US3895970A - Sealing rinse for phosphate coatings of metal - Google Patents

Abstract

This invention provides a new sealing rinse and process for sealing phosphate coated metals against porosity and to promote the adhesion of subsequent paint coatings. The new sealing rinse is an acidic aqueous solution containing simple or complex fluoride ions. In the application of the sealing rinse, steel, galvanized steel, aluminum or zinc are given a phosphate coating and then wetted with an acid aqueous solution containing fluoride ion.

Description

United States Patent [1 1 Blum et al.

[451 July 22, 1975 SEALING RINSE FOR PHOSPHATE COATINGS OF METAL [73] Assignee: Pennwalt Corporation, Philadelphia,

[22] Filed: June 11, 1973 [21] Appl. No.: 365,019

[52] US. Cl. l48/6.l5 R; 148/6.l6 [5]] Int. Cl. C23f 7/08 [58] Field of Search l48/6.l5 R, 6.15 Z, 6.16

[56] References Cited UNITED STATES PATENTS 2,795,5[8 6/1957 Carroll et al 148/6.16

2.798,829 7/1957 Newhard et al. l48/6.l6 X

Primary Examiner-Ralph S. Kendall Assistant Examiner-Charles R. Wolfe, Jr. Attorney, Agent, or FirmRobert G. Danehower [57] ABSTRACT This invention provides a new sealing rinse and process for sealing phosphate coated metals against porosity and to promote the adhesion of subsequent paint coatings. The new sealing rinse is an acidic aqueous solution containing simple or complex fluoride ions. In the application of the sealing rinse, steel, galvanized steel, aluminum or zinc are given a phosphate coating and then wetted with an acid aqueous solution containing fluoride ion.

5 Claims, No Drawings SEALING RINSE FOR PHOSPHATE COATINGS OF METAL BACKGROUND OF THE INVENTION Phosphate coatings on metals are widely known as useful adhesion promoters for paint, varnish, lacquer and the like, and their application is one of the standard procedures of the metal finishing industry. Besides adhesion, the phosphate coatings also provide some protection against underpaint corrosion, but normally not enough. It was found long ago that the underpaint corrosion protection of phosphate coatings is greatly enhanced when the phosphate coated metal is wetted with a dilute acid chromate rinse solution prior to paint application, and almost every proprietary phosphate coating process specifies these chromate rinses which are also called sealing rinses or chromate seals.

An early disclosure of chromium salts used as sealing rinses is contained in US. Pat. No. 2,067,215 granted in 1937. Later developments in aqueous sealing rinses for phosphate coated metals combined hexavalent chromium and a fluorine bearing compound derived from hydrofluoric acid or its simple salts as disclosed in US. Pat. No. 2,798,829.

I Complex fluorides such as the fluosilicates, fluoborates, fluozirconates, fluotitanates and fluostannates were combined with hexavalent chromium in aqueous solution and used as a sealing rinse as disclosed in US. Pat. No. 2,795,518.

These chromate sealing rinses have two disadvantages. Since chromates are toxic to animal life, depleted chromate solutions must now be treated before they can be discharged into streams or waste systems to render them non-toxic. A major short-coming of the chromate rinse is that uneven accumulations of the chromate rinse, when dried on the metal surface, cause discloloration under the paint or cause blister-type failures of the painted surfaces. Water rinsing alleviates the blistering but removes most of the chromate coating along with most of its sealing ability.

The fluoride sealing rinses which are the subject of this invention do not contain chromates and hence do not suffer from the above disadvantages. The fluoride sealing rinse also has the advantage that the chemical ingredients are relatively inexpensive.

BRIEF DESCRIPTION OF INVENTION We have now discovered that acidic aqueous solutions of certain fluoride ions when used to wet the surface of phosphate coated metals such as steel, galvanized steel, zinc and aluminum are effective sealing agents for phosphate coated metals. The fluoride sealing rinses provided by this invention perform as well as the conventional chromate sealing rinse used in the prior art for the same purpose.

The fluorides useful in forming the fluoride sealing rinses of our invention are obtained from calcium fluoride, zinc fluoride, zinc aluminum fluoride, titanium fluoride, zirconium fluoride, chromic fluoride, chromic zirconium fluoride, nickel fluoride, ammonium fluoride, hydrofluoric acid and fluoboric acid. Mixtures of one or more fluorides can also used.

