WO1993012268A1

WO1993012268A1 - A process and composition for treating the surface of tin-plated steel

Info

Publication number: WO1993012268A1
Application number: PCT/US1992/010161
Authority: WO
Inventors: Tomoyuki Aoki; Masayuki Yoshida; Akio Shimizu
Original assignee: Henkel Corporation
Priority date: 1991-12-12
Filing date: 1992-12-01
Publication date: 1993-06-24
Also published as: AU3147993A; ZA929156B; MX9207072A; JPH05163584A

Abstract

The invention is a novel bath and process for treating the surface of tin-plated steel that generates only small quantities of sludge in wastewater treatment and imparts, by a low-temperature treatment, excellent corrosion resistance and paint adherence to the surface of tin-plated steel prior to the painting or printing of same. The bath for treating the surface of tin-plated steel is an aqueous solution with pH 1.5 to 3.5 which contains 10 to 500 ppm phosphate, 10 to 500 ppm fluoride as fluorine, and 5 to 500 ppm tin.

Description

A PROCESS AND COMPOSITION FOR TREATING THE SURFACE OF TIN- PLATED STEEL

Field of the Invention

The invention relates to a novel process and aqueous bath for treating the surface of tin-plated steel and particularly tin-plated steel drawn and ironed (DI) cans by a low temperature treatment, which imparts excellent corrosion resistance and paint adherence to the tin-plated surface prior to the painting or printing of the tin-plated steel surface.

The process is particularly useful for treating the surface formed by the draw-ironing of tin-plated steel sheet. In addition, the new process and bath produces very little sludge in wastewater treatment.

Background of the Invention

Before applying an organic coating to a tin-plated steel surface, the tin plated surface is first cleaned and a conversion coating formed on the surface. The device for cleaning and applying a conversion coating to a tin-plated steel surface, particularly the surface of a DI can, is referred to as a "washer". In the washer, the DI can is continuously treated while inverted, with a degreasing bath and a conversion coating bath. Washers are preferably organized to carry out six process steps: 1) preliminary degreasing, 2) degreasing, 3) water rinsing, 4) conversion treatment, 5) water rinsing, and 6) de-ionized water rinsing. The liquid effluent from the washer is subjected to a wastewater treatment process and then discharged as effluent. In recent years, as the regulatory standards for effluents have become increasingly stringent in response to environmental problems, reducing wastewater treatment loads has become a major objective . Related Art

JapanesePatentApplicationKokai 1-100281 (100,281/1989) discloses a bath for treating the surface of tin-plated steel

DI cans. The bath for the treatment of metal surfaces has a pH of 2 to 6 and contains 1 to 50 grams/liter phosphate ion, 0.2 to 20.0 grams/liter oxyacid ion, 0.01 to 5.0 grams/liter tin ion, and 0.01 to 5.0 grams/liter condensed phosphate ion.

Treatment of a tin-plated steel surface with this conversion treatment bath forms a highly corrosion-resistant phosphate film on the surface of tin-plated steel DI can. In its practical applications, the process disclosed in

Kokai 1-100,281 does not generally require an extensive and complex water effluent treatment; however, a large amount of solid material (hereinafter referred to as sludge) is nevertheless produced during effluent treatment. At present the sludge is discarded as an industrial waste. It is desirable to reduce the quantity of sludge in order to reduce costs and protect the environment. In addition, the treatment temperature in the above-referenced invention is generally 60^βC, and a conversion agent capable of low-temperature treatment would also be desirable. Brief Summary of the Invention

The conversion coating bath of the invention imparts excellent corrosion resistance and paint adherence to the surface of tin-plated steel particularly tin-plated steel DI cans by a low-temperature treatment, that generates a low effluent treatment load, and produces only small amounts of sludge.

According to the present invention, the surface of tin- plated steel is contacted with a novel bath for treating the surface of the tin-plated steel at a low temperature. The treatment imparts excellent corrosion resistance and paint adherence to the surface of tin-plated steel prior to the painting or printing on the surface, and generates only small amounts of sludge in wastewater effluent treatment.

