IE61097B1

IE61097B1 - Internally oxidized pipes

Info

Publication number: IE61097B1
Application number: IE221789A
Authority: IE
Inventors: Achim Baukloh; Ulrich Reiter; Christian Triquet
Original assignee: Kabelmetal Ag
Priority date: 1988-08-12
Filing date: 1989-07-10
Publication date: 1994-09-21
Also published as: NO177688C; ATE76175T1; FI90136B; FI90136C; ES2036763T3; MX173263B; AR247013A1; CS8904206A2; YU118689A; ZA896043B; PL161517B1; EP0356732A1; PT91428A; CZ280990B6; TNSN89088A1; GR3004809T3; JPH0261054A; PT91428B; IL91145A; DE58901399D1

Abstract

Corrosion damage to tubes made of copper or copper alloys by local pitting is usually triggered by the residues of drawing oil located on the inner surface and/or by oxide layers of poor adhesion. To achieve an especially high resistance of internally oxidised tubes to pitting or to prevent an uneven formation or detachment of the oxides located on the inner surface, according to the invention the thickness of the copper-oxide layer adhering to the basic metal is limited to values of between 0.01 and 0.2 mu m. Preferably, the copper-oxide crystals have a maximum grain size of 0.5 mu m and possess an oriented structure. The good adhesion of the copper-oxide layer formed on the inner surface of the tube preferably by continuous annealing is preserved even when the tubes have been subjected to a machining reducing their cross-section, for example by bending or drawing.

Description

The Invention relates to a pipe which is resistant to pitting corrosion and which is composed of copper or a copper alloy with an oxide layer on the inner pipe surface, in particular for use in the plumbing sector. The invention further relates to a process for the production of such a pipe.

Pipes made of copper or copper alloys are used as pipe-lines in the plumbing sector, for example for cold and hot water, but also in condensers and heat exchangers. To avoid corrosion damage, in particular due to the occurrence of local pitting, it has already 0 been proposed that the drawing oil residues which are present on the inner surface of the pipe and which tend to form carbon deposits be substantially removed, prior to the annealing, by degreasing means, for example by organic solvents such as tri- or perchloroethylene.

Further processes provide that the annealing be carried out in a reducing atmosphere and that the inner surface of the pipes then be freed by abrasive means of the carbon film which has formed. Here the abrasive means are introduced Into the pipe either by means of compressed air or compressed water.

Finally DE-A-30 04 455 and FR-A-2 500 S13 describe internally, oxidized pipes made of copper ox copper alloys, the oxide layer of which, which adheres to the base metal, has a thickness range of 0.1 to 3 ya and simultaneously possesses a low residual carbon content of 0.05 mg/dms or less. These characteristic values are obtained in that the thermal treatment is carried out, following the degreasing of the pipes, in an atmosphere containing oxygen, such as for example a specified gas mixture of oxygen, helium and ergon. However, an the case of thermal treatment under oxidizing conditions, in particular when this is carried out at a stationary location, the danger exists that the oxide layers which form will adhere poorly, 3Q will possess an increased thickness and in certain cases will be porous, with the result that negative effects, i.a. upon the corrosion behaviour, cannot be avoided. Furthermore, the oxide layers which are described in the exemplary embodiments of DS-A-30 04 455 and which have an average thickness of 1 ym can easily become cracked or detached during the subsequent processing of the pipes, for example in the case of bending.

Similar problems occur when, following thermal treatment under oxidizing conditions, in order to produce the semi-hard state the pipes must be additionally subjected to cross-section-reducing processing. The deformation forces then likewise lead to crack formation and to detachment of the oxide layer which has formed on the inner surface. The detached oxides can than lead to disturbances within individual units of a plumbing installation.

The aim of the present invention is to make available an internally oxidized pipe made of copper or a copper alloy with, in particular, high resistance to pitting corrosion, where the oxides present on the inner surface of said pipe do not lead, as a result of unfavourable formations or detachment, to negative effects upon the corrosion resistance of the pipes or the operational reliability of the installation.

The aim is fulfilled. In accordance with the invention, in that the thickness of the oxide layer which adheres to the base metal and substantially consists of copper oxide crystals is in the range of 0.01 to 0.09 μα, and the copper oxide crystals, which are orientated preferably with a (1,1,1) structure, possess s maximum grain size of 0.05 μη. Advantageous further developments of the invention are described in the sub-claims.

A preferred process for the production of an internally oxidized pipe is described in the characterising part of Claim 5.

The internally oxidized pipes produced in accordance with this process possess a copper oxide layer thickness which is preferably in the range fro® 0.03 to 0.09 μ®. By means of the process it is possible, with a very high degree of accuracy, to set virtually any value within the given range by varying the process parameters. This enables production technicians to specify the respective operating conditions for the annealing treatment. In particular the duration of the annealing treatment under oxidizing conditions as well as the composition and pressure of the gas mixture required for this purpose. For an economical· production of the pipes and for a uniform formation of the oxide coating on the inner surface of the pipes, continuous annealing treatment constitutes an essential process feature.

Surprisingly, Investigations have shown that, in contrast to previous technical opinion, even very small oxide layer thicknesses on the inner surface of the pipes, even In aggressive water, ensure adequate protection against pitting corrosion. Even after cross-sectional deformation of up to 20 % or after extreme bending by up to 180°C . no impairment of the corrosion behaviour occurs. it can easily be detected with the naked eye whether the oxide layer on the inner surface of the pipes has suffered damage due to cracks or detachment. For these investigations pipes were divided la the longitudinal direction, having previously been deformed, for example bent by up to 180°C. The oxide layer can be described . as adhering to the base metal when, following the deformation, the inner surface of th© pipes exhibits no sign of damage due to cracks or detachment.

