LU83165A1 - TUBES FOR CONDENSERS OR HEAT EXCHANGERS OF CORROSION RESISTANT COPPER ALLOYS AND METHOD FOR THE PRODUCTION THEREOF - Google Patents

Description

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Tubes for condensers or heat exchangers made of corrosion-resistant copper alloys and process for their production.

The present invention relates, as new products, to tubes for condensers or heat exchangers made of copper alloys, having good corrosion resistance properties. According to a particular embodiment, it relates to tubes made of a material other than copper, for example stainless steel or a suitable non-copper metal or alloy, internally lined with a copper alloy made resistant to corrosion, these composite tubes can be used in particular for condensers. 10 of the secondary circuit of nuclear power plants. The invention also relates to a method for manufacturing the above-mentioned single or composite tubes.

Condensers and heat exchangers installed in power plants, petroleum refineries, on board ships or in the chemical industry, desalination plants, etc. must meet severe operating conditions and in particular resist corrosion. The use of copper alloys, used in principle in the annealed state, for the manufacture of the tubes of these condensers and heat exchangers is particularly attractive, in particular because the relative toxicity of these alloys prevents the attachment of crustaceans or other living organisms from sea, estuary, or river water used for cooling. Among these alloys, mention may be made, for example, of brass 70 Cu - 30 Zn, 70 Cu - 29 Zn - 1 Sn, 76 Cu - 22 Zn -2 Al, cupronickels 90 Cu - 10 Ni + 1 Fe, 66 Cu - 30 Ni -k Fe and aluminum bronzes 92 Cu - 6 Al - 2 Ni or 8 * f Cu - 15 Al - 1 As.

It is known that various forms of corrosion can reach the internal surface of the tubes for exchangers, in particular erosion by sand particles, generalized corrosion due to corrosive chemical agents contained in water, localized corrosion caused by turbulence in the cooling water or by pitting from the sedimentation of particles foreign to the tube, for example iron particles carried by the cooling water. Pitting corrosion is also sometimes observed due to the deposition of carbon particles from the distillation of the drawing oils during the annealing of the tubes.

10 A problem of pitting corrosion of this kind also occurs in nuclear power plants using the WESTINGHOUSE process and using tubes of copper alloys for cooling the fluid of the secondary circuit by means of seawater, estuary or river. This water passes inside the condenser tubes and cools the fluid of the secondary circuit; this then passes through the steam generator which is heated by a primary, radioactive circuit fluid, which circulates inside a bundle of inconel (Ni-Cr-Fe) hairpin tubes. Traces of copper 20 (above 5 pph (parts per billion) which dissolve from the outside of the copper alloy condenser tubes in the secondary circuit of the power plant contribute to the formation of deposits in the steam generator and corrosion under these deposits The inconel tubes, inside which the radioactive circuit is located, can thus be attacked.

Until now, this problem has not been solved by continuously purifying> the water in the secondary circuit or by avoiding the use of copper alloys as condenser tubes in WESTINGHOUSE power plants, despite the advantages that provide these 30 alloys for the use of sea, estuary or river water.

Various means have been studied in an attempt to avoid corrosion of condenser tubes or heat exchangers made of reduced copper alloys. It is generally accepted /

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- 3 - Λ. that it is desirable that a protective layer of salts be formed in service, inside these tubes, which must be as adherent as possible to the base metal. The insufficiency of the natural protective layer of tubes made of copper alloys is compensated in many cases by a treatment with ferrous sulphate which is injected at low dose at the inlet of the condensers, but it has been found that this technique does not does not prevent pitting corrosion due to carbon deposition.

- It has also been proposed to expose the tubes, during their annealing in manufacturing, to an oxidizing atmosphere to burn the carbon and form a protective film of metal oxide.

This technique provides sufficient protection for the transport and storage of the tubes, but it is found that in service the protective effect ceases fairly quickly.

13 The Applicant has therefore endeavored to find a means of producing condenser tubes or heat exchangers made of copper alloys free from the above-mentioned defects, not causing pitting or other corrosion and even, depending on particular embodiment of composite tubes, to be used in nuclear power plants of the WESTINGHOUSE type.

The present invention aims to provide tubes for condensers or heat exchangers made of copper alloys having an internal oxide coating, having good properties of resistance to corrosion, in particular to corrosion by pitting in aggressive waters, characterized by a carbon content of 0.03 mg / dm or less, preferably 0.03 mg / dm, and by the presence of an internal coating with a thickness of 0.1 to J p consisting of oxides adhering to the base metal.

