JPS63171685A - Formation of anticorrosive white chromate film - Google Patents

Abstract

PURPOSE:To easily obtain the title white chromate film having excellent corrosion resistance, by coating an aq. acidic soln. contg. chromic acid with the Cr<3+>/(Cr<3+>+Cr<6+>) ratio specified, an inorg. colloidal compd., and an inorg. anion on the surface of a metal, and then heating and drying the soln. CONSTITUTION:The aq. soln. contg. the chromic acid with 0.1-0.7 ratio of Cr<3+>/(Cr<3+>+Cr<6+>), an inorg. collidal compd., and an inorg. anion is coated on the surface of a metal such as a galvanized steel sheet, heated, and dried. An anticorrosive white chromate film can be formed on the metal surface in this way. In this case, a silica sol is used as the inorg. colloidal compd., and a phosphoric ion or a condensed phosphoric acid ion is used as the inorg. anion.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for chromate treatment of zinc or zinc alloy plated steel sheets and aluminium-plated steel sheets with white color having excellent corrosion resistance.

(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, it is necessary to keep the increase in material prices within the product cost, so there has been an active movement to replace parts that were conventionally used as painted parts with corrosion-resistant surface-treated steel sheets (= = unpainted). In addition to primary rust prevention from manufacturing to user, which was the mission of chromate, high corrosion resistance after user is also required.Steel manufacturers have developed various devised chromates to meet this demand. However, the most important thing for road-saving coating applications is the appearance. Conventional chromate does not always ensure high corrosion resistance, uniform color tone,
It is not possible to obtain a uniform appearance, and users of home appliances are demanding a more luxurious and highly corrosion-resistant chromate.

The present invention provides a method for treating chromate that meets these objectives.

A uniform appearance is easily obtained with white products, and this type of chromate product includes chromate-treated electrogalvanized plating called dinichrome plating.

This method uses chromic anhydride to coat bright electrogalvanized
A colored chromate film is formed by immersion in an etching chromate bath containing sulfuric acid/nitric acid as the main component, washed with water, and then immersed in an alkaline solution to form a soluble chromate component (a colored film component mainly composed of Cr'+). ) is colorless except for Although Unichrome plating has an excellent appearance, it has insufficient corrosion resistance due to the low Cr6+ content.

Chromate treatment can be broadly divided into etching chromate, which is washed with water after chromate treatment, electrolytic chromate, which is washed with water after electrolysis, and coated chromate, which is heated and cured after application. Among these, coated chromate has the advantage of simple equipment and reduced drainage and water treatment, and is superior in quality, so it has become the mainstream chromate treatment for continuous plating lines.

A known technique is Japanese Patent Publication No. 42-14050, which uses an aqueous solution of silica sol and chromic acid. JP-A-52-1: Application of silica powder and partially reduced aqueous solution to metal surface and drying
7340 and JP-A-52-17341, JP-A-61-1508 which specifies the particle size of silica, and JP-A-53-92339 which uses white carbon. Although these known techniques are excellent in preventing white rust during storage or transportation,
The level of demand for chromate in recent years is unsatisfactory.

(Means for Solving the Problems) The present invention provides a method for treating white-based chromate with excellent corrosion resistance for use in road saving applications, and can be summarized as the following method.

(1) Cr3+/(Cr3''+cr6+) ratio is 0.
Corrosion-resistant white chromate treatment method characterized by applying an acidic aqueous solution containing 1 to 0.7 chromic acid, an inorganic colloidal compound, and an inorganic anion to a metal surface and then heating and drying (2) Silica sol as an inorganic anion as an inorganic colloidal compound Corrosion-resistant white chromate treatment method according to claim 1 using phosphate ions or condensed phosphate ions (3) As inorganic anions, sulfate groups, nitrate groups, chloride ions, fluorine ions, fluorine complex ions, sulfonic acid groups Corrosion-resistant white chromate treatment method (4) according to claim 1, using any one of the following: Cr3+/(Cr”
It is characterized by applying an acidic aqueous solution containing chromic acid, an inorganic colloidal compound, an inorganic anion, and a divalent or higher valent metal ion with a ratio of 0.1 to 0.7 on the metal surface and then heating and drying it. Corrosion-resistant white chromate treatment method (5) M , 2 + , ca as metal ions
2 + , s r2 +.

Ba”, Ail, Zn”, Ni”, Co”
, Fe.

A corrosion-resistant white chromate treatment method according to claim 4, which contains one or more types of Sn2+.

The whitening mechanism will be described in the section on action, and the structure of the invention will be explained in detail here.

