JP2019090092A - Copper pipe excellent in ant nest-like corrosion resistance - Google Patents

Abstract

To provide a copper pipe excellent in corrosion resistance, capable of being preferably used as a heat transfer pipe or a refrigerant piping or the like in an air-conditioning machine or refrigerating-machine, and hot water supply water supply machine, capable of reducing P content in whole copper pipe while securing excellent corrosion resistance to nest-like corrosion of ant, and to advantageously provide a copper pipe of which properties such as processability are improved while maintaining excellent ant nest-like corrosion resistance.SOLUTION: There is provided a copper pipe consisting of a copper alloy containing phosphorus of 0.15 to 0.40 mass% and phosphorus concentration is changed in a thickness direction of a pipe wall of the copper pipe.SELECTED DRAWING: None

Description

  TECHNICAL FIELD The present invention relates to a copper pipe excellent in ants-like corrosive resistance, and in particular, it is an ant-like corrosion of a copper pipe suitably used for a heat transfer pipe, a refrigerant pipe, etc. Relates to the technology for improving the corrosion resistance to

  Conventionally, pipes such as air conditioners, refrigeration equipment, heat transfer pipes in hot water supply and water supply equipment, refrigerant piping (in-machine piping) exhibit excellent characteristics in corrosion resistance, brazing property, thermal conductivity, bending workability, etc. Phosphorus (P) deoxidized copper tubes (JIS-H3300-C1220) are mainly used.

  And, in the phosphorus-deoxidized copper pipe which is a pipe material used for such air conditioners, refrigeration equipment and hot water supply / water supply equipment, abnormal corrosion which proceeds in the thickness direction of the pipe from the pipe surface in the form of an ant nest, so-called ant It is recognized that there is a possibility of occurrence of nest-like corrosion. This ant's nest corrosion is considered to occur in a wet environment using a lower carboxylic acid such as formic acid or acetic acid as a corrosive medium, and it is also possible to use a chlorinated organic solvent such as 1,1,1-trichloroethane or a certain lubricating oil The same occurrence of corrosion has been confirmed even in the presence of formaldehyde and the like. In particular, it is known that when it is used as a conduit for causing condensation in an air conditioner, a refrigerator, or a hot water supply / water supply device, the occurrence is remarkable. And if this ant's nest corrosion generate | occur | produces, corrosion will progress rapidly and will progress by the time it penetrates a copper pipe in a short time, and it will cause the problem that an apparatus will become unusable.

  For this reason, WO 2014/148127 is characterized in that P (phosphorus) is contained at a ratio of 0.05 to 1.0% by weight, and the balance is made of Cu (copper) and Cu material to be an unavoidable impurity. It has been shown that highly corrosion resistant copper tubes have been proposed, whereby the corrosion resistance to ant nest corrosion can be improved. That is, there is practically advantageously obtained a copper pipe which can further improve the corrosion resistance to ant-like corrosion in a region where the P content is larger than a conventional pipe made of phosphorus-deoxidized copper. The fact that it can be done is pointed out.

  However, in a copper tube having such an increased P content, as the P content is increased, problems such as cracking tend to be caused in the production of the copper tube, and the tube as a tube There is a problem that the processability is lowered and it becomes difficult to smoothly perform bending for use in heat transfer tubes, refrigerant piping, etc. Furthermore, the increase in the amount of expensive P used increases the tube manufacturing cost. The problem is also inherent.

WO2014 / 148127

  Here, the present invention is made on the background of such circumstances, and the problem to be solved by the present invention is that the entire copper pipe is secured while securing excellent corrosion resistance against nest-like corrosion of ant. It is to provide a copper pipe excellent in corrosion resistance, which can be suitably used as a heat transfer pipe or refrigerant piping in air conditioners, refrigeration equipment, hot water supply / water supply equipment, and the like which can reduce P content as Another object of the present invention is to advantageously provide a copper tube having improved properties such as processability while maintaining excellent resistance to dovetail corrosion.

