JPH0230771A - Production of surface-treated steel sheet having high corrosion resistance - Google Patents

Abstract

PURPOSE:To form a chromate film having excellent corrosion resistance while preventing the elution of Cr by applying a chromate soln. having a specified composition on the surface of a steel sheet plated with Zn (alloy), then applying a solvent-type org. polymeric resin thereon, and baking the material. CONSTITUTION:The chromate soln. contains 5-100g/l chromic acid, 0.5-20g/l phosphate ion, 0.2-4g/l fluorozirconium ion, and 0.2-7g/l Zn ion, and the weight ratios of the Cr<6+> to Cr<3+> and chromic acid to fluorozirconium ion are controlled respectively to 3/4-3/2 and 10/1-100/1. The chromate soln. is applied on the surface of a steel sheet plated with zinc (alloy), and then dried. A solvent-type org. polymeric resin is applied on the formed chromate film, and then baked. By this method, a chromated steel sheet with reduced elution of Cr even in low-temp. drying and having excellent corrosion resistance is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for manufacturing a highly corrosion-resistant surface-treated steel sheet suitable for automobile bodies, outer panels of home appliances, and the like.

[Conventional technology]

Chromate-treated steel sheets are widely used as chemical conversion treated steel sheets for the purpose of rust prevention of zinc-based plated steel sheets. Generally, chromate treatment methods are broadly classified into three types: electrolytic type, reaction type, and coating type.

Among these, the electrolytic type produces a film mainly composed of Cr. This film has a high degree of perfection, is hardly soluble in water, and has an excellent anchoring effect as a paint base, but Cr
It has the disadvantage of poor corrosion resistance because of its low corrosion resistance.

In addition, in the reactive type, a chromate film is reduced and precipitated on the plating surface through the dissolution of the base metal by acid and the chemical reaction with Cr6+ ions, so, like the electrolytic type, only a film consisting of Cr6 ÷ main body is obtained, and the amount of Cr deposited is large. Although it is easy to do so, we cannot expect much improvement in corrosion resistance.

On the other hand, in the coating type, a treatment solution consisting of a basic bath mainly composed of chromic acid anhydride and inorganic additives such as silica is applied to the surface of the plated steel sheet and dried.
Since it contains a relatively large amount of r6+, it has the best corrosion resistance among the three, and exhibits high corrosion resistance depending on the amount of cr attached. However, because Cr8+ is soluble in water, Cr8+ is
is eluted, and there is a limit to the effective amount of Cr deposited.

Therefore, as a conventional method for improving the Cr elution property of coated chromate, a method was used in which chromate treatment solution was applied, dried, and then washed with water (including hot water washing) to elute soluble Cr in advance (Japanese Patent Application Laid-open No. 62 -202083 and Japanese Patent Application Laid-Open No. 62-202084).

It is also known that the Cr elution property of a general coating type chromate solution as shown in Japanese Patent Publication No. 45-38891 can be improved by drying it at high temperature or by increasing the drying time (CAMP -I8IJ Vol (1
98g) 680).

[Problem to be solved by the invention]

However, if the chromate treatment solution is applied and dried and then washed with water, Cr6+, which has a self-repairing effect, is lost, resulting in a significant deterioration in corrosion resistance.

In contrast to this water washing method, C in the chromate bath
A method of reducing the ratio of r8+ can be considered.

To reduce the proportion of Cr6+ in the chromate bath, organic or inorganic reducing agents such as sugars and alcohols are generally used, but if the weight ratio of Cr"/Cr3+ is less than so/so, the chromate solution will turn into a gel within a short period of time.

Cr/Cr that allows stable use of chromate solution
The lower limit of the ratio has conventionally been set at about 60/40, but even if a chromate bath reduced to this degree is used, the degree of completion of the chromate film cannot be improved, and no improvement in Cr elution can be expected.

