JPH01280545A - Organic resin coated steel pipe with excellent cathode peeling resistance at high temperature - Google Patents

Abstract

PURPOSE:To improve the cathode peeling resistance in a high temperature by applying and baking on the outer surface of a steel pipe, a chromate treatment agent consisting of a mixture in which the mixture solution of phosphoric acid and chromic acid anhydride is partially reduced by a highmolecular reducing agent, and phosphoric acid and silica particles are added thereto, after that, giving an organic resin coating. CONSTITUTION:A chromate treatment agent layer obtained in heat-baking the mixture wherein the mixture solution of phosphoric acid and chromic acid anhydride are partially reduced by a highmolecular reducing agent of A or B, and silica particles are added thereto, and an organic resin coating layer are laminated in order of the inside to the outer surface of a steel pipe. A is dextrin in which an average molecular weight is in the range of 50,000-250,000, and B is partially saponified polyvinyl acetate in which the molecular weight is in the range of 60,000-140,000, and has a molecular structure shown in the formula, and further the saponification degree is in the range of 80-90%.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to an organic resin-coated steel pipe, and more particularly to an organic resin-coated steel pipe that has excellent high-temperature cathodic peelability.

(Prior art) Organic resin-coated steel pipes, including powder epoxy resin-coated steel pipes, have excellent anti-corrosion properties, and are therefore
It is increasingly being used in pipelines for transporting natural gas, etc. Usually, such pipelines are used for a long time once they are laid, so cathodic protection is usually used in combination. Steel pipes coated with organic resins such as powdered epoxy resins are exposed to rocks, gravel, etc. during transportation or handling during installation, causing penetration damage to the coating, so cathodic protection is applied to prevent corrosion of the underlying parts. Ru. However, especially in the case of excessive corrosion protection, a phenomenon called cathodic peeling occurs in which soil moisture is electrolyzed by the anticorrosion current and the coating is peeled off by the generated hydrogen and alkali, resulting in a reduction in the corrosion protection of the pipeline.

In addition, in recent years, in order to improve the transport efficiency of heavy oil, the material being conveyed in the pipe becomes high temperature due to measures such as heating the heavy oil to lower its viscosity before transporting it, and cathodic peeling resistance ( From now on, improving the "high temperature cathode peelability") is becoming an important issue.

Conventionally, in order to improve the cathodic peeling resistance of an organic resin-coated steel pipe, a method has been proposed in which the surface of the steel pipe is previously subjected to chromate treatment.

For example, in JP-A-52-14392, as shown in FIG. 3, a compound of hexavalent chromium and an amino acid, an acid amide, a lactam, a saturated polycarboxylic acid, an unsaturated polycarboxylic acid, etc. are added to the surface of the steel pipe 1. There is a proposal for a coated steel pipe in which a powder coating film 5 of a thermoplastic or thermosetting resin composition is laminated after applying and baking a chromate treatment agent No. 4 containing an organic component.

Further, as shown in FIG. 4, the above chromate treatment agent layer 4
Instead of 0, chromic acid (Cry) is reduced to Cr6+/total Om=0.35-0. A coated steel pipe laminated with a chromate treatment agent layer 6 obtained by reducing the chromate treatment agent at a ratio of 65% and adding fine silica to the reduced product, and further, as shown in FIG. There is a coated steel pipe in which a layer 7 of a chromate treatment agent is laminated with fine silica and a nonionic water-soluble resin such as polyvinyl alcohol, methylcellulose, polyethylene oxide, or hexamethoxymethylmelamine-modified polyhydroxyethyl acrylate.

(Problems to be Solved by the Invention) The coated steel pipes shown in Figs. 3, 4, and 5 have relatively good cathode peeling resistance at room temperature, but the resistance to cathode peeling at temperatures exceeding 60°C is poor. High temperature cathode removability is extremely poor. The present inventors have described the coated steel pipes shown in FIGS. 3, 4, and 5.
Judging from the results of structural analysis of the chromate treatment agent layer and measurement of the amount of chromium ions eluted into hot water over 60°C, it is thought that high-temperature cathode peeling resistance is poor for the following reasons. .

First, since the chromate treatment agent layer shown in Figure 3 contains a large amount of soluble hexavalent chromium, when a cathodic peel test is performed at temperatures exceeding 60°C, hexavalent chromium ions are easily removed from the chromate coating by hot water entering from the uneven coating penetration part. The coating peels off due to elution.

In the chromate treatment agent layer shown in Figure 4, the bond between the chromium compound in the layer and the particulate silica is weak, so when a cathodic peel test is performed at temperatures exceeding 60°C, the particulate silica in the chromate coating is removed by hot water that enters from the uneven part of the coating. The coating adsorbs water on its surface and swells, causing the coating to peel off, resulting in poor high-temperature cathode peelability.

