JP3020336B2 - Corrosion prevention method of steel by controlling flow velocity in neutral liquid - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for preventing corrosion of steel equipment used by being immersed in a neutral liquid such as natural water of rivers and lakes. In the specification, the neutral or neutral region means a range of about pH 5 to 9 unless otherwise specified, and the potential is represented by SHE standard.

[0002]

2. Description of the Related Art As a method of preventing corrosion of steel equipment immersed in natural waters of rivers and lakes, there have conventionally been coating protection methods and cathode protection methods. The coating anticorrosion method is a method in which water is not brought into direct contact with steel by applying a coating (plating or painting) or a lining (metal spraying or enamel) of a corrosion inhibitor on the surface of the steel. Usually, the coating is intended to shield the steel from the outside world, but there are also coatings that provide localized cathodic protection, such as galvanization and cadmium plating.

Generally, when electrochemically discussing corrosion and corrosion prevention of steel, a potential-pH diagram shown in FIG. 5 is used. In this potential-pH diagram, the lower part is an inactive area A,
The upper right side is a passivation zone B, the upper left side is a corrosion zone C, and there is a slight corrosion zone D on the right side of the boundary between the inactive zone A and the passivation zone B. In this potential-pH diagram, the boundary between the inactive region A and the corrosion region C is the equilibrium potential of the Fe ion / Fe redox couple.

The cathodic protection method is a method of sufficiently lowering the potential E of steel and keeping it in an inactive region A of the potential-pH diagram shown in FIG. And a galvanic anode system (sacrificial anode system). The external power supply method is a method in which a steel is connected to the negative electrode of an external DC power supply and a corrosion prevention current that lowers the potential E of the steel is applied, and the galvanic anode method is a low-potential sacrificial anode electrode made of a metal having a high tendency to ionize steel. Are electrically connected to generate an anticorrosion current that lowers the potential E of the steel.

Further, there is an anodic protection method in a strongly acidic solution such as concentrated sulfuric acid. In this anode corrosion protection method, an oxide film is formed on the surface of steel by connecting the steel to the positive electrode of an external DC power supply and a corrosion protection current is applied, and the potential E of the steel is maintained in a passive region B shown in FIG. This method has the advantage that once the oxide film is formed, the corrosion can be prevented with only a small corrosion protection current for maintaining the formed oxide film.

[0006]

According to the coating anticorrosion method, it is difficult to apply a coating or lining of an anticorrosion agent without any defect, and corrosion starts from a defective portion of the construction, and penetrates between the anticorrosion agent and steel to form a coating. There was a problem that it spreads to the part where it is. In addition, since corrosion inhibitors cannot be protected from destruction and deterioration over time due to external forces such as collisions, periodic recoating is necessary, and in this case, maintenance including repair of corrosion of defective parts of the construction There was a problem that costs were high. In addition, when recoating, there is little problem with relatively small and easily removable steel equipment, but with large steel equipment that can not be easily removed, the entire equipment has to be shut down and major work is required. ,
There has been a problem that a commercial loss may occur due to the stoppage of the device.

The external power supply method of cathodic protection requires not only an external power supply, but also a large anticorrosive current which must always flow to maintain the steel potential in the inactive region A of FIG. There was a problem that (electricity bill) increased. The cathodic protection method of the galvanic anode method requires a sacrificial anode electrode that is electrically connected to steel, and this anode electrode wears out instead of steel, so it needs to be replaced periodically. There was a problem.

[0008] In the anode corrosion protection method, once an oxide film is formed, the corrosion can be prevented with a small current just enough to maintain the oxide film. ), But there is a problem that it can be used only in a strongly acidic solution such as concentrated sulfuric acid. Although the exact reason has not been elucidated, if a large anticorrosive current is applied in a neutral solution, the elution rate due to ionization of Fe (steel) will be faster than the formation rate of the oxide film. It seems that it cannot be formed.

The present invention provides a method for preventing corrosion of steel in a neutral solution without applying a coating or lining of a corrosion-resistant material and without requiring an external power supply device or a sacrificial anode electrode. It is an object of the present invention to provide an anodic corrosion protection method that can be used in steel.

[0010]

DISCLOSURE OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and immerses steel in a neutral solution and contacts the steel with the neutral solution at a high relative flow rate. By doing so, the formation of an oxide film on the surface of the steel is promoted, and the corrosion potential is raised to the passivation region. Further, when a relative velocity that is high enough to raise the corrosion potential of the steel to the passivation region cannot be obtained, in addition to a certain relative velocity, an anode corrosion protection method of simultaneously connecting a positive electrode of an external DC power supply to the steel is used. It further promotes the formation of an oxide film on the surface of the steel and raises the corrosion potential to the passivation region.

