DESCRIPTIC»T AQUEOUS COATING COMPOSITION AND RUST PREVENTION METHOD USING THE AQUEOUS COATING COMPOSITION 5 Technical Field [0001] The present invention relates to aqueous coating composition, andmoreparticularlyto an aqueous coating composition (aqueous binder composition), that is a binder composition, or an aqueous coating 10 composition (aqueous rust-preventive coating composition), that is a rust-preventive coating composition, [0002] Further, the present invention relates to a rust prevention coating method for a base such as a steel material, using the aqueous 15 coating composition such as an aqueous rust-preventive coating composition. BackgroTUid Art [0003] 20 Conventional solvent-based coating compositions include various volatile organic solvents (organic solvents) in order to improve dispersibility of a binder such as a polymer or in order to control viscosities of the compositions. In the preparation of a polymer-based component of a binder, an organic solvent is sometimes SF-2426 2 used as a solvent. On that account, the organic solvent is sometimes included together with the polymer-based component that is a binder in the coating compositions. Also in coating compositions including a hydrolysis condensate 5 (polyorganosiloxane) of a silane compound or a metal oxide as a binder, solvent-based compositions are the mainstream coating compositions. For example, a solvent-based rust-preventive coating composition including tetraethyl silicate (binder), zinc dust (rust-preventive pigment) and a large amount of an organic solvent is known, 10 In such a solvent-based coating composition, the organic solvent can dissolve the binder homogeneously, and therefore, excessive thickening or occurrence of a non-dissolved substance (gel) can be reduced. That is to say, it is thought that the organic solvent contributes to inhibiting local deterioration reaction or 15 decomposition reaction of the coating composition, said reactionbeing liable to take place in the excessively thickened portion or the non-dissolved substance (gel). Thus, the organic solvent can chemically stabilize the coating composition. Moreover, the organic solvent has a function of maintaining the low viscosity of the 20 composition by virtue of dilution effect, and hence, application of the composition can be easily carried out by using publicly known various methods. Further, when the solvent-based coating composition is applied to a base, the organic solvent vaporizes efficiently from the composition applied, and hence, the coating composition is cured SF-2426 3 in a relatively short time to form a coating film. [0004] The solvent-based coating compositions including a hydrolysis condensate of a silane compound as a binder are favorably used as 5 solvent-based binder compositions or solvent-based rust-preventive coating compositions. Uses of these compositions extend over many divergencies. [0005] Examples of main uses of the solvent-based binder compositions 10 include uses as coating agents used for surfaces of bases, such as mica plate, glass laminated sheet, glass sleeve, rubber, plastic and leather, binders for asbestos, anti-scattering curing agents or surface reinforcing agents for asbestos-containing spray materials, anti-scattering primary curing agents used in removal of 15 asbestos-containing spray materials, metal-based rust-preventive pigments or pollution inhibitors or surface protective agents, stone material-based antifouling treatment agents or water repellants, heat-resistant coating materials, additives therefor, and binders and additives for various coating materials. 20 [0006] Examples of main uses of the solvent-based rust-preventive coating compositions including a binder include uses as coating compositions for primary rust prevention and thick-coating type inorganic zinc coating compositions (dry film thickness: e.g., 50 SF-2426 4 to 200 pm). [0007] The primary rust-preventive coating compositions are used for the purpose of temporarily preventing rusting on the surface of steel 5 used for automobiles, domestic electrical appliances, large steel structures (ships, bridges, plants, etc.)A containers for transportation, land tanks, etc., during manufacturing or construction. Examples of the primary rust-preventive coating compositions include organic primary rust-preventive coating 10 compositions, such as wash primer, non-zinc epoxy primer and epoxy zinc-rich primer, and inorganic zinc primary rust-preventive coating compositions. Of these, inorganic zinc primary rust-preventive coating compositions having excellent weldability are widely used. [0008] 15 The thick-coating type inorganic zinc coating compositions are used for cargo tanks and ballast tanks of ships such as chemical tankers and ships for exclusive use of methanol. These cargo tanks and ballast tanks are required to have resistance to cargos such as petroleum refining products and chemical products, resistance to corrosion by 20 tank wash water (fresh water, seawater), mechanical properties that can sufficiently cope with strain of a steel plate caused by water pressure during pouring of ballast water or expansion and contraction of a steel material caused by temperature change, etc., and the thick-coating type inorganic zinc coating compositions exert these SF-2426 5 excellent properties. The thick-coating type inorganic zinc coating compositions are favorably used particularly for ballast tanks that are required to have long-teim resistance to corrosion by seawater. The solvent-based thick-coating type inorganic zinc coating 5 compositions including polyorganosiloxane are inferior to epoxy resin-based coating compositions in workability, properties such as coating film strength and flexibility, and characteristics such as long-term corrosion resistance. On that account, the epoxy resin-based coating compositions are widely used as the thick-coating 10 type inorganic zinc coating compositions at present. The solvent-based coating compositions including a metal oxide as a binder are also used as various coating agents though they are not used so much as the coating compositions including polyorganosiloxane as a binder 15 [0009] On the other hand, VOC countermeasures (reduction of environmental burden) have been recently required in many countries. On that account, non-solvent or low-solvent coating compositions have been desired. 20 As a means to obtain the non-solvent or low-solvent coating compositions, for example, technique to obtain high solid coating compositions, such as increase in solids content of a binder such as a polymer or use of a reactive monomer as a diluent, has been proposed. Further, technique to obtain aqueous coating compositions SF-2426 6 utilizing a method of using, as a diluent, water or a surface active agent instead of an organic solvent (water dispersion method, such as emulsifying method or suspending method) has been studied. In this method, the surface active agent has functions of facilitating 5 dispersing of a binder in water and favorably maintaining the dispersed state for a certain period of time. The coating composition obtained by dispersing the binder in water by a compulsive method such as a method of using mechanical pressure in the presence of a surface active agent as above has good stability in the points that the degree of 10 sedimentation of components such as a binder in the coating composition is low and the degree of thickening of the coating composition is low, but when a coating film is formed from the composition, a surface active agent remains in the film, and hence, topcoating adhesion properties are sometimes deteriorated. Accordingly, the binder is 15 desired to be water-soluble so that no surface active agent shall need to be used. On the other hand, such a coating composition is required to have, in addition to the water solubility, properties such as good curability and drying properties when the composition is formed into 20 a film, but compatibility of these properties is extremely difficult. Further, a coating film formed from the composition is also required to have not only good water