In addition, in the chromate treatment agent layer shown in Figure 5, the bond between the chromium compound in the chromate coating and the fine particle silica, and the bond between the resin and the fine particle silica, is weak, so if a cathodic peel test is performed at a high temperature exceeding 60°C. The hot water that enters through the uneven part of the coating breaks the bond between the chromium compound and the particulate silica, and between the resin and the particulate silica, and the particulate silica absorbs water on its surface and swells, causing the coating to peel off. Poor resistance and high temperature cathode peelability.

Due to the above-mentioned problems, it has been difficult to obtain an organic resin-coated steel pipe with excellent cathode peeling resistance in a high temperature range exceeding 60° C. using the conventional techniques.

The present invention provides an organic resin-coated steel pipe that has excellent cathode peeling resistance in a high temperature range exceeding 60°C.

(Means for Solving the Problems) As a result of intensive studies to solve the above-mentioned problems, the present inventors applied a mixed aqueous solution of phosphoric acid and chromic anhydride as a polymer reducing agent to the outer surface of a steel pipe. After applying and baking a chromate treatment agent consisting of a mixture of phosphoric acid and silica-based fine particles that has been partially reduced with phosphoric acid and then baked, an organic resin coating with an organic resin coating is applied that has excellent cathodic peeling resistance at high temperatures. It was discovered that a coated steel pipe based on the above-mentioned method can be obtained, leading to the present invention.

That is, as shown in FIG. 1, the present invention applies a mixture of a mixed aqueous solution of phosphoric acid and chromic anhydride partially reduced with a polymer reducing agent and silica-based fine particles added to the outer surface of a steel pipe 1 from the inside. This invention relates to an organic resin-coated steel pipe having excellent cathodic peeling resistance at high temperatures, characterized in that a chromate treatment agent layer 2 obtained by heating and baking and an organic resin coating layer 3 are combined.

Hereinafter, the present invention will be explained in detail.

The steel pipe used in the present invention is a pipe made of alloy steel such as carbon steel and stainless steel. ! This also includes plated steel pipes and double pipes, which have improved corrosion resistance on the inner surface of steel pipes. Examples include plated steel pipes in which the inner surface of the steel pipe is coated with zinc, aluminum, chromium, nickel, zinc-nickel, zinc-nickel-cohalt chrome, etc., and double-walled pipes in which the following metals are bonded to the inner surface. The outer layer of the double tube is a steel pipe, and the inner layer is made of copper, aluminum, titanium, stainless steel, aluminum-magnesium alloy, nickel-chromium-iron alloy, nickel-mo! J Butene-based alloy, nickel-molbutene-chromium
It may be made of a metal or alloy such as a tungsten alloy, a titanium-palladium alloy, a titanium-molybten-zirconium alloy, or a titanium-aluminum-vanadium alloy, as long as the outer layer is a steel pipe.

Next, the chromate treatment agent used for forming the chromate treatment layer of the present invention will be explained.

The chromate treatment agent used in the present invention is an aqueous solution in which phosphoric acid and chromic anhydride (Cry) are dissolved in distilled water, and is partially reduced with a polymer reducing agent to produce phosphoric acid and hexavalent chromium ions. Although trivalent chromium ions and silica-based fine particles are mixed therein, part of the phosphoric acid can be replaced with condensed phosphoric acid such as pyrophosphoric acid or tripolyphosphoric acid, if necessary.

As the polymeric reducing agent used for partial reduction of chromium from hexavalent to trivalent, starch containing a large amount of amylopectin,
For example, corn starch is partially hydrolyzed with a hydrolytic enzyme such as amyloglucosidase, and dextrin (average molecular weight 5oooo to 250,000) or OHOCOCH3, which is easily converted into phosphoric acid ester by reaction with phosphate ions, is prepared. Partially saponified polyvinyl acetate having a diameter of 60,000 to 40,000 is used.

Since the above polymer reducing agent has an extremely high molecular weight, it is difficult to dissolve in a mixed aqueous solution of phosphoric acid and chromic acid at room temperature, so the mixed aqueous solution is heated to 80 to 100° C. and added to completely dissolve. In the above method, the polymer reducing agent dissolved in the mixed aqueous solution of phosphoric acid and chromic acid is chromate treatment agent and its heat baking is inferred from the analysis results of the film and the measurement results of the amount of chromium ions eluted from hot water. At the same time, part of the polymeric reducing agent decomposes and reduces hexavalent chromium to trivalent chromium.
Chromium ions are coordinated with the remaining polymeric reducing agent, and phosphoric acid is further bonded to the chromium ions, which has a remarkable effect on making the chromate film insoluble in hot water.

