Metallographic Sample Preparation : Chemical Etching

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1 Metallographic Sample Preparation : Chemical Etching Related terms Etching, Crystallographic structure, Micrographic inspection, Microscopy, Metallographic inspection Method Chemical etching is the most common method for contrasting polished metal surfaces to reveal structural details of pure metals and alloys. The precondition for a good result in etching is a carefully polished and clean surface. The experiment describes the basic procedure, gives some recipes and presents a few pictures as possible results. Equipment Metallurgical microscope, magn With Micrometer eyepiece Light bulb 6V/30W, replacement Video camera, 5 MPixel, w. software Press for polished sections Plasticine, 10 rods Microscope slides, pkg. of Balance, DENVER DLT-411, 400g/0.1g Power supply for DLT balances Hot-air blower, 1200W Marking pencils, set, waterproof Labels, blank, 40 pcs Protective gloves Safety goggles Wash bottle, plastic, 250 ml Pasteur pipettes, 3 ml, PE, 500 pcs Crystallising dish, 190 ml Dish, PVC, white, 300x240x65 mm Beaker, low, 250 ml Glass rod, l = 200 mm, d = 3 mm Spoon w. spatula end, PA, l = 180 mm Sieve, plastic, dia. 60 mm Set of eight raw samples consisting of: brass (CuZn40Pb2F44), copper (E-CuF25), aluminium (Al99zh), Al alloy (AlMgSiPbF28), steel (9s20k), alloyed steel (x12crmos17) steel, 750 C heat treated, brass, 600 C heat treated Additionally required: PC or notebook, WindowsXP or higher Continued on next page Fig. 1: Equipment for viewing etched metal specimens (etching utensils not shown) P PHYWE Systeme GmbH & Co. KG All rights reserved 1

2 Metallographic Sample Preparation : Chemical Etching Chemicals: Nitric acid, 65%, 500m ml Hydrochloric acid, 30%, 500 ml Ammonia solution, 25%, 250 ml Hydrogen peroxide, 30%, 250 ml Sodium hydroxide, 500 g Zinc chloride, 250 g Iron-III chloride, 250 g Ethyl alcohol, denatured, 1000 ml Propanol, 1000 ml Attention! Please obey all safety rules in handling hazardous chemicals. Wear gloves and safety glasses when working with caustic solutions. Recommended accessories: Metallographic sample preparation: Grinding and Polishing of Metals, P Task 1. Check the six metal specimens polished according to Experiment Section 1.1 by means of the microscope to see if any macroscopic or microscopic structural features can be noticed. 2. Prepare the etching solutions and etch the specimens according to the instructions. 3. Examine the specimen surfaces as to whether the structural details have been satisfactorily revealed. Experimental Procedure and Results Assemble the metallurgical microscope and the video camera according to the Operating Instructions, connect the illumination power supply and the additionally required PC, and install the included software on your PC according to the instructions. Place the equipment together with the press for polished sections and the polished samples(see Fig. 1) on a clean laboratory table. Put the utensils needed for etching at a sufficient distance from the microscope equipment on another table which should be close to a water tap and a ceramic sink. Place a 250 ml beaker on the balance platform, pour the required quantity of distilled water and/or ethyl alcohol into the beaker, and cautiously add, by means of a disposable pipette, the required volume of acid according to the prescriptions below. Larger quantities of liquids required can be dosed directly from the reagent storage bottles and the final dosage be made with a Pasteur pipette. Solid substances are added by means of the plastic spoon. Stir well with the glass rod until all substances have been completely dissolved. Only prepare a quantity provided for use at the same day since most of the solutions are not very stable. If you prepare several solutions for different samples simulataneously, stick labels and hazard symbols on the beakers to avoid confusion. It is recommended to wear protective gloves and goggles while preparing the solutions and during the etching procedure. Choose one of the polished samples from the set and inspect it under the microscope as to whether the polished surface is virtually free of scratches, corrosion and other defects. Any traces of oil (e.g. from the final polishing) or grease and dirt should be removed with a clean soft cloth wetted with propanol. Polished soft metal surfaces (e.g. aluminium) should not be rubbed, not even with a soft cloth. Place both crystallising dishes side by side in the white plastic dish and pour the contents of the beaker, i.e. about 150 g of the etchant designed for the sample concerned, into one of the crystallising dishes and about 150 g of tap water into the other. Hold the sieve in an inclined position and put the dry sample into the sieve as shown in Fig. 2, with the polished side up. Immerse the sample with the sieve into the etching solution so that the sample surface is nearly horizontal and is fully covered by the liquid. Visually observe the sample surface while immersed. Leave the sample in the solution for about half the maximum time specified in the corresponding recipe and ensure a uniform surface attack by continuously moving the sieve. After that time, lift the sieve with the specimen out of the liquid and submerge it immediately in the water in the second crystallising dish in order to stop the etching process. Then rinse the sample in running tap water and finally with some propanol from the wash bottle. Use the hot-air blower to dry the sample in a stream of warm air. Fig. 2: Schematic representation of how to immerse the sample into the etching solution Place the etched sample on the microscope stage. If the two circular faces of the sample are not exactly 2 PHYWE Systeme GmbH & Co. KG All rights reserved P

