Corrosion resistant copper alloy, copper alloy pipe, and heat exchanger

Abstract

To provide a copper alloy which improves corrosion resistance against stress corrosion cracks, and a copper alloy pipe and a heat exchanger using the same.SOLUTION: A copper alloy is a copper alloy added with a base metal element (for example, Mg and Mn) having a standard electrode potential equal to or lower than the potential of Mn. A copper alloy pipe is formed of a copper alloy added with a base metal element having a standard electrode potential equal to or lower than the potential of Mn. A heat exchanger uses a copper alloy pipe formed of a copper alloy added with a base metal element having a standard electrode potential equal to or lower than the potential of Mn.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a corrosion-resistant copper alloy having improved corrosion resistance against stress corrosion cracking, and a copper alloy tube and a heat exchanger using the same. [Background technology]

[0002] Phosphorus-deoxidized copper tubes are widely used as refrigerant piping and heat exchanger piping in refrigeration and air conditioning equipment. There are two types of phosphorus-deoxidized copper: C1201, which is low phosphorus deoxidized copper, and C1220, which is high phosphorus deoxidized copper, as specified in JIS H3300:2018.

[0003] Copper is a material with excellent thermal conductivity, bending workability, brazing properties, etc. In addition, due to its high standard electrode potential, it has excellent corrosion resistance in non-oxidizing environments. Phosphorus-deoxidized copper contains phosphorus but no oxygen, so it is known to be less susceptible to hydrogen embrittlement, just like oxygen-free copper. Phosphorus-deoxidized copper has a lower manufacturing cost than oxygen-free copper, so it is widely used in applications where it is exposed to high temperatures, such as brazing.

[0004] Corrosion-resistant materials such as copper-based materials can suffer from stress corrosion cracking (SCC) due to the overlap of certain factors. Cases of SCC in copper-based materials such as pure copper and copper alloys in ammonia environments have long been reported. SCC in copper-based materials was particularly notable in brass. However, in recent years, cases have also been reported in phosphorus-deoxidized copper pipes.

[0005] SCC of copper-based materials occurs under special conditions such as an ammonia environment, but it does not progress in ammonia alone, but in the coexistence of ammonia and moisture. When moisture adheres to copper-based materials, ammonia dissolves in the water phase, causing erosion. SCC occurs when residual stress or external stress concentrates at the site where the grain boundary has been eroded. SCC is a phenomenon that leads to cracks due to tensile stress, but the corrosion mode in phosphorus-deoxidized copper is characterized by intergranular corrosion.

[0006] In fields such as refrigerant piping and heat exchanger piping, refrigerant leakage due to SCC has become a problem, and measures to suppress SCC are required. If the progression of SCC causes piping to break, it can lead to refrigerant leakage, making it impossible to maintain the functionality of equipment or compromising the reliability of the equipment. There are also concerns about the impact of refrigerant leakage on global warming. As a measure to suppress SCC, from the perspective of material factors, there is a method of making the crystal grains fine. Also, from the perspective of mechanical factors, there is a method of reducing residual stress and external stress.

[0007] However, methods to reduce residual stress and external stress are difficult to realize in actual materials, and are not realistic as measures to suppress SCC. In addition, methods to refine crystal grains do not allow the material to be annealed sufficiently, which significantly impairs workability and limits applications. Under these circumstances, measures to suppress SCC from the perspective of chemical composition, etc. are being considered.

[0008] Patent Document 1 describes a copper tube that is excellent in corrosion resistance against ant nest corrosion and excellent in corrosion resistance against SCC. This copper tube is made of a copper material containing 0.10 to 1.0 weight % of P, with the remainder being Cu and unavoidable impurities, and the P concentration (P1) at the crystal grain boundaries of the copper material is less than 5.0 times the P concentration (P0) within the crystal grains of the copper material. By optimizing the final heat treatment conditions, the concentration of P at the crystal grain boundaries is suppressed, thereby reducing the SCC susceptibility (see paragraph 0011).

[0009] Patent Document 2 describes a copper alloy having high electrical conductivity and excellent stress relaxation resistance. The copper alloy contains more than 0.001 mass% and not more than 0.01 mass% of Mg, and not more than 0.001 mass% of P. The H content is also not more than 0.001 mass%, the O content is not more than 0.01 mass%, and the C content is not more than 0.001 mass%.

[0010] Patent Document 3 describes pitting corrosion resistant copper and copper alloy tubes that can prevent the occurrence of pitting corrosion. It also describes that when lithium bromide is used as an absorbing solution, ammonia inevitably remains in the refining process, and that the stress corrosion cracking susceptibility of phosphorus-deoxidized copper increases with increasing phosphorus content (see paragraph 0002).

[0011] The mechanism by which P added to copper promotes SCC has not been fully elucidated. In general, the following factors (1) to (3) are assumed to be involved in the mechanism by which P promotes SCC.

[0012] (1) Grain boundaries are essentially locations where impurities and added P tend to concentrate. (2) P dissolves from the copper phase into the aqueous phase, which in turn lowers the pH of the aqueous phase and dissolves Cu, and the copper ions are more stable in the chemical form than the oxide or hydroxide of the dissolved Cu. (3) Distributions of pH and other parameters occur in areas where corrosion progresses.

[0013] In addition, the following factor (4) has been newly presumed in Copper and Copper Alloys, Vol. 53, No. 1 (2014), p. 128-p. 133 (Electrochemical approach to clarify the mechanism of ant nest corrosion). As ant nest corrosion progresses, (4) a complex formation reaction occurs in which the dissolved P and copper ions form complex ions. From the viewpoint of free energy, this reaction is considered to drive the dissolution of Cu from the copper phase to the aqueous phase. This copper dissolution reaction caused by the dissolution of P can work regardless of whether the corrosion-promoting substance is formic acid or ammonia. Therefore, in SCC, the dissolution of P is considered to lead to further dissolution of copper ions by reaction (4), which is expected to cause more serious SCC progression. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] JP 2022-056871 A [Patent Document 2] Patent Publication No. 2022-022637 [Patent Document 3] JP 2007-154221 A Summary of the Invention [Problem to be solved by the invention]

[0015] With conventional copper-based materials, it is currently difficult to prevent hydrogen embrittlement while suppressing SCC. In order to prevent hydrogen embrittlement in copper-based materials, it is necessary to reduce the amount of oxygen. When copper containing oxygen is exposed to high temperatures in a hydrogen atmosphere, it undergoes hydrogen embrittlement, resulting in a decrease in strength and toughness. In applications such as furnace brazing in a hydrogen atmosphere, hydrogen diffuses in the high-temperature environment when the material is heated, reducing copper oxide and generating water vapor voids at the grain boundaries, resulting in a decrease in strength and toughness.

[0016] Adding P to copper-based materials can suppress this type of hydrogen embrittlement by deoxidizing them, but the addition of P can cause the initiation and progression of SCC, which is a problem.

[0017] Oxygen-free copper is a material that is not only less susceptible to hydrogen embrittlement due to its low oxygen and phosphorus concentrations, but also has low SCC susceptibility. However, oxygen-free copper requires special casting equipment such as vacuum melting and casting, which poses issues in terms of manufacturing costs and price.

