EP0724488A1

EP0724488A1 - Process for treating zinciferous surfaces

Info

Publication number: EP0724488A1
Application number: EP94927906A
Authority: EP
Inventors: Kensuke Mizuno; Takao Ogino; Jyun Kawaguchi
Original assignee: Henkel Corp
Current assignee: Henkel Corp
Priority date: 1993-08-14
Filing date: 1994-08-11
Publication date: 1996-08-07
Also published as: EP0724488A4; TW247321B; ZA946049B; CA2169193A1; CN1102442A; KR950006022A; JPH0754156A; AU7714194A; JP3288152B2; KR100311062B1; WO1995005247A1

Abstract

A solid 10-200 mg/m2, calculated as chromium metal, film is formed on a zinciferous surface by coating the surface with an acidic aqueous liquid composition that contains hexavalent chromium ions, trivalent chromium ions, at least one of sulfate ions and nitrate ions, and zinc, nickel, cobalt, and/or aluminum ions, in which liquid composition the trivalent chromium ion/hexavalent chromium ion molar ratio is 1/9 to 4/1, the (nitrate + sulfate + phosphate)/(hexavalent chromium ion + trivalent chromium ion) molar ratio is 0.1 to 2.0 and the metal ion/inorganic acid molar ratio is 0.05 to 1, and subsequently drying the applied coating. The chromate film formed has both an excellent resistance to blackening and an excellent resistance to white rust.

Description

PROCESS FOR TREATING ZINCIFEROUS SURFACES

Technical Field

The invention relates to a process for treating zinciferous surfaces to give them an excellent resistance to both blackening and white rusting. The process, which uses a one-step chromate treatment that does not require a preliminary flash treatment with, e.g., Ni or Co, is particularly useful for the manufacture of zinciferous-plated steel sheet. As used herein, "zinciferous-plated" refers to elec- trogalvanization, hot-dip zinc plating, hot-dip 5% Al/Zn plating, hot-dip 55% Al/Zn plating, and the like. The process is largely described below with respect to zin¬ ciferous-plated steel sheet, but is equally applicable to any other surface with the same chemical composition. Background Art

The use of zinc plating to provide iron and steel with sacrificial corrosion resistance has proven generally to be the most effective and economical ap¬ proach, and for this reason 10 % of Japan's annual raw steel output of 100 mil- lion tons, or 10 million tons, is produced as galvanized steel sheet. Galvanized steel sheet is used in a variety of applications, for example, as a construction ma¬ terial, in automobiles, and for household electrical appliances. The sacrificial cor¬ rosion resistance conferred by zinc is based on the formation of a galvanic cell by bringing two different metals into contact, in this case zinc and steel (or iron). The zinc, as the baser metal, forms an anode and thereby cathodizes the iron. This inhibits corrosion of the steel or iron by preventing the anodic dissolution due to local cell formation that occurs in the case of iron by itself. Thus, since the anticorrosion activity ceases when the zinc in contact with the iron or steel has been consumed, corrosion of the zinc layer must also be inhibited if its sacri- ficial anticorrosion activity is to be maintained for long periods of time. One coun- termeasure to corrosion of the zinc layer consists of the execution of a chromate treatment after plating.

This chromating-based anticorrosion strategy is, however, not without its problems. While the chromate treatment of zinc-plated steel sheet does lead to a substantial inhibition of white rust development, it still permits the development of aesthetically undesirable black rust (also known as blackening) during storage and transport. It has been observed that this blackening phenomenon occurs more readily for steel sheet subjected to a post-galvanization skin pass and for Zn/AI-plated (several % Al) steel sheet.

