GB1575271A - Paste solder for use in joining stainless steel pipes - Google Patents

Description

(54) PASTE SOLDER FOR USE IN JOINING STAINLESS STEEL PIPES (71) We, SENJU METAL INDUSTRY CO., LTD., and NISSHIN STEEL CO., LTD., Corporations organized and existing under the laws of the Japan, of No. 23, Senju Hashidocho, Adachi-ku, Tokyo, Japan and 4-1, Marunouchi 3-chome, Chiyoda-ku, Tokyo, Japan respectively, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention is directed to paste solder useful in joining stainless steel pipes.

It has been difficult to effect soldering of stainless steel. In particular, it is extremely difficult to provide a stainless steel pipe joint satisfactory from a practical viewpoint for use in water or hot water supply piping or other fluid supply piping where the soldered pipe joint must have high resistance to corrosion and a good hermetic seal. Some of the reasons for difficulty are: (1) Stainless steels are covered with a peculiar oxide film formed by the chromium contained therein. This oxide film is tenacious and hardly dissolvable and special flux is required to completely dissolve the oxide film during the soldering operation.

The thermal conductivity of stainless steel is as low as from 0.03 to 0.06 cal/cm. sec"C, which is very low as compared with the thermal conductivity of 0.94 cal/cm.sec C for copper. Therefore, it is not unusual that, when the conventional soldering method is used, the temperature distribution at the pipe joint during soldering operation becomes nonuniform, resulting in the formation of voids in the resulting joints due to uneven spreading of the molten solder which tends to flow to the high temperature areas. Therefore, it has been very difficult to solder the stainless steel pipes without the formation of big voids in the soldered joints. However, such voids in a soldered pipe joint are fatal defects in soldered pipe joints to be used in water or hot water supply piping in which resistance to relatively high pressure is required, because they adversely affect the durability against leakage.

(3) In soldering stainless steels, an aggressive flux comprising a metal chloride in combination with hydrochloric acid, etc. must be used in order to remove the hard-to-dissolve oxide film. (Japanese Patent Publications Nos. 3465/1950, 1114/ 1963 and 2787/ 1968) However, since, the flux residue cannot be completely washed off from the interior of the soldered joint and it has a strong corrosive action, the occurrence of pitting corrosion of the stainless steel material cannot be avoided. Therefore, the chloride-containing flux should not be used in soldering stainless steel.

(4) Noncorrosive fluxes such as phosphoric acid base flux or phosphoric acid base flux containing a small quantity of copper salts or tin salts can be used in the end-feed soldering method, in which a bar or wire-shaped solder is fed into the joint clearance during soldering operation, for soldering stainless steel pipes. (Japanese Patent Publications Nos. 2314/ 1961, 8421/1961. 15008/1961 and Japanes Patent Disclosure No. 59/1972). U.S. Patent 3,597,285 discloses a flux composition for use in soldering stainless steel, which comprises orthophosphoric acid or phosphorous acid and at least one member of finely divided copper and copper salts. However, a major portion of such a flux composition is comprised of phosphoric acid. and it contains copper or copper salts as an essential ingredient. Because of the narrow activation temperature range of such noncorrosive fluxes and the low thermal conductivity of stainless steel, the resulting pipe joint has many big voids and is unsatisfactory. It is extremely difficult to determine the most suitable time to feed solder into the joint clearance.

(5) It is difficult to provide proper accuracy of shape and size with the stainless steel pipe and fitting because of peculiar mechanical properties such as poor formability and springback action of stainless steels. Therefore, it is also difficult to realize a proper clearance ranging from 0.1 to 0.2 mm with close tolerance in the pipe joint.

It is critical for molten solder to penetrate deep into the pipe joint, but when the joint clearance is too wide or varied, big voids form therein, and degrade the soldered pipe joint.

