An efficient metallographic electrolytic etchant for Hastelloy and a detection method

Abstract

The present invention discloses an efficient metallographic electrolytic etchant for Hastelloy and a detection method. The electrolytic etchant comprises: 160 parts by weight of copper sulfate, 55 - 60 parts by weight of sodium chloride, 80 - 100 parts by weight of potassium permanganate, 50 - 150 parts by weight of nitric acid, and 4500 - 4700 parts by weight of distilled water. The detection method provided by the present invention uses a Hastelloy sample block to be tested as the anode, a stainless steel plate or a platinum electrode as the cathode, and the above-mentioned efficient metallographic electrolytic etchant as the electrolyte for electrolytic etching, so that the Hastelloy sample block to be tested shows the grain boundaries. The metallographic electrolytic etchant provided by the present invention has rich raw material sources, low cost, and a simple preparation method. It is used for the metallographic detection of Hastelloy, with simple and convenient operation. After electrolytic etching, the metallographic structure is clear and the grain boundaries are distinct.

Description

Technical Field

[0001] The present invention relates to metallographic inspection technology, and particularly to an efficient metallographic electrolytic etchant for Hastelloy and an inspection method thereof. Background Art

[0002] Hastelloy is a nickel-based corrosion-resistant alloy, mainly divided into two categories: nickel-chromium alloy and nickel-chromium-molybdenum alloy. Hastelloy has good corrosion resistance and thermal stability, and is mostly used in the aviation industry, chemical field, etc.

[0003] Nickel-based corrosion-resistant alloys are widely used in strongly corrosive environments such as oil and gas due to their excellent corrosion resistance and outstanding mechanical properties. Because of their superior performance and high economic benefits, they have become the objects of competing research.

[0004] However, the problem that it is not easy to prepare nickel-based metallography accompanied by excellent corrosion resistance has brought difficulties to the optimization research work of nickel-based alloy materials. Especially Hastelloy, which is the most corrosion-resistant type among nickel-based alloys, is a tungsten-containing nickel-chromium-molybdenum alloy with extremely low silicon and carbon contents, and is considered a universal corrosion-resistant alloy. Hastelloy is mainly resistant to wet chlorine, various oxidizing chlorides, chloride solutions, sulfuric acid and oxidizing salts, and has good corrosion resistance in low-temperature and medium-temperature hydrochloric acid. Due to the excellent corrosion resistance of this alloy, the corrosion detection of its grains and phase structure has always been a difficult problem.

[0005] However, the currently disclosed corrosion methods and etchants for nickel-based alloys have unsatisfactory corrosion effects on Hastelloy metallography. Summary of the Invention

[0006] The purpose of the present invention is to solve the problem of unsatisfactory Hastelloy metallographic corrosion effect, and provide an electrolytic etchant as the electrolyte to realize the corrosion detection of grains and phase structure through the electrolytic corrosion method.

[0007] In order to achieve the above purpose, the present invention provides an efficient metallographic electrolytic etchant for Hastelloy, which comprises:

[0008] 160 parts by weight of copper sulfate

[0009] 55 - 60 parts by weight of sodium chloride

[0010] 80 - 100 parts by weight of potassium permanganate

[0011] 50 - 150 parts by weight of nitric acid

[0012] 4500 - 4700 parts by weight of distilled water.

[0013] Optionally, the pH value of the electrolytic etchant is 1.5 - 3.

[0014] The present invention also provides a method for detecting the corrosion of the grains and phase structure of Hastelloy, and the method includes:

[0015] Step 1, prepare the above-mentioned highly efficient metallographic electrolytic etchant for Hastelloy;

[0016] Step 2, use the Hastelloy sample block to be tested as the anode, a stainless steel plate or a platinum electrode as the cathode, and the above-mentioned highly efficient metallographic electrolytic etchant as the electrolyte to conduct electrolytic etching, so that the Hastelloy sample block to be tested shows grain boundaries.

[0017] Optionally, in Step 2, the electrode current for electrolytic etching is 1 - 6 A, and the voltage is 15 - 25 V. More preferably, the electrode current for electrolytic etching is 1 - 3 A.

[0018] Optionally, in Step 2, the electrolytic etching time is 5 - 8 minutes.

[0019] Optionally, before the electrolytic etching of the Hastelloy sample to be tested, it needs to be pretreated by polishing, and the polishing includes any one or any combination of two or more of mechanical polishing, chemical polishing, and mechanical chemical polishing.

[0020] Further, the polishing is mechanical polishing.

[0021] Compared with the prior art, the beneficial effects of the present invention at least include:

[0022] According to the influence of the reaction coefficient K, the present invention adjusts the ratio of nitric acid to potassium permanganate, provides the metallographic electrolytic etchant as the electrolyte, uses an inert electrode as the cathode, and the Hastelloy sample block to be tested as the anode. Under specific voltage and current, electrolytic etching for a certain time can make the Hastelloy sample block to be tested corrode quickly to present clear grain boundaries and grains. The metallographic electrolytic etchant provided by the present invention has rich raw material sources, low cost, and simple preparation method. It is used for the metallographic detection of Hastelloy, is simple and convenient to use, and the metallographic structure is clear and the grain boundaries are distinct after electrolytic etching. Description of the Drawings

[0023] Figure 1a-1b It is a scanning electron microscope image of the Hastelloy sample block after corrosion treatment in Example 1 of the present invention.

