JPH1030196A - Method for preventing microorganismic corrosion of stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for preventing the occurrence of the microorganismic corrosion in the stainless steels in contact with fresh water having a possibility of intrusion of the microorganisms of river, lake and swamp water, etc., for example, the stainless steel pipes built into circulation systems, such as cooling water pipings or closed systems at the time of pressure resistant tests of pipings, etc. SOLUTION: One kind of any among sulfuric acid ions, phosphoric acid ions, molybdenic acid ions, vanadic acid ions or tungstenitic acid ions which are oxyanions or >=2 kinds thereof are added to the water in contact with the stainless steels. The molar concn. of any one kind or the total of the molar concn. of >=2 kinds is specified to >=0.1 in the ratio to the molar concn. of the chlorine ions included in the water.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to the microbial corrosion of stainless steel, for example, the microbial corrosion of stainless steel pipes incorporated in circulating systems such as cooling water pipes in air conditioning systems and various plants, or in closed systems after a pressure test. Prevention methods.

[0002]

2. Description of the Related Art Natural water usually contains chloride ions. However, when stainless steel is used in an aqueous solution environment containing chloride ions, local corrosion such as pitting and crevice corrosion is a problem. Sometimes. Regarding the occurrence of pitting corrosion and crevice corrosion of stainless steel in an aqueous solution containing these chloride ions, the concentration of the chloride ion in the solution and the temperature of the solution have been considered as main factors.

[0003] However, recently, cases in which so-called microbial corrosion is a problem have been reported. Microbial corrosion is corrosion induced by the action of microorganisms present in the environment. For example, the case of promoting corrosion of carbon steel by sulfate-reducing bacteria in an anaerobic environment is well known.

[0004] In a test in a solution in which a corrosive environment is artificially reproduced by adjusting a chloride ion concentration or the like using a reagent, stainless steel having sufficient corrosion resistance is found to be used in natural environment water such as actual seawater or freshwater. In some cases, local corrosion may occur, which is also thought to be due to the influence of microorganisms existing in natural environment water. That is, in the natural environment water, the corrosion potential of stainless steel shifts to the noble side due to the action of microorganisms present therein, and exceeds the local corrosion occurrence potential, leading to the occurrence of local corrosion. This is equivalent to shifting the standard of corrosion resistance made based on an artificial reagent to a direction in which corrosion is more likely to occur.

To support such a phenomenon, recently,
Microorganisms may occur in stainless steel pipes in pipes that use rivers, lakes, or marsh water containing microorganisms as cooling water, or when drainage of pressure tests performed after piping is insufficient or water is retained. There are known cases where corrosion occurs.

[0006] As a method of preventing corrosion in piping and the like, a method of adding an inhibitor (anticorrosive) has been conventionally known. For example, JP-A-55-8947
No. 9 discloses that a metal such as carbon steel, copper, and aluminum is protected by adding a heterocyclic nitrogen compound to an alkylenebiscarboxylic acid to prepare an aqueous alkaline solution and adding it to an aqueous environment.

In Japanese Patent Application Laid-Open No. 5222537/1982, a SUS316L stainless steel material used in a wet flue gas desulfurization apparatus or the like has a PO ₄ concentration of 10 ppm or more in order to prevent corrosion by dilute sulfuric acid in which SO ₂ is dissolved. A method of including a phosphate ion in such a manner is described.

A method for preventing stress corrosion cracking of stainless steel in contact with high-temperature and high-pressure water by adding ethylenediaminetetraacetate as an inhibitor is disclosed in Japanese Patent Application Laid-Open No.
No. 1677782.

However, the above-mentioned environment is a method for coping with severe corrosive environment such as high temperature, and is not a method for coping with normal-temperature cooling water which is the object of the present invention.
Stainless steel is considered to have sufficient corrosion resistance by conventional common sense for cooling water or pressure test water, which is fresh water near normal temperature, which is the object of the present invention, and is particularly suitable for stainless steel piping. It has never been considered to add an inhibitor to prevent corrosion. However, when the conditions for the growth of microorganisms in stainless steel pipes are met,
In recent years, it has become clear that damage due to microbial corrosion may occur, and there is a need for a method for preventing microbial corrosion of stainless steel used in a relatively mild environment.

[0010]

SUMMARY OF THE INVENTION The present invention relates to a stainless steel in contact with freshwater which may be contaminated with microorganisms such as rivers and lakes, for example, a circulating system such as a cooling water pipe, or a closed pipe after a pipe pressure test. It is an object of the present invention to provide a method for preventing the occurrence of microbial corrosion in stainless steel piping incorporated in a system.

