WO2024075797A1 - Corrosion-resistant copper alloy, copper alloy pipe, and heat exchanger - Google Patents

Abstract

Provided are a copper alloy that has improved corrosion resistance against stress corrosion cracking, and a copper alloy pipe and a heat exchanger in which said copper alloy is used. The copper alloy is a copper alloy in which a base metal element (such as Mg or Mn, for example) having a potential that is less than or equal to the potential of Mn at a standard electrode potential has been added. The copper alloy pipe is formed from the copper alloy in which a base metal element having a potential that is less than or equal to the potential of Mn at a standard electrode potential has been added. The heat exchanger uses the copper alloy pipe that is formed from the copper alloy in which a base metal element having a potential that is less than or equal to the potential of Mn at a standard electrode potential has been added.

Description

Corrosion-resistant copper alloys, copper alloy tubes and heat exchangers

The present invention relates to a corrosion-resistant copper alloy with improved corrosion resistance against stress corrosion cracking, and to a copper alloy tube and a heat exchanger using the same.

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

Copper is a material with excellent thermal conductivity, bending workability, and brazing properties. In addition, its high standard electrode potential gives it 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 lower manufacturing costs than oxygen-free copper, so it is widely used in applications where it is exposed to high temperatures, such as brazing.

Corrosion-resistant materials such as copper-based materials can suffer from stress corrosion cracking (SCC) due to the combination 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 noticeable in brass. However, in recent years, cases have also been reported in phosphorus-deoxidized copper pipes.

SCC of copper-based materials occurs under special conditions, such as in an ammonia environment, but does not progress with ammonia alone; it progresses in the presence of both ammonia and moisture. When moisture adheres to a copper-based material, ammonia dissolves in the water phase, causing erosion. SCC occurs when residual stress or external stress concentrates at the site of grain boundary erosion. SCC is a phenomenon in which tensile stress leads to cracking, but the form of corrosion in phosphorus-deoxidized copper is characterized by intergranular corrosion.

In areas 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 that refrigerant leakage could have an impact on global warming. As a measure to suppress SCC, from the perspective of material factors, one possible method is to make the crystal grains fine. Also, from the perspective of mechanical factors, one possible method is to reduce residual stress and external stress.

However, methods to reduce residual stress and external stress are difficult to implement in actual materials, and are not realistic measures to suppress SCC. Furthermore, methods to refine crystal grains do not allow the material to be annealed sufficiently, which significantly impairs workability and places significant limitations on applications. Under these circumstances, measures to suppress SCC are being considered from the perspective of chemical composition, etc.

Patent Document 1 describes a copper tube that has excellent corrosion resistance against ant nest corrosion and excellent corrosion resistance against SCC. This copper tube is made of a copper material containing 0.10 to 1.0 weight % P, with the remainder being Cu and unavoidable impurities, and the P concentration (P1) at the 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, P is prevented from concentrating at the grain boundaries, reducing the susceptibility to SCC (see paragraph 0011 of Patent Document 1).

Patent Document 2 describes a copper alloy that has high electrical conductivity and excellent stress relaxation resistance. This copper alloy has a Mg content of more than 0.001 mass% and not more than 0.01 mass%, and a P content of not more than 0.001 mass%. In addition, the H content is 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%.

Patent Document 3 describes pitting-resistant copper and copper alloy tubes that can prevent the occurrence of pitting corrosion. It also describes that when lithium bromide is used as the absorption liquid, 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 of Patent Document 3).

　Until now, 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.

(1) Grain boundaries are essentially sites where impurities and added P are likely to concentrate.
(2) P dissolves from the copper phase into the aqueous phase, the pH of the aqueous phase decreases as a result of the dissolution of P, and Cu dissolves therefrom. Of the chemical forms of Cu, copper ions become more stable than oxides or hydroxides.
(3) Distribution of pH, etc. occurs in areas where corrosion progresses.

