JPH03146676A - Production of chromated steel sheet having superior resistance to stain by moisture absorption - Google Patents

Abstract

PURPOSE:To improve resistance to stain by moisture absorption by coating a galvanized steel sheet with a treating agent prepd. by blending chromic acid with an inorg. colloidal compd. and a silane coupling agent in specified ratios and regulating the amt. of soluble Cr in the resulting coating film by drying. CONSTITUTION:A treating agent having a compsn. contg. 5-100g/l chromic acid having 0-40 ratio of Cr<3+> to total Cr, an inorg. colloidal compd. in 0.1-20 ratio to total Cr (expressed in terms of chromic acid) and a silane coupling agent in 0.5-5 ratio to total Cr (expressed in terms of chromic acid) is prepd. A galvanized steel sheet is coated with the treating agent and the amt. of soluble Cr in the resulting coating film is reduced to <=10mg/m<2> by drying. The occurrence of spotlike stains by moisture absorption due to soluble Cr<6+> in the coating film is prevented at the time of storage over a long period of time and a chromated steel sheet having superior quality can stably be obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a chromate-treated steel sheet having excellent moisture absorption and stain resistance.

Strain chromate treatment has conventionally been widely used to improve the corrosion resistance of galvanized steel sheets, such as galvanized steel sheets, zinc alloy-plated steel sheets, and aluminized steel sheets, and as a base treatment for painting.

This type of chromate treatment involves forming a chromate film using a reaction accelerator in the treatment agent or electrolysis, and then washing away the soluble components while leaving the film behind. It is broadly divided into coating type chromate treatment, which is heated and dried to form a chromate film. Among these, the coating type chromate treatment has many advantages such as simple equipment and a small amount of waste water, and is widely used for continuous treatment of galvanized steel sheets.

In the above coating type chromate treatment, it is already known to mix silica in the form of powder or colloidal silica with the chromate treatment agent in order to improve the corrosion resistance of the coating. For example, a method using silica sol and chromic acid aqueous solution as a chromate treatment agent (Japanese Patent Publication No. 42-140
50), a method using silica sol and a reduced chromic acid aqueous solution (Japanese Patent Publication No. 61-58552@), a method using a condensed phosphoric acid aqueous solution together with silica sol and a chromic acid aqueous solution (Japanese Patent Publication No. 60-18751), silica sol, A method using an aqueous solution containing cobalt ions together with a chromic acid aqueous solution and a phosphoric acid aqueous solution (Japanese Unexamined Patent Publication No. 57-174469), a method using silica powder and reduced chromic acid (Japanese Unexamined Patent Application Publication No. 52-17340 and JP-A-52-1988) -17341) etc. are known. Furthermore, it is also known to specify the particle size of the silica used (Japanese Patent Publication No. 1508/1982), and to use white carbon (Japanese Patent Application Laid-open No. 92339/1983).

However, in all of the conventional chromate treatment methods described above, the treatment agent contains a large amount of Cr, and as a result, the resulting chromate film has a high Cr content. . The present inventors have discovered that a chromate film containing such a large amount of Cr causes minute green spot-like stains to appear on the surface when the treated steel sheet is stored in coil or sheet form for a long period of time. This speckled stain, depending on its size and amount, can significantly impair the commercial value of the chromate-treated steel sheet.

As a result of extensive research on such speckled stains, the present inventors found that they are compounds consisting of Cr'', Cr'' and zinc, and are crystallized on the surface. Spotted stains often occur when steel plates are placed in an environment where high humidity with dew condensation and dryness are alternately repeated.The cause of this is that Cr in the chromate film absorbs moisture and dissolves. It was determined that the cause of the corrosion was localized corrosion of the plated zinc.

Furthermore, the present inventors have found that the above-mentioned spotty stains or hygroscopic stains occur when the surface is It is difficult to occur in inert plated steel sheets, but on the other hand, it occurs in electrogalvanized copper sheets, hot-dip galvanized steel sheets that are skin-pass rolled after plating, etc.
It has been found that this phenomenon is more likely to occur in active-plated steel sheets.

