For a layperson, a polished metal surface — is just a shiny mirror. However, for a physical metallurgy engineer or process engineer this shine is deceptive. Behind a perfectly smooth surface lies a highly complex microstructural universe:
grain and subgrain boundaries;
brittle and ductile phases;
deformation and recrystallization twins;
segregation inhomogeneity;
traces of thermal and thermomechanical treatment.
Exactly this hidden structure determines strength, ductility, magnetic properties, corrosion resistance, and durability of an alloy.
It can be seen using metallography — a scientific discipline combining methods of sample preparation, microscopic analysis, and structure interpretation. The central stage of metallographic study is etching of sections — a process that turns a featureless surface into an informative microstructure map.
Before reaching the microscope, an alloy sample goes through a long preparation path. It includes:
sequential grinding with abrasives of decreasing grit;
finish polishing with aluminum oxide or diamond powder suspensions.
As a result, a perfectly smooth surface with minimal roughness is obtained. However, such a surface practically contains no visually distinguishable structural elements. Etching performs a key function — selectively reveals metal microstructure.
The physicochemical etching mechanism is based on differences in electrochemical activity of structural constituents. The reagent dissolves metal unevenly:
regions with elevated defectiveness have higher free energy and etch faster;
different phases have different electrochemical potentials;
grain boundaries dissolve more intensely, forming microgrooves;
carbides, intermetallics, and inclusions create contrast due to different dissolution rates.
As a result, a microrelief forms that becomes visible in reflected light of an optical or electron microscope.
In modern metallography a wide range of reagents is used, differing in chemical composition, action mechanism, and application area.
From the physicochemical principle of action, all etchants fall into two main groups:
chemical etchants, whose action is based solely on chemical interaction of the reagent with the metal;
electrolytic etchants, when using which electric current is applied to the sample, accelerating anodic metal dissolution and providing more controlled structure revelation.
Choice of a specific etchant depends on:
alloy chemical composition;
degree of alloying;
required etching depth;
type of structure examined;
subsequent analysis method (optical or electron microscopy).
For various groups of precision alloys at PZPS, specialized etching methods are used that ensure result reproducibility and compliance with metallographic inspection standards. Let’s examine the most common solutions.
Nital — a universal metallographer’s tool
The most common reagent for etching structural steels is a 4% alcoholic solution of nitric acid, known as nital.
The reagent’s action mechanism includes two key components:
nitric acid acts as a strong oxidizer, converting iron into a soluble ionic form;
ethyl alcohol ensures uniform surface wetting and stabilizes reaction rate, preventing local overloads.
Nital effectively reveals:
pearlite (dark regions);
ferrite (light regions);
cementite structures;
banding after rolling;
actual austenite grain size.
Additionally, nital is used to assess:
decarburization depth;
structural uniformity;
degree of recrystallization after heat treatment.
Typical etching time is 5–20 seconds at room temperature.
Marble’s reagent and the ion substitution mechanism
Highly alloyed steels containing significant amounts of chromium and nickel have high corrosion resistance due to formation of a passivating oxide film.
Therefore standard etchants such as nital are of little effect. In such cases Marble’s reagent is used, which is a mixture of:
hydrochloric acid;
copper sulfate;
alcohol or water.
The etching mechanism is based on an electrochemical substitution process: copper ions are reduced on the metal surface, while iron or nickel ions pass into solution.
This reagent reliably reveals:
austenite grain boundaries;
carbides at grain boundaries;
sensitization zones;
deformation and recrystallization twins.
Use of this reagent is especially important when inspecting welded joints and heat-treated parts.
Aqua regia and the acid synergy effect
For chemically inert alloys with high corrosion resistance and structural stability, more active reagents are used. One of the most effective is diluted aqua regia — a mixture of hydrochloric and nitric acids.
Historically the name is linked to the ability to dissolve noble metals, including gold and platinum. However, the key interest for metallography is the synergistic acid interaction mechanism.
The process proceeds as follows: nitric acid oxidizes the metal, converting it into ionic form: Ni²⁺, Cr³⁺ or Fe³⁺.