A deposit of oxides is described as adherent to the base metal when it is able to withstand bending at b3 ° without showing any visible crack on visual examination.

The copper alloys used for the production of said tubes preferably consist of annealed alloys - b - in an oxidizing atmosphere after drawing. As mentioned above, these are brasses, cupronickels or aluminum bronze.

In the case of condenser tubes which can be used for cooling the fluid of the secondary circuit in a nuclear power plant of the WESTINGHOUSE type, the external part of the tubes consists of a material other than copper, for example inconel, and is lined internally with bronze 2 of annealed aluminum, containing not more than 0.05 mg / dm of carbon and bearing an internal coating with a thickness of 0.1 to 3 microns made of aluminum oxide.

The conditions necessary for the invention therefore include cleaning the inside of the tubes to lower the carbon content to a value equal to or less than 0.05 mg / dm 15 and performing an oxidizing annealing to form a adherent metal oxide layer 0.1 to 3 microns thick.

Tests carried out according to the method described by F.J. Cornell, G. Wildsmith and P.T. Gilbert "Pitting Corrosion in Copper Tubes in Cold Water Service" - Br. Corros. J., 1973i September 20 8i, show that there is indeed a relationship between the susceptibility to the appearance of bites isolated from certain cold waters and the evolution over time of the potential difference which is established between the internal wall of the tube and an electrode disposed inside this tube.

The Applicant's tests were carried out with a silver / silver chloride electrode placed axially in the tube * The water used had the following characteristics: pH 7.5

TH 40 ° F

30 TA 0 ° F

è

TAC 26 "F

SO ^ k5 mg / l

Cl "70-135 mg / l CO U mg / l Λ 35 Conductivity 650 to 800 μΛ / cm.

/// i - 5 -

The results of the tests carried out using tubes of Cu 89 Ni 10 Fe 1 alloys, on the one hand cleaned and then annealed in an oxidizing atmosphere, on the other hand produced by the conventional route (reducing annealing) are the following: Tube cleaned Tube not oxidized, days (0.03 mg C / dm) reducing annealing and oxidized 0.06 mg C / dm ^

0 54 mV 160 mV

3 mV 169 mV

5 50 mV 192 mV

10 7 mV 209 mV

Visual examination of the tubes tested confirmed that the cleaned and oxidized tubes showed no pitting, while the tubes which had undergone a reduction annealing bore numerous primers of corrosion.

The table of results also shows that the cleaned / oxidized tube has a high corrosion resistance and a small potential difference (for example kk mV), while the conventional tube has a reduced pitting corrosion resistance and a difference high potential (for example greater than 170 mV). In general, it is advantageous that, when the test is carried out under the prescribed conditions (pH> 7 and conductivity 650-800 ρΛ / cm), the potential difference between the axial electrode and the internal surface of the tube remains less than 100 mV; no more bites are then observed, even in the presence of very aggressive water.

The above results could not be obtained using oxidized tubes with a carbon content greater than 0.1 mg / dm. The same was true, as seen in the table, for a 90/10 cupronickel tube having a low carbon content (<0.1 mg / dm) and not oxidized.

We conclude that to resist pitting corrosion, the tubes must both meet the following conditions: - have an adherent oxide layer - the oxide layer must be thin (0.1 to 3 µm) / -¼ - 6 - - have a carbon content less than or equal to 2 2 0.05 mg / dm, preferably between 0.00 mg / dm and 2 0.03 mg / dm.

The oxide layer has a very marked effect on the adhesion of the salts which form in service. It has been found that an oxidized condenser tube always causes in formation the formation of a protective film of salts adhering to the base metal, while an unoxidized tube also leads in service to the formation of salts, but these these are not adherent and are removed in places by the circulating water which locally causes corrosion, where the metal is exposed.

Finally, it has also been found that when foreign particles, for example iron, are present on the internal surface of the condenser tubes, these particles can induce pitting corrosion on tubes normally annealed in a reducing atmosphere but do not induce corrosion when the tubes have been cleaned and annealed in an oxidizing atmosphere under the conditions prescribed by the invention.

As regards the particular application of composite condenser tubes 20 usable in WESTINGHOUSE type nuclear power plants, it is advantageous - according to the present invention - to put into service a tube made of a metallic material other than copper, for example nickel or stainless steel, internally lined with cleaned aluminum bronze and coated with an adherent layer of aluminum oxide 0.1 to 3 µm thick.

The present invention also relates to the process used to produce the corrosion-resistant copper alloy tubes described above.