Also, in Copper and Copper Alloys, Vol. 53, No. 1 (2014), pp. 128-133 (Electrochemical Approach to Elucidating the Mechanism of Ant Nest Corrosion), the following factor (4) is newly presumed. 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, even in SCC, the dissolution of P is considered to lead to further dissolution of copper ions through reaction (4), which is expected to lead to the progression of more serious SCC.

Japanese Patent Publication No. 2022-056871 Japanese Patent Publication No. 2022-022637 Japanese Patent Publication No. 2007-154221

　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, reducing its 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 problem of reduced strength and toughness.

Adding P to copper-based materials deoxidizes them, which helps prevent this type of hydrogen embrittlement. However, the addition of P causes problems, as it can lead to the initiation and progression of SCC.

Oxygen-free copper has low oxygen and phosphorus concentrations, making it not only less susceptible to hydrogen embrittlement, but also less susceptible to SCC. However, oxygen-free copper requires special casting equipment, such as vacuum melting and casting methods, which creates issues in terms of manufacturing costs and price.

On the other hand, although phosphorus-deoxidized copper is less susceptible to hydrogen embrittlement, it does contain P that is added during the melting process for deoxidation. Phosphorus-deoxidized copper also exists as low-phosphorus deoxidized copper, which has a P content of 0.004% by mass or more but less than 0.015% by 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, meaning that there are significant restrictions in terms of production costs and price.

In Patent Document 1, the P concentration at the grain boundaries is reduced in order to reduce the SCC susceptibility of copper material. However, this method adjusts the ratio of the P concentration at the grain boundaries to the P concentration within 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 thought to pose practical issues in terms of manufacturing efficiency and manufacturing equipment.

Patent Document 2 specifies the ratio of Mg to the sum of S, P, Se, Te, Sb, Bi, and As, as well as the content of H, O, and C, for copper alloys. However, this copper alloy contains only trace amounts of P and is considered to be equivalent to oxygen-free copper. Such a P content poses problems in terms of manufacturing costs and price. Furthermore, H can cause defects in the structure, which may ultimately promote SCC, but it is not directly involved in SCC in copper-based materials.

Under these circumstances, there is a demand for technology that can suppress SCC while controlling costs and preventing hydrogen embrittlement 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 eliminate the effects of P and directly suppress SCC in copper-based materials.

The present invention aims to provide a copper alloy with improved corrosion resistance to stress corrosion cracking, and a copper alloy tube and heat exchanger using the same.

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

The present invention provides a copper alloy with improved corrosion resistance against stress corrosion cracking, and a copper alloy tube and heat exchanger using the same.

FIG. 1 is a schematic diagram showing an example of a heat exchanger equipped with copper alloy tubes. FIG. 2 is a diagram showing a method of pretreatment of a test material formed of a copper alloy. 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 portion of FIG. FIG. 5 is a graph showing the relationship between crack depth due to stress corrosion cracking and P concentration. FIG. 6 is a graph showing the relationship between the Mg concentration and the P concentration and the crack depth due to stress corrosion cracking.

The following describes a copper alloy according to one embodiment of the present invention, as well as a copper alloy tube and a heat exchanger that use the same.

The copper alloy according to this embodiment is a copper alloy to which a base metal element is added that has a standard electrode potential equal to or lower than that of Mn. The base metal element is preferably a phosphorus compound-forming element that reacts with P to form a phosphorus compound. A preferred form of this copper alloy is one in which P is greater than 0% and not greater than 0.040% by mass, the total of the base metal elements is 0.01% by mass or more and 1.5% by mass or less, and the remainder is Cu and unavoidable impurities.

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

Copper containing oxygen is known to 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 action of the generated water vapor forms voids at the grain boundaries, causing a decrease in strength and toughness due to hydrogen embrittlement.

In addition, although rare, copper-based materials used for refrigerant piping and heat exchanger piping can sometimes suffer from ant nest corrosion. Ant nest corrosion is a type of corrosion in which tiny corrosion holes on the surface of a material cause ant nest-like erosion in the material. Ant nest corrosion occurs in the presence of oxygen and moisture, with corrosive agents being carboxylic acids such as formic acid and acetic acid, aldehydes such as formaldehyde and acetaldehyde, and alcohols.