Flash treatment, for example, with an aqueous solution containing Ni²⁺ and/or Co²⁺ as described in, e.g., Japanese Laid Open [Kokai or Unexamined] Patent Application Number Sho 59-177381 [177,381/1984], is currently regarded as a very effective countermeasure for solving this post-chromating black rust problem. Here and below, flash treatment is taken to mean the chemical deposi¬ tion of very small quantities of metal. In its essential features, the above-refer¬ enced invention concerns a means for inhibiting black rust through the pre-chro- mating flash treatment of the surface of zinc-plated or zinc alloy-plated steel sheet. In this particular flash treatment, the surface of the steel sheet is flash- treated with an aqueous solution (pH 1 to 4 or 11 to 13.5) that contains Ni ⁺ and/or Co²⁺ metal ions. This treatment results in the deposition of these metal ions on the plating surface as the metal or oxide. This treatment is followed by a water wash and then the formation of a chromate film. The mechanism for black rust inhibition on zinc-plated or zinc alloy-plated steel sheet that has been flash-treated with Ni and/or Co and then chromated has yet to be clearly established. However, according to the report on pages 150 to 151 of Abstracts of the 60th Scientific Lecture Conference of Kinzoku Hyomen Gijutsu Kyokai [Metal Finishing Society of Japan], the flashed metal is largely de- posited at the zinc crystal grain boundaries. It is further reported that the chromi¬ um compounds laid down in a follow-on coating-type chromate treatment are, like the flashed metal, also distributed at the grain boundaries. Based on this, it is presumed that some type of interaction, which results in adsorption and immob¬ ilization of the chromium compounds on the flashed metal, occurs between the flashed metal and the chromium compounds. Disclosure of the Invention

Problems to Be Solved bv the Invention Zinc black rust, like the corresponding white rust, is thought to be a basic zinc carbonate (ZnC0₃)_x [Zn(OH)₂]_y; however, it differs from white rust in that it is stoichiometrically oxygen deficient. Black rust is therefore a corrosion product occurring under oxygen-deficient conditions, and in particular it is thought that black rust is formed in association with the development of corrosion from the grain boundaries. As a result, it can be postulated that the chromium compounds concentrated at the grain boundaries by the flashed metal contribute to an inhibi- tion of black rust development by inhibiting corrosion originating from the grain boundaries. It is in this way that the Ni and/or Co flash treatment of zinc-plated steel sheet prior to the chromate treatment thereof is believed to be able very effectively to counter the problem of black rust. However, despite its ability to in¬ hibit black rust, this pre-chromating flash treatment results in the rather facile de- velopment of white rust. It is precisely this problem that the present invention seeks to solve.

Summary of the Invention The invention offers a one-step process that consists of only a chromate treatment and that does not employ a pretreatment, but is able to simultaneously inhibit both blackening and white rust. This one-step process renders unneces¬ sary the two-step prior art process of a flash treatment with Ni and/or Co plus a chromate treatment. This process yields zinciferous-plated steel sheet that ex¬ hibits a satisfactory corrosion resistance, blackening resistance, and white rust resistance. Moreover, being a one-step process, it avoids the drawbacks in the prior art due to the use of a two-step treatment process (pretreatment followed by chromate treatment). This novel one-step chromate treatment process is the subject of Japanese Patent Application Number Hei 4-164083 [164,083/1992]. The invention in Japanese Patent Application Number Hei 4-164083 re¬ lates to a process for fabricating zinciferous-plated steel sheet that has an excel- lent resistance to both blackening and white rusting, wherein said process is characterized by the formation on zinciferous-plated steel sheet of a 10 - 200 mg/m² (calculated as chromium metal) film by: (I) application to zinciferous-plated steel sheet of a chromate bath that contains hexavalent chromium ion, trivalent chromium ion, and inorganic acid comprising at least one selection from nitric acid, boric acid, and sulfuric acid, and in which the trivalent chromium ion/hexavalent chromium ion weight ratio is 1/9 to 4/1 and the inorganic acid/total chromi¬ um ion (hexavalent chromium ion + trivalent chromium ion) weight ratio is 0.3 to 2.0, and

(II) subsequently drying the applied coating.

