EP0673445A1

EP0673445A1 - Composition and process for treating tinplate

Info

Publication number: EP0673445A1
Application number: EP94902385A
Authority: EP
Inventors: Tomoyuki Aoki; Masayuki Nihon Parkerizing Kibogaoka Yoshida
Original assignee: Henkel Corp
Current assignee: Henkel Corp
Priority date: 1992-12-09
Filing date: 1993-12-01
Publication date: 1995-09-27
Anticipated expiration: 2013-12-01
Also published as: EP0673445A4; ZA939171B; JPH06173024A; WO1994013855A1; DE69325035T2; DE69325035D1; AU5677594A; EP0673445B1; JP3256009B2

Abstract

To impart an excellent corrosion resistance and adherence to the surface of tinplate while avoiding the production of sludge in the treatment bath during continuous treatment, a bath is used thatcontains phosphate ions, from 0.1 to 5.0 g/L of chelating agent, and tin ions; has a pH from 2.0 to 4.5; is essentially free of oxidizing agent and ferric ion; and has an oxidation-reduction potential of ≤450 mV more oxidizing than a silver-saturated silver chloride reference electrode.

Description

Description COMPOSITION AND PROCESS FOR TREATING TINPLATE

Technical Field

The invention relates to a phosphate containing composition (often de¬ noted hereinafter as a "bath" for brevity) for treating the surface of tinplate (i.e., tin-plated steel) and to a treatment process for tinplate. More specifically, the present invention relates to an improvement of a treatment that is already used, prior to the painting or printing of surfaces of tinplate sheet, strip, and formed objects, e.g., cans and the like, to provide such surfaces with an excellent cor¬ rosion resistance and paint adherence. In particular, the treatment bath and treatment process of the present invention are well adapted for treating sur- faces of tinplate that has been formed by Dl (i.e., drawing-and-ironing) process¬ ing. Thus, the present invention concerns a novel technology for treating tin- plate surfaces, a technology that may be used to provide tinplate surfaces with an excellent corrosion resistance and paint adherence, but which is free or very nearly free of the insoluble salts (hereinafter referred to as "sludge") that are produced by the tin ions and iron ions that elute into the bath during treatment. This sludge reduces the productivity of tinplate surface treatment lines. Background Art

The cleaning and surface treatment of tinplate is frequently conducted by a spray process. For example, the surface treatment equipment for tinplate Dl can is generally called a washer. Molded Dl can is inverted and continuous¬ ly treated in the washer with a cleaning bath and a surface treatment bath. Ex¬ isting washers normally use 6 steps (pre-cleaning, cleaning, water wash, sur¬ face treatment, water wash, and wash with de-ionized water), and treatment is conducted entirely by spraying. Compositions of phosphate ion, tin ion, and oxidizing agent are already known as surface treatment baths for tinplate Dl can. As discussed by the present inventors in Nihon Parkerizing Giho, 89, No. 2, page 6, the mechanism of conversion film formation by these components consists of tin and iron elu- tion reactions (anodic reactions) and the precipitation of insoluble phosphate salts (cathodic reaction).

Furthermore, in Japanese Patent Application Laid Open [Kokai or Unex- amined] Number Hei 1-100281 [100,281/1989]), there has already been pro¬ posed a composition for the purpose of inverting the tin-iron potential in the conversion bath, i.e., the tin region becomes the anode and the iron region be¬ comes the cathode. This particular invention consists of a conversion coating bath for the treatment of metal surfaces. This bath has a pH of 2 to 6 and con¬ tains 1 to 50 grams per liter (hereinafter often abbreviated "g/L") of phosphate ions, 0.2 to 20.0 g/L of oxyacid ions, 0.01 to 2.0 g/L of tin ion, and 0.01 to 5.0 g/L of condensed phosphate ions. Treatment with this conversion treatment bath forms a highly corrosion-resistant, highly paint-adherent phosphate film on the surface of tinplate Dl can. The oxyacid ion is an oxidizing agent that func¬ tions to oxidatively remove the hydrogen that is produced by the anodic reac¬ tions. When the aforesaid invention is practiced on a continuous basis, it is in fact capable of initially providing an excellent surface treatment. However, it has been found that the referenced invention gradually generates a phosphate salt sludge, which is produced by the reaction of the phosphate ions present in the bath with the tin ions and iron ions that elute from the tinplate. It has also been determined that iron ions elute from the tinplate in the divalent state; that gradual oxidation by the oxidizing agent (oxyacid ion, etc.) produces the trivalent state in the surface treatment bath at a level of approximately 0.05 g/L; and that this is the cause of sludge production.

