JPS5834177A - Chromate treatment for plated steel plate - Google Patents

Abstract

PURPOSE:To form a chromate film having excellent properties by using a chromating soln. of slow aging prepd. by controlling a Cr<+3>/Cr<+6> molar ratio in an aq. CrO3 soln. then adding an F compd. consisting of F<-> or fluorine complex ions by treating a plated steel plate with said treating soln. CONSTITUTION:A reducing agent (e.g.; aq. PVA soln. of 500 polymn. degrees) is added to an aq. CrO3 son. contg. 10-50g/l CrO3 in terms of its concn. and the mixture is heated for a constant time to control its Cr<+3>/Cr<+6> molar ratio to 0.1-0.5. Ba<+2> alone as cations and a fluorine compd. consisting of F<-> or fluorine complex ions (e.g.; BaF 2 BaSiF6, etc.) as anions are added to the aq. CrO3 soln. thus prepd. in such a way that the same are contained at >=0.04g/l in terms of F elements. A plated steel plate is treated of sufaces with the resultant chromating soln., whereby the chromate films which are hard to dissolve in water and have excellent corrosion resistance, workability, adhesive strength of paint films, etc. are formed.

Description

[Detailed description of the invention] Conventionally, galvanized steel sheets plated with zinc, aluminum, etc. have been used for various purposes either uncoated or coated. The surface of plated steel sheets is treated with a chromate treatment solution containing chromic anhydride as the main component in order to improve corrosion resistance. Chromate treatment can also be carried out by an electrolytic treatment method in which electrolysis is performed in a treatment solution using the base material to be treated as an electrode, or by a coating method in which the treatment solution is applied to the surface of the base material to be treated, but in general, the base material to be treated is treated. Chromate treatment (hereinafter referred to as "reaction") involves forming a chromate film with etching through a chemical reaction between the surface of the base material and the treatment solution, using the immersion method in which the treatment solution is immersed in the treatment solution or the spray method in which the treatment solution is sprayed onto the treatment solution. chromate treatment) is performed. In the case of reactive chromate treatment, the treatment solution is used repeatedly, and as shown in the linear equation, metals (zinc, aluminum, etc.) on the surface of the base material to be treated dissolve into the chromate treatment solution and become metal ions, and at the same time, the treatment solution The hydrogen ions inside are consumed O Me+ nH2Cr207 →Me”+HCr207
+H2↑ (where n is 2 or 6) As a result, the free acid concentration in the liquid decreases depending on the amount of material to be processed, and the chromate film formation reaction gradually becomes difficult to occur, until it becomes normal and the chromate film is formed. The processing solution that is no longer formed and has aged must be disposed of as waste and replaced with a new processing solution.

This aging phenomenon of the treatment solution is generally a phenomenon that is observed when the object to be treated is repeatedly treated with a fixed amount of solution, but especially in chromate treatment, the following steps are required to increase the treatment effect. Since various treatment methods such as these are used, the aging of the treatment solution becomes particularly significant. In other words, in an attempt to obtain high corrosion resistance through chromate treatment, the chromate concentration in the chromate treatment solution is increased to increase the amount of chromium deposited, but in this case, the chromate film may easily dissolve in water or become colored. , chromic acid stains may occur, and the amount of chromium deposited may be uneven, resulting in poor appearance. Therefore, on the one hand, an etching agent is added to the chromate treatment solution in order to promote the chromate reaction. Mineral acid is used as an etching agent.

Organic acids or salts thereof are used, but fluorine compounds such as NH4HF2 and N28iF'6, which have high solubility, are also being used. Even when using a chromate treatment solution containing such etching agents, it is necessary to add a large amount of these etching agents to the chromate treatment solution in order to increase the amount of chromium deposited and obtain sufficient corrosion resistance. However, when a large amount of etching agent is added, the reaction between the chromate treatment solution and the surface of the plated steel sheet, which is the base material to be treated, progresses rapidly, causing rapid aging of the treatment solution and shortening the lifespan of the treatment solution. In addition, there is a limit to the amount of etching agent that can be added because the surface of the base material to be treated may be colored and chromic acid stains may occur. 11 Therefore, it is not easy to extend the life of the treatment solution by strictly controlling the concentration of the etching agent, and furthermore, the corrosion resistance of the chromate film is still insufficient.

It is also known to perform chromate treatment by adding a film forming agent such as silica sol or alumina sol to the chromate treatment solution, but the effect is not sufficient unless it is added together with an etching agent.