As a further improvement of this previous invention, the subject of this ap- plication is a process characterized by the formation on zinciferous-plated steel sheet of a solid 10 - 200 mg/m² (calculated as chromium metal) film by: (I) application to zinciferous-plated steel sheet, so as to form a liquid coating thereon, of a chromate bath that comprises, preferably consists essential¬ ly of, or still more preferably consists of: (A) hexavalent chromium ions,

(B) trivalent chromium ions,

(C) at least one of nitrate ions and sulfate ions, and

(D) metal ions selected from the group consisting of Zn, Ni, Co, and Al ions and mixtures of any two or more thereof, and, optionally, one or more of:

(E) phosphate ions from an inorganic acid and

(F) colloidal silica, in which:

(i) the trivalent chromium ion/hexavalent chromium ion molar ratio is 1/9 to 4/1,

(ii) the total of nitrate + sulfate + phosphate ions/total chromium ions (i.e., hexavalent chromium ions + trivalent chromium ions) molar ratio is 0.1 to 2.0, (iii) the total of Zn + Ni + Co + Al ions/the total of nitrate + sulfate + phosphate ions molar ratio is 0.05 to 1 , and

(iv) the molar ratio of phosphate ions/total chromium ions does not ex¬ ceed 0.3; and (II) subsequently drying the applied coating.

For the purposes of this description, the entire stoichiometric equivalent as phos¬ phate ions of any phosphoric acid and/or anions produced by ionization thereof that are added to the chromate bath are counted as phosphate ions, irrespective of the actual degree of ionization of the added acid. Preferably, the acid added is orthophosphoric acid H₃P0₄, but other inorganic phosphoric acids such as HP0₃, H₄P₂0₇, H₅P₃O₁₀ may also be used. Also, it is to be understood that the constituents specified in ionic form must be accompanied by counterions so that the composition as a whole is electrically neutral. Such counterions preferably are chosen from other specified constituents of the invention to the extent possi¬ ble, and otherwise may be freely chosen, except for avoiding counterions that are detrimental to the purposes of the invention.

Description of Preferred Embodiments The trivalent chromium ion/hexavalent chromium ion molar ratio in the subject chromate treatment bath is preferably 1/9 to 4/1 and more preferably is 1/4 to 7/3. Trivalent chromium ion/hexavalent chromium ion molar ratios below 1/9 are undesirable because such values usually result in excessive chromium ion elution in corrosive environments and thus in pollution of the environment. When the trivalent chromium ion/hexavalent chromium ion molar ratio exceeds , 4/1 , the corrosion resistance becomes inadequate.

Sulfate and/or nitrate ions as required for the chromating treatment ac¬ cording to the invention are preferably added at least partially in the form of the corresponding acids. These anions are added in a quantity that yields values preferably of 0.1 to 2.0 and more preferably of 0.2 to 1.6 for the sulfate + nitrate + phosphate anions/total chromium ions molar ratio. The blackening resistance is usually inadequate when the inorganic anions/total chromium ions molar ratio is less than 0.1. Inorganic anions/total chromium ions molar ratios in excess of 2.0 are undesirable because such values usually result in a degraded corrosion resistance due to a deficient chromium immobilization ratio in the resulting chro- mate film.

The metal ion/inorganic anion molar ratio in the subject chromate bath is to be 0.05 to 1. The pH of the chromate bath is too low when the metal ion/inor- ganic anion molar ratio is below 0.05. As a consequence, the reaction is too vig¬ orous when the chromate bath and zinciferous-plated steel sheet are brought into contact, so that there is a whitening of the zinciferous-plated steel sheet and a degraded corrosion resistance due to an elevated chromium immobilization ratio in the resulting chromate film. When this ratio has a value in excess of 1 , the chromate bath has a reduced stability and precipitation readily occurs.

