JPS6260882A - Structure for preventing caustic cracking of steel material - Google Patents

Abstract

PURPOSE:To prevent the caustic cracking of a steel material without generation of H2 and remarkable elution loss of an electrode member by specifying the composition and surface area of an alloy steel of a cathode member for a galvanic anode system which is brought into electrical-contact with the steel material. CONSTITUTION:The cathode member 2 for a galvanic anode system contg., by weight, 15.0-65.0% Ni, 16.0-30.0% Cr as required, 0.005-10.0% of >=1 kind among Mo, Cu, N, Ti, Nb, W, B, V, Ca, Ce, Mg and Mn as required and the balance iron is used. The member 2 is formed so that >=80% of the surface area of the steel material 1 to be protected is occupied by the member and the member 2 is electrically bonded to the steel material 1 by a galvanic bonding means such as bolt-nut joint. Consequently, the corrosion potential of the steel material 1 is transferred to the potential nobler than the critical corrosion potential for preventing the caustic cracking of the steel material 1 and the corrosion rate can be decreased.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a caustic cracking prevention structure for preventing stress corrosion cracking of steel materials used in a caustic soda environment.

<Prior art> Steel materials such as carbon steel and low alloy steel are treated with 1 (-,
-4(,1,1,,,H' l-77y* +jl
*Rin r -m) + pulpit p, -h
It has been known that when f, , -1, intergranular corrosion cracking occurs. For this reason, when steel materials are used in an alkaline aqueous solution environment, it is necessary to place the steel material at a temperature lower than the critical temperature for corrosion cracking (approximately 60° C.) in order to eliminate the influence of corrosion cracking as much as possible.

However, equipment for steam turbine power generation, for example, is exposed to high temperature and high pressure (approximately 300°C, 70 kg/Cm2) water vapor, and sodium contained as an impurity in the water vapor may be present at the joint between the turbine blades and the turbine shaft. The problem is that this caustic soda is said to cause caustic cracking in the steel parts due to the huge stress generated when the turbine is driven, and there is no way to solve this problem. It has been particularly requested in recent years.

By the way, caustic cracking is a type of electrochemical phenomenon, and the first
As shown in the figure, the potential range (-0, 8V V
It is known that this occurs only when s SCE ~-1, 2V Vs 5CE). Therefore, in order to prevent caustic cracking of steel materials, it is important to polarize the steel material to a more base or negative potential than the potential range corresponding to the transition region.

Conventionally, methods for achieving such polarization include an external power supply method in which the steel material is forcibly polarized by passing an electric current through it from the outside, and a metal that has a predetermined potential difference with the potential of the steel material to be protected. A galvanic method that polarizes by making electrical contact is known. Here, the galvanic method is divided into a galvanic anode method and a galvanic cathode method depending on whether the electric potential of the steel material to be protected is an anode potential or a cathode potential with respect to the metal in contact.

<Problems to be solved by the invention> However, the method of preventing caustic cracking using an external power supply method is to attach a counter electrode and a reference electrode near the steel material to be protected, create a liquid junction between each electrode with an environmental aqueous solution, and constantly Since it is necessary to conduct electricity at a specific electrode density, for example, if the steel material to be protected is linked to rotational operation, such as the steam turbine equipment mentioned above, or if the surface of the steel material is locally wet. In the usage environment F, where there are many types of devices, it is extremely difficult to conduct electricity and it is difficult to adopt the device.

On the other hand, the galvanic current method electrically joins the steel material to be protected and a metal that has a predetermined potential difference with respect to the steel material, so it avoids the problems of the external power method method, but Among the galvanic current methods, in the case of the FL cathode method, hydrogen generation reactions are likely to occur on the surface of the steel material to be protected in a caustic cracking environment, resulting in the steel material becoming brittle and brittle due to hydrogen absorption, and causing damage to the steel material. Since the anode electrode member used as the metal to be joined is limited to zinc-based or aluminum-based materials, there are problems such as the electrode member being extremely prone to melting down in a caustic cracking environment and requiring much effort to replace it. Yes, it is difficult to hire.

Means for Solving the Problems This invention has been made by focusing on the above problems,
As the cathode electrode member for the FtZ 7-node type, in the case of the first invention, an alloy steel containing nickel of 15.0 to 65.0% by weight, with the balance being iron and unavoidable impurities,
In the case of the second invention, 6.0 to 65.0% nickel,
Using alloy steel containing 16.0 to 30,0 tons of 5% chromium, the balance being iron and unavoidable impurities,
The third invention is 6.0 to 65.0% by weight of nickel.

Each contains 16.0 to 30.01 ffi% chromium, and molybdenum, copper, nitrogen, and titanium.

