JP2007263400A - Water heater casing made of coated steel plate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a water heater casing made of a coated steel plate capable of keeping good corrosion resistance even under wet environment where corrosion often occurs because of dew condensation water. <P>SOLUTION: The casings 16, 17 surrounding a heating unit 11 and a hot water storage tank 12 are made of coated steel plates provided with a hydrophilic coating film on inner surfaces opposed to the heating unit 11 and the hot water storage tank 11 as heat sources. The hydrophilic coating film is adjusted to have a water contact angle of 60 degrees or less by a hydrophilicity imparting agent, and non-chrome rust proof pigment such as magnesium phosphate, magnesium hydrogen phosphate, zinc phosphate, aluminum dihydrogen tripolyphosphate, and calcium silicate is blended. As the hydrophilicity imparting agent, partially hydrolyzed condensate of tetraalkoxy silane is used. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention uses a coated steel sheet provided with a coating film that promotes drying of the inner surface of a water heater outer box, which is likely to cause corrosion due to condensed water, as an outer box material, and improves the corrosion resistance on the inner surface side. Regarding the box.

The outdoor stationary water heater connected between the heating unit 11 and the hot water storage tank 12 with pipes 13 and 14, fed cold water from the water supply port 15 to the hot water storage tank 12, and flowed into the heating unit 11 from the cold water pipe 13 Hot water obtained by heating the cold water is returned to the hot water storage tank 12 via the hot water pipe 14 (FIG. 1). Hot water is sent from the hot water storage tank 12 through the hot water supply pipe 15 to the consumption place.
If the heating unit 11 and hot water storage tank 12 in the high temperature state are exposed, it is not only dangerous, but energy waste due to heat dissipation cannot be ignored, so the heating unit 11 is in the outer box 16, and the hot water storage tank 12 is in the outer box 17. Covering. Since both the outer boxes 16 and 17 are required to have corrosion resistance, a coated steel plate having a coating film on a plated steel plate is used. Among pre-coated steel sheets, pre-coated steel sheets that can be processed and cut into a predetermined shape and do not require an on-site coating process are often used.

The outer surfaces of the outer boxes 16 and 17 are exposed to rainwater and sunlight, and enter the consumer's field of view. In many cases, a two-layer coating film is provided. On the other hand, on the inner surface side of the outer boxes 16 and 17, a simple coating film that is usually called “service coat” is provided. However, in use as the water heater outer boxes 16 and 17, corrosion that cannot be dealt with by coating of a service coat is sometimes observed.
Corrosion on the inner surface side is caused by the fact that condensed water is generated on the inner surfaces of the outer boxes 16 and 17 due to a temperature difference between the inner side where the heat source is located and the outer side exposed to the outside air. Condensed water is generated at night and in winter when the temperature difference increases, and when the condensed water contacts the inner surface of the outer casings 16 and 17 for a long time, the corrosive component penetrates the coating film and causes corrosion on the coated steel sheet. In particular, the ceiling inner surface, the lower end processed portion, the cut end surface, and the like where condensed water tends to remain are easily corroded.

Corrosion is suppressed by adding chromium-based rust preventive pigments such as calcium chromate and strontium chromate to the coating (Patent Document 1). It is being replaced. It is also a drawback of chromium-based anticorrosive pigments that the coating film tends to yellow.
Japanese Patent Laid-Open No. 7-185452 [0010]

Non-chromium rust preventive pigments include ion exchange silica, phosphate, etc., but all have low rust preventive ability compared to chromium rust preventive pigments. Further, in an environment where condensed water adheres, the effect of the rust preventive pigment is not sufficiently exhibited, and in extreme cases, pitting corrosion that penetrates the outer boxes 16 and 17 may occur.
The present inventors investigated and examined various mechanisms for the occurrence of corrosion caused by condensed water on the inner surface of the water heater outer box. As a result, it was found that the corrosion caused by condensed water is effectively suppressed when the surface of the coated film is dried early by suppressing the growth of condensed water adhering to the surface of the coated film and promoting rapid evaporation.

