JPH0478420A - Method for preventing corrosion of stainless steel for stack gas desulfurizer - Google Patents

Abstract

PURPOSE:To form an anticorrosive coating film consisting essentially of oxidative manganese oxide on the surface of the stainless steel and to prevent the corrosion of the steel by adding >=1mg/l of manganese salt while blowing a gas contg. oxygen into a liq. absorbent. CONSTITUTION:The solid Mn compd. in a liq. absorbent is filtered off, and the Mn<++> ion in the filtrate is controlled to >=2mg/l. The absorbent thus obtained and contg. the manganese compd. is intermittently or continuously injected into the circulating tank 2 from a pipeline 5. Austenitic SUS304 is used as the material for the tank 2, air blowing line 3 and pipeline 7 for pumping the absorbent into an absorption tower 1. The desulfurizer is continuously operated and overhauled after 700hr and 8,000hr. A coating film of dark brown manganese oxide is firmly formed after 8,000hr, the oxide is cleaned off, and the surface of the material is investigated. According to the result, the corrosion after 8,000hr is just the same as that after 700hr, and the corrosion does not proceeds.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for preventing corrosion of stainless steel used in flue gas desulfurization equipment using the lime plaster method.

[Conventional technology]

Chlorine compounds contained in flue gas or chlorides contained in supply water (e.g. industrial water, seawater) accumulate in the absorption liquid of flue gas desulfurization equipment using the lime plaster method, causing pitting corrosion of stainless steel, deposit attack, and Crevice corrosion has occurred, which has become a problem. To prevent this, conventionally, high-grade stainless copper, which is less affected by chloride corrosion, nickel-based alloys such as Hastelloy, titanium-based alloys, or rubber linings or resin linings have been used.

[Problem to be solved by the invention]

Among conventional technologies, high-grade stainless steel, nickel-based alloys,
Materials such as titanium alloys are expensive and equipment construction costs are high. That is, the higher the chloride concentration in the absorption liquid, the more expensive metal materials need to be selected and used. On the other hand, rubber and resin linings often lack reliability in terms of durability, such as defects caused by poor construction and slurry wear.

In order to eliminate such drawbacks, the present invention attempts to propose a corrosion prevention method that can prevent pitting corrosion, crevice corrosion, etc. even when inexpensive stainless steel is used.

[Means to solve the problem]

The present invention provides a corrosion prevention method that forms an oxidizing film on the surface of stainless steel that comes into contact with the weakly acidic absorption liquid, thereby preventing pitting corrosion, deposit attack, and crevice corrosion even on low-grade stainless steel copper. Therefore, in the present invention, while blowing a gas containing oxygen into the absorption liquid,
This is a method of preventing corrosion by adding the above manganese salt to form an anticorrosive coating mainly composed of oxidizing manganese oxide, which has excellent anticorrosive properties, on the surface of stainless steel.

[Effect]

In general, pitting corrosion, deposit attack, and crevice corrosion of stainless steel in weakly acidic solutions containing chlorides are caused by the non-uniformity of the passive film formed on the surface of the stainless steel, which is made of oxides of metal components. This occurs due to the formation of an oxygen concentration battery, an oxygen concentration battery due to oxygen deficiency in the deposit area, and an oxygen concentration battery due to oxygen deficiency in the gap, but if chloride is included, C1- ions diffuse into the corroded area. However, it is said that due to further diffusion of H+, the pH of approximately one corroded area gradually decreases, and corrosion is accelerated.

The manganese oxide film of the present invention has the following effect on preventing the above-mentioned corrosion.

Manganese oxide has an oxidizing property, and by supplying oxygen to the oxygen-deficient portion, it has the effect of strengthening the passive film on the surface of the stainless steel and preventing the formation of an oxygen concentration battery. Furthermore, the main component of the manganese oxide film formed in the present invention is MnL, and also contains Mn2D3, MnO, etc., but after supplying oxygen to the oxygen-deficient area, for example, MnL is reduced and becomes MnJs, MnO. In places where corrosion is likely to occur, Mn20s, Mn[l
increases, but Mn2O3 is weakly basic and Mn[] is an oxide that shows basicity, and C1- diffuses into the corroded area, resulting in p)l
It has a neutralizing effect and a basicizing effect and suppresses corrosion.

6. The manganese oxide film of the present invention is formed by blowing oxygen-containing gas into the film, whereby the added manganese salt is oxidized and insoluble manganese oxide is precipitated.

