What Is an Etchant? Etching Mechanisms for Revealing Metallographic Microstructures, Selection Methods, and Applications of Related Wet-Etching Reagents

1 Why Etchants Matter

1.1 A Polished Sample Does Not Necessarily Reveal a Clear Microstructure

In metallographic analysis, a sample usually needs to undergo cutting, mounting, grinding, and polishing to obtain a flat observation surface with minimal scratches. After polishing, the metal surface may appear bright and mirror-like, but this does not mean that grains, grain boundaries, phase constituents, precipitates, or heat-treatment microstructures are already clearly visible.

The reason is that many microstructural features do not naturally produce sufficient light–dark contrast in the mirror-polished state. Differences in crystallographic orientation between grains, differences in chemical composition between phases, differences in energy at grain boundaries, and local structural differences caused by heat treatment or processing often require further treatment before they can be converted into observable contrast under a microscope. This is precisely the role of an etchant. It is not used simply to corrode the material; rather, it produces controlled chemical or electrochemical reactions that cause different regions of the material surface to respond differently, thereby revealing microstructural information that would otherwise remain hidden.

Note: Metallographic microstructure refers to the features presented by metals and alloys at the microscopic scale, including grains, grain boundaries, phase constituents, second phases, precipitates, inclusions, and microstructural characteristics formed by heat treatment or mechanical processing.

1.2 The Core Value of an Etchant Is “Selective Revelation”

The value of an etchant lies in whether it can act selectively on the target microstructure. A suitable etchant should clearly reveal the microstructural features of interest while minimizing interference from irrelevant information. For example, when observing ferrite grain boundaries, cementite, pearlite, or martensite in steel, the required etching effect is not exactly the same in each case. When observing grain orientation in aluminum alloys, it may be necessary to use electrolytic film formation followed by polarized-light observation, rather than relying on ordinary chemical etching.

2 What Is an Etchant?

2.1 Definition of an Etchant

An etchant is a type of chemical or electrochemical reagent used for observing material microstructures. It is usually applied to the surface of metals, alloys, or other materials after grinding and polishing. Through selective dissolution, oxidation, complexation, electrochemical reactions, film formation, or coloring, it causes microstructural features such as grain boundaries, phase boundaries, second phases, precipitates, segregation regions, and deformation zones to form differences in brightness, color, reflectivity, or microscopic surface morphology.

In metallographic analysis, the primary purpose of an etchant is not to remove a large amount of material, but to reveal the material microstructure. In other words, an etchant is a reagent system used to reveal microstructural features.

2.2 Difference Between Etching and General Corrosion

General corrosion is usually non-targeted damage that occurs when a material interacts with its environment, whereas metallographic etching is a deliberately controlled surface reaction. Both may involve chemical or electrochemical processes, but their purposes are completely different.

Comparison Item	General Corrosion	Metallographic Etching
Purpose	Material damage or failure process	Revealing the microstructure
Degree of control	In most cases, uncontrolled or difficult to control	Controlled through reagents, time, voltage, current, and other conditions
Result	Surface degradation, pitting, oxidation, cracking, etc.	Clearer grain boundaries, phase structures, precipitates, or color contrast
Evaluation criterion	Generally, the less unintended corrosion or damage, the better	The target microstructure is clearly revealed, background interference is low, and overetching is avoided

Metallographic etching emphasizes reaction selectivity and reproducibility. A suitable etchant can enhance the target microstructural features; an unsuitable etchant may result in unclear microstructures, surface darkening, pitting, false structures, or overetching.

3 Why Etchants Can Reveal Microstructures

3.1 Selective Dissolution: Different Regions React at Different Rates

A material surface is not chemically uniform throughout. Grain boundaries, phase boundaries, areas around inclusions, areas around precipitates, and deformation-affected regions often have different energy states, compositional distributions, or electrochemical activities. When an etchant contacts the sample surface, these regions may react with the etchant at different rates.

