Method for testing residual carbon on the inner surface of copper and copper alloy tubes

Abstract

A kind of copper and copper alloy pipe inner surface residual carbon detection method, main steps include (1) sample preparation;(2) nitric acid solution immersion;(3) preliminary determination;(4) acid solution heating, filtration and filter material drying;(5) detection object extraction;(6) residual carbon determination: using energy dispersive spectrometer or electron probe, under the condition that test parameters are: working voltage 15-20kV, working distance 10-12mm, if composition test shows that the mass fraction of C in these black substances is not less than 70%, it is determined that residual carbon is determined.The method of the application has the advantages of accuracy, reliability, simple operation, short test cycle, strong implementability, etc., can accurately judge the residual carbon on the inner surface of copper and copper alloy pipe, can reduce the corrosion perforation risk of copper and copper alloy pipe caused by residual carbon, improve the reliability of equipment operation, reduce the cost of equipment maintenance management and overhaul.

Description

Technical Field

[0001] This invention belongs to the field of metal material testing technology, and relates to a test method for detecting residual carbon on the inner surface of copper and copper alloy tubes. Background Technology

[0002] Copper alloys are important materials for marine engineering, especially for coastal power plants, ship heat exchangers, and condenser tubes, due to their excellent resistance to seawater corrosion, fouling, and marine organism attachment. However, residual carbon is easily generated during the manufacturing process of copper tubes, particularly during the cleaning and oxidation annealing processes before extrusion and annealing.

[0003] Numerous cases have demonstrated that the presence of residual carbon can trigger corrosion and perforation in copper pipes. On one hand, residual carbon can lead to electrochemical corrosion of the copper pipe. The residual carbon on the pipe surface shifts the corrosion potential of the copper to a positive value, making the potential of the carbon film (cathode) more positive and the copper substrate (anode) more negative. This forms a localized electrical couple between the residual carbon and the copper substrate, creating potential corrosion points that easily lead to corrosion. On the other hand, the corrosion resistance of copper pipes depends on the presence of a passive film on their surface. A clean surface and uniform structure are prerequisites for the formation of a uniform and dense passivation film. The presence of residual carbon film or localized blocky carbon film on the copper pipe surface will affect the structural uniformity of the copper pipe surface, impacting the density and uniformity of the film formation. Previous tests have shown that even localized carbon films at the hundred-micron level can cause localized corrosion of copper pipes. Therefore, residual carbon has a significant impact on the corrosion resistance of copper pipes, thereby affecting the reliability and safety of equipment. Thus, residual carbon in copper pipes used in critical equipment should be strictly controlled, and rigorous residual carbon testing should be conducted before the pipes are put into use.

[0004] Currently, the main method for detecting residual carbon film is based on the residual carbon film test method in Appendix A of GB / T 18033-2017 "Seamless Copper Water Pipes and Copper Gas Pipes". However, this method has many ambiguities. (1) The method does not define the film and micro-substances, which may lead to different testers having different opinions on the identification of the film and micro-substances. (2) The morphology and size of the film are not clearly defined. (3) The size of the micro-substances is not clearly defined. Previous testing work found that even when there is an obvious black carbon film on the surface of the copper pipe sample, the substance floating in the solution does not conform to the morphology of the film, but is a small black block floating object. Moreover, these black substances do not undergo obvious changes after heating and boiling. Based on these phenomena and the corresponding judgment conditions, it is impossible to determine whether these black substances are carbon film. Therefore, a more accurate method for detecting residual carbon is needed. Summary of the Invention

[0005] The purpose of this invention is to provide an effective method for detecting residual carbon on the inner surface of copper and copper alloy tubes.

[0006] This invention primarily involves immersing the copper tube sample in a 30-40% (volume fraction) nitric acid solution. Once the solution turns blue, the sample is removed. If black floating matter is observed in the solution, it is heated to boiling for 5 minutes. If black floating matter is still visible after heating, the solution is filtered and dried. The black substance is then placed on a low-melting-point alloy surface and allowed to solidify. Compositional analysis is performed using energy dispersive spectroscopy (EDS) or electron probe microanalysis. If the carbon (C) mass fraction of the black substance reaches 70%, it can be identified as residual carbon.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A method for detecting residual carbon on the inner surface of copper and copper alloy tubes, comprising the following steps:

[0009] (1) Cleaning the outer surface: Take a certain length of tube sample from the copper tube to be tested and plug both ends of the tube with a waterproof plug; rinse the outer surface with anhydrous ethanol, and then place it in nitric acid solution to clean the outer surface of the copper tube; when the solution turns blue and the sample surface is free of contaminant residue and turns silver-gray, it can be taken out and cleaned with deionized water.

