JPH0254423B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for preventing corrosion of metals exposed to high temperature gases, such as combustion gases, containing HCl.

BACKGROUND ART In recent years, the amount of waste containing chloride from households and industrial facilities, such as garbage containing plastics such as vinyl chloride, has increased dramatically, and how to dispose of it has become a problem. Currently, there are two common disposal methods: landfilling and incineration, but the former has pollution problems such as soil contamination, and the proportion of incineration is increasing due to the depletion of land for landfill. As the amount of waste increases, the amount of plastic contained in the waste also increases, and the energy contained in the waste increases, making effective recovery of the waste an urgent issue. From this perspective, various devices have been researched and developed in recent years that effectively utilize the calorific value of the high-temperature combustion gas generated when chloride-containing substances, such as the above-mentioned wastes, are combusted, and some of them have already been put into practical use. . In these devices, corrosive gases such as HCl, Cl ₂ and SOx are present in the high-temperature combustion gas, and ash consisting of unburned substances and alkali metal salts, heavy metal salts and other solid substances produced as a result of combustion is inevitable. coexist with Metals exposed to such high-temperature combustion gases will undergo very severe corrosion (high-temperature corrosion) if ash adheres to their surfaces and HCl coexists in the atmosphere to which they are exposed, and the corrosion will increase as the temperature rises. It is known that the speed increases.
In other words, high-temperature corrosion occurs when the surface temperature of metal exceeds 300℃, and corrosion becomes particularly severe when the surface temperature exceeds 400℃. There is nothing that can withstand high temperature corrosion.

Traditionally, two methods have been used to protect the metal materials of combustion furnaces from this type of high-temperature corrosion. One is to neutralize the acidic corrosive gas generated during combustion by reacting with alkaline substances such as soda carbonate, or to incinerate alkaline substances together with the waste when burning waste that generates HCl such as PVC. There is a method of using a so-called neutralizing agent, which is supplied to the combustion atmosphere. However, although these methods are mainly effective for treating corrosive gases in combustion gas, they are effective against high-temperature corrosion that occurs when ash adheres to the metal surface and HCl coexists in the atmospheric gas. This method was insufficient as a corrosion prevention method. In other words, metals that undergo high-temperature corrosion depend on various factors such as the geometric relationship between the furnace internal structure and the metal surface where high-temperature corrosion occurs, the flow state of the combustion gas, and the mixed contact state of the corrosive gas and the neutralizing agent. This is because there are areas on the surface where carbonate and the like do not reach a sufficient amount for corrosion protection.

Another method is to lower the temperature of the metal material in the combustion furnace. That is, since this high-temperature corrosion progresses more rapidly when the metal surface temperature exceeds 400°C, the metal surface temperature is kept below 400°C. Therefore, when waste combustion heat is recovered as steam in a boiler, waste heat boiler, etc., the boiler's heat transfer tube wall temperature must be maintained at 400℃ or less, and as a result, the generated steam is 20℃ at the design value.
Kg/cm ² G, below 300℃, 16 under operating conditions of practical equipment
~20Kg/cm ² G, the temperature will be around 200℃. In such medium-pressure steam, for example, when energy is converted into electricity, which is the most common method, the power generation efficiency is considerably lower than the latest heavy oil-fired thermal power generation, and the power generation cost is also high.

The present invention provides a method that prevents the above-mentioned high-temperature corrosion and, as one result, makes it possible to economically obtain high-pressure steam.

In this invention, chromium and nickel are added to metal surfaces exposed to HCl gas, such as incinerator parts made of a wide variety of metal materials from carbon steel to heat-resistant alloys, depending on the required heat resistance. The first of the mixtures () containing at a weight ratio of 2/8 to 8/2
The layer is first applied by thermal spraying and then a second layer of a different composition than the first layer is applied thereon by thermal spraying.

