JP6289382B2 - Primary rust preventive paint composition and painted steel structure coated with the same - Google Patents

Description

  The present invention relates to an inorganic zinc shop primer composition and a coated steel structure obtained by coating it on a steel structure. More specifically, the present invention is excellent in weldability and anticorrosion properties, and is also excellent in fusing properties. The present invention relates to an inorganic zinc shop primer composition capable of forming an inorganic zinc shop primer coating film on a steel structure, and a painted steel structure obtained by coating the steel structure with the inorganic zinc shop primer composition.

  Conventionally, steel structures used for the construction of ships, bridges, tanks, plants, etc. are pre-drying on the steel surface in advance to prevent the occurrence of rust during processing and assembly, and anticorrosion, heat resistance, It is common to coat a primary rust-preventing primer with excellent properties such as flexibility and welding properties, so-called inorganic zinc shop primer, and after finishing the processing and assembly, perform secondary surface treatment, and then apply undercoating, intermediate coating and overcoating. Is. This inorganic zinc shop primer is characterized by having excellent anticorrosive properties based on the electrochemical action between the zinc dust contained in the coating film and the surface of the raw iron.

  Inorganic zinc shop primer, when welding and fusing for processing assembly, due to the combustion gas of the organic component contained in the primer, welding defects such as pits and blowholes (bubbles) are generated in the welded part. There has been a problem that it becomes large and the width of the thermally damaged portion of the primer coating film becomes large. Moreover, the steel structure coated with the inorganic zinc shop primer sometimes has a poor corrosion resistance at the part subjected to the heat load on the painted back surface of the welded portion. In the present specification, the anticorrosion property of the coated back surface may be expressed as “welding portion back surface anticorrosion property”.

  As a method for reducing the above-mentioned welding defects, studies have been made to reduce the average dry film thickness of the inorganic zinc shop primer to a thin film of less than 15 μm. For example, Patent Document 1 discloses a method for improving a welding defect by setting the film thickness to about 6 to 10 μm. However, a coated steel structure coated with such a thin-film inorganic zinc shop primer is disclosed in Patent Document 1. A new problem arises that red rust tends to occur. In order to suppress the occurrence of red rust in the above thin film, if the zinc powder in the inorganic zinc shop primer composition is increased, a lot of white rust is generated, and it is necessary to remove this during overcoating, increasing the number of man-hours Became a problem.

  Patent Document 2 discloses tetraalkoxysilicates, alkyltrialkoxysilicates and / or their hydrolysis initial condensates (A), acidic water-dispersed colloidal silica (B), zinc dust (C) and silica powder, A primary rust preventive paint for steel containing a powder (D) having a loss on ignition of 2% by weight or less selected from rutile powder, synthetic rutile powder, chromium powder and zircon powder is disclosed. By using this paint, the weldability is greatly improved. However, for example, when only silica powder is used as the powder (D), there is a problem that the fusing property is inferior.

JP-A-6-210240 JP-A-3-79675

  An object of the present invention is an inorganic zinc shop primer composition capable of forming a coating film excellent in weldability and corrosion resistance in a thin film, and the fusing property of a coated steel structure on which the coating film is formed is also good. An inorganic zinc shop primer composition and a coated steel structure obtained by coating the primer composition on a steel structure.

  The present inventors have intensively studied to achieve the above object, an inorganic zinc shop primer composition containing a zinc powder having an average particle diameter in a specific range and an extender pigment exhibits good weldability and corrosion resistance, It has been found that a coated steel structure obtained by coating the steel composition with the primer composition is also excellent in fusing property. The present invention has been completed based on such knowledge.

  The present invention provides the following inorganic zinc shop primer composition and a coated steel structure obtained by coating the primer composition on a steel structure.

Item 1. Based on the total mass of the silica-based mass of the inorganic binder component (A), the mass of the zinc dust (B) and the mass of the extender pigment (C), 9-19% by mass of the inorganic binder component (A ), An inorganic zinc shop primer composition comprising 61 to 79% by weight of zinc dust (B), and 8 to 23% by weight of extender pigment (C),
The inorganic binder component (A) has a general formula of (R ¹ ) _n —Si— of 64 to 95% by mass in terms of silica based on the total mass in terms of silica of the inorganic binder component (A). (OR ² ) _4-n (wherein R ¹ is an alkyl or phenyl group having 1 to 18 carbon atoms which may be substituted with an epoxy group or a mercapto group, and R ² has 1 to 6 carbon atoms) An organosilicate represented by an alkyl group, and n is an integer of 0 to 2) and / or its condensate component (a1), and 5 to 36% by mass of colloidal silica (a2) in terms of silica. Including
The zinc dust (B) has an average particle size of 0.5 to 10 μm,
The extender pigment (C) is based on the total mass of the extender pigment (C), 50 to 80% by mass of silica powder (c1) having an average particle diameter of 1 to 10 μm, 20 to 50% by mass of feldspar and kaolin An inorganic zinc shop primer composition comprising at least one extender pigment (c2) having an average particle diameter of 10 μm or less selected from the group consisting of:

