JPH0140108B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a corrosion prevention method that effectively prevents corrosion of metals, especially steel materials, that come into contact with water, without causing environmental pollution problems such as toxicity and eutrophication in closed water bodies. It concerns a method of control. [Prior art] In water systems such as open circulation cooling water systems, heat exchangers,
Steel materials such as carbon steel are mainly used as base materials for piping and the like. Steel materials immersed in cooling water are
It is corroded by dissolved oxygen, chlorine ions, sulfate ions, etc. in water, causing pitting corrosion. However, such pitting corrosion also occurs in corrosion-resistant materials such as copper, copper alloys, and stainless steel. Therefore, anticorrosive agents have conventionally been used to suppress corrosion of metal materials that come into contact with water systems. Corrosion inhibitors used in cooling water systems are soluble in water themselves, but they form an oxide film or a film that is insoluble or poorly soluble in water on the surface of the metal.
Corrosion reactions are controlled by preventing the elution of metal ions or the reduction reaction of dissolved oxygen. As anticorrosive agents, heavy metal salts such as chromates and zinc salts, (poly)phosphates, organic phosphorus compounds, and the like are generally used. Furthermore, oxycarboxylic acids such as citric acid and tartaric acid have been proposed. [Problems to be Solved by the Invention] Among these conventionally used anticorrosive agents, chromate has an excellent anticorrosive effect, but has the disadvantage that local corrosion tends to occur when used in low concentrations. Furthermore, chromate is highly toxic, and its discharge is strictly regulated, making wastewater treatment and recovery treatment essential. Zinc salts have some problems in terms of anti-corrosion effects, and there are regulatory problems if they are discharged as wastewater. Furthermore, since phosphates are a cause of red tide, the discharge of phosphorus has recently been regulated from the perspective of preventing eutrophication in closed bodies of water. For this reason, there is currently a strong demand for a method that can suppress corrosion of metals such as steel materials that come into contact with water systems without causing environmental pollution problems such as toxicity and eutrophication. On the other hand, although oxycarboxylic acid does not contain any regulated substances, it has the disadvantage that it is easily decomposed by microorganisms and cannot maintain its anticorrosion effect. [Means for Solving the Problems] In view of the above circumstances, the present invention has been made to provide a method for effectively preventing or controlling corrosion of metals in contact with water systems without causing environmental pollution problems. Then, in an aqueous system with a calcium hardness of 25 to 250 mg/molybdic acid and/or its salt and a water-soluble silicate, each was added at a weight ratio of 11: Mo equivalent: SiO ₂ equivalent:
The gist of the present invention is to provide a method for preventing corrosion, characterized in that the ratio is 1 to 1:1. Conventionally, it has been known to use molybdates or water-soluble silicates as water-based metal corrosion inhibitors (Japanese Patent Publication No. 57-44753, Japanese Patent Publication No. 52-34574, etc.). However, although molybdate has a good anti-corrosion effect, it is relatively expensive, which limits its practical use. On the other hand, although water-soluble silicates are relatively inexpensive, they have a weak anticorrosion effect, and cannot be said to be good anticorrosive agents at practical concentrations. As a result of intensive studies on the characteristics of such molybdates and water-soluble silicates, the present inventors have determined that molybdic acid or its salts and water-soluble silicates can be identified in an aqueous system with a specific calcium hardness. The present inventors have completed the present invention by discovering that in the presence of a certain amount, a specific synergistic effect is produced and an excellent anticorrosion effect can be obtained. The present invention will be explained in detail below. The method of the present invention involves adding molybdic acid and/or molybdate and a water-soluble silicate as anticorrosive agents to a water system to be treated, such as an open cooling water system, a closed cooling water system, or a brine water system. The calcium hardness of the target water system is 25 to 250 mg/, preferably 50 to 150 mg/. If the calcium hardness of the water system is less than 25mg, a good anticorrosive film of calcium molybdate may not be formed on the metal surface, and conversely, if the calcium hardness is less than 25mg/
If it exceeds mg/, calcium molybdate precipitates in water, which is not efficient. In the case of cooling water, the calcium hardness value of 25 to 250 mg/ is sometimes within this range due to the calcium concentration present in industrial water, and may also be within this range due to concentration operation.