The useful concentration of the above fluorides in water as sealing rinses ranges from about 0.01 gram/- liter to about 25 grams/liter expressed as fluoride. De ionized water is a preferred source of water. The fluorides are useful as sealing rinses for phosphate coated metals at a pH ranging from about 3 to about 7. The application temperature of the sealing rinses may range from ambient to about 200F. The time of contact with the metal can range from about 2 seconds to about 1 hour.

After the fluoride sealing rinses are applied to the phosphate coated metal, the metal may then be washed with water, preferably deionized water an then the metal is dried. Preferably, the metal wetted with the fluoride sealing rinse is dried without any water rinse, or the metal wetted with the fluoride sealing rinse is first dried, then rinsed with water, preferably deionized water and then redried. The latter two procedures give better under-paint corrosion resistance than the first procedure embodying the water rinse prior to drying. The drying of the rinsed panels is done by any conventional way currently employed in the metal treating industry. The drying temperatures are not critical and will vary from room temperature to about 180F. or higher as measured on the treated metal surfaces.

The phosphate coated metals which have been sealed with our fluoride sealing rinse are then ready for the application of siccative organic coatings such as plastic coatings, paints, enamels and the like.

DETAILED DESCRIPTION OF THE INVENTION The fluoride rinses of our invention are useful in sealing phosphate coatings on any metal substrate which carries a phosphate coating to increase resistance to corrosion and to enhance the bonding of paint or lacquer coatings. Generally, the substrate will be steel, galvanized steel, zinc or aluminum. The metal substrates must be clean for application of the phosphating solutions and the metal surfaces are first cleaned by physical and/or chemical means well known in the art to remove surface dirt, grease or oxides.

The fluoride sealing rinses of this invention are applicable to either the heavy-weight coating phosphates derived from aqueous zinc or manganese phosphate solutions or the light-weight phosphate coatings, generally called iron phosphate coatings, derived from aqueous solutions of acid sodium, potassium, and ammonium phosphates. The heavy-weight zinc phosphate coatings may be prepared and applied to steel, galvanized steel, zinc and aluminum metals as is disclosed in U.S. Pat. Nos. 3,203,835 and 3,619,300 and these patents are incorporated by reference. The use and application of the iron phosphate coatings to these metals are shown in US. Pat. Nos. 3,l29,121 and 3,152,018 and these patents are incorporated by reference.

The phosphating solutions described above are applied by immersion, spraying, wiping and/or by roller coating as is well known in the art. After application of the phosphate coating, the coated metal is rinsed with water and then wetted with the fluoride sealing rinse of our invention. If it is necessary to store the phosphate coated metal before sealing, it is preferable to dry the water rinsed metal rinsed metal by conventional methods.

The fluorides useful in sealing the phosphate coated metals are obtained from calcium fluoride, titanium fluoride, zirconium fluoride, chromic fluoride, chromic zirconium fluoride, nickel fluoride, ammonium fluoride, hydrofluoric acid and fluoboric acid. Mixtures of the above fluorides are also useful.

When the fluorine acids are used as the source of fluoride ion, the acids must be adjusted with a base to bring them within the proper pH range. Ammonia and water soluble amines such as triethanolamine and nmethyl morpholine are desirable for this purpose since the amines are volatilized at the paint curing temperatures.

The reason why the fluorides are effective in the application is not wholly understood at the present time. We assume that some complex fluoride-containing compounds formed on the surface of the zinc phosphate crystals, making them less soluble, similar to the effect fluoride has on calcium phosphates. It seems that in order to be effective in a rinse, the cationic part of the fluoride has to be either volatile like ammonium or amines (i.e., volatile at or below the curing temperature of the paint used) or it has to be able to form insoluble phosphates on the coated surface, which all other effective cations do. Some other fluorides, like lithium, sodium or potassium fluoride, are not only not effective, but are in fact detrimental to the performance of the phosphate coating.

The fluoride materials useful in forming the sealing rinses of our new process are obtainable from commercial sources or else they can be prepared by processes well known in the art.