In the conventional soldering of a pipe joint made of a material which is relatively easily soldered, such as copper and the like, two different methods are used: one is to heat a-flux applied to the bonding surface of a joint to a soldering temperature at which temperature a soldering alloy is supplied on the bonding area of the joint; and the other is to apply a flux onto the bonding surfaces then to place a solder alloy between the bonding surfaces at ambient temperature, and thereafter to carry out soldering by heating. However, for the reasons given hereinbefore, a satisfactory stainless steel pipe joint for use in piping such as water or hot water supply piping has not yet been obtained. Therefore, another method has also been proposed which comprises pre-plating Cu, Ni or Sn-Pb onto the bonding surfaces of stainless steel pipes and joining the pipes by using a copper joint. This method, however, is not practical in view of the complicated working steps it requires and the resulting high cost.

The inventors of the present invention have disclosed in U.S. Patent 3,963,162, for example, a method of soldering stainless steel pipes without using an additional copper-made joint to provide the soldered area substantially free of voids, in which paste solder comprising a flux component predominantly of phosphoric acid and ammonium dihydrogenphosphate and the powder solder of Sn-Pb alloy is applied onto the bonding surfaces of stainless steel pipes having a joint clearance of 0.05 - 1.0 mm, said paste solder is molten by heating and immediately afterwards additional solder alloy in wire form is fed into said clearance (hereinafter referred to as two-stage soldering). The two-stage soldering was found practically satisfactory for joining stainless steel pipes for use in water or hot water supply piping.

According to the method, the following advantages are obtained.

(1) Stainless steel pipes can be soldered by using a simple heating means, such as an infrared ray heating apparatus, a resistance heating apparatus, a torch lamp, a gas burner, etc.

(2) The chromium oxide film of the stainless steel can easily be dissolved leaving an activated surface at a soldering temperature.

(3) After completion of the soldering, a residual halide corrosive to stainless steel such as metal halide, hydrochloric acid, ammonium chloride, etc. can not be found.

(4) A satisfactory void-free joint can be obtained even with a non-corrosive phosphoric acid base flux having an activation temperature in a narrower range.

(5) It is possible to heat and activate the bonding surfaces of the pipe joint uniformly, so that soldering is possible with a relatively large clearance e.g. 0.05 - 1.0 mm.

Thus. the method disclosed in the U.S. patent provided, for the first time, a practical method of soldering stainless steel pipes. The composition of the paste solder used comprises from 20 to 80% by weight of a flux component and from 80 to 20% by weight of powder solder alloy. and said flux component comprises, on the basis of the total weight of the flux component. from 5 to 50% by weight of phosphoric acid (H3PO4) from 95 to 50% by weight of ammonium dihydrogen-phosphate (NH4H2PO4) and 0 to 20%by weight of other additives in a vehicle such as water, alcohols, glycols, etc.

If a solvent such as water, alcohol is employed as a vehicle, the resulting paste solder will stay in a creamy state for a while after the preparation thereof, and during this time it is possible to apply it with a brush to the bonding surface. However. after several hours of storage. since the viscosity is relatively low, the flux component and the solder may separate, and sometimes turn into a state just like sand dispersed in water. In such a case, re-stirring is necessary prior to use. In addition. if the amount of phosphoric acid is small, it is difficult to apply the paste solder onto the bonding surface due to its weak adhesive force. Further, if stored for a long period of time, the flux component and the solder may react with each other to solidify the paste solder making it useless.

In general. the solder used in soldering stainless steels tends to wet preferentially just the surface area where the flux has reacted with the stainless steel. This tendency is distinctive for a phosphoric acid-containing flux. Therefore, it is very important to apply the paste solder uniformly onto the surface area to be bonded. For example, in case of soldering stainless steel pipes. a paste solder is supplied onto the bonding area each of a male pipe and a female pipe, and then the male pipe is inserted into the female pipe. However, if the paste solder does not remain in a uniform pasty state or if it does not have a sufficient adhesive force, the solder coating is easily peeled off from the bonding surface upon insertion of the male pipe into the female pipe. causing the formation of numerous voids even with paste solder.

