JPH0364432A - High temperature corrosion-resistant heat transfer tube and its manufacture - Google Patents

Abstract

PURPOSE:To improve the high temp. corrosion resistance of the heat transfer tube used in a high temp. corrosion environment caused by an eutectic salt based on the sulfate and chloride of alkaline metals by forming of an alloy steel having specified content of Ni and Cr. CONSTITUTION:The heat transfer tube for high temp. and high pressure type- recoverying boilers, trash incineration furnaces or the like in a paper mill is formed of the alloy steel contg., by weight, 12 to 30% Ni, 18 to 30% Cr, >=2% Mo and the balance Fe. In this way, the corrosion of the alloy steel caused by the above eutectic salt can drastically be reduced. Thus, the high temp. and high pressure-type recoverying boilers stably capable of feeding high temp. steam of 500 deg.C which has not been attained by a conventional technique can be offered.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to corrosion-resistant heat exchanger tubes containing Ni and Cr used in high-temperature corrosive environments with eutectic salts based on alkali metal sulfates and chlorides. Regarding high temperature corrosion resistant heat exchanger tubes.

[Conventional technology]

Recovery boilers in paper mills are the main equipment that burns black liquor, which is cooking waste, and recovers chemicals and steam. Recently, in order to make boilers, which are the main power source in factories, even more efficient, the combustion of highly concentrated black liquor and the use of high-temperature, high-pressure recovery boilers have been promoted.

However, when adopting this new type of boiler,
Conventionally, the maximum steam temperature was 480°C, but
In order to raise the temperature to 00-505°C, the following serious problems will arise.

That is, when the steam temperature is raised to the above temperature, the surface temperature of the boiler superheater tube rises to nearly 530 to 540°C, causing high-temperature corrosion peculiar to recovery boilers.

This high-temperature corrosion is caused by a large amount of alkaline compounds being scattered when black liquor is burned in the recovery boiler and deposited on the surface of the superheater tubes. To explain in more detail, the main component of the deposit is NazS○, but with the addition of compounds contained in wood chips and NaCl mixed in from outside the system, the eutectic point is in the range of 500 to 600°C. Due to the formation of eutectic material, the corrosion reaction of the metal under the molten salt progresses significantly. Figure 1 shows an example in which deposits on the surface of superheater tubes of various recovery boilers were collected and thermally analyzed. It varies depending on each deposit, but 514 to 570'C
It can be seen that it has a eutectic point. If such a eutectic point is equal to or exceeds the tube surface temperature of the superheater tube, severe corrosion due to molten salt will proceed.

Similar corrosion occurs in municipal and industrial waste incinerators, which are attempting to raise the steam temperature and improve the efficiency of waste heat recovery.

The above-mentioned problems cannot be addressed with conventional heat transfer tube materials, so currently each equipment manufacturer selects its own corrosion-resistant material and applies it to actual equipment. However, taking a recovery boiler as an example, in order to achieve high temperature and high pressure, a predetermined 50%
As a result of maintaining the steam temperature above 0°C for a long time, it was confirmed that the corrosion of the heat transfer tubes had progressed more than expected. Currently, it is operated at a lower steam temperature.

As described above, an economical heat exchanger tube material with excellent corrosion resistance in a corrosive environment caused by alkali salts has not yet been developed. The main reason for this is that the corrosive environment is not uniform. In other words, the composition of deposits differs depending on the boiler or incinerator, and also varies greatly depending on the operating conditions. As a result, plants are faced with difficult problems such as the inability to set a corrosive medium for material evaluation.

[Problem to be solved by the invention]

Since the above-mentioned conventional technology is operated without solving the problem of corrosion prevention technology against high-temperature corrosion in a high-temperature, high-pressure recovery boiler, there is a problem that a predetermined high-temperature steam cannot be stably supplied.

An object of the present invention is to provide a high-temperature, high-pressure recovery boiler that can solve the above problems and stably supply high-temperature steam.

[Means for Solving the Problems] The above object is to provide a high temperature corrosion resistant heat exchanger tube containing Ni and Cr that is used in a high temperature corrosion environment due to eutectic salts based on alkali metal sulfates and chlorides.
~30wt%, Cr 18~30wt%, Mo 2wt
% or more, with the remainder substantially consisting of Fe, and the heat exchanger tube surface is coated with a high Ni-base alloy having a Ni content of 58% or more by weight. This is achieved by using high-temperature corrosion-resistant heat exchanger tubes that are characterized by:

[Effect]

In the case of corrosion due to eutectic salts of alkali metal sulfates and chlorides, each salt will react with each alloying element.

