The corrosivity of sulfuric acid depends on many factors including temperature, concentration, the presence of oxidizing or reducing impurities, velocity effects, and solids in suspension. According to the MTI publication "Materials Selector for Hazardous Chemicals, Vol. 1: Sulfuric Acid, 2nd Edition", neither 304 or 316L (alone) are recommended for concentrated sulfuric acid at 45 °C. For concentrated acid (93% or >) at 40 to 60 °C and pipe diameters > 3 inches, the recommended materials are 304L with anodic protection (AP), 14% silicon-iron, Alloy B-2, Alloy C-276, glass, PVDF or PTFE lined pipe, and high-silicon stainless steel. AP is a technique used to control the corrosion of a metal surface by making it the anode of an electrochemical cell and controlling the electrode potential in a zone where the metal is passive. According to the same MTI publication, practically indefinite equipment life can be achieved with AP, even in hot sulfuric acid, for strongly passivating alloys like 304, 316, and 904L.