In this way, aging of the treatment solution is the fate of reactor chromate treatment, especially recently with the tightening of pollution regulations.
Since the treatment of chromic acid waste liquid has become extremely difficult, extending the life of the chromate treatment liquid and increasing its treatment effect are the biggest challenges in the treatment of reactive chromate treatment liquids. In view of this, there has been a desire for a chromate treatment method that uses a treatment solution that is easy to control concentration and that ages slowly and contains an etching agent that forms a strong chromate film with excellent corrosion resistance even at low concentrations.

As a result of intensive research, the present inventors have found that one part of silovalent chromium is reduced to trivalent chromium using a reducing agent in an aqueous chromic anhydride solution, or by further adding an aqueous chromic acid anhydride solution, fluorine is added to the resulting solution as an etching agent. A chromate treatment solution containing fluorine compounds consisting only of ions, fluorocarbon chain ions, and barium ions can form a chromate film that ages slowly, is poorly soluble in water, and has excellent corrosion resistance, processability, and coating adhesion. After investigating this, the present invention was completed.

An object of the present invention is to provide a method for chromate treatment of a plated steel sheet in which a chromate treatment solution is slow in aging and a chromate film with excellent physical properties can be formed in the reactive chromate treatment of a plated steel sheet.

That is, the present invention adds a reducing agent to an aqueous chromic anhydride (croH) solution to partially and completely reduce the chromic anhydride, or further adds an aqueous chromic anhydride solution to reduce the molar ratio of hexavalent chromium to hexavalent chromium ( Cr4δ/Cr◆'3k 0.1~0.5
A fluorine compound consisting of only barium ions as cations and fluorine ions or fluorine chain ions as anions was added to the solution to bring the concentration of anhydrous chromium to 10 to 5.
The present invention relates to a method for surface treatment of a plated steel sheet, characterized in that the plated steel sheet is surface treated with a chromate treatment solution in which the concentration of the fluorine compound is adjusted to 0.04171 or more expressed as the fluorine element.

The present invention will be explained in more detail below using all the drawings.

Figure 1 shows the fluorine compounds (hereinafter referred to as
As an example of a barium-containing fluorine compound (barium-containing fluorine compound),
Graph showing the effect of barium tsulfide on S-solubilizing the chromate film in comparison with that of other fluorine compounds, ・Second
The figure shows various molar ratios (Or,' ZCr, Cr, hexavalent chromium and pentavalent chromium mixed water** saturation concentration of barium 7silicate (J number of solutes in a saturated solution 1) t- The graph shown in Figure 16 shows the various molar ratios (Cr''' shown in Figure 2).
In treatment with various chromate treatment solutions in which a mixed aqueous solution of
It is a graph showing (a) the amount of etching and (o) the corrosion resistance of the steel plate to be treated.

The chromic anhydride (Crys) used in the present invention is one that has been conventionally used for surface treatment of steel sheets. The optimum concentration of chromic anhydride in the chromate treatment solution is 10 to 50/l/1 in order to form a chromate film containing chromic anhydride as the main component on the surface of a fully plated steel sheet. If the chromic anhydride concentration is less than 10g/1, a chromate film with excellent corrosion resistance cannot be obtained, and if it exceeds 50gZt, the chromate film takes on a yellowish tinge, and as the treatment solution rises, chromate stains are likely to occur, impairing the appearance. This is not only undesirable, but also increases consumption of chromate, making it uneconomical.

Hexavalent chromium 6 along with hexavalent chromium in the total chromate treatment solution
The reason why some of the valent chromium is completely reduced and mixed as hexavalent chromium is to increase the proportion of hexavalent chromium in the chromate film that is formed, thereby reducing its solubility in water and improving corrosion resistance. It is. A preferred method for reducing a portion of hexavalent chromium to form hexavalent chromium and mixing it with hexavalent chromium is to add a reducing agent to a highly concentrated aqueous chromic acid anhydride solution to achieve a predetermined molar ratio (Cr4p &/Cr◆
6) A method of reducing and then diluting is used.

Another method is to perform reduction at a predetermined molar ratio (Cr0/cr◆6
) or more, and then further adding an aqueous chromic acid anhydride solution to adjust the molar ratio to a predetermined value (Cr0/cr◆6).