[0011]

The steel is immersed in a neutral solution, and the steel and the neutral solution are brought into contact at a high relative flow rate. Then, the formation of an oxide film on the surface of the steel is promoted, and the corrosion potential of the steel increases to the passivation region. By maintaining this high relative flow rate for a certain period of time or more, an oxide film sufficient for anticorrosion is formed on the surface of the steel. Further, when a high relative flow velocity that cannot raise the corrosion potential of the steel to the passivation region is not obtained, oxidation to the steel surface is performed by adding an anodic protection method for connecting the positive electrode of an external DC power supply to the steel. The film formation is further accelerated, and the corrosion potential is raised to the passivation zone.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described with reference to FIGS. FIG. 3 is a schematic view for briefly explaining an embodiment of the present invention. (1) is a water turbine made of ordinary steel having a diameter of 7.2 cm, and this water wheel (1) is a neutral water stored in a water tank (2). It is immersed in the liquid (3).

[0013] In this configuration, the turbine (1) is rotated by a motor or the like (not shown) to start the initial operation, and the rotation speed R of the turbine (1) is increased by the rotation of the turbine (1) and the neutral liquid (3). Is controlled to be at least 4 m / s or more, and the rotational speed R is maintained for at least 5 hours or more, and the initial operation of the water turbine (1) is continued. Incidentally, in the case of the water turbine (1) having the above-described size, the relative flow velocity S can be obtained at a rotation speed R of about 2000 rpm. At this time, a load may or may not be applied as long as the relative speed S between the water turbine (1) and the neutral liquid (3) can be maintained.

[0014] Relative flow velocity S of water turbine (1) and neutral liquid (3)
By performing the initial operation at a speed of 4 m / s or more, an oxide film is formed on the surface of the water turbine (1) made of ordinary steel. After the oxide film required for anticorrosion is formed, the rotation speed R may be changed, and if the relative flow speed S does not drop extremely, 4 m /
s or less.

FIG. 2 shows the potential of ordinary steel in the neutral region.
It is a pH diagram. The lower part is an inactive area A, the upper right side is a passive area B, and the middle left is a corrosion area C. As can be seen from FIG. 2, the ordinary steel is inactivated and does not corrode when the corrosion potential E is about -0.62 V or less. This is a conventional cathodic protection method. Further, in a neutral solution (3) having a pH of 7, when an oxide film is formed on the surface of ordinary steel and passivated, the corrosion potential E becomes about -0.13 V or more, and the steel does not corrode.

FIG. 1 is a characteristic diagram showing the relationship between the relative flow velocity S of steel and the neutral liquid and the corrosion potential E of the steel in the neutral liquid (pH 7). Relative velocity S (m
/ S), and the vertical axis represents the corrosion potential E (V) of steel. As shown by the solid line in FIG. 1, the corrosion potential E of the steel is about -0.5 V when the relative flow rate S is zero.
And rises with an increase in the relative flow velocity S, and is substantially saturated at about -0.13 V when the relative flow velocity S is about 6 m / s or more. That is, at about 6 m / s or less, the higher the relative flow rate S between the steel and the neutral liquid, the more the oxide film formation is promoted and the corrosion potential E increases, and the lower the slower, the lower the corrosion potential E.

The corrosion potential E at the lower limit of the passivation range of ordinary steel in a neutral solution of pH 7 shown in FIG. 2 is -0.1.
3V is indicated by a broken line in FIG. The ordinary steel is corroded because the corrosion potential E is in the corrosion zone C below the broken line, and is in the passivation zone B above the broken line, so that it is passivated by forming an oxide film and does not corrode. That is, when the relative flow velocity S is about 3.7 m / s or more, ordinary steel forms an oxide film and is passivated, and the corrosion potential E becomes -0.13 V or more. The oxide film once formed does not disappear unless the relative flow rate S decreases extremely.

In the range of about 3.7 m / s or more and about 6 m / s or less, the higher the relative flow velocity S, the faster the formation rate of the oxide film and the higher the corrosion potential E. Conversely, the lower the relative flow rate S, the more the oxide film Is slowed down and the corrosion potential E is lowered.
Incidentally, in the above embodiment, as shown in FIG.
It is considered that the corrosion potential E and the polarization conductance hardly changed after time, and the formation rate and dissolution rate of the oxide film reached a steady state equilibrium, and the thickness of the oxide film did not change.

