JPH0366391B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for producing a white galvanized steel sheet by chromate treatment that has a uniform appearance. (Prior Art) Chromate treatment is widely used to prevent corrosion of galvanized steel sheets and aluminized steel sheets and as a base treatment for painting. Particularly in recent years, due to the need to keep rising material prices within product costs, there has been an active movement to replace conventionally painted parts with corrosion-resistant surface-treated plates, omit pre-treatment, and go unpainted. Therefore, in addition to primary rust prevention from manufacturing to user, which was the mission of conventional chromate, high corrosion resistance after user is also required. Steel manufacturers have developed various devised chromates to increase the amount of chromate deposited in response to this demand. However, the most important thing in applications where painting is omitted is the appearance. Conventional chromate does not necessarily provide a uniform color tone while ensuring high corrosion resistance.
It is not possible to obtain a uniform appearance, and users of home appliances are demanding a more luxurious and highly corrosion-resistant chromate. A uniform appearance is easily obtained with white products, and this type of chromate product includes a chromate-treated electrogalvanized product called Unichrome Plating. This method involves immersing bright electrogalvanized plating in an etching chromate bath containing anhydrous chromic acid, sulfuric acid, and nitric acid to form a colored chromate film, washing it with water, and then immersing it in an alkaline solution. , which has been made colorless by removing the soluble chromate component (colored coating component mainly composed of Cr ⁶⁺ ).
Although Unichrome plating has an excellent appearance, it has insufficient corrosion resistance due to its low Cr ⁶⁺ content. Known techniques for waterless wash-type applied chromate include Japanese Patent Publication No. 42-14050, which uses an aqueous solution of silica sol and chromic acid, and Japanese Patent Publication No. 1983-14050, which uses silica powder and a partially reduced aqueous solution to be applied to a metal surface and dries it.
There are Japanese Patent Publication No. 17340 and Japanese Patent Publication No. 52-17341, Japanese Patent Publication No. 61-1508 which specifies the particle size of silica, and Japanese Patent Application Laid-Open No. 53-92339 which uses white carbon. (Problems to be Solved by the Invention) The conventional techniques described above have characteristics depending on the type of chromate, but it is difficult to satisfy the appearance and corrosion resistance, and improvements are required. For example, in the case of the above-mentioned coated chromate, the chromate film contains a large amount of soluble hexavalent chromium (hereinafter referred to as Cr ⁶⁺ ), so it has excellent corrosion resistance, but it is easily colored and becomes uneven due to variations in the amount of coating. easy. Furthermore, although the quality after coating is improved by regulating the particle size of silica particles, no effect has been obtained with respect to surface properties, particularly uniform white appearance, and no examples have been disclosed. The present invention provides a high-productivity chromate treatment method that provides coated chromate with excellent uniformity in appearance, corrosion resistance, and paintability that could not be obtained using conventional methods. (Means for Solving the Problems) The technical means of the present invention for solving the above problems is to use chromic acid and chromic acid with a Cr ³⁺ /(Cr ³⁺ +Cr ⁶⁺ ) ratio of 0.1-0.8. phosphoric acid compound with a weight ratio of 0.5-2.0 and a particle size of 1-100 mμ and a particle size of 500-
A chromate solution consisting of a total of 3000 mμ of inorganic colloids and a weight ratio of 0.75-0.3 to the total of chromic acid and phosphoric acid compounds was applied to the surface of the plated steel plate with Cr.
This is a method for producing a white chromate-treated galvanized steel sheet, which is characterized by applying the coating in an amount of 10 to 100 mg/m ² and heating and drying it, and also uses silica sol, white carbon, and mica as inorganic colloids. The composition of the chromate solution of the present invention will be described. The first component of the chromate used in the present invention is partially reduced chromic acid. The return rate is Cr ³⁺ / (Cr ³⁺
+Cr ⁶⁺ ), organic compounds such as starch, polyhydric alcohol compounds such as sucrose, oxycarboxylic acid, oxalic acid, etc. are used as reducing agents, and hypophosphorous acid, hydrogen peroxide, etc. are used as inorganic compounds. be done. Alternatively, it may be added in the form of a chromium phosphate compound. The concentration varies depending on the application method, but total chromium is 10g/~100 in terms of chromic anhydride (CrO ₃ ).