The process of the invention comprises contacting the tin-plated surface with a bath which comprises an aqueous solution with pH 1.5 to 3.5 containing 10 to 500 ppm phosphate group, 10 to 500 ppm fluoride as fluorine, and 5 to 500 pp tin. The bath is at a temperature of from about ambient t about 60° C. The contact time is from 2 seconds to about 6 seconds. Detailed Description of the Invention

The surface treatment bath used in the practice of th present invention comprises an acidic aqueous solution tha contains the phosphate group, fluoride, and tin as it essential components. The phosphate group can be supplied by phosphoric aci (H₃PO_A) , trisodium phosphate (Na₃P0₄) monosodium phosphat (NaH₂P0₄) , disodium phosphate (Na₂HP0 ) and the like. Th phosphate ion (P0₄≡) content is in the range of about 10 to about 500 ppm and preferably within the range of about 20 to about 90 ppm. At less than 10 ppm, the reactivity is poor and conversion coating formation is not satisfactory. While a high-quality conversion coating is formed at concentration above of 500 ppm, the higher concentrations are generally associated with the production of large quantities of sludge in wastewater effluent treatment.

The fluoride source can be, for example, hydrofluoric acid (HF) or a salt thereof such as sodium fluoride (NaF) , fluorozirconic acid (H_j>ZrF₆) , fluorotitanic acid (H-,TiF₆) or a salt thereof. The fluoride content preferably falls within the range of about 10 to about 500 ppm as fluorine and more preferably within the range of 20 to 90 ppm as fluorine. At less than 10 ppm, the reactivity is poor and conversion coating formation is not satisfactory. A good coating is formed at fluoride concentrations in excess of 500 ppm, but etching is increased and appearance of the tin-plated surface is degraded.

The tin can be introduced into the conversion coating bath by dissolving tin salts such as stannous chloride (SnCl₂) , stannic chloride (SnCl₄) , stannous fluoride (SnF₂) , stannic fluoride (SnF , in the solution, or the tin can be introduced by dissolution of tin metal or tin from the tin- plated steel. The tin content preferably falls within the range of about 5 to about 500 ppm and more preferably within the range of about 10 to about 100 ppm. A highly corrosion- resistant film is not formed at concentrations less than 5 ppm. Although a good coating is formed at concentrations in excess of 500 ppm, at such levels the tin readily precipitates as the hydroxide, and impairs bath stability. An organic acid such as gluconic acid or oxalic acid can be added to the conversion coating bath when the stability of the bath is impaired by iron and tin ions eluting from the tin-plated steel.

The pH of the treatment bath should be adjusted to about 1.5 to about 3.5. Etching is severe and film formation is impaired at a pH less than about 1.5. The formation of a strongly corrosion-resistant coating is difficult at a pH above about 3.5. Thus, the pH must be adjusted into the range of about 1.5 to about 3.5 and preferably to the range of about 2.3 to about 3.0. The pH can be controlled through the use of an acid such as phosphoric acid, nitric acid, hydrofluoric acid, and the like, or alkali such as sodium hydroxide, sodium carbonate, ammonium hydroxide, and the like.

The surface treatment bath is prepared by dissolving the specified quantities of phosphate, fluoride, and tin in water with thorough stirring. The pH is then adjusted by addition of an acid or alkaline material as required. The coating which is formed by the surface treatment bath on the tin-plated surface in accordance with the invention is an inorganic coating. The coating generally contains tin hydroxide and tin oxide along with other components formed by the treatment. The process will be illustrated by a treatment of tin- plated DI cans by application of the surface treatment bath in accordance with the present invention. A preferred embodiment of the process is as follows.

