JP7426641B1 - How to remove rust from existing steel materials - Google Patents

Abstract

An object of the present invention is to provide a method for removing rust from existing steel materials, which can effectively remove rust using a rust remover that does not use an inorganic acid. [Solution] An electrochemical treatment process in which an exposed part of the base metal iron is formed on a part of the rusted surface of the existing steel material and used as an anode reaction promoting part, and a hydroxy acid aqueous solution is applied to the rusted surface having the anode reaction promoting part. The method includes an electrochemical reaction step in which rust is dissolved and iron ions are eluted from the base metal iron, and a rust removal step in which rust is removed from the area where the hydroxy acid aqueous solution is applied. As the hydroxy acid, citric acid, malic acid, or acid salts thereof are preferred. The hydroxy acid aqueous solution may further contain a viscosity imparting agent and an aliphatic polyhydric alcohol or carbohydrate as a syneresis inhibitor. [Selection diagram] None

Description

The present invention relates to a method for removing rust from existing steel materials by electrochemical cleaning using a rust remover, which is carried out as a surface preparation when repainting existing steel materials.

When repainting steel materials used in existing structures such as steel buildings, structures, ships, and vehicles (hereinafter collectively referred to as "existing steel materials"), the base material must be coated before repainting. As an adjustment, rust and deteriorated existing paint film will be removed. Conventionally, the main method of preparing the substrate is physical cleaning, and chemical cleaning using a paint film remover is also used to remove existing paint films, but it cannot be used to remove rust, so it is a separate method. , it is necessary to use physical cleaning to remove rust.

Physical cleaning includes those using large-scale incidental equipment such as blasting, and those using power cleaning tools and manual cleaning tools (hereinafter, power cleaning tools and manual cleaning tools are collectively referred to as power tools, etc.). Although large-scale physical cleaning by blasting or the like can provide a high-quality base for repainting, it is not suitable for small-scale construction because it is expensive. On the other hand, physical burring using power tools, etc. is cheaper than blasting and easier to apply to small-scale construction, but on the other hand, it requires a huge amount of labor to expose the base metal using power tools, etc. Some rust remains. In addition, it is actually difficult to expose the base metal using a power tool or the like in hollow areas of thick rust layers such as floating rust, layered rust, and scaly rust, and in narrow areas of bolts and members. In this way, it is unavoidable that rust remains even after cleaning, and it is difficult to obtain sufficient cleaning precision (base quality), which may adversely affect the durability of the repainted protective coating, and the residual rust in the remaining rust may be difficult to obtain. Salt makes corrosion more likely to occur again.

Until now, when repainting existing steel materials, chemical cleaning using a chemical conversion treatment using a rust remover to remove rust during surface preparation has not been widely used. The reason for this is that inorganic acids such as hydrochloric acid and sulfuric acid are often used as active ingredients in general rust removers, but since these inorganic acids are strong acids, they cannot be used particularly at construction sites due to safety and environmental concerns. There are some things that are not desirable. There is also a concern that the strength of the steel material may decrease due to thinning of the steel material and hydrogen embrittlement. Furthermore, even if rust is removed using hydrochloric acid, rust immediately occurs (returning rust), so it is not suitable for use at construction sites. Therefore, as a technique for removing rust using a rust remover containing an organic acid as an active ingredient, there are, for example, the following Patent Document 1 and Patent Document 2.

Special table 2014-508220 publication Japanese Patent Application Publication No. 2016-11436

However, even if a rust remover containing an organic acid as an active ingredient is used in the same manner as a conventional rust remover containing an inorganic acid as an active ingredient, organic acids are less effective at removing rust than inorganic acids (see below). It was not practical because its rust removal properties (referred to as rust removal properties) were significantly inferior.

SUMMARY OF THE INVENTION Therefore, the present invention aims to solve the above problems and provides a method for removing rust from existing steel materials, which can effectively remove rust using a rust remover containing an organic acid as an active ingredient.

As a means for that purpose, the present invention is a method for removing rust from existing steel materials, which is carried out as a surface preparation when repainting existing steel materials, and in which the exposed portion of the base metal iron is removed on a part of the rusted surface of the existing steel materials. an electrochemical treatment step to form an anode reaction promoting region, and applying a hydroxy acid aqueous solution to the rust surface having the anode reaction promoting region to proceed with dissolving the rust and eluting iron ions from the base metal iron. and a rust removal step to remove rust from the area to which the hydroxy acid aqueous solution has been applied.

<Principle of rust removal using hydroxy acid>
Iron in contact with water forms a local battery through an anodic reaction, which is the elution of iron ions (Fe ²⁺ ), and a cathodic reaction, such as reduction of dissolved oxygen and hydrogen gasification of hydrogen ions, producing a corrosion current. An electrochemical reaction proceeds. In an acidic atmosphere, this electrochemical reaction proceeds more quickly, and the external elution of iron ions by dissolving rust and the external elution of iron ions from the base metal iron are promoted. When an aqueous solution of hydroxy acid is applied to the rusted surface of existing steel, it penetrates into the interior of the rust and slightly erodes the interface between the rust layer and the base steel, although it is not as strong as strong acids such as inorganic acids. This reduces the adhesion of rust to the base metal iron, making it possible to easily remove the rust.

