JPH06173085A - Treatment before coating of stainless steel - Google Patents

Abstract

PURPOSE:To form the film excellent in adhesion with an org. material coating layer by impressing a square wave electric potential on the stainless steel immersed in an electrolyte bath. CONSTITUTION:Cu-containing stainless steel is immersed in the acidic electrolyte bath such as a sulfric acid bath, and the square wave electric potential vibrating from the electric potential range EL which is lower than the passivation electric potential E1 of Cr to the higher electric potential range EH which is the potential range from the natural electric potential E2 of Fe to passivation completed electric potential E3 of Fe, is impressed repeatedly. In this way, the metallic ion eluted from the stainless steel by electrolysis in the higher electric potential range EH is deposited on the surface of the stainless steel by electrolysis in the lower electric potential range EL, thus a film is formed. In the process of the elution-deposition, the Cu included in the stainless steel is condensed in the film, and the surface layer high in affinity with gum, resin, coating etc., is obtained.

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 coating having excellent adhesion to an organic coating on the surface of stainless steel.

[0002]

2. Description of the Related Art Stainless steel sheets are used in a wide range of fields such as building materials, structural members for various plants, kitchen supplies, etc. due to their excellent corrosion resistance. Along with the diversification of this field of use, various coloring methods, organic coating methods, etc. have been developed in order to improve the designability and product value of stainless steel. Prior to the organic coating of stainless steel, pretreatment such as degreasing, surface roughening and chemical conversion treatment is performed. Further, degreasing, surface roughening, chemical conversion treatment and the like are appropriately combined to adjust the surface condition of the stainless steel to be suitable for organic coating.

Degreasing using an alkaline solution, an organic solvent or the like removes contaminants such as residual oil and fat on the surface of stainless steel to obtain a clean surface. The roughening treatment makes the surface shape of the stainless steel uneven so as to improve the anchoring action to the organic coating layer and improve the adhesiveness of the organic coating layer. The roughening treatment includes physical methods such as sandblasting and dull roll rolling, and chemical methods using hydrofluoric acid and the like. The adhesion of the organic coating layer is also improved by exposing the active coating by dissolving the passivation coating on the surface of stainless steel. Further, the adhesion of the organic coating layer is also improved by forming a film having a high affinity for the organic coating layer on the surface of stainless steel by chemical conversion treatment.

[0004]

The strong passivation film formed on the surface of stainless steel is necessary for ensuring the corrosion resistance of the base material, but it lowers the affinity for the organic coating layer. Therefore, the organic coating layer formed on the surface of stainless steel has poorer adhesion to the base steel than the organic coating layer formed on the surface of ordinary steel, and peeling and cracks are likely to occur during processing and use. . Only by degreasing or roughening, the removal of the passivation film is insufficient, and primary bonding between the base steel and the constituent resin of the organic coating layer cannot be expected, resulting in poor adhesion. Even with the silane coupling treatment, which is one of the chemical conversion treatment methods, it is not possible to obtain a strong adhesive force as in the case of FRP glass fiber.

As a pretreatment for obtaining high adhesion, coating type chromate treatment, electrolytic chromate treatment and the like are known. However, since chromic acid, dichromic acid, etc. are used, there is a problem in terms of environmental protection, and it is expected that their use will be greatly restricted in the future. By the way, in the coating pretreatment by alternating current electrolysis represented by JP-A-2-4999, the insulating property of the coating becomes higher as it grows,
The electrolysis applied voltage increases rapidly. As a result, the electrode reaction taking place on the stainless steel surface changes. In this respect, the potential control is more effective than the current control as a method of causing an intended electrode reaction in a material having a strong potential dependency such as stainless steel. The present invention has been devised to solve such a problem, and by applying a pulse potential of a specific waveform to stainless steel, dissolution-precipitation using each alloying element contained in stainless steel. The purpose of the reaction is to heat the stainless steel to a surface state having excellent adhesion to the organic coating layer.

