JPH05195242A - Treatment for highly corrosion-resistant chromate coating on galvanized steel sheet - Google Patents

Abstract

PURPOSE:To provide a production method of galvanized steel sheet having improved chromium elution resistance, top coating property and high corrosion resistance by making a chromate coating film hardly soluble. CONSTITUTION:A chromate compsn. is applied on the surface of galvanized steel sheet by an amt. containing 5-150mg/m<2> metal chromium as a solidified film and dried at the max. sheet temp. of 50-200 deg.C. This chromate compsn. consists of a chromate liquid having 5-30g/l total chromium concn., 40-80% reduction rate, and 0.1-1.0g/l hydrofluoric acid, added with phosphomolybdic acid by 0.1-1.0 weight ratio to hexavalent chromium and an inorg. inhibitor as a second additive by 0.1-0.5 weight ratio to the phosphomolybdic acid. This inorg. inhibitor consists of a mixture of silicotungstic acid, hydrosilicofluoric acid, zirconium potassium fluoride, and carbonates of Co and Ni. By the chromate coating treatment of this invention, the chromium elution resistance of a galvanized steel sheet is improved and the corrosion resistance and coating property of the seet are markedly improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for coating a galvanized steel sheet with high corrosion resistance and applying chromate.

[0002]

2. Description of the Related Art In recent years, the market needs for higher functionality such as high corrosion resistance of zinc-based plated steel sheets for automobiles and home appliances and cost reduction have increased more and more, and research and development of new products in response to this have recently been active. Has been done in. In this,
With respect to the chromate treatment of galvanized steel sheets, there is a strong demand for the development of a refractory chromate treatment method that improves chromium elution resistance without deteriorating other performance from the viewpoint of eliminating pollution. Japanese Patent Laid-Open No. 50-158535 discloses an example of a conventional technique in which the chromate film is made difficult to dissolve. This discloses a chromate solution of chromic anhydride-phosphoric acid-aqueous polymer compound, and the hexavalent chromium ions in the treated solution are reduced by 70% or more with a reducing agent such as ethylene glycol. However, since the chromate film formed by this example contains a polymer,
Although it is excellent in poor solubility, corrosion resistance, and paintability, it has a drawback in stable production that it is difficult to obtain stable performance in a line having a short life as a treatment liquid and high productivity.

The chromate disclosed in Japanese Patent Publication No. 61-58552 is a chromic acid-chromium reduction product-silica sol system. However, in the chromate film obtained by this method, hexavalent chromium is likely to be eluted by alkali washing or water washing before coating, and there is a problem that corrosion resistance is reduced and wastewater treatment and other complicated work are required. Next, JP-A-58-22383 and JP-A-62-834 which disclose the use of a silane coupling agent for reducing hexavalent chromium ions in a chromate treatment liquid.
No. 78 is cited. The chromate films formed by these methods all have excellent paint adhesion, but have poor alkali resistance. When the effect of each component of the prior art in the above chromate treatment method on the performance was examined, organic polymers and silica increase the corrosion resistance, but a tendency to decrease the alkali resistance was recognized, and a silane cup having a chromium reducing action was also observed. It is recognized that the ring agent tends to reduce the corrosion resistance. As described above, although various methods have been disclosed, each method has advantages and disadvantages as a coating chromate film, and particularly strongly depends on the drying conditions after chromate treatment.

Furthermore, an aqueous solution prepared by mixing chromic acid or a chromium compound having a high rust preventive power with a water-dispersible resin is applied to the surface of a galvanized or zinc-based alloy plated steel sheet to achieve high rust prevention. JP-B-55-51032, JP-A-59-162278, JP-A-61-584 and the like have been proposed, but all of them have high rust-preventing power, but chromium elutes in dew condensation or an aqueous treatment liquid. However, there is a problem in practical use. Further, as a material obtained by blending a water-dispersible resin with an organic composite silicate (silica sol, silane coupling agent) and applying a thin film, there are JP-A-60-149786 and JP-A-61-50181. Although these do not have the problem due to the elution of chromium as described above, ammonium ions and the like are present as alkali ions for containing silica in a stable colloidal form, and this is a problem because there is a decrease in water resistance among the physical properties of the coating film. There is.

