So much of human progress in recent history has been made possible by our knowledge and understanding of metals, their composition, and how different elements affect the physical and mechanical properties of a metal.
Common Metal Types
Metals fall into two main categories: ferrous and non-ferrous. Ferrous metals, made primarily of iron, are strong but heavier, making them ideal for applications requiring durability. Non-ferrous metals, which don’t contain iron, are lighter, more corrosion-resistant, and often better for applications where weight and resistance to the elements are key.
Ferrous Metals
It is estimated that 90% of materials in use today are ferrous metals. Ferrous metals are those whose primary element is iron. (Ferrous comes from the Latin word “ferrum,” meaning iron.) Ferrous metals are typically heavier than non-ferrous metals (due to their higher density) and are typically magnetic. Stainless steels are a notable exception to magnetic testing. They are classified as ferrous metals but, due to their lower iron content, usually have little to no magnetic attraction.
Iron
Iron is typically heavy and has a rough texture. All types of iron have a high degree of magnetic attraction and are very hard but have limited ductility.
Steel
Steel is iron alloyed with other elements, primarily with carbon, but also with manganese, cobalt, chromium, sulfur, silicone, and many other elements. The choice of element produces different types of steel with varying strengths, hardness, ductility, and corrosion resistance. Steel is likewise heavy and has a hard, smooth surface. Most steels have a magnetic attraction.
Stainless Steel
Chromium and nickel combined with a lower carbon content gives stainless steel its anti-corrosive properties. Stainless is usually identifiable by its relatively shiny, silver-colored hue. Like other types of steel, stainless steel is relatively heavy due to its high density.
Non-Ferrous Metals
Non-ferrous metals do not contain iron and therefore have no magnetic attraction.
Aluminum
Aluminum is widely used for its light weight. It is usually a dull, silver color and very soft. It bends easily and is easily dented and deformed. It produces no sparks when subjected to a spark test.
Copper
Copper is lightweight and widely used for its thermal and electrical conductivity. It is shiny and bright when polished but turns a distinctive shade of green when oxidized. It is soft and highly malleable.
Brass
Brass is the alloy formed when zinc is added to copper. It is lightweight, malleable, and soft and can be polished to a bright, almost golden, hue.
Bronze
Bronze is similar to brass, as both are copper-based alloys. It is made when copper is alloyed with tin. Bronze is relatively lightweight but slightly less malleable and more brittle than brass. It is more reddish-brown than brass.
Zinc
Zinc, in its pure elemental form, is white with a slightly bluish tint. It is very brittle at ambient temperatures but becomes highly malleable when heated to between 100 and 150 degrees Celsius. Zinc has excellent anti-corrosion properties and is commonly used in coating applications for galvanizing, where it is identifiable by its dull, silvery-gray appearance.
Nickel
Nickel is widely used for its excellent corrosion resistance as well as its conductivity of thermal and electrical energy. It is highly malleable and identifiable by its silvery-white appearance.
Titanium
Titanium is known for its exceptional lightweight and high-strength properties, which makes it well-suited for aviation and aerospace applications. (The strength of titanium is nearly equivalent to some low-grade steels, although at a significantly lighter weight.) Titanium typically has a smooth surface with a shiny, gray-silver hue.
Techniques for Identifying Metals
There are a number of ways to identify common metals, ranging in complexity from a simple visual inspection to high-tech spectroscopy.
Physical Properties of Metals
A metal’s physical properties determine how it reacts to heat, pressure, and the environment. These properties affect how a metal cuts, welds, bends, or holds up over time.
Ductility
Ductility refers to how well a metal can stretch or be pulled without snapping. Metals like copper and aluminum have high ductility and they can be drawn into wires or stretched without breaking. On the other hand, metals like cast iron are much more brittle and tend to crack under tension.
Malleability
Malleability is a metal’s ability to withstand compressive stress or how well it can be bent or rolled into shape without breaking. Metals like copper are both malleable and ductile, meaning they can handle being stretched and shaped. However, some metals, like lead, are highly malleable but lack ductility, making them easy to shape but prone to snapping or breaking under tension.
