CA1160824A - Coating composition and method - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

An aqueous acidic composition provides improved corrosion resistance to a metal, e.g., ferrous, zinc or aluminum surface upon contact. The composition contains dissolved zirconium with or without hafnium and fluoride and preferably a vegetable tannin compound.

Description

Canadi~n Application No. 321,927 broadly discloses an acidic aqueous composition containing both dissolved hafnium and fluoride in amounts sufficient, when contacted with a metal surface, to impart corrosion resistance to the metal surface, and wherein the dissolved hafnium and fluoride are each present in an amount of at least 1 ppm, and the pH value of the composi tion is on the acid.
There is also disclosed a process for forming a corrosion resistant paint receptive coating on a metal surfa~e by contacting the surface with the composition defined above, for a time sufficient to produce a coating thereon.
An object of the present invention is to further improve the art of treating metal surfaces for example 9 ferrous, zinc, or aluminum and the method of treating metal sur~aces to produce an adherent corrosion resistant coating thereon which is receptive to organic or siccative coatings.
In the treatment of aluminum surfaces~ and particularly the surfaces of drawn and ironed aluminum beverage containers, it is important to provide the surfaces of the container with a protective corrosion resistant coating which is substantially colorless in nature and does not impair the taste characteristics of the food or beverages coming in contact with the coating. It is also important that the coating be adherent and receptive to subsequently applied finishes such as paint, varnish, lacquer, etc. to the coated surface. In normal practice, after treatment of the aluminum container, the exterior of the can is decorated and overvarnished on the sidewalls thereof but the exterior bottom

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of the container receives no organic finish. Accordingly, the only protection afforded to the exterior bottom of the container is the chemical coating.
It is conventional practice after the containers are filled with a beverage such as beer, for example, and sealed, to pasteurize the sealed containers in order to destroy bacteria.
This pasteurization process conventionally comprises immersing the filled and sealed cans in WatQr heated at about 150 to about 160F for a period of about 30 minutes. The pasteurization treat-ment does not affect the overvarnished sidewalls of the containerbut the unvarnished exterior bottom of the container has in many instances undergone severe discoloration during pasteurization which is highly objectionable.
It is also conventional for quality control to subject spot samples of the chemically treated containers to a high temp-erature test to make certain that an adequate chemical coating has been ~ormed thereon. This test usually comprises placing a treated container in a muffle furnace at 1000F~ for a pericd of 5 minutes, Evidence o~ a satisfactory coating is visually ascertained by the formation of a dark gold color. Coatin~s of the type heretofore known have in many instances failed to produce a satisfactory visual color change during the muffle furnace test to enable accurate quality control determination.
It is also desirable in the chemical treatment of such containers that the chemical coating produced is substantially colorless to avoid detracting from the subsequently applied decorative coatings and varnish. Many of the coating systems in accordance with prior art practice result in coatings of a light yellow color which is objectionable, particularly, when the treatment o~ the containers in the coating solution is prolonged due to line stoppages or the like.
The aqueous acidic coating composition and method of the present invention overcomes man~ of the problems associated with prior art compositions and practices achieving a substantially colorless, adherent corrosion protective coating on aluminum surfaces which is receptive to subsequently applied organic finishes and which composition and method is effective for forming a coating of the requisite thickness in comparatively short time periods thereby achieving increased throughput and efficiency in rnetal processing.
The beneflts and advantages of the present invention are achieved in accordance with the composition aspects of the present invention by forming an aqueous acidic treating com-position containing as its essentia:L constituents, dissolved zirconium ions, fluoride ions and, preferably, a bath soluble vegetable tannin compound present in amounts effective to produce a corrosion resistant adherent coating on aluminum surfaces. The zirconium ions are present in an amount of at least about 1 part per million ~ppm) to amounts as high as 5000 ppmor greater, the fluoride ions are present in an amount ranging from at least about 1 ppm up to about 6000 ppm or ~reater, and the vegetable tannin when it is a constituent of the composition is present in an amount ranging from about 1 ppm, preferably at least about 25 ppm, up to a level corresponding to the solubility of the tannin compound in the aqueous acidic solution.