These corrosive agents are thought to be generated by hydrolysis and deterioration reactions caused by moisture such as condensation in lubricants, 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 in specific locations, accelerating the corrosion process and causing it to progress in the thickness direction and penetrate the material in a short period of time.

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. Because 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. From the perspective of improving SCC resistance and suppressing other corrosion phenomena, it is necessary to eliminate the effects of P.

The mechanism by which P added to Cu promotes SCC in phosphorus-deoxidized copper-based materials has not been fully elucidated. However, the form of corrosion that causes 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 susceptible to corrosion under alkaline or non-oxidizing conditions, and it is presumed that oxygen is involved in the cathodic reaction.

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)
O ₂ + 2H ₂ O + 4e ⁻ → 4OH ⁻ (2) (cathode reaction)

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 → ^Cu2 ++4OH ^-... (3)
Cu ²⁺ + 4OH ⁻ ⇔ Cu(OH) ₂ + 2OH ⁻ ... (4)

In addition, it is said that the reaction represented by the following formula (5) occurs in an ammonia environment: Cu dissolved in the aqueous phase forms an aqua complex ion, which is believed to generate tetraamminecopper(II) ions through the equilibrium reaction represented by formula (5).
[Cu( _H2O ) ₄ ] ²⁺ + _4NH3 ⇔ [Cu( _NH3 ) ₄ ] ²⁺ + _4H2O ... (5)

The amount of Cu eluted from the copper phase to the aqueous phase is thought to depend on the dissolved oxygen concentration in the aqueous phase, the copper(II) hydroxide concentration, and the pH of the aqueous phase, when equations (3) and (4) are taken into consideration. Also, when equation (5) is taken into consideration, it is thought to depend on the ammonia concentration. Since equation (5) shifts the equilibrium of equation (4), there is concern that Cu elution will continue.

Based on these findings, the 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. Possible means of suppressing Cu dissolution include suppressing the anodic reaction that dissolves Cu, detoxifying P in the copper phase, and detoxifying O, which is involved in the cathodic reaction. Possible means of suppressing the formation of complex ions include removing complex-forming components such as ammonia and inhibiting complex formation reactions.

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.

As a result, the inventors discovered a method to address the problem of SCC accelerated by P by adding a specific additive element to Cu to eliminate the effects of P, leading to the completion of the present invention, which suppresses SCC. The inventors considered that the following characteristics (1) and (2) of the additive element would be effective.

(1) It is a base metal element with a lower potential than H at 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 act to suppress a decrease in the 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.

(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 immobilize P in the copper phase. When P is immobilized in the copper phase, the elution of P from the copper phase into the aqueous phase is suppressed, making it difficult for the pH of the aqueous phase to decrease. This pH suppression effect can suppress the elution of Cu from the copper phase into the aqueous phase.

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

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

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

As the base metal element, a phosphorus compound-forming element that reacts with P to form a phosphorus compound is preferred. Examples of phosphorus compound-forming elements include Mn, Mg, and Ca. These elements immobilize P in the copper phase, making it possible to suppress the elution of P from the copper phase into 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, making it possible to effectively suppress the elution of Cu from the copper phase into the aqueous phase.

As the base metal element, one type of element or multiple types 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 phosphorus compound forming elements may be added, or a combination of phosphorus compound forming elements and non-phosphorus compound forming elements may be added.

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. Such an amount of base metal elements can provide the 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.

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, and 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 high, 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 copper pipe. However, with such an amount of base metal elements, it is possible to suppress the raw material cost and the difficulty of casting while avoiding the effects 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.

As the base metal element, it is preferable to add Mg or Mn, and it is preferable to add at least one of Mg and Mn. In addition, both Mg and Mn may be added as the base metal element. 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 a decrease in the pH of the aqueous phase due to the elution of P can be suppressed. In addition, good workability and brazing properties can be obtained.