[0010] (2) Sample block preparation: Cut the sample tube longitudinally into two sample blocks. After cutting, polish the cut surface with sandpaper to remove any contaminants that may have been introduced during the cutting process. After polishing until the cutting marks are completely removed, rinse the sample with deionized water and dry it. After drying, soak the sample in acetone for degreasing treatment.

[0011] (3) Soaking: Place the sample block after (2) degreasing treatment with the inner surface facing up into a beaker containing nitric acid solution, and observe the color change of the solution and the floating of substances on the surface of the sample block.

[0012] (4) Preliminary judgment: If no substance is observed to float when the solution turns blue, it is determined that there is no residual carbon in the sample block; if visible black substance floats out during the soaking process, when the solution turns blue, the sample block is picked up and any visible black substance on the surface of the sample block is rinsed into the beaker.

[0013] (5) Extraction of analyte: Take out the rinsed sample block, heat the beaker containing the black substance from (4) and boil for 5 minutes. If the substance disappears after heating, it is determined that there is no residual carbon in the sample block. If the substance does not disappear after heating, filter and dry the solution after cooling to obtain the black substance in the solution;

[0014] (6) Determination of residual carbon: Take the black substance in (5) and place it on the surface of a low melting point metal alloy. After it solidifies, place it under an energy dispersive spectroscopy instrument or an electron probe for composition analysis.

[0015] (7) The test parameters are: working voltage 15-20kV, working distance 10-12mm; if the composition test shows that the mass fraction of C in these black substances is not less than 70%, then it is determined to be residual carbon.

[0016] In the above preparation method, (1) the volume fraction of the nitric acid solution used is 25% to 40%. The higher the concentration, the faster the reaction and the shorter the time required for cleaning.

[0017] In the above preparation method, (1) a tube sample with a length of not less than 30 mm is taken from the copper tube to be tested.

[0018] In the above preparation method, (2) a water-cooled brown corundum abrasive wheel cutter or a clean band saw is used for cutting.

[0019] In the above preparation method, (3) the soaking uses a nitric acid solution with a volume fraction of 30-40%.

[0020] In the above preparation method, after soaking in nitric acid solution, (5) if there is obvious black floating matter that does not disappear after heating to boiling, the solution is filtered and dried; the black substance obtained by filtration is analyzed by energy dispersive spectroscopy or electron probe microanalysis.

[0021] In the above preparation method, the molten low-melting-point alloy mentioned in (6) is an alloy containing tin or lead.

[0022] Advantages of this invention:

[0023] This invention overcomes the difficulties in defining minute substances and film-like substances in Appendix A of GB / T 18033-2017 "Seamless Copper Water Pipes and Copper Gas Pipes". It utilizes a testing method combining acid leaching and component analysis to further determine residual carbon using a more precise detection method, making the testing and judgment standards for residual carbon films clearer. This can provide a reference for the acceptance of copper and copper alloy pipes. Attached Figure Description

[0024] Figure 1 The acid solution is the result of soaking the copper tube sample #1 in a 35% (v / v) nitric acid solution.

[0025] Figure 2 The acid solution obtained by soaking the copper tube sample #2 in a 35% (v / v) nitric acid solution and heating it to boiling for 5 minutes.

[0026] Figure 3 for Figure 2 Scanning electron microscope images of the extract.

[0027] Figure 4 for Figure 2 Energy dispersive spectroscopy (EDS) results of the extract. Detailed Implementation

[0028] The present invention will be further described in conjunction with the embodiments, but the content of the present invention is not limited to the following embodiments.

[0029] In the embodiments of the present invention, the "copper tube" used is an industrial product, and the chemical reagent used is an industrially pure product.

[0030] Example 1: A method for detecting residual carbon on the inner surface of a cupronickel tube.

[0031] (1) Select B30 white copper tubes from different batches, labeled as 1# and 2#. Cut 30mm long sample tubes using a tube cutter, and seal both ends with soft rubber stoppers. Perform preliminary cleaning of the outer surface with anhydrous ethanol to remove dust and other impurities. Prepare a 30% (v / v) nitric acid solution and place the preliminarily cleaned copper tubes in the nitric acid solution. When the marks on the outer surface are completely clear and silvery-gray, remove the sample, rinse with deionized water, and dry.

[0032] (2) Remove the waterproof rubber stopper and use a water-cooled brown corundum cutter to cut the sample tube in half to make two samples. Soak the cut sample pieces in acetone solution for 5-10 minutes to degrease them.

[0033] (3) Prepare a new 35% nitric acid solution. Place the degreased sample block with its inner surface facing up into a beaker containing a 35% nitric acid solution and observe the color change of the solution and the floating of substances from the surface of the sample block.

[0034] (4) Figure 1 As shown, no black substance was observed to float to the surface in the solution after soaking sample tube #1, indicating that there was no residual carbon at that sampling location of the sample tube.