  The present invention is based on the knowledge that the surface state of the coating film affects the dew condensation water that causes corrosion, and by forming a back coating film from a paint containing a hydrophilicity imparting agent in addition to a non-chromium rust preventive pigment. An object of the present invention is to provide a water heater outer box that suppresses the condensation of condensed water on a part of the film surface of the back coating film, maintains the coating film in a dry state, and improves the corrosion resistance.

  The water heater outer box of the present invention is made of a coated steel plate provided with a non-chromium rust preventive pigment and a hydrophilicity imparting agent on a plated steel plate, and faces a heat source such as a heating unit or a hot water storage tank. A back coating film is arranged on the inner surface side. The back coating film is formed from a paint in which a non-chromium rust preventive pigment is blended with a resin base such as polyester, alkyd, or epoxy.

  Non-chromium rust preventive pigments include magnesium phosphate, magnesium hydrogen phosphate, zinc phosphate, aluminum dihydrogen triphosphate, calcium silicate, etc., and resin solid content: 2 to 150 parts by mass with respect to 100 parts by mass Blended in proportions. As the hydrophilicity imparting agent, a partially hydrolyzed condensate of tetraalkoxysilane is used and blended at a ratio of 0.5 to 50 parts by mass so as to obtain a water contact angle of 60 degrees or less.

In the present invention, corrosion due to condensed water is suppressed by imparting hydrophilicity to the back surface coating film. However, hydrophilicity and hydrophobicity are particularly taken for an outer box ceiling where corrosion due to condensed water tends to occur. Explain the effect on corrosion.
In the outer boxes 16 and 17 incorporating the heating unit 11 and the hot water storage tank 12, the heating unit 11 and the hot water storage tank 12 serve as heat sources, and the temperature difference between the inner surface and the outer surface becomes large, so that condensation is likely to occur.
Since the coating film on the ceiling inner surface to which the condensed water adheres is made hydrophilic, the condensed water spreads along the coating film surface, and even if the ceiling inner surface is slightly inclined, it flows down to the lower side, and further the side wall It flows down to the ground along the inner surface. Therefore, even in the late stage of dew condensation, it remains a thin water film, and the coating film dries relatively early. Therefore, the contact time with the dew condensation water containing a corrosive component is shortened, and the ceiling surface is protected from corrosion. (Left column in Fig. 2)

  On the other hand, the condensed water adhering to the hydrophobic coating film aggregates due to the action of surface tension and grows into large-sized water droplets. Even if the ceiling surface is inclined, the fact that the condensed water flows to the lower side due to the hydrophobicity is significantly less than that of the hydrophilic coating film. Large water droplets take time to evaporate, and the water droplets remain on the hydrophobic coating film for a longer time. (Figure 2, right column)

  If water droplets containing corrosive components remain on the surface of the coating film for a long time, the coated steel sheet 20 is easily corroded. The coated steel plate 20 is coated with a back coating film 23 on the plating layer 22 provided on the base steel 21, and is provided with corrosion resistance with a rust preventive pigment 24 blended with the back coating film 23. The rust preventive pigment 24 is likely to elute in the condensed water 25 (Fig. 3a). When the cavity 26 is generated in the back coating 23 due to the elution of the rust preventive pigment 24, the corrosive component 27 penetrates the back coating 23 from the condensed water 25 toward the cavity 26 (FIG. 3b). The corrosive component 27 that has penetrated the back coating 23 reacts with the plating layer 22 to generate a corrosion product 28 at the interface between the plating layer 22 and the back coating 23 (FIG. 3c). The corrosion product 28 causes the coating film 29 to swell and the coating film to peel off, further promoting the corrosion reaction.

  As described above, the hydrophilicity or hydrophobicity of the back surface coating film 23 has a great influence on the residual state of the dew condensation water 25 and consequently the degree of drying of the coating film surface. As a result, corrosion due to condensed water is unavoidable in hydrophobic coatings, whereas the occurrence of corrosion is suppressed over a long period of time in hydrophilic coatings that are easily maintained in a dry state.

