FI66209B - FOERFARANDE FOER FOERBAETTRING AV KORROSIONSBESTAENDIGHETEN FOR INSTALLATIONSROER AV KOPPAR - Google Patents

Description

χ 66209

Method for improving the corrosion resistance of copper installation pipes. - Förfarande för förbättring av korrosions-beständigheten för installationsrör av koppar.

The present invention relates to a method for improving the corrosion resistance of soft-annealed pipes of phosphorus-oxidized copper, wherein the softening annealing of a hard-drawn pipe drawn to its final dimensions to burn lubricant residues is carried out in an oxidizing atmosphere and the formed oxide layer is removed.

Seamlessly drawn phosphorus deoxidized copper pipes are manufactured as hard bar pipes, and especially in the case of rings twisted into rings, by softening annealing and are widely used for the production of cold and hot water pipes in apartment buildings.

Experience has shown that especially in cold, aggressive pipe water, soft-annealed pipes are in some way susceptible to pit corrosion (Loch corrosion). A typical shape, referred to in the professional literature as "type 1 corrosion," are bumpy occurrences on the inside of tubes filled with defects in color, pimple-like deposits of alkaline copper salts.

According to the generally accepted theory today, the carbonaceous residues left over from the lubricating oil used to draw the pipe play a crucial role in the formation of these corrosion damage. This is a very thin layer, which is formed when the tubes are softened annealed at temperatures of about 600 ° C in an inert gas after the last drawing. In this case, the adhering lubricating oil does not completely evaporate. The part cracks, leaving above all a surface with a high carbon content inside the pipe. Although the quantities can be considered relatively small by production engineering measures, ie 2 2 66209 less than 1 mg / dm, this surface cannot be completely avoided. On the other hand, even in this order of magnitude, according to current theory, this amount is sufficient to make the pipe susceptible to corrosion, for example compared to a hard-drawn pipe with similar water conditions.

In the method described in German Application Publication No. 2,337,022 of the type described above, softening annealing is performed in an oxidizing atmosphere and the formed oxide layer must be removed by pickling treatment. The method of subjecting the pipes to subsequent cleaning by the pickling method cannot, for a number of reasons, be brought to the desired end. First, pickling long, narrow pipes is very complicated and expensive because it involves several sub-steps. Second, it poses hazards to the product and the environment (environmental contamination) because copper as a semi-precious metal requires oxidizing acids (e.g., nitric acid) to provide a clear, clean surface.

The object of the invention is to provide a method in which the susceptibility to corrosion can be reliably prevented by removing said disadvantages in particular.

According to the invention, the task is solved in such a way that in connection with softening annealing there are oxidizing conditions inside the pipes while the pipe is surrounded on the outside by a shielding gas atmosphere and that the inner part of the pipe is then subjected to sandblasting.

However, it is previously known to subject copper tubes softened in an inert gas to sandblasting from the inside in order to improve the corrosion resistance. However, this method has so far been used exclusively for short and straight pipes with a length of about 5 to 10 m. Studies have also shown that in this way the carbon coating is significantly reduced and 3 66209 corrosion resistance can also be improved. However, more detailed analyzes have shown that in this way it is not possible to completely remove the carbon coating on the inner surface. In this case, amounts of about 0.1 to 0.2 mg / dm are still present. This is due in part to the fact that the carbonaceous layer is soft and firmly adhering due to the presence of small amounts of tarry substances. The sandblasting method, on the other hand, has the property that, in addition to abrasion and abrasion, it also, to a certain extent, plastically presses on the surface by adhering substances adhering thereto.

According to the invention, this disadvantage is avoided by carrying out a previous annealing treatment by oxidizing the inside of the tubes. In this case, the remaining oil is burned and the copper surface is oxidized to form copper oxide.

The accompanying blowing completely removes the brittle oxide layer. The combination of these treatment steps corresponds to the removal of the copper tube surface to a depth of about 10 to 50. The surface thus released is completely clean and has no traces of adhering blowing material, which in any case is chosen in such a way that it cannot be adversely affected in any way.

Still further, it has been known from previous attempts that oxidatively annealed copper tubes are more resistant to corrosion in a given water than inert gas annealed ones. However, in order to reduce the risk of corrosion, this annealing method is not used so far because of the disadvantages per se. Copper oxide tends to appear as a brittle okeid layer that easily breaks and crumbles when the pipe is bent.

It is clear that such an inhomogeneous surface is less corrosion chemically disadvantageous than a metallically pure copper surface. Quite surprisingly, the carbon residues that often remain on such oxidized inner surfaces of pipes can also be detected analytically. Quite obviously, the oxidation of 4 66209 copper takes place faster than the oxidation of carbon and the carbon particles are then submerged in the oxide layer.

According to a preferred embodiment of the invention, an oxygen-containing gas is fed into the pipe in an amount sufficient to completely incinerate the lubricant waste. In this case, for example, it is not sufficient for the tube to be annealed in place with constant air. For adequate oxidation, it is suitable to either purge the tube with air or to fill the circumferentially twisted tube with oxygen.

The annealing itself is carried out at suitable temperatures of 500 to 700 ° C.