<Principle of promoting electrochemical reactions by electrochemical treatment>
If a hydroxy acid aqueous solution is simply applied to the rusted surface of the existing steel material, the rust layer will become an obstacle, inhibiting the aforementioned electrochemical reaction and making it difficult to proceed. On the other hand, as an electrochemical treatment, an exposed part of the base metal iron is formed in advance on a part of the rusted surface to serve as an anode reaction promoting part. Here, the rusted surface includes the vicinity of the rust-generated region, and is the range within which the electrochemical treatment is effective. Then, a hydroxy acid aqueous solution is applied to the rusted surface including the anode reaction promoting part. In the anode reaction promotion section, the external elution of iron ions is not inhibited, so that the cathode reaction on the entire rust surface due to the corrosion current is promoted. As a result, the anode reaction is promoted not only in the anode reaction promotion area but also on the entire rust surface, the electrochemical reaction progresses at an accelerated pace, and the adhesion of rust to the base metal rapidly decreases, making it easy to remove rust. Able to carry out work. Hereinafter, the anode reaction promoting section will be abbreviated as "reaction promoting section" for convenience.

The hydroxy acid is preferably at least one selected from citric acid, malic acid, or acid salts thereof.

It is preferable that the hydroxy acid aqueous solution further contains a viscosity imparting agent and an aliphatic polyhydric alcohol or a carbohydrate as an anti-syringe agent.

<Adding viscosity to hydroxy acid aqueous solution, reducing viscosity after application and suppressing syneresis phenomenon>
When removing rust from downward, vertical, or inclined surfaces, a viscosity imparting agent is added to the hydroxy acid aqueous solution in order to prevent the hydroxy acid aqueous solution from sagging after application and to maintain the hydroxy acid aqueous solution for a certain period of time. However, as the electrochemical reaction progresses, due to the interaction between electrolytes such as iron ions that are rapidly eluted and the polar groups of the viscosity imparting agent, the excluded volume of the viscosity imparting agent becomes smaller and the viscosity of the coating liquid decreases. There is. Furthermore, free water that is not restrained by the viscosity imparting agent increases, causing a "water separation phenomenon" in which the free water seeps out from the coating area to the surrounding area, which may contaminate the area around the coating area. Therefore, by blending an aliphatic polyhydric alcohol or a carbohydrate with good water retention together with a viscosity imparting agent into an aqueous hydroxy acid solution, it is possible to prevent the reduction in the excluded volume of the viscosity imparting agent, thereby reducing the viscosity of the hydroxy acid aqueous solution and causing water syneresis. The phenomenon can be suppressed.

The present invention removes rust by electrochemically treating the rusted surface of existing steel to form an exposed portion of the base metal iron that will serve as a reaction accelerator, and then applying a hydroxy acid aqueous solution as a chemical conversion treatment. , is a new substrate preparation method (hereinafter referred to as electrochemical cleaning). As a result, rust removal work (hereinafter referred to as rust removal work) when repainting existing steel materials can be carried out with high quality and at low cost, and the durability of the repainted protective coating film is improved. In addition, most of the salt present in the rust is also removed during rust removal, which effectively contributes to preventing early recurrence of corrosion due to rust after repainting.

That is, by performing electrochemical treatment in advance and applying an aqueous hydroxy acid solution as a chemical conversion treatment, electrochemical cleaning with excellent rust removal properties can be performed. Since a hydroxy acid aqueous solution is used, rust removal work can be carried out more safely than with strong inorganic acids. Furthermore, the erosion of the base metal iron is so slight that it cannot be visually perceived, and there is almost no concern about thinning of the steel material due to inorganic acids during pickling or reduction in strength of the steel material due to hydrogen embrittlement. The electrochemical treatment carried out in advance is a simple operation of simply forming a minute exposed portion of the base metal iron, and is low in cost. As described above, the electrochemical cleaning of the present invention enables higher quality and lower cost rust removal work than the rust removal work in which base metal iron is exposed by physical cleaning using a power tool or the like.

If the hydroxy acid is citric acid, malic acid, or an acid salt thereof, the health hazards and corrosive hazard risks that occur when oxalic acid is used, for example, can be avoided.

Furthermore, by incorporating a viscosity imparting agent and a syneresis inhibitor into the hydroxy acid aqueous solution, it is possible to suppress the decrease in viscosity and the syneresis phenomenon after application of the hydroxy acid aqueous solution, and it is possible to prevent rust even on downward facing, vertical, and inclined surfaces. Work becomes possible.

Existing steel materials to which the present invention is applied include steel materials used in buildings such as buildings and plants, structures such as steel towers and bridges, existing structures such as ships, and vehicles. The surface of such existing steel materials is coated with paint to form a protective coating film for the purpose of improving weather resistance. However, the protective coating also deteriorates over time due to exposure to ultraviolet rays, wind and rain, and the like. Therefore, it is necessary to periodically reapply the protective coating. At this time, if new paint is applied while the deteriorated existing paint film or rust remains, this will induce early deterioration of the protective paint film that has been repainted. Therefore, when repainting, it is necessary to prepare the surface in advance to remove rust as well as the existing paint film that has deteriorated. In the present invention, when removing this rust, at least an electrochemical treatment step, an electrochemical reaction step, and a rust removal step are performed in this order.