3 Metallographic Sample Preparation : Chemical Etching parallel due to repeated grinding and polishing, sharp definition of the microscopic image is not maintained when scanning the sample surface by means of the XY adjustment of the mechanical stage. In this case, use the press for polished sections to ensure parallelism. Place a microscope slide on the base of the press, put a small piece of plasticine on the centre of the slide, and place the sample with its rear side on the plasticine. Protect the upper (polished and etched) surface by a thin clean piece of paper. Then press the piston down on the sample until the piston face fully touches the paper-covered sample surface. Release the piston and place the sample with the slide on the microscope stage. Adjust the illumination and the coarse and fine focus controls until you see a bright and clear image of the sample surface through the binocular tube. By turning the XY adjustment knobs you can scan the whole surface of the sample to evaluate the result of etching. If the surface does not seem to be sufficiently etched, i.e. the microstructure is not satisfactorily developed, re-immerse the specimen into the etching solution for the rest of the maximum time specified in the respective recipe. When you are satisfied with the result of your first etching experiment, repeat the procedure with the other polished samples in combination with the corresponding etching solutions as given below. Wash the crystallising dishes carefully before you change from one to another etching solution. Also clean the white plastic dish from any spilled etchants. The last two samples specified in the equipment list are identical to the steel and brass samples mentioned before, but they have been heat treated at the specified temperature for some hours and then slowly cooled down before grinding and polishing. This can result in considerable changes in the microstructure (phase composition, grain orientation, grain size), which can be detected by the microscope. Recipes for the preparation of etching solutions: Solution No. 1 (etching of steel) 150 g Distilled water 15 g Iron-III chloride Immersion time: 30 to 100 seconds Solution No. 3 (etching of alloyed steel) 110 g Distilled water 45 g Hydrochloric acid 8 g Iron-III chloride Immersion time: 20 to 30 seconds Solution No g Distilled water 35 g Hydrochloric acid 5 g Iron-III chloride Immersion time: 10 seconds Solution No g Distilled water 90 g Ammonia solution 3 g Hydrogen peroxide solution Immersion time: 15 to 20 seconds Solution No. 2 (etching of steel) 150 g Ethyl alcohol 6 g Nitric acid Immersion time: 20 to 30 seconds Solution No g Distilled water 60 g Ethyl alcohol 35 g Hydrochloric acid 6 g Iron-III chloride Immersion time: 10 to 40 seconds Solution No g Distilled water 60 g Ammonia solution 2 g Hydrogen peroxide solution Immersion time: 10 seconds Solution No. 8 (etching of aluminium and aluminium alloys) 150 g Distilled water 30 g Sodium hydroxide 1.5 g Zinc chloride Immersion time: 1 to 4 minutes P PHYWE Systeme GmbH & Co. KG All rights reserved 3

4 Metallographic Sample Preparation : Chemical Etching Sample pictures of etched specimen surfaces: Fig. 3: Steel, etched in sol. 1, selective staining of microconstituents, magnification approx. 100x Fig. 4: Steel, etched in sol. 2, grain boundary etching, magnification approx. 250x Fig. 5: Alloyed steel, etched in sol. 3, selective phase etching, magnification approx. 100x Fig. 6: Brass, etched in sol. 4, β phase selectively etched, magnification approx. 100x Fig. 7: Brass, etched in sol. 6, grain boundary etching, magnification approx. 100x Fig. 8: Brass, etched in sol. 7, grain contrast etching, magnification approx. 250x Fig. 9: Copper, etched in sol. 5, grain contrast/precip. etching, magnification approx. 100x Fig. 10: Aluminium, etched in sol. 8, magnification approx. 100x Fig. 11: Aluminium alloy (AlMgSiPb), etched in sol. 8, magnification approx. 100x Fig. 12: Steel, same as in Fig. 4, but heat treated at 750 C, magnification approx. 100x Fig. 13: Steel, same as in Fig. 4, but heat treated at 750 C, magnification approx. 250x Fig. 14: Brass, same as in Fig. 6, but heat treated at 600 C, magnification approx. 100x 4 PHYWE Systeme GmbH & Co. KG All rights reserved P