Any metal ion can be used that is capable of dissolving in the chromate bath of the present invention, but Zn, Ni, Co, and Al are particularly preferred. Alkali metals such as Na and K are undesirable because they cause a substan- tial decline in the chromium immobilization ratio of the chromate film. The addi¬ tional presence of phosphoric acid as inorganic acid in the subject chromate treatment bath functions to raise the chromium immobilization ratio of the chrom¬ ate film. When phosphoric acid is added, it is preferably added in a quantity such that the phosphate ions/total chromium ions molar ratio does not exceed 0.3. While phosphate ions/total chromium ions molar ratios above 0.3 do provide an increased chromium immobilization ratio for the resulting chromate film, such val¬ ues are nevertheless undesirable because they cause the resistance to blacken¬ ing to deteriorate.

The instant chromate treatment bath may of course optionally contain col- loidal silica as known from the art, for example, silica sol, fumed silica, and the like.

The chromate film is formed by coating the coating-type blackening-resist- ant chromate bath prepared as described above on the surface of zinciferous- plated steel sheet followed by drying without a water wash. The film weight pref- erably falls in the range of 10 to 200 mg/m² calculated as chromium metal. A sat¬ isfactory corrosion resistance is not usually obtained when the chromate film weight falls below 10 mg/m² as chromium metal. At the other extreme, the anti¬ corrosion performance is saturated at chromate film weights in excess of 200 mg/m² as chromium metal, and such values are therefore uneconomical. The first essential step in treatment according to the invention begins by applying the treatment bath to the surface of the zinciferous plating, e.g., by spraying the treatment bath on the workpiece or immersing the workpiece in the treatment bath, followed in either case by a roll or air wipe, or by applying the treatment bath to the surface of the zinciferous plating by known coating meth¬ ods, for example, by roll coating. After forming a liquid film of substantially uni¬ form thickness of the treatment bath over the surface being treated, the final pro- tective film is formed by drying the liquid film without any water wash or rinse. The drying conditions should be sufficient to evaporate the water in the coating, and drying is generally carried out at a sheet temperature of 40° C to 100° C. The function and activity of each of the constituent components of the in¬ vention are not known with certainty. However, the sulfate and/or nitrate anions present in the coating-type chromate treatment bath appear to suppress blacken¬ ing by inhibiting the production of basic zinc carbonate (ZnC0₃)_x [Zn(OH)₂]_y through the formation of the corresponding salts, for example, Zn(N0₃)₂, etc., between zinc and the inorganic acid.

With regard to the metal ion additive and particularly the Co ion, it has been reported (Tetsu to Hagane, Volume 76, pp. 383-390 (1990)) that this addi¬ tive can inhibit blackening by inhibiting cathodic reactions on the plating surface and retarding zinc corrosion (oxidation). The metal ion added in the present in¬ vention presumably exercises the same effect.

Thus, the instant coating-type chromate bath containing hexavalent chromium ion, trivalent chromium ion, inorganic acid, and metal ion is able to pro¬ vide both a blackening-inhibiting activity (based on the inorganic acid and metal ion) and a white rust-inhibiting activity (based on the hexavalent chromium ion and trivalent chromium ion).

The mechanism underlying the white rust-inhibiting activity of chromate films is generally thought to consist of an inhibitor effect in which the hexavalent chromium ion inhibits zinc corrosion. Another mechanism, as reported in Zairyo to Kankyo, Volume 41 , pp. 244-245 (1992), consists of a self-repair activity exer¬ cised by hexavalent chromium ion that elutes from the film. In this mechanism, the eluted hexavalent chromium ion repairs damaged regions in the film. Accord- ingly, when the chromium immobilization ratio is higher than necessary, the ab¬ solute amount of hexavalent chromium ion available to function as inhibitor will be inadequate. In addition, the inhibitor effect cannot be maintained, i.e., is not durable, when the chromium immobilization ratio is too low. The white rust-inhib¬ iting activity is inadequate in either case. These phenomena make it clear that the chromium immobilization ratio will have an optimal range.

