JPH03229878A - Re-corrosion protecting method for steel product - Google Patents

Abstract

PURPOSE:To satisfactorily apply re-corrosion protecting treatment to a steel product used, e.g. for pipeline laid underground by treating the surface of a steel product by using an electrolytic solution containing specific percentage of citric acid. CONSTITUTION:At the time of applying re-corrosion protection to a steel product, e.g. for pipeline laid underground, the surface of the steel product is covered and held with a cloth, etc., impregnated with electrolyte (in which Na2SO4, K2SO4, etc., are used as an electrolyte) containing 10-40wt.% citric acid, by which red rust on the surface of steel product is converted into black rust. Further, it is preferable to regulate the treating temp. at this time to >= about 20 deg.C, and the treating time is properly controlled according to the size of the material to be treated, the concentration of the electrolytic solution, etc. By this method, superior corrosion protecting property can be provided to the surface of the steel product.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a re-corrosion prevention method for steel materials, and more particularly to a re-corrosion prevention method for steel materials used for underground pipelines, various marine structures, and the like.

(Prior art) In general, steel materials such as steel pipes used for underground pipelines and offshore structures are coated with various types of anti-corrosion paints or pasted with anti-corrosion tape. It is being processed. However, all of these anti-corrosion materials have a service life and do not maintain their original applied state forever, and after a long period of time, there is a phenomenon in which the paint or tape peels off from the steel surface and floats up. arise. When such a phenomenon occurs, it is necessary to remove the anticorrosion material in this area to expose the steel surface, and then apply anticorrosion treatment to this exposed surface again.

At the point when such re-corrosion prevention treatment becomes necessary,
A layer of red rust represented by γ-FeOOH may be formed on the surface of the steel material, and red rust may be newly formed before the anticorrosion treatment is performed again.

This red rust has poor adhesion to the surface of the steel material, and in its original state, it is difficult to apply anticorrosive treatment using ordinary anticorrosive paints or anticorrosive tapes. Therefore, the following three methods have been tried as anti-corrosion treatment methods for such rusted surfaces, in which a paint containing a component that chemically reacts with red rust is applied to the rusted surface to impart anti-corrosion ability to the red rust layer. ing.

The first method is to apply a paint made of low-viscosity processed fish oil or low-molecular-weight petroleum resin as an undercoat to the surface where red rust occurs, and allow it to penetrate deep into the red rust layer.

The second method is to apply a paint containing a pigment to the rusted surface and react the pigment with red rust to convert it into chemically stable magnetite.

The third method is to apply a paint containing a complex to the rusted surface, converting the iron ions in the red rust into a stable chelate compound, and suppressing the further growth of the red rust. .

(Problems to be Solved by the Invention) In the case of the first method, even if the paint penetrates deep into the red rust layer, the adhesion between the red rust layer and the steel surface can be sufficiently improved. It won't be. Therefore, this method does not end up imparting anti-corrosion ability.

In the case of the second and third methods, it cannot be said that the adhesion between the red rust layer and the steel surface has been sufficiently improved, and the reaction between the iron component of the red rust and the pigment or complex does not proceed completely, so the side bottom This is not a countermeasure that can be put to practical use.

In this way, conventionally, the various paints described above were applied to the red rust layer to form an undercoat layer, and then the anticorrosion material was applied again on top of the undercoat layer. However, in this method, the anticorrosion performance of the undercoat layer is poor, and it is insufficient as a re-corrosion protection treatment method. Therefore, in order to complete the re-corrosion prevention treatment, the red rust on the surface of the steel material may be removed in advance and then the corrosion prevention treatment performed, but this is not a very practical method.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to solve the above-mentioned problems and to provide a method that can perform simple and excellent re-corrosion treatment without removing red rust during re-corrosion treatment of steel materials.

(Means and effects for solving the problem) In order to achieve the above object, the re-corrosion prevention method for steel materials of the present invention includes treating the surface of the steel materials with an electrolytic solution containing 10 to 40% by weight of citric acid. Configure.

