Why Don't Stainless Steel Pots and Sinks Stick to Magnets?
― It's mostly iron. What changed is how the atoms are arranged
About 70% of stainless steel by weight is iron. Yet hold a magnet to a kitchen sink and it barely sticks. What decides whether a magnet sticks is not whether iron is present. It is the pattern the iron atoms are arranged in.
The kitchen timer you just bought has a magnet on the back. It snaps neatly onto the fridge door.
But put it against the stainless steel wall of the sink and it slides right off. It is the same kind of metal, and iron is supposed to be in there, yet it won't stick.
Stranger still, around the corners of the sink or the bent rim, the magnet may catch a little. It is the same sheet, but the result changes from place to place.
There are only two reasons
Each iron atom is a tiny magnet. In ordinary iron, they all line up and point the same way. Add nickel to stainless steel and the atoms settle into a different pattern, and they can no longer line up.
In a sharply bent spot, the atoms are forced out of place. Some of them shift into the arrangement that magnets stick to. That is why a magnet catches only at the corners of a sink.
In short, magnetic pull depends less on the atoms' "ingredients" than on their "teamwork". Let's go through it one step at a time.
Tiny atomic magnets only matter when they line up
Inside an iron atom, electrons spin like tops. Because of this spin, each atom acts like a tiny compass needle. In ordinary iron, neighbouring atoms pull strongly on each other to point the same way. So trillions of atoms line up, and the iron pulls hard on a magnet brought close.
What matters here is the arrangement. The atoms of ordinary iron sit one at each corner of a cube and one in its centre. In "18-8", the standard stainless steel (18% chromium, 8% nickel), nickel changes this. The atoms sit at the corners and at the centre of each face, a more tightly packed pattern. In this pattern, neighbouring atoms can't easily push each other into the same direction. The directions stay random, so the magnetic effect of the whole almost cancels out (Figure 1).
Here is the odd part: nickel itself is a metal that magnets stick to. Mix magnetic nickel into magnetic iron, and the result is not magnetic. That is the clearest evidence that magnetic pull is decided not by adding up the ingredients but by the arrangement.
Even among stainless steels, some types and some spots are magnetic
There is also a stainless steel with no nickel, called "18-0" (18% chromium only). Its atoms are arranged just like ordinary iron's, so magnets stick to it firmly. It is often used for cheap spoons and for the base of pots that work on induction (IH) cookers. An induction cooker heats by driving swirling electric currents inside a magnetic metal, so a pot that magnets don't stick to heats poorly.
Even with 18-8, parts that have been bent sharply or pressed into shape are a different story. When force pushes the atoms out of their pattern, some of them switch to the arrangement that magnets stick to. That is why a magnet catches weakly at sink corners, at the bent neck of a spoon, or at a pot's rim. Slide a magnet along and you can feel a "snag" only where the bending was strongest.
You may have heard that "if a magnet sticks, it isn't stainless steel", but that is wrong. Some stainless steels, like 18-0, are strongly magnetic. Rust resistance comes mainly from chromium, which forms an extremely thin film on the surface that protects the metal beneath. Whether a magnet sticks is a separate property from rust resistance.
Even iron stops sticking to magnets when it gets hot
Whether the atoms line up also depends on temperature. As it gets hotter, the atoms jiggle harder and work against the pull that aligns them with their neighbours. In iron, above about 770°C the alignment collapses completely and magnets no longer stick. This temperature is called the "Curie temperature". Try the slider in Figure 2 to change the temperature. The degree of alignment drops sharply from around 700°C and reaches 0 at 770°C.
Heat it further, past about 910°C, and the iron atoms rearrange into the same tightly packed pattern as 18-8 stainless steel. You could think of 18-8 as a metal in which nickel holds the "high-temperature iron arrangement" in place all the way down to room temperature.
Summary
Most stainless steel doesn't stick to magnets, and not because the iron has gone. Adding nickel changed the atoms' arrangement, so their tiny magnets can no longer line up. 18-0, which has the same arrangement as iron, is magnetic, and even 18-8 is slightly magnetic where it has been bent. Iron itself also stops being magnetic once it passes 770°C and the alignment breaks down.
Magnetic pull depends not on how many atoms there are, but on how they line up.