Combustion furnace members are usually made of various steel materials, iron-based alloys, nickel-chromium alloys, etc., as required, so the present invention uses the above-mentioned nickel-chromium mixture, which has a sufficiently strong affinity for these materials and has good heat resistance. A first layer is formed on the surface of the incinerator member by thermal spraying. The mixture () consists only of nickel and chromium, or contains up to 3% of other elements based on the total of both. The reason why the mixture () has a chromium/nickel ratio of 20/80 to 80/20 is that the adhesion to the base material has good durability against heating cycles.
20/80 is often used in plasma spraying, but in order to further improve the corrosion resistance of the second layer etc. when it contains a mixture (2), it is recommended to increase the amount of chromium, such as 50/50, within the above range. Further, other elements are added to take advantage of the properties of the mixture of the two to further improve strength, acid resistance, oxidation resistance, etc. Typical examples include niobium, tantalum, molybdenum, tungsten, aluminum, and silicon.

Since the first layer of the mixture () does not have sufficient corrosion resistance for a combustion furnace member exposed to HCl, in this invention, a second layer having sufficient affinity for the first layer and having better corrosion resistance is added to the first layer. Form on top.

One second layer is the mixture () i.e. the first layer in weight ratio.
30 to 70% of layer components and at least one kind of alkaline substance are formed. If a high degree of heat resistance is required to partially withstand high temperatures of 800 to 1000℃, the second layer must have a mixture () of 30% or more, otherwise the second layer will lack affinity for the first layer. Therefore, peeling between the first layer and the second layer tends to occur, and it is often difficult to form a stable second layer. This tendency is particularly remarkable in the alkaline material of the present invention compared to ceramics conventionally known for this type of use. On the other hand, in order to provide such a high degree of heat resistance, alkaline substances must be
% or more, the corrosion resistance of the second layer is generally insufficient.

As described above, the present invention started by coating the first layer with a second layer having a specific composition consisting of the components of the first layer and a material with better corrosion resistance to provide the above-mentioned high heat resistance. According to research conducted by researchers, it is surprising that ceramics such as alumina have a heat resistance of up to 30°C, especially in cases such as steam-generating municipal waste incinerators, which only require heat resistance up to about 600°C.
Between the above-mentioned cases, the second layer may be a mixture of the substantial ceramics containing the first layer component and the alkaline substance in a weight ratio of 7:3 to 3:7, preferably 7:3 to 5:5. It was found that there was no significant difference in performance. However, in whole or in part, e.g. 800 to 1000
Considering heat resistance at higher temperatures such as °C, the second
When the layer is made of a mixture of the first layer component and the corrosion-resistant material having the above-mentioned specific composition, there is less risk of peeling or the like.

Various thermal spraying methods can be used as necessary for the thermal spraying used in the present invention, but plasma spraying is particularly representative and is useful for forming a dense coating layer that is useful for corrosion resistance. It is also possible to reduce the oxide coating by mixing hydrogen into an inert gas used in plasma spraying or the like.

The ceramics that can be used in this invention are silicon,
There are oxides such as aluminum, magnesium, calcium, and zirconium, carbides such as silicon, boron, and titanium, and nitrides such as silicon, boron, and aluminum, and at least one of them is used.
Aluminum oxide is very typical.

It is sufficient that each layer used in this invention is mixed during thermal spraying to form a sufficiently uniform layer, but it is of course possible to create a more uniform layer by preparing a uniform mixture of alloys, etc., before thermal spraying, if desired. is easy to obtain and is generally preferred.

In the present invention, the first layer with an intermediate composition is interposed between the base material and the second layer, but an intermediate layer may also be interposed between the base material and each layer and the layer with an intermediate composition. In general, if this is done, improved durability against thermal cycles can be expected.

According to the method of the present invention, even if HCl and ash approach the high-temperature metal surface when the furnace starts operating, the alkaline substances in the layer containing a sufficient amount of alkaline substances already present will react with the HCl. By turning into neutral salt ash and preventing high-temperature corrosion on metal surfaces, it prevents oxidation at high temperatures, corrosion due to HCl at high temperatures, or severe high-temperature corrosion when HCl and ash of alkali metal salts, heavy metal salts, etc. coexist. In the case of coating layers consisting of a mixture of ceramics and alkaline substances, this layer has the effect of both forming neutral salt ash and exhibiting corrosion resistance. By developing this, high-temperature corrosion that reaches the base metal surface does not occur.