In particular, sulfate reacts with Ni, and chloride reacts with Cr. Therefore, by optimizing the proportions of these elements, corrosion of alloy steel due to eutectic salt can be significantly reduced.

〔Example〕

Specific embodiments of the present invention will be explained, but before that,
First, it is necessary to use experimental data to explain how complex the corrosive environments in the recovery boilers and garbage incinerators are.

FIG. 1 shows the results of differential thermal analysis of the deposits on the superheater tubes of the recovery boiler, and in all cases, a eutectic point is observed at a temperature of 600°C or less. The eutectic point in such a temperature range is also seen in high-temperature corrosion in garbage incinerators (for example, Thermal Power Generation No. 164, May 1972), and the common feature of both is that Na deposits on the heat transfer tube surface Contains sulfate and chloride of K and K.

Table 1 shows the results of chemical analysis and thermal analysis of the deposits on the recovery boiler.

Margins below Table 1 For each boiler, deposits were analyzed by dividing them into the tube wall side and the gas side, but the largest difference in component content between these divisions was at 0°C; It can be seen that there are many on the wall side.

The division between the tube wall side and the gas side is as shown in Fig. 13, and the reason why there is a large amount of Ci on the tube wall side is due to Na CP in the furnace.
, is due to the selective adhesion of what is in an airy state to being cooled on the surface of the heat exchanger tube, becoming a liquid, and then becoming a solid.

The table also shows the eutectic point of each sample, and there is no difference in this depending on the classification of deposits, indicating that there is a large difference between the boilers.

Next, the results of a test to determine the degree of corrosion caused by these deposits on steel materials will be explained.

Figure 2 shows that the test piece is 5TBA24S (length 10mm
55 mm (width IG mm x thickness 2 mm), and the various deposits shown in Table 1 were applied as a corrosive medium to this surface.
After heating for 20 hours in an electric furnace maintained at 0°C, corrosion products were removed, and the results of determining the amount of corrosion of each sample are shown.

Comparing the amount of corrosion of the 5TBA24 material due to various deposits, it can be seen that the corrosion of boiler B has not progressed much compared to boilers A and C4. This is because the deposits on boiler B have a eutectic point in the 590°C range, which is higher than the test temperature of 550°C, which clearly shows that this corrosion is due to molten salt.

Furthermore, even when the deposits on the boilers A and C, where corrosion has progressed, are used as the corrosive medium, it is noted that the amount of corrosion caused by the deposits on the tube wall side is larger. As shown in Table 1 above, this means that there is no CI! , is deeply related to the fact that it was contained in large amounts.

If the corrosion caused by these deposits is due to molten salt, then the absolute amount of molten salt contained in the deposits should be different between the gas side and the tube wall side. In order to prove this, the results of a thermal analysis of the deposits in boiler A are shown in FIG. As is clear from the figure, the endothermic peak indicating melting at around 520°C is larger for the deposits on the pipe wall side, which clearly shows the difference in corrosiveness with respect to steel materials. Such a difference also makes Ci
In other words, there is no doubt that the chloride content plays a large role.

As mentioned above, it was found that the corrosiveness of steel materials changes greatly depending on the composition of the deposits.
This has made it extremely difficult to select materials that are resistant to high-temperature corrosion for recovery boilers and garbage incinerators.

Therefore, in the past, material evaluations have been conducted without fully recognizing the complex corrosive environment described above, resulting in technical flaws such as not being able to select appropriate materials and not being able to obtain the desired high-temperature steam. I was holding it.

Accordingly, the inventors have discovered a new material that can be put to practical use by discovering new facts and selecting a material based on the findings through the experimental examples described below.

Experimental Example 1 FIG. 4 shows the results of corrosion tests on steel materials using various synthetic dusts. This is because the deposits on heat transfer tubes in actual boilers are extremely inconsistent, as explained earlier, and cannot be used as a corrosive medium for corrosion resistance evaluation. This is what was adopted. In the synthesis of the medium, the reagent Naz SO4,
K, SO, , NaCj2 and KCI were used. The preparation method is to make a mixture by changing the mixing ratio of each reagent so that the eutectic point is below 540°C (tube wall temperature), melt each reagent, cool it, crush it, and collect the bottom dust. And so. Table 2 shows the composition ratios and co-solubilization points of the seven types of bottom dust used in the corrosion test.