  Therefore, in order to solve the problems in the prior art as described above, the present inventors conducted intensive studies on toroidal corrosion in copper tubes as pipes used in air conditioners, refrigeration equipment, etc. In a copper tube made of a copper alloy containing 0.15 to 0.40% by mass, the P concentration in the thickness direction of the tube wall is changed to increase the P concentration area and the P concentration area Even if the P content of the copper tube as a whole is reduced, the properties such as the processability of the copper tube can be advantageously improved while sufficiently securing the corrosion resistance against the nest-like corrosion of the anvil. The facts to be obtained were found, and the present invention was completed.

  That is, the present invention has been completed based on the above-mentioned findings, and the gist of the present invention is a copper tube made of a copper alloy containing P in a proportion of 0.15 to 0.40% by mass. Such a copper tube is characterized by varying P concentration in the thickness direction of the tube wall of the copper tube.

  In one of the desirable modes of the copper tube according to the present invention as described above, it is preferable that the P concentration is increased from the outer surface of the tube toward the central portion in the thickness direction of the tube wall. In particular, the average P concentration in the central portion in the thickness direction of the tube wall is 0.2 to 0.5% by mass, and the average P concentration in the outer surface portion of the tube is 0. An arrangement of 1 to 0.3% by weight is advantageously employed.

  Moreover, in another one of the preferable embodiments of the copper tube excellent in ants-like corrosion resistance according to the present invention, the P concentration is directed toward the central portion in the thickness direction of the tube wall. An arrangement which decreases from the outer surface of the tube may be suitably adopted, and in particular, the average P concentration at the central portion in the thickness direction of the tube wall is 0.3 mass% or less, A configuration in which the average P concentration of the surface sites is 0.2 to 0.5% by mass is advantageously employed.

  In addition, in the present invention, preferably, the copper pipe is formed of a cast pipe obtained by a cast and roll method.

  Furthermore, such ant-resistant copper tube according to the present invention is advantageously placed in a wet environment, and a corrosive medium consisting of lower carboxylic acid allows the dovetail in the thickness direction of the tube from the tube surface. It will be used as a copper tube which is exposed to the corrosive effect progressing in the form of nests.

  Further, in the present invention, the heat transfer pipe and the refrigerant pipe (in-machine pipe) in the air conditioner, the refrigeration apparatus, the hot water supply and the water supply apparatus, which are made of the above-mentioned copper pipe excellent in nested corrosion resistance as described above The point is that.

  In addition, according to the present invention, the lower carboxylic acid, which is generated from the surface of a copper pipe which is used in an air conditioner, a refrigerator, or a hot water supply device and is placed in a wet environment, is used as a corrosive medium. In a method of improving the corrosion resistance against ants' nesting corrosion that occurs in a wet environment, a copper tube made of a copper alloy containing P in an amount of 0.15 to 0.40% by mass is used as the copper tube. The gist of the present invention is also a method of improving the corrosion resistance characterized by using one in which the P concentration is changed in the thickness direction of the pipe wall of the copper pipe.

  As described above, in the copper tube excellent in the ant-like corrosion resistance according to the present invention, the region where the P concentration is high and the region where the P concentration is low exist in the thickness direction of the tube wall. By the presence of such a high P concentration region, it is possible to ensure effective corrosion resistance against ant-like corrosion, while in the region where such P concentration is low, pipe workability It is possible to greatly improve the properties like the above, and, by suppressing the P content (concentration) of the whole tube, it is possible to advantageously exhibit the anticorrosion resistance like ants. The reduction of the P content, and hence the amount of P raw material used, can also be effectively achieved, which can advantageously contribute to the reduction of the copper tube manufacturing cost.

