JPS6099380A - Corrosion inhibiting method of seawater desalination apparatus - Google Patents

Abstract

PURPOSE:To form a film on the bottom plate of an evaporation chamber made of stainless steel, and to prevent accurately the clearance corrosion by sprinkling a corrosion inhibitor of a specified composition. CONSTITUTION:A mixture of zinc powder and a water-soluble dispersant or a mixture of zinc powder, a water-soluble dispersant, and a water-soluble binder is used as a corrosion inhibitor. The clearance corrsion can be prevented by sprinkling said corrosion inhibitor into an evaporation chamber made of stainless steel, and setting the inhibitor on the bottom plate to form a film. Since the film is the water-soluble deposit of zinc hydroxide, etc., the deposit is completely eluted in a hydraulic test and in the normal operation stage, and the difficulties due to the remaining substance do not occur.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in corrosion prevention methods for seawater desalination equipment.

Expansion of evaporation chamber in multi-stage flash desalination equipment, often 8
It is made of stainless steel such as UF3316L, 5US317L or stainless steel clad copper plate, and is designed to prevent corrosion in the high temperature degassing pline during steady operation.

However, even in the stainless steel part, the following (1) to (4)
Corrosion occurs frequently under these conditions.

(If foreign matter adheres during the manufacturing, transportation, and on-site installation stages, crevice corrosion will occur at those locations.

(2) During the water pressure test, crevice corrosion occurs on the bottom plate and the area where foreign matter adheres.

(3) During the trial run stage, when the operation is stopped and the air pressure in the evaporation chamber is released, oxygen will dissolve in the pline, but since the pline is not flowing, crevice corrosion will occur where foreign matter adheres. .

(4) When desalination equipment is stopped for a long time and stored, the inside of the evaporation chamber is washed with water with low salt concentration before it is stopped and then dried to prevent corrosion. Incomplete cleaning may cause corrosion of the stainless steel on the bottom plate.

Among these, inorganic and organic inhibitors have been conventionally used to prevent crevice corrosion during hydraulic tests. However, due to local circumstances, water with various quality properties is used as pressure-resistant water for hydraulic pressure tests, which has the following drawbacks.

(1) Addition of an inhibitor does not provide a corrosion protection effect of 1009 g, and it is not possible to sufficiently prevent crevice corrosion etc. for materials such as stainless steel where local corrosion is the main cause.

(2) When testing the evaporation chamber of a multi-stage flood seawater desalination equipment, a large amount of pressure-resistant water of 4,000 to 6,000 m is required, and if 1 to 3% inhibitor is added to this water, it will pollute the ocean during drainage. will occur.

(3) Even if an inhibitor is added, little corrosion prevention effect can be expected in the lower part of the snowboard and the lower part of the deposit.

Therefore, the present inventor conducted intensive research to solve the problem of crevice corrosion on the bottom plate of a stainless steel evaporation chamber, and as a result, found that a large number of crevice corrosion occurred in the part where foreign matter was attached. Although it is not possible to completely prevent corrosion, corrosion problems did not occur in pieces B1 other than the bottom plate even when no additives were used. Therefore, it was found that crevice corrosion can be prevented by locally applying anti-corrosion treatment to only the bottom plate.

Corrosion prevention treatment methods include a galvanic anode type cathodic protection method that provides cathodic protection through electrochemical action using zinc, which has a base potential on steel, a zinc-rich paint coating method, and corrosion protection using crevice corrosion inhibitors. methods, etc. have been put into practical use for some time.

However, in the cathodic protection method, if the conductivity of the water used in the hydraulic test is low, the current reach range is extremely narrow. For this reason, zinc cannot be used in practice, and it is necessary to use magnesium or the like, which is not appropriate.

In addition, the zinc rich paint coating method uses a coating liquid containing a binder such as ethyl silicate or epoxy resin mixed with zinc powder, and the amount of zinc in the dry film is 87 to 87%.
It is around 93%.

This coating film has sufficient anticorrosion effect during the hydraulic test.

However, it has been experimentally confirmed that when the zinc is eluted during subsequent steady-state operation, the binder remains on the stainless steel surface and that this accelerates crevice corrosion.

In addition, a method using a crevice corrosion inhibitor may be used, with 10 to 50% of polyethylene or ethylene glopylene rubber
This method uses a mixture of zinc powder in the form of a paste or sheet.

As with the zinc-rich paint coating method, the binder is not water-soluble, so if the zinc is eluted, the binder remains and the residue accelerates crevice corrosion.

The present invention has been made in view of the above circumstances, and its purpose is to provide a corrosion prevention method for seawater desalination equipment that can prevent crevice corrosion in the bottom plate of an evaporation chamber made of mineral or stainless steel. It is something to do.

