Triazinethiol-containing anticorrosive coating composition, anticorrosion treatment method and anticorrosion treated metal materials

Abstract: A solution and a process for non-chromium rust proof coating is disclosed which has superior corrosion resistance to a rust proof solution including chromium. The rust proof coating solution includes a composite of which main components are an aqueous resin and water, with 0.2-20g/l of triazine thiol compound and 0.1/5g/l phosphoric acid contained therein. The rust proof process is a process for coating a zinc-coated steel or a non-coated steel with the rust proof coating solution as described above.

Description: BACKGROUND OF THE INVENTION: Field of the Invention: This invention relates to a triazinethiol-containing anticorrosive coating composition, anticorrosion treatment method, and anticorrosion-treated metal material, and, more specifically, to a non-chromium anticorrosive coating composition with superior anticorrosion ability to that of known chromium-containing anticorrosion agents, to a corresponding anticorrosion treatment method, and to a corresponding anticorrosion-treated metal material. Description of the Related Arts: When zinc coated steel plate or alloyed zinc coated steel plate are exposed to an atmosphere containing saline or brine, or are exposed to high temperature and high humidity, white rust forms on the surface. This white rust greatly damages both the outward appearance and the ability to prevent corrosion of the base iron surface. To prevent white rust, chromium based anticorrosion agents are widely used. For example, Japanese Laid-open Publication No. Hei 3-131370 discloses a resin type treatment agent wherein a water dispersible chromium compound and water dispersible silica are incorporated in an olefin-α, β-ethylenic unsaturated carboxylic acid polymer resin dispersion. However, even this type of chromium-containing resin type treatment agent does not necessarily have sufficient anticorrosion properties, and, if exposed to brine or to a high temperature and high humidity atmosphere for an extended time, white rust forms even when this treatment agent is used. Also, it has recently been understood that chromium is highly toxic, and consequently, the use of chromium on equipment or in places where coated objects come in direct contact with workers has been limited. The demand for non-chromium anticorrosion treatment agents has therefore been increasing, leading the inventors in the present application to find that sulfide ion reacts with zinc so as to form a stable ZnS coating, and their invention of a non-chromium anticorrosion treatment agent using sulfides and sulfur, as dislosed in Japanese Laid-open Publication No. Hei 8-239776 and Japanese Laid-open Publication No. Hei 8-67834. However some sulfides emit a peculiar odor, and their handling is not always easy. An anticorrosion treatment agent using a triazinethiol compound containing sulfur atoms, and without any foul odor or toxicity, has also been proposed. For example, the "water-soluble anticorrosion coating" of Japanese Laid-open Publication No. Sho 53-31737 discloses a water-soluble anticorrosion coating to which a dithiol-S-triazine derivative is added. Also, the "metal-reactive emulsion" of Japanese Laid-open Publication No. Sho 61-223062 discloses a metal-reactive emulsion obtained by mixing a triazinethiol compound with an organic compound that is difficultly soluble or insoluble in water. However the water-soluble anticorrosion coating disclosed in Japanese Laid-open Publication No. Sho 53-31737 is intended to prevent corrosion of soft steel, copper, brass, or copper wire, and is prepared in order to adsorb particularly well to a base material of copper or brass. This agent is therefore inadequate as an anticorrosion agent for zinc or other metal surfaces. Also, the reactive emulsion disclosed by Japanese Laid-open Publication No. Sho 61-223062 reacts with copper, nickel, tin, cobalt, aluminum, and their alloys, and was inadequate as an anti-corrosion agent for zinc and other metal surfaces. WO 97/22423 and the corresponding European patent application EP-A-0 870 549 which represent state of the art according to Article 54(3, 4) EPC disclose a treatment liquid for forming a rust-preventive organic layer. This treatment liquid comprises a layer forming resin dissolved or dispersed in an aqueous medium and a colloid or micelle of a hardly water-soluble organic corrosion inhibitor dispersed in said aqueous medium. The above-mentioned hardly water-soluble organic corrosion inhibitor can be selected from seven different types of compounds, inter alia derivative of triazinethiol. This document further teaches that in preferred embodiments at least one acid selected from ortho-phosphoric acids, polyphosphoric acids and meta-phosphoric acids can be added as passivating layer forming aid. In the examples, 20 g/l ortho-phosphoric acid were used. SUMMARY OF THE INVENTION: This invention, which was conceived in view of the above problems, aims to provide a non-chromium type anticorrosive coating composition and