  There are cold-rolled steel sheets, galvanized steel sheets, zinc-aluminum-plated steel sheets, zinc-aluminum-magnesium-plated steel sheets-aluminum-plated steel sheets-stainless steel sheets, and the like. Prior to painting, pre-coating treatments such as pickling, degreasing, surface conditioning, and non-chromic treatment are applied to the original coating as necessary.

The coating method is not particularly limited, but a roll coating method is common.
First, an undercoat paint film is formed by applying and baking an undercoat paint on the surface of a coating original plate, and then an overcoat paint film is provided on the front side and a back coat film is provided on the back side by applying and baking the top coat paint and the back paint. As the baking conditions, for example, the maximum reached plate temperature for baking of the undercoat paint: 200 to 220 ° C., the heating time: 30 seconds, the highest reached plate temperature for baking of the top coat and the back coating: 210 to 240 ° C., the heating time: 40 seconds. Is adopted.

  The back coating is a resin base such as polyester, alkyd, epoxy, etc., partially hydrolyzed with anticorrosive pigments such as magnesium phosphate, magnesium hydrogen phosphate, zinc phosphate, aluminum dihydrogen triphosphate, calcium silicate, and tetraalkoxysilane. It is formed by directly applying and baking a paint containing a condensate (hydrophilicity imparting agent) on the back surface of the steel sheet. Various additives such as organic aggregates, inorganic aggregates, metallic pigments, fungicides, and matting agents can be blended in the back coating as necessary.

Since a partially hydrolyzed condensate (hydrophilicity imparting agent) of tetraalkoxysilane is blended, a hydrophilic coating film is obtained. Above all, when the blending amount of the hydrophilicity-imparting agent is adjusted so that the water contact angle of the back coating film is 60 degrees or less, the rainwater adhering to the coated steel sheet becomes a thin water film and spreads on the coating film surface, and is dried. Time to completion is greatly reduced.
As the partially hydrolyzed condensate of tetraalkoxysilane, commercially available products such as methyl silicate 51, ethyl silicate 40, ethyl silicate 48 (manufactured by Colcoat Co.), MKC silicate MS51, MS56 (manufactured by Mitsubishi Chemical Corporation) can be used. Alternatively, it can also be obtained by adding water and a catalyst to a monomer such as tetramethoxysilane, tetraethoxysilane, or tetrapropoxysilane to cause hydrolysis and condensation.

  The partial hydrolysis-condensation product of tetraalkoxysilane is blended at a ratio of 0.5 to 50 parts by mass with respect to 100 parts by mass of the coating resin. If the blending amount is less than 0.5, the water contact angle of the resin coating film does not become 60 degrees or less, but if it exceeds 50 parts by mass, the processability of the coating film decreases or cracks occur in the coating film. There is. Even when a partially hydrolyzed condensate of tetraalkoxysilane is blended, since the base resin of the coating film is hydrophobic, the function of shielding rainwater and corrosive ions and protecting the underlying metal has the function of partially hydrolyzing tetraalkoxysilane. The same performance as that of the resin coating film not containing the decomposition condensate is maintained.

  The water adhering to the hydrophilic steel plate member falls without growing into large water droplets, or the water droplets are spread over a relatively wide area, so that the evaporation area is increased and water evaporation is promoted. As a result, the rust preventive component is prevented from being eluted from the undercoat coating film and the back coating film to the moisture adhering to the steel sheet surface such as the cut end face, and the corrosion resistance is maintained.

  The back coating film is preferably formed with a film thickness of 3 μm or more in consideration of the anticorrosion of the base, adhesion of the coating film, and concealment. In any of the paints of polyester, alkyd, and epoxy resin, a coating film having good corrosion resistance, hiding property, and adhesion is formed at a film thickness of 3 μm or more. However, in the case of a thick film exceeding 20 μm, improvement in corrosion resistance corresponding to the increase in film thickness cannot be expected, resulting in an increase in cost. Preferably, the film thickness of the back coating film is adjusted in the range of 5 to 8 μm.

