A corrosion method for revealing the metallographic structure of alloy samples

Abstract

The application discloses a kind of corrosion methods for revealing alloy sample metallographic structure, comprising: step S1, preparation electrolytic etchant: using oxalic acid, sodium bicarbonate and concentrated nitric acid preparation final concentration is 0.25-0.5mol / L sodium oxalate and 0.15-0.3mol / L nitric acid mixed aqueous solution;Step S2, electrolytic etching of corrosion piece: electrolytic etchant is poured into electrolytic etching instrument, and the alloy sample to be corroded is placed in electrolytic etching liquid and electrolytic etching is carried out.The electrolytic etchant of the application uses oxalic acid, sodium bicarbonate and concentrated nitric acid as reactant, and is prepared by chemical ion reaction.After the preparation of etchant is finished, a small amount of carbonate ions and hydrogen ions are still stored in the etchant, so that a small amount of carbonate ions and hydrogen ions will continue to react when the alloy sample is electrolyzed, and micro-bubbles are attached to the alloy sample, so that the metallographic photo taken has stereoscopic imaging effect, and the grain boundary is clear and identifiable.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of metallographic electrolytic etching, in particular to an etching method for showing metallographic structure of alloy sample. BACKGROUND

[0002] The austenitic stainless steel refers to the stainless steel with austenitic structure at normal temperature, which is characterized by non-magnetic and high toughness and plasticity but low strength. The stainless steel material has excellent corrosion resistance, formability, compatibility and high strength and toughness in a wide temperature range, so it is widely used in heavy industry, light industry, household goods industry and building decoration industry.

[0003] At present, the industry generally uses 10% oxalic acid aqueous solution electrolytic etching to show the structure of the austenitic stainless steel after solid solution, but the grain structure obtained by this method will appear twin crystal, and the grain boundary is not complete, the twin crystal will obviously interfere with the measurement and evaluation of the grain size, and the incomplete grain boundary cannot clearly evaluate the grain size. The prior art mainly aims at showing the details of the structure of the austenitic stainless steel after solid solution, and comprehensively analyzing the structure properties; but the effect of grain size evaluation is not ideal. SUMMARY

[0004] The purpose of the present application is to provide an etching method for showing the metallographic structure of alloy sample, which will not appear twin crystal in the grain structure, the grain boundary is complete and clear, and the metallographic structure photo taken has a stereoscopic imaging effect.

[0005] In order to achieve the above purpose, the present application provides an etching method for showing the metallographic structure of alloy sample, which comprises the following steps:

[0006] Step S1, preparing electrolytic etchant: using oxalic acid, sodium bicarbonate and concentrated nitric acid to prepare sodium oxalate with a final concentration of 0.25-0.5 mol / L and nitric acid mixed aqueous solution with a concentration of 0.15-0.3 mol / L;

[0007] Step S2, electrolytic etching of the corrosion part: pouring the electrolytic etchant into the electrolytic etching instrument, and placing the alloy sample to be etched in the electrolytic etching solution for electrolytic etching.

[0008] Preferably, in step S1, the preparation method of the electrolytic etchant is as follows: weighing 45-90g of oxalic acid, 42-84g of sodium bicarbonate and 400-500mL of deionized water, slowly adding them into a beaker and stirring to dissolve, slowly adding 5-15mL of concentrated nitric acid after waiting for the bubbles to be calm, and finally using deionized water to make up to 1000ml.

[0009] Preferably, in step S2, the cathode material of the electrolytic etching instrument is platinum or stainless steel, and the anode is the alloy sample to be etched.

[0010] Preferably, the voltage of the electrolytic corrosion in step S2 is 15-25V, the current is 1-2A, and the time is 5-10s.

[0011] Preferably, before the electrolytic corrosion of the to-be-corroded member in step S2, the to-be-corroded member needs to be pretreated, and the pretreatment method is: sequentially using 400#, 600#, 800# and 1000# sandpaper to polish the to-be-corroded alloy sample, and then using diamond polishing agent for polishing treatment; the polishing includes any one or a combination of two or more of mechanical polishing, chemical polishing and mechanical-chemical polishing.

[0012] Preferably, after the electrolytic corrosion of the alloy sample, the alloy sample is cleaned by anhydrous ethanol and then air-dried.

[0013] Compared with the prior art, the present application has the following beneficial effects:

[0014] (1) The grain structure obtained by using the electrolytic corrosion agent of the present application to corrode the metallographic structure of the alloy sample does not appear twin crystal, and the grain boundary is complete and clear.

[0015] (2) The electrolytic corrosion agent of the present application uses nitric acid and sodium oxalate as main components, and the passivation effect of nitric acid and the alkaline adjustment of sodium oxalate make the alloy sample of the present application not easy to be over-corroded.