[0018] On the other hand, although phosphorus-deoxidized copper is less susceptible to hydrogen embrittlement, it contains P that was added in the melting process for deoxidization. Phosphorus-deoxidized copper also includes low-phosphorus deoxidized copper with a P content of 0.004 mass% or more and less than 0.015 mass%. However, even with low-phosphorus deoxidized copper, the effects of P cannot be completely eliminated, and use in highly corrosive environments is restricted. In addition, the proportion of recycled raw materials containing P used in the production of low-phosphorus deoxidized copper must be reduced, which places significant restrictions on production costs and price.

[0019] In Patent Document 1, the P concentration in the grain boundaries is reduced in order to reduce the SCC susceptibility of the copper material. However, this method adjusts the ratio of the P concentration in the grain boundaries to the P concentration in the grains, and does not directly render the P in the copper material harmless. In addition, this method requires a special final heat treatment, and is therefore considered to have practical problems in terms of production efficiency, production equipment, and the like.

[0020] Patent Document 2 specifies the ratio of Mg to the total of S, P, Se, Te, Sb, Bi, and As, and the contents of H, O, and C for copper alloys. However, this copper alloy contains only a small amount of P and is considered to be equivalent to oxygen-free copper. Such a P content causes problems in terms of manufacturing costs and price. In addition, H may cause defects in the structure, which may ultimately promote SCC, but it is not directly involved in SCC of copper-based materials.

[0021] Under these circumstances, there is a demand for technology that can suppress SCC while preventing hydrogen embrittlement and reducing costs for copper-based materials equivalent to phosphorus-deoxidized copper or oxygen-free copper. Even when P, which promotes SCC, is added, it is desirable to directly suppress SCC in copper-based materials by eliminating the effects of P.

[0022] Therefore, an object of the present invention is to provide a copper alloy having improved corrosion resistance against stress corrosion cracking, and a copper alloy tube and a heat exchanger using the same. [Means for solving the problem]

[0023] In order to solve the above problems, the copper alloy according to the present invention is a copper alloy to which a base metal element having a standard electrode potential equal to or lower than that of Mn is added. The copper alloy tube according to the present invention is formed from the above copper alloy. The heat exchanger according to the present invention uses the copper alloy tube formed from the above copper alloy. Effect of the Invention

[0024] According to the present invention, it is possible to provide a copper alloy having improved corrosion resistance against stress corrosion cracking, and a copper alloy tube and a heat exchanger using the same. [Brief description of the drawings]

[0025] [Figure 1] FIG. 1 is a diagram illustrating an example of a heat exchanger including a copper alloy tube. [Diagram 2] FIG. 2 is a diagram showing a method of pretreatment of a test material made of a copper alloy. [Diagram 3] FIG. 1 is a diagram showing a method for measuring the crack depth due to stress corrosion cracking. [Figure 4] FIG. 3 is an enlarged view of a main part of FIG. 2. [Diagram 5] FIG. 1 is a graph showing the relationship between crack depth due to stress corrosion cracking and P concentration. [Figure 6] FIG. 1 is a graph showing the relationship between the Mg concentration and the P concentration and the crack depth due to stress corrosion cracking. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] Hereinafter, a copper alloy according to one embodiment of the present invention, and a copper alloy tube and a heat exchanger using the same will be described.

[0027] The copper alloy according to the present embodiment is a copper alloy to which a base metal element having a standard electrode potential equal to or lower than that of Mn is added. As the base metal element, a phosphorus compound forming element that reacts with P to form a phosphorus compound is preferable. In a preferred embodiment of this copper alloy, P is more than 0% and 0.040% by mass or less, the total of the base metal elements is 0.01% by mass or more and 1.5% by mass or less, and the balance is Cu and unavoidable impurities.

[0028] The copper alloy according to this embodiment is improved in corrosion resistance against stress corrosion cracking (SCC) by adding base metal elements. P may be added to this copper alloy in order to reduce the amount of O, which is a cause of hydrogen embrittlement. Although P is a factor that promotes SCC, the occurrence and progression of SCC can be suppressed by the base metal elements even if P is added for deoxidation.

[0029] It is known that copper containing oxygen can become embrittled when exposed to high temperatures in a hydrogen atmosphere. Hydrogen penetrates and diffuses between the lattices of the copper phase. When the diffused hydrogen reacts with copper oxide in the copper phase, it reduces the copper oxide and generates water vapor. As a result, even though hydrogen itself does not directly cause an embrittlement reaction, the generated water vapor forms voids at the grain boundaries, causing a decrease in strength and toughness due to hydrogen embrittlement.

[0030] In addition, copper-based materials used for refrigerant pipes, heat exchanger pipes, etc., may rarely suffer from ant nest corrosion. Ant nest corrosion is corrosion that causes ant nest-like erosion from tiny corrosion holes on the surface of the material toward the inside. Ant nest corrosion occurs in the presence of oxygen and moisture, using carboxylic acids such as formic acid and acetic acid, aldehydes such as formaldehyde and acetaldehyde, and alcohols as corrosive agents.

[0031] These corrosive media are thought to be generated by hydrolysis or deterioration reactions caused by moisture such as condensation water in lubricating oils, processing oils, and organic solvents used in pipe manufacturing and heat exchanger assembly processes, as well as substances contained in the usage environment. Once ant nest corrosion occurs, its characteristic shape causes the anodic reaction to concentrate at specific points, accelerating the corrosion process and causing it to progress in the thickness direction and penetrate the wall in a short period of time.

[0032] Copper-based materials are often used in applications where brazing of refrigerant pipes, heat exchanger pipes, etc. is performed in a furnace in a hydrogen atmosphere. Since the brazing process involves exposure to high temperatures in a hydrogen atmosphere, it is desirable to reduce the amount of O, which is a cause of hydrogen embrittlement. However, ant nest corrosion and stress corrosion cracking are affected by P, which is necessary for deoxidation in the casting process. It is necessary to eliminate the effects of P from the viewpoint of improving SCC resistance and suppressing other corrosion phenomena.

[0033] In phosphorus-deoxidized copper-based materials, the mechanism by which P added to Cu promotes SCC has not been fully elucidated. However, the form of corrosion in SCC in phosphorus-deoxidized copper is intergranular corrosion. If an anodic reaction occurs that dissolves Cu, it is believed that a corresponding cathodic reaction is also occurring. Cu is not easily corroded under alkaline or non-oxidizing conditions, and it is speculated that oxygen is involved in the cathodic reaction.