1. The surface of the tin-plated DI can be cleaned: one or more degreasing steps including contact with mildly alkaline cleaner;

2. water rinse to remove alkali cleaner;

3. coating formation by contact with the treating bath of the invention at a temperature of room temperature to 50^βC, by spraying the bath onto the DI cans for 2 to 60 seconds; 4. water rinse to remove the aqueous treating bath;

5. rinse with de-ionized water; and

6. dry.

The treatment temperature for contact of the tin-plated surface with the treatment bath is room temperature to 60°C, preferably room temperature to about 50°C and more preferably about 30 to about 35°C. The temperatures can be substantially lower than the known treatment (60^βC) disclosed in Kokai 1- 100,281. The spray time is preferably 2 to 60 seconds. A satisfactory reaction does not occur at times below 2 seconds and a highly corrosion-resistant film is not formed. No further increase in performance is obtained at times in excess of 60 seconds. Accordingly, suitable treatment times fall in the range of 2 to 60 seconds. The tin-plated steel surface can be contacted with the treating solution by means generally used to contact surfaces with liquids. The tin-plated surface can be contacted with the bath solution by dipping, spraying, flow-coating, brushing, roller-coating, wiping or any other means for contacting a surface with a liquid. Preferably the surface is contacted with the liquid by means such as spraying, flow- coating or dipping. The following examples illustrate the treatment bath and the process of the invention. The utility of the invention is illustrated by comparison with comparison examples.

Corrosion resistance was evaluated by the iron exposure value (IEV) measured as disclosed in United States Patent Number 4,332,646. Lower IEV values are indicative of a better corrosion resistance. IEV values below 150 are generally rated as excellent. Paint adherence was evaluated by the peel strength as follows: an epoxy-urea can paint was coated to a paint film thickness of 5 to 7 micrometers on the surface of a treated can; the can was baked for 4 minutes at 215^βC; the can was then cut into 5 x 150 mm rectangular strips; a test specimen was prepared by the hot-press application of a polyamide film. The test specimen was peeled using the 180° peel test technique. In this case, higher peel strength values are indicative of a better paint adherence. Peel strength values of at least 1.5 kgf/5 mm width are generally rated as excellent.

The wastewater treatability was evaluated by the following method. The treatment bath was diluted 1/10 (in practical operations, the treatment bath is diluted to this level in wastewater treatment), calcium hydroxide (Ca(0H)₂) was added to provide a pH of about 10, aqueous aluminum sulfate solution was then added to give 20 ppm aluminum in the solution, the pH was again adjusted to 10 using calcium hydroxide. The bath was permitted to stand for 1 hour and any solid was separated from the liquid. The phosphorus, fluorine, and tin concentrations in the liquid were measured and precipitation of these components was confirmed. Then, the solids (sludge) separated from the liquid was dried at 110°C for 24 hours, cooled and weighed. The wastewater treatability was evaluated based on this weight. In this case, the production of smaller amounts of sludge is desired. Example 1

A tin-plated DI can was prepared from a tin-plated steel sheet. The DI can was cleaned with a hot 1% aqueous solution of a weakly alkaline degreaser (Fine Cleaner 4361A, registered brandname of Nihon Parkerizing Company, Limited) and rinsed with tap water. The can was contacted with Surface Treatment Bath 1 heated to 30"C by spraying for 30 seconds, rinsed with tapwater, rinsed by spray for 10 seconds with de-ionized water (>3,000,000 ohm-cm), and finally drying in a hot-air drying oven for 3 minutes at 180°C. The corrosion resistance and adherence of the treated surfaces were evaluated. The wastewater treatability of Surface Treatment Bath 1 was also evaluated.

Surface Treatment Bath 1 75% phosphoric acid (H_jPO 20% hydrofluoric acid (HF) stannic chloride (SnCl₄.5H₂0)

pH 2.5 (adjusted with aqueous ammonia)

Example 2 A tin-plated DI can was cleaned as in Example 1 and was then treated by spraying for 30 second spray with the Surface Treatment Bath 2 heated to 30°C. After treatment, water washing and drying were conducted under the same conditions a in Example 1. The corrosion resistance and adherence of th treated can were subsequently evaluated, and the wastewate treatability of Surface Treatment Bath 2 was evaluated.