[0016] (3) The corrosion method of the present application for showing the metallographic structure of the alloy sample has simple formula, short required corrosion time, small required current and good corrosion effect.

[0017] (4) The corrosion effect of the present application is high, has stereoscopic imaging effect, and the metallographic structure photo has obvious layering and clear grain boundary. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 The alloy metallographic structure corroded by the present application example 1 has a magnification of 100 times;

[0019] Figure 2 The alloy metallographic structure corroded by the present application example 2 has a magnification of 200 times;

[0020] Figure 3 The alloy metallographic structure corroded by the present application example 3 has a magnification of 100 times;

[0021] Figure 4 The alloy metallographic structure corroded by the present application example 4 has a magnification of 200 times;

[0022] Figure 5 The alloy metallographic structure corroded by the present application example 5 has a magnification of 100 times;

[0023] Figure 6 The metallographic structure of the alloy corroded for the embodiment 6 of the present application, the magnification is 200 times;

[0024] Figure 7 The metallographic structure of the alloy corroded for the comparative example 1 of the present application, the magnification is 100 times;

[0025] Figure 8 The metallographic structure of the alloy corroded for the comparative example 2 of the present application, the magnification is 200 times. DETAILED DESCRIPTION

[0026] The technical solutions of the present application are further described below in combination with the drawings and examples.

[0027] At present, 10% oxalic acid aqueous solution is generally used for electrolytic corrosion of austenitic stainless steel in the industry, and the grain structure obtained by this method will appear twin crystal, and the grain boundary is not complete, which will obviously interfere with the measurement and evaluation of the grain size, because in the grain size measurement standard, the calculation standard has made provisions for twin crystal, and if there is twin crystal in the grain, the twin crystal volume will not be counted in the calculation. Therefore, if twin crystal appears after the corrosion of the austenitic stainless steel, it will seriously interfere with the evaluation and calculation of the grain size.

[0028] In order to solve the technical problems that twin crystal and unclear grain boundary will appear in the corrosion by using the existing corrosion agent, the present application provides an electrolytic corrosion agent for showing the metallographic structure of an alloy sample. The electrolytic corrosion agent for showing the metallographic structure of an alloy sample comprises: 0.25-0.5 mol / L of sodium oxalate and 0.15-0.3 mol / L of nitric acid aqueous solution. The present application proves by experiments that: ①the grain structure obtained by using the electrolytic corrosion agent of the present application will not appear twin crystal, and the grain boundary is complete and clear; ②the present application uses nitric acid and sodium oxalate as the main components of the electrolytic corrosion agent, and the passivation effect of nitric acid plus the alkalinity adjustment of sodium oxalate makes the alloy sample of the present application not easy to be excessively corroded; ③since the ion reactions occur during the preparation of the electrolytic corrosion agent of the present application, the reaction efficiency will not reach 100%, and therefore a small amount of carbonate ions and hydrogen ions still exist in the corrosion agent after the preparation is completed, and the trace amount of carbonate ions and hydrogen ions will continue to react when the alloy sample is electrolyzed, and micro-bubbles will be generated and attached to the alloy sample, and when the metallographic structure photo is taken, the micro-bubbles act as contrast agents to achieve the effect of stereoscopic imaging, and the taken photo has obvious level sense and clear grain boundary.

[0029] The following will be described in combination with specific examples.

[0030] Example 1

[0031] The present embodiment provides an electrolytic corrosion agent for showing the metallographic structure of an alloy sample, and the components of the electrolytic corrosion agent comprise: 0.5 mol / L of sodium oxalate and 0.3 mol / L of nitric acid.

[0032] The embodiment also provides a method for etching an alloy sample by using the electrolytic etchant, and the method comprises the following steps:

[0033] In step S1, the electrolytic etchant is prepared by weighing 90 g of oxalic acid, dissolving the oxalic acid in deionized water, weighing 84 g of sodium bicarbonate, slowly adding the sodium bicarbonate into the oxalic acid solution and closing the beaker, stirring and mixing, heating the solution to a slight boil after waiting for the bubbles to subside, stirring and mixing again until the sodium oxalate is completely dissolved, then slowly adding 15 ml of concentrated nitric acid by using a pipette, and finally using deionized water to make the solution to 1000 ml.

[0034] In step S2, the etching part is pretreated by polishing the alloy sample to be etched by using 400#, 600#, 800# and 1000# water sandpaper in sequence and then using diamond polishing agent for polishing.

[0035] In step S3, the etching part is electrolytically etched by pouring the electrolytic etchant prepared in step S1 into an electrolytic etching instrument, using platinum as the cathode material of the electrolytic etching instrument, using the polished alloy sample as the anode, placing the alloy sample and the cathode in parallel with the test surface of the alloy sample facing the cathode, setting the electrolytic etching direct current voltage to 25 V, the current to 2 A, the electrolytic etching temperature to room temperature, and the electrolytic etching time to 10 s.

