JP4552746B2 - Weather-resistant structural steel with excellent long-term durability in high-flying chloride environments - Google Patents

Description

  The present invention suppresses long-term corrosion in atmospheric corrosive atmospheres, such as in high air chloride environments such as coastal areas where salinity flies and areas where anti-freezing agents such as rock salt are sprayed. The present invention relates to a weather-resistant structural steel material having excellent long-term durability.

  The weather-resistant structural steel material of the present invention means that it can be used as a minimum maintenance material even in a high-flying chloride environment with a large amount of incoming salt, such as a beach area or an area where snow melting salt / freezing agents are sprayed. Weather resistance (seaside weather resistance), and the weather resistance lasts for a long time.

  In general, when weathering steel is exposed to an atmospheric corrosive environment, a protective rust layer is formed on the surface, and the subsequent corrosion of steel is suppressed, thereby exhibiting weather resistance. Therefore, the weather-resistant steel is used for structures such as bridges as a minimum maintenance steel that can be used as it is without being painted.

  However, in environments where there is a large amount of incoming salt (that is, under high-flying chloride environments), such as beach areas and areas where snow melting salt is scattered even in the inland areas, the surface of the weathering steel is protected. The layer is not formed, and the effect of suppressing corrosion is not exhibited. For this reason, it was not possible to use a weather-resistant steel material that has not been painted on the beach.

Also in the standard of Japanese Industrial Standard (JIS) weathering steel (JIS G3114: weathering hot rolled steel for welded structure), the amount of incoming salt is 0.05 mg / dm ² / day (0.05 mdd) or more as NaCl. In areas such as beach areas and areas where snowmelt salt / freezing agents are sprayed (hereinafter collectively referred to as beach areas), corrosion loss due to the occurrence of scale-like rust, layered rust, etc. is so large that it cannot be used without painting. (Ministry of Construction Public Works Research Institute, Steel Club, Japan Bridge Construction Association: Joint Research Report on the Application of Weatherproof Steel to Bridges (XX)-Design of Uncoated Weatherproof Bridges・ Guidelines for construction (revised version-1993.3)).

  For this reason, in the beach area, it is common to use ordinary steel that is coated with ordinary steel. However, in bridges constructed in the coastal area near the river mouth and roads such as mountainous areas where snow melt is salted, the corrosion is significant and the coating film deteriorates due to corrosion, so it is necessary to repaint every 10 years. Since repainting takes a lot of man-hours and enormous costs for maintenance, there is a strong demand for steel materials with excellent beach weather resistance that can be used without painting even in the beach area.

  Recently, a Ni-based high weathering steel added with about 1 to 3% of Ni has been developed and put into practical use. For example, see Patent Document 1 below. However, it has been found that application of Ni alone to such steels is difficult in areas where the amount of incoming salt exceeds 0.3 to 0.4 mdd, resulting in a high incoming chloride environment.

  Since corrosion of steel materials becomes more severe as the amount of flying salt increases, a weather-resistant steel material corresponding to the amount of flying salt is required from the viewpoint of corrosion resistance and economy. Moreover, even if it is called a bridge, the corrosive environment of steel materials is not the same by the place and site | part used. For example, outside the girders, they are exposed to rainfall, condensed water and sunlight. On the other hand, inside the girders, they are exposed to condensed water, but there is no rain. In general, in an environment with a large amount of incoming salt, it is said that corrosion is more severe inside the girders than outside the girders that are washed with rain.

  In addition, in an environment where snow melting salt is sprinkled on the road, the salt is wound up on a running car and adheres to the bridge supporting the road, creating a severe corrosive environment. Furthermore, the eaves under the eaves a little away from the coast are also exposed to severe salt damage environments, and in such areas, the amount of incoming salt becomes a severe corrosive environment with 1 mdd or more.