Furthermore, even when the film is dried at high temperature, although the Cr elution property is improved to some extent, it still remains water-soluble Cr6.
At the same time, cracks are generated in the chromate film, resulting in a problem of deterioration of corrosion resistance. Furthermore, if an attempt is made to improve Cr elution through high temperature drying, high temperature drying exceeding 200° C. is required, which poses a problem in terms of manufacturing costs.

The present invention was made in view of such conventional problems,
It is an object of the present invention to provide a method for manufacturing a highly corrosion-resistant surface-treated steel sheet, which produces a romate film with good corrosion resistance and less Cr leakage even when drying at low temperatures, and which has excellent overall corrosion resistance.

[Means to solve the problem]

The present inventors conducted studies on chromate coatings and resin composition coatings in terms of Cr elution properties, corrosion resistance, etc., and came to the following conclusion.

(t) When lowering the Cr6-Cr3+ ratio in the chromate bath, phosphate ions are effective in preventing gelation of cr.

Additionally, if zirconium fluoride ions are present in the chromate bath, they will form a complex compound with Cr8+,
It has the effect of suppressing the elution of 6+. Further, Zn ions in the bath convert chromate ions into zinc chromate and improve the elution of %CrCr.

Therefore, by appropriately adjusting these components, Cr elution can be effectively suppressed.

(11) Corrosion resistance and adhesion in a humid environment are better for solvent-based resins than for water-soluble or water-dispersible resins. Furthermore, water-based resins cannot avoid elution of Cr6+ from the chromate film during the coating process, and the eluted Cra+ ions cause the water-based resin to gel, resulting in poor workability. Therefore, in this sense as well, solvent-type resins are preferable.

The present invention has been made based on these points, and on the surface of a zinc-plated or zinc alloy-plated steel sheet, chromic acid = 5 to 1 oor/z phosphate ion 20.5 to 201/l zirconium fluoride ion: 02 ~4g/l Zn ion: 0.2~rt/l, and the weight ratio of the following components in the bath is Cr''/Cr
” = 3/4 to 3/2 chromate solution adjusted to chromic acid/zirconium fluoride ion = 10/1 to 100/1 is applied and dried to perform chromate treatment, and then the chromate film is removed without washing with water. A solvent-type organic polymer resin is applied to the upper part and then baked.

The present invention will be explained in detail below.

The plated steel plates used in the present invention include Zn plated steel plates, Zn-Fe alloy plated steel plates, Zn-Ni alloy plated steel plates, Zn-Mn alloy plated steel plates, and Zn-α
Nl%F is added to the plating components of alloy-plated steel sheets, Zn-Co-Cr alloy-plated steel sheets, and any of these steel sheets.
es Mns Mo s Co .

Examples include those to which one or more elements such as At and Cr are added. Alternatively, it may be a composite plated steel sheet having two or more layers of the same or different types of plating as described above. As a plating method for these plated steel sheets, any practicable method among electrolytic method, melting method, vapor phase method, etc. can be adopted.

However, among these methods, the 'dL method is advantageous as a plating method because it does not select the material of the underlying cold-rolled steel sheet.

The above-mentioned zinc-based plated steel sheet is first subjected to chromate treatment using a chromate solution. The chromate solution is: Chromic acid: 5~1oor/z Phosphate ion 20,5~zot/l Zirconium fluoride ion = 0.2~4 y/lZn
ions: o, z~7t/l, and the weight ratio of the following components in the bath is Cr"/C%"
= 374 to 3/2 chromate/zirconium fluoride ion = 10/1 to 100/1, and the chromate solution is applied to a plated steel plate and dried.

Here, when the concentration of the chromic acid is less than 5y/,
The amount of chromate film formed on the surface of the treated object is small, resulting in poor corrosion resistance.

On the other hand, if the chromic acid content exceeds 1 oor/l, the amount of chromate film deposited becomes too large, significantly deteriorating weldability.

