JPS63143292A - Production of electrolytically chromated steel sheet having excellent corrosion resistance - Google Patents

Abstract

PURPOSE:To efficiently obtain electrolytic chromate having excellent corrosion resistance and adhesiveness by plating a cold rolled steel sheet then subjecting the steel sheet to a cathodic or AC electrolytic treatment in an aq. acidic soln. controlled specifically in Cr<3+> and Cr<5+>. CONSTITUTION:The cold rolled steel sheet is degreased and pickled and is then subjected to Ni-Zn alloy plating and thereafter, the steel sheet is subjected to the cathodic or AC electrolytic treatment in the aq. acidic soln. having 0.1-0.7 ratio of Cr<3+>/(Cr<3+>+Cr6+). The efficient deposition of the chromate film is not possible and the corrosion resistance is not obtainable in terms of quality if the above-mentioned ratio (chromium reduction ratio) is <0.1. The chromate film having the poor corrosion resistance is just obtd. if the above-mentioned ratio is >=0.7. The chromium reduction ratio is adjusted by adding a reducing agent to an aq. chromic anhydride soln. An org. reducing agent is applicable as the reducing agent. An effect of eliminating the need for a costly current rectifier is obtd. in the case of AC electrolysis.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for manufacturing plated steel sheets subjected to electrolytic chromate treatment, which is carried out for the purpose of improving surface properties such as corrosion resistance and paintability of various plated steel sheets.

(Prior Art) Generally, chromate is widely used in zinc, aluminum, and alloy plated steel sheets containing these metals as main components. Chromate treatment includes the coating chromate method, in which a chromate treatment solution is applied to the metal plating surface and then dried or baked; the reactive chromate method, in which the metal plating is immersed or sprayed in a chromate treatment solution, then washed and dried; and It can be broadly divided into electrolytic chromate methods, in which plated steel sheets are immersed and electrolyzed. Each method has advantages and disadvantages, and is selected and used depending on the desired quality, processing equipment, and plated metal.

The present invention mainly deals with electrolytic chromates that are treated with cathode or alternating current, and the known techniques related to the present invention will be described.

Conventional cathodic electrolytic chromate is basically prepared using the famous Sargent bath (Cr03/H2SO4=250/2.5 as described in Metal Surface Technology Handbook 347-359).
y/z). Fluorine ion baths, which are diluted Sargent baths, are in practical use.

Most of the chromium plating is applied to chromate plating, such as fluorine silicon ion bath and 5RH8 bath. In both cases, an insoluble chromate compound mainly composed of metallic chromium and trivalent chromium (hereinafter referred to as "Or") is formed by reducing hexavalent chromium (hereinafter referred to as "Or") on the cathode surface. It is explained that the reduction reaction of chromium proceeds through the addition of a catalyst such as sulfuric acid formed on the surface of the cathode.

(Problem to be solved by the invention) In the conventional method, the chromate film formed is metallic chromium (
Since it is mainly composed of C'0) and trivalent chromium (Or')1, the corrosion resistance that depends on Or' is non-luminous. O
r' There are almost no electrolytic chromates found in living organisms, much less O
There is no known technology for electrolytic chromate that controls r"10r6+.Also, since the chromate treatment solution contains sulfate ions, fluorine ions, silicon-fluoride ions, etc. and is a strongly acidic solution with TiH of 2 or less, it cannot be used with plating metals. Because the chemical reaction occurs at the same time as or before or after the electrolytic reaction, it is important to finish the outer surface uniformly.Furthermore, the surface condition of the plated metal beak is significantly affected by alloy components, etc. Several processing baths and processing conditions must be provided.

There are no examples of practical use of lightning and dechromating using alternating current, and there are only examples of treatment with alkaline chromate and phosphoric acid.

The present invention solves the poor corrosion resistance caused by the conventional electrolytic chromate coating being a biological chromate coating.
! ! ! The present invention provides a method for efficiently obtaining electrolytic chromate with excellent adhesion properties.