[0034] Copper pipes used as refrigerant pipes, heat exchanger pipes, etc. may come into contact with moisture such as condensed water. Considering the Pourbaix diagram, which shows the stable state of chemical forms for each potential and pH, the reactions related to the dissolution of Cu in an aqueous solution can be expressed by the following formulas (1) and (2). Cu → Cu 2+ +2e - (1) (anode reaction) O2+2H2O+4e - → 4OH - (2) (Cathode reaction)

[0035] Using formulas (1) and (2), the dissolution of Cu from the copper phase to the aqueous phase can be expressed by the following formulas (3) and (4). The Cu dissolved in the aqueous phase can be considered to form copper(II) hydroxide through the equilibrium reaction represented by formula (4). Cu+O2+2H2O → Cu 2+ +4OH - (3) Cu 2+ +4OH - ⇔ Cu(OH)2+2OH - (4)

[0036] In addition, in an ammonia environment, the reaction represented by the following formula (5) is said to occur. It is believed that Cu dissolved in the aqueous phase forms an aqua complex ion, which generates tetraamminecopper(II) ions through the equilibrium reaction represented by formula (5). [Cu(H2O)4] 2+ +4NH3⇔[Cu(NH3)4] 2+ +4H2O···(5)

[0037] The amount of Cu eluted from the copper phase to the aqueous phase is considered to depend on the dissolved oxygen concentration in the aqueous phase, the concentration of copper(II) hydroxide, and the pH of the aqueous phase, when considering equations (3) and (4). Also, when considering equation (5), it is considered to depend on the ammonia concentration. Since the equilibrium of equation (4) shifts due to equation (5), there is a concern that Cu elution will continue.

[0038] Based on these findings, the present inventors considered that suppressing the dissolution of Cu from the copper phase into the aqueous phase and suppressing the formation of complex ions involving Cu would be effective in suppressing SCC of copper-based materials. As a means for suppressing the dissolution of Cu, the suppression of the anode reaction that dissolves Cu, the detoxification of P in the copper phase, and the detoxification of O involved in the cathode reaction are candidates. As a means for suppressing the formation of complex ions, the removal of complex-forming components such as ammonia and the inhibition of complex formation reactions are candidates.

[0039] The inventors also took into consideration that SCC does not occur in oxygen-free copper that does not contain P, that P in the copper phase is likely to dissolve into the aqueous phase and dissolves into the aqueous phase, lowering the pH, that at low pH, among the chemical forms of Cu, copper ions are more stable than oxides and hydroxides, promoting the dissolution of Cu from the copper phase into the aqueous phase, and that when P is contained in the copper phase, the electrode potential of the copper phase decreases, promoting the dissolution of Cu from the copper phase into the aqueous phase.

[0040] As a result, the inventors discovered a method for eliminating the effect of P by adding a specific additive element to Cu to address the problem of SCC accelerated by P, and completed the present invention, which suppresses SCC. It was believed that the following characteristics (1) and (2) of the additive element would be effective.

[0041] (1) It is a base metal element with a lower potential than H in terms of the standard electrode potential. Such base metal elements cause an anodic reaction at a lower potential than the anodic reaction that dissolves Cu. In other words, they cause an anodic reaction that is a counter reaction to the cathodic reaction involving oxygen. In addition, because their potential is lower than the hydrogen electrode potential, they suppress the decrease in pH of the aqueous phase even if P dissolves from the copper phase into the aqueous phase. Therefore, this anodic reaction and pH suppression effect can suppress the dissolution of Cu from the copper phase into the aqueous phase.

[0042] (2) It is a phosphorus compound-forming element that reacts with P in the copper phase to form a phosphorus compound. The phosphorus compound-forming element acts to fix P in the copper phase. When P is fixed in the copper phase, the elution of P from the copper phase into the aqueous phase is suppressed, and the pH of the aqueous phase is less likely to decrease. This pH-suppressing effect can suppress the elution of Cu from the copper phase into the aqueous phase.

[0043] Here, the chemical composition of the copper alloy according to this embodiment will be described in more detail. In the following description, the notation "%" means mass % unless otherwise specified.

[0044] (base metal element) As the base metal element, an element having a standard electrode potential lower than H is preferable, and an element having a potential equal to or lower than that of Mn is more preferable. With such an element, even if P dissolves from the copper phase into the aqueous phase, the effect of suppressing the decrease in pH of the aqueous phase can be obtained. In addition, if the standard electrode potential is equal to or lower than that of Mn, the effect of phosphorus compound forming elements such as Mn can also be obtained.

[0045] Specific examples of base metal elements include Mn, Al, Group 1 elements, and Group 2 elements. Examples of Group 1 elements include Li, Na, K, etc. Examples of Group 2 elements include Mg, Ca, Ba, etc. These elements cause an anodic reaction at a lower potential than Cu, so that the anodic reaction that dissolves Cu can be suppressed. In addition, even if P dissolves from the copper phase into the aqueous phase, the decrease in pH of the aqueous phase can be suppressed. These pH suppressing effects can effectively suppress the dissolution of Cu from the copper phase into the aqueous phase.

[0046] As the base metal element, a phosphorus compound forming element that reacts with P to form a phosphorus compound is preferable. Examples of phosphorus compound forming elements include Mn, Mg, and Ca. These elements can immobilize P in the copper phase and suppress the elution of P from the copper phase to the aqueous phase. Since the decrease in pH of the aqueous phase is suppressed, among the chemical forms of Cu, oxides and hydroxides become more stable than copper ions, and the elution of Cu from the copper phase to the aqueous phase can be effectively suppressed.

[0047] As the base metal element, one kind of element or a plurality of kinds of elements may be added to the copper base material. As the base metal element, only phosphorus compound forming elements may be added to the copper base material, only non-phosphorus compound forming elements other than the phosphorus compound forming elements may be added, or a combination of phosphorus compound forming elements and non-phosphorus compound forming elements may be added to the copper base material.

[0048] The amount of base metal elements is preferably 0.01% or more, more preferably more than 0.01%, even more preferably 0.03% or more, even more preferably 0.05% or more, and even more preferably 0.10% or more, in terms of the total amount of base metal elements. With such an amount of base metal elements, it is possible to obtain an effect of suppressing the dissolution of Cu from the copper phase into the aqueous phase. The amount of base metal elements may be 0.15% or more, or 0.20% or more, in terms of the total amount of base metal elements.

[0049] The amount of base metal elements is preferably 1.5% or less, more preferably 1.0% or less, even more preferably 0.5% or less, even more preferably 0.25% or less, even more preferably 0.20% or less, even more preferably 0.15% or less, in terms of the total amount of base metal elements. If the amount of base metal elements is too large, the raw material cost increases and the difficulty of casting increases. In addition, the mechanical properties and electrical properties change, making it difficult to use as a copper tube. However, with such an amount of base metal elements, the raw material cost and the difficulty of casting can be suppressed while avoiding the influence on mechanical properties, brazing properties, etc. The amount of base metal elements may be 0.10% or less, or 0.05% or less, in terms of the total amount of base metal elements.

[0050] As the base metal element, it is preferable to add Mg or Mn. These base metal elements are elements that form phosphorus compounds. By adding these base metal elements, P in the copper phase is fixed as a phosphorus compound, and the decrease in pH of the aqueous phase due to the elution of P can be suppressed. In addition, good workability and brazing properties can be obtained.