Regions that react faster are preferentially dissolved, forming slight depressions or contours; regions that react more slowly remain relatively intact. Under the microscope, differences in surface height and reflectivity are converted into light–dark contrast, making grain boundaries, phase boundaries, or microstructural morphology visible.

For example, in ferrous materials, ferrite, pearlite, cementite, martensite, and other microstructures respond differently to etchants. Nital, a nitric acid–alcohol etchant, is commonly used for carbon steels and low-alloy steels and can reveal ferrite grain boundaries, pearlite, martensite, and heat-treatment microstructures. Picral, a picric acid–alcohol etchant, is particularly representative for revealing cementite, carbide morphology, fine pearlite lamellae, and tempered carbides, but it is generally not used alone to reveal ferrite grain boundaries. If ferrite grain boundaries and carbide morphology need to be observed simultaneously, the etching effects of Nital and Picral may be compared or used in combination.

3.2 Electrochemical Differences: Different Phases Have Different Potentials and Reaction Tendencies

Many alloys are composed of different phases, and these phases may differ in chemical composition and electrode potential. In an etching solution, such differences may generate microscopic electrochemical reactions, causing some phases to dissolve preferentially while others remain relatively stable. Grain boundaries, precipitates, segregation regions, and heat-affected zones may also exhibit different reaction rates due to local compositional or structural differences.

This mechanism is especially important in materials such as stainless steels, high-alloy steels, nickel-based alloys, and titanium alloys. These materials are inherently corrosion-resistant, so ordinary etchants may react insufficiently and fail to reveal the microstructure. However, overly aggressive etchants may cause pitting or microstructural distortion. Therefore, such materials often require more targeted etchants or electrolytic etching.

Electrolytic etching uses an external power supply to control anodic dissolution or film formation on the sample surface. The sample is usually used as the anode. The etching result depends not only on the electrolyte, but also on voltage, current density, time, sample area, agitation, electrode spacing, and other conditions. Compared with ordinary chemical etching, electrolytic etching is more suitable for certain difficult-to-etch materials or for microstructure revelation that requires fine control.

3.3 Film Formation and Coloring: Color Contrast Produced by Thin-Film Differences

Not all etchants reveal microstructures primarily through dissolution. Some color etchants or electrolytic etchants form thin films on the sample surface. Different phases, grain orientations, or compositional regions may form films with different thicknesses and optical properties, resulting in color differences. These colors usually arise from light interference in the thin film. When the film thickness differs, the color of the reflected light also changes. Under the microscope, different grains or phases may therefore appear in different colors.

A typical example is Barker’s etching for aluminum alloys. Barker’s etching is an electrolytic film-forming method commonly used for observing grain structures in aluminum and aluminum alloys. After an anodic oxide film forms on the sample surface, grains with different orientations may show different colors under polarized light.

4 Main Types of Etchants and Representative Products

4.1 Chemical Etchants

Chemical etchants are common metallographic etchants. They react with the sample surface through immersion or swabbing, causing different microstructural regions to develop selective dissolution, changes in reflectivity, or differences in surface relief.

Chemical etchants are suitable for most routine metallographic observations and are relatively straightforward to use. However, they are highly sensitive to material type, etching time, sample surface condition, and cleaning conditions. If the etching time is insufficient, the microstructure will not be clearly revealed. If the etching time is too long, overetching, darkening, pitting, or loss of fine details may occur.

Representative Etchant	Main Applicable Materials	Typical Features Revealed
Nital	Carbon steels, low-alloy steels	Ferrite grain boundaries, pearlite, martensite, heat-treatment microstructures
Picral	Medium- and high-carbon steels, spheroidized annealed steels, quenched-and-tempered steels, and other ferrous materials	Cementite, carbide morphology, fine pearlite lamellae, or tempered carbide distribution
Keller’s reagent	Aluminum and aluminum alloys	Grains, phase structures, cast or worked microstructures
Kroll’s reagent	Titanium and titanium alloys	Alpha phase, beta phase, grain boundaries, and hot-worked microstructures
Kalling’s reagent	Stainless steels, nickel-based alloys, etc.	Austenite, ferrite, weld microstructures, corrosion-resistant alloy microstructures
Murakami’s reagent	High-alloy steels, cemented carbides, etc.	Carbides, certain second phases, or microstructural differences