That is, the present invention is characterized in that a water-soluble corrosion inhibitor mainly composed of zinc powder is sprinkled on the stainless steel evaporation chamber of the seawater desalination equipment, and is deposited on the bottom plate to prevent crevice corrosion. shall be.

The present invention will be explained in detail below.

The corrosion prevention method of the present invention is a method in which a water-soluble corrosion inhibitor mainly consisting of zinc powder is sprinkled on a stainless steel evaporation chamber of a seawater desalination apparatus. This corrosion inhibitor may be composed of zinc powder and a water-soluble dispersant, or it may be composed of zinc powder, a water-soluble dispersant, and a water-soluble binder.

By sprinkling this corrosion inhibitor, a film is formed on the bottom plate to prevent corrosion. Since this film is a water-soluble precipitate such as zinc and zinc hydroxide, it is completely eluted during the hydraulic test and steady-state operation stage, and no damage is caused by residual substances.

Note that the method of the present invention is applicable not only to water pressure tests but also as a corrosion prevention method during marine transportation and during long-term suspension. Next, an experimental example of the present invention will be explained.

In this experimental example, in order to confirm the effectiveness of this corrosion prevention method, we studied the electrochemical corrosion prevention effect and pH controllability on stainless steel (Sl: IS 316L) using simulated hydraulic water containing 2250 ppm NaC. .

Experimental Example 1 (Basic Corrosion Prevention Test) Corrosion prevention effects were investigated using the following corrosion inhibitors of the present invention and for comparison. The results are shown in Table 1.

From the above table, it was confirmed that stainless steel M (8US 316L) has the following electrochemical anti-corrosion effect due to adsorption of zinc powder.

(a) It is known that the pitting corrosion potential and protection potential are increased #1, and the crevice corrosion prevention effect is improved. The method of this invention has an effect comparable to that of chlorine nitrite inhibitors.

(b) The corrosion potential of SUS 316L increases, and the polarization towards the negative polarization side increases. # Once this occurs, pitting corrosion and crevice corrosion become less likely to occur. This method is the best -0.65V vs SCE.

(c) As an evaluation value of pitting corrosion resistance, (pitting corrosion potential) - (oxidation-reduction potential) is used, and this value is also the best in the method of this invention.

(a) The pH value near the surface of SUs 3i6L is
The closer it is to alkalinity, the more difficult it is for crevice corrosion to occur. The method of this invention has a pH of 9,
It can be controlled to 2.

(c) Concentration distribution of anticorrosion additive In the case of the comparative method (chlorine nitrite inhibitor), the concentration is constant in all parts of the beaker because it is completely dissolved.

On the other hand, in the method of the present invention, when stirring is stopped after addition, most of the zinc powder settles at the bottom of the bead. Some of the zinc powder was dispersed in the liquid and installed in the center of the beer force.
It is adsorbed on the surface of the test piece.

Due to this sedimentation and floating effect, an environment with a high concentration of zinc and zinc compounds (zinc hydroxide) can be formed near the bottom.

Experimental Example 2 (Modeled Corrosion Test) As shown in Table 2, the method of the present invention has excellent corrosion protection even for complex gap structures such as sputter deposition.

Next, examples of the present invention will be described.

The seawater desalination equipment consists of 24 stages of evaporation chambers.
SUS 316L blades are used in stages 1 to 6 and stage 24. Furthermore, the bottom plate portion of each stage has dimensions of 3m x 18m.

Before passing water through the water pressure test of this equipment, the following treatments were carried out to investigate the corrosion resistance.

(1st to 6th tier) After flowing water with the same properties as hydraulic water to the bottom plate of each tier, immediately apply metal zinc powder (manufactured by Mitsubishi Metals KK) to all tiers of 6 to 1oKft.
- Spread evenly.

The brushed zinc and water were then mixed.

Then, on the fifth day after the treatment, hydraulic test water was passed through the water.

(24th stage) After cleaning the bottom plate part with a vacuum cleaner, water pressure test water was passed on the third day.

The time from water flow to drying of the drain was 32 days. After drying the drain, we investigated the condition of the bottom plate of each tier, and found that tiers 1 to 6 treated with zinc powder had no signs of corrosion, including areas where spatter had adhered. No corrosion ore was detected.

On the other hand, on the 24th stage, corrosion had occurred in a total of 12 places on the lid and smooth parts of the snow and ivy.

As is clear from the above results, the present invention has a remarkable effect of reliably preventing corrosion of the bottom plate portion using a simple method.

Claims (1)

[Claims] Corrosion prevention for seawater desalination equipment, characterized in that a water-soluble corrosion inhibitor mainly composed of zinc powder is sprinkled on the stainless steel evaporation chamber of the seawater desalination equipment and precipitated on the bottom plate to prevent corrosion. Method.