[0051] (Mg: 0.01% or more, less than 0.25%) The Mg content is preferably 0.01% or more, more preferably 0.017% or more, even more preferably 0.03% or more, even more preferably 0.05% or more, and even more preferably 0.10% or more. With such an Mg content, the effect of suppressing the dissolution of Cu from the copper phase to the water phase is obtained, and good mechanical properties such as tensile strength and wettability of the brazing material are obtained. Furthermore, when the Mg content is 0.017% or more, sufficient SCC resistance can be obtained even with a P content of 0.015%, which is the upper limit of the standard for low phosphorus deoxidized copper. Furthermore, when the Mg content is 0.10% or more, sufficient SCC resistance can be obtained even with a P content of 0.04%, which is the upper limit of the standard for high phosphorus deoxidized copper.

[0052] The Mg content is preferably 0.25% or less, more preferably 0.20% or less, and even more preferably 0.15% or less. If the Mg content is 0.29% or more, the wettability of the brazing material is impaired. However, if the Mg content is 0.25% or less, good wettability of the brazing material can be obtained, and mechanical properties and high electrical conductivity can be appropriately ensured.

[0053] (Mn: 0.01% or more and 1.5% or less) The Mn content is preferably 0.01% or more, more preferably 0.03% or more, even more preferably 0.05% or more, even more preferably 0.10% or more, even more preferably 0.50% or more, even more preferably 1.0% or more, even more preferably 1.2% or more. With such a Mn content, the effect of suppressing the dissolution of Cu from the copper phase into the aqueous phase is obtained, and good mechanical properties such as tensile strength are obtained.

[0054] The Mn content is preferably 1.5% or less, more preferably 1.4% or less, and even more preferably 1.3% or less. If the Mn content exceeds 1.5%, the yield strength increases and the bending workability decreases. However, if the Mn content is 1.5% or less, good workability can be obtained and high electrical conductivity can be appropriately ensured.

[0055] (P: over 0% and up to 0.040%) P is added mainly for deoxidation. If the P content is too high, toughness and workability decrease, and SCC susceptibility and ant nest corrosion susceptibility increase. Therefore, the P content is preferably 0.040% or less. Depending on the allowable O content, etc., the P content may be more than 0% and less than 0.0003%, more than 0.0003% and less than 0.001%, more than 0.001% and less than 0.004%, 0.004% or more and less than 0.015%, or 0.015% or more and 0.040% or less.

[0056] (Inevitable impurities) The inevitable impurities are derived from substances that need to be added in the manufacture of copper alloys and copper alloy pipes, or substances that are difficult to completely separate and remove, and refer to elements of impurities that are inevitably mixed in from raw materials or during the manufacturing process. The copper alloy preferably contains more than 0% and not more than 0.040% by mass of P, 1.5% by mass or less of the total of base metal elements, and the remainder being Cu and inevitable impurities.

[0057] Specific examples of the unavoidable impurities include elements other than base metal elements, such as O, H, S, Pb, Bi, Se, Te, As, and Sb. The amount of the unavoidable impurities is preferably 0.1% or less, more preferably 0.05% or less, and even more preferably 0.01% or less, in terms of the total amount of each element. If the total amount of the unavoidable impurities is 0.1% or less, the effect of the present invention is not impaired.

[0058] (O: 0.01% or less) In a high-temperature environment such as furnace brazing under a hydrogen atmosphere, O reacts with H to generate water vapor, which causes hydrogen embrittlement. In addition, O forms oxides that reduce workability. Therefore, the O content is preferably 0.01% or less, more preferably 0.005% or less, and even more preferably 0.001% or less.

[0059] (Other elements) The amount of S is preferably 0.0018% or less. The amounts of Pb, Bi, Se, Te and C are each preferably 0.001% or less. The amounts of Zn, Cd and Hg are each preferably 0.0001% or less.

[0060] (Cu:98.5% or more) Cu constitutes the remainder of the copper alloy, excluding base metal elements and inevitable impurities. The amount of Cu is not particularly limited as long as it constitutes the remainder of the copper alloy. However, from the viewpoint of obtaining properties of the copper alloy equivalent to pure copper, such as thermal conductivity, bending workability, brazing property, etc., the amount of Cu is preferably 98.5% or more, more preferably 99.0% or more, even more preferably 99.5% or more, even more preferably 99.80% or more, and even more preferably 99.90% or more.

[0061] Next, the corrosion resistance against stress corrosion cracking (SCC) of the copper alloy according to this embodiment will be described.

[0062] The corrosion resistance against stress corrosion cracking (SCC) of the copper alloy according to this embodiment can be evaluated by the crack depth measured after applying a predetermined stress after exposing the copper alloy to a saturated environment with an aqueous ammonia solution in accordance with the technical standard JBMA T-301-1981 of the Japan Copper and Brass Association. The crack depth due to SCC is defined as the shortest distance from the surface of the copper alloy to the deepest point of the crack.

[0063] Ammonia testing in accordance with JBMA T-301-1981 is performed using a 14% by weight aqueous ammonia solution. The aqueous ammonia solution is prepared by diluting at least 25% by weight aqueous ammonia solution with an equal amount of pure water. The test environment temperature is room temperature, and the container containing the aqueous ammonia solution is kept in a room controlled at room temperature of 20°C ± 5°C. The test material is placed in a 10L test container containing the aqueous ammonia solution, so as not to come into direct contact with the aqueous ammonia solution. The test material is placed 100 mm away from the liquid surface of the aqueous ammonia solution. The exposure conditions to the ammonia atmosphere in the test container are 72 hours at room temperature.

[0064] The copper alloy test material for the ammonia test is either plate or tube material. After the exposure test to the ammonia atmosphere, in the case of plate material, as shown in Figure 2, the material is bent 180 degrees with the center line parallel to the rolling direction as the bending axis to apply external stress. In the case of tube material, the material is crushed from the radial direction to less than half of its outer diameter to apply external stress. In the case of plate material, the observation point for cracks due to SCC is the surface on the crest side when bent. In the case of tube material, the observation point is the outer surface of the bent part when crushed.

[0065] Surface defects may form on the surface of copper pipes during pipe manufacturing. Generally, the maximum depth of cracks caused by surface defects is about 0.03 mm. It is difficult to distinguish between shallow cracks caused by SCC and cracks caused by surface defects. Therefore, even if the crack depth caused by SCC exceeds 0.03 mm, if it is 0.05 mm or less, it can be determined that the corrosion resistance against SCC is good. Also, if it is 0.03 mm or less, it can be said that SCC has not actually occurred.

[0066] In the copper alloy according to the present embodiment, when P is 0.0065% by mass or more and 0.040% by mass or less, the copper alloy satisfies the following formula (I) when the Mg concentration in the copper alloy is Y [%] and the P concentration in mass % is X [%], and in accordance with JBMA T-301-1981, after exposure to a 14 mass% aqueous ammonia solution at a distance of 100 mm in a room controlled at room temperature of 20±5°C for 72 hours, the copper alloy sheet is bent 180 degrees, or the copper alloy pipe is crushed to half or less of its outer diameter, and the crack depth measured is preferably 0.05 mm or less. Y≧2X-0.0130 (0.0065≦X≦0.0400)···(I)

[0067] In the case where P is 0.0065% by mass or more and 0.040% by mass or less, if the Mg concentration satisfies the condition of formula (I), the crack depth due to SCC can be suppressed to 0.05 mm or less in an ammonia test conforming to JBMA T-301-1981. Since the progress of SCC is suppressed to a small extent in an ammonia environment in which SCC is promoted, a copper alloy with excellent SCC resistance can be obtained.