Surface Treatment Bath 2

75% phosphoric acid (H₃P0₄) 110 ppm (P0₃: 80 ppm) 20% fluorozirconic acid (H₂ZrF₆) 500 ppm (F: 40 ppm) stannic chloride (SnC- .5H₂0) 30 ppm (Sn: 10 ppm) pH 3.0 (adjusted with aqueous ammonia) Example 3

A tin-plated DI can was cleaned using the same conditions as in Example 1 and was then treated with a 3 second spray of Surface Treatment Bath 3 heated to 30^βC. After treatment, water washing and drying were conducted under the same conditions as in Example 1. The corrosion resistance and adherence of the treated can were subsequently evaluated, and the wastewater treatability of Surface Treatment Bath 3 was evaluated.

Surface Treatment Bath 3 75% phosphoric acid (H₃P0₄) 20% hydrofluoric acid (HF) tin metal (Sn)

pH 2.7 (adjusted with sodium hydroxide)

Example 4 A tin-plated DI can was cleaned using the same conditions as in Example 1 and was then treated by a 30 second spray of Surface Treatment Bath 4 heated to 30°C. After treatment, water washing and drying were conducted under the same conditions as in Example 1. The corrosion resistance and adherence of the treated can were subsequently evaluated, and the wastewater treatability of Surface Treatment Bath 4 was evaluated.

Surface Treatment Bath 4 75% phosphoric acid (H₃P0₄) 110 ppm (P0 : 80 ppm) 20% fluorozirconic acid (H₂ZrF₆) 500 ppm (F: 55 ppm) stannous chloride (SnCl₂.2H₂0) 19 ppm (Sn: 10 ppm) pH 2.5 (adjusted with sodium hydroxide)

Example 5 A tin-plated DI can was cleaned using the same conditions as in Example 1 and was then treated by a 30 second spray of Surface Treatment Bath 5 heated to 60°C. After treatment, water washing and drying were conducted under the same conditions as in Example 1. The corrosion resistance and adherence of the treated can were subsequently evaluated, and the wastewater treatability of Surface Treatment Bath 5 was evaluated.

Surface Treatment Bath 5 trisodium phosphate (Na₃P0₄) 86 ppm (P0₄: 50 ppm) 20% hydrofluoric acid (HF) 210 ppm (F: 40 ppm) stannous chloride (SnCl₂.2H₂0) 19 ppm (Sn: 10 ppm) pH 2.5 (adjusted with nitric acid) Example 6 A tin-plated DI can was cleaned using the same conditions as in Example 1 and was then treated by a 30 second spray of Surface Treatment Bath 6 heated to 30°C. After treatment, water washing and drying were conducted under the same conditions as in Example 1. The corrosion resistance and adherence of the treated can were subsequently evaluated, the wastewater treatability of Surface Treatment Bath 6 was evaluated. Surface Treatment Bath 6

75% phosphoric acid (H₃P0₄) 20% hydrofluoric acid (HF) stannic chloride (SnCl₄.5H₂0)

pH 2.5 (adjusted with aqueous ammonia) Example 7

A tin-plated DI can was cleaned using the same conditions as in Example 1 and was then treated by a 30 second spray of Surface Treatment Bath 7 heated to 30^βC. After treatment, water washing and drying were conducted under the same conditions as in Example 1. The corrosion resistance and adherence of the treated can were subsequently evaluated, and the wastewater treatability of surface treatment bath 7 was evaluated.