《Preliminary process》
Before starting the first electrochemical treatment process, it is preferable to remove some of the existing paint film, floating rust, layered rust, scaly rust, brittle rust, etc. using a power tool or the like. Since this step is only carried out as a preliminary step, there is no need to remove the rust layer until the base metal is exposed, and the rust layer is removed at the limit of the rust thickness at which the rust removal effect by electrochemical cleaning of the present invention is effective. do it. Specifically, the thickness of the rust layer is preferably 100 μm or less.

《Electrochemical treatment process》
As an electrochemical treatment, an exposed part of the base metal iron is formed on a part of the rusted surface to serve as a reaction accelerating part. Specifically, electrochemical treatment involves scratching (denting) a portion of the rusted surface using a drilling tool such as a drill, center punch, or skin plow, or a grinding tool such as a disc sander, electric wire brush, or belt sander. Just wear them. The depth of the reaction promoting portion may be as long as it extends beyond the surface rust layer to reach the base metal iron, and does not need to be deeper than necessary. The area of the reaction promoting portion does not need to be large, and the electrochemical reaction can be promoted even if the exposed area is small. However, if the area of the reaction promoting part is too small, the electrochemical reaction may be slowed down, so the total area of the reaction promoting part is preferably 3 mm ² or more.

The number of reaction accelerators to be formed may be one or more than one per rust surface, as long as the area where the rust occurs is relatively small (for example, less than 6 cm ² ). The electrochemical reaction tends to be promoted more easily when reaction promoting parts are formed in a plurality of places, but if the area where the rust surface occurs is relatively small, even one place can function satisfactorily. The reaction promoting portion may be formed anywhere within the rust surface (rust generation area). That is, it does not necessarily need to be formed in the center of the rusted surface, but may be formed in the peripheral part.

On the other hand, if the area where the rust occurs is relatively large (for example, 6 cm ² or more), it is preferable to form reaction accelerators at a plurality of locations on one rust surface. This is because if the distance between the reaction promoting parts exceeds 6 cm, the electrochemical reaction tends to slow down. However, it is not necessary to form the reaction promoting parts at very close intervals. Further, it may be formed not only within the rusted surface but also in the vicinity (for example, within 5 cm) of the rusted surface.

《Electrochemical reaction process》
After the reaction promoting portion is formed in the electrochemical treatment step, a hydroxy acid aqueous solution is then applied to the rusted surface having the reaction promoting portion to allow the electrochemical reaction to proceed on the entire rusted surface. At this time, the electrochemical reaction may be further promoted by applying a DC voltage to the hydroxy acid aqueous solution and the base metal iron.

<Hydroxy acid aqueous solution>
Hydroxy acids suitable for use in the present invention are preferably aliphatic hydroxy acids that have high solubility in water, high boiling points, low odor, and low health hazards. Examples of such aliphatic hydroxy acids include malic acid, which is chain-like with 4 to 6 carbon atoms and has one hydroxy group and two carboxyl groups in one molecule, or malic acid, which has one hydroxy group and two carboxyl groups in one molecule. Citric acid, which has 1 hydroxyl groups and 3 carboxy groups, is preferred because it can be expected to promote the elution of iron ions through its chelating action. Among them, citric acid is more preferable because it is relatively low cost and easily available. Further, as the aliphatic hydroxy acid, a partially neutralized salt (hereinafter referred to as an acid salt) of an aliphatic hydroxy acid with a base component such as monovalent Li, K, Na, or NH ₃ can also be used. Among these, NH ₃ is preferable as the base component because it does not volatilize and remain after the electrochemical reaction step using an aqueous hydroxy acid solution.

The content of the hydroxy acid or its acid salt in the hydroxy acid aqueous solution is preferably 5 to 35% by weight, more preferably 10 to 30% by weight, and even more preferably 15 to 25% by weight. If the content of the hydroxy acid or its acid salt is too low, the electrochemical reaction tends to proceed insufficiently. On the other hand, if the content of the hydroxy acid or its acidic salt is too large, the water content will conversely decrease, thereby inhibiting the electrochemical reaction. Since hydroxy acids have high solubility in water, a hydroxy acid aqueous solution can be easily prepared in a short time by mixing and stirring water and hydroxy acids.

The pH of the hydroxy acid aqueous solution is adjusted to 1.0 to 6.0. Even if the hydroxy acid aqueous solution has a high concentration, it is difficult for the pH to become less than 1.0, and when the pH is higher than 6.0, the electrochemical reaction becomes difficult to proceed. Monovalent Li, K, Na, _NH3 , etc. are suitable as the base component for adjusting the pH. When neutralized with a divalent or higher base component, dissolution of the hydroxy acid is inhibited and precipitation of the hydroxy acid is likely to occur. Further, instead of adjusting the pH after dissolving the hydroxy acid in water, the pH may be adjusted by using an acidic salt of the hydroxy acid with these base components.