[0068] In the copper alloy according to the present embodiment, when P is 0.040% by mass or less, the copper alloy satisfies the following formula (II) where the Mg concentration in the copper alloy is Y [%] and the P concentration in mass % is X [%], and in accordance with JBMA T-301-1981, after exposure to a 14% by mass aqueous ammonia solution at a distance of 100 mm in a room controlled at room temperature of 20±5°C for 72 hours, when a copper alloy sheet is bent 180 degrees or when a copper alloy pipe is crushed to half or less of its outer diameter, the crack depth measured is more preferably 0.03 mm or less. Y≧2X (X≦0.0400) (II)

[0069] When P is 0.040 mass% or less and the Mg concentration satisfies the condition of formula (II), the crack depth due to stress corrosion cracking can be suppressed to 0.03 mm or less in an ammonia test conforming to JBMA T-301-1981. When P is less than 0.0065 mass%, SCC is unlikely to progress even when no base metal elements are added. However, even in this range, copper alloys still exhibit SCC susceptibility. Therefore, if a higher level of SCC resistance is to be guaranteed, the condition of formula (II) is required.

[0070] Cases where a higher level of SCC resistance is required include cases where copper alloys are used under conditions that accelerate the initiation or progression of SCC, such as when an ammonia source is present in a sealed environment such as a factory or a house, and cases where damage to copper alloys due to SCC must be strictly restricted, such as when copper alloys are used as materials for refrigerant piping, heat exchanger piping, etc., and a flammable refrigerant flows inside the copper alloy tube.

[0071] Next, applications of the copper alloy and copper alloy tube according to this embodiment, and methods for producing them will be described.

[0072] The copper alloy according to the present embodiment can be used as a material for various copper products. Examples of the copper products include pipe materials, plate materials, rod materials, wire materials, and formed materials of other shapes. The copper alloy is particularly preferably used as a material for copper alloy pipes.

[0073] The copper alloy tube may be an internally grooved tube with grooves formed on the inner surface, or a smooth tube with no grooves formed on the inner surface. The internally grooved tube has, for example, spiral grooves at a predetermined interval or linear grooves arranged parallel to each other over the entire circumference of the inner surface of the tube. When manufacturing an internally grooved tube as a copper alloy tube, the number of grooves, the bottom width of the grooves, the wall thickness of the grooves, the height of the fins between the grooves, the apex angle of the fins, the lead angle of the grooves with respect to the central axis of the tube, and the like can be set as appropriate conditions.

[0074] Copper alloy pipes contain base metal elements, so even if P is added for deoxidation, the occurrence and progression of SCC can be suppressed. Therefore, compared to oxygen-free copper, costs can be reduced, hydrogen embrittlement can be prevented by deoxidation with P, and SCC accelerated by P can be suppressed. Since the amount of P is less likely to have an effect, there is a greater degree of freedom in material selection.

[0075] In particular, the inner grooved pipe increases the surface area of ​​the copper alloy pipe and the grooves allow the fluid flowing through the copper alloy pipe to be agitated. Therefore, in applications where a refrigerant is passed through the pipe, high energy efficiency and high energy saving performance can be obtained. In addition, the increase in surface area and the improvement in energy efficiency allow the pipe through which the refrigerant passes to be made smaller.

[0076] The copper alloy tube can be manufactured by a manufacturing method including a casting step, a soaking step, a hot extrusion step, a rolling and drawing step, and an annealing step. The internally grooved tube can be manufactured by manufacturing a copper alloy tube, followed by a rolling process step and a final annealing step.

[0077] (Casting process) In the casting process, the raw material of the copper alloy is melted in a reducing atmosphere, a deoxidizer is added to adjust the amount of P, and then an ingot of a predetermined size is cast. As the raw material, electrolytic copper, ingots containing base metal elements, etc. can be used. As the deoxidizer, phosphorus copper, etc. can be used. As the casting method, billets, etc. can be cast using a semi-continuous casting method, etc.

[0078] (Soaking process) In the soaking process, the ingot such as the billet is homogenized by heat treatment. By homogenization, segregation of P and the like is removed and the added base metal elements are diffused. The heat treatment temperature is, for example, 680°C or higher and 950°C or lower. If the temperature is 680°C or higher, segregation of P and the like can be sufficiently removed. If the temperature exceeds 950°C, the homogenization effect reaches a plateau, but if the temperature is 950°C or lower, the heat treatment cost can be suppressed. The heat treatment time is, for example, 15 minutes to 2 hours.

[0079] (Hot extrusion process) In the hot extrusion process, a heated ingot such as a billet is hot extruded into a die with a mandrel inserted therein to form a blank pipe. The temperature of the hot extrusion is, for example, 680°C to 950°C. The processing rate in the hot extrusion can be any appropriate condition as long as cracks, surface defects, etc. are not generated. The blank pipe after extrusion is cooled, for example, by natural cooling. Note that piercing and rolling using a plug and roll die may be performed instead of the hot extrusion and rolling.

[0080] (Rolling and drawing process) In the rolling and drawing process, the formed mother tube is rolled using a mandrel and drawn to form a drawn mother tube. The processing rate in the rolling and drawing processes can be set to any suitable condition, but is preferably 95% or less from the viewpoint of reducing cracks, surface defects, etc. The drawing process can be performed in an appropriate number of passes using a continuous drawing machine using a plug, etc. The processing rate in each pass is preferably 40% or less from the viewpoint of reducing cracks, surface defects, etc.

[0081] (Annealing process) In the annealing step, the processed drawn blank pipe is annealed by heat treatment. By annealing, processing strain is removed and the pipe is softened. Annealing can be performed using a roller hearth furnace, a high-frequency induction heating furnace, or the like. The heat treatment temperature is, for example, 350°C or higher and 700°C or lower, and preferably 350°C or higher and 500°C or lower. If the temperature is 350°C or higher, processing strain can be appropriately removed. The heat treatment time is, for example, 5 minutes to 2 hours.

[0082] By the above steps, a smooth copper alloy tube can be manufactured. An internally grooved tube can be manufactured by performing groove rolling on a drawn blank tube.

[0083] (Rolling process) In the rolling process, the drawn blank tube is subjected to groove rolling to form grooves on the inner surface of the drawn blank tube. The groove rolling process can be performed, for example, by roll rolling using a grooved plug. A grooved plug having a reverse groove formed therein for transferring the groove is inserted into the blank tube. The blank tube is then pulled out while being pressed by a rotating roll die, forming grooves on the inner surface of the blank tube. The groove rolling process can also be performed continuously from the diameter reduction pass of the drawing process, using a grooved plug connected to a diameter reduction plug. Also, a ball die with a bearing structure can be used instead of the roll die.

[0084] (Final annealing process) In the final annealing step, the grooved pipe is subjected to final annealing by heat treatment. The final annealing can be performed in a roller hearth furnace, a high-frequency induction heating furnace, or the like, as in the annealing step. The temperature of the heat treatment is, for example, 350°C or higher and 700°C or lower, preferably 350°C or higher and 500°C or lower. The time of the heat treatment is, for example, 5 minutes or higher and 2 hours or lower.