  Item 2. A coated steel structure obtained by coating the steel structure with the inorganic zinc shop primer composition of Item 1 so that the average film thickness of the dried coating film is 5 to 15 μm.

  The inorganic zinc shop primer composition of the present invention exhibits good weldability and corrosion resistance, and a coated steel structure obtained by coating the primer composition on a steel structure is excellent in fusing property. In particular, the inorganic zinc shop primer of the present invention is characterized by excellent corrosion resistance even in a thin film having an average dry film thickness of about 5 to 15 μm.

  The inorganic zinc shop primer composition of the present invention has a silica equivalent mass of 9 based on the total mass of the inorganic binder component (A), the silica equivalent mass, the zinc dust (B) mass, and the extender pigment (C) mass. -19% by mass of inorganic binder component (A), 61-79% by mass of zinc powder (B) having an average particle size of 0.5-10 μm, and 8-23% by mass of extender pigment (C). It is characterized by that. Hereinafter, the inorganic zinc shop primer composition of the present invention will be described in detail.

Inorganic binder component (A)
The inorganic binder component (A) is 9 to 19% by mass in terms of silica, preferably based on the total mass of component (A) in terms of silica, component (B) and component (C), preferably It can be contained in a range of 10 to 17% by mass, more preferably 11 to 15% by mass. When the amount of the inorganic binder component (A) is less than 9% by mass, the anticorrosion property of the obtained coating film may be deteriorated. May decrease.

The inorganic binder component (A) is a general formula of (R ¹ ) _n —Si— (64 to 95% by mass in terms of silica based on the total mass in terms of silica of the inorganic binder component (A). OR ² ) _4-n (wherein R ¹ is an alkyl or phenyl group having 1 to 18 carbon atoms which may be substituted with an epoxy group or a mercapto group, and R ² is an alkyl having 1 to 6 carbon atoms) And n is an integer of 0 to 2) and / or its condensate component (a1), and 5 to 36% by mass of colloidal silica (a2) in terms of silica. .

  Examples of the organosilicate and / or its condensate component (a1) include organosilicates such as tetraalkoxysilicate, alkyltrialkoxysilicate, and dialkyldialkoxysilicate and / or condensate components thereof. Examples of the tetraalkoxysilicate include tetramethoxysilicate, tetraethoxysilicate, tetrapropoxysilicate, tetraisopropoxysilicate, tetrabutoxysilicate, tetraisobutoxysilicate, and the like. Furthermore, examples of the alkyltrialkoxysilicate include methyltrimethoxysilicate, methyltriethoxysilicate, methyltripropoxysilicate, ethyltrimethoxysilicate, ethyltriethoxysilicate, and the like. Examples of the dialkyl dialkoxysilicate include dimethyldimethoxysilicate, dimethyldiethoxysilicate, diethyldimethoxysilicate, diethyldiethoxysilicate, and the like. These can be contained alone or in admixture of two or more. Moreover, these organosilicates may contain what advanced the condensation reaction to arbitrary reaction rates following the hydrolysis reaction.

  Examples of commercially available products of such an organosilicate condensation reaction include ethyl silicate 28 (manufactured by Nippon Colcoat), ethyl silicate 40 (manufactured by Nippon Colcoat), ethyl silicate 48 (manufactured by Nippon Colcoat), and the like. Can do. The hydrolysis reaction can be performed by a reaction between an appropriate amount of water and an organosilicate, but may be performed in the presence of water or a colloidal silica (a2) described later dispersed in a solvent containing water. In the hydrolysis reaction and / or condensation reaction, an appropriate compound such as an acid or a base can be used as a catalyst. The organosilicate and / or its condensate component (a1) is 64 to 95% by mass in terms of silica, more preferably 70 to 90% by mass based on the total mass of silica in terms of the inorganic binder component (A). % Can be included. When the content of (a1) is less than 64% by mass, the obtained coating film becomes brittle and the corrosion resistance may be lowered. When the content is more than 95% by mass, the weldability of the obtained coating film is low. May be inferior. In this specification, in the welding of a steel structure coated with a shop primer, the performance of preventing weld defects such as pits and blowholes (bubbles) in the welded portion is referred to as “weldability of the shop primer coating film”. ".