Luster
Luster is how shiny or reflective a metal is. It depends on things like surface finish and oxidation. Metals like gold, silver, and stainless steel can be polished to a high shine, reflecting lots of light. On the other hand, metals with rough or oxidized surfaces, like iron or lead, reflect very little light, giving them a duller appearance.
Density
Density is a measure of mass divided by volume. The more dense a material is, the more it exists in a smaller volume. For example, one pound of iron is significantly smaller than the same mass of aluminum.
Conductivity
How efficiently thermal or electrical energy can move through a piece of material is the measure of conductivity. Copper has a high degree of electrical conductivity, which has led it to become the most commonly used metal in electrical wires and connectors.
Hardness
Hardness is best described as the capability of the metal to withstand deformation from impact, abrasion, or mechanically applied forces.
Melting Point
The melting point of a metal is the point at which it transitions from a solid to a liquid. Lead has a low melting point (328 degrees Celsius), whereas tungsten has a high melting point (3300 degrees Celsius).
Chemical Properties of Metals
Reactivity
Reactivity is the measure of how easily a metal becomes susceptible to material loss (loss of electrons) in the presence of oxygen, water, or acids. Sodium and potassium are highly reactive metals and essentially dissolve in water. At the other end of the spectrum, gold, silver, and platinum are non-reactive and undergo little to no change when placed in contact with water and acids.
Oxidation States
The oxidation state of a metal measures its capability to withstand the loss of electrons in the presence of oxygen. Copper turns a characteristic green in an oxygen-rich environment, while steel begins to rust when exposed to moisture (from the oxygen in water), which is a slow process of oxidation.
Corrosion Resistance
Corrosion resistance defines how well a material can withstand exposure to certain chemicals and compounds without incurring significant loss of material. Stainless steel is an example of a metal that was designed for exceptional corrosion resistance.
Visual Inspection
The simplest and most cost-effective method of identifying metals is the visual inspection method. This requires a skill that inspectors should use daily to ensure the material is the correct choice for the project.
Color and Luster
The first characteristics used to identify a material are color and luster. Is it shiny and silver (which might indicate stainless steel) or is it a dark and dull gray (which might be indicative of regular mild steel)? If a metal is a dark reddish-brown, it might be copper, but if it’s more of a shiny, golden color, it may be brass.
Surface Texture and Patterns
Oftentimes, the texture of the metal gives clues to its identity. In its natural state, iron has a rough surface with minute indentations. Steel is typically much smoother.
Visual Markings and Labels
In most cases, raw materials are labeled and have some identifying marks on them to help with identification. Pipe, for example, usually has the material specification and grade painted on the outside with a stencil. The heat number (which signifies the specific batch or lot of the material) is often a requirement of quality control procedures.
Physical Tests
Visual inspection only goes so far in the identification of metals. To help narrow down the metal in question, physical tests are useful.
Magnetism Test
The magnet test is a quick and easy way to help tell metals apart, especially when trying to separate carbon steel from stainless. If a magnet sticks strongly, you’re probably looking at carbon steel. If there’s little to no pull, it’s likely stainless steel.
Just keep in mind: some stainless steels (like the ferritic 300 series) are slightly magnetic, so this test isn’t foolproof. Use it as a starting point—not your only method.
Spark Test
The spark test is performed by removing material from a test piece and visually observing the characteristics of the sparks produced to determine the material’s identity. For example, bright, white sparks indicate the presence of steel, while iron produces a reddish, orange color.
Density Measurement
There are a number of methods to perform density measurements, but the most common is the simple water displacement method. Since metals have a specific density, knowing the density of a material helps with making an accurate identification.
Hardness Test
Brinell hardness, Rockwell hardness, and Charpy V-notch tests (to name just a few options) help determine the identity of a material by its hardness. In the Rockwell hardness test, a small steel ball is pressed into the metal under a controlled load. The size and depth of the indentation are then compared to known hardness values.
Chip Test
A chip test is usually used to determine the machinability of a metal for manufacturing processes. During the machining process, a tool is brought in contact with the metal to remove a small portion of the metal, forming a “chip.” The size, shape, and surface finish of the chip are then analyzed and compared to known standards.