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m e trea-ting solutions of the present in~ention must be adjusted in pH value to yield a p~I on the acid side. Best results are obtained at pH values of less than 5 and a pH
value is preferably at least 2. When the metal ion in the bath comprises zirconium or predominantly zirconium, a p~I of about 3 to about 4.5 is preferred.
In accordance with a preferred embodiment of the present invention, phosphate ions such as introduced by monoammonium phosphate are incorporated in the aqueous acidic treating solution which effectively inhibits discoloration or yellowing of the chemical coating in spite of prolonged treat- -ment times of the aluminum surface at high bath temperatures.
It is also contemplated that additional metal ions such as titanium, lithium, or mixtures thereof, can be employed in the 1$ bath. The presance of such optional metal ions, however, is not necessary to achieve the benefits of the present invention.
In accordance with the process aspects of the present invention, aluminum surfaces are coated employing the herein-above described aqueous acidic coating composition by contacting cleaned surfaces with the solution at a temperature of about room temperature (70F) up to the boiling point of the solution, preferably temperatures ranging from a~out 100F to about 160F
for periods of time ranging from about 0.1 seconds up to about 10 minutes with time periods ranging from about 2 seconds to about 1 minute being more typical. The fo~mation of the coating is a function of concentration of the solution, temperature and contact time such that as the temperature and/or concentration of the solution is increased, the contact time can be corres-pondingly reduced to achieve the requisite coating.
In accordance with the composition aspects of the present invention, the aqueous acidic coating composition contains as its essential constituents, controlled effective amounts of zirconium ions, fluoride ions and preferably a bath soluble vegetable tannin compound. The zirconium ions can be introduced into the bath by any zirconium compound which is soluble in the aqueous acidic medium and which does not contribute deleterious components to the coating solution. For example, suitable bath soluble zirconium compounds whieh can be employed include fluozirconic aeid, ammonium and alkali metal fluozixconates, zirconium fluoride, zirconium nitrate, æirconium sulfate, or the like. The use of an alkali metal fluozireonate, such as 7 for example, potassium fluozireonate (K2Zr~6) is usually preferred in that it simultaneously introduces zirconium and fluoride ions into the bath composition. The concentration of the zirconium ion can broadly range from as low as about 1 ppm to 5000 ppm and even higher with amounts ran~ing from about 4 ppm to about 100 ppm being preferred. A particularly satisfactory concentration o~ zirconium is about 50 ppm.
The composition may also contain hafnium and the source of hafnium employed may be any hafnium compound which is soluble in the aqueous acidic medium and which ~oes not contribute deleterious components to the coating bath. Examples of available hafnium compounds are set forth in the ~Iandbook Gf _hemist1y and P~hyslcs, 55th Editiony CRC_Press, Inc., Clevelan_, Ohio (1974~. Preferred sources of hafnium are hafnium oxide and acids or salts based upon hafnium or hafnyl nitrate, fluoride or chloride. The hafnium compound should be included to pro~ide a hafnium content of at least one part per million. Preferably, the hafnium compound is present so as to supply hafnium in a concentration of between 4 and 100 parts per million.
The treatiny solution may contain only zirconium ions or mixtures of zirconium and hafnium. In accordance with a preferred embodiment of the present invention, the treating solution contains zirconiurn ions or predominantly zirconium ions. In other words, when the composition contains hafnium and zirconium, the zirconium concentration exceeds the hafnium concentration.
The fluoride ion can be introduced into the composi-tion in the form of a simple or complex fluoride compound such as hydrofluoric acid or a simple or bifluoride salt of an alkali metal or ammonium or as a comple~ fluoride acid or salt based upon an element such as boron, silicon, titanium, zirconium, and the like. The fluoride concentration can range from as low as about 1 ppm up to 6000 ppm or higher with amounts ranging from about 4 to about 100 ppm being preferred. ~ particularly satisfactory fluoride concentration is about 60 ppm. The particular fluoride ion concentration is preferably controlled in relationship to the quantity of hafnium and/or zirconium ions present. Preferably, the fluoride ion is present at a weight ratio of fluoride to zirconium of at least about 1025:1. The ma~imum fluoride ion concentration is controlled at a level below that at which an objectionable etching of the aluminum surface occurs. This maximum fluoride concentration i5 a function of the nature of the aluminum surface being treated, the temperature of the bath and the duration of the treatment time.
When a vegetable tannin i9 employed, it is preferably present in an amount of at least l ppm, and more preferably, in an amount of at least 25 ppm up to the solubility of the compound in the bath with upper concentrations of about 500 ppm being satisfactory~ Concentrations of about 70 ppm of the tannin compound are particularly satisfactory.
I~e treating solution of the present invention must be adjusted in pH value to yield a pH on the acid side.
Best results are obtained at pH values of less than 5 and the pH value is preferably at least 2. When the metal ion in the treating solution is zirconium or predominantly zirconium, the pH is preferably at least 3 up to about 4.5.
Depending on the raw material compounds employed to supply the zirconium and fluoride components, the pH value may be within an acceptable range without any further adjustment being necessary. On the other hand J if an adjustment of pH
is necessary, any of the inorganic or organic compounds commonly used for pH adjustment may be employed. Among these materials are the mineral acids including hydrofluoric, sulfuric, nitric and phosphoric acids, as well as the alkali metal and ammonium hydroxides, carbonates and bicarbonates, oxides and silicates~
Other adjuvants may be included in the composition to modify one or more of the qualities of the coating obtained with the bath of the present invention. Among these possible adjuvants are nitrate compounds, phosphate compounds~ citrate compounds and compounds containing titanium, lithium 9 or resinous materials. When employed, the adjuvants will normally be present in minor amounts.
Of the foregoing adjuvants, the bath can optionally but preferably contain phosphate ions in an amount of about 10 up to about 200 ppm, preferably frorn about 25 to about 75 ppm with amounts of about 45 to about 55 ppm being particularly satisfactory. m e inclusion of pho~3phate ions in the bath has been found to inhibit discoloration or yellowing of the chemical coating formed as a result of prolonged treatment times and also improves depth and intensity of the gold color developed ~0 on processed aluminum cans during the muf~le furnace test. The inclusion of phosphate ions in the bath, however, has been found to cause discoloration of the unvarnished exterior bottom of a treated aluminum container during the pasteurization step and it is necessary in such instances in which discoloration is un-desirable, to incorporate free fluoride ions in the bath to prevent such discoloration. Concentrations of phosphate ions below about 10 ppm are usually undesirable due to decreased inhibition of discoloration during excessive treatment times such as occasioned during line stoppages and also a reduction in the color change during the muffle furnace test. On the other hand, concentrations of phosphate ions in excess of about 200 ppm is undesirable due to the passivating effect thereof and a reduction in the coating action. Additionally, such higher phosphate ion concentrations require an increase in the free fluoride content to avoid discoloration of the treated surface during the pasteurization treatment~ Phosphate ion concentra-tions within the preferred range of about 25 to about 75 ppm provide the desired results in addition to ease of control of a bath during commercial operation.
As previously indicated, the fluoride concentration in the bath is controlled in relationship to the concentration of the zirconium ions ~with or without hafnium i~ns) present so as to provide a stoichiometric ratio of at least 6 mols fluoride for each mol of the metal ion present. The inclusion of additional fluoride in the bath to prevent discoloration during pasteurization is controlled to provide a free fluoride content as a function of the ph~sphate ion concentration. The free fluoride concentra-tion in the bath is conveniently measured by a specific fluorideion electrode in terms of millivolts ~mv) which will vary depend-ing upon the specific composition and concentration of the bath constituents and on the pH thereof. For any particular bath at a substantially constant pH, a correlation can readily be made of the millivolt reading and the fxee fluoride content which provides satisfactory bath operation and prevents discoloration during the pasteurization treatment. Such millivolt reading serves as a simple commercial control of the bath. For example, a satisfactory bath containing phosphate ions at a pH of about