  Steel materials that prevent corrosion in environments with a high amount of incoming salt have also been developed. For example, Patent Document 2 discloses a method for growing stable rust at an early stage by coating an organic resin paint containing 1 to 65% by mass of chromium sulfate or copper sulfate. In Patent Document 3, an organic resin coating film having a dry film thickness of 5 to 50 μm containing 0.1 to 15% by mass of chromium sulfate in the lower layer is formed, and a dry film thickness of 5 to 20 μm not containing chromium sulfate in the upper layer. A treatment method for forming an organic resin coating film is disclosed. Both of these methods have been demonstrated to facilitate the formation of a protective rust layer and to exhibit high corrosion resistance at an early stage, so that weatherability can be significantly improved.

Patent Document 4 discloses a weather-resistant steel material in which a coating having a dry film thickness of 5 μm or more and 150 μm or less is formed on the surface of a steel material or a rust layer of the steel material with an organic resin paint containing 1 to 65% by mass of aluminum sulfate in a dry mass. It is disclosed. Prior to the formation of the coating film, a base treatment may be performed in which an aqueous solution containing Al and optionally one or more metal ions of Cu, Fe, P, Cr, and Ni is applied and dried.
JP-A-11-172370 JP-A-6-226198 JP 2001-81575 A JP-A-8-13158

  The present invention aims to solve the problems of conventional weathering steel and the like, and is in an environment with a large amount of incoming salt (especially exceeding 1 mdd) such as a beach area or an area where snowmelt salt is sprayed. For use in low-cost civil engineering, construction, and oil tanks that exhibit excellent weather resistance for a long period of time, even in an environment where crude oil is stored. It aims at providing applicable structural steel materials.

As already reported by one of the present inventors ("Materials and the Environment", Vol. 43 (1994), No. 1, page 26), the rust layer has a protective property in weathering steel. This is because a rust layer composed of fine α- (Fe _1-X Cr _x ) OOH in which part is replaced with Cr is formed. However, the addition of Cr to the steel to promote the formation of the rust layer is effective in improving the weather resistance in an environment where the amount of incoming salt is relatively small, but in an environment where the amount of incoming salt is as high as 1 mdd or more, Conversely, it has been found that the weather resistance deteriorates. On the other hand, it has been considered that the addition of Ni is effective for improving the weather resistance in an area where the amount of incoming salt is large.

Based on these findings, the present inventors examined corrosion in an environment with a large amount of incoming salt. As a result, in such an environment, the FeCl ₃ solution was repeatedly dried and wet (this solution was dried and redissolved with water). of repetition) is an essential condition, while pH is lowered by hydrolysis of Fe ^3+, and by the Fe ³⁺ acts as an oxidizing agent, it has been found that the corrosion is accelerated. The corrosion reaction at this time is expressed by the following equation.

Cathode reaction: Fe ³⁺ + e ⁻ → Fe ²⁺ (reduction reaction of Fe ³⁺ )
Of course, besides this reaction,
2H ₂ O + O ₂ + 2e ⁻ → 4OH ⁻
2H ^{+ +} 2e ^- → H ₂
The cathodic reaction also occurs.

On the other hand, for the above reduction reaction, anode reaction: Fe → Fe ²⁺ + 2e ⁻ (Fe dissolution reaction)
Also happens. Therefore, the overall reaction of corrosion is
2Fe ³⁺ + Fe → 3Fe ²⁺ ... Reaction 1
It becomes.

The Fe ²⁺ generated by the reaction 1 is oxidized to Fe ³⁺ by air oxidation, and the generated Fe ³⁺ again accelerates corrosion as an oxidant. At this time, the reaction rate of air oxidation of Fe ²⁺ is generally slow in a low pH environment, but is accelerated in a concentrated chloride solution, and Fe ³⁺ is easily generated. It was found that due to such a cyclic reaction, in an environment where the amount of incoming salt is very large, Fe ³⁺ is constantly supplied, the corrosion of steel is accelerated, and the corrosion resistance is significantly deteriorated.

  In this way, in an environment where the amount of incoming salt is very large, protection by the rust layer cannot be expected, so it is effective to slow the anodic dissolution reaction of the steel itself. That is, in an environment where the amount of flying salt is very large, the steel containing Cr accelerates the anode reaction (Fe dissolution reaction), so the weather resistance deteriorates, whereas the steel containing Ni does not react with the anode reaction. It is estimated that the weather resistance is improved.

