The 309 stainless steel foil has an unpolished (mill) surface and a standard tolerance. The 309 stainless steel grade provides the same corrosion resistance as general-purpose 304 stainless steel but has higher chromium and nickel content to better retain its properties in high temperatures. An austenitic stainless steel, 309 is typically nonmagnetic but may become slightly magnetic when cold worked.
Stainless steel is an iron alloy with resistance to staining and rusting in many environments where steel would typically degrade. The chemical composition of each grade creates a grain structure that falls into one of five classes: austenitic, duplex, ferritic, martensitic, and precipitation hardening. The austenitic class contains the most formable, weldable, and corrosion-resistant stainless steel grades, but they cannot be heat treated.
Tensile strength, used to indicate the material’s overall strength, is the peak stress it can withstand before it breaks. Corrosion resistance describes the material's ability to prevent deterioration caused by atmosphere, moisture, or other medium. Magnetism characterizes how much the material is repelled by or attracted to a magnet.