This sludge can cause problems by adhering to the tinplate surface and degrading the paint adherence. In addition, the sludge can clog the piping and nozzles of the spray equipment and can thereby prevent a high quality surface treatment. This has necessitated the implementation of periodic maintenance in order to clean the piping and nozzles of the spray equipment and has result¬ ed in unstable quality characteristics. Since productivity enhancements and im- provements in quality stability have recently become critical issues, a surface treatment bath is desired that carries a reduced cleaning burden and that offers stable quality characteristics, i.e., that is free of sludge production in the bath even during continuous service. Disclosure of the Invention

Problemfs) to Be Solved bv the Invention Accordingly, the present invention takes as its object the introduction of a bath and process for treating tinplate surfaces that solves the problems de¬ scribed above and that enhances quality stability and leads to improvements in productivity (easy maintenance and the like).

Summary of the Invention As a result of extensive research into the problems described above, it was determined that sludge production is particularly significantly influenced by the oxidation state (divalent or trivalent) in the treatment composition of the iron ions present therein, which normally elute from the tinplate during treatment with the composition. With respect to a bath for treating tinplate surfaces that comprises, preferably consists essentially of, or more preferably consists of, water, acidity, phosphate ions, chelating agent, and tin ions, it was also deter¬ mined that an excellent corrosion resistance and paint adherence could be ob¬ tained without sludge production — even during continuous treatment — by such a bath for treating tinplate surfaces that has a pH in the range of 2.0 to 4.5 and a concentration of chelating agent in the range of 0.1 to 5.0 g/L and that essentially does not contain ferric iron or an oxidizing agent sufficiently strong to oxidize ferrous to ferric ions. The present invention was achieved based on these findings.

In addition, the iron ions eluting from tinplate often undergo spontaneous oxidation to the trivalent state when the surface treatment process employs the surface treatment bath on a continuous basis. With the objective of maintaining the iron ions in the divalent state, the use of the oxidation-reduction potential to monitor the oxidation state of the iron ions was therefore examined. As a re¬ sult, with respect to the treatment of tinplate surfaces by contacting tinplate with an acidic surface treatment bath that contains at least phosphate ion, chelating agent, and tin ion, a method for treating tinplate surfaces was discovered whose characteristic features are a pH in the surface treatment bath in the range of 2.0 to 4.5 and control of the oxidation-reduction potential of the sur- face treatment bath to < 450 mV by the addition of reducing agent on an as-re¬ quired basis. The present invention was also achieved based on this discovery. The structure of the present invention is explained in detail below. Description of Preferred Embodiments Phosphoric acid (H₃PO₄), sodium phosphate (Na₃P0₄), and the like can be used to provide the phosphate ion, and this component should be used in quantities sufficient to bring about tin phosphate precipitation. The reactivity is low when phosphate ion is present at less than 1 g/L, and this prevents satis¬ factory formation of the coating under ordinary treatment conditions. While a good quality coating is formed at values in excess of 30 g/L, the corresponding high cost of the treatment bath becomes economically disadvantageous. Thus, the phosphate ion is present preferably in the range of 1 to 30 g/L and more preferably in the range of 4 to 8 g/L.

The present invention requires that the bath contain chelating agent in a quantity sufficient to bring about a satisfactory etching, selective conversion film formation on exposed iron regions, and a satisfactory tin ion stabilization. Preferred chelating agents that meet these requirements are exemplified by condensed phosphate ions, tartaric acid, oxalic acid, and citric acid. Particularly preferred chelating agents comprise at least one selection from the condensed phosphate ions. This is because the condensed phosphate ions gradually de¬ compose to phosphoric acid and therefore have little to no adverse effect on waste water treatment. The acid or salt can be used to provide condensed phosphate ion. For example, pyrophosphoric acid (H₄P₂O₇), sodium pyrophos- phate (Na₄P₂O₇), and so forth can be used to provide pyrophosphate ion. The etching activity is weak and film formation is unsatisfactory at a chelating agent concentration of less than 0.1 g/L. On the other hand, the etching activity is too strong and the film-formation reactions are inhibited at more than 5 g/L of che¬ lating agent. The chelating agent content therefore preferably falls in the range of 0.1 to 5 g/L and particularly preferably falls in the range of 0.2 to 1.0 g/L. Since tinplate Dl can has been subjected to Dl processing, its surface presents both tin-plated regions and iron regions that have been exposed by the processing, and the corrosion resistance is generally poor when large areas of iron are exposed. For this reason, the generation of uniform coverage of the exposed iron regions by the conversion coating is a crucial issue from the standpoint of improving the corrosion resistance. Because the surface treat¬ ment bath of the present invention contains a chelating agent, it is able to se- 5 lectively and uniformly cover the exposed iron regions with a conversion coat¬ ing, whereas a very poor conversion is produced at these exposed iron regions in the absence of chelating agent. This makes possible the production of a highly corrosion-resistant conversion film. Moreover, the chelating agent and particularly the condensed phosphates function to stabilize the eluted tin ions 0 in the bath and therefore also act to inhibit sludge production.