The sealing rinses are prepared by adding the fluorine containing material to water. If the fluorine containing material is a solid, agitation of the water will assist in solution of the material. For best sealing results deionized water is preferred because of the absence of water impurities which could interfere with the sealing process.

The pH of the sealing rinse is adjusted within the range of about 3 to about 7. A preferred pH range is from about 4 to about 6 for the metal fluorides and fluorine acids while the preferred range for ammonium fluoride is about 4 to about 7. Adjustments in pH can be made with suitable acids and/or bases such as ammonia, zinc oxide, n-methyl morpholine, triethanolamine, hydrofluroic acid fluoboric acid.

The concentration of the fluoride in the sealing rinse and the thickness of the fluoride coating on the metal surfaces will be the same as for the conventional chromic acid sealing rinse and coating as taught in US. Pat. Nos. 3,116,178 and 2,882,189 and the teachings of these patents are incorporated by reference. Generally, the concentration will vary from about 0.01 grams to about 25 grams per liter expressed as fluoride. Higher amounts than 25 grams/liter can be used without any significant benefit in corrosion prevention or paint adhesion.

The time of contact or wetting of the sealing rinse with the phosphated coated metal will vary with the temperature of the solution, the type of phosphate coating and the desired thickness of the fluoride coating. Contact or wetting time may vary from 2 seconds as in spraying to about 1 hour as by immersion. Coating weight will range from less than about 0.5 milligrams per square foot to as high as about 10 milligrams per square foot. The coating weight can be influenced by many factors including the nature of the phosphate undercoat. A heavy zinc phosphate coating will absorb more of the fluoride rinse than an iron phosphate coating. Also, if the fluoride rinse is followed by a water rinse without drying of the fluoride rinse, the coating weight will be quite low.

The temperature of the sealing rinse may range from room temperature to boiling temperatures, that is, from about F. to about 212F. Where the sealing rinse is to be dried on the phosphate coating without intermediate water rinse, a hot fluoride solution will leave a substantial amount of heat in the metal which will assist in the drying operation. Bath temperatures of about to F. are generally encountered in the processing lines and this range is preferred.

The best manner of applying the fluoride sealing rinses is to contact the phosphate coated substrate with the sealing rinse, dry the metal wetted with fluoride containg solution, then water rinse the sealed phosphate coating, preferably with deionized water, and then redry the metal. This manner of sealing the phosphate coatings gives the highest ratings in the salt spray and paint adhesion tests.

In another sealing procedure the fluoride rinse is allowed to dry without any water rinse. In still another finishing procedure, the fluoride sealing rinse is followed by a water rinse, preferably a deionized water rinse and then the metal is dried. This last procedure gives the lowest ratings in the salt spray and paint adhesion tests.

The fluoride sealing rinse may be sold as an aqueous acid concentrate which can be diluted with water for application to the phosphate coated metal substrate. These concentrates will contain from 20 to about 200 grams per liter of fluoride ion.

The best mode of carrying out our invention will be apparent from a consideration of the following exam ples. In these examples the resistance to under-paint corrosion was observed by salt spray tests as set forth in A.S.T.M. Standard B117 and A.S.T.M. D1654 evaluation procedure. In both the salt spray test and underpaint corrosion tests referred to above, the phosphate coated metal is treated with the fluoride sealing rinse after which the metal is painted before being tested. In the A.S.T.M. D1654 rating, a scale from 0 to 10 indicates the severeness of effect of the salt spray on the panels. Zero indicates a complete failure, ten a completely unimpaired panel.

EXAMPLE 1.

Mild steel panels (SAE-l0l0) were cleaned in a proprietary alkaline hot soak cleaner, water rinsed, spray coated with a proprietary chlorate accelerated zinc phosphate coating compound and water rinsed again. The coated panels were then divided into three equal parts. The first part was dried immediately. The panels of the second part were rinsed with a proprietary chromate rinsing compound before drying. This compound consisted mainly of acid calcium chromates, having a CrO (crhomic acid) concentration of 0.47 g./l. The third part of the panels received a rinse of chromic fluoride (CrF before drying. This rinse was adjusted to a concentration of 0.27 g./l. fluoride. The rinse solution was prepared by placing an excess of a commercial grade chromium trifluoride (CrF .3 /2 H O) into cold water, stirring for three days and then filtering the solution from the undissolved excess. The concentrated solution contained 43.6 g./l. Cr (chromium) and 47.9 g./l. F (fluorine) and was diluted further prior to application.