0 Table 2 Using these synthetic dusts, 5TBA24 (2-1/
The amount of corrosion was investigated for four steel types: 4Cr) A, 5US347 (18C'r8Ni), 5US310 (25Cr-2ONi) C, and 30Cr-5ONi steel II. The test results at 550°C for 20 hours are shown in Figure 4. Looking at the results by steel type, 5TB
The amount of corrosion of A2'4 material is greater than that of SUS material.
This is particularly noticeable in synthetic dusts C and D. Between the SUS material and the 30Cr-5ONi steel 2, the latter has better corrosion resistance, but the synthetic steel 1-B has an abnormally large number of corrosion gaps. Such a tendency is also seen in synthetic dust B for 5US347 material. These results demonstrate that errors can occur in the selection of corrosion-resistant materials depending on the setting and selection of the corrosive medium, that is, the dust composition.

Experimental Example 2 As mentioned above, the composition of synthetic dust as a corrosive medium is important in selecting a corrosion resistant material.

Therefore, when we look at the composition of each synthetic dust shown in Table 3, we find that synthetic dust (B), which was highly corrosive to SUS materials, has a higher amount of chloride than sulfate. However, other synthetic dusts have a high content of sulfates. Judging from these results, it can be seen that roughly two types of synthetic dust should be employed as corrosive media in the evaluation of SUS-based materials. From this point of view, from the synthetic dusts shown in Table 2 as corrosive media, we selected synthetic 12 dust A, which contains a lot of sulfates, and synthetic dust B, which contains a lot of chlorides, and used various commercially available SUS materials. We also carried out corrosion resistance tests on alloy steel.

FIG. 5 shows the results of a corrosion test using synthetic dust 1-A.

The amount of corrosion of the test steel material is shown in relation to the amount of Cr in the steel material. Synthetic steel I-A containing a large amount of sulfate shows a tendency that the higher the Cr content, the better the corrosion resistance.

On the other hand, FIG. 6 shows a corrosion test using synthetic dust B. Similarly to Figure 5, the amount of corrosion in the steel material is shown in relation to the amount of Cr in the steel material, but in this case, when the amount of Cr in the steel material is about 30% or more, the amount of corrosion increases. A rapidly increasing phenomenon is emerging. This indicates that since there are many chlorides in the dust, the Cr component in the steel material is corroded. Next, these results are rearranged according to the amount of Ni in the steel material, as shown in FIGS. 7 and 8.

Figure 7 shows the case of composite steel (A). As the amount of Ni in the steel material increases, corrosion tends to decrease, but when the amount of Ni is 30
% or more, it cannot necessarily be said that the corrosion resistance is good. The same thing can be seen in the synthetic dust I-B shown in FIG. 8, and in this case, the tendency for corrosion to increase when the Ni content is 30% or more is more pronounced than in the synthetic dust A. This is thought to be due to the large amount of chloride in the dust. This shows that in the case of corrosion caused by eutectic salts of Na and K sulfates and chlorides, the amounts of Cr and Ni in the steel material can be limited to around 25% each from the viewpoint of corrosion resistance.

However, in a severely corrosive environment like this system, the above
Corrosion resistance is not maintained by the ingredients alone, but the third alloy ingredient in particular plays an important role. In FIGS. 7 and 8, the data indicated by black circles are steel types that contain 1% or more of Mo as a component next to Cr and Ni. It can be seen that there are many steel types that have better corrosion resistance than those that do not contain MO, which are indicated by white circles.

The effect of Mo addition is effective for both the sulfate-rich synthetic dust holes and the chloride-rich synthetic dust B34, which is clear from the results shown in FIGS. 7 and 8.

Experimental Example 3 As is clear from the above experimental examples, in a corrosive environment due to eutectic salts of Na and K sulfates and chlorides, it is possible to limit the amount of Cr and Ni in the steel material to around 25%.
The addition of MO was found to be effective as the third precept. Therefore, based on these results, a new alloy steel mainly composed of the above three components was prepared and its corrosion resistance was evaluated.

FIG. 9 shows the results of a corrosion test using synthetic dust B on alloy steels in which the amount of Mo added as the third component was arbitrarily changed for various combinations of Ni and Cr.

