Qfbpera'tion may be achieved by substitutinganor'ganic phosphatizing liquid for theadu'eous solution above'described, while at the same time modifying the process in a number of particulars later discussed. The active phosphatizing agent "may be the commercial phosphoric acid known as the syrup, and the zinc phosphateand the accelerator of the aqueous solution'are unnecessary, although they may bej usedif desired-.- According to the preferred form of ourinvention-,-the phosphoric acid syrup is dissolved in avolatile organic solvent, preferably"a*50 50 mixture of acetone and carbon tetrachloride; The'concentration of syrup in the solventmay*range'from 1% to 7%, but we prefer 3%;a'lthough' there is not much difierence inresults' anywhere in the range from 2% to 4%. Immediately following the conventional degreasing' step,"the organic phosphatizing liquid is applied-bydippin -or by spraying. After being dipped or sprayed;themetal article dries in air at room temperature almost instantly, due to the volatility of the solvent 'employed, leaving a very thin film of metal phosphate on the metal article. "A conventional solvent recovery system mayfiadvantageously" be employed, connected either to a hood over "the dipping tank or to the usual spraybootm thus enabling the acetone and carbon tetrachloride to be recovered and reused. The"meta1ajrticle may be painted immediately, or, if more convenient, may be stored for a reasonable period before painting without requiring any special precautionsin storage. Tests have shown that polishedsteel panels so phosphated may be stored for as long as one month under ordinary "room conditions without exhibiting any signs of rustand without impairing the subsequent paint finish. As is customary with aqueous phosphati'zing; the'paint is preferably sprayed on the phosphatized article and. then baked,

The extreme simplicity of this organic phosphatizing process as compared to the aqueous phosphatizing processes now in use will be at once apparent. The nine steps of the full aqueous process described above are reduced to three: (1) degreasing, (2) phosphatizing by dipping or spraying and practically simultaneous air drying, and (3) painting. No elevated temperatures are required in the combined phosphatizing and drying step. As we shall show hereinafter, it is even possible under somecircumstances to eliminate the degreasingstep.

Metal articles phosphatized by our organic phosphatizing method, and thenpainted, Show superior results in the standard salt spray aocelerated test as compared to similar articles produced by either the full or the abbreviated aqueous phosphatizing methods described above. With our process, the paint does not peel along the edges of the conventional scratch, nor is there any evidence of creep. The paint remaining after the salt spray tests shows superior adhesion with our process, this having been determined by subjecting the various panels to a prescribed brushing procedure.

7 While the organic vehicle we employ is more expensive than the aqueous vehicle of. former methods, the superior product obtained and the savings effected by. the elimination of stepsand the simplification of the remaining steps, more than counterbalance the increased material cost, which cost may be greatly lessenedby using conventional solvent recovery methods as above described. Moreover, the fact that our organic phosphatizing liquid may beapplied at ordinary room temperature and the accompanying drying may also be efiected at room temperature, enables the necessary apparatus to be decidedly simplified, although heat may be used in our process if desired.

. In the conventional aqueous process, the hydrogen which is liberated as a result of thereaction between the phosphoric acid and the metal, apparently tends to adhere to the metal and prevent further reaction. One purpose of the oxidizing agents or accelerators is to react with this hydrogen and thus speed up the phosphatizing reaction by removing the hydrogen from the reaction zone In our process. nosuch hydrogen difficulty is experienced, making it Imv necessary to employ oxidizing agents, although, as stated above, they may be used withoutdoing any harm. We have found that zinc phosphate is entirely unnecessary in our process, but, like the accelerators, it may be employed if desired. The phosphate coating obtained by our preferred process is so much better than that ob.- tained by the conventional aqueous phosphatizing processes, that, even without the, paint finish, it affords a very substantial degree of protection to the metal. The chromic acid passivation treatment, designed to prevent creep, is rendered unnecessary by the superior phosphate coating we obtain.

One of the advantages of our preferred process as described above, is the wide applicability of the process to differentmetals. We have obtained excellent results not only with steel articles, but with articles made of cast iron, zinc diecast metal, aluminum-zinc die-.cast metal containing only a small proportion of zinc, and Ger-.- man silver. ,Pure aluminum, and cadmiumplated articles cannot be satisfactorily phospl a d, her by ur process or by conventional aqueous methods.