[0005]

One of the drawbacks of the above-mentioned conventional chromate treatment methods is that the chromate film containing soluble hexavalent chromium is hardly soluble (improvement in chromium elution resistance) and is insufficient. , Which causes deterioration of corrosion resistance after coating (blister resistance of coating film), and long-term treatment liquid for producing coated chromate-treated galvanized steel sheet with low cost and stable performance under high productivity line. Lack of stability and incompatibility of suitable drying conditions after chromate treatment in view of stability of applied chromate film. That is, the present invention is intended to provide a coating chromate treatment method for solving various problems of the prior art in the coating chromate treatment method for these zinc-based plated steel sheets.

[0006]

In the present invention, in order to solve the above-mentioned problems, the following studies were conducted on the aqueous chromate solution to be applied first. That is, in order to make the applied chromate film hardly soluble, the conversion of soluble hexavalent chromium ions to trivalent chromium ions must basically be achieved. For that purpose, the interfacial reaction of the chromate film and the underlying plating layer in the dry solidification process thereof. It is possible to increase the reactivity of the base metal and the positively reacting compound of the base metal ion and the hexavalent chromium ion which is inevitably eluted with the insoluble trivalent chromium ionization of the hexavalent chromium ion due to the reduction current when the base metal is dissolved. It is intended to achieve high corrosion resistance as the zinc-based plated steel sheet by blocking the corrosive factor by the barrier effect due to the addition of the reaction compound and the inhibitor by forming it at the interface physically. A study was conducted.

Clarification of the appropriate reduction rate in the chromate treatment liquid for the purpose of improving the chromium elution resistance of the applied chromate film and thereby improving the adhesive strength with the underlying plating. In order to stabilize the adhesion of the chromate film to the plating surface, it is necessary to add a function that promotes the uniform reaction of the chromate film at the plating interface, and an appropriate etching accelerator for that purpose should be investigated. Examination of inorganic inhibitors to exert a sufficient barrier effect against corrosion factors.

[0008] As a result of the above investigations, an appropriate chromium reduction rate and activation of the plating interface reaction by hydrofluoric acid resulted in uniform formation of a chromate film with excellent adhesion, and an inorganic inhibitor consisting of phosphomolybdic acid and an auxiliary additive. It has been found that the combined action with and suppresses the excessive elution of hexavalent chromium, and at the same time exerts the barrier effect of corrosive factors, making it possible to form a chromate film with unprecedented solubility and high corrosion resistance. The present invention has led to the proposal of the present invention. Furthermore, under the high productivity line,
Upon stabilizing the appearance and quality of the chromate treatment, the present invention was proposed by specifying the drying conditions after the chromate treatment.

The constitution of the chromate treatment method according to the present invention will be described first. A coating chromate composition having the bath composition of [A] below is coated on the surface of zinc plating, zinc alloy plating, and zinc composite plating steel sheet as a solid coating in an amount of 5 to 150 mg / m ² in terms of metallic chromium, Immediately after that, the maximum temperature of the outlet plate of the furnace was 50 to 200.
It is a method for chromate treatment of zinc-based plated steel sheet, which is characterized in that it is dried so as to reach ℃. [A] Coating Chromate Composition Total Chromium Concentration (g / l); 5-30 Chromium Reduction Rate (%); 40-80 Hydrofluoric Acid (g / l); 0.1-1.0 Phosphomolybdic Acid (g / l); [-] x0.
1 to 1.0 Sub-additive (g / l); x 0.1 to 0.
5 [The sub-additive refers to a mixed inorganic inhibitor composed of silicotungstic acid, silicofluoric acid, potassium zirconium fluoride, nickel carbonate and cobalt carbonate. ]

That is, the essence of the present invention is that the film form of the coating chromate formed is basically hydrofluoric acid, phosphomolybdic acid as an inorganic inhibitor and a chromic chromate mainly containing soluble hexavalent chromium by employing an auxiliary additive. It is characterized in that the structure is changed to a refractory fluoride-based chromium compound-dominant type, and in addition, the adhesion of the chromate film is improved by specifying the chromium reduction rate and improving the uniform reactivity at the plating interface with hydrofluoric acid. By improving the corrosion resistance of the bare plate, the corrosion resistance (perforation resistance) of the bare plate is improved. Further, in producing the quality including the finished appearance of the chromate stably at a high level, the dry plate temperature after the chromate treatment is specified. The galvanized steel sheet applied to the present invention includes an electroplated Zn-plated steel sheet and a Zn-based alloy-plated steel sheet containing at least one of Zn and Ni, Cr, and Fe. May be used. Moreover, in the electrodispersive plating system, SiO ₂ and Ti are based on Zn-Ni and Zn-Fe.
A Zn-based dispersion alloy plated steel sheet obtained by uniformly dispersing a metal oxide such as O ₂ , ZrO ₂ , or BaCrO ₄ may be used. Further, in the hot dipping system, zinc iron plate, Zn
-Al-based alloy plated steel sheets, and their alloyed galvanized steel sheets may be applied.