To reduce 6@+i chromium in an aqueous solution of chromic anhydride, a known method may be used, for example, an aqueous solution of chromic anhydride (
For example, an aqueous solution of polyvinyl alcohol (polymerization degree 500) is added to the reducing agent (1001/l), heated for a certain period of time, e.g., 4 hours at 80 to 90°C to reduce the reducing agent to a predetermined molar ratio (Zcr). 6) p An example of a method for adjusting the chromic anhydride concentration is as follows.

Organic reducing agents include ethylene glycol,
10 Glycols such as pyrene glycol.

Monohydric alcohols such as methanol and ethanol.

Pyrogallol, saccharose, mannidol natonopolyhydric alcohol, and silane coupling agents that produce alcohols such as methanol and ethanol by hydrolysis are used, and inorganic reducing agents such as hypophosphorous acid and hydrogen peroxide are used. nru. The reason why the ratio t molar ratio of pentavalent chromium to hexavalent chromium (Cr"/cr@) is set to 0.1 to 0.5 is that if it is less than 0.1, the corrosion resistance and workability of the chromate film will deteriorate.
This is because no improvement in performance such as coating film adhesion is expected, and if it exceeds 0.5, the stability of the treatment liquid may become poor or the appearance of the film may be impaired.

The barium-containing fluorine compound is used as an etching agent. This barium-containing fluorine compound is a fluorine compound having fluorine ions or fluorine chain ions as anions and only barium as a cation, and is the most distinctive feature of the present invention.

:゛As such a barium-containing fluorine compound, BaFl
, BaSiF6, BaSiF6, BaTi
F6 etc. are used. The amount of the barium-containing fluorine compound contained in the treatment liquid 1 is 0.00% expressed as elemental fluorine.
That's all for 04I/.

This barium-containing fluorine compound brings about the following important effects in the present invention. The first effect is to make the formed μMate film difficult to dissolve and provide a high degree of corrosion resistance. Figure 1 shows that 50% of 42.859r chromic anhydride (Cr05) has been reduced and 6 is mixed in the liquid.
11 valent chromium, hexavalent chromium and various fluorine compounds! A galvanized steel plate was treated with a chromate treatment solution containing Toga 1 for 1 meter, and the resulting chromate treated steel plate (adhered chromium amount 55 WLfl/-z) was placed in distilled water at 40°C. The sample was immersed in water for 2 minutes and the amount of chromium eluted t' was measured.

In the drawing, (a) is: sa eiF6 (Fte0,41
Contains J9], (w) HaNap 511F6 (contains IF Te0.611), (c) is NH4B i F4 (F
(2) is the case in which each fluorine compound is contained (with te0-641), and (2) is the case in which no fluorine compound is contained.From Figure 1, fluorine ion (9) is the case with respect to the anion of the fluorine chain ion. It can be seen that a fluorine compound whose cation is only a barium ion has a greater effect of making the chromate film less soluble than other highly soluble heptadium compounds whose cation is a sodium or ammonium ion.

The effect of the barium-containing fluorine compound on making the chromate film less soluble is fully manifested at a concentration of 0.049 or more expressed as fluorine element per chromate treatment solution 1, as shown in Examples below. And this concentration is [1,04,9
As far as the effect of making the Chronaut film difficult to solubilize is concerned, it is not necessary to have a saturation concentration as described as the fifth effect.

The second effect of the barium-containing fluorine compound is that its etching effect is moderately small, which prevents the appearance from being damaged by excessive etching, and it slows down the aging of the chromate treatment solution, extending the life of the treatment solution. It is to let

The effect of O6, a barium-containing fluorine compound, is that the solubility of barium-containing fluorine ions in water is moderately low, so the plated steel sheet is treated with a chromate treatment solution in which a barium-containing fluorine compound is dissolved in the chromate treatment solution until it reaches saturation. In such a case, the etching agent has an appropriate etching power to form a romate film even when excessively etched, thereby making it unnecessary to control the concentration of the etching agent. Let me explain the reason. First, Figure 2 [Note; Bast Fb ('/l) in the figure
1.2 and 3.4 are expressed as fluorine element and are respectively 0.
41, 0.82, 1.25, 1.65 <11/
1)], the larger the molar ratio (''/cr+6) of the mixed aqueous solution of trivalent chromium and hexavalent chromium, the more barium fluorosilicate (BaSiF6) the nuclear mixed aqueous solution becomes.
Although the saturation concentration of Cr increases, the molar ratio (Cr
Even the maximum value of +5Zcr+6) is 1 expressed as fluorine element.
．． It can be seen that the value is approximately 651/. Curve (a) in Figure 3 shows the amount of etching when a mixed aqueous solution of each molar ratio -(cr+'/are) is saturated with barium silicate and then a galvanized steel plate is chromate treated with a chromate treatment solution. The amount of etching decreases as the percentage of hexavalent chromium increases, which is dangerous as shown by the loss of corrosion after immersion in bath water at 50° C. for 10 minutes. Also, from the curve (b) in Figure 3, the same treated steel plate as in the case of the corrosion resistance [curve (a)] was
2371, the state of white rust occurrence was observed using the salt spray test method in accordance with 2371], the value of the molar ratio (Cr+l/cr+4) was 0.
．． It can be seen that a value of about 2 to 0.4 is best. Therefore, even if the barium-containing fluorine compound is dissolved in the chromate treatment solution in a saturated state, the amount of etching is moderately small and a film with excellent corrosion resistance can be obtained. The dissolved barium-containing fluorine compound, which is contained in excess solid form and is consumed as the chromate treatment progresses, is constantly replenished by the successive dissolution of solids and can be maintained at a saturated concentration, that is, a constant concentration. It can be done.