The above-mentioned phosphate ion is effective in preventing gelation of Cr3+, and if its concentration is less than o, sy/, gelation of CrS+ cannot be prevented.
At this ratio, the chromate bath becomes unstable and precipitates. On the other hand, when it exceeds zoy/l, the pH of the bath decreases,
The decomposition of the zinc plating, etc. to be treated is promoted, and the corrosion resistance is deteriorated.

The above-mentioned zirconium fluoride ion forms a complex compound with Cr6+ and has the effect of suppressing the elution of Cr6+, but if the concentration is less than o, 2y/, the effect will not be sufficiently obtained and the corrosion resistance will be poor.

On the other hand, if the concentration exceeds 42/l, the surface of the object to be treated, such as zinc plating, will be excessively etched, and as a result, the Zn concentration in the chromate solution will increase, promoting gelation of the solution.

The above-mentioned Zn ion has the effect of improving the Cr elution property by using zinc chromate as the chromate ion, and if the concentration is less than o,2y/, the effect of improving the Cr elution property cannot be achieved. On the other hand, if the concentration exceeds 72/l, the chromate solution tends to gel, which is not preferable.

Furthermore, if the weight ratio of Cr"/Cr"+ is less than 3/4, the chromate solution becomes unstable, the repair effect of Cr is not sufficient, and the corrosion resistance is poor. On the other hand, if the weight ratio exceeds 3/2, a film containing a large amount of chromium is formed, and the adhesion of the paint tends to deteriorate.

Chromate/zirconium fluoride ion weight ratio is 10
/1, Cr6 due to zirconium fluoride ions
Because the complex compound formation reaction with + proceeds excessively, Cr6
+ inhibits the self-repairing effect and deteriorates corrosion resistance. On the other hand, if the ratio exceeds 100/1, the improvement in Cr elution by zirconium fluoride ions will not be sufficient, and the desired corrosion resistance will not be obtained.

Chromic acid in the chromate solution can be obtained by adding chromic anhydride, and Cr6-Cr3+ can be adjusted by adding oxalic acid,
This is carried out by reducing Cr6+ to Cr3+ in a bath using a reducing agent such as tannic acid, starch, alcohol, or hydrazine. Further, phosphate ions can be obtained by adding orthophosphoric acid, ammonium phosphate, etc. Zirconium fluoride ions are preferably added in the form of ZrF6, (NH4)2 zrF'6. H2ZrF6
It can be obtained by adding etc.

The amount of chromate film deposited is 10 in terms of metallic chromium.
~2001n9/rn", preferably 30-150 I
It is appropriate to set it to n97m2. The amount of M with chrome is 2
If it exceeds 00 rtv;

The application of the chromate solution in the present invention is carried out using a roll coater.
Any method may be used, such as a method, a dipping method, a spray method, etc.

After the chromate treatment as described above, a solvent-type organic polymer resin is applied to the top of the chromate film without washing with water (including hot water washing), and then baking treatment is performed.

In the present invention, solvent-type organic polymer resins include vinyl-based resins such as vinyl acetate, vinyl chloride, and vinylidene chloride, and copolymers thereof, acrylic acid, methacrylic acid, acrylic esters, methacrylic esters, hydroxyacrylic acid, and hydroxyl. Acrylic type and its copolymer such as acrylic acid ester, alkyd type, epoxy type, urea type, fluorine type, urethane type, ester type, styrene type, olefin type and their copolymer, synthetic rubber type such as butadiene and Examples include resins such as natural polymers. A curing agent such as melamine or an inorganic substance such as silica can be added to this organic polymer resin as necessary.

When a water-based organic polymer resin is applied onto a chromate film, it is inevitable that a small amount of Cr will leak out from the chromate film into the resin liquid, which will inhibit the stabilization of the resin liquid and cause the liquid to gel. This results in the problem of deterioration. For this reason, a solvent type organic polymer resin is used.

The organic polymer resin as described above is applied by a method such as roll squeezing, roll coater-air knife, or the like.