[0011]

First, the mechanism of microbial corrosion of stainless steel in a natural environment will be briefly described.

When stainless steel is immersed in natural environment water in which microorganisms are present, a film by an attached microorganism, a so-called biological film, is quickly formed on the surface. The presence of this biofilm shifts the corrosion potential of stainless steel to the noble side.
Here, the corrosion potential is a potential shown when stainless steel is naturally immersed, and is also referred to as a natural potential.
Further, the pitting potential of stainless steel described later means a potential at which pitting occurs in stainless steel. In addition, the electrochemical behavior on the stainless steel surface with the biofilm attached was examined, and it was found that the cathode reaction (oxygen reduction reaction) was significantly accelerated. This is because an active oxygen species such as hydrogen peroxide is generated as an intermediate product of the metabolic reaction of the microorganism, and its oxidative influence is caused.

In an environment where chloride ions are present due to such an increase in corrosion potential, a part of the passive film on the stainless steel surface is destroyed in association with the chloride ion concentration, and local corrosion such as pitting corrosion is caused. Corrosion is likely to occur.

Thus, judging from temperature or chloride ion concentration, even in an environment considered to be relatively mild,
Corrosion may occur due to the presence of microorganisms. It is known that corrosion due to the influence of such microorganisms is generally likely to occur in welds.

On the other hand, oxyanions such as sulfate ion, phosphate ion, molybdate ion, vanadate ion, tungstate ion and the like are oxidizing substances, which facilitate passivation to metal materials and act as inhibitors. Is known to do. In order to promote the passivation by adding these oxyanions and obtain the anticorrosion effect, an oxyanion of a certain concentration (so-called critical concentration) or more is necessary, and the environmental conditions used (temperature, chloride ion concentration) It changes depending on. The present inventors have limited the environment to be subjected to the method of the present invention to an environment in which so-called microbial corrosion is a problem, and repeated the experiment by changing the concentration of oxyanion variously depending on the ratio to the chloride ion concentration.

In the experiment, a test piece was prepared by performing TIG welding on a SUS304 steel base material using SUSY308 as a welding material, and the effect of the oxyanion concentration on the pitting potential in the weld was examined.

FIG. 1 shows the effect of the concentration of SO ₄ ²⁻ on the pitting potential when a sulfate ion (SO ₄ ²⁻ ) is added as an example of the oxyanion. In an environment where microorganisms are present, as described above, the corrosion potential of stainless steel increases due to the influence of the metabolic reaction of microorganisms, and 300 to 350 mV vs. 300 mV. SC
E (saturated calomel electrode). If the pitting potential is lower than this corrosion potential, there is a risk of pitting occurring. Therefore, according to FIG. 1, it is understood that pitting corrosion may occur in an environment where microorganisms are present unless SO ₄ ^2- is added. On the other hand, the ratio of the SO ₄ ²⁻ concentration to the Cl ⁻ concentration is 0.
When the value is 1 or more, the pitting potential shifts in the noble direction and exceeds the corrosion potential, and thus it has been found that there is no risk of pitting occurring in an environment where microorganisms are present.

FIG. 1 has been found for the first time by the present inventors in relation to microbial corrosion, and specifically illustrates the basic technical concept of the present invention.

Based on the knowledge obtained on the mechanism of the occurrence of microbial corrosion, the present inventors have extended the SO ₄ ^2- ion exemplified in FIG. The gist is a method for preventing microbial corrosion by adding an inhibitor as described below.

That is, one or more of oxyanion sulfate, phosphate, molybdate, vanadate or tungstate ions are added to water in contact with stainless steel. A method for preventing microbial corrosion of stainless steel wherein the molar concentration of any one or two or more of the molar concentrations is 0.1 or more as a ratio to the molar concentration of chloride ions contained in the water. It is.

In the above method, stainless steel covers stainless steel in general. Among stainless steels, it is particularly applicable to stainless steel piping. The stainless steel pipe refers to a stainless steel pipe that is welded and incorporated as a pipe, or a stainless steel pipe that is incorporated as a pipe with an appropriate joint or the like. Stainless steel pipe as material
A seamless steel pipe may be used, or a stainless steel strip may be welded into a pipe shape.

The water to be filled is industrial water such as river water, and tap water is also applicable regardless of the circulation system and the closed system if the residence time is long. Although the temperature of the cooling water is around room temperature, the case where the temperature exceeds room temperature and becomes about warm water is also included.

The ion is added as a salt such as a sodium salt, but is not necessarily limited to a salt.

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the reasons for limiting each factor of the present invention will be described.