[0006]

In order to achieve the object, the coating pretreatment method of the present invention is performed by immersing a Cu-containing stainless steel in an acidic electrolysis bath to obtain a base potential range lower than the passivation potential E ₁ of Cr. A square wave potential oscillating between E _L and a noble potential range E _H from the natural potential E ₂ of Fe to the passivation completion potential E ₃ of Fe is applied to the Cu-containing stainless steel to obtain a monovalent It is characterized in that a film containing Cu is formed on the surface of the Cu-containing stainless steel.

As the Cu-containing 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. In the electrolytic treatment according to the present invention, stainless steel having a Cu content of 0.05% by weight or more is preferable because Cu is concentrated from the base steel to the surface layer. The contained Cu may be either unavoidably mixed in stainless steel or positively added as an alloying element for improving corrosion resistance, workability and the like. As the acidic electrolytic bath, an acidic solution prepared by mixing a nitric acid solution, a phosphoric acid solution, etc. can be used, including a sulfuric acid bath. Since metal ions are eluted from the stainless steel by electrolysis in the noble potential range E _H, it is not necessary to separately add metal ions as a film forming material to this acidic electrolytic solution.

[0008]

[Operation] S, which is a typical material of stainless steel,
US304 will be described as an example. The present inventors investigated and studied how the structure of a film formed on the surface of stainless steel is affected by the waveform of the applied pulse potential when the stainless steel is electrolyzed by the application of the pulse potential. In the process of investigation / research, the spontaneous electric potential of Cr E ₁
Base range E from the passivation start potential E ₂ of Cr to Cr
_L and the noble potential range E from the passivation starting potential E _{3 of} Fe to the potential E _{4 at} which the transpassive dissolution current of Cr rapidly increases.
It was found that when a square wave pulse potential oscillating between _H and _H was applied to stainless steel immersed in an electrolytic bath, a film containing chromium hydroxide with excellent coating adhesion was formed on the stainless steel surface. Filed as Japanese Patent Application No. 4-69155.

As a result of further investigation, although the film formed on the surface of the stainless steel differs in composition from that proposed in the previous application, a new electrolytic condition was found that can give excellent adhesion to the surface of the stainless steel. The present invention has been completed. In the present invention, the natural potential E ₂ of Fe to the passivation completion potential E _{3 of} Fe is defined as the noble potential E _H , and Cr
The passivation potential E _{1 of} E is the base potential range E _L and the noble potential E
A square wave potential that oscillates with _H is applied to the stainless steel plate. For example, when the total application time exceeds 1 minute when the temperature of the electrolytic bath is 25 ° C., a film having good adhesion to the organic coating layer is formed on the surface of stainless steel. The formed film contains a monovalent Cu compound such as Cu ₂ O and CuS, and improves the adhesion to the organic film by a mechanism different from that proposed in the previous application.

The SUS304 series stainless steel has a potential-current density relationship which shows the polarization curve of FIG. Note that FIG. 1 also shows polarization curves of Fe, Cr, and Ni. As is apparent from FIG. 1, the polarization curve of SUS304 series stainless steel is a superposition of the polarization curves of Fe, Cr and Ni. And 0.5 kmol / m ³
In aqueous sulfuric acid solution, the potential range E _L of noble -0.42
The potential is V (vs SHE) or less, and the noble potential range E
_H is -0.28 to 0.78 V (vs SHE).
Select an appropriate electrolytic potential E _L 'and E _H' from the potential range E _L and the potential range E _H respective noble more negative, the potential E _L 'and E
_{When H'is} alternately applied to stainless steel to perform electrolysis, the passivation film is removed from the steel surface, and cleaning and homogenization of activity are achieved. In addition, melting of metals contained in steel −
The deposition reaction proceeds and the surface state has a strong affinity for the organic coating layer.

In electrolysis in the noble potential range E _H, the metal elements contained in stainless steel are eluted. Mainly, Fe and Cr are actively dissolved, and Ni is also actively dissolved at a natural potential or lower. Metal ions such as Fe, Cr, and Ni eluted in the electrolytic bath serve as a supplementary source of the film-forming material formed by electrolysis in the base potential range E _L. At this time, Cu contained in the stainless steel is melted in the noble potential range E _H and is deposited on the surface of the stainless steel in the base potential range E _L , like Fe, Ni and the like. As a result, Cu is preferentially concentrated on the surface of stainless steel rather than other metal ions, and the Cu concentration ratio may reach 100 times as high as the Cu content of the base steel.