If the water quality of the water system to be treated is such that the calcium hardness is outside the above range, an appropriate ingredient such as CaCl ₂ may be added as necessary, either in advance or at the same time as the addition of the active ingredient according to the present invention. Adjust the water quality by In the present invention, molybdic acid and/or a molybdate salt and a water-soluble silicate are added to such an aqueous system. Known water-soluble salts such as salts and ammonium salts can be mentioned. In addition, examples of water-soluble silicates include alkali metal salts of silicic acid, and examples of alkali metals include:
From the viewpoint of economy and toxicity, sodium and potassium are preferred. When using sodium silicate, its
_Na2O / _SiO2 (weight ratio) is preferably 4/1 to 1/4, particularly preferably 1/1 to 1/4. The ratio of molybdic acid and/or molybdate to water-soluble silicate is the weight ratio of Mo in the molybdic acid and/or molybdate to _SiO2 in the water-soluble silicate.Mo:SiO ₂ = 11:1~
1:1, preferably Mo: _SiO2 = 3:1 to 1:1
shall be. When such a blending ratio is used, an extremely high anticorrosive effect can be obtained due to an extremely good synergistic effect. In the present invention, molybdic acid and/or molybdate and water-soluble silicate are present in the water system to be treated at the above-mentioned mixing ratio, and the form of addition is not particularly limited. The active ingredients may be mixed in advance at a predetermined ratio and added to the aqueous system, or each may be added separately and mixed in the aqueous system. Note that when a predetermined amount of water-soluble silicate is present in the aqueous system, only molybdic acid or a salt thereof may be added. The concentration of these active ingredients used is determined depending on the properties of the water system to be treated and the degree of its corrosive environment, but in general, the concentration of molybdic acid and/or molybdate and water-soluble silicate in the water system is determined. The total concentration is
It is preferably added in an amount of 500 to 5000 mg/, particularly 100 to 2000 mg/. In the present invention, a sufficient anticorrosive effect can be obtained with the above-mentioned active ingredients, but if necessary, other anticorrosive agents such as thiazoles such as mercaptobenzothiazole; azoles such as benzotriazole; and hydrazines may be used. ; Water-soluble amines such as cyclohexylamine, alkylamine, alkanolamine, and polyamine; Imines such as ethyleneimine, pyrrolidine, piperidine, piperazine, and ketimine; Hydroxamic acids such as formhydroxamic acid, acetohydroxamic acid, and benzhydroxamic acid; Catechol Tannins; Lignins; Phosphonic acids; Organic compounds such as oxycarboxylic acids, nitrites, various phosphates, borates, zinc salts, nickel salts, aluminum salts, aluminates, tungsten salts, vanadium salts, etc. Other inorganic salts, scale inhibitors, slime removers, etc. may be used in combination. [Function] In the present invention, calcium hardness of 25 to 250 mg/
The following is thought to be the mechanism for the extremely excellent anti-corrosion effect achieved by coexisting molybdic acid and/or molybdate and water-soluble silicate in the aqueous system. In other words, molybdate and water-soluble silicate each have an action as an anode anticorrosive agent, but furthermore, in an aqueous system with a calcium hardness of 25 to 250 mg/molybdate, molybdate or molybdate has a good anticorrosive effect on calcium molybdate. A film is formed on the surface of the object to be protected, such as metal (cathode protection). On the other hand, the water-soluble silicate also becomes calcium silicate, which has the effect of sealing between the anticorrosive coatings of calcium molybdate. Therefore, in the present invention, by using molybdic acid and/or molybdic acid in combination with a water-soluble silicate, an extremely strong anti-corrosion film is formed, and an extremely excellent synergistic effect can be obtained. Presumed. [Examples] The present invention will be described in more detail below with reference to Examples and Comparative Examples, but the present invention is not limited to the following Examples unless it exceeds the gist thereof. Examples 1 to 4, Comparative Examples 1 to 6 Add calcium chloride solution to pure water at 50 mg/(as
CaCO ₃ ), 50 mg/sodium bicarbonate solution
(as CaCO ₃ ), 30 mg/magnesium carbonate solution
(as CaCO ₃ ), 50 mg/sodium chloride solution
(as Cl ^- ) and 50mg/in sodium sulfate solution
(as SO ₄ ^2- ) was added to prepare a test solution. The obtained test solution was placed in one beaker, and various chemicals shown in Table 1 were added (no addition was made in Comparative Example 1).
One mild steel test piece was immersed in this solution, and a rotary corrosion test was conducted at 30° C. and 160 rpm for 5 days to examine the corrosion rate. The results are also listed in Table 1. From Table 1, according to the present invention, by using molybdate and water-soluble silicate in combination, extremely excellent anticorrosion effect can be achieved at a lower additive concentration than when each agent is added alone. It is clear that a synergistic effect can be obtained.