  The said colloidal silica (a2) can use what was disperse | distributed to either the organic solvent, water, or the mixture of water and an organic solvent. The colloidal silica (a2) has an average particle diameter of about 1 nm to 100 nm, preferably 5 nm to 50 nm. As the colloidal silica (a2), those dispersed in water can be used as they are, but those obtained by chemically modifying the surface of the colloidal silica particles with an organic resin, an organic compound, an inorganic compound, a polysiloxane compound or the like are used. Also good. Colloidal silica (a2) is contained in the range of 5 to 36% by mass, more preferably 10 to 30% by mass in terms of silica, based on the total mass in terms of silica of the inorganic binder component (A). it can. When the content of the colloidal silica (a2) is less than 5% by mass, the weldability of the obtained coating film may be inferior, and when it is more than 36% by mass, the obtained coating film may become brittle. is there. Colloidal silica (a2) has a smaller amount of pyrolysis gas due to a high-temperature arc during welding than organosilicate and / or its condensate component (a1). It is estimated that

The silica equivalent mass of the inorganic binder component (A) or the silica equivalent mass of the organosilicate and / or its condensate component (a1) is all silica atoms contained in the component (A) or component (a1). The mass is calculated on the assumption that the composition is changed to [SiO ₂ ] in the composition formula. In the present specification, the mass value is determined by adding 10% by mass of water, 30% by mass of isopropanol, and 0.01% by mass of 2N hydrochloric acid to the total mass of the component (A) or the component (a1). By holding at 40 ° C. for 1 hour with stirring, the hydrolysis reaction of the silicate contained in component (A) or component (a1) and the condensation reaction of the silanol produced are carried out and volatilized at 105 ° C. for 3 hours. The mass of the solid content remaining after the components are removed.

Zinc powder (B)
In a conventional ordinary inorganic zinc shop primer composition, zinc powder having an average particle diameter larger than 10 μm is often used. In the present invention, however, the average particle is used from the viewpoint of the anticorrosive property of the resulting coating film. A diameter of 0.5 to 10 μm, preferably 2 to 8 μm, more preferably 3 to 6 μm can be used. The average film thickness of the dried coating film was 5 to 15 μm by using one within the above particle diameter range and further using an extender pigment (C) described later having an average particle diameter within a specific range. However, it is possible to obtain a coating film excellent in long-term corrosion resistance. In the present specification, the particle size of zinc powder is a value measured by a Beckman Coulter LS particle size distribution measuring device.

  The zinc dust (B) is 61 to 79% by mass based on the silica-based mass of the inorganic binder component (A), the mass of the component (B) and the total mass of the component (C), and further 64 It is preferable to contain within the range of -76 mass%. If the zinc dust (B) is less than 61% by mass, the resulting coating film may be inferior in corrosion resistance, and if it is more than 79% by mass, the weldability of the obtained coating film may be reduced, The amount of rust generated may increase.

Extender pigment (C)
The extender pigment (C) contains silica powder (c1) having an average particle diameter of 1 to 10 μm and at least one extender pigment (c2) having an average particle diameter of 10 μm or less selected from the group consisting of feldspar and kaolin. The extender pigment (c2) is particularly preferably kaolin from the viewpoint of both performance of the welded portion back surface anticorrosive property and fusing property. In the present specification, the evaluation of the roughness of the melted section of the steel structure coated with the shop primer is referred to as “melting property” of the shop primer coating film.

  The silica powder (c1) is contained in the range of 50 to 80% by mass, more preferably 55 to 75% by mass, and further preferably 60 to 70% by mass based on the total mass of the extender pigment (C). it can. When the silica powder (c1) is less than 50% by mass, the corrosion resistance at the back of the welded portion may be inferior, and when it is more than 80% by mass, the fusing property may be inferior. Further, the extender pigment (c2) is contained in the range of 20 to 50% by mass, more preferably 25 to 45% by mass, and further preferably 30 to 40% by mass based on the total mass of the extender pigment (C). Can do. If the extender pigment (c2) is less than 20% by mass, the fusing property may be inferior, and if it is more than 50% by mass, the corrosion resistance at the back of the welded part may be inferior.