The baking process is performed at 50 to 200°C, preferably 60 to 200°C.
The process is carried out at a temperature of 150°C, and a dry film is obtained by maintaining this temperature for several seconds to several minutes. This baking is usually done by supplying hot air, but is not limited to this. In the present invention, a desired film can be obtained by baking at a relatively low temperature.

Here, if the baking temperature is less than 50°C, a large amount of Cr will be eluted, which is problematic, and a baking temperature of 60°C or higher is preferable from the viewpoint of Cr elution.

On the other hand, if the baking temperature exceeds 200°C, not only will economic efficiency be impaired, but the corrosion resistance will also deteriorate. This is because when baking at a high temperature exceeding 200°C, the moisture contained in the chromate film components evaporates and the dehydration condensation reaction between hydroxyl groups (-cr-OH) progresses rapidly, causing cracks in the chromate film. Destruction of the chromate film progresses, and C
This is presumed to be due to the fact that the reduction of r6+ progresses and the passivation effect of Cr6+ is reduced. Preferably, the baking temperature is 150° C. or lower, which is advantageous in terms of corrosion resistance and economy. In addition, when applying the present invention method to a high tensile strength steel plate (so-called BH steel plate) having bake hardenability,
The following baking temperatures are preferred:

In addition, the resin composition film is applied on the chromate film by 0.2 to 2
．． 5 y/m'', preferably 0.5 to 2.0 f/-.
If it is less than 2y/rn2, sufficient corrosion resistance cannot be obtained; If it exceeds sy/m2, weldability (especially continuous multi-point weldability) will deteriorate, and it will be 0.2 to 2.517.
m'' range is particularly suitable for highly corrosion-resistant surface-treated steel sheets for automobiles.

According to the method of the present invention, the above-described treatments can be applied to both or one side of a plated steel sheet to produce, for example, a surface-treated steel sheet in the following manner.

(1) One side...Plated film - Chromate film - Resin composition film One side... Fe side (2) One side... Plated film - Chromate film - Resin composition film One side... Plated film (3) Both sides ... Plated film - Chromate film - Resin composition film The highly corrosion-resistant surface-treated steel sheet produced by the method of the present invention can be used not only for automobiles but also for home appliances, building materials, etc.

〔Example〕

Various plated steel plates are degreased with alkaline, washed with water and dried to be used as steel plates for the inner surface of automobile bodies.
Phosphate ion concentration, zirconium fluoride ion concentration,
A chromate solution containing various Zn ion-concentrated K, Cr''/Cr3+ weight ratio, and chromic acid/zirconium fluoride ion weight ratio was applied using a roll coater and dried. Next, a solvent-based organic polymer resin is coated with a roll coater and baked, and the resulting copper plate has corrosion resistance,
Tests were conducted for paint adhesion and Criminality. The results are shown in Tables 1-8 to 1-c.

In addition, as one example of softness, an example was given in which chromate treatment was performed by electrolytic chromate treatment, but the electrolytic chromate treatment was performed using Cr0150? /L%H2So, o, sy/
1, the current density is 4.9 A/dm in a bath with a bath temperature of 5oC.
, ', and the '1 electrolytic interval was set according to the target Cr deposition amount.

In the corrosion resistance test, the edges and back surfaces of each specimen were sealed with tape, and then a composite corrosion test was performed in an unpainted state, with the above cycle being one cycle, up to 100 cycles, and the area where red rust occurred was evaluated. Note that a cross cut was made on the entire surface of the sample using a cutter, and a corrosion resistance test was conducted.

In the paint adhesion test, each sample material was treated with phosphate using PB-L3020 manufactured by Nippon Pariki Reizink Co., Ltd., and then treated with cationic electrodeposition paint Power Top U-100 manufactured by Nippon Paint Co., Ltd.
Electrodeposition coating was performed to a thickness of 20 μm, and a 30 μm spray coating of Lugabake B531 White manufactured by Kansai Paint Co., Ltd. was applied. The primary adhesion and secondary adhesion of the sample materials coated under these conditions were then tested.

