Strictly speaking, stainless steel is a trade name, an alloy originally patented in 1916 by English metallurgist Harry Brearley, containing a maximum of 0.70% carbon and 9-16% chromium. In the United States, American Stainless Steel Co. of Pittsburgh, PA, produces a chrome-iron alloy under the "stainless steel" patent which it owns. In more general terms, stainless steel is a corrosion resistant steel containing at least 10% chromium. According to the American Iron and Steel Institure (AISI), a steel is considered "Stainless" if it contains 4% or more chromium. Stainless Steels are characterized by their resistance to organic acids, weak mineral acids, and atmospheric oxidation, and their ability to retain their strength at high temperatures. The most common grades of stainless steel in the US are: Type 304, austenitic (chromium-nickel); Type 316, austenitic with 2%-3% molybdenum; Type 409, ferritic (low chromium) for high-temperature use; Type 410, heat-treatable martensitic (medium chromium) with a high strength level; and Type 430, a ferritic general-purpose grade with some corrosion resistance.