3.7 is achieved by providing a free-fluoride concentration to provide a millivolt reading of about -30 mv calibrated against a standard solution measured at 0 mv containing 4907 ppm 5 H2S04(100%), 40 ppm F added as NaF and 558 F added as NH4HF2.
The appropriate millivolt reading of the free fluoride con-centration can readily be ascertained for any bath by simple experimentation to achieve the desired results~
It is also desirable to employ fluoboric acid in the bath in such instances to provide a reservoir source of free fluoride to supplement the free-fluoride concentration as it is consumed in the complexing of aluminum ions formed during the coating reaction.
A particularly satisfactory bath composition according to the present invention contains zirconium ions in a concentration of about 50 ppm, a total fluoride ion concentration of about 180 ppm, a tannin concentration calculated on a weight equivalent basis to tannic acid of about 70 ppm and a phosphate ion concentration of about 50 ppm.
The time of contact and the temperature at which the treating solution is maintained are interdependent variables.
Employing higher temperatures will normally shorten the contact time required. Furthermore, the time of contact is somewhat dependent upon the method of application employed. Typically, the duration of contact should be from 0.1 seconds to 10 minutes and is preferably between 2 seconds and 1 minute. In the treat-ment of aluminum beverage containers, for example~ production facilities and requirements normally dictate contact times ranging 2~