In the primary density test, 100
The secondary adhesion test was carried out by carving out a number of gongs and applying and peeling adhesive tape to these goblets.The secondary adhesion test was carried out by immersing the scion sample material in warm water (pure water) at 40°C for 120 hours after painting. After taking out the product, within 30 minutes thereafter, cuts were made at two tone intervals, and an adhesive tape was attached to and peeled off from the cuts.

In the Cr elution test, the sample material was degreased by 0.6 m in 1 ton of degreasing solution using the degreaser FC-4410 manufactured by Nippon Parkerizing Co., Ltd. under standard conditions, and the amount of Cr in the solution was determined by atomic absorption. By measuring *1 See Table 2 *2 The total amount of Cr in the chromate bath was expressed as a concentration in terms of CrO3.

*3 Phosphate ion was adjusted by adding orthophosphoric acid and expressed as the concentration of PO43-.

*4 Zirconium fluoride ion was adjusted by adding H2ZrF4 and shown as the concentration of ZrF62-.

*5 Zn ions are adjusted by adding ZnO.
It is shown as the concentration of zn2+.

*6 Weight ratio of Cr6+ and Cr3+ in chromate bath *7
The weight ratio of chromic acid and zirconium fluoride ions in the chromate bath is expressed as cr03/zrF62-.

*8 The amount of attached chromate film obtained was measured by FX,
Shown in terms of metal cr.

*9 See Table 3 *1O See Table 4 *11 Weight ratio of solid content of base resin/silica *12 Amount of resin composition adhered *13 Indicated by the final plate @ (pM'r: °C) of the sample material Ta.

*14 Evaluation criteria for corrosion resistance are as shown below.

◎ : No red rust O + : Less than 5% red rust ○ : Red rust 5q6 or more and less than 10 inches ○ - : Red rust 10 inches or more and less than 20 inches △ : Red rust 20 inches or more and less than 50 inches × : Red rust 50 inches or more * 15 paints (2 Coat) The evaluation criteria for the adhesion test are as shown below.

◎: Peeling area 0% ○＋: Peeling area less than 5 inches ○: Peeling area 5 inches or more and less than 10 inches 〇-: Peeling and accumulation 10 inches or more and less than 20 inches △: Peeling area 20 inches or more and less than 5 inches X: Peeling surface a50cm or more *16 Evaluation criteria for cr dissolution are as shown below.

◎: Cr in the degreasing liquid is less than 2 ppm ○: Cr in the degreasing liquid is 2 ppm or more and less than 6 ppm △ ×: Cr in the degreasing liquid is 61) 9 m or more, less than 1,2 ppH: Cr in the degreasing liquid is 12 ppm or more Table 1 [Effects of the invention] According to the method of the present invention described above, Cr6 in the chromate solution
By being able to lower the +/Cr3+ ratio without gelling the solution and by incorporating an ingredient effective in removing Cr into the chromate solution, the Cr elution property can be significantly improved compared to the conventional method. Moreover, the repair effect of CR6+ can be sustained over a long period of time, and a film having excellent corrosion resistance can be obtained.

In addition, the chromate film does not require high temperature drying, and excellent Cr elution properties and corrosion resistance can be obtained even with low temperature drying, so it is an economically advantageous method.

Claims (1)

[Claims] On the surface of a galvanized or zinc alloy plated steel sheet, chromic acid: 5 to 100 g/l, phosphate ion: 0.5 to 20 g/l, zirconium fluoride ion: 0.2 to 4 g/l Zn ion: 0.2 to 7 g/l, and the weight ratio of the following components in the bath is: Cr^6^+/Cr^3^+ = 3/4 to 3/2 chromic acid/zirconium fluoride ion = 10 Chromate treatment is performed by applying and drying a chromate solution adjusted to a ratio of /1 to 100/1, then a solvent-based organic polymer resin is applied to the top of the chromate film without washing with water, and then baking treatment is performed. A method for producing a highly corrosion-resistant surface-treated steel sheet.