JP3065830B2 - Pretreatment method for coating stainless steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a coating pretreatment method for forming a film having excellent adhesion to an organic coating layer on the surface of stainless steel.

[0002]

2. Description of the Related Art Stainless steel has been used in a wide range of fields such as building materials, structural members for various plants, kitchenware, etc., taking advantage of its excellent corrosion resistance. With the diversification of fields of use, various coloring methods, organic coating methods and the like have been developed in order to enhance the design of stainless steel and increase its commercial value. Prior to the organic coating of stainless steel, pretreatments such as degreasing, roughening, and chemical conversion are performed. In addition, stainless steel is adjusted to a surface state suitable for organic coating by appropriately combining degreasing, surface roughening, and chemical conversion treatment.

[0003]

The coating type chromate treatment is the most widespread pretreatment among these pretreatments.
Form a film with excellent adhesion to the organic coating layer. Hexavalent and trivalent Cr contained in the coating type chromate treatment liquid are as follows:
Since it has a function of forming a strong film by cross-linking with the organic coating layer and self-polymerization, a high adhesive strength to the organic coating layer is exhibited. By the way, in the case of stainless steel having an extremely stable and poorly reactive surface state, an appropriate base treatment is required to form a chromate film on the stainless steel with good adhesion. As a practical base treatment method, surface cleaning with alkali, immersion in hydrofluoric / nitric acid, immersion in sulfuric acid, activation by immersion in phosphoric acid, and the like are employed. However, when processing or heating is performed after the formation of the organic coating, sufficient adhesion to the coating type chromate film cannot be maintained.

[0004] In a zinc-based plated steel sheet, Zn, Fe,
A method of treating with a treatment solution containing ions of Ni, Co and the like, displacing and depositing these metals on the surface, and then applying a coating-type chromate treatment to promote adsorption and fixation of the chromate film has been performed. See JP-A-4-198488). However, in stainless steel having an inert surface, the same substitutional precipitation does not occur, so that sufficient performance cannot be obtained by treatment with a metal ion-containing solution. The present invention
It has been devised to solve such a problem, and is contained in steel by applying a square wave potential of a specific waveform to stainless steel as a base adjustment before performing coating type chromate treatment. An object of the present invention is to cause a dissolution-precipitation reaction while using Cr as a coating material supply source to form a coating-type chromate film having excellent adhesion containing a hydroxide of Cr on a stainless steel surface.

[0005]

Coating pretreatment method of the present invention, in order to solve the problems] In order to achieve its objectives, the potential range of less noble from spontaneous potential E ₁ of Cr to passivation onset potential E ₂ of Cr E _L
And a noble potential range E from the passivation onset potential E ₃ of Fe to the potential E _{4 at} which the overpassive dissolution current of Cr rapidly increases.
_H is applied to a stainless steel immersed in an electrolytic bath to form a coating containing chromium hydroxide on the surface of the stainless steel, and then applying a coating type chromate treatment. Features.

As the stainless steel, there are austenitic stainless steel, ferritic stainless steel, duplex stainless steel and the like, and stainless steel having a Cr content of 10% or less can also be used. As the acidic electrolytic bath, an acidic solution obtained by mixing a nitric acid solution, a phosphoric acid solution, and the like, such as a sulfuric acid bath, can be used. This acidic electrolyte contains
Since metal ions are eluted from stainless steel by electrolysis in a low potential range, there is no need to separately add metal ions to be a film forming material.

[0007]

[Operation] The following is a typical stainless steel material, SUS3.
A description will be given taking O4 as an example. The present inventors have investigated and studied how the structure of a film formed on the surface of stainless steel is affected by the pulse potential waveform when stainless steel is electrolyzed by applying a pulse potential.
The nature and potential range E _L of less noble potential from E ₁ to passivate onset potential E ₂ of Cr and Cr, over-passive dissolution current from passivation onset potential E ₃ of Cr Fe increases rapidly up potential E ₄ as potential range E _H noble, when applying a square wave voltage swinging between the potential range E _L and noble potential range E _H of less noble stainless steel, for example, temperature of the electrolytic bath 25 When the total application time exceeds 30 seconds at a temperature of ° C., it has been found that a film having good adhesion to the chromate film is formed on the stainless steel surface.

