JP2005194553A - Hexavalent chromium-free black rust preventive film, surface treatment liquid and treatment method - Google Patents

Abstract

A hexavalent chromium-free black rust preventive film having both a black appearance comparable to a chromate film and good corrosion resistance is formed on the surface of a zinc-based plated steel material. In particular, it provides an effective means not only when electroplating is performed in a chloride bath but also in a zincate bath.
A mass ratio of phosphate ions / sulfate ions in an aqueous solution comprising a trivalent chromium compound, a cobalt compound, oxalic acid, phosphoric acid, and sulfuric acid, and an acidic aqueous solution that does not contain hexavalent chromium and nitrate ions. When a surface treatment solution having a surface roughness greater than 1 is used to perform a reactive immersion treatment on a zinc-based plated steel material, a black anticorrosive film having excellent corrosion resistance is formed. This coating consists of three layers: a black layer containing a low-order zinc oxide at the interface with the plating surface, a phosphate layer thereon, and a chromium / phosphorus oxide layer not containing zinc as the uppermost layer. The film structure is as follows. An organic or inorganic overcoat layer may be formed thereon.

Description

  The present invention provides a uniform, black, beautiful and high-grade black rust coating on the surface of various zinc-based plated steel materials, including steel plates, small articles such as bolts and nuts, without using hexavalent chromium compounds. Relates to forming technology. More specifically, the present invention relates to a hexavalent chromium-free blackened zinc-based plated steel material having such a black anticorrosive film, and a surface treatment for forming such a black anticorrosive film on zinc-based plating. The present invention relates to a liquid and a processing method.

  There is a chromate treatment using a treatment liquid containing hexavalent chromium as a rust prevention treatment of a zinc-based plated steel material including a galvanized steel material and a zinc alloy plated steel material. In the case of zinc-based plating, long-term corrosion resistance cannot be obtained because white rust due to zinc oxidation occurs immediately. However, by applying chromate treatment after plating, the corrosion resistance of galvanized steel is dramatically improved, and the corrosion resistance of 96 to 120 hours before white rust generation in the salt spray (SST) test is achieved in galvanized steel. Is granted. Therefore, chromate treatment has been widely used for galvanized steel materials.

  Chromate treatment is a hexavalent chromium compound containing chromic acid (or dichromic acid) containing various mineral acids and other additives. By changing its composition, blue, yellow, green・ It is possible to form a film with four different colors (chromate film). Therefore, for example, an automobile manufacturer uses this color tone difference to identify parts to be attached to the left and right. Therefore, the color of the chromate film not only improves the aesthetic appearance of giving a high-class feeling, but also helps to identify parts having similar shapes.

  The main component of the chromate film is chromium chromate, which is a compound of trivalent chromium and hexavalent chromium. This chromium chromate is reduced to a trivalent chromium ion by dissolution (ie, oxidation) of zinc during the chromate treatment, and this trivalent chromium ion is accompanied by a pH increase near the zinc surface. It is also produced by combining with chromic acid. In order to promote the formation of a chromate film, a partially reduced chromate treatment liquid in which a portion of hexavalent chromium in the chromate treatment liquid is reduced to trivalent chromium in advance is also used. The film is essentially composed of chromium chromate, and it becomes a film containing both trivalent chromium and hexavalent chromium.

  However, hexavalent chromium compounds are carcinogenic and harmful to the human body and the environment, so their use has been regulated. Therefore, the rust prevention treatment of zinc-based plated steel materials is also being switched to the use of a hexavalent chromium-free treatment solution that does not contain a hexavalent chromium compound. Such hexavalent chromium-free treatment solutions include an acidic treatment solution using a trivalent chromium compound, an inorganic (eg, silicic acid) or organic (resin) treatment solution that does not use any chromium compound. There is. In terms of improving the corrosion resistance of the galvanized steel material, which is the purpose of the treatment, an acidic treatment liquid containing a trivalent chromium compound can generally form a film with higher corrosion resistance.

  However, most of the conventional acid treatment liquids containing trivalent chromium compounds, when used for the surface treatment of galvanized steel, the formed anticorrosive film has a colorless or light color tone, especially a yellow color tone, Since the surface of the zinc-based plating was visible, the product could not be given a high-class feeling.