  As a result of studying the influence of various alloy elements on the weather resistance based on the above-described corrosion mechanism in an environment with a large amount of incoming salt, it was found that Sn is effective in suppressing the corrosion of steel materials by the above mechanism. Furthermore, it discovered that long-term weather resistance, ie, long-term durability, could be imparted to the structural steel material by coating the Sn-containing film with a thick resin film, and the present invention was completed.

The present invention is Sn, which is an acid-soluble Sn ion source material in an amount of 1 to 54% by mass in terms of metal Sn in a binder resin on the surface of a structural steel material or on a rust layer formed on the steel material. It is a weather resistant structural steel material comprising a compound- containing Sn-containing layer having a thickness of 5 to 50 μm and an organic resin layer having a thickness of 100 μm or more as an upper layer.

The Sn-containing layer may further contain a metal compound that is at least one acid-soluble additional metal ion source material that serves as a source of Cu ²⁺ ions, Ni ²⁺ ions, or Cr ³⁺ ions. In that case, the total amount of these additional metal ion source materials and the Sn ion source material in terms of metal is 65% by mass or less.

“Acid-soluble Sn ion source material” means a material that can be dissolved in an acidic solution to supply one or both of Sn ²⁺ ions and Sn ⁴⁺ ions. Such Sn ion source materials specifically include divalent Sn compounds, tetravalent Sn compounds, and metal Sn. As will be described below, in an acidic solution containing iron ions generated by contact of water with steel in a chloride environment, reactions in both directions of Sn⇔Sn ²⁺ ions⇔Sn ⁴⁺ ions can occur.

The inhibitory effect of Sn on the corrosion of steel materials in a high flying chloride environment is considered to be due to the following mechanism.
(a) When Sn is dissolved as Sn ²⁺ , the concentration of Fe ³⁺ is decreased by a reaction of 2Fe ³⁺ + Sn ²⁺ → 2Fe ²⁺ + Sn ⁴⁺ , thereby suppressing the reaction 1 described above. Sn also has an effect of suppressing anodic dissolution.

(b) Of the Sn dissolved as Sn ²⁺ , the excess not receiving the reaction described in (a) above is deposited as metal Sn on the steel material surface.
(c) The precipitated Sn significantly suppresses the reaction of 2H ⁺ + 2e ⁻ → H ₂ , which is a counter reaction (cathode reaction) of the steel elution reaction (anode reaction). This is thought to be due to high hydrogen overvoltage.

(d) Since precipitation of Sn concentrates on the elution part of steel, the eluted Sn ^<2+ > precipitates as Sn efficiently only in the corroded part.
(e) Once the deposited Sn is corroded directly under or near the deposited portion, it re-elutes as Sn ^{2+ and} precipitates as Sn only on the corroded portion, thereby suppressing the corrosion. To do.

(f) Sn ⁴⁺ also becomes Sn ^{2+ in the} vicinity of the steel surface due to the reaction of Sn ⁴⁺ + 2e ⁻ → Sn ²⁺ , and thus has the effects (c) to (e).
Therefore, if a Sn-containing layer containing a Sn ion source material that serves as a Sn ²⁺ ion and / or Sn ⁴⁺ ion source is formed on the surface of the steel material, the corrosion of the steel material is repeated without depletion of Sn ions. It can be suppressed and exerts its effect semi-permanently.

  The weather-resistant structural steel material of the present invention exhibits long-term durability capable of maintaining corrosion resistance for a long time even in a highly corrosive high-flying chloride environment in which a large amount of chlorides fly. In particular, when the present invention is applied as a steel material for civil engineering or building structures or oil tanks constructed in a beach area, the maintenance cost related to the corrosion prevention of the steel material is remarkably reduced, so the economic effect of the present invention is high.

As described above, in the concentrated chloride environment where thickening of Cl, wet and dry repetition of FeCl ₃ solution is an essential condition, while pH is lowered by hydrolysis of Fe ^3+, and Fe ³⁺ oxidation By acting as an agent, corrosion is accelerated. That is, when water contacts the steel material interface, Fe as a base material is eluted to produce Fe ²⁺ , and a part thereof is oxidized to Fe ³⁺ . If chloride ions are present there, hydrolysis is accelerated and the pH is significantly reduced.