4.2 Electrolytic Etchants

Electrolytic etchants must be used with an electrolytic apparatus. The sample is placed in the electrolyte as an electrode and undergoes a controlled surface reaction under an applied current or voltage. Electrolytic etching is often used for materials with strong corrosion resistance whose microstructures are difficult to reveal by ordinary chemical etching. It is also suitable for certain analytical scenarios where the influence of mechanical polishing needs to be reduced or microstructural contrast needs to be improved.

Representative System	Main Applicable Materials	Typical Function
Oxalic acid electrolytic etching	Austenitic stainless steels, etc.	Reveals grain boundaries and may be used for microstructural features related to sensitization or intergranular corrosion susceptibility screening
Barker’s electrolytic etching	Aluminum and aluminum alloys	Forms an anodic film and reveals grain orientation when used with polarized light
Certain acidic or alkaline electrolytes	High-alloy steels, nickel-based alloys, titanium alloys, etc.	Reveals corrosion-resistant alloy microstructures or improves interphase contrast

The key control factors in electrolytic etching are not limited to the electrolyte itself. Voltage, current density, time, temperature, sample area, electrode position, and surface cleanliness all affect the final result. Therefore, electrolytic etching places greater emphasis on the controllability of process parameters.

4.3 Color Etchants

Color etchants produce different colors in different microstructural regions by forming thin films, deposited layers, or selective color reactions on the sample surface. They are often used for phase differentiation, grain-orientation observation, image analysis, and identification of complex microstructures.

Representative Etchant	Main Application Direction	Typical Characteristics
Beraha’s reagent	Steels, stainless steels, and certain alloys	Improves phase-structure contrast through coloring
Klemm’s reagent	Steels, copper alloys, etc.	Produces color differences to assist in identifying phases or grains
Weck’s reagent	Aluminum alloys, etc.	Improves grain or microstructure color contrast
Barker’s reagent/method	Aluminum and aluminum alloys	Uses electrolytic film formation followed by polarized-light observation of grain orientation; an electrolytic film-forming color observation method

Color etchants are especially suitable for samples where simple light–dark contrast is insufficient. However, these methods require high surface quality. If a deformed layer, scratches, or contaminants remain after polishing, the colors may become uneven and may even lead to misinterpretation.

5 How to Select a Suitable Etchant

5.1 First Determine the Material System

The first consideration in selecting an etchant is the material type. Steels, aluminum alloys, titanium alloys, copper alloys, stainless steels, and nickel-based alloys differ in chemical stability, phase composition, and surface-film characteristics, and therefore respond differently to etchants.

For example, carbon steels are commonly examined with Nital to observe general microstructures and ferrite grain boundaries, while Picral may be used as a supplementary etchant to observe cementite, fine pearlite, or carbide morphology. Aluminum alloys commonly use systems such as Keller’s reagent, Barker’s etching, and Weck’s reagent. Titanium alloys commonly use Kroll-type etchants. Because stainless steels and nickel-based alloys have strong corrosion resistance, they often require Kalling’s reagent, Glyceregia, oxalic acid electrolytic etching, or color etching systems.

5.2 Then Clarify the Observation Target

An easily overlooked question in etchant selection is: what exactly needs to be observed? In the same material, different observation targets may require different sources of contrast. To observe grain boundaries, there must be a clear difference between the grain boundary and the grain interior. To observe second phases, the reaction difference between phases must be distinct. To observe carbides, the carbides must be clearly distinguishable from the matrix. To observe heat-treatment microstructures, martensite, bainite, pearlite, ferrite, and other morphologies must have sufficient contrast.