The electrolyte solution used in the present invention contains citric acid as a component imparting anticorrosion ability in the electrolyte aqueous solution.

The content of citric acid in the electrolyte is 10 to 40% by weight. If this content is less than 10% by weight, the effect of imparting anticorrosion ability will be insufficient, and if it exceeds 40% by weight, the excessive amount of acid will cause corrosion of the steel rather than the expression of anticorrosion effect. The preferred content ratio of citric acid is 20-3
It is 5% by weight.

The electrolyte constituting the electrolytic solution preferably has high ionic conductivity, and includes, for example, sodium sulfate, potassium sulfate, and sodium carbonate.

The content ratio of the electrolyte can be appropriately set so that the ionic conductivity becomes large and the re-corrosion prevention treatment can be performed sufficiently.

The method of treating the surface of the steel material with an electrolytic solution is not particularly limited as long as the method allows the steel material and the electrolytic solution to be kept in contact for a certain period of time. For example, the surface of the steel material is coated and held for a certain period of time with a cloth impregnated with an electrolytic solution, or the steel material is wrapped with a plastic cover so that a gap is left at a certain distance from the outer surface.
A method can be applied in which an electrolytic solution is poured into the gap and held for a certain period of time.

The treatment time can be adjusted as appropriate depending on the size of the object to be treated, the concentration of the electrolytic solution, etc.

Although the treatment temperature is not particularly limited, it is preferably 20° C. or higher in order to quickly proceed with the re-corrosion prevention treatment.

After such re-corrosion prevention treatment with the electrolytic solution, after washing with water and drying treatment as required, it is possible to perform further known corrosion prevention treatment such as applying an anti-corrosion paint or pasting an anti-corrosion tape.

In the re-corrosion treatment of the present invention, the following reaction proceeds due to the action of citric acid and the action of various ions in the electrolyte,
Red rust (7-FeOOH) is converted to black rust (Fe3s4).

Fe" + 87-Fe00H+2e--+ 3Fez
On 4H20 This black rust is a chemically stable compound, and since it adheres strongly to the surface of the steel material, it will not peel off, and the conversion of this red rust to black rust gives the steel material anti-corrosion ability. be able to.

(Example) Examples 1 to 3 and Comparative Example 1.2 A 300A steel pipe (JIS G3452) was left outdoors for one month. There was a thin layer of red rust on the surface of the steel pipe. This steel pipe was subjected to anti-corrosion treatment again in the following manner.

This steel pipe was surrounded by an FRP cover, and the inside was filled with an electrolytic solution having the composition shown in Table 1, and then held at 15°C for 1 hour (Example 1.2 and Comparative Example 1.2) . Further, a 0.7 mm thick polyester nonwoven fabric impregnated with an electrolytic solution having the composition shown in Table 1 was wound around the steel pipe twice and held at 15° C. for 1 hour (Example 3).

After the anticorrosive treatment, it was washed with water and dried at 15°C for 24 hours.

Regarding the steel pipe after treatment, the state of rust on the surface and 180
The adhesive strength of the vinyl was measured by the following method.

The results are shown in Table 1.

Surface condition of steel pipe: Visually observed.

+80° Peel Adhesive Strength - The adhesive strength was measured when a tape coated on a polyethylene sheet using a butyl rubber adhesive or a roll coater method was attached to the surface of a steel pipe and then peeled off.

(The following is a blank space) (Effects of the Invention) The re-corrosion prevention method of the present invention can convert red rust generated on the surface of a steel material into black rust that is chemically stable and has good adhesion to the surface of the steel material by the action of citric acid.

Therefore, in the re-corrosion treatment of steel materials, excellent corrosion prevention ability can be imparted by a simple method without removing red rust generated on the surface of the steel materials.

Claims (1)

[Claims] A re-corrosion prevention method for steel materials, which comprises treating the surface of steel materials with an electrolytic solution containing 10 to 40% by weight of citric acid.