from about 10 seconds to about 30 seconds with 20 seconds being usualO In the treatment of aluminum articles such as extrusions, for example, longer treatment times are feasible providing for a corresponding reduction in the concentration and/or temperature of the treating solution.
The treating solution as applied to the surface to be treated may range from as low as room temperature (70F, 21C~
up to the boiling point of the solution with temperatures ranging from about 100F to about 160F (37 to 71C) being preferred.
In the treatment of aluminum containers r temperatures ranging from about 100 to about 120F are typical.

A standard or control so:Lution designated as Test Solution 1 is prepared by adding 0.28 gm K2~rF6, 3.4 ml 7~/O
nitric acid and 26 ml of a l~/o ammonium bicarbonate solution to water to yield 6 liters of Test ,Solution 1. Similar solutions are prepared in 6 liter quantities to which 0.25 gm, 0.5 gm and 0.75 gm per 6 liters of tannic acid is added to yield Test 5O1utions 2-4, respectively. Test Solutions 1 through 4 contain an equivalent of 15 ppm zirconium ions, 18.7 ppm fluoride ions and 555 ppm nitrate ions and have a pH of about 2.5. Test Solution 2 contains an equivalent of 42 ppm tannic acid; Test Solution 3 contains an equivalent of 84 ppm tannic acid while Test Solution 4 contains an equivalent of 126 ppm tannic acid.
Each of Test Solutions 1-4 is employed ~or treating aluminum containers in accordance with the previously described test procedure whereafter the treated containers are subjected to the pasteurization test employing the TR-4 water solution at 2~

a temperature of 165F for a time period of 1 hour and 2 hours, respectively. The results obtained from these tests are set forth in Table 1.

TR-4 Pasteuriza~ion Test Results TestTannic Acid TR-4 Test Ratinq Solution ppm 1 Hr. at 165F2 Hrs. at 165F

4 126 It is clear from the results as set forth in Table 1 that the Test Solution 1 comprising the control and devoid of any tannic acid underwent a dark go:Ld discoloration during the pasteuriz~tion test resulting in a :L0 rating. On the other hand, Test Solutions 2-4 containing varying amounts of tannic acid evidenced no or little discoloration evidencing the formation of a commercially acceptable coating.

Six liters of a test solution designated as ~o. 5 is prepared containing 12~75 ppm zirconium ions, 123.6 ppm fluoride ions, 67.5 ppm tannic acid~ 124.5 ppm nitrate ions, 14.7 ppm boron ions, and 20.5 ppm of a chelating agent based t~a d~ in ~ rl~
on ethylenediaminetetraacetic acid sold under the~ n~-~r Versene. An identical solution of the same composition designated as Test Solution 6 is prepared to which 0.36 grams per 6 liters of NH~H2PO4 is added to provide a phosphate ion con-centration of about 50 ppm.