In general, a two-layer structure of undercoat and topcoat is employed for the coating on the surface side.
For the undercoat film, magnesium phosphate, magnesium hydrogen phosphate, zinc phosphate, aluminum dihydrogen phosphate tribasic, calcium silicate, etc. are blended as a rust preventive pigment in epoxy, epoxy urethane, urethane, polyester, acrylic, etc. Film is formed from paint.
In order to improve the anticorrosion, concealment, and adhesion of the topcoat film of the undercoat, it is desirable to form the undercoat film with a film thickness of 3 μm or more. Film thickness: Less than 3 μm, corrosion resistance, concealment, and adhesion are not sufficient, but providing an undercoating film that is too thick causes not only an increase in cost but also a decrease in workability, so an undercoating film with a film thickness of around 5 μm Is preferably formed.

  The top coat film is formed by applying and baking a paint in which a color pigment is blended with a resin base such as polyester, silicone polyester, or polyvinylidene fluoride. Various additives such as organic aggregates, inorganic aggregates, metallic pigments, fungicides, and matting agents may be blended in the top coat as necessary. The top coat film is preferably formed with a film thickness of 3 μm or more in consideration of rust prevention and coating film adhesion of the base steel. However, in the case of a thick film exceeding 30 μm, it is not possible to expect an improvement in corrosion resistance commensurate with the increase in film thickness, and the coating cost is increased. In particular, a film thickness of 8 to 20 μm for a polyester or silicone polyester system and 15 to 25 μm for a polyvinylidene fluoride system is effective in improving corrosion resistance, concealment and adhesion.

  Painted steel sheets for water heater outer plates are cut according to the specifications of the front plate, back plate, side plate, and top plate, bent by a press or roll forming machine, punched by punching, etc., screwed, rivets It is assembled into an outer box shape by fastening, welding, etc. At this time, the back surface coating film having hydrophilicity is disposed on the inner surface side of the outer box facing the water heater and the heating unit.

A hot-dip galvanized steel sheet (thickness 0.5 mm, zinc adhesion amount Z12) was prepared as a coating raw sheet, and a non-chromium pretreatment was performed. For comparison, a sample with chromium-based pretreatment was also prepared.
Two types of paints based on polyester resins were prepared for the back paint.
The first paint is a paint prepared by blending 50 parts by mass of tripolyaluminum dihydrogen phosphate and calcium silicate and 6 parts by mass of methyl silicate (hydrophilicity imparting agent) with respect to 100 parts by mass of the coating film resin component.
The second coating material is a coating material resin component: a coating material containing 100 parts by mass of aluminum dihydrogen phosphate, magnesium phosphate, magnesium hydrogen phosphate and zinc phosphate in total 10 parts by mass and 6 parts by mass of methyl silicate. It is.

  In addition, as a conventional product, based on a polyester-based resin, a coating resin component: a paint in which 50 parts by mass of aluminum dihydrogen phosphate and calcium silicate are combined with 100 parts by mass, aluminum dihydrogen phosphate, magnesium phosphate , A paint containing 10 parts by mass of a total of magnesium hydrogen phosphate and zinc phosphate and a paint containing 10 parts by mass of strontium chromate were prepared.

The undercoat paint was based on an epoxy resin and was prepared with the same kind and amount of rust-preventive pigment as the back paint.
A general polyester-based paint was used as the top coat.
First, an undercoat was applied to the surface of the coating original plate and baked at a maximum attained plate temperature: 200 ° C. for 30 seconds to form an undercoat film having a dry film thickness of 5 μm. Subsequently, the back coating material was applied at a coating amount that resulted in a dry coating thickness of 5 μm, and the top coating material was applied at a coating amount that resulted in a dry coating thickness of 15 μm, and baked at a maximum plate temperature of 215 ° C. for 40 seconds.

Test pieces were cut out from each coated steel sheet and investigated for hydrophilicity and corrosion resistance.
As a result of measuring the contact angle with water on the back surface, as shown in Table 1, the conventional coating film containing no methyl silicate is around 80 degrees, whereas the coating film of the present invention example containing methyl silicate was used. Showed a small value of less than 50 degrees, confirming hydrophilicity of the coating film surface.