  For example, Patent Document 1 below describes that a zinc-based plated steel material is surface-treated with an acidic treatment liquid containing a trivalent chromium compound, oxalic acid, and Co ions, but there is no description regarding color tone. . It is specified that the rust preventive film which does not contain hexavalent chromium and contains trivalent chromium formed on zinc or a zinc alloy described in Patent Document 2 below is transparent and basically colorless.

Regarding the black rust preventive film, Patent Document 3 listed below describes a treatment solution that uses a trivalent chromium compound to form a black chemical conversion film that does not contain hexavalent chromium. This treatment solution contains nitric acid (ion) in such an amount that the molar ratio of nitrate ion to trivalent chromium (NO ₃ ⁻ / Cr ³⁺ ) is less than 0.5, and trivalent chromium is an aqueous complex with a chelating agent. It exists in a form and further contains Co and / or Ni ions in a state that does not form a sparingly soluble salt with the chelating agent.

  However, when the present inventors re-executed the example of Patent Document 3, there is a case where a black rust preventive film is surely formed, but even in that case, the formed rust preventive film is uniformly black as a whole. In many cases, the film did not become a film, and the degree of blackness was insufficient, blackness was thin, and the coloration was uneven. Compared to zincate bath, when zinc-plated steel material electroplated with a chloride bath is treated, the anti-corrosion film is less likely to be colored, and the plating bath used for the underlying electrogalvanization is a zincate bath There was a large difference in the black coloration of the rust preventive film between the case of using a chloride bath.

  Patent Document 4 listed below describes a surface treatment agent for forming a black film on a metal surface such as zinc, including a sulfite ion source and an oxidizing substance source (eg, nitric acid, peroxide, etc.). Has been. The surface treatment agent can further contain various metal cations including trivalent chromium. However, the film formed from this surface treatment agent is basically not a rust preventive film, and is inferior in corrosion resistance. In addition, even when a trivalent chromium compound is contained, nitric acid or other oxidizing agent coexists, so that it is not possible to form a beautiful black film like the acidic treatment liquid described in Patent Document 3. Conceivable.

The molybdate method and the chlorate method (hereinafter referred to as the molybdate method, etc.) have long been known as methods for changing the color of zinc to black. According to these methods, a uniform black film can be formed on the zinc surface. However, since the film is formed by substitution deposition of another metal on zinc, the corrosion resistance does not reach far from that of the chromate film, and cannot be said to be a rust-proof film.
Japanese Patent No. 3332373 JP 2000-509434 Gazette JP 2003-268562 A JP 2003-213446 A

  As described above, the surface of the zinc-plated steel material has a colored anticorrosive film that has a clear color tone that enables identification of parts without using hexavalent chromium and corrosion resistance comparable to that of a chromate film. It has been desired to develop a treatment solution that can be formed in a simple manner.

  In particular, it is possible to form a rust-proof coating with a black appearance on the surface of zinc-based plated steel because it is a color that gives a sense of quality and weight in addition to enabling identification. There was a strong demand from steel users. In this regard, the chemical conversion film formed on the surface of the zinc-based plated steel material from the treatment liquid described in Patent Document 3 has a slightly thin blackness and non-uniformity, and is obtained by a conventional molybdate method or the like. Compared with the uniformly black black film formed, the blackness was insufficient, and the coloring was particularly light when galvanized in a chloride bath.

  An object of the present invention is to provide a hexavalent chromium-free black rust-preventing film having a beautiful black appearance at the same level as that formed by a conventional molybdate method and an excellent corrosion resistance comparable to a chromate film. Is to develop a technology to form a surface of zinc-plated steel.

  Another object of the present invention is to develop a technique capable of forming a beautiful black rust preventive film not only on a zincate bath but also on a zinc-based plated steel material using a chloride bath.

  In the hexavalent chromium-free acidic surface treatment liquid containing a trivalent chromium compound that is known to be capable of forming a film having good corrosion resistance, the present inventors have determined the types and amounts of components other than the trivalent chromium compound. By selecting, the plating surface of zinc-plated steel material plated by various plating methods including zincate bath and chloride bath has a beautiful black appearance that looks black like the molybdate method and a chromate film. It has been found that a black rust preventive film having good corrosion resistance comparable to that can be formed.