(Mg: 0.01% or more and 0.25% or less)
When Mg is included as a base metal element, 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 aqueous 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.

When Mg is included as a base metal element, the Mg content is preferably 0.25% or less, more preferably less than 0.25%, more preferably 0.20% or less, and even more preferably 0.15% or less. If the Mg content exceeds 0.25%, and in particular is 0.29% or more, the wettability of the brazing filler metal is impaired. However, if the Mg content is 0.25% or less, good brazing filler metal wettability can be obtained, and mechanical properties and high electrical conductivity can be appropriately ensured.

(Mn: 0.01% or more and 1.5% or less)
When Mn is contained as a base metal element, 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, and even more preferably 1.2% or more. With such an 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.

When Mn is included as a base metal element, 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 bending workability decreases. However, if the Mn content is 1.5% or less, good workability is obtained and high electrical conductivity can be appropriately ensured.

(P: more than 0% and 0.040% or less)
P is added mainly for deoxidation. If the P content is too high, toughness and workability are reduced, and SCC susceptibility and ant nest corrosion susceptibility are increased. Therefore, the P content is preferably 0.040% or less. 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, depending on the allowable O content, etc.

(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 0.040% by mass or less of P, 1.5% by mass or less of the total of base metal elements, and the balance being Cu and inevitable impurities.

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

(O: 0.01% or less)
O reacts with H to generate water vapor in a high-temperature environment such as furnace brazing in a hydrogen atmosphere, which causes hydrogen embrittlement. It also 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.

(Other elements)
The S content is preferably 0.0018% or less. Each of the Pb content, the Bi content, the Se content, the Te content, and the C content is preferably 0.001% or less. Each of the Zn content, the Cd content, and the Hg content is preferably 0.0001% or less.

(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 the 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.

Next, we will explain the corrosion resistance of the copper alloy according to this embodiment to stress corrosion cracking (SCC).

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 specified stress after exposing it to a saturated environment with an aqueous ammonia solution in accordance with the Japan Copper and Brass Association technical standard JBMA T-301-1981. 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.

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 10 L 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.

The copper alloy test material for the ammonia test is either plate or tube. After the exposure test to the ammonia atmosphere, in the case of plate material, as shown in Figure 2, the center line parallel to the rolling direction is bent 180 degrees (180°) as the bending axis to apply external stress. In the case of tube material, the tube 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, it is the outer surface of the bent part when crushed.

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 to SCC is good. Also, if it is 0.03 mm or less, it can be said that SCC has not actually occurred.

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, when the Mg concentration of the copper alloy is Y [%] and the P concentration is X [%] in mass %, it is preferable that the copper alloy satisfies the following formula (I), and when 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 room temperature of 20±5° C. in accordance with JBMA T-301-1981, the copper alloy sheet is bent 180 degrees, or the copper alloy tube 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 (0.0065≦X≦0.0400) (I)

When 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. In an ammonia environment in which SCC is promoted, the progression of SCC is suppressed to a small extent, so a copper alloy with excellent SCC resistance can be obtained.

In the copper alloy according to the present embodiment, when P is 0.040% by mass or less, and the Mg concentration in the copper alloy is Y [%] and the P concentration in mass % is X [%], it is more preferable that the copper alloy satisfies the following formula (II), and that after being exposed 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 in accordance with JBMA T-301-1981, when a copper alloy sheet is bent 180 degrees or when a copper alloy tube is crushed to half or less of its outer diameter, the crack depth measured is 0.03 mm or less.
Y≧2X (X≦0.0400) (II)

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 you want to guarantee a higher level of SCC resistance, the condition of formula (II) is required.

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

Next, we will explain the uses of the copper alloy and copper alloy tubes according to this embodiment, as well as the manufacturing methods thereof.

The copper alloy according to this embodiment can be used as a material for various copper products. Examples of copper products include pipes, plates, bars, wires, and other shaped materials. The copper alloy is particularly preferably used as a material for copper alloy pipes.