According to the present invention, the lower surface film in contact with the iron surface or rust layer of the steel material is an Sn-containing layer capable of supplying Sn ²⁺ ions and / or Sn ⁴⁺ ions, so that moisture can be passed through the Sn-containing layer. In this case, Sn ²⁺ ions and / or Sn ⁴⁺ ions are generated from the Sn ion source material in the Sn-containing layer and are preferentially deposited as Sn on the surface of the corroded portion. On the deposited Sn, the hydrogen reduction reaction (cathode reaction) is remarkably suppressed, and corrosion is suppressed. Furthermore, some of the Sn ²⁺ ions react with the Fe ³⁺ ions reduces the concentration of Fe ³⁺ ions, suppressing the reaction 1.

  In order to acquire the said effect by Sn, the quantity of Sn ion supply source material in the lower Sn content layer must be 1 mass% or more in metal Sn conversion amount. On the other hand, when this amount exceeds 54% by mass, the amount of the binder resin that binds the Sn ion source material becomes insufficient, the Sn-containing layer becomes brittle, and through-holes that reach the steel surface from the Sn-containing layer surface are formed. This causes flow rust. Therefore, the amount of the Sn ion source material is in the range of 1 to 54% by mass in terms of metal Sn. A preferred range for this amount is 2-40% by weight.

When the Sn ion source material is a Sn compound, the metal Sn equivalent can be calculated according to the following formula:
(Addition amount of Sn compound) × [(Sn atomic weight) / (Molecular weight of Sn compound)].

The Sn ion source material used is an Sn compound that dissolves and ionizes in a low pH region to generate Sn ²⁺ ions or Sn ⁴⁺ ions, that is, acid-soluble divalent and tetravalent Sn compounds, and metal Sn. At least one selected from the above may be used. A divalent Sn compound is preferable. The divalent Sn compound decreases the Fe ³⁺ concentration directly by the reaction of 2Fe ³⁺ + Sn ²⁺ → 2Fe ²⁺ + Sn ⁴⁺ without going through the reaction of Sn ⁴⁺ + 2e ⁻ → Sn ²⁺ . This is because the reaction 1 can be suppressed. Specific examples of the divalent Sn compound include tin (II) sulfate, tin (II) oxide, and tin (II) pyrophosphate. Halides such as tin (II) chloride are acid-soluble, but are not preferred because they are sources of corrosive chloride ions or other halogen ions. Among them, tin (II) oxide is difficult to dissolve in the neutral region and dissolves in the low pH region, so it dissolves only in the chloride-concentrated portion and exerts its effect and is used in the present invention. Particularly preferred Sn ion source material.

In addition to the Sn ion source material, the Sn-containing layer has an additional metal ion source that serves as a source of metal cations that exhibit a weather resistance improving effect, such as Cu ²⁺ ions, Ni ²⁺ ions, Cr ³⁺ ions Substances can coexist and are suitable. An acid-soluble metal compound (or metal) is also used for the metal ion source material. Specific examples of such a metal compound include Cu (NO ₃ ) ₂ , CuSO ₄ , Ni (NO ₃ ) ₂ , NiSO ₄ , Cr (NO ₃ ) ₃ , Cr ₂ (SO ₄ ) _{3 and the} like.

  These additional metal ion source materials for improving the weather resistance can be used singly or in combination of two or more, and the added amount is 1 to 20% by mass of the Sn-containing layer in terms of the total added amount in terms of metal. It is preferable to set the amount within the range. In this case, if the total amount of the additional metal ion source material and the Sn ion source material is too large, the amount of the binder resin is insufficient and the strength of the Sn-containing layer is insufficient. It is preferable to make it 65 mass% or less in metal conversion amount based on the minute.

  The surface treatment agent used for forming the Sn-containing layer may further contain pigments and various additives in addition to the Sn ion source material, the above-described optional additional metal ion source material, and the binder resin. . The solvent for the surface treatment agent can be either water or an organic solvent, and one or more solvents can be used. The resin may be dissolved in a solvent or may be in an emulsion state.