Change the arrangement, and even iron forgets its magnetism.
For more on the nature of magnets themselves, see If you cut a magnet in half, can you get a magnet with only a north pole? For the colour changes of heated iron, see How can blacksmiths tell the temperature of iron without a thermometer? For a story linked to why stainless steel resists rust, see Why does iron rust?
- Take one magnet off the fridge and hold it to a flat part of the sink. It should barely stick. Next, slide it slowly over the sink's corners and bent rims and look for spots where it catches.
- Try it on the handles of spoons and forks. Some will attract it, some won't, and some will catch a little only at the bent neck. If there is an "18-8" or "18-0" stamp on the back of the handle, compare it with your result.
- Try the bottoms of pots and frying pans too. A pot where only the base attracts a magnet often has a magnetic metal layer bonded on so it works on induction cookers.
A strong magnet (neodymium) may pull slightly on 18-8 as well. A small fridge magnet shows the difference more clearly. Don't test a pot that has just come off the heat.
For those who want more ― terms, formulas and links to textbooksEach part is labelled with its level, from lower secondary science to university courses
- Middle schoolCovered in lower secondary science
- High schoolCovered in upper secondary physics and chemistry
- High school+Advanced upper secondary material, or a textbook sidebar
- UniversityUniversity-level courses not taught in high school (solid-state physics, metallurgy)
- ResearchNot yet settled even at university level; researchers are still working on it
Middle schoolTerms: this phenomenon has names
- Ferromagnetism: the property, seen in iron, nickel and cobalt, that atomic magnets line up on their own and are strongly attracted to magnets.
- Body-centred cubic and face-centred cubic lattices: names for atom arrangements. The former has atoms at the corners and centre of a cube, the latter at the corners and the centre of each face. Ordinary iron is the former, and 18-8 stainless steel is the latter.
- Austenitic stainless steel: stainless steel with a face-centred cubic arrangement. 18-8 (for example SUS304 in the JIS standard) is the typical case, and magnets barely stick to it.
- Curie temperature: the temperature at which ferromagnetism disappears. For iron it is said to be about 770°C.
Middle schoolHigh schoolChecking with a formula: how different is the pull on iron and on 18-8?
Let's estimate the pull on a 1 cubic centimetre lump of metal brought close to the surface of a strong magnet. The number for how easily a material becomes magnetic is called "magnetic susceptibility" (symbol χ). The pull is directly proportional to this number.
| In symbols | F ＝ （χ × V ÷ μ0） × B × （ΔB ÷ Δx） |
| In words | Pull ＝ susceptibility × volume ÷ permeability of vacuum × field strength × rate at which the field weakens with distance |
| Where it comes from | An object magnetised in a magnetic field has lower magnetic energy where the field is stronger. The object is pushed toward the lower energy, that is, toward the magnet. The slope of that "energy hill" is the force. |
| Meaning of symbols and units | F is force (N), V is volume (m³), B is field strength (tesla), Δx is distance (m) |
| Field B at the surface of a strong magnet | About 0.4 tesla |
| Rate at which the field weakens (assuming it reaches 0 at 1 cm) | 40 tesla per metre |
| 1 ÷ μ0 (reciprocal of the permeability of vacuum) | About 800000 |
| Volume V (1 cubic centimetre) | 0.000001 m³ |
| Effective susceptibility of an iron plate | About 1 (the iron's own magnetism cancels itself out and reaches a ceiling set by its shape) |
| Susceptibility of 18-8 | About 0.003 to 0.01 (0.005 is used here) |
| Field × rate of weakening | 0.4 × 40 ＝ 16 |
| Divide by μ0 (multiply by 800000) | 16 × 800000 ＝ 12800000 |
| Multiply by the volume of 1 cubic centimetre | 12800000 × 0.000001 ＝ 12.8 |
| Pull on iron (susceptibility 1) (N) | 12.8 × 1 ＝ 12.8 |
| Convert to weight (kg) | 12.8 ÷ 9.8 ≒ 1.3 |
| Pull on 18-8 (susceptibility 0.005) (N) | 12.8 × 0.005 ＝ 0.064 |
| Convert to weight (kg) | 0.064 ÷ 9.8 ≒ 0.0065 |
| How many times stronger for iron than 18-8 | 12.8 ÷ 0.064 ＝ 200 |
For a lump of the same size, iron is pulled with a force equal to about 1.3 kg, while 18-8 is pulled with only about 6.5 g. The difference is about 200 times. A fridge magnet is much weaker, so on 18-8 it can't hold its own weight and falls. These numbers are rough guides to help you grasp the mechanism.