[0085] By the above steps, a copper alloy tube having an inner groove can be manufactured. The manufactured copper alloy tube can be subjected to processing such as straightening, chamfering, and cutting, and can also be subjected to an appearance inspection. After drawing and before annealing, the drawn blank tube can also be subjected to straightening, chamfering, and flaw detection inspection.

[0086] Copper alloy pipes can be used for various purposes. Examples of the uses of copper alloy pipes include heat exchanger pipes, refrigerant pipes, hot water supply pipes, and water supply pipes. Examples of heat exchanger pipes include pipes that are connected to fins or the like to form a heat exchanger. Examples of refrigerant pipes include pipes that allow refrigerants to flow. Examples of hot water supply pipes and water supply pipes include pipes that allow hot water, warm water, and cold water to flow.

[0087] These pipes can be provided in refrigeration and air conditioning equipment, heat exchangers, water heaters, etc. Refrigeration and air conditioning equipment includes air conditioners, refrigerators, vapor compression refrigerators, absorption refrigerators, etc. Absorption refrigerators include ammonia types and lithium bromide types. In the lithium bromide type, ammonia may inevitably remain during the refrigerant refining process. Ammonia is one of the environmental factors that promote SCC. The copper alloy tube according to this embodiment is particularly effective for applications used in an ammonia environment.

[0088] The copper alloy tube is preferably used as a material for a heat exchanger. Examples of the heat exchanger include various types of heat exchangers such as fin tube type, corrugated tube type, and double tube type. The copper alloy tube may be used in a straight pipe section, or in a curved pipe section such as a U-shaped bend section or a spirally wound section around a main pipe. The heat exchanger using the copper alloy tube may be used in, for example, an air conditioner, a refrigeration showcase, a refrigerator, an oil cooler, a radiator, etc.

[0089] FIG. 1 is a schematic diagram showing an example of a heat exchanger equipped with copper alloy tubes. 1, the heat exchanger 30 includes a plurality of fins 10 and a heat transfer tube 20. The fins 10 are arranged at predetermined intervals, and air passages are formed between the fins 10. The heat transfer tube 20 is bent into a U-shape at multiple locations, and is inserted into through holes on the fins 10 so as to pass through the plurality of fins 10, and is brazed.

[0090] The heat transfer tube 20 is formed of a copper alloy tube to which the base metal element is added. The copper alloy tube may be an inner grooved tube with grooves formed on the inner surface of the tube, or a smooth tube with no grooves formed on the inner surface of the tube. In a heat exchanger using a copper alloy tube, the base metal element suppresses SCC, so that a heat exchange medium such as a refrigerant is unlikely to leak for a long period of time, and a highly reliable heat exchanger can be obtained. EXAMPLES

[0091] The present invention will be specifically described below with reference to examples of the present invention, although the technical scope of the present invention is not limited thereto.

[0092] Test specimens of copper alloys containing base metal elements were prepared and their effects on stress corrosion cracking (SCC) and other material properties were evaluated. Other material properties evaluated included termite nest corrosion resistance, tensile strength, and brazing filler metal wettability. Test specimens of copper alloys containing different amounts of base metal elements were also prepared and the crack depth due to SCC was measured for each content.

[0093] The test materials were plates or tubes made by adding only the specified base metal elements to a copper alloy equivalent to phosphorus deoxidized copper. The plates were made by adjusting the chemical composition and casting the raw material, followed by hot rolling, cold rolling, and annealing, in that order. The tubes were made by adjusting the chemical composition and casting the raw material, followed by hot extrusion, cold drawing, and annealing, in that order.

[0094] (Evaluation of resistance to ant nest corrosion) The evaluation of ant nest corrosion resistance was carried out using the plate material as the test material in the following procedure. First, a 200 mm long test material was placed in a test container filled with a corrosive liquid. A plastic bottle was used as the test container, and a hole was made in the lid of the plastic bottle, and a silicon plug was inserted into the hole to hold the plate material. The plate material was placed at a height where it would not come into direct contact with the corrosive liquid, and a 100 mm long section of the plate material was exposed to the test environment inside the plastic bottle. In order to unify the direction of corrosion occurring on the plate material, the plate material was covered with silicon resin except for the observation surface. The test container was then sealed and placed in a drying oven with a specified heat cycle, and the plate material was left to stand while the heat cycle was repeated for the specified test time. The test material was then embedded in acrylic resin or epoxy resin, and the ant nest corrosion occurring on the test material was observed by cross-sectional observation.

[0095] The conditions for evaluating the resistance to termite nest corrosion are as follows. - Dimensions of test material: Width 10~13mm x Length 200mm x Thickness 1.0mm (Part of the test material was covered with rubber material, so that only one side was exposed to the corrosive environment inside the test vessel.) Test container: 2L plastic container Etching solution: 500mL of 0.5% by volume formic acid solution Test atmosphere: The replacement gas was oxygen gas taken from a cylinder of industrial oxygen (purity 99.5 vol.% or more) via indoor dedicated piping and a connected silicon tube. Oxygen gas was introduced as replacement gas at a flow rate of 1 L / min for 5 minutes through a silicon tube inserted 100 mm or more into a 2 L plastic container, creating an oxygen atmosphere inside the container. Temperature conditions (heat cycle conditions of the drying oven): Repeatedly hold at 20°C for 2 hours, then hold at 40°C for 22 hours Test duration: 60 days

[0096] The evaluation of ant nest corrosion resistance was based on the following criteria. The maximum corrosion depth due to ant nest corrosion was measured as the distance from the surface of the test material to the deepest point of corrosion. Three cross sections (with an interval of 1 mm or more between cross sections) were observed for each test material, and the maximum distance among these was calculated as the maximum corrosion depth. 〇: Maximum corrosion depth 0.25mm or less → Good resistance to termite nest corrosion ×: Maximum corrosion depth exceeds 0.25 mm → Poor resistance to ant nest corrosion

[0097] (Evaluation of tensile strength) The evaluation of tensile strength was carried out under the following conditions using plate materials as test materials: The tensile strength was measured using a tensile testing machine.

[0098] - Dimensions of test material: Width 10mm x Length 200mm x Thickness 0.1mm Test method: Compliant with JIS Z2241:2011 Metallic material tensile test method, using rectangular test pieces.

[0099] The tensile strength evaluation is based on the following criteria: 〇: Tensile strength 280N / mm 2 Above → Maintains workability such as bending ×: Tensile strength 280N / mm 2 Over → Workability such as bending is reduced

[0100] (Evaluation of stress corrosion cracking resistance) The evaluation of stress corrosion cracking resistance was performed by using ammonia test according to JBMA T-301-1981 with plate or pipe material as the test material, as follows. First, the test material was placed horizontally above the middle plate in the test container containing the corrosive liquid, at a height where it did not come into direct contact with the corrosive liquid. Figure 2 shows the sampling method for the test material in the case of plate material. When using plate material as the test material, as shown in Figure 2, the plate material was cut out from the rolled material to have a width of 10 to 13 mm, a length of 25 mm, and a thickness of 1 mm, and placed above the middle plate so that the front and back faces in the up-down direction. In the case of pipe material, a length of 20 mm was cut out. Resin-coated copper wires with a diameter of 2.5 mm were placed between both ends of the test material and the middle plate to prevent direct contact between the test material and the middle plate. Next, the test container was sealed and left to stand for a specified test time. The test material was then taken out of the test vessel, pickled with sulfuric acid, and subjected to an external stress as a pretreatment.