The chromium immobilization ratio and the Cr'VCr⁶* ratio are closely re- lated, and it is known that the presence of trivalent chromium ions is essential for inhibiting hexavalent chromium ion elution and raising the chromium immobiliza¬ tion ratio. However, in the case of an aqueous solution containing only trivalent chromium ions and hexavalent chromium ion, the Cr'VCr⁶* molar ratio must be held below 2/3 in order for Cr³* ions to remain in the aqueous solution as ions without precipitation. On the other hand, Cr'VCr^6* ratios in excess of 2/3 are de¬ sirable in order to improve the white rust-inhibiting activity. The addition of an acid component other than chromic acid is a tactic that can be considered for the purpose of raising the Cr³7Cr⁶⁺ ratio.

The chromium immobilization ratio is also affected by the reactivity pre- vailing when the chromate bath and zinciferous-plated steel sheet are brought into contact (abbreviated below as the contact reactivity). The addition of even very small quantities of the nitric acid or sulfuric acid that are added to the chromate bath of the present invention causes a sharp drop in the pH of the chromate bath, which increases the contact reactivity and ultimately results in a higher than necessary chromium immobilization ratio. Thus, the chromate bath pH will clearly have an optimal range. The following methods can be considered for adjusting the pH of a chromate bath whose pH has been reduced by the addi¬ tion of inorganic acid: (i) the addition of a metal oxide, hydroxide, or carbonate to the reduced pH chromate bath; (ii) in the case of addition of nitric acid or sul- furic acid to the chromate bath, the addition of metal salts of these acids.

The inventors carried out detailed investigations into the effects obtained when the specified inorganic anions and metal ions stipulated in the present specification are added to chromate baths at particular molar ratios relative to the total chromium ions. These investigations determined that the addition of both inorganic anions and metal ions as reported below resulted in the same activity as for Ni or Co flash treatment followed by chromate treatment. The reasons for this have yet to be satisfactorily resolved, but it is thought that an anti-blackening activity is produced by the concentration of chromium compounds at the grain boundaries of the zinc crystals and by the formation of Zn(N0₃)₂, etc. An anti- blackening effect also appears to be produced by an inhibition of cathodic reac¬ tions on the galvanized surface by the metal ion. White rust prevention is thought to be based on a persistent or continuing elution of hexavalent chromium ion that is induced by holding the chromium immobilization ratio within an optimal range. Thus, the process of the present invention is able, as described above, to render these two effects or activities compatible.

The present invention will be explained in greater detail below through working examples, but these are provided simply to explain the invention and should not be construed as limiting the present invention in any manner.

Examples General Procedure

Each sample test sheet in the examples and comparative examples was alkaline degreased, water washed, and dried as described in items (2), (3), and (4) below. The sample test sheet was then subjected to a chromate treatment using the procedures outlined in items (5) and (6) below. Each sample test sheet, after treatment as specified above, was subjected to accelerated black rust testing and accelerated white rust testing as described below. The stability of the treatment baths was also tested.

(1 ) sample test sheets: hot-dip zinc-plated steel sheet hot-dip 5% Al/Zn-plated steel sheet hot-dip 55% Al/Zn-plated steel sheet (2) alkaline degreasing:

30 second spray at 60° C using a weakly alkaline degreaser (Pal- klin™ 342 from Nihon Parkerizing Kabushiki Kaisha, 2 % aqueous solu¬ tion)

(3) cleaning: 10 second spray with tap water (4) drying: drying in an air current

(5) chromate treatment:

The chromate film weight was controlled through the wet pick-up in roll coating.

(6) drying: maximum attained sheet temperature = 100° C, drying time = 5 seconds

(7) chromium immobilization ratio:

The chromate-treated zinciferous-plated steel sheet was alkali- washed under the conditions given below, and the chromium add-on in mg/m² was measured using an X-ray fluorescence analyzer both before and after this alkali wash. The chromium immobilization ratio was then calculated from the equation given below (a higher percentage indicates a higher chromium immobilization ratio), Chromium immobilization ratio = (A/A,,) x 100 (%), where:

A chromium film weight after the alkali wash, in mg/m²

A,, chromium film weight before the alkali wash, in mg/m²

The alkali wash consisted of a 2 minute spray at 60° C with a 2 % aqueous solution of a sodium silicate-based alkaline degreaser (Palklin™

N364S from Nihon Parkerizing Kabushiki Kaisha).