On the other hand, when the present inventors investigated the uniform black film formed by the conventional molybdate method etc. using the measuring instrument of L ^* a ^* b ^* color system, this kind of black color was measured. In the film, it was found that the value of the color index L ^* of the color system (hereinafter referred to as L value, the darker the film increases as the L value is smaller) is always less than 15.

  Moreover, when the black film formed according to the Example of the said patent document 3 was investigated similarly, L value was 15 or more, and it might be 20 or more. Furthermore, in the example of Patent Document 3, electrozinc plating is performed using a zincate bath, but when a black film is similarly formed on a galvanized steel sheet electroplated with a chloride bath, the L value is at least 20. It was very large, exceeding 30 in some cases. The L value in the black film correlates well with the blackness seen with the naked eye. When the L value is less than 15, particularly 14 or less, the black film looks black, and when the L value is 20 or more, the color becomes gray. It was found that when the value is 30 or more, the coloring is very light.

  Then, about said black rust preventive film which the present inventors developed, when L value was measured similarly, L value of this film | membrane is less than 15 similarly to the black film formed by the molybdate method etc., Most were below 14.

Here, the present invention is as follows.
(1) An acidic surface treatment solution for forming a black anticorrosive film on the surface of a zinc-based plated steel material, comprising a trivalent chromium compound, a cobalt compound, an organic acid capable of forming a chelate, phosphoric acid, and sulfuric acid. A surface treatment liquid comprising an acidic aqueous solution containing hexavalent chromium and nitrate ions, wherein the mass ratio of phosphate ions / sulfate ions in the aqueous solution is 1 or more.

(2) The surface treatment liquid according to (1) above, wherein the organic acid is oxalic acid.
(3) The surface treatment liquid according to (1) or (2) above, wherein the pH of the liquid is in the range of 2 to 3.
(4) A zinc-based plated steel material is immersed in the surface treatment solution according to any one of (1) to (3) above, and then dried to form a black rust preventive film on the plated surface. , Blackening treatment method for galvanized steel.

(5) The method for blackening a galvanized steel material according to the above (4), wherein an organic or inorganic overcoat layer is formed on the formed black rust preventive film.
(6) A blackened zinc-based plated steel material having a black rust-preventing film containing a trivalent chromium compound and not containing hexavalent chromium and having an L value of less than 15 on the surface of an electrogalvanized plating using a chloride bath.

 (7) The zinc-based plating surface is formed by dipping using the surface treatment liquid according to any one of (1) to (3) above, and does not contain hexavalent chromium but contains trivalent chromium. Blackened zinc-based plated steel material with a black rust preventive film with a value of less than 15.

 (8) The blackened zinc-based plated steel material according to (6) or (7) above, further having a colorless inorganic or organic overcoat layer on the black rust preventive film.

  According to the present invention, a hexavalent chromium-free compound having a corrosion resistance comparable to that of a chromate film containing hexavalent chromium using a treatment liquid containing a trivalent chromium compound as a main film-forming component and not containing hexavalent chromium. Can be formed on the plated surface of the zinc-based plated steel material.

  This trivalent chromium-based black rust preventive film has an L value of less than 15 and is uniformly black when viewed with the naked eye, similar to a black film formed by a conventional molybdate method or the like. Further, in the technique described in Patent Document 3, the steel material electrogalvanized using a chloride bath was less likely to have a black color compared to the zincate bath. However, the black rust preventive film of the present invention is a chloride bath and zincate. Even when the zinc plating is a chloride bath, a black and beautiful black anticorrosive film similar to the case of the zincate bath can be formed on the plating surface.

  As a result, it is possible to identify the parts made of galvanized steel that should be attached to the left and right with the naked eye by forming the conventional colorless anticorrosive film on one side and the black anticorrosive film of the present invention on the other side. , They can be easily identified, making the mounting work easier, and preventing mounting errors.

  In addition, the black anticorrosive film of the present invention can give a thick and high-grade black appearance to a zinc-based plated steel material, and the underlying zinc-based plated metallic surface is completely invisible. Therefore, this black rust preventive film is particularly suitable for application to parts and products that are visible to the outside, and imparts a beautiful black appearance to them, thereby increasing its commercial value.

The present invention will be described more specifically below. In this specification, “%” means “% by mass” unless otherwise specified.
The present invention relates to a technique for forming a hexavalent chromium-free black rust preventive film on a zinc-based plated steel material.