The first term is basic to the present invention, and the chromate bath is composed of a chromic acid bath in which partially reduced Cr'' and Cr'+ coexist, and further composed of an inorganic colloid compound and an inorganic anion.

Partially reduced chromic acid is prepared by adding a reducing agent to an aqueous chromic acid solution using a known method. The reducing agent is preferably an organic compound that does not easily leave decomposition reaction products. For example, starch, sugars, and alcohols are added little by little to an aqueous solution containing concentrated chromic anhydride and reduced while decomposing them into water and carbon dioxide gas. The return rate is C
r3+/(Cr3+++Cr6+) in weight ratio of 0.1 to 0
．． Adjust to 7. The most preferred range is 0.3-0.5. If it exceeds 0.7, the liquid tends to solidify or precipitate, and corrosion resistance also deteriorates due to a decrease in Cr'+. 0.1
If the amount is less than that, the film obtained will be colored in proportion to the amount of coating.
There are problems with moisture absorption and chromate elution. Or”/(c
r"+cr") of 0.3 to 0.5 is excellent in terms of quality and bath stabilization. However, a chromium compound alone does not lead to highly corrosion-resistant white chromate; an inorganic colloid compound and an inorganic anion are required. As the inorganic colloid, silica sol is preferred, but alumina sol,
Even titanium compound sols and zirconia sols have a quality similar to that of silica sol. These compounds may be combined.

The amount added is to replace all chromium ions with chromic anhydride (Cr0
3) 0.5 to 3.0 is preferable with respect to 1.

This is because a value over 3 shows a tendency for deterioration in adhesive workability. As inorganic anions, phosphate ions, condensed phosphate ions, sulfate ions, nitrate ions, and chloride ions are used. In particular, condensed phosphate ions are excellent.

Condensed phosphoric acid compounds are P2O7+ in which two or more P are bonded.
P2oll + P30105″″
, P O'-+ PaO+s
Acids called phosphoric acids, divalent or higher metal salts, i.e. alkaline earth metal salts such as calcium salts, magnesium salts, strontium salts, barium salts and other metal salts (At", Zn", Fe ” ,
Co”.

2+Ni, Sn). In addition, metal ions can be prepared by adding oxides, hydroxides,
It may be added in the form of carbonate or dissolved. Phytic acids in which organic and phosphoric acids are combined are also included in the present invention. The amount of inorganic anions added varies depending on the amount of chromium deposited and the color tone, but it is preferably 1 in terms of total chromium ions as chromic anhydride (CrO3).
In the case of phosphate ion, condensed phosphate ion, p, o
, in terms of 0.5 to 3. The other sulfate ions, nitrate ions, and chloride ions are preferably 0.05 to 1.0. If the amount of inorganic anions is too large, the chromate film becomes water-soluble (==), which causes moisture absorption, poor corrosion resistance, color change over time, and uneven reaction with plating during processing. It becomes difficult to obtain a white coating, which is one of the

The amount of chromate deposited is determined based on the desired appearance and corrosion resistance. Generally, the total amount of Cr deposited is 10 to 5o.
rn9/rrl Desirably 20-40 with a good balance of whiteness and corrosion resistance In9/rr? is appropriate.

The coating method can be any conventional method. Examples include a roll coater method, a squeeze roll method, an air knife method, a bar coater method, a flow coating method, a mist method, and the like.

The target plated steel sheets include electro-galvanized steel sheets and electro-zinc alloy plated steel sheets, such as Ni-Zn.

Alloy plating such as Fe-Zn, Co-Zn, Zn-8n, etc., hot-dip galvanized steel sheet, hot-dip zinc alloy plated steel sheet such as Zn-Mg Zn-Fe
, Zn-Ml or other alloy-plated steel sheets, hot-dip aluminium-plated steel sheets, turn-plated steel sheets, etc.

(Function) The object of the present invention is to obtain a white chromate with excellent corrosion resistance. Therefore, the present invention will be described from the viewpoint of whitening and corrosion resistance as the mechanism of action.

Figure 1 shows partial reduction (Cr”/ (Cr”+Cr) with starch.
A chromate solution obtained by adding birophosphoric acid to an acidic aqueous solution containing 50 t/L of chromic anhydride and 10 Of,/z of commercially available silica sol was electrolyzed using a roll coater. Galvanized (basis weight 20 y/n?
) Shows the effect of the amount of birophosphoric acid added on the yellowness (YI value; RS standard) of a chromate-treated galvanized steel sheet obtained by coating it on a steel sheet and drying it with heat at a sheet temperature of 60 ° C without washing with water. (curve C).