Normally, in order to obtain a chromate-treated steel sheet with good corrosion resistance, a Cr coating amount of 30 mg/rd or more is required. However, according to research by the present inventors, soluble Cr' in the chromate film When the degree exceeds 10 .mu./d, hygroscopic stains tend to occur; on the other hand, when it is less than 10 .mu./M, hardly any hygroscopic stains occur. In addition, soluble Cr6” is 10
(2) Even when it is less than /d, when the ratio of soluble Cr'' to the total amount of Cr deposit exceeds 30%, it is likely to occur similarly.

Conventionally, the amount of Cr" that can be stably present in a chromate treatment agent is usually limited to 50%, and at this ratio, it is difficult to make the Cr" in the film satisfy the above conditions. be. On the other hand, when a reducing agent such as Polymer II or alcohol is added to the chromate treatment agent so that the Cr6+ in the resulting chromate film satisfies the above conditions, the film has poor corrosion resistance.

On the other hand, it is already known that Cr'' can be reduced by incorporating a fluorine compound into the chromate treatment agent. However, fluorine compounds have the effect of coagulating and precipitating colloidal silica, and are not suitable for use in industrial romate treatment. Furthermore, after applying a chromate treatment agent to a steel plate, Cr'' in the coating can be reduced or made insolubilized by heating the steel plate to a temperature of 150°C or higher, but in high-speed production equipment, extensive heating is required. It requires a furnace and is industrially disadvantageous.

The amount of chromate-treated steel sheet that is intended to be reduced The present invention was made to solve the above-mentioned various problems in the production of conventional chromate-treated steel sheets. The purpose of the present invention is to provide a method for manufacturing treated steel sheets.

The first method of manufacturing a chromate-treated steel sheet with excellent moisture absorption and stain resistance according to the present invention for preparing bones is as follows: (a) Cr3+/total chromium 0 to 40% chromic acid 5
~100g/l, Cbl 061~ for total chromic acid equivalent value 1
A treatment agent containing an inorganic colloid compound of 20 and a silane coupling agent of 0.5 to 5 per chromic acid equivalent value of 1 of total Cr was applied to a blood lead-plated steel plate, dried, It is characterized in that the soluble Cr1l in the coating is 10 .mu./d or less.

The second method for producing a chromate-treated steel sheet with excellent moisture absorption and stain resistance according to the present invention is that, in addition to the above, a phosphoric acid compound is contained in an amount of 0.05 to 0.5 based on the chromic acid equivalent value l of total Cr. It uses a processing agent that

In the method of the present invention, the chromic acid used is an aqueous solution of chromic acid anhydride or an aqueous solution of reduced chromic acid with a reduction rate of 40% or less. Its concentration is chromic acid (crow, hereinafter referred to as
5 to 100 g/l in terms of ) conversion value is preferred. The reduced chromic acid used is one obtained by reducing chromic acid anhydride with an inorganic or organic reducing agent according to a conventional method. Reduced chromic acid is
When the reduction rate exceeds 40%, when a silane coupling agent, which will be described later, is added to the treatment agent, it becomes difficult to uniformly apply the treatment agent, resulting in uneven coating and deterioration of surface quality.

In the method of the present invention, inorganic colloidal compounds in the treatment agent are used to impart high corrosion resistance to the steel sheet. As the inorganic colloid compound, silica sol is preferably used, but alumina sol, titania sol, zirconia sol, etc. can also be used.

Moreover, these may be used in combination.

The amount of the inorganic colloid compound added to the treatment agent is preferably in the range of 0.1 to 20 (weight ratio, hereinafter referred to as μ) to 1 of the total Cr in terms of chromic acid. When the amount of the inorganic colloid compound added is less than 0.1 per chromic acid equivalent value of total Cr, no improvement in corrosion resistance can be expected, while on the other hand, when it exceeds 20, film forming properties deteriorate.

As the silane coupling agent, one having a methoxy group on a silicon atom is preferably used.