  The steel material used as the plating base material may be a primary processed product such as a steel plate, a steel pipe, a wire, a bar, or a deformed material, or a part, a semi-processed product, or a product obtained by further processing them. Examples of the latter include small parts such as bolts and nuts, press-formed products, stamped products, forged products, cast products, and the like, but are not limited thereto.

  In the present invention, the galvanized steel material means to include both galvanized steel material and zinc alloy plated steel material. Examples of zinc alloy plating include Zn-Ni alloy plating, Zn-Fe alloy plating (including alloyed hot dip galvanizing), Zn-Al alloy plating, and the like. The zinc plating method may be electroplating or hot dip plating, or may be a special plating method such as vapor phase plating. The plating adhesion amount is not particularly limited as long as the necessary corrosion resistance can be imparted to the steel material.

  As is well known to those skilled in the art, a plating bath for electrogalvanizing can be selected depending on the type of steel material. For example, a zincate bath is often used for plating steel sheets, and a chloride bath is often used for small parts, but is not limited thereto. As described above, unlike the technique described in Patent Document 3, in the present invention, a beautiful black anticorrosive film can be formed even on a zinc-based plated steel material that has been electroplated using a chloride bath. it can. Therefore, since it can be applied to small parts that are often applied with a chloride bath, for example, both corrosion resistance and a black appearance can be imparted to bolts and nuts at the same time, and the commercial value thereof can be significantly increased. it can.

  The surface treatment liquid used to form a black rust preventive film on the surface of the zinc-based plated steel material in the present invention contains a trivalent chromium compound, a cobalt compound, an organic acid capable of forming a chelate, phosphoric acid, and sulfuric acid, It consists of an acidic aqueous solution that does not contain hexavalent chromium and nitrate ions.

  The trivalent chromium compound is a main film forming component of the rust preventive film. In the case of the present invention, the trivalent chromium compound mainly forms a film having excellent corrosion resistance by reacting with phosphate ions in the treatment liquid to form an insoluble phosphate. As the trivalent chromium compound, a compound soluble in an acidic aqueous solution may be used. Examples of such trivalent chromium compounds include inorganic acid salts such as chromium chloride and chromium sulfate, and organic acid salts such as chromium acetate and chromium oxalate.

  Similarly, the cobalt compound may be a compound that is soluble in an acidic aqueous solution. Examples of such cobalt compounds include inorganic acid salts such as cobalt chloride and cobalt sulfate, and organic acid salts such as cobalt acetate and cobalt oxalate. The mechanism of the cobalt compound is unknown, but is involved in the blackening of the film. If the surface treatment liquid does not contain a cobalt compound, the formed film is colorless to light yellow.

  The chelate-forming organic acid includes dicarboxylic acid and hydroxycarboxylic acid, but is preferably an aliphatic dicarboxylic acid having 6 or less carbon atoms such as oxalic acid, malonic acid, and succinic acid. Most preferred. Even if the surface treatment liquid does not contain this type of organic acid, a film is formed, but it also becomes a colorless to light yellow film and does not become black. In addition, this organic acid also affects the corrosion resistance of the film, and if the surface treatment liquid does not contain this type of organic acid, the film has very poor corrosion resistance. Hereinafter, the present invention will be described with an organic acid capable of forming a chelate represented by oxalic acid.

  Both phosphoric acid and sulfuric acid may be used for the preparation of the treatment liquid after being diluted to an appropriate concentration. When the surface treatment liquid does not contain phosphoric acid and instead contains only nitric acid, sulfuric acid and / or hydrochloric acid, a film with high blackness is not formed. Instead, an unsatisfactory (smooth) film is often formed with black wrinkle-like particles attached to the surface.

  The amount of phosphate ion and sulfate ion in the aqueous solution is not limited to the amount added as phosphoric acid and sulfuric acid. For example, when phosphate or sulfate is used as a trivalent chromium compound or cobalt compound, The amount includes phosphate ions and sulfate ions supplied as anions from such metal salts.