resistance, corrosion resistance and topcoating adhesion properties but also good weldability and cutting properties, but compatibility of them is also difficult. SF-2426 7 In the existing circumstances, the coating compositions having aqueous system (aqueous coating compositions) cannot exhibit properties equal to those of the solvent-based coating compositions. Such aqueous coating compositions have been disclosed in patent 5 literatures 1 to 9. The coating compositions of the patent literatures enumerated below can be broadly divided into compositions in which the binder is a hydrolysis condensate (polyorganosiloxane) of a silane compound (patent literatures 1 to 6) and compositions in which the binder is 10 a metal oxide sol (patent literatures 7 to 9). In the patent literatures 1 to 5, the binder included in the coating compositions is a hydrolysis condensate prepared from an alkoxysilane, and in the patent literature 6, the binder included in the coating composition is a hydrolysis condensate prepared from an amino group-containing 15 silane coupling agent (amino group-containing silane compound). In the patent literature 1 (JP-A-2002-121485), an aqueous composition for anticorrosive coating, which comprises organic functional silane including an alkoxy group and capable of being diluted with water and a wetting agent and which is used as a rust 20 preventive for a metal substrate such as a steel fastener, has been disclosed. In the patent literature 2 ( JP-A-2000-144020), a rust-preventive coating agent in which phosphoric acid, phosphorous acid or hypophosphorous acid ion, and a sulfur-containing compound SF-2426 8 comprising a thiocarbonyl compound, a thiol compound, a sulfide or persulfuric acid ion are included in an aqueous solution including a silane coupling agent (e.g., specifically, various alkoxysilanes) and its hydrolysis condensate has been disclosed. 5 In the patent literature 3 (JP-A-2002-105401), an aqueous two-component type protective coating agent for steel materials, which includes a first component of water dispersibility including a binder obtained by reacting an amino group-containing alkoxysilane with an epoxy group-containing alkoxysilane in the presence of a relatively 10 strong acid, and a pigment, and a second component comprising finely pulverized zinc dust, has been disclosed. In the patent literature 4 (JP-A-2008-528741), a siloxane polymer (binder composition) having N-formyl group or the like, which is obtained by the reaction of an amino group-containing alkoxysilane 15 with a formic acid ester, and a rust-preventive coating composition including the binder composition and an anticorrosive pigment such as zinc have been disclosed. In the patent literature 5 (WO2008/003695), a two-component type coating composition including, in an aqueous solution, (a) an 20 aqueous binder (first component) including a polymer or oligomer compound formed from a tetra-substituted alkoxysilane compound, a functional tri-substituted alkoxysilane compound, and if necessary, a colloidal silica water dispersing agent, and (b) a reactive filler (second component) such as zinc dust has been disclosed. SF-2426 9 In the patent literature 6 (JP-A-2008-150537), a coating composition including, as a binder, a hydrolysis condensate prepared by the reaction of an amino group-containing silane coupling agent (amino group-containing silane compound) has been described, and 5 specifically, an aqueous rust-preventive coating material including a water-soluble or hydrolyzable amino group-containing silane coupling agent as a binder and a metal pigment comprising at least one of tin and tin-containing alloys having white rust prevention properties and zinc has been disclosed. 10 Next, examples using a metal oxide sol as a binder are described. In the patent literature 7 (JP-B-1981-29904), an inorganic film-forming composition comprising water- dispersible colloidal silica, urea or thiourea, water-soluble lithiumpolysilicate and zinc dust has been disclosed. 15 In the patent literature 8 (WO2008/128932), a coating composition including i) zinc dust and/or zinc alloy and ii) colloidal silica having been surface-treated with a silane compound under the specific conditions has been disclosed. In the patent literature 9 (JP-B-2005-510584) , an aqueous shop 20 primer composition including an aqueous silica sol as a binder, and zinc dust or zinc alloy has been disclosed. Citation List Patent Literature SF-2426 10 [0010] Patent literature 1: JP-A-2002-121485 Patent literature 2: JP-A-2000-144020 Patent literature 3: JP-A-2002-105401 5 Patent literature 4: JP-A-2008-528741 Patent literature 5: WO2008/003695 Patent literature 6: JP-A-2008-150537 Patent literature 7: JP-B-1981-29904 Patent literature 8: WO2008/128932 10 Patent literature 9: JP-A-2005-510584 Smmnaxy of Invention Technical Problem [0011] 15 In any of the aqueous coating compositions of these patent literatures, however, there are problems of various properties, for example, a problem of drying properties such that long time curing or heating is necessary, as described below, and the range of use applications is limited. 20 First, when the binder is a hydrolysis condensate of an alkoxysilane compound, a by-product formed after the hydrolysis is an alcohol, and the by-product has low corrosion properties and irritation properties and has high volatility, so that it can be easily removed by distillation or the like, and the hydrolysis condensate SF-2426 11 can be easily purified, as described in the patent literatures 1 to 5. Further, there is an advantage that the hydrolysis condensate can be prepared by a reaction container or a reaction device in which storage or hydrolysis reaction is simply carried out, but in obtaining 5 a hydrolysis condensate of a high molecular weight, the degree of freedom in the design of a product is not so high because the reactivity of the silane compound is low. That is to say, in the case of hydrolysis reaction of an alkoxysilane compound only, the number of alkoxysilyl groups undergoing reaction in one molecule is small, and therefore, 10 there are problems in the molecular design such that increase of a molecular weight is difficult (number-average molecular weight: about several hundred) and even if a co-hydrolyzate having a narrow molecular weight distribution is intended to be prepared from several kinds of alkoxysilanes as starting substances, the molecular weight 15 distribution is widened and the ratio of constituent units of the co-hydrolyzate largely differs from the blending ratio of the staring substances. Hence, it is difficult to obtain a co-hydrolyzate having a desired molecular structure. Further, because of such problems, any of the resulting aqueous coating compositions cannot acquire 20 well-balanced excellent properties of drying properties (curing properties), water resistance, corrosion resistance, coating film strength, etc. Furthermore, the aqueous rust-preventive coating compositions (aqueous coating compositions for primary rust prevention, aqueous thick-coating type inorganic zinc coating SF-2426 12 compositions) including such a binder can exhibit only insufficient properties in not only the above properties but also rust prevention properties, properties of topcoating film (topcoating properties) and weldability. 5 When the binder is a hydrolysis condensate of an amino group-containing alkoxysilane, the amino group actions as a reaction catalyst to thereby excessively promote hydrolysis condensate-forming reaction in the preparation of the binder (hydrolysis condensate), as disclosed in the patent literature 3. 