High schoolHigh school+Crystal structure, and magnetism created by processing
High schoolIn chemistry you learn the crystal lattices of metals: body-centred cubic, face-centred cubic and hexagonal close-packed. The face-centred cubic lattice has a packing fraction of about 74%, tighter than the body-centred cubic's 68%. At about 910°C, iron undergoes an "allotropic transformation" from body-centred to face-centred cubic.
High school+When 18-8 is bent hard at room temperature, part of the face-centred cubic structure changes into a nearly body-centred cubic structure called "deformation-induced martensite". This part is ferromagnetic, so it is weakly attracted to magnets. Heat treatment (solution annealing) is said to return it to face-centred cubic, and it stops sticking to magnets again.
UniversityExchange interaction and the Curie temperature
The force that lines up the atoms' magnets is the "exchange interaction" between electrons. It is an effect that comes from the Pauli exclusion principle in quantum mechanics, and is said to be far stronger than the classical force between magnets. In metallic iron, electrons roam between atoms, so band theory (itinerant-electron magnetism) is used, in which ferromagnetism depends on whether the "Stoner criterion" is met. When the spacing or arrangement of atoms changes, this condition can fail. Above the Curie temperature, the "Curie–Weiss law" holds: susceptibility falls in inverse proportion to temperature. The curve in Figure 2 is drawn with Kuz'min's approximate formula, which fits measured values well.
📖 Derivations of the formulas and further reading: Curie temperature (Wikipedia, in Japanese) ／ Ferromagnetism (Wikipedia, in Japanese)
ResearchWhat is still not well understood
- What is the magnetism of face-centred cubic iron really like? Pure face-centred cubic iron is thought to be close to "antiferromagnetic" at low temperature, with the directions alternating. But its properties change a lot with the spacing of the atoms, and the full picture is still debated.
- How much bending makes how much magnetism? How deformation-induced martensite forms varies greatly with temperature, the speed of working and tiny differences in composition. Work continues on models that predict it accurately.
- Can the shape of the curve of magnetisation falling with temperature be derived from theory alone? The approximate formula used in Figure 2 is fitted to measurements. Calculating the curve for a metal like iron accurately from first principles is still said to be a hard problem.
So what this article says is also "an explanation of what is understood so far". Even in the sink you touch every day, an unsolved question of physics lies asleep.
Links to textbooks (by level)
| Level | Subject and unit | Where in this article |
|---|---|---|
| Middle school | Science: magnets and magnetic fields, properties of metals | Metals that do and don't stick to magnets |
| High school | Chemistry: crystal lattices of metals / Physics: magnetic fields | The different arrangements in Figure 1, the pull formula |
| High school+ | Advanced chemistry: alloys and allotropic transformation | 18-8 versus 18-0, magnetism at bent spots |
| University | Solid-state physics, metallurgy | Exchange interaction, Stoner criterion, Curie temperature |
| Research | Magnetism physics, materials science | Magnetism of face-centred cubic iron, predicting deformation-induced transformation |
| ― | Links to daily life | Choosing induction-ready pots, reading stamps on tableware |
- Stainless steel (Wikipedia, in Japanese)
- Austenite (Wikipedia, in Japanese)
- Japan Stainless Steel Association (ステンレス協会), ed., Handbook of Stainless Steel (ステンレス鋼便覧), Nikkan Kogyo Shimbun
- Chikazumi Soshin (近角聰信), Physics of Ferromagnetic Materials, Vol. 1 (強磁性体の物理（上）), Shokabo (裳華房)
- M. D. Kuz'min, "Shape of Temperature Dependence of Spontaneous Magnetization of Ferromagnets: Quantitative Analysis", Physical Review Letters 94, 107204 (2005)
※This article is a general-audience science explainer. The numbers given are rough guides to help you understand the mechanism. For a product's material or whether it works with induction cookers, check the product's own labelling.