Observation Target	Selection Focus
Grains and grain boundaries	Select an etchant that preferentially reveals grain boundaries or grain-orientation differences
Second phases or precipitates	Select an etchant sensitive to chemical differences between phases
Carbides	Select an etchant that highlights carbide morphology or distribution
Weld microstructures	Select an etchant that distinguishes the weld zone, fusion line, and heat-affected zone
Heat-treatment microstructures	Select an etchant that distinguishes martensite, bainite, pearlite, ferrite, and other microstructures
Grain orientation	Prefer color etching, electrolytic film formation, or polarized-light observation methods

For example, low-carbon steel treated with Nital, Picral, or Beraha-type color etchants may reveal different information. Nital is commonly used to reveal ferrite grain boundaries and general microstructural morphology. Picral is more representative for revealing carbides. Beraha-type reagents can help distinguish microstructures through color contrast. The selection should be based on the analytical purpose.

5.3 Match the Observation Method

The observation method after etching also affects etchant selection. Bright-field observation with an ordinary optical microscope requires clear light–dark or morphological contrast. Polarized-light observation is suitable for certain film-forming methods or anisotropic materials. Differential interference contrast, or DIC, is sensitive to slight surface-height differences. Scanning electron microscopy, or SEM, focuses more on surface morphology and compositional differences.

If subsequent image analysis is required, the etchant should produce stable, uniform, and reproducible microstructural contrast while avoiding excessive irrelevant detail. Some inclusions, pores, or cracks may actually be easier to identify in the unetched condition. Excessive etching can introduce additional background information and affect quantitative analysis.

5.4 Check the Quality of Sample Preparation

Poor etching results do not necessarily mean that the wrong etchant was selected. Grinding scratches, polishing-induced deformation layers, mounting contamination, insufficient cleaning, polishing-agent residues, or surface oxide films can all cause uneven etching, abnormal coloration, unclear grain boundaries, or pitting.

A satisfactory etching result depends on good preparation before etching. The sample surface should be as flat and clean as possible, with no obvious scratches or smearing. For soft metals, work-hardening-prone materials, or multiphase alloys, if mechanical polishing produces a deformed layer, further fine polishing, electrolytic polishing, or adjustment of the preparation process may be required.

5.5 Optimize from Mild Conditions

Etching should follow the principle of starting with mild conditions and then gradually increasing intensity. Usually, a short etching time or mild condition should be used first for trial etching. After observing the result, additional etching can be performed step by step. If the sample is underetched, the microstructure is insufficiently revealed and further adjustment is still possible. If the sample is overetched, surface details may be damaged, and repolishing is often required. Etching results can be evaluated using the following three states:

State	Typical Appearance	Treatment
Underetched	Grain boundaries or phase boundaries are unclear; microstructural contrast is weak	Appropriately extend the etching time or adjust the conditions
Properly etched	Target microstructure is clear, background interference is low, and details are intact	Record the conditions and maintain reproducibility
Overetched	Surface darkening, pitting, overly deep contours, or loss of detail	Usually requires repolishing followed by re-etching

An excellent etching result is not achieved by making the reaction as strong as possible, but by clearly revealing the target information, minimizing background interference, and preserving the true microstructural morphology.

6 Typical Applications of Etchants

6.1 Metallographic Microstructure Observation

Metallographic microstructure observation is the most fundamental application of etchants. By selecting a suitable etchant, it is possible to reveal grain size, grain-boundary morphology, phase distribution, precipitates, structures around inclusions, and microstructural changes after processing or heat treatment. In quality control, etchants help determine whether the material microstructure meets requirements. In research and development, etchants help analyze the effects of composition design, heat-treatment schedules, and processing routes on microstructure.

6.2 Heat-Treatment Quality Analysis

Heat treatment changes the microstructural state of metallic materials, such as forming martensite, bainite, pearlite, ferrite, tempered structures, or precipitation-strengthened phases. Different microstructures respond differently to etchants, so etchants can be used to determine whether heat treatment has achieved the expected result.