A concentration of g/g is preferred. The usable reduction rate is 0.1 to 0.8, and the preferable reduction rate is 0.3 to 0.6. When the reduction rate is less than 0.1, coloring due to Cr ⁶⁺ occurs. or
If it exceeds 0.8, corrosion resistance deteriorates and the bath tends to gel. The second component is a phosphoric acid compound. As the phosphoric acid compound, orthophosphoric acid, condensed phosphoric acid, and salts thereof can be used. The concentration of these phosphoric acid compounds is related to the concentration of chromic acid, and the usable range is 0.5 to 2.0 with respect to chromic acid. That is, if the weight ratio is less than 0.5, a colorless appearance cannot be obtained, and corrosion resistance and adhesion are insufficient. If it exceeds 2.0, it is undesirable because it easily dissolves in water, reduces corrosion resistance, and tends to discolor over time. The third component is an inorganic colloid. Colloids contain two types of particles: thin, ultrafine particles called sol, and slightly coarser particles. The inorganic colloid compounds are hydrophilic water-dispersible silicone,
Particles of aluminum, titanium, zirconium oxides or hydroxides are preferable, and barium,
Strontium, calcium, magnesium,
Slightly soluble inorganic compounds such as lead and tin, hydrophilic plastic particles, and metal particles are included in the present invention. Shapes include spherical, flake-like, irregularly shaped, and thread-like. In particular, as for the particle size described later, fine particles are 1~
100mμ, preferably 1~50mμ, coarse particles 500~
It is 3000 mμ, preferably 500 to 1500 mμ. The concentration of the inorganic colloid is preferably such that the ratio of coarse particles to fine particles is 0.5 to 0.2. Also, the ratio of the total concentration of particles to the total concentration of chromic acid and phosphoric acid compounds is
Use between 0.75 and 3.0. If the diameter of the fine particles is less than 1 mμ, the bath tends to gel due to particle adsorption and particle growth. Moreover, if it exceeds 100 mμ, the particles are too large to be fixed with a chromate film, and the film is likely to become powdered (hereinafter referred to as powdering). On the other hand, the diameter of coarse particles is
If it is less than 500 mμ, the appearance tends to have low whiteness (Hunter whiteness according to JIS regulations). Moreover, if it exceeds 3000 mμ, powdering tends to occur. As will be described later, the present invention is configured to form a chromate film using fine particles and obtain whiteness using coarse particles. Therefore, the concentration ratio of inorganic colloids having different particle sizes as described above is desirable. In addition, other components act as a color vehicle for this chromate film,
When the ratio of particles to other components is more than 3/1, it is difficult to fix coarse particles. Conversely, if it is less than 0.75, the white appearance and performance will be insufficient. The most preferred inorganic colloid in the present invention is silica. Sols are commercially available as fine particles. The coarse particles are preferably white carbon whose primary or secondary particle diameter is optimal for the present invention. The chromate solution of the present invention contains metal ions of divalent or higher valence (abbreviated as Men ⁺ ) such as Ca ²⁺ , Mg ²⁺ , Sr ²⁺ ,
One or more of Ba ²⁺ , Zn ²⁺ , and Sn ²⁺ may be added. Feeds include phosphoric acid compounds or carbonates, oxides, hydroxides, and metal powders. The amount added is preferably 1 or less in molar ratio of [Me ⁿ⁺ /phosphoric acid compound]. The processing method of the present invention will be described below. The plated steel sheets to which the present invention is applied include zinc alloy plated steel plates and aluminum plated steel plates such as zinc plated steel plates, zinc aluminum plated steel plates, zinc-nickel alloy plated steel plates, zinc-iron alloy plated steel plates, etc.
It is a plated steel plate such as an aluminum silicon alloy plated steel plate. These plated steel sheets include those obtained by electroplating, hot-dip plating, and vacuum plating. In the present invention, the above-mentioned chromate solution is applied to an active plated steel sheet immediately after being plated by a known method, at a rate of 10 to 100 mg/10 to 100 mg of total Cr deposited after drying.