An alloy steel containing 0.005 to 10.0% by weight of at least one of niobium, tungsten, boron, vanadium, calcium, cerium, magnesium, manganese, etc., with the balance being iron and unavoidable impurities is used. On the other hand, the cathode electrode member is formed to have a surface area that accounts for at least 80% of the surface area of the steel material to be protected, and the cathode electrode member is further bonded to the steel material by galvanic bonding means such as bolt-nut bonding. It is characterized in that it is electrically connected.

Effect> If steel containing nickel or steel containing nickel and chromium is used for the cathode electrode member and sleeve, the corrosion potential of the steel material to be protected will be -0.8V Vs SCE
The potential changes to a more negative one, and the corrosion rate also decreases.

<Example> As shown in Fig. 2, the material of the cathode electrode member, which transfers the corrosion potential of the steel material to be protected in an aqueous caustic soda solution environment, is steel containing nickel (Ni) (shown as a solid line). Steels containing N1-Cr (see broken line fL1) and N1-Cr (chromium) (see broken lines connected by broken lines) are preferable, and the corrosion potential of steel with a high content of Ni and Cr is I can understand how to move wisely. Furthermore, what can be understood from the same figure is that the alloy composition that makes the corrosion potential more critical than -0.8V Vs SCE, which is the critical corrosion potential for preventing caustic cracking, is in the case of Ni-containing steel.

15. It is necessary that the steel contains 1% or more of Ni, and in the case of steel containing Ni-Cr, it is 16.0% by weight.
The point is that it is necessary to contain Cr of less than 6.0% by weight and Ni of 6.0% by weight or more, and the effect of adding Ni is the same at both temperatures, as it reduces the corrosion potential. However, if the Ni content exceeds 65.0% by weight, a saturation phenomenon occurs.

In addition, for Ni-Cr containing steel, molybdenum (Mo), copper (Cu), nitrogen (N), titanium (Ti), niobium (Nb), and tungsten (W) are used as described below.
, boron (B), vanadium (V), calcium (Ca
), cerium (Ce), and magnesium (Me) in an amount of 10.0% by weight or less does not affect the caustic cracking effect.

In view of the above points, the compositions of the Ni-containing steel and the Ni-Cr-containing steel used as the cathode electrode member according to the present invention are as follows.

■Ni-containing steel: Ni is an effective element for preventing caustic cracking, but 15. If the content is less than 65.0% by weight, the expected effect cannot be obtained, and if the content exceeds 65.0% by weight, the effect is saturated, and further addition of 1- is uneconomical because Ni is expensive. be. Therefore, the Ni content is 15.0 to 65
．． 0% by weight.

Contains Ni-Cr! S: Cr is an element that is effective in preventing caustic cracking only when added simultaneously with Ni, but if the weight is less than 16.0%, the expected effect cannot be obtained, and as shown in Table 1. 3
If the content exceeds 0.0% by weight, hot workability will be significantly deteriorated, making it difficult to manufacture plate-shaped electrode members. Therefore, the Cr content was set to 16.0 to 30.0% by weight.

2) Ni is an effective element for preventing caustic cracking, and when added simultaneously with Cr, it has a combined effect on caustic cracking, but 6.0
If it is less than % by weight, the expected effect cannot be obtained, and
When the content exceeds 65.0% by weight, the effect is saturated,
Adding more than that is uneconomical because Ni is effective.

Therefore, the Ni content was set to 6.0 to 65.0% by weight.

3) MOlCu, N, Ti, Nb, W, B, V, (',
It is effective in improving the strength and hot bending properties of 2 rp Mp, but if one or more of them is contained in excess of lO, OqT u%, it becomes caustic. The crack prevention IF effect deteriorates. Therefore, the content of these elements was set to 10,0Bq% or less.

Next, the galvanic bonding means for electrically connecting the cathode electrode member and the steel material to be protected will be described. As will be described later, the joining can be done by any means that can realize electrical connection between the two, such as thermal spraying of the cathode electrode member, tactile welding, explosion bonding of plates, welding, bolt-nut joining, etc. The size of the cathode electrode member needs to have a surface area of 8.0 or more compared to the surface area of the steel material to be protected that is exposed to the environment in which it is used, and if the size is less than this, it will not be effective in preventing caustic cracking. I can't wait.

Tables 1 and 2 show the composition and hot workability of the steel material to be protected and the cathode electrode member. The steel material to be protected is a commercially available steel plate (2 to 5 mm thick), and the cathode electrode member is as follows: Commercially available steel plate (plate thickness 3mm), filler rod (φ3m
m) and powder (100 to 300 g), or a 50 kg steel ingot is melted in a high frequency furnace according to a conventional method, hot rolled (plate thickness 3 mm), and machined.

The hot workability shown in Table 1 was evaluated based on the presence or absence of cracks in the steel plate during hot rolling.