For example, in ferrous materials, etching can be used to observe whether the quenched structure is sufficient, whether the tempered structure is uniform, whether pearlite lamellae are clearly visible, whether grains have coarsened, and whether abnormal microstructures are present. For heat-treatment failure analysis, etching results often provide direct microstructural evidence.

6.3 Weld Joint Analysis

A welded joint usually consists of a weld zone, a fusion line, and a heat-affected zone. Different regions experience different thermal cycles, resulting in different microstructures and properties. Etchants can create clear boundaries between these regions, helping analyze welding heat input, grain coarsening, segregation, crack paths, and microstructural transformations.

In the inspection of welded components, etching is used not only for microstructural analysis but also commonly for macrostructural revelation. With suitable macroetching, weld bead morphology, penetration depth, fusion condition, and defect distribution can be observed.

6.4 Analysis of Cast, Forged, and Worked Microstructures

Cast materials commonly contain dendrites, segregation, shrinkage cavities, inclusions, and eutectic structures. Forged, rolled, extruded, and otherwise worked materials often exhibit deformation flow lines, recrystallized structures, and differences in grain orientation. Etchants can reveal these microstructural features and are used to evaluate the solidification process, degree of plastic deformation, and microstructural uniformity. For materials such as aluminum alloys, titanium alloys, and nickel-based alloys, suitable etching methods can help identify grain morphology, phase distribution, and hot-worked microstructures, providing a basis for process optimization.

6.5 Failure Analysis

In failure analysis, etchants can be used to evaluate crack propagation paths, microstructural abnormalities, overheating, decarburization, corrosion damage, grain-boundary embrittlement, or improper heat treatment. The microstructural information obtained after etching can be cross-validated with hardness testing, compositional analysis, fracture-surface analysis, and mechanical property testing.

It should be noted that failure analysis does not necessarily begin with etching. Pores, cracks, inclusions, and corrosion products may sometimes be easier to observe in the unetched condition. A reasonable approach is to first observe the polished condition, and then select the etchant and etching method according to the problem, so as to avoid prematurely etching the sample and obscuring the original failure features.

7 Common Problems and Safety Precautions

7.1 Unclear Microstructure

An unclear microstructure usually has three types of causes: insufficient etching, an unsuitable etchant, or inadequate sample preparation. If grain boundaries and phase boundaries appear generally faint, the etching time may be appropriately extended. If the target microstructure remains indistinct, it is necessary to reassess whether the etchant is suitable for the material and observation target. If local unevenness, smearing, or abnormal spots appear, the polishing and cleaning quality should be checked first.

7.2 Surface Darkening or Pitting

Surface darkening, pitting, or loss of detail is usually associated with overetching, but it may also be related to excessive etchant concentration, overly long etching time, delayed cleaning, or local corrosion sensitivity of the material. Overetching can damage the true microstructural morphology. In severe cases, the sample must be repolished before etching again.

7.3 Uneven Coloration

Uneven color etching is often related to surface contamination, polishing residues, uneven oxide films, or unstable operating time. Color etching depends on the film-formation process and requires high surface cleanliness and polishing quality. If localized abnormal coloration occurs, the first priority should be to check whether the sample has been thoroughly cleaned and whether the deformed layer has been removed during polishing.

7.4 Safety Precautions

Many etchants may contain strong acids, strong oxidizing agents, fluorides, picric acid, chromates, or other highly hazardous chemicals. Before use, the safety data sheet, or SDS, should be consulted. Appropriate personal protective equipment should be worn, operations should be carried out in a fume hood, and laboratory safety rules and waste-disposal requirements should be followed.

The preparation, storage, and waste disposal of etchants should comply with relevant standards, regulations, and institutional safety procedures. Etchant systems containing hydrofluoric acid, picric acid, chromates, strong oxidizing agents, or ferricyanides should be handled only by trained personnel, and arbitrary self-preparation or mixing should be avoided. Systems containing hydrofluoric acid or fluorides must not be used in ordinary glass containers. Picric acid and its dried residues present an explosion hazard. Ferricyanide-containing systems should not be mixed casually with strong acids. Alcohol-based etchants such as Nital and Picral should be kept away from ignition sources and managed according to specified concentration, container, and storage-duration requirements.