<Viscosifier>
Viscosity can be imparted to the hydroxy acid aqueous solution by adding a viscosity imparting agent that increases the viscosity in an acidic aqueous solution as necessary. The viscosity (23°C) of the hydroxy acid aqueous solution when imparted with viscosity is preferably 1,000 to 40,000 mPa·s, more preferably 5,000 to 35,000 mPa·s, and 10, 000 to 30,000 mPa·s is more preferable. If the viscosity of the hydroxy acid aqueous solution is too low, it is likely to fall on a downward surface or sag on a vertical or inclined surface. On the other hand, if the viscosity of the hydroxy acid aqueous solution is too high, it will be difficult to manufacture and difficult to apply.

A water-soluble polymer can be used as the viscosity imparting agent. Water-soluble polymers include non-electrolyte polymers that increase in viscosity due to hydrogen bonding (hydration) between polar groups and water, and electrolyte polymers that increase in viscosity by forming an electric double layer due to repulsion between ionic dissociative groups. Further, electrolyte polymers include anionic polymers having anionic groups, cationic polymers having cationic groups, and amphoteric polymers having both groups. There are also water-soluble polymers that have the characteristics of both non-electrolyte polymers and electrolyte polymers.

Specific viscosity-imparting agents include guar gum, carrageenan, xanthan gum, tamarind seed gum, methylcellulose, hydroxyethylcellulose, methylhydroxypropylcellulose, carboxymethylcellulose, alginate, starch, polyvinyl alcohol, polyvinylpyrrolidone, acrylic thickener, water-soluble polyurethane, cationized modified products thereof, chitosan, and the like. Since these water-soluble polymers have different pH ranges in which they can thicken, they should be able to maintain a thickened state in the acidic range of pH 1.0 to 6.0. Furthermore, in order to prevent falling on a downward surface or hanging on a vertical or inclined surface, it is preferable to use a material that can impart thixotropic viscosity. Examples include "Cosmote A-40S" manufactured by Senka Co., Ltd., which is an acrylic thickener. The water-soluble polymers to be added to the hydroxy acid aqueous solution can be used alone or in combination of two or more.

The content of the viscosity imparting agent may be within a range that allows the viscosity of the hydroxy acid aqueous solution to be adjusted within the aforementioned range. Specifically, the solid content is preferably 0.1 to 5.0% by weight. If the content of the tackifying agent is too small, the viscosity imparting will be insufficient, and if the content is too large, the viscosity will be extremely high, making it difficult to manufacture and difficult to apply. The viscosity imparting agent contained in the hydroxy acid aqueous solution may be directly mixed as it is, or may be dissolved or swollen in water in advance to form a thickened liquid and then mixed indirectly.

Furthermore, when a hydroxy acid aqueous solution is applied to the rusted surface of existing steel materials, not only iron ions are eluted from the rust layer and base metal iron into the hydroxy acid aqueous solution, but also hydroxide ions are generated and eluted by the cathodic reaction, further rusting. Cationic electrolytes such as sodium ions derived from the salt inherent in water and anionic electrolytes such as chloride ions are eluted. For this reason, the water-soluble polymer contained in the hydroxy acid aqueous solution has a reduced excluded volume due to contact with and absorption of the electrolyte, and is likely to cause a decrease in viscosity and a syneresis phenomenon.

<Anti-synaptic agent>
When imparting viscosity by blending a viscosity imparting agent, a water syneresis inhibitor can also be blended at the same time to prevent viscosity reduction and water syneresis. As the anti-syringe agent, an aliphatic polyhydric alcohol or a carbohydrate can be used. In non-electrolyte polymer aqueous solutions, aliphatic polyhydric alcohols or carbohydrates have a high affinity for both non-electrolyte polymers and water, so they have the effect of retaining hydration water within the excluded volume of water-soluble polymers. . Also, in an electrolyte polymer aqueous solution, aliphatic polyhydric alcohols or carbohydrates have a high affinity with both the electrolyte polymer and water, so the external electrolyte compresses the electric double layer formed by the ionic groups in the electrolyte polymer. It has the effect of preventing water-soluble polymers from occurring and maintaining the excluded volume of water-soluble polymers.

The aliphatic polyhydric alcohol is preferably a liquid having 2 to 6 carbon atoms that has high solubility in water, such as dihydric alcohols such as ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, and dipropylene glycol; Examples include glycerol, which is a hydric alcohol.

Examples of carbohydrates include monosaccharides such as glucose, fructose, and galactose; disaccharides such as sucrose, lactose, and maltose; oligosaccharides such as oligosaccharides; polysaccharides such as dextrin; sugar alcohols xylitol, Examples include sorbitol, erythritol, pentaerythritol, dipentaerythritol, and the like.

Liquid aliphatic polyhydric alcohols maintain the sol state of the aqueous hydroxy acid solution for a long time even after application, and are therefore preferred over solid carbohydrates, and among them glycerol, which is less hazardous, is more preferred. The aliphatic polyhydric alcohol or carbohydrate contained in the hydroxy acid aqueous solution can be used alone or in combination of two or more.

The content of the syneresis inhibitor is preferably 5 to 35% by weight, more preferably 10 to 30% by weight. If the content of the syneresis inhibitor is too small, the effect of reducing viscosity and inhibiting syneresis will be insufficient. On the other hand, if the content of the syneresis inhibitor is too high, the content of water will decrease, and the electrochemical reaction will be inhibited. When a hydroxy acid aqueous solution contains a liquid aliphatic polyhydric alcohol or a powdered carbohydrate, the liquid aliphatic polyhydric alcohol or powdered carbohydrate is added while stirring the hydroxy acid aqueous solution that has been given viscosity in advance. Preferably, it is added in small portions and completely dissolved.