[0036] In step S4, after the etching is completed, the sample is taken out from the etchant, cleaned by using anhydrous ethanol and then air dried.

[0037] The etched surface is observed by using a metallographic microscope, and the metallographic structure is shown in Figure 1 .

[0038] Embodiment 2

[0039] The electrolytic etchant of the embodiment is different from that of embodiment 1 in that:

[0040] The components of the electrolytic etchant of the embodiment include 0.25 mol / L of sodium oxalate and 0.15 mol / L of nitric acid.

[0041] The method for etching an alloy sample by using the electrolytic etchant of the embodiment is different from that of embodiment 1 in that:

[0042] In step S1, the electrolytic etchant is prepared by weighing 45 g of oxalic acid, dissolving the oxalic acid in deionized water, weighing 42 g of sodium bicarbonate, slowly adding the sodium bicarbonate into the oxalic acid solution and closing the beaker, stirring and mixing, heating the solution to a slight boil after waiting for the bubbles to subside and stirring and mixing again, then slowly adding 7.5 ml of concentrated nitric acid by using a pipette, and finally using deionized water to make the solution to 1000 ml. The etched surface is observed by using a metallographic microscope, and the metallographic structure is shown in Figure 2 .

[0043] Embodiment 3

[0044] The electrolytic etchant of the present example is different from that of Example 1 in that:

[0045] The components of the electrolytic etchant of the present example include: 0.25 mol / L sodium oxalate and 0.3 mol / L nitric acid.

[0046] The method for etching the alloy sample by the electrolytic etchant of the present example is different from that of Example 1 in that:

[0047] In Step S1, the method for preparing the electrolytic etchant is as follows: 45 g of oxalic acid is weighed and dissolved in deionized water, 42 g of sodium bicarbonate is weighed and slowly added into the oxalic acid solution, and the beaker is closed and stirred to mix uniformly. After the bubbles are calm, the solution is heated to a slight boil and stirred again to mix uniformly. Then, 15 ml of concentrated nitric acid is slowly added by using a pipette, and finally, deionized water is used to make the volume to 1000 ml. The corrosion surface is observed by a metallographic microscope, and it is found that Figure 3 .

[0048] Example 4

[0049] The electrolytic etchant of the present example is different from that of Example 1 in that:

[0050] The components of the electrolytic etchant of the present example include: 0.5 mol / L sodium oxalate and 0.15 mol / L nitric acid.

[0051] The method for etching the alloy sample by the electrolytic etchant of the present example is different from that of Example 1 in that:

[0052] In Step S1, the method for preparing the electrolytic etchant is as follows: 90 g of oxalic acid is weighed and dissolved in deionized water, 84 g of sodium bicarbonate is weighed and slowly added into the oxalic acid solution, and the beaker is closed and stirred to mix uniformly. After the bubbles are calm, the solution is heated to a slight boil and stirred again to mix uniformly. Then, 7.5 ml of concentrated nitric acid is slowly added by using a pipette, and finally, deionized water is used to make the volume to 1000 ml. The corrosion surface is observed by a metallographic microscope, and it is found that Figure 4 .

[0053] Example 5

[0054] The electrolytic etchant of the present example is the same as that of Example 1, and the method for etching the alloy sample by the electrolytic etchant is different from that of Example 1 in that:

[0055] In Step S3, electrolytic etching of the corrosion piece: the electrolytic etchant prepared in Step S1 is poured into an electrolytic etching instrument, the cathode material of the electrolytic etching instrument is platinum, the polished alloy sample is used as an anode, the test surface of the alloy sample faces the cathode, and the two are placed in parallel. The direct current voltage for electrolytic etching is 15 V, the current is 1 A, the electrolytic etching temperature is room temperature, and the electrolytic etching time is 10 s. The corrosion surface is observed by a metallographic microscope, and it is found that Figure 5 .

[0056] Example 6

[0057] The electrolytic etchant of this embodiment is the same as that of embodiment 1. The difference between the method of corroding the alloy sample by the electrolytic etchant and that of embodiment 1 is as follows:

[0058] Step S3, electrolytic corrosion of the workpiece: Pour the electrolytic corrosive agent prepared in step S1 into the electrolytic corrosion instrument. The cathode material of the electrolytic corrosion instrument is platinum. The polished alloy sample is used as the anode. The test surface of the alloy sample faces the cathode, and the two are placed parallel. The electrolytic corrosion DC voltage is 15V, the current is 1A, the electrolytic corrosion temperature is room temperature, and the electrolytic corrosion time is 8s. The corroded surface is observed under a metallographic microscope. Figure 6 .