(Means for Solving the Problems) The present invention provides a method for plating cold-rolled steel sheets and then plating them.
This is a method for producing an electrolytic chromate-treated plated steel sheet with excellent corrosion resistance, which is characterized by carrying out cathodic or AC electrolytic treatment in an acidic aqueous solution with a ratio of (Or" + Or") of 0.1 to 0.7. Totally 0 to 0 in acidic aqueous solution
．． It is preferable that the acidic aqueous solution contains an inorganic anion of 01-0.1 and one or more of divalent or higher metal salts, inorganic polymer compounds, bound polymer compounds, and chelate compounds.

(Production) The manufacturing method of the present invention will be described below.

The acidic aqueous solution used in the present invention is Cr3. Ors, and inorganic anions (indicated as [:Arn-)) and additives are added as necessary.

Cr3+ and Or'+ beam OA reduction ratio (Or3"/
(calculated as OIJ”+0.6+) is 0.1
~0.7.

Me of total Om (Or"-1-Or") is preferably 5 to 100/l, and optimally 10 to 309/l. The chromium reduction ratio is the most important factor in the present invention. If it is less than 0.1, a chromate film cannot be deposited efficiently as described below, and corrosion resistance cannot be obtained in terms of quality. Moreover, if it is 0.7 or more, only a chromate film with poor corrosion resistance can be obtained. When the chromium reduction ratio is 0.2 to 0.4, a chromate film of excellent quality can be obtained most efficiently. Comparison of chromium reduction A reducing agent is added to a chromic acid anhydride water bath solution. As the reducing agent, organic reducing agents can be used, and organic reducing agents such as slags, alcohols, phenols, oxycarboxylic acids, calgenic acids, etc., which do not leave behind residues, are preferable. Further, an inorganic compound of 0r-1+ may be added within the allowable range of the inorganic anion/total chromium ratio.

The acidic aqueous solution used in the present invention includes a bath containing inorganic anions.

Inorganic anion is 80.2'', Or r No3
- + F-, fluorine complex ion, phosphoric acid, sulfonic acid,
It is boric acid. The amount of inorganic anions is preferably defined so that the inorganic anion/total chromium ratio is 0.01 to 0.1. These inorganic anions are added to improve the adhesion of the chromate film due to the electroconductivity of the acidic water bath solution and the reducing action that occurs at the interface of the plated metal. Its effect is 0
．． A value of 0.01 or higher is desirable, and if it exceeds 0.1, 8-conductor defects are likely to occur due to chemical reactions other than electrolysis.

Additional subcomponents may be added to the acidic aqueous solution used in the present invention for quality reasons. Sub-components include metal salts with a valence of 02 or higher, and (2) inorganic polymer compounds.

■Organic polymer compounds, ■chelate compounds.

These compounds precipitate together with the chromate film and improve the corrosion resistance, paint adhesion, and post-painting corrosion resistance of the film.

Each compound will be described below.

Metal salts with a valence of 2 or more are Mg2'', Oa2+, Zr
t2”, di”.

Salts of Fe", Co", Ni", Sn2+ and Mo042-, WO4'-Mn04
These metal ions precipitate as chromate, east chromate, or oxide during chromate electrodeposition, fix hexavalent chromium, and improve corrosion resistance.The amount added is Me (0rzOy). C-Kyu Me”*
Mar'' expressed as (A''-)(n7m)' is added as an upper limit value.

The inorganic polymer compound also includes sols having a particle size of 100 mμ or less, such as partially cationized sols with silica sol, zirconia sol, titanium oxide sol, and alumina sol. These sols eutectoid or surface concentrate on the chromate film, improving corrosion resistance and paint adhesion. The amount added is preferably 0.1 to 1.0 based on the total chromium.

The organic polymer compound is a water-soluble or water-dispersible polymer compound containing a carboxyl group, a hydroxyl group, an amide group, an amine group, or a sulfonic acid group as a hydrophilic group.

These polymers form a chelate bond with Or'', giving the film its own corrosion resistance and fine fF workability, and also have the effect of fixing 0-+.The amount added is 0.1 to 1.0 with respect to the total chromium. is desirable.