  The silica powder (c1) may have an average particle diameter of 1 to 10 μm, preferably 2 to 8 μm, more preferably 3 to 6 μm. When the average particle diameter of the silica powder (c1) is larger than 10 μm, the meltability and long-term corrosion resistance of the obtained coating film may be inferior. When the average particle diameter is smaller than 1 μm, the obtained inorganic zinc Cracks or peeling may occur easily in the shop primer coating. The extender pigment (c2) may have an average particle size of 10 μm or less, preferably 0.5 to 5 μm, more preferably 1 to 3 μm. When the average particle diameter of the extender pigment (c2) is larger than 10 μm, the fusing property and long-term corrosion resistance of the obtained coating film may be inferior. In addition, the average particle diameter of extender pigment (C) is the value measured with the laser diffraction type particle size distribution measuring apparatus (use apparatus name: SALD-200V ER Shimadzu Corporation make). There are two types of kaolin, wet kaolin and calcined kaolin, depending on the production method, and calcined kaolin is preferred in terms of weldability.

  If necessary, the extender pigment (C) may contain other extender pigments (c3) in a range of 30% by mass or less based on the total mass of the extender pigment (C). Examples of other extender pigments (c3) include clay, calcined clay, mica, calcined mica, talc, barium sulfate, calcium carbonate, aluminum hydroxide, zinc oxide, barite, zirconium oxide, iron oxide and the like. . These may be used alone or in combination of two or more, among which fired clay and fired mica are preferred from the viewpoints of weldability and fusing properties. When the other extender pigment (c3) is used in an amount exceeding 30% by mass, the corrosion resistance of the back surface of the welded portion of the obtained coating film may be inferior. Further, the average particle diameter of the other extender pigment (c3) is preferably 10 μm or less from the viewpoint of long-term corrosion resistance.

  The extender pigment (C) is 8 to 23% by mass, further 10 to 21 based on the total mass of the inorganic binder component (A) in terms of silica, the component (B) and the component (C). It is preferable to contain within the range of mass%. When the amount of extender pigment (C) is less than 8% by mass, the corrosion resistance at the back of the welded portion of the obtained coating film may be inferior, and when it is more than 23% by mass, the fusing property of the obtained coating film is deteriorated. Sometimes.

  In the present invention, if necessary, as a pigment component other than those described above, ordinary rust preventive pigments and colored pigments can be used in combination to the extent that the object of the present invention is not impaired. Examples of rust preventive pigments and colored pigments include titanium oxide, graphite, iron phosphide, MIO, lead cyanamide, zinc chromate, zinc phosphate, calcium phosphate, barium metaborate, zinc molybdate, aluminum molybdate, bengara, and cyanine coloring. Examples thereof include pigments, carbon black, and zircon powder. About these rust preventive pigments and colored pigments, it is usually desirable that the average particle diameter is less than 4 μm from the viewpoint of forming a dense coating film.

  In the primer composition of the present invention, additives such as resins, solvents and anti-settling agents used in ordinary paints other than the above essential components and pigments may be blended in appropriate amounts. As said resin, well-known resin, such as polyvinyl alcohol resin, an acrylic resin, an epoxy resin, a butyral resin, a polyester resin, an alkyd resin, a polysiloxane resin, can be used individually or in mixture.

  The inorganic zinc shop primer composition of the present invention can be prepared according to a conventional method. For example, a liquid component containing an inorganic binder component (A), a powder of zinc dust (B) and extender pigment (C), and as necessary Correspondingly, the powder-containing component containing a solvent, an additive and the like can be stored in a separate container, and both can be mixed immediately before use.

  The inorganic zinc shop primer composition of the present invention is preferably applied to the surface of the steel structure so that the average film thickness of the dry film thickness is 5 to 15 μm. When the average film thickness is thinner than 5 μm, the anticorrosive property for a long time may be insufficient. In addition, the inorganic zinc shop primer of the present invention is excellent in corrosion resistance and fusing property as compared with the conventional inorganic zinc shop primer composition even when the average film thickness is greater than 15 μm. The primer composition can be applied by conventionally known means such as air spray, airless spray, and brush.