  And, by using a copper pipe exhibiting such characteristics as a heat transfer pipe in an air conditioner, a refrigerator, a hot water supply / water supply device, a refrigerant pipe (in-machine piping), etc., the life of those devices is further effectively improved. Characteristics such as to be obtained can be exhibited advantageously.

BRIEF DESCRIPTION OF THE DRAWINGS It is a cross-sectional schematic explanatory drawing which shows an example of the horizontal casting apparatus suitably used for manufacture of the copper tube excellent in the nest-corrosion-proof property according to this invention. It is cross-sectional explanatory drawing which shows the outline | summary of the corrosion resistance test apparatus used in the Example.

  By the way, in the copper tube excellent in the nest corrosion resistance according to the present invention, the P concentration (content) is changed in the thickness direction of the tube wall to increase the P concentration. From the point where it becomes possible to exert effective dovetail corrosion resistance, the P content of the copper alloy as a whole constituting such a copper tube is relatively Therefore, in the present invention, such P content is advantageously selected within the range of 0.15 to 0.40% by mass. In the present invention, a copper tube as an object of the present invention is a selective corrosion form in which the corrosion proceeds in ant-like manner when the P content is less than 0.15% by mass. The P content in the copper alloy to give N should be at least 0.15 mass%. On the other hand, when such P content increases and exceeds 0.40% by mass, the processability is lowered in the production of the target copper tube, and problems such as cracking are easily caused. In addition to the above, the upper limit of the P content is preferably 0.40% by mass, particularly about 0.35% by mass, and further about 0.30% by mass, because the processability in the pipe is lowered. It is desirable to

  Moreover, the copper tube excellent in the dovetail-like corrosion resistance according to the present invention has a P content as described above and, in addition, is generally made of a material in which the balance is Cu and impurities. In the present invention, in particular, the content of the specific impurity element group consisting of Cr, Mn, Fe, Co, Zr and Mo among such impurities is, in total, 0.01% by mass or less. In this case, the corrosion resistance of the copper tube is further improved. These specific impurity element groups are likely to form a compound with P by heat treatment such as annealing, and the resulting precipitate of the P-based compound will reduce the corrosion resistance of the copper tube.

  Further, the impurities to be contained together with the above-described Cu include elements such as S, Si, Ti, Ag, Pb, Se, Te, Bi, Sn, Sb, As, etc. Although it is also present as an unavoidable impurity, it is desirable that the total amount of such unavoidable impurities is generally adjusted to be 0.005% by mass or less.

  Incidentally, as a Cu material having a reduced content of such a specific impurity element group and other unavoidable impurity elements as described above, industrial pure copper whose purity has been enhanced by a conventionally known smelting and refining technique, for example, Cu-containing An electrolytic copper whose purity is increased to an amount of 99.99% by mass or more is advantageously used.

  Moreover, as described above, the ants-proof corrosive copper tube according to the present invention is composed of a Cu alloy containing a specific amount of P and the balance Cu and impurities as described above, and further, the ants-proof nest of such a copper tube. In order to further improve the corrosion resistance and the like, it is also effective to positively contain a predetermined alloy component in the copper alloy constituting the same. For example, at least one element of at least one of Ti and Sb is contained in a total amount of 0.01 to 3.0% by mass, together with or instead thereof, Zn, Sn, Ni and By including at least one element selected from the group consisting of Al in a total amount of 0.01 to 5.0% by mass, the corrosion resistance to ant-like corrosion can be more advantageously enhanced. I can do it.

  And, in the present invention, in the copper tube produced using the Cu alloy having the composition as described above, the P concentration gradually changes in the thickness direction of the tube wall, for example, continuously or It is configured to be gradually changed, thereby achieving the object of the present invention described above. Here, the change in P concentration is advantageously realized in the form of increasing or decreasing from the outer surface of the tube toward the central portion in the thickness direction of the tube wall, whereby the thickness direction of the tube wall is obtained. In the above, the region where the concentration of P is large can be advantageously formed, so that excellent characteristics can be secured in terms of the resistance to vesicular corrosion. At the same time, a region with low P concentration will be formed on the outer surface or central portion of the tube in the thickness direction of the tube wall, and by the presence of such a region with low P concentration It is possible to advantageously improve the heat transfer characteristics and the heat transfer characteristics.