anticorrosion treatment method having superior anticorrosion properties to those of chromium-containing anti-agents. It further provides a non-chromium corrosion-protected metal material having superior anticorrosion properties. In order to achieve the aforesaid objectives, the anticorrosive coating composition according to this invention comprises 0.2-20 g/l of a triazinethiol compound and 0.1-5 g/l of phosphate ion contained in a composition comprising an water-soluble or water-dispersible resin and water as its principal components. The anticorrosion treatment method according to this invention is a method of coating the aforesaid anticorrosion agent onto zinc coated steel or uncoated steel. The anticorrosion-treated metal materials according to this invention are metal materials which have been coated with the aforesaid anticorrosive coatings. Generally, in order for an anticorrosion treatment coating agent to be effective, it must possess the following properties: (1) prevents permeation of corrosive liquids, (2) forms an anticorrosive film which adsorbs strongly to the base metal, (3) causes passivation of the base metal due to anticorrosion ions, and (4) forms an anticorrosive film which is water-resistant, acid-resistant and alkali-resistant. An agent that is inadequate in any of these properties does not have sufficient anticorrosion properties. Conventional chromium-type anticorrosion agents are superior as regards (3) passivation properties. In this context, the meaning of passivation is that a metal or alloy becomes inert, even when placed in an environment which is chemically or electrochemically active from a thermodynamic or chemical viewpoint. In the same way as chromic acids, sulfides easily adsorb to metal surfaces, and, as they have excellent oxidizing power, they can also be used to render metal surfaces passive. Triazinethiol compounds, which are a type of sulfide, therefore have anticorrosion properties with respect to zinc coated metals. When triazinethiol is added together with phosphoric acid ions (PO 4 3- ) to an anticorrosive coating composition comprising an aqueous resin, its anticorrosion properties increase remarkably, and an anticorrosive coating composition superior to conventional chromium-containing resin anticorrosion agents is thereby obtained. This enhancement comes as a result of synergistic effects of the triazinethiol compound and phosphate ion. The actual mechanism of these effects is thought to be as follows. (1) thiol base ions in triazinethiol adsorb to active sites on a zinc surface when the anticorrosion agent is applied, and exhibit an anticorrosion effect. Triazinethiol cannot adsorb to inactive sites on the zinc surface (e.g. where oxide is present), however phosphoric acid ions react with these inactive sites forming zinc phosphate so that these sites become active. Triazinethiol then adsorbs to these activated sites also, thereby exhibiting an enhanced anticorrosion effect. (2) The triazinethiol compound and phosphate ion both act to promote cross-linking of the resin film. As a result of these two synergistic effects, there are fewer micropores in the resin film, and water or harmful ions such as chlorine ion are effectively blocked. This invention will next be described in more detail, with the triazinethiol-containing anticorrosive coating composition according to this invention first being described. The triazinethiol compound has the following chemical structure: In the above general formula, R is -NR' 2 , -NHR', or SA; R' is hydrogen or a lower alkyl group comprising 1-5 carbon atoms, phenyl, benzyl, naphthyl or cyclohexyl; which groups may be substituted with one or more groups selected from -OH, -COOH, and -NH 2 ; and A is sodium, potassium, hydrogen, or an amine group. Typical triazinethiol compounds are 2,4,6-trimercapto-s-triazine, 2,4,6-trimercapto-s-triazine monosodium salt, 2,4,6-trimercapto-s-triazine trisodium salt, 2-dibutylamino-4,6-dimercapto-s-triazine, and 2-anilino-4,6-dimercapto-s-triazine. Of the above compounds, those which are not directly soluble in water were first dissolved in an alkaline solution, and then incorporated in the anticorrosive coating composition. Any water-soluble or water-dispersible resin may be used as the resin of this invention. Specific examples are polyolefin resins, polyurethane resins, acryl resins, polycarbonate resins, polyester resins, alkyd resins, phenol resins and other heat-curing resins. Resins capable of cross-linking are particularly preferred. The aforesaid resins may be blended in any desired proportion; two or more of the resins may also be blended together. The anticorrosive coating composition of the present invention comprises 1-80 weight parts of an water-soluble resin or water-dispersible resin expressed as solids, 99-20 weight parts of water, 0.2-20 g/l but more preferably 0.8-5 g/l of a triazinethiol compound relative to coating agent, and 0.1-5 g/l of phosphate ion relative to coating agent. The pH is adjusted to be in the range 3-12. Herein, when the amount of triazinethiol compound is less than 0.2 g/l, anticorrosion