  In order to form a black rust preventive film, the mass ratio of phosphate ions / sulfate ions in the aqueous solution is set to 1 or more. When this mass ratio was smaller than 1 and the amount of sulfate ions was larger than phosphate ions, the rust preventive film did not become black, but became gray or burned in the same manner as when phosphoric acid was not contained. It can be a thing. Even if this mass ratio is too large, the blackness may decrease, so this mass ratio is preferably 5 or less.

  The surface treatment liquid of the present invention does not contain nitrate ions. The surface treatment liquid described in Patent Document 3 contains nitric acid, and the treatment liquid described in Patent Document 4 contains an oxidizing agent. When the surface treatment liquid of the present invention contains nitrate ions, the blackness of the formed rust-preventive film is lowered, as in the case of the surface treatment liquid described in Patent Document 3, and this tendency tends to occur particularly on the galvanized surface by a chloride bath. Get higher. Accordingly, in the present invention, not only free nitric acid but also nitrates such as chromium nitrate and cobalt nitrate are not used as components used for preparing the surface treatment liquid.

  The pH of the surface treatment liquid is preferably in the range of 2-3. If the pH is too low, elution of galvanizing during processing increases, and the corrosion resistance of the galvanized steel material decreases. When the pH is too high, film formation hardly occurs and the blackness tends to decrease.

  This surface treatment liquid may contain other components not containing hexavalent chromium in addition to the above components as long as a black rust preventive film having an L value of less than 15 can be formed on the surface of the zinc-based plating bath. Examples of such components include compounds for adjusting the pH of the liquid. In addition to the cobalt compound, a nickel compound may be used in combination.

  The treatment liquid described in Patent Document 3 above can further contain compounds of various metal species including colloidal silica and titanium trichloride in order to improve the corrosion resistance. Additional metal compounds. Such an additional metal compound can also be contained in the surface treatment liquid of the present invention, but depending on the type, the properties of the film may be slightly altered. For example, when colloidal silica is contained in the surface treatment liquid, the gloss of the film is slightly increased, but the high-grade feeling as a black film is somewhat lost. In the case of the surface treatment liquid of the present invention, the zinc-based plated steel material can be provided with excellent corrosion resistance similar to that of a chromate film without containing an additional metal compound such as colloidal silica.

  What is necessary is just to select content of the said each component in the surface treatment liquid of this invention so that the black membrane | film | coat excellent in corrosion resistance may be formed. The content of trivalent chromium ions in the treatment liquid is preferably in the range of 0.5 to 30 g / l, more preferably in the range of 1.5 to 20 g / l.

  Oxalic acid or other organic acid having chelating ability is contained in such an amount that the trivalent chromium compound and cobalt compound, and if present, other metal compounds do not precipitate. Usually, the molar ratio with respect to trivalent chromium ions is preferably in the range of 0.2 to 4, and this molar ratio is more preferably in the range of 0.4 to 2.

  The cobalt compound content is preferably in the range of 0.1 to 10 g / l as cobalt ions. This amount is more preferably 0.3 to 5 g / l and even more preferably 0.5 to 3 g / l.

As described above, the phosphate ion and the sulfate ion are contained in such an amount that the mass ratio of phosphate ion / sulfate ion becomes 1 or more and the pH of the surface treatment solution becomes a preferable value.
The surface treatment liquid of the present invention is used for reactive chemical conversion treatment of galvanized steel materials. Accordingly, the surface treatment can be carried out by immersing the zinc-based plated steel material in the surface treatment solution for a sufficient time to form a desired film and then drying.

  Treatment conditions such as immersion time (treatment time) and treatment temperature (surface treatment solution bath temperature) may be selected so as to form a rust-proof film with sufficient corrosion resistance and black appearance, depending on the composition of the surface treatment solution. However, the appropriate processing conditions are different. The surface treatment liquid may be stirred by an appropriate means (eg, rocking) during the immersion so that the film formation by the treatment proceeds uniformly throughout the galvanized steel material.

  In general, the surface treatment solution containing chromium chloride, which is a strong acid salt, is more likely to react than the surface treatment solution containing chromium acetate, which is a weak acid salt, so that the treatment time is shortened and / or The processing temperature can be lowered. For example, in the surface treatment liquid containing chromium chloride, it is preferable that the treatment time is 30 to 120 seconds and the treatment temperature is 20 to 70 ° C. On the other hand, in the case of a surface treatment liquid containing chromium acetate (or other trivalent chromium organic acid salt), the treatment time is preferably 60 to 180 seconds and the treatment temperature is 40 to 70 ° C. However, the treatment conditions may be outside this range as long as a black rust preventive film having an L value of less than 15 and excellent in rust prevention can be formed.