[0011]

The action limits for the constituent factors of the present invention will be described below. (1) Chromate composition The coated chromate film of the present invention is on the surface of a plated steel sheet and has high adhesion to the undercoat and high chromium elution resistance,
In addition, it is an important film for improving the corrosion resistance of the zinc-based plated steel sheet by improving the adhesion to the top coating film. In particular, in order to dramatically improve the properties such as water resistance, acid resistance, and alkali resistance, it is necessary to make the chromate film insoluble at a high level, and in order to achieve this without lowering the high corrosion resistance function, It is necessary to manage the bath composition.

Total Chromium Concentration and Its Reduction Rate The present chromate composition uses water as a solvent, and in order to stably maintain the corrosion resistance and various performances of the zinc-based plated steel sheet, the total chromium concentration (hereinafter referred to as T.Cr). Is required to be 5 to 30 g / l, and the chromium reduction rate (hereinafter referred to as R) is required to be 40 to 80%. T. When Cr is less than 5 g / l and / or R is less than 40%, a chromate film mainly composed of soluble hexavalent chromium becomes a chromate film, so that chromium necessary for high corrosion resistance is easily extracted into water excessively. Along with the deterioration of the corrosion resistance, the bare corrosion resistance of the zinc-based plated steel sheet, the coating property of the overcoat, and the corrosion resistance after the overcoat are shortened. In addition, since the liquid film contains a large amount of water, uneven appearance of the treatment is likely to occur during the drying process of the film, which greatly impairs the appearance quality, and wastewater treatment with eluted chromium is necessary. Not. Also, T. If Cr exceeds 30 g / l and / or R exceeds 80%, the viscosity of the treatment liquid increases,
It lacks stability in terms of control of chromate deposit and bath life.
It is not always advantageous in terms of quality and cost under a high productivity line. Therefore, the total chromium concentration is 5 to 30 g /
1 and preferably 10 to 20 g / l. The chromium reduction rate is 40 to 80%, preferably 45 to 60%. The reducing agent for chromium may be any known method using alcohol, hydrogen peroxide or the like, but it is better to prevent undecomposition of the reducing agent from the stability of the treatment bath.

Hydrofluoric Acid Concentration In the present invention, the hydrofluoric acid concentration is 0.1 to 1.0 g / l. The function of this hydrofluoric acid is the excess and deficiency of metal oxides and metal hydrates [ZnO, Zn (OH) ₂ , Al ₂ O ₃ etc.] that form on the surface of the undercoat and hinder the wettability of the chromate treatment solution. The reason is that it is removed to promote the interfacial reaction between the plating surface and the chromate treatment liquid as appropriate, thereby stably obtaining a coated chromate film with high adhesion. If hydrofluoric acid is less than 0.1 g / l, the etching action is small and the effect of removing the oxide on the plating surface is not sufficient. Therefore, the liquid of chromate is repelled, resulting in deterioration of the finished appearance, and the interfacial reaction. Due to the lack, only a chromate film with poor adhesion is formed. Also, hydrofluoric acid is 1.0 g / l
Beyond the above, accumulation of plating components of the underlayer that is eluted by excessive etching leads to shortening of the life of the chromate treatment liquid, stabilization of various performances including corrosion resistance as a zinc-based plated steel sheet, and soaring costs. It is not a good idea because it invites Therefore, the hydrofluoric acid concentration in the present invention is 0.1 to 1.0 g / l, preferably 0.2.
~ 0.7 g / l is preferred.