properties are inadequate. Conversely, when the amount is greater than 20 g/l, not only is there no enhancement of anticorrosion properties so that there is no economic advantage, but also depending on the type of aqueous resin used, the resin may gelate so that coating is impossible. The phosphate ions form a layer of phosphate on the surface of the base metal, passivate the metal and enhance cross-linking reactions of the water-soluble or water-dispersible resin in the resin film. Due to the fineness of the anticorrosion film which is thereby formed, anticorrosion properties are enhanced. When the amount of phosphate ion is less than 0.1 g/l, corrosion protection is inadequate. When on the other hand this amount exceeds 5 g/l, anticorrosion properties decline, the resin gels, and the stability of the anticorrosive coatings in storage is poorer. Anticorrosion additives may also be added to the anticorrosive coating composition of this invention. One example of such an anticorrosion additive is water-dispersible silica. When water-dispersible silica is added to the anticorrosive coating composition of this invention, drying properties, anti-scratch properties and coating adhesion are improved. There is no particular limitation on the aforesaid water-dispersible silica provided that it does not contain sodium or other impurities, and provided that it is weakly alkaline. Examples include a commercial silica gel or commercial aerosil of powdered silica particles such as "Snowtex N" (Nissan Chemical Industries) or "Adelite AT-20N" (Asahi Electrochemical Industries). The amount of the aforesaid water-dispersible silicas is preferably in the range 2-600 g/l relative to anticorrosive coatings. When the amount of water-dispersible coating agent is less than 2 g/l, corrosion protection is inadequate, whereas when this amount exceeds 600 g/l, anticorrosion properties decline. Additionally, the anticorrosive coating composition according to the present invention may be blended with other components, such as, for example, a pigment or surfactant. Silane coupling agents may also be blended in order to improve affinity between the water-soluble or water-dispersible resin, and silica particles or pigment, and to improve adhesion between the water-soluble or water-dispersible resin and zinc or iron phosphate layer. The aforesaid pigment may for example be an inorganic pigment such as titanium oxide (TiO 2 ), zinc oxide (ZnO), zirconium oxide (ZrO), calcium carbonate (CaCO 3 ), barium sulfate (BaSO 4 ), alumina (Al 2 O 3 ), kaolin clay, carbon black, or iron oxide (Fe 2 O 3 , Fe 3 O 4 ), or a coloring pigment, including organic pigments and the like. The aforesaid silane coupling agent may, for example, be γ-aminopropyltrimethoxysilane, γ-aminopropyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane, γ-methacryloxypropyltriethoxysilane, or N-[2-(vinylbenzylamino)ethyl]-3-aminopropyltrimethoxysilane. A solvent may also be added to the anticorrosive coating composition of this invention so as to improve the film-forming properties of the water-soluble or water-dispersible resin, and form a more uniform, smoother film. Any solvent generally used in paints may be used, including, for example, an alcohol, ketone, or ether. According to this invention, the aforesaid anticorrosive coating composition may be used as a treatment to protect zinc coated steel or uncoated steel from corrosion. In this treatment, the anticorrosion agent of the invention may be applied to an object to be coated, and the object may then be heated and dried in a current of warm air. Alternatively, the object to be coated may first be heated, the anticorrosion agent of this invention applied to the object while it is hot, and the surplus heat used to dry the object. In either of the aforesaid methods, the heating temperature is 50-250°C. When the temperature falls below 50°C, the water evaporation rate is slow and adequate film-forming properties are not obtained, so corrosion protection is poor. Conversely, when the temperature exceeds 250°C, the water-soluble or water-dispersible resin thermally decomposes, salt-spray test (SST) performance and water resistance decline, and the surface color changes to yellow. The temperature must therefore lie within the above range, and more preferably within the range 70-100°C. When the object is heated and dried in a current of warm air after coating, the drying time is preferably 1 second - 5 minutes. In the aforesaid anticorrosion treatment, the coating thickness of the anticorrosion agent of this invention after drying is preferably equal to or greater than 0.1 µm. When it is less than 0.1 µm, corrosion protection is inadequate. Conversely when the dry film thickness is too thick, coating of the substrate is uneconomical and is also difficult to perform. The thickness therefore preferably lies in the range 0.1-20 µm, and more preferably in the range 0.1-10 µm. However when the coating is used as an aqueous anticorrosion paint, the film thickness may be equal to 0.1 µm or more. There is no particular limitation on the method used to apply the