When using a polymeric reducing agent other than the above, for example, a polysaccharide that is not partially hydrolyzed by an enzyme such as starch, phosphoric acid and chromic acid are used, compared to when using the above-mentioned dextrin or partially saponified polyvinyl acetate. Even if it is added to a mixed aqueous solution, the effect of making the chromate film insoluble in hot water is small because the coordination of chromium ions becomes insufficient.

In addition, as a polymer reducing agent, a water-soluble resin other than the above-mentioned partially saponified polyvinyl acetate, such as -+-c)1. -C
When using polyvinyl alcohol having a molecular structure of H-)-nH, all substituents are active hydroxyl groups, so
When added to a mixed solution of phosphoric acid and chromic acid, it is susceptible to oxidative decomposition by the chromic acid, and the effect of making the chromate film insoluble in hot water is reduced.

In addition, when using a low-molecular reducing agent such as methyl alcohol, succinic acid, or sorbitol instead of a high-molecular reducing agent, most of it is decomposed when added to a mixed aqueous solution of phosphoric acid and chromic acid. It has almost no effect of making it insoluble in water.

Among the above polymer reducing agents, dextrin having an average molecular weight in the range of 50,000 to 250,000 is used. If the average molecular weight of IJ is less than 50,000, it will have little effect on making the chromate film insoluble in hot water, and if it exceeds 250,000, it will be difficult to dissolve in a mixed aqueous solution of phosphoric acid and chromic acid, and the resulting film will be difficult to coat. This is undesirable because it impairs smoothness.

Further, the above partially saponified polyvinyl acetate has a molecular weight of 60
000 to 140,000 and a saponification degree of 80 to 90%. If the molecular weight of the partially saponified polyvinyl acetate is less than 60,000, the effect of making the chromate film insoluble in hot water is small;
If it exceeds 0, it is not desirable because it is difficult to dissolve in a mixed aqueous solution of phosphoric acid and chromic acid and the smoothness of the coating obtained by coating is impaired.

In addition, if the degree of saponification of partially saponified polyvinyl acetate is less than 80 degrees, the proportion of hydroxyl groups added to the molecular chain is small, resulting in less phosphoric acid ester formation; Since the ratio of the number of hydroxyl groups present is too high, when added to a mixed aqueous solution of phosphoric acid and chromic acid, it is easily decomposed and the effect of making the chromate film insoluble in hot water is reduced.

The above dextrin and partially saponified polyvinyl acetate are used in amounts necessary to maintain the ratio of hexavalent chromium to total chromium at a desired ratio. The desired ratio is a weight ratio of hexavalent chromium to total chromium in the range of 0.35 to 0.65.

Regarding this ratio, if the weight ratio of hexavalent chromium to total chromium is less than 0.35, the adhesion between the steel pipe surface and the chromate coating will decrease, and if the weight ratio exceeds 0.75, the chromate coating will become resistant to hot water. The solubilizing effect is significantly reduced.

In addition, the weight ratio of hexavalent chromium to the total chromium mentioned above is 0.
The amount of dextrin required to achieve a range of 35 to 0.65 is o, as a weight ratio to the total solid content in the chromate treatment solution.
oos to 0.05B, and the amount of partially saponified polyvinyl alcohol is in the range of 0.009 to 0.062 as a weight ratio to the total solid content in the chromate treatment liquid.

In addition, the phosphoric acid added to the chromate treatment agent mentioned above is estimated from the analysis results of the chromate treatment agent and its coating film, and the measurement results of the amount of chromium ions dissolved in hot water. ■ Undecomposed dextrin or parts It binds to a coordination compound in which chromium ions are coordinated to saponified polyacetate vinyl.

(2) It binds to a hydroxyl group on the surface of the silica-based fine particles, and the phosphoric acid group bound to this hydroxyl group further binds to free chromium ions and dextrin or partially saponified polyvinyl acetate to which the above-mentioned chromium ions are coordinated.

Due to these effects, the chromate film is integrated, and some of the phosphate groups bonded to dextrin or partially saponified polyvinyl acetate, in which free phosphoric acid and chromium ions are coordinated, are combined with free hexavalent chromium ions. It reacts with the surface of the steel pipe to generate an iron chromium phosphate compound that firmly adheres the chromate film to the surface of the steel pipe, making the chromate film insoluble in hot water and preventing the generation of alkali during cathodic peeling tests. It is also thought that the coating becomes difficult to peel off.

The amount of phosphoric acid added is such that the weight ratio of Po43- to total chromium is in the range of 0.5 to 2.0. Linba addition amount is 0
．． If it is less than 5, there is almost no effect described above, and if it is more than 2.0, free soluble phosphoric acid remains in the chromate coating, and the effect of making the chromate coating insoluble in hot water decreases.