Two 100 mm × 100 mm test pieces 31 and 32 were sampled from the prepared coated steel sheet, attached to the ceiling and side walls of the test chamber 30 with the back coating film 23 facing inward, and subjected to a dew condensation test.
A humidifier 34 that is heated by a heater 33 is arranged in the test chamber 30. When the water contained in the humidifier 34 is heated and evaporated, the inside of the test chamber 30 becomes a moist atmosphere, and the test comes into contact with the outside air. The test pieces 31 and 32 are exposed to an environment in which condensation is likely to occur due to a temperature difference with the outer surface of the chamber 30.

  In the dew condensation test, the wet atmosphere inside the test chamber 30 was maintained for 2 hours, then the test chamber 30 was opened to the atmosphere for 1 hour to make the inside dry, and wet-drying was repeated 200 cycles. Remove the test pieces 31 and 32 after the dew condensation test from the test chamber 30. For the ceiling test piece 31, the number and average diameter of the film swell, and for the side test piece 32, the length of the film swell from the lower cut end face. investigated.

  As can be seen from the results of the investigation in Table 1, in the example of the present invention in which a hydrophilicity imparting agent was blended in the back coating film 23 to lower the contact angle with water, even a sample using a non-chromium anticorrosive pigment was used as a ceiling test piece. No film bulge 29 is observed at 31, and the film bulge 29 of the side surface test piece 32 is relatively small, indicating that corrosion due to the film dew condensation water hardly occurs. The effect of improving the corrosion resistance was almost the same as that of the comparative example using the chromium-based rust preventive pigment.

On the other hand, in the comparative example (corresponding to the conventional product) provided with a hydrophobic coating film having a water contact angle of 78 degrees, a coating film swell 29 occurs on the ceiling test piece 31 and a large coating film swell 29 on the side test piece 32 Had occurred. Moreover, white rust was detected on the surface of the coating film on a part of the ceiling test piece 31.
From this comparison, it can be seen that by imparting hydrophilicity to the back surface coating film 23, the surface of the coating film is easily dried, and corrosion caused by condensed water is suppressed.

  As described above, when a coated steel plate is used for the outer boxes 16 and 17 surrounding the heating unit 11 and the hot water storage tank 12 serving as a heat source, the inner surfaces of the outer boxes 16 and 17 facing the heating unit 11 and the hot water storage tank 12 are used. By imparting hydrophilicity to the coating film to be provided, the inner surface of the outer box is easily maintained in a dry state, corrosion caused by condensed water is suppressed, and a hot water heater outer box with good durability is obtained.

Schematic cross section of hot water supply equipment The hydrophilicity and hydrophobicity of the coating film contribute to the generation and growth of condensed water. Chart explaining the effect Explanatory diagram of the mechanism of corrosion caused by condensed water Explanatory drawing of the condensation test adopted in the examples

Explanation of symbols

10: Hot water supply equipment 11: Heating unit 12: Hot water storage tank 13: Cold water piping 14: Hot water piping 15: Hot water supply piping 16: Outer box of heating unit 17: Outer box of hot water storage tank
20: Painted steel plate 21: Base steel 22: Plating layer 23: Back coating 24: Rust prevention pigment 25: Condensation water 26: Cavity 27: Corrosive component 28: Corrosion product 29: Coating swelling
30: Condensation test chamber 31: Ceiling test piece 32: Side test piece 33: Heater 34: Humidifier

Claims (2)

  Partially hydrolyzed tetraalkoxysilane as hydrophilicity-imparting agent and one or more non-chromium rust preventive pigments selected from magnesium phosphate, magnesium hydrogen phosphate, zinc phosphate, tripolyaluminum dihydrogen phosphate, calcium silicate A water heater outer box made from a coated steel sheet having a back coating film mixed with a condensate, characterized in that the back coating film is arranged on the inner surface side of the outer box facing the water heater and the heating unit. Painted steel plate water heater outer box.   The coated steel sheet water heater outer box according to claim 1, wherein the coated steel sheet which hits the outer surface side of the outer box is provided with an undercoat film containing a rust preventive pigment and an overcoat film containing a color pigment.

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