Observation of the oxide layer present on the base metal using a scanning electron microscope has indicated that the grain size of the copper oxide crystals does not exceed a value of 0.05 μη. In contrast fco the previously investigated pipe inner surfaces, the visual appearance of the oxide layer is characterised by a very uniform surface. The oxide layer has a light red colour and exhibits a high reflective capacity under Incident light. It was also established that the crystals of the oxide layer consist of CuaO (cuprite) and preferably possess an orientated (1, 1, 1) structure. The Figure illustrates a 10,000-fold enlargement of the CuaO-layer adhering to the pipe inner surface, where in particular the extremely uniform surface of the layer and the lack of roughness of the surface ., can be seen.

In the past It was assumed that a simple relationship exists ; 5 between the thickness or the oxide layer and the residual carbon ί content on the inner surface of the pipes: The thinner the oxide layer, the lower is the residual carbon content. A reduction In the residual carbon content to a value or less than 0.03 mg/drn3 could previously be obtained, however, only by extremely costly degreasing of the pipe inner surface prior to the annealing treatment under oxidising conditions. The oxidising annealing treatment Itself required to be carried out in an atmosphere containing approximately 85¾ of an Inert gas mixture composed of helium and argon.

The internally oxidized pipes in accordance with the invention now show that a residual carbon content of 0.05 mg/dma or less is not necessarily required to avoid corrosion damage. Rather, it Is uniformity and small thickness of the oxidation which are of Importance, the layer thickness ranging between 0.01 and 0.09 pm, and the copper oxide crystals preferably possessing an orientated (1,1,1) structure and a maximum grain size of 0.05 pm.

For the production of an internally oxidized pipe in accordance with the invention, firstly the inner surface of a ring-shaped copper pipe, for example composed of phosphorus-deoxidized copper,, is degreased by a process as described im DB-&-32 07 135. The residual grease content on the pipe inner surface-prior fco the oxidizing annealing treatment was less than 0.4 mg/dm2. Individual pipe lengths of the copper pipes, connected to each other at their ends by gas-permeable connecting pieces, were annealed in continuous operation by resistance- or Inductive heating at a temperature in the range of 600 to 730°C, where a monitored gas mixture was Introduced Into the interior of the pipes. The respective atmosphere within the pipe length was set in dependence upon the flow speed, specified at between 50 and 220 at/min, and upon the cross-section of the pipes to be annealed. Preferably the gas Mixture to be used consists of 5 to approximately 15 vol.t oxygen and 85 to 95 vol Λ nitrogen gas. By constant monitoring of the parameters of temperature and speed of the continuous annealing and of the oxygen content of the gas atmosphere, it was possible to set ep an extremely uniform copper oxide layer, the thickness of which has a value within the desired range of 0.01 fco 0.09 pm.

As a result of a series of experiments It was established that the preferred layer thickness of the copper oxide adhering to the base metal substantially ranges between 0.03 and 0.09 pm.

Complete adherence of this thin copper oxide coating is maintained , even when the copper pipe was deformed so as to result in a cross-sectional reduction of up to 20¾ or was bent by up to 180°C.

\ Following the deformation, detachments or cracks in the oxide The good adherence of the copper oxide is also of particular importance in the event that semi-hard copper pipes are to be produced. In order to produce the sen!-hard state, soft-annealed copper pipes must be deformed so as to result in a cross-sectional reduction, being drawn, for example, fro» 18mm to 15mm diameter. Hard-draws copper pipes are normally deformed to the desired final dimensions without the interposition of recrystallisation annealing steps between the drawing steps, in order to for® a thin copper oxide addition on the inner surface of the pipes, these axe then thermally created at temperatures of approximately 250°C for a sufficiently short period of time to prevent any disadvantageous change in the mechanical properties.

Claims

1. e&MMs

1. a pipe which is resistant to pitting corrosion and is made of copper or a copper alloy with an oxide layer on the inner surface of the pipe, in particular for use in the plumbing sector, characterised in that tlie'^lchness of the oxide layer which adheres to the base metal and subsetially consists of copper oxide crystals is in the range of ©-©1 0¾. p®, and the copper oxide crystals, which are orientated preferably with a (1,1,11 structure, have a maximum grain size ©£ Ό.05 we/’

2. A pipe which is resistant to pitting corrosion as claimed in Claim 1, characterised in that it exhibits a soft-annealed state following the last deformation· step.

3. A pipe which is resistant to pitting corrosion as claimed in Claim 1, characterised an that it exhibits a semi-hard state.

4. A pipe which is resistant to pitting corrosion as claimed in Claim 1, characterised ia that -it exhibits a hard-drawn state.

5. A process for the production of an internally oxidized pipe as claimed in one of Claims 1 to 4, characterised by the following process steps: degreasing with a solvent continuous anneal lag treatment in the temperature range fro© approximately 600 to 730°C and a flow speed of 5© to 220 ss/rnin, where in dependence upon the pipe diameter and the speed of the continuous annealing treatment Inside the pipe an annealing atmosphere is set which comprises 1 to 2S v©!. preferably 5 to less thati 15 w’. % oxygen and 75 to 99 vol. t,, preferably approximately 85 to 95 vol. % nitrogen

6. The use of an internally oxidized pipe as claimed in one'of Claims 1 to < as pitting-resistant plumbing pipe having a residual carbon content of ®.0S to 0.15 mg/dm* and an oxide layer thickness of 0.03 to ¢.09 pa.

7. A pipe as claimed in Claim 1,- substantially as hereinbefore described.

8. A process as claimed in Claim 5, substantially as hereinbefore described.