The chelate compounds are Amii7, pyrazoles, phenols, condensed phosphates, triazoles, and carboxylic acids. These chelate compounds chelate with Or”+ in the film to improve the quality of the film, and also improve the quality of the film.
'Beneficial for stabilization.

The pHI of the acidic aqueous solution is preferably in the range of 1 to 5.

If the pH is less than 1, a chemical reaction occurs due to dissolution of the plating, and high quality chromate cannot be obtained. Moreover, at pH exceeding 5, the chromate precipitation efficiency deteriorates significantly. Acidic water bath temperature is 20
The treatment can be carried out at a temperature of ~60°C, which is the temperature at which ordinary electrolytic treatment is carried out.

As the electrolysis method, a method in which electricity is applied using a plated steel plate as a cathode, and a method in which alternating current electrolysis is performed can be adopted.

The present invention is based on a method of electrolyzing in an acidic aqueous solution, washing with water, and drying, but a method of squeezing with waterless washing can also be adopted as necessary.

The film formation according to the method of the present invention is similar to that of conventional electrolytic chromate, which is based on the catalytic action of inorganic anions.
In contrast to forming a film by electrolytic reduction represented by r −+Cr, this method is characterized in that it mainly precipitates components in the chromate bath on the cathode or electrode surface.

Therefore, in contrast to conventional coatings in which the deposition efficiency differs depending on Cr or Cr0 and the underlying metal, the coating of the present invention is composed of Cr, Cr and additive components controlled by the bath composition, and is suitable for all metals. It is characterized by efficient precipitation.

Figure 1 shows chromic anhydride (total chromium concentration 15 P/l) with a chromium reduction ratio of 0.2 and sulfuric acid 1.2? /l (inorganic anion/total chromium ratio O, OS) acidic aqueous solution (pH=
2) Inside electrogalvanized steel sheet (EG), 12% Ni-Z
Current density 3 A/ using n-alloy plated steel (EA) as the cathode
Amount of current (coulomb/d-) and amount of chromate film (total chromium adhesion i1: T, Cr/) when electrolyzed with dy♂
-) is the example shown. For comparison, the result of electrolysis of EA in a 5-fold dilution of a known Sargent bath is shown as P. EA and EC treated by the treatment method of the present invention exhibit almost the same behavior, and a high adhesion amount can be obtained with a comparatively small amount of current applied to known P. The coating is extremely uniform and has excellent adhesion and corrosion resistance. Particularly regarding the adhesion, no peeling occurred during T-bending. This is a dramatic improvement compared to films obtained from conventional chromates.

Figure 2 shows Cr/(Cr + Cr ・) = 0.
In an aqueous solution, 12% Ni-
This figure shows the relationship between the current density and the amount of chromium deposited when a Zn alloy plated steel plate (EA) was treated at the current density shown in the figure at a current flow rate of 20 coulombs/8R. In this method, a low current density is preferable, and the shaded area where the current density exceeds 10 A/d- has an uneven appearance and becomes difficult to form chromate suddenly.

Figure 3 shows a 12% Ni-Zn alloy plated steel plate subjected to alternating current electrolysis in an acidic aqueous solution with a chromium reduction ratio of 0.4, a total chromium concentration of 2 s t/l, and a colloidal silica of 50 s/l, p) (= 1.8). (symbol A) and cathodic electrolysis (symbol D) and the relationship between the amount of current applied and the amount of adhesion. A, D
Both methods deposit a chromate film with good current efficiency, but the cathode method deposits a chromate film with higher efficiency. The coatings in both A and D had a brown uniform appearance, and compared to known methods, coatings with a high Cr content were uniformly obtained.

(Example) Examples of the present invention will be shown below.

Symbols and evaluation methods in Examples are shown below.

The total amount of chromium deposited was dissolved in 5% HCt and analyzed using the Atsuko absorption 9 degree method.■/? It was expressed as F/.

Corrosion resistance of unpainted products was measured by a salt spray test, and the evaluation was expressed as the time until white rust appeared.

The adhesion of the painted plate was indicated by T-bending taping, and good results were indicated by "◎" and peeling was indicated by "x".