As such a silane coupling agent, for example, T
-glycidoxypropyltrimethoxysilane, r-(2
-aminoethyl)aminopropyltrimethoxysilane,
γ-(2-aminoethyl)aminopropylmethyldimethoxysilane, β-(3°4-epoxycyclohexyl)
Ethyltrimethoxysilane or the like is used. these are,
It can be used alone or as a mixture of two or more.

In the method of the present invention, the silane coupling agent liberates methanol in an amount that exceeds the chemical equivalent of Cr'' remaining in the coating during the drying process after applying the chromate treatment agent to the steel sheet. Soluble Cr in
Furthermore, the simultaneously generated silanol groups are crosslinked with silica, Cr'' and/or remaining Cr'' coexisting in the coating, thus forming a strong coating with high corrosion resistance.

In the present invention, the amount of silane coupling agent added to the treatment agent is 0.5 to 1 per chromic acid equivalent value of total Cr.
The range is 5. The total amount of silane coupling agent is C
When r is less than 0°5 relative to the chromic acid equivalent value of 1, the above-mentioned effect of suppressing soluble Cr in the coating is insufficient; on the other hand, even when r exceeds 5, the above effect is not achieved. is saturated and is economically disadvantageous.

Furthermore, in the method of the present invention, in order to further improve the corrosion resistance of the copper plate, the treatment agent has a total Cr equivalent value of 1
may contain 0.05 to 0.5 of the phosphoric acid compound. When the blending amount of phosphoric acid is less than 0.05 to 1 of the total Cr converted to chromic acid, there is no effect on further improving corrosion resistance. However, when it exceeds 0.5, the hygroscopicity of the coating increases and the corrosion resistance deteriorates.

As the phosphoric acid, orthophosphoric acid, various condensed phosphoric acids, ammonium salts and alkaline earth metal salts thereof are preferably used. These phosphoric acid compounds produce chromium phosphate, which is incorporated into the crosslinks formed by the silane coupling agent, and is thought to further improve the corrosion resistance of the chromate coating.

However, alkali metal salts are not suitable for use in the method of the present invention because they promote the formation of spotty stains.

In the method of the present invention, no fluorine compound is added to the processing agent. This is because, as described above, fluorine compounds inhibit the dispersion of the inorganic colloid compound in the treatment agent and deteriorate the quality of the coated surface.

The amount of chromate film deposited is determined based on the appearance and corrosion resistance of the treated steel sheet, but it is usually in the range of 20 to 100 ffv/m, preferably 30 to 60 ffv/m, which provides an excellent balance between appearance and corrosion resistance. The range is M.

In the method of the present invention, the method of applying the chromate treatment agent to the mouth plate is not limited in any way; for example, conventionally known roll coater method, shower ringer method, air nuff method, etc. It's fine. For subsequent drying, for example, hot air drying, gas or electric heating, infrared heating, induction heating, etc. can be employed according to conventionally known methods. The board temperature during these drying is 60
It is preferable that the temperature is at least °C.

According to the method of the present invention, as described above, a chromate treatment agent containing chromic acid or reduced chromic acid, an inorganic colloidal compound, and a silane coupling agent is applied to a galvanized steel sheet, and then dried. , the amount of soluble Cr in the film was 10■
/d or less, while taking advantage of the advantages of coating-type chromate treatment, it is possible to prevent the occurrence of spot-like hygroscopic stains during long-term storage due to soluble Cr in the coating,
Chromate-treated steel sheets of excellent quality can be stably obtained.

Furthermore, according to the method of the present invention, by adding a phosphoric acid compound to the chromate treatment agent in addition to the above, -layer,
A chromate-treated steel sheet with excellent corrosion resistance can be obtained.

EXAMPLES The present invention will be explained below with reference to Examples, but the present invention is not limited to these Examples in any way.

An aqueous chromic acid anhydride solution was prepared, and methanol was added to the aqueous chromic acid anhydride solution to prepare reduced chromic acid with various reduction rates. In addition to this, silica sol and silane coupling,
A chromate treatment agent was prepared by adding phosphoric acid and, depending on the case, the proportions shown in Table 1.

