JPH0248629B2 - TETSUKEIKINZOKUNOFUSHOKUBOSHIHOHO - Google Patents

Description

[Detailed description of the invention]

(Purpose of the Invention) Industrial Application Field The present invention relates to a method for corrosion-proofing the metal surfaces of various equipment that come into contact with water, and more specifically to a method for corrosion-proofing the surfaces of ferrous metals in boilers, heat exchangers, piping systems, etc. Regarding the method. BACKGROUND OF THE INVENTION When equipment made of ferrous metals is used in contact with water, problems such as surface corrosion may occur, impairing the functionality of the equipment or shortening its lifespan. The main factors causing corrosion of ferrous metal surfaces by water are the pH of the water and dissolved oxygen, which have a particularly significant effect on equipment that handles high-temperature water systems, such as boilers. In order to remove such corrosive factors from metal surfaces, chemical treatment methods involving the addition of chemicals are usually employed, except in special cases. In other words, PH is controlled to be within a range that minimizes the corrosion rate depending on the material of the metal material and the environmental conditions in which it is used. For example, when used in high-temperature water-based equipment such as boilers, Since the corrosion rate of iron-based metal materials decreases when the pH is in the range of 11 to 12, it is necessary to add alkaline agents such as sodium hydroxide or sodium carbonate to the water to maintain the pH within this range. is normal. Regarding dissolved oxygen, there is a method of deoxygenating water using a reducing agent such as hydrazine or sodium sulfite, but since oxygen in the air dissolves in water, it is necessary to replenish the reducing agent as it is consumed. is necessary. On the other hand, it is known that the surface of iron reacts with water in a temperature range of 100°C or higher, forming a film of magnetite (Fe ₃ O ₄ ) through the schikorr reaction, resulting in corrosion resistance. Fe→Fe ²⁺ +2e ^- Fe ²⁺ +2OH ^- →Fe(OH) ₂ 3Fe(OH) ₂ →Fe ₃ O ₄ +H ₂ +2H ₂ O However, this reaction occurs relatively quickly in the high temperature range of 200℃ or higher. However, at lower temperatures, the reaction rate is slow and a magnetite film with sufficient anti-corrosion properties will not be formed unless it takes a long time. At this time, there is a problem that when there is dissolved oxygen in the water, oxygen and ferrous hydroxide react to form ferric hydroxide, which prevents the formation of a magnetite film. 4Fe(OH) ₂ +O ₂ +2H ₂ O→4Fe(OH) ₃ Under these circumstances, in order to quickly form a stable corrosion-resistant film on the surface of iron-based metals, an alkalizing agent and a reducing agent are added to the water. Addition of such chemicals to high-temperature treatment has been carried out, but such chemicals are less effective if used in small amounts, and if concentrated in boiler water, etc., there is a risk of causing pitting corrosion. . Furthermore, there is a problem that a stable and strong anti-corrosion film cannot be formed unless the temperature is sufficiently high, such as 200°C or higher, and that the anti-corrosion film is unstable and easily destroyed at 100 to 200°C. Problems to be Solved by the Invention Based on the above, we have developed a method for reliably forming a stable anti-corrosion film on the surface of iron-based metals in a short period of time, even if the temperature is not as high as 200°C or higher. The present invention aims to provide a method for preventing corrosion of iron-based metals by forming an anti-corrosion film that can meet such demands. (Structure of the Invention) Means for Solving the Problems As a result of intensive research to solve the above-mentioned problems, the present inventors have discovered that iron-based metals fall within a specific range.
When it comes into contact with a divalent tin salt aqueous solution having a PH value,
The present inventors have discovered that a strong anticorrosive film containing this tin oxide is formed at the same time as a layer of stannic oxide is formed on a metal surface, leading to the completion of the present invention.
That is, the present invention is an aqueous solution containing a divalent tin salt, the pH of which is expressed by the following formulas (), (), and () PH≧10.5 () PH≧2.8log ₁₀ [Sn] + 19 () PH≧ −9.8log ₁₀ [Sn] −25 () However, [Sn] is the molar concentration of divalent tin (mol/
) is shown. This is a method for preventing corrosion of iron-based metals, which is characterized in that a material that is within a range that satisfies the following is brought into contact with the surface of iron-based metals at a reaction temperature. The relationship between the concentration of divalent tin and PH is shown in FIG. 1, and the shaded area surrounded by three lines in the figure represents the range suitable for implementing the present invention. There is. In the present invention, the divalent tin salt contained in the aqueous solution only needs to be dissolved in water, and may be a tin ion or a complex ion. Such tin salts include stannous chloride, stannous sulfate, stannous nitrate, etc., but are not particularly limited and may also be stannous salts of alkali metals. Further, the reaction temperature is preferably 100°C or higher, more preferably 150°C or higher. It goes without saying that when carrying out the method of the present invention, appropriate agents may be used in combination as long as they do not inhibit the reaction between the tin ions and the surface of the iron-based metal. Example: To dealkalized soft water obtained from Tokyo Metropolitan Water and having the properties shown in Table 1, stannous chloride dihydrate was added to the specified tin concentration, and potassium hydroxide was used to adjust the pH to the specified value. Test water was obtained by adjusting the water as follows. Pour test water into each autoclave with an internal volume of 3, and place a test piece of polished mild steel plate (30 mm x
15 mm x 2 mm) was hung and sealed, and then heat treated at 170°C for 48 hours with stirring to react.
After the treatment was completed, the test pieces were taken out and examined for appearance and presence of pitting corrosion. The results are shown in Table 2. The surface of the specimen treated by the method of the present invention was colored blue to black, and when the surface was examined by X-ray diffraction, it was found that a film containing tin dioxide as well as iron was formed. . Further, when the surface film was subjected to Auger electron spectroscopy, it was confirmed that it was a tin oxide film containing iron. Next, the treated test piece was suspended in corrosion test water 1, which was prepared by adding 1 mg of sodium nitrite, 15 mg of glucose, and 10 mg of potassium hydroxide to the same dealkalized soft water as above, and left it open. Corrosion tests were conducted by immersing at 40°C for 72 hours with stirring. On the other hand, as a blank test, a corrosion test was conducted on a test piece of an untreated mild steel plate in exactly the same manner. Through these tests, the corrosion rate (mdd: corrosion loss mg per 1dm ² per day) was determined.
The following calculation formula: Corrosion protection rate = mdd of blank - mdd of sample / md of blank
Corrosion protection rate was calculated by d×100. The results of these corrosion tests are also shown in Table 2.