<Other additives>
A gelling agent may be added to the hydroxy acid aqueous solution as necessary for the purpose of delaying water evaporation and film formation. Among them, tamarind seed gum, which is a water-soluble polymer, is preferable as a gelling agent because it has the property of gelling when the concentration increases at a certain ratio due to interaction with the syneresis inhibitor. A hydroxy acid aqueous solution containing an anti-syringe agent will not gel even if it contains tamarind seed gum below a certain concentration, but if the concentration of tamarind seed gum increases as the water evaporates in the hydroxy acid aqueous solution after application, water retention will increase. It is possible to form a gel-like film with low strength while leaving the film on, thereby delaying water evaporation and film formation. Moreover, since this gel-like film has weak adhesion to rust, it can be easily removed.

Since tamarind seed gum is in powder form, it is preferable to dissolve it in advance to a concentration of 0.5 to 2.0% by weight to thicken it and then mix it into the hydroxy acid aqueous solution. In this way, the gelling agent may be mixed at the time of preparing the hydroxy acid aqueous solution, or may be applied to the rusted surface in advance before applying the hydroxy acid aqueous solution, and further, the gelling agent may be mixed during the preparation of the hydroxy acid aqueous solution. The gelling agent may be applied in layers to the surface.

In addition, if organic substances such as oil are attached to existing steel materials, penetration of the hydroxy acid aqueous solution into the interior of the rust will be inhibited, so a penetrating wetting agent may be added as necessary to increase the penetration rate. You can also. Penetrating wetting agents dissolve or disperse in water and lower the surface tension of water, making it easier for water to penetrate inside the rust. Types of rust-penetrating wetting agents include nonionic surfactants, silicone surfactants, anionic surfactants, and cationic surfactants. Not limited.

Examples of nonionic surfactants include polyoxyethylene alkyl ether, polyoxyethylene alkylphenyl ether, polyoxyalkylene derivative, polyoxyethylene sorbitan fatty acid ester, polyoxyethylene sorbitol fatty acid ester, glycerin fatty acid ester, sorbitan fatty acid ester, Examples include polyoxyethylene fatty acid ester, polyoxyethylene hydrogenated castor oil, polyoxyethylene alkylamine, and alkyl alkanolamide. Among these, those with an HLB (Hydrophilic-Lipophilic Balance) of 12 or more have excellent water permeability into the rust. Examples of silicone surfactants include polyether-modified silicones, polyester-modified silicones, aralkyl-modified silicones, and the like. Examples of the anionic surfactant include fatty acid salts, sulfonate salts, sulfate ester salts, naphthalene sulfonic acid formalin condensates, polycarboxylate salts, and the like. Examples of cationic surfactants include alkylamine salts and quaternary ammonium salts. Examples of amphoteric surfactants include alkyl betaines and alkyl amine oxides. These penetrating wetting agents can be used alone or in combination of two or more.

When a penetrating wetting agent is added, its content is preferably 0.01 to 2% by weight, more preferably 0.05 to 1% by weight. If the content of the penetrating wetting agent is too small, it will be difficult to obtain the wetting promoting effect of the aqueous wetting agent. On the other hand, if the content of the penetrating wetting agent is too high, there is a concern that a large amount of the penetrating wetting agent will remain on the surface of the base steel even after rust is removed, reducing the water resistance and corrosion resistance of the repainted protective coating. There is.

Furthermore, to the hydroxy acid aqueous solution, preservatives, fungicides, antifoaming agents, initial rust preventive agents, optical brighteners, and drying retardant solvents may be added as necessary to the extent that they do not inhibit the effects of the present invention. You can also. When these additives are included, the additives are added little by little while stirring the hydroxy acid aqueous solution that has been given viscosity in advance, and are completely dissolved or sufficiently dispersed.

<Application of hydroxy acid aqueous solution>
The method of applying the hydroxy acid aqueous solution is not particularly limited, and typical examples include roller application using a flocked roller, porous sponge roller, etc., brush application, and jet application using a spray or jetting machine. . The coating amount is preferably 0.1 to 2.5 kg/m ² , more preferably 0.5 to 2.3 kg/m ² , even more preferably 1.0 to 2.0 kg/m ² . If the coating amount is too small, the coating solution tends to dry and the electrochemical reaction tends to proceed insufficiently. On the other hand, if the coating amount is too large, it becomes difficult to maintain the shape at the time of coating, and the coating tends to fall on downward surfaces or sag on vertical or inclined surfaces.

After applying the hydroxy acid aqueous solution, if the solution temperature is 23° C., the adhesion of rust to the base metal iron can be reduced by leaving it for about 1 hour or more and less than 24 hours. If the standing time is too short, the progress of the electrochemical reaction tends to be insufficient. On the other hand, if the standing time is too long, the coating liquid containing the water-soluble polymer will dry and solidify, making it difficult to remove the coating liquid. Note that the standing time for the electrochemical reaction process can be shortened by applying a DC voltage to the hydroxy acid aqueous solution and the base metal iron. Additionally, as the electrochemical reaction progresses, iron ions are eluted into the coating solution, which is further oxidized and turns dark brown, so the degree of discoloration cannot be used as a guide to the progress of the electrochemical reaction. can.