  In addition, such a high P concentration region is partially present in the thickness direction of the pipe wall, and thus it is possible to advantageously ensure the desired excellent anti-tribe corrosion resistance. It is not necessary to allow P to be present at a high concentration over the entire region in the thickness direction of the tube wall, and therefore, the amount of P used is more effective than in the case where P is present at a predetermined constant concentration throughout the tube wall. Thus, it can advantageously contribute to the reduction of the manufacturing cost of the copper tube.

  By the way, it is desirable that such a change in the P concentration in the tube wall thickness direction be realized at least between the outer surface of the tube and the central portion of the tube wall, and such a central portion of the tube wall and the tube surface (copper tube In the region between the inner surface and the inner surface, it is possible to make the P concentration equal to or greater than or equal to that of the central portion of the tube wall. For example, in the case where the P concentration increases from the outer surface of the tube toward the central portion of the tube wall, the P concentration between the central portion of the tube wall and the inner surface of the tube is generally equal to or greater than that of the central portion of the tube wall In the (high) aspect is adopted, and in the opposite case, the P concentration between the central portion of the pipe wall and the inner surface of the pipe is generally as low as, or lower than, that of the central portion of the pipe wall (lower) Will be adopted.

  As described above, in the case where the P concentration increases from the outer surface of the tube toward the central portion in the thickness direction of the tube wall, the average at the central portion in the thickness direction of the tube wall The P concentration is preferably 0.2 to 0.5% by mass, and the average P concentration at the outer surface of the tube is preferably 0.1 to 0.3% by mass. As described above, by increasing the average P concentration at the central portion in the thickness direction of the tube wall, excellent corrosion resistance against ant nest corrosion can be exhibited, and the tube strength is also greatly improved. On the other hand, by lowering the P concentration at the outer surface of the tube, the processability and thermal conductivity of the tube can be advantageously improved. Here, the average P concentration refers to the average value of the amount of P contained in a certain thickness region in the thickness direction of the pipe wall, and in general, Equally divided, the P concentration in the entire 1/3 area of the central area is taken as the average P concentration in the central area, while on the outside of the central area in the thickness direction of the pipe wall (outside the outer surface of the pipe) The total P concentration of the region of the located 1/3 portion will be used as the average P concentration of the outer surface portion of the tube.

  In the case where the P concentration decreases from the outer surface of the tube toward the central portion in the thickness direction of the tube wall, the average P concentration at the central portion in the thickness direction of the tube wall It is desirable that the average P concentration at the outer surface of the tube be 0.2 to 0.5% by mass, while it is configured to be 0.3% by mass or less. Thus, by increasing the average P concentration in the outer surface portion of the tube, which is a partial region in the thickness direction of the tube wall, it is possible to more effectively exhibit the nest-like corrosion resistance of ants. Therefore, it is possible to lower the average P concentration at the central portion in the thickness direction of the tube wall, and it is also possible to make it 0 mass% at the extreme, which makes it possible to process the tube Etc. can be effectively enhanced. Here, the average P concentration has the same meaning as described above, and indicates the P content of the entire predetermined portion in the thickness direction of the pipe wall.

  And, when manufacturing a copper tube in which the P concentration changes in the thickness direction of the tube wall according to the present invention as described above, advantageously, a Cu molten metal is used, using a Cu raw material giving a Cu alloy having the composition described above. Is cast, and the pipe material is cast by a cast-and-roll method (casting-type rolling method), and this cast pipe material is subjected to drawing, annealing, etc. to produce a copper pipe of the desired size. It becomes. The cast-and-roll method is disclosed in, for example, JP-A-8-33949 and JP-A-2009-114493, a horizontal mold is used to form a pipe from molten Cu. The pipe forming method is a combination of horizontal continuous casting in which the ingots are continuously cast and a rolling operation in which the obtained pipe-like ingots are rolled to a predetermined size by a predetermined rolling mill.

