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Selected Application Recommendations Concentrated Acids Containment of acids is one of the most popular uses of corrosion grade composites. Polyesters and vinyl esters display excellent acid resistance, and almost all acids can be accommodated in dilute form. However, there are some concentrated acids which can be quite aggressive or deserve special attention. Sulfuric Acid Sulfuric acid below 75% concentration can be handled at elevated temperatures quite easily in accordance with the material selection guide. However, because of the strong affinity of SO3 toward water, concentrated sulfuric acid (76-78%) is a powerful oxidizing agent that will spontaneously react with polymers and other organic materials to dehydrate the resin and yield a characteristic black carbonaceous char. Effectively, composites behave in an opposite manner to many metals. For very concentrated sulfuric acid, including oleum (fuming sulfuric acid) it is common to use steel or cast iron for shipment and containment, but even very dilute sulfuric acid can be extremely corrosive to steel. Hydrochloric Acid Although resins employed with hydrochloric acid are by themselves resistive, HCI is sterically a relatively small molecule which can diffuse into the structural reinforcement by mechanisms which depend in some part on the glass and sizing chemistry. This osmosis can induce a gradual green color to the composite, although this does not necessarily denote a problem or failure. Wicking or blistering is also sometimes observed. While elevated temperature and increased concentration accelerates the attack by HCI, tanks made from premium resins have provided service life of 20 years or more with concentrated (37%) acid at ambient temperature. Muriatic acid and other dilute forms can be handled up to 200° F with no blistering or wicking. The osmosis or diffusion effects can result in localized formation of water soluble salts, which in turn form salt solutions. This creates a concentration gradient, and the salt solutions effectively try to dilute themselves with water diffusing from a salt solution of lower concentration. The diffusing water thus creates osmotic pressure with effects such as blistering. Since osmotic effects are based on concentration differences it is advisable to always use the tank with the same concentration of acid and the tank should not be cleaned unless necessary. The cleaning should never be done with water. If cleaning is necessary, some owners will employ a slightly alkaline salt solution, typically I% caustic and 10% NaCl. Low grades of hydrochloric acid are often produced via a byproduct recovery process and may contain traces of chlorinated hydrocarbons. These high density organic compounds are immiscible and may settle to the bottom of the tank and gradually induce swelling of the composite. For example, this is a common problem with rubber -lined railcars transporting low grade HCI. Purity should thus be carefully evaluated in specifying the equipment.