The panels were then painted with one coat of a proprietary high bake alkyd enamel spray paint and exposed to 5% salt spray according to A.S.T.M. B117. After 240 hours, the panels were inspected and rated according to A.S.T.M. D1654 with the results shown in Table l.

Mild steel panels were cleaned as above. One series of panels was coated with the chlorate accelerated zinc phosphate compound of the first example. A second series was coated with a proprietary nitrite accelerated zinc phosphate coating compound. The coated panels were again divided into three parts. The first part received only a deionized water rinse prior to drying. The second and third parts were again rinsed respectively with the chromate and chromic fluoride rinse of the first example, but this time one-half of the panels were washed off with deionized water immediately following the rinse application and then dried. On the other half, the rinse was dried on first and then rinsed with deionized water and then dried again. Finally the panels were painted and tested as in Example 1. The observations are set forth in Table 2.

Table 2 HF were adjusted to a pH of 5.5; all the other ones either were at a pH of 4.2-4.5 or rinses TiF ZrF and Cr (ZrF were adjusted to this pH with zinc oxide. The results appear in Table 3.

Table 3 Result of Salt Spray Rated According to A.S.T.M. Dl654 Mild steel panels were cleanedand coated. withpthe proprietary nitrite accelerated zinc phosphate coating compound used in Examples 2 and 3. After water rinse, again some control panels were dried immediately without any sealing treatment. The other panels were rinsed with chromic trifluoride solutions of a variety of concentrations. One-third of the panels was washed with deionized raw water immediately after coming out -of the fluoride solution. In the second group of panels,

Chlorate Accelerated Coating Nitrite Accelerated Coating Sealing Rinsed lmmedi Rinsed After Rinsed lmmedi- Rinsed After Rinse ately with De- Drying with ately'with De- Drying with Deionized H 0 and Deionized ionized H 0 and ionized H O Dried H 0 and Re Dried and Redried dried None 5.5 5.0 Chromate 5.5 5.0 5.0 Fluoride X 5 7.5 9.5

EXAMPLE 3. the chromic fluoride sealing rinse was dried on the pan- Mild steel panels were cleaned as before and coated with the nitrite accelerated zinc phosphate compound of the second example. After water rinsing, the followingsealing rinses were applied: NH F, HF, CaF ZnF AlF TiF... ZrF Cr (ZrF NiF The concentrations were 0.27 g./l. fluoride, except rinse CaF (calcium fluoride) which was a saturated solution (0.016 g./l.). After application, the sealing rinse was washed immediately with deionized water, the panels dried, painted and salt spray tested as in Example 1. Rinses NH F and els and then they were rinsed with deionized water. In the third set of panels the fluoride sealing rinse was dried without any water rinse. The pH of the rinse solutions at the various concentrations are shown in C01- umn 2 of;Table 4.

After the steel panels were treated as described above, the panels were spray painted as in Example 1 and one-half of the panels were exposed to salt spray as in Examples 1 to 3. The other half was exposed for 500 hours to humidity at F. The results appear in Table 4.

Table 4 Chromic Fluoride Sealing Rinse Sealing Rinse Results of Salt Spray Rated According to A. S.T.M. Dl654 Concentration Panels washed Sealing Rinse Dried Sealing Rinse Table 4 Continued Chromic Fluoride Sealing Rinse Sealing Rinse Results of Salt Spray Rated According to A.S.T.M. D1654 Concentration Panels wmahed Sealing Rinse Dried Sealing Rinse grams/liter pH immediately with on panels Ind then dried on Panels. deionized water rinsed with deionized No water 9inse water Result Humidity (A.S.T.M. D714):

All 10 (no blisters) except the control panels, which were 9F (few very small blisters).

EXAMPLE 5.

rinse. The panels were spray painted and exposed to salt spray and humidity as in the previous examples.

Hot dip galvanized steel panels were cleaned and 20 The results are shown in Table 5.