The smaller the YI value, the lower the degree of yellowing.

The black dot numbers represent the total amount of Cr deposited in the chromate film (In9
/- one side). The addition of birophosphoric acid lowers the yellowness of the chromate, and when used on a bath containing birophosphoric acid, almost no treatment is applied to electrogalvanized plating (go
) It has a normal appearance (straight line b). In addition, a commercially available white type electrogalvanized steel plate (EG-low chromium) with a low chromium coating amount (however, corrosion resistance is insufficient) that has been subjected to primary rust prevention romate is at the level of straight line a.

The mechanism of whitening is that the reaction of Cr'+ in the underlying zinc-methoxychromic acid is accelerated by inorganic anions, and the colored Cr'+ is reduced to Cr3+, and that the ionized zinc ions are reduced as the interfacial pH increases. Forms water-insoluble phosphate (white) or condensed phosphate (colorless), which is adsorbed to the silica sol in the film to form an inorganic white film.

Cr in the film becomes insoluble chromium chromate, e.g. Cr(OH) due to the increase in pH due to the reaction at the interface.
, CrO4, and forms a water-insoluble chromate with a colorless to swirly green color while retaining corrosion-resistant ions (CR). The chromate coating is a composite compound of chromium chromate, phosphate, and silica.

Comparing the corrosion resistance shown in Figure 1 using a salt spray test (JIS Z2371 continuous method), the one without the addition of P, O, showed 20% white rust in the 72-hour test; The one with P2O520t/l- had 1% white rust, and the one with P2O,5O, 75fil had no white rust. Under severe corrosive environments such as salt spray, when the coating weight of zinc plating is about 3 microns, pitting-like black spot rust often occurs from pinholes in the plating, even if a high amount of chromate is applied. The electrogalvanized steel sheet treated by the method of the present invention has the characteristic that black spot rust does not occur, and each component of the bath of the present invention has the effect of forming a film having a pore-sealing effect with the metal substrate.

As described above, according to the present invention, it is possible to obtain a chromate-treated galvanized steel sheet having excellent corrosion resistance and which is extremely easily whitened by coating-type chromate.

Example 1 A cold-rolled steel sheet was degreased and pickled using a known method, and then galvanized with sulfate (fabric weight 2o9./m'). After washing with water, apply the chromate solution shown in Table 1 using roll coating, and then immediately apply hot air (200°C) to the plate temperature at 60°C.
Dry to reach.

CrO3 in Table 1 is Cr"" / (Cr" + C
r”) = 0.4 starch partially reduced chromic acid, the number is (r/4). 5i02 is a commercially available 7Ricasol with a particle size of 10 to 20 mμ, P2O is a commercially available POIJ IJ It is sold as an acid.

H-P means polyphosphoric acid, Zn-P means ZnO/P
20゜=13150, N1-P is NiCO5/
P2O5” 10/50, M-P is M(OH), /P
, O, = 10150, Mp-P is M f(OH)t
/p2o, := 10150, Ba-P is Ba(OH)
2/ P, O, = 1 o150, Ca-P is CaCO
5/P2O5” 10/50, Co-P is C
Metal ions are added at a ratio of oCO3/P2O, = 2150.

The adhesion amount is T, Cr (IrLJi) as the total Cr adhesion amount.
/rr? ). Corrosion resistance was expressed as a percentage of the white rust area after 72 hours of the salt spray test (indicated as SST 72h). Also, moisture test (JIS Z022B) 168
The white rust occurrence rate after hours (indicated as HCT 168h) is expressed as a percentage.

Furthermore, in order to investigate discoloration over time, the color change after storage at room temperature 70℃ and humidity 95℃ for 2 hours (displayed as 70'C72h) was evaluated in 4 stages ◎: No abnormality, rather whiteness increased ○: Almost no change Δ: Darkened ×: Discolored to a blackish color.

Yellowness was determined as X using a commercially available computer-equipped color difference meter.

Y and Z were measured, and the yellowness YI value was shown. The same applies to brightness (L). 810□/CrO3 ratio, P, O,/C
The rO3 ratio is the respective non-volatile ratio in the bath. However, C
rO3 is a value when Cr'' and Cr'+ are all converted into CrO3.

N[LP-0 is a comparative example and is a chromate containing no P2O. It is colored and its corrosion resistance is somewhat lacking. P-1~P
-4 was added to the PzOs k P -0 bath;
The color becomes white in proportion to the P2O, and the corrosion resistance is improved.

P-5 to P-12 were obtained from a bath in which metal ions were added to P2O, and the yellowness was lower than that of p, o alone, and the corrosion resistance was also good.