Both Test Solutions 5 and 6 have a pH of 3.78.
Cleaned aluminum cans in accordance with the afore-mentioned process sequence are treated in Test Solutions 5 and 6 for a period of 20 seconds at 120F after which they are dried. The surfaces of the treated containers are visually inspected to evaluate any noticeable color on the aluminum surface and thereafter are subjected to a muffle furnace test for a period of 5 minutes at 1000F.
m e containers treated in accordance with Test Solu-tion 5 evidenced a very slight pale yellow appearance in the formed coating and were of a very pale yellow upon removal from the muffle ~urnace~ In contrast, the containers treated with Test Solution 6 exhibited no discernible color in the formed coating and produced a deep gold color upon extraction ~rom the mu~fle furnace- These tests evidence the advantages obtain~d by the addition of controlled quantities of phosphate ions in accordance with a preferred embodiment of the present invention in preventing coating discoloration when subjected to prolonged treatment times and also the formation of a discern-ible discoloration ~or quality control purposes of the depositedcoating when subjected to the muffle furnace test. The 20 second treatment time employing Solutions 5 and 6 at the concentrations of the constituents and the temperature employed is considered excessive in that satisfactory coatings can be fonmed in time periods of as little as 10 seconds.

~ 2 A control solution is prepared in accordance with an embodiment of the present invention devoid of any phosphate ions containing 0.125 g/l nitrate ions, 0.015 g~l boron, 0~02 g/l Versene sequestrant, 0~04 g/l ammonia ions, 0.068 gjl tannic aeid and sufficient potassium zirconium fluoride salt and hydrofluoric acid to provide a zirconium ion concentration of 0.013 gJl, potassium ions of 0.01 g/l and 0.124 g/l fluoride ions. The pEI of the control Test Solution 9 is adjusted with ammonium bicarbonate to a nominal pH ranging from 3.7 to 3.8 and averaging 3.75. Control Test Solution 9 is spray applied for a period of 20 seconds at 100F to aluminum containers and thereater is subjected to a TR-4 Pasteurization Test for a period of 30 mimltes at a temperature of 155F to evaluate bare corrosion resistance and discoloration. The TR-4 test results reveal a colorless coating after the TR~4 test having a rating of 1. However, the control Test Solution 9 is susceptible to forming a light yellow eolor on the aluminum container as a result of excessive treating times and also does not provide 2~

a deep, distinct color on the container during the muffle furnace test. As previously indicated, the addition of controlled amounts of phosphate ions inhibits coating discoloration in spite of excessive treatment times and also provides a deep distinct gold color during the muffle furnace test. ~t the same time, however, the addition of such phosphate ions detracts from the TR-4 Pasteuri~ation Test results causing discoloration in many instances.
In order to evaluate the effect of two different levels of phosphate ion concentrations in control Solution 9 and the effect of the addition of supplemental zirconium and/or fluoride ions to the bath, Test Solutions 9.1 through 9.5 are prepared. Test Solution 9.1 is identical to Test Solution 9 but further contains the addition of 25 ppm and 100 ppm phosphate ions. Test Solutlon 9.2 is identical to Test Solution 9~1 but further conta:ins 0.12 g/l of potassium zirconium fluoride. Test Solution 9.3 is identical to Test Solution 9.1 but further contains 0.18 g/l of zirconium nitrate pentahydrate to provide a zirconium ion concentration identical to that in Test Solution 9.2. Test Solution 9.4 is identical to Test Solution 9~3 but further contains 0.05 g/l hydrofluoric acid to provi~e additional free-fluoride concentration in an amount equal to the additional fluoride ions added to Test Solution 9.2~ Finally, Test Solution 9.5 is identical to Test Solution 9.1 but further containing 0.05 g/l of 10~,' hydro-fluoric acid equivalent to that added to Test Solution 9.~.
Each of Test Solutions 9.1 through 9.5 is employed for treating the bottoms of aluminum containers by spray application for a period of 20 seconds at a temperature of 100F. The treated container ~ottoms are thereafter subjected to a TR~4 Pasteurization Test for a period of 30 minutes at 155F in a manner identical to that employed on the container bottoms treated with control Test Solution 9. The TR-4 test results and the Millivolt readings of the Test Solutions as indicative of free-fluoride ion concentration are set forth in Table 3.