Thus, in order to join stainless steel pipes without formation of voids. paste solder which will satisfy the following requirements has been highly desired: i) The paste solder should remain in a homogenous pasty state and be easily applied onto the stainless steel surface. ii) The paste solder. upon application. should show good spreadability and adhesiveness and not leave lumps in an applied coating. iii) During storage for a long period of time. its properties and its pasty state should he maintained without separation or solidification.

Thus. the object of the present invention is to provide an improved paste solder having the above-mentioned characteristics for use in the two-stage soldering mentioned hercinbefore.

The inventors have found a paste solder satisfying the above requirements and completed the present invention. This paste solder compriss s (A) 20-80% by xb weight of a flux component and (B) 80-20Nc by weight of powder solder. such as Sn-Pb system powder solder. said flux component comprising. on the basis of the total weight of the flux component. 2()-8()of bv weight of phosphoric acid (H3PO4). 7.5-70% by weight of ammonium dihydrogenphosphatc (NH4H2PO4), 0.5-10% by weight of copper carboxylate and 2-15% by weight of carboxylic acid amide having at least 8 carbon atoms. The flux component, if desired, may further comprise up to 30% by weight of a glycol ether.

The solding according to the present invention is superior to the paste solder disclosed in U.S. Patent 3.963.162 in its application properties. compatibility of flux with powder solder and storage stability. Such improvement in properties was obtained by modifying the flux component by incorporating in the phosphoric acid-containing flux a carboxylic acid amide having 8 or more atoms which. upon heating. provides the flux in a pasty state with thixotropy.

The carboxylic acid amide having 8 or more carbon atoms is defined as a derivative of an organic carboxylic acid in which the hydroxyl moiety (-OH) of carboxyl group (0 = C-OH) has been replaced by amino group. The resulting amide group (O =C-NH2) does not show the acidity due to carboxylic acid nor alkalinity due to amine group. but is essentiallv neutral.

Examples of the carboxylic acid amide include: octane amide (CH3(CH2)CONH2), decant amide (CH3(CH2)8CONH2), dodecane amide (CH3(CH2)10CONH2), tetradecane amide (CH3(CH2)12CONH2), hexadecane amide (CH3(CH2)14CONH62), octadecane amide (CH3(CH2)16CONH2), 9-octadecane amide (CH3(CH2)7CH=CH(CH7)7CONH2), etc.

Such carboxylic acid amides are in part compatible with phosphoric acid and the addition thereof to the phosphoric acid makes is like a soft wax. providing a pasty flux useful in preparing paste solder.

The composition of the paste solder of the present invention and the reasons for the restriction thereof will be described in more detail.

Phosphoric acid is used to fuse and reduce the chromium oxide film on the surface of the stainless steel at a soldering temperature. Though phosphoric acid of 40-60% may be used, phosphoric acid of 85-86% is preferred. In general, "phosphoric acid" refers to a 85-86% orthophosphoric acid. But, it is not intended to use the phosphoric acid only as a flux component, since the phosphoric acid itself, when heated to a soldering temperature, would promptly decompose to metaphosphoric acid and polyphosphoric acid, which do not give sufficient fluidity as a flux, and it difficult to be replaced by molten solder. So, to remove such a disadvantage. the combination of phosphoric acid with ammonium dihydrogenphosphate is employed in the presence of copper carboxylate. The ammonium dihydrogenphosphate is used in combination with phosphoric acid for the purposes of suppressing the decomposition of the phosphoric acid and maintaining the heat-resistance of the flux at a soldering tempera- ture. Namely, the ammonium dihydrogenphosphate decomposes at the soldering temperature to deliver ammonium with the remaining phosphoric acid showing a fluxing action. Thus. since the decomposition of phosphoric acid to methaphosphoric acid does not occur until the decomposition of ammonium dihydrogenphosphate is completed. activation of the flux component is maintained for as long a time as it required to melt the solder added, resulting in a satisfactory flow of solder. In addition. since in accordance with the present invention the paste solder comprises powder solder incorporated in a pasty flux, the melting of the solder takes place at once when heated at a soldering temperature, and the need of the flux for resistance to heat is not so severe as in the case wherein flux and solder are separately applied.