The tin ions can be supplied by tin metal or a tin salt, for example, tin chloride, but the tin source is not specifically restricted. In the case of continu¬ ous treatment, supplemental additions are not specifically required due to elu- tion of tin ion from the tinplate. The tin ion content should be selected so as s to yield the formation of a satisfactory tin phosphate coating, and preferably falls into the range of 0.01 to 2.0 g/L, more preferably into the range of 0.1 to 1.0 g/L, and particularly preferably into the range of 0.2 to 0.6 g/L. The range of 0.01 to 2.0 g/L yields a highly corrosion resistant film and avoids the precipi¬ tation of sludge. 0 The pH of the treatment bath should be maintained at 2.0 to 4.5. Strong etching and an inhibition of film formation are obtained at below 2.0. The anod¬ ic reaction conditions suffer from substantial deterioration when the pH exceeds 4.5 because the development of the anodic reactions is inhibited due to the es¬ sential absence of oxidizing agent from the treatment bath in accordance with 5 the present invention. Accordingly, the pH must be held in the range of 2.0 to 4.5, and is preferably held in the range of 2.5 to 3.5 and more preferably in the range of 2.7 to 3.3. The pH may be adjusted through the use of an acid such as phosphoric acid, sulfuric acid, and the like or through the use of an alkali such as sodium hydroxide, sodium carbonate, ammonium hydroxide, and the o like.

A characteristic feature of the treatment bath in accordance with the present invention is that essentially it contains neither ferric iron ions nor any oxidizing agent that will oxidize any substantial amount of ferrous iron ions to ferric iron ions. Preferably, the concentration of ferric ions in any surface treat¬ ment bath according to this invention is not greater than 7 mg/L, more prefer¬ ably not greater than 3 mg/L, still more preferably not greater than 2.0 mg/L, or most preferably not greater than 1.1 mg/L

Although prior surface treatment baths have contained oxidizing agent, the surface treatment bath in accordance with the present invention essentially does not contain an oxidizing agent such as oxyacid ion or the like, that is, does not contain oxidizing agent which substantially removes the hydrogen pro- duced by anodic reactions. Given that trivalent iron ion facilitates the occur¬ rence of sludge precipitation, the reason for omitting the oxidizing agent is that the presence of oxidizing agent leads to a condition in which both divalent and trivalent iron ions are present.

The absence of oxidizing agent from tinplate surface treatment baths has heretofore resulted in unstable conversion characteristics and in particular in an inability to obtain a uniform conversion at exposed iron regions, and for these reasons the absence of oxidizing agent has heretofore been considered unde¬ sirable. However, the continuous execution of conversion while still maintaining a good quality conversion film is made possible even in the absence of oxidiz- ing agent by holding the pH and chelating agent concentration within the ranges specified above.

Another crucial point in the treatment process in accordance with the present invention is that the oxidation-reduction potential of the treatment bath is to be controlled to < 450 mV during treatment. No specific restrictions apply to the electrodes used to measure the oxidation-reduction potential. The poten¬ tials provided in the present invention were obtained using a platinum electrode as the oxidation- reduction electrode and a silver-saturated silver chloride elec¬ trode as the reference electrode. When the oxidation-reduction potential is < 450 mV during this measurement, the iron ion is present almost entirely in the divalent state and the production of sludge is inhibited.