[0059] Comparative Example 1

[0060] The electrolytic etchant used in this comparative example is 10% oxalic acid. The method for corroding the alloy sample using the electrolytic etchant is as follows:

[0061] Step S1, pretreatment of the part to be corroded: the alloy sample to be corroded is polished with 400#, 600#, 800#, and 1000# water-abrasive sandpaper in sequence, and then polished with a diamond polishing agent.

[0062] Step S2, electrolytic corrosion of the workpiece to be corroded: pour 10% oxalic acid (commercially available) into an electrolytic corrosion instrument, the cathode material of the electrolytic corrosion instrument is platinum, and the polished alloy sample is used as the anode. The test surface of the alloy sample faces the cathode, and the two are placed parallel. The electrolytic corrosion DC voltage is 25V, the current is 2A, the electrolytic corrosion temperature is room temperature, and the electrolytic corrosion time is 10s.

[0063] Step S3: After the etching is completed, the sample is taken out from the etching agent, rinsed with anhydrous ethanol and then air-dried.

[0064] The corrosion surface was observed under a metallographic microscope. Figure 7 .

[0065] Comparative Example 2

[0066] The difference between this comparative example and comparative example 1 is:

[0067] Step S2 of the method for corroding the alloy sample with an electrolytic corrosive agent in this comparative example is: pouring 10% oxalic acid (commercially available) into an electrolytic corrosion instrument, the cathode material of the electrolytic corrosion instrument is platinum, and the polished alloy sample is used as the anode. The test surface of the alloy sample faces the cathode, and the two are placed parallel to each other. The electrolytic corrosion DC voltage is 15V, the current is 1A, the electrolytic corrosion temperature is room temperature, and the electrolytic corrosion time is 10s.

[0068] like Figures 1-6 As shown in the metallographic photograph, there is no twin boundary and the grain boundary is complete, such as Figure 7 andFigure 8 As shown, the metallographic photograph contains twin grain boundaries, and the grain boundaries are not complete.

[0069] In summary, the present application provides an electrolytic etchant for revealing the metallographic structure of an alloy sample and an etching method, the electrolytic etchant has a simple formula, and nitric acid and sodium oxalate are used as main components, the passivation effect of nitric acid and the alkaline adjustment of sodium oxalate make the alloy sample not easy to be excessively corroded. Moreover, when the alloy sample is etched by using the electrolytic etchant, the metallographic structure of the alloy sample has no twin grain boundaries, and the grain boundaries are complete, so that the grain size evaluation of the alloy sample is not disturbed.

[0070] Although the content of the present application has been described in detail by the above preferred embodiments, it should be recognized that the above description should not be considered as a limitation to the present application. After reading the above content, various modifications and substitutions of the present application will be apparent to those skilled in the art. Therefore, the protection scope of the present application should be defined by the appended claims.

Claims

1. A corrosion method for revealing the metallographic structure of an austenitic stainless steel sample, characterized in that: The steps include: Step S1, preparing an electrolytic corrosive agent: using oxalic acid, sodium bicarbonate, and concentrated nitric acid to prepare a mixed aqueous solution of sodium oxalate and nitric acid with a final concentration of 0.25-0.5 mol / L; Specifically, weigh 45-90g of oxalic acid and dissolve it in deionized water. Weigh 42-84g of sodium bicarbonate and slowly add it to the oxalic acid solution and seal the beaker. Stir and mix thoroughly. After the bubbles subside, heat the solution to a slight boil to completely dissolve the sodium oxalate. Stir and mix again. Then, slowly add 5-15ml of concentrated nitric acid with a pipette. Finally, use deionized water to make up to 1000ml. Step S2, electrolytic corrosion of the workpiece to be corroded: pouring the electrolytic corrosive agent into an electrolytic corrosion apparatus, placing the austenitic stainless steel sample to be corroded in the electrolytic corrosion solution for electrolytic corrosion; the electrolytic corrosion causes the grains and grain boundaries of the austenitic stainless steel to be revealed; In step S2, the voltage of the electrolytic corrosion is 15-25V, the current is 1-2A, and the time of the electrolytic corrosion is 5-10s.

2. The etching method according to claim 1, wherein In step S2, the cathode material of the electrolytic corrosion instrument is platinum or stainless steel, and the anode is the austenitic stainless steel sample to be corroded.

3. The etching method according to claim 1, wherein: Before the electrolytic corrosion of the workpiece in step S2, the workpiece needs to be pretreated. The pretreatment method is: use 400#, 600#, 800#, and 1000# sandpaper to grind the austenitic stainless steel sample to be corroded in sequence, and then use diamond polishing agent to polish it; the polishing includes any one of mechanical polishing, chemical polishing and mechanical chemical polishing, or a combination of any two or more.

4. The etching method according to claim 1, wherein: After the austenitic stainless steel sample was electrolytically corroded, the sample was rinsed with anhydrous ethanol and then air-dried.

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