Whether a steel tank is magnetic depends entirely on the specific type of steel used in its construction. Steel is an alloy primarily composed of iron and carbon, and since iron is a naturally magnetic element, most steels retain this property. However, the addition of other elements fundamentally changes the metal’s internal structure, which can disrupt its magnetic nature. Therefore, a tank built from standard low-carbon steel will be strongly magnetic, while one made from certain stainless steel alloys may be practically non-magnetic.
Understanding Ferromagnetism in Metals
The potential for a material like steel to be magnetic stems from the fundamental physics known as ferromagnetism. Ferromagnetism represents the strongest form of magnetism and is a characteristic shared by only a few elements at room temperature, notably iron, nickel, and cobalt. This property arises from the alignment of electron spins within the material’s atoms. In these elements, the electrons in the outer shells are unpaired, and their magnetic moments are naturally aligned in the same direction, a condition known as spontaneous magnetization.
These aligned atomic magnets group together into microscopic regions called magnetic domains. In an unmagnetized piece of steel, these domains are oriented randomly, causing their individual magnetic fields to cancel each other out, resulting in no overall net field. When an external magnetic field is applied, the domain walls shift, and the individual domains rotate to align with the external field. This alignment causes the material to be strongly attracted to a magnet and allows it to become magnetized itself.
The Role of Steel Composition
The difference between a magnetic and non-magnetic steel tank is determined by the specific alloying elements added to the iron and carbon mixture. Different combinations of elements force the steel to adopt distinct atomic lattice structures, which either support or inhibit the formation and alignment of magnetic domains. The resulting crystal structure of the alloy is the primary determinant.
Standard carbon steels and low-alloy steels, which are commonly used for general-purpose storage tanks, are strongly magnetic. These steels maintain a body-centered cubic (BCC) crystal structure, known as ferrite or martensite. This structure readily allows the magnetic domains of the iron atoms to align, meaning they exhibit robust ferromagnetic properties and will be strongly attracted to a magnet.
In contrast, certain grades of stainless steel, such as the 300-series (e.g., 304 and 316), are considered non-magnetic. These steels are made non-magnetic by the addition of high percentages of nickel and chromium, which force the steel into a face-centered cubic (FCC) structure, known as austenite. The austenitic structure physically disrupts the stable alignment of the magnetic domains, making the material either non-magnetic or only very weakly magnetic.
How Tank Structure and Use Influence Magnetism
Even if a steel tank is built from a magnetic grade of steel, its final magnetic state can be influenced by manufacturing processes and its operational environment. When a ferromagnetic material is exposed to a strong external magnetic field, such as from magnetic lifting cranes or high-current welding equipment, it can retain some of that magnetization. This remnant field is called residual magnetism, meaning the finished tank itself acts like a weak, temporary magnet.
Residual magnetism can cause practical issues, such as attracting metal shavings and debris to the tank’s surface. During the welding process, a field strength of 40 Gauss or above can interfere with the electric arc, causing a condition known as “arc blow” that compromises the weld quality. Furthermore, mechanical stress applied during fabrication, such as bending or cold-working the steel plates, can sometimes induce weak magnetism even in nominally non-magnetic austenitic stainless steels. This physical deformation can locally alter the crystal structure enough to create small magnetic regions.