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LABTT | Metallographic Sample Preparation Method
2026-04-24
Metallographic analysis is a core method for metal material research, quality inspection, and failure analysis, and clearly presenting metallographic structures is a prerequisite for analysis work. In metallographic observation, a key prerequisite is that the difference in reflectivity between metallographic structures must be more than 10% to reflect light of different intensities, allowing for clear observation and identification. However, in practical operations, the surface of metallographic specimens after polishing treatment reflects incident light almost uniformly, resulting in the "concealment" of metallographic structures and making direct distinction impossible.
To address this issue and achieve a clear and distinct appearance of the metallographic structure, targeted metallographic treatment is required for the polished samples. Based on years of experience in metallographic equipment research and development and practical operations, Naibo has compiled commonly used metallographic sample processing methods and common metal etchants to assist relevant practitioners in efficiently completing sample preparation and accurately conducting metallographic analysis. The commonly used metallographic processing methods mainly include the following categories:
chemical erosion
8CrNiMo7-6 steel, Beraha's 10/3 etching. Image source: publicly available online
The chemical etching of pure metals and single-phase alloys is a process of chemical dissolution. Due to the irregular atomic arrangement at grain boundaries, which possess higher free energy, these boundaries are prone to corrosion, resulting in grooves that reveal the microstructure. Under a microscope, polygonal grains can be observed. If the etching is deep, due to the different orientations of the grains, the dissolution rates of different crystal faces vary. The angle between the etched microplane and the original grinding surface is different, and under vertical light irradiation, different amounts of light are reflected into the objective lens, resulting in grains with varying shades.
Chemical corrosion of magnesium-aluminum alloy
The chemical etching of multiphase alloys involves the dissolution of various phases to varying degrees by the etchant during the corrosion process. It is essential to use an appropriate etchant. If one etchant cannot reveal all the structures, two or more etchants should be employed in sequence to gradually expose the phase structures. This method is also known as selective etching. Another approach is the thin-film staining method. This method utilizes the chemical reaction between the etchant and the phases on the grinding surface to form a layer of unevenly thick film (or reaction precipitate). Under white light, the interference of light causes the phases to exhibit different colors, thereby achieving the purpose of identifying the phases.
electrolytic etching
409 stainless steel, oxalic acid electrolytic corrosion
Chemical etching operates without the influence of an external power source, whereas electrolytic etching involves immersing the polished sample in a solution of a suitable chemical reagent (electrolytic etchant) and etching it using a small direct current. The operating voltage and current for electrolytic etching are typically low, with the operating voltage generally ranging from 2 to 6V and the operating current approximately 0.05 to 0.3A/cm2. Electrolytic etching is primarily used for alloys with high chemical stability, such as stainless steel, heat-resistant steel, and nickel-based alloys, which are difficult to obtain clear microstructures using chemical etching.
Stable potential etching:
An improved method of electrolytic etching is called stable potential etching. Usually, due to changes in electrolyte concentration, there are different current loads, causing the potential of the sample to change frequently. By using a potential stabilizer to maintain a constant potential, clear contrasts can be obtained that cannot be achieved by other etching methods.
The constant potential etching deposition method begins by determining the polarization curve of a certain metal in a specific electrolyte. Based on the polarization curve, an appropriate etching potential is selected. Then, utilizing the difference in film formation rates of various phases in the alloy, a constant potential instrument is employed to facilitate the completion of the entire etching deposition process under the influence of this external constant potential. Due to the varying lattice energies of different phases, the film formation rates and thicknesses differ at a certain potential, resulting in distinct interference colors.
hot corrosion
Grain structure of annealed pure titanium after hot corrosion
Thermal corrosion is a method of heating and corroding polished specimens without inlaying them in a furnace. This method exhibits excellent effects on the microstructure of ceramic materials. The temperature for thermal corrosion depends on the heat treatment system of the material itself. Generally, the temperature should be high enough to allow the etching process required for grain boundary and surface diffusion to occur fully. For most materials, the appropriate temperature is within the range of 100~250 ℃ below the sintering temperature. During thermal corrosion, some materials may also undergo precipitation and phase transformation reactions, and the glass phase may melt, and even the grain boundary phase may evaporate.
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