However. the combination of phosphoric acid with ammonium dihydrogenphosphate is preferable for soldering stainless steel pipes by means of a torch lamp and for obtaining a good solder plating as well as a pipe joint substantially free of voids.

According to the present invention, the contents of the phosphoric acid (H3PO4) and ammonium dihydrogenphosphate (NH4H2PO4) are limited to 20-80% by weight and 17.570% by weight, respectively. The phosphoric acid content is based on the 85% phosphoric acid. This is because less than 20%by weight of phosphoric acid increases the relative amount of ammonium dihydrogenphosphate, giving less fluxing action at a melting temperature of the solder (183-240 C). In such a case, upon heating at the soldering temperature, the solder does not show wetting action against the stainless steel, but forms a discrete bead-like extension on the surface thereof. If the flux contains more than 70% by weight of ammonium dihydrogenphosphate, making small the amount of phosphoric acid, though it is possible to make the resulting flux pasty due to the presence of carboxylic acid amide by adding glycol ethers, the resulting paste solder cannot be applied uniformly onto the surface of the stainless steel with a brush, and the paste solder does not wet the stainless steel. That is, the surface of the stainless steel repels the paste solder, and the brush just passes over the surface thereof.

Therefore, it is impossible to provide a pipe joint substantially free from voids. On the other hand, when the phosphoric acid exceeds 80 % by weight, the applicability of the resulting flux is improved due to the decrease in the amount of ammonium dihydrogenphosphate, but the heat-resistance of the resulting flux is impaired due to the decrease in the amount of the ammonium dihydrogenphosphate, making it difficult to obtain a soldered joint substantially free from voids.

Thus, the amount of the phosphoric acid and ammonium dihydrogenphosphate are restricted as in the present invention.

Copper carboxylate is added for improving the flow of solder upon soldering, since the copper carboxylate decomposes at a soldering temperature to deposit a copper plating on the stainless steel surface which has been previously cleaned.

It has been known in the art to add to a flux an ingredient such as stannous chloride, copper chloride, stannous sulfate, copper sulfate etc., so as to deposit a metal on the stainless steel surface. However, chlorides are not desirable, since chlorides cause pitting corrosion of the stainless steel. The stannous sulfate and copper sulfate tend to adversely affect the spreadability of molten solder due to the introduction of sulfuric acid. In addition, copper sulfate reacts with the solder to oxidize the solder. Since the formation of an oxide film on the surface of solder prevents the solder from wetting the stainless steel surface, soldering is not satisfactory in the presence of copper sulfate. It has also been proposed to add to flux composition a copper-containing additive, such as basic copper carbonate (CuCO3Cu(OH)2-H20) and copper powder, but the paste solder containing any one of them does not show additional improvement with respect to the wetting action compared with that of a flux free of them.

The copper carboxylate used in the present invention is the copper salt obtained by reacting copper with a carboxylic group. Examples of the copper carboxylate include cupric formate (Cu(CHO0)2 4H20), cupric acetate (Cu(CH3CO0)2 H20), cupric tartrate (CuC4H406 3H20), cupric oxalate (CUC204.1/2H20) etc. The copper carboxylate is added in an amount of 0.5-10% by weight on the basis of the weight of flux. Copper carboxylate in an amount less than 0.5% by weight does not improve the spreadability of solder. On the other hand, more than 10% by weight does not effect additional improvement.

Carboxylic acid amide having 8 or more carbon atoms, as hereinbefore described, is added for providing a paste solder which does not separate into flux and solder during storage and which can be easily applied onto the bonding surface with a brush. However, less than 2% by weight of the amide does not change the phosphoric acid base flux to paste solder which can be uniformly and satisfactorily applied onto the bonding surface, and the resulting paste solder tends to separate into flux and solder at a high phosphoric acid content. More than 15 % by weight of the amide makes the final flux so stiff that it cannot be applied with a brush, and also, when heated with a torch lamp or a gas burner, carburization of flux excessively occurs leaving a large amount of residual flux, which is difficult to remove by washing. Thus, according to the present invention the carboxylic acid amide content of the flux is limited to 2-15 by weight, preferably to 3-9%by weight for the best results. The addition of carboxylic acid amide having less than 7 carbon atoms does not make the flux pasty. The preferred carboxylic acid amide is octadecane amide having 18 carbon atoms.