[0024] Figure 2a-2b It is a scanning electron microscope image of the Hastelloy sample block after corrosion treatment in Example 2 of the present invention.

[0025] Figure 3a-3b It is a scanning electron microscope image of the Hastelloy sample block after corrosion treatment in Example 3 of the present invention.

[0026] Figure 4 It is a scanning electron microscope image of the Hastelloy sample block after corrosion treatment in Comparative Example 1 of the present invention.

[0027] Figure 5 SEM image of the Hastelloy alloy sample block after the corrosion treatment of Comparative Example 2 of the present invention.

[0028] Figure 6 SEM image of the Hastelloy alloy sample block after the corrosion treatment of Comparative Example 3 of the present invention.

[0029] Figure 7 SEM image of the Hastelloy alloy sample block after the corrosion treatment of Comparative Example 4 of the present invention. Detailed implementation manners

[0030] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0031] In the description of the present invention, it should be noted that the orientation or positional relationship indicated by the terms "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings. It is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus cannot be understood as a limitation to the present invention. In addition, the terms "first", "second", "third" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0032] In the description of the present invention, it should be noted that unless otherwise clearly specified and limited, the terms "installation", "connection", "connection" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection; it can be directly connected, or indirectly connected through an intermediate medium, and it can be the communication inside two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific situations.

[0033] The difference in the reflection ability of the metallographic structure must be at least 10% or more in order to reflect light of different intensities and be observed. However, the surface of the polished specimen cannot show the metallographic structure because the incident light is almost evenly reflected back. Therefore, in order to see clearly, it is usually necessary to create a contrast in the structure. To obtain this contrast, the specimen usually needs to be subjected to metallographic treatment. Common treatment methods include: chemical etching, electrolytic etching, cathode vacuum etching, thermal corrosion, and thin film interference method.

[0034] Chemical etching is carried out without an external power source, while electrolytic etching is to immerse the polished specimen in a solution of a suitable chemical reagent (electrolytic etchant) and carry out etching through a small direct current. The working voltage and working current of electrolytic etching are usually small. The working voltage is generally between 2 and 6 V, and the working current is about 0.05 - 0.3 A / cm2. Electrolytic etching is mainly used for alloys with relatively high chemical stability, such as stainless steel, heat-resistant steel, nickel-based alloys, etc. It is difficult to obtain clear microstructures for these alloys by chemical etching.

[0035] Example 1

[0036] Prepare 5 L of Hastelloy metallographic electrolytic corrosion agent: Take 160 g of copper sulfate, dissolve it in distilled water, then slowly add 55 g of sodium chloride, heat it to a gentle boil with stirring, and then slowly cool it to room temperature. Add 80 g of potassium permanganate, make up the volume to 4000 mL with distilled water, and then slowly add about 50 ml of concentrated nitric acid with a pipette. Use a pH meter to adjust the pH value to be between 1.5 and 3. Continue to make up the volume to 5000 mL with distilled water.

[0037] Polish the experimental surface of the Hastelloy sample block. The polishing includes any one or any combination of two or more of mechanical polishing, chemical polishing, and mechanical chemical polishing methods. In this example, mechanical polishing is used.

[0038] Use the Hastelloy sample block as the anode and the platinum electrode as the cathode, with the experimental surface of the Hastelloy sample block facing the cathode; use the above-prepared electrolytic corrosion solution as the electrolyte for electrolytic corrosion. The current is about 0 - 1.5 A, the voltage is about 15 - 25 V, and the immersion electrolysis time is 5 - 8 minutes. Take out the Hastelloy sample block, clean the surface and then carry out drying treatment. The scanning electron microscope images are as shown in Figure 1a 、 Figure 1b shown, and the grain boundaries are clearly presented for evaluation.

[0039] Example 2

[0040] Same as Example 1, except that the current is 1 - 3 A, the voltage is about 15 - 25 V, and the immersion electrolysis time is 5 - 8 minutes. Take out the Hastelloy sample block, clean the surface and then carry out drying treatment. The scanning electron microscope images are as shown in Figure 2a 、 Figure 2b shown, and the grain boundaries are clearly presented for evaluation.

[0041] Example 3

[0042] Same as Example 1, except that the current is 3 - 6 A, the voltage is about 15 - 25 V, and the immersion electrolysis time is 5 - 8 minutes.

[0043] Take out the Hastelloy sample block, clean the surface and then carry out drying treatment. The scanning electron microscope images are as shown in Figure 3a 、 Figure 3b shown, and the grain boundaries are clearly presented for evaluation.

[0044] Comparative Example 1

[0045] Take the Hastelloy sample block as the anode and the platinum electrode as the cathode, with the experimental surface of the Hastelloy sample block facing the cathode; 10 wt% oxalic acid is used as the electrolyte for electrolytic corrosion. The current is about 1 - 2 A, the voltage is about 15 - 25 V, and the immersion electrolysis time is 10 - 15 minutes.