(8) accelerated black rust testing ("ABRT"):

Multiple 70 x 150 mm test coupons were cut from each sample test sheet. The test surfaces of two test coupons were laid against each other to give a pair, and 5 to 10 pairs were stacked. The stack was wrapped with vinyl-coated paper, the four corners were tightened down with bolts, and a 0.67 kgf»cm load was applied with a torque wrench. This assembly was held in a humidistat for 240 hours at 70° C/RH 80 %. After removal, blackening of the counterfacing areas was visually evaluated. The evalu- ation standards are given below. 5: no blackening

4: very slight graying

3: less than 25 % blackening

2: blackening from 25 % to less than 50 % 1 : 50 % or more blackening.

(9) accelerated white rust testing ("AWRT"):

A 70 x 150 mm test coupon was cut from each sample test sheet and submitted to salt-spray testing as specified in JIS Z2371. The area of white rust development was visually inspected after 72 hours for the hot-dip zinc-plated steel sheet, after 240 hours for the 5 % Al/Zn-plated steel sheet, and after 600 hours for the 55 % Al/Zn-plated steel sheet. The evaluation standards are given below. 5: no white rust

4: less than 5 % area of white rust development 3: area of white rust development from 5 % to less than 25 %

2: area of white rust development from 25 % to less than 50 % 1 : area of white rust development of 50 % or more. Tables 1 - 4 report the composition of the chromate treatment baths, the component ratios for the treatment baths, the pH, the film weight, the chromium immobilization ratio, and the results of accelerated black rust testing and acceler¬ ated white rust testing.

Table 1: RESULTS FOR HOT-DIP ZINC-PLATED STEEL SHEETS

Example or Comparison Film Mass, Chromium Example No. Milligrams/ Immobiliza¬ ABRT AWRT or the Chro- Square Meter tion Ratio, % mating Bath

Ex. 1 12 70.2 5 5

Ex. 2 13 75.3 5 ^" 5

Ex. 3 12 77.2 5 5

Ex. 4 50 65.9 5 5

Ex. 5 23 67.4 5 5

Ex. 6 12 83.5 5 5

Ex. 7 11 78.4 5 5

C. Ex. 1 12 26.7 2 1

C. Ex. 2 12 98.1 4 1

C. Ex. 3 13 86.0 1 5

C. Ex. 4 12 92.5 2 4

Notes for Table 1

"AWRT" = Accelerated White Rust Testing; "ABRT" = Accelerated Black Rust

Testing; "Ex." = Example; "C. Ex. = Comparison Example.

The compositions of the chromating treatment baths are given in Table 4.

The wet pick-up mass of the treatment bath was about 1.2 milliiiters/meter².

The film mass was measured as its stoichiometric equivalent as chromium metal.

Table 2

RESULTS FOR HOT-DIP 5 % ALUMINUM/ZINC-PLATED STEEL SHEETS

Example or Comparison Film Mass, Chromium Example No. Milligrams/ Immobiliza¬ ABRT AWRT for the Chro- Square Meter tion Ratio, % ating Bath

Ex. 1 25 73.4 5 5

Ex. 2 26 76.7 5 5

Ex. 3 24 75.5 5 5

Ex. 4 110 62.1 5 5

Ex. 5 54 65.0 5 5

Ex. 6 25 80.1 5 5

Ex. 7 23 85.5 5 5

C. Ex. 1 26 26.0 2 1

C. Ex. 2 25 99.0 4 1

C. Ex. 3 28 90.0 1 5

C. Ex. 4 25 91.5 1 4

Notes for Table 2

The wet pick-up mass of the treatment bath was about 2.5 milliliters/meter².

Other notes are the same as for Table 1.