Next, a second embodiment of the present invention will be described. In the same configuration as the first embodiment, when the relative speed S between the water turbine (1) and the neutral liquid (3) cannot be set to 4 m / s or more, the anode corrosion protection method can be combined. For example, when the relative speed S between the water turbine (1) and the neutral liquid (3) is only 3 m / s, the water wheel (1)
Has a corrosion potential E of -0.16 V and a potential -pH shown in FIG.
Since it is in the corrosion area C in the figure, it is corroded. However, if an anti-corrosion current is passed by connecting the positive electrode of an external DC power supply to the water turbine (1) made of ordinary steel, an oxide film is formed and polarization occurs.
The corrosion potential E moves to the passive area.

Of course, the relative flow rate S between the water turbine (1) and the neutral liquid (3) is zero or low, and the corrosion potential E before connection to the power supply is obtained.
Is too low, a large anticorrosion current must be passed, so that the elution rate due to ionization of Fe (steel) becomes faster than the formation rate of the oxide film, as in the prior art, and the oxide film cannot be formed. After the oxide film is formed, corrosion can be prevented by passing a current sufficient to maintain the oxide film, as in the conventional anode corrosion protection method.

In the above embodiment, a water turbine that is actively rotated in water to generate a relative flow velocity with a neutral liquid has been described as an example of a steel device, but the present invention is not limited to this. Any equipment made of steel that is immersed in a neutral liquid and that can generate a relative flow rate of a certain degree or more between the steel equipment and the neutral liquid It can be applied. For example, the present invention can be applied to a water turbine that is passively rotated by the flow of water, such as a hydroelectric generator. Further, the device need not be a rotating device.
For example, it can be applied to something like a waterway.

[0022]

Since the present invention is constructed as described above, there is no need to apply a coating or lining of a corrosion-resistant material, and if a sufficient relative flow rate between the steel equipment and the neutral liquid can be obtained, the external The present invention has an effect that steel equipment can be protected in a neutral solution without requiring a power supply device, a sacrificial anode electrode, and a maintenance cost (electricity cost) for flowing a large anticorrosion current.

Further, even when a sufficient relative flow rate between the equipment made of steel and the neutral solution cannot be obtained, the anodic protection method can be performed in the neutral solution. If you use the anode protection method,
Corrosion can be prevented only by applying an extremely small amount of anticorrosion current as compared with the cathodic protection method, and the maintenance cost (electricity cost) can be reduced.

[Brief description of the drawings]

FIG. 1 is a characteristic diagram showing a relationship between a relative flow rate of steel in a neutral solution and a neutral solution and a corrosion potential of the steel.

FIG. 2 is a potential-pH diagram of ordinary steel in a neutral region.

FIG. 3 is a schematic view of a water turbine according to an embodiment of the present invention.

FIG. 4 is a diagram showing a corrosion potential-polarization conductance trajectory when the turbine of FIG. 3 is initially operated at 2000 rpm.

FIG. 5 is a potential-pH diagram of Pourbaix.

[Explanation of symbols]

 (1) ... water wheel, (2) ... water tank, (3) ... neutral liquid.

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Okada et al. 2940 Shinyoshida-cho, Kohoku-ku, Yokohama-shi, Kanagawa Prefecture Nippon Koei Co., Ltd. Yokohama business headquarters (58) Field surveyed (Int.Cl. ⁷ , DB name) C23F 13/00-15/00

Claims (4)

(57) [Claims] The steel is immersed in a neutral solution, and the steel and the neutral solution are brought into contact at a high relative flow rate to promote the formation of an oxide film on the surface of the steel, thereby reducing the corrosion potential of the steel. A corrosion prevention method for steel by controlling a flow rate in a neutral liquid, wherein the steel is raised to a dynamic range. 2. The method according to claim 1, wherein the steel is brought into contact with the neutral liquid at a relative flow rate of 4 m / s or more for 5 hours or more. 3. A method in which a steel is immersed in a neutral solution, and the steel and the neutral solution are brought into contact with each other at a relative flow rate to promote the formation of an oxide film on the surface of the steel to increase the corrosion potential of the steel. At the same time, a flow rate control in a neutral liquid characterized by further promoting the formation of an oxide film by an anodic corrosion protection method in which a positive electrode of an external DC power supply is connected to the steel to raise the corrosion potential of the steel to a passive region. Corrosion protection method of steel. 4. After completion of the formation of the oxide film, a positive electrode of an external DC power supply is connected to the steel to supply a current necessary for holding the formed oxide film. The method for preventing corrosion of steel by controlling the flow rate in a neutral liquid according to claim 2 or 3.