  Specifically, in the manufacture of a copper tube by the cast and roll method, for example, it will be manufactured using a typical horizontal casting apparatus as shown in FIG. That is, in FIG. 1, the Cu molten metal 4 of a predetermined composition contained in the holding furnace 2 is introduced into the annular space of the graphite mold 6 and solidified by cooling by the cooling jacket 8 through which primary cooling water is made to flow. Further, after leaving the graphite mold 6, the cylindrical casting pipe 12 is formed by cooling with secondary cooling water in the cooling tank 10, and then rolled by a planetary rolling mill or the like to form a cast pipe having a required diameter. It is assumed. And this cast pipe material is further processed by drawing, annealing, etc. as needed, and it is set as the copper pipe of the target size.

  And, in the manufacture of such a copper tube by the cast-and-roll method, by controlling the solidification rate at the time of casting, the P concentration is effectively changed in the thickness direction of the tube wall of the obtained tube material It is possible to For example, by slowing the solidification rate at the time of casting, a change form in which the P concentration increases toward the central part in the thickness direction of the pipe wall is revealed, and thereby the central part in the thickness direction of the pipe wall While forming a region of high P concentration, a region of low P concentration can be easily formed on the outer surface site of the tube. Further, by adjusting in such a direction as to accelerate the solidification speed at the time of such casting, the solidification interface becomes near the casting outlet of the graphite mold 6 and new molten Cu metal 4 flows into the outer peripheral surface side of the solidification site to solidify first. Since solidification is slower in the outer surface side than in the central portion of the tube wall, P is segregated to the surface, and the cast pipe 12 having a high P concentration in the outer surface of the tube is formed. is there. That is, by increasing the solidification rate at the time of casting, the P concentration at the outer surface of the tube is increased, and the P concentration gradually decreases from the high P concentration, and the P concentration at the central region in the thickness direction of the tube wall Can be made the lowest.

  In addition, sizes, such as the outer diameter of the copper pipe according to this invention obtained as mentioned above, and thickness, will be suitably selected according to the use. Furthermore, in the case where the copper tube according to the present invention is used as a heat transfer tube, in addition to adopting a smooth inner surface and an outer surface, as is well known, various known inner surface processing and outer surface processing are performed. It is also effective to use a heat transfer tube in which inner surface grooves (protrusions) and outer surface grooves (protrusions) of various forms are provided. In addition, in the case of being used as a refrigerant pipe, in general, the inner surface and the outer surface are used as a smooth copper tube.

  Moreover, the copper pipe according to the present invention can be suitably used as a heat transfer pipe, refrigerant piping, etc. in an air conditioner, and similarly can be suitably used as a heat transmission pipe, refrigerant piping (in-machine piping), etc. in a refrigeration equipment or hot water supply / water supply equipment. It is a thing.

  In the following, some examples according to the present invention will be shown to clarify the present invention more specifically, but the present invention is not limited by any description of such examples. It goes without saying. In addition to the embodiments described below, the present invention also includes various changes and modifications based on the knowledge of those skilled in the art without departing from the spirit of the present invention other than the specific description described above. It should be understood that it is possible to add improvements, etc.

