Method for reducing copper solubility at the inner surface of a copper tube

Abstract

In a method for reducing copper solubility at the inner surface of a copper tube, the process parameters surface treatment (degreasing and pickling), flow conditions (flow speed<1 m/s), temperature (50.degree. C. to 80.degree. C.) and time (1 min to 10 min) are purposefully adjusted to one another, so as to achieve a uniformly directed crystal growth during the course of the tin coating. In particular, the planes (101) of the copper crystals and the tin crystals are aligned parallel to one another and the directions [101] are aligned perpendicular to one another.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates to a method for reducing copper solubility at the inner surface of a copper tube.

[0003] 2. Description of Related Art

[0004] It is known that one can coat the inner surface of a copper tube with tin, in order to prevent, with the aid of such a tin layer, copper ions dissolved from the copper from going over into drinking water, if such a copper tube is being used as a component of a drinking water line. In this connection, the European guideline for drinking water regulations should be noted.

[0005] In the methods known up to this point for depositing a tin layer on the inner surface of a copper tube, only random crystals are formed. The packing density of the tin crystals was therefore unsatisfactory. Thus, copper ions are able to go over into the drinking water by way of the tin layer.

SUMMARY OF THE INVENTION

[0006] It is an object of the invention to create a method that will ensure a clear reduction of copper solubility at the inner surface of the copper tube.

[0007] These and other objects of the invention are achieved by a method for reducing copper solubility at the inner surface of a copper tube in which the process parameters surface treatment (degreasing and pickling), flow conditions (flow speed<1 m/s), temperature (50.degree. C. to 80.degree. C.) and time (1 min to 10 min) are purposefully adjusted to one another, so as to achieve a uniformly directed crystal growth during the course of the tin coating and the formation of a copper/tin phase.

DETAILED DESCRIPTION OF THE INVENTION

[0008] The invention recognized that a high packing density of tin crystals is able to be reached in the case of a directed crystal growth during the course of the tin coating. The high packing density of the tin layer leads to a very homogeneous, evenly formed and most stable copper/tin phase. In addition to that, the tin layer is deposited evenly and having a thickness of an order of magnitude of about 0.1 .mu.m to 3 .mu.m, at a low pore number. The directed crystal growth is achieved by purposeful setting of the process parameters surface pretreatment (degreasing and pickling), flow conditions (flow speed<1 m/s), temperature (50.degree. C. to 80.degree. C.) and time (1 min to 10 min).

[0009] In spite of the fact that, even after a relatively short operating time, metallic tin can no longer be detected at the tube surface, still, the property of the low copper solubility because of the extremely stable Cu/Sn phase is maintained. Tube surfaces treated according to the invention distinguish themselves by having a very low copper solubility and a high stability.

[0010] Ascertaining the crystal structure of tin is preferably done using the results of X-ray diffraction experiments. During the interaction of the X-rays and the monocrystals, the incident waves are diffracted in discrete spatial directions on account of the lattice-like structure of the tin crystals. Whereas the position of a diffraction direction is determined by the orientation of the crystal lattice to the primary beam, the lattice dimensions and the wavelength used, the intensity of the diffracted beam depends on the distribution of the tin atoms in the unit cell.

[0011] For this purpose, a length section was cut off as a sample from the copper tube coated with tin according to the invention. This sample was then slit in the longitudinal direction, and the longitudinally slit sample was subsequently bent open to form a flat material. This flat material with the tin coating facing upwards was then irradiated with X-rays having lambda(FeK.sub.a)=1.9373 .ANG., the X-ray being directed at the flat sample at various diffraction angles. The results show that, at various angles, a high X-ray diffraction intensity was detected, which proves that the X-rays were reflected close to 100%. It follows that the tin atoms have a very high packing density. A satisfactory barrier layer is formed for the copper ions against transition into the drinking water.

[0012] In an advantageous further design of the basic idea according to the invention, plane (101) of the tin crystal is aligned parallel to the inner surface of the copper tube.

[0013] In this connection, it is then further advantageous, if plane (101) of the tin crystals is aligned parallel to plane (101) of the copper crystals.

[0014] A further improvement going beyond this is achieved, if planes (101) of the copper and tin crystals are aligned parallel to each other and directions [101]are aligned perpendicular to each other.

[0015] The advantageous tin coating was confirmed by the following internal comparison investigation:

[0016] A copper tube coated according to the invention was brought into contact with drinking water for 15 months. As a result of this contact, a tin hydroxide layer (Sn.sub.3O.sub.2(OH).sub.2) was formed on the inner surface. From this copper tube a lengthwise section was removed, cut open in the longitudinal direction, bent flat, and then submitted to an X-ray irradiation having lambda(FeK.sub.a)=1.9373 .ANG.. It was thereby determined that the reflection of the X-rays (X-ray diffraction intensity) was clearly less than 100%. Thus, one might have assumed that the object of the present invention was not attained.

[0017] The tin hydroxide layer was subsequently completely removed, so that the original coating state was present again. That is, on the copper there was a copper/tin phase (Cu.sub.6Sn.sub.5).

[0018] Then an X-ray irradiation was again carries out, at which time an almost 100% reflection was found. Thereby the method according to the invention was fully confirmed in its advantageousness.

Claims

What is claimed is:

1. A method for reducing copper solubility at the inner surface of a copper tube, comprising adjusting: surface treatment; flow conditions, including flow speed<1 m/s; temperature between 50.degree. C. and 80.degree. C., and time between 1 min and 10 min with respect to one another, so as to achieve a uniformly directed crystal growth during the course of a tin coating and formation of a copper/tin phase.

2. The method according to claim 1, wherein the surface treatment adjusted includes degreasing and pickling.

3. The method according to claim 1, in which the plane (101) of the tin crystals is aligned parallel to the inner surface of the copper tube.

4. The method according to claim 2, in which the plane (101) of the tin crystals is aligned parallel to the inner surface of the copper tube.

5. The method according to claim 1, in which the plane (101) of the tin crystals is aligned parallel to the plane (101) of the copper crystals.

6. The method according to claim 2, in which the plane (101) of the tin crystals is aligned parallel to the plane (101) of the copper crystals.

7. The method according to claim 3, in which the plane (101) of the tin crystals is aligned parallel to the plane (101) of the copper crystals.

8. The method according to claim 4, in which the plane (101) of the tin crystals is aligned parallel to the plane (101) of the copper crystals.

9. The method according to claim 1, in which the planes (101) of the copper crystals and the tin crystals are aligned parallel to each other and directions [101] are aligned perpendicular to each other.

10. The method according to claim 2, in which the planes (101) of the copper crystals and the tin crystals are aligned parallel to each other and directions [101] are aligned perpendicular to each other.

11. The method according to claim 3, in which the planes (101) of the copper crystals and the tin crystals are aligned parallel to each other and directions [101] are aligned perpendicular to each other.

12. The method according to claim 4, in which the planes (101) of the copper crystals and the tin crystals are aligned parallel to each other and directions [101] are aligned perpendicular to each other.

13. The method according to claim 5, in which the planes (101) of the copper crystals and the tin crystals are aligned parallel to each other and directions [101] are aligned perpendicular to each other.

14. The method according to claim 6, in which the planes (101) of the copper crystals and the tin crystals are aligned parallel to each other and directions [101] are aligned perpendicular to each other.

15. The method according to claim 7, in which the planes (101) of the copper crystals and the tin crystals are aligned parallel to each other and directions [101] are aligned perpendicular to each other.

16. The method according to claim 8, in which the planes (101) of the copper crystals and the tin crystals are aligned parallel to each other and directions [101] are aligned perpendicular to each other.