Phosphomolybdic Acid Addition Concentration In the chromate film of the present invention, phosphomolybdic acid suppresses the short-term elution of soluble hexavalent chromium, and acts as a barrier function for exhibiting the corrosion-resistant life of the zinc-based plated steel sheet for a long period of time. In order to fully demonstrate its function, including the adhesion as a chromate film,
It is necessary to properly control the concentration of the processing liquid. The concentration of phosphomolybdic acid in the present invention is the difference between the total chromium concentration (T.Cr = hexavalent Cr + trivalent chromium) and trivalent chromium (T.Cr x chromium reduction rate), that is, hexavalent chromium. In contrast, the ratio is preferably 0.1 to 1.0. If this ratio is less than 0.1, the effect of suppressing chromium elution as a chromate film is small, and therefore high corrosion resistance as a zinc-based plated steel sheet cannot be expected in many cases.
On the other hand, if the ratio exceeds 1.0, the adhesion of the chromate film to the plating base is reduced, which is not practical. Therefore, the suitable ratio of phosphomolybdic acid to hexavalent chromium is 0.1 to 1.0, preferably 0.2 to
0.7 is good. The phosphomolybdic acid applied to the present invention is a water-soluble salt of a heteropoly acid made of phosphorus and molybdenum, and compounds having an oxidation number of phosphorus of 3 and 5 may be used. A compound having a phosphorus oxidation number of 3 means M ₄ [P ₂ Mo ₁₂ O ₄₁ ] .nH ₂ O (M; Li, N
a, K, NH ₄ , CN ₃ H _6, etc.) and 2M ₂ O · P ₂ O ₃ ·
5MoO ₃ · nH ₂ O (M; Na, K, NH ₄ )
Further, a compound having a phosphorus oxidation number of 5 means M ₃ [PO ₄ Mo ₁₂
O ₃₆ ] .nH ₂ O (M; H, Li, Na, K, NH ₄ , C
N ₃ H ₆ etc.) dodecamolybdate.

Additive Concentration of Sub-Additive In the present invention, the sub-additive refers to a mixed inorganic inhibitor consisting of silicotungstic acid, silicofluoric acid, potassium zirconium fluoride, nickel carbonate and cobalt carbonate. The purpose is to suppress the short-term elution of hexavalent chromium from the contained chromate film and to provide the chromate film with an auxiliary barrier effect for exhibiting the corrosion resistance life of the zinc-based plated steel sheet for a longer period. Therefore, the amount of the sub-additive added is naturally a small amount so as to supplement the barrier effect of phosphomolybdic acid, and the ratio to the above-mentioned amount of phosphomolybdic acid added is 0.1.
Applied at ~ 0.5. In the present invention, when the addition amount of the sub-additive is less than 0.1 in terms of the addition amount ratio with the phosphomolybdic acid, the effect of suppressing chromium elution from the chromate film is not sufficient, and high corrosion resistance as a galvanized steel sheet is obtained. Can not expect much. On the other hand, if the ratio exceeds 0.5, the adhesion of the chromate film is liable to be lowered, and not only the corrosion resistance is improved but also the adhesion of the top coating film is deteriorated, which is not practical. Therefore, the addition amount of the sub-additive in the present invention is 0.1 to 0.5, preferably 0.15 to 0.4, in terms of the addition amount ratio to the phosphomolybdic acid.

(2) Drying plate temperature after chromate treatment One of the features of the present invention is that the drying plate temperature of the chromate film after coating chromate treatment is formed in a relatively wide range. Increased chromium elution resistance of the chromate film,
In order to improve the corrosion resistance as a zinc-based plated steel sheet, especially the corrosion resistance after topcoating, the steel sheet has the drying conditions of the chromate film after applying a predetermined amount of the chromate composition, and in particular, the maximum dry sheet temperature. It is necessary to control within an appropriate range while considering the mechanical strength of the material. The dry plate temperature after the coating chromate treatment in the present invention is 5 to 30 as a heat pattern.
It needs to be heated and dried so that the plate temperature becomes maximum on the outlet side of the drying furnace in seconds. When the maximum plate temperature is less than 50 ° C., the excess water contained in the chromate film is merely evaporated and dried, and the air oxidation polymerization required for high corrosion resistance is promoted to change the structure to a chromate film having an appropriate network structure. Since it cannot be expected, there is a decrease in performance such as the adhesion of the chromate film and the chromium elution resistance, which is not practical for stable production of the target quality. Further, when the maximum plate temperature exceeds 200 ° C., the chromate film is accompanied by a dehydration reaction of crystal water which is originally required as a film, and the adhesion is largely hindered due to the embrittlement of the film, and the corrosion resistance is remarkably lowered. Not practical. Therefore, the dry plate temperature after the chromate treatment in the present invention is 50 to 200 ° C. at the maximum plate temperature on the furnace outlet side, and preferably 8
0-150 degreeC is good.