Table 3 RESULTS FOR HOT-DIP 55 % ALUMINUM/ZINC-PLATED STEEL SHEETS

Example or Comparison Film Mass, Chromium Example No. Milligrams/ Immobiliza¬ ABRT AWRT for the Chro- Square Meter tion Ratio, % mating Bath

Ex. 1 40 69.7 5 5

Ex. 2 41 70.4 5 5

Ex. 3 40 71.5 5 5

Ex. 4 165 60.1 5 5

Ex. 5 124 62.2 5 5

Ex. 6 42 75.0 5 5

Ex. 7 41 85.8 5 5

C. Ex. 1 41 24.9 3 1

C. Ex. 2 42 95.0 4 1

C. Ex. 3 40 90.5 1 5

C. Ex. 4 40 96.0 2 4

Notes for Table 3

The wet pick-up mass of the treatment bath was about 4 milliliters/meter². Other notes are the same as for Table 1.

Table 4: COMPOSITIONS OF THE CHROMATING TREATMENTS

Compositional Characteristic In Example No.: In Comparison Example No.

1 2 3 4 5 6 7 1 2 3 4

Cr⁺³, grams per liter 2.1 3.1 3.9 19.2 11.0 6.8 7.5 2.1 5.5 3.9 5.5

Cr⁺⁶, grams per liter 7.9 6.9 6.1 20.8 9.0 3.2 2.5 7.9 6.5 6.1 4.5

Co(NO₃)₂, grams per liter 5.3

HNO₃, grams per liter 12.1 34.0 9.7 21.8 21.8

Basic Nickel Carbonate, grams per liter 19.5 17.0 11.7

H₂SO₄, grams per liter 68.0

ZnO, grams per liter 6.26

Silica, grams per liter 60

H₃BO₃, grams per liter 9.6

Basic Cobalt Carbonate, grams per liter 30.8 2.6

H₃PO₄, grams per liter 4.2 3.0 7.0 10.

Al(OH)₃, grams per liter 7.5 1.5

H₂ZrF₆, grams per liter 1.0 -

CoSO₄, grams per liter 5.9

Ni(NO₃)₂, grams per liter 7.0 pH 1.6 2.3 2.6 1.2 1.2 1.5 1.4 2.6 0.5 1.3 1.1

Table continued on next page

Compositional Characteristic In Example No.: In Comparison Example No.

1 2 3 4 5 6 7 1 2 3 4

Molar Ratio of Cr÷VCr⁴* 0.27 0.45 0.64 0.92 1.22 2.13 3.0 0.27 1.22 0.64 1.2

Molar Ratio of NO₃7(Cr⁴³ + Cr⁴*) 0.3 1.0 1.4 1.2 1.8 1.8

Molar Ratio of Co⁺²/NO₃ ^" 0.5

Molar Ratio of Ni⁺²/NO₃ ^" 0.5

Molar Ratio of SO₄ (Cr⁺³ + Cr⁴⁶) 0.9 0.9 0.2

Molar Ratio of Zn⁺7SO₄ ^"2 0.44

Weight Ratio of SiO^Cr⁴³ + Cr⁴⁶) 6

Molar Ratio of BO₃7(Cr⁴³ + Cr⁴⁶) 0.2 σι

Molar Ratio of Co⁴²/(BO₃ ³ + SO₄ ^"2) 0.27

Molar Ratio of (Al⁴³ + Ni⁺²)/(Cr⁴³ + Cr⁴⁶) 0.20

Molar Ratio of PO₄ ^"3/(Cr⁴³ + Cr⁴⁶) 0.11 0.16 0.37 0.5

Molar Ratio of Al⁺³/(PO₄ ^"3 + NO₃ ) 0.17

Weight Ratio of r^ZrF^Cr⁴³ + Cr⁴*) 0.1

Molar Ratio of (Co⁴² + Ni⁺²)/(NO₃ ^" + SO₄ ^"2) 0.29

Molar Ratio of Co⁴² PO₄ ⁴³) 0.1

Notes for Table 4

The basic nickel carbonate used has the chemical formula 3 NiCO₃ • Ni(OH)₂ • 20 H₂O and contains 29 % Ni.