  The applied primer composition can be dried and cured at room temperature or under heating. When drying and curing, the hydrolysis reaction of the organosilicate and / or its condensate component in the primer composition is accelerated by moisture in the atmosphere, and further proceeds by the subsequent condensation reaction. The drying and curing time at room temperature is usually 7 days or more after the primer composition is applied. Further, the heating temperature in drying and curing under heating is preferably 40 to 120 ° C., and the drying and curing time can be appropriately adjusted.

  Prior to the coating of the inorganic zinc shop primer composition of the present invention, the surface of the steel structure is subjected to a treatment such as shot blasting or sand blasting to remove black skin or red rust, and then coated with the primer composition. The Usually, this blasting and painting are performed continuously.

  Since the object to be coated with the inorganic zinc shop primer coating is mainly a large steel structure such as a ship or a bridge or a steel material for the steel structure, the coating is used for welding, fusing, etc. in the construction process. The main purpose is to protect the steel structure or the steel material for steel structure from rusting during the construction period without any hindrance. In addition, after the construction is completed, anticorrosion paints such as epoxy, tar epoxy, chlorinated rubber, oily, epoxy ester, acrylic, vinyl and inorganic zinc can be overcoated. The primer coating film is excellent in adhesion with such a top coating film.

  EXAMPLES Hereinafter, although an Example and a comparative example are given and this invention is demonstrated further more concretely, this invention is not limited only to an Example. In the following examples, “part” and “%” mean “part by mass” and “% by mass”, respectively.

[Manufacture of inorganic zinc shop primer]
Among the raw materials shown in Table 1-1 and Table 1-2, the remaining raw materials excluding 2N hydrochloric acid and colloidal silica (“Snowtex O” or “Snowtex 20”) are placed in a reaction vessel, and the reaction vessel Was maintained at 50 ° C., and hydrochloric acid and colloidal silica were added dropwise over about 1 hour with stirring. After completion of the dropping, stirring was continued for 1 hour to obtain a solution or dispersion of the inorganic binder component (A). Table 2-1-1, Table 2-1-2, Table 2-2-1 and Table 2-2-2, and the above-mentioned solution or dispersion liquid, zinc dust (B), extender pigment ( C) was mixed immediately before coating, stirred, and isopropyl alcohol was added as necessary to adjust the viscosity to obtain various inorganic zinc shop primer compositions. In Table 2-1-1, Table 2-1-2, Table 2-2-1 and Table 2-2-2, the organosilicate derived from the inorganic binder component (A) and / or its condensation component The silica equivalent mass of (a1) and colloidal silica (a2) is shown, and the total mass of the silica equivalent mass, the zinc dust (B) mass, and the extender pigment (C) mass is 100. . In Table 2-1-1, Table 2-1-2, Table 2-2-1 and Table 2-2-2, the numbers in parentheses in the columns of zinc powder (B) and extender pigment (C) Each represents an average particle diameter (for example, “particle diameter 4 μ” represents “average particle diameter 4 μm”).

  The following tests were performed on each of the obtained inorganic zinc shop primers. In addition, it coated so that the average dry film thickness of each inorganic zinc shop primer in the following test might be 10 micrometers except 6 micrometers in Example 18, 14 micrometers in Example 19, and 30 micrometers in a reference example. In the following description of the tests and evaluations, the average dry film thickness is 10 μm, but tests and evaluations with different average dry film thicknesses were also performed in the same manner. Test results and evaluation are shown in Table 2-1-1, Table 2-1-2, Table 2-2-1 and Table 2-2-2.

[Preparation of test panels for outdoor exposure and evaluation of corrosion resistance]
A steel plate having a length of 300 mm, a width of 100 mm, and a thickness of 3 mm was shot blasted, and each inorganic zinc shop primer was applied thereto so as to have a dry film thickness of 10 μm to obtain a test coating plate. About one type of coated board, about 4 months and 6 months exposed outdoors, each rust generation state was compared with the rust generation standard board of ASTM-D610 / SSPC-VIS2, and evaluated as follows.
A: Rust area is less than about 0.03% of the total painted area B: Rust area is about 0.03% or more of the total painted area and less than 0.3% X: Rust area is about 0.3 of the total painted area %that's all