In order to confirm the effect of Mo, 3 from 12NilSCr system
A wide range of materials were prepared, including 4Ni2SCr, and in the evaluation of corrosion resistance, it was found that materials containing 2% or more of Mo were superior for each steel type. In the previous Experimental Example 2, the results of evaluating commercially available materials showed that Ni and Cr were each 25
% was expected to be preferable, but it is clear that the content of MO, which is the third commandment, plays an important role in a corrosive environment with eutectic salts of Na and K sulfates and chlorides. It is.

In particular, 12Ni18Cr2.2M shown in FIG.

The steel is similar to 5US316 material, but this 5US
316 material once attracted attention as a superheater tube material for heavy oil-fired boilers containing a mixture of Na, SO, and vanadium compounds, but when it was actually used, Mo as an alloying element impeded corrosion resistance. It is hardly used now.

In addition, in the case of 35N + 25Cr system with Ni content of 30% or more, the MO content must be 4.9% to maintain corrosion resistance.
It turns out that the above is necessary.

Experimental Example 4 Using a commercially available high Ni-based alloy steel, we confirmed that in order to increase the amount of Ni in alloy steel to further improve its corrosion resistance, it was necessary to add even more MO. 10
The corrosion test results are shown in the figure.

Although the corrosion resistance of Hastelloy C, Hastelloy H, and Inconel 625 has been considerably improved, they are very expensive materials, with 8% or more of Mo added and about 60% of Ni.

Experimental Example 5 Next, FIG. 11 shows the results of a long-term corrosion test comparing the corrosion resistance of a material conventionally used as a superheater tube for a recovery boiler and a material according to the present invention.

A in the figure is 5US347 material, B is 13Ni2SCr1
It is a Mo-based material. These have a proven track record in boilers in Japan, and the latter in particular is known for its excellent corrosion resistance against hydrogen chloride and the like at high temperatures. C is a 34N, i2aCr1Mo material, which is equivalent to Incoloy 800 and is used in Canadian recovery boilers, to which MO is added.

In contrast to the comparative materials A, B and C, the material according to the invention has D as 12Ni 18Cr2.2Mo material and E as 2
0 N i 20 Cr 2.5 Mo material and F
is 25 N + 25 Cr 2. It is ZMo material. Comparing each of the above materials, the amount of corrosion increases over time for the conventional materials, whereas the increase in the amount of corrosion over time for the materials containing 2% or more of Mo is extremely slight, except for D. It can be seen that it shows excellent corrosion resistance.

Furthermore, as shown in Figure 9, when the Ni content exceeds 30%, it is necessary to increase the amount of Mo added, and from the viewpoint of corrosion resistance and economy, it is necessary to use eutectic salts of Na and K sulfates and chlorides. As a corrosion-resistant alloy steel in a corrosive environment, Ni1
An alloy containing 2% or more and 30% or less, Cr 18% or more and 30% or less, and Mo 2% or more is preferable, and in addition, it is an alloy to ensure workability, weldability, toughness, etc. as a heat exchanger tube used in a high-temperature environment. Naturally, the addition of ingredients is not prohibited.

Experimental Example 6 In the example shown in Fig. 11 shown earlier, M
It has been revealed that the corrosion resistance of Ni-Cr alloy steel containing 2% or more of o is excellent even in long-term evaluations. This indicates that a stronger corrosion-resistant 7 8 film was formed in the latter case. Therefore, the first
It can be considered that a strong corrosion-resistant coating is also formed in the case of the high Ni-base alloy steel shown in Figure 0. From this, it is expected that corrosion resistance will be improved by processing the surface of the alloy steel with a material that forms a strong corrosion-resistant coating. Figure 12 shows that a material equivalent to high Ni-based alloy steel with a Ni content of 58% or more shown in Figure 10 is thermally sprayed (approximately 300 μm) onto the surface of various alloy steels to remove highly corrosive synthetic dust B. The corrosion resistance of these media was evaluated over a long period of 100 hours.

A indicated by a broken line in the figure is the same as A in Figure 11. 5US34
The amount of corrosion of 7 materials (A in Figure 11) is shown. Similarly, broken line B is Inconel 62 with 5US347 material as the base material.
5 (60,3N i -22, I Cr 8.5
7Mo) equivalent material was thermally sprayed.