  First, using high-purity electric copper having a Cu content of 99.999% by mass or more, P is added and contained so as to have a ratio (total P content) shown in Table 1 below. The molten copper having the following composition is melted and cast by a cast-and-roll method at a casting speed shown in Table 1 below using a horizontal pipe casting apparatus as shown in FIG. Various cast pipe materials were produced, and then, through processes such as drawing, various test copper pipes having an outer diameter of 7.00 mm and a wall thickness (pipe wall thickness) of 0.23 mm were produced. Test copper tube No. The casting speed employed in the production of No. 10 is a normal casting speed, and when it falls below this casting speed, the proportion of P remaining in the molten Cu at the solidification interface increases, and the P concentration in the central portion of the tube wall Becomes higher (center line segregation), and conversely, when it becomes faster than the normal casting speed, the solidification interface separates from the mold, and the inner Cu molten metal continuously or intermittently on the ingot surface It leaks so as to cover, and a portion with a high P concentration is formed on the ingot surface.

  Moreover, the test copper pipe No. Regarding the content of impurities in 1 to 10, the respective test copper tubes are dissolved in an acid (aqua regia), and the content of elements contained as impurities is determined by high-frequency inductively coupled plasma emission spectrometry (ICP -Determined by analysis by OES). As a result, the total content of specific impurity element groups (Cr, Mn, Fe, Co, Zr and Mo) in any of the test copper tubes is less than 0.01% by mass, and such specific impurities Even if it was in other unavoidable impurities other than an element group, it confirmed that the total content was less than 0.005 mass%.

  Furthermore, the obtained test copper pipe No. From 1 to 10, the distribution of P concentration in the thickness direction of the tube wall was examined. That is, in order to check the P concentration of the pipe outer peripheral portion (outer surface surface portion) and the central portion in the thickness direction of the pipe wall, one-third of the thickness direction (pipe wall thickness direction) is cut from the pipe outer peripheral surface The chips obtained by this process and the chips obtained by cutting one third of the wall thickness including the central portion in the thickness direction are dissolved and then solidified. Each P concentration of the obtained samples was measured according to IPC emission spectroscopy defined in JIS-H1058, and the results are shown in Table 1 below.

  Then, the various test copper pipe No. which were prepared in this way. For the test pieces 1 to 10, the nest corrosion test of ant was carried out using the test apparatus shown in FIG. In FIG. 2, reference numeral 22 denotes a 2 L poly container which can be sealed with a cap 24. A test copper pipe 30 is provided so as to penetrate the silicon plug 26 which penetrates the cap 24 and is attached. The lower end opening of the test copper tube 30 is closed by a silicon plug 28 while being inserted into the poly container 22 to a predetermined depth. The entire length of the test copper pipe was 18 cm, and the length of the exposed portion in the poly container 22 was 15 cm. Further, 100 ml of a formic acid aqueous solution having a predetermined concentration is accommodated in the poly container 22 in a form not to be in contact with the test copper tube 30.

  In the ant nest corrosion test adopted here, a predetermined test copper pipe 30 was set in a poly container 22 containing the formic acid aqueous solution 32 at a concentration of 0.1% of the formic acid aqueous solution 32. In the state, it is left in the constant temperature bath at 40 ° C., taken out of the bath for 2 hours / day, and kept at room temperature (15 ° C.), and the temperature difference causes the surface of the test copper tube 30 to Urges for dew condensation. And the corrosion test under such conditions was conducted for 80 days.

  Then, for each of the test copper pipes subjected to such corrosion test, among the portions exposed in the poly container 22 of FIG. 2, the cross section perpendicular to the axial direction of the pipe is checked at any five positions; The maximum corrosion depth from the tube surface was determined, and the results are shown in Table 2 below.

  In addition, the thermal conductivity of each test copper pipe is described in "The Proceedings of the Annual Meeting of the Japan Refrigerating and Air Conditioning Society of Japan 2005", A206-2, "Verification of heat transfer performance evaluation method" in the right column. The respective methods were determined, and the results are shown in Table 2 below.

  Furthermore, the workability of each test copper tube is evaluated by 180 ° hairpin bending, which is a forming process adopted when using each test copper tube as a heat transfer tube, The severe radius of curvature: 9 mm, in which case the tube is buckled or the bent top of the pipe is split to make it "X", while in such bending any problems are encountered Those that did not occur were evaluated as “o”, and the results are shown together in Table 2 below.