TR-4_PAST2URIZATI0~ TEST RESULTS
Test Solution No.
Solution 9 Avg. Rating rnv Reading -8 Phosphate ion concentration, ppm 25 pprn 100 ppm Solution 9.1 Avg. Rating8.5 8 mv Reading -7 -11 Solution 9.2 Avg. Rating3 6~5 mv Reading-16 -25 Solution 9.3 Avg. Rating10 10 mv Reading+14 +14 Solution 9.4 Avg. Rating3.5 5.5 mv Reading+1 -9 TABLE ~ (continued) Phosphate ion concentration, ppm 25 ppm 100 ppm Solution 9.5 Avg~ Rating 2 4.5 mv Reading -40 -35 It i.s clear from the results of Table 3 that the addition of 25 and 100 ppm phosphate ions to control Test Solution 9 as evidenced by the ratings obtained on Test Solu-tion 9.1 results in an unacceptable discoloration of the container bottoms producing ratings of about 8. The addition of additional zirconium and fluoride ions to such solution as evidenced by the results obtained on Test Solution 9~2 ~ffects an improvement in the TR-4 results at the 25 ppm phosphate ion level but remains unacceptable at an average rating of 6.5 at -the 100 ppm phosphate ion concentration level~ The addition of an equivalent amount of zirconi~m ions as evidenced by the results obtained on Test Solution 9.3 to the additional zircon-ium ions added to Test Solution 9.2 produces TR-4 test results which are entirely unacceptable at average ratings of 10. On the other hand, the further addition of free-~luoride in combina-tion with zirconium nitrate as represented by Test Solution 9.4 produces a distinct improvement providing an everage rating of 3.5 at the 25 ppm phosphate ion level and a rating of 5.5 at the 100 ppm phosphate ion level. By the addition o~ only free-fluoride as evidenced by Test Solution 9.5, very acceptable TR-4 results are obtained at an average rating of 2 at the 25 ppm -18~ g~
phosphate ion concentration level while a rating of 4.5 is obtained at the higher phosphate ion concentration.
These data clearly substantiate the necessity of providing a controlled free-fluoride concentration in treating baths also containing phosphate ions to counteract the dis-coloration effect of such phosphate ions during a TR-4 Pasteuri-zation Test of the treated aluminum surface. A Millivolt reading of -40 of the specific bath composition represented by Test Solution 9.5 provides a control for achieving coatings which will satisfactorily pass a TR-4 Pasteurization Test in terms of free-fluoride concentration. This Millivolt reading compares to a Millivolt reading of -8 on control Test Solution 9 devoid of any phosphate ions and which contains fluoride ions which for the most part are complexed with the zirconium ions and boron ions present in the bath.
EXAMPLE _ The interrelationship of tannic acid concentration and treatment times to achieve coatings having a satisfactory TR-4 test performance is demonstrated by the Test Solutions o~ this examp]e. A control Test Solution desi~nated as 10 is prepared containing 25 p~m zirconium ions, 138.9 ppm fluoride ions, 25 ppm phosphate ions, 124.5 ppm nitrate ions, 14.7 ppm boron and 19~5 ppm ~ersene ~a chelating agent ~ased on ethylenediaminetetraacetic acid). The pH of the bath is adjusted to a value of 3 7. A
series o~ Test Solutions based on control Solution 10 is prepared incorporatin~ varying amounts of tannic acid. Test Solution 10.1 contains 17 ppm tannic acid, Test Solution 10.2 contains 33 ppm tannic acid, Test Solution 10~3 contains 50 ppm tannic acid and Test Solution 10.4 contains 66 ppm tannic acid.
Each of Test Solutions 10 through 10.4 is employed for treating aluminum cans by spray application at a solution temperature of 115F at alternate processing times of 10 and 20 seconds, respectively. Following each treatment, the coated cans are cold water rinsed ~or a period of 15 seconds followed by a 5 second deionized water rinse and are oven dried at 380F
for a period of 5 minutes. Each of the treated aluminum cans is subjected to a TR-4 test procedure for a period of ~0 minutes at 165F. The test results obtained are set forth in Table 4.

