JPS5942751B2 - Method for preventing local corrosion of stainless steel structures - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for preventing localized corrosion of stainless steel structures.

As is well known, stainless steel has good corrosion resistance due to the formation of a so-called passive film, which is a protective oxide film, on its surface, so it is widely used as a material for structures in corrosive environments such as seawater utilization equipment. There is.

However, even stainless steel has the disadvantage that when it is used for a long period of time in an environment such as high-temperature seawater or a solution containing high concentration of dissolved oxygen, the protective film is broken in a certain part of the member, causing localized corrosion. . Examples of this include crevice corrosion and pitting corrosion that occur in material joints with small gaps, and pitting corrosion and stress corrosion cracking that occur in the heat affected zone of welded joints, both of which are caused by batteries between active and passive states on the steel surface. This is due to formation. In order to prevent the above-mentioned local corrosion, a method is usually taken to improve the corrosion resistance by changing the alloy composition of the entire steel. For pitting corrosion and crevice corrosion, increasing the amount of Cr and Ni or adding Mo, stress corrosion, etc. To prevent cracks, in addition to the above, reduction of C or addition of stabilizing elements such as Ti and Nb are carried out.

This method has the problem that stainless steel is already expensive, and since it requires increasing or adding an even more expensive alloying element, it becomes extremely expensive and impractical from an economic point of view. The purpose of the present invention is to solve the above-mentioned drawbacks.
o, W, V film or foil with a thickness of 0.1 μm or more, or Mo, W, V on the surface of the heat affected zone of the welded joint.
By attaching the film or foil to a thickness of 0.1 μm or more, local corrosion can be prevented easily and inexpensively and stably over a long period of time. The present invention will be explained in detail below.

The present inventors have focused on Mo, W, and V, which are highly effective alloying elements for preventing local corrosion of stainless steel, and have focused on Mo, W, and V, which are highly effective as alloying elements for preventing local corrosion of stainless steel. As a result of detailed research on the corrosion prevention mechanism,
Mo, W, and V are each M.

042-, WO42-, H2VO ion is eluted into the solution and prevents chloride ions, which are a factor in accelerating corrosion, from penetrating into or concentrating at locally corroded areas, and prevents the steel surface of the locally corroded areas from being re-infiltrated. It has been found that the above-mentioned crevice corrosion, pitting corrosion, stress corrosion cracking, etc. can be prevented by promoting mobilization.

Therefore, Mo is applied to the material joints of stainless steel structures.
If a film or foil of Mo, W, or V is interposed, or a film or foil of Mo, W, or V is attached to the surface of the heat-affected zone of a welded joint, localized corrosion will be concentrated only at the location in a corrosive environment. The concentration of M0042LWO42-, H2VO7 ions is increased to promote repassivation of the steel surface, and the total amount of alloying elements added is greatly reduced compared to the conventional method of changing the alloy composition of the entire steel, making it cheaper. This makes it possible to prevent local corrosion. Mo, W,
The V film or foil may be applied by attaching a vacuum-deposited film, a thermal sprayed film, an electroplated layer, or a spread foil of Mo, W, and V to the surface of the steel. The operation of adhering to the steel surface is extremely simple using conventional methods.

In the present invention, the thickness of the Mo, W, or V film or foil interposed in the material joint or attached to the surface of the weld heat affected zone is set to O. The reason why it is limited to 1 μm or more is because O. If it is less than 1 μm, the M of the joint surface of the material joint or the surface of the heat affected zone of the weld joint
This is because the effect of repassivation by 0042-, WO42LH2VO'4 ions cannot be sufficiently obtained.

Note that when a Mo, W, or V film or foil is interposed on the bonding surface of the material bonding portion, the Mo, W, or V film or foil may be provided on either one surface or both surfaces of the bonding portion.

Next, the effects of the present invention will be explained with reference to Examples. Example
1 test material, length 15 x width 15 x as shown in Figure 5.
Test pieces 1 and 2 of 0 SUS304 with a thickness of 2 mm were placed on the bonding surfaces of the test pieces 1 and 2. Vacuum deposited films of Mo, W, ■ with various thicknesses in the range of 0.05 to 10 μm 3 or thickness of 100 μm
It was used by interposing a μm foil 3 and tightening it with Teflon bolt nuts 4.

Each of the above test materials was treated with 3% NaCl + 1/20 MNa at 30°C.
It was immersed in a solution of 2SO4, and the potential as it was immersed was -50, 0, and 1, respectively, with respect to the agaric electrode (standard electrode).
After holding the potential at 0.00, 200 mV and continuously applying current for 30 days, the occurrence of crevice corrosion at the joints of the test materials was evaluated using the area ratio of the crevice corrosion occurrence area, and the thickness of the Mo film or foil and the The relationship of area ratio is shown in FIG. 1, the same relationship of V is shown in FIG. 2, and the same relationship of W is shown in FIG. 3. In Figures 1 to 3, the vertical axis shows the crevice corrosion occurrence area rate (%), and the horizontal axis shows Mo, ■,
The thickness (μm) of the W film or foil is measured, and the relationship between the thickness of the film or foil and the crevice corrosion occurrence area rate at the potential while immersed is indicated by ○-, and the same at a potential of -50 mV with respect to the agaric electrode. The relationship is...△..., and the same relationship at the potential of Om■ with respect to the sweet tooth electrode is ---]--
1. The same relationship at a potential of 100 mV with respect to the sweet tooth electrode is shown as -------, and the same relationship at a potential of 200 mV with respect to the sweet tooth electrode is shown as ----1◎---1---.