The secondary adhesion after painting was determined by making artificial scratches on the coated plate, and after 168 hours of a salt spray test, the coating was peeled off from the artificial scratches by taping (NR).

The paint was a commercially available melamine alkyd resin paint with a target of 25μ and evaluated.

Example 1 A cold-rolled steel sheet was degreased and pickled by a known method, and Ni-Zn alloy plating containing 12% Ni was plated on both sides at 20 f/l.
,5A/aW? A cathodic electrolytic treatment was carried out at a current flow rate of 20 coulombs/drrt, followed by immediately washing with water to produce an electrolytic chromate-treated plated steel sheet.

For flats 1 to 4, Cr / (Cr + Cr) is 0.1
This is an example produced by the method of the present invention with a .about.0.70.

°3+ Cr / (Cr + Cr) up to 0.30 is Cr
The amount of adhesion increases in proportion to the amount of 3+ Cr, and the corrosion resistance is good.

When 3+Cr/(Cr+Cr) is 0.70, corrosion resistance decreases. 46,7 is Cr/(Cr-1-C
r) is constant and [Am--n total Crt-0.016 and 0.08 were obtained from acidic aqueous solutions. Also, A8
This is an example of increasing the pH of the acidic aqueous solution of Haya 2 with aqueous ammonia. Sample No. 9 is a comparative example using an acidic aqueous solution in which Cr/(Cr + Cr) is smaller than the range required by the present invention, and the amount of Cr deposited is small and the corrosion resistance is not good.

Example 2 An electrogalvanized steel sheet (symbol EG) was prepared by degreasing a cold rolled steel sheet using a known method, pickling, and electrolytic galvanizing to 3 μm.

3μ 12% NL-Zn alloy coated steel sheet (EA) and 10μ hot dip galvanized steel sheet (EA)
GI) was subjected to cathodic electrolysis treatment using an acidic aqueous solution reduced with glycerin shown in Table 2. After washing with water, it was dried to produce a chromate-treated plated steel plate.

In Examples 10 to 13, barriers were created by changing the amount of current applied to a Ni-Zn alloy plated steel plate while keeping the current density constant. The finished appearance was uniform, the amount of Cr deposited increased in proportion to the amount of current applied, and the corrosion resistance and adhesion were good. Examples 14-17
This is an example in which electrolytic galvanized steel sheets were treated, and good corrosion resistance was obtained for an electrolytic chromate. Examples 18 to 20 are examples in which Ni--Zn alloy plated steel sheets were electrolyzed at different current densities. All had a uniform romate appearance and good corrosion resistance and paint adhesion. A21 is an example treated with hot-dip galvanizing. A22-27 are comparative examples, (A
”-)/total Cr ratio of 0.02 and 0.08+7:7.The amount of Cr deposited is small and uneven.In terms of quality, the paintability (adhesion, corrosion resistance) is good, but there is no The corrosion resistance of painted plates is poor.

Example 3 After electroplating N i -Z n alloy plating containing 12% Ni, cathodic electrolytic chromate treatment was performed using an aqueous solution shown in Table 3, which is obtained by adding sulfuric acid and additional ions to a chromic acid aqueous solution partially reduced with glucose. and tested it. As added ions, PO45-CL −+ F″′lS i F
6 was counted as an inorganic anion in CAm-)/total Cr as described above.

A28 is an example in which phosphoric acid was added, and the amount of Cr deposited is somewhat low, but the corrosion resistance is good. Boiled 29 is an example of adding boric acid,
430 is an example in which a water-soluble polymer compound is added, and corrosion resistance is improved in both cases. A31 to A35 are examples of addition of silica, alumina sol, and metal ions, which improve corrosion resistance. The adhesion of samples 33 and 35 to which metal ions were added was slightly lower.

Shallow 36 is an example using -qCt- as an inorganic anion, and a high Cr deposition amount with good quality was obtained. Also, ya 37
．． No. 38 is an example containing fluorine ions as fluorine and fluorine complex salts, and an alloy-plated steel plate with a romate film having a uniform appearance and good corrosion resistance was obtained.

