JP2615167B2 - Heat resistant anticorrosion coating - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention adheres to the surface of a heat transfer tube of a heavy oil fired boiler, a boiler for power generation, a soda recovery boiler, a refuse incineration boiler, etc., particularly the surface of a heat transfer tube at a high temperature. The present invention relates to a heat-resistant anticorrosion film formed by the method.

[Conventional technology]

In recent years, due to factors such as the increase in the temperature of the furnace upper part due to the improvement of the combustion method for NOx countermeasures and the adoption of high-sulfur fuel due to the spread of desulfurization equipment, the combustion ash adhering to the high-temperature part As a result, corrosion of the high-temperature heat exchanger tubes is occurring.

The combustion ash adhering to the surface of this high temperature part heat transfer tube is V, Na,
It is known that the substance is a highly corrosive V ₂ O ₅ —Na ₂ SO ₄ substance containing S and O as main components. Here, the high temperature portion is a temperature range where the surface temperature of the attached combustion ash is 200 to 1500 ° C.

As a well-known measure against corrosion caused by the adhered combustion ash, it is roughly classified into the following: 1) Improvement of the material of the high temperature part heat transfer pipe 2) Reforming of the fuel ash adhering to the surface of the high temperature part heat transfer pipe (low S) , Conversion to V fuel and addition of anticorrosive to fuel) 3) Methods such as surface treatment of high temperature heat exchanger tubes are being researched and developed in various fields.
According to the above-mentioned classification, it relates to improvement of 3) by application of 2), and describes Mg, Ca, Ba, Be on a surface of a high-temperature section heat transfer tube of a boiler or the like described in JP-A-59-4661. There is a heat-resistant anticorrosion coating formed by adhering and forming a mixture containing at least one kind of alkaline earth metal carbonates with a silicate-based binder.

[Problems to be solved by the invention]

What is described in JP-A-59-4661 is
Alkaline earth metal carbonates such as Mg, Ca, Ba, Be, etc. are simply adhered to the surface of high-temperature heat transfer tubes such as boilers with a silicate-based binder. There was a disadvantage that cracks and peeling were caused by repeated thermal shocks.
In particular, when the high-temperature heat exchanger tube is exposed to high temperatures, the anticorrosion coating is relatively hard to peel off for ferrite-based steel and those having a small coefficient of thermal expansion such as martensite-based steel,
A steel having a large thermal expansion coefficient, such as austenitic steel, was peeled off due to a difference in thermal expansion coefficient, and a film could not be formed.

[Means for solving the problem]

The present invention has been made in order to eliminate such disadvantages in the conventional method, and relates to an improvement of a heat-resistant anticorrosion coating described in the above-mentioned JP-A-59-4661, and relates to a boiler and the like. An anticorrosion method in which a mixture containing calcium carbonate and at least one kind of frit used for ceramic coating, ceramics, etc. is adhered to the surface of a heat transfer tube with a silicate-based and / or aluminate-based binder. A heat-resistant anticorrosion coating in which the proportion of calcium carbonate in the anticorrosion coating is 26.7 to 46.7 wt% and the proportion of frit is 20 to 40 wt%.

(Operation)

Calcium carbonate is well dispersed in silicate-based and / or aluminate-based binders and adheres uniformly and densely to the surface of the heat transfer tube during application. By dispersing the powder of the frit used for near-ceramic coating, porcelain, etc., the difference in the coefficient of thermal expansion from the tube material is reduced, and cracks and peeling due to thermal shock do not occur.

An embodiment of the present invention will be described below with reference to FIG. In FIG. 1, reference numeral 1 denotes a metal on a surface of a high-temperature part heat transfer tube,
Reference numeral 2 denotes a heat-resistant anticorrosive coating containing calcium carbonate formed on the surface of a metal 1 by using a silicate-based and / or aluminate-based binder, and 3 denotes a high-temperature portion heat transfer dispersed inside the heat-resistant anticorrosive coating 2. Frit powder used for ceramic coatings, ceramics, etc. having a thermal expansion coefficient close to that of the tube 1, 4 denotes voids existing inside the heat-resistant anticorrosion coating 2, 5 denotes corrosiveness adhering to the surface of the heat-resistant anticorrosion coating 2 Strong V ₂ O ₅ −
Na ₂ SO ₄ scale.

In FIG. 1, when the heat-resistant anticorrosive film 2 is a heat-resistant anticorrosive film containing no Ca compound, the scale 5 is melted when heated to a high temperature and penetrates the voids 4 existing inside the heat-resistant anticorrosive film 2. Eventually, it reaches the surface of the metal 1. Here, the metal surface is locally oxidized and sulfided according to the reaction formulas 1 and 2 described later, and the metal surface expands and destroys the boundary between the metal 1 and the heat-resistant anticorrosion coating 2 and corrosion proceeds.

