JPH0754193A - Production of high corrosion resistant electrogalvanized steel sheet excellent in chemical convertibility - Google Patents

Abstract

PURPOSE:To provide a producing method for improving the chemical convertibility of a high Cr-contg. electrogalvanized steel sheet without affecting any influence on its excellent corrosion resistance in the method for producing a high Cr-contg. electrogalvanized steel sheet. CONSTITUTION:An electrogalvanized steel sheet contg., by weight, 5-30% Cr and 0.1-10% iron group metal is brought into contact with a mixed aq. soln. of nitric acid and nitrous acid, by which a dense and uniform chemically converted film can be obtd. The influence on its corrosion resistance is eliminated, and its performance after coating, particularly, its coating film adhesion can be improved. As the iron group metal, Ni is most preferably used.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a galvanized steel sheet with high corrosion resistance, which has excellent chemical conversion treatability and is used in automobiles, home appliances, building materials and the like.

[0002]

2. Description of the Related Art It is well known that electrogalvanized steel sheets are widely used as surface-treated steel sheets which improve the corrosion resistance of cold-rolled steel sheets and the corrosion resistance after coating and can be mass-produced without impairing workability. In recent years, the use of galvanized steel sheets has been attempted as an anticorrosion measure for automobiles against salt spray for preventing road freezing in winter in cold regions, and high corrosion resistance in severe corrosive environments is required.

In order to meet the demand for improving the corrosion resistance of galvanized steel sheets, there is a means of increasing the zinc coating amount (adhesion amount), but this has many problems in terms of weldability and workability. Therefore, many alloy platings have been proposed as a method of suppressing the dissolution of zinc itself and extending the life of zinc plating. Among them, Zn-based alloy plating containing an iron group metal such as Fe, Co, or Ni as an alloy component has been recognized for its good bare corrosion resistance and post-coating corrosion resistance, and has been put to practical use. Further, a technique for precipitating a large amount of Cr, which has been difficult to co-deposit with Zn in the past, to improve the corrosion resistance is also disclosed in JP-A-1-1971.
It is disclosed in Japanese Patent Application Laid-Open No. 97 and Japanese Patent Application Laid-Open No. 1-290798.

[0004]

Zn containing a large amount of Cr
It is noted that the system-based plating has much more excellent corrosion resistance than Zn-based alloy plating that has been put into practical use such as Zn-Ni and Zn-Fe, and is remarkable, but it is often used in automobiles, home appliances, etc. However, there is a problem that the chemical conversion processability (phosphate processability), which is regarded as important as a pretreatment for coating, is poor. As a method of improving the chemical conversion treatability of the Cr-containing Zn-based plating, there is a method of forming the plating layer into two layers and providing a plating layer having good chemical conversion treatability such as Zn or Zn-Ni, Zn-Fe on the upper layer. It is also described in the above publication.

However, the two-layer plating is not only disadvantageous in terms of cost, but also the existence of the upper layer plating may impair the excellent corrosion resistance originally possessed by the Cr-containing Zn-based plating. In view of the above problems, the present invention, in the production of a Cr-containing Zn-plated steel sheet, by modifying the surface layer without resorting to the means of applying an upper layer plating, does not affect the excellent corrosion resistance at all, and performs the chemical conversion treatment. It is intended to provide a method for improving the sex.

[0006]

The gist of the present invention is (1)
High corrosion resistance electrogalvanizing excellent in chemical conversion treatment, characterized in that an electrogalvanized steel sheet containing 5 to 30% by weight of Cr and 0.1 to 10% by weight of iron group metal is brought into contact with a mixed aqueous solution of nitric acid and nitrous acid. A method for producing a steel sheet, and (2) a method for producing a high corrosion-resistant galvanized steel sheet having excellent chemical conversion treatability, wherein the iron group metal is Ni (1).

[0007]

[Function] The formation process of the chemical conversion coating (phosphate coating) is
Zn ions are eluted from the plating layer in the weakly acidic chemical conversion treatment solution, and as a result, hydrogen ions are consumed as a reaction, resulting in an increase in the pH of the interface and reaction products of Zn ions and free phosphoric acid in the treatment solution. Some zinc phosphate is deposited on the plating as a film. The finer and denser the chemical conversion treatment film is, the better the performance after coating is. Therefore, a surface conditioning step is usually provided before the chemical conversion treatment to adsorb Ti colloid on the plating surface. As a result, a micro battery is formed on the plating surface in the chemical conversion treatment solution, Zn is uniformly eluted, and the Ti colloid becomes a nucleus for zinc phosphate precipitation, so that a fine and dense chemical conversion treatment film is obtained.