Cu deposited on the surface of stainless steel exists in the film as monovalent and divalent oxides, sulfates, sulfides and the like. Oxides, especially Cu ₂ O, are effective in bonding with rubber by primary bonding through sulfur, as can be seen from the fact that brass plating is applied as a pretreatment when directly bonding rubber to metal materials. To work. Further, although the detailed adhesion mechanism is unknown, excellent adhesion to the organic coating layer due to, for example, covalent bonding of the unsaturated double bond part and the copper sulfide in the commonly used organic resin paint. Present. The action of the copper compound contained in the coating is confirmed by the fact that the surface of the stainless steel treated according to the present invention adheres well to rubber, organic paint and the like. In order to form a film exhibiting good adhesion on the surface of stainless steel, the Cu content of stainless steel is 0.05% by weight.
The above is preferable.

Similarly, Fe deposited on the surface of stainless steel,
Ni oxides, hydroxides, sulfates, sulfides, etc. are Cu
It forms a film mixed with the oxides and sulfides of. It is considered that these metal compounds other than Cu are also a factor in forming a film having excellent adhesion to the organic coating layer. By alternately repeating electrolysis in the base potential range E _L and electrolysis in the noble potential range E _H , in other words, repeatedly applying a square wave potential to the stainless steel, the amount of dissolution of the metal increases and the film Also grow thicker. The formed film uses metal ions eluted from stainless steel into the electrolytic bath by electrolysis in the noble potential range E _H as a replenishing source, and thus does not require the metal ions serving as a coating material in the electrolytic bath.

The excellent affinity is effective not only in baking coating using a coating material containing an ordinary organic coating film forming material but also in cationic electrodeposition coating, and an organic coating layer excellent in adhesion is obtained. It is formed. For example, in the cationic electrodeposition coating using the adherend as a cathode, the coating is electrodeposited while maintaining the characteristics of the coating, and then three-dimensional crosslinking is performed by baking the coating. At the time of baking for this coating, a condensation reaction between the film and the organic layer occurs as in the case of ordinary baking, and a strong coating film having excellent adhesion is formed. The electrolysis in the base potential range E _L needs to be performed at a potential lower than the passivation potential E ₁ of Cr. If the electrolysis potential is higher than the passivation potential E ₁ of Cr, a sufficient reduction reaction does not occur and the metal ions eluted in the noble potential range E _H cannot be deposited. However, if the base potential range E _L is too low, the hydrogen generating reaction occurs preferentially, and the efficiency of the film forming reaction decreases, so −1.50 V (vs SHE) or more is preferable.

The electrolysis in the noble potential range E _H is higher than the natural potential E _{2 of} Fe, and the passivation completion potential E ₃ of Fe is higher.
It is necessary to carry out at a lower electric potential. Natural potential E ₂
In electrolysis at a lower potential, active dissolution of Fe does not occur and only dissolution of Cr progresses preferentially. Therefore, when the base potential E _L is maintained, no metal is deposited and a film having excellent adhesion is not formed. Noble potential range E _H is Fe
When the potential is higher than the natural potential E _{2 of} , even if electrolysis is performed at a potential equal to or lower than the natural potential of Ni and Cu, not only Fe and Cr but also Ni and Cu are dissolved. As a result, when the base potential E _L is maintained, a deposited film composed of Fe, Ni and Cu is formed. As described above, in the present invention, the base potential range E _L and the noble potential range E _H are controlled without controlling the electrolytic current.
By setting the electrolytic potential in each case, the electrochemical reaction is arbitrarily controlled, and a film suitable as an underlayer of the organic coating layer is formed on the stainless steel surface. In order to make the formed film have an affinity for the organic coating layer,
It is preferable that the application time of the electrolytic potential in each of the base potential range E _L and the noble potential range E _H is set to 0.01 to 10 seconds and the number of repetitions of the pulse potential is set to 4 times or more.