The polarization curve of a SUS3O4 stainless steel sheet has a potential-current density relationship shown in FIG. In addition,
FIG. 1 also shows the polarization curves of Fe, Cr and Ni. As is clear from FIG. 1, the polarization curve of the SUS304 stainless steel sheet is obtained by superposing the polarization curves of Fe, Cr and Ni. And 0.5kmol /
In m ³ sulfuric acid solution, -0.48～-0. l4
V (vs SHE) is a base potential range E _L , 0.80 to 0.8
l. 3OV (vs SHE) is the potential range E _H of Guizhou. The electrolyte in the potential range E _L of noble metal element contained in the stainless steel is eluted. For example, the potential range E _L of less noble is lower than the natural potential of Fe, only Cr is active dissolution. When the potential range E _L of less noble is higher than the natural potential of Fe, both Cr and Fe becomes active dissolution.
Cr dissolved in the electrolytic bath, metal ions such as Fe is a supply source of the film-forming material formed by electroless the potential range E _H noble.

[0009] In the electrolyte in a potential range E _H noble, part of the dissolved Cr ions of metal ions are precipitated as a film material. That is, Fe forms a precipitation film, and Cr is oxidized from divalent to trivalent and precipitates as hydroxide. The film formed at this time contains a large amount of a hydroxide of Cr having a high affinity for the coating type chromate film, unlike a passivation film formed naturally in the air. This film is
The compatibility with the coating type chromate film is high, and the coating type chromate film is crosslinked by dehydration and condensation at the same time as the heating and film formation, so that the chromate film can be formed firmly. By repeating noble of an electrolyte in a potential range E _H of electrolyte and noble in potential range E _L alternately,
In other words, the film grows thick by repeatedly applying the square wave potential. The formed film has a base potential range E _L
Since the metal ion eluted from the stainless steel in the electrolytic bath in the electrolysis in step (1) is used as a replenishing source, an electrolyte serving as a film material is not required in the electrolytic bath. Therefore, an electrolyte solution such as a sulfuric acid solution, a nitric acid solution, and a phosphoric acid solution can be used.

The film formed on the stainless steel surface contains a hydroxyl group as described above, and has a high affinity for a chromate film. The hydroxyl group firstly binds to the chromium hydroxide or the hydroxyl group of the silica particles contained in the chromate film, and further causes a dehydration condensation reaction at the time of film formation by heating to form a primary bond. Therefore, the adhesion of the chromate film is improved by forming a strong film containing a hydroxyl group on the stainless steel surface. However, if the content of the hydroxide is too high, the volume shrinkage during the dehydration condensation becomes too large, and the adhesion of the chromate film is hindered by the residual shrinkage stress.
Therefore, the amount of hydroxyl groups in the film in consideration of adhesion is preferably 50 to 95 mol% of the ligands bonded to the metal.

The excellent affinity of the coating type chromate film with the organic coating layer is effective not only in the case of laminating ordinary organic materials and baking paint, but also in the case of cationic electrodeposition coating. Thus, an organic coating layer having excellent adhesion is formed. That is, in the cationic electrodeposition coating using an object to be coated as a cathode, a paint is electrodeposited while maintaining the properties of the film, and three-dimensional crosslinking is performed by subsequent baking of the coating. It is presumed that a dehydration-condensation reaction between the coating type chromate film and the coating film occurs during the coating baking in the same manner as in normal baking coating, and a strong coating film having excellent adhesion is formed. Electrolyte in a potential range E _L of less noble is higher than the natural potential E ₁ of Cr, passivation onset potential of Cr E ₂
It is necessary to perform at a lower potential. When electrolysis potential is lower than the spontaneous potential E ₁ of Cr, not metal elution such as Cr, Cr ions is not obtained as a film-forming material during electrolysis in the potential range E _H noble. On the other hand, the electrolytic potential when exceeding passivation onset potential E ₂ of Cr, dense passivation film covering the stainless steel surface. As a result, the elution suppression of metal from stainless steel, can not be obtained metal ions of film-forming material during electrolysis in the potential range E _H noble.

The electrolysis in the noble potential range E _H needs to be performed at a potential higher than the passivation start potential E ₃ of Fe and lower than the potential E _{4 at} which the overpassive dissolution current of Cr rapidly increases. It is. When it is lower than the passivation onset potential E ₃ , Fe is actively dissolved. As a result, the formed coating has a fragile, porous structure that is unsuitable as a coating base and is easily broken. The over-passive region of Cr which electrolysis potential exceeds the potential E _4, the dissolution reaction of Cr is performed severely, the film does not generate. As the coating type chromate treatment, a commonly used coating type chromate treatment agent can be used. Specifically, the trivalent ratio is 0. Total Cr content of 1 to 0.7, 5 to 8
A solution containing Og / l and SiO ₂ , an organic polymer, phosphoric acid, a silane coupling agent and the like as main components is used as a treatment agent. A treating agent is applied to the surface of stainless steel electrolyzed at a square wave potential so that the amount of Cr attached is 10 to 15 mg / m ^2, and then dried to form a film.