[0035] (5) After soaking sample tube #2, obvious black substances were visible floating in the solution. All substances on the surface of the sample tube were rinsed into a beaker, and the solution was heated to boiling for 5 minutes. After cooling, the solution showed... Figure 2 As shown, obvious black substances are still visible.

[0036] (6) After the solution cools, filter and dry it to obtain the following: Figure 2 The black substance in the solution is shown. The black substance was placed on the surface of a tin alloy, and after solidification, its composition was analyzed using an energy dispersive spectroscopy (EDS) or electron probe microanalysis (EPMA). The test parameters were: operating voltage 15-20 kV, operating distance 10-12 mm.

[0037] (7) The results of component detection in step (6) are as follows Figure 3 As shown, the energy dispersive spectroscopy (EDS) component test results are as follows: Figure 4 As shown, the black substance floating in the solution of sample tube #2 has a carbon content of 92%, which is determined to be residual carbon.

[0038] Example 2: A method for detecting residual carbon film on the inner surface of a brass tube

[0039] Cut a 30mm long brass tube sample using a tube cutter, and seal both ends with soft rubber stoppers. Perform a preliminary cleaning of the outer surface with anhydrous ethanol to remove dust and other impurities. Prepare a 40% (v / v) nitric acid solution and immerse the pre-cleaned copper tube in the solution. When the outer surface is completely clean and silvery-gray, remove the sample, rinse with deionized water, and dry.

[0040] After splitting the sample tube in half with a band saw, polish the cut surface on sandpaper with water until it is shiny to remove impurities caused by cutting, and then degrease it with acetone for 10-15 minutes. The subsequent steps are the same as above (3)-(6), but the concentration of the nitric acid solution used for soaking in step (3) is 30%. The electron probe microanalysis results show that the mass fraction of C in the floating substance in the solution is 83%, which is identified as carbon film.

[0041] As can be seen from the results of the above embodiments, the present invention has the advantages of accuracy and reliability, simple operation, short test cycle, and strong feasibility. It can accurately determine the residual carbon on the inner surface of copper and copper alloy pipes. This reduces the risk of corrosion and perforation of copper and copper alloy pipes caused by residual carbon, improves the reliability of equipment operation, and reduces the cost of equipment maintenance and repair.

[0042] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method for detecting residual carbon on the inner surface of copper and copper alloy tubes, comprising the following steps: (1) Cleaning the outer surface: Take a tube sample with a length of not less than 30 mm from the copper tube to be tested, and plug both ends of the tube with a waterproof plug; rinse the outer surface with anhydrous ethanol, and then place it in nitric acid solution to clean the outer surface of the copper tube; when the solution turns blue and the sample surface is free of contaminant residue and turns silver-gray, it can be taken out and cleaned with deionized water. (2) Sample block preparation: Cut the sample tube longitudinally into two sample blocks. After cutting, polish the cut surface with sandpaper to remove any contaminants that may have been introduced during the cutting process. After polishing until the cutting marks are completely removed, rinse the sample with deionized water and dry it. After drying, soak the sample in acetone for degreasing treatment. (3) Soaking: Place the sample block after (2) degreasing treatment with the inner surface facing up into a beaker containing nitric acid solution, and observe the color change of the solution and the floating of substances on the surface of the sample block. (4) Preliminary judgment: If no substance is observed to float when the solution turns blue, it is determined that there is no residual carbon in the sample block; if visible black substance floats out during the soaking process, when the solution turns blue, the sample block is picked up and any visible black substance on the surface of the sample block is rinsed into the beaker. (5) Extraction of the analyte: Take out the sample block after rinsing, heat the beaker containing the black substance in (4) and boil for 5 minutes. If the substance disappears after heating, it is determined that there is no residual carbon in the sample block. If the substance does not disappear after heating, filter and dry the solution after cooling to obtain the black substance in the solution. (6) Determination of residual carbon: Take the black substance in (5) and place it on the surface of a low melting point metal alloy. After it solidifies, place it under an energy dispersive spectrometer or an electron probe for composition analysis. (7) The test parameters are: working voltage 15-20kV, working distance 10-12mm; if the composition test shows that the mass fraction of C in these black substances is not less than 70%, then it is determined to be residual carbon.

2. The detection method as described in claim 1, (1) the volume fraction of the nitric acid solution used to clean the outer surface is 25%~40%.

3. The detection method as described in claim 1, (3) the soaking is done with a nitric acid solution with a volume fraction of 30-40%.

4. According to the detection method described in claim 1, after soaking in nitric acid solution, (5) if there is obvious black floating matter that does not disappear after heating and boiling, the solution is filtered and dried; the black substance obtained by filtration is analyzed by energy dispersive spectroscopy or electron probe microanalysis.

5. The detection method as described in claim 1, (6) the low melting point metal alloy is: an alloy containing tin or lead.

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