The copper alloy tube may be an internally grooved tube, which has grooves formed on the inner surface, or a smooth tube, which has no grooves formed on the inner surface. An internally grooved tube has, for example, spiral grooves at a specified interval around the entire circumference of the inner surface of the tube, or linear grooves arranged parallel to each other. 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 grooved portion, the height of the fins between the grooved portions, the apex angle of the fins, the lead angle of the grooves relative to the central axis of the tube, etc. can be set as appropriate.

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, and hydrogen embrittlement can be prevented by deoxidation with P, while SCC, which is accelerated by P, can also be suppressed. As it is less affected by the amount of P, there is greater freedom in material selection.

In particular, inner grooved pipes increase the surface area of the copper alloy pipe and the grooves allow the fluid flowing through the copper alloy pipe to be agitated. This allows for high energy efficiency and high energy saving performance in applications where refrigerants are passed through. Furthermore, the increased surface area and improved energy efficiency make it possible to reduce the size of the pipes through which the refrigerant passes.

Copper alloy tubes can be manufactured by a manufacturing method including a casting process, a soaking process, a hot extrusion process, a rolling and drawing process, and an annealing process. Inner grooved tubes can be manufactured by manufacturing copper alloy tubes, followed by a rolling process and a final annealing process.

(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 P content, 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, a billet, etc. can be cast using a semi-continuous casting method, etc.

(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.

(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 rolling using a plug and roll die may be performed instead of the hot extrusion and rolling.

(Rolling and drawing process)
In the rolling and drawing process, the formed mother tube is subjected to rolling using a mandrel and drawing to form a drawn mother tube. The working ratio in the rolling and drawing processes can be set to appropriate conditions, 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 working ratio in each pass is preferably 40% or less from the viewpoint of reducing cracks, surface defects, etc.

(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. The annealing can be performed using a roller hearth furnace, a high-frequency induction heating furnace, or the like. The temperature of the heat treatment is, for example, 350° C. or more and 700° C. or less, and preferably 350° C. or more and 500° C. or less. If the temperature is 350° C. or more, processing strain can be appropriately removed. The time of the heat treatment is, for example, 5 minutes to 2 hours.

The above process allows the manufacture of smooth copper alloy tubes. Inner grooved tubes can be manufactured by carrying out groove rolling on drawn bare tubes.

(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.

(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 more and 700° C. or less, preferably 350° C. or more and 500° C. or less. The time of the heat treatment is, for example, 5 minutes or more and 2 hours or less.

By using the above process, a copper alloy tube with 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 visual inspection. In addition, after drawing and before annealing, the drawn blank tube can also be subjected to straightening, chamfering, flaw detection, etc.

Copper alloy pipes can be used for a variety of purposes. Applications of copper alloy pipes include heat exchanger piping, refrigerant piping, hot water supply piping, water supply piping, etc. Examples of heat exchanger piping include piping that is connected to fins or the like to form a heat exchanger. Examples of refrigerant piping include piping that carries refrigerant. Examples of hot water supply piping and water supply piping include piping that carries hot water, warm water, and cold water.

These pipes can be installed in refrigeration and air conditioning equipment, heat exchangers, water heaters, etc. Refrigeration and air conditioning equipment includes air conditioners, freezers, vapor compression freezers, absorption freezers, etc. Absorption freezers include ammonia types and lithium bromide types. In lithium bromide types, ammonia may inevitably remain during the refrigerant refining process. Ammonia is a type of environmental factor that promotes SCC. The copper alloy tube of this embodiment is particularly effective for applications used in an ammonia environment.

Copper alloy tubes are preferably used as a material for heat exchangers. Examples of heat exchangers include fin tube type, corrugated tube type, double tube type, and various other types. Copper alloy tubes may be used in straight pipe sections, or in curved pipe sections such as U-shaped bends and helical wound sections around the main pipe. Heat exchangers using copper alloy tubes can be used, for example, in air conditioners, freezer showcases, refrigerators, oil coolers, radiators, and the like.