As described above, in the present invention, the electrochemical reaction is arbitrarily controlled by setting the electrolytic potential in each of the base potential range E _L and the noble potential range E _H without controlling the electrolytic current. Then, a coating type chromate film is firmly formed on the surface of the stainless steel, and is formed on the surface of the stainless steel suitable as an organic coating base. In order for the film formed by the square wave potential electrolysis to exhibit the function as a coating type chromate treatment base, the application time of the electrolytic potential in each of the base potential range E _L and the noble potential range E _H must be set at 0. It is preferable to set the number of repetitions of the pulse potential to four times or more for 01 to 10 seconds.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A 6 mm SUS304 stainless steel plate (Cr: 17.76%) was cut into a size of 100 mm square to obtain a test piece. After the surface of the test piece was degreased with a dichloromethane and liquid, electrolysis was performed by applying a square wave potential. 0.5k maintained at 25 ° C as electrolytic bath
A mol / m ³ sulfuric acid solution was used. FIG. 2 shows the square wave potential used for the electrolysis. Electrolysis time t _L in the electrolytic potential E _L of electrolysis time t _H, and baser in the electrolytic potential E _H noble
Was set to 0.1 second in each case. The coating type chromate treatment uses a treatment liquid containing 35 g / l of chromic acid reduced to a Cr ³⁺ ratio of 0.5 and SiO ₂ 20 g / l,
r Apply so that the adhesion amount becomes 30 mg / m ² ,
And dried.

A fluororesin paint containing an acrylic resin was applied and baked on the pretreated test piece so that the film thickness after baking was 20 μm. A notch reaching the base steel from the coating film surface was cut into the test piece with a cutter such that 100 meshes at 1 mm intervals were formed. Next, the test piece was immersed in boiling water for 24 hours. After removing the test piece from the boiling water, the water was wiped off, and the adhesive tape was immediately adhered to the surface of the cut test piece and peeled off. The meshes on the surface of the test piece from which the coating film was peeled off by peeling off the adhesive tape were counted. The coating film adhesion was evaluated by the number of the squares. The evaluation results, electrolysis potential is in the potential range E _H noble E _H '
And noble Table l with electrolysis potential E _L 'in the potential range E _L of
Shown in The application time was represented by the total energization time including the electrolysis potential E _H ′ and the electrolysis potential E _L ′ alternately and alternately.

[Table 1]

As apparent from Table 1, the noble electrolytic potential E
_The coating film formed on the surface of the test piece of the present invention, which was subjected to electrolysis with a square wave potential repeatedly changing between _H ′ and the base electrolytic potential E _L ′ and then subjected to coating type chromate treatment, It showed excellent coating film adhesion, and the base steel of the test piece was not exposed after peeling off the adhesive tape. In addition, excellent coating film adhesion was exhibited by electrolysis for a relatively short time. On the other hand, after performing electrolysis with a square wave potential in which either the base electrolytic potential E _L ′ or the noble electrolytic potential E _H ′ is out of the range specified in the present invention, a comparison was performed in which the coating type chromate treatment was performed. No coating film having sufficient adhesion was formed on the surface of the test piece of the example.

[0017]

As described above, according to the present invention, in the present invention, by applying a square wave voltage swinging between the potential range E _L and noble potential range E _H of less noble stainless steel,
A film rich in Cr hydroxide that adheres firmly to the coating type chromate film is formed on the stainless steel surface with good reproducibility. Then, by laminating and cross-linking the electrolytic film and the coating type chromate film densely formed on the stainless steel surface, a surface suitable for organic coating can be formed. This electrolytic film formation utilizes a reaction in which the surface layer of stainless steel is dissolved and precipitated, and there is no need to use an electrolytic bath containing an electrolyte serving as a film material. As described above, according to the present invention, a surface-treated steel sheet having excellent adhesion to an organic coating layer can be manufactured with good reproducibility, and exterior construction materials excellent in design, durability, and corrosion resistance, and severe building materials such as around water can be obtained. A material that can be used in the environment is obtained.

[Brief description of the drawings]

FIG. 1 SUS304 stainless steel, Fe, Cr, N
Polarization curve of i

FIG. 2 shows a waveform of a square wave potential applied to a test piece in an embodiment of the present invention.

────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-57-161069 (JP, A) JP-A-3-271376 (JP, A) JP-A-2-250998 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) C23C 22/00-22/86 C23C 28/00 C25D 11/38 302

Claims (1)

(57) [Claims] 1. The base potential range E _L from the natural potential E ₁ of Cr to the passivation start potential E ₂ of Cr and the overpassive dissolution current of Cr from the passivation start potential E ₃ of Fe are sharply increased. A square wave potential oscillating between a noble potential range E _H up to a potential E ₄ and a stainless steel immersed in an electrolytic bath is applied to the surface of the stainless steel containing a chromium hydroxide. And forming a coating-type chromate treatment after forming the coating.