At this stage a passivating film that slows the reaction could form. However, hydrochloric acid contains chloride ions that:
form stable complex compounds;
bind metal ions;
remove reaction products from the surface.
As a result, passivation does not occur and metal dissolution continues continuously.
Etching time for such alloys is extremely short — usually from 2 to 10 seconds. Overexposure can lead to overetching of the structure, surface destruction, and distortion of analysis results.
The first stage of assessing metallurgical purity
Analysis of non-metallic inclusions — is an important stage of metal product quality control, since inclusions often become initiators of material failure. Therefore inclusion examination is carried out before etching.
At this stage the section is examined in the unetched state to determine:
sulfide inclusions;
oxide inclusions;
silicate inclusions;
carbonitride phases.
However, some inclusions, for example titanium nitride, have characteristic morphology and color that become clearly visible only after light pre-etching.
Such pre-etching performs an auxiliary function: it removes the work-hardened surface layer formed during mechanical sample preparation.
Despite the apparent simplicity of the process, etching is a highly sensitive operation requiring strict adherence to process discipline. In practice, quality of structure revelation is determined by a number of critically important factors.
Even minimal contamination can completely change etching results.
For example, a grease film from finger contact blocks reagent contact with the metal and causes local structure defects.
Therefore mandatory degreasing is performed before etching — with alcohol, acetone, or isopropanol. This stage ensures reaction uniformity over the entire sample surface.
Chemical stability of the etchant directly affects result reproducibility. For example, nital has a limited shelf life.
Signs of reagent degradation:
color change (reddish tint);
appearance of precipitate;
reduced activity.
Using old reagent leads to uneven etching, spotting, and microstructure distortion.
Thermal and mechanical effects during polishing can significantly change the surface layer structure.
Especially dangerous are:
local overheating;
plastic deformation;
work hardening.
During etching such a layer may appear as a false structure, distorted grain structure, and anomalous contrast. Therefore observing the polishing regime is a mandatory condition for reliable metallographic analysis.
In a number of cases multi-step etching is used to obtain complete information about the structure.
For example, first a weak reagent is used to reveal grain boundaries. After re-polishing a stronger reagent is used to reveal the carbide network, segregation zones, and phase transformations. This approach makes it possible to examine complex multicomponent alloys in detail.
Metallographic inspection is an integral part of the metal product quality assurance system. It makes it possible not only to confirm chemical composition compliance but also to assess the actual state of the material structure.
When purchasing an industrially produced precision alloy, the customer receives not merely a composition certificate. Each material batch undergoes comprehensive metallographic checking, including structure analysis after etching.
Within such inspection key parameters are assessed:
structural uniformity across the section;
grain size;
phase distribution;
presence of defects of metallurgical origin.
Exactly these parameters determine product reliability, service life, and stability of operating characteristics.
At PZPS you can purchase cold-rolled strip of specialized materials intended for critical technical applications.
Precision soft magnetic alloys (49K2FA, 27KKh, 50N, 50NP, 79NM, 81NMA) are used in transformers, sensors, relays, and control systems requiring high magnetic permeability and low losses. They feature stable magnetic characteristics and high structural uniformity.
Precision alloys with specified elastic properties (40KKhNM, 36NKhTYu, 17KhNGT) are used in spring elements, membranes, sensitive mechanisms, and measuring instruments. Their key advantage — is a combination of high strength and a stable elastic modulus.
Corrosion-resistant steels (12Kh18N9, 12Kh18N10T, 10Kh17N13M3T) are intended for service in aggressive environments, including the chemical, food, and power industries. They have high resistance to intergranular corrosion and good manufacturability in welding and forming.
Alloys with a specified temperature coefficient of linear expansion (29NK, 36N, 42N) are used in high-precision instruments requiring geometric dimensional stability under temperature change. These alloys are widely used in instrument making, aircraft engineering, and electronics.
Etching of sections — is not merely a laboratory operation. It is a tool for understanding the nature of metal. Every revealed grain contour, every phase-boundary line, and every included particle — is information about the material’s process history: how it was melted, how deformed, how cooled, and how processed.
That is why modern metallography remains one of the most accurate methods of quality control for metallic materials.
We do not merely look at metal. We read its structure.
