corrosion testing. Acidic solutions can be put into two general categories: reducing acids, such as hydrochloric acid and dilute sulfuric acid, and oxidizing acids, such as nitric acid and concentrated sulfuric acid. Increasing chromium and molybdenum content provides increased resistance to reducing acids while increasing chromium and silicon content provides increased resistance to oxidizing acids. Sulfuric acid is one of the most-produced industrial chemicals. At room temperature, type 304 stainless steel is only resistant to 3% acid, while type 316 is resistant to 3% acid up to and 20% acid at room temperature. Thus type 304 SS is rarely used in contact with sulfuric acid. type 904L and Alloy 20 are resistant to sulfuric acid at even higher concentrations above room temperature. Concentrated sulfuric acid possesses oxidizing characteristics like nitric acid, and thus silicon-bearing stainless steels are also useful. Hydrochloric acid damages any kind of stainless steel and should be avoided. All types of stainless steel resist attack from phosphoric acid and nitric acid at room temperature. At high concentrations and elevated temperatures, attack will occur, and higher-alloy stainless steels are required. In general, organic acids are less corrosive than mineral acids such as hydrochloric and sulfuric acid. As the molecular weight of organic acids increases, their corrosivity decreases. Formic acid has the lowest molecular weight and is a weak acid. Type 304 can be used with formic acid, though it tends to discolor the solution. Type 316 is commonly used for storing and handling acetic acid, a commercially important organic acid. Type 304 and type 316 stainless steels are unaffected by weak bases such as ammonium hydroxide, even in high concentrations and at high temperatures. The same grades exposed to stronger bases such as sodium hydroxide at high concentrations and high temperatures will likely experience some etching and cracking. Increasing chromium and nickel contents provide increased resistance. All grades resist damage from aldehydes and amines, though in the latter case type 316 is preferable to type 304; cellulose acetate damages type 304 unless the temperature is kept low. Fats and fatty acids only affect type 304 at temperatures above and type 316 SS above , while type 317 SS is unaffected at all temperatures. Type 316L is required for the processing of urea. Localized Localized corrosion can occur in several ways, e.g. pitting corrosion and crevice corrosion. These localized attacks are most common in the presence of chloride ions.