5 Metallographic Sample Preparation : Chemical Etching There are many etchant recipes to be found in the literature. To avoid excessive dangers to safety and health, highly hazardous chemicals, such as hydrofluoric acid and others, have been deliberately excluded. All etching processes are temperature dependent. For reasons of simplicity, the etchants selected are for use at normal room temperature. Please note, however, that any considerable changes in the temperature of the solutions will have an influence on the speed and intensity of the attack. Moreover, local concentration fluctuations within the solutions during etching could dramatically affect the appearance of the etched surface. Therefore, the intensity of movement of the sample in the etching solution as well as maintaining the temperature and time values are important factors in order to obtain reproducible results. A few sample images of specimen surfaces etched with the above solutions are shown in Figs. 3 to 14. Basic Theory Metals, when immersed in an aqueous solution, have a more or less strong tendency to release electrons, i.e. to become oxidized, sending positively charged ions into the solution. The released electrons are taken up at the solid/liquid phase boundary by the available hydrogen ions or other oxidisers (electron acceptors), which are thereby reduced, e.g.: Zn Zn e - or Cu Cu e - and 2H + + 2e - H 2 or ½ O 2 + H 2 O + 2e - 2OH - The metal/metal ion and the H + /H 2 or O 2 /OH redox couples constitute the anode (electrode being oxidized) and cathode (electrode being reduced) of an electrochemical cell. The tendency of an individual metal to release electrons, i.e. its individual potential, cannot be measured directly. What can be measured is the potential difference between two redox couples. To define a common reference, the potential of a hydrogen electrode (SHE) under standard conditions (1 bar, 25 C, 1 mol/l) has been arbitrarily set to zero. The potential differences between the SHE and the individual metals, measured under these conditions, are called Standard Potentials and are tabulated in the Electrochemical Series. Since the standard potential of zinc, for example, in the electrochemical series is more negative than that of hydrogen, which is zero, zinc will be readily dissolved by a hydrochloric acid solution, whereas copper, with a positive standard potential as compared to hydrogen, is insoluble in hydrochloric acid because it cannot be oxidized by H + ions. It can be dissolved, however, in the presence of stronger oxidizing agents such as nitric acid or hydrogen peroxide. In etching processes, we frequently use acid solutions containing hydrochloric acid, hydrofluoric acid, nitric acid, or phosphoric acid in aqueous and/or alcoholic media. Ethanol diminishes the ionization constant and thereby makes the attack more uniform and more controllable. The metal to be etched constitutes the anode of a galvanic cell, the cathode being the H + /H 2 couple or some other electron accepting couple like O 2 /OH -, HNO 3 /HNO 2, Fe 3+ /Fe 2+, and others. Actually, the situation is far more complicated. A metal or alloy surface represents a system of many "micro-cells" due to its highly inhomogeneous character. Among the heterogeneities are the differently composed phases, intermetallic compound particles, phase boundaries, segregations, cracks, stress zones, lattice defects, dislocations, etc. Even the different orientations of individual grains of pure metals in the polished section plane give rise to a different behaviour in etching. The inhomogeneities act as the anodes and cathodes of miniature corrosion cells electrically short-circuited through the metal contact. The result of the corrosion processes is a preferential dissolution of the anodic sites. The selective or preferential dissolution of individual macrostructural or microstructural elements produces a relief and/or a differential roughening of parts of the surface. In some cases, with suitably composed etchants, it is also possible that selective surface layers are produced by reaction of dissolution products with other constituents of the etching solution ("precipitation etching"). Such selective surface layers on certain structural elements appear darker than uncovered sites due to increased light absorption or thin-layer interference. The surface roughening leads to the scattering of incident light with a resultant darkening, in contrast to the bright specular reflection characteristic of unaffected polished surface areas. Fig. 15 The etching process producing an increased contrast of certain grains in comparison with other grains is called grain contrast etching. Fig.15 shows the schematic representation of the cross-section of two adjacent grains. Grain a has been roughened by etching, whereas grain b has remained unaffected. Light incident perpendicularly on the surface would P PHYWE Systeme GmbH & Co. KG All rights reserved 5

6 Metallographic Sample Preparation : Chemical Etching be reflected back in the opposite direction by grain b, while it would be scattered by the rough surface of grain a. For an observer viewing the surface in the direction of the incident light, grain b would appear bright, while grain a would look considerably darker. This is, in a very simplified form, the basic principle of grain contrast generation as it can be detected by reflected light bright-field microscopy. Conclusion Chemical etching processes involve complex composition and structure selective dissolution and precipitation phenomena on metallic surfaces, leading to a revealment of structural details. The various possibilities of modifying the composition of the etching solutions, trying new recipes, and varying the working parameters (mainly etching time, temperature and sample movement) will provide a wide field for experimentation and research. 6 PHYWE Systeme GmbH & Co. KG All rights reserved P