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 plurality of 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 thereto.

The heat transfer tube 20 is made of a copper alloy tube to which the above-mentioned base metal elements have been added. The copper alloy tube may be an internally 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 elements suppress SCC, so that it is difficult for the heat exchange medium such as a refrigerant to leak over a long period of time, and a highly reliable heat exchanger can be obtained.

The present invention will be specifically explained below with examples. However, the technical scope of the present invention is not limited to these examples.

　Copper alloy test specimens containing added base metal elements were prepared and their effects on stress corrosion cracking (SCC) and other material properties were evaluated. Other material properties evaluated included resistance to termite nest corrosion, tensile strength, and wettability of the brazing filler metal. In addition, copper alloy test specimens containing different amounts of base metal elements were prepared and the crack depth due to SCC was measured for each content.

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

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

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

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 (observed 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.
○: Maximum corrosion depth 0.25 mm or less → Good resistance to ant nest corrosion ×: Maximum corrosion depth over 0.25 mm → Poor resistance to ant nest corrosion

(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.

Dimensions of test material: Width 10 mm x Length 200 mm x Thickness 0.1 mm
Test method: The test was performed in accordance with JIS Z2241:2011 Metallic material tensile test method using a rectangular test piece.

The tensile strength evaluation is based on the following criteria:
○: Tensile strength of 280 N/ ^mm2 or more → bending workability etc. is maintained ×: Tensile strength of more than 280 N/ ^mm2 → bending workability etc. is deteriorated

(Evaluation of stress corrosion cracking resistance)
The evaluation of stress corrosion cracking resistance was performed by the following procedure using ammonia test according to JBMA T-301-1981 using plate or pipe material as the test material. First, the test material was stored horizontally above the middle plate in a test vessel containing a corrosive liquid, and placed at a height that did not directly contact the corrosive liquid. Figure 2 shows the sampling method for the test material in the case of plate material. When a plate material was used 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 and 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 vessel was sealed and left to stand for a predetermined test time. Then, the test material was taken out of the test vessel, pickled with sulfuric acid, and then external stress was applied as a pretreatment.

Fig. 2 is a diagram showing a method of pretreatment of a test material formed of a copper alloy. The upper left diagram of Fig. 2 shows a copper alloy ingot before rolling used to prepare the test material. The upper right diagram of Fig. 2 shows a rolled material obtained by rolling the copper alloy ingot and the cutting position of the test material. The lower diagram of Fig. 2 shows a bending position of the test material to apply stress.
As shown in FIG. 2, when the test material was a plate material, it was bent 180 degrees (180°) with the center line parallel to the rolling direction as the axis so that the upper surface during the exposure to the test environment was on the outside. On the other hand, for the tube 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 presence or absence of cracks was observed from the appearance of the crushed surface side with an optical microscope (×69 magnification). A cross section of a part with severe cracks from the appearance was cut out and embedded in acrylic resin or epoxy resin, and the cracks on the cross section were observed with an optical microscope (×150 magnification). When there were multiple points with severe cracks, the test material was divided and the cross section was observed.

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.

As shown in Figure 3, points B and B' were set on the outer surface at 45 degrees (45°) 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 a virtual surface with point A as the center. A virtual line C passing through point A and the deepest point of the crack was drawn, and the intersection of virtual line C and the arc-shaped virtual curve was obtained. The shortest distance from this intersection 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.

The conditions for evaluating the stress corrosion cracking resistance are as follows.
・Plate dimensions: Width 10-13 mm x Length 25 mm x Thickness 1.0 mm
・Pipe dimensions: outer diameter 9.52 mm x thickness 0.8 mm x length 20 mm
Test container: 10 L desiccator Corrosive solution: 100 mL of 14% by mass ammonia water (a commercially available 25% by mass or higher 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 in which the test container was held was controlled to within 20° C.±5° C. by an air conditioner.
Exposure conditions: 100 mm from the surface of the corrosive liquid
・Test time: Maximum 72 hours

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 a surface defect during pipe manufacturing.
◎: Maximum crack depth is 0.03 mm or less → Excellent stress corrosion cracking resistance ○: Maximum crack depth is more than 0.03 mm and less than 0.05 mm → Good stress corrosion cracking resistance ×: Maximum crack depth is more than 0.05 mm → Poor stress corrosion cracking resistance

(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 (90°) 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 predetermined heating conditions and then cooled. Then, the longitudinal length of the brazing filler metal that had spread over the surface of the test material was measured.