8 Classification Table of Raw Materials, Formulation Components, and Related Wet-Etching Reagent Products for Metallographic Etchants

Note: The following products include commonly used raw materials, formulation components, auxiliary reagents for metallographic etchants, as well as wet-etching reagents related to material surface treatment. The acids, bases, oxidizing agents, complexing agents, and solvents listed in the tables are mostly formulation components or auxiliary reagents, and do not necessarily mean that they can be used alone as metallographic etchants. In actual use, the mixing ratio, concentration, and operating conditions should be determined based on the material system, observation target, established formulations, SDS, COA, and laboratory safety requirements.

Table 1 Acids, Bases, and Solvent Reagents Related to Metallographic Etching

Category	CAS No.	Aladdin Item No.	Name	Specification or Purity	Product Features and Applications
Alcohol solvent	67-56-1	M1522475	Methanol	Electronic grade, UPS, ≥99.5%	Used in alcohol-based metallographic etching systems. It can serve as a solvent for nitric acid–alcohol, picric acid–alcohol, and certain high-alloy-material etching systems, and is suitable for cleaning and dehydration treatment before and after etching polished samples.
Acid etching reagent	7647-01-0	H1520896	Hydrochloric acid (regulated precursor chemical)	Electronic grade, UPSS	Used in etching systems related to stainless steels, high-alloy steels, nickel-based alloys, and aluminum alloys. It can participate in the revelation of grain boundaries, phase structures, and weld microstructures.
Alcohol solvent	64-17-5	E111965	Ethanol	Moligand™, electronic grade, total metal impurities ≤ 20 ppm	Used in ferrous metallographic etching systems such as nitric acid–alcohol and picric acid–alcohol. It is a commonly used solvent for revealing heat-treatment microstructures, grain boundaries, and pearlitic structures in steels.
Acid etching reagent	7697-37-2	N116243	Nitric acid (regulated explosive precursor)	Electronic grade, ≥70%	Used in etching systems for steels, aluminum alloys, titanium alloys, and high-alloy materials. It can participate in selective oxidation and surface dissolution, helping reveal grain boundaries, phase structures, and heat-treatment microstructures.
Alkaline etching reagent	1336-21-6	A112083	Ammonia solution	Electronic grade, ≥28% NH3 in H2O	Used in ammoniacal etching systems for copper and copper alloys. It can work with oxidizing agents to produce complexation-assisted dissolution, and is suitable for studies of copper-alloy grains, phase structures, and surface reactions.
Acid auxiliary reagent	64-19-7	A116171	Glacial acetic acid	Moligand™, electronic grade, ≥99.7%	Used in etching systems for aluminum and certain metals. It can adjust the etching reaction rate and wettability, and is suitable for thin-film material etching and auxiliary metallographic treatment.
Alkaline etching reagent	1310-73-2	S163080	Sodium hydroxide	Electronic grade, ≥99.9% metals basis	Used for alkaline etching of aluminum alloys, alkaline cleaning, and certain alkaline metallographic etching systems. It can be used for oxide-film removal, surface activation, and microstructural pretreatment.
Fluorine-containing etching reagent	7664-39-3	H116237	Hydrofluoric acid	PrimorTrace™ Ultra, electronic grade, ≥99.99998% metals basis, 49 wt.% in H2O	Used in etching systems for aluminum alloys, titanium alloys, and materials with stable oxide films. It can participate in oxide-film breakdown and microstructure revelation.
Polyol auxiliary reagent	56-81-5	G116203	Glycerol	AR, ≥99%	Used in etching systems related to high-alloy steels, stainless steels, and nickel-based alloys. It can adjust solution viscosity, wettability, and reaction uniformity.
Alkaline etching reagent	1310-58-3	P431767	Potassium hydroxide	Anhydrous grade, ≥99.95% metals basis	Used as a component in alkaline etching systems such as Murakami-type reagents. It can be combined with potassium ferricyanide and other components to reveal carbides or second phases in cemented carbides and high-alloy steels.
Acidic electrolytic etching reagent	144-62-7	O107179	Anhydrous oxalic acid	Anhydrous grade, ≥99%	Used in oxalic acid electrolytic etching systems for austenitic stainless steels. It can be used for grain-boundary revelation and rapid screening related to sensitization, but should not be generalized as a universal microstructure-revealing reagent for all stainless steels.
Oxidizing acid reagent	7664-38-2	P112025	Phosphoric acid	HPLC grade, ≥85%	Used in electrolytic polishing, surface treatment, and thin-film wet-etching systems for aluminum and certain metals. It may also serve as a formulation component in some metallographic or material surface-treatment systems.

