If you are choosing between stainless steel and aluminum and only remember one thing, make it this: stainless steel buys corrosion resistance and strength at roughly three times the weight and price, while aluminum buys lightness and conductivity at a lower cost but with far less resistance to chlorides and acids. For a structural or marine part, choose stainless steel. For a heat sink, a body panel or anything where weight or thermal transfer matters more than strength, choose aluminum. The two metals are not substitutes — they sit on opposite sides of the density, cost and corrosion map — so the decision should be made once, against the part's environment and load, before a single quote is written.
The numbers that separate them
| Property | Stainless steel (304) | Aluminum (6061-T6) |
|---|---|---|
| Density | 7.93 g/cm³ | 2.70 g/cm³ |
| Tensile strength, typical | 515 MPa min | 290 MPa min |
| Yield strength, typical | 205 MPa min | 240 MPa min |
| Thermal conductivity | ~16 W/m·K | ~167 W/m·K |
| Electrical conductivity | ~2.4% IACS | ~40% IACS |
| Relative cost (bar/plate) | higher | lower |
Values are typical published ranges, not acceptance limits. The number that governs your part is the one on the mill certificate for your heat.
Where stainless steel wins
Chloride and acid service. Stainless steel's chromium oxide passive film resists salt water, chlorinated cleaners and most food acids. Aluminum relies on a different oxide that breaks down quickly in chloride or strong-alkaline environments, which is why a stainless part lasts where an aluminum one pits.
Strength and rigidity. Stainless steel carries a higher tensile and yield strength at a comparable section, and it keeps its strength at higher temperature. A structural bracket, pressure vessel or pump shaft that must not deflect is normally stainless.
Hygiene and cleanability. Food, pharmaceutical and medical equipment use stainless because the surface is non-porous, easy to clean and does not shed metal. Aluminum oxidizes and can leave a grey residue in contact with food.
Where aluminum wins
Weight. At roughly one-third the density of stainless steel, aluminum is the default when mass is the design driver: transport bodies, aerospace frames, ladders, enclosures and anything lifted or moved.
Heat transfer. Aluminum conducts heat roughly ten times better than stainless steel. A heat sink, fin stock or cooling plate is almost always aluminum; stainless steel is a poor conductor and is chosen for its surface, not its thermal path.
Cost and machinability. Aluminum is cheaper per part for a given volume, machines faster, and is easier to form, weld and finish. For high-volume, non-structural, non-corrosive parts, aluminum wins on price.
The decision in one pass
Choose stainless steel when the part sees chlorides, acids, sustained load, high temperature, or must be hygienic — and the extra weight is acceptable.
Choose aluminum when weight, heat transfer or cost per piece is the driver, and the environment is dry and non-corrosive.
Where both seem to fit, the tie-breaker is usually corrosion: if the part lives outdoors near salt or is washed with chemicals, the safe answer is stainless. If it is an indoor, dry, non-structural part, aluminum is the cheaper correct answer.
What this means for your order
Because the two metals are quoted and certified differently, name the grade, temper and environment together when you ask for a quote. A stainless part is usually certified to EN 10204 3.1 with heat analysis; an aluminum part is usually certified to the ASTM or EN standard with the temper called out. State the service environment — salt air, food contact, load — in the first message and the correct metal, grade and certificate type come back in the same reply.