(3) Coating Chromate Adhesion Amount The chromate composition [A] of the present invention is applied by, for example, a roll coater and dried to form a film, but in order to obtain stable performance such as corrosion resistance. However, it is necessary to control the adhesion amount within an appropriate range. If the amount of the applied chromate coating is less than 5 mg / m ^{2 in} terms of metal chromium, the bare plate as a zinc-plated steel sheet and the corrosion resistance after top coating are not sufficient, and if it exceeds 150 mg / m ⁵ , it is uniform. Since it is difficult to control the coating, it is not a good idea in terms of stable production of appearance such as chromate treatment, corrosion resistance and coating adhesion. Therefore, the amount of applied chromate in the present invention is
It is 5 to 150 mg / m ² in terms of metal chromium, and preferably 10 to 80 mg / m ² . Hereinafter, the present invention will be described in more detail with reference to Examples.

[0018]

[Example] In a high productivity continuous plating line, a low carbon steel plate having a plate thickness of 0.8 mm at a line speed of 150 m / min was subjected to zinc-based plating shown in Tables 1 to 7 using a known plating method. Immediately thereafter, a predetermined chromate composition was immediately applied by a roll coater method, followed by drying so that the maximum plate temperature on the outlet side of the hot-air drying furnace became the predetermined plate temperature, and then the plate temperature was adjusted to 40 ° C. or lower by air cooling. Wind up into a coil. The performance of the coated chromate-treated zinc-based plated steel sheet of the present invention thus obtained is summarized in Tables 8 to 14. First,
Regarding the proper range of the total chromium concentration and the chromium reduction rate, which are the basic compositions of the present coating chromate composition, the examples of the present invention are given. 1-No. 6, and No. 6 9-No. No. 12 and the comparative example No. 7-No. 8 and No. 1
3 to No. 14 shows. As is clear from this, when the total chromium concentration and the chromium reduction rate, which are the basic compositions of the chromate composition of the present invention, are deviated, the uniformity of the treated appearance, the corrosion resistance, the top coatability, and the like decrease in performance. Acknowledged, it will be difficult to obtain a high level of stable performance in a balanced manner.

Next, the interfacial reaction with the base plating is promoted,
Hydrofluoric acid is required to form a coated chromate film having excellent adhesion. This embodiment according to the invention is N
o. 2 and No. 15-No. No. 20, and the comparative example when the present invention is deviated from No. 20. 21-No. 22. As is apparent from the above, the addition of an appropriate amount of hydrofluoric acid in the examples of the present invention resulted in the formation of chromium fluoride, etc., and thus the chromate film in which the content ratio of the hardly soluble trivalent chromium was increased from the conventional chromate film mainly containing hexavalent chromium. It can be considered that the morphological change is caused by the above, but the chromium elution resistance and the adhesion of the chromate film itself are improved by this, and it is understood that this leads to high corrosion resistance or improvement of top coatability. Further, when the examples of the present invention were deviated, it was found that, as in the comparative example, the chromate film and the underlying plating interface reaction was lowered or excessively reacted, which impaired quality and productivity, and was not practical.

Next, the coated chromate film of the present invention is made more difficult to dissolve, and the effect of blocking the corrosion factor from the corrosive environment (barrier effect) is sufficiently exerted to stabilize the long-term corrosion resistance of the galvanized steel sheet. In order to secure it, it is preferable to use an inorganic inhibitor composed of phosphomolybdic acid and an auxiliary additive in combination. The amount of these inhibitors added is in comparison with the concentration of soluble hexavalent chromium in the chromate composition. First,
Regarding the proper ratio of the phosphomolybdic acid and the auxiliary additive to hexavalent chromium in the present invention, the examples thereof are described. 2 and No. 23-No. No. 63, and No. 63 as a comparative example when the present invention is deviated. 64-No. 67. As is apparent from this, within the examples of the present invention, the combined use of phosphomolybdic acid and the sub-additive moderately suppresses the elution of chromium in the chromate film, which further enhances the barrier effect from the corrosion factor. It can be seen that the corrosion resistance life as a galvanized steel sheet can be dramatically improved. Therefore, in the chromium composition of the present invention, the addition ratio of phosphomolybdic acid to hexavalent chromium is 0.1 to 1.0, preferably 0.2 to 0.7. Further, with respect to high corrosion resistance of the steel sheet, the addition ratio of the auxiliary additive used as a mixed inhibitor used in combination is 0.1 to 0.1 in terms of the addition amount ratio of phosphomolybdic acid.
0.5, preferably 0.15 to 0.4.