In addition, as the silicate-based fine particles added to the chromate treatment agent, for example, Aerosil 2 manufactured by Nippon Aerosil Co., Ltd.
00, Aerosil 300, Aerosil 380, Aerosil 0X50. Nip Seal L 30 from Nippon Sirikani Gyosha
0, chive 7' seal N 30OA, 27 but seal g
200. Silica particles such as 27 Pseal E200A, Aerosil C0K84 manufactured by Nippon Aerosil Co., Ltd. Silica-alumina fine particles such as Aerosi/L'MOX80 and Aerosil MOX170, Snowtex O, Snowtex OL, Snowtex OS, Snowtex OML manufactured by Nichikagaku Kogyo Co., Ltd., and Cataloid manufactured by Catalysts and Chemicals Co., Ltd.
SN, Cataloid SA.

Colloidal silica such as Cataloid 520L, Alumina Sol 100, Alumina Sol 2 manufactured by Nichijo Kagaku Kogyo Co., Ltd.
Use one type or a mixture of two or more types of silica alumina granite such as 00.

Inferred from the analysis results of the chromate treatment agent and its heated coating, and the measurement results of the elution amount of phosphate ions and chromium ions in hot water, the silica-based fine particles mentioned above have hydroxyl groups on their surface. The chromate treatment agent combines with dextrin or partially saponified polyvinyl acetate coordinated with phosphate ions and chromium ions in the chromate treatment agent, so the coating obtained by applying the chromate treatment agent to the steel pipe surface is integrated, and the phosphoric acid from the coating is combined with the chromate treatment agent. It is believed that this prevents the elution of ions and chromium ions and significantly improves the high-temperature cathode peeling resistance.

The amount of silica-based fine particles added is such that the weight ratio of silica-based fine particles to the total chromium in the chromate treatment solution is 0.5 to 2.5.
Add so that it falls within the range of . If the amount of silica-based fine particles added is less than 0.5, the above effect will hardly be achieved, and if the amount added is more than 2.5, the fluidity of the chromate treatment agent will deteriorate significantly, and the smoothness of the coating obtained by applying it to the steel pipe surface will deteriorate. undesirable because it inhibits

If the adhesion between the chromate film and the organic resin coating is insufficient, metals such as ammonium metavanadate, dialuminum hydrogen phosphate, ammonium molybutate, cobalt carbonate, manganese carbonate, etc. may be added to the chromate treatment agent. After appropriately selecting from phosphorus/oxyacids such as salts, phosphomolybdic acid, phosphotungstic acid, and phosphovanadic acid,
Added.

Before applying the above-mentioned chromate treatment agent to the surface of the steel pipe, scale etc. on the pipe surface are removed by pickling, sandblasting, grid blasting, shot blasting or the like.

When a chromate treatment agent is applied to the surface of a steel pipe from which scale has been removed, hexavalent chromium is reduced by the oxidation effect on the pipe surface and the heating of the pipe after application, resulting in a dehydration condensation reaction between the surface of the silica-based fine particles and phosphoric acid. The dehydration condensation reaction between the phosphoric acid groups of the dehydrating fastener and chromium ions and chromium ion-coordinated dextrin or partially saponified polyvinyl acetate is promoted, resulting in a chromate coating that is poorly soluble in hot water and has excellent alkali resistance. is generated.

The baking temperature of the chromate treatment agent is the steel pipe surface temperature.
A temperature of 0 to 300°C is suitable. If the steel pipe surface temperature is less than 120°C, it will take a very long time to insolubilize the chromate treatment agent layer, making it unsuitable for practical use. Since bond cleavage occurs and free soluble phosphoric acid is produced, the insolubility of the chromate film is rather reduced and the hot water immersion resistance is deteriorated.

Further, the amount of the chromate treatment agent deposited is preferably 100 to 900/d as the total chromium weight. This amount of adhesion is 10
If it is less than 0 m9/rrl, the effect of the chromate treatment agent will not be exhibited, and if it exceeds 900 rn9/rrl, a strong film will not be formed and the adhesiveness will decrease.

Next, the organic resin coating used in the present invention will be explained.

The organic resin coating in the present invention includes epoxy resin, epoxy silicone resin, polyimide epoxy resin, polyphenylene sulfide resin, polyether sulfone resin, polyurethane resin, modified polyethylene, modified polypropylene, modified ethylene propylene copolymer, It is a coating made by applying and curing a powder coating whose main component is an organic resin such as a modified polyamide/propylene copolymer, a solvent-based coating diluted with a solvent, or a liquid solvent-free coating.

