Sheet metal is metal formed into thin, flat sections, typically through industrial rolling. Whether this material is magnetic depends entirely on its chemical makeup. Magnetism is a property of the atomic structure, not its shape, meaning a thin sheet has the same magnetic properties as a thick bar of the same composition. Sheet metal can exhibit strong attraction, weak attraction, or slight repulsion, depending on the specific elements used to create it.
Why Magnetism Depends on Metal Type
The magnetic behavior of any metal originates at the atomic level, specifically with the movement and spin of its electrons, which generate tiny magnetic fields called magnetic moments. In most elements, these moments are randomly oriented or paired up, causing their effects to cancel out, resulting in no net magnetism. For a metal to be strongly magnetic, it must contain elements like iron, nickel, or cobalt. These elements allow the magnetic moments of adjacent atoms to spontaneously align in the same direction, creating microscopic regions called magnetic domains that amplify the collective magnetic force. A metal’s composition is the sole determinant of its magnetic potential.
Three Categories of Magnetic Behavior
Materials are scientifically categorized into three main groups based on how they interact with an external magnetic field. This classification is determined by the internal arrangement of their electron spins and magnetic domains, providing the framework for predicting how any sheet metal will behave near a magnet.
Ferromagnetism
Ferromagnetism is the strongest form of magnetic interaction and is the behavior most people associate with magnets. Ferromagnetic materials, like iron, have magnetic domains that align strongly and permanently when exposed to an external magnetic field. Once magnetized, these materials can retain their magnetic properties even after the external field is removed, which is why they are strongly attracted to common magnets.
Paramagnetism
Paramagnetism describes a much weaker form of magnetic attraction, where materials only display magnetism in the presence of an external field. Atoms in these substances have unpaired electrons, which create small magnetic moments that align very loosely with the external field. When the magnet is taken away, the alignment instantly disappears, meaning the material loses all induced magnetism.
Diamagnetism
Diamagnetism is the weakest magnetic behavior, where the material is actually repelled by a magnetic field. This slight repulsion occurs because the external field causes a minor shift in the electron orbits, inducing a temporary magnetic moment that opposes the applied field. All materials have a diamagnetic component, but it is only observable in substances where the stronger ferromagnetic or paramagnetic effects are absent.
Magnetic Properties of Common Sheet Metals
Carbon steel is one of the most common sheet metals and is reliably ferromagnetic due to its high iron content. Since steel is an alloy made primarily of iron, the material has the necessary atomic structure for strong magnetic attraction. Low-carbon or mild steel, widely used in construction and appliances, will readily stick to a magnet.
The magnetic behavior of stainless steel is more complex and depends entirely on its specific alloy series. The 400 series of stainless steel, known as ferritic or martensitic, is magnetic because its internal crystalline structure allows the iron atoms to align. The 300 series, however, which includes common grades like 304 and 316, is non-magnetic because the addition of nickel disrupts the iron’s crystal lattice, preventing magnetic domain alignment.
Sheet metals like aluminum and copper are considered non-magnetic for practical, everyday purposes. Aluminum is a paramagnetic material, meaning it is very weakly attracted to a strong magnet, an effect that is practically undetectable without specialized instruments. Copper is a diamagnetic material, displaying a slight repulsion from a magnetic field, though this force is also far too weak to be noticed outside of a laboratory setting.