As shown in FIG. 3, the stress corrosion cracking test piece 1 is made of a steel material to be protected (for example, thickness t: 2 mm, radius W: 20 mm).
, length n: 190 mm (7) A plate material is bent with a curvature of 10 mm) to form a horseshoe shape, and as shown in FIG.
The cathode electrode member 2 shown in the table, made of, for example, 5VS304, was electrically connected to test No. (1) using, for example, a port 3 and a nut 4 made of 5vS304, and subjected to an assembly experiment.

Stress corrosion cracking resistance is determined by immersing a test piece in a degassed 3O, OI aqueous solution of caustic soda at 150°C for two weeks, and then taking it out to examine the presence or absence of stress corrosion cracking and the crack depth using an optical microscope. Investigated and evaluated.

Table 3 shows examples of Ni-containing steel according to the first invention of the present invention, and Table 4 shows examples of Ni-Cr-containing steel according to the second and third inventions of the present invention. .

As is clear from Tables 3 and 4.

As the cathode electrode member according to the present invention, carbon steel and low alloy steel, which are made by joining Ni-containing steel and Ni-Cr-containing steel and are subjected to anti-corrosion measures using a galvanic anode method, are both sound and do not cause stress corrosion cracking. However, the results show that stress corrosion cracking occurs in the comparative example.

FIG. 5 shows another example of galvanic bonding means for stress corrosion cracking test piece 1 of steel material to be protected and cathode electrode member 2, in which both ends of Teflon-coated conductive wire 5 are connected to stress corrosion cracking test piece 1. 1 and a cathode electrode member 2 are joined by spot welding. In this way, the stress corrosion cracking test piece 1 and the cathode electrode member 2 may be separated by a predetermined distance.

FIG. 6 shows a case in which the cathode electrode member 2 is attached to the outer surface of the stress corrosion cracking test piece 1 by thermal spraying as fILt bonding means.

The thermal spraying was performed on stress corrosion cracking test pieces l as shown in Figure 7.
It may be applied to a part of the outer surface.

<Effect of the invention> According to the first invention, 15.0 to 65.0% by weight of Ni
As the cathode electrode member is made of alloy steel containing be.

Further, according to the second invention, 6.0 to 65.0% by weight (
7) Since alloy steel containing Ni and 16.0 to 30.0 wt% ty) Cr is used as the cathode electrode member, in addition to the effect of the first invention, it has excellent hot workability. A cathode electrode member can be provided.

Furthermore, according to the third invention, an alloy steel containing elements such as Mo in addition to the composition of Ni and Cr is used as the cathode electrode member, so that the effect of the second invention is achieved. In addition, hot workability, general corrosion resistance,
In addition, a cathode electrode member with excellent mechanical strength can be provided.

[Brief explanation of the drawing]

Fig. 1 is a graph showing the positive polarization curve of steel material, Fig. 2 is a graph illustrating the corrosion potential of the cathode electrode member according to the present invention for Ni-containing specimens, and Fig. 3 is a graph illustrating the shape of the stress corrosion cracking test piece. 4 is a perspective view illustrating a galvanic joining means using bolts and bolts, FIG. 5 is a perspective view illustrating a galvanic joining means using spot welding, and FIG. 6 is a perspective view illustrating a galvanic joining means using spot welding. FIG. 7 is a perspective view illustrating a galvanic bonding means partially formed by thermal spraying. l... Stress corrosion cracking test piece, Nikai force sword electrode member. Figure 1 Figure 3 Figure 4 Figure 5 Figure 6

Claims (3)

[Claims] (1) Contains 15.0 to 65.0% by weight of nickel,
A cathode electrode member for a galvanic anode type, the remainder of which is iron and unavoidable impurities, and which has a surface area that accounts for at least 80% of the surface area of the steel material to be protected, is connected to the steel material by galvanic current such as bolt and nut joints. A structure for preventing caustic cracking of steel material, characterized in that it is electrically connected via a joining means. (2) 6.0-65.0% nickel by weight, 16.0-65.0% by weight
A cathode electrode member for a galvanic anode type, each containing 30.0% by weight of chromium, the balance being iron and unavoidable impurities, and having a surface area that accounts for at least 80% of the surface area of the steel material to be protected. A structure for preventing caustic cracking of a steel material, characterized in that the steel material is electrically connected to the steel material via galvanic bonding means such as bolt-nut bonding. (3) 6.0-65.0% nickel by weight, 16.0-65.0% by weight
Each contains 30.0% by weight of chromium, and 0.0% of at least one of molybdenum, copper, nitrogen, titanium, niobium, tungsten, boron, vanadium, calcium, cerium, magnesium, manganese, etc.
0.005 to 10.0% by weight, with the balance being iron and unavoidable impurities, a cathode electrode member for galvanic anode type, the member having a surface area occupying at least 80% of the surface area of the steel material to be protected, A structure for preventing caustic cracking of a steel material, characterized in that the steel material is electrically connected to the steel material via a galvanic joining means such as a bolt-nut joint.