《Rust removal process》
After reducing the adhesion of rust to the base metal iron in the electrochemical reaction process, the rust is finally removed from the area where the hydroxy acid aqueous solution has been applied. Specifically, rust whose adhesion to the base metal iron has decreased is removed by scraping it off with a power tool or the like. It does not matter whether or not it is necessary to remove the hydroxy acid aqueous solution before removing the rust, but before removing the rust, the hydroxy acid aqueous solution may be wiped off with a spatula, skin scraper, rag, etc., or washed away with high pressure water. Further, when scraping off rust whose adhesion to the base metal iron has decreased with a brush, skin scraper, electric brush, etc., it may be scraped off while applying a cream to prevent scattering of rust pieces. After removing rust, if there is any dirt on it, wash it with water or a cleaning agent and dry it quickly to prevent it from returning to rust.

≪Rust removal using hydroxy acid aqueous solution≫
<Preparation of hydroxy acid aqueous solution>
Hydroxy acid aqueous solutions were prepared according to the following procedure, and hydroxy acid aqueous solutions of Examples 1 to 6 and Comparative Examples 1 to 2 were prepared in order to evaluate that the rust removal effect of hydroxy acids was promoted by electrochemical treatment. did. Table 1 shows these raw materials and their blending amounts (wt%).

As each raw material shown in Table 1, the following were used.
Citric acid: Citric acid (crystalline) manufactured by Fuji Film Wako Pure Chemical Industries, Ltd.
Diammonium hydrogen citrate: Diammonium hydrogen citrate manufactured by Kanto Chemical Co., Ltd. Malic acid: DL-malic acid manufactured by Marubeni Shokai Co., Ltd.

The hydroxy acid aqueous solution shown in Table 1 was prepared by weighing a powdered hydroxy acid into a 150 cc polycup, adding water, and dissolving it by mixing and stirring. The pH of each hydroxy acid aqueous solution is also shown in Table 1.

<Procedure for producing test base material>
In evaluating the rust removal properties of the present invention, the surface of a rusted steel plate on which rust had formed due to outdoor exposure was prepared and used as a test base material. The specific procedure for producing the test base material is shown below.
Step 1: Using structural steel plate SS400 (size: 300 mm long x 90 mm wide x 3.2 mm thick) specified in JIS G 3101, blast treatment on both sides according to 7.14 (cycle corrosion) of JIS K 5551. did.
Step 2: The above-mentioned steel plate was installed vertically (lengthwise) outdoors in a coastal area (Pacific Ocean side in Chiba Prefecture) and left for about one year to produce a roughened rusted steel plate.
Step 3: The surface of the rusted steel plate was rubbed with an electric wire brush (cup wire) to remove brittle rust adhering to the surface until a metallic luster appeared, and the surface was adjusted. Even after the adjustment, the surface had a rough appearance, and the rust thickness before adjustment was about 50 to 120 μm, and the rust thickness after adjustment was about 20 to 80 μm. To measure the rust thickness, an electromagnetic film thickness meter was used to measure 10 points on the surface of the rusted steel plate.

<Rust removal evaluation procedure>
The rust removal properties of the hydroxy acid aqueous solution by electrochemical treatment shown in Table 1 were evaluated by the following procedure.
Step 1: As an electrochemical treatment, indentations (exposed area approximately 3 mm ^{2 ) were made in the test locations of Examples 1 to 6 of the test substrate so that the base metal iron was exposed in a circular shape with a diameter of 2 mm (exposed area approximately 3 mm 2} ). A reaction promoting part) was punctured in one place. No recesses (reaction promoting areas) were made in the test locations of Comparative Examples 1 and 2.
Procedure 2: Based on the test method of JIS K 5675 7.11 (acid resistance) (hereinafter referred to as spot immersion), a hard polyvinyl chloride pipe (inner diameter 40 mm, height 35 mm) was soaked in white vaseline as a test base material. (in Examples 1 to 6, the depression was centered), and 15 g of each hydroxy acid aqueous solution was injected. The upper part of the hard polyvinyl chloride tube was covered with a transparent plastic film ("Saran Wrap" (registered trademark) manufactured by Asahi Kasei Home Products Co., Ltd.) and sealed to prevent evaporation of the liquid.
Step 3: After being left at 23°C for a predetermined time, the hard polyvinyl chloride pipe, the hydroxy acid aqueous solution and white petrolatum therein were removed, and the spot immersed area was washed with water while being rubbed lightly with a sponge to remove rust.
Step 4: After drying, the rate at which the rust in the spot immersion portion was removed and the base metal iron was exposed was evaluated.

<Rust removal evaluation criteria>
Rust removal performance was evaluated based on the following criteria.
◎: Exposure of base metal iron is 60% or more ○: Exposure of base metal iron is 20% or more, less than 60% △: Exposure of base metal iron is 1% or more, less than 20% ×: Exposure of base metal iron is 1 Less than % (including no exposure)

<Rust removal evaluation results>
Table 1 also shows the evaluation results of the rust removal properties of Examples 1 to 6 and Comparative Examples 1 to 2. As a result, those that were electrochemically treated in advance showed good rust removal properties (each example), and those that were not electrochemically treated did not show rust removal properties (comparative examples 1 and 2). .