[Welding test]
Shot blasting is performed on a steel plate with a length of 1000 mm, a width of 150 mm, and a thickness of 12 mm. Each inorganic zinc shop primer is coated to a dry film thickness of 10 μm, dried for 7 days, and then subjected to a welding test. It was. For welding, a welding wire SF-1 (Nippon Sumikin Welding Co., Ltd.) was used, and a horizontal fillet welding test was conducted at a welding speed of 80 cm / min by the carbon dioxide shielded arc welding method. The total number of pits generated in the weld bead portion and the number of blow holes generated was evaluated per 1 m length.
◎: Less than 10 ○: 10 or more and less than 30 ×: 30 or more

[Weld back surface anticorrosion]
Shot blasting was performed on both sides of a steel plate having a length of 1000 mm, a width of 150 mm, and a thickness of 12 mm, and each inorganic zinc shop primer was coated on both sides so that the dry film thickness was 10 μm and dried for 7 days. On one side, welding was performed in the same manner as in the welding test. Compared with the standard rust generation plate of ASTM-D610 / SSPC-VIS2, the rust generation state of the surface opposite to the welded surface was compared with the rust generation standard plate of the obtained coated plate exposed for 3 months outdoors. Evaluation was made in the same manner as [Preparation of plate and evaluation of anticorrosion].

[Fusing property]
Shot blasted on a steel plate (SM50A) 300 mm long, 100 mm wide and 12 mm thick, and each inorganic zinc shop primer was coated to a dry film thickness of 10 μm and dried at room temperature for 7 days for testing. A coated plate was obtained. Using a laser cutting machine “TF2500” (manufactured by Tanaka Seisakusho) for each test plate, a fusing test is performed at a cutting speed of 1 m / min. Evaluation was carried out as follows according to the standard of “roughness (R)” shown in the standard WES2801.
First grade: 50 s or less Second grade: More than 50 s and less than 100 s Third grade: More than 100 s and less than 200 s

(Note 1) Made by Colcoat, hydrolyzed and condensed with ethyl silicate, and the silica (SiO ₂ ) concentration in the product is 28% by mass.
(Note 2) Made by Colcoat, hydrolyzed and condensed with ethyl silicate, the silica (SiO ₂ ) concentration in the product is 40% by mass.
(Note 3) Colcoat, hydrolyzed and condensed ethyl silicate, the silica (SiO ₂ ) concentration in the product is 48% by mass.
(Note 4) Nissan Chemical Industries, Ltd., acidic type silica sol, particle size of 10-20 nm, silica (SiO ₂ ) concentration in the product is 20% by mass.
(Note 5) Made by Nissan Chemical Industries, Na neutralization type silica sol, particle diameter 10-20 nm, silica (SiO ₂ ) concentration in the product is 20% by mass.

(Note 6) Silica conversion mass of organosilicate and / or its condensate component (a1) and silica conversion mass of colloidal silica (a2) (total mass 100).
(Note 7) Mass of each component when the total mass of silica powder (c1), feldspar and / or kaolin (c2), and other extender pigments (c3) is 100.
(Note 8) Calcined kaolin (water content 0.7% or less) is used for kaolin (c2).

Claims (2)

Based on the total mass of the silica-based mass of the inorganic binder component (A), the mass of the zinc dust (B) and the mass of the extender pigment (C), 9-19% by mass of the inorganic binder component (A ), An inorganic zinc shop primer composition comprising 61 to 79% by weight of zinc dust (B), and 8 to 23% by weight of extender pigment (C),
The inorganic binder component (A) has a general formula of (R ¹ ) _n —Si— of 64 to 95% by mass in terms of silica based on the total mass in terms of silica of the inorganic binder component (A). (OR ² ) _4-n (wherein R ¹ is an alkyl or phenyl group having 1 to 18 carbon atoms which may be substituted with an epoxy group or a mercapto group, and R ² has 1 to 6 carbon atoms) An organosilicate represented by an alkyl group, and n is an integer of 0 to 2) and / or its condensate component (a1), and 5 to 36% by mass of colloidal silica (a2) in terms of silica. Including
The zinc dust (B) has an average particle size of 0.5 to 10 μm,
The extender pigment (C) is based on the total mass of the extender pigment (C), 50 to 80% by mass of silica powder (c1) having an average particle diameter of 1 to 10 μm, 20 to 50% by mass of feldspar and kaolin An inorganic zinc shop primer composition comprising at least one extender pigment (c2) having an average particle diameter of 10 μm or less selected from the group consisting of:   A coated steel structure obtained by coating the steel structure with the inorganic zinc shop primer composition according to claim 1 so that the average film thickness of the dried coating film is 5 to 15 µm.