The organic vehicle employed in our process should be one in which phosphoric acid is soluble in the proportion in which it is to be employed, and the vehicle should preferably also be sufliciently volatile so that the remaining film will evaporate promptly at room temperature from the metal article after clipping or spraying. The commercial phosphoric acid syrup contains about 15% of water, and if this material is to be used as the source of phosphoric acid, which is desirable from the cost standpoint, the organic vehicle employed must also be compatible with the resulting small amount of water in the final mixture. It is probable also that a small amount of water is necessary in the phosphatizing liquid in order to permit the phosphoric acid to ionize.

Acetone ideally meets all of the foregoing requirements, and may be used alone with entirely satisfactory results, but its low flash point (-10 0.) makes its use rather dangerous in commercial operations, creating a possible explosion hazard. We therefore prefer tomix carbon tetrachloride with the acetone in order to raisethc flash po n a c ase the hazard. The carbon tetrachloride is used solely as a flash inhibitor. its function being to raise thefiash point of the mixture to the desired extent. is necessary, carbon tetrachloride is compatible with acetone without destroying the ability of the acetone to dissolve the phosphoric acid. The phosphoric acid syrup is not soluble in carbon tetrachloride alone, but is sufficiently soluble in a mixture of acetone and carbon tetrachloride.

Mixtures of acetone and carbon tetrachloride ranging from 60-40 to 491-56 by volume have a fla h po nt of about54 C sl h y above the fl s Point of ordi ary kerosene. Such mixtures may therefore be Safely'handled'with only the usual precau ons e pl yed i p t spray ng oper tions. The proportions of acetone and carbon tetrachloride may thus be considerably varied with ut substantia y changing the flas p int, of the mixture. If the carbon tetrachloride b reduced o 30% of t e, m xtu e the l h po nt dro s to &9" 0.. slightly less than that. o ke se e. a d this would appearto be about the lower limit of safety. Acetone and carbon tetrachloride by themse v s are completely miscible in all proporions, bu due to the wa inc dedin t e com rn roial pho phor c acid that we p efer to emp proportion of carbon tetrachloride cannot be increased beyond 56%, or two liqu d ases bpear. mak ng sprayin i act a W ha e een able to. o serve n c b v difi rc o o ll' as PhOSPhatiZingresults are concerned, with mixtures of acetone and .carbon tetrachloride ran ing fr m 30 a b t rachl e up to 56% carbon tetrachloride, as compared wi h the u e of acetone alone. For reasons of, economy and convenience, we therefo'reprefer to use as our organic phosphatizing l quid, a mixture of equal volumes of acetone and carbon tetrachloride, in which isd ssolved 3% of comm c l p sph r acid 85% syrup.

It is possible to substitute other organic liqnids forthe actone-carbon tetrachloride mixture described above. We have found that 3% of commcrci l. hosph ic, c y up w l d o v inthe following vola e o ganic s lv nts: methy al ohol, amyl acetate, methyl acetate, normalrbutyl alcohol, s condary-bow alcohol. s p y alcohol, denatured ethyl c o -prowl a ooho and d xanc. A of t s solven s r 9 1 merc lly obtainable, and, l e aceto e. may be used either with or without arbon tetrachloride,

but,- 1 for the sake ;,-of'.saiety, a: flash inhibitor is eminently, desirable. rThGSe solvents are listed above in their decreasing .order of. effectiveness, methyl alcohol and amylacetate producing a slightly less desirable phosphate coating (judged by salt spray paint tests) than our preferred 50-50, acetone-carbon@tetrachloride.mixture, and the othersabeingz-i successively less effective, but all ofithem produce a phosphate coating superior toa,,con,ventional aqueous :phosphatizingprocess used;-as a comparisonstandardn I v Solvents. having: a boiling point substantially higher than that of amyl acetate (148. (3.) may be regarded; for ,thepurposes of this-invention as non-{volatile We haveg'successfully used several such nonevolatileorganic solvents, such as octyl alcohol (B. P. 195 C.), cyclohexanol (B. P. 161.5 C.) and tri-butyl phosphate (13. P. 178 C.), in all of which phosphoricacid syrup is solubleto the extent'of 3%; These non-volatile solvents donot require a flashinhibitor, but have he disadvantage that heatshbiild be used to accelerate the drying step, for example, by blowing hot air over the metal articles to be dried. They also produce somewhat less desirable phosphate coatings than the ivolatile solvents. However, all of the nonvolatile solvents named above produced better phosphate coatings (judged by salt spray paint tests). than aconventional aqueous phosphatizing process-used as a comparison standard, and we therefore do;-.not wish to exclude their usefrom the scopeofpur invention. I i