In addition to deliberately added oxidizing agent, atmospheric oxygen al¬ so can oxidize the divalent iron ions in the treatment bath. The tendency for the divalent iron ions to be oxidized by atmospheric oxygen varies as a function of the precise nature of the equipment, the spray conditions, and the like. The oxidation-reduction potential may in some cases exceed 450 mV when the present invention is implemented on a continuous basis under conditions in which air tends to be taken up and the difficult-to-avoid removal of bath by the treatment substrate requires only minor renewal of the surface treatment bath. Because sludge will be produced under such circumstances and quality and equipment maintenance will then again become problematic, reducing agent must be added on a preliminary basis or when the oxidation-reduction potential becomes elevated in order thereby to maintain the oxidation-reduction potential at < 450 mV. No specific restrictions apply to this reducing agent, but sub¬ stances that inhibit conversion film formation on the tinplate by the surface treatment bath should be avoided. Viewed from this perspective, phosphorous acid and hypophosphorous acid are preferred as reducing agents, because the main component of the surface treatment bath is phosphate ion and both phos¬ phorous acid and hypophosphorous acid are converted into phosphate ion in fulfilling their function as reducing agent. Thus, adverse effects due to an ac¬ cumulation of their decomposition product are completely avoided.

Phosphorous acid and hypophosphorous acid can be added as the acid or salt. The quantity of addition will vary as a function of the treatment condi¬ tions, but is preferably as small as possible from the standpoint of economics. Thus, the presence or addition of the minimum quantity that maintains the oxi¬ dation-reduction potential at < 450 mV is sufficient. In other words, the quantity of addition of the reducing agent can be regulated based on the oxidation-re- duction potential. When the reducing agent is supplied so as to maintain the oxidation-reduction potential at < 450 mV, substantially all of the iron ions in the composition are maintained in the divalent state and the production of sludge in the surface treatment bath can be prevented even during continuous treat¬ ment over long periods of time. The conversion film that is formed will now be briefly considered. The conversion film that is formed by a phosphate surface treatment bath for tin- plate is generally a phosphate salt whose principal component is tin phosphate, and the basic mechanism for its formation is believed to be the same even for the present invention. Thus, the tinplate substrate is etched by the phosphate ions and chelating agent (particularly condensed phosphate ions); a local in¬ crease in the pH at the interface occurs at this time; and a phosphate conver- sion film (principally of tin phosphate) precipitates on the surface.

One difference between prior phosphate films and the phosphate film of the present invention is the fact that the prior films are produced in the pres¬ ence of chelating agent and oxidizing agent while in the present invention pro¬ duction occurs in the presence of chelating agent and (optionally) reducing agent, i.e., the iron ions are only in the divalent state and production occurs es¬ sentially in the absence of trivalent ferric ions. A second difference is that the "sludge skin" is then presumably negligible for the film of the present invention. "Sludge skin" refers to the adhesion of a relatively poorly adherent, sediment¬ like substance in the vicinity of the tin phosphate film proper. Moreover, be- cause the phosphate film formed on tin-plated steel sheet in the case of tinplate Dl can is usually extremely thin, approximately 10 to 20 Angstroms, in both the tin-plated regions and the exposed iron regions, the sludge skin is not suscept¬ ible in this case to visual evaluation, in contrast to ordinary zinc phosphate films, for which the areal density is approximately 1 to 10 g/m² and the corre- sponding thickness from 1 ,000 to 8,000 Angstroms. The exact situation has therefore yet to be elucidated.

The treatment of tinplate using the surface treatment bath of the present invention is briefly explained below. The treatment bath of the present in¬ vention is used, preferably as part of the following sequence, which is provided as a preferred example:

Tinplate cleaning: degreasing (a weakly alkaline degreaser is typically used) Water wash

Surface treatment (application of treatment bath of the present invention) Treatment temperature: 30° C to 70° C Treatment technique: spray or immersion

Treatment time: 2 to 40 seconds

Water wash Wash with de-ionized water Drying.

The treatment temperature with the surface treatment bath of the present invention is preferably 30° C to 70° C, and heating the bath generally to 40° C to 60° C for use is particularly preferred. The preferred treatment time is 2 to 40 seconds. At below 2 seconds, the reaction is inadequate and a highly corro¬ sion-resistant film will not normally be formed. On the other hand, the perform¬ ance does not improve at treatment times in excess of 40 seconds, and there¬ fore optimal treatment times fall in the range of 2 to 40 seconds. While the treatment technique can be either immersion or spray, as dis¬ cussed above the present invention gives particularly good effects when used with spray equipment.