The sandblasting treatment is suitably carried out for 10 to 40 seconds, in which case the grain size of the blowing agent is selected to be between 0.1 and 0.3 mm. According to a preferred embodiment, corundum is used as the blowing agent.

The invention is explained in more detail below with the following working examples.

Example 1

A tube of phosphorus deoxidized copper, drawn to a final length of 15 x 1 mm / 50 m in length, is drawn continuously / through an annealed heating zone at a speed of 50 m / min. and is then gradually heated to a softening annealing temperature. The temperature of the annealing zone rises to 600 ° C. In this area, up to the inlet of the associated cooling area, the pipe is surrounded on the outside by inert gas. Air flows in the opposite direction through the inside of the tube.

As soon as the pipe has left the annealing zone, it is connected to the nozzle of the sandblasting device and treated for 20 seconds with blown corundum having a grain size of 120 5 66209 (0.1 to 0.125 mm). The blowing agent is sucked out of the pipe end in a known manner and, after passing through the cleaning device, is fed back into the blowing device. The pressure jet finally releases the pipe from the detergent residues.

The inner surface of the pipe treated in this way is metallically clean and opaque in appearance. The average surface roughness depth reaches up to 0.3. Quantitative measurement of the carbon content of the waste on the inner surface gives a value below 0.1 mg / dm, ie below the analytical limit.

To determine the corrosion state, the electrochemical polarization resistance is also measured. For this purpose, the tube section is filled with chloride-containing borate buffer solution, pH 8, and a platinum wire and a reference electrode (Kalomel electrode, 3.5 n KCl) are introduced. The buffer solution then acts as a model for water, which promotes pit weathering. The polarization resistance is obtained from the curvature of the cathodic current density / voltage curve, which is almost straight in the exit region. From this, conclusions can be drawn about the layer of foreign substances on the surface of the pipe. A very low polarization resistance is typical of a metallically pure copper surface.

2

In the embodiment, it was found: R = 1.5 k ohm * cm.

P

This value corresponds practically to the value of a copper surface completely purified by pickling in nitric acid, measured up to = 1.4 k ohm 'cm. On the other hand, it is known from corrosion tests that a pickled, metallically clean pipe surface is not exposed to pit corrosion even in aggressive or concentrated water.

If, on the other hand, the polarization resistance is measured with softening-annealed tubes which have not been produced by the method according to the invention, the following is obtained: 6 66209

Manufacturing conditions in Polar - Carbon residue resistance R /.2 _p mg / dm k ohm * cnr

Conventional method 1 annealed in an inert atmosphere 56 0.25 2 as 1, this ice is blown with corundum 5.8 0.20 3 oxidative annealing 40 0.15

Method 4 according to claims 1 according to claims 1-3 and 5-8 1.5 £ 0.10

Despite the very low carbon deposition, Option 3 shows a high polarization resistance, which should be understood as indicating that the corrosion properties are less advantageous than in the method according to the invention, which gives very low values for both carbon content and polarization resistance.

It is further apparent from this table that the combination of oxidative annealing and sandblasting gives the pipe an internal surface with excellent corrosion properties.

Example 2

A copper pipe of 50 m length, drawn to the final dimensions of 15 x 1 mm and wound into a ring, is filled with oxygen and pressed together at the ends to the extent that pressure equalization is possible at all times. The tubes are sent through an annealing furnace under the influence of an inert gas and of conventional construction and softened. The cooled softening-7 66209 annealed tube is clean on the outside and oxidized to black on the inside. Carbon analysis of the oxidized surface 2 gives a value of about 0.15 mg / dm.

The pipes are opened by sawing the ends and connected as an annular pipe to a sandblasting device. The tube is blown for 20 seconds with corundum having a grain size of 120 (0.1-0.125 mm) and then blown clean.

The inner surface of this tube is then metallically clean. It is somewhat opaque and slightly stronger on the outer surface of the ring rotations than on the inner surface due to the centrifugal force acting on the blowing agent.

The determination of the carbon content gives a value of less than 0.1 mg / dm for the entire inner surface. The measurement of the polarization resistance gives 7 results of 1.1 k ohm * cm.

Claims (8)

8 66209 A method for improving the corrosion resistance of softened-annealed pipes of phosphorus deoxidized copper, wherein the softening annealing of a permanently drawn hard tube for burning lubricant residues is carried out in an oxidizing atmosphere and the oxide layer is then removed during is surrounded by a shielding gas atmosphere, after which the inside of the pipe is subjected to a sandblasting treatment. A method according to claim 1, characterized in that an oxygen-containing gas is fed into the pipe in such an amount that the gas is sufficient to completely burn the remaining lubricant. Method according to Claim 2, characterized in that the pipe is thoroughly flushed with air. A method according to claim 2, characterized in that the tube twisted into a ring is filled with oxygen. Process according to one of Claims 1 to 4, characterized in that the softening annealing is carried out at temperatures of 500 to 700 ° C. Method according to one of Claims 1 to 5, characterized in that the sandblasting treatment is carried out in 10 to 40 seconds. Method according to one of Claims 1 to 6, characterized in that the grain size 66209 of the sandblasting agent is up to 0.1 to 0.3 mm. Method according to Claim 6 or 7, characterized in that corundum is used as the sandblasting agent.