The blown oxygen not only acts on the precipitation of the manganese oxide, but also the manganese oxide formed on the stainless steel surface changes into a low-value oxide by oxidation of the stainless steel, and reacts with this low-value oxide and regenerates. It also has the effect of converting it into expensive oxides. Therefore, by constantly blowing oxygen-containing gas into the stainless steel, the oxidizing properties of the manganese oxide film formed on the surface of the stainless steel can be maintained.

It is desirable to add manganese salt at a high concentration in the early stage of equipment operation in order to quickly form a manganese oxide film, but after the manganese oxide film is formed, the anticorrosion effect can be maintained by adding about 1 ppm. Manganese salt can be added continuously or intermittently.

〔Example〕

The present invention will be specifically explained using an embodiment shown in FIG.

In FIG. 1, an exhaust gas a containing SO3 is subjected to gas-liquid contact treatment with an absorption liquid containing a calcium compound in an absorption tower 1, is desulfurized, and is discharged from the absorption tower 1 as a clean exhaust gas b. The absorption liquid that has absorbed SO3 as sulfite in the absorption tower 1 falls into the circulation tank 2 at the bottom of the absorption tower 1. Air is blown into the tank 2 from an air supply line 3 in order to oxidize the sulfite and turn it into gypsum.

In the wet flue gas desulfurization apparatus of the above aspect, 5Oa1200
Combustion exhaust gas containing ppm at a temperature of 110°C is generated at 20°C per hour.
．． 0008m' is introduced into the absorption tower 1, and the calcium compound L
The absorption liquid containing 2 mol/i and Mn ~ ion 2 /1 is sprayed every hour into 350 m3 of absorption ifI for desulfurization, and the volume 15 in which the absorbed liquid falling from the absorption tower 1 is stored is
30 m3 per hour from the air blowing line 3 to the circulation tank 2.
0 m' of air is blown into the absorption tower 1 to oxidize the sulfite produced by absorption of SO2. Furthermore, in the absorption tower circulation tank 2, control analysis is performed so that the Mn''+ ions in the filtrate excluding the solid phase Mn compounds of the absorption liquid stored in said tank 2 are 2■/1 or more, and manganese is A solution of the compound was injected through pipe 5. During steady state operation, the amounts of make-up water and water taken out were adjusted so that the CI concentration in the absorption liquid was 25.000 ppm.

In the example of the embodiment operated under the above conditions, the material of the circulation tank 2, the air blowing line 3, and the piping 7 serving as the pumping line for the absorption liquid to the absorption tower 1 in FIG. Continuous operation was performed, and overhaul inspections were conducted after approximately 700 hours and 8,000 hours had passed. After about 700 hours, an open inspection revealed that a very thin layer of dark brown manganese oxide had been deposited on the inner walls of the circulation tank 2, the piping 7, and the surface of the air blowing line 3.
Pitting corrosion of approximately 1 mm was observed. Subsequently, an open inspection after 8,000 hours of operation revealed that a dark brown manganese oxide film had formed firmly, and after washing and removing the manganese oxide, the condition of the material surface was investigated and the condition after approximately 700 hours had elapsed. The corrosion progress was not observed. In addition,
In FIG. 1, 6 is a circulation pump.

The same equipment as in the above example was operated continuously under the same conditions except that the manganese compound was not injected, and an inspection was performed after about 700 hours. As a result, the inner walls of the circulation tank 2 piping 7 and the air blowing line were Severe pitting corrosion was observed in material No. 3.

(The depth of pitting corrosion reached 5 mm in some cases.)
In addition, as a result of an open inspection after 1000 hours of operation using a device with the same specifications as in the above example, injecting a manganese compound but not blowing air, no adhesion of manganese oxides to the material surface was observed. The corroded state of the material was pitting corrosion equivalent to the case where no manganese was added.

〔Effect of the invention〕

As shown in the examples, when the corrosion prevention method of the present invention is not applied, it is difficult to resist pitting corrosion, deposit attack, or crevice corrosion unless the material used for the device is a high-quality material such as Hastelloy C276. In the present invention, 5IJS 3
It becomes possible to use inexpensive stainless steels such as 04, SO83] and 6, which has a large economical effect.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram for explaining the present invention in detail. Figure 1

Claims (1)

[Claims] While blowing oxygen-containing gas into the absorption liquid circulation tank of the flue gas desulfurization equipment using the lime plaster method, 1 mg of oxygen was added to the absorption liquid.
1. A method for preventing corrosion of stainless steel for flue gas desulfurization equipment, the method comprising: adding manganese salt in an amount of /l or more to form a film containing manganese oxide as a main component on the surface of the stainless steel used for the flue gas desulfurization equipment.