Test Tannin T~-4 Ratin~
SolutionConc;, p~m 10 Seconds 20 Seconds 10.1 17 5 10.2 33 10.3 50 10.4 6~ 1 1 It is evident from the data presented in Table 4 that Test Solution 10 devoid of any tannic acid produced unacceptable ratings due to the severe discoloration incurred at treating periods of both 10 seconds and 20 seconds. At a tannic acid concentration of only 17 ppm as represented by Test Solution 10.1, no discoloration of the coating was observed at the conclusion of the TR-4 test at a treating time of 20 seconds whereas an unacceptable rating of 5 was obtained for this same solution at a treatment time of only 10 seconds. ~t the higher Z~

tannin concentrations as represented by Test Solutions 10.2, 10.3 and 10.4, no discoloration occurred during the TR-4 test.
The foregoing data clearly substantiate the effectiveness of the tannin constituent in an operating bath and the faet that very low concentrations thereof do provide a significant improvement but require substantially longer treating times in order to achieve a coating resistant to discoloration on TR-4 testing equivalent to that obtained at higher tannin concentra-tions.
The operating bath is conveniently prepared employing a make-up concentrate containing the several constituents in appropriate amounts whieh can be diluted with water to the final desired operating concentration. The broad useable as well as preferred concentrations of a bath make-up concentrate is set forth in Table 5.

Pe:rcent by Wei~_ Constltuent Broad Preferred Zr with or without Hf 0.01 - 2.0 0.15 - 0.25 F 0.0355 - 7.1 0.5 - 0.9 NO-3= 0.025 - 5.0 0.35 - 0.65 P04 0.009 - 1.8 0.13 - 0.23 Boron 0.00295 - .59 0.04 - 0.08 EDTA 0.0039 - .78 0.06 - 0~1 Tannic Acid 0.0135 - 2.7 0.2 - 0.35 NH3 0.0125 - 2.5 0.2 - 0.3 The make up concentrate is usually in concentration so as to provide a dilution thereof employing one part con-centrate and 39 parts water to form an operating bath contain-ing 2 1/2% of the concentrate~ The zirconium and hafnium ions are preferably introduced in the form of potassium zirconium fluoride and hafnyl fluoride, respectively 9 which concurrently supplies some of the fluoride ions in the bath. The remaining fluoride concentration is preferably introduced in the form of hydrofluoric acid and a 49% aqueous solution of fluoboric acid (HBF4)- The phosphate ions are preferably introduced in the form of monoammonium phosphate and the ammonium ions as indicated in Table 5 are usually present as a result of the use of ammonium hydro~ide for pH adjustment. The tannic acid constituent can be introduced as such or as tannin extracts employing a quantity to provide a weight equivalent basis equal to that of tannic acid.
The EDTA or equivalent complexing or sequestering agent is advantageously employed and will vary in amount depending upon the hardness of the tap water employed in formulating the operat-ing bath. ~he sequesterant is effective to complex hard water salts including calcium, magnesium, iron 9 etc. ions present in the make-up water.
A particularly satlsfactory make-up concentrate for dilution to a final concentration of 2.5% contains 0.2% zirconium ions, 0.71% total fluoride ions, 0.5% nitrate ions, 0.18%
phosphate ions, 588 ppm boron, 0.078% EDTA ~ersene), 0~27%
tannic acid and 0.25% ammonia. Such concentrate when diluted with deionized water exhibits a pH of about 3.1. Upon dilution, the zirconium ion concentration in the operating bath is about 50 ppm.
Similarly, concentrates containing the bath con-stituents to effect a replenishment thereof during use can satisfactorily be prepared which are added directly to the operating bath without prior dilution.

Claims (29)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. An acidic aqueous composition containing in dissolved form 1) zirconium and 2) fluoride, in amounts sufficient, when contacted with a metal surface, to impart corrosion resistance to the metal surface, wherein the zirconium and fluoride are each present in an amount of at least 1 ppm, and the pH value of the composition is on the acid side.

2. An acidic aqueous chromium-free composition of pH
value less than 5 containing in dissolved form zirconium, fluoride and a vegetable tannin in amounts, at least 1 ppm of each, sufficient, when contacted with a metal surface, to impart corrosion resistance to the metal surface.