As discussed hereinbefore, the oxidation state of the iron ions that have eluted from the tinplate significantly affects sludge production. Iron ions are be- lieved to elute from the tinplate as divalent ferrous ions. In the treatment bath in accordance with the present invention, the iron ions are typically present as ferrous ions at a concentration of about 0.005 to about 0.025 g/L when the line is running, while ferric ions are essentially not present. In contrast to this, the ferrous ions are almost entirely oxidized in prior art treatment baths to yield ferric ions or colloid in a concentration typically on the level of 0.05 g/L. Sludge is produced because this ferric ion and the phosphate ion form an insoluble salt that also traps the tin and phosphate ions that are present. In other words, sludge production in the surface treatment bath can be suppressed by maintain¬ ing the iron ion eluted from the tinplate in the divalent state. By essentially omitting the oxidizing agent that has been used in prior-art treatment baths, the iron ions in the present invention consist almost completely of divalent iron ions. It is thought that this occurs because both divalent tin ions and tetravalent tin ions are present and the divalent tin ions rapidly reduce tri¬ valent iron ions to divalent iron ions. The oxidation- reduction potential of a composition is measured by the equilibrium electrode potential of an inert oxidation-reduction electrode in contact with the composition, and it represents the magnitude of the oxidizing power or reducing power of the composition. The following equation gives the oxidation-reduction potential E_e for the half-reaction oxidation of ferrous ion to ferric ion according to the chemical equation Fe²⁺ → Fe³⁺ + e^". E_e = E₀-(RT/i)ln([Fe⁺²]/[Fe⁺³]), where R = the gas constant, T = the absolute temperature, jf = Faraday's con¬ stant, square brackets indicate activities of the chemical species within the brackets, and E₀ = the standard electrode potential for the reaction. Larger val¬ ues of E_e correspond to a higher oxidizing power and thus to a higher ferric ion/ferrous ion ratio; smaller values of the oxidation-reduction potential indicate fewer ferric ions. Accordingly, the average oxidation state of the eluted iron ions can be controlled by controlling the oxidation-reduction potential.

Examples The utility of the surface treatment bath of the present invention is ex¬ plained below through a comparison of several working examples with compari- son examples. In these examples, the tinplate substrates consisted of tinplate Dl cans fabricated by the Dl processing of tin-plated steel sheet. The corrosion resistance after surface treatment was evaluated using the iron exposure value ("IEV"). The IEV was measured in accordance with United States Patent Num¬ ber 4,332,646. Lower IEV values correspond to a better corrosion resistance, and values < 150 generally correspond to an excellent corrosion resistance.

The paint adherence was evaluated through the peel strength. An epoxy/urea can paint was coated on the surface of the treated can to a paint film thickness of 5 to 7 micrometers ("μM") followed by baking for 4 minutes at

215° C. Each can was subsequently cut into 5 x 150 mm strips, and a test specimen was prepared by hot pressing polyamide film onto a strip. The test specimen was then peeled in a 180° peel test and the peel strength was mea¬ sured. In this case, larger peel strength values indicate a better paint adher¬ ence, and values of 1.5 kilograms force ("kgf")/5 mm-width or more are gener¬ ally regarded as excellent. Sludge production was evaluated as follows. 0.05 g/L of iron ions from ferrous chloride was added to the particular surface treatment bath as de¬ scribed in the working or comparison example, the pH was adjusted, the bath was allowed to stand for 1 day, and the status of the bath was then inspected. A bath that was transparent and free of precipitate or the like was judged as essentially free of ferric ion. The oxidation-reduction potential was measured after standing using a platinum electrode as the oxidation-reduction electrode and a silver-saturated silver chloride electrode as the reference electrode.

In order to evaluate sludge production during continuous treatment, a continuous treatment was run using freshly prepared surface treatment bath as reported in the particular example or comparison example. The continuous treatment used 2 liters ("L") of treatment bath, and a 30-second treatment was conducted on a total of 360 cans. The bath quantity and pH were maintained at their initial values through the addition of the particular surface treatment bath and phosphoric acid, respectively. The bath status and oxidation-reduction potential ("ORP") were evaluated after the continuous test.