In general, this process essentially comprises a cleaning of the internal surface of the tubes then an annealing operation under an oxidizing atmosphere giving an adherent layer of oxide of 0.1 to 3 μm in thickness.

The internal cleaning of the tubes, whatever. 35 brass, cupronickel or aluminum bronze, is carried out 11 * r * - 7 - advantageously by -injection of water vapor into the tubes, this operation being followed by an introduction of dry air and finally d 'exposure to condensing vapors of trichlorethylene.

5 According to a suitable embodiment, the internal cleaning with steam and dry air is carried out before the final stretching, which is then carried out with the minimum quantity of lubricant and then followed by thorough cleaning. using trichlorethylene vapors.

Annealing in an oxidizing atmosphere is then carried out so as to produce the formation of an oxide layer having from 0.1 to 3 microns thick.

, When the tube is made of brass, this result can be obtained by injecting a mixture of dry air and 35% by volume 13 of pure oxygen inside the tube, crushing the ends of the tube and passing it this at a speed of 14 m / hour in a passage oven heated to 500 ° C and whose atmosphere consists of a mixture of natural gas and compressed air, under pressure.

When the tube is made of cupronickel or aluminum bronze, the oxidizing annealing is advantageously carried out by injecting a mixture of dry air and 35% by volume of pure oxygen, crushing the ends of the tube and by passing it at a speed of 1k m / hour for cupronickels and 25 8 m / hour for aluminum bronzes in a pass-through oven heated to 720-800 ° C for cupronickels and 610 ° C for aluminum bronzes, and whose atmosphere consists of airless natural gas.

To better illustrate the invention, 30 - by way of nonlimiting examples - will be given a more detailed description of the method of manufacturing tubes according to the invention.

Manufacturing process

The process for treating copper alloy tubes, in order to give them good corrosion resistance, including pitting corrosion, comprises two main stages: cleaning the internal surface of the tubes and the annealing operation in an oxidizing atmosphere.

1. Cleaning of the internal surface 5 A burner (using natural gas for example) heats a copper coil inside which a stream of water is injected. Water vaporizes instantly. The temperature is about 120 ° C and the pressure 12 kg / cm. By means of a pump, this vapor is introduced inside the tubes to be treated when they are still in the form of rolls before the last drawing pass (reduction of wall thickness and reduction of the outside diameter). For a roll of * f00 m „in length, steam is introduced for 40 seconds.

Then, dry air is introduced inside the roller 15 for 30 seconds.

After this cleaning, the rollers undergo the last drawing pass with the minimum quantity of lubricant.

The tubes are then sawn to the desired length (400 mm longer than the finished length) and erected, after which they undergo a final thorough cleaning: they are bound in bundles and suspended for 20 minutes above liquid trichlorethylene heated to 200 ° C in the trichlorethylene vapor which overcomes this liquid. The boot is inclined by 15 ° with respect to the horizontal to facilitate the circulation of the condensed solvent on the internal wall of the tubes and its return to the trichlorethylene tank, entraining therein the last traces of drawing oil.

2. Annealing in an oxidizing atmosphere for the interior of the tubes According to the type of alloy treated, there are two variants: a) for brasses (76 Cu-22 Zn-2 Al or 70 Cu-29 Zn-1 Sn for example): - a mixture of dry air and 55% by volume of pure oxygen is injected inside the tubes - - while the gas is passing inside the tubes, the two ends are crushed and they are closed over a length of 200 mm each / - the tubes are then placed "in bed" over a width of 1500 mm on the conveyor floor of a passage oven heated to 500 ° C; the speed of passage of the tubes is 14 m / hour (they remain approximately 25 min in the hot zone, 5 to 500 ° C); the atmosphere injected into the oven, in contact with the outside of the tubes to be treated, consists of a mixture of natural gas (at the pressure of 2.5 kg / cm) and compressed air (at the pressure of 7 kg / cm) - after passing through the oven, the tubes are sawn to the finished length.

b) for cupronickels and aluminum bronzes - a mixture of dry air and 35% by volume of pure oxygen is injected inside the tubes - while the gas is passing inside the tubes, crushes the two ends and they are closed over a length of 200 mm each - the tubes are then placed "in bed" over a width of 1500 mm on the conveyor floor of a passage oven; the temperatures and the speeds of passage of the tubes in the furnace depend on the alloy treated: ^ _Alliage ._Temperature Speed of passage

Cupronickel: 10% Ni 720 ° C 14 m / h 20% Ni 750 ° C 14 m / h 30% Ni 800 ° C 14 m / h

Aluminum bronze: G% kl 610 ° C 8 m / h 25 - the atmosphere injected into the oven, in contact with the exterior. of the tubes to be treated, consists of natural gas without air.