Therefore, as long as a barium-containing fluorine compound is used as an etching agent and is added to the chromate treatment solution in excess of its saturation level, there is no need for extensive work to control the concentration of the etching agent in the treatment solution. Simply replenishing to maintain this excess will be enough.

The chromate treatment solution used in the present invention can be prepared by adding a reducing agent to an aqueous solution of chromic anhydride (hexavalent chromium compound) to reduce it, or by further adding an aqueous solution of chromic anhydride (a hexavalent chromium compound) to prepare the chromate treatment solution at a predetermined 0 molar ratio ( A barium-containing fluorine compound is added to a solution in which hexavalent chromium and trivalent chromium are mixed in Cr''/cr@-b), and each component is adjusted to a predetermined concentration.

The plated steel sheet to be treated in the present invention is a plated steel sheet prepared by hot-dipping or electroplating with zinc, aluminum, etc., and the galvanized steel sheet is a plated steel sheet that is simply galvanized. In addition, zinc-plated steel sheets are also used, which are subjected to alloying treatment by heating at high temperatures after galvanization (in the present invention, each steel sheet to be treated is referred to as the steel sheet). (collectively called steel plates).

Chromate treatment 11 used in A of the present invention on plated steel sheet
To perform surface treatment using Et, follow the commonly practiced method. After degreasing and washing the steel plate with water as necessary to make the surface at-clean, apply a treatment solution at an appropriate temperature and treatment time. For example, dipping or spraying at 50 to 70°C for 5 to 10 seconds under specific treatment conditions. ! After surface treatment, if necessary, excess treatment liquid adhering to the steel plate to be treated may be removed with a rubber roll or the like, and then dried. The amount of the chromate film deposited is preferably 10 to 5 O-A- in terms of chromium.

The present invention will be further explained below with reference to Examples and Comparative Examples.

Examples 1 to 8 Comparative Examples 1 to 6 Various plated steel plates having a thickness of 0.4 mm were subjected to continuous surface treatment using a treatment solution having a composition shown in Table 1 and treatment conditions by a dipping method or a spray method.

A test piece was taken from each of the obtained chromate-treated steel plates, and tested for (1) coloration, (2) corrosion resistance, and (3) coating performance, as will be explained in detail later.

The test method for each test item of the chromate-treated test piece is as follows.

(1) Coloring condition The coloring condition of the surface of the treated steel sheet after chromate treatment was observed with the naked eye.

(2) Corrosion resistance JIS test specimens under six conditions as shown below.
A salt spray test was conducted in accordance with Z 2371 (salt spray test method), and the state of white rust occurrence was observed with the naked eye.

(a) Flat part: A flat test piece was subjected to the test as it was, and a salt spray test was conducted for 240 hours.

(b) Eriksen Extrusion part: By Erichsen extrusion testing machine 4mm extruded test piece was subjected to a salt spray test. was carried out for 150 hours.

(c) A test piece bent at 1180 degrees at the bent portion was subjected to a test, and a salt spray test was conducted for 72 hours in the dark.

(3) Paint film performance test piece was coated with Acrylic II paint and baked and dried at 650° C. for 55 seconds to form a paint film with a thickness of 20 μm. This painted test piece was subjected to (a) goblin test and (b) DuPont impact test according to the test method of JIS G3312 (Colored galvanized iron plate).

and (c) a 600-hour salt water spray test was conducted, and the condition after the test was observed with the naked eye.