10 On that account, there is a possibility of occurrence of troubles such as gelation during the reaction or storage. Thus, it is difficult to control a molecular weight of the hydrolysis condensate obtained by the use of the amino group-containing alkoxysilane. Moreover, when the molecular weight is too high, it is difficult to disperse 15 the resulting hydrolysis condensate in water. On the other hand, when the molecular weight is low, or when the concentration of a binder component, which is obtained by promoting the hydrolysis reaction in a low concentration of raw material silane in order to reduce occurrence of the gelation, is low, there occurs a problem such that 20 the resulting hydrolysis condensate cannot exhibit satisfactory curing properties/drying properties and water resistance of a coating film. When the binder is not a hydrolysis condensate of an alkoxysilane compoundbut a hydrolysis condensate of amino group-containing silane. SF-2426 13 the interaction between an amino group, which is derived from the amino group-containing silane and is present in the binder, and a rust-preventive pigment such as zinc becomes strong, as described in the patent literature 6. It is said that when the coating object 5 is a steel material or the like, an iron atom is ionized to form rust, but zinc is ionized by an acid, and the zinc ion formed inhibits ionization of the iron atom to thereby exhibit rust prevention action. The amino group derived from the amino group-containing silane and included in the binder undergoes interaction with zinc (it is thought 10 that zinc that is an amphoteric metal probably forms a complex or a substance analogous to it together with the amino group) to thereby inhibit efficient ionization of zinc, and as a result, the possibility of impairing the rust prevention action of zinc becomes high. That is to say, the hydrolysis condensate that is obtained from the amino 15 group-containing alkoxysilane in order to impart water solubility has an advantage of good water solubility, but because of use of the amino group-containing alkoxysilane, the rust prevention effect that is the original object is lowered, far from being improved. Hence, it cannot be necessarily said that such composition is effective. 20 It is thought that when the binder is an inorganic oxide sol, there is room for improvement in properties, such as drying properties, curing properties, water resistance, anticorrosion properties and coating film strength, as disclosed in the patent literatures 6 to 9. SF-2426 14 Particularly when a surface of a coating film (coating layer) formed from such an aqueous coating composition including an alkali metal silicate (mixture of silicon oxide and alkali metal oxide) as disclosed in the patent literature 7 is further coated with a topcoating 5 material to form a topcoating film, various problems attributable to the action (alkalinity) of an alkali metal remaining on the coating layer surface occur. For example, a blister of the topcoating film occurs, or when zinc is included in the aqueous coating composition, white rust due to corrosion of zinc occurs. Because of such problems, 10 removal work to remove the coating film of the aqueous coating composition from base is separately needed prior to coating with the topcoating material. Presence of this removal work is very disadvantageous because one step surely increases in the topcoating process. Moreover, for customers who generally use a method in which 15 a coating material that is a high-performance coating composition and can inhibit occurrence of white rust is preferentially adopted and the removal work is not adopted, the above composition cannot be said to be a composition sufficiently satisfying the customers' requirements. 20 Also in the case where the binder is water glass or an aqueous silica sol such as a colloidal silica water dispersing agent (e.g., patent literatures 8 and 9) and pH of the binder is on the alkaline side because an excess ammonium compound or a large amount of an alkali metal oxide is included in the binder, the same problems as above SF-2426 15 occur. In order to cope with such conventional problems, the present inventors have earnestly studied, and as a result, they have found that the conventional problems can be solved by using, as a binder 5 in a coating composition, a hydrolysis condensate obtained by subjecting a specific silane compound (as raw material silane), such as acetoxysilane, ketoxime silane or chlorosilane which is rather avoided because it generates irritant and corrosive hydrogen chloride gas during the reaction though its reactivity is extremely high, to 10 hydrolysis reaction and condensation reaction under the specific pH conditions. Thus, the present invention has been accomplished. That is to say, it is an object of the present invention to provide an aqueous coating composition capable of exerting an effect that the composition, while using water as a diluting solvent, can 15 form a coating film having well-balanced excellent drying properties, curing properties and coating film properties (coating film strength) nearly equal to those of a coating film formed from a conventional solvent-based coating composition using an organic solvent as a diluent, and said effect being unable to be achieved by conventional aqueous 20 coating compositions. In particular, there can be provided a binder composition and a rust-preventive coating composition each of which can be expected to exhibit interaction between a residual functional group of a hydrolysis condensate and a metal oxide sol when the composition SF-2426 16 includes the hydrolysis condensate obtained by subjecting a specific silane compound such as chlorosilane to hydrolysis reaction and condensation reaction in the presence of a metal oxide sol, and each of which is capable of forming a coating film having higher strength. 5 Solution to Problem [0012] The aqueous coating composition of the present invention comprises a binder (A) and water (B), wherein the binder (A) comprises 10 a hydrolysis condensate obtained by subjecting a silane raw material (a) including a silane compound (al) represented by the following formula (I) to hydrolysis reaction and condensation reaction under the conditions of pH of 0.4 to 8.0; [0013] 15 R^^SiR^^R^^- (Ym-SiR^^R^^) p-R^^ (I) [0014] wherein R'"'^ to R^^ are each independently a hydrogen atom, a halogen atom, an alkyl group of 1 to 10 carbon atoms, an alkenyl group of 2 to 5 carbon atoms, an aryl group of 6 to 12 carbon atoms, Z-R^^- 20 (Z is a halogen atom, a hydroxyl group, an epoxy group, an acryloxy group, a methacryloxy group or a polyoxyalkylene group represented by R^O-(R^O)c- (R^ is a hydrogen atom or an alkyl group of 1 to 10 carbon atoms, R^ is an alkylene group of 2 to 4 carbon atoms, and c is an integer of 1 to 15), and R^^ is an alkylene group of 1 to SF-2426 17 10 carbon atoms), or -OR (R is a hydrogen atom, an alkyl group of 1 to 10 carbon atoms or an alkenyl group of 1 to 10 carbon atoms), at least one of R"^^ to R^^ is a halogen atom, [0015] 5 Y is an oxygen atom or an alkylene group of 1 to 10 carbon atoms, [0016] m is 0 or 1, and p is an integer of not less than 0. In the aqueous coating composition of the present invention, the silane raw material (a) preferably comprises a silane compound 10 (a2) represented by the following general formula (II) in addition to the silane compound (al); [0017] R^'^SiR^'^R^''- (Y'n>--SiR''V^)p'-R* (II) [0018] 15 wherein R''''^ to R^ are each independently a hydrogen atom, an alkyl group of 1 to 10 carbon atoms, an alkenyl group of 2 to 5 carbon atoms, an aryl group of 6 to 12 carbon atoms, Z-R^'^- (Z is a hydroxyl group, an epoxy group, an acryloxy group, a methacryloxy group or a polyoxyalkylene group represented by R^'o- (R^'O) c'- (R^' is a hydrogen 20 atom or an alkyl group of 1 to 10 carbon atoms, R'^' is an alkylene group of 2 to 4 carbon atoms, and c' is an integer of 1 to 15), and R^"^ is an alkylene group of 1 to 10 carbon atoms), -OR®'' (R^*^ is an alkyl group of 1 to 10 carbon atoms or an alkenyl group of 1 to 10 carbon atoms), a hydroxyl group or an acyl group. SF-2426 18 at least one of R to R is a hydroxyl group, a -OR group (R^'^ is an alkyl group of 1 to 10 carbon atoms), Z-R^''- (Z is an epoxy group, and R''^ is an alkylene group of 1 to 10 carbon atoms), or an acyl group, 5 [0019] Y' is an oxygen atom or an alkylene group of 1 to 10 carbon atoms, [0020] m' is 0 or 1, and p' is an integer of not less than 0. 