In addition to the organic resin as the main component, pigments, filler reinforcing agents, etc. can be added to the above-mentioned organic resin paint. Pigments include silica, silica/alumina, rutile titanium oxide, glass, mapico, zirconium silicate, magnesium silicate, talc, barium sulfate, alumina, zinc chromate, strontium chromate, lead cinnamide,
General commercially available fine powder, flake or scale pigments such as lead zinc oxide, zinc phosphate, aluminum phosphate, calcium phosphate, silicomolybutenic acid, silicotungstic acid, zinc phosphomolybdate, etc., but they require aesthetic appearance. In some cases, cadmium yellow, polyazo yellow, quinophthalone yellow, isoindolinone yellow,
Add color pigments such as quinacridone yellow, red red, polyazo brown, azo lake yellow, perylene red, phthalocyanine blue, phthalocyanine clean, red red, cobalt aluminate, aniline black, carbon blank, ultramarine blue, fine aluminum powder, etc. You can also do that.

Further, the filling strength agent is an inorganic fiber filler such as glass, slag, silicon carbide, carbon, boron, poron nitride, alumina, etc., and an organic fiber filler such as nylon, polyester, vinylon, aramid, Kepler, etc.

The organic resin-coated steel pipe according to the present invention can be obtained, for example, by the manufacturing method shown in FIG. That is, the chromate treatment agent according to the present invention is applied to the surface of the steel pipe 1 from which scale and the like have been removed using the chromate treatment agent application device 9, and baked using the heating device 10. Next, an organic resin coating is applied to the surface thereof by an organic resin coating device 11 to produce an organic resin coated steel pipe.

The organic resin coating device 11 includes an electrostatic powder coating machine, solvent-based and solvent-free epoxy resin paints, epoxy-silicon resin paints, and polyphenylene sulfide-based paints when powdered epoxy resin paint is used as the organic resin-based coating. When using a paint, a polyurethane resin paint, a polyimide/epoxy resin paint, etc., it can be appropriately selected from conventionally known methods such as a spray paint machine and a roll coater.

In order to specifically explain the present invention, a preparation example of the chromate treatment agent according to the present invention and JP-A-52-143934 will be described below.
Comparative formulation examples of chromate treatment agents and manufacturing examples of organic resin-coated steel pipes that are compatible with the above publications are listed below.

Preparation example 1 of chromate treatment agent The following solutions ■, ■, ■ were prepared.

(2) Mixed aqueous solution of phosphoric acid and chromic acid 49.22 F of phosphoric acid and 76.8 F of chromic anhydride were dissolved in 247.6 F of distilled water.

05% by weight partially saponified polyvinyl acetate aqueous solution molecular weight 5s
Partially saponified polyvinyl acetate having a saponification degree of 87 degrees was added to distilled water and left to stand for 2 hours to swell. Next, this aqueous solution was heated to 98° C. to completely dissolve it, thereby creating an aqueous solution containing 5% by weight of partially saponified polyvinyl acetate.

010% by weight Aerosil 200 aqueous solution Aerosil 200 manufactured by Nippon Aerosil Co., Ltd. was used as the silica-based fine particles. Add Aerosil 200 to distilled water,
The mixture was stirred and dispersed using a high-speed mixer (rotation speed: 30 o rpm) to prepare an aqueous solution containing 10% by weight of Aerosil 200.

Next, the mixed aqueous solution 373 of phosphoric acid and chromic acid mentioned above
．． 6F was added with 106 liters of the 5% by weight partially saponified polyvinyl acetate aqueous solution (2) and heated to 90°C to reduce some of the hexavalent chromium ions to trivalent chromium ions.

The weight ratio of hexavalent chromium to total chromium in the aqueous solution is 0.
．． 60, the weight ratio of PO43- to total chromium is 1.1
It was 6. Next, add 10 of the above (①) to the above reduced aqueous solution.
A chromate treatment agent A according to the present invention was prepared by adding and dispersing a wt% Aerosil 200 aqueous solution 51562.

5i02 relative to the total chromium in the chromate treatment agent A
(Aerosil 200) had a weight ratio of 1.29.

Preparation Example 2 of Chromate Treatment Agent Chromate treatment agent B according to the present invention was prepared by adding dextrin with an average molecular weight of 120,000 instead of the partially saponified polyvinyl acetate as the polymer reducing agent used in the preparation of the above-mentioned chromate treatment agent A. did. The weight ratio of trivalent chromium to total chromium in chromate treatment agent B was 0.38, the weight ratio of Po and 3- to total chromium was 1.20, and the weight ratio of 5i02 to total chromium was 1.60.

Comparative Preparation Example 1 of Chromate Treatment Agents As comparative materials, Chromate treatment agents C, D, and E corresponding to JP-A-52-143934 were prepared.

[Chromate treatment agent C]

76.8 P of chromic anhydride and 2 succinic acid in 700 ml of distilled water
8.8F and added Barbir d19.2 to the aqueous solution.
y was added and dispersed, and distilled water was further added to prepare 1 t of chromate treatment agent C.