  The galvanized steel material to be treated is preferably pretreated in the same manner as in the chromate treatment. This pretreatment can be performed, for example, by first washing the zinc-based plated steel with water, then immersing it in an aqueous solution of a mineral acid such as hydrochloric acid, nitric acid, and sulfuric acid, activating the plating surface, and then washing with water again.

  The zinc-based plated steel material that has been dipped using the surface treatment liquid of the present invention is then preferably washed with water and then dried in order to remove excess components on the surface and form a normal film. This drying may be room temperature drying or heat drying.

  Thus, according to the present invention, the surface of the galvanized steel material has a black anticorrosive film formed by reactive dipping treatment using a treatment liquid containing a trivalent chromium compound and not containing hexavalent chromium. A zinc-based plated steel material is provided. Naturally, the black rust preventive film formed on the surface of the zinc-based plating does not contain hexavalent chromium and is a film containing trivalent chromium.

  The reason why this black rust preventive film is black was measured using an Auger spectroscopic analyzer and the results obtained were examined. As a result, it contained low-order oxides of zinc at the interface with the plating surface. It was concluded that a black layer had formed and that the entire coating appeared black because of the black layer at this plating interface (ie, the bottom layer of the coating).

As is well known, zinc oxide (ZnO) is a white substance, and the surface of the active zinc-based plating immediately oxidizes and becomes zinc oxide white (white rust is generated). However, in the case of the surface treatment solution of the present invention, zinc is only incompletely oxidized on the surface of the zinc plating, and remains in the state of a low-order zinc oxide (ZnO _x : x <0.8). And since the low-order zinc oxide contained in the lowermost layer of the film exhibits a black color, the entire film looks black.

The reason why this black zinc low-order oxide is formed on the plating surface is not completely understood, but the surface treatment solution of the present invention does not contain an oxidizing agent such as nitric acid and contains cobalt ions. Is considered to be related. That is, since there is no strong oxidizing agent, the zinc ions generated by the dissolution of zinc in contact with the acidic surface treatment liquid are not completely oxidized when they are deposited again on the plating surface as oxides. It is speculated that it remains in the state of lower oxides of zinc. It is known that a low-order zinc oxide in which _x of ZnO _x is less than 0.8 is black. In addition, it is considered that cobalt ions are involved in creating an environment in which a low-order zinc oxide is generated. Ions of nickel and iron, which are iron-based metals other than cobalt, are somewhat effective, but the effect of cobalt ions is the highest. In addition, the ratio of phosphoric acid and sulfuric acid and oxalic acid are also involved in the blackening of the film, but the details are unknown.

  As a result of Auger spectroscopic analysis, the insolubility of chromium and zinc on the first layer is formed on the black layer of the lowermost layer (first layer) mainly composed of low-order zinc oxide formed at the plating interface. A layer mainly composed of phosphate (second layer) and a chromium / phosphorus oxide layer (third layer) not containing zinc in the surface layer were formed. The film composed of this three-layer structure has, for example, a first layer having a thickness of 200 nm, a second layer having a thickness of 500 nm, and a third layer having a thickness of 200 nm. The thickness of the third layer was relatively small. In this three-layered film, corrosion resistance is ensured by the second layer and the third layer, and since the third layer does not contain zinc in particular, it is presumed that high corrosion resistance can be exhibited.

  However, as described above, the blackening of the film is a low-order oxide of zinc contained in the first layer formed at the plating interface, and if this black layer is formed at the plating interface, The upper layer may be a layer different from the above as long as it has corrosion resistance.

  The total thickness of the black rust preventive film is not particularly limited, but is preferably in the range of 300 to 5000 nm. Below 300 nm, the corrosion resistance is insufficient. Forming a thick film of 5000 nm or more requires a very long processing time and is very expensive.

  An overcoat layer may be formed on the black rust preventive film in order to further improve the corrosion resistance. This overcoat layer is conventionally formed on a chromate film or other chemical conversion treatment film, and can be formed using a known method or coating solution. The overcoat layer is preferably transparent so as not to impair the color of the black anticorrosive film below it.