10 In the aqueous coating composition of the present invention, the binder (A) preferably comprises a hydrolysis condensate and a metal oxide represented by the following chemical fonnula (III), wherein the hydrolysis condensate is obtained by sub j ecting the silane raw material (a) to hydrolysis reaction and condensation reaction 15 in the presence of a metal oxide sol (C) comprising the metal oxide; [0021] MpOr (III) wherein M is Si, Al or Ti, and p and r are each independently an integer of 1 to 3. 20 In the aqueous coating composition of the present invention, at least one of R'^^ to R^^ in the formula (I) is preferably a chlorine atom from the viewpoints that it is excellent in hydrolysis reactivity and dehydrocondensation reactivity and an aqueous coating composition having desired effects is finally obtained. SF-2426 19 [0022] In the aqueous coating composition of the present invention, at least one of R^'" to R^*" in the general formula (II) is preferably a glycidoxy group from the viewpoints of vyater solubility and storage 5 stability. [0023] In the aqueous coating composition of the present invention, Y in the formula (I) is preferably an alkylene group of 1 to 10 carbon atoms. 10 [0024] In the aqueous coating composition of the present invention, the metal oxide sol (C) is preferably a silicic anhydride sol. [0025] In the aqueous coating composition of the present invention, 15 the silicic anhydride sol is preferably a fumed silica sol. [0026] In the aqueous coating composition of the present invention, it is preferable that the metal oxide sol (C) is a colloidal silica water dispersing agent, and the colloidal silica water dispersing 20 agent has pH of not more than 7 and a Na20 content of not more than 400 ppm. [0027] In the aqueous coating composition of the present invention, the metal oxide sol (C) is preferably a sol comprising fumed alumina SF-2426 20 or fumed titania. [0028] It is preferable that the aqueous coating composition o£ the present invention further comprises a pigment and a ratio 5 ((PVC) / (CPVC)) of a pigment volume concentration (PVC) to a critical pigment volume concentration (CPVC) in the aqueous coating composition is preferably higher than 1. [0029] The aqueous coating composition of the present invention 10 preferably further comprises a rust-preventive pigment (D). [0030] In the aqueous coating composition of the present invention, the rust-preventive pigment (D) preferably includes zinc powder and/or zinc alloy powder having a mean particle diameter of 2 to 20 yiia. 15 [0031] The aqueous coating composition of the present invention preferably further comprises metallic molybdenum and/or a molybdenum compound as a white rust inhibiter (E). [0032] 20 The aqueous coating composition of the present invention is preferably used as a binder composition, [0033] The aqueous coating composition of the present invention is preferably used as a rust-preventive coating composition. SF-2426 21 [0034] The aqueous coating composition of the present invention is preferably a coating composition for primary rust prevention. [0035] 5 The aqueous coating composition of the present invention is preferably a thick-coating type inorganic zinc coating composition. [0036] The primary rust prevention coating method for a steel material of the present invention comprises applying the coating composition 10 for primary rust prevention to a surface of a steel material and then curing the applied coating composition to form a primary rust-preventive coating film. [0037] The rust prevention coating method for a steel material of the 15 present invention comprises applying the thick-coating type inorganic zinc coating composition to a surface of a steel material and then curing the applied coating composition to form a thick-coating type inorganic zinc rust-preventive coating film. [0038] 20 The steel structure of the present invention has, on a surface of a steel material, a primary rust-preventive coating film formed from the coating composition for primary rust prevention. [0039] The steel structure of the present invention has, on a surface SF-2426 22 of a steel material, a thick-coating type inorganic zinc rust-preventive coating film formed from the thick-coating type inorganic zinc coating composition. 5 Advantageous Effects of Invention [0040] The aqueous coating composition of the present invention can exhibit an effect that the composition, while using water as a diluting solvent, can form a coating film having well-balanced excellent drying 10 properties, curing properties and coating film properties (coating film strength) nearly equal to those of a coating film formed from a conventional solvent-based coating composition using an organic solvent as a diluent, and said effect being unable to be achieved by conventional aqueous coating compositions. 15 [0041] When the aqueous coating composition of the present invention is used as a rust-preventive coating composition, the rust-preventive coating composition can exhibit an effect that the composition, while using water as a diluting solvent, provides a rust-preventive coating 20 film which has both of excellent weldability and excellent cutting properties nearly equal to those of a coating film formed from a conventional solvent-based rust-preventive coating composition, while maintaining rust prevention properties inherent in a rust-preventive pigment such as zinc, and said effect being unable SF-2426 23 to be achieved by coating films formed from conventional aqueous coating compositions. [0042] The rust prevention method of the present invention can form, 5 on a surface of a base such as a steel material, a rust-preventive coating film which has both of excellent weldability and excellent cutting properties nearly equal to those of a rust-preventive coating film formed from a solvent-based rust-preventive coating composition, while maintaining rust prevention properties inherent in a 10 rust-preventive pigment such as zinc. Brief Description of Drawings [0043] Fig. 1 is a view showing an IR chart (after dehydration) of 15 a binder composition of Example 1, Fig. 2 is a view showing a GPC chart (after dehydration) of a binder composition of Example 1. Fig. 3 is a view showing a Si^^NMR chart of a binder composition of Example 1. 20 Fig. 4 is a view showing an IR chart (after dehydration) of a binder composition of Example 2. Fig. 5 is a view showing a GPC chart (after dehydration) of a binder composition of Example 2. Fig. 6 is a view showing an IR chart (after dehydration) of SF-2426 24 a binder composition of Example 3. Fig. 7 is a view showing a GPC chart (after dehydration) of a binder composition of Example 3. Fig. 8 (a) is a group of views showing sandblasted plates (upper 5 plate and lower plate) (steel plates for welding test and coated portions) used in a weldability test of a topcoating film formed from a primary rust-preventive coating composition of an example or a comparative example, and Fig. 8(b) is a group of views showing an embodiment of welding (outline of welding method) carried out in a 10 weldability test of a topcoating film. Description of Embodiments [0044] The agueous coating composition of the present invention 15 comprises a binder (A) and water (B), wherein the binder (A) comprises a hydrolysis condensate obtained by subjecting a silane raw material (a) comprising the below-described specific silane compound (al) to hydrolysis reaction and condensation reaction under the conditions of pH of 0.4 to 8.0. 20 [0045] Binder (A) The binder (A) comprises a hydrolysis condensate obtained by subjecting a silane raw material (a) comprising a silane compound (al) represented by the following general formula (I) to hydrolysis SF-2426 25 reaction and condensation reaction under the conditions of pH of 0.4 to 8.0. Here, the binder (A) may comprise, in addition to the hydrolysis condensate, arbitrary components except water, when needed, or may be a binder composed of the hydrolysis condensate only. 5 [0046] The silane raw material (a) may comprise, in addition to the silane compound (al), such silane compounds other than the silane compound (al) as described later (silane compound (a2), silane compound (a3), etc.), when needed. 