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[Table] * Comparative example
** Average value of 20 corrosion tests on untreated steel sheets The above results show that excellent corrosion resistance can be obtained by treating the surface of ferrous metals by the method of the present invention. (Effects of the Invention) The present invention is capable of treating iron-based metals by bringing them into contact with an aqueous solution containing a divalent tin salt, the pH of which is within a specific range relative to the molar concentration of divalent tin. It imparts excellent corrosion resistance to the surface of the metal, and has the feature of being able to reliably form a stable anti-corrosion film in a short period of time.

[Brief explanation of drawings]

FIG. 1 is a graph showing the range required for the tin concentration and pH of the aqueous treatment solution used in the method for preventing corrosion of iron-based metals of the present invention.

Claims (1)

[Claims] 1. An aqueous solution containing a divalent tin salt, which
PH is the following formula (), () and () PH≧10.5 () PH≧2.8log ₁₀ [Sn]+1.9 () PH≧−9.8log ₁₀ [Sn]−25 () However, [Sn] is 2 molar concentration of tin (mol/
) is shown. 1. A method for preventing corrosion of ferrous metals, the method comprising bringing a substance within a range that simultaneously satisfies the following into contact with the surface of ferrous metals at a reaction temperature. 2. The method for preventing corrosion of ferrous metals according to claim 1, wherein the reaction temperature is 100°C or higher.