  Specifically, the overcoat layer is made of an inorganic material using a metal oxide (or precursor thereof) or phosphate such as colloidal silica (silicate sol) or titania sol, and a thin resin film (eg, polyester). , An acrylic resin, an epoxy resin, a phenol resin, a polyurethane, a melamine resin, a fluororesin, and the like, and any of them may be used. Although there is no restriction | limiting in particular also about the thickness, Usually, it is about 0.1-30 micrometers. The overcoat layer is generally formed by application of a treatment liquid and drying, and the application may be performed by appropriate means such as dipping, spraying, roll application, etc. according to the shape of the steel material. Drying is usually heat drying.

  A cold rolled steel sheet or bolt was subjected to electrogalvanizing (film thickness: 8 μm) using a commercially available zinc or zinc-iron alloy plating solution. The electrogalvanization was performed using a zincate bath for steel plates and a chloride bath for bolts. Electro zinc-iron alloy plating was performed using a zincate bath. This plated material (steel plate or bolt) was washed with water, and then immersed in an acid aqueous solution containing 3 ml / L of 67.5% nitric acid at room temperature for 5 seconds for activation treatment. This activated plating material is washed with water, and then immersed and swung using the surface treatment liquid for forming the black rust preventive film of the present invention, which contains a trivalent chromium compound and does not contain hexavalent chromium. And finally washed with water. Table 1 shows the composition of the surface treatment liquid used and the treatment conditions (immersion treatment time and treatment bath temperature).

In this example, no overcoat layer was formed on the black rust preventive film.
As a comparative example, a blackening treatment was performed using a surface treatment liquid containing a trivalent chromium compound and not containing a hexavalent chromium compound but having a composition outside the scope of the present invention. Specifically, a surface treatment solution that does not contain any of oxalic acid, phosphoric acid, or cobalt compound, or has a phosphate ion / sulfate ion mass ratio of less than 1 was used. Table 2 shows the treatment liquid composition and treatment conditions.

  In Tables 1 and 2, “acid ratio” means a mass ratio of phosphate ion / sulfate ion. In addition to free sulfuric acid, the amount of sulfate ion is an amount including the amount of sulfate ion when the cobalt compound is cobalt sulfate.

Moreover, as a conventional example, the blackening process was performed using the well-known process liquid for blackening of the zinc-type plating surface. The composition of the treatment liquid and the treatment conditions are shown in Table 3.
The appearance (blackness) and corrosion resistance of the zinc-based plated material surface-treated as described above were evaluated as follows. The test results are also shown in Table 1 (Example), Table 2 (Comparative Example) and Table 3 (Conventional Example). In addition, when the galvanized material is a bolt, the appearance was evaluated with the bolt material. Used.

Appearance: Visually determined according to the following criteria. Only is a good black film.
A: Black (uniformly black),
○: Gray (including some black but not uniform),
Δ: Blackish candy-like substance is attached to the surface,
X: White to light yellow.

Lightness: Using a digital color meter TC-3600 manufactured by Tokyo Denshoku Co., Ltd., the L value (L ^* a ^* b ^* L ^* value in the color system) was measured, and the lightness was evaluated according to the following criteria:
A: L value <15
○: 15 ≦ L value <20
X: L value ≧ 20.

  Corrosion resistance: A salt spray test (SST) according to JIS Z 2371 was performed for 120 hours, and the white rust generation status (area ratio) after the test was determined according to the following criteria. The reason for setting the test time to 120 hours is to compare with the corrosion resistance of the chromate film in which white rust does not occur in the 96 to 120 hour salt spray test.

A: No white rust in 120 hours,
○: The area ratio of white rust generated in 120 hours is less than 10%.
×: White rust generation area rate in 120 hours is 10% or more.

  As shown in Table 3, in the conventional blackening treatment using molybdate or chlorate, an even black film was formed with an L value of less than 15. However, the film was extremely inferior in corrosion resistance.

  On the other hand, as shown in Table 1, in the examples in which the black rust preventive film was formed using the surface treatment liquid of the present invention, in all examples, the appearance was completely black as in the conventional example (that is, A beautiful black film having a high-class feeling with an L value of less than 15 was formed. More specifically, the L value ranged from 12 to 14, with a maximum of 13.8. Moreover, this black film also showed excellent corrosion resistance comparable to that of the chromate film. In addition, when the electrogalvanizing bath of the treatment material is a chloride bath, a black black rust-preventing film having an L value of 14 or less is formed exactly as in the case of the zincate bath. There was no difference in coloring.