The amount of corrosion in a short time shows corrosion resistance comparable to Inconel 625, but the amount of corrosion in 100 hours shows an increasing tendency. On the other hand, C is 12Ni-18Cr-2
, 2Mo alloy steel (D in Figure 11), D is the material of C with Hastelloy C (58,2N i -16
, I Cr -16,6M o ) equivalent material was thermally sprayed, and E
Similarly, a material equivalent to Inconel 625 was thermally sprayed. Comparing these, even if a material equivalent to high Ni alloy steel with a Ni content of 58% or more is thermally sprayed to form an excellent corrosion-resistant coating, if the base material has a corrosion resistance of about 5US347, it will not be able to withstand long-term use. At least, as is clear from the examples, the effect cannot be expected unless a base material having corrosion resistance higher than 12Ni-18Cr-2,2Mo alloy steel is used.

As a known example related to the present invention, there is a presentation on the high-temperature corrosion behavior of austenitic stainless steel in a recovery boiler (Preliminary of the 32nd Corrosion Prevention Symposium fiP3).
62). In this example, as a result of comparing the corrosive properties of materials using various synthetic dusts, it was found that the addition of Mo in an amount of 0.5% or more was particularly effective. However, 1%
The effect is small above 2%, and tends to decrease below 2%.

90 On the other hand, in the present invention, there is no effect unless Mo is added in an amount of 2% or more, which is fundamentally different from the above.

This may be due to whether or not an evaluation method was adopted that takes into account the actual state of boilers, that is, the constantly changing composition of deposits on superheater tubes as a corrosive environment.

Furthermore, in this corrosive environment, the composition of heat transfer tube deposits is limited to Na and K sulfates and chlorides, but in reality carbonates are also included. However, sulfates and chlorides are more corrosive to steel materials, and this should not lead to incorrect evaluation and selection of materials.

On the other hand, in addition to deposits, the influence of atmospheric gas within the boiler furnace cannot be ignored. In particular, it is well known that HCn and SOX accelerate high-temperature corrosion due to sulfates and chlorides contained in large amounts in deposits. However, in this case too,
There is only a slight increase in the amount of corrosion, and it does not significantly affect material selection.

〔Effect of the invention〕

According to the present invention, it is possible to provide a boiler heat exchanger tube that can stably supply steam at 500°C, which could not be achieved with conventional technology, in a high-temperature corrosive environment caused by eutectic salts based on alkali metal sulfates and chlorides. The effect is that the turbine efficiency is improved by increasing the steam temperature, thereby ensuring economic efficiency in terms of electric power.

In addition, the amount of Ni and Cr in the heat exchanger tube material is 30% each.
Since the following can be achieved, there is no significant increase in material costs, and the material costs can be recovered within at least one year due to the benefits of increasing the steam temperature.

[Brief explanation of drawings]

Figure 1 is a diagram showing an example of thermal analysis of recovery boiler superheater tube deposits, Figure 2 is a side view of a corrosion test of low alloy steel due to various deposits,
Figure 3 is a thermal analysis diagram of deposits on the gas side and pipe side, Figure 4 is a diagram showing examples of corrosion tests on steel materials using various types of synthetic dust, and Figure 5 is the results of corrosion tests using synthetic dust holes in steel materials. Figure 6 is a diagram showing the corrosion test results using synthetic dust B, similar to Figure 5, and Figures 7 and 8 are the results of the tests in Figures 5 and 6 for steel materials. Fig. 9 is a diagram showing the results of corrosion tests using synthetic dust I-B of new alloy steels with various amounts of Mo,
Figure 10 is a comparison diagram of corrosion resistance of commercially available high Ni-Cr alloy steel.
Figure 11 shows the results of a long-term corrosion resistance comparison test between conventional materials and new alloy steel, and Figure 12 shows the high N content on the surface of various alloy steels.
Figure 1 showing the results of corrosion tests for materials sprayed with i-based alloys.
FIG. 3 is a diagram showing the state of dust adhesion on the recovery boiler superheater tube.

Claims (2)

[Claims] (1) In high-temperature corrosion-resistant heat exchanger tubes containing Ni and Cr that are used in high-temperature corrosive environments due to eutectic salts based on alkali metal sulfates and chlorides, Ni is 12 to 30%
18 to 30 wt% of Cr, 2 wt% or more of Mo, and the remainder substantially consists of Fe. (2) In the high temperature corrosion resistant heat exchanger tube according to claim (1),
A high-temperature corrosion-resistant heat exchanger tube characterized in that the surface of the heat exchanger tube is coated with a high Ni-based alloy having a Ni content of 58% by weight or more.