It is also desirable that the organic vehicle be one that has grease and oil penetrating properties. This is true of ourpreferredacetoneecarbon tetrachloride m'ixturem Where theorganic vehicle has this property, it is possible to apply our phosphate coating even through a thin oil film. Thus, we have successfully coated polished steel panels dipped in either lubricating oil or a, com mercial slushinig oil, drained for twenty-four hours, and then sprayed with our preferred organic phosphatizing solvent, without cleaning or degreasing the panels in any way after the oil dip. The panels were then painted in the usual manner. Clean control panels were similarly treated except for the oil dip. The phosphate coatings on the oil-dipped panels, before painting, had a poor appearance, but, judged by salt spray tests after painting, were almost the equal of those obtained on the clean control panels. This suggests the possibility of eliminating the degreasing step in commercial practice where the metal articles to be phosphatized are not excessively coated with oil or grease.

Our above-described low temperature, rapid process results in the formation of an extremely thin phosphate film. This film is frequently so thin that it would easily escape notice by the uninitiated. In the usual phosphatizing processes, the phosphate coating produced is of substantial thickness, being measurable in thousandths of an inch, and is plainly evident to the naked eye. It may very well be that the exceptional results produced by our process are due in part to the extreme thinness of the resulting phosphate film. It seems quite probable that the adhesion between the metal and the metal phosphate and the adhesion between the metal phosphate and the paint are both greater than the adhesion between superposed molecules of the metal phosphate; and, if this be true, the ideal coating would be a uniform layer of metal phosphate just one molecule thick. Another possible explanation is that the thick prior art coatings are suificiently porous" to hold enough water to affect the aint adherence adversely. Regardless of the correctness of these theories, the fact remains,

as pointed out above, thatour process produces superior paint adherence, judged by the standardsalt spray test, as compared to previous aqueous phosphatizing methods.

Although we have thus described our invention in considerable detail in the best form of which weare aware, in accordance with the patent statutes, it will be evident that various changes and modifications may be made by those skilled in the art without departing from the spirit of our. invention. Accordingly, we desire to. be limited.

only by the scope of the appended claims.

We claim: 1

i l. The rocess of phosphate coating an article made of metal of the class capable of reacting with phosphoric acid inform a metal phosphate, whioh lccmpriseswetting the metal article .With

a phosphatizing liquid consisting, essentially of from one to seven percent of concentrated ph0s-.

ing the organic solvent from the Wetted article.

by evaporation, thereby producing on the metal article a thin and substantially dry film of metal phosphatecapable of serving directly as a paintbonding medium. a

2 The process;as claimed in claim 1 in which the organic solvent is acetone.

i 3. Theprocess of phosphate coating an articl made of metal of the class capable of reacting with phosphoric acid to form a metal phosphate, which comprises wetting the metal article with a phosphatizing liquid consisting essentially of from one to seven percent of concentrated phosphoric acid dissolved in a volatile organic solvent therefor, and carbon tetrachloride; said carbon tetrachloride being present in sufficient amount to raise the flash point of the phosphatizing liquid materially above that of said volatile organic solvent, said phosphatizing liquid having no aqueous phase and containing only a small proportion of water based on its phosphonic acid content; and removing the organic solvent and carbon tetrachloride from the wetted article by evaporation at not substantially above room temperature, thereby producing on the metal article a thin and substantially dry film of metal 131105- phate capable of serving directly as a paintbonding medium.