≪Rust removal using hydroxy acid aqueous thickener≫
<Preparation of hydroxy acid aqueous thickening liquid>
In order to evaluate that the rust removal effect of a viscosity-imparted hydroxy acid aqueous solution (hereinafter referred to as hydroxy acid aqueous thickener) is promoted by electrochemical treatment, a hydroxy acid aqueous thickener was prepared using the following procedure. . First, 15% by weight of citric acid ("Citric Acid (Crystal)" manufactured by Fuji Film Wako Pure Chemical Industries, Ltd.) was weighed into a 150cc polycup, 61% by weight of water was added, and the mixture was dissolved by stirring with a mixer. Next, while stirring with a mixer, 20% by weight of glycerol (“Glycerin” manufactured by Kenei Pharmaceutical Co., Ltd.) is added to make a homogeneous solution. A citric acid aqueous thickener was prepared by adding 4% by weight of a thickener ("Cosmote A-40S" manufactured by Senka Co., Ltd.) little by little and stirring with a mixer for about 1 hour. The obtained hydroxy acid aqueous thickening liquid had a viscosity of 16,900 mPa·s, which was thixotropic viscosity. The viscosity was measured using a B8H type viscometer (rotation speed: 20 rpm, measurement time: 2 minutes) after adjusting the temperature of the citric acid aqueous thickener to 23°C. The pH was 1.7, and the temperature of the citric acid aqueous thickener was adjusted to 23°C, and the pH was measured using a glass electrode pH meter.

<Procedure for producing test base material>
The test base material was produced using the same procedure as the above-mentioned <<Rust removal using hydroxy acid aqueous solution>>.

<Evaluation procedure for rust removal effect>
The rust removal effect of the hydroxy acid aqueous thickening liquid by electrochemical treatment shown in Table 2 was evaluated by the following procedure.
Step 1: Electrochemical treatment is applied to the test locations of Examples 7 to 15 on the test base material so that the base metal iron is exposed in a circular shape with a diameter of 2 mm (exposed area approximately 3 mm ² ), or the base metal is One recess (reaction promotion part) was made with a drill so that the iron was exposed in a circular shape with a diameter of 4.4 mm (exposed area approximately 15 mm ² ). No recess (reaction promotion part) was made in the test location of Comparative Example 3.
Procedure 2: Apply hydroxy acid aqueous thickener to the test area in an amount of 1.5 kg/m ² (Examples 7 to 13 and Comparative Example 3) or 0.5 kg/m ² (Examples 14 and 15) I applied it with a brush and let it stand horizontally. At this time, in Examples 7 to 11 and Comparative Example 3, the coating was applied in a square (4 cm in length and width) so that the position of the recess was in the center, and in Examples 12 to 15, the position of the recess was 1 cm from the short side. It was applied to a rectangle (length: 8 cm, width: 2 cm).
Step 3: After leaving the coating at 23° C. for a predetermined time and evaluating browning of the coating solution, the coating solution was removed with a spatula, and the coating area was washed with water while being lightly rubbed with a sponge to remove rust.
Step 4: After drying, the ratio of exposed base metal iron due to removal of rust in the applied area and the effective range of rust removal were evaluated.

<Evaluation criteria for rust removal effect>
The rust removal effect was evaluated based on the following criteria.
(Browning of coating solution)
A: Dark brown B: Translucent brown C: Translucent light yellow (rust removal properties)
A: Base metal iron is exposed in a planar manner over more than half of the coating area B: Base metal iron is exposed in dots over the entire coating area C: Base metal iron is exposed in dots in localized areas of the coating area D: Base metal No exposed iron (rust removal effective range)
In the application area, the length (cm) from the dent forming part (reaction promoting part) to the point where the rust removal effect is effective

<Evaluation results of rust removal effect>
Table 2 shows the evaluation results of the rust removal effect of Examples 7 to 15 and Comparative Example 3.

At a coating amount of 1.5 kg/ ^m2 of hydroxy acid aqueous thickener, when electrochemical treatment is applied, browning of the coating solution progresses within 5 to 8 hours after application, and rust appears in dots all over the coated area. It was successfully removed (Examples 7 to 10). Furthermore, 24 hours after application, the coating solution turned dark brown, and rust could be removed planarly from more than half of the coated area (Example 11). On the other hand, when electrochemical treatment was not performed, the browning of the coating solution did not progress even after 16 hours had passed after coating the hydroxy acid aqueous thickening solution, and rust could not be removed (Comparative Example 3). When the coating amount of the hydroxy acid aqueous thickening liquid was 1.5 kg/m ² , the coating liquid turned dark brown 16 hours after application, regardless of whether the exposed area of the base metal iron in the reaction accelerator was 3 mm ² or 15 mm ² . Rust could be removed planarly from more than half of the coated area. Furthermore, the effective rust removal range covered the entire length of the application area (Examples 12 and 13). On the other hand, when the coating amount of the hydroxy acid aqueous thickening liquid is 0.5 kg/ ^m2 , the coating liquid turns pale yellow 16 hours after application, regardless of whether the exposed area of the base metal iron in the reaction accelerator is 3 ^mm2 or 15 ^mm2 . Although the rust could be removed locally in spots, the rust removal performance was lower than when the coating amount was 1.5 kg/m ² . Moreover, the effective rust removal range was about 6 cm (Examples 14 and 15).