As the above-mentioned alkaline substances, any substance that can sufficiently exhibit the above-mentioned effects can be used, and at least one kind of them is used.Of these, carbonates and alkali silicates of alkali metals and alkaline earth metals are representative. And as the alkaline earth metal carbonate, at least one type of carbonate such as Na, K, Ca, or Mg is used, and sodium carbonate is usually most suitable from the viewpoint of anticorrosion performance and cost.
The alkali silicate is typically sodium, potassium or lithium silicate.

In the above, the field of application of the present invention was typically explained using a combustion furnace, but as mentioned above, fuel furnaces are subject to severe corrosive conditions due to the coexistence of ash. Therefore, the present invention applies to other parts exposed to high temperature gas containing HCl, not only when ash coexists, but also under milder conditions where ash does not coexist. It is also applicable to various members etc.

The present invention will be clarified with reference to Examples below, but the present invention is not limited thereto.

Example: 30 mm long and 50 mm wide from a rolled plate of alloy steel (SCMV4) consisting of 2.2% chromium, 1% molybdenum, 0.15% or less carbon (all weight%), and the balance iron.
A large number of plate-shaped specimens with a diameter of 40 mm and a thickness of 40 mm were cut out, and each test material was coated with various methods and then subjected to a corrosion resistance test.

The test material was coated using a commercially available plasma spraying machine with 60% argon and 40% helium as the spray gas.
%, (all % by volume) using a mixture of DC voltage
Used under the conditions of 40 volts and a discharge current of 800 amperes,
One surface of the test material with an area of 30 mm x 50 mm was coated. A corrosion resistance test was conducted using this coated test material as a test piece. For the corrosion resistance test, ash collected from an actual garbage incinerator was placed uniformly on the surface of the test piece on which the coating layer was formed using a tubular electric furnace, and water vapor was placed inside the tube for 30 minutes.
%, CC ₂ 10%, HCl 1000ppm, SO ₂ 20ppm (all % by volume).The test piece was placed in a tubular electric furnace through which a high-temperature atmosphere gas having the composition of the remaining air was flowing, and the test piece surface temperature was maintained at 600℃. I went. Five sides of the specimen other than the 30 x 50 mm area being tested were protected from corrosion by fine alumina powder and fibrous insulation. The composition of the ash collected from the garbage incinerator and placed on the surface of the test piece was 7.1% aluminum, 3.8% sodium, and 2.8% potassium.
%, calcium 14.5%, magnesium 1.5%, iron
6.1%, silicon 15.2%, chlorine 2.1%, total sulfur 1.1
%, water content 0.1% (both weight %), and the remainder is mainly oxygen bonded to the above elements.

Corrosion resistance tests were conducted under the conditions described above for up to 10 days, and SCMV4 without coating was also tested for comparison. After conducting the corrosion resistance test, the test piece was completely purged with _N2 gas in the tubular electric furnace, and then taken out from the electric furnace and left to cool at room temperature for about 5 hours.Then, the ash on the surface of the test piece was removed by brushing, and the ash was further removed as an inhibitor.
The test piece was immersed in an aqueous hydrochloric acid solution (8 wt% HCl) containing 0.5% hexamethylenetetramine for 30 minutes at room temperature to remove corrosion products, then washed with water, dried, measured its weight, and conducted a corrosion resistance test. The so-called weight loss was determined by comparing the weight of the test piece that had been previously measured. FIG. 1 is a diagram showing the results of this corrosion resistance test, where the horizontal axis is the number of test days and the vertical axis is the weight loss of the test piece (mg/effective surface area 15 cm ² ).

In addition, the line numbers in the figure are the test piece numbers and also indicate the differences in the content of the coating layer, and a coating layer with the structure described below is used for each number.

Test piece 1: SCMV4 steel without coating layer Test piece 2: 1st layer chromium-nickel alloy (20% Cr - 80% Ni) 2nd layer on SCMV4 steel
Na ₂ CO ₃ 50% and chromium-nickel alloy (20%
A mixed layer of 50% Cr-80%Ni was coated with a thickness of 150μ each by plasma spraying.

Test piece 3: The same first layer as test piece 2 was applied on SCMV4 steel, and the second layer was made of 50% alumina and chromium.
A 50% mixed layer of nickel alloy (20% Cr-80% Ni) was coated with a thickness of 100 μm each by plasma spraying.

Test piece 4: On SCMV4 steel, the first and second layers were the same as those for test piece 3, and the third layer was coated with alumina to a thickness of 50μ by plasma spraying.