  On the other hand, as shown in Table 2, in Comparative Examples 1 to 8, in which the composition of the surface treatment liquid to be used is outside the scope of the present invention, a completely black appearance is not obtained, and the corrosion resistance is also different in degree. Tended to decline. The results of these comparative examples also show that the appearance and the L value are well correlated.

  As another comparative example, a black rust preventive film was formed on the surface of a galvanized steel sheet according to Examples 1 to 6 of Patent Document 3 (Japanese Patent Laid-Open No. 2003-268562) described above. In Examples 1 to 6, surface treatment was performed on an electrogalvanized steel sheet using a zincate bath, but in this example, electrogalvanization was carried out by both a chloride bath and a zincate bath. The composition of the treatment liquid and the treatment conditions (temperature, time) were the same as those shown in Table 1 of this publication. However, for the treatment liquid containing oxalic acid, it was unclear whether this oxalic acid was an anhydride or a dihydrate. Therefore, when oxalic acid anhydride was used at the indicated concentration and oxalic acid 2 Two types of treatment liquids were prepared when the hydrate was used.

The appearance and lightness (L value) of the formed black rust preventive film were evaluated in the same manner as described above. The results were as follows.
(1) According to Examples 1 to 6 of this publication, when a galvanized steel sheet electrogalvanized in a zincate bath was surface-treated, in Examples 4 and 6, a black film having an L value of 15 was formed. The L value in the examples was 20 or more except that the L value was 18 in one example (both when oxalic acid is an anhydride and when it is a dihydrate) Including).

 (2) When the surface treatments of Examples 1 to 6 of this publication were applied to a galvanized steel sheet electrogalvanized with a chloride bath, completely different results were obtained from the case of the zincate bath. That is, even when the L value was the lowest, it was 22, and most of the L values exceeded 30. When the L value exceeded 30, the film was a pale yellow color or it was a dark color and not gray.

  From these results, the surface treatment liquid described in Patent Document 3 may be able to form a black rust-preventive film for the electrogalvanized steel material by the zincate bath, but the zinc-based plated steel material electrogalvanized by the chloride bath. On the other hand, a black rust preventive film cannot be formed, and it can be seen that there is a significant difference in the formed film between the zincate bath and the chloride bath. That is, this surface treatment liquid cannot be applied to a galvanized steel material that has been electrogalvanized using a chloride bath.

Claims (8)

  An acidic surface treatment liquid for forming a black anticorrosive film on the surface of a zinc-based plated steel material, containing a trivalent chromium compound, a cobalt compound, an organic acid capable of forming a chelate, phosphoric acid, and sulfuric acid, A surface treatment solution comprising an acidic aqueous solution not containing hexavalent chromium and nitrate ions, wherein a mass ratio of phosphate ions / sulfate ions in the aqueous solution is 1 or more.   The surface treatment liquid according to claim 1, wherein the organic acid is oxalic acid.   The surface treatment liquid of Claim 1 or 2 whose pH of a liquid is the range of 2-3.   A zinc-based plated steel material, wherein the zinc-based plated steel material is dipped in the surface treatment solution according to any one of claims 1 to 3 and then dried to form a black anticorrosive film on the plated surface. Blackening treatment method.   The method for blackening a galvanized steel material according to claim 4, wherein an organic or inorganic overcoat layer is formed on the formed black rust preventive film. Black with a lightness index (L ^* a ^* b ^* L ^* value of the color system) of less than 15 that contains a trivalent chromium compound and does not contain hexavalent chromium on the electrogalvanized surface using a chloride bath. Blackened zinc-based plated steel with an antirust coating.   A black color having a lightness index of less than 15 and not containing hexavalent chromium formed on the surface of the zinc-based plating by immersion treatment using the surface treatment liquid according to any one of claims 1 to 3 Blackened zinc-based plated steel with an antirust coating. The blackened zinc-based plated steel material according to claim 6 or 7, further comprising a colorless inorganic or organic overcoat layer on the black anticorrosive film.