[0101] Figure 2 is a diagram showing the method of pretreatment of test materials made of copper alloy. The upper left diagram of Figure 2 shows the copper alloy block before rolling used to prepare the test materials. The upper right diagram of Figure 2 shows the rolled material obtained by rolling the copper alloy block and the cutting positions of the test materials. The lower diagram of Figure 2 shows the bending positions of the test materials to apply stress. As shown in Figure 2, when the test material was a plate, it was bent 180 degrees around the center line parallel to the rolling direction so that the surface that was on the top during exposure to the test environment was on the outside. On the other hand, for pipe material, it was crushed uniaxially along the radial direction, which is the rolling direction, until the outer diameter was reduced to half or less, for example, about 4 mm in the case of Φ9.52 mm. The appearance of the crushed surface side was observed with an optical microscope (x69 magnification) for the presence or absence of cracks. A cross section of the area with severe cracks from the outside was cut out and embedded in acrylic resin or epoxy resin, and the cracks on the cross section were observed with an optical microscope (x150 magnification). When there were multiple areas with severe cracks, the test material was divided and the cross section was observed.

[0102] Fig. 3 is a diagram showing a method for measuring the crack depth due to stress corrosion cracking. Fig. 4 is an enlarged view of a main part of Fig. 3. Fig. 4 corresponds to an enlarged view of a rectangular area S in Fig. 3. As shown in Figure 3, the original outer surface of the test material does not exist at the location where stress corrosion cracking occurred. Therefore, a cross-sectional image of the test material was taken, and a virtual line representing the outer surface of the test material was inserted by image processing to measure the maximum crack depth due to stress corrosion cracking.

[0103] As shown in Figure 3, points B and B' were set on the outer surface at 45 degrees to the left and right of point A on the valley side as the center. Then, a circular arc passing through points B and B' was interpolated as an imaginary surface with point A as the center. An imaginary line C passing through point A and the deepest point of the crack was drawn, and the intersection point between imaginary line C and the arc-shaped imaginary curve was obtained. The shortest distance from this intersection point to the deepest point of the crack was measured as the crack depth due to SCC. The maximum corrosion depth was taken as the maximum crack depth among the cracks observed between arcs B-B' in the cross section observed for a specified number of measurements.

[0104] The conditions for evaluating the stress corrosion cracking resistance are as follows. Plate dimensions: Width 10~13mm x Length 25mm x Thickness 1.0mm Tube dimensions: outer diameter 9.52mm x thickness 0.8mm x length 20mm Test container: 10L desiccator Corrosive solution: 100 mL of 14% ammonia water (commercially available 25% or more ammonia water solution diluted with an equal amount of pure water) Temperature conditions: The test temperature was room temperature, and the room temperature in the room where the test container was kept was controlled within 20℃±5℃ by an air conditioner. Exposure conditions: 100mm from the surface of the corrosive liquid Test duration: up to 72 hours

[0105] The evaluation of stress corrosion cracking resistance was based on the following criteria: When the maximum crack depth was 0.03 mm or less, it was not considered to be a crack caused by SCC because it was difficult to distinguish it from surface defects during pipe manufacturing. ◎: Maximum crack depth is 0.03 mm or less → Excellent stress corrosion cracking resistance 〇: Maximum crack depth is over 0.03mm and 0.05mm or less → Good stress corrosion cracking resistance ×: Maximum crack depth exceeds 0.05 mm → poor stress corrosion cracking resistance

[0106] (Evaluation of wettability of brazing filler metal) The wettability of the brazing filler metal was evaluated under the following conditions using a plate material as the test material. First, the test material was bent 90 degrees along the center line in the longitudinal direction. Then, a rod-shaped brazing filler metal was placed in the center of the valley side of the test material. The test material with the brazing filler metal placed thereon was heated under specified heating conditions and then cooled. After that, the longitudinal length of the brazing filler metal that had spread over the surface of the test material was measured.

[0107] The conditions for evaluating the wettability of the brazing material are as follows. - Dimensions of test material: Width 30mm x Length 100mm x Thickness 1.0mm Brazing material type: Phosphorus copper brazing BCuP-2 (diameter 1.6mm x length 20mm) Heating equipment: Infrared gold image furnace (ULVAC) Heating atmosphere: Nitrogen gas atmosphere Heating conditions: Heat from room temperature to 850℃ at a rate of 850℃ / 5 minutes Holding conditions: 850℃ for 5 minutes ·Cooling conditions: natural cooling

[0108] The wettability of the brazing material is evaluated according to the following criteria. 〇: The length of the brazing filler metal in the longitudinal direction is 100 mm or more → The brazing filler metal has good wettability ×: The length of the brazing filler metal in the longitudinal direction is less than 100 mm → The wettability of the brazing filler metal is poor

[0109] (Analysis of the chemical composition of the test material) The chemical composition of the test material was analyzed using a PDA-7000 optical emission spectrometer (manufactured by Shimadzu Corporation) under the following conditions in accordance with "5. Spark discharge optical emission spectroscopic analysis" of JIS K0116:2014 General rules for optical emission spectroscopic analysis.

[0110] The conditions for the analysis of the chemical composition are as follows: Analysis atmosphere: High-purity argon gas atmosphere (99.9995% by volume) Electrode spacing: 7mm (discharge gap distance) Quantitative method: Fixed-time integration by intensity ratio method Pre-discharge: 1500 pulses Main discharge: 1200 pulses (discharge time used as integration time)

[0111] Chemical composition analysis was performed at three random points on the surface of the test material after annealing. For plate materials, measurements were taken on the smooth main surface. For tube materials, measurements were taken on the smooth outer surface after crushing the tube material. The average of the measurements taken at each point was calculated as the measurement result for each test material.

[0112] The measurement wavelengths of the spark discharge optical emission spectrometry used in the analysis of the chemical composition and the measurement sensitivity at each wavelength are as follows: Al: Wavelength 396.1nm, Measurement sensitivity 44 Cu: wavelength 296.1nm, measurement sensitivity 24 Mg: Wavelength 285.2nm, Measurement sensitivity 24 P: wavelength 178.3nm, measurement sensitivity 52

[0113] Table 1 shows the chemical composition of the test material (target value of Mg content) and the evaluation results of termite nest corrosion resistance, tensile strength, stress corrosion cracking resistance, and brazing filler metal wettability. The overall judgment is a comprehensive evaluation of these.

[0114] [Table 1]

[0115] As shown in Table 1, in Examples 1 and 2, the Mg concentration was 0.01 mass% and 0.25 mass%, and the wettability of the brazing material met the standard. In Comparative Example 1, the Mg concentration was 0.29 mass%, and the wettability of the brazing material did not meet the standard. From the viewpoint of the wettability of the brazing material, it can be said that the Mg concentration is preferably 0.25 mass% or less.