- after passing through the oven, the tubes are sawn to the finished length.

30 Application of the product in the case of condenser tubes for nuclear power plants of the WESTINGHOUSE type

The tube is made of two separate materials.

Inside, in contact with the cooling water of the estuary or the sea, resistance to corrosion, erosion by sand and pitting is ensured by an internally oxidized tube in aluminum bronze (92 Cu-6 Al-2 Ni) containing less than 2 0.05 mg / dm of carbon and covered inside with a layer of hard alumina which resists abrasion, forms in ser -5 defect in protective films adhering to the base metal and opposes the deposition of shellfish and crustaceans (anti-fouling).

The outside of the tube is formed by a material not containing copper, for example nickel deposited by electro-lysis or a stainless steel tube placed by co-stretching with the inner tube.

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Claims (6)

1. Tubes for condensers or heat exchangers made of copper alloys having an internal coating of oxides, 5 having good corrosion resistance properties, in particular corrosion by pitting in aggressive waters, characterized by a lower carbon content or equal to 2 0.05 mg / dm and the presence of an internal coating with a thickness of 0.1 to 3 microns consisting of oxides adhering to the base metal. 2. Tubes for condensers or heat exchangers made of copper alloys according to claim 1, characterized in that they are formed of copper alloys annealed in an oxidizing atmosphere after drawing. 15 3 · Tubes for condensers or heat exchangers made of copper alloys according to either of Claims 1 and 2, characterized in that they are formed of copper alloys consisting of brasses, cupronickels or aluminum bronzes. 20 km Tubes for condensers or heat exchangers according to either of Claims 1 and 2, characterized in that they consist of a tube or of a metallic material other than copper, for example nickel or stainless steel, internally lined with aluminum bronze cleaned 25 and covered with a layer of aluminum oxide having 0.1 to 3 μm thick, used in particular in condensers of the secondary circuit of thermal power plants with nuclear reactors. 5 · Tubes for condensers or heat exchangers 30 according to any one of claims 1 to h characterized in that the axial electrode potential difference - internal surface of the tube according to the test described in "Pitting Corrosion in Copper Tubes in Cold Water Service "- FJ Cornwell, G. Wildsmith and PT Gilbert in British Corrosion Journal, 35 1973" Vol. September 8, remains below 100 mV, even in /> Z * - 12 - presence of cold, hard and aggressive water whose pH is equal to or greater than 7 and the electrical conductivity between 6 50 and 800 ρΛ / cm . 6. A method of manufacturing tubes according to any one-5 conch of claims 1 to 5 characterized in that it essentially comprises a cleaning of the internal surface of the tubes and then an annealing operation in an oxidizing atmosphere giving an adherent layer of oxide from 0.1 to J p thick. 10 7 · A method of manufacturing tubes according to claim 6 characterized in that the cleaning of the tubes is carried out by injecting steam into the tubes, this operation being followed by the introduction of dry air and finally from exposure to condensing vapors of trichlorethylene. 8 8 »A method of manufacturing tubes according to claim 7 characterized in that the internal cleaning with steam and dry air takes place before the final drawing, which is then carried out with the minimum amount of lubricant then followed by thorough cleaning by means of trichlorethylene vapors. 9 · A method of manufacturing tubes according to any one of claims 6 to 8 characterized in that, when applied to brass tubes, annealing in an oxidizing atmosphere is carried out by injection of a mixture of dry air and 55% by volume of pure oxygen inside the tubes, crushing of the ends of the tubes and passage of tr. tubes at a speed of 1 ^ m / hour in a passage oven heated to 500 ° C and the atmosphere of which consists of a mixture of natural gas and compressed air, under pressure. 10. A method of manufacturing tubes according to any conch of claims 6 to 8 characterized in that, when applied to tubes made of cupronickels or aluminum bronzes, annealing in an oxidizing atmosphere is carried out by injection of a mixture of dry air and J> 3% by volume of pure oxygen - 13 - inside the tubes, crushing of the ends of the tubes and passage of the tubes at a speed of 14 m / hour for cupronickels and 8 m / hour for aluminum bronzes in a pass-through oven heated to 720-800 ° C for cupronickels and 3 to 610 ° C for aluminum bronzes and whose atmosphere consists of natural gas without air. / Π