Table 2 Reagents Related to Oxidation, Complexation, Coloring, and Second-Phase Revelation

Category	CAS No.	Aladdin Item No.	Name	Specification or Purity	Product Features and Applications
Metal complexing etching reagent	7681-11-0	P1511709	Potassium iodide	Electronic grade, ≥99.5%	Used in iodine/iodide gold etching systems. It can participate in the selective dissolution of gold thin films, gold electrodes, and precious-metal layers.
Oxidizing color reagent	7722-64-7	P112383	Potassium permanganate (regulated precursor chemical) (regulated explosive precursor)	ACS, ≥99%, low in mercury	Used in oxidizing and color-etching systems. It can participate in color-contrast formation and surface-reaction control in aluminum alloys, non-ferrous metals, and alkaline etching systems.
Oxidizing etching reagent	7727-54-0	A112447	Ammonium persulfate (APS)	PrimorTrace™, ≥99.99% metals basis	Used in etching systems for copper and copper alloys. It can also be used in metallographic etching and thin-film copper removal experiments, and is suitable for oxidative dissolution of copper surfaces.
Metal complexing etching reagent	7553-56-2	I116354	Iodine	ACS, ≥99.8%	Used in iodine/iodide gold etching systems. It can work with potassium iodide to participate in the etching of gold thin films and gold electrode materials.
Color etching reagent	7681-57-4	S433809	Sodium metabisulfite	Anhydrous grade, high-purity grade, reagent grade, ≥99%	Used in color etching and phase-differentiation etching systems. It can participate in color-contrast revelation of microstructures in steels, stainless steels, and certain alloys.
Oxidizing etching reagent	7722-84-1	H112517	Hydrogen peroxide solution (regulated explosive precursor)	GR, 30 wt.% in H2O	Used in etching systems for copper and copper alloys, non-ferrous metals, and composite oxidative etching systems. It can participate in surface oxidative dissolution and microstructure revelation.
Metal salt etching reagent	7705-08-0	I112066	Anhydrous ferric chloride	PrimorTrace™, ≥99.99% metals basis	Used in etching systems for copper, copper alloys, and certain metal materials. It can achieve selective metal-surface dissolution through oxidation reactions.
Second-phase revelation reagent	13746-66-2	P111564	Potassium ferricyanide	AR, ≥99.5%	Used in alkaline metallographic etching systems. It can participate in revealing carbides, second phases, or microstructural differences in high-alloy steels, tool steels, and cemented carbides.
Color etching reagent	16731-55-8	P299315	Potassium metabisulfite	≥97%	Used in color-etching systems. It can participate in color-contrast formation and phase differentiation in steels, stainless steels, and certain non-ferrous alloys.
Metal salt etching reagent	7447-39-4	C433487	Copper(II) chloride	PrimorTrace™, ≥99.999% metals basis	Used in etching systems for stainless steels, high-alloy steels, and nickel-based alloys. It can participate in revealing grain boundaries, second phases, and weld microstructures.