  The binder resin is not particularly limited, and various organic resins used for paints can be used. Specific examples include epoxy resin, urethane resin, vinyl resin, polyester resin, acrylic resin, alkyd resin, phthalic acid resin, butyral resin, melamine resin, phenol resin, and the like. These may be in the state of either a solution or an emulsion. If necessary, a curing agent is added to the binder resin. In particular, a butyral resin alone or a mixture of a butyral resin and another resin compatible with the butyral resin (for example, melamine resin or phenol resin) is preferable from the viewpoint of moisture permeability of the formed film. An inorganic resin that can form a metal oxide type film such as ethyl silicate resin can also be used as the binder resin.

  The amount of the binder resin as a solid content is preferably at least 25% by mass, more preferably at least 30% by mass, based on the total solid content of the surface treatment agent, from the viewpoint of securing the strength of the Sn-containing layer. is there.

  In addition to the above components, the surface treatment agent includes colored pigments such as bengara, titanium dioxide, carbon black, phthalocyanine blue, α-FeOOH, and iron oxide; and extender pigments such as talc, silica, mica, barium sulfate, and calcium carbonate. One or more of each can be added.

  Further, as a known anti-rust pigment, an anti-corrosion pigment such as chromium oxide, zinc chromate, lead chromate, basic lead sulfate and the like, further chromium compounds such as chromium sulfate, and nickel compounds such as nickel sulfate are included. It is not excluded. However, considering the environmental load, the amount of these compounds added (the total amount in the case of two or more) is desirably 20% by mass or less based on the total solid content of the surface treatment agent.

In addition, conventional additives such as thixotropic agents, dispersants, antioxidants, etc. may be added. Moreover, it is also possible to contain phosphoric acid, which is effective for preventing the initial flow rust outflow.
The concentration of the surface treatment agent may be adjusted by diluting with a suitable solvent so that the viscosity of the surface treatment agent is suitable for the coating operation. The coating on the steel surface can be performed according to a conventional method. For example, in the case of an existing structure, an air spray method, an airless spray method, a brush coating method, or the like is appropriate. When painting at the factory, other coating methods such as roll coating and dipping can be employed.

  Since the solvent is preferably evaporated by natural drying after coating, it is preferable to select such a solvent. The coating is performed so that a Sn-containing layer having a thickness of 5 to 50 μm is formed after drying. When the thickness of the Sn-containing layer is in the range of 5 to 50 μm, Sn ions are appropriately supplied to the steel material surface even in a high-flying chloride environment, and the weather resistance of the steel material is improved. The thickness of the Sn-containing layer is preferably in the range of 20 to 50 μm.

On top of the Sn-containing layer containing the Sn ion source material that supplies Sn ²⁺ ions and / or Sn ⁴⁺ ions thus formed, the organic resin layer has a dry film thickness of 100 μm or more as an upper layer. To form. If the thickness of the organic resin layer is 100 μm or more, the organic resin layer completely covers the steel material surface, and it is possible to ensure long-term durability that can guarantee long-term weather resistance by improving corrosion resistance. It becomes. The upper limit of the thickness of the upper organic resin layer is not particularly limited, but from the viewpoint of economy, the dry film thickness is preferably 1000 μm or less.

  By forming a Sn-containing layer having a dry film thickness of 5 to 50 μm containing 1 to 54 mass% of Sn ion source material in terms of metal Sn on the surface of the steel material or the rust layer formed on the steel material Further, peeling of the organic resin layer formed thereon becomes extremely small, which also contributes to ensuring long-term durability.

  The upper organic resin layer is not particularly limited. For example, an epoxy resin, a urethane resin, a vinyl resin, a polyester resin, an acrylic resin, an alkyd resin, a phthalic resin, a butyral resin, a melamine resin, a phenol resin, or the like is used as the organic resin layer. Can be used for forming. Also, coloring pigments such as bengara, titanium dioxide, carbon black, phthalocyanine blue, α-FeOOH, and iron oxide; and one or more extender pigments such as talc, silica, mica, barium sulfate, and calcium carbonate are added. be able to.