In order to improve the spreadability and stability of homogeneous paste solder the person skilled in the art would think of adding polyethylene glycols, glycerine, various kinds of fats and oils such as palm oil, lanolins, carnauba wax etc. instead of said carboxylic acid amide.

However, the addition of them even in an amount equal to the amount of carboxylic acid amide does not result in paste solder having good spreadability. If the resulting paste solder is applied to the bonding surface, carburization of the flux occurs upon soldering, preventing the solder from wetting the stainless steel surface. It is also difficult to wash off the carburized residual flux. In addition, since the various kinds of fats and oils mentioned above are not compatible with the phosphoric acid base flux, the resulting paste solder is not in a stable creamy state.

Glycol ethers may be added to the flux component of the present invention so as to adjust the consistency of the paste solder which sometimes might become too thick and solidify in part even with the addition of carboxylic acid amide due to a higher content of ammonium dihydrogenphosphate and a lower content of phosphoric acid. The addition of glycol ethers is preferred. if the phosphoric acid is less than 40% by weight.

Thus, the addition of a glycol ether is necessary if the phosphoric acid is contained in a relatively small amount. But if the phosphoric acid is more than 40% by weight in the flux, there is no need to add the glycol ether.

Suitable glycol ethers are those containing an ether group as well as a hydroxyl group in a molecule; they are less volatile and are compatible with most liquids containing water.

Examples of such glycol ethers include: ethylene glycol monoethyl ether (C2H5-O C2H40H), ethylene glycol monobutyl ether (n-C4H9-O-C2H4OH), diethylene glycol monoethyl ether (C2H5-O-C2H4-O-C2H4OH), diethylene glycol monobutyl ether (n-C4H9-O- C6H4-O-C2H4-OH) etc.

Preparation of the paste solder of the present invention is carried out as in the following.

Predetermined amounts of ammonium dihydrogenphosphate and a copper carboxylate are charged into a mixing and milling machine, and are thoroughly mixed and milled. Predetermined amounts of phosphoric acid and/or a glycol ether as well as a carboxylic acid amide are heated to disperse the amide in the phosphoric acid and the resulting mixture is charged into said mixing and milling machine. To the resulting mixed mass of flux component is added a predetermined amount of powder solder to prepare the paste solder. Alternatively, predetermined amounts of phosphoric acid and/or a glycol ether as well as a carboxylic acid amide are heated to disperse the amide in the phosphoric acid, and the resulting mixture is charged into a mixing and milling machine. To the thoroughly mixed mass is added a predetermined amount of powder solder and then a predetermined amount of previously throughly mixed and milled ammonium dihydrogenphosphate and a copper carboxylate is added to the contents of the mixing and milling machine to prepare the paste solder.

It is to be understood that the present invention is in no way limited by the procedure for preparation of paste solder.

Proportions of the flux component and powder solder are so determined that the powder solder comprises 20-80% by weight of the paste solder. Less than 20% by weight of solder does not provide the bonding surface with a sufficient amount of solder. More than 80% by weight of the solder makes the paste solder rather hard. Even with the addition of the amide, spreadability of the resulting paste solder is impaired. Therefore, the powder solder content is limited to 20-80%by weight. With respect to the composition of the solder itself, the present invention is not restricted thereby, and solder alloys such as Sn-Pb, Sn-Ag Sn-Pb-Ag and Sn-Sb alloys may be used. In general, according to the present invention, a soft solfer is used for joining stainless steeel pipes. A solder alloy finer than 100 mesh may be used successfully.