anticorrosive coating composition of this invention in the aforesaid anticorrosion treatment. This may done by any of the methods generally used such as roll coat, air spray, airless spray or dipping. The material coated by the anticorrosive coating composition of this invention may be zinc coated steel or uncoated steel, as stated hereabove. The anticorrosive coating composition of this invention may be used as a paint undercoat and aqueous anticorrosion paint, as mentioned hereabove, or as a primary anticorrosive agent. DESCRIPTION OF ACTUAL EXAMPLES: Next, this invention will be described more specifically by means of examples and comparative examples. However, it shall be understood that these examples are given to help illustrate and explain the invention, and are not intended to limit the invention in any way. Examples 1-9 and Comparative Examples 1-5 (1) Test method: a) SST test (SST performance) 5% brine was sprayed on a coated surface at 35°C, and the extent of white rust was evaluated on a 10 point scale after 48 hours. The following point scale was used: 10 points: No abnormality 9 points: (Between 10 and 8) 8 points: Slight white rust 7-6 points: (Between 8 and 5) 5 points: White rust over half the surface area 4-2 points: (Between 5 and 1) 1 point: White rust over entire surface b) Hot water test (humidity test) The extent of white rust was evaluated on a 10 point scale after immersion in hot water at a constant temperature of 40°C for 20 days. The following point scale was used: 10 points: No abnormality 9 points: (Between 10 and 8) 8 points: Slight swelling in coating film 7-6 points: (Between 8 and 5) 5 points: Swelling over half the surface area 4-2 points: (Between 5 and 1) 1 point: Swelling over entire surface (2) Anticorrosion treatment conditions Example 1 2 g/l of 2,4,6-trimercapto-s-triazine and 1.25 g/l ammonium phosphate were dissolved in pure water, a polyolefin resin as described below) was added to give a 20% weight parts concentration expressed as resin solids, and 25 g/l of "Snowtex-N" (Nissan Chemical Industries) was added. The mixture was dispersed with stirring for 30 minutes in a Disper, and the pH was adjusted to 8.0 so as to obtain an anticorrosive coating composition. The anticorrosive coating composition obtained was coated by bar coater No. 5 onto a commercial hot dipp galvanized steel (Z-27, Japan Test Panel Inc., 70x150x1.6mm) that had been preheated (i.e. heated before coating) to a plate temperature of 80°C, so that the dry coating film had a thickness of 2-3 µm. After the surface of the hot dipp galvanized steel was polished with Scotchbrite, it was alkali degreased ("Surfcleaner 53", Nippon Paint Co., Ltd.), washed with water, and dried. Evaluation as explained above was then performed. The evaluation results are shown in Table 1. Example 2 The same procedure as that of Example 1 was followed, except that the addition concentration of the triazinethiol compound was 20 g/l, and the addition concentration of phosphate ion was 0.1 g/l. The test results are shown in Table 1. Example 3 The same procedure as that of Example 1 was followed, except that the addition concentration of the triazinethiol compound was 0.2 g/l, and the addition concentration of phosphate ion was 5.0 g/l. The test results are shown in Table 1. Examples 4-9 The same procedure as that of Example 1 was followed, except that the type of aqueous resin, type of triazinethiol compound, their concentrations, and the concentration of phosphate ion, were varied. The test results are shown in Table 1. Comparative Example 1 The same procedure as that of Example 1 was followed, except that 100 weight parts of the polyolefin resin in Example 1 below expressed as solids, 70 weight parts of "Snowtex N" expressed as solids and 5 weight parts of strontium chromate were blended together. The test results are shown in Table 1. Comparative Examples 2-6 The same procedure as that of Example 1 was followed, except that the concentration of the triazinethiol compound and the addition concentration of phosphate ion were varied. The test results are shown in Table 1. From these results, it was found that anticorrosion properties and corrosion protection were improved by the triazinethiol-containing anticorrosive coatings and anticorrosion treatment of this invention as compared to the conventional agents and methods. As described above, according to the triazinethiol-containing anticorrosive coating composition and anticorrosion treatment agent of this invention, by blending a triazinethiol and phosphate ion which have low toxicity with an water-soluble or water-dispersible resin, superior anticorrosion properties to those of conventional chromate-containing aqueous resin anticorrosion agents are obtained. This invention therefore provides an environmentally safe, non-chromate anticorrosive coating composition with excellent anticorrosion properties. On materials coated with the triazinethiol-containing anticorrosive coating composition according to this invention, a film having superior anticorrosion properties is formed, and corrosion is therefore suppressed.