Table 3 Wet Etchants for Metal Thin Films and Microelectronics

Category	CAS No.	Aladdin Item No.	Name	Specification or Purity	Product Features and Applications
Aluminum etchant	——	A487579	Aluminum Etchant Type A	——	Used for wet etching of aluminum metallization layers in silicon devices and microelectronic processes. It can be used for preparing aluminum contacts, interconnect structures, and metal patterns.
Aluminum etchant	——	A487581	Aluminum Etchant Type D	——	Used for wet etching of aluminum metallization layers in microelectronic devices. It is suitable for aluminum-layer etching related to compound semiconductor devices such as gallium arsenide and gallium phosphide devices, as well as nickel-chromium thin-film resistors.
Aluminum etchant	——	A487590	Aluminum Etchant Type F	——	Used for wet etching of aluminum or aluminum-silicon metallization layers in silicon-device and integrated-circuit applications. It can be used for preparing aluminum contacts, interconnect structures, and high-resolution metal patterns.
Nitride thin-film etchant	——	T487601	Tantalum Nitride Etchant	——	Used for etching tantalum nitride thin films. It can be used for barrier layers, functional thin films, and micro/nano-structure patterning.
Precious-metal etchant	——	G433796	Gold Etchant, Standard	——	Used for selective wet etching of gold thin films, gold electrodes, and precious-metal layers. It can be used for post-photolithography patterning, micro/nano electrodes, and sensor structure fabrication.
Precious-metal etchant	——	G487585	Gold Etchant, Nickel-Compatible	——	Used for etching gold layers in structures containing nickel underlayers or nickel-related structures. It can be used for multilayer metal thin films, electrodes, and micro/nano device fabrication.
Tungsten etchant	——	T487587	Tungsten Etchant	——	Used for etching tungsten thin films and tungsten-based materials. It can be used for metal interconnects, functional film layers, and micro/nano-structure fabrication.
Copper etchant	——	C487577	Copper Etchant	——	Used for etching copper thin films, copper electrodes, and copper-based materials. It can be used for circuit patterning, thin-film removal, and copper-alloy surface treatment.
Chromium etchant	——	C476102	Chromium Etchant	Standard	Used for etching chromium thin films and chromium mask layers. It can be used for photomasks, metal adhesion layers, and micro/nano patterning.
Nickel-Chromium etchant	——	N476103	Nickel-Chromium Etchant	Standard	Used for etching nickel-chromium alloy thin films. It can be used for resistor films, functional metal layers, and microelectronic device patterning.

Table 4 Wet Etchants Related to Silicon, Oxides, and Ceramic Materials

Category	CAS No.	Aladdin Item No.	Name	Specification or Purity	Product Features and Applications
Silicon anisotropic etchant	——	A487594	Anisotropic Silicon <100> Etchant	——	Used for anisotropic wet etching of silicon materials with <100> crystal orientation. It can be used for microstructures, trenches, cavities, and silicon-based device fabrication.
Oxide etchant	——	B487576	Buffered Oxide Etchant (BOE) 10:1	——	Used for etching silicon dioxide, thermal oxide layers, and certain oxide thin films. It can be used for silicon-based processes, oxide-layer window opening, and surface treatment.
Oxide etchant	——	B487578	Buffered Oxide Etchant (BOE) 6:1	——	Used for wet etching of silicon oxide thin films. It can be used for removing oxide layers on silicon wafers, pattern transfer, and micro/nano fabrication.
Oxide etchant	——	B487586	Buffered Oxide Etchant (BOE) 6:1 with Surfactant	——	Used for etching silicon oxide thin films. It can improve liquid spreading and wetting in patterned areas, and is suitable for microstructure window opening and oxide-layer removal.
Ceramic etchant	——	C487583	Ceramic Etchant A	——	Used for surface etching and microstructure revelation of ceramic materials. It can be used for surface treatment, microscopic observation, and interface studies of inorganic non-metallic materials.

Note: The products listed above are representative Aladdin products related to scientific research and formulation studies. For more product specifications, grades, and COA information, search by “product name/CAS/item number” on the Aladdin website.

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