The basic cobalt carbonate used is believed to contain cobalt carbonate and cobalt hydroxide along with water and contains 44 % Co.

The silica used was SNOTEX™ O, a colloidal silica commercially supplied by Nissan Kagaku Kabushiki Kaisha.

As reported in Tables 1 to 3, execution of a chromate treatment of the present invention on galvanized steel sheet is able to improve the chromium im¬ mobilization ratio, blackening resistance, and white rust resistance of various types of hot-dip galvanized steel sheet. Moreover, this is achieved even without the use of a metal flash treatment, and a good balance is also obtained among the aforesaid properties. Thus, the process of the invention makes possible a one-step treatment, which consists of only a chromate treatment, that can re¬ place the two-step process of a pretreatment followed by a chromate treatment. This clearly has important industrial effects, for example, providing an efficient continuous operation. In contrast to this, while the composition of Comparative Example 2 does provide an anti-black rust activity, white rust resistance is poor. Comparative Examples 3 and 4 produce a moderate activity with regard to the chromium immobilization ratio and white rust resistance, but they lack any anti- black rust activity. The composition in Comparative Example 1 performs poorly in all areas (chromium immobilization ratio, anti-black rust activity, and anti-white rust activity) Benefits of the Invention

As has been discussed in detail above, the chromate treatment process of the present invention offers very substantial practical results for zinciferous- plated steel sheet. Not only does the invention process improve both the resist¬ ance to black rust and the resistance to white rust, it also stabilizes and improves the chromium immobilization ratio and the appearance of the chromate film.

Claims

1. A process for treating a zinciferous surface to increase the resistance of the surface to both black and white rusting, wherein a solid surface layer that has from 10 - 200 mg/m², calculated as chromium metal, is formed on the zinciferous surface by:

(I) coating on the zinciferous surface a layer of an aqueous liquid chromate bath composition that consists essentially of water and:

(A) hexavalent chromium ions,

(B) trivalent chromium ions, (C) at least one of nitrate ions and sulfate ions, and

(E) phosphate ions from an inorganic acid and (F) colloidal silica, and in which:

(i) the trivalent chromium ion/hexavalent chromium ion molar ratio is

1/9 to 4/1, (ii) the total of nitrate + sulfate + phosphate ions/total chromium ions (i.e., hexavalent chromium ions + trivalent chromium ions) molar ratio is 0.1 to 2.0, (iii) the total of Zn + Ni + Co + Al ions/the total of nitrate + sulfate + phosphate ions molar ratio is 0.05 to 1 , and (iv) the molar ratio of phosphate ions to total chromium ions does not exceed 0.3; and

(II) subsequently drying into place on the zinciferous surface the coating formed in step (I).

2. A process according to claim 1 , wherein (i) the trivalent chromium ion/ hexavalent chromium ion molar ratio is 1/4 to 7/3 and (ii) the total of nitrate + sulfate + phosphate ions/total chromium ions molar ratio is 0.2 to 1.6.

3. A process according to claim 2, wherein the aqueous liquid chromate bath composition contains phosphate ions.

4. A process according to claim 1 , wherein the aqueous liquid chromate bath composition contains phosphate ions.

5. A process according to claim 4, wherein step (II) is performed at a treated surface temperature from 40 to 100 ° C.

6. A process according to claim 3, wherein step (II) is performed at a treated surface temperature from 40 to 100 ° C.

7. A process according to claim 2, wherein step (II) is performed at a treated surface temperature from 40 to 100 ° C.

8. A process according to claim 1 , wherein step (II) is performed at a treated surface temperature from 40 to 100 ° C.

9. A process according to any one of claims 1 - 8, wherein the zinciferous surface treated is that of zinciferous-plated steel sheet.