Claims: An anticorrosion coating composition comprising as main components a water-soluble or water-dispersible resin and water, with 0.2-20g/l of triazinethiol compound and 0.1-5g/l phosphate ion contained therein, both relative to the anticorrosion coating composition. The anticorrosive coating composition as defined in Claim 1, wherein said triazinethiol compound has the following chemical formula (I): wherein, in the aforesaid general formula, R is -NR' 2 , - NHR' or SA, R' is hydrogen or a lower alkyl group comprising 1-5 carbon atoms, phenyl, benzyl, naphthyl or cyclohexyl; which groups may be substituted with one or more groups selected from -OH, -COOH, and -NH 2 ; and A is sodium, potassium, hydrogen, or an amine group. The anticorrosive coating composition as defined in Claim 2, wherein said triazinethiol compound is at least of 2,4,6-trimercapto-s-triazine, 2,4,6-trimercapto-s-triazine monosodium salt, 2,4,6-trimercapto-s-triazine trisodium salt, 2-dibutylamino-4,6-dimercapto-s-triazine, and 2-anilino-4,6-dimercapto-s-triazine. The anticorrosive coating composition as defined in Claim 1, wherein said resin source is one or more resin sources chosen from the group including polyolefin resin, polyurethane resin, acryl resin, polycarbonate resin, polyester resin, alkyd resin, phenol resin, and heat-cured resin. The anticorrosive coating composition as defined in Claim 4, wherein said resin may be blended in any desired proportion, or comprises two or more resins which may be blended in any desired proportion. The anticorrosive coating composition as defined in any of Claims 1-5, wherein said resin comprises 1-80 weight parts expressed as solids, 99-20 weight parts of water, and further comprises 0.2-20 g/l of said triazinethiol compound and 0.1-5 g/l phosphoric acid ion relative to said anticorrosive coating composition. The anticorrosive coating composition as defined in any of Claims 1-5, wherein said resin comprises 1-80 weight parts expressed as solids, 99-20 weight parts of water, and further comprises 0.8-5 g/l of said triazinethiol compound and 0.1-5 g/l phosphoric acid ion relative to said anticorrosive coating composition. The anticorrosive coating composition as defined in any of Claims 1-7, wherein the pH is adjusted to 3-12. The anticorrosive coating composition as defined in any of Claims 1-7, wherein an anticorrosion additive is added. The anticorrosive coating composition as defined in Claim 9, wherein said anticorrosion additive is water-dispersible silica. The anticorrosive coating composition as defined in claim 10, wherein the amount of said water-dispersible silica is 2-600 g/l relative to said anticorrosive coating composition. The anticorrosive coating composition as defined in any of Claims 1-7, further comprising at least one of a pigment, surfactant, or silane coupling agent. The anticorrosive coating composition as defined in Claim 12, wherein said pigment is at least one inorganic pigment such as titanium oxide (TiO 2 ), zinc oxide (ZnO), Zirconium oxide (ZrO), calcium carbonate (CaCO 3 ), barium sulfate (BaSO 4 ), alumina (Al 2 O 3 ), kaolin clay, carbon black, or iron oxide (Fe 2 O 3 , Fe 3 O 4 ), or an organic coloring pigment. The anticorrosive coating composition as defined in claim 12, wherein said silane coupling agent is, for example, at least one of γ-aminopropyltrimethoxysilane, γ-aminopropyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane, γ-methacryloxypropyltriethoxysilane, or N-[2-(vinylbenzylamino)ethyl] -3-aminopropyltrimethoxy-silane. An anticorrosion treatment method whereby the anticorrosion coating composition of claim 1 is applied to a zinc-coated steel or a non-coated steel. The anticorrosion treatment method as defined in Claim 15, wherein said anticorrosive coating composition is coated on an object to be coated and said object is heated and dried in a current of warm air after coating. The anticorrosion treatment method as defined in Claim 15, wherein the object to be coated is heated first, said anticorrosive coating composition is applied while said object is hot, and said object is dried by excess heat. The anticorrosion treatment method as defined in Claim 16 or 17, wherein said heating temperature is 50-250°C. An anticorrosion treatment method as defined in any of Claims 15 to 17, wherein the coating thickness of said anticorrosive coating composition is such that the dry film thickness is equal to or greater than 0.1 µ m. An anticorrosion treatment method as defined in any of Claims 15 to 17, wherein the coating thickness of said anticorrosive coating composition is such that the dry film thickness is in the range 0.1-20 µ m. An anticorrosion treatment method as defined in any of Claims 15 to 17, wherein the coating thickness of said anticorrosive coating composition is such that the dry film thickness is in the range 0.1-10 µ m. A metal which has been coated with the anticorrosion coating composition of claim 1.

Inventors: ONODERA YOUSUKE, SHIMAKURA TOSHIAKI, ONODERA, YOUSUKE, SHIMAKURA, TOSHIAKI
Applicants: NIPPON PAINT CO LTD, NIPPON PAINT CO., LTD.
Application year: 1998
Main classification: C09D5
IPC codes: C23F11, B05D7, B05D3, C09D5