The present invention will be further illustrated in conjunction with the following Working Examples, Comparative Examples and Reference Example. Parts (percents) are shown by weight. The phosphoric acid used in the following examples is the 85 %phosphoric acid of the first class grade chemicals.

Example 1 Preparation of Paste Solder: phosphoric acid (85%) 67 parts ammonium dihydrogenphosphate 20 parts cupric formate 3 parts Armid-HT (octadecane amide) 10 parts powder solder (Sn50-Pb50, 150 mesh) 100 parts Predetermined amounts of ammonium dihydrogenphosphate and cupric formate were charged into a mixing and milling machine and throughly mixed and milled. Apart from this, predetermined amounts of phosphoric acid and Armid-HT (trade name of octadecane amide sold by Lion-Armour Co.. Ltd.) were placed in an enamelled container and were heated therein to dissolve the amide in part and to disperse the amide in the phosphoric acid. The resulting mixture was charged into said mixing and milling machine containing said ammonium dihydrogenphosphate and cupric formate in mixture and the resulting mixture was thoroughly mixed and milled in said machine to provide a pasty flux, to which, additionally. a predetermined amount of powder solder was added, and, after thoroughly mixing, to recover paste solder. The resulting paste solder showed good storage stability.

Soldering Operation: Two stainless steel pipes of AISI 304 stainless steel having an outer diameter of 22 mm and wall thickness of 1 mm were worked to form a pipe joint having a joint clearance 0.1-0.2 mm and an overlap of 16-17 mm. The joint clearance means a difference between the outer diameter of the inner pipe and the inner diameter of the outer pipe. These male and female pipes were degreased with trichloroethylene available under the trade nambe of TRIC LENE. and the surfaces to be solder-bonded were polished with sand paper. The paste solder obtained in previous steps was applied onto the surfaces by means of a brush. After assembling the stainless steel pipe joint, the pipe joint was heated uniformly by a torch lamp.

Upon observing that the flux was boiling and the powder solder in the flux was molten, wire-shaped Sn50-Pb50 solder having a diameter of 1.6 mm was fed into the clearance while it was being heated. The test results of the soldered pipe joint are summarized in the Table.

Satisfactory soldering was carried out without formation of voids in the soldering area.

Example 2 Preparation of Paste Solder: phosphoric acid (85%) 60 parts ammonium dihydrogenphosphate 25 parts cupric acetate 9 parts Armid-HT (octadecane amide) 6 parts powder solder (Sn60-Pb40, 150 mesh) 100 parts Predetermined amounts of phosphoric acid and Armi HT were placed in an enamelled container and heated therein to dissolve in part the amide and to disperse the amide in the phosphoric acid. The resulting mixture was charged into a mixing and milling machine and a predetermined amount of powder solder was also charged thereto, and then the contents were thoroughly mixed and milled. Thereafter, predetermined amounts of ammonium dihydrogenphosphate and cupric acetate were crushed and mixed thoroughly and then charged into the mixing and milling machine. The contents were thoroughly mixed and milled to recover paste solder. The resulting paste solder showed good storage stability.

Soldering operation was carried out in the same manner as in Example 1 and spreadability, plating properties and formation of voids in the pipe joint were examined. Satisfactory results were obtained with respect to all the items above.

Example 3 Preparation of Paste Solder: phosphoric acid (85%) 50 parts ammonium dihydrogenphosphate 35 parts cupric formate 9 parts Armid-HT (octadecane amide) 6 parts powder solder (Sn50-Pb50, 150 mesh) 100 parts The preparation of paste solder and soldering operation were carried out in the same manner as in Example 1, and storage stability and spreadability of the paste solder, plating properties and formation of voids in the pipe joint were examined. Satisfactory results were obtained with respect to all the items above.