Next, regarding the proper amount of chromate deposition for the high corrosion resistance of the zinc-based plated steel sheet, no. 2 and No. 68-No. No. 75, and the comparative example No. 76-No. 77. As is clear from this, if the chromate deposition amount in the example of the present invention, the uniformity of the treated appearance as a zinc-based plated steel sheet,
It can be seen that it is possible to manufacture with high quality such as corrosion resistance and top coatability and with high productivity. As in the comparative example,
If the adhesion amount is too small, the basic corrosion resistance will be impaired, and if it is too large, there will be practical problems such as non-uniform appearance of the treatment and cohesive failure of the chromate film, resulting in deterioration of paint adhesion. Therefore, the proper adhesion amount of the chromate film made of the chromate composition of the present invention is 5 to 150 in terms of metallic chromium.
mg / m ² , preferably 10 to 80 mg / m ^2.
Is good. Finally, the dry plate temperature after chromate treatment is one of the factors that largely influence the effect of the chromate film in the present invention, and a heat pattern that maximizes the plate temperature on the outlet side of the drying furnace is preferable.

The embodiment of the present invention is No. 2 and No. 78
~ No. No. 86, and the comparative example No. 87-No.
88. As is clear from this, the maximum plate temperature is 5
If the temperature is lower than 0 ° C, even if the water content in the chromate film flies away, it is not possible to achieve the network structure of the film due to the oxidative polymerization reaction. It is not preferable because it causes high corrosion resistance, occurrence of blister after coating, and deterioration of coating adhesion. On the other hand, when the maximum plate temperature exceeds 200 ° C,
From the dehydration reaction of water of crystallization necessary for the chromate film to exist stably, the adhesion function of the film is basically lost,
It is not practical because there is an extreme decrease in corrosion resistance and paint adhesion. Therefore, in drying the chromate film in the present invention, the maximum plate temperature on the outlet side of the furnace is 50 to 200 ° C, preferably 80 to 150 ° C. In the examples of the present invention, No. 2 and No. 89-No. As shown in No. 99, it can be seen that the zinc-based plated steel sheet applicable to the present invention may have any plating system.

The reason why the chromate film of the present invention obtained as described above is moderately insoluble, and the reason why it effectively works for high corrosion resistance, has not been fully clarified yet, but ESCA and AES It can be considered as follows from the surface analysis such as. The chromate film form is basically composed of trivalent chromium and hexavalent chromium such as chromium hydroxide and fluoride chromium. In addition, Mo, Si, P, which is a component added as an inorganic inhibitor as a sub-component,
It is presumed that this is a chromate film mainly containing a sparingly soluble trivalent chromium which also contains a multi-component chromium compound containing Zr, F, Co and Ni, and it is considered that this further exerts a barrier action from a corrosive environment. Such a chromate film is expected to undergo further morphological change depending on the drying plate temperature, and by drying at a plate temperature within the dehydration limit of crystal water of the film, the chromate film is basically all-bonded (Cr-O).
It is considered that the morphology changed from H) to a film having a network structure mainly composed of oxo bonds (Cr-O), and as a result, this further promoted insolubilization of the film.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

[Table 3]

[0027]

[Table 4]

[0028]

[Table 5]

[0029]

[Table 6]

[0030]

[Table 7]

[0031]

[Table 8]

[0032]

[Table 9]

[0033]

[Table 10]

[0034]

[Table 11]

[0035]

[Table 12]

[0036]

[Table 13]

[0037]

[Table 14]

(Note) * 1. Plating system EZ: Electric zinc plating ZN: Electric Zn-Ni alloy plating (Ni; 11.5%) EC: Electric Zn-Cr-Ni alloy plating (Cr; 10)
%, Ni; 2%) EF : Electrical Zn-Fe alloy plating (Fe; 15%) ZNS: Electrical _{Zn-Ni-SiO 2 (Ni} ; 12%, S
iO _2; 3%) ZNT: Electrical _{Zn-Ni-TiO 2 (Ni} ; 12%, T
iO _2; 3%) ZNZ: Electrical _{Zn-Ni-ZrO 2 (Ni} ; 12%, Z
rO _2; 3%) ZNB: Electrical _{Zn-Ni-BaSO 4 (Ni} ; 12%,
BaSO _4; 3%) ZFS: Electrical _{Zn-Fe-SiO 2 (Fe} ; 10%, S
iO ₂ ; 3%) GZ: hot-dip Zn plating GA: hot-dip Zn-Al plating (Al; 5%) GF: alloyed hot-dip Zn plating (Fe; 8-11%)