  Furthermore, it does not exclude the inclusion of rust preventive pigments such as chromium oxide, zinc chromate, lead chromate and basic lead sulfate as known rust preventive pigments in the organic resin layer. However, considering the environmental load, the addition amount is preferably 20% by mass or less of the organic resin layer. In addition, additives commonly used in paints such as thixotropic agents, dispersants, antioxidants and the like may be included in the organic resin layer. When the organic resin layer contains one or more kinds of pigments and other solid particles as described above, the total amount of the solid particles is preferably 30% by mass or less in order to ensure the film strength.

  The organic resin layer may be diluted with an organic solvent as necessary so that the viscosity becomes suitable for the coating work before coating, after adding optional components to the organic resin liquid (which may be a solution or an emulsion) as desired. Can be adjusted and used for painting. Painting can be performed according to a conventional method, and in the case of an existing structure, methods such as air spray, airless spray, and brush coating are suitable. When painting at a factory, other methods such as roll coating and dipping can be employed. Thereafter, the solvent is evaporated to form an organic resin layer. About the solvent to be used, it is the same as that of the case of the surface area agent used for formation of Sn content layer.

  The structural steel material in which the lower Sn-containing layer and the upper organic resin layer are formed according to the present invention is not particularly limited in the steel type. Although ordinary steel may be used, it is advantageous from the viewpoint of long-term durability if it is a weather resistant steel or a low alloy steel containing Ni, Al, Sn or the like. The form of the steel material is not particularly limited, and may be any form including a plate, a rod, a shaped steel, a pipe, a cast product, and the like. Here, the structural steel material means a steel material that can be used for civil engineering and architectural purposes. Further, since it can sufficiently withstand use in a crude oil tank environment, it can also be applied to an oil tank. As described above, the structural steel material may be an existing steel structure.

  When applied to an existing structural steel material such as an existing bridge or building, the Sn-containing layer and the upper organic resin layer according to the present invention are coated by coating on the rust layer generated on the surface of the steel material. Alternatively, the rust layer may be removed by an appropriate method such as shot blasting before coating. The same applies to the case where rust has already occurred in the steel material when painting at the factory.

  In addition, as steel materials surface-treated with Sn, there are the following steel materials in the prior art, but these are not related to structural steel materials, and the purpose of the surface treatment is not imparting weather resistance, so the present invention and Have different technical ideas.

  (1) As antifouling paints for ship hulls and fish nets, paints containing organotin compounds, that is, tin compounds such as TBT (tributyltin) and TPT (triphenyltin) have been used. However, since this is added for the purpose of antifouling due to adhesion of shellfish or the like, the technical idea is different from the present invention.

(2) Although there is surface treatment by Sn plating performed on the inner surface of canned foods, the steel material is not a structural steel material, and the purpose of the surface treatment is different.
(3) As a surface treatment of electrical steel sheets used for generators, transformers, motors, etc., it is known to form a coating of Sn-treated surface treatment agent on steel, but the steel is not structural. Further, the purpose of the surface treatment is to prevent nitriding of the steel material from the atmosphere during annealing to prevent deterioration of magnetic characteristics, and the technical idea is different from the present invention.

  Four types of test steel materials (all 100 × 60 × 3 mm thick plate materials) having the chemical composition shown in Table 1 were removed by shot blasting and used for coating. The steel materials [1] in Table 1 are so-called weather resistant steels (JIS 3114, SMA), [2] is ordinary steel, [3] is high Ni weather resistant steel, and [4] is Sn-added corrosion resistant steel.

The steel material used for coating was subjected to any of the following pretreatments.
X: Steel material that was exposed to an industrial zone in Amagasaki City, Hyogo Prefecture, 10 meters from the coast after shot blasting, for 30 days and formed a rust layer;
Y: Steel material without rust layer removed by shot blasting.