Table 5 Results of Salt Spray Rated According to A.S.T.M. D1654 Sealing Rinse Rinsed Immediately with Deionized Water and Dried Sealing Rinse Dried on Panels. No Water Rinse None (Control) C hromate Fluoride Result of Humidity Test Rated According to A.S.T.M. D7 l4 Sealing Rinse Rinsed Immediately with Sealing Rinse Dried on Deionized Water and Dried Panels. No Water Rinse None (Control) C hromate Fluoride Few to Medium Number 8 Blisters l0 No Blisters coated with a proprietary zinc phosphate coating compound in the manner of the last three examples with the exception that no accelerator was used, because galvanized steel will coat well without an accelerator. After water rinse, a few panels were dried again without further treatment for use as controls. The remaining panels were rinsed with either a. The proprietary chromate rinse of Examples 1 and 2 or,

b. The 0.25 g./l. chromic fluoride containing rinse of Example 4. One half of the so-treated panels were rinsed with deionized water immediately and then dried, the other half was dried without a final water EXAMPLE 6.

Table 6 High Quality Steel Sealing Rinse Rinsed Immediately with Sealing Rinse Dried on Sealing Rinse Dried on Component Concentration pH Deionized Water and Dried Panels and Then Rinsed Panels. No Water Rinse Grams/liter With Deionized Water None (control) 6.5

NH,.F 0.25 3.5 6.0 6.0 6 5 NH. F 0.25 4.2 7.0 7.5 810 NH F 0.25 5.5 7.5 8.0 9 0 CrF 2.7 3.8 9.0 9.0 910 CrF 2.7 5.0 6.5 7.5 9.0

Table 6 Continued I High Quality Steel Sealing Rinse Rinsed Immediately with Sealing Rinse Dried on Sealing Rinse-Dried on Component Concentration pH Deionized Water and Dried Panels and Then Rinsed Panels. No Water Rinse Grams/liter g w With Deionized Water H I Quality Steel (Ratings as above) None (control) 4.0 NH F 0.25 5.5 5.0 6.0 6.0 NH F 0.25 7.l 5.0 6.0 6.0 CrF 0.25 4.3 6.0+ 6.0 6.0+ CrF 0.25 3.0 6.0- 6.0 6.0

EXAMPLE 7. taining fluoride ion at a concentration ranging from Various simple and complex fluorides were tested as sealing rinses using the procedure and test panels as described in Example 3. The fluoride rinse solutions were adjusted to a fluoride concentration of 0.27 grams/liter. The pH of all solutions was at 4.2 after adjustment with hydrofluoric acid or zinc oxide. All of the sealing rinses failed in the salt spray evaluation tests. The results appear in Table 7.

Table 7 about 0.01 to about grams/liter which is supplied by at least one of the members selected from the group consisting of calcium fluoride, zinc fluoride, aluminum fluoride, titanium fluoride, zirconium fluoride, chromic fluoride, chromic zirconium fluoride, nickel fluoride, ammonium fluoride, hydrofluoric acid and fluoboric acid and then drying the said fluoride solution on the said wetted metal.

Results of Salt Spray Rated According to A.S.T.M. D1654 EXAMPLE 8.

Fluoboric acid and zinc fluoborate were tested on mild steel panels using the procedure of Example 3. The fluoride concentration was 0.5 grams/liter. The pH of the fluoboric acid was adjusted with N-methyl morpholine to 6.7 and the zinc fluoborate to 4.3. The panels were spray painted as in the prior examples and then subjected to the salt spray tests. The results appear in Table 8.

Table 8 2. The process of claim 1 in which the fluoride ion is obtained from chromic fluoride.

3. The process of sealing a phosphate coating on a metal comprising wetting the phosphate coated metal with the composition consisting essentially of an acidic aqueous fluoride solution free of chromates and having a pH within the range of about 3 to about 7 and containing fluoride ion at a concentration ranging from about 0.01 to about 25 grams/liter which is supplied by Results of Salt Spray Rated According to A.S.T.M. D1654 Sealing Rinsed Immediately Sealing Rinse Dried Sealing Rinse Rinse with Deionized Water on Panels and then Dried on Panels Rinsed with Dionized No Water Rinse Water None (control) 6.5 HB 7.0 8.0 8.0 Zn(BF.,) 6.5 7.0 6.5