However, if a Ca compound is present in the heat-resistant anticorrosive coating 2 described above, the scale 5 is heated to a high temperature in FIG. Is the gap 4 while passing through the gap 4.
The Ca compound on the surface and uptake _{reaction, CaO-V 2 O 5 -Na} 2 SO
_Since the scale changes to a _4- system scale, the melting point of the scale 5 rises and solidifies in the voids.

However, a conventional heat-resistant anticorrosion coating having no frit powder used for ceramic coating or ceramics close to the heat-transfer tubing 1 in the heat-resistant anticorrosion coating 2 (Japanese Unexamined Patent Publication No.
4661), cracks, peeling, etc. occur due to repeated thermal shocks such as rapid heating, rapid cooling, etc., from which strong corrosive V ₂ O ₅ −Na ₂ SO ₄
The scale 5 of the system enters and corrosion proceeds.

However, a mixture containing calcium carbonate and at least one kind of frit powder used for ceramic coating, ceramics, etc. having a thermal expansion coefficient close to that of the high temperature part heat transfer tube 1 is added to the metal 1 on the surface of the high temperature part heat transfer tube. In the case of the heat-resistant anti-corrosion coating of the present invention formed by bonding with a silicate-based and / or aluminate-based binder, even if the high-temperature part heat transfer tube 1 has an austenitic steel having a large coefficient of thermal expansion, the high-temperature part heat transfer Frit powder used for ceramic coating or ceramics having a thermal expansion coefficient close to that of the pipe material 1 is dispersed in the heat-resistant anticorrosion coating 2, so that the metal 1 on the surface of the heat transfer tube and the heat-resistant anticorrosion coating are used. The difference in the coefficient of thermal expansion of No. 2 is reduced, and cracking does not occur even after repeated thermal shocks such as rapid heating and rapid cooling, and peeling does not occur.

Next, phosphate-based and silicate-based binders are known as binders for adhering and forming inorganic substances such as Al, Si, and Zn on metal surfaces. The present inventors have studied various methods for depositing and forming the above-mentioned Ca compound on the surface of a high-temperature heat transfer tube such as a boiler for a silicate-based and / or aluminate-based binder. As a result, calcium carbonate is well dispersed in a silicate-based and / or aluminate-based binder such as an aqueous solution of sodium silicate or sodium aluminate, and the adhesion, uniformity and denseness of the coating film after coating are good. I found it. On the other hand, other compounds of the alkaline earth metal, such as calcium hydroxide and calcium sulfate, solidify when mixed with sodium silicate or sodium aluminate aqueous solution, and cannot be applied and formed on the metal surface.

As described above, according to the present invention, the corrosive scale 5 reacts with the calcium carbonate to close the voids 4 in the heat-resistant anticorrosion coating 2 and cut off the contact between the corrosive scale 5 and the metal to prevent corrosion. Using a silicate-based and / or aluminate-based binder as a method of applying and molding calcium carbonate on a metal surface; and for a ceramic coating, a ceramic, or the like having a thermal expansion coefficient close to that of the heat transfer tube 1 at a high temperature. It is characterized in that the thermal shock resistance of rapid heating, rapid cooling and the like is improved by dispersing the frit powder used.

Further, in the present invention, Al, Si, Z
Even if a metal such as n or Zr or an oxide thereof is added, the effect is maintained. The calcium carbonate used in the present invention, limestone containing these compounds (CaCO _3), chalk (CaCO _3), calcite (CaCO _3), aragonite (CaCO _3),
Ore such as marble (CaCO ₃ ) and dolomite (CaCO ₃ · MgCO ₃ ) may be used.

In addition, silicate and aluminate binders include sodium silicate and sodium aluminate,
It goes without saying that silicates and aluminates such as Ca, Al, Zn, CO, Fe, Ba, Be, Mg, Mn, and Li and combinations of two or more thereof may be used.

Reaction formula Na ₂ SO ₄ + 3R + M → Na ₂ O + 3RO + MS (Formula 2) where R: Unspecified reducing agent M: Metal [Example] Next, the present invention will be specifically described by way of examples.

The scale attached to the heat transfer tube of the heavy oil boiler, which caused the corrosion accident, was collected and crushed, and the magnetic crucible filled with this corrosive scale was filled with No. 1 to No. 10 in Table 1. The test piece was immersed and subjected to a corrosion test in an electric furnace.