[Chromate treatment agent]

76.8 F of chromic anhydride was dissolved in 823 Y of distilled water, 8.42 Y of wheat starch was added thereto, and the mixture was heated and boiled for 1 hour to partially reduce the hexavalent chromium ions. The weight ratio of trivalent chromium to total chromium in the reduced aqueous solution is 0.
It was 38. Add the above Aerosil $20 to the reduced aqueous solution.
0 was added and stirred and dispersed to prepare a chromate treatment agent.

[Chromate treatment agent E]

7.6.8 F of chromic anhydride was dissolved in 5 ooy of distilled water, 3.42 g of corn starch was added thereto, and the mixture was heated and boiled for 1 hour to partially reduce the hexavalent chromium ions. The weight ratio of trivalent chromium to total chromium in the reduced aqueous solution was 0.22. After stirring and dispersing the aforementioned Aerosil $200 232 and talc powder (magnesium silicate) 23f in the reduced aqueous solution, polyvinyl alcohol (manufactured by Nippon Gosei Kagaku Kogyo Co., Ltd., Gohsenol NM-11) was added.
) was added and dispersed to prepare chromate treatment agent E.

Production Example 1 An ilIi pipe (200AX5500w length xls+m thickness) was subjected to grid blasting treatment, and the above-mentioned chromate treatment agents A to E were applied to its surface at a rate of 280m9/rr in terms of total chromium adhesion amount.
? Apply, heat to 230°C, bake, and immediately apply powder epoxy resin paint (Scotch Coat 206N, manufactured by 3M).
was electrostatically coated to a film thickness of 400μ, heated and cured for 3 minutes, and then heated to a surface temperature of 240μ using a far-infrared heater.
℃ and then heated for 4 minutes to produce a powder epoxy coated steel pipe.

The above powder epoxy coated steel pipe was subjected to a high-temperature cathode peeling test (test temperature 80°C, electrolyte 3 t NaCJ, voltage -1,
5,,;,V (Cu/Cu5O,, standard electrode),
The test results were shown in Table 1.

Table 1 From the results shown in Table 1, a significant difference in peeling distance was observed, especially the chromate treatment according to the present invention in which partially saponified polyvinyl acetate was used as a reducing agent and phosphoric acid and silica-based fine particles were added as a surface treatment agent. When agent A is used and when chromate treatment agent B according to the present invention, which uses dextrin as a reducing agent and contains phosphoric acid and silica-based fine particles, is disclosed in JP-A-52-1
Chromate treatment agents C and D corresponding to Publication No. 43934,
It was confirmed that much better results were obtained compared to the case where E was used.

Production Example 2 The above powder epoxy coated steel pipe was produced in the same manner as in Production Example 1 by changing the weight ratio of po,''-(phosphate ions) to total chromium in chromate treatment agent A. FIG. 6 shows the results of the high temperature cathode peeling test described above on the epoxy coated steel pipe.

From the results shown in FIG. 6, good high-temperature cathodic peelability is exhibited when the weight ratio of PO43- to total chromium in chromate treatment agent A is in the range of 0.5 to 2.0. Therefore, it is necessary to add phosphoric acid to the chromate treatment agent used in the present invention so that the weight ratio of po to total chromium is in the range of 0.5 to 2.0.

Production Example 3 In the same manner as Production Example 1, 115 weight of the phosphoric acid added to chromate treatment agent A was replaced with (1) pyrophosphoric acid (2) dialuminum hydrogen phosphate to produce the above-mentioned powder epoxy coated steel pipe.

Table 2 shows the results of the high-temperature cathode peel test performed on this powder epoxy coated steel pipe.

From the results shown in Table 2, even if 115 parts by weight of phosphoric acid added to chromate treatment agent A is replaced with birophosphoric acid or dialuminum hydrogen phosphate, good high-temperature cathodic peelability can be obtained.

Production Example 4 In the same manner as in Production Example 1, the above powder epoxy coated steel pipes were produced by changing the weight ratio of StO, (Aerosil $E200) to the total chromium in chromate treatment agent A. The above-mentioned high-temperature cathode peeling test was conducted on this powder epoxy coated steel pipe, and the results are shown in FIG.

From the results shown in FIG. 1, good high-temperature cathodic peelability was obtained when the weight ratio of SiO□ to total chromium in chromate treatment agent A was in the range of 0.5 to 2.5. Therefore, the chromate treatment agent used in the present invention needs to be added with 7-Liner fine particles such that the weight ratio of 5102 to the total chromium is in the range of 0.5 to 2.5.

Production Example 5 The above-mentioned powder epoxy coated steel pipe was produced in the same manner as in Production Example 1, except that the silica-based fine particles added to chromate treatment agent A were changed as follows.

■Silica fine particles (manufactured by Nippon Aerosil Co., Ltd., Aerosil Son 2)
00) ■Silica/alumina fine particles (manufactured by Nippon Aerosil Co., Ltd., C0
K84) ■ Colloidal silica (manufactured by Nichikagaku Co., Ltd., Snowtex 0
) Titanium oxide fine particles (Titan Kogyo, K R310) These powder epoxy coated steel pipes were subjected to the above-mentioned high temperature cathode peel test, and the results are shown in Table 3.

