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 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 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. Higher chloride levels require more highly-alloyed stainless steels. Localized corrosion can be difficult to predict because it is dependent on many factors, including:  Pitting corrosion is considered the most common form of localized corrosion. The corrosion resistance of stainless steels to pitting corrosion is often expressed by the PREN (pitting resistance equivalent number), obtained through the formula:  where the terms correspond to the proportion of the contents by mass of chromium, molybdenum, and nitrogen in the steel. For example, if the steel consisted of 15% chromium formula_2would be equal to 15. The higher the PREN, the higher the pitting corrosion resistance. Thus, increasing chromium, molybdenum, and nitrogen contents provide better resistance to pitting corrosion. Though the PREN of a certain steel may be theoretically sufficient to resist pitting corrosion, crevice corrosion can still occur when poor design has created confined areas (overlapping plates, washer-plate interfaces, etc.) or when deposits form on the material. In these select areas, the PREN may not high enough for the service conditions. Good design and fabrication techniques combined with correct alloy selection can prevent such corrosion. Stress corrosion cracking (SCC) is a sudden cracking and failure of a component without deformation.