[0046] Take out the Hastelloy sample block, clean the surface and then perform drying treatment. The scanning electron microscope image is as Figure 4 shown. The corrosion is too shallow and the grain boundaries are not obvious, so it cannot be used for evaluation.

[0047] Comparative Example 2

[0048] Take the Hastelloy sample block as the anode and the stainless steel plate as the cathode, with the experimental surface of the Hastelloy sample block facing the cathode; 15 wt% chromic acid is used as the electrolyte for electrolytic corrosion. The current is about 1 - 2 A, the voltage is about 15 - 25 V, and the immersion electrolysis time is 10 - 15 minutes.

[0049] Take out the Hastelloy sample block, clean the surface and then perform drying treatment. The scanning electron microscope image is as Figure 5 shown. The corrosion is too shallow and the grain boundaries are not obvious, so it cannot be used for evaluation.

[0050] Comparative Example 3

[0051] Immerse the Hastelloy sample block in a solution prepared with 10 ml of sulfuric acid, 100 ml of hydrochloric acid, and 10 g of anhydrous copper sulfate for 15 - 25 minutes.

[0052] Take out the Hastelloy sample block, clean the surface and then perform drying treatment. The scanning electron microscope image is as Figure 6 shown. There are obvious corrosion pits, so it cannot be used for evaluation.

[0053] Comparative Example 4

[0054] Immerse the Hastelloy sample block in a solution prepared with 75 ml of hydrochloric acid and 25 ml of nitric acid for 15 - 25 minutes.

[0055] Take out the Hastelloy sample block, clean the surface and then perform drying treatment. The scanning electron microscope image is as Figure 7 shown. There are corrosion pits, so it cannot be used for evaluation.

[0056] By comparing the metallographic structure pictures of the crystal phase structures of Example 1 and Comparative Examples 1 - 4 above, it can be seen that the detection method provided by the present invention has the following advantages: short corrosion time; relatively easy control of current and voltage; not easy to over - corrode, with fewer corrosion pits; and clearer grains. Among them, Example 2 has the best effect.

[0057] In summary, according to the influence of the reaction coefficient K, the present invention adjusts the ratio of nitric acid to potassium permanganate, provides the metallographic electrolytic etchant as the electrolyte, uses an inert electrode as the cathode, and the sample block of Hastelloy to be tested as the anode. Under specific voltage and current, electrolytic etching is carried out for a certain time, which can make the sample block of Hastelloy to be tested corrode rapidly to present clear grain boundaries and grains, and can be used for the metallographic inspection of Hastelloy. It is simple and convenient to use, and the metallographic structure is clear and the grain boundaries are distinct after electrolytic etching.

[0058] Although the content of the present invention has been introduced in detail through the above preferred embodiments, it should be recognized that the above description should not be considered as a limitation of the present invention. After those skilled in the art have read the above content, various modifications and substitutions of the present invention will be obvious. Therefore, the protection scope of the present invention should be defined by the appended claims.

Claims

1. An efficient metallographic electrolytic etchant for Hastelloy, characterized in that, The electrolytic etchant contains: 160 parts by weight of copper sulfate 55 - 60 parts by weight of sodium chloride 80 - 100 parts by weight of potassium permanganate 50 - 150 parts by weight of nitric acid 4500 - 4700 parts by weight of distilled water The pH value of the electrolytic etchant is 1.5 - 3.

2. A method for detecting the corrosion of the grain and phase structure of Hastelloy, characterized in that The method includes: Step 1, preparing a high - efficiency metallographic electrolytic etchant for Hastelloy as described in claim 1; Step 2, using the Hastelloy sample block to be tested as the anode, an inert electrode as the cathode, and the high - efficiency metallographic electrolytic etchant as the electrolyte for electrolytic etching, so that the Hastelloy sample block to be tested shows grain boundaries.

3. The method for detecting the grain and phase structure corrosion of Hastelloy according to claim 2, characterized in that, In Step 2, the electrode current for electrolytic etching is 1 - 6 A, and the voltage is 15 - 25 V.

4. The method for detecting the grain and phase structure corrosion of Hastelloy according to claim 3, characterized in that, In Step 2, the electrode current for electrolytic etching is 1 - 3 A.

5. The method for detecting the grain and phase structure corrosion of Hastelloy according to claim 2, characterized in that, In Step 2, the electrolytic etching time is 5 - 8 minutes.

6. The method for detecting the grain and phase structure corrosion of Hastelloy according to claim 2, characterized in that, Before the electrolytic etching, the Hastelloy sample block to be tested needs to be pretreated by polishing, and the polishing includes any one or any combination of two or more of mechanical polishing, chemical polishing, and mechanical - chemical polishing methods.

7. The method for detecting the grain and phase structure corrosion of Hastelloy according to claim 6, characterized in that, The polishing is mechanical polishing.

8. The method for detecting the grain and phase structure corrosion of Hastelloy according to claim 2, wherein The inert electrode includes a stainless - steel plate or a platinum electrode.

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