10 [0047] R^^SiR^^R^^- (yn,-SiR^^R^^)p-R^^ (I) [0048] In the formula (I), R'^^ to R^^ are each independently a hydrogen atom, a halogen atom, an alkyl group of 1 to 10 carbon atoms, an alkenyl 15 group of 2 to 5 carbon atoms, an aryl group of 6 to 12 carbon atoms, Z-R^^- (Z is a halogen atom, a hydroxyl group, an epoxy group (e.g., glycidoxy group, 3, 4-epoxycyclohexyl group or the like), an acryloxy group, a methacryloxy group or a polyoxyalkylene group represented by R^O-(R''0)C- (R^ is a hydrogen atom or an alkyl group of 1 to 10 20 carbon atoms, R'^ is an alkylene group of 2 to 4 carbon atoms, and c is an integer of 1 to 15) , and R^^ is an alkylene group of 1 to 10 carbon atoms), or -OR^^ (R^^ is a hydrogen atom, an alkyl group of 1 to 10 carbon atoms or an alkenyl group of 1 to 10 carbon atoms), and SF-2426 26 at least one of R'^^ to R^^ is a halogen atom. [0049] Y is an oxygen atom or an alkylene group of 1 to 10 carbon atoms. [0050] 5 m is 0 or 1, and p is an integer of not less than 0. [0051] The silane raw material (a) may be one composed of a silane compound (al) only (that is, in 100 parts by weight of the silane raw material (a), the silane compound (al) maybe included in an amount 10 of 100 parts by weight) , as described above. However, in order that (1) rapid progress of hydrolysis condensation reaction caused by excessively high reactivity between the silane compound (al) and water and formation of water-insoluble products such as a gel-like substance may be prevented, or in order that (2) appropriate reactivity may 15 be obtained and introduction of an appropriate functional group may be achieved, or from the viewpoint that (3) a compound having appropriate molecular weight and structure can be obtained and an aqueous coating composition capable of forming a coating film that can finally exhibit desired effects (e.g., improvement in adhesion 20 to base and coating film strength) is obtained, the silane raw material preferably comprises the later-described silane compound (a2) in addition to the silane compound (al). [0052] From the viewpoints of improvement in dispersibility of the SF-2426 27 binder (A) and a pigment in each other, reinforcement of a coating film formed from the coating composition and cost reduction by decreasing the amount of a relatively expensive silane compound used, the binder (A) preferably comprises a hydrolysis condensate and the 5 metal oxide, wherein the hydrolysis condensate obtained by subjecting the silane raw material (a) to hydrolysis reaction and condensation reaction in the presence of the later-described metal oxide sol (C). [0053] When the silane raw material (a) includes the silane compound 10 (al) only, the resulting hydrolysis condensate is a single hydrolysis condensate having constituent units derived from the silane compound (al) . When the silane raw material (a) includes the silane compound (a2), etc., in addition to the silane compound (al), the resulting hydrolysis condensate is a co-hydrolysis condensate having 15 constituent units derived from the silane compounds (al) and (a2), etc. When the hydrolysis condensate is prepared by subjecting the silane raw material (a) to hydrolysis/condensation reaction in the presence of the later-described metal oxide sol (C), the binder (A) is a composite substance comprising the resulting hydrolysis 20 condensate and a metal oxide that is included in the metal oxide sol (C). [0054] The preparation product comprising the hydrolysis condensate prepared is usually a transparent aqueous solution (in the case where SF-2426 28 the hydrolysis condensate is water-soluble) or a white opaque water dispersion (in the case where the hydrolysis condensate is slightly water-soluble and water-dispersible). [0055] 5 In the hydrolysis reaction and the condensation reaction of the silane raw material (a), pH of the system is 0.4 to 8.0, and from the viewpoint that drying properties and curability of the coating film and coating film properties (coating film strength) of the dry coating film are further improved, pH is preferably 0.7 to 6.0, more 10 preferably 0.9 to 5.0. Here, these pH conditions indicate that the silane raw material (a) undergoes hydrolysis/condensation reaction at pH of the above specific range constantly through the reaction, and mean that the pH does not deviate from the above range from the beginning to the end of the reaction. 15 [0056] In the case where the binder (A) comprises a metal oxide and a hydrolysis condensate obtained by subjecting the silane raw material (a) to hydrolysis reaction and condensation reaction in the presence of the later-described metal oxide sol (C), and in the case where 20 the pH in the hydrolysis reaction and the condensation reaction of the silane raw material (a) deviates from the above range and is lower than the lower limit, aggregation of colloidal particles of the metal oxide tends to take place. Also in the case where the pH deviates from the above range and is higher than the upper limit, aggregation SF-2426 29 of colloidal particles of the metal oxide tends to take place. If the pH in the hydrolysis reaction and the condensation reaction of the silane raw material (a) deviates from the above range, aggregation of colloidal particles of the metal oxide tends to take place, as 5 described above, and this phenomenon is attributed to lowering of stability (i.e., storage stability) of colloidal particles of the metal oxide, and as a result, stability (i.e., storage stability) of the binder is sometimes deteriorated. [0057] 10 The present invention is described below by taking, as an example, a process in which the silane raw material (a) comprising methyl trichlorosilane as the silane compound (al) and comprising y-glycidoxypropyltrimethoxysilane as the later-described silane compound (a2) is subjected to hydrolysis reaction and condensation 15 reaction to form a binder (A) that is a hydrolysis condensate. The reactions indicated by the following chemical reaction formulas (i) to (iii) correspond to reactions in the later-described Example 2. [0058] As indicated by the following chemical reaction formulas (i) 20 and (ii), methyltrichlorosilane and y-glycidoxypropyltrimethoxysilane are each hydrolyzed to form a silanol group (Si-OH group). It is thought that these hydrolysis reactions do not proceed at once but proceed stepwise generally. [0059] SF-2426 30 CI + H2O OH C I — S i - C H g - ^ - ^ HO-Si-CHs+3HCI (i) CI OH 9*^2 CH2 ? o (")) is usually 140 to 500,000, preferably 200 to 100,000, more preferably 300 to 30,000. When the molecular weight is in such 20 a range, the hydrolysis condensate can exhibit, as a binder, satisfactory level of drying properties and curability of a coating film while maintaining solubility in water. [0072] When the reaction product comprising the hydrolysis condensate SF-2426 36 comprises a metal oxide derived from the metal oxide sol (C), the mean particle diameter of a composite substance comprising the metal oxide and the hydrolysis condensate (measurement by dynamic light scattering method (specifically, see'M4) Measurement of mean particle 5 diameter" of the later-described "")) is usually 1 to 500 nm, preferably 4 to 300 nm, more preferably 5 to 200 nm. When the mean particle diameter is in such a range, the composite substance stably exists as colloidal particles, and there is no fear of gelation of the hydrolysis condensate and 10 sedimentation of the components, so that the composite substance exhibits good dispersibility as a binder. Moreover, drying properties and curability of a coating film formed from the coating composition are good. The composite substances comprise not only a substance (mixture) in which the metal oxide and the hydrolysis 15 condensate are simply mixed but also a substance in which the metal oxide andthe hydrolysis condensate are chemically bondedor physically bonded. [0073] (Silane compound (al), silane compound (a2) and metal oxide sol (C) ) 20 The silane compounds (al) to (a4) and the metal oxide sol (C) are described below. [0074] (Silane compound (al)) The silane compound (al) included in the silane raw material SF-2426 37 (a) is representeci t»y the following general formula (I) (structural formula: the following structural formula (I'))- [0075] RlasjR2apj3a.