Example 4 12%Ni-Zn alloy plated steel plate with Cr/(Cr +
Partially reduced chromic anhydride 25y with Cr3”=0.4
/l and an aqueous solution of silica sol 50 f/A (pH=2
) cathodic electrolysis and alternating current electrolysis were performed. Current density is 6A
/dm' and the amount of current is 20.40.60 coron/dm'
It was electrolyzed with The amount of deposited chromium is shown in Figure 3. A is alternating current and D is direct current. Both chromate-treated coatings had good adhesion, and the corrosion resistance of uncoated plates in salt spray tests was T, Cr 1501 nq/m' (D), T
, Cr 200 'F/m' (A) treated material was tested for 172 hours, but no white rust was observed.

Example 5 Electrogalvanized steel plate was coated with Cr/(Cr”-)-C
r'') = 0.3 partially reduced chromic anhydride 25 f/
After carrying out cathodic electrolysis treatment (current density 5 A/dyr/, 40 coulombs/d-) in an aqueous solution (p) I=1.8>, it was washed with water and dried with hot air. The amount of chromium attached is 2
00%'1? /No peeling was observed in 11 bending processes. Further, no white rust was observed after 72 hours of salt water spraying.

(Effects of the Invention) As described above, the present invention can form a high-performance chromate film, especially a coating with excellent corrosion resistance, on a single galvanized or zinc alloy plate. For the chromate coating, there has been a desire for a technique to quantitatively and uniformly form a coating with a well-balanced combination of corrosion resistance imparted by Cr and hardly soluble by Cr. To date, many coated chromates, electrolytic chromates, and reactive chromates have been developed, but each has advantages and disadvantages, and it is necessary to develop a perfect one in the future. The present invention can easily produce a chromate film with a much improved uniformity compared to known chromates using current processes.
This is a technology that is one step closer to complete ROMET.

Also, due to the structure of the uninvented bath, I! It is possible to double-coat the solution and the coating, which is advantageous in terms of 6D quality and process.

Furthermore, the present invention is a chromate treatment method that can be sufficiently applied to other metals such as aluminum, iron, copper, etc., since chromate precipitation does not depend on the type of metal surface.

Furthermore, the present invention allows processing by direct current electrolysis or current electrolysis; in the case of alternating current electrolysis, an expensive rectifier is not required.

[Brief explanation of the drawing]

Figure 1 shows electrolytic zinc plating (EG) with chromic acid aqueous solution of Cr /Cr +Cr) = 0.2, 12%Ni.
- Relationship between the amount of current applied and the amount of chromate deposited in terms of T and Cr when a Zn alloy plated (EA) steel plate is subjected to cathode electrolysis, and the well-known Cr03-H2SO4 bath CP) as a comparative example.
Figure 2 is a diagram showing the relationship between current density and chromate deposition amount when treated with an acidic aqueous solution that meets the requirements of the present invention, and Figure 3 is a diagram showing the relationship between the current density and the amount of chromate deposited when Cr / (Cr + Cr) = 0.4. It is a figure which shows the example which chromate-treated the 12% Ni-Zn alloy plating steel plate by the alternating current electrolysis method in the chromate solution comprised from chromic acid and silica sol. Agent Patent Attorney Masamitsu Aki Sawa and 1 other person 7t1 Figure J Electricity (Coulombs/dnz)

Claims (3)

[Claims] (1) After plating a cold rolled steel plate, Cr^3^^+
/(Cr^3^^+++Cr^6^^+) ratio is 0.1~
A method for producing an electrolytic chromate-treated plated steel sheet with excellent corrosion resistance, which comprises cathodic or alternating current electrolysis treatment in an acidic aqueous solution of less than 0.7. (2) 0.01 to 0.0% of total chromium in acidic aqueous solution.
2. The method of claim 1, comprising one inorganic anion. (3) The method according to claim 1, wherein the acidic aqueous solution contains one or more of divalent or higher metal salts, inorganic polymer compounds, organic polymer compounds, and chelate compounds.