  As apparent from the results of Table 2, the test copper tube No. 1 according to the present invention. No. 1 and 2 and sample copper pipe No. In 5 to 7, it is possible to ensure excellent ants' nest-like corrosion resistance even though the total P content is small, and at the same time to show excellent tube workability and effective thermal conductivity. It became clear. On the other hand, the test copper pipe No. which concerns on a comparative example. No. 3, 4 and copper tube No. In the cases of No. 8 and No. 9, in addition to the fact that the total P content increased and the manufacturing cost increased, the properties such as the processability of the pipe were inferior. Moreover, the test copper pipe No. In No. 10, there is almost no change in P concentration in the thickness direction of the pipe wall, and therefore, in addition to the fact that the P content does not have a sufficient contribution to the nest corrosion resistance, the pipe workability Also in the test copper tube according to the present invention was found to be inferior to.

DESCRIPTION OF SYMBOLS 2 holding furnace 4 Cu molten metal 6 graphite mold 8 cooling jacket 10 cooling tank 12 casting raw pipe 22 poly container 24 cap 26, 28 silicon stopper 30 test copper pipe 32 formic acid aqueous solution

Claims (10)

  A copper tube made of a copper alloy containing phosphorus in a proportion of 0.15 to 0.40% by mass is characterized in that the phosphorus concentration is changed in the thickness direction of the tube wall of the copper tube. Copper tube with excellent ant-like corrosion.   2. The copper tube excellent in anti-corrosion resistance as set forth in claim 1, wherein the phosphorus concentration increases from the outer surface of the tube toward the center in the thickness direction of the tube wall.   The average phosphorus concentration at the central portion in the thickness direction of the pipe wall is 0.2 to 0.5 mass%, and the average phosphorus concentration at the outer surface portion of the pipe is 0.1 to 0.3 mass% The copper tube excellent in the dovetail-like corrosion resistance according to claim 2 characterized by the above-mentioned.   2. The copper tube excellent in anti-corrosion resistance according to claim 1, wherein the phosphorus concentration decreases from the outer surface of the tube toward the center in the thickness direction of the tube wall.   The average phosphorus concentration at the central portion in the thickness direction of the pipe wall is 0.3 mass% or less, and the average phosphorus concentration at the outer surface portion of the pipe is 0.2 to 0.5 mass%. The copper tube excellent in the dovetail-like corrosion resistance of Claim 4.   The anticorrosion resistance to termites according to any one of claims 1 to 5, wherein the copper pipe is formed of a cast pipe obtained by a cast and roll method. Excellent copper tube.   A copper tube which is disposed in a wet environment and which is exposed to a corrosive action progressing like a nest of ant from a tube surface to a tube thickness direction from a tube surface by a corrosive medium comprising a lower carboxylic acid. The copper tube excellent in the nest-like corrosion resistance according to any one of claims 6.   A heat transfer pipe in an air conditioner, a refrigerator, or a hot water supply / water supply device, comprising the copper tube excellent in nest corrosion resistance as set forth in any one of claims 1 to 7.   A refrigerant pipe in an air conditioner, a refrigerator, or a hot water supply / water supply device, characterized by comprising a copper tube excellent in the nest corrosion resistance of the ant according to any one of claims 1 to 7.   Copper tubes used in air conditioners, refrigeration equipment, and hot water supply and supply equipment, placed in a wet environment, ant-like corrosion that occurs in the wet environment using lower carboxylic acid as a corrosive medium, which originates from the surface In the method of improving the corrosion resistance to copper, as the copper tube, a copper tube made of a copper alloy containing phosphorus in a proportion of 0.15 to 0.40% by mass, in the thickness direction of the tube wall of such copper tube What is claimed is: 1. A method for improving corrosion resistance, comprising using a material obtained by changing the phosphorus concentration.

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