  In the types and blending ratios shown in Table 2, resin, Sn ion source material (Sn added species), other metal ion source material coexisting in some cases (coexisting cation species), pigment [barium sulfate / talc (by mass ratio) 4/1) Mixture and iron oxide (Bengara)], and other additives (thixotropic agent, anti-settling agent) may be added with an appropriate amount of solvent so that the viscosity (B-type viscometer measurement) is 200 to 1000 cps. By stirring, the surface treating agent used for formation of the lower Sn content layer was produced. Content (mass%) of each component in a table | surface is the mass% based on the total solid of the surface treating agent except the solvent.

As the coexisting cation source materials, Cu ²⁺ ions were copper sulfate (II), Ni ²⁺ ions were nickel nitrate (II), and Cr ³⁺ ions were chromium (III) nitrate. As the solvent, a mixed solvent in which toluene and xylene as aromatic hydrocarbon solvents and isopropyl alcohol as an alcohol solvent were mixed at a volume ratio of 2/1 was used.

The upper resin layer consisted essentially of a resin and contained no metal compound, pigment, or other additives.
The prepared surface treatment agent for forming the Sn-containing layer is applied to the entire surface of a predetermined test steel material by air spray so as to have a predetermined dry thickness, and the coating is dried by allowing the solvent to evaporate to leave the test steel material. An Sn-containing layer was formed on the surface. On top of that, an upper organic resin layer was formed by air spray coating using commercially available paints shown in Table 2.

A cross-cut reaching the steel substrate was put into the test material thus prepared, and then evaluated by SAE (Society of Automotive Engineers) J2334 accelerated corrosion test. SAE J2334 test: wet: 50 ° C., 100% RH, 6 hours, salt adhesion: 0.5% NaCl, 0.1% CaCl ₂ , 0.075% NaHCO ₃ aqueous solution, 0.25 hour, dry: 60 It is a corrosion acceleration test with one cycle (24 hours) at ℃, 50% RH, 17.75 hours, and is said to be similar to the atmospheric exposure test (Hiroo Nagano, Masato Yamashita, Hitoshi Uchida) : Environmental Materials Science, Kyoritsu Shuppan (2004), p.74).

  This test is a corrosion acceleration test that simulates a severe corrosive environment in which the amount of incoming salt exceeds 1 mdd. When there is a thick resin film on the upper layer, it is difficult to evaluate by outdoor exposure. For this reason, a cross cut was put in and evaluated by an accelerated corrosion test, but this test condition is considered to be a very severe test from the viewpoint of peeling of the scratches on the coating film. After 40 cycles of this accelerated corrosion test, the peel distance of the coating film from the crosscut portion was measured. The results are also shown in Table 2.

  In Test Nos. 1 to 10 according to the present invention, peeling from the scratch portion (cross cut portion) was remarkably suppressed. On the other hand, as seen in Test No. 11, when there was no lower Sn-containing layer, peeling of the upper coating film (organic resin layer) proceeded remarkably. In addition, as shown in Test Nos. 12 and 13, when the Sn ion source material in the lower resin layer was 0 or its content was too small, the effect of suppressing the peeling of the upper coating film was not observed. As shown in Test No. 14, when the lower Sn-containing layer was thin, the effect of improving the weather resistance by Sn was not sufficiently obtained. When the content of the Sn ion source material in the Sn-containing layer exceeds 60% by mass in terms of metal as in test number 15, there is almost no adhesion of this Sn-containing layer immediately after coating, and the coating film has been applied since before the test. The test was not carried out due to floating.

Claims (2)

On the surface of the structural steel material or on the rust layer formed on the steel material, the binder resin contains an Sn compound that is an acid-soluble Sn ion source material in an amount of 1 to 54% by mass in terms of metal Sn. A weatherable structural steel material comprising a Sn-containing layer having a thickness of 5 to 50 μm and an organic resin layer having a thickness of 100 μm or more as an upper layer. The Sn-containing layer further contains a metal compound that is at least one additional acid-soluble metal ion source material that serves as a source of Cu ²⁺ ions, Ni ²⁺ ions, or Cr ³⁺ ions. The weatherable structural steel material according to claim 1, wherein the total amount of the compound and the Sn compound in terms of metal is 65% by mass or less.