3. The composition of claim 2 in which said zirconium is present in an amount of about 4 to about 100 ppm.

4. The composition of claim 2 in which said fluoride is present in a weight ratio of F:Zr of at least 1.25:1.

5. The composition of claim 2 in which said tannin is present in an amount of at least 25 ppm calculated as a weight equivalent to tannic acid.

6. The composition of claim 2 in which said tannin is present in an amount up to about 500 ppm calculated as a weight equivalent to tannic acid.

7, The composition of claim 2 additionally containing phosphate ions in an amount of about 10 to about 200 ppm.

8. The composition of claim 7 in which said phosphate ions are present in an amount of about 25 to about 75 ppm.

9. The composition of claim 7 in which said phosphate ions are present in an amount of about 45 to about 55 ppm.

10. The composition of claim 7 containing about 50 ppm zirconium, about 180 ppm total fluoride, about 70 ppm tannin calculated on a weight equivalent to tannic acid, about 50 ppm phosphate ions, said composition of a pH value of about 3 to about 4.5.

11. The composition of claim 2 additionally containing nitrate ions.

12. The composition of claim 2 additionally containing a titanium compound.

13. The composition of claim 2 additionally containing a lithium compound.

14. The composition of claim 2 additionally containing a boron compound.

15. The composition of claim 14 in which said boron compound comprises fluoboric acid.

16. The composition of claim 2 additionally containing a sequestering agent present in an amount sufficient to complex at least a portion of the hard water salts including calcium, magnesium and iron.

17. The composition of claim 16 in which said sequestering agent comprises EDTA.

18. The composition of claim 2 additionally containing hafnium 9 wherein the zirconium concentration exceeds the hafnium concentration.

19. An aqueous concentrate adapted to be diluted with water to form an operating bath for treating metal surfaces comprising an aqueous solution containing on a weight percent basis about 0.1 to about 2% of zirconium ion, about 0.0355 to about 7.1% fluoride ions 9 about 0.025 to about 5% nitrate ions 9 about 0.009 to about 1.8% phosphate ions, about 0.00295 to about 0.59% boron, about 0.0039 to about 0.78% EDTA, about 0.0135 to about 2.7% of a tannin compound calculated as tannic acid equivalent, and about 0.125 to about 2.5% ammonia.

20. The aqueous concentrate as defined in claim 19 contain-ing about 0.15 to about 0.25% of said zirconium ion, about 0.5 to about 0.9% fluoride ions, about 0.35 to about 0.65% nitrate ions, about 0.13 to about 0.23% phosphate ions, about 0.04 to about 0.08% boron, about 0.06 to about 0.1% EDTA, about 0.2 to about 0.35% of said tannin compound and about 0.2 to about 0.3%
ammonia.

21. The aqueous concentrate as defined in claim 19 containing about 0.2% of said zirconium ion, about 0.71%
fluoride ions, about 0.5% nitrate ions, about 0.18%
phosphate ions, about 0.0588% boron, about 0.078% EDTA, about 0.57% of said tannin compound and about 0.25%
ammonia.

22. An aqueous concentrate adapted to be diluted with water to form an operating bath for treating metal surfaces comprising an aqueous solution containing on a weight percent basis about 0.1 to about 2% of zirconium ion, about 0.0355 to about 7.1% fluoride ions, and about 0.0135 to about 2.7% of a tannin compound calculated as tannic acid equivalent.

23. The concentrate as defined in claim 22 further containing about 0.025 to about 5% nitrate ions.

24. The concentrate as defined in claim 22 further containing about 0.009 to about 1.8% phosphate ions.

25. The concentrate as defined in claim 22 further containing about 0.00295 to about 0.59% boron.

26. The concentrate as defined in claim 22 further containing about 0.0039 to about 0.78% of a sequestrant.

27. The concentrate as defined in claim 26 in which said sequestrant comprises EDTA.

28. The concentrate as defined in claim 22 further containing about 0.125 to about 2.5% ammonia.

29. A process for forming a corrosion resistant paint receptive coating on a metal surface comprising contacting the surface with the composition of claim 1, 2 or 18.