[0016]

[Example] Stainless steel plate SUS30 having a plate thickness of 0.6 mm
4 (Cu content 0.20% by weight) was cut into a 100 mm square and used as a test piece. After degreasing the surface of the test piece with a liquid and vapor of dichloromethane, it was immersed in an electrolytic bath and electrolysis was performed by applying a square wave potential. As an electrolytic bath, 25
A 0.5 kmol / m ³ sulfuric acid solution kept at ℃ was used. The potential waveform used for electrolysis is shown in FIG. The field time t _L electrolysis at time t _H, and baser potential range E _L in the potential range E _H noble, both set to 0.1 seconds. A fluororesin coating material containing an acrylic resin was applied to the electrolytically treated test piece so that the coating film thickness after baking was 20 μm and baked. The adhesion of the formed coating film was examined by the following test method.

The cut from the surface of the coating film to the base steel is 1
The test piece was put into the test piece with a cutter so that 100 squares at mm intervals were formed. Then, the test piece was immersed in boiling water for 2 hours. After the test piece was taken out of the boiling water, the water was wiped off, and an adhesive tape was immediately adhered to the surface of the test piece having a notch and peeled off. The squares on the surface of the test piece where the coating film was peeled off by peeling off the adhesive tape were counted. The coating film adhesion was evaluated by the number of squares. The evaluation results are shown in Table 1 together with the electrolytic potential E _H ′ in the noble potential range E _H and the electrolytic potential E _L ′ in the base potential range E _L. The electrolysis with the electrolysis potential E _H 'and the electrolysis potential _EL ' was alternately repeated, and the application time was expressed by the total energization time including both. In addition, Table 1 also shows the ratio of Cu ⁺ in all the cations when the concentration of the element contained in the film over a depth of about 50 Å from the surface is investigated by X-ray photoelectron spectroscopy. It is considered that Cu ⁺ is present in the film in the form of Cu ₂ O, CuS or the like.

[Table 1]

As is clear from Table 1, the noble electrolysis potential E
_H 'and baser the electrolysis potential E _L' repeatedly varying square wave potential by coating film formed on the surface of the test piece of the present invention example was electrolytically between are all showed excellent adhesion, adhesive tape The base steel of the test piece was not exposed after peeling. Moreover, excellent coating film adhesion was exhibited by electrolysis in a relatively short time. On the other hand, the surface of the test piece of the comparative example, which is electrolyzed with a square wave potential in which either the noble electrolytic potential E _H 'or the base electrolytic potential E _L ' is outside the range specified in the present invention, is sufficient. A coating film with good adhesion was not formed.

[0019]

As described above, according to the present invention, a square wave pulse potential oscillating between the base potential range E _L and the noble potential range E _H is applied to the Cu-containing stainless steel. , Cu, Fe, Ni and other oxides, hydroxides, sulfides, etc. are formed on the surface of stainless steel. This film formation utilizes a reaction in which the surface layer of stainless steel self-dissolves and precipitates, so that it is not necessary to use an electrolytic bath containing an electrolyte as a film material. Therefore, unlike the conventional chromate treatment using chromic acid, dichromic acid, etc., it does not adversely affect the environmental protection. In addition, a film having excellent adhesion to the organic coating layer is reproducibly formed on the surface of stainless steel. 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 an exterior building material excellent in designability, durability, and corrosion resistance, and a harsh environment such as water A material that can also be used is obtained.

[Brief description of drawings]

FIG. 1 Polarization curves of stainless steel sheets SUS304, Fe, Cr and Ni

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

Claims (1)

[Claims] 1. A Cu-containing stainless steel is dipped in an acidic electrolytic bath to provide a base potential range E _{L which} is lower than the passivation potential E ₁ of Cr.
And the natural potential E ₂ of Fe to the passivation completion potential E of Fe.
A square wave potential oscillating between noble potential ranges E _H up to ₃ is applied to the Cu-containing stainless steel to form a coating containing monovalent Cu on the surface of the Cu-containing stainless steel. Pretreatment method for coating stainless steel.