This treatment agent was applied to an electrogalvanized steel plate using a roll coater, and then heated and dried at a plate temperature of 60°C to obtain a test material.

Regarding these test materials, the total amount of Cr deposited and the amount of soluble Cr were measured, and a speckled stain generation test and a salt spray test were conducted. The results are shown in Table 1.

The test method is as follows.

Gold nail #51 Cut out a 34n x 34n test piece from the sample material, and
The amount of Cr deposited was determined by line analysis.

The above test piece on which the total Cr content was measured was immersed in boiling water for 1 minute to elute and remove soluble Cr, washed with water, and dried.The remaining Cr content was determined by fluorescent X-ray analysis. The difference from the amount was defined as soluble Cr. That is, soluble Cr amount - total Cr amount - residual CrttC / d).

A test piece measuring 70 m x 150 m was cut out from the star-stained, normal-light, raw-wiping specimen, and a tape with a width of 10 cm and a thickness of 50 μm was pasted along one end in the longitudinal direction. Ten sheets of this were stacked, and further, one sheet of dummy material was placed on top and bottom of the dummy material, and both ends of the image in the longitudinal direction were fixed with tape to prepare a test sample.

As shown in FIG. 1, this sample was subjected to a cycle of repeated dew condensation and drying (3 cycles/day). The samples were released after 15 cycles (5 days), 30 cycles (10 days), and 45 cycles (15 days), and the presence or absence of speckled stains on the test materials was examined.

In Table 1, ◎ indicates no spotty staining after 45 cycles, O indicates occurrence after 45 cycles, Δ indicates occurrence after 30 cycles, and × indicates occurrence after 15 cycles.

■A 70m x 150wI test piece was cut out from the paper and wiped fold specimen material, tape was pasted on the edge, and paint was applied and sealed. A salt spray test was conducted for 168 hours to determine the area where white rust had occurred. I checked the rate.

As is clear from the results shown in Table 1, the chromate-treated steel sheets obtained by the method of the present invention do not exhibit any abnormality in surface quality even when left for long periods of time under severe environmental conditions of high temperature and humidity. , has excellent corrosion resistance.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the dew condensation and drying cycle used in the moisture absorption stain generation test.

Claims (5)

[Claims] (1) (a) Cr^3^+/total Cr ratio 0 to 40% chromic acid 5 to 100 g/l, (b) total Cr chromic acid equivalent value 1 to 0.1 to 20
A treatment agent containing an inorganic colloidal compound of A method for producing a chromate-treated steel sheet with excellent moisture absorption and stain resistance, characterized in that the amount of soluble Cr is 10 mg/m^2 or less. (2) The method for producing a chromate-treated steel sheet according to claim 1, wherein the inorganic colloid compound is silica sol, alumina sol, titania sol, or zirconia sol. (3) (a) Cr^3^+/total Cr ratio 0 to 40% chromic acid 5 to 100 g/l, (b) total Cr chromic acid equivalent value 1 to 0.1 to 20
an inorganic colloidal compound, (c) a silane coupling agent of 0.5 to 5 per chromic acid equivalent value of total Cr, and (d) 0.05 to 0 per chromic acid equivalent value of total Cr of 1
．． A treatment agent containing the phosphoric acid compound of No. 5 is applied to a galvanized steel sheet and dried to reduce the amount of soluble Cr in the coating to 10 mg/m^2 or less.It has excellent moisture absorption and stain resistance. A method for producing chromate-treated steel sheets. (4) The method for producing a chromate-treated steel sheet according to claim 3, wherein the inorganic colloid compound is silica sol, alumina sol, titania sol, or zirconia sol. (5) The method for producing a chromate-treated steel sheet according to claim 3, wherein the phosphoric acid compound is orthophosphoric acid, condensed phosphoric acid, an ammonium salt thereof, or an alkaline earth metal salt thereof.