(Mine) The numbers in the figure are the number of grams expressed as fluorine element (1) above.
The observation results for each item (2) l and (3) were evaluated in four stages as shown in Table 2, and symbols (◎lotΔ, ×) were determined to represent each stage.

Table 2 The above test results are shown in Table 5.

As can be seen from Table 6, the steel plates treated with chromate according to the chromate treatment method of the present invention have better coloration, corrosion resistance in various parts, and coating performance than those treated with the conventional chromate treatment method. ing.

In Table 4 of Table 15, in each Example, the aging of the treatment solution was slow, the service life was extended, and the amount of steel plate processed was large. And Example 9゜11 and 12
When barium-containing fluorine compounds were added in excess, as in the case of fluorine compounds containing barium, only addition was required from time to time to maintain the excess state during processing, and concentration control did not require as much effort as in other cases.

The chromate treatment method according to the present invention uses a treatment solution containing a barium-containing fluorine compound as an etching agent, and further contains a hexavalent chromium compound together with chromic anhydride (hexavalent chromium compound) as a main component. It is strong and has excellent corrosion resistance and coating adhesion, and the aging of the treatment solution is slow and has a long life.Furthermore, the process can be easily controlled, making it easy to produce surface-treated plated steel sheets with excellent performance. It can be supplied at low cost and has great industrial benefits.

[Brief explanation of drawings]

Figure 1 is a graph showing the effect of barium silicofluoride, an example of the barium-containing fluorine compound used in the present invention, in making the chromate film less soluble in comparison with that of other fluorine compounds. Figure 2 is a graph showing the molar ratio (Cr+s/ Cr")
A graph showing the saturation concentration of barium fluorosilicide (the number of grams of solute in the saturated solution 11) for a mixed aqueous solution of hexavalent chromium and trivalent chromium at various strength values, Figure 6 is a graph showing the various molar concentrations shown in Figure 2. It is a graph showing (a) the amount of etching and (b) the corrosion resistance of the treated steel sheet in treatments with various chromate treatment solutions in which a mixed aqueous solution with a ratio (Cr+6/Cr+6) is saturated with barium fluorosilicate.

Claims (1)

[Claims] 1 A reducing agent is added to an aqueous solution of chromic anhydride (Cr03) to reduce a part of chromic anhydride, or a chromic anhydride aqueous solution is further added to convert trivalent chromium and hexavalent chromium into monomers. In a solution with an A/ratio (Cr+1'/Cr") of 0.1 to 0.5,
A fluorine compound consisting of only barium ions as cations and fluorine ions or fluorine ions as anions is added, and the concentration of chromic acid anhydride is 10 to 50K. The concentration of the fluorine compound expressed as elemental fluorine is 0.04
1! 1. A method for surface treatment of a plated steel sheet, comprising subjecting the plated steel sheet to surface treatment with a chromate treatment solution adjusted to 1/1 or higher. 2. The method for chromate treatment of a plated steel sheet according to claim 1, using a chromate treatment solution in which the fluorine compound is BaF2. 6. The method for chromate treatment of a plated steel sheet according to claim 1, which uses a chromate treatment solution in which the fluorine compound is BaSiF6. 4. The method for chromate treatment of a plated steel sheet according to claim 1, using a chromate treatment solution in which the fluorine compound is BaSiF6. 5. The method for chromate treatment of a plated steel sheet according to claim 1, using a chromate treatment solution in which the fluorine compound is B and 1TiF6. 6 Fluorine compounds expressed as fluorine element are 0.041/
5. The method for chromate treatment of a plated steel sheet according to any one of claims 1 to 5, which uses a chromate treatment solution containing 1 or more chromate at a saturation concentration or higher. 7. The method for chromate treatment of a plated steel sheet according to any one of claims 1 to 6, using glycols as a reducing agent. 8. A method for chromate treatment of a plated steel sheet according to any one of claims 1 to 6, in which a monohydric alcohol is used as a reducing agent. 9. The method for chromate treatment of a plated steel sheet according to any one of claims 1 to 6, using a polyhydric alcohol as a reducing agent. 10. The method for chromate treatment of a plated steel sheet according to any one of claims 1 to 6, which uses a silane coupling agent as a reducing agent. 11. The method for chromate treatment of a plated steel sheet according to any one of claims 1 to 6, using hypophosphorous acid as a reducing agent. 12. The method for chromate treatment of a plated steel sheet according to any one of claims 1 to 6, using hydrogen peroxide as a reducing agent.