Table 3 From the results in Table 3, chromate treatment agent A has 1. When using silica-based fine particles (silica fine particles, silica/alumina fine particles, and colloidal silica) as fine particles to add 'tM, good high-temperature cathode peelability can be obtained in all cases.
When titanium oxide fine particles are used, peeling becomes large.

Therefore, it is necessary to add silica-based fine particles to the chromate treatment agent used in the present invention.

Production Example 6 In the same manner as in Production Example 1, the above powder epoxy coated steel pipes were produced by changing the molecular weight of the partially saponified polyvinyl acetate used in the preparation of chromate treatment agent A. FIG. 8 shows the results of the Takahata cathode peeling test and testing performed on this powder epoxy coated steel pipe.

From the results shown in Figure 8, the molecular weight of the partially saponified polyvinyl acetate used for preparing chromate treatment agent A is 60,000 to 14.
Good high temperature cathode peelability can be obtained within the range of 0,000.

Therefore, the molecular weight of the partially saponified polyvinyl acetate added to the chromate treatment agent used in the present invention is 60,000 to 140.
It needs to be in the range of 000.

Production Example 7 In the same manner as in Production Example 1, the above-mentioned powder epoxy coated steel pipes were produced by changing the degree of saponification of the partially saponified polyvinyl acetate used in the preparation of chromate treatment agent A. FIG. 9 shows the results of the above-mentioned high-temperature cathode peeling test performed on this powder epoxy coated steel pipe.

From the results shown in FIG. 9, good high-temperature cathodic peelability can be obtained when the degree of saponification of the partially saponified polyvinyl acetate added to chromate treatment agent A is in the range of 80 to 90%. Therefore, it is necessary that the degree of resaponification of the partially saponified polyvinyl acetate added to the chromate treatment agent used in the present invention is in the range of 80 to 90 degrees.

Production Example 8 In the same manner as in Production Example 1, the partially saponified polyacetylated vinyl used in the preparation of chromate treatment agent A was replaced with the following reducing agent to produce the above-mentioned powder epoxy coated steel pipe.

Partially saponified polyvinyl acetate (molecular weight 5sooo, degree of saponification 87%) ■Dextrin (average molecular weight 120,000) ■Wheat starch ■Corn starch ■Succinic acid ■Methyl alcohol ■Polyvinyl alcohol Regarding these powdered epoxy-coated steel pipes, the above-mentioned The results of the high temperature cathode peeling test are shown in Table 4.

From the results shown in Table 4, good high-temperature cathodic peelability was obtained only when partially saponified polyvinyl acetate or dextrin was used as the reducing agent in the Chronaut treatment agent. Therefore, it is necessary to use partially saponified polyvinyl acetate or dextrin as the reducing agent for the chromate treatment agent according to the present invention.

Mounting Example 9 Steel pipes (20 pieces AX5500 length x 5°8I51 thickness) were glysothoblasted, and the chromate treatment agent A was applied to the surface to give a total chromium deposit of 380 m9/n? Apply,
It was heated and baked at 150°C. The following organic resin paint was applied to the outer surface of this chromate-treated steel pipe and cured by heating to produce an organic resin-coated steel pipe according to the present invention.

■Solvent-free epoxy resin paint (film thickness 1.5+e+) ■Solvent-type epoxy resin paint (film thickness 600μ) (0100 above)
(mix 20 parts by weight of thinner) ■Epoxy/silicone resin paint! (M thickness 1.8w)
■Epoxy silicone resin paint■ (film thickness 2.5 rm
)■Epoxy/silicone resin paint I (film thickness 600μ
)■Polyimide/epoxy resin paint (film thickness 600μ)■
Polyphenylene sulfide resin paint (film thickness 400μ)
■Polyether sulfone resin paint (film thickness 400μ)■
Polyurethane Resin Paint (Film Thickness 2.5.) Table 5 shows the results of the high temperature cathodic peel test described above for these organic resin coated steel pipes.

Table 5 From the results shown in Table 5, good high-temperature cathodic peeling resistance can be obtained no matter which organic resin paint is used, and good results can be obtained when the chromate treatment agent of the present invention is used for the surface treatment of steel pipes. It will be done.

Production Example 10 A steel pipe (200AX 5500am length x 5.8+o+thickness) was subjected to grid blast treatment, the above-mentioned chromate treatment agent B was applied to its surface so that the total chromium coating amount was 590F-9/m', and the tube was heated to 200°C. It was heated and baked. Then,
An organic resin-coated steel pipe according to the present invention was manufactured by electrostatically applying the following organic resin powder coating.

