IE63028B1

IE63028B1 - Process for producing pitting-resistant hard-drawn pipes of copper or copper alloys

Info

Publication number: IE63028B1
Application number: IE270788A
Authority: IE
Inventors: Ulrich Reiter; Achim Baukloh
Original assignee: Kabel Metallwerke Ghh
Priority date: 1987-09-10
Filing date: 1988-09-07
Publication date: 1995-03-22
Also published as: PL274547A1; EP0306810A3; ATE103225T1; DK495488A; FI94496B; DK493288D0; GR3033568T3; DE3730367A1; YU46572B; YU171088A; DK170250B1; ES2050684T5; DE3730367C2; PT88473A; EP0306810B2; ES2050684T3; IE882707L; EP0306810B1; FI884071A0; DE3888605D1

Abstract

The corrosion damage, particularly that due to local pitting, which occurs on hard-drawn copper tubes used in the sanitary sector is generally caused by carbon-containing films but also by oxide films. A carbon film on the inside of the tube is formed, for example, if the drawing compound added when drawing the tube is subjected to excessive heat. To avoid these damaging films and to improve the pitting resistance of hard tubes made of copper or copper-based alloys, the tubes are first of all degreased. In a further process step, the inner surface of the tubes is then treated with a blasting abrasive in order to establish a minimum roughness. A mean roughness of 0.8 to 1.0 mu m has proven particularly advantageous. The hard-drawn tubes treated by the method according to the invention have none of the films which promote pitting, even after brazing and/or hot bending.

Description

Process for producing pitting-resistant hard-drawn pipes of copper or copper alloys The invention relates to a process for the production of pitting-resistant, hard-drawn pipes consisting of copper or a copper alloy for the sanitary sector, wherein the pipes are degreased following the cold forming and the inner surface of the pipes is then treated with an abrasive. The invention further relates to the use of pipes produced in accordance with this process in the sanitary sector.

It is known that, after the last drawing operation, soft-annealed copper pipes are susceptible to corrosion in aggressive tap waters. A particularly characteristic form of manifestation of corrosion damage is caused by pitting corrosion, in which local pitting sites are produced on the inside of the pipe. The production of. such corrosion damage is attributed to carbon-containing residues which occur, for example, as a result of thermal decomposition of the drawing compound used in pipe drawing.

But even in the case of hard-drawn copper pipes which are not subjected to any annealing process, a more or less continuous carbon film can be produced on the inside of the pipes if the pipes are, for example, joined together by hard soldering or are heated to facilitate r I bending.

It has been proved that the carbon film which forms i as a result of the heating is virtually harmless if less than 0.1 mg/dm^ drawing oil is present on the inner surface of the copper pipe.

To avoid all risks, copper pipe for use in the clinical sector, in particular pipes for the transportation of medicinal gases, must exhibit an absolutely clean and grease-free inner surface. To enable these stringent requirements to be met, the jq document British Hospital Technical Memorandum 22, Edition May 1972, describes a multi-stage cleaning procedure wherein the interior of the pipe is firstly degreased with steam or an organic solvent, dried, treated with an abrasive, and then blown through with medicinally pure air.

It is also already known in pipe production to use degreasing agents, for example organic solvents such as perchloro- or trichloroethylene, to remove the film of drawing oil. Yet another process provides that the 2q drawing oil be evaporated and the drawing oil vapours evacuated.

A further form of treatment consists in performing the annealing in a reducing atmosphere and removing the formed carbon film by means of an abrasive. In this case an abrasive is introduced into the pipe either by ·? means of pressurised water or compressed air.

Finally a process is known wherein, for example, a jet of sand particles is used to clean oil-containing deposits from the inner surface of installed pipelines (EP-A-0 180 228). A r The aim of the invention is to prevent the formation of carbon- and/or oxide films in the hard drawing or hot bending of hard-drawn pipes composed of copper or copper alloys and thus to improve the resistance to pitting of these pipes.

This aim is fulfilled in a process of the type described in the introduction, in accordance with the invention, in that for degreasing the pipes are immersed in an organic solvent bath, whereupon the inner surface of the pipes is then treated with a jet of sand or corundum particles for a time interval of approximately 3 to 30 s in such manner that the inner surface of the abrasively treated pipes possesses a mean roughness value of 0.4 to 1.5 pm.

Advantageous further developments are specified in the subclaims.

The process according to the teaching of the invention differs from the hitherto known processes, which relate essentially to soft-annealed copper pipes, by a combination of chemical and mechanical treatment stages.