Example 4 Preparation of Paste Solder: phosphoric acid (85%) 39 parts ammonium dihydrogenphosphate 39 parts cupric oxalate 6 parts Armid-O (9-octadecane amide) 6 parts diethylene glycol monobutyl ether 10 parts (Butyl Carbitol, Union Carbide) powder solder (Sn50-Pb50. 15B mesh) 100 parts In this example Example 2 was repeated except that diethylene glycol monobutyl ether was added to phosphoric acid and then the amide was added. Armid-O is the trade name of 9-octadecane amide sold by Lion-Armour Co., Ltd. Soldering operation was carried out in the same manner as in Example 1, and storage stability, spreadability and plating properties of the paste solder and formation of voids were examined. Satisfactory results were obtained with respect to all the items above.

Example 5 Preparation of Paste Solder: phosphoric acid (85%) 26 parts ammonium dihydrogenphosphate 40 parts cupric tartrate 6 parts Armid-C (dodecane amide) 3 parts diethylene glycol monoethyl ether 25 parts (Carbitol. Union Carbide) powder solder (Sn50-Pb50, 150 mesh) 100 parts In this example. Example 4 was repeated to prepare paste solder. Armid-C is the trade name of docecane amide sold by Lion-Armour Co., Ltd. Soldering operation was carried out as in Example l and st and formation of voids were examined. Satisfactory results were obtained with respect to all the items above.

Comparative Example 1 Preparation of Paste Solder: phosphoric acid (85%) 65 parts ammonium dihydrogenphosphate 20 parts palm oil 15 parts powder solder (Sn50-Pb50, 150 mesh) 100 parts Predetermined amounts of phosphoric acid, ammonium dihydrogenphosphate and palm oil were charged into a mixing and milling machine and thoroughly mixed and milled, and then a predetermined amount of powder solder was aded thereto to recover paste solder.

Soldering operation was carried out in accordance with the procedure of Example 1. The paste solder of this example separated into solder and flux immediately after the preparation thereof. and the flux applied was easily carburized during soldering.

Comparative Example 2 Preparation of Paste Solder: phosphoric acid (85%) 60 parts ammonium dihydrogenphosphate 23 parts cupric formate 5 parts polyethylene glycol (molecular weight 1500) 12 parts powder solder (Sn50-Pb50, 150 mesh) 100 parts In this example, Comparative Example 1 was repeated to prepare paste solder. Soldering operation was carried out in accordance with the procedure of Example 1. It was very difficult to apply the paste solder of this example onto the stainless steel surface with a brush and the flux applied was easily carburized during soldering so that solder did not wet the surface of the stainless steel successfully.

Comparative Example 3 Preparation of Paste Solder: phosphoric acid (85%) 60 parts ammonium dihydrogenphosphate 23 parts cupric formate 5 parts lanolin 12 parts powder solder (Sn50-Pb50. 150 mesh) 100 parts In this example. Comparative Example 1 was repeated to prepare paste solder. Soldering operation was carried out as in Example 1. It was very difficult to apply the paste solder of this example onto the stainless steel surface and the flux applied was carburized during soldering so that solder did not wet the surface of the stainless steel successfully.

Comparative Example 4 Preparation of Paste Solder: phosphoric acid (85%) 78 parts ammonium dihydrogenphosphate 10 parts cupric formate 6 parts Armid-HT 6 parts powder solder (Sn50-Pb50. 150 mesh) 100 parts In this example. Example 1 was repeated to prepare paste solder and soldering operation was carried out in the same manner as in Example 1. The resulting paste solder was easily applied onto the bonding surfaces. But the resistance to heat is low so that additional solder of Sn50-Pb50 in wire form had to be fed into a joint clearance in a very short time after the solder in the paste solder was molten. Practice required highly skilled techniques.

Comparative Example 5 Preparation of Paste Solder: ammonium dihydrogenphosphate 60 parts cupric formate 5 parts Armid-HT 5 parts diethylene glycol monobutyl ether 30 parts (Butyl Carbitol, Union Carbide) powder solder (Sn50-Pb50, 150 mesh) 100 parts In this example, diethylene glycol monobutyl ether was used instead of phosphoric acid.

Soldering operation was carried out in accordance with the procedure of Example 1. The resulting paste solder of this example did not show adhesiveness at all, so that it can hardly be applied onto the bonding surface with a brush. In addition, since the wetting action of molten solder exhibited time lag, the molten solder tended to form beads which fell in droplet. The formation of numerous voids was noted.