A bath that was transparent and free of precipitate or the like was judged to be essentially free of ferric ion. In addition, the iron ion concentration in the treatment bath after continuous treatment was measured by atomic absorption. When a precipitate had been produced, analysis was run by dissolving the pre¬ cipitate by the addition of hydrochloric acid. Example 1 Tinplate Dl cans (fabricated by the Dl processing of tin-plated steel sheet) were (1) thoroughly cleaned using a hot 1 % aqueous solution of a weakly alkaline degreaser (FINECLEANER™ 4488 from Nihon Parkerizing Company, Limited); (2) sprayed for 20 seconds with surface treatment bath 1 heated to 60° C; (3) washed with tap water; (4) sprayed with deionized water (with a specific resistance > 3 Mohm-cm) for 10 seconds; and (5) dried in a hot-air drying oven for 3 minutes at 180° C. The treated cans were evaluated for corrosion resistance and paint adherence, and surface treatment bath 1 was evaluated for sludge production. Surface treatment bath 1 75% phosphoric acid (H₃P0₄) 10.0 g/L (PO^: 7.2 g/L)

Sodium pyrophosphate (Na₄P₂O₇ • 10 H₂O) 1.0 g/L (P₂0₇ ^4": 0.4 g/L) SnCI₄ - 5 H₂O 0.6 g/L (Sn⁴⁺: 0.2 g/L) FeCI₃ • 6 H₂O 4.8 mg/L (Fe³⁺: 1.0 mg/L)

Phosphorous acid (H₃PO₃) 0.01 g/L pH 3.0 (adjusted with sodium carbonate) The ferric chloride was added in order to examine the effect of trivalent iron ion on sludge production. Example 2

Tinplate Dl can was cleaned using the same conditions as in Example 1 , sprayed for 10 seconds with surface treatment bath 2 heated to 40° C, and then washed with water and dried under the same conditions as in Example 1. The treated can was evaluated for corrosion resistance and paint adherence, and surface treatment bath 2 was evaluated for sludge production. Surface treatment bath 2

75 % Phosphoric acid (H₃PO₄) 5.0 g/L (PO,^3": 3.6 g/L)

Sodium pyrophosphate (Na₄P₂O₇ • 10 H₂O) 2.0 g/L (P₂O₇ ^4": 0.8 g/L) SnCI₄ - 5 H₂O 1.2 g/L (Sn⁴⁺: 0.4 g/L) pH 2.8 (adjusted with phosphoric acid) Example 3

Tinplate Dl can was cleaned using the same conditions as in Example 1 , sprayed for 40 seconds with surface treatment bath 3 heated to 60° C, and then washed with water and dried under the same conditions as in Example 1. The treated can was evaluated for corrosion resistance and paint adherence, and surface treatment bath 3 was evaluated for sludge production. Surface treatment bath 3

75 % Phosphoric acid (H₃PO₄) 5.0 g/L (PO^: 3.6 g/L) Sodium pyrophosphate (Na₄P₂O₇ • 10 H₂O) 2.0 g/L (P₂0₇ ^4": 0.8 g/L) SnCI₄ - 5 H₂O 0.10 g/L (Sn⁴⁺: 0.03 g/L)

Hypophosphorous acid (H₃PO₂) 0.01 g/L pH 4.0 (adjusted with sodium hydroxide) Example 4 Tinplate Dl can was cleaned using the same conditions as in Example

1 , sprayed for 10 seconds with surface treatment bath 4 heated to 40°C, and then washed with water and dried under the same conditions as in Example 1. The treated can was evaluated for corrosion resistance and paint adherence, and surface treatment bath 4 was evaluated for sludge production.

Surface treatment bath 4

75 % Phosphoric aςid (H₃PO₄) 15.0 g/L (PO : 10.8 g/L) Sodium pyrophosphate (Na₄P₂O₇ • 10 H₂O) 2.0 g/L (P₂0₇ ^4": 0.8 g/L)

Sodium tripolyphosphate (Na₅P₃O₁₀) 1.0 g/L (P-β^: 0.6 g/L)

SnCI₄ • 5 H₂0 1.2 g/L (Sn⁴⁺: 0.4 g/L)

Phosphorous acid (H₃PO₃) 0.01 g/L

Hypophosphorous acid (H₃PO₂) 0.01 g/L pH 3.0 (adjusted with sodium carbonate)

Example 5

Tinplate Dl can was cleaned using the same conditions as in Example

1 , sprayed for 30 seconds with surface treatment bath 5 heated to 50° C, and then washed with water and dried under the same conditions as in Example 1. The treated can was evaluated for corrosion resistance and paint adherence, and surface treatment bath 5 was evaluated for sludge production.