Example 2 After galvanizing the flat head of Example 1, the partial reduction rate (Cr / (Cr -) - Cr )) was reduced to 0.
Chromic anhydride 50 f/l, birophosphoric acid (P2O,
Using a roll coater, apply a chromate bath containing 50 l/l and a commercially available atomized silica sol particle size of 5 to 10 mμ 1009/l to a total Cr coating amount of 25 m9/l, and bring the plate temperature to 60°C with hot air. It was heated and dried. The YI value of the obtained chromate-treated galvanized steel sheet and the white rust occurrence rate after 53T72 hours were measured as the partial reduction rate. The results are shown in Table 2. Return rate 0.1CP-13) - Return rate 0.7
The yellowness decreases in the order of (P-18), but the corrosion resistance decreases in the order of P-18.
It tends to decrease from around 17.

Example 3 Under the conditions of IMP-4 in Example 1, zirconia sol, alumina sol, and titanium oxide sol were used instead of silica sol.

However, the concentration was 3 of/l. The results are shown in Table 2.

The sol concentration is low due to bath stability, and P-19 to P-2
Although Sample No. 1 had a slightly higher Y value than silica sol, it had a practical appearance and corrosion resistance as a white chromate.

Example 4 The experiment was carried out under the conditions of Example 1 and mP-4 using a chromate bath in which inorganic anions were controlled to the types and concentrations shown in Table 3.

Table 3 P-23 is an example of orthophosphoric acid. White chromate is obtained.

P-24 is an example of aluminum biphosphate, and P-25 is an example of phytic acid, both of which yielded corrosion-resistant white chromate.

P-26 to P-28 are examples of sulfuric acid, nitric acid, and hydrochloric acid, and the inorganic anions are soluble in ice, resulting in slightly deteriorated corrosion resistance, but the yellowness of chromate is reduced.

Example 5 Example I A hot-dip aluminized steel plate and a Ni-Zn alloy electroplated steel (rn 12-chi) were processed under the conditions of FnP-4. A plated steel sheet was obtained that had a white chromate film that was slightly grayish with almost no yellow color. As for corrosion resistance, no white rust was observed after 5ST and 16B hours.

(Effects of the Invention) By the method of the present invention, a white chromate with excellent corrosion resistance and a uniform appearance can be obtained through a simple process.

The chemicals used are popular and inexpensive, so the cost is low and can meet market demands.

In terms of equipment, it is a coating chromate that can be fully applied to conventional processes, and by changing the inorganic anion and reduction rate, highly corrosion-resistant chromate treatment from pale yellow to yellow can be achieved.

Further, in the present invention, since the applied chromate solution reacts with the plating substrate depending on the temperature, there is an effect that the dependence of conventional applied chromate on the substrate is low.

[Brief explanation of the drawing]

Figure 1 shows the reduction rate (Cr" / (Cr"-4-Cr"
) shows the amount of polyphosphoric acid (expressed as P2O3) and the yellowness of the chromate film of a chromate-treated electrogalvanized steel sheet obtained from a bath of chromic acid anhydride, silica sol, and polyphosphoric acid with 0.4. Figure 1 P2Os O/l) Procedural amendment March 5, 1986

Claims (5)

[Claims] (1) Cr^3^+/(Cr^3^++Cr^6^+)
A corrosion-resistant white chromate treatment method characterized by applying an acidic aqueous solution containing chromic acid, an inorganic colloid compound, and an inorganic anion with a ratio of 0.1 to 0.7 to a metal surface, and then heating and drying the solution. (2) Corrosion-resistant white chromate treatment method according to claim 1, which uses silica sol as the inorganic colloid compound and phosphate ion or condensed phosphate ion as the inorganic anion. (3) Corrosion-resistant white chromate treatment method according to claim 1, which uses any one of a sulfate group, a nitrate group, a chlorine ion, a fluorine ion, a fluorine complex ion, and a sulfonic acid group as the inorganic anion. (4) Cr^3^+/(Cr^3^++Cr^6^+)
Chromic acid with a ratio of 0.1 to 0.7, an inorganic colloid compound,
A corrosion-resistant white chromate treatment method characterized by applying an acidic aqueous solution containing an inorganic anion and a divalent or higher valence metal ion to a metal surface and then heating and drying it. (5) Mg^2^+, Ca^2^+, as metal ions
Sr^2^+, Ba^2^+, Al^3^+, Zn^2
^+, Ni^2^+, Co^2^+, Fe^2^+, S
Corrosion-resistant white chromate treatment method according to claim 4, which includes one or more types of n^2^+