* 2. Coating chromate treatment Chromic acid: Nippon Denko's product is reduced with methanol and used. Hydrofluoric acid: used by Morita Chemical Industries, 46%. PM: Phosphomolybdic acid (manufactured by Komune Chemical Co., Ltd.), weight ratio to hexavalent chromium, [molecular formula; P ₂ O ₅ · 24MoO ₃ ·
nH ₂ O (used as n = 1)] MI: auxiliary additive consisting of an equal amount of inorganic inhibitor,
Compared to PM weight Tungstosilicic acid (manufactured by Komune Chemical), hydrosilicofluoric acid (manufactured by Kanto Chemical) potassium zirconium fluoride (manufactured by Kanto Chemical), nickel carbonate (Kanto Chemical), cobalt carbonate (manufactured by Kanto Chemical). T. Cr deposit: Fluorescent X-ray analysis method

* 3. Appearance of coating chromate treatment ◎: Uniform without unevenness, ○: Very slight unevenness, △: Light linear pattern, ×: Butyric or linear pattern * 4. Chromium elution resistance L4410 Alkaline degreasing (Nippon Parker Rising)
And under the standard treatment conditions of PB3020 chemical conversion treatment (manufactured by Nippon Parker Rising Co., Ltd.), the total amount of chromium eluted in the treatment liquid including the washing step was analyzed and determined by a fluorescent X-ray analysis method. ⊚: No elution at all, ∘: Very slight elution, Δ: Slight elution, ×: Significant elution * 5. Bare corrosion resistance Salt spray test (JIS Z-2371) 500 hours ◎: White rust 5% or less, ○: White rust 10% or less, △: Red rust 5%
Below, x: Red rust over 5%

* 6. Paint corrosion resistance Melamine alkyd paint, 20μm, 120 ℃ × 30
Minute baking, evaluation of coating film blisters on the cross cut portion, salt spray test (JIS Z-2371) 1000 hours ◎: no blisters, ○: slight blisters, Δ: slightly large blisters, ×: full blisters * 7. Top coat paintability Primary adhesion: Melamine alkyd paint, 20 μm, 1
20 ° C x 30 minutes baking 1 mm x 100 x grid cross-section taping peeling test Secondary adhesion; same as after primary adhesion, wet test (50 ° C, RH 98% or more, 7 days) Cross-cut test ◎: No peeling of coating film, ○: 5% or less of peeling, Δ: 10% or less of peeling, ×: More than 10% of peeling

[0042]

As described above, according to the present invention, by applying the coating chromate treatment of the present invention, the elution of chromium resistance of the zinc-based plated steel sheet is enhanced, which brings about high corrosion resistance and top coatability. The present invention provides the market with an epoch-making coating chromate treatment technology that simultaneously solves the difficulty of coating chromate coating and the high corrosion resistance of the conventional technology.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Koji Tanimura 1 Kimitsu, Kimitsu-shi, Chiba Nippon Steel Corporation Stock of Kimitsu Works

Claims (2)

[Claims] 1. Zinc, characterized in that the chromate composition of the following [A] is formed as a solid film on the surface of a zinc or zinc-based alloy plated steel sheet to form a total chromium amount of 5 to 150 mg / m ^{2 on} one side. Highly corrosion resistant coating chromate treatment method for plated steel sheet. [A] Chromate composition Total chromium concentration (g / l); 5-30 Chromium reduction rate (%); 40-80 Hydrofluoric acid (g / l); 0.1-1.0 Phosphomolybdic acid (g / l) ); [−] × 0.
1 to 1.0 Sub-additive (g / l); x 0.1 to 0.
5 [The sub-additive refers to a mixed inorganic inhibitor composed of silicotungstic acid, silicofluoric acid, potassium zirconium fluoride, nickel carbonate and cobalt carbonate. ] 2. The method for chromate treatment with high corrosion resistance according to claim 1, wherein the drying plate temperature after coating chromate treatment is dried so that the maximum plate temperature on the outlet side of the furnace is 50 to 200 ° C.