The process according to the invention succeeds, in a K surprising manner, in avoiding the formation of detrimental films, for example carbon-containing films or oxide films, Λ on the interior surface of hard-drawn copper pipes. These films, which act, according to theory, as bipolar electrodes, 5 are, as a rule, produced at the temperatures which are normal for hard soldering or for hot bending copper pipes. Detailed investigations have shown that it is possible to avoid the detrimental films essentially as a result of the interior surface of the copper pipes, which has been largely freed of drawing oil, having a certain minimum roughness after the additional blasting treatment.

The important point is therefore that a particular surface structure which facilitates only a very low adhesion for the films produced during a heat treatment is achieved at the interior surface of the pipes. Stable corrosion elements which cause the production of pitting corrosion are therefore unable to form. According to a preferred embodiment, the blasting medium treat-ment is carried out in a manner such that the interior surface of the pipes has a mean peak-to-valley height of between 0.8 ym and 1.0 ym.

The invention is explained in yet more detail below with reference to exemplary embodiments.

Example 1 m longpipes of SF Cu hard-drawn to final dimen, 25 sion having a diameter of 15 mm and a wall thickness of 1 mm were immersed in a solvent bath of trichloro-ethylene to remove the drawing oil in a first treatment stage. In a further treatment stage, the copper pipes were connected to the blasting nozzle of a blasting system and treated for 5 s with corundum of grain size 54 (corresponding to a mean particle diameter of 300 to 350 ym).

The mean blasting pressure was about 0.55 MPa. In order « to clean the interior surface of the pipes, depending on subsequent application, of corundum particles, a felt plug, for example, which fills the pipe cross-section can be introduced and the felt plug, possibly impregnated with a cleaning agent, can then be transported by means of pressure over the pipe length. Frequently, even a simple blowing out with compressed air will be completely adequate to remove corundum particles adhering to the inside wall of the pipe. The mean peak-to-va11ey height Ra was determined as 0.8 ym. At < 0.03 mg/dm2, the residual carbon content was below the limit of detection.

Long-term trials in various corrosion-promoting domestic waters did not show any significant development of pitting corrosion. Equally, no corrosion damage occurred after hard soldering and/or hot bending.

Example 2 Pipes measuring 22 x 1 mm drawn to final dimension were used in accordance with the process conditions specified in Example 1. As a modification of the process, the pipes were treated for 10 s with corundum. The mean « peak-to-va11ey height Ra was 1.0 ym after the treatment with blasting medium. The residual carbon or residual lubricant content was again below the limit of detection.

Example 3 As a further modification of the process conditions specified hitherto, pipes drawn to a final dimension of 15 x 1 mm were first degreased with trichloroethane, dried and then immersed for a short time in a nitric acid bath.

The further process conditions for the treatment with 5 blasting medium corresponded to those of Example 1. Longterm trials of five test stands operated realistically in areas with waters which promote pitting did not reveal any indication of pitting attack. The inferior surface of the pipes had a bright metallic appearance. The water flow rate during the flowthrough periods was about 1.5 Ι/min., which corresponds to a mean flow velocity of about 0.2 m/s in the test pipe.

Claims

Claims :

1. A process for the production of pitting-resistant, hard-drawn pipes consisting of copper or a copper alloy for the sanitary sector, wherein the ·♦ pipes are degreased after the cold forming and the inner surface of the pipes is then treated with an abrasive, ‘ characterised in that for degreasing the pipes are immersed in an organic solvent bath, whereupon the inner surface of the pipes is treated with a jet of sand- or corundum particles for a time interval of approximately 3 to 30 s in such manner that the inner surface of the abrasively treated pipes possesses a mean roughness value of 0.4 to 1.5 pm.

2. A process as claimed in Claim 1, characterised in that the abrasive treatment amounts to 5 to 10 s.

3. A process as claimed in Claim 1 or 2, characterised in that the inner surface of the pipes is treated with a particle jet which possesses a pressure of at least 0.5 MPa.

4. A process as claimed in one of Claims 1 to 3, characterised in that the particles of sand or corundum possess an average particle diameter of approximately 200 to 350 pm.

5. A process as claimed in Claim 1, characterised in that at least the inner surface of the pipes is pickled in addition to the degreasing treatment.

6. A process as claimed in one of Claims 1 to 5, characterised in that the mean roughness value of the inner surface of the abrasively treated pipes amounts to 0.8 to 1.0 pm.

7. The use of pipes produced by the process according to one of Claims 1 to 6 in the sanitary sector. «

8. A process according to Claim 1 for producing a pitting-resistant hard-drawn pipe of copper or copper alloy, substantially as hereinbefore described with particular reference to the accompanying Examples.

9. A pitting-resistant hard-drawn pipe of copper or copper alloy, whenever produced by a process claimed in a preceding claim.

10. Use according to Claim 7, substantially as hereinbefore described.