Chemical Tests
Chemical tests show how a metal acts when subjected to various stimuli to help determine its identity.
Acid Test
In an acid test, a small piece of metal is introduced to the acid (typically hydrochloric or nitric acid), either by applying the acid to the material’s surface or by immersing the metal sample in the acid. The chemical interaction and results are then compared to known standards to determine the material's identity. For example, hydrochloric acid forms silver chloride when mixed with silver, which appears as a white substance.
Precipitation and Solubility Tests
Much like acid tests, precipitation and solubility testing are performed by analyzing the chemical reactions that take place after applying various chemical compounds to metal samples. Chemicals like water, acetone, and ethanol generate chemical reactions. The results are then compared to known standards.
Flame Test
The flame test is performed by introducing a metal specimen to a flame and observing the results. For example, copper produces a flame with a blue/green flame. Sodium generates a yellow flame, whereas lithium produces a dark-red, almost crimson, flame.
Non-Destructive Testing (NDT)
There are several ways to determine a metal's identity by utilizing non-destructive testing (NDT) methods.
Ultrasonic Testing
Ultrasonic testing (UT) involves sending an acoustic sound wave through a metal and assessing its reflection. The rate at which the sound energy travels says a lot about the metal's grain structure and density.
Radiographic Testing
Radiography has long been utilized in welding inspections on pipe and other materials to look for weld defects. Radiography transmits energy through a material, which is then captured on a film or digital device. The degree to which this radiation energy passes through the material is determined by the density of the material.
Eddy Current Testing
Eddy current testing uses electromagnetic induction to examine welds for surface and near-surface flaws. The strength and distribution of the eddy currents vary depending on the material’s conductivity and permeability. The resistance and reaction of the electromagnetic energy in the material reveal the material’s identifying characteristics.
Advanced Identification Methods
The most definitive form of material identification involves analyzing a material’s characteristics at the atomic level.
X-Ray Fluorescence (XRF)
X-ray fluorescence is a non-destructive way to identify what metals are in a material. It works by hitting the sample with high-energy X-rays, which makes the atoms give off their own unique X-ray signals. The machine reads those signals to tell you exactly which elements are present.
Scanning Electron Microscopy (SEM)
Scanning electron microscopy involves scanning a material’s surface with a highly focused electron beam. When the electrons hit the surface, they interact with the atoms, producing signals that are reflected back. These signals are used to create high-resolution images, often magnified up to several hundred thousand times, allowing you to closely examine the material’s surface and composition.
Atomic Absorption Spectroscopy (AAS)
Atomic absorption spectroscopy starts by dissolving a metal sample in an acid solution. This solution is then atomized using a flame to vaporize the metal. Light is passed through the vapor, and some of it is absorbed by the metal. The amount of light absorbed correlates directly with the concentration of metal in the sample. By comparing this absorption to known standards, you can determine the precise concentration of elements in the material.
Optical Emission Spectroscopy (OES)
Optical emission spectroscopy uses a high-energy spark or plasma arc to excite and vaporize the atoms in a metal sample. As the atoms vaporize, they emit light energy, which is directly tied to the metal’s composition. This emitted light is then measured and compared to known standards to identify the elements in the sample. It’s a reliable method for precise material identification, especially when dealing with alloys.
Energy Dispersive X-Ray Spectroscopy (EDS)
Energy-dispersive X-ray spectroscopy is often paired with scanning electron microscopy to give a detailed analysis of a metal’s composition. A focused electron beam is aimed at the metal’s surface, causing the atoms to release X-rays. These X-rays are then analyzed and compared to known standards to identify the elements in the material. It’s a precise method used when you need to know exactly what’s in your metal, down to the trace elements.
Mass Spectrometry
Mass spectroscopy calculates the exact molecular weight of compounds within a sample. Mass spectroscopy requires three things: an ionization source, a mass analyzer, and an ion detection system. First, molecules are converted to ions. The ions are then sorted according to their mass-to-charge ratios. Lastly, the now-separated ions are measured to calculate the exact molecular weight of the elements in the sample.
Conductivity Tests
Conductivity is a physical property of metal directly determined by the composition of the elements in the metal. Measuring the conductivity of a metal helps with material identification.