We claim: at least one of the members selected from the group 1. The process of sealing a phosphate coating on a metal comprising wetting the phosphate coated metal with the composition consisting essentially of an acidic aqueous fluoride solution free of chromates and having a pH within the range of about 3 to about 7 and conconsisting of calcium fluoride, zinc fluoride, aluminum fluoride, titanium fluoride, zirconium fluoride, chromic fluoride, chromic zirconium fluoride, nickel fluoride, ammonium fluoride, hydrofluoric acid and fluoboric acid, drying the said fluoride solution on the said wettaining fluoride ion at a concentration ranging from about 0.01 to about 25 grams/liter which is supplied by at least one of the members selected from the group consisting of calcium fluoride, zinc fluoride, aluminum fluoride, titanium fluoride, zirconium fluoride, chromic fluoride, chromic zirconium fluoride, nickel fluoride, ammonium fluoride, hydrofluroic acid and fluoboric acid and thereafter rinsing the wetted metal with water. l

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION- 3,895,970 Iuly 22, 1975 Patent No.

Dated Inventor 5 William A. Blum and Kurt Goltz It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 58, delete "zinc" (second occurrence) Column 2, line 58, delete "rinsed metal" (first occurrence) Column 3, line 40, correct spelling of "hydrofluoric" Column 4, line 53, correct spelling of "chromic" Columns 9 and 10, Table 6 Continued, delete "High" and insert Low prior to "Quality Steel".

Signed and Scaled this th' I [h [SEAL] D3) Of September 1975 Arrest:

RUTH C. MASON (nmmissivnvr uj'lalems and Trademurkx

Claims (5)

1. The process of sealing a phosphate coating on a metal comprising wetting the phosphate coated metal with the composition consisting essentially of an acidic aqueous fluoride solution free of chromates and having a pH within the range of about 3 to about 7 and containing fluoride ion at a concentration ranging from about 0.01 to about 25 grams/liter which is supplied by at least one of the members selected from the group consisting of calcium fluoride, zinc fluoride, aluminum fluoride, titanium fluoride, zirconium fluoride, chromic fluoride, chromic zirconium fluoride, nickel fluoride, ammonium fluoride, hydrofluoric acid and fluoboric acid and then drying the said fluoride solution on the said wetted metal.

2. The process of claim 1 in which the fluoride ion is obtained from chromic fluoride.

3. THE PROCESS OF SEALING A PHOSPHATE COATING ON A METAL COMPRISING WETTING THE PHOSPHATE COATED METAL WITH THE COMPOSITION CONSISTING ESSENTIALLY OF AN ACIDIC AQUEOUS FLOURIDE SOLUTION FREE OF CHROMATES AND HAVING A PH WITHIN THE RANGE OF ABOUT 3 TO ABOUT 7 AND CONTAINING FLOURIDE ION AT A CONCENTRATION RANGING FROM ABOUT 0.01 TO ABOUT 25 GRAMS/LITER WHICH IS SUPPLIED BY AT LEAST ONE OF THE MEMBERS SELECTED FROM THE GROUP CONSISTING OF CALCIUM FLORIDE, ZINC FLOURIDE, ALUMINUM FLOURIDE, TIANIUM FLOURIDE, ZIRCOMIUM FLOURIDE, CHROMIC FLOURIDE, CHROMIC ZIRCONIUM FLUORIDE,NICKEL FLUORIDE, CHROMIC FLUOFLUORIDE, HYDROFLUORIC ACID AND FLUORIC ACID, DRYING THE SAID FLUORIDE SOLUTION OF THE SAID WETTED METAL AND THEREAFTER RISING THE METAL WITH WATER AND REDRYING THE METAL.

4. The process of claim 3 in which the fluoride ion is obtained from chromic fluoride.

5. The process of sealing a phosphate coating on a metal comprising wetting the phosphate coated metal with the composition consisting essentially of an acidic aqueous fluoride solution free of chromates and having a pH within the range of about 3 to about 7 and containing fluoride ion at a concentration ranging from about 0.01 to about 25 grams/liter which is supplied by at least one of the members selected from the group consisting of calcium fluoride, zinc fluoride, aluminum fluoride, titanium fluoride, zirconium fluoride, chromic fluoride, chromic zirconium fluoride, nickel fluoride, ammonium fluoride, hydrofluroic acid and fluoboric acid and thereafter rinsing the wetted metal with water.