As is clear from FIGS. 1 to 3, the film thicknesses of Mo, V, and W are O. When the thickness was 1 μm or more, no crevice corrosion occurred at all, indicating that the method of the present invention is effective in preventing crevice corrosion.

Example 2 As shown in Figure 6, the test materials were bent into a U shape, each having a length of 75 x width of 10 x thickness of 2 mm, heated at 650°C for 3 hours.
SUS304 test pieces 5 and 6 that have been subjected to air cooling heat treatment (heat treatment equivalent to welding heat affected zone) are placed on the joint surfaces of the test pieces 5 and 6 with an O. A double U-bend test material was used in which a vacuum-deposited film 7 of Mo, W, and V with various thicknesses in the range of O5 to 10 μm or a foil 7 with a thickness of 100 μm was interposed and tightened with stainless steel bolt nuts 8. .

After immersing each of the test materials in a 3L circulation autoclave at 250°C and 8 ppm dissolved oxygen for 500 hours,
The presence or absence of stress corrosion cracking was investigated. Stress corrosion cracking is determined based on the maximum crack depth measured using an optical microscope at the center longitudinal section of the U-bend joint side of the inner side 5 of the double U-bend specimen, and the relationship between the maximum crack depth and film thickness is This is shown in the diagram in Figure 4.

In Figure 4, the vertical axis shows the maximum crack depth (4), and the horizontal axis shows the film thickness (μ
), and the relationship between the film thickness and the maximum crack depth for Mo is shown as -○1, ■... Δ..., and W -D---.

As shown in the figure, the film thickness of Mo, V, and W is 0.1 μm.
As a result, the maximum crack depth was 25μ or less, indicating that the stress corrosion cracking resistance was significantly improved.

Example 3 As a test material, a test piece having the same dimensions as Example 2 and cut from a welded SUS304 material instead of the inner test piece 5 shown in FIG. 6 was used, as shown in FIG. The inner test piece 52 was bent into a U-shape, and the outer surface was coated with MO sprayed coatings 71 with various thicknesses by the wire gas blasting method. A double U-bend test material having the same shape as the test material shown in FIG. 6 was used in combination with test piece 6.

The test material was immersed for 500 hours in a pure water 31 circulating autoclave at 250° C. and 8 ppm of dissolved oxygen, and then the presence or absence of stress corrosion cracking was investigated. Stress corrosion cracking was determined based on the maximum crack depth measured using an optical microscope in the longitudinal section of the heat-affected zone near the weld 9 on the U-bend joining side of the inner test piece 5' of the double U-bend test material. , maximum crack depth and MO
The relationship between sprayed film thickness is shown in the diagram of FIG.

Figure 8 shows the maximum crack depth (μ) on the vertical axis and the film thickness (μ) on the horizontal axis.
) is taken. As shown in the figure, if MO spraying is not performed, intergranular stress corrosion cracking of 1401 tm will occur in the weld heat affected zone, but if the MO spray film thickness is 0.1 μm, the maximum crack depth will be less than 25 μm, and stress corrosion resistant cracking will occur. performance is significantly improved.

Example 4 As a test material, length 7 including welded part 9 was used as shown in FIG.
MO of 10 mm x 30 pieces with a thickness of 100 μm was placed on the weld heat affected zone of a test piece of 0 x width 30 x 5 mm thick ($US3O4).
The foil 7 was attached using bolts and nuts 8 made of SUS3O4.

This test material was immersed in a 3% NaCl solution at 30° C. for 100 days, but no abnormalities were observed and no local corrosion occurred. On the other hand, when MO foil 7 was not attached, pitting corrosion occurred in the weld heat affected zone under the same conditions. It has been confirmed that the same effect can be obtained by using W or V foil instead of MO foil. Example 5 In the salt production process of a seawater desalination equipment,
1) The flange joints of stainless steel valves that are immersed in NaCl solution usually develop crevice corrosion after about 3 months of continuous use and become unusable. As a result of interposing a vacuum-deposited film with a thickness of 2 μm, no crevice corrosion occurred at the flange joint even after one year of continuous use.

[Brief explanation of the drawing]

Figure 1 is a chart showing the relationship between the thickness of the MO film or foil and the crevice corrosion occurrence area rate, Figure 2 is a chart showing the relationship between the same thickness and the same area rate for V, and Figure 3 is the same for W. A chart showing the relationship between the thickness and the same area ratio, Figure 4 is a chart showing the relationship between the thickness of MO, V, and W films or foils and the maximum crack depth, and Figure 5 is the first test of the crevice corrosion test. Figure 6 is a cross-sectional view of the test material for stress corrosion cracking test 1. Figure 7 is a cross-sectional view of a test piece showing another example of the inner test 1 test piece in Figure 6. Figure 8 is a chart showing the relationship between the sprayed MO film thickness and the maximum crack depth in the weld heat affected zone, and Figure 9 is a perspective view of the test material with MO, W, and V foils attached to the weld heat affected zone. be. 1, 2, 5, 5', 6: 1 test piece, 3, 7: membrane or foil,
7': Thermal spray coating, 4: Teflon bolt nuts, 8: Stainless steel bolt nuts, 9: Welded parts.

Claims (1)

[Claims] 1 Stainless steel structures are bonded by interposing a Mo, W or V film or foil with a thickness of 0.1 μm or more at the material joints, or Mo, W or V is applied to the surface of the heat-affected zone of welded joints. A method for preventing local corrosion of stainless steel structures, which comprises depositing a film or foil to a thickness of 0.1 μm or more.