m ² by a known method such as roll coating, dipping, spraying and squeezing roll. after that,
Dry immediately using hot air or other methods. Heating conditions are board temperature 50-200℃, ideally 60℃
The above is preferable. (Function) The present invention promotes the chemical reaction of the coated chromate with the plating metal to form a chromate film in which a poorly water-soluble chromate compound is combined with the added colloid and phosphate ions. In the present invention, a small amount of chromate liquid applied to the surface of the plating reacts with the plating interface by applying temperature, so that the reaction between the plating and the chromate liquid interface is uniform, and as the pH of the interface increases, the chromate liquid is hardly soluble in water. A film can be formed efficiently, and the film components can be provided with a function. The reduction rate of chromic acid has important implications for the formation of poorly soluble films. That is, since the bath composition already has a composition that forms a poorly soluble chromate film, it efficiently becomes poorly soluble in water by increasing the pH at the interface between the chromate and the plating. However, the desired quality cannot be obtained by the reduction rate alone. As mentioned above, the phosphoric acid compound is an etching component at the interface between the chromate solution and the plating, and plays an important role in determining the reaction rate. During film formation, some of it is taken in in a water-soluble form, but in the case of phosphoric acid and condensed phosphoric acid, water-insoluble phosphoric acid compounds are often formed, which has a pore-sealing effect on pinholes, It greatly contributes to improving adhesion, whitening of chromate, paint adhesion, and corrosion resistance after painting. Colloidal compounds form a stable water-coordinated sol in the state of chromate solution, but the stable charge balance is disrupted due to reactions on the surface of the matte, pH increase, evaporation of water, and adsorption of chromate, resulting in the aggregation of silica particles. It forms a film in which chromium ions, zinc ions, phosphoric acid compounds, etc. are adsorbed. This tendency is more likely to occur as the particle size is smaller, and the above-mentioned reaction is more likely to occur in the thin fine particles used in the present invention. On the other hand, coarse particles reduce the gloss of the chromate coating,
It scatters light and gives a white image to the eye. The fine particles actually have the effect of increasing gloss. The white appearance mechanism, which is a major objective of the present invention, is as follows. A luxurious white appearance is achieved by the composition in which relatively coarse particles are dispersed in a transparent, glossy, amorphous chromate film of phosphate, which is mainly composed of Cr ³⁺ . FIG. 1 is a sectional view showing the structure of the coating of the present invention, where Z is a galvanized layer, ZP is a reaction layer of zinc and chromate, P is a chromate layer close to the solution composition,
SS is a thin inorganic colloid layer with a particle size of 1 to 100 mμ, LS
indicates a coarse inorganic colloid with a particle size of 500 to 3000 mμ.
In the present invention, P and ZP are light-colored chromium compounds mainly composed of Cr ³⁺ (phosphates, oxides, hydroxides, chromic acid compounds) and phosphates and fine sol (SS).
It is a transparent coating that transmits light. FIG. 2 shows a schematic diagram of light scattering in the chromate-treated galvanized steel sheet of the present invention. Light entering from the outside (incident light) passes through the transparent chromate layer, reaches the plating surface, and is reflected. During the incident and reflection processes, light scattering particles are dispersed in the coating, causing the light to be refracted and scattered. This effect makes the white color of the plating more visible to the eye as a semi-gloss appearance, giving it a strong white tone. (Example) Examples will be described below. Example 1 A cold-rolled steel sheet was electrogalvanized (Zn 20 g/m ² ) by a known method, then the chromate solution shown in Table 1 was applied by roll coating, and the sheet temperature was heated to 80°C using hot air at 200°C. It was dried by heating at ℃. The coating amount was set at a target value of 30 to 35 mg/m ² as total chromium.