■Maleic anhydride-modified low-density polyethylene ■Maleic anhydride-modified linear low-density polyethylene ■Maleic anhydride-modified high-density polyethylene ■Maleic anhydride ethylene/propylene copolymer ■Maleic anhydride-modified polyamide/propylene copolymer ■Anhydrous Maleic acid-modified polypropylene - Vinyl silane-modified linear low-density polyethylene - Vinyl silane-modified ethylene propylene copolymer These organic resin-coated steel pipes were subjected to the high temperature cathode peel test described above, and the results are shown in Table 6.

Table 6 From the results shown in Table 6, good high-temperature cathodic peeling resistance can be obtained no matter which organic resin powder coating is used, and good high-temperature cathodic peeling resistance can be obtained when the chromate treatment agent of the present invention is used for the base treatment of steel pipes. Get results.

(Effect of xA Ming) As is clear from the manufacturing examples, the organic resin-coated steel pipe according to the present invention has a chromate treatment agent layer on the base that has excellent hot water resistance compared to conventional organic resin-coated steel pipes. ,
Because it has extremely high-temperature cathode peeling resistance, it has become possible to provide an organic resin-coated steel pipe with unprecedented durability.

[Brief explanation of the drawing]

FIG. 1 shows a partial cross section of an organic resin coated steel pipe according to the present invention.
Fig. 2 is a schematic explanatory diagram showing an example of the present invention; Fig. 3;
Figures 4 and 5 are partial cross-sectional views of organic resin-coated steel pipes coated with conventional methods, and Figure 6 is the weight ratio of PO43- to total chromium in chromate treatment agent A of powder epoxy-coated steel pipes and high-temperature cathode peeling. A graph showing the relationship with the exam. FIG. 7 is a graph showing the relationship between the weight ratio of 5102 to total chromium in the chromate treatment agent for powder epoxy-coated steel pipes and high-temperature cathode peeling tests. FIG. 8 is a graph showing the relationship between the molecular weight of partially saponified polyvinyl acetate, which is chromate treatment agent A for powder epoxy coated steel pipes, and the hot cathode peel test. FIG. 9 is a graph showing the relationship between the degree of saponification of partially saponified polyvinyl acetate used as chromate treatment agent A for powder epoxy-coated steel pipes and high-temperature cathode peeling test. 1... Steel pipe 2... Chromate treatment agent layer 3... in which a mixed aqueous solution of phosphoric acid and chromic acid is partially reduced with dextrin or partial polyvinyl acetate, silica-based fine particles are added, and heat baked. Organic plum fat-based coating layer 4: Hexavalent chromium compound and organic components (amino acids, acid amides, lactams, saturated and unsaturated polycarboxylic acids)
Chromate treatment layer 5 consisting of... Powder coating film 6 of thermoplastic or thermosetting resin composition... Chromate (CrO3) is combined with a reducing agent (saccharides, polyhydric alcohols, -hydric fluorols, alkylolamines) , aromatic polyhydric alcohol, phosphorous acid] and added fine powder silica to this. 7...Chromic acid (CrO3') is partially reduced with a reducing agent, This is combined with finely divided silica and nonionic water-soluble resin (
Chromate treatment agent layer 8 to which polyvinyl alcohol, methylcellulose, polyethylene oxide, hexamethoxymethylmelamine-modified polyhydroxyethyl acrylate was added...Powder epoxy coating film 9...Chromate treatment agent coating device 10...Heating device 11... Organic resin coating equipment Figure 1 Figure 2 n Figure 3 Factor 4 Section 5 Figure 6 Factor 7 SiO2/total chromium C ratio) Figure 8 Molecule l Figure 9 Degree of saponification (Z)

Claims (2)

[Claims] (1) Heat and bake a mixture of a mixed aqueous solution of phosphoric acid and chromic anhydride partially reduced with the following polymer reducing agent A or B and silica-based fine particles added to the outer surface of the steel pipe from the inside. An organic resin-coated steel pipe with excellent high-temperature cathode peeling resistance, characterized by laminating the resulting chromate treatment agent layer and an organic resin coating. (A) Dextrin whose average molecular weight is in the range of 50,000 to 250,000 (B) whose molecular weight is in the range of 60,000 to 140,000, has a molecular structure of (m+n)×100(%) is 80-90%
Partially saponified polyvinyl acetate in the range of . (2) Organic resin coatings include epoxy resin, epoxy/silicone resin, polyimide/epoxy resin, polyphenylene sulfide resin, polyether sulfone resin, polyurethane resin, modified polyethylene, modified polypropylene, modified ethylene/propylene copolymer, and modified polyamide.・An organic resin with excellent high-temperature cathode peelability as described in claim (1), which uses organic resin-based powder coatings such as propylene copolymers, solvent-based coatings, and solvent-free coatings. Resin coated steel pipe.