Surface treatment bath 5

75 % Phosphoric acid (H₃P0₄) 1.0 g/L (PO^: 0.7 g/L)

Sodium pyrophosphate (Na₄P₂0₇ • 10H₂O) 2.0 g/L (P₂0₇ ^4": 0.8 g/L) SnCI₄ - 5H₂0 1.2 g/L (Sn⁴⁺: 0.4 g/L)

Phosphorous acid (H₃P0₃) 0.01 g/L (H₃P0₃: 0.01 g/L) pH 3.0 (adjusted with phosphoric acid)

Example 6

Tinplate Dl can was cleaned using the same conditions as in Example 1 , sprayed for 20 seconds with surface treatment bath 6 heated to 50°C, and then washed with water and dried under the same conditions as in Example 1.

The treated can was evaluated for corrosion resistance and paint adherence, and surface treatment bath 6 was evaluated for sludge production.

Surface treatment bath 6

75 % Phosphoric acid (H₃PO₄) 5.0 g/L (PO,,^3": 3.6 g/L)

Sodium pyrophosphate (Na₄P₂O₇ • 10H₂O) 2.0 g/L (P₂0₇ ^4': 0.8 g/L) Tin (by dissolution of tin metal) 0.2 g/L (Sn²⁺: 0.2 g/L) Phosphorous acid (H₃PO₃) 0.01 g/L (H₃PO₃: 0.01 g/L) pH 3.0 (adjusted with phosphoric acid) Example 7

Tinplate Dl can was cleaned using the same conditions as in Example 1 , sprayed for 2 seconds with surface treatment bath 7 heated to 70° C, and then washed with water and dried under the same conditions as in Example 1. The treated can was evaluated for corrosion resistance and paint adherence, and surface treatment bath 7 was evaluated for sludge production. Surface treatment bath 7 75 % Phosphoric acid (H₃P0₄) 30.0 g/L (PO ^": 21.6 g/L)

Sodium pyrophosphate (Na₄P₂O₇ • 10H₂O) 2.0 g/L (P₂0₇ ^4": 0.8 g/L) Sodium tripolyphosphate (Na₅P₃O₁₀) 1.0 g/L 0.6 g/L)

SnCI₄ • 5H₂O) 1.2 g/L (Sn⁴⁺: 0.4 g/L)

Phosphorous acid (H₃P0₃) 0.01 g/L Hypophosphorous acid (H₃PO₂) 0.01 g/L pH 2.0 (adjusted with phosphoric acid) Comparison Example 1

Tinplate Dl can was cleaned using the same conditions as in Example 1 , sprayed for 30 seconds with surface treatment bath 8 heated to 40° C, and then washed with water and dried under the same conditions as in Example 1. The treated can was evaluated for corrosion resistance and paint adherence, and surface treatment bath 8 was evaluated for sludge production. Surface treatment bath 8

75 % Phosphoric acid (H₃P0₄) 10.0 g/L (PO^: 7.2 g/L) SnCI₄ • 5H₂0 0.6 g/L (Sn⁴⁺: 0.2 g/L)

Phosphorous acid (H₃P0₃) 0.01 g/L pH 3.0 (adjusted with sodium carbonate) Comparison Example 2

Tinplate Dl can was cleaned using the same conditions as in Example 1 , sprayed for 30 seconds with surface treatment bath 9 heated to 50° C, and then washed with water and dried under the same conditions as in Example 1.

The treated can was evaluated for corrosion resistance and paint adherence, and surface treatment bath 9 was evaluated for sludge production. Surface treatment bath 9

75 % Phosphoric acid (H₃PO₄) 10.0 g/L (PO;^3": 7.2 g/L)

Sodium pyrophosphate (Na₄P₂O₇ • 10H₂O) 1.0 g/L (P₂0₇ ^4": 0.4 g/L) 5 SnCI₄ • 5H₂O 0.6 g/L (Sn⁴⁺: 0.2 g/L)

Phosphorous acid (H₃P0₃) 0.01 g/L pH 4.6 (adjusted with sodium hydroxide) Comparison Example 3

Tinplate Dl can was cleaned using the same conditions as in Example o 1 , sprayed for 30 seconds with surface treatment bath 10 heated to 50° C, and then washed with water and dried under the same conditions as in Example 1. The treated can was evaluated for corrosion resistance and paint adherence, and surface treatment bath 10 was evaluated for sludge production. Surface treatment bath 10 5 75 % Phosphoric acid (H₃PO₄) 1.33 g/L (PO ^": 0.97 g/L)

Sodium pyrophosphate (Na₄P₂O₇ • 10H₂O) 1.0 g/L (P₂0₇ ^4": 0.4 g/L) SnCI₄ - 5H₂0 0.6 g/L (Sn⁴⁺: 0.2 g/L)