Surface Treatment Bath 7 75% phosphoric acid (H₃P0₄) 110 ppm (P0₄: 80 ppm) 20% fluorozirconic acid (H₂ZrF₆) 500 ppm (F: 55 ppm) 20% hydrofluoric acid (HF) 210 ppm (F: 40 ppm)

F: total 95 ppm stannic chloride (SnCl₄.5H₂0) 30 ppm (Sn: 10 ppm) pH 2.5 (adjusted with aqueous ammonia) Example 8

A tin-plated DI can was cleaned using the same conditions as in Example 1 and was then treated by a 30 second spray of Surface Treatment Bath 1 heated to 50°C. After treatment, water washing and drying were conducted under the same conditions as in Example 1. The corrosion resistance and adherence of the treated can were subsequently evaluated.

Example 9 A tin-plated DI can was cleaned using the same conditions as in Example 1 and was then treated by a 10 second spray of Surface Treatment Bath 1 heated to 30°C. After treatment, water washing and drying were conducted under the same conditions as in Example 1. The corrosion resistance and adherence of the treated can were subsequently evaluated.

The results of the tests are reported in Table 1. The results demonstrate that the surface treatment baths of the present invention produce a highly corrosion-resistant, highly adherent film by treatment at low temperatures. In addition, the treatment baths demonstrate a low sludge production. The improved process, sludge production and coating can be seen by the comparison examples. Table 1 Test Results

ame as Example 1

Comparison Example 1 A tin-plated DI can was cleaned using the same conditions as in Example 1 and was then treated by a 30 second spray of Surface Treatment Bath 8 heated to 30^βC. After treatment, water washing and drying were conducted under the same conditions as in Example 1. The corrosion resistance and adherence of the treated can were subsequently evaluated, and the wastewater treatability of Surface Treatment Bath 8 was evaluated. The pH of the Surface Treatment Bath 8 was above 3.5. Surface Treatment Bath 8 trisodium phosphate (Na₃P0₄) 86 20% hydrofluoric acid (HF) 210 stannous chloride (SnCl₂.2H₂0) 19

pH 4.0 (adjusted with sodium hydroxide)

Comparison Example 2 A tin-plated DI can was cleaned using the same conditions as in Example 1 and was then treated by a 30 second spray of Surface Treatment Bath 9 heated to 50°C. After treatment, water washing and drying were conducted under the same conditions as in Example 1. The corrosion resistance and adherence of the treated can were subsequently evaluated, and the wastewater treatability of Surface Treatment Bath 9 was evaluated. The Surface Treatment Bath 9 did not contain tin within the range 5-500 ppm. Surface Treatment Bath 9 trisodium phosphate (Na₃P0₄) 86 ppm (P0₄: 50 ppm) 20% hydrofluoric acid (HF) 210 ppm (F: 40 ppm) pH 3.0 (adjusted with nitric acid) Comparison Example 3

A tin-plated DI can was cleaned using the same conditions as in Example 1 and was then treated by a 30 second spray of Surface Treatment Bath 10 heated to 30^βC. After treatment, water washing and drying were conducted under the same conditions as in Example 1. The corrosion resistance and adherence of the treated can were subsequently evaluated, and the wastewater treatability of Surface Treatment Bath 10 was evaluated.

The pH of Surface Treatment Bath 10 was below 1.5. Surface Treatment Bath 10

75% phosphoric acid (H₃P0₄) 69 ppm (P0₄: 50 ppm) 20% hydrofluoric acid (HF) 210 ppm (F: .40 ppm) stannous chloride (SnCl₂.2H₂0) 19 ppm (Sn: 10 ppm) pH 1.4 (adjusted with nitric acid) Comparison Example 4

A tin-plated DI can was cleaned using the same conditions as in Example 1 and was then treated by a 30 second spray of Surface Treatment Bath 11 heated to 30^βC. After treatment, water washing and drying were conducted under the same conditions as in Example 1. The corrosion resistance and adherence of the treated can were subsequently evaluated, and the wastewater treatability of Surface Treatment Bath 11 was evaluated.