The test conditions were: oxygen concentration: 5 vol%, carbon dioxide concentration: 15
vol%, sulfur dioxide gas concentration: 0.5vol%, remaining nitrogen gas atmosphere, test temperature: 650 ° C, total test time: 200 hours, during which a magnetic crucible in which test pieces were immersed every 24 hours was put into the room from an electric furnace. The thermal shock of removal, rapid heating and rapid cooling was repeatedly applied.

Also, as test specimens, STBA24 and SUS30 of 20 mm × 50 mm × 5 mm
4, a 50 wt% aqueous solution of sodium silicate or sodium aluminate was applied to 50 wt%, and a solution of 50 wt% of calcium carbonate powder was applied and molded, and a 50 wt% aqueous solution of sodium silicate or sodium aluminate of 50 wt% was applied. A solution prepared by applying a solution obtained by mixing 25 wt% of calcium carbonate powder and 25 wt% of frit powder used for ceramic coating, porcelain, and the like, and applying and forming the mixture was used.

Tables 2 to 4 show the composition and properties of the frit used in this example.

[Effects of the Invention] As a result, as shown in Table 5, the calcium carbonate according to the present invention and the powder of the frit used for ceramic coating, porcelain, etc. are adhered and formed using a silicate-based and / or aluminate-based binder. The heat-resistant type anticorrosion coating (specimen No. 6 to No. 10) is a coating of inorganic paint obtained by adhering and forming a conventional alkaline earth metal carbonate such as Ca with a silicate binder (specimen No. 3). ~ No. 5), the corrosion loss is about 1/10, and it can be seen that the corrosion resistance is remarkably improved.

In addition, as a result of examining the test pieces after the corrosion test, corrosive substances such as Va and V were found on the test piece surfaces of the test pieces of No. 3 to No. 5, and the test pieces of No. 6 to No. 10 Then it was not recognized. This also indicates that the inorganic coating of the present invention is resistant to thermal shock and is effective as a corrosion-resistant coating.

Next, the composition range of the heat-resistant anticorrosive film of the present invention will be described.

1) Regarding the amount of the binder: 50% by weight of an aqueous solution of 20-60% by weight of sodium silicate or sodium aluminate, 25% by weight of calcium carbonate powder, and a frit for ceramics as a frit used for ceramic coating, ceramics, etc., powder of product number PN-5400N ( (See Table 4) A solution mixed with 25 wt% was applied to a SUS304 test piece and formed by adhesion molding, and a corrosion test similar to that described in the example was conducted. The result was as shown in Table 5.

From the above results, the concentration of the aqueous solution of sodium silicate or sodium aluminate is preferably 35 to 60% by weight. Therefore, the content of sodium silicate or sodium aluminate of 26% to 37.5% by weight in the anticorrosion film after the evaporation of water is good. It is valid.

2) About the quantity ratio of CaCO ₃ to the frit used for ceramic coating, ceramics, etc .: 50wt% sodium silicate or sodium aluminate aqueous solution, 50wt%, calcium carbonate powder 10-40wt%, used for ceramic coating, ceramics, etc. As a frit, a mixture of a ceramic frit and a powder of PN-5400N of 10 to 40% by weight was applied to a test piece of SUS304 and adhered and molded. As a result of the same corrosion test as described above, the results were as shown in Table 6. Atsuta.

From the above results, the ratio of CaCO ₃ and frit is 20 ~ for CaCO ₃
Effective in the range of 35 wt% and frit in the range of 15 to 30 wt%, CaCO ₃ is 26.7 to 46.7 wt% in the anticorrosion coating after moisture evaporates.
% And a frit component in the range of 20-40% by weight were effective.

As described above, the effective range of the present invention is that the content of sodium silicate or sodium aluminate as a binder in a state of an anticorrosion film in which water is evaporated is 26 to 37.5 wt%,
The effective range was 26.7 to 46.7% by weight of CaCO _{3 and 20} to 40% by weight of the frit component.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram as one embodiment of the present invention. 1 ... metal, 2 ... heat-resistant anticorrosion coating, 3 ... powder of frit, 4 ... voids, 5 ... scale.

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-60-11863 (JP, A) JP-A-50-126035 (JP, A) JP-A-59-4661 (JP, A) JP-A-1- 167382 (JP, A) JP-A-1-108274 (JP, A)

Claims (1)

(57) [Claims] An anticorrosion coating comprising a mixture containing calcium carbonate and a frit adhered to a surface of a heat transfer tube such as a boiler with a silicate and / or aluminate binder. The proportion of calcium carbonate in the coating is 26.7-46.7wt%, and the proportion of frit is 20-40wt%
A heat-resistant anticorrosive coating characterized by the following.