Reference Example (U.S.P. 3,963,162) Preparation of Paste Solder:

phosphoric acid (85%) 25% by weight ammonium dihydrogen- 70% by @ 72% by phosphate weight 70% by weight copper formate 5% by # # 50% by weight weight ethylene glycol mono ethyl ether (Ethyl 28% by Cellosolve, Union Carbide) weight powder solder (Sn50-Pb50, 150 mesh) 50% by weight The flux components mentioned above were charged into a mixing and milling machine and mixed and stirred therein. A predetermined amount of powder solder was charged into the mixing and milling machine and then the contents thereof were mixed and stirred thoroughly to provide paste solder. Example 1 was repeated to carry out soldering with this paste solder.

Just after the preparation of paste solder, the paste solder showed good spreadability and soldering properties. but one day after the preparation the flux components had begun to separate and the paste solder had to be stirred again prior to the use thereof. One month after the preparation. the solder had oxidized and the paste solder had started to solidify.

Test results of the foregoing examples are summarized in the Table below.

Table Example spreadability1) soldering2) void area3) operation in pipe joint (%) remarks Example 1 good good 4 - 8 present invention pasty after 2 weeks, no solidification found one month after Example 2 good good 4 - 8 " Example 3 good good 4 - 8 " Example 4 good good 4 - 8 " Example 5 good good 4 - 8 " Reference difficult to brush. Solder turned beady, 25 - 30** flux separated one Example 1 brush passed over the flux carburized day after surface Reference difficult to brush " 30-50** Example 2 Reference " " 30-50** Example 3 Reference less heat resistance, good Example 4 solder hardly pene- 10 - 20 pasty after 2 weeks trated Reference difficult to brush solder turned beady 20 - 30 Example 5 Comparative flux separated one good good 4 - 8 Example day after, solidification began one month after NOTE: ') Spreadability of paste solder: Paste solder was examined as to whether the paste solder may be coated on the surface of a stainless steel pipe uniformly and smoothly with a painting brush.

2) Soldering workability: Paste solder was examined as to whether the solder turns silvery upon heating the paste solder coating on the surface of a stainless steel pipe and as to whether a wire-shaped solder easily penetrate into the joint clearance.

3) Void area as percentage of soldered pipe joint area: Proportion of void area with respect to the total area of joint was examined by mechanically breaking the soldered joint.

* Voids extend through from one end to the other end of the joint. A void area less than 20% is tolerable from a practical viewpoint.

As is apparent from the foregoing, according to the present invention, a paste solder satisfactory from a practical viewpoint for use in making stainless steel pipe joint can be obtained. This is because, instead of applying a flux and solder separately as in the prior art, the present invention paste solder comprises a non-corrosive phosphoric acid base flux with a soft solder incorporated therein, and the paste solder has good spreadability and storage stability.

Thus, the present invention provides an improved paste solder useful for joining stainless steel pipes for use in water or hot water supply piping or other fluid supply piping.

Claims (5)

WHAT WE CLAIM IS:

1. Paste solder for use in joining stainless steel pipes comprising (A) 20-80 % by weight of a flux component; and (B) 80-20% by weight of powder solder, and said flux component comprising on the basis of the total weight of the flux component 20-80% by weight of phosphoric acid (H3PO4), 17.5-70% by weight of ammonium dihydrogenphosphate (NH4H2PO4),0.5-10%by weight of a copper carboxylate and 2-15% by weight of a carboxylic acid amide having at least 8 carbon atoms.

2. Paste solder as defined in Claim 1, in which said powder solder is an Sn-Pb based solder.

3. Paste solder as defined in Claim 1, in which said flux component further comprises up to 30% by weight of a glycol ether.

4. Paste solder as defined in Claim 1, in which 85-86% by weight of said phosphoric acid is orthophosphoric acid.

5. Paste solder substantially as herein described with reference to each one of Examples 1 to 5.