Surface Treatment Bath 11 contained more than 500 ppm P0₄. The coating formed was acceptable but the wastewater treatability formed an excessive amount of sludge. Surface Treatment Bath 11

75% phosphoric acid (H₃P0₄) 12,000 ppm (P0₄: 8,724 ppm) 20% hydrofluoric acid (HF) 210 ppm (F: 40 ppm) stannous chloride (SnCl₂.2H₂0) 76 ppm (Sn: 40 ppm) pH 3.0 (adjusted with sodium hydroxide)

Comparison Example 5 A tin-plated DI can was cleaned using the same conditions as in Example 1 and was then treated by a 30 second spray of Surface Treatment Bath 12 heated to 30°C. After treatment, water washing and drying were conducted under the same conditions as in Example 1. The corrosion resistance and adherence of the treated can were subsequently evaluated, and the wastewater treatability of Surface Treatment Bath 12 was evaluated. Treatment Bath 12 did not contain fluoride. Surface Treatment Bath 12

75% phosphoric acid (H₃P0₄) 10,000 ppm (P0₄: 7,270 ppm) sodium tripolyphosphate (Na₅P₃O₁₀) 400 ppm (P₃Oι₀ ^{: 75} PP∞) stannous chloride (SnCl₂.2H₂0) 76 ppm (Sn: 40 ppm) pH 3.0 (adjusted with sodium hydroxide)

The treatment of tin-plated steel and particularly tin- plated DI can with a surface treatment bath in accordance wit the present invention accrues the following highly desirabl effects:

(1) through a low-temperature treatment, the surface of th tin-plated steel is provided with excellent corrosion resistance and paint adherence prior to the painting or printing thereof; and

(2) very little sludge is produced in wastewater effluent treatment.

Claims

1. A bath for improving the corrosion resistance and paint adherence of a tin-plated steel surface which comprises an aqueous solution at a pH of 1.5 to 3.5 containing about 10 to about 500 ppm P0₄≤, about 10 to about 500 ppm fluoride as fluorine and about 5 to about 500 ppm Sn.

2. A bath of Claim 1 containing about 20 to about 90 ppm P0₄≡.

3. A bath of Claim 1 containing abut 20 to about 90 ppm fluoride as fluorine.

4. A bath of Claim 1 containing about 10 to about 100 ppm Sn.

5. A bath of Claim 2 containing about 20 to about 90 ppm fluoride as fluorine.

6. A bath of Claim 5 containing about 10 to about 100 ppm Sn.

7. A bath of Claim 2 containing 10 to 100 ppm Sn.

8. A bath of Claim 1 having a pH of 2.3 to 3.0.

9. A bath of Claim 6 having a pH of 2.3 to 3.0. 10. A process for improving the corrosion resistance and paint adherence of a tin-plated steel surface which comprises contacting a clean tin-plated steel surface with an aqueous solution comprising 10 to 500 ppm P0₄≡,

10-500 ppm fluoride as fluorine, and 5 to 500 ppm Sn, at a temperature of from room temperature to 60^βC and a pH of from 1.5 to 3.5 for from 2 to 60 seconds, rinsing the solution from the surface and drying the surface.

11. A process of claim 10 wherein the pH is from about 2.3 to about 3.0.

12. A process of Claim 10 wherein the P0₄≡ is present at from about 20 to about 90 ppm.

13. A process of Claim 10 wherein the fluoride as fluorine is present at from about 20 to about 90 ppm.

14. A process of Claim 10 wherein the Sn is present at from about 10 to about 100 ppm.

15. A process of Claim 12 wherein the fluoride as fluorine is present at from about 20 to about 90 ppm.

16. A process of Claim 12 wherein Sn is present at from about 10 to about 100 ppm.

17. A process of Claim 16 wherein the fluoride as fluorine is present at from abut 20 to about 90 ppm.

18. A process of Claim 10 wherein the aqueous solution is at a temperature of from about 30^βC to about 50:

19. A process of Claim 10 wherein the tin-plated steel surface is contacted with the aqueous solution by spraying.