≪Suppression of syneresis after applying hydroxy acid aqueous thickener to rusted surface≫
<Preparation of hydroxy acid aqueous thickening liquid>
In order to evaluate the effect of containing an aliphatic polyhydric alcohol or carbohydrate on the viscosity decrease and syneresis phenomenon that occur over time in a hydroxy acid aqueous thickening solution after being applied to a rusted surface. Hydroxy acid aqueous thickeners of Examples 16-22 and Comparative Example 4 were prepared. Table 3 shows these raw materials and their blending amounts (% by weight).

Among the raw materials shown in Table 3, the following raw materials were used, except that the same ones as in <<Rust Removal with Hydroxy Acid Aqueous Thickener>> were used.
Propylene glycol: Propylene glycol manufactured by Ando Parachemy Co., Ltd. Glucose: Glucose manufactured by Margo Corporation Sucrose: Granulated sugar manufactured by Itochu Sugar Co., Ltd. Oligosaccharide: Fructooligosaccharide manufactured by Nippon Garlic Co., Ltd. Dextrin: Bubble Star ( Indigestible dextrin sorbitol manufactured by Tokyo Chemical Industry Co., Ltd.: "D-Sorbitol" manufactured by Tokyo Chemical Industry Co., Ltd.

In preparing the hydroxy acid aqueous thickening liquid shown in Table 3, first, a predetermined amount of water was measured into a 150 cc polycup, 15% by weight of citric acid was added, and the mixture was dissolved by stirring with a mixer. Next, while stirring with a mixer, add 20% by weight of each of the aliphatic polyhydric alcohols or carbohydrates shown in Table 3 to make a uniform solution, and further add 4% by weight of an acrylic thickener little by little while stirring with a mixer. Each hydroxy acid aqueous thickening solution was prepared by stirring with a mixer for about 1 hour. The viscosity and pH of each citric acid aqueous thickener are also shown in Table 3.

<Procedure for producing test base material>
The test base material was produced by the same procedure as described above in <<Rust Removal with Hydroxy Acid Aqueous Thickener>> except that Step 3 was not performed.

<Evaluation procedure for syneresis>
The syneresis phenomenon of the hydroxy acid aqueous thickening liquid in Table 3 was evaluated by the following procedure.
Procedure 1: Rust on the surface of the test substrate was ground using an electric orbital sander (abrasive paper #120) to remove brittle rust and adjust the surface. The rust thickness after adjustment was approximately 14 to 60 μm. To measure the rust thickness, an electromagnetic film thickness meter was used to measure 10 points on the surface of the rusted steel plate.
Procedure 2: One recess (reaction accelerator) was drilled at the test location as an electrochemical treatment so that the base metal iron was exposed in a circular shape with a diameter of 2 mm (exposed area approximately 3 mm ² ).
Procedure 3: A hydroxy acid aqueous thickener was applied to the test area with a brush at a coating amount of 1.9 kg/m ² in a square (4 cm in length and width) so that the depression was in the center, and the test area was left to stand horizontally.
Procedure 4: After being left at 23° C. for 16 hours and evaluating the water separation phenomenon of the coating solution, the coating solution was removed with a spatula, and the coating area was washed with water while being lightly rubbed with a sponge to remove rust.
Step 5: After drying, the rate at which the rust in the applied area was removed and the base material was exposed was evaluated.

<Evaluation criteria for syneresis>
The syneresis phenomenon was evaluated according to the following criteria, and the rust removal property was evaluated using the same criteria as the above-mentioned <<rust removal using hydroxy acid aqueous thickening liquid>>.
(hydrolysis phenomenon of coating liquid)
A: 1 mm or less B: More than 1 mm and less than 5 mm C: More than 5 mm and less than 1 cm D: More than 1 cm

<Evaluation results of syneresis phenomenon>
Table 3 also shows the evaluation results of the syneresis phenomenon of Examples 16 to 22 and Comparative Example 4. The syneresis phenomenon of the coating solution occurred significantly in Comparative Example 4, but was suppressed in Examples 16 to 22, and in particular, almost no syneresis phenomenon was observed in Examples 18 to 22.

Claims (3)

A method for removing rust from existing steel materials that is carried out as a surface preparation when repainting existing steel materials,
An electrochemical treatment process that forms an exposed part of the base metal iron on a part of the rusted surface of the existing steel material and uses it as an anode reaction promotion part,
an electrochemical reaction step in which a hydroxy acid aqueous solution is applied to the rusted surface having the anode reaction promoting portion to promote dissolution of the rust and elution of iron ions from the base iron;
a rust removal step of removing rust from the area where the hydroxy acid aqueous solution is applied;
How to remove rust from existing steel, including: The method for removing rust from existing steel materials according to claim 1, wherein the hydroxy acid is at least one selected from citric acid, malic acid, or acid salts thereof. The method for removing rust from existing steel materials according to claim 1 or 2, wherein the hydroxy acid aqueous solution further contains a viscosity imparting agent and an aliphatic polyhydric alcohol or a carbohydrate as a syneresis inhibitor.