Electrical Conductivity
To measure electrical conductivity, electrical energy is passed through a metal sample. The resistance of the electrical energy is then measured, which determines its conductivity. This conductivity is compared to known standards.
Thermal Conductivity
To use thermal energy to identify metals, a sample of the metal is heated at a controlled rate. Since the rate of heating and cooling is determined by the metal’s constituent elements, the rate of heating and cooling can be compared to known standards for identification.
Safety Precautions During Testing
The safety hazards involved in materials testing and identification are as varied as the identification techniques themselves.
Personal Protective Equipment (PPE)
The most basic forms of PPE that should be worn at all times include:
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Safety glasses.
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Gloves. These may need to be chemical- or acid-resistant. Check all manufacturers’ recommendations and OSHA requirements when working with acids and potentially harmful chemicals.
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Respiratory protection. Required when grinding, sanding, or polishing or when chemical reactions are likely to produce fumes.
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Full-length work pants and work shirts, preferably cotton.
If you're using high-end testing methods like X-ray fluorescence or anything involving radiation, you may also need lead shielding, barriers, or designated isolation zones to keep others safe. These setups should only be run by trained operators following all radiation safety protocols.
Handling Chemicals Safely
If you’re using chemicals for material testing, safety comes first. Some of these substances can cause serious harm like burns, breathing problems, and other health risks if you don’t handle them right. Spills and improper storage can also lead to environmental damage, like contaminating soil or water. Always follow the chemical manufacturer’s instructions and stay up to date with OSHA safety requirements..
Proper Disposal of Test Materials
If you're using chemicals or creating waste during metal testing, you should handle that waste the right way. Some materials can be hazardous, especially those used in advanced testing. To keep your shop safe and protect the environment, make sure you’re following proper disposal guidelines. Many local recycling centers and waste services (including ones that handle hazardous materials) can help you get rid of testing byproducts the right way.
FAQ
Why is metal identification important in manufacturing and construction?
Proper metal identification is crucial for ensuring a material’s performance in specific applications. For instance, using a piece of metal that’s thought to be steel but is actually iron can result in failure under load, as iron lacks steel’s ductility. Similarly, using a metal that’s assumed to be stainless steel in a pipe system requiring high corrosion resistance could lead to service failure and costly leaks.
Metal identification also affects compatibility with welding rods, electrodes, and filler metals. Using the wrong filler for a metal could cause weld failure, compromising the integrity of the structure or equipment.
How can I identify metals by their physical properties?
It is often possible to identify metals visually by their color and luster. For example, a smooth, silvery, shiny surface could indicate that the metal in question is stainless steel.
Lifting the metal and determining its weight relative to its size will provide clues to its density. If the piece is hefty, it may be stainless steel. If it is lightweight, it could be a polished piece of aluminum.
If you next subject the piece to a magnetic test and observe strong magnetic attraction, it may be that you have highly polished carbon steel. If there is no magnetic attraction, the possibility of aluminum or stainless remains.
If the sparks generated by grinding are soft and white, the metal could be aluminum. If the sparks are a brighter white and yellow, the metal is more likely to be stainless steel.
Finally, strike the metal with a hammer or chisel or even a center punch. If this makes a deep indentation, it is probably a soft metal like aluminum. If the indentation is shallow and slight, it is more likely stainless steel. This simple step-by-step deductive reasoning will help identify with reasonable certainty the type of metal.
What are the safest and most practical physical or chemical tests for identifying metals?
Acid and spark tests are safe and cost-effective methods commonly used to identify metals. Other methods pose greater safety risks. Always wear gloves, and goggles, and work in a well-ventilated area when handling chemicals.
What is non-destructive testing (NDT), and why is it used in metal identification?
NDT methods are commonly used to identify materials because, as the name implies, these procedures don’t damage the base metal, meaning they are appropriate for performing examinations on completed products and components. Secondly, the use of acoustical energy in UT scanning or radiation energy (as in radiography) calculates a material’s density. Since the density of a material is determined by its composition, density measurements taken with NDT can positively identify the material.
Featured photo courtesy of Dan Cristian Pădureț // Unsplash
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