^Y^.5Jp,4ap5aj^.R6a (|) R"-si4-Y„-si-4-R=" (r) 5 [0076] In the general formula (I) and the structural formula {I'), R-^^ to R^^ are each independently a hydrogen atom, a halogen atom, an alkyl group of 1 to 10 carbon atoms, an alkenyl group of 2 to 5 carbon atoms, an aryl group of 6 to 12 carbon atoms, Z-R''^- (Z is 10 a halogen atom, a hydroxyl group, an epoxy group (e.g., glycidoxy group, 3,4-epoxycyclohexyl group or the like), an acryloxy group, a methacryloxy group or a polyoxyalkylene group represented by R^O-(R''0)C- (R^ is a hydrogen atom or an alkyl group of 1 to 10 carbon atoms, R^ is an alkylene group of 2 to 4 carbon atoms, and c is an 15 integer of 1 to 15) , and R^^ is an alkylene group of 1 to 10 carbon atoms) , or -OR®^ (R®^ is a hydrogen atom, an alkyl group of 1 to 10 carbon atoms or an alkenyl group of 1 to 10 carbon atoms), and at least one of R-^^ to R^^ is a halogen atom. [0077] 20 Y is an oxygen atom or an alkylene group of 1 to 10 carbon atoms, SF-2426 38 [0078] m is 0 or 1, and p is an integer of not less than 0. At least one of functional groups (R-^^ to R^^ in the general formula (I) and the structural formula (I')) directly bonded to Si 5 atoms is a halogen atom that is a hydrolyzable group, so that the silane compound (al) has good reactivity and is easily available. [0079] Y is an oxygen atom or an alkylene group of 1 to 10 carbon atoms, and from the viewpoints of easy availability and good reactivity, 10 Y is preferably an alkylene group of 1 to 10 carbon atoms, more preferably an alkylene group of 2 to 5 carbon atoms. [0080] From the viewpoints of excellent hydrolysis reactivity and dehydrocondensation reactivity, it is preferable that in the silane 15 compound (al), the functional groups (R""-^ to R^^ in the general foimula (I) and the structural formula (I') ) directly bonded to Si atoms are each independently a halogen atom (preferably chlorine atom), an alkyl group of 1 to 10 carbon atoms or -OR®^ (R®^ is an alkyl group of 1 to 10 carbon atoms), at least 3 of the functional groups (R'^^ to R^^ 20 in the general formula (I) and the structural fo3:mula (I') ) directly bonded to Si atoms are each a halogen atom (preferably chlorine atom), andp is an integer of 0 to 4 (preferably 0 or 1, particularly preferably p=0) . When p is an integer of 1 or 2, it is preferable that m is 1 and Y is an alkylene group of 1 to 10 carbon atoms (preferably an SF-2426 39 alkylene group of 2 to 5 carbon atoms). [0081] Examples of the silane compounds (al) represented by the following general formula (I) (structural formula: the following 5 Structural formula (I')) are given below. [0082] Examples of Chlorosilanes include tetrachlorosilane, trichlorosilane, methyltrichlorosilane, ethyltrichlorosilane, vinyltrichlorosilane, n-propyltrichlorosilane, 10 i-propyltrichlorosilane, n-butyltrichlorosilane, sec-butyltrichlorosilane, t-butyltrichlorosilane, n-pentyltrichlorosilane, n-hexyltrichlorosilane, n-octyltrichlorosilane, methyldichlorosilane, dimethyldichlorosilane, methylethyldichlorosilane, 15 methylvinyldichlorosilane, dimethylchlorosilane, diethylchlorosilane, trimethylchlorosilane, vinyldimethylchlorosilane, vinyldiethylchlorosilane, allyltrichlorosilane, methylallyldichlorosilane, Y-acryloxypropyltrichlorosilane and 20 y-methacryloxypropyltrichlorosilane. [0083] The silane compounds (al) may be silane compounds wherein a chlorine atom in the above chlorosilanes is replaced with fluorine, bromine or iodine, that is, fluorosilanes, bromosilanesor iodosilanes. SF-2426 40 such as trimethylfluorosilane, trimethylbromosilane or trimethyliodosilane. The silane compounds wherein a chlorine atom in the above chlorosilanes is replaced with a fluorine atom, a bromine atom or an iodine atom are on the market. 5 [0084] A silane compound (al) having an alkoxy group aS a functional group directly bonded to a Si atom together with the halogen atom is also useful, and examples of such silane compounds (al) include methyldichloromethoxysilane, methylchlorodiethoxysilane, 10 methyldimethoxychlorosilane, methyldi-i-propoxychlorosilane, ethyl-n-butoxydibromosilane, dimethoxychlorosilane, Y-chloropropyltrimethoxysilane, ychloropropyltriethoxysilane, y-chloropropyltripropoxysilane, y-chloropropylmethyldimethoxysilane, 15 y-chloropropylmethyldiethoxysilane, y-chloropropyldimethylethoxysilane, y-chloropropylethyldimethoxysilane and y-chloropropylethyldiethoxysilane. [0085] 20 As the silane compounds (al), polysilanes such as disilanes (in the general formula (I) and the structural formula (I'), m is 0 and p is an integer of not less than 1) are also useful. Examples of such polysilanes include alkylene group-containing disilyl compounds, e.g., disilanes, such as trimethyltrichlorodisilane. SF-2426 41 dimethyltetrachlorodisilane, trimethyltrimethoxydisilane and dimethyltetramethoxydisilane. Further, disilylethanes and disilylpropanes, which ai^^ produced by the addition reaction of disilane compounds with silane 5 compounds such as vinylchlorosilane, allylchlorosllane and hydrogen atom-containing chlorosilane using platinum or a platinum compound as a catalyst, as indicated by the following formulas, are also useful compounds as the silane compounds (al). In the following chemical formulas, description of functional groups directly bonded to Si atoms 10 other than -CH=CH2, -CH2-CH=CH2, -CH2-CH2- and -CH2-CH2-CH2- is omitted. [0086] sSi-CH=CH2+H-Si= - =Si-CH2-CH2-Si= (Platinum catalyst) HSi-CH2-CH=CH2+H-Si= -* =Si-CH2-CH2-CH2-Si= 15 (Platinum catalyst) [0087] Examples of the disilane compounds as the silane compounds (al) include disilylalkyls (e.g., disilylethanes, disilylpropanes and disilylbutanes). 20 [0088] Examples of the disilylethanes include compounds, such as Cl3SiCH2CH2SiCl3, (CH30)2ClSiCH2CH2SiCl3, (CH3O) 3SiCH2CH2SiCl3, CI (CH3O) 2SiCH2CH2Si (CH3O) CI2, CI (C2H5O) 2SiCH2CH2Si (C2H5O) CI2, (CH3O) 3SiCH2CH2Si (C2H5O) 3, CI2 (CH3) SiCH2CH2SiCl3, SF-2426 42 (CH3O) zClSiCHzCHsSi (CH3) CI2, (CH3O) 2 (CH3) SiCH2CH2Si (CH3) CI2, Cl2(C6H5)SiCH2CH2SiCl3, (CH30)2(C6H5) SiCH2CH2Si (CH30)Cl2 and CI2 (CgHs) SiCH2CH2Si (C2H5O) 3. [0089] 5 Examples of t h e d i s i l y l p r o p a n e s i n c l u d e compounds, such as (CgHs) (CH3) ClSiCH2CH2CH2Si (CH3) CI2, (C2H5O) 2 (CH3) SiCH2CH2CH2Si {CH3) CI2, (C2H5O) 3SiCH2CH2CH2Si (CH3) CI2, (CH3O) 3SiCH2CH2CH2Si (C2H5O) 3, (C2H50)3SiCH2CH2CH2Si(C2H50)Cl2, (C2H50)3SiCH2CH2CH2SiCl3, Cl3SiCH2CH2CH2SiCl3, (CH3O) 3SiCH2CH2CH2SiCl3, 10 CI (CH3O) 2SiCH2CH2CH2Si (CH3O) CI2, CI (C2H5O) 2SiCH2CH2CH2Si (C2H5O) CI2, (CH3O) 3SiCH2CH2CH2Si (C2H5O) 3, CI2 (CH3) SiCH2CH2CH2SiCl3, (CH3O) 2ClSiCH2CH2CH2Si (CH3) CI2, (CH3O) 2 (CH3) SiCH2CH2CH2Si (CH3) CI2, CI2 (CeHs) SiCH2CH2CH2SiCl3, (CH3O) 2 (CeHs) SiCH2CH2CH2Si (CH3O) CI2 and CI2 (CeHs) SiCHzCHzCHzSi (C2H5O) 3. 15 [0090] Examples of t h e d i s i l y l b u t a n e s i n c l u d e compounds, such as (CH3O) 2ClSiCH2CH2CH2CH2SiCl3, {C2H5O) 3SiCH2CH2CH2CH2SiCl3, CI (CH3O) 2SiCH2CH2CH2CH2Si (CH3O) CI2, (C2H5O) 3SiCH2CH2CH2CH2Si (C2H5O) CI2, (CH3O) 3SiCH2CH2CH2CH2Si (C2H5O) 3/ (C2H5O) 3SiCH2CH2CH2CH2Si (CH3) CI2, 20 (C2H50)2(CH3)SiCH2CH2CH2CH2Si(CH3)Cl2 and (CgHs) (CH3) ClSiCH2CH2CH2CH2Si (CH3) CI2. [0091] By t h e p r e p a r a t i o n u s i n g a s i l a n e compound h a v i n g a l o n g er a l k y l e n e group t h a n t h a t of t h e a f o r e s a i d u n s a t u r a t e d g r o u p - c o n t a i n i ng SF-2426 43 silane, a disilyl compound including an alkylene group of a longer chain is obtained. [0092] Examples of the silane compounds (al) have been enumerated above, 5 and of these, chlorosilanes (that is, at least ong of R^^ to R^^ in the general formula (I) and the structural formula (I') is a chlorine atom) are preferable when it is taken into consideration that drying properties and curing properties of a coating film of the resulting coating composition and coating film strength and rust prevention 10 properties of a dry coating film formed from the composition are good, that they are easily available and that handling thereof is also relatively easy. Of the chlorosilanes, more preferable are tetrachlorosilane and methyltrichlorosilane. [0093] 15 (Silane compound (a2)) The silane raw material (a) may comprise a silane compound (a2) representedby the following general fojnnula (II) (structural formula: the following structural formula (II')) in addition to the silane compound (al), 20 [0094] SF-2426 44 plb5Jp2bp3b_(Y'^.