Test piece 5: SCMV4 steel was coated with the same first layer as test piece 2, and alumina was coated on the second layer to a thickness of 50μ by plasma spraying.

Test piece 6: On SCMV4 steel, the first layer is chromium-nickel alloy (50% Cr-50% Ni), the second layer is 30% alumina, chromium-nickel alloy (50%
%Cr-50%Ni) 70% mixed layers were coated by plasma spraying to a thickness of 100μ each.

Test piece 7: On SCMV4 steel, the first layer is chromium-nickel alloy (85% Cr-15% Ni), the second layer is 30% alumina, chromium-nickel alloy (50%
%Cr-50%Ni) 70% mixed layers were each coated with a thickness of 100μ by plasma spraying.

Test piece 8: 1st layer of chromium on SCMV4 steel
Silicon-nickel alloy (20%Cr-10%Si-
70% Ni), 30% alumina in the second layer, chromium
A 70% mixed layer of nickel alloy (50% Cr-50% Ni) was coated with a thickness of 100 μm each by plasma spraying.

As shown by the line in Figure 1, the weight loss, or the amount of corrosion, increases over time for test specimens without anti-corrosion coatings, and if this line continues as it is, the corrosion rate will increase over the first year. is 1.5mm
That's all.

On the contrary, test piece wire 2, which is an example of the conventional technique using ceramics, is a test piece wire 2 in which the present invention is applied and an effective coating layer is formed for corrosion prevention.
In samples No. 5 and No. 6, there was almost no weight loss and it was below the measurable limit value. In other words, there was almost no corrosion, proving the effectiveness of the present invention and the effectiveness of conventionally known ceramics when suitably applied. In addition, in test specimen 7 indicated by line 7, when the chromium content of the first layer increases beyond the appropriate range in the conventional technology, peeling of the coating layer occurs;
If the layer contains as much as 10% of a third element other than chromium or nickel, such as silicon, the coating layer will peel off, resulting in no anticorrosion effect.

Furthermore, in specimen 2, instead of Na ₂ CO ₃
A similar corrosion resistance test was conducted using test piece 9 obtained using Na ₂ SiO ₃ and test piece 10 obtained using a mixture of alumina and Na ₂ CO ₃ at a weight ratio of 1:1 instead of alumina. As a result, the same results as Test Piece 2 and Test Piece 5 were obtained.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the results of Examples.

Claims (1)

[Claims] 1. A mixture of chromium and nickel with a chromium/nickel weight ratio of 2/8 to 8/2 is deposited on a metal surface exposed to a gas atmosphere of 400°C or higher containing HCl gas. ), and a second layer having a composition of any one of A to B below is formed thereon by thermal spraying. A: A mixture consisting of 30 to 70% by weight of the first layer component and 70 to 30% of at least one type of alkaline substance. B: less than 30% by weight of ceramics
Mixture containing layer components 70-30% and alkaline substances
A mixture consisting of 30-70%. 2. Apply a mixture of chromium and nickel with a chromium/nickel weight ratio of 2/8 to 8/2 on a metal surface exposed to a gas atmosphere containing HCl gas at a temperature of 400°C or higher, with a weight ratio of 3% or less to this mixture. The first layer of the mixture () is a mixture containing at least one other element of niobium, tantalum, molybdenum, tungsten, aluminum, and silicon in a proportion of
A corrosion prevention method characterized in that a second layer having a composition of one of the following A-B is formed thereon by thermal spraying. A: A mixture consisting of 30 to 70% by weight of the first layer component and 70 to 30% of at least one type of alkaline substance. B: less than 30% by weight of ceramics
Mixture containing layer components 70-30% and alkaline substances
A mixture consisting of 30-70%. 3. The method according to any one of claims 1 to 2, wherein the thermal spraying is plasma spraying. 4. The method according to any one of claims 1 to 3, wherein the alkaline substance is at least one of an alkali metal carbonate, an alkaline earth metal carbonate, or an alkali silicate. 5. Claims 1 to 4 in which the ceramic is at least one of oxides of silicon, aluminum, magnesium, calcium, or zirconium; carbides of silicon, boron, or titanium; and nitrides of silicon, boron, or aluminum. Any method described.