The obtained chromate-treated galvanized steel sheet was evaluated as follows. Appearance: Visually evaluate uniformity (○: good, △:
Slightly uneven, ×: poor) Yellowness (Y1)...Measured with a color difference meter (JIS
K7103) Y1=100 (1.28X-1.06Z)/Y Whiteness (W)...Measured with a color difference meter (JIS
Hunter whiteness was calculated from K7103) and (Lab). Chromate adhesion amount...Total Cr on the plating surface is measured in mg/ ^m2 using fluorescent X-ray method. Corrosion resistance...Salt spray test (JIS Z2371 continuous method) White rust occurrence % (area ratio) after 72 hours In addition, the compounds used in the chromate solution are indicated by the following abbreviations. Chromic acid R-0 (reduction rate 0.0), R-10
(reduction rate 0.1)... Phosphate compounds include H-P (orthophosphoric acid H ₂ PO ₄ ), P-P (condensed phosphoric acid), Al-P (monomagnesium phosphate), Mg-P (orthophosphoric acid H 2 PO 4 ), Al-P (monomagnesium phosphate), etc. Magnesium monophosphate), Zn-P (zinc monophosphate), Al-
PP (aluminum biphosphate), Mg-PP (means magnesium biphosphate. Inorganic colloids are as follows: S-Si (silica sol with a particle size of 2 to 5 mμ), M-
A1 (alumina sol with a particle size of 20 mμ), M-Si (silica sol with a particle size of 20 mμ), M-Zr (zirconia sol with a particle size of 20 mμ), L-Si (silica sol with a particle size of 50 mμ), M-Ti (silica sol with a particle size of 20 mμ), titaniya sol), WC
(particle size 1000-1500 mμ) L-A1 (alumina sol with particle size 750 mμ), BaSO ₄ (particle size 2000 mμ), Al (scaly aluminum powder with maximum particle size 2000 mμ), TF (Teflon particles 1000 mμ), MN (max. Scale-like synthetic mica sol with a particle diameter of 150 mμ) No. 1 to No. 5 have a reduction rate of Cr of Cr ³⁺ / (Cr ³⁺ +
This is an example of a chromate solution using phosphoric acid, silica sol, and white carbon, with the Cr ⁶⁺ ) ratio varying from 0.2 to 0.7, and the higher the reduction rate, the lower the Y value and the higher the W value, showing a white appearance. These samples were all superior in terms of whiteness, uniformity of appearance, and corrosion resistance compared to Nos. 6 and 7 added as comparative examples. No. 8 to 10 are the amount of phosphoric acid relative to chromic acid.
This is an example in which the amount of phosphoric acid was changed from 0.6 to 3.0, and an appearance with strong whiteness was obtained in proportion to the amount of phosphoric acid. No.10 (over time) is
This is an example that was stored for 3 months in summer, and the W value was lower than that of No. 10, and the color was slightly discolored. Nos. 11 to 15 are examples in which silica, alumina, zirconia, and titani sols were used as fine particulate sols, and although the uniformity was slightly inferior to that of silica sol, overall white quality and excellent corrosion resistance were obtained. No. 16 is an example in which barium sulfate was used as coarse particles, and No. 17 is an example in which Teflon particles were dispersed, and a white plated steel sheet with excellent corrosion resistance was obtained. Nos. 18 to 23 are examples in which various phosphates were used as phosphoric acid compounds, and plated steel sheets were obtained that were whiter and had better corrosion resistance than comparative examples. No. 24 is an example in which aluminum powder with fine flakes was used as the coarse particles, and an appearance with strong whiteness was obtained. Nos. 25 and 26 are examples in which synthetic mica and coarse alumina sol were used, and plated steel plates with a white appearance were obtained. Nos. 24 to 26 had slightly better adhesion workability than No. 3, and powdering occurred at the folded portion, but this was at a level that would pose no problem in practical use.

【table】

[Table] (Effects of the invention) By the method of the present invention, a chromate-treated galvanized steel sheet with excellent corrosion resistance and a uniform appearance can be obtained, and it can be applied to applications where part of the coated product is omitted, reducing the cost of the product. It can contribute to the reduction. In terms of equipment, it is sufficient to install a water-washing immersion layer in the conventional process, and existing processes can be used. Above all, the present invention provides a product with a uniform appearance and high practical utility value.

[Brief explanation of drawings]

FIG. 1 is a schematic cross-sectional view of the surface coating of a white chromate galvanized steel sheet obtained by the present invention, and FIG. FIG.

Claims (1)

[Claims] 1 Chromic acid with a Cr ³⁺ /(Cr ³⁺ +Cr ⁶⁺ ) ratio of 0.1-0.8, a phosphoric acid compound with a weight ratio of 0.5-2.0 to chromic acid, and a particle size of 1-100 mμ and particle size 500−
A chromate solution consisting of a total inorganic colloid of 3000 mμ and a weight ratio of 0.75 to 3.0 to the total of chromic acid and phosphoric acid compounds was applied to the surface of the plated steel plate with Cr.
A method for producing a white chromate-treated galvanized steel sheet, which comprises applying the coating in an amount of 10 to 100 mg/m ² and heating and drying it. 2. The method for producing a white chromate-treated galvanized steel sheet according to claim 1, using silica sol, white carbon, and mica as inorganic colloids.