-SiR4^R^^)p.-R^^ (II) [0095] In the above formulas (the general formula (II) and the structural formula (II') ), R^'' to R^ are each independently a hydrogen 5 atom, an alkyl group of 1 to 10 carbon atoms, an alkenyl group of 2 to 5 carbon atoms, an aryl group of 6 to 12 carbon atoms, Z-R^''- (Z is a hydroxyl group, an epoxy group (e.g., glycidoxy group, 3,4-epoxycyclohexyl group or the like), an acryloxy group, a methacryloxy group or a polyoxyalkylene group represented by 10 R^'o- (R^'O) c'- (R^' is a hydrogen atom or an alkyl group of 1 to 10 carbon atoms, R^' is an alkylene group of 2 to 4 carbon atoms, and c' is an integer of 1 to 15), and R^*^ is an alkylene group of 1 to 10 carbon atoms), -OR^'^ (R^'' is an alkyl group of 1 to 10 carbon atoms or an alkenyl group of 1 to 10 carbon atoms), a hydroxyl group or an acyl 15 group, at least one of R"'''' to R^^ is a hydroxyl group, a -OR^"^ group {R^^ is a alkyl group of 1 to 10 carbon atoms) , Z-R'''^- (Z is an epoxy group, and R^'^ is an alkylene group of 1 to 10 carbon atoms), or an acyl group. 20 [0096] SF-2426 45 Y' is an oxygen atom or an alkylene group of 1 to 10 carbon atoms. [0097] m' isOorl, andp' is an integer of not less than 0 (for example, 5 m' is 1, and p' is an integer of 0 to 10 (preferably 0 to 5)). [0098] As examples of such silane compounds (a2), various silane compounds are enumerated below, and of these, silane compounds (a2) having an epoxy group (in the general formula (II) and the structural 10 formula (II') , at least one of R^'^ to R* is Z-R^"^- (Z is an epoxy group, and R^'^ is an alkylene group of 1 to 10 carbon atoms) ) are preferable as the silane compounds (a2) because they have good water solubility and storage stability. The epoxy group is more preferably a glycidoxy group (that is, silane compounds (a2) having a glycidoxy group is 15 more preferable). [0099] When coating film strength of a dry coating film formed from a coating composition comprising the resulting binder (A) and drying properties (curing properties) of a coating film formed from the 20 coating composition are taken into consideration, y-glycidoxypropyltrimethoxysilane is more preferable. [0100] Examples of the silane compounds (a2) having a hydroxyl group (-0H) include trimethylsilanol, triethylsilanol, SF-2426 46 triisopropylsilanol and tri-n-butylsilanol. [0101] Examples of the silane compounds (a2) having an alkoxy group (-OR, R: alkyl group of about 1 to 5 carbon atoms) include 5 trimethoxysilane, triethoxysilane, tri-n-propoxysilane, tri-i-propoxysilane, n-butoxysilane, methyltrimethoxysilane, methyltriethoxysilane, methyltriisopropoxysilane, methyltributoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, ethyltriisopropoxysilane, 10 vinyltrimethoxysilane, vinyltriethoxysilane, vinyltriisopropoxysilane, dimethyldimethoxysilane, dimethyldiethoxysilane, dimethyldiisopropoxysilane, dimethylmethoxyethoxysilane, dimethylmethoxyisopropoxysilane, methylethyldimethoxysilane, methylethyldiethoxysilane, 15 trimethylmethoxysilane, triethyltriethoxysilane, triisopropylmethoxysilane, vinyldimethylmethoxysilane, allyltrimethoxysilane and allylmethyldiethoxysilane. [0102] Examples of the silane compounds {a2) having an epoxy group 20 (e.g., glycidoxy group, 3, 4-epoxycyclohexylgrouporthelike) include Y-glycidoxypropyltrimethoxysilane, y-glycidoxypropyltriethoxysilane, y-glycidoxypropyltri-i-propoxysilane, y-glycidoxypropylmethyldimethoxysilane. SF-2426 47 Y-glycidoxypropylmethyldiethoxysilane, Y-giycidoxypropyldimethylethoxysilane, Y-glycidoxypropylethyldimethoxysilane, y-glycidoxypropylethyldiethoxysilane, 5 p-(3,4-epoxyGyclohexyl)ethyltrimethoxysilane, P-(3,4-epoxycyclohexyl)ethyltriethoxysilane, (i- (3,4-epoxycyclohexyl) ethyltri-i-propoxysilane, (3- (3, 4-epoxycyclohexyl) ethylmethyldimethoxysilane, (3- (3, 4-epoxycyclohexyl) ethylmethyldiethoxysilane and 10 (3- (3, 4-epoxycyclohexyl) ethylmethyldi-i-propoxysilane. [0103] Examples of the silane compounds (a2) having an acyl group include methyltriacetoxysilane, ethyltriacetoxysilane, vinyltriacetoxysilane, dimethyldiacetoxysilane, 15 triethylmethoxysilane and tetraacetoxysilane. [0104] The silane compound (a2) may be not such a monomer type compound (in the general formula (II) and the structural formula (II'), p'=0) as above but an oligomer or polymer type compound (in the general 20 formula (II) and the structural formula (II'), p'^0). [0105] Examples of the silane compounds (a2) of oligomer type include tetramethoxydisiloxane, tetraethoxydisiloxane, dimethyltetramethoxydisiloxane, methylethyltetramethoxydisiloxane. SF-242'6 48 trimethylpentamethoxytrisiloxane and tetramethyltetramethoxycyclotetrasiloxane, [0106] Examples of the silane compounds (a2) of polymer type include 5 hydrolysis condensates (alkyl silicates) of orthoethoxysilane, Orthoniethoxysilane, orthoisopropoxysilane, methyltrimethoxysilane, methyltriethoxysilane, etc. [0107] Examples of commercial products of the above silane compounds 10 include Ethyl Silicate 40, Ethyl Silicate 45, Methyl Silicate 51 and Methyl Silicate 56 (all manufactured by Tama Chemicals, Ltd.). Of these, a hydrolysis condensate of methyltrimethoxysilane or methyltriethoxysilane is preferable from the viewpoints of ease of obtaining, reactivity, etc. 15 [0108] The content of the silane compound (a2) based on 100 parts by weight of the silane compound (al) is preferably 10 to 5000 parts by weight, more preferably 30 to 3000 parts by weight, still more preferably 50 to 2000 parts by weight. If the content of the silane 20 compound (a2) is less than the lower limit of the above range, the number of the polymerization reaction sites (OH groups formed by hydrolysis) derived from the silane compound (a2) to the number of the polymerization reaction sites (OH groups formed by hydrolysis) derived from the silane compound (al) is too small. Therefore, the SF-2426 49 proportion of constituent units derived from the silane compound (a2) to the constituent units in the resulting hydrolysis condensate is lowered, and it sometimes becomes difficult to obtain appropriate molecular weight and structure. I£ the content exceeds the upper 5 limit of the above range, the number of the polymerization reaction sites (OH groups formed by hydrolysis) derived from the silane compound (al) to the number of the polymerization reaction sites (OH groups formed by hydrolysis) derived from the silane compound (a2) is too small. Therefore, the proportion of constituent units derived from 10 the silane compound (al) to the constituent units in the resulting hydrolysis condensate is lowered, and appropriate molecular designing becomes impossible. [0109] In the silane compounds (a2) , the amount of the tetrafunctional 15 silane compound based on 100 parts by weight of the trifunctional silane compound is preferably 0.1 to 200 parts by weight, more preferably 1 to 100 parts by weight, still more preferably 5 to 80 parts by weight. If the content of the tetraf unctinal silane compound is less than the lower limit of the above range, the number of the 20 polymerization reaction sites (OHgroups formedbyhydrolysis) derived from the tetrafunctional silane compound to the number of the polymerization reaction sites (OH groups formedby hydrolysis) derived from the trifunctional silane compound is too small, and therefore, it is difficult to obtain appropriate molecular weight and structure. SF-2426 50 If the content is more than the upper limit of the above range, the molecular weight becomes too high, and as a result, solubility is deteriorated to cause sedimentation or gelation. [0110] 5 The trifunctional silane compound and the tetrafunctional silane compound indicate a silane compound having, as R'^'^ to R^'' in the general formula (II) and the structural formula (II'), afunctional groups selected from the group consisting of a hydroxyl group, a -OR^'^ group (R®^ is an alkyl group of 1 to 10 carbon atoms) and an acyl 10 group, and a silane compound having 4 functional groups selected from them, respectively. [0111] By appropriately designing molecular structure (number of crosslinkages, molecular weight distribution, type of end group, 15 presence or absence of end group having hydrolysis reactivity and condensation reactivity, molecular weight) of a hydrolysis condensate prepared, water solubility and storage stability of the hydrolysis condensate can be improved, or curability of a coating film formed from the coating composition can be controlled. 