FeCI₃ - 6H₂0) 48 mg/L (Fe³⁺: 10 mg/L) pH 4.0 (adjusted with sodium carbonate) o Comparison Example 4

Tinplate Dl can was cleaned using the same conditions as in Example 1 and was then sprayed for 30 seconds with a 4 % aqueous solution (heated to 50° C) of a commercial tinplate Dl can surface treatment agent (PALFOS™ K3466 from Nihon Parkerizing Company, Limited). This was followed by wash- 5 ing with water and drying under the same conditions as in Example 1. The treated can was evaluated for corrosion resistance and paint adherence, and the treatment bath was evaluated for sludge production. Comparison Example 5

Tinplate Dl can was cleaned using the same conditions as in Example o 1 and was then sprayed for 30 seconds with a 4 % aqueous solution (heated to 50° C) of a commercial tinplate Dl can surface treatment agent (PALFOS™ K3482 from Nihon Parkerizing Company, Limited). This was followed by washing with water and drying under the same conditions as in Example 1. The treated can was evaluated for corrosion resistance and paint adherence, and the treatment bath was evaluated for sludge production.

The results are reported in Table 1. Benefits of the Invention

As discussed in the preceding, treating the surface of tinplate (tin-plated steel) sheet, strip, or shaped objects (cans or the like) with the surface treat¬ ment bath of the present invention accrues the highly desirable effects of imparting an excellent corrosion resistance and adherence to the tinplate sur- face and avoiding sludge production in the treatment bath when treatment is run on a continuous basis.

Claims

1. An aqueous liquid composition suitable for treating tinplate surfaces, said composition having a pH in the range from 2.0 to 4.5 and an oxidation- reduction potential not greater than 450 mV more oxidizing than a silver-saturated silver

5 chloride reference electrode, and consisting essentially of water and:

(A) from 1 to 30 g/L of phosphate ions,

(B) from 0.1 to 5.0 g/L of chelating agent, and

(C) from 0.01 to 2.0 g/L of tin ions.

2. A composition according to claim 1 , wherein the chelating agent is selected o from condensed phosphate ions.

3. A composition according to claim 2, wherein the concentration of tin ions is from 0.1 to 1.0 g/L and the pH is from 2.5 to 3.5.

4. A composition according to claim 3, wherein the concentration of tin ions is from 0.2 to 0.6 g/L and the pH is from 2.7 to 3.3. ⁵ 5. A composition according to claim 1 , wherein the concentration of tin ions is from 0.1 to 1.0 g/L and the pH is from 2.5 to 3.

5.

6. A composition according to claim 5, wherein the concentration of tin ions is from 0.2 to 0.6 g/L and the pH is from 2.7 to 3.3.

7. A composition according to claim 6, wherein the concentration of phosphate o ions is within the range from 4 to 8 g/L and the concentration of chelating agent is within the range from 0.2 to 1.0 g/L.

8. A composition according to claim 5, wherein the concentration of phosphate ions is within the range from 4 to 8 g/L and the concentration of chelating agent is within the range from 0.2 to 1.0 g/L. 5

9. A composition according to claim 4, wherein the concentration of phosphate ions is within the range from 4 to 8 g/L and the concentration of chelating agent is within the range from 0.2 to 1.0 g/L.

10. A composition according to claim 3, wherein the concentration of phosphate ions is within the range from 4 to 8 g/L and the concentration of chelating agent is within the range from 0.2 to 1.0 g/L.

11. A composition according to claim 2, wherein the concentration of phosphate 5 ions is within the range from 4 to 8 g/L and the concentration of chelating agent is within the range from 0.2 to 1.0 g/L.

12. A composition according to claim 1 , wherein the concentration of phosphate ions is within the range from 4 to 8 g/L and the concentration of chelating agent is within the range from 0.2 to 1.0 g/L. ⁰

13. A process for treating tinplate to form a protective coating thereon, comprising contacting the tinplate with a composition according to any one of claims 1 - 12 at a temperature within the range from 30 to 70 ° C for a time within the range from 2 to 40 seconds.

14. A process according to claim 13, wherein a reducing agent is added to the s initial composition as the latter is used, in an amount sufficient to maintain the oxidation-reduction potential of the composition not more than 450 mV more oxidizing than a silver-saturated silver chloride reference electrode.

15. A process according to claim 14, wherein the reducing agent is selected from the group consisting of phosphorous acid, hypophosphorous acid, their salts, o and mixtures of any two or more of said acids and their salts.