1. Oxyanion An oxyanion such as a sulfate ion, a phosphate ion, a molybdate ion, a vanadate ion, and a tungstate ion is an oxidizing substance, which promotes passivation of a metal material and acts as an inhibitor. The effect of each of these oxyanions was confirmed by experiments. The effect was recognized, although the effect was strong or weak. In particular, the effect of sulfate ion was remarkable as illustrated in FIG.

All of these oxyanions are added as a sodium salt or the like, as shown in Examples described later.

2. Oxyanion Concentration In order to promote the passivation by adding the oxyanion to the above-mentioned environment targeted by the present invention and to obtain an anticorrosion effect, the oxyanion must have a certain concentration (so-called critical concentration) or more. is required.

Here, when the ratio of the total molar concentration of the oxyanion to the molar concentration of the Cl ^- ion in the water used is smaller than 0.1, it is more than the corrosion potential reached by the influence of microorganisms. The pitting potential cannot be shifted in a noble direction, and sufficient microbial corrosion resistance cannot be imparted. In order to further ensure the anticorrosion effect,
Preferably it is 0.2 or more. The upper limit is not particularly specified, but the corrosion prevention effect due to the addition of the oxyanion tends to be saturated, and the excess concentration is uneconomical, so that the upper limit is preferably 2.0 or less.

In the cooling water piping, etc., in order to prevent contamination and blockage of the inner surface of the tube due to propagation of microorganisms, oxidizing disinfectants such as sodium hypochlorite and chlorine dioxide, and non-oxidizing agents such as organic nitrogen-sulfur. A sex bactericide may be added, but there is no particular problem when used in combination with the addition of the oxyanion of the present invention.

[0030]

EXAMPLES Next, the effects of the present invention will be described using examples.

As a test material, SUS304 steel generally used for piping such as cooling water was selected, and a commercially available rolled steel plate (JIS compliant) was used.

FIG. 2 shows the shape of the test piece used in the crevice corrosion test. As shown in FIGS. 2 (a) and 2 (b), a plate-shaped test piece 1 having a width of 25 mm, a length of 30 mm and a thickness of 3 mm and a width of 12 mm and a length of 3 mm were respectively formed from a SUS304 steel plate as shown in FIGS. A 30 mm thick, 3 mm thick plate-shaped test piece 2 is cut out, superimposed as shown in FIG. 3C, and fastened with a Teflon bolt 3 and a nut 4 to produce a crevice corrosion test piece 5. It was subjected to a crevice corrosion test.

In the crevice corrosion test, water collected at a point 1 km from the lower estuary estuary was used in order to select an environment where microorganisms exist and to provide an environment having a high chloride ion concentration. As a result of water quality analysis, chlorine ion was 1
It was found that the content was 50 ppm.

Table 1 shows the drug added to the aqueous solution, the drug concentration in the aqueous solution, and the ratio of the molar concentration of oxyanion to the molar concentration of chloride ion.

[0035]

[Table 1]

A 1000 ml aqueous solution shown in the table was placed in a beaker, and the crevice corrosion test piece 5 was immersed in the beaker at 30.degree.
A 0-hour immersion test was performed to determine whether crevice corrosion occurred.

Table 1 shows the results of the immersion test. Test number 1
When no oxyanion was added as in 2, the occurrence of crevice corrosion was observed due to the influence of microorganisms, but no crevice corrosion was observed in Test Nos. 1 to 9 of the present invention, indicating an excellent anticorrosion effect. You can see that there is. On the other hand, as shown in Test Nos. 10 and 11 of Comparative Examples, when the concentration of the oxyanion is less than the range limited by the present invention, it is understood that a sufficient anticorrosion effect cannot be obtained.

[0038]

According to the present invention, it is possible to provide a method of preventing microbial corrosion by a simple method of adding oxyanions to a stainless steel piping system incorporated in an air conditioning system or the like using natural water such as a river. It is very beneficial for related industries and for people's lives.

[Brief description of the drawings]

FIG. 1 Sulfate ion (SO ₄ ^2-
2 is a drawing showing the effect of ionic concentration on pitting potential.

FIG. 2 shows the shape of a crevice corrosion test piece. (A) and (b) are plan views of a plate-like test piece before assembly, and (c) is a side view of a crevice corrosion test piece after assembly.

[Explanation of symbols]

 1, 2 ... plate-like test piece 3 ... bolt 4 ... nut 5 ... crevice corrosion test piece

Claims (1)

[Claims] (1) To water in contact with stainless steel, one or more of oxyanion sulfate, phosphate, molybdate, vanadate or tungstate ions are added. Wherein the molar concentration of any one type or the total of two or more types of molar concentrations is 0.1 or more in a ratio to the molar concentration of chloride ions contained in the water. How to prevent microbial corrosion.