4. The process as claimed in claim 3 in which the volatile organic solvent is acetone.

5. The process of phosphate coating an article made of metal of the class capable of reacting with phosphoric acid to form a metal phosphate, which comprises wetting the metal article with a phosphatizing liquid consisting essentially of from two to four percent of concentrated phosphoric acid dissolved in acetone and carbon tetrachloride; said carbon tetrachloride being present in sufficient amount to raise the flash point of the phosphatizing liquid materially above that of the acetone, said phosphatizing liquid having no aqueous phase and containing only a small proportion of water based on its phosphoric acid content; and removing the acetone and carbon tetrachloride from the wetted article by evaporation at not substantially above room temperature, thereby producing on the metal article a thin: and substantially dryfilm of-metal phosphate capable of serving directly as a paint-bonding. medium.

6-.- A- phosphatizing liquid for phosphate coatingaarticles: made of metal of the class capable of reacting with phosphonic acid to form a metal phosphate, consisting essentially of from one to seven: percent of concentrated phosphoric acid dissolved in an organic sol-vent therefor, said organic solvent being capable ofbeingevaporated at a moderate temperature, and said phosphatizing liquid having no aqueous phase and containing only a small proportionof water based onitsphosphoric acidcontent.

7. A phosphatizing liquid for phosphate coating articles made of metal of the class capable of reacting with phosphoric acid to form a metal phosphate, consisting essentially of fromone to seven percent ofconcentrated phosphoric acid dissolved in acetone, and said phosphatizing liquid having no aqueous phase and containing only a small proportion of water based on its phosphoric acid content.

8-. A phosphatizing liquidfor phosphate coating=v articles madeof metal of the class capable of reacting with phosphoric acid to form a metal phosphate, consisting essentially of from one to seven; percent of concentrated phosphoric acid dissolved in a volatile organic solvent therefor and; carbon tetrachloride, said carbon tetrachlo- 9; A phosphatizing liquidasa claimedzin claim 8 in which the volatile organic sdlventis acetone.

102 A phosphatizing liquid for phosphate coating articlesma'd'e of;meta1- of the classcapable-oi reacting with phosphoric acid. tutor-mt aametal phosphate, consisting essentially of from two to four percent of: concentrated phosphoric acid dis solved. in acetone and carbon tetrachloride; said carbon tetrachloride being present in sumcient:

; amount toaraisethe flashpointofi the phosphatizing liquid materially above that of the acetone,- and said phosphatizing liquid having no aqueous phase and containing only: a small proportion of; water based; on its phosphoric acidcmrtent. GREY VERNER;

S1 WOGD;

S; G ED The following references are of record in the fiil'e ofthis' patent:

UNITED sT'A Es PATENTS Number Name Date 1,398,507 Gravell Nov. 29; 1921 1,428,085 Gravelle, Sept-.15; 1922" 1 ,549,442 Brown Y a Aug; n, 1925; 1,592,102 Gravell- July 13; 1926 1,673,951 Rogers June 19',- 1928. 1,765,331 Gravell June v1'7, 1930' 1,837,430 Gravel-l; Dec. 22; 1931 1,995,954 Albrecht, Mar. 26, 1935 2,142,024 Hall Dec. 27=, 1938* FOREIGN PATEN'IS Number Country Date 499,046 Great Britam o Jan. 18-, 1 939,

Claims (1)

1. 6. A PHOSPHATIZING LIQUID FOR PHOSPHATE COATING ARTICLES MADE OF METAL OF THE CLASS CAPABLE OF REACTING WITH PHOSPHORIC ACID TO FORM A METAL PHOSPHATE, CONSISTING ESSENTIALLY OF FROM ONE TO SEVEN PERCENT OF CONCENTRATED PHOSPHORIC ACID DISSOLVED IN AN ORGANIC SOLVENT THEREFOR, SAID ORGANIC SOLVENT BEING CAPABLE OF BEING EVAPORATED AT A MODERATE TEMPERATURE, AND SAID PHOSPHATIZING LIQUID HAVING NO AQUEOUS PHASE AND CONTAINING ONLY A SMALL PROPORTION OF WATER BASED ON ITS PHOSPHORIC ACID CONTENT.