20 [0112] In the present invention, the silane raw material (a) comprises the silane compound (al), preferably the silane compound (al) and the silane compound (a2), but if necessary, it may comprise a silane compound which is different from the silane compound (al) and the SF-2426 51 silane compound (a2) and is hydrolyzable and condensable (silane compound which undergoes hydrolysis reaction and condensation reactiontogetherwiththe silane compound (al) andbecomes constituent units of a hydrolysis condensate (silane compound (a3)). As the silane 5 compound (a3), a silane coinpound Wherein at least one Icetoxime group (e.g., the following functional group) is bonded to a Si atom (oxime-based silane compound) can be mentioned, and the oxime-based silane compound is, for example, methyltris(methylethylketoxime)silane. CLAIMS 1. Aqueous coating composition comprising a binder (A) and water (B); the said binder (A) comprising a hydrolysis condensate obtained 5 by subjecting a silane raw material (a) comprising a silane compound (al) represented by the following general formula (I) to hydrolysis reaction and condensation reaction under the conditions of pH of 0.4 to 8.0; R^^SiR^^R^^- (Ym-SiR''^R^^)p-R^^ (I) 10 wherein R'"'^ to R^^ are each independently a hydrogen atom, a halogen atom, an alkyl group of 1 to 10 carbon atoms, an alkenyl group of 2 to 5 carbon atoms, an aryl group of 6 to 12 carbon atoms, Z-R^^- (Z is a halogen atom, a hydroxyl group, an epoxy group, an acryloxy group, a methacryloxy group or a polyoxyalkylene group represented 15 by R^O-(R'^0)C- (R^ is a hydrogen atom or an alkyl group of 1 to 10 carbon atoms, R"^ is an alkylene group of 2 to 4 carbon atoms, and c is an integer of 1 to 15), and R^^ is an alkylene group of 1 to 10 carbon atoms) , or -OR®^ (R®^ is a hydrogen atom, an alkyl group of 1 to 10 carbon atoms or an alkenyl group of 1 to 10 carbon atoms) , 20 at least one of R'^^ to R^^ is a halogen atom, Y is an oxygen atom or an alkylene group of 1 to 10 carbon atoms, m is 0 or 1, and p is an integer of not less than 0. SF-2426 148 2. The aqueous coating composition as claimed in claim 1, wherein the said silane raw material (a) comprises a silane compound (a2) represented by the following general formula (II) in addition to the silane compound (al); 5 R^^SiR^^R^^- (Y^--SiR^^R^^)p-R^ (11) wherein R''''' to R^^ are each independently a hydrogen atom, an alkyl group of 1 to 10 carbon atoms, an alkenyl group of 2 to 5 carbon atoms, an aryl group of 6 to 12 carbon atoms, Z-R^'^- (Z is a hydroxyl group, an epoxy group, an acryloxy group, a methacryloxy group or 10 a polyoxyalkylene group represented by R^'O-(R'''0)C'- (R^' is a hydrogen atom or an alkyl group of 1 to 10 carbon atoms, R'^' is an alkylene group of 2 to 4 carbon atoms, and c' is an integer of 1 to 15), and R^"^ is an alkylene group of 1 to 10 carbon atoms), -GR^"^ (R^'' is an alkyl group of 1 to 10 carbon atoms or an alkenyl group of 1 to 10 15 carbon atoms), a hydroxyl group or an acyl group, at least one of R''"'^ to R^*^ is a hydroxyl group, a -OR^'^ group (R^'' is a alkyl group of 1 to 10 carbon atoms), Z-R^"^- (Z is an epoxy group, and R^^ is an alkylene group of 1 to 10 carbon atoms) , or an acyl group, 20 Y' is an oxygen atom or an alkylene group of 1 to 10 carbon atoms, m' is 0 or 1, and p' is an integer of not less than 0. SF-2426 149 3. The aqueous coating composition as claimed in claim 1 or 2, wherein the said binder (A) comprises a hydrolysis condensate and a metal oxide represented by the following chemical formula (III), the said hydrolysis condensate obtained by subjecting the silane raw 5 material (a) to hydrolysis reaction and condensation reaction in the presence of a metal oxide sol (C) comprising the metal oxide; MpOr (III) wherein M is Si, Al or Ti, and p and r are each independently an integer of 1 to 3. 10 4. The aqueous coating composition as claimed in any one of claims 1 to 3, wherein at least one of R-^^ to R^^ in the formula (I) is a chlorine atom. 15 5. The aqueous coating composition as claimed in claim 2 or 3, wherein at least one of R'"'^ to R^'^ in the general foimula (11) is a glycidoxy group. 6. The aqueous coating composition as claimed in any one of claims 20 1 to 5, wherein Y in the formula (I) is an alkylene group of 1 to 10 carbon atoms. 7. The aqueous coating composition as claimed in any one of claims 3 to 6, wherein the said metal oxide sol (C) is a silicic anhydride SF-2426 150 sol. 8. The aqueous coating composition as claimed in claim 1, wherein the said silicic anhydride sol is a fumed silica sol. 5 9. The aqueous coating composition as claimed in any one of claims 3 to 6, wherein the said metal oxide sol (C) is a colloidal silica water dispersing agent, and the said colloidal silica water dispersing agent has pH of not more than 7 and a Na20 content of not more than 10 400 ppm. 10. The aqueous coating composition as claimed in any one of claims 3 to 6, wherein the said metal oxide sol (C) is a sol comprising fumed alumina or fumed titania. 15 11. The aqueous coating composition as claimed in any one of claims 1 to 10, which further comprises a pigment, and a ratio ( (PVC) / (CPVC) ) of a pigment volume concentration (PVC) to a critical pigment volume concentration (CPVC) in the aqueous coating composition is higher 20 than 1. 12. The aqueous coating composition as claimed in any one of claims 1 to 11, which further comprises a rust-preventive pigment (D). SF-2426 151 13. The aqueous coating composition as claimed in claim 12, wherein the said rust-prgventivQ pipont (D) comprisQS 2inc powdor and/or zinc alloy powder having a mean particle diameter of 2 to 20 pm. 5 14. The aqueous coating composition as claimed in any one of claims 1 to 13, which further comprises metallic molybdenum and/or a molybdenum compound as a white rust inhibiter (E). 15. The aqueous coating composition as claimed in any one of claims 10 1 to 11, which is used as a binder composition. 16. The aqueous coating composition as claimed in any one of claims 12 to 14, which is used as a rust-preventive coating composition. 15 17. The aqueous coating composition as claimed in claim 16, which is a coating composition for primary rust prevention. 18. The aqueous coating composition as claimed in claim 16, which is a thick-coating type inorganic zinc coating composition. 20 19. A primary rust prevention coating method for a steel material, comprising applying the coating composition for primary rust prevention as claimed in claim 17 to a surface of a steel material and then curing the applied coating composition to form a primary SF-2426 • 152 ,, rust-preventive coating film. '20. A rust prevention coating method for a steel material, comprising applying the thick-coating type inorganic zinc coating 5 composition as claimed in claim 18 to a surface of a steel material and then curing the applied coating composition to form a thick-coating type inorganic zinc rust-preventive coating film. 21. A steel structure having, on a surface of a steel material, 10 a primary rust-preventive coating film formed from the coating composition for primary rust prevention as claimed in claim 17. 22. A steel structure having, on a surface of a steel material, a thick-coating type inorganic zinc rust-preventive coating film 15 formed from the thick-coating type inorganic zinc coating composition as claimed in claim 18.