In conventional Zn-Ni, Zn-Fe alloy plating, a chemical conversion coating is formed in the same process,
Ni and Fe ions, which are dissolved in a trace amount together with Zn, are taken into the phosphate film and rather improve the quality of the phosphate film. However, in Zn-based plating containing Cr, Z
Since n is difficult to elute, a sufficient amount of chemical conversion coating cannot be formed within a practical time. Further, even if Zn is easily eluted by elongating the treatment time or increasing the liquid temperature, it is difficult to elute Zn uniformly, so that the crystal becomes coarse and the scale not covered with the crystal occurs.

The present inventors presume that in order to improve the chemical conversion treatability of the Zn-based plating containing Cr, it is necessary to promote the elution of Zn in the chemical conversion treatment solution and to carry out the process uniformly. As a result of various studies, it was found that it is extremely effective to use a Zn-plated steel sheet containing Cr and an iron group metal as a raw material and to bring this into contact with a mixed aqueous solution of nitric acid and nitrous acid. The mechanism of improving the chemical conversion treatability by this method is presumed as follows. The plating surface is oxidized in a very short time by coming into contact with a mixed aqueous solution of nitric acid and nitrous acid, and is uniformly coated with an oxide of Zn and Cr.

As a result, the affinity with the Ti colloid is improved in the surface adjustment performed before the chemical conversion treatment, and the uniform adsorption of the Ti colloid is promoted, so that Zn is eluted quickly and uniformly in the chemical conversion treatment solution. , A fine and fine chemical conversion coating is formed. The effect of such treatment on corrosion resistance is to oxidize only the surface layer, so
It is a level that can be ignored.

The plated steel sheet used in the present invention is a Zn-plated steel sheet containing Cr and an iron group metal. Cr is 5 to 30% by weight from the viewpoint of corrosion resistance and workability, and the iron group metal is chemical conversion treatability. From the viewpoint of corrosion resistance, the range of 0.1 to 10% by weight is preferable. Even if Cr is less than 5%, the corrosion resistance is improved as compared with zinc plating, but if it is 5% or more, not only zinc plating but also high corrosion resistance exceeding conventional alloy plating such as Zn-Ni and Zn-Fe is exhibited. Red rust does not easily occur even if the salt spray test is performed for 500 hours or more. If it exceeds 30%, the workability is deteriorated, which is not preferable.

The iron group metals are Fe, Co and Ni, and one or more of them are contained. Among them, Ni is the most preferable in terms of chemical conversion treatability and corrosion resistance. If the iron group metal is less than 0.1%, the effect of improving the chemical conversion treatability is not remarkable, and if it exceeds 10%, the properties of the iron group metal become strong and the corrosion resistance may be deteriorated. Considering the workability as well, the total amount of Cr and the iron group metal is more preferably 30% or less. Further, a small amount of an organic additive for Cr precipitation described later may be contained.

Next, the treatment with a mixed aqueous solution of nitric acid and nitrous acid is carried out.
A processing condition of about 05 to 0.2 μ is preferable. Specifically, the total concentration of nitric acid and nitrous acid is 0.1 to 10%,
It is preferable that the ratio of nitrous acid to the total concentration is 0.1 or more, the liquid temperature is from room temperature to 60 ° C., and the treatment time is 0.1 to 10 seconds. Although a single aqueous solution of nitric acid or nitrous acid is effective, it is more effective if treated with an aqueous solution in which these are mixed, and a good chemical conversion film is easily obtained even if the chemical conversion treatment time is shortened.

As a treatment method, the above-mentioned plated steel sheet may be brought into contact with a mixed aqueous solution of nitric acid and nitrous acid.
Any of the spraying processes such as spraying may be used. Immediately after the treatment, washing with water is performed. As the step of performing the treatment, after washing with water after forming the plating layer or washing with water and drying,
Or during degreasing and surface preparation before chemical conversion treatment,
In any case, the same effect is exhibited. Thus, C
Even in a Zn-based plated steel sheet containing a large amount of r, a dense and uniform chemical conversion treatment film can be formed, which is comparable to conventional zinc plating and Zn-based alloy plating such as Zn-Ni and Zn-Fe. As a result, it is possible to remarkably improve the post-painting performance, particularly the coating film adhesion.

The method for producing a Zn-plated steel sheet containing Cr and an iron group metal, which is the object of the present invention, is as follows. The plating bath contains Zn ²⁺ ions, Cr ³⁺ ions, iron group metal divalent ions, and Cr precipitation additive as essential components. In addition to these, Na, K, NH to increase the conductivity
Salts such as ₄ may be included. The type of plating bath is not particularly limited, but a sulfate bath, a chloride bath, or a mixed bath thereof is preferable. The total metal ion concentration of the plating component is preferably 30 to 200 g / l, and if it is less than 30 g / l, the supply of ions is likely to be insufficient at a high current density, and as a result, plating burn is likely to occur. When it exceeds 200 g / l, the waste of ions due to dragout cannot be ignored, and there is a concern of crystallization when the liquid temperature becomes low.