【table】

[Table] Examples 5 to 12, Comparative Examples 7 to 10 Using a solution of primary ethylene glycol dissolved in pure water at a ratio of 30% by volume, it was dissolved in calcium chloride solution.
50 mg/(as CaCO ₃ ), 82 mg/(as CaCO ₃ ) in sodium bicarbonate solution, 500 mg/(as Cl ^- ) in sodium chloride solution, 500 mg/(as SO ₄ ^2- ) in sodium sulfate solution. This was used as the test solution. A mild steel test piece (SPCC, 31 cm ² ) was immersed in this test solution for 3 days at 30°C without the addition of any chemicals to prepare an already rusted surface, and then various chemicals shown in Table 2 were added (comparison). Example 7 is without additives), after adjusting the pH to 7, at 10℃
A rotary corrosion test was conducted at 180 rpm for 7 days to examine the corrosion weight loss. The results are shown in Table 2. The corrosion weight loss when the rusted surface was formed for 3 days without the addition of chemicals was 82.8 mg.

【table】

[Table] From Table 2, it can be seen that the combined use of the molybdate of the present invention and water-soluble silicate can also be used on already rusted surfaces.
It is clear that an excellent anticorrosive effect that could not be obtained by adding each agent alone can be obtained. Examples 13 to 17, Comparative Examples 11 and 12 Corrosion tests were conducted in the same manner as in Example 8, except that the amount of calcium chloride solution added was changed and the calcium hardness of the test solution was changed as shown in Table 3. The corrosion weight loss was investigated. The results are shown in Table 3. From Table 3, it is clear that a good anticorrosion effect can be obtained by molybdate and water-soluble silicate in the calcium hardness range of 25 to 250 mg/.

[Table] [Effects of the Invention] As detailed above, the corrosion prevention method of the present invention involves adding a specific proportion of molybdic acid and/or molybdate and a water-soluble silicate to an aqueous system having a calcium hardness of 25 to 250 mg/. Due to the excellent synergistic effect of the two active ingredients, an extremely high anticorrosive effect can be obtained, and an excellent anticorrosion suppressing or preventing effect is achieved on metals such as steel materials that come into contact with water. According to the present invention, a sufficient effect can be obtained with a small amount of the chemical used, so the cost of anticorrosion treatment can be reduced, and furthermore, an excellent anticorrosion effect can be achieved even on already rusted surfaces. In addition, the wastewater does not contain heavy metal salts, so there is no toxicity problem, and it does not contain phosphorus.
Since it is COD, there is no problem of eutrophication. Therefore, according to the present invention, metal corrosion can be effectively prevented without causing environmental pollution problems.

Claims (1)

[Claims] 1. In an aqueous system with a calcium hardness of 25 to 250 mg/
Molybdic acid and/or its salt and water-soluble silicate, respectively, in the weight ratio of Mo equivalent: SiO ₂ equivalent
A corrosion prevention method characterized in that the ratio is 11:1 to 1:2. 2. The method according to claim 1, wherein the calcium hardness of the aqueous system is 50 to 150 mg/. 3. A patent claim characterized in that molybdic acid and/or its salt and a water-soluble silicate are present in a weight ratio of 3:1 to 1:1.4 in terms of Mo: _SiO2 , respectively. The method according to scope 1 or 2. 4. The method according to any one of claims 1 to 3, wherein the molybdate is a sodium salt, potassium salt, or ammonium salt of molybdic acid. 5. The method according to any one of claims 1 to 4, wherein the water-soluble silicate is an alkali metal salt of silicic acid.