[0116] Table 2 shows the chemical composition of the test material (analytical values ​​of Mg content, Mn content, and P content), the measurement results of the maximum crack depth due to stress corrosion cracking, and the evaluation results of stress corrosion cracking resistance.

[0117] [Table 2]

[0118] Fig. 5 is a diagram showing the relationship between crack depth due to stress corrosion cracking and P concentration. In Fig. 5, the vertical axis indicates the crack depth [μm] measured in the test material, and the horizontal axis indicates the P concentration [mass%] of the test material. The ◯ plots are the results of the test material according to the embodiment in which the Mg content is 0.1 mass%. The ◇ plots are the results of the test material according to the comparative example in which no base metal element is added.

[0119] As shown in Figure 5, when the P concentration of the copper alloy is low, cracks due to stress corrosion cracking are suppressed regardless of whether base metal elements are added. When the crack depth is 30 μm or less, it is difficult to distinguish it from surface defects during pipe manufacturing, so it can be said that cracks due to SCC do not occur. Therefore, it can be said that the addition of base metal elements is particularly effective when the P concentration is 0.0065 mass% or more.

[0120] Fig. 6 is a diagram showing the relationship between Mg concentration and P concentration versus crack depth due to stress corrosion cracking. In Fig. 6, the vertical axis shows the Mg concentration [mass%] of the test material, and the horizontal axis shows the P concentration [mass%] of the test material. The plots marked with black circles show the results for test materials with a maximum crack depth of 30 µm or less. The plots marked with a triangle show the results for test materials with a maximum crack depth of more than 30 µm and less than 50 µm. The plots marked with black circles show the results for test materials with a maximum crack depth of more than 50 µm.

[0121] 6, the upper dashed line indicates a straight line Y=2X-0.0130 expressed by formula (I) when the Mg concentration is Y [%] and the P concentration is X [%]. The lower dashed line indicates a straight line Y=2X expressed by formula (II) when the Mg concentration is Y [%] and the P concentration is X [%]. These straight lines were determined as linear boundary conditions based on the results of Examples 1-1 to 1-20 shown in Table 2.

[0122] 6, Examples 1-1 to 1-14 satisfied the relationship represented by formula (II): Y≧2X (X≦0.0400), and the amount of base metal elements was appropriate relative to the amount of P, so that the stress corrosion cracking resistance met the criterion of 0.03 mm or less. Example 1-13 obtained good stress corrosion cracking resistance due to 0.041 mass% Mg relative to 0.02 mass% P, and the stress corrosion cracking resistance met the criterion of 0.03 mm or less.

[0123] Since Example 1-15 did not contain P, the stress corrosion cracking resistance satisfied the standard of 0.03 mm or less even though no base metal elements were added.

[0124] In Example 1-16, the stress corrosion cracking resistance did not meet the standard of 0.03 mm or less because of the trace amount of P, but met the standard of 0.05 mm or less even though no base metal elements were added. However, compared to Example 1-15, the crack depth increased with increasing P.

[0125] Example 1-17 did not satisfy the relationship represented by formula (II): Y≧2X (X≦0.0400), but met the criterion of 0.05 mm or less in stress corrosion cracking resistance because of the trace amount of P. Examples 1-18 to 1-19 satisfied the relationship represented by formula (I): Y≧2X-0.0130 (0.0065≦X≦0.0400), and the amount of base metal elements was appropriate relative to the amount of P, so that the stress corrosion cracking resistance met the criterion of 0.05 mm or less.

[0126] In Example 1-21, the base metal element was Mn, but the amount of the base metal element was appropriate relative to the amount of P, so the stress corrosion cracking resistance met the standard of 0.03 mm or less. It was revealed that even if the base metal element was Mn, the growth of cracks due to stress corrosion cracking was suppressed.

[0127] In Comparative Examples 1-1 to 1-5, the crack depth after the stress corrosion cracking test exceeded 0.05 mm, and did not satisfy the relationship represented by formula (I) or formula (II). Comparative Examples 1-2 and 1-3 did not contain base metal elements, and Comparative Example 1-2 in particular corresponds to low phosphate copper (JIS H3300 C1201), but did not satisfy the standard of 0.05 mm or less in stress corrosion cracking resistance. In Comparative Examples 1-1 and 1-4 to 1-5, the amount of base metal elements was insufficient relative to the amount of P, and no effect of improving corrosion resistance against stress corrosion cracking was observed.

Claims

1. A copper alloy to which a base metal element having a standard electrode potential equal to or lower than that of Mn is added, wherein P is more than 0 mass % and not more than 0.040 mass %, the total amount of the base metal elements is 0.059 mass % or more and 1.5 mass % or less, the balance being Cu and unavoidable impurities, the base metal elements being one or more of Mn, Al, Group 1 elements, and Group 2 elements, and when the base metal elements include Mn, Mn is 1.0 mass % or more and 1.3 mass % or less.

2. 2. The copper alloy of claim 1, The copper alloy contains a base metal element that forms a phosphorus compound with phosphorus.

3. 3. The copper alloy of claim 2, A copper alloy in which the base metal element is Mg or Mn.

4. 4. The copper alloy of claim 3, When the base metal element contains Mg, the copper alloy has Mg: 0.25 mass% or less.

5. 5. The copper alloy of claim 4, P: 0.0065% by mass or more and 0.040% by mass or less; When the Mg concentration of the copper alloy is Y [%] and the P concentration is X [%], the following formula (I) is satisfied: A copper alloy in which, in accordance with JBMA T-301-1981, the copper alloy is exposed to a 14 mass % aqueous ammonia solution at a distance of 100 mm for 72 hours in a room controlled at a room temperature of 20±5°C, and then a sheet material of the copper alloy is bent at 180 degrees, or a pipe material of the copper alloy is crushed to half or less of its outer diameter, and the crack depth measured is 0.05 mm or less. Y≧2X-0.0130...(I)

6. 5. The copper alloy of claim 4, P: 0.040% by mass or less, When the Mg concentration of the copper alloy is Y [%] and the P concentration is X [%], the following formula (II) is satisfied: A copper alloy in which, in accordance with JBMA T-301-1981, the copper alloy is exposed to a 14 mass % aqueous ammonia solution at a distance of 100 mm for 72 hours in a room controlled at a room temperature of 20±5°C, and then a plate material of the copper alloy is bent at 180 degrees, or a pipe material of the copper alloy is crushed to half or less of its outer diameter, and the crack depth measured is 0.03 mm or less. Y≧2X (II)

7. 5. The copper alloy of claim 4, When the base metal element contains Mn, the copper alloy has Mn: 1.2 mass % or more and 1.3 mass % or less.

8. A copper alloy tube formed from the copper alloy according to any one of claims 1 to 7.

9. The copper alloy tube according to claim 8, A copper alloy pipe with grooves formed on the inside surface.

10. A heat exchanger using a copper alloy tube formed from the copper alloy according to any one of claims 1 to 7.

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