As the Cr deposition additive, a cationic polymer such as polyamine or polyamine sulfone or a polyoxyalkylene derivative such as polyethylene glycol is effective, and the addition amount is 0.01 to 20 g / l.
Is appropriate. Other plating conditions are not particularly limited, but pH 0.5 to 3, bath temperature 40 to
70 ° C., liquid flow rate 1 m / min or more, current density 20 A / d
m ² or more is a preferable range, and the optimum conditions can be adopted in consideration of the intended plating composition and current efficiency. There is essentially no restriction on the coating weight from the viewpoint of chemical conversion treatment, but from the viewpoint of corrosion resistance and economic efficiency
A practical range is 60 g / m ² .

[0017]

EXAMPLE A cold rolled steel sheet was degreased with alkali, pickled with a 5% aqueous solution of sulfuric acid, electrogalvanized containing various compositions of Cr and an iron group metal, and immersed in a mixed aqueous solution of nitric acid and nitrous acid. Spray processing was performed. About 1 part, the said process was performed between degreasing and surface adjustment when evaluating chemical conversion treatability, and compared with the case where it performed after plating. The plating amount was 20 g / m ² . The corrosion resistance, chemical conversion treatability and coating adhesion of the thus obtained plated steel sheet were evaluated. The results are shown in Table 1 together with the plating composition and the treatment conditions with the nitric acid aqueous solution. The corrosion resistance, chemical conversion treatability and coating adhesion were evaluated as follows.

[0018]

[Table 1]

(1) Corrosion resistance A salt spray test (according to JIS Z2371) was carried out for 500 hours to evaluate the red rust generation area. 4: Red rust 1% or less, 3: Red rust 10% or less, 2: Red rust 50
% Or less, 1: Red rust over 50%

(2) Chemical conversion treatment The immersion type phosphate treatment was carried out for 30 seconds, which is shorter than the standard conditions, and 2 minutes, which is the standard conditions, to evaluate the uniformity and crystal size of the phosphate film. 4: Uniform in processing time of 30 seconds and 2 minutes, average size 10μ
Below, 3: Processing level 2 minutes, 4 levels, 30 seconds non-uniform, average size over 10μ 2: Even processing time 2 minutes non-uniform, average size over 10μ 1: Processing time 2 min scale

(3) Adhesion of coating film An immersion type phosphate treatment was performed, and then cationic electrodeposition coating (20μ), intermediate coating (35μ), and topcoat coating (35μ) were applied, and the mixture was left at room temperature for 24 hours. After 40 ℃
Was immersed in distilled water for 500 hours, and immediately thereafter, a 2 mm cross-cut test was performed to evaluate the degree of coating film peeling. 4: No peeling of coating film 3: Slightly peeling of corners of cross-cut grid 2: Number of cross-cut peeling 1-5 pieces 1: Cross-cut peeling number of 6 or more

In Table 1, Comparative Example 1 is an example containing no iron group metal and not treated with a nitric acid / nitrous acid mixed aqueous solution, and the chemical conversion treatment property and the coating film adhesion are poor. Comparative Example 2 is an example containing an iron group metal but not treated with a nitric acid / nitrous acid mixed aqueous solution, and Comparative Example 3 is an example containing no iron group metal but not treated with a nitric acid / nitrous acid mixed aqueous solution. In each case, the chemical conversion treatability and the coating film adhesion are slightly better than those of Comparative Example 1, but they have not reached a sufficient level.

Further, in Comparative Example 4, since the Cr content is low,
Comparative Example 5 has a high iron group metal content and therefore has poor corrosion resistance. Comparative Example 6 is Zn-Ni alloy plating, and although the chemical conversion treatability and the coating film adhesion are good, the corrosion resistance is lower than that of the present invention. On the other hand, Examples 1 to 16 of the present invention have good corrosion resistance, chemical conversion treatability, and coating adhesion. In Examples 1 to 12 of the present invention, the treatment with the nitric acid / nitrous acid mixed aqueous solution was performed between degreasing and surface preparation before the chemical conversion treatment, but the same results as when the treatment was performed after plating were obtained. .

[0024]

As described above, according to the present invention, the chemical conversion treatability, which was a drawback of the Cr-containing electrogalvanized steel sheet having a high degree of corrosion resistance, can be improved to the level of the conventional plated steel sheet. The method is also practical and simple, and is extremely effective when the Cr-containing electrogalvanized steel sheet is put to practical use in automobiles, home appliances and the like.

Claims (2)

[Claims] 1. A Cr content of 5 to 30% by weight and an iron group metal of 0.1.
A method for producing a highly corrosion-resistant galvanized steel sheet having excellent chemical conversion treatment properties, which comprises contacting an electrogalvanized steel sheet containing 10% by weight with a mixed aqueous solution of nitric acid and nitrous acid. 2. The method for producing a highly corrosion-resistant galvanized steel sheet having excellent chemical conversion treatability according to claim 1, wherein the iron group metal is Ni.