The conditions for evaluating the wettability of the brazing material are as follows.
Dimensions of test material: Width 30 mm x Length 100 mm x Thickness 1.0 mm
・Type of brazing material: Phosphorus copper brazing BCuP-2 (diameter 1.6 mm x length 20 mm)
Heating equipment: Infrared gold image furnace (ULVAC, Inc.)
Heating atmosphere: Nitrogen gas atmosphere Heating conditions: Heat from room temperature to 850°C at a heating rate of 850°C/5 minutes Holding conditions: Hold at 850°C for 5 minutes Cooling conditions: Natural cooling

The wettability of the brazing material is evaluated according to the following criteria.
○: The length of the brazing material in the longitudinal direction is 100 mm or more → the wettability of the brazing material is good. ×: The length of the brazing material in the longitudinal direction is less than 100 mm → the wettability of the brazing material is poor.

(Analysis of Chemical Composition of Test Materials)
The chemical composition of the test material was analyzed using an optical emission spectrometer PDA-7000 (manufactured by Shimadzu Corporation) in accordance with "5 Spark discharge optical emission spectroscopic analysis" of JIS K0116:2014 General rules for optical emission spectroscopic analysis. It was performed under the following conditions.

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: 7 mm (discharge gap distance)
Quantitative method: Fixed-time integration using the intensity ratio method Pre-discharge: 1500 pulses Main discharge: 1200 pulses (discharge time used as integration time)

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. The average values measured at each point were calculated and used as the measurement results for each test material.

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 396.1 nm, measurement sensitivity 44
Cu: Wavelength 296.1 nm, measurement sensitivity 24
Mg: Wavelength 285.2 nm, measurement sensitivity 24
P: Wavelength 178.3 nm, measurement sensitivity 52

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

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

Table 2 shows the chemical compositions of the test materials (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.

Figure 5 shows the relationship between crack depth due to stress corrosion cracking and P concentration. In Figure 5, the vertical axis shows the crack depth [μm] measured in the test material, and the horizontal axis shows the P concentration [mass%] of the test material. The plots marked with ○ are the results for the test material according to the embodiment in which the Mg content is 0.1 mass%. The plots marked with ◇ are the results for the test material according to the comparative example in which no base metal elements are added.

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 this from surface defects during pipe manufacturing, so it can be said that cracks due to SCC are not occurring. Therefore, the addition of base metal elements is particularly effective when the P concentration is 0.0065 mass% or more.

Figure 6 shows the relationship between Mg concentration and P concentration versus crack depth due to stress corrosion cracking. In Figure 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 ● show the results for test materials with a maximum crack depth of 30 μm or less. The plots marked with ▲ 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 ● show the results for test materials with a maximum crack depth of more than 50 μm.

In Figure 6, the upper dashed line indicates the straight line expressed by formula (I): Y = 2X - 0.0130 when the Mg concentration is Y [%] and the P concentration is X [%]. The lower dashed line indicates the straight line expressed by formula (II): Y = 2X 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.

As shown in Table 2 and Figure 6, Examples 1-1 to 1-14 satisfied the relationship expressed by formula (II): Y ≥ 2X (X ≤ 0.0400), and the amount of base metal elements was appropriate relative to the amount of P, so 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.

Example 1-15 does not contain P, so even though no base metal elements are added, the stress corrosion cracking resistance met the standard of 0.03 mm or less.

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

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

In Example 1-21, the base metal element is Mn, but because the amount of the base metal element is appropriate relative to the amount of P, the stress corrosion cracking resistance met the standard of 0.03 mm or less. It was revealed that even when the base metal element is Mn, the propagation of cracks due to stress corrosion cracking is suppressed.

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 expressed 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.

Although various embodiments have been described above, it goes without saying that the present invention is not limited to these examples. It is clear that a person skilled in the art can come up with various modified or revised examples within the scope of the claims, and it is understood that these also naturally fall within the technical scope of the present invention. Furthermore, the components in the above embodiments may be combined in any manner as long as it does not deviate from the spirit of the invention.

This application is based on a Japanese patent application (Patent Application No. 2022-162811) filed on October 7, 2022, the contents of which are incorporated by reference into this application.

As described above, this specification discloses the following:

(1) A copper alloy to which a base metal element has been added whose standard electrode potential is lower than that of Mn.
(2) The copper alloy according to (1) above,
The base metal element is a phosphorus compound forming element that forms a compound with P in the copper alloy.
(3) The copper alloy according to (1) or (2) above,
The copper alloy contains at least one of Mg and Mn as the base metal element.
(4) The copper alloy according to (3) above,
A copper alloy, in which, when Mg is contained as the base metal element, Mg is 0.25 mass% or less, and when Mn is contained as the base metal element, Mn is 1.5 mass% or less.
(5) The copper alloy according to any one of (1) to (4) above,
P: 0.0065% by mass or more;
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, which has a crack depth of 0.05 mm or less, measured when a sheet material of the copper alloy is bent at 180 degrees or a tube material of the copper alloy is crushed to half or less of its outer diameter after being 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 in accordance with JBMA T-301-1981.
Y≧2X−0.0130 (I)
(6) The copper alloy according to any one of (1) to (5) above,
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, which has a crack depth of 0.03 mm or less, measured when a sheet material of the copper alloy is bent at 180 degrees or a tube material of the copper alloy is crushed to half or less of its outer diameter after being 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 in accordance with JBMA T-301-1981.
Y≧2X (II)
(7) The copper alloy according to any one of (1) to (6) above,
The base metal element includes Mn,
A copper alloy containing 1.2 mass % or more and 1.5 mass % or less of Mn.
(8) A copper alloy tube formed from the copper alloy according to any one of (1) to (7) above.
(9) The copper alloy tube according to (8) above,
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 (1) to (7) above.

Claims (10)

1. A copper alloy to which base metal elements have been added that have a standard electrode potential lower than that of Mn.
2. 2. The copper alloy of claim 1,
   The base metal element is a phosphorus compound forming element that forms a compound with P in the copper alloy.
3. The copper alloy according to claim 1 or 2,
   The copper alloy contains at least one of Mg and Mn as the base metal element.
4. 4. The copper alloy of claim 3,
   A copper alloy, in which, when Mg is contained as the base metal element, Mg is 0.25 mass% or less, and when Mn is contained as the base metal element, Mn is 1.5 mass% or less.
5. The copper alloy according to any one of claims 1 to 4,
   P: 0.0065% by mass or more;
   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, which has a crack depth of 0.05 mm or less, measured when a sheet material of the copper alloy is bent at 180 degrees or a tube material of the copper alloy is crushed to half or less of its outer diameter after being 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 in accordance with JBMA T-301-1981.
   Y≧2X−0.0130 (I)
6. The copper alloy according to any one of claims 1 to 5,
   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, which has a crack depth of 0.03 mm or less, measured when a sheet material of the copper alloy is bent at 180 degrees or a tube material of the copper alloy is crushed to half or less of its outer diameter after being 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 in accordance with JBMA T-301-1981.
   Y≧2X (II)
7. The copper alloy according to any one of claims 1 to 6,
   The base metal element includes Mn,
   A copper alloy